%VAL() Construct
%REF() Construct
%DESCR() Construct
REAL() and AIMAG() of Complex
CMPLX() of DOUBLE PRECISION
PARAMETER Statements
SELECT CASE on CHARACTER Type
RECURSIVE Keyword
READONLY Keyword
FLUSH Statement
FORMAT Statements
TYPE and ACCEPT I/O Statements
STRUCTURE, UNION, RECORD, MAP
OPEN, CLOSE, and INQUIRE Keywords
ENCODE and DECODE
AUTOMATIC Statement
POSIX Standard
DO Variable
This manual documents how to run, install and port g77, as well as its new features and incompatibilities, and how to report bugs. It corresponds to the GCC-3.3.4 version of g77.
Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
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The precise terms and conditions for copying, distribution and modification follow.
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
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Copyright (C) year name of author
This program is free software; you can redistribute it and/or modify
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Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.
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The hypothetical commands show w and show c should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than show w and show c; they could even be mouse-clicks or menu items—whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
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The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
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If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
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Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
In addition to James Craig Burley, who wrote the front end, many people have helped create and improve GNU Fortran.
libf2c library (combined from the libF77 and
libI77 libraries) provided as part of f2c, available for
free from netlib sites on the Internet.
INTEGER*1, INTEGER*2, and
LOGICAL*1.
This inspired Craig to add further support,
even though the resulting support
would still be incomplete.
(These, in turn, had made their way into the egcs
version of the compiler, and do not exist in gcc
version 2.8 or versions of g77 based on that version
of gcc.)
If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don't be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn't give a basis for comparison.
Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright © 1994 Free Software Foundation, Inc.
Verbatim copying and redistribution of this section is permitted
without royalty; alteration is not permitted.
James Craig Burley (craig@jcb-sc.com), the original author of g77, stopped working on it in September 1999 (He has a web page at http://world.std.com/%7Eburley/.)
GNU Fortran is currently maintained by Toon Moene (toon@moene.indiv.nluug.nl), with the help of countless other volunteers.
As with other GNU software, funding is important because it can pay for needed equipment, personnel, and so on.
The FSF provides information on the best way to fund ongoing development of GNU software (such as GNU Fortran) in documents such as the “GNUS Bulletin”. Email gnu@gnu.org for information on funding the FSF.
Another important way to support work on GNU Fortran is to volunteer to help out.
Email gcc@gcc.gnu.org to volunteer for this work.
However, we strongly expect that there will never be a version 0.6 of g77. Work on this compiler has stopped as of the release of GCC 3.1, except for bug fixing. g77 will be succeeded by g95 - see http://g95.sourceforge.net.
See Funding Free Software, for more information.
If you don't need help getting started reading the portions of this manual that are most important to you, you should skip this portion of the manual.
If you are new to compilers, especially Fortran compilers, or new to how compilers are structured under UNIX and UNIX-like systems, you'll want to see What is GNU Fortran?.
If you are new to GNU compilers, or have used only one GNU compiler in the past and not had to delve into how it lets you manage various versions and configurations of gcc, you should see G77 and GCC.
Everyone except experienced g77 users should see Invoking G77.
If you're acquainted with previous versions of g77, you should see News About GNU Fortran. Further, if you've actually used previous versions of g77, especially if you've written or modified Fortran code to be compiled by previous versions of g77, you should see Changes.
If you intend to write or otherwise compile code that is not already strictly conforming ANSI FORTRAN 77—and this is probably everyone—you should see Language.
If you run into trouble getting Fortran code to compile, link, run, or work properly, you might find answers if you see Debugging and Interfacing, see Collected Fortran Wisdom, and see Trouble. You might also find that the problems you are encountering are bugs in g77—see Bugs, for information on reporting them, after reading the other material.
If you need further help with g77, or with freely redistributable software in general, see Service.
If you would like to help the g77 project, see Funding GNU Fortran, for information on helping financially, and see Projects, for information on helping in other ways.
If you're generally curious about the future of g77, see Projects. If you're curious about its past, see Contributors, and see Funding GNU Fortran.
To see a few of the questions maintainers of g77 have, and that you might be able to answer, see Open Questions.
GNU Fortran, or g77, is designed initially as a free replacement for, or alternative to, the UNIX f77 command. (Similarly, gcc is designed as a replacement for the UNIX cc command.)
g77 also is designed to fit in well with the other fine GNU compilers and tools.
Sometimes these design goals conflict—in such cases, resolution often is made in favor of fitting in well with Project GNU. These cases are usually identified in the appropriate sections of this manual.
As compilers, g77, gcc, and f77 share the following characteristics:
How these actions are performed is generally under the control of the user. Using command-line options, the user can specify how persnickety the compiler is to be regarding the program (whether to diagnose questionable usage of the language), how much time to spend making the generated machine code run faster, and so on.
g77 consists of several components:
libg2c run-time library.
This library contains the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the g77
compilation phase.
libg2c is just the unique name g77 gives
to its version of libf2c to distinguish it from
any copy of libf2c installed from f2c
(or versions of g77 that built libf2c under
that same name)
on the system.
The maintainer of libf2c currently is
dmg@bell-labs.com.
f771.
Note that f771 does not generate machine code directly—it
generates assembly code that is a more readable form
of machine code, leaving the conversion to actual machine code
to an assembler, usually named as.
gcc is often thought of as “the C compiler” only, but it does more than that. Based on command-line options and the names given for files on the command line, gcc determines which actions to perform, including preprocessing, compiling (in a variety of possible languages), assembling, and linking.
For example, the command gcc foo.c drives the file
foo.c through the preprocessor cpp, then
the C compiler (internally named
cc1), then the assembler (usually as), then the linker
(ld), producing an executable program named a.out (on
UNIX systems).
As another example, the command gcc foo.cc would do much the same as
gcc foo.c, but instead of using the C compiler named cc1,
gcc would use the C++ compiler (named cc1plus).
In a GNU Fortran installation, gcc recognizes Fortran source
files by name just like it does C and C++ source files.
It knows to use the Fortran compiler named f771, instead of
cc1 or cc1plus, to compile Fortran files.
Non-Fortran-related operation of gcc is generally unaffected by installing the GNU Fortran version of gcc. However, without the installed version of gcc being the GNU Fortran version, gcc will not be able to compile and link Fortran programs—and since g77 uses gcc to do most of the actual work, neither will g77!
The g77 command is essentially just a front-end for
the gcc command.
Fortran users will normally use g77 instead of gcc,
because g77
knows how to specify the libraries needed to link with Fortran programs
(libg2c and lm).
g77 can still compile and link programs and
source files written in other languages, just like gcc.
The command g77 -v is a quick
way to display lots of version information for the various programs
used to compile a typical preprocessed Fortran source file—this
produces much more output than gcc -v currently does.
(If it produces an error message near the end of the output—diagnostics
from the linker, usually ld—you might
have an out-of-date libf2c that improperly handles
complex arithmetic.)
In the output of this command, the line beginning GNU Fortran Front
End identifies the version number of GNU Fortran; immediately
preceding that line is a line identifying the version of gcc
with which that version of g77 was built.
The libf2c library is distributed with GNU Fortran for
the convenience of its users, but is not part of GNU Fortran.
It contains the procedures
needed by Fortran programs while they are running.
For example, while code generated by g77 is likely to do additions, subtractions, and multiplications in line—in the actual compiled code—it is not likely to do trigonometric functions this way.
Instead, operations like trigonometric
functions are compiled by the f771 compiler
(invoked by g77 when compiling Fortran code) into machine
code that, when run, calls on functions in libg2c, so
libg2c must be linked with almost every useful program
having any component compiled by GNU Fortran.
(As mentioned above, the g77 command takes
care of all this for you.)
The f771 program represents most of what is unique to GNU Fortran.
While much of the libg2c component comes from
the libf2c component of f2c,
a free Fortran-to-C converter distributed by Bellcore (AT&T),
plus libU77, provided by Dave Love,
and the g77 command is just a small front-end to gcc,
f771 is a combination of two rather
large chunks of code.
One chunk is the so-called GNU Back End, or GBE,
which knows how to generate fast code for a wide variety of processors.
The same GBE is used by the C, C++, and Fortran compiler programs cc1,
cc1plus, and f771, plus others.
Often the GBE is referred to as the “gcc back end” or
even just “gcc”—in this manual, the term GBE is used
whenever the distinction is important.
The other chunk of f771 is the
majority of what is unique about GNU Fortran—the code that knows how
to interpret Fortran programs to determine what they are intending to
do, and then communicate that knowledge to the GBE for actual compilation
of those programs.
This chunk is called the Fortran Front End (FFE).
The cc1 and cc1plus programs have their own front ends,
for the C and C++ languages, respectively.
These fronts ends are responsible for diagnosing
incorrect usage of their respective languages by the
programs the process, and are responsible for most of
the warnings about questionable constructs as well.
(The GBE handles producing some warnings, like those
concerning possible references to undefined variables.)
Because so much is shared among the compilers for various languages, much of the behavior and many of the user-selectable options for these compilers are similar. For example, diagnostics (error messages and warnings) are similar in appearance; command-line options like -Wall have generally similar effects; and the quality of generated code (in terms of speed and size) is roughly similar (since that work is done by the shared GBE).
A GNU Fortran installation includes a modified version of the gcc command.
In a non-Fortran installation, gcc recognizes C, C++, and Objective-C source files.
In a GNU Fortran installation, gcc also recognizes Fortran source files and accepts Fortran-specific command-line options, plus some command-line options that are designed to cater to Fortran users but apply to other languages as well.
See Compile C; C++; Objective-C; Ada; Fortran; or Java (Using the GNU Compiler Collection (GCC)), for information on the way different languages are handled by the GNU CC compiler (gcc).
Also provided as part of GNU Fortran is the g77 command. The g77 command is designed to make compiling and linking Fortran programs somewhat easier than when using the gcc command for these tasks. It does this by analyzing the command line somewhat and changing it appropriately before submitting it to the gcc command.
Use the -v option with g77 to see what is going on—the first line of output is the invocation of the gcc command.
The g77 command supports all the options supported by the gcc command. See GCC Command Options (Using the GNU Compiler Collection (GCC)), for information on the non-Fortran-specific aspects of the gcc command (and, therefore, the g77 command).
All gcc and g77 options are accepted both by g77 and by gcc (as well as any other drivers built at the same time, such as g++), since adding g77 to the gcc distribution enables acceptance of g77 options by all of the relevant drivers.
In some cases, options have positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections.
-fversion -fset-g77-defaults -fno-silent
-ff66 -fno-f66 -ff77 -fno-f77 -fno-ugly
-ffree-form -fno-fixed-form -ff90
-fvxt -fdollar-ok -fno-backslash
-fno-ugly-args -fno-ugly-assign -fno-ugly-assumed
-fugly-comma -fugly-complex -fugly-init -fugly-logint
-fonetrip -ftypeless-boz
-fintrin-case-initcap -fintrin-case-upper
-fintrin-case-lower -fintrin-case-any
-fmatch-case-initcap -fmatch-case-upper
-fmatch-case-lower -fmatch-case-any
-fsource-case-upper -fsource-case-lower
-fsource-case-preserve
-fsymbol-case-initcap -fsymbol-case-upper
-fsymbol-case-lower -fsymbol-case-any
-fcase-strict-upper -fcase-strict-lower
-fcase-initcap -fcase-upper -fcase-lower -fcase-preserve
-ff2c-intrinsics-delete -ff2c-intrinsics-hide
-ff2c-intrinsics-disable -ff2c-intrinsics-enable
-fbadu77-intrinsics-delete -fbadu77-intrinsics-hide
-fbadu77-intrinsics-disable -fbadu77-intrinsics-enable
-ff90-intrinsics-delete -ff90-intrinsics-hide
-ff90-intrinsics-disable -ff90-intrinsics-enable
-fgnu-intrinsics-delete -fgnu-intrinsics-hide
-fgnu-intrinsics-disable -fgnu-intrinsics-enable
-fmil-intrinsics-delete -fmil-intrinsics-hide
-fmil-intrinsics-disable -fmil-intrinsics-enable
-funix-intrinsics-delete -funix-intrinsics-hide
-funix-intrinsics-disable -funix-intrinsics-enable
-fvxt-intrinsics-delete -fvxt-intrinsics-hide
-fvxt-intrinsics-disable -fvxt-intrinsics-enable
-ffixed-line-length-n -ffixed-line-length-none
-fsyntax-only -pedantic -pedantic-errors -fpedantic
-w -Wno-globals -Wimplicit -Wunused -Wuninitialized
-Wall -Wsurprising
-Werror -W
-g
-malign-double
-ffloat-store -fforce-mem -fforce-addr -fno-inline
-ffast-math -fstrength-reduce -frerun-cse-after-loop
-funsafe-math-optimizations -ffinite-math-only -fno-trapping-math
-fexpensive-optimizations -fdelayed-branch
-fschedule-insns -fschedule-insn2 -fcaller-saves
-funroll-loops -funroll-all-loops
-fno-move-all-movables -fno-reduce-all-givs
-fno-rerun-loop-opt
-Idir -I-
-fno-automatic -finit-local-zero -fno-f2c
-ff2c-library -fno-underscoring -fno-ident
-fpcc-struct-return -freg-struct-return
-fshort-double -fno-common -fpack-struct
-fzeros -fno-second-underscore
-femulate-complex
-falias-check -fargument-alias
-fargument-noalias -fno-argument-noalias-global
-fno-globals -fflatten-arrays
-fbounds-check -ffortran-bounds-check
Compilation can involve as many as four stages: preprocessing, code generation (often what is really meant by the term “compilation”), assembly, and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of program is contained in the file—that is, the language in which the program is written is generally indicated by the suffix. Suffixes specific to GNU Fortran are listed below. See Options Controlling the Kind of Output (Using the GNU Compiler Collection (GCC)), for information on suffixes recognized by GNU CC.
.f.for.FORSuch source code cannot contain any preprocessor directives, such
as #include, #define, #if, and so on.
You can force .f files to be preprocessed by cpp by using -x f77-cpp-input. See LEX.
.F.fpp.FPPNote that preprocessing is not extended to the contents of
files included by the INCLUDE directive—the #include
preprocessor directive must be used instead.
.rUNIX users typically use the file.f and file.F nomenclature. Users of other operating systems, especially those that cannot distinguish upper-case letters from lower-case letters in their file names, typically use the file.for and file.fpp nomenclature.
Use of the preprocessor cpp allows use of C-like
constructs such as #define and #include, but can
lead to unexpected, even mistaken, results due to Fortran's source file
format.
It is recommended that use of the C preprocessor
be limited to #include and, in
conjunction with #define, only #if and related directives,
thus avoiding in-line macro expansion entirely.
This recommendation applies especially
when using the traditional fixed source form.
With free source form,
fewer unexpected transformations are likely to happen, but use of
constructs such as Hollerith and character constants can nevertheless
present problems, especially when these are continued across multiple
source lines.
These problems result, primarily, from differences between the way
such constants are interpreted by the C preprocessor and by a Fortran
compiler.
Another example of a problem that results from using the C preprocessor is that a Fortran comment line that happens to contain any characters “interesting” to the C preprocessor, such as a backslash at the end of the line, is not recognized by the preprocessor as a comment line, so instead of being passed through “raw”, the line is edited according to the rules for the preprocessor. For example, the backslash at the end of the line is removed, along with the subsequent newline, resulting in the next line being effectively commented out—unfortunate if that line is a non-comment line of important code!
Note: The -traditional and -undef flags are supplied to cpp by default, to help avoid unpleasant surprises. See Options Controlling the Preprocessor (Using the GNU Compiler Collection (GCC)). This means that ANSI C preprocessor features (such as the # operator) aren't available, and only variables in the C reserved namespace (generally, names with a leading underscore) are liable to substitution by C predefines. Thus, if you want to do system-specific tests, use, for example, #ifdef __linux__ rather than #ifdef linux. Use the -v option to see exactly how the preprocessor is invoked.
Unfortunately, the -traditional flag will not avoid an error from anything that cpp sees as an unterminated C comment, such as:
C Some Fortran compilers accept /* as starting
C an inline comment.
See Trailing Comment.
The following options that affect overall processing are recognized by the g77 and gcc commands in a GNU Fortran installation:
-fversionegcs version 1.1,
that internal consistency checks in the f771 program are run.
This option is supplied automatically when -v or --verbose is specified as a command-line option for g77 or gcc and when the resulting commands compile Fortran source files.
In GCC 3.1, this is changed back to the behavior gcc displays for .c files.
-fset-g77-defaultsegcs
version 1.1.
The effect is instead achieved
by the lang_init_options routine
in gcc/gcc/f/com.c.
Set up whatever gcc options are to apply to Fortran compilations, and avoid running internal consistency checks that might take some time.
This option is supplied automatically when compiling Fortran code via the g77 or gcc command. The description of this option is provided so that users seeing it in the output of, say, g77 -v understand why it is there.
Also, developers who run f771 directly might want to specify it
by hand to get the same defaults as they would running f771
via g77 or gcc
However, such developers should, after linking a new f771
executable, invoke it without this option once,
e.g. via ./f771 -quiet < /dev/null,
to ensure that they have not introduced any
internal inconsistencies (such as in the table of
intrinsics) before proceeding—g77 will crash
with a diagnostic if it detects an inconsistency.
-fno-silentstderr) the names of the program units as
they are compiled, in a form similar to that used by popular
UNIX f77 implementations and f2c
See Options Controlling the Kind of Output (Using the GNU Compiler Collection (GCC)), for information on more options that control the overall operation of the gcc command (and, by extension, the g77 command).
The following options serve as “shorthand” for other options accepted by the compiler:
-fuglySpecify that certain “ugly” constructs are to be quietly accepted. Same as:
-fugly-args -fugly-assign -fugly-assumed
-fugly-comma -fugly-complex -fugly-init
-fugly-logint
These constructs are considered inappropriate to use in new or well-maintained portable Fortran code, but widely used in old code. See Distensions, for more information.
-fno-ugly -fno-ugly-args -fno-ugly-assign -fno-ugly-assumed
-fno-ugly-comma -fno-ugly-complex -fno-ugly-init
-fno-ugly-logint
See Distensions, for more information.
-ff66The -fno-f66 option is the inverse of -ff66. As such, it is the same as -fno-onetrip -fno-ugly-assumed.
The meaning of this option is likely to be refined as future versions of g77 provide more compatibility with other existing and obsolete Fortran implementations.
-ff77The meaning of this option is likely to be refined as future versions of g77 provide more compatibility with other existing and obsolete Fortran implementations.
-fno-f77The meaning of this option is likely to be refined as future versions of g77 provide more compatibility with other existing and obsolete Fortran implementations.
The following options control the dialect of Fortran that the compiler accepts:
-ffree-form-fno-fixed-form-ff90This option controls whether certain Fortran 90 constructs are recognized. (Other Fortran 90 constructs might or might not be recognized depending on other options such as -fvxt, -ff90-intrinsics-enable, and the current level of support for Fortran 90.)
See Fortran 90, for more information.
-fvxtThe default is -fno-vxt. -fvxt specifies that the VXT Fortran interpretations for those constructs are to be chosen.
See VXT Fortran, for more information.
-fdollar-ok-fno-backslashFor example, with -fbackslash in effect, A\nB specifies three characters, with the second one being newline. With -fno-backslash, it specifies four characters, A, \, n, and B.
Note that g77 implements a fairly general form of backslash processing that is incompatible with the narrower forms supported by some other compilers. For example, 'A\003B' is a three-character string in g77 whereas other compilers that support backslash might not support the three-octal-digit form, and thus treat that string as longer than three characters.
See Backslash in Constants, for information on why -fbackslash is the default instead of -fno-backslash.
-fno-ugly-argsSee Ugly Implicit Argument Conversion, for more information.
-fugly-assignSee Ugly Assigned Labels, for more information.
-fugly-assumedFor example, DIMENSION X(1) is treated as if it had read DIMENSION X(*).
See Ugly Assumed-Size Arrays, for more information.
-fugly-commaFor example, CALL FOO(,) is treated as CALL FOO(%VAL(0), %VAL(0)). That is, two null arguments are specified by the procedure call when -fugly-comma is in force. And F = FUNC() is treated as F = FUNC(%VAL(0)).
The default behavior, -fno-ugly-comma, is to ignore a single trailing comma in an argument list. So, by default, CALL FOO(X,) is treated exactly the same as CALL FOO(X).
See Ugly Null Arguments, for more information.
-fugly-complexCOMPLEX
type other than COMPLEX(KIND=1)—usually
this is used to permit COMPLEX(KIND=2)
(DOUBLE COMPLEX) operands.
The -ff90 option controls the interpretation of this construct.
See Ugly Complex Part Extraction, for more information.
-fno-ugly-initPARAMETER and DATA statements), and
use of character constants to
initialize numeric types and vice versa.
For example, DATA I/'F'/, CHRVAR/65/, J/4HABCD/ is disallowed by -fno-ugly-init.
See Ugly Conversion of Initializers, for more information.
-fugly-logintINTEGER and LOGICAL variables and
expressions as potential stand-ins for each other.
For example, automatic conversion between INTEGER and
LOGICAL is enabled, for many contexts, via this option.
See Ugly Integer Conversions, for more information.
-fonetripDO loops are to be executed at
least once each time they are reached.
ANSI FORTRAN 77 and more recent versions of the Fortran standard
specify that the body of an iterative DO loop is not executed
if the number of iterations calculated from the parameters of the
loop is less than 1.
(For example, DO 10 I = 1, 0.)
Such a loop is called a zero-trip loop.
Prior to ANSI FORTRAN 77, many compilers implemented DO loops
such that the body of a loop would be executed at least once, even
if the iteration count was zero.
Fortran code written assuming this behavior is said to require
one-trip loops.
For example, some code written to the FORTRAN 66 standard
expects this behavior from its DO loops, although that
standard did not specify this behavior.
The -fonetrip option specifies that the source file(s) being compiled require one-trip loops.
This option affects only those loops specified by the (iterative) DO
statement and by implied-DO lists in I/O statements.
Loops specified by implied-DO lists in DATA and
specification (non-executable) statements are not affected.
-ftypeless-bozINTEGER(KIND=1).
You can test for yourself whether a particular compiler treats
the prefix form as INTEGER(KIND=1) or typeless by running the
following program:
EQUIVALENCE (I, R)
R = Z'ABCD1234'
J = Z'ABCD1234'
IF (J .EQ. I) PRINT *, 'Prefix form is TYPELESS'
IF (J .NE. I) PRINT *, 'Prefix form is INTEGER'
END
Reports indicate that many compilers process this form as
INTEGER(KIND=1), though a few as typeless, and at least one
based on a command-line option specifying some kind of
compatibility.
-fintrin-case-initcap-fintrin-case-upper-fintrin-case-lower-fintrin-case-any-fmatch-case-initcap-fmatch-case-upper-fmatch-case-lower-fmatch-case-any-fsource-case-upper-fsource-case-lower-fsource-case-preserve-fsymbol-case-initcap-fsymbol-case-upper-fsymbol-case-lower-fsymbol-case-any-fcase-strict-upper-fcase-strict-lower-fcase-initcap-fcase-upper-fcase-lower-fcase-preserve-fbadu77-intrinsics-delete-fbadu77-intrinsics-hide-fbadu77-intrinsics-disable-fbadu77-intrinsics-enable-ff2c-intrinsics-delete-ff2c-intrinsics-hide-ff2c-intrinsics-disable-ff2c-intrinsics-enable-ff90-intrinsics-delete-ff90-intrinsics-hide-ff90-intrinsics-disable-ff90-intrinsics-enable-fgnu-intrinsics-delete-fgnu-intrinsics-hide-fgnu-intrinsics-disable-fgnu-intrinsics-enable-fmil-intrinsics-delete-fmil-intrinsics-hide-fmil-intrinsics-disable-fmil-intrinsics-enable-funix-intrinsics-delete-funix-intrinsics-hide-funix-intrinsics-disable-funix-intrinsics-enable-fvxt-intrinsics-delete-fvxt-intrinsics-hide-fvxt-intrinsics-disable-fvxt-intrinsics-enable-ffixed-line-length-nPopular values for n include 72 (the standard and the default), 80 (card image), and 132 (corresponds to “extended-source” options in some popular compilers). n may be none, meaning that the entire line is meaningful and that continued character constants never have implicit spaces appended to them to fill out the line. -ffixed-line-length-0 means the same thing as -ffixed-line-length-none.
See Source Form, for more information.
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there might have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.
These options control the amount and kinds of warnings produced by GNU Fortran:
-fsyntax-only-pedanticValid ANSI FORTRAN 77 programs should compile properly with or without this option. However, without this option, certain GNU extensions and traditional Fortran features are supported as well. With this option, many of them are rejected.
Some users try to use -pedantic to check programs for strict ANSI conformance. They soon find that it does not do quite what they want—it finds some non-ANSI practices, but not all. However, improvements to g77 in this area are welcome.
-pedantic-errors-fpedantic-w-Wno-globalsAlso inhibit warnings about inconsistent invocations and/or definitions of global procedures (function and subroutines). Such inconsistencies include different numbers of arguments and different types of arguments.
-WimplicitIMPLICIT NONE statement
in every program unit.
(Some Fortran compilers provide this feature by an option
named -u or /WARNINGS=DECLARATIONS.)
-Wunused-WuninitializedThese warnings are possible only in optimizing compilation, because they require data-flow information that is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.
These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for arrays, even when they are in registers.
Note that there might be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data-flow analysis before the warnings are printed.
These warnings are made optional because GNU Fortran is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:
SUBROUTINE DISPAT(J)
IF (J.EQ.1) I=1
IF (J.EQ.2) I=4
IF (J.EQ.3) I=5
CALL FOO(I)
END
If the value of J is always 1, 2 or 3, then I is
always initialized, but GNU Fortran doesn't know this. Here is
another common case:
SUBROUTINE MAYBE(FLAG)
LOGICAL FLAG
IF (FLAG) VALUE = 9.4
...
IF (FLAG) PRINT *, VALUE
END
This has no bug because VALUE is used only if it is set.
-WallThe remaining -W... options are not implied by -Wall because they warn about constructions that we consider reasonable to use, on occasion, in clean programs.
-WsurprisingA revealing example is the constant expression 2**-2*1., which g77 evaluates to .25, while others might evaluate it to 0., the difference resulting from the way precedence affects type promotion.
(The -fpedantic option also warns about expressions having two arithmetic operators in a row.)
An example of an expression producing different results
in a surprising way is -I*S, where I holds
the value -2147483648 and S holds 0.5.
On many systems, negating I results in the same
value, not a positive number, because it is already the
lower bound of what an INTEGER(KIND=1) variable can hold.
So, the expression evaluates to a positive number, while
the “expected” interpretation, (-I)*S, would
evaluate to a negative number.
Even cases such as -I*J produce warnings, even though, in most configurations and situations, there is no computational difference between the results of the two interpretations—the purpose of this warning is to warn about differing interpretations and encourage a better style of coding, not to identify only those places where bugs might exist in the user's code.
DO loops with DO variables that are not
of integral type—that is, using REAL
variables as loop control variables.
Although such loops can be written to work in the
“obvious” way, the way g77 is required by the
Fortran standard to interpret such code is likely to
be quite different from the way many programmers expect.
(This is true of all DO loops, but the differences
are pronounced for non-integral loop control variables.)
See Loops, for more information.
-Werror-W“Extra warnings” are issued for:
See Options to Request or Suppress Warnings (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by g77 gcc and other GNU compilers.
Some of these have no effect when compiling programs written in Fortran:
-Wcomment-Wformat-Wparentheses-Wswitch-Wswitch-default-Wswitch-enum-Wtraditional-Wshadow-Wid-clash-len-Wlarger-than-len-Wconversion-Waggregate-return-Wredundant-declsGNU Fortran has various special options that are used for debugging either your program or g77
-gA sample debugging session looks like this (note the use of the breakpoint):
$ cat gdb.f
PROGRAM PROG
DIMENSION A(10)
DATA A /1.,2.,3.,4.,5.,6.,7.,8.,9.,10./
A(5) = 4.
PRINT*,A
END
$ g77 -g -O gdb.f
$ gdb a.out
...
(gdb) break MAIN__
Breakpoint 1 at 0x8048e96: file gdb.f, line 4.
(gdb) run
Starting program: /home/toon/g77-bugs/./a.out
Breakpoint 1, MAIN__ () at gdb.f:4
4 A(5) = 4.
Current language: auto; currently fortran
(gdb) print a(5)
$1 = 5
(gdb) step
5 PRINT*,A
(gdb) print a(5)
$2 = 4
...
One could also add the setting of the breakpoint and the first run command to the file .gdbinit in the current directory, to simplify the debugging session.
See Options for Debugging Your Program or GCC (Using the GNU Compiler Collection (GCC)), for more information on debugging options.
Most Fortran users will want to use no optimization when developing and testing programs, and use -O or -O2 when compiling programs for late-cycle testing and for production use. However, note that certain diagnostics—such as for uninitialized variables—depend on the flow analysis done by -O, i.e. you must use -O or -O2 to get such diagnostics.
The following flags have particular applicability when compiling Fortran programs:
-malign-doubleNoticeably improves performance of g77 programs making
heavy use of REAL(KIND=2) (DOUBLE PRECISION) data
on some systems.
In particular, systems using Pentium, Pentium Pro, 586, and
686 implementations
of the i386 architecture execute programs faster when
REAL(KIND=2) (DOUBLE PRECISION) data are
aligned on 64-bit boundaries
in memory.
This option can, at least, make benchmark results more consistent across various system configurations, versions of the program, and data sets.
Note: The warning in the gcc documentation about this option does not apply, generally speaking, to Fortran code compiled by g77
See Aligned Data, for more information on alignment issues.
Also also note: The negative form of -malign-double is -mno-align-double, not -benign-double.
-ffloat-storeThis option is effective when the floating-point unit is set to work in IEEE 854 `extended precision'—as it typically is on x86 and m68k GNU systems—rather than IEEE 754 double precision. -ffloat-store tries to remove the extra precision by spilling data from floating-point registers into memory and this typically involves a big performance hit. However, it doesn't affect intermediate results, so that it is only partially effective. `Excess precision' is avoided in code like:
a = b + c
d = a * e
but not in code like:
d = (b + c) * e
For another, potentially better, way of controlling the precision, see Floating-point precision.
-fforce-mem-fforce-addr-fno-inline-ffast-math-funsafe-math-optimizations-ffinite-math-onlyThis option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math-fstrength-reduce-frerun-cse-after-loop-fexpensive-optimizations-fdelayed-branch-fschedule-insns-fschedule-insns2-fcaller-saves-funroll-loopsDO loops by
unrolling them and is probably generally appropriate for Fortran, though
it is not turned on at any optimization level.
Note that outer loop unrolling isn't done specifically; decisions about
whether to unroll a loop are made on the basis of its instruction count.
Also, no `loop discovery'1 is done, so only loops written with DO
benefit from loop optimizations, including—but not limited
to—unrolling. Loops written with IF and GOTO are not
currently recognized as such. This option unrolls only iterative
DO loops, not DO WHILE loops.
-funroll-all-loopsDO WHILE loops by
unrolling them in addition to iterative DO loops. In the absence
of DO WHILE, this option is equivalent to -funroll-loops
but possibly slower.
-fno-move-all-movables-fno-reduce-all-givs-fno-rerun-loop-opt-fmove-all-movables and -freduce-all-givs will enable loop optimization to move all loop-invariant index computations in nested loops over multi-rank array dummy arguments out of these loops.
-frerun-loop-opt will move offset calculations resulting from the fact that Fortran arrays by default have a lower bound of 1 out of the loops.
These three options are intended to be removed someday, once loop optimization is sufficiently advanced to perform all those transformations without help from these options.
See Options That Control Optimization (Using the GNU Compiler Collection (GCC)), for more information on options to optimize the generated machine code.
These options control the C preprocessor, which is run on each C source file before actual compilation.
See Options Controlling the Preprocessor (Using the GNU Compiler Collection (GCC)), for information on C preprocessor options.
Some of these options also affect how g77 processes the
INCLUDE directive.
Since this directive is processed even when preprocessing
is not requested, it is not described in this section.
See Options for Directory Search, for
information on how g77 processes the INCLUDE directive.
However, the INCLUDE directive does not apply
preprocessing to the contents of the included file itself.
Therefore, any file that contains preprocessor directives
(such as #include, #define, and #if)
must be included via the #include directive, not
via the INCLUDE directive.
Therefore, any file containing preprocessor directives,
if included, is necessarily included by a file that itself
contains preprocessor directives.
These options affect how the cpp preprocessor searches
for files specified via the #include directive.
Therefore, when compiling Fortran programs, they are meaningful
when the preprocessor is used.
Some of these options also affect how g77 searches
for files specified via the INCLUDE directive,
although files included by that directive are not,
themselves, preprocessed.
These options are:
-I--IdirINCLUDE directive
(as well as of the #include directive of the cpp
preprocessor).
Note that -Idir must be specified without any
spaces between -I and the directory name—that is,
-Ifoo/bar is valid, but -I foo/bar
is rejected by the g77 compiler (though the preprocessor supports
the latter form).
Also note that the general behavior of -I and
INCLUDE is pretty much the same as of -I with
#include in the cpp preprocessor, with regard to
looking for header.gcc files and other such things.
See Options for Directory Search (Using the GNU Compiler Collection (GCC)), for information on the -I option.
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing no- or adding it.
-fno-automaticSAVE statement was specified
for every local variable and array referenced in it.
Does not affect common blocks.
(Some Fortran compilers provide this option under
the name -static.)
-finit-local-zeroSince there is a run-time penalty for initialization of variables
that are not given the SAVE attribute, it might be a
good idea to also use -fno-automatic with -finit-local-zero.
-fno-f2cThe f2c calling conventions require functions that return
type REAL(KIND=1) to actually return the C type double,
and functions that return type COMPLEX to return the
values via an extra argument in the calling sequence that points
to where to store the return value.
Under the GNU calling conventions, such functions simply return
their results as they would in GNU C—REAL(KIND=1) functions
return the C type float, and COMPLEX functions
return the GNU C type complex (or its struct
equivalent).
This does not affect the generation of code that interfaces with the
libg2c library.
However, because the libg2c library uses f2c
calling conventions, g77 rejects attempts to pass
intrinsics implemented by routines in this library as actual
arguments when -fno-f2c is used, to avoid bugs when
they are actually called by code expecting the GNU calling
conventions to work.
For example, INTRINSIC ABS;CALL FOO(ABS) is rejected when -fno-f2c is in force. (Future versions of the g77 run-time library might offer routines that provide GNU-callable versions of the routines that implement the f2c intrinsics that may be passed as actual arguments, so that valid programs need not be rejected when -fno-f2c is used.)
Caution: If -fno-f2c is used when compiling any source file used in a program, it must be used when compiling all Fortran source files used in that program.
-ff2c-librarylibg2c (or the original libf2c)
is required.
This is the default for the current version of g77
Currently it is not
valid to specify -fno-f2c-library.
This option is provided so users can specify it in shell
scripts that build programs and libraries that require the
libf2c library, even when being compiled by future
versions of g77 that might otherwise default to
generating code for an incompatible library.
-fno-underscoringWith -funderscoring in effect, g77 appends two underscores to names with underscores and one underscore to external names with no underscores. (g77 also appends two underscores to internal names with underscores to avoid naming collisions with external names. The -fno-second-underscore option disables appending of the second underscore in all cases.)
This is done to ensure compatibility with code produced by many UNIX Fortran compilers, including f2c which perform the same transformations.
Use of -fno-underscoring is not recommended unless you are experimenting with issues such as integration of (GNU) Fortran into existing system environments (vis-a-vis existing libraries, tools, and so on).
For example, with -funderscoring, and assuming other defaults like -fcase-lower and that j() and max_count() are external functions while my_var and lvar are local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of user-defined names while debugging and when interfacing g77 code with other languages.
Note that just because the names match does not mean that the interface implemented by g77 for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by g77 to link to code produced by some other compiler using this or any other method can be only a small part of the overall solution—getting the code generated by both compilers to agree on issues other than naming can require significant effort, and, unlike naming disagreements, linkers normally cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended underscores introduces the very real possibility that a user-defined external name will conflict with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases—they might occur at program run time, and show up only as buggy behavior at run time.
In future versions of g77 we hope to improve naming and linking issues so that debugging always involves using the names as they appear in the source, even if the names as seen by the linker are mangled to prevent accidental linking between procedures with incompatible interfaces.
-fno-second-underscoreThis option has no effect if -fno-underscoring is in effect.
Otherwise, with this option, an external name such as MAX_COUNT is implemented as a reference to the link-time external symbol max_count_, instead of max_count__.
-fno-ident-fzerosAs of version 0.5.18, g77 normally treats DATA and
other statements that are used to specify initial values of zero
for variables and arrays as if no values were actually specified,
in the sense that no diagnostics regarding multiple initializations
are produced.
This is done to speed up compiling of programs that initialize large arrays to zeros.
Use -fzeros to revert to the simpler, slower behavior that can catch multiple initializations by keeping track of all initializations, zero or otherwise.
Caution: Future versions of g77 might disregard this option (and its negative form, the default) or interpret it somewhat differently. The interpretation changes will affect only non-standard programs; standard-conforming programs should not be affected.
-femulate-complexCOMPLEX arithmetic via emulation,
instead of using the facilities of
the gcc back end that provide direct support of
complex arithmetic.
(gcc had some bugs in its back-end support
for complex arithmetic, due primarily to the support not being
completed as of version 2.8.1 and egcs 1.1.2.)
Use -femulate-complex if you suspect code-generation bugs,
or experience compiler crashes,
that might result from g77 using the COMPLEX support
in the gcc back end.
If using that option fixes the bugs or crashes you are seeing,
that indicates a likely g77 bugs
(though, all compiler crashes are considered bugs),
so, please report it.
(Note that the known bugs, now believed fixed, produced compiler crashes
rather than causing the generation of incorrect code.)
Use of this option should not affect how Fortran code compiled by g77 works in terms of its interfaces to other code, e.g. that compiled by f2c
As of GCC version 3.0, this option is not necessary anymore.
Caution: Future versions of g77 might ignore both forms of this option.
-falias-check-fargument-alias-fargument-noalias-fno-argument-noalias-globalThese options specify to what degree aliasing
(overlap)
is permitted between
arguments (passed as pointers) and COMMON (external, or
public) storage.
The default for Fortran code, as mandated by the FORTRAN 77 and Fortran 90 standards, is -fargument-noalias-global. The default for code written in the C language family is -fargument-alias.
Note that, on some systems, compiling with -fforce-addr in effect can produce more optimal code when the default aliasing options are in effect (and when optimization is enabled).
See Aliasing Assumed To Work, for detailed information on the implications of compiling Fortran code that depends on the ability to alias dummy arguments.
-fno-globalsFurther, this option disables such inlining, to avoid compiler crashes resulting from incorrect code that would otherwise be diagnosed.
As such, this option might be quite useful when compiling existing, “working” code that happens to have a few bugs that do not generally show themselves, but which g77 diagnoses.
Use of this option therefore has the effect of instructing g77 to behave more like it did up through version 0.5.19.1, when it paid little or no attention to disagreements between program units about a procedure's type and argument information, and when it performed no inlining of procedures (except statement functions).
Without this option, g77 defaults to performing the potentially inlining procedures as it started doing in version 0.5.20, but as of version 0.5.21, it also diagnoses disagreements that might cause such inlining to crash the compiler as (fatal) errors, and warns about similar disagreements that are currently believed to not likely to result in the compiler later crashing or producing incorrect code.
-fflatten-arraysARRAY_REF construct
to handle all array references.
Note: This option is not supported. It is intended for use only by g77 developers, to evaluate code-generation issues. It might be removed at any time.
-fbounds-check-ffortran-bounds-checkThe current implementation uses the libf2c
library routine s_rnge to print the diagnostic.
However, whereas f2c generates a single check per reference for a multi-dimensional array, of the computed offset against the valid offset range (0 through the size of the array), g77 generates a single check per subscript expression. This catches some cases of potential bugs that f2c does not, such as references to below the beginning of an assumed-size array.
g77 also generates checks for CHARACTER substring references,
something f2c currently does not do.
Use the new -ffortran-bounds-check option to specify bounds-checking for only the Fortran code you are compiling, not necessarily for code written in other languages.
Note: To provide more detailed information on the offending subscript,
g77 provides the libg2c run-time library routine s_rnge
with somewhat differently-formatted information.
Here's a sample diagnostic:
Subscript out of range on file line 4, procedure rnge.f/bf.
Attempt to access the -6-th element of variable b[subscript-2-of-2].
Aborted
The above message indicates that the offending source line is line 4 of the file rnge.f, within the program unit (or statement function) named bf. The offended array is named b. The offended array dimension is the second for a two-dimensional array, and the offending, computed subscript expression was -6.
For a CHARACTER substring reference, the second line has
this appearance:
Attempt to access the 11-th element of variable a[start-substring].
This indicates that the offended CHARACTER variable or array
is named a,
the offended substring position is the starting (leftmost) position,
and the offending substring expression is 11.
(Though the verbage of s_rnge is not ideal
for the purpose of the g77 compiler,
the above information should provide adequate diagnostic abilities
to it users.)
See Options for Code Generation Conventions (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by g77 gcc and other GNU compilers.
Some of these do not work when compiling programs written in Fortran:
-fpcc-struct-return-freg-struct-returnlibg2c with which
you will be linking all code compiled by g77 with the
same option.
-fshort-double-fno-common-fpack-structlibg2c library,
at the very least, even if it is built with the same option.
GNU Fortran currently does not make use of any environment variables to control its operation above and beyond those that affect the operation of gcc.
See Environment Variables Affecting GCC (Using the GNU Compiler Collection (GCC)), for information on environment variables.
Changes made to recent versions of GNU Fortran are listed below, with the most recent version first.
The changes are generally listed in order:
This order is not strict—for example, some items involve a combination of these elements.
Note that two variants of g77 are tracked below.
The egcs variant is described vis-a-vis
previous versions of egcs and/or
an official FSF version, as appropriate.
Note that all such variants are obsolete as of July 1999 -
the information is retained here only for its historical value.
Therefore, egcs versions sometimes have multiple listings
to help clarify how they differ from other versions,
though this can make getting a complete picture
of what a particular egcs version contains
somewhat more difficult.
For information on bugs in the GCC-3.3.4 version of g77, see Known Bugs In GNU Fortran.
An online, “live” version of this document (derived directly from the mainline, development version of g77 within gcc) is available at http://gcc.gnu.org/onlinedocs/g77/News.html.
The following information was last updated on 2003-05-18:
GCC 3.3 versus GCC 3.2:183239246286636764916742711372367278738473888587903892631019710726GCC 3.2 versus GCC 3.1:768183089258GCC 3.1 (formerly known as g77-0.5.27) versus GCC 3.0:94737433807395742794730475248855122539754735837610661386304 PROGRAM PROG
DIMENSION A(140 000 000)
END
with the message:
prog.f: In program `prog':
prog.f:2:
DIMENSION A(140 000 000)
^
Array `a' at (^) is too large to handle
because 140 000 000 REALs is larger than the largest bit-extent that can be
expressed in 32 bits. However, bit-sizes never play a role after offsets
have been converted to byte addresses. Therefore this check has been removed,
and the limit is now 2 Gbyte of memory (around 530 000 000 REALs).
Note: On GNU/Linux systems one has to compile and link programs that occupy
more than 1 Gbyte statically, i.e. g77 -static ....
SUBROUTINE SUB(A, N)
DIMENSION N(2)
DIMENSION A(N(1),N(2))
A(1,1) = 1.
END
Note the use of array elements in the bounds of the adjustable array A.
string(1:0).
libf2c library is now able to read and write files larger than
2 Gbyte on 32-bit target machines, if the operating system supports this.
GCC 3.0 versus GCC 2.95:ftruncate OS function. Thanks go to the GAMESS developers
for bringing this to our attention.
libf2c as of 2000-12-05.
This fixes a bug where a namelist containing initialization of LOGICAL items and a variable starting with T or F would be read incorrectly.
TtyNam intrinsics now set Name to all spaces (at run time)
if the system has no ttyname implementation available.
libf2c as of 1999-06-28.
This fixes a bug whereby
input to a NAMELIST read involving a repeat count,
such as K(5)=10*3,
was not properly handled by libf2c.
The first item was written to K(5),
but the remaining nine were written elsewhere (still within the array),
not necessarily starting at K(6).
GCC 2.95 (EGCS 1.2) versus EGCS 1.1.2:REAL or COMPLEX constant expressions
to type INTEGER(KIND=2)
(often referred to as INTEGER*8).
For example, INTEGER*8 J; J = 4E10 now works as documented.
INTEGER(KIND=2)
(usually INTEGER*8)
subscript expressions when evaluating array references
on systems with pointers widers than INTEGER(KIND=1)
(such as Alphas).
COMPLEX variable or array
that partially overlaps one or more of the sources
of the same assignment
(a very rare construction).
It now assigns through a temporary,
in cases where such partial overlap is deemed possible.
libg2c (libf2c) no longer loses track
of the file being worked on
during a BACKSPACE operation.
libg2c (libf2c) fixes a bug whereby
input to a NAMELIST read involving a repeat count,
such as K(5)=10*3,
was not properly handled by libf2c.
The first item was written to K(5),
but the remaining nine were written elsewhere (still within the array),
not necessarily starting at K(6).
Date intrinsic now returns the correct result
on big-endian systems.
INTEGER values,
such as IOSTAT=j,
where j is other than default INTEGER
(such as INTEGER*2).
Instead, it issues a diagnostic.
INTEGER, such as INTEGER*2,
instead of producing a spurious diagnostic.
Also fix DATA (A(I),I=1,N),
where N is not default INTEGER
to work instead of crashing g77.
libg2c now supports building as multilibbed library,
which provides better support for systems
that require options such as -mieee
to work properly.
CTime, DTime, ETime, and TtyNam
intrinsics has been swapped.
The argument serving as the returned value
for the corresponding function forms
now is the second argument,
making these consistent with the other subroutine forms
of libU77 intrinsics.
libg2c has been changed to increase the likelihood
of catching references to the implementations of these intrinsics
using the EXTERNAL mechanism
(which would avoid the new warnings).
See Year 2000 (Y2K) Problems, for more information.
COMPLEX data type.
EQUIVALENCE areas
and not SAVE'd.
COMPLEX operands
instead of generating a run-time call to
the libf2c routines c_div or z_div,
unless the -Os option is specified.
errno,
a C-language concept,
when performing operations such as the SqRt intrinsic.
libf2c as of 1999-05-10.
There is no g77 version 0.5.24 at this time, or planned. 0.5.24 is the version number designated for bug fixes and, perhaps, some new features added, to 0.5.23. Version 0.5.23 requires gcc 2.8.1, as 0.5.24 was planned to require.
Due to EGCS becoming GCC
(which is now an acronym for “GNU Compiler Collection”),
and EGCS 1.2 becoming officially designated GCC 2.95,
there seems to be no need for an actual 0.5.24 release.
To reduce the confusion already resulting from use of 0.5.24
to designate g77 versions within EGCS versions 1.0 and 1.1,
as well as in versions of g77 documentation and notices
during that period,
“mainline” g77 version numbering resumes
at 0.5.25 with GCC 2.95 (EGCS 1.2),
skipping over 0.5.24 as a placeholder version number.
To repeat, there is no g77 0.5.24, but there is now a 0.5.25. Please remain calm and return to your keypunch units.
EGCS 1.1.2 versus EGCS 1.1.1:IDate intrinsic (VXT) (in libg2c)
so the returned year is in the documented, non-Y2K-compliant range
of 0-99,
instead of being returned as 100 in the year 2000.
See IDate Intrinsic (VXT), for more information.
Date_and_Time intrinsic (in libg2c)
to return the milliseconds value properly
in Values(8).
LStat intrinsic (in libg2c)
to return device-ID information properly
in SArray(7).
EGCS 1.1.1 versus EGCS 1.1:libg2c so it performs an implicit ENDFILE operation
(as appropriate)
whenever a REWIND is done.
(This bug was introduced in 0.5.23 and egcs 1.1 in
g77's version of libf2c.)
libg2c so it no longer crashes with a spurious diagnostic
upon doing any I/O following a direct formatted write.
(This bug was introduced in 0.5.23 and egcs 1.1 in
g77's version of libf2c.)
Rand intrinsic on some systems.
if constructs
for the completion code to be set properly).
EGCS 1.1 versus EGCS 1.0.3:libU77 intrinsic HostNm
that wrote one byte beyond the end of its CHARACTER
argument,
and in the libU77 intrinsics
GMTime and LTime
that overwrote their arguments.
This bug is not known to have existed in any
recent version of gcc.
It was introduced in an early release of egcs.
EXTERNAL,
passing that external as a dummy argument
without explicitly giving it a type,
and, in a subsequent program unit,
referencing that external as
an external function with a different type
no longer crash g77.
CASE DEFAULT no longer crashes g77.
ENTRY statements.
INTEGER expression.
ENTRY can be stepped through, line by line,
in gdb.
REAL argument to intrinsics
Second and CPU_Time.
tempnam, if available, to open scratch files
(as in OPEN(STATUS='SCRATCH'))
so that the TMPDIR environment variable,
if present, is used.
libf2c separates out
the setting of global state
(such as command-line arguments and signal handling)
from main.o into distinct, new library
archive members.
This should make it easier to write portable applications
that have their own (non-Fortran) main() routine
properly set up the libf2c environment, even
when libf2c (now libg2c) is a shared library.
libf2c library from netlib they
wish to use on a case-by-case basis.
See the installation documentation for more information.
netlib they
wish to use on a case-by-case basis.
See the installation documentation for more information.
libg2c.a instead of libf2c.a,
to ensure that a version other than the one built and
installed as part of the same g77 version is picked up.
install-info now used to update the directory of
Info documentation to contain an entry for g77
(during installation).
OPEN, INQUIRE, READ, and
WRITE statements,
and about truncations of various sorts of constants.
FORMAT expressions so that
a null byte is appended to the last operand if it
is a constant.
This provides a cleaner run-time diagnostic as provided
by libf2c for statements like PRINT '(I1', 42.
libf2c as of 1998-06-18
should fix a variety of problems, including
those involving some uses of the T format
specifier, and perhaps some build (porting) problems
as well.
EGCS 1.1 versus g77 0.5.23:DNRM2 routine.
The x87 coprocessor stack was being
mismanaged in cases involving assigned GOTO
and ASSIGN.
EQUIVALENCE and COMMON
aggregates that, due to “unnatural” ordering of members
vis-a-vis their types, require initial padding.
Previously, g77 treated these expressions as denoting special “pointer” arguments for the purposes of filewide analysis.
COMPLEX arithmetic
(especially multiplication).
Generally, this affects only local variables and arrays
having the SAVE attribute
or given initial values via DATA.
libf2c (libg2c).
This new information allows, for example,
which __g77_length_a to be used in gdb
to determine the type of the phantom length argument
supplied with CHARACTER variables.
This information pertains to internally-generated
type, variable, and other information,
not to the longstanding deficiencies vis-a-vis
COMMON and EQUIVALENCE.
Date_and_Time intrinsic now is
supported.
System_Clock intrinsic allows
the optional arguments (except for the Count
argument) to be omitted.
libf2c as of 1998-06-18.
Features that have been dropped from this version of g77 due to their being implemented via g77-specific patches to the gcc back end in previous releases include:
__restrict__ keyword,
the options -fargument-alias, -fargument-noalias,
and -fargument-noalias-global,
and the corresponding alias-analysis code.
(egcs has the alias-analysis
code, but not the __restrict__ keyword.
egcs g77 users benefit from the alias-analysis
code despite the lack of the __restrict__ keyword,
which is a C-language construct.)
(egcs supports these options.
g77 users of egcs benefit from them even if
they are not explicitly specified,
because the defaults are optimized for g77 users.)
Note that the gcc/f/gbe/ subdirectory has been removed from this distribution as a result of g77 no longer including patches for the gcc back end.
libU77 intrinsic HostNm
that wrote one byte beyond the end of its CHARACTER
argument,
and in the libU77 intrinsics
GMTime and LTime
that overwrote their arguments.
CASE DEFAULT no longer crashes g77.
EXTERNAL,
passing that external as a dummy argument
without explicitly giving it a type,
and, in a subsequent program unit,
referencing that external as
an external function with a different type
no longer crash g77.
libf2c library from netlib they
wish to use on a case-by-case basis.
See the installation documentation for more information.
netlib they
wish to use on a case-by-case basis.
See the installation documentation for more information.
libg2c.a instead of libf2c.a,
to ensure that a version other than the one built and
installed as part of the same g77 version is picked up.
ENTRY statements.
libf2c separates out
the setting of global state
(such as command-line arguments and signal handling)
from main.o into distinct, new library
archive members.
This should make it easier to write portable applications
that have their own (non-Fortran) main() routine
properly set up the libf2c environment, even
when libf2c (now libg2c) is a shared library.
install-info now used to update the directory of
Info documentation to contain an entry for g77
(during installation).
OPEN, INQUIRE, READ, and
WRITE statements,
and about truncations of various sorts of constants.
libf2c as of 1998-04-20.
This should fix a variety of problems, including
those involving some uses of the T format
specifier, and perhaps some build (porting) problems
as well.
DO loops that
have one or more references to the iteration variable,
or to aliases of it, in their control expressions.
For example, DO 10 J=2,J now is compiled correctly.
DNRM2 routine.
The x87 coprocessor stack was being
mismanaged in cases involving assigned GOTO
and ASSIGN.
DTime intrinsic so as not to truncate
results to integer values (on some systems).
Signal intrinsic so it offers portable
support for 64-bit systems (such as Digital Alphas
running GNU/Linux).
NAMELIST on 64-bit
machines such as Alphas.
libf2c so it no longer
produces a spurious I/O recursion diagnostic at run time
when an I/O operation (such as READ *,I) is interrupted
in a manner that causes the program to be terminated
via the f_exit routine (such as via C-c).
CASE statement with
an omitted lower or upper bound.
CPU_Time
intrinsic.
COMPLEX arithmetic
(especially multiplication).
DOUBLE COMPLEX.
INTEGER expression.
ENTRY can be stepped through, line by line,
in gdb.
REAL argument to intrinsics
Second and CPU_Time.
Int2 and Int8.
tempnam, if available, to open scratch files
(as in OPEN(STATUS='SCRATCH'))
so that the TMPDIR environment variable,
if present, is used.
restrict to
__restrict__, to avoid rejecting valid, existing,
C programs.
Support for restrict is now more like support
for complex.
libf2c
so it is more likely that the printing of the
active format string is limited to the string,
with no trailing garbage being printed.
(Unlike f2c, g77 did not append
a null byte to its compiled form of every
format string specified via a FORMAT statement.
However, f2c would exhibit the problem
anyway for a statement like PRINT '(I)garbage', 1
by printing (I)garbage as the format string.)
FORMAT expressions so that
a null byte is appended to the last operand if it
is a constant.
This provides a cleaner run-time diagnostic as provided
by libf2c for statements like PRINT '(I1', 42.
libf2c.
libf2c as of 1997-09-23.
This fixes a formatted-I/O bug that afflicted
64-bit systems with 32-bit integers
(such as Digital Alpha running GNU/Linux).
EGCS 1.0.2 versus EGCS 1.0.1:CASE statement with
an omitted lower or upper bound.
DOUBLE COMPLEX.
EGCS 1.0.1 versus EGCS 1.0:NAMELIST on 64-bit
machines such as Alphas.
EGCS 1.0 versus g77 0.5.21:egcs
contains several regressions against
version 0.5.21 of g77,
due to using the
“vanilla” gcc back end instead of patching
it to fix a few bugs and improve performance in a
few cases.
Features that have been dropped from this version of g77 due to their being implemented via g77-specific patches to the gcc back end in previous releases include:
restrict keyword.
Note that the gcc/f/gbe/ subdirectory has been removed
from this distribution as a result of g77
being fully integrated with
the egcs variant of the gcc back end.
DO loops that
have one or more references to the iteration variable,
or to aliases of it, in their control expressions.
For example, DO 10 J=2,J now is compiled correctly.
DTime intrinsic so as not to truncate
results to integer values (on some systems).
egcs versions of gcc.
Int2 and Int8.
libf2c
so it is more likely that the printing of the
active format string is limited to the string,
with no trailing garbage being printed.
(Unlike f2c, g77 did not append
a null byte to its compiled form of every
format string specified via a FORMAT statement.
However, f2c would exhibit the problem
anyway for a statement like PRINT '(I)garbage', 1
by printing (I)garbage as the format string.)
libf2c as of 1997-09-23.
This fixes a formatted-I/O bug that afflicted
64-bit systems with 32-bit integers
(such as Digital Alpha running GNU/Linux).
EQUIVALENCE with a
DATA statement that follows
the first executable statement (or is
treated as an executable-context statement
as a result of using the -fpedantic
option).
DATA
or similar to initialize a COMPLEX variable or
array to zero.
AND, OR,
or XOR intrinsics.
COMMON
or EQUIVALENCE variable
as the target of an ASSIGN
or assigned-GOTO statement.
FTell or
FPutC) as such and as the name of a procedure
or common block.
Such dual use of a name in a program is allowed by
the standard.
SAVE or the -fno-automatic option
is in effect.
This avoids a compiler crash in some cases.
DOUBLE PRECISION optimally on Pentium and
Pentium Pro architectures (586 and 686 in gcc).
The default is to issue such warnings, which are new as of this version of g77.
The default is to issue such diagnostics and flag the compilation as unsuccessful. With this option, the diagnostics are issued as warnings, or, if -Wno-globals is specified, are not issued at all.
This option also disables inlining of global procedures, to avoid compiler crashes resulting from coding errors that these diagnostics normally would identify.
DATA statement,
and the second specification was an implied-DO list.
COMPLEX
arithmetic (especially multiplication) don't appear to
take forever to compile.
gcc-2.7.2.3.tar.gz
distribution.)
libU77 routines that accept file and other names
to strip trailing blanks from them, for consistency
with other implementations.
Blanks may be forcibly appended to such names by
appending a single null character (CHAR(0))
to the significant trailing blanks.
CHMOD intrinsic to work with file names
that have embedded blanks, commas, and so on.
SIGNAL intrinsic so it accepts an
optional third Status argument.
IDATE() intrinsic subroutine (VXT form)
so it accepts arguments in the correct order.
Documentation fixed accordingly, and for
GMTIME() and LTIME() as well.
libU77 intrinsics to
support existing code more directly.
Such changes include allowing both subroutine and
function forms of many routines, changing MCLOCK()
and TIME() to return INTEGER(KIND=1) values,
introducing MCLOCK8() and TIME8() to
return INTEGER(KIND=2) values,
and placing functions that are intended to perform
side effects in a new intrinsic group, badu77.
libU77 so it is more portable.
restrict keyword in gcc
front end.
INT2 and INT8 intrinsics.
CPU_TIME intrinsic.
ALARM intrinsic.
CTIME intrinsic now accepts any INTEGER
argument, not just INTEGER(KIND=2).
libf2c build procedure to re-archive library
if previous attempt to archive was interrupted.
libf2c as of 1997-08-16.
libf2c to consistently and clearly diagnose
recursive I/O (at run time).
stderr instead of stdout.
libf2c that come
from netlib.bell-labs.com; give any such files
that aren't quite accurate in g77's version of
libf2c the suffix .netlib.
INTEGER(KIND=0) for future use.
This option specifies that non-decimal-radix
constants using the prefixed-radix form (such as Z'1234')
are to be interpreted as INTEGER(KIND=1) constants.
Specify -ftypeless-boz to cause such
constants to be interpreted as typeless.
(Version 0.5.19 introduced -fno-typeless-boz and its inverse.)
See Options Controlling Fortran Dialect, for information on the -ftypeless-boz option.
Some programs might use names that clash with
intrinsic names defined (and now enabled) by these
options or by the new libU77 intrinsics.
Users of such programs might need to compile them
differently (using, for example, -ff90-intrinsics-disable)
or, better yet, insert appropriate EXTERNAL
statements specifying that these names are not intended
to be names of intrinsics.
ALWAYS_FLUSH macro is no longer defined when
building libf2c, which should result in improved
I/O performance, especially over NFS.
Note: If you have code that depends on the behavior
of libf2c when built with ALWAYS_FLUSH defined,
you will have to modify libf2c accordingly before
building it from this and future versions of g77.
See Output Assumed To Flush, for more information.
libU77 has been
added to the version of libf2c distributed with
and built as part of g77.
g77 now knows about the routines in this library
as intrinsics.
See VXT Fortran, for more information on the constructs recognized when the -fvxt option is specified.
If you used one of these deleted options, you should re-read the pertinent documentation to determine which options, if any, are appropriate for compiling your code with this version of g77.
See Other Dialects, for more information.
(Enabling all the -fugly-* options is unlikely to be feasible, or sensible, in the future, so users should learn to specify only those -fugly-* options they really need for a particular source file.)
See Ugly Assumed-Size Arrays, for more information.
LOC()
intrinsic and %LOC() construct now return
values of INTEGER(KIND=0) type,
as defined by the GNU Fortran language.
This type is wide enough (holds the same number of bits) as the character-pointer type on the machine.
On most machines, this won't make a difference,
whereas, on Alphas and other systems with 64-bit pointers,
the INTEGER(KIND=0) type is equivalent to INTEGER(KIND=2)
(often referred to as INTEGER*8)
instead of the more common INTEGER(KIND=1)
(often referred to as INTEGER*4).
COMPLEX arithmetic in the g77 front
end, to avoid bugs in complex support in the
gcc back end.
New option -fno-emulate-complex
causes g77 to revert the 0.5.19 behavior.
COMMON areas when any of
these are defined (assigned to) by Fortran code.
This can result in faster and/or smaller programs when compiling with optimization enabled, though on some systems this effect is observed only when -fforce-addr also is specified.
New options -falias-check, -fargument-alias, -fargument-noalias, and -fno-argument-noalias-global control the way g77 handles potential aliasing.
See Aliasing Assumed To Work, for detailed information on why the new defaults might result in some programs no longer working the way they did when compiled by previous versions of g77.
CONJG() and DCONJG() intrinsics now
are compiled in-line.
libf2c has no aliasing problems in
its implementations of the COMPLEX (and
DOUBLE COMPLEX) intrinsics.
The libf2c has been changed to have no such
problems.
As a result, 0.5.20 is expected to offer improved performance over 0.5.19.1, perhaps as good as 0.5.19 in most or all cases, due to this change alone.
Note: This change requires version 0.5.20 of
libf2c, at least, when linking code produced
by any versions of g77 other than 0.5.19.1.
Use g77 -v to determine the version numbers
of the libF77, libI77, and libU77
components of the libf2c library.
(If these version numbers are not printed—in
particular, if the linker complains about unresolved
references to names like g77__fvers__—that
strongly suggests your installation has an obsolete
version of libf2c.)
See Ugly Assigned Labels, for more information.
FORMAT and ENTRY statements now are allowed to
precede IMPLICIT NONE statements.
SELECT CASE on
CHARACTER type, instead of crashing, at compile time.
libf2c archive
(libf2c.a) so that members are added to it
only when truly necessary, so the user that installs
an already-built g77 doesn't need to have write
access to the build tree (whereas the user doing the
build might not have access to install new software
on the system).
libf2c as of 1997-02-08, and
fix up some of the build procedures.
gcc/).
INTEGER(KIND=2)
(often referred to as INTEGER*8)
available in
libf2c and f2c.h so that f2c users
may make full use of its features via the g77
version of f2c.h and the INTEGER(KIND=2)
support routines in the g77 version of libf2c.
libf2c so that g77 -v
yields version information on the library.
SNGL and FLOAT intrinsics now are
specific intrinsics, instead of synonyms for the
generic intrinsic REAL.
REALPART, IMAGPART,
COMPLEX,
LONG, and SHORT.
gnu, has been added
to contain the new REALPART, IMAGPART,
and COMPLEX intrinsics.
An old group, dcp, has been removed.
DOUBLE COMPLEX (or any
complex type other than COMPLEX), unless
-ff90 option specifies Fortran 90 interpretation
or new -fugly-complex option, in conjunction with
-fnot-f90, specifies f2c interpretation.
(Hence the menu item M for the node
Diagnostics in the top-level menu of
the Info documentation.)
Information on previous versions is archived
in gcc/gcc/f/news.texi
following the test of the DOC-OLDNEWS macro.
This chapter describes changes to g77 that are visible to the programmers who actually write and maintain Fortran code they compile with g77. Information on changes to installation procedures, changes to the documentation, and bug fixes is not provided here, unless it is likely to affect how users use g77. See News About GNU Fortran, for information on such changes to g77.
Note that two variants of g77 are tracked below.
The egcs variant is described vis-a-vis
previous versions of egcs and/or
an official FSF version, as appropriate.
Note that all such variants are obsolete as of July 1999 -
the information is retained here only for its historical value.
Therefore, egcs versions sometimes have multiple listings
to help clarify how they differ from other versions,
though this can make getting a complete picture
of what a particular egcs version contains
somewhat more difficult.
For information on bugs in the GCC-3.3.4 version of g77, see Known Bugs In GNU Fortran.
The following information was last updated on 2003-05-18:
GCC 3.3 versus GCC 3.2:183239246286636764916742711372367278738473888587903892631019710726GCC 3.2 versus GCC 3.1:768183089258GCC 3.1 (formerly known as g77-0.5.27) versus GCC 3.0:94737433807395742794730475248855122539754735837610661386304 PROGRAM PROG
DIMENSION A(140 000 000)
END
with the message:
prog.f: In program `prog':
prog.f:2:
DIMENSION A(140 000 000)
^
Array `a' at (^) is too large to handle
because 140 000 000 REALs is larger than the largest bit-extent that can be
expressed in 32 bits. However, bit-sizes never play a role after offsets
have been converted to byte addresses. Therefore this check has been removed,
and the limit is now 2 Gbyte of memory (around 530 000 000 REALs).
Note: On GNU/Linux systems one has to compile and link programs that occupy
more than 1 Gbyte statically, i.e. g77 -static ....
SUBROUTINE SUB(A, N)
DIMENSION N(2)
DIMENSION A(N(1),N(2))
A(1,1) = 1.
END
Note the use of array elements in the bounds of the adjustable array A.
string(1:0).
libf2c library is now able to read and write files larger than
2 Gbyte on 32-bit target machines, if the operating system supports this.
GCC 3.0 versus GCC 2.95:ftruncate OS function. Thanks go to the GAMESS developers
for bringing this to our attention.
GCC 2.95 (EGCS 1.2) versus EGCS 1.1.2:libg2c now supports building as multilibbed library,
which provides better support for systems
that require options such as -mieee
to work properly.
CTime, DTime, ETime, and TtyNam
intrinsics has been swapped.
The argument serving as the returned value
for the corresponding function forms
now is the second argument,
making these consistent with the other subroutine forms
of libU77 intrinsics.
libg2c has been changed to increase the likelihood
of catching references to the implementations of these intrinsics
using the EXTERNAL mechanism
(which would avoid the new warnings).
See Year 2000 (Y2K) Problems, for more information.
COMPLEX data type.
errno,
a C-language concept,
when performing operations such as the SqRt intrinsic.
There is no g77 version 0.5.24 at this time, or planned. 0.5.24 is the version number designated for bug fixes and, perhaps, some new features added, to 0.5.23. Version 0.5.23 requires gcc 2.8.1, as 0.5.24 was planned to require.
Due to EGCS becoming GCC
(which is now an acronym for “GNU Compiler Collection”),
and EGCS 1.2 becoming officially designated GCC 2.95,
there seems to be no need for an actual 0.5.24 release.
To reduce the confusion already resulting from use of 0.5.24
to designate g77 versions within EGCS versions 1.0 and 1.1,
as well as in versions of g77 documentation and notices
during that period,
“mainline” g77 version numbering resumes
at 0.5.25 with GCC 2.95 (EGCS 1.2),
skipping over 0.5.24 as a placeholder version number.
To repeat, there is no g77 0.5.24, but there is now a 0.5.25. Please remain calm and return to your keypunch units.
EGCS 1.1.2 versus EGCS 1.1.1:EGCS 1.1.1 versus EGCS 1.1:EGCS 1.1 versus EGCS 1.0.3:INTEGER expression.
ENTRY can be stepped through, line by line,
in gdb.
REAL argument to intrinsics
Second and CPU_Time.
tempnam, if available, to open scratch files
(as in OPEN(STATUS='SCRATCH'))
so that the TMPDIR environment variable,
if present, is used.
libf2c separates out
the setting of global state
(such as command-line arguments and signal handling)
from main.o into distinct, new library
archive members.
This should make it easier to write portable applications
that have their own (non-Fortran) main() routine
properly set up the libf2c environment, even
when libf2c (now libg2c) is a shared library.
libg2c.a instead of libf2c.a,
to ensure that a version other than the one built and
installed as part of the same g77 version is picked up.
OPEN, INQUIRE, READ, and
WRITE statements,
and about truncations of various sorts of constants.
EGCS 1.1 versus g77 0.5.23:Previously, g77 treated these expressions as denoting special “pointer” arguments for the purposes of filewide analysis.
Generally, this affects only local variables and arrays
having the SAVE attribute
or given initial values via DATA.
libf2c (libg2c).
This new information allows, for example,
which __g77_length_a to be used in gdb
to determine the type of the phantom length argument
supplied with CHARACTER variables.
This information pertains to internally-generated
type, variable, and other information,
not to the longstanding deficiencies vis-a-vis
COMMON and EQUIVALENCE.
Date_and_Time intrinsic now is
supported.
System_Clock intrinsic allows
the optional arguments (except for the Count
argument) to be omitted.
Features that have been dropped from this version of g77 due to their being implemented via g77-specific patches to the gcc back end in previous releases include:
__restrict__ keyword,
the options -fargument-alias, -fargument-noalias,
and -fargument-noalias-global,
and the corresponding alias-analysis code.
(egcs has the alias-analysis
code, but not the __restrict__ keyword.
egcs g77 users benefit from the alias-analysis
code despite the lack of the __restrict__ keyword,
which is a C-language construct.)
(egcs supports these options.
g77 users of egcs benefit from them even if
they are not explicitly specified,
because the defaults are optimized for g77 users.)
libg2c.a instead of libf2c.a,
to ensure that a version other than the one built and
installed as part of the same g77 version is picked up.
libf2c separates out
the setting of global state
(such as command-line arguments and signal handling)
from main.o into distinct, new library
archive members.
This should make it easier to write portable applications
that have their own (non-Fortran) main() routine
properly set up the libf2c environment, even
when libf2c (now libg2c) is a shared library.
OPEN, INQUIRE, READ, and
WRITE statements,
and about truncations of various sorts of constants.
Signal intrinsic so it offers portable
support for 64-bit systems (such as Digital Alphas
running GNU/Linux).
INTEGER expression.
ENTRY can be stepped through, line by line,
in gdb.
REAL argument to intrinsics
Second and CPU_Time.
Int2 and Int8.
tempnam, if available, to open scratch files
(as in OPEN(STATUS='SCRATCH'))
so that the TMPDIR environment variable,
if present, is used.
restrict to
__restrict__, to avoid rejecting valid, existing,
C programs.
Support for restrict is now more like support
for complex.
EGCS 1.0.2 versus EGCS 1.0.1:EGCS 1.0.1 versus EGCS 1.0:EGCS 1.0 versus g77 0.5.21:egcs
contains several regressions against
version 0.5.21 of g77,
due to using the
“vanilla” gcc back end instead of patching
it to fix a few bugs and improve performance in a
few cases.
Features that have been dropped from this version of g77 due to their being implemented via g77-specific patches to the gcc back end in previous releases include:
restrict keyword.
Int2 and Int8.
The default is to issue such warnings, which are new as of this version of g77.
The default is to issue such diagnostics and flag the compilation as unsuccessful. With this option, the diagnostics are issued as warnings, or, if -Wno-globals is specified, are not issued at all.
This option also disables inlining of global procedures, to avoid compiler crashes resulting from coding errors that these diagnostics normally would identify.
libU77 routines that accept file and other names
to strip trailing blanks from them, for consistency
with other implementations.
Blanks may be forcibly appended to such names by
appending a single null character (CHAR(0))
to the significant trailing blanks.
CHMOD intrinsic to work with file names
that have embedded blanks, commas, and so on.
SIGNAL intrinsic so it accepts an
optional third Status argument.
libU77 intrinsics to
support existing code more directly.
Such changes include allowing both subroutine and
function forms of many routines, changing MCLOCK()
and TIME() to return INTEGER(KIND=1) values,
introducing MCLOCK8() and TIME8() to
return INTEGER(KIND=2) values,
and placing functions that are intended to perform
side effects in a new intrinsic group, badu77.
INT2 and INT8 intrinsics.
CPU_TIME intrinsic.
ALARM intrinsic.
CTIME intrinsic now accepts any INTEGER
argument, not just INTEGER(KIND=2).
stderr instead of stdout.
This option specifies that non-decimal-radix
constants using the prefixed-radix form (such as Z'1234')
are to be interpreted as INTEGER(KIND=1) constants.
Specify -ftypeless-boz to cause such
constants to be interpreted as typeless.
(Version 0.5.19 introduced -fno-typeless-boz and its inverse.)
See Options Controlling Fortran Dialect, for information on the -ftypeless-boz option.
Some programs might use names that clash with
intrinsic names defined (and now enabled) by these
options or by the new libU77 intrinsics.
Users of such programs might need to compile them
differently (using, for example, -ff90-intrinsics-disable)
or, better yet, insert appropriate EXTERNAL
statements specifying that these names are not intended
to be names of intrinsics.
ALWAYS_FLUSH macro is no longer defined when
building libf2c, which should result in improved
I/O performance, especially over NFS.
Note: If you have code that depends on the behavior
of libf2c when built with ALWAYS_FLUSH defined,
you will have to modify libf2c accordingly before
building it from this and future versions of g77.
See Output Assumed To Flush, for more information.
libU77 has been
added to the version of libf2c distributed with
and built as part of g77.
g77 now knows about the routines in this library
as intrinsics.
See VXT Fortran, for more information on the constructs recognized when the -fvxt option is specified.
If you used one of these deleted options, you should re-read the pertinent documentation to determine which options, if any, are appropriate for compiling your code with this version of g77.
See Other Dialects, for more information.
(Enabling all the -fugly-* options is unlikely to be feasible, or sensible, in the future, so users should learn to specify only those -fugly-* options they really need for a particular source file.)
See Ugly Assumed-Size Arrays, for more information.
LOC()
intrinsic and %LOC() construct now return
values of INTEGER(KIND=0) type,
as defined by the GNU Fortran language.
This type is wide enough (holds the same number of bits) as the character-pointer type on the machine.
On most machines, this won't make a difference,
whereas, on Alphas and other systems with 64-bit pointers,
the INTEGER(KIND=0) type is equivalent to INTEGER(KIND=2)
(often referred to as INTEGER*8)
instead of the more common INTEGER(KIND=1)
(often referred to as INTEGER*4).
COMPLEX arithmetic in the g77 front
end, to avoid bugs in complex support in the
gcc back end.
New option -fno-emulate-complex
causes g77 to revert the 0.5.19 behavior.
COMMON areas when any of
these are defined (assigned to) by Fortran code.
This can result in faster and/or smaller programs when compiling with optimization enabled, though on some systems this effect is observed only when -fforce-addr also is specified.
New options -falias-check, -fargument-alias, -fargument-noalias, and -fno-argument-noalias-global control the way g77 handles potential aliasing.
See Aliasing Assumed To Work, for detailed information on why the new defaults might result in some programs no longer working the way they did when compiled by previous versions of g77.
See Ugly Assigned Labels, for more information.
FORMAT and ENTRY statements now are allowed to
precede IMPLICIT NONE statements.
INTEGER(KIND=2)
(often referred to as INTEGER*8)
available in
libf2c and f2c.h so that f2c users
may make full use of its features via the g77
version of f2c.h and the INTEGER(KIND=2)
support routines in the g77 version of libf2c.
libf2c so that g77 -v
yields version information on the library.
SNGL and FLOAT intrinsics now are
specific intrinsics, instead of synonyms for the
generic intrinsic REAL.
REALPART, IMAGPART,
COMPLEX,
LONG, and SHORT.
gnu, has been added
to contain the new REALPART, IMAGPART,
and COMPLEX intrinsics.
An old group, dcp, has been removed.
DOUBLE COMPLEX (or any
complex type other than COMPLEX), unless
-ff90 option specifies Fortran 90 interpretation
or new -fugly-complex option, in conjunction with
-fnot-f90, specifies f2c interpretation.
Information on previous versions is archived
in gcc/gcc/f/news.texi
following the test of the DOC-OLDNEWS macro.
GNU Fortran supports a variety of extensions to, and dialects of, the Fortran language. Its primary base is the ANSI FORTRAN 77 standard, currently available on the network at http://www.fortran.com/fortran/F77_std/rjcnf0001.html or as monolithic text at http://www.fortran.com/fortran/F77_std/f77_std.html. It offers some extensions that are popular among users of UNIX f77 and f2c compilers, some that are popular among users of other compilers (such as Digital products), some that are popular among users of the newer Fortran 90 standard, and some that are introduced by GNU Fortran.
(If you need a text on Fortran, a few freely available electronic references have pointers from http://www.fortran.com/F/books.html. There is a `cooperative net project', User Notes on Fortran Programming at ftp://vms.huji.ac.il/fortran/ and mirrors elsewhere; some of this material might not apply specifically to g77.)
Part of what defines a particular implementation of a Fortran system, such as g77, is the particular characteristics of how it supports types, constants, and so on. Much of this is left up to the implementation by the various Fortran standards and accepted practice in the industry.
The GNU Fortran language is described below. Much of the material is organized along the same lines as the ANSI FORTRAN 77 standard itself.
See Other Dialects, for information on features g77 supports that are not part of the GNU Fortran language.
Note: This portion of the documentation definitely needs a lot of work!
Relationship to the ANSI FORTRAN 77 standard:
Extensions to the ANSI FORTRAN 77 standard:
The purpose of the following description of the GNU Fortran language is to promote wide portability of GNU Fortran programs.
GNU Fortran is an evolving language, due to the fact that g77 itself is in beta test. Some current features of the language might later be redefined as dialects of Fortran supported by g77 when better ways to express these features are added to g77, for example. Such features would still be supported by g77, but would be available only when one or more command-line options were used.
The GNU Fortran language is distinct from the GNU Fortran compilation system (g77).
For example, g77 supports various dialects of Fortran—in a sense, these are languages other than GNU Fortran—though its primary purpose is to support the GNU Fortran language, which also is described in its documentation and by its implementation.
On the other hand, non-GNU compilers might offer support for the GNU Fortran language, and are encouraged to do so.
Currently, the GNU Fortran language is a fairly fuzzy object. It represents something of a cross between what g77 accepts when compiling using the prevailing defaults and what this document describes as being part of the language.
Future versions of g77 are expected to clarify the definition of the language in the documentation. Often, this will mean adding new features to the language, in the form of both new documentation and new support in g77. However, it might occasionally mean removing a feature from the language itself to “dialect” status. In such a case, the documentation would be adjusted to reflect the change, and g77 itself would likely be changed to require one or more command-line options to continue supporting the feature.
The development of the GNU Fortran language is intended to strike a balance between:
One of the biggest practical challenges for the developers of the GNU Fortran language is meeting the sometimes contradictory demands of the above items.
For example, a feature might be widely used in one popular environment, but the exact same code that utilizes that feature might not work as expected—perhaps it might mean something entirely different—in another popular environment.
Traditionally, Fortran compilers—even portable ones—have solved this problem by simply offering the appropriate feature to users of the respective systems. This approach treats users of various Fortran systems and dialects as remote “islands”, or camps, of programmers, and assume that these camps rarely come into contact with each other (or, especially, with each other's code).
Project GNU takes a radically different approach to software and language design, in that it assumes that users of GNU software do not necessarily care what kind of underlying system they are using, regardless of whether they are using software (at the user-interface level) or writing it (for example, writing Fortran or C code).
As such, GNU users rarely need consider just what kind of underlying hardware (or, in many cases, operating system) they are using at any particular time. They can use and write software designed for a general-purpose, widely portable, heterogeneous environment—the GNU environment.
In line with this philosophy, GNU Fortran must evolve into a product that is widely ported and portable not only in the sense that it can be successfully built, installed, and run by users, but in the larger sense that its users can use it in the same way, and expect largely the same behaviors from it, regardless of the kind of system they are using at any particular time.
This approach constrains the solutions g77 can use to resolve conflicts between various camps of Fortran users. If these two camps disagree about what a particular construct should mean, g77 cannot simply be changed to treat that particular construct as having one meaning without comment (such as a warning), lest the users expecting it to have the other meaning are unpleasantly surprised that their code misbehaves when executed.
The use of the ASCII backslash character in character constants is an excellent (and still somewhat unresolved) example of this kind of controversy. See Backslash in Constants. Other examples are likely to arise in the future, as g77 developers strive to improve its ability to accept an ever-wider variety of existing Fortran code without requiring significant modifications to said code.
Development of GNU Fortran is further constrained by the desire to avoid requiring programmers to change their code. This is important because it allows programmers, administrators, and others to more faithfully evaluate and validate g77 (as an overall product and as new versions are distributed) without having to support multiple versions of their programs so that they continue to work the same way on their existing systems (non-GNU perhaps, but possibly also earlier versions of g77).
GNU Fortran supports ANSI FORTRAN 77 with the following caveats. In summary, the only ANSI FORTRAN 77 features g77 doesn't support are those that are probably rarely used in actual code, some of which are explicitly disallowed by the Fortran 90 standard.
g77 disallows passing of an external procedure
as an actual argument if the procedure's
type is declared CHARACTER*(*). For example:
CHARACTER*(*) CFUNC
EXTERNAL CFUNC
CALL FOO(CFUNC)
END
It isn't clear whether the standard considers this conforming.
g77 disallows passing of a dummy procedure
as an actual argument if the procedure's
type is declared CHARACTER*(*).
SUBROUTINE BAR(CFUNC)
CHARACTER*(*) CFUNC
EXTERNAL CFUNC
CALL FOO(CFUNC)
END
It isn't clear whether the standard considers this conforming.
The DO variable for an implied-DO construct in a
DATA statement may not be used as the DO variable
for an outer implied-DO construct. For example, this
fragment is disallowed by g77:
DATA ((A(I, I), I= 1, 10), I= 1, 10) /.../
This also is disallowed by Fortran 90, as it offers no additional capabilities and would have a variety of possible meanings.
Note that it is very unlikely that any production Fortran code tries to use this unsupported construct.
An array element initializer in an implied-DO construct in a
DATA statement must contain at least one reference to the DO
variables of each outer implied-DO construct. For example,
this fragment is disallowed by g77:
DATA (A, I= 1, 1) /1./
This also is disallowed by Fortran 90, as FORTRAN 77's more permissive requirements offer no additional capabilities. However, g77 doesn't necessarily diagnose all cases where this requirement is not met.
Note that it is very unlikely that any production Fortran code tries to use this unsupported construct.
(The following information augments or overrides the information in Section 1.4 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 1 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
The definition of the GNU Fortran language is akin to that of the ANSI FORTRAN 77 language in that it does not generally require conforming implementations to diagnose cases where programs do not conform to the language.
However, g77 as a compiler is being developed in a way that is intended to enable it to diagnose such cases in an easy-to-understand manner.
A program that conforms to the GNU Fortran language should, when compiled, linked, and executed using a properly installed g77 system, perform as described by the GNU Fortran language definition. Reasons for different behavior include, among others:
Despite these “loopholes”, the availability of a clear specification of the language of programs submitted to g77, as this document is intended to provide, is considered an important aspect of providing a robust, clean, predictable Fortran implementation.
The definition of the GNU Fortran language, while having no special legal status, can therefore be viewed as a sort of contract, or agreement. This agreement says, in essence, “if you write a program in this language, and run it in an environment (such as a g77 system) that supports this language, the program should behave in a largely predictable way”.
(The following information augments or overrides the information in Section 1.5 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 1 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
In this chapter, “must” denotes a requirement, “may” denotes permission, and “must not” and “may not” denote prohibition. Terms such as “might”, “should”, and “can” generally add little or nothing in the way of weight to the GNU Fortran language itself, but are used to explain or illustrate the language.
For example:
“The FROBNITZ statement must precede all executable
statements in a program unit, and may not specify any dummy
arguments. It may specify local or common variables and arrays.
Its use should be limited to portions of the program designed to
be non-portable and system-specific, because it might cause the
containing program unit to behave quite differently on different
systems.”
Insofar as the GNU Fortran language is specified,
the requirements and permissions denoted by the above sample statement
are limited to the placement of the statement and the kinds of
things it may specify.
The rest of the statement—the content regarding non-portable portions
of the program and the differing behavior of program units containing
the FROBNITZ statement—does not pertain the GNU Fortran
language itself.
That content offers advice and warnings about the FROBNITZ
statement.
Remember: The GNU Fortran language definition specifies both what constitutes a valid GNU Fortran program and how, given such a program, a valid GNU Fortran implementation is to interpret that program.
It is not incumbent upon a valid GNU Fortran implementation to behave in any particular way, any consistent way, or any predictable way when it is asked to interpret input that is not a valid GNU Fortran program.
Such input is said to have undefined behavior when interpreted by a valid GNU Fortran implementation, though an implementation may choose to specify behaviors for some cases of inputs that are not valid GNU Fortran programs.
Other notation used herein is that of the GNU texinfo format, which is used to generate printed hardcopy, on-line hypertext (Info), and on-line HTML versions, all from a single source document. This notation is used as follows:
COMMON, INTEGER, and
BLOCK DATA.
Note that, in practice, many Fortran programs are written in lowercase—uppercase is used in this manual as a means to readily distinguish keywords and sample Fortran-related text from the prose in this document.
Generally, uppercase is used for all Fortran-specific and Fortran-related text, though this does not always include literal text within Fortran code.
For example: PRINT *, 'My name is Bob'.
“The INTEGER ivar statement specifies that
ivar is a variable or array of type INTEGER.”
In the above example, any valid text may be substituted for the metasyntactic variable ivar to make the statement apply to a specific instance, as long as the same text is substituted for both occurrences of ivar.
See Kind Notation, for information on the relationship
between Fortran 90 nomenclature (such as INTEGER(KIND=1))
and the more traditional, less portably concise nomenclature
(such as INTEGER*4).
(The following information augments or overrides the information in Chapter 2 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 2 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
(Corresponds to Section 2.2 of ANSI X3.9-1978 FORTRAN 77.)
In GNU Fortran, a symbolic name is at least one character long,
and has no arbitrary upper limit on length.
However, names of entities requiring external linkage (such as
external functions, external subroutines, and COMMON areas)
might be restricted to some arbitrary length by the system.
Such a restriction is no more constrained than that of one
through six characters.
Underscores (_) are accepted in symbol names after the first character (which must be a letter).
(Corresponds to Section 2.3 of ANSI X3.9-1978 FORTRAN 77.)
Use of an exclamation point (!) to begin a trailing comment (a comment that extends to the end of the same source line) is permitted under the following conditions:
Use of a semicolon (;) as a statement separator is permitted under the following conditions:
IF statement nor a non-construct
WHERE statement (a Fortran 90 feature) may be
followed (in the same, possibly continued, line) by
a semicolon used as a statement separator.
This restriction avoids the confusion that can result when reading a line such as:
IF (VALIDP) CALL FOO; CALL BAR
Some readers might think the CALL BAR is executed
only if VALIDP is .TRUE., while others might
assume its execution is unconditional.
(At present, g77 does not diagnose code that violates this restriction.)
(Corresponds to Section 2.9 of ANSI X3.9-1978 FORTRAN 77.)
Included in the list of entities that have a scope of a program unit are construct names (a Fortran 90 feature). See Construct Names, for more information.
(The following information augments or overrides the information in Chapter 3 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 3 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
(Corresponds to Section 3.1 of ANSI X3.9-1978 FORTRAN 77.)
Letters include uppercase letters (the twenty-six characters of the English alphabet) and lowercase letters (their lowercase equivalent). Generally, lowercase letters may be used in place of uppercase letters, though in character and Hollerith constants, they are distinct.
Special characters include:
Note that this document refers to <SPC> as space, while X3.9-1978 FORTRAN 77 refers to it as blank.
(Corresponds to Section 3.2 of ANSI X3.9-1978 FORTRAN 77.)
The way a Fortran compiler views source files depends entirely on the implementation choices made for the compiler, since those choices are explicitly left to the implementation by the published Fortran standards.
The GNU Fortran language mandates a view applicable to UNIX-like text files—files that are made up of an arbitrary number of lines, each with an arbitrary number of characters (sometimes called stream-based files).
This view does not apply to types of files that are specified as having a particular number of characters on every single line (sometimes referred to as record-based files).
Because a “line in a program unit is a sequence of 72 characters”, to quote X3.9-1978, the GNU Fortran language specifies that a stream-based text file is translated to GNU Fortran lines as follows:
EOF) also serves to end the line
of text that precedes it (and that does not contain a newline).
For the purposes of the remainder of this description of the GNU Fortran language, the translation described above has already taken place, unless otherwise specified.
The result of the above translation is that the source file appears, in terms of the remainder of this description of the GNU Fortran language, as if it had an arbitrary number of 72-character lines, each character being among the GNU Fortran character set.
For example, if the source file itself has two newlines in a row, the second newline becomes, after the above translation, a single line containing 72 spaces.
(Corresponds to Section 3.2.3 of ANSI X3.9-1978 FORTRAN 77.)
A continuation line is any line that both
A continuation character is any character of the GNU Fortran character set other than space (<SPC>) or zero (0) in column 6, or a digit (0 through 9) in column 7 through 72 of a line that has only spaces to the left of that digit.
The continuation character is ignored as far as the content of the statement is concerned.
The GNU Fortran language places no limit on the number of continuation lines in a statement. In practice, the limit depends on a variety of factors, such as available memory, statement content, and so on, but no GNU Fortran system may impose an arbitrary limit.
(Corresponds to Section 3.3 of ANSI X3.9-1978 FORTRAN 77.)
Statements may be written using an arbitrary number of continuation lines.
Statements may be separated using the semicolon (;), except
that the logical IF and non-construct WHERE statements
may not be separated from subsequent statements using only a semicolon
as statement separator.
The END PROGRAM, END SUBROUTINE, END FUNCTION,
and END BLOCK DATA statements are alternatives to the END
statement.
These alternatives may be written as normal statements—they are not
subject to the restrictions of the END statement.
However, no statement other than END may have an initial line
that appears to be an END statement—even END PROGRAM,
for example, must not be written as:
END
&PROGRAM
(Corresponds to Section 3.4 of ANSI X3.9-1978 FORTRAN 77.)
A statement separated from its predecessor via a semicolon may be labeled as follows:
A statement may have only one label defined for it.
(Corresponds to Section 3.5 of ANSI X3.9-1978 FORTRAN 77.)
Generally, DATA statements may precede executable statements.
However, specification statements pertaining to any entities
initialized by a DATA statement must precede that DATA
statement.
For example,
after DATA I/1/, INTEGER I is not permitted, but
INTEGER J is permitted.
The last line of a program unit may be an END statement,
or may be:
END PROGRAM statement, if the program unit is a main program.
END SUBROUTINE statement, if the program unit is a subroutine.
END FUNCTION statement, if the program unit is a function.
END BLOCK DATA statement, if the program unit is a block data.
Additional source text may be included in the processing of
the source file via the INCLUDE directive:
INCLUDE filename
The source text to be included is identified by filename, which is a literal GNU Fortran character constant. The meaning and interpretation of filename depends on the implementation, but typically is a filename.
(g77 treats it as a filename that it searches for in the current directory and/or directories specified via the -I command-line option.)
The effect of the INCLUDE directive is as if the
included text directly replaced the directive in the source
file prior to interpretation of the program.
Included text may itself use INCLUDE.
The depth of nested INCLUDE references depends on
the implementation, but typically is a positive integer.
This virtual replacement treats the statements and INCLUDE
directives in the included text as syntactically distinct from
those in the including text.
Therefore, the first non-comment line of the included text
must not be a continuation line.
The included text must therefore have, after the non-comment
lines, either an initial line (statement), an INCLUDE
directive, or nothing (the end of the included text).
Similarly, the including text may end the INCLUDE
directive with a semicolon or the end of the line, but it
cannot follow an INCLUDE directive at the end of its
line with a continuation line.
Thus, the last statement in an included text may not be
continued.
Any statements between two INCLUDE directives on the
same line are treated as if they appeared in between the
respective included texts.
For example:
INCLUDE 'A'; PRINT *, 'B'; INCLUDE 'C'; END PROGRAM
If the text included by INCLUDE 'A' constitutes a PRINT *, 'A' statement and the text included by INCLUDE 'C' constitutes a PRINT *, 'C' statement, then the output of the above sample program would be
A
B
C
(with suitable allowances for how an implementation defines its handling of output).
Included text must not include itself directly or indirectly, regardless of whether the filename used to reference the text is the same.
Note that INCLUDE is not a statement.
As such, it is neither a non-executable or executable
statement.
However, if the text it includes constitutes one or more
executable statements, then the placement of INCLUDE
is subject to effectively the same restrictions as those
on executable statements.
An INCLUDE directive may be continued across multiple
lines as if it were a statement.
This permits long names to be used for filename.
cpp output-style # directives
(see C Preprocessor Output (The C Preprocessor))
are recognized by the compiler even
when the preprocessor isn't run on the input (as it is when compiling
.F files). (Note the distinction between these cpp
# output directives and #line input
directives.)
(The following information augments or overrides the information in Chapter 4 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 4 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
To more concisely express the appropriate types for
entities, this document uses the more concise
Fortran 90 nomenclature such as INTEGER(KIND=1)
instead of the more traditional, but less portably concise,
byte-size-based nomenclature such as INTEGER*4,
wherever reasonable.
When referring to generic types—in contexts where the
specific precision and range of a type are not important—this
document uses the generic type names INTEGER, LOGICAL,
REAL, COMPLEX, and CHARACTER.
In some cases, the context requires specification of a particular type. This document uses the KIND= notation to accomplish this throughout, sometimes supplying the more traditional notation for clarification, though the traditional notation might not work the same way on all GNU Fortran implementations.
Use of KIND= makes this document more concise because g77 is able to define values for KIND= that have the same meanings on all systems, due to the way the Fortran 90 standard specifies these values are to be used.
(In particular, that standard permits an implementation to
arbitrarily assign nonnegative values.
There are four distinct sets of assignments: one to the CHARACTER
type; one to the INTEGER type; one to the LOGICAL type;
and the fourth to both the REAL and COMPLEX types.
Implementations are free to assign these values in any order,
leave gaps in the ordering of assignments, and assign more than
one value to a representation.)
This makes KIND= values superior to the values used in non-standard statements such as INTEGER*4, because the meanings of the values in those statements vary from machine to machine, compiler to compiler, even operating system to operating system.
However, use of KIND= is not generally recommended when writing portable code (unless, for example, the code is going to be compiled only via g77, which is a widely ported compiler). GNU Fortran does not yet have adequate language constructs to permit use of KIND= in a fashion that would make the code portable to Fortran 90 implementations; and, this construct is known to not be accepted by many popular FORTRAN 77 implementations, so it cannot be used in code that is to be ported to those.
The distinction here is that this document is able to use specific values for KIND= to concisely document the types of various operations and operands.
A Fortran program should use the FORTRAN 77 designations for the
appropriate GNU Fortran types—such as INTEGER for
INTEGER(KIND=1), REAL for REAL(KIND=1),
and DOUBLE COMPLEX for COMPLEX(KIND=2)—and,
where no such designations exist, make use of appropriate
techniques (preprocessor macros, parameters, and so on)
to specify the types in a fashion that may be easily adjusted
to suit each particular implementation to which the program
is ported.
(These types generally won't need to be adjusted for ports of
g77.)
Further details regarding GNU Fortran data types and constants are provided below.
(Corresponds to Section 4.1 of ANSI X3.9-1978 FORTRAN 77.)
GNU Fortran supports these types:
INTEGER)
REAL)
COMPLEX)
LOGICAL)
CHARACTER)
(The types numbered 1 through 6 above are standard FORTRAN 77 types.)
The generic types shown above are referred to in this document using only their generic type names. Such references usually indicate that any specific type (kind) of that generic type is valid.
For example, a context described in this document as accepting
the COMPLEX type also is likely to accept the
DOUBLE COMPLEX type.
The GNU Fortran language supports three ways to specify a specific kind of a generic type.
The GNU Fortran language supports two uses of the keyword
DOUBLE to specify a specific kind of type:
DOUBLE PRECISION, equivalent to REAL(KIND=2)
DOUBLE COMPLEX, equivalent to COMPLEX(KIND=2)
Use one of the above forms where a type name is valid.
While use of this notation is popular, it doesn't scale well in a language or dialect rich in intrinsic types, as is the case for the GNU Fortran language (especially planned future versions of it).
After all, one rarely sees type names such as DOUBLE INTEGER,
QUADRUPLE REAL, or QUARTER INTEGER.
Instead, INTEGER*8, REAL*16, and INTEGER*1
often are substituted for these, respectively, even though they
do not always have the same meanings on all systems.
(And, the fact that DOUBLE REAL does not exist as such
is an inconsistency.)
Therefore, this document uses “double notation” only on occasion for the benefit of those readers who are accustomed to it.
The following notation specifies the storage size for a type:
generic-type*n
generic-type must be a generic type—one of
INTEGER, REAL, COMPLEX, LOGICAL,
or CHARACTER.
n must be one or more digits comprising a decimal
integer number greater than zero.
Use the above form where a type name is valid.
The *n notation specifies that the amount of storage
occupied by variables and array elements of that type is n
times the storage occupied by a CHARACTER*1 variable.
This notation might indicate a different degree of precision and/or range for such variables and array elements, and the functions that return values of types using this notation. It does not limit the precision or range of values of that type in any particular way—use explicit code to do that.
Further, the GNU Fortran language requires no particular values
for n to be supported by an implementation via the *n
notation.
g77 supports INTEGER*1 (as INTEGER(KIND=3))
on all systems, for example,
but not all implementations are required to do so, and g77
is known to not support REAL*1 on most (or all) systems.
As a result, except for generic-type of CHARACTER,
uses of this notation should be limited to isolated
portions of a program that are intended to handle system-specific
tasks and are expected to be non-portable.
(Standard FORTRAN 77 supports the *n notation for
only CHARACTER, where it signifies not only the amount
of storage occupied, but the number of characters in entities
of that type.
However, almost all Fortran compilers have supported this
notation for generic types, though with a variety of meanings
for n.)
Specifications of types using the *n notation always are interpreted as specifications of the appropriate types described in this document using the KIND=n notation, described below.
While use of this notation is popular, it doesn't serve well in the context of a widely portable dialect of Fortran, such as the GNU Fortran language.
For example, even on one particular machine, two or more popular
Fortran compilers might well disagree on the size of a type
declared INTEGER*2 or REAL*16.
Certainly there
is known to be disagreement over such things among Fortran
compilers on different systems.
Further, this notation offers no elegant way to specify sizes
that are not even multiples of the “byte size” typically
designated by INTEGER*1.
Use of “absurd” values (such as INTEGER*1000) would
certainly be possible, but would perhaps be stretching the original
intent of this notation beyond the breaking point in terms
of widespread readability of documentation and code making use
of it.
Therefore, this document uses “star notation” only on occasion for the benefit of those readers who are accustomed to it.
The following notation specifies the kind-type selector of a type:
generic-type(KIND=n)
Use the above form where a type name is valid.
generic-type must be a generic type—one of
INTEGER, REAL, COMPLEX, LOGICAL,
or CHARACTER.
n must be an integer initialization expression that
is a positive, nonzero value.
Programmers are discouraged from writing these values directly into their code. Future versions of the GNU Fortran language will offer facilities that will make the writing of code portable to g77 and Fortran 90 implementations simpler.
However, writing code that ports to existing FORTRAN 77 implementations depends on avoiding the KIND= construct.
The KIND= construct is thus useful in the context of GNU Fortran for two reasons:
The values of n in the GNU Fortran language are assigned using a scheme that:
The assignment system accomplishes this by assigning to each “fundamental meaning” of a specific type a unique prime number. Combinations of fundamental meanings—for example, a type that is two times the size of some other type—are assigned values of n that are the products of the values for those fundamental meanings.
A prime value of n is never given more than one fundamental meaning, to avoid situations where some code or system cannot reasonably provide those meanings in the form of a single type.
The values of n assigned so far are:
KIND=0The planned future use is for this value to designate,
explicitly, context-sensitive kind-type selection.
For example, the expression 1D0 * 0.1_0 would
be equivalent to 1D0 * 0.1D0.
KIND=1REAL, INTEGER, LOGICAL, COMPLEX,
and CHARACTER, as appropriate.
These are the “default” types described in the Fortran 90 standard, though that standard does not assign any particular KIND= value to these types.
(Typically, these are REAL*4, INTEGER*4,
LOGICAL*4, and COMPLEX*8.)
KIND=2REAL(KIND=2) is DOUBLE PRECISION (typically REAL*8),
COMPLEX(KIND=2) is DOUBLE COMPLEX (typically COMPLEX*16),
These are the “double precision” types described in the Fortran 90 standard, though that standard does not assign any particular KIND= value to these types.
n of 4 thus corresponds to types that occupy four times as much storage as the default types, n of 8 to types that occupy eight times as much storage, and so on.
The INTEGER(KIND=2) and LOGICAL(KIND=2) types
are not necessarily supported by every GNU Fortran implementation.
KIND=3CHARACTER type,
which is the same effective type as CHARACTER(KIND=1)
(making that type effectively the same as CHARACTER(KIND=3)).
(Typically, these are INTEGER*1 and LOGICAL*1.)
n of 6 thus corresponds to types that occupy twice as much storage as the n=3 types, n of 12 to types that occupy four times as much storage, and so on.
These are not necessarily supported by every GNU Fortran
implementation.
KIND=5(Typically, these are INTEGER*2 and LOGICAL*2.)
n of 25 thus corresponds to types that occupy one-quarter as much storage as the default types.
These are not necessarily supported by every GNU Fortran
implementation.
KIND=7INTEGER(KIND=7) and
denotes the INTEGER type that has the smallest
storage size that holds a pointer on the system.
A pointer representable by this type is capable of uniquely
addressing a CHARACTER*1 variable, array, array element,
or substring.
(Typically this is equivalent to INTEGER*4 or,
on 64-bit systems, INTEGER*8.
In a compatible C implementation, it typically would
be the same size and semantics of the C type void *.)
Note that these are proposed correspondences and might change in future versions of g77—avoid writing code depending on them while g77, and therefore the GNU Fortran language it defines, is in beta testing.
Values not specified in the above list are reserved to future versions of the GNU Fortran language.
Implementation-dependent meanings will be assigned new, unique prime numbers so as to not interfere with other implementation-dependent meanings, and offer the possibility of increasing the portability of code depending on such types by offering support for them in other GNU Fortran implementations.
Other meanings that might be given unique values are:
For example, some compilers offer options that cause
INTEGER types to occupy the amount of storage
that would be needed for INTEGER(KIND=2) types, but the
range remains that of INTEGER(KIND=1).
INTEGER(KIND=1).
These could permit, conceptually, use of portable code and
implementations on data files written by existing systems.
Future prime numbers should be given meanings in as incremental a fashion as possible, to allow for flexibility and expressiveness in combining types.
For example, instead of defining a prime number for little-endian IEEE doubles, one prime number might be assigned the meaning “little-endian”, another the meaning “IEEE double”, and the value of n for a little-endian IEEE double would thus naturally be the product of those two respective assigned values. (It could even be reasonable to have IEEE values result from the products of prime values denoting exponent and fraction sizes and meanings, hidden bit usage, availability and representations of special values such as subnormals, infinities, and Not-A-Numbers (NaNs), and so on.)
This assignment mechanism, while not inherently required for future versions of the GNU Fortran language, is worth using because it could ease management of the “space” of supported types much easier in the long run.
The above approach suggests a mechanism for specifying inheritance of intrinsic (built-in) types for an entire, widely portable product line. It is certainly reasonable that, unlike programmers of other languages offering inheritance mechanisms that employ verbose names for classes and subclasses, along with graphical browsers to elucidate the relationships, Fortran programmers would employ a mechanism that works by multiplying prime numbers together and finding the prime factors of such products.
Most of the advantages for the above scheme have been explained above. One disadvantage is that it could lead to the defining, by the GNU Fortran language, of some fairly large prime numbers. This could lead to the GNU Fortran language being declared “munitions” by the United States Department of Defense.
(Corresponds to Section 4.2 of ANSI X3.9-1978 FORTRAN 77.)
A typeless constant has one of the following forms:
'binary-digits'B
'octal-digits'O
'hexadecimal-digits'Z
'hexadecimal-digits'X
binary-digits, octal-digits, and hexadecimal-digits are nonempty strings of characters in the set 01, 01234567, and 0123456789ABCDEFabcdef, respectively. (The value for A (and a) is 10, for B and b is 11, and so on.)
A prefix-radix constant, such as Z'ABCD', can optionally be treated as typeless. See Options Controlling Fortran Dialect, for information on the -ftypeless-boz option.
Typeless constants have values that depend on the context in which they are used.
All other constants, called typed constants, are interpreted—converted to internal form—according to their inherent type. Thus, context is never a determining factor for the type, and hence the interpretation, of a typed constant. (All constants in the ANSI FORTRAN 77 language are typed constants.)
For example, 1 is always type INTEGER(KIND=1) in GNU
Fortran (called default INTEGER in Fortran 90),
9.435784839284958 is always type REAL(KIND=1) (even if the
additional precision specified is lost, and even when used in a
REAL(KIND=2) context), 1E0 is always type REAL(KIND=2),
and 1D0 is always type REAL(KIND=2).
(Corresponds to Section 4.3 of ANSI X3.9-1978 FORTRAN 77.)
An integer constant also may have one of the following forms:
B'binary-digits'
O'octal-digits'
Z'hexadecimal-digits'
X'hexadecimal-digits'
binary-digits, octal-digits, and hexadecimal-digits are nonempty strings of characters in the set 01, 01234567, and 0123456789ABCDEFabcdef, respectively. (The value for A (and a) is 10, for B and b is 11, and so on.)
(Corresponds to Section 4.8 of ANSI X3.9-1978 FORTRAN 77.)
A character constant may be delimited by a pair of double quotes (") instead of apostrophes. In this case, an apostrophe within the constant represents a single apostrophe, while a double quote is represented in the source text of the constant by two consecutive double quotes with no intervening spaces.
A character constant may be empty (have a length of zero).
A character constant may include a substring specification, The value of such a constant is the value of the substring—for example, the value of 'hello'(3:5) is the same as the value of 'llo'.
(The following information augments or overrides the information in Chapter 6 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 6 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
%LOC() Construct%LOC(arg)
The %LOC() construct is an expression
that yields the value of the location of its argument,
arg, in memory.
The size of the type of the expression depends on the system—typically,
it is equivalent to either INTEGER(KIND=1) or INTEGER(KIND=2),
though it is actually type INTEGER(KIND=7).
The argument to %LOC() must be suitable as the
left-hand side of an assignment statement.
That is, it may not be a general expression involving
operators such as addition, subtraction, and so on,
nor may it be a constant.
Use of %LOC() is recommended only for code that
is accessing facilities outside of GNU Fortran, such as
operating system or windowing facilities.
It is best to constrain such uses to isolated portions of
a program—portions that deal specifically and exclusively
with low-level, system-dependent facilities.
Such portions might well provide a portable interface for
use by the program as a whole, but are themselves not
portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
Do not depend on %LOC() returning a pointer that
can be safely used to define (change) the argument.
While this might work in some circumstances, it is hard
to predict whether it will continue to work when a program
(that works using this unsafe behavior)
is recompiled using different command-line options or
a different version of g77.
Generally, %LOC() is safe when used as an argument
to a procedure that makes use of the value of the corresponding
dummy argument only during its activation, and only when
such use is restricted to referencing (reading) the value
of the argument to %LOC().
Implementation Note: Currently, g77 passes
arguments (those not passed using a construct such as %VAL())
by reference or descriptor, depending on the type of
the actual argument.
Thus, given INTEGER I, CALL FOO(I) would
seem to mean the same thing as CALL FOO(%VAL(%LOC(I))), and
in fact might compile to identical code.
However, CALL FOO(%VAL(%LOC(I))) emphatically means “pass, by value, the address of I in memory”. While CALL FOO(I) might use that same approach in a particular version of g77, another version or compiler might choose a different implementation, such as copy-in/copy-out, to effect the desired behavior—and which will therefore not necessarily compile to the same code as would CALL FOO(%VAL(%LOC(I))) using the same version or compiler.
See Debugging and Interfacing, for detailed information on how this particular version of g77 implements various constructs.
(The following information augments or overrides the information in Chapter 8 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 8 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
NAMELIST Statement
The NAMELIST statement, and related I/O constructs, are
supported by the GNU Fortran language in essentially the same
way as they are by f2c.
This follows Fortran 90 with the restriction that on NAMELIST
input, subscripts must have the form
subscript [:subscript [:stride]]
i.e.
&xx x(1:3,8:10:2)=1,2,3,4,5,6/
is allowed, but not, say,
&xx x(:3,8::2)=1,2,3,4,5,6/
As an extension of the Fortran 90 form, $ and $END may be
used in place of & and / in NAMELIST input, so that
$&xx x(1:3,8:10:2)=1,2,3,4,5,6 $end
could be used instead of the example above.
DOUBLE COMPLEX Statement
DOUBLE COMPLEX is a type-statement (and type) that
specifies the type COMPLEX(KIND=2) in GNU Fortran.
(The following information augments or overrides the information in Chapter 11 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 11 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
The DO WHILE statement, a feature of both the MIL-STD 1753 and
Fortran 90 standards, is provided by the GNU Fortran language.
The Fortran 90 “do forever” statement comprising just DO is
also supported.
The END DO statement is provided by the GNU Fortran language.
This statement is used in one of two ways:
DO loop started with a DO statement
that specifies no termination label.
DO loops, all of which start with a
DO statement that specify the label defined for the
END DO statement.
This kind of END DO statement is merely a synonym for
CONTINUE, except it is permitted only when the statement
is labeled and a target of one or more labeled DO loops.
It is expected that this use of END DO will be removed from
the GNU Fortran language in the future, though it is likely that
it will long be supported by g77 as a dialect form.
The GNU Fortran language supports construct names as defined by the Fortran 90 standard. These names are local to the program unit and are defined as follows:
construct-name: block-statement
Here, construct-name is the construct name itself;
its definition is connoted by the single colon (:); and
block-statement is an IF, DO,
or SELECT CASE statement that begins a block.
A block that is given a construct name must also specify the same construct name in its termination statement:
END block construct-name
Here, block must be IF, DO, or SELECT,
as appropriate.
CYCLE and EXIT StatementsThe CYCLE and EXIT statements specify that
the remaining statements in the current iteration of a
particular active (enclosing) DO loop are to be skipped.
CYCLE specifies that these statements are skipped,
but the END DO statement that marks the end of the
DO loop be executed—that is, the next iteration,
if any, is to be started.
If the statement marking the end of the DO loop is
not END DO—in other words, if the loop is not
a block DO—the CYCLE statement does not
execute that statement, but does start the next iteration (if any).
EXIT specifies that the loop specified by the
DO construct is terminated.
The DO loop affected by CYCLE and EXIT
is the innermost enclosing DO loop when the following
forms are used:
CYCLE
EXIT
Otherwise, the following forms specify the construct name
of the pertinent DO loop:
CYCLE construct-name
EXIT construct-name
CYCLE and EXIT can be viewed as glorified GO TO
statements.
However, they cannot be easily thought of as GO TO statements
in obscure cases involving FORTRAN 77 loops.
For example:
DO 10 I = 1, 5
DO 10 J = 1, 5
IF (J .EQ. 5) EXIT
DO 10 K = 1, 5
IF (K .EQ. 3) CYCLE
10 PRINT *, 'I=', I, ' J=', J, ' K=', K
20 CONTINUE
In particular, neither the EXIT nor CYCLE statements
above are equivalent to a GO TO statement to either label
10 or 20.
To understand the effect of CYCLE and EXIT in the
above fragment, it is helpful to first translate it to its equivalent
using only block DO loops:
DO I = 1, 5
DO J = 1, 5
IF (J .EQ. 5) EXIT
DO K = 1, 5
IF (K .EQ. 3) CYCLE
10 PRINT *, 'I=', I, ' J=', J, ' K=', K
END DO
END DO
END DO
20 CONTINUE
Adding new labels allows translation of CYCLE and EXIT
to GO TO so they may be more easily understood by programmers
accustomed to FORTRAN coding:
DO I = 1, 5
DO J = 1, 5
IF (J .EQ. 5) GOTO 18
DO K = 1, 5
IF (K .EQ. 3) GO TO 12
10 PRINT *, 'I=', I, ' J=', J, ' K=', K
12 END DO
END DO
18 END DO
20 CONTINUE
Thus, the CYCLE statement in the innermost loop skips over
the PRINT statement as it begins the next iteration of the
loop, while the EXIT statement in the middle loop ends that
loop but not the outermost loop.
(The following information augments or overrides the information in Chapter 15 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 15 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
%VAL() Construct%VAL(arg)
The %VAL() construct specifies that an argument,
arg, is to be passed by value, instead of by reference
or descriptor.
%VAL() is restricted to actual arguments in
invocations of external procedures.
Use of %VAL() is recommended only for code that
is accessing facilities outside of GNU Fortran, such as
operating system or windowing facilities.
It is best to constrain such uses to isolated portions of
a program—portions the deal specifically and exclusively
with low-level, system-dependent facilities.
Such portions might well provide a portable interface for
use by the program as a whole, but are themselves not
portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
Implementation Note: Currently, g77 passes all arguments either by reference or by descriptor.
Thus, use of %VAL() tends to be restricted to cases
where the called procedure is written in a language other
than Fortran that supports call-by-value semantics.
(C is an example of such a language.)
See Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.
%REF() Construct%REF(arg)
The %REF() construct specifies that an argument,
arg, is to be passed by reference, instead of by
value or descriptor.
%REF() is restricted to actual arguments in
invocations of external procedures.
Use of %REF() is recommended only for code that
is accessing facilities outside of GNU Fortran, such as
operating system or windowing facilities.
It is best to constrain such uses to isolated portions of
a program—portions the deal specifically and exclusively
with low-level, system-dependent facilities.
Such portions might well provide a portable interface for
use by the program as a whole, but are themselves not
portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
Do not depend on %REF() supplying a pointer to the
procedure being invoked.
While that is a likely implementation choice, other
implementation choices are available that preserve Fortran
pass-by-reference semantics without passing a pointer to
the argument, arg.
(For example, a copy-in/copy-out implementation.)
Implementation Note: Currently, g77 passes
all arguments
(other than variables and arrays of type CHARACTER)
by reference.
Future versions of, or dialects supported by, g77 might
not pass CHARACTER functions by reference.
Thus, use of %REF() tends to be restricted to cases
where arg is type CHARACTER but the called
procedure accesses it via a means other than the method
used for Fortran CHARACTER arguments.
See Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.
%DESCR() Construct%DESCR(arg)
The %DESCR() construct specifies that an argument,
arg, is to be passed by descriptor, instead of by
value or reference.
%DESCR() is restricted to actual arguments in
invocations of external procedures.
Use of %DESCR() is recommended only for code that
is accessing facilities outside of GNU Fortran, such as
operating system or windowing facilities.
It is best to constrain such uses to isolated portions of
a program—portions the deal specifically and exclusively
with low-level, system-dependent facilities.
Such portions might well provide a portable interface for
use by the program as a whole, but are themselves not
portable, and should be thoroughly tested each time they
are rebuilt using a new compiler or version of a compiler.
Do not depend on %DESCR() supplying a pointer
and/or a length passed by value
to the procedure being invoked.
While that is a likely implementation choice, other
implementation choices are available that preserve the
pass-by-reference semantics without passing a pointer to
the argument, arg.
(For example, a copy-in/copy-out implementation.)
And, future versions of g77 might change the
way descriptors are implemented, such as passing a
single argument pointing to a record containing the
pointer/length information instead of passing that same
information via two arguments as it currently does.
Implementation Note: Currently, g77 passes
all variables and arrays of type CHARACTER
by descriptor.
Future versions of, or dialects supported by, g77 might
pass CHARACTER functions by descriptor as well.
Thus, use of %DESCR() tends to be restricted to cases
where arg is not type CHARACTER but the called
procedure accesses it via a means similar to the method
used for Fortran CHARACTER arguments.
See Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.
The ANSI FORTRAN 77 language defines generic and specific intrinsics. In short, the distinctions are:
Typically, a generic intrinsic has a return type that is determined by the type of one or more of its arguments.
The GNU Fortran language generalizes these concepts somewhat,
especially by providing intrinsic subroutines and generic
intrinsics that are treated as either a specific intrinsic subroutine
or a specific intrinsic function (e.g. SECOND).
However, GNU Fortran avoids generalizing this concept to the point where existing code would be accepted as meaning something possibly different than what was intended.
For example, ABS is a generic intrinsic, so all working
code written using ABS of an INTEGER argument
expects an INTEGER return value.
Similarly, all such code expects that ABS of an INTEGER*2
argument returns an INTEGER*2 return value.
Yet, IABS is a specific intrinsic that accepts only
an INTEGER(KIND=1) argument.
Code that passes something other than an INTEGER(KIND=1)
argument to IABS is not valid GNU Fortran code, because
it is not clear what the author intended.
For example, if J is INTEGER(KIND=6), IABS(J)
is not defined by the GNU Fortran language, because the programmer
might have used that construct to mean any of the following, subtly
different, things:
INTEGER(KIND=1) first
(as if IABS(INT(J)) had been written).
INTEGER(KIND=1)
(as if INT(ABS(J)) had been written).
The distinctions matter especially when types and values wider than
INTEGER(KIND=1) (such as INTEGER(KIND=2)), or when
operations performing more “arithmetic” than absolute-value, are involved.
The following sample program is not a valid GNU Fortran program, but might be accepted by other compilers. If so, the output is likely to be revealing in terms of how a given compiler treats intrinsics (that normally are specific) when they are given arguments that do not conform to their stated requirements:
PROGRAM JCB002
C Version 1:
C Modified 1999-02-15 (Burley) to delete my email address.
C Modified 1997-05-21 (Burley) to accommodate compilers that implement
C INT(I1-I2) as INT(I1)-INT(I2) given INTEGER*2 I1,I2.
C
C Version 0:
C Written by James Craig Burley 1997-02-20.
C
C Purpose:
C Determine how compilers handle non-standard IDIM
C on INTEGER*2 operands, which presumably can be
C extrapolated into understanding how the compiler
C generally treats specific intrinsics that are passed
C arguments not of the correct types.
C
C If your compiler implements INTEGER*2 and INTEGER
C as the same type, change all INTEGER*2 below to
C INTEGER*1.
C
INTEGER*2 I0, I4
INTEGER I1, I2, I3
INTEGER*2 ISMALL, ILARGE
INTEGER*2 ITOOLG, ITWO
INTEGER*2 ITMP
LOGICAL L2, L3, L4
C
C Find smallest INTEGER*2 number.
C
ISMALL=0
10 I0 = ISMALL-1
IF ((I0 .GE. ISMALL) .OR. (I0+1 .NE. ISMALL)) GOTO 20
ISMALL = I0
GOTO 10
20 CONTINUE
C
C Find largest INTEGER*2 number.
C
ILARGE=0
30 I0 = ILARGE+1
IF ((I0 .LE. ILARGE) .OR. (I0-1 .NE. ILARGE)) GOTO 40
ILARGE = I0
GOTO 30
40 CONTINUE
C
C Multiplying by two adds stress to the situation.
C
ITWO = 2
C
C Need a number that, added to -2, is too wide to fit in I*2.
C
ITOOLG = ISMALL
C
C Use IDIM the straightforward way.
C
I1 = IDIM (ILARGE, ISMALL) * ITWO + ITOOLG
C
C Calculate result for first interpretation.
C
I2 = (INT (ILARGE) - INT (ISMALL)) * ITWO + ITOOLG
C
C Calculate result for second interpretation.
C
ITMP = ILARGE - ISMALL
I3 = (INT (ITMP)) * ITWO + ITOOLG
C
C Calculate result for third interpretation.
C
I4 = (ILARGE - ISMALL) * ITWO + ITOOLG
C
C Print results.
C
PRINT *, 'ILARGE=', ILARGE
PRINT *, 'ITWO=', ITWO
PRINT *, 'ITOOLG=', ITOOLG
PRINT *, 'ISMALL=', ISMALL
PRINT *, 'I1=', I1
PRINT *, 'I2=', I2
PRINT *, 'I3=', I3
PRINT *, 'I4=', I4
PRINT *
L2 = (I1 .EQ. I2)
L3 = (I1 .EQ. I3)
L4 = (I1 .EQ. I4)
IF (L2 .AND. .NOT.L3 .AND. .NOT.L4) THEN
PRINT *, 'Interp 1: IDIM(I*2,I*2) => IDIM(INT(I*2),INT(I*2))'
STOP
END IF
IF (L3 .AND. .NOT.L2 .AND. .NOT.L4) THEN
PRINT *, 'Interp 2: IDIM(I*2,I*2) => INT(DIM(I*2,I*2))'
STOP
END IF
IF (L4 .AND. .NOT.L2 .AND. .NOT.L3) THEN
PRINT *, 'Interp 3: IDIM(I*2,I*2) => DIM(I*2,I*2)'
STOP
END IF
PRINT *, 'Results need careful analysis.'
END
No future version of the GNU Fortran language
will likely permit specific intrinsic invocations with wrong-typed
arguments (such as IDIM in the above example), since
it has been determined that disagreements exist among
many production compilers on the interpretation of
such invocations.
These disagreements strongly suggest that Fortran programmers,
and certainly existing Fortran programs, disagree about the
meaning of such invocations.
The first version of JCB002 didn't accommodate some compilers'
treatment of INT(I1-I2) where I1 and I2 are
INTEGER*2.
In such a case, these compilers apparently convert both
operands to INTEGER*4 and then do an INTEGER*4 subtraction,
instead of doing an INTEGER*2 subtraction on the
original values in I1 and I2.
However, the results of the careful analyses done on the outputs of programs compiled by these various compilers show that they all implement either Interp 1 or Interp 2 above.
Specifically, it is believed that the new version of JCB002
above will confirm that:
If you get different results than the above for the stated compilers, or have results for other compilers that might be worth adding to the above list, please let us know the details (compiler product, version, machine, results, and so on).
REAL() and AIMAG() of Complex
The GNU Fortran language disallows REAL(expr)
and AIMAG(expr),
where expr is any COMPLEX type other than COMPLEX(KIND=1),
except when they are used in the following way:
REAL(REAL(expr))
REAL(AIMAG(expr))
The above forms explicitly specify that the desired effect
is to convert the real or imaginary part of expr, which might
be some REAL type other than REAL(KIND=1),
to type REAL(KIND=1),
and have that serve as the value of the expression.
The GNU Fortran language offers clearly named intrinsics to extract the real and imaginary parts of a complex entity without any conversion:
REALPART(expr)
IMAGPART(expr)
To express the above using typical extended FORTRAN 77,
use the following constructs
(when expr is COMPLEX(KIND=2)):
DBLE(expr)
DIMAG(expr)
The FORTRAN 77 language offers no way
to explicitly specify the real and imaginary parts of a complex expression of
arbitrary type, apparently as a result of requiring support for
only one COMPLEX type (COMPLEX(KIND=1)).
The concepts of converting an expression to type REAL(KIND=1) and
of extracting the real part of a complex expression were
thus “smooshed” by FORTRAN 77 into a single intrinsic, since
they happened to have the exact same effect in that language
(due to having only one COMPLEX type).
Note: When -ff90 is in effect,
g77 treats REAL(expr), where expr is of
type COMPLEX, as REALPART(expr),
whereas with -fugly-complex -fno-f90 in effect, it is
treated as REAL(REALPART(expr)).
See Ugly Complex Part Extraction, for more information.
CMPLX() of DOUBLE PRECISION
In accordance with Fortran 90 and at least some (perhaps all)
other compilers, the GNU Fortran language defines CMPLX()
as always returning a result that is type COMPLEX(KIND=1).
This means CMPLX(D1,D2), where D1 and D2
are REAL(KIND=2) (DOUBLE PRECISION), is treated as:
CMPLX(SNGL(D1), SNGL(D2))
(It was necessary for Fortran 90 to specify this behavior
for DOUBLE PRECISION arguments, since that is
the behavior mandated by FORTRAN 77.)
The GNU Fortran language also provides the DCMPLX() intrinsic,
which is provided by some FORTRAN 77 compilers to construct
a DOUBLE COMPLEX entity from of DOUBLE PRECISION
operands.
However, this solution does not scale well when more COMPLEX types
(having various precisions and ranges) are offered by Fortran implementations.
Fortran 90 extends the CMPLX() intrinsic by adding
an extra argument used to specify the desired kind of complex
result.
However, this solution is somewhat awkward to use, and
g77 currently does not support it.
The GNU Fortran language provides a simple way to build a complex value out of two numbers, with the precise type of the value determined by the types of the two numbers (via the usual type-promotion mechanism):
COMPLEX(real, imag)
When real and imag are the same REAL types, COMPLEX()
performs no conversion other than to put them together to form a
complex result of the same (complex version of real) type.
See Complex Intrinsic, for more information.
The GNU Fortran language includes the MIL-STD 1753 intrinsics
BTEST, IAND, IBCLR, IBITS,
IBSET, IEOR, IOR, ISHFT,
ISHFTC, MVBITS, and NOT.
The bit-manipulation intrinsics supported by traditional
f77 and by f2c are available in the GNU Fortran language.
These include AND, LSHIFT, OR, RSHIFT,
and XOR.
Also supported are the intrinsics CDABS,
CDCOS, CDEXP, CDLOG, CDSIN,
CDSQRT, DCMPLX, DCONJG, DFLOAT,
DIMAG, DREAL, and IMAG,
ZABS, ZCOS, ZEXP, ZLOG, ZSIN,
and ZSQRT.
(Corresponds to Section 15.10 of ANSI X3.9-1978 FORTRAN 77.)
The GNU Fortran language adds various functions, subroutines, types, and arguments to the set of intrinsic functions in ANSI FORTRAN 77. The complete set of intrinsics supported by the GNU Fortran language is described below.
Note that a name is not treated as that of an intrinsic if it is
specified in an EXTERNAL statement in the same program unit;
if a command-line option is used to disable the groups to which
the intrinsic belongs; or if the intrinsic is not named in an
INTRINSIC statement and a command-line option is used to
hide the groups to which the intrinsic belongs.
So, it is recommended that any reference in a program unit to
an intrinsic procedure that is not a standard FORTRAN 77
intrinsic be accompanied by an appropriate INTRINSIC
statement in that program unit.
This sort of defensive programming makes it more
likely that an implementation will issue a diagnostic rather
than generate incorrect code for such a reference.
The terminology used below is based on that of the Fortran 90 standard, so that the text may be more concise and accurate:
OPTIONAL means the argument may be omitted.
INTENT(IN) means the argument must be an expression
(such as a constant or a variable that is defined upon invocation
of the intrinsic).
INTENT(OUT) means the argument must be definable by the
invocation of the intrinsic (that is, must not be a constant nor
an expression involving operators other than array reference and
substring reference).
INTENT(INOUT) means the argument must be defined prior to,
and definable by, invocation of the intrinsic (a combination of
the requirements of INTENT(IN) and INTENT(OUT).
KIND.
CALL Abort()
Intrinsic groups: unix.
Description:
Prints a message and potentially causes a core dump via abort(3).
Abs(A)
Abs: INTEGER or REAL function.
The exact type depends on that of argument A—if A is
COMPLEX, this function's type is REAL
with the same KIND= value as the type of A.
Otherwise, this function's type is the same as that of A.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the absolute value of A.
If A is type COMPLEX, the absolute
value is computed as:
SQRT(REALPART(A)**2+IMAGPART(A)**2)
Otherwise, it is computed by negating A if it is negative, or returning A.
See Sign Intrinsic, for how to explicitly compute the positive or negative form of the absolute value of an expression.
Access(Name, Mode)
Access: INTEGER(KIND=1) function.
Name: CHARACTER; scalar; INTENT(IN).
Mode: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Checks file Name for accessibility in the mode specified by Mode and
returns 0 if the file is accessible in that mode, otherwise an error
code if the file is inaccessible or Mode is invalid.
See access(2).
A null character (CHAR(0)) marks the end of
the name in Name—otherwise,
trailing blanks in Name are ignored.
Mode may be a concatenation of any of the following characters:
AChar(I)
AChar: CHARACTER*1 function.
I: INTEGER; scalar; INTENT(IN).
Intrinsic groups: f2c, f90.
Description:
Returns the ASCII character corresponding to the code specified by I.
See IAChar Intrinsic, for the inverse of this function.
See Char Intrinsic, for the function corresponding to the system's native character set.
ACos(X)
ACos: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the arc-cosine (inverse cosine) of X in radians.
See Cos Intrinsic, for the inverse of this function.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AdjustL to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AdjustR to use this name for an external procedure.
AImag(Z)
AImag: REAL function.
This intrinsic is valid when argument Z is
COMPLEX(KIND=1).
When Z is any other COMPLEX type,
this intrinsic is valid only when used as the argument to
REAL(), as explained below.
Z: COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the (possibly converted) imaginary part of Z.
Use of AIMAG() with an argument of a type
other than COMPLEX(KIND=1) is restricted to the following case:
REAL(AIMAG(Z))
This expression converts the imaginary part of Z to
REAL(KIND=1).
See REAL() and AIMAG() of Complex, for more information.
AInt(A)
AInt: REAL function, the KIND= value of the type being that of argument A.
A: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns A with the fractional portion of its magnitude truncated and its sign preserved. (Also called “truncation towards zero”.)
See ANInt Intrinsic, for how to round to nearest whole number.
See Int Intrinsic, for how to truncate and then convert
number to INTEGER.
CALL Alarm(Seconds, Handler, Status)
Seconds: INTEGER; scalar; INTENT(IN).
Handler: Signal handler (INTEGER FUNCTION or SUBROUTINE)
or dummy/global INTEGER(KIND=1) scalar.
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Causes external subroutine Handler to be executed after a delay of
Seconds seconds by using alarm(1) to set up a signal and
signal(2) to catch it.
If Status is supplied, it will be
returned with the number of seconds remaining until any previously
scheduled alarm was due to be delivered, or zero if there was no
previously scheduled alarm.
See Signal Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL All to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Allocated to use this name for an external procedure.
ALog(X)
ALog: REAL(KIND=1) function.
X: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of LOG() that is specific
to one type for X.
See Log Intrinsic.
ALog10(X)
ALog10: REAL(KIND=1) function.
X: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of LOG10() that is specific
to one type for X.
See Log10 Intrinsic.
AMax0(A-1, A-2, ..., A-n)
AMax0: REAL(KIND=1) function.
A: INTEGER(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MAX() that is specific
to one type for A and a different return type.
See Max Intrinsic.
AMax1(A-1, A-2, ..., A-n)
AMax1: REAL(KIND=1) function.
A: REAL(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MAX() that is specific
to one type for A.
See Max Intrinsic.
AMin0(A-1, A-2, ..., A-n)
AMin0: REAL(KIND=1) function.
A: INTEGER(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MIN() that is specific
to one type for A and a different return type.
See Min Intrinsic.
AMin1(A-1, A-2, ..., A-n)
AMin1: REAL(KIND=1) function.
A: REAL(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MIN() that is specific
to one type for A.
See Min Intrinsic.
AMod(A, P)
AMod: REAL(KIND=1) function.
A: REAL(KIND=1); scalar; INTENT(IN).
P: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MOD() that is specific
to one type for A.
See Mod Intrinsic.
And(I, J)
And: INTEGER or LOGICAL function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER or LOGICAL; scalar; INTENT(IN).
J: INTEGER or LOGICAL; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Returns value resulting from boolean AND of pair of bits in each of I and J.
ANInt(A)
ANInt: REAL function, the KIND= value of the type being that of argument A.
A: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns A with the fractional portion of its magnitude eliminated by rounding to the nearest whole number and with its sign preserved.
A fractional portion exactly equal to .5 is rounded to the whole number that is larger in magnitude. (Also called “Fortran round”.)
See AInt Intrinsic, for how to truncate to whole number.
See NInt Intrinsic, for how to round and then convert
number to INTEGER.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Any to use this name for an external procedure.
ASin(X)
ASin: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the arc-sine (inverse sine) of X in radians.
See Sin Intrinsic, for the inverse of this function.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Associated to use this name for an external procedure.
ATan(X)
ATan: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the arc-tangent (inverse tangent) of X in radians.
See Tan Intrinsic, for the inverse of this function.
ATan2(Y, X)
ATan2: REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
Y: REAL; scalar; INTENT(IN).
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the arc-tangent (inverse tangent) of the complex number (Y, X) in radians.
See Tan Intrinsic, for the inverse of this function.
BesJ0(X)
BesJ0: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the first kind of order 0 of X.
See bessel(3m), on whose implementation the function depends.
BesJ1(X)
BesJ1: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the first kind of order 1 of X.
See bessel(3m), on whose implementation the function depends.
BesJN(N, X)
BesJN: REAL function, the KIND= value of the type being that of argument X.
N: INTEGER not wider than the default kind; scalar; INTENT(IN).
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the first kind of order N of X.
See bessel(3m), on whose implementation the function depends.
BesY0(X)
BesY0: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the second kind of order 0 of X.
See bessel(3m), on whose implementation the function depends.
BesY1(X)
BesY1: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the second kind of order 1 of X.
See bessel(3m), on whose implementation the function depends.
BesYN(N, X)
BesYN: REAL function, the KIND= value of the type being that of argument X.
N: INTEGER not wider than the default kind; scalar; INTENT(IN).
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Calculates the Bessel function of the second kind of order N of X.
See bessel(3m), on whose implementation the function depends.
Bit_Size(I)
Bit_Size: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar.
Intrinsic groups: f90.
Description:
Returns the number of bits (integer precision plus sign bit) represented by the type for I.
See BTest Intrinsic, for how to test the value of a bit in a variable or array.
See IBSet Intrinsic, for how to set a bit in a variable to 1.
See IBClr Intrinsic, for how to set a bit in a variable to 0.
BTest(I, Pos)
BTest: LOGICAL(KIND=1) function.
I: INTEGER; scalar; INTENT(IN).
Pos: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns .TRUE. if bit Pos in I is
1, .FALSE. otherwise.
(Bit 0 is the low-order (rightmost) bit, adding the value or 1, to the number if set to 1; bit 1 is the next-higher-order bit, adding or 2; bit 2 adds or 4; and so on.)
See Bit_Size Intrinsic, for how to obtain the number of bits in a type. The leftmost bit of I is BIT_SIZE(I-1).
CAbs(A)
CAbs: REAL(KIND=1) function.
A: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ABS() that is specific
to one type for A.
See Abs Intrinsic.
CCos(X)
CCos: COMPLEX(KIND=1) function.
X: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of COS() that is specific
to one type for X.
See Cos Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Ceiling to use this name for an external procedure.
CExp(X)
CExp: COMPLEX(KIND=1) function.
X: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of EXP() that is specific
to one type for X.
See Exp Intrinsic.
Char(I)
Char: CHARACTER*1 function.
I: INTEGER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the character corresponding to the code specified by I, using the system's native character set.
Because the system's native character set is used, the correspondence between character and their codes is not necessarily the same between GNU Fortran implementations.
Note that no intrinsic exists to convert a numerical
value to a printable character string.
For example, there is no intrinsic that, given
an INTEGER or REAL argument with the
value 154, returns the CHARACTER
result '154'.
Instead, you can use internal-file I/O to do this kind of conversion. For example:
INTEGER VALUE
CHARACTER*10 STRING
VALUE = 154
WRITE (STRING, '(I10)'), VALUE
PRINT *, STRING
END
The above program, when run, prints:
154
See IChar Intrinsic, for the inverse of the CHAR function.
See AChar Intrinsic, for the function corresponding to the ASCII character set.
CALL ChDir(Dir, Status)
Dir: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets the current working directory to be Dir.
If the Status argument is supplied, it contains 0
on success or a nonzero error code otherwise upon return.
See chdir(3).
Caution: Using this routine during I/O to a unit connected with a non-absolute file name can cause subsequent I/O on such a unit to fail because the I/O library might reopen files by name.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See ChDir Intrinsic (function).
CALL ChMod(Name, Mode, Status)
Name: CHARACTER; scalar; INTENT(IN).
Mode: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Changes the access mode of file Name according to the
specification Mode, which is given in the format of
chmod(1).
A null character (CHAR(0)) marks the end of
the name in Name—otherwise,
trailing blanks in Name are ignored.
Currently, Name must not contain the single quote
character.
If the Status argument is supplied, it contains 0 on success or a nonzero error code upon return.
Note that this currently works
by actually invoking /bin/chmod (or the chmod found when
the library was configured) and so might fail in some circumstances and
will, anyway, be slow.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See ChMod Intrinsic (function).
CLog(X)
CLog: COMPLEX(KIND=1) function.
X: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of LOG() that is specific
to one type for X.
See Log Intrinsic.
Cmplx(X, Y)
Cmplx: COMPLEX(KIND=1) function.
X: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Y: INTEGER or REAL; OPTIONAL (must be omitted if X is COMPLEX); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
If X is not type COMPLEX,
constructs a value of type COMPLEX(KIND=1) from the
real and imaginary values specified by X and
Y, respectively.
If Y is omitted, 0. is assumed.
If X is type COMPLEX,
converts it to type COMPLEX(KIND=1).
See Complex Intrinsic, for information on easily constructing
a COMPLEX value of arbitrary precision from REAL
arguments.
Complex(Real, Imag)
Complex: COMPLEX function, the exact type being the result of cross-promoting the
types of all the arguments.
Real: INTEGER or REAL; scalar; INTENT(IN).
Imag: INTEGER or REAL; scalar; INTENT(IN).
Intrinsic groups: gnu.
Description:
Returns a COMPLEX value that has Real and Imag as its
real and imaginary parts, respectively.
If Real and Imag are the same type, and that type is not
INTEGER, no data conversion is performed, and the type of
the resulting value has the same kind value as the types
of Real and Imag.
If Real and Imag are not the same type, the usual type-promotion
rules are applied to both, converting either or both to the
appropriate REAL type.
The type of the resulting value has the same kind value as the
type to which both Real and Imag were converted, in this case.
If Real and Imag are both INTEGER, they are both converted
to REAL(KIND=1), and the result of the COMPLEX()
invocation is type COMPLEX(KIND=1).
Note: The way to do this in standard Fortran 90
is too hairy to describe here, but it is important to
note that CMPLX(D1,D2) returns a COMPLEX(KIND=1)
result even if D1 and D2 are type REAL(KIND=2).
Hence the availability of COMPLEX() in GNU Fortran.
Conjg(Z)
Conjg: COMPLEX function, the KIND= value of the type being that of argument Z.
Z: COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the complex conjugate:
COMPLEX(REALPART(Z), -IMAGPART(Z))
Cos(X)
Cos: REAL or COMPLEX function, the exact type being that of argument X.
X: REAL or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the cosine of X, an angle measured in radians.
See ACos Intrinsic, for the inverse of this function.
CosH(X)
CosH: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the hyperbolic cosine of X.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Count to use this name for an external procedure.
CALL CPU_Time(Seconds)
Seconds: REAL; scalar; INTENT(OUT).
Intrinsic groups: f90.
Description:
Returns in Seconds the current value of the system time.
This implementation of the Fortran 95 intrinsic is just an alias for
second See Second Intrinsic (subroutine).
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL CShift to use this name for an external procedure.
CSin(X)
CSin: COMPLEX(KIND=1) function.
X: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SIN() that is specific
to one type for X.
See Sin Intrinsic.
CSqRt(X)
CSqRt: COMPLEX(KIND=1) function.
X: COMPLEX(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SQRT() that is specific
to one type for X.
See SqRt Intrinsic.
CALL CTime(STime, Result)
STime: INTEGER; scalar; INTENT(IN).
Result: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Converts STime, a system time value, such as returned by
TIME8(), to a string of the form Sat Aug 19 18:13:14 1995,
and returns that string in Result.
See Time8 Intrinsic.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See CTime Intrinsic (function).
CTime(STime)
CTime: CHARACTER*(*) function.
STime: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Converts STime, a system time value, such as returned by
TIME8(), to a string of the form Sat Aug 19 18:13:14 1995,
and returns that string as the function value.
See Time8 Intrinsic.
For information on other intrinsics with the same name: See CTime Intrinsic (subroutine).
DAbs(A)
DAbs: REAL(KIND=2) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ABS() that is specific
to one type for A.
See Abs Intrinsic.
DACos(X)
DACos: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ACOS() that is specific
to one type for X.
See ACos Intrinsic.
DASin(X)
DASin: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ASIN() that is specific
to one type for X.
See ASin Intrinsic.
DATan(X)
DATan: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ATAN() that is specific
to one type for X.
See ATan Intrinsic.
DATan2(Y, X)
DATan2: REAL(KIND=2) function.
Y: REAL(KIND=2); scalar; INTENT(IN).
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ATAN2() that is specific
to one type for Y and X.
See ATan2 Intrinsic.
CALL Date_and_Time(Date, Time, Zone, Values)
Date: CHARACTER; scalar; INTENT(OUT).
Time: CHARACTER; OPTIONAL; scalar; INTENT(OUT).
Zone: CHARACTER; OPTIONAL; scalar; INTENT(OUT).
Values: INTEGER(KIND=1); OPTIONAL; DIMENSION(8); INTENT(OUT).
Intrinsic groups: f90.
Description:
Returns:
Programs making use of this intrinsic might not be Year 10000 (Y10K) compliant. For example, the date might appear, to such programs, to wrap around (change from a larger value to a smaller one) as of the Year 10000.
On systems where a millisecond timer isn't available, the millisecond value is returned as zero.
DbesJ0(X)
DbesJ0: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESJ0() that is specific
to one type for X.
See BesJ0 Intrinsic.
DbesJ1(X)
DbesJ1: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESJ1() that is specific
to one type for X.
See BesJ1 Intrinsic.
DbesJN(N, X)
DbesJN: REAL(KIND=2) function.
N: INTEGER not wider than the default kind; scalar; INTENT(IN).
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESJN() that is specific
to one type for X.
See BesJN Intrinsic.
DbesY0(X)
DbesY0: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESY0() that is specific
to one type for X.
See BesY0 Intrinsic.
DbesY1(X)
DbesY1: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESY1() that is specific
to one type for X.
See BesY1 Intrinsic.
DbesYN(N, X)
DbesYN: REAL(KIND=2) function.
N: INTEGER not wider than the default kind; scalar; INTENT(IN).
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of BESYN() that is specific
to one type for X.
See BesYN Intrinsic.
Dble(A)
Dble: REAL(KIND=2) function.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns A converted to double precision
(REAL(KIND=2)).
If A is COMPLEX, the real part of
A is used for the conversion
and the imaginary part disregarded.
See Sngl Intrinsic, for the function that converts to single precision.
See Int Intrinsic, for the function that converts
to INTEGER.
See Complex Intrinsic, for the function that converts
to COMPLEX.
DCos(X)
DCos: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of COS() that is specific
to one type for X.
See Cos Intrinsic.
DCosH(X)
DCosH: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of COSH() that is specific
to one type for X.
See CosH Intrinsic.
DDiM(X, Y)
DDiM: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Y: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of DIM() that is specific
to one type for X and Y.
See DiM Intrinsic.
DErF(X)
DErF: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of ERF() that is specific
to one type for X.
See ErF Intrinsic.
DErFC(X)
DErFC: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of ERFC() that is specific
to one type for X.
See ErFC Intrinsic.
DExp(X)
DExp: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of EXP() that is specific
to one type for X.
See Exp Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Digits to use this name for an external procedure.
DiM(X, Y)
DiM: INTEGER or REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
X: INTEGER or REAL; scalar; INTENT(IN).
Y: INTEGER or REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns X-Y if X is greater than Y; otherwise returns zero.
DInt(A)
DInt: REAL(KIND=2) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of AINT() that is specific
to one type for A.
See AInt Intrinsic.
DLog(X)
DLog: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of LOG() that is specific
to one type for X.
See Log Intrinsic.
DLog10(X)
DLog10: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of LOG10() that is specific
to one type for X.
See Log10 Intrinsic.
DMax1(A-1, A-2, ..., A-n)
DMax1: REAL(KIND=2) function.
A: REAL(KIND=2); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MAX() that is specific
to one type for A.
See Max Intrinsic.
DMin1(A-1, A-2, ..., A-n)
DMin1: REAL(KIND=2) function.
A: REAL(KIND=2); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MIN() that is specific
to one type for A.
See Min Intrinsic.
DMod(A, P)
DMod: REAL(KIND=2) function.
A: REAL(KIND=2); scalar; INTENT(IN).
P: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MOD() that is specific
to one type for A.
See Mod Intrinsic.
DNInt(A)
DNInt: REAL(KIND=2) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ANINT() that is specific
to one type for A.
See ANInt Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Dot_Product to use this name for an external procedure.
DProd(X, Y)
DProd: REAL(KIND=2) function.
X: REAL(KIND=1); scalar; INTENT(IN).
Y: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns DBLE(X)*DBLE(Y).
DSign(A, B)
DSign: REAL(KIND=2) function.
A: REAL(KIND=2); scalar; INTENT(IN).
B: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SIGN() that is specific
to one type for A and B.
See Sign Intrinsic.
DSin(X)
DSin: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SIN() that is specific
to one type for X.
See Sin Intrinsic.
DSinH(X)
DSinH: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SINH() that is specific
to one type for X.
See SinH Intrinsic.
DSqRt(X)
DSqRt: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SQRT() that is specific
to one type for X.
See SqRt Intrinsic.
DTan(X)
DTan: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of TAN() that is specific
to one type for X.
See Tan Intrinsic.
DTanH(X)
DTanH: REAL(KIND=2) function.
X: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of TANH() that is specific
to one type for X.
See TanH Intrinsic.
CALL DTime(TArray, Result)
TArray: REAL(KIND=1); DIMENSION(2); INTENT(OUT).
Result: REAL(KIND=1); scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Initially, return the number of seconds of runtime since the start of the process's execution in Result, and the user and system components of this in TArray(1) and TArray(2) respectively. The value of Result is equal to TArray(1) + TArray(2).
Subsequent invocations of DTIME() set values based on accumulations since the previous invocation.
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See DTime Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL EOShift to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Epsilon to use this name for an external procedure.
ErF(X)
ErF: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the error function of X.
See erf(3m), which provides the implementation.
ErFC(X)
ErFC: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the complementary error function of X:
ERFC(R) = 1 - ERF(R) (except that the result might be more
accurate than explicitly evaluating that formulae would give).
See erfc(3m), which provides the implementation.
CALL ETime(TArray, Result)
TArray: REAL(KIND=1); DIMENSION(2); INTENT(OUT).
Result: REAL(KIND=1); scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Return the number of seconds of runtime since the start of the process's execution in Result, and the user and system components of this in TArray(1) and TArray(2) respectively. The value of Result is equal to TArray(1) + TArray(2).
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See ETime Intrinsic (function).
ETime(TArray)
ETime: REAL(KIND=1) function.
TArray: REAL(KIND=1); DIMENSION(2); INTENT(OUT).
Intrinsic groups: unix.
Description:
Return the number of seconds of runtime since the start of the process's execution as the function value, and the user and system components of this in TArray(1) and TArray(2) respectively. The functions' value is equal to TArray(1) + TArray(2).
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
For information on other intrinsics with the same name: See ETime Intrinsic (subroutine).
CALL Exit(Status)
Status: INTEGER not wider than the default kind; OPTIONAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Exit the program with status Status after closing open Fortran
I/O units and otherwise behaving as exit(2).
If Status is omitted the canonical `success' value
will be returned to the system.
Exp(X)
Exp: REAL or COMPLEX function, the exact type being that of argument X.
X: REAL or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns e**X, where e is approximately 2.7182818.
See Log Intrinsic, for the inverse of this function.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Exponent to use this name for an external procedure.
CALL FDate(Date)
Date: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Returns the current date (using the same format as CTIME())
in Date.
Equivalent to:
CALL CTIME(Date, TIME8())
Programs making use of this intrinsic might not be Year 10000 (Y10K) compliant. For example, the date might appear, to such programs, to wrap around (change from a larger value to a smaller one) as of the Year 10000.
See CTime Intrinsic (subroutine).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See FDate Intrinsic (function).
FDate()
FDate: CHARACTER*(*) function.
Intrinsic groups: unix.
Description:
Returns the current date (using the same format as CTIME()).
Equivalent to:
CTIME(TIME8())
Programs making use of this intrinsic might not be Year 10000 (Y10K) compliant. For example, the date might appear, to such programs, to wrap around (change from a larger value to a smaller one) as of the Year 10000.
See CTime Intrinsic (function).
For information on other intrinsics with the same name: See FDate Intrinsic (subroutine).
CALL FGet(C, Status)
C: CHARACTER; scalar; INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Reads a single character into C in stream mode from unit 5
(by-passing normal formatted output) using getc(3).
Returns in
Status 0 on success, −1 on end-of-file, and the error code
from ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FGet Intrinsic (function).
CALL FGetC(Unit, C, Status)
Unit: INTEGER; scalar; INTENT(IN).
C: CHARACTER; scalar; INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Reads a single character into C in stream mode from unit Unit
(by-passing normal formatted output) using getc(3).
Returns in
Status 0 on success, −1 on end-of-file, and the error code from
ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FGetC Intrinsic (function).
Float(A)
Float: REAL(KIND=1) function.
A: INTEGER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of REAL() that is specific
to one type for A.
See Real Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Floor to use this name for an external procedure.
CALL Flush(Unit)
Unit: INTEGER; OPTIONAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Flushes Fortran unit(s) currently open for output. Without the optional argument, all such units are flushed, otherwise just the unit specified by Unit.
Some non-GNU implementations of Fortran provide this intrinsic as a library procedure that might or might not support the (optional) Unit argument.
FNum(Unit)
FNum: INTEGER(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the Unix file descriptor number corresponding to the open Fortran I/O unit Unit. This could be passed to an interface to C I/O routines.
CALL FPut(C, Status)
C: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Writes the single character C in stream mode to unit 6
(by-passing normal formatted output) using putc(3).
Returns in
Status 0 on success, the error code from ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FPut Intrinsic (function).
CALL FPutC(Unit, C, Status)
Unit: INTEGER; scalar; INTENT(IN).
C: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Writes the single character Unit in stream mode to unit 6
(by-passing normal formatted output) using putc(3).
Returns in
C 0 on success, the error code from ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FPutC Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Fraction to use this name for an external procedure.
CALL FSeek(Unit, Offset, Whence, ErrLab)
Unit: INTEGER; scalar; INTENT(IN).
Offset: INTEGER; scalar; INTENT(IN).
Whence: INTEGER; scalar; INTENT(IN).
ErrLab: *label, where label is the label of an executable statement; OPTIONAL.
Intrinsic groups: unix.
Description:
Attempts to move Fortran unit Unit to the specified Offset: absolute offset if Whence=0; relative to the current offset if Whence=1; relative to the end of the file if Whence=2. It branches to label ErrLab if Unit is not open or if the call otherwise fails.
CALL FStat(Unit, SArray, Status)
Unit: INTEGER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the file open on Fortran I/O unit Unit and
places them in the array SArray.
The values in this array are
extracted from the stat structure as returned by
fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
If the Status argument is supplied, it contains 0 on success or a nonzero error code upon return.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See FStat Intrinsic (function).
FStat(Unit, SArray)
FStat: INTEGER(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the file open on Fortran I/O unit Unit and
places them in the array SArray.
The values in this array are
extracted from the stat structure as returned by
fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
Returns 0 on success or a nonzero error code.
For information on other intrinsics with the same name: See FStat Intrinsic (subroutine).
CALL FTell(Unit, Offset)
Unit: INTEGER; scalar; INTENT(IN).
Offset: INTEGER(KIND=1); scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets Offset to the current offset of Fortran unit Unit (or to −1 if Unit is not open).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See FTell Intrinsic (function).
FTell(Unit)
FTell: INTEGER(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the current offset of Fortran unit Unit (or −1 if Unit is not open).
For information on other intrinsics with the same name: See FTell Intrinsic (subroutine).
CALL GError(Message)
Message: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Returns the system error message corresponding to the last system
error (C errno).
CALL GetArg(Pos, Value)
Pos: INTEGER not wider than the default kind; scalar; INTENT(IN).
Value: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets Value to the Pos-th command-line argument (or to all
blanks if there are fewer than Value command-line arguments);
CALL GETARG(0, value) sets value to the name of the
program (on systems that support this feature).
See IArgC Intrinsic, for information on how to get the number of arguments.
CALL GetCWD(Name, Status)
Name: CHARACTER; scalar; INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Places the current working directory in Name.
If the Status argument is supplied, it contains 0
success or a nonzero error code upon return
(ENOSYS if the system does not provide getcwd(3)
or getwd(3)).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See GetCWD Intrinsic (function).
GetCWD(Name)
GetCWD: INTEGER(KIND=1) function.
Name: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Places the current working directory in Name.
Returns 0 on
success, otherwise a nonzero error code
(ENOSYS if the system does not provide getcwd(3)
or getwd(3)).
For information on other intrinsics with the same name: See GetCWD Intrinsic (subroutine).
CALL GetEnv(Name, Value)
Name: CHARACTER; scalar; INTENT(IN).
Value: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets Value to the value of environment variable given by the
value of Name ($name in shell terms) or to blanks if
$name has not been set.
A null character (CHAR(0)) marks the end of
the name in Name—otherwise,
trailing blanks in Name are ignored.
GetGId()
GetGId: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the group id for the current process.
CALL GetLog(Login)
Login: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Returns the login name for the process in Login.
Caution: On some systems, the getlogin(3)
function, which this intrinsic calls at run time,
is either not implemented or returns a null pointer.
In the latter case, this intrinsic returns blanks
in Login.
GetPId()
GetPId: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the process id for the current process.
GetUId()
GetUId: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the user id for the current process.
CALL GMTime(STime, TArray)
STime: INTEGER(KIND=1); scalar; INTENT(IN).
TArray: INTEGER(KIND=1); DIMENSION(9); INTENT(OUT).
Intrinsic groups: unix.
Description:
Given a system time value STime, fills TArray with values
extracted from it appropriate to the GMT time zone using
gmtime(3).
The array elements are as follows:
CALL HostNm(Name, Status)
Name: CHARACTER; scalar; INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Fills Name with the system's host name returned by
gethostname(2).
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return
(ENOSYS if the system does not provide gethostname(2)).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
On some systems (specifically SCO) it might be necessary to link the
“socket” library if you call this routine.
Typically this means adding -lg2c -lsocket -lm
to the g77 command line when linking the program.
For information on other intrinsics with the same name: See HostNm Intrinsic (function).
HostNm(Name)
HostNm: INTEGER(KIND=1) function.
Name: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Fills Name with the system's host name returned by
gethostname(2), returning 0 on success or a nonzero error code
(ENOSYS if the system does not provide gethostname(2)).
On some systems (specifically SCO) it might be necessary to link the
“socket” library if you call this routine.
Typically this means adding -lg2c -lsocket -lm
to the g77 command line when linking the program.
For information on other intrinsics with the same name: See HostNm Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Huge to use this name for an external procedure.
IAbs(A)
IAbs: INTEGER(KIND=1) function.
A: INTEGER(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of ABS() that is specific
to one type for A.
See Abs Intrinsic.
IAChar(C)
IAChar: INTEGER(KIND=1) function.
C: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: f2c, f90.
Description:
Returns the code for the ASCII character in the first character position of C.
See AChar Intrinsic, for the inverse of this function.
See IChar Intrinsic, for the function corresponding to the system's native character set.
IAnd(I, J)
IAnd: INTEGER function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER; scalar; INTENT(IN).
J: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns value resulting from boolean AND of pair of bits in each of I and J.
IArgC()
IArgC: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the number of command-line arguments.
This count does not include the specification of the program name itself.
IBClr(I, Pos)
IBClr: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Pos: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns the value of I with bit Pos cleared (set to zero). See BTest Intrinsic, for information on bit positions.
IBits(I, Pos, Len)
IBits: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Pos: INTEGER; scalar; INTENT(IN).
Len: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Extracts a subfield of length Len from I, starting from bit position Pos and extending left for Len bits. The result is right-justified and the remaining bits are zeroed. The value of Pos+Len must be less than or equal to the value BIT_SIZE(I). See Bit_Size Intrinsic.
IBSet(I, Pos)
IBSet: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Pos: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns the value of I with bit Pos set (to one). See BTest Intrinsic, for information on bit positions.
IChar(C)
IChar: INTEGER(KIND=1) function.
C: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the code for the character in the first character position of C.
Because the system's native character set is used, the correspondence between character and their codes is not necessarily the same between GNU Fortran implementations.
Note that no intrinsic exists to convert a printable
character string to a numerical value.
For example, there is no intrinsic that, given
the CHARACTER value '154', returns an
INTEGER or REAL value with the value 154.
Instead, you can use internal-file I/O to do this kind of conversion. For example:
INTEGER VALUE
CHARACTER*10 STRING
STRING = '154'
READ (STRING, '(I10)'), VALUE
PRINT *, VALUE
END
The above program, when run, prints:
154
See Char Intrinsic, for the inverse of the ICHAR function.
See IAChar Intrinsic, for the function corresponding to the ASCII character set.
CALL IDate(TArray)
TArray: INTEGER(KIND=1); DIMENSION(3); INTENT(OUT).
Intrinsic groups: unix.
Description:
Fills TArray with the numerical values at the current local time. The day (in the range 1–31), month (in the range 1–12), and year appear in elements 1, 2, and 3 of TArray, respectively. The year has four significant digits.
Programs making use of this intrinsic might not be Year 10000 (Y10K) compliant. For example, the date might appear, to such programs, to wrap around (change from a larger value to a smaller one) as of the Year 10000.
For information on other intrinsics with the same name: See IDate Intrinsic (VXT).
IDiM(X, Y)
IDiM: INTEGER(KIND=1) function.
X: INTEGER(KIND=1); scalar; INTENT(IN).
Y: INTEGER(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of DIM() that is specific
to one type for X and Y.
See DiM Intrinsic.
IDInt(A)
IDInt: INTEGER(KIND=1) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of INT() that is specific
to one type for A.
See Int Intrinsic.
IDNInt(A)
IDNInt: INTEGER(KIND=1) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of NINT() that is specific
to one type for A.
See NInt Intrinsic.
IEOr(I, J)
IEOr: INTEGER function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER; scalar; INTENT(IN).
J: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns value resulting from boolean exclusive-OR of pair of bits in each of I and J.
IErrNo()
IErrNo: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the last system error number (corresponding to the C
errno).
IFix(A)
IFix: INTEGER(KIND=1) function.
A: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of INT() that is specific
to one type for A.
See Int Intrinsic.
Imag(Z)
Imag: REAL function, the KIND= value of the type being that of argument Z.
Z: COMPLEX; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
The imaginary part of Z is returned, without conversion.
Note: The way to do this in standard Fortran 90
is AIMAG(Z).
However, when, for example, Z is DOUBLE COMPLEX,
AIMAG(Z) means something different for some compilers
that are not true Fortran 90 compilers but offer some
extensions standardized by Fortran 90 (such as the
DOUBLE COMPLEX type, also known as COMPLEX(KIND=2)).
The advantage of IMAG() is that, while not necessarily
more or less portable than AIMAG(), it is more likely to
cause a compiler that doesn't support it to produce a diagnostic
than generate incorrect code.
See REAL() and AIMAG() of Complex, for more information.
ImagPart(Z)
ImagPart: REAL function, the KIND= value of the type being that of argument Z.
Z: COMPLEX; scalar; INTENT(IN).
Intrinsic groups: gnu.
Description:
The imaginary part of Z is returned, without conversion.
Note: The way to do this in standard Fortran 90
is AIMAG(Z).
However, when, for example, Z is DOUBLE COMPLEX,
AIMAG(Z) means something different for some compilers
that are not true Fortran 90 compilers but offer some
extensions standardized by Fortran 90 (such as the
DOUBLE COMPLEX type, also known as COMPLEX(KIND=2)).
The advantage of IMAGPART() is that, while not necessarily
more or less portable than AIMAG(), it is more likely to
cause a compiler that doesn't support it to produce a diagnostic
than generate incorrect code.
See REAL() and AIMAG() of Complex, for more information.
Index(String, Substring)
Index: INTEGER(KIND=1) function.
String: CHARACTER; scalar; INTENT(IN).
Substring: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the position of the start of the first occurrence of string Substring as a substring in String, counting from one. If Substring doesn't occur in String, zero is returned.
Int(A)
Int: INTEGER(KIND=1) function.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns A with the fractional portion of its
magnitude truncated and its sign preserved, converted
to type INTEGER(KIND=1).
If A is type COMPLEX, its real part is
truncated and converted, and its imaginary part is disregarded.
See NInt Intrinsic, for how to convert, rounded to nearest whole number.
See AInt Intrinsic, for how to truncate to whole number without converting.
Int2(A)
Int2: INTEGER(KIND=6) function.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: gnu.
Description:
Returns A with the fractional portion of its
magnitude truncated and its sign preserved, converted
to type INTEGER(KIND=6).
If A is type COMPLEX, its real part
is truncated and converted, and its imaginary part is disregarded.
See Int Intrinsic.
The precise meaning of this intrinsic might change in a future version of the GNU Fortran language, as more is learned about how it is used.
Int8(A)
Int8: INTEGER(KIND=2) function.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: gnu.
Description:
Returns A with the fractional portion of its
magnitude truncated and its sign preserved, converted
to type INTEGER(KIND=2).
If A is type COMPLEX, its real part
is truncated and converted, and its imaginary part is disregarded.
See Int Intrinsic.
The precise meaning of this intrinsic might change in a future version of the GNU Fortran language, as more is learned about how it is used.
IOr(I, J)
IOr: INTEGER function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER; scalar; INTENT(IN).
J: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns value resulting from boolean OR of pair of bits in each of I and J.
IRand(Flag)
IRand: INTEGER(KIND=1) function.
Flag: INTEGER; OPTIONAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns a uniform quasi-random number up to a system-dependent limit.
If Flag is 0, the next number in sequence is returned; if
Flag is 1, the generator is restarted by calling the UNIX function
srand(0); if Flag has any other value,
it is used as a new seed with srand().
See SRand Intrinsic.
Note: As typically implemented (by the routine of the same name in the C library), this random number generator is a very poor one, though the BSD and GNU libraries provide a much better implementation than the `traditional' one. On a different system you almost certainly want to use something better.
IsaTty(Unit)
IsaTty: LOGICAL(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns .TRUE. if and only if the Fortran I/O unit
specified by Unit is connected
to a terminal device.
See isatty(3).
IShft(I, Shift)
IShft: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Shift: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
All bits representing I are shifted Shift places. Shift.GT.0 indicates a left shift, Shift.EQ.0 indicates no shift and Shift.LT.0 indicates a right shift. If the absolute value of the shift count is greater than BIT_SIZE(I), the result is undefined. Bits shifted out from the left end or the right end are lost. Zeros are shifted in from the opposite end.
See IShftC Intrinsic, for the circular-shift equivalent.
IShftC(I, Shift, Size)
IShftC: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Shift: INTEGER; scalar; INTENT(IN).
Size: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
The rightmost Size bits of the argument I are shifted circularly Shift places, i.e. the bits shifted out of one end are shifted into the opposite end. No bits are lost. The unshifted bits of the result are the same as the unshifted bits of I. The absolute value of the argument Shift must be less than or equal to Size. The value of Size must be greater than or equal to one and less than or equal to BIT_SIZE(I).
See IShft Intrinsic, for the logical shift equivalent.
ISign(A, B)
ISign: INTEGER(KIND=1) function.
A: INTEGER(KIND=1); scalar; INTENT(IN).
B: INTEGER(KIND=1); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of SIGN() that is specific
to one type for A and B.
See Sign Intrinsic.
CALL ITime(TArray)
TArray: INTEGER(KIND=1); DIMENSION(3); INTENT(OUT).
Intrinsic groups: unix.
Description:
Returns the current local time hour, minutes, and seconds in elements 1, 2, and 3 of TArray, respectively.
CALL Kill(Pid, Signal, Status)
Pid: INTEGER; scalar; INTENT(IN).
Signal: INTEGER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sends the signal specified by Signal to the process Pid.
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return.
See kill(2).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See Kill Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Kind to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL LBound to use this name for an external procedure.
Len(String)
Len: INTEGER(KIND=1) function.
String: CHARACTER; scalar.
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the length of String.
If String is an array, the length of an element of String is returned.
Note that String need not be defined when this intrinsic is invoked, since only the length, not the content, of String is needed.
See Bit_Size Intrinsic, for the function that determines the size of its argument in bits.
Len_Trim(String)
Len_Trim: INTEGER(KIND=1) function.
String: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: f90.
Description:
Returns the index of the last non-blank character in String.
LNBLNK and LEN_TRIM are equivalent.
LGe(String_A, String_B)
LGe: LOGICAL(KIND=1) function.
String_A: CHARACTER; scalar; INTENT(IN).
String_B: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns .TRUE. if String_A.GE.String_B, .FALSE. otherwise. String_A and String_B are interpreted as containing ASCII character codes. If either value contains a character not in the ASCII character set, the result is processor dependent.
If the String_A and String_B are not the same length, the shorter is compared as if spaces were appended to it to form a value that has the same length as the longer.
The lexical comparison intrinsics LGe, LGt,
LLe, and LLt differ from the corresponding
intrinsic operators .GE., .GT.,
.LE., .LT..
Because the ASCII collating sequence is assumed,
the following expressions always return .TRUE.:
LGE ('0', ' ')
LGE ('A', '0')
LGE ('a', 'A')
The following related expressions do not always return .TRUE., as they are not necessarily evaluated assuming the arguments use ASCII encoding:
'0' .GE. ' '
'A' .GE. '0'
'a' .GE. 'A'
The same difference exists
between LGt and .GT.;
between LLe and .LE.; and
between LLt and .LT..
LGt(String_A, String_B)
LGt: LOGICAL(KIND=1) function.
String_A: CHARACTER; scalar; INTENT(IN).
String_B: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns .TRUE. if String_A.GT.String_B, .FALSE. otherwise. String_A and String_B are interpreted as containing ASCII character codes. If either value contains a character not in the ASCII character set, the result is processor dependent.
If the String_A and String_B are not the same length, the shorter is compared as if spaces were appended to it to form a value that has the same length as the longer.
See LGe Intrinsic, for information on the distinction
between the LGT intrinsic and the .GT.
operator.
CALL Link(Path1, Path2, Status)
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Makes a (hard) link from file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return.
See link(2).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See Link Intrinsic (function).
LLe(String_A, String_B)
LLe: LOGICAL(KIND=1) function.
String_A: CHARACTER; scalar; INTENT(IN).
String_B: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns .TRUE. if String_A.LE.String_B, .FALSE. otherwise. String_A and String_B are interpreted as containing ASCII character codes. If either value contains a character not in the ASCII character set, the result is processor dependent.
If the String_A and String_B are not the same length, the shorter is compared as if spaces were appended to it to form a value that has the same length as the longer.
See LGe Intrinsic, for information on the distinction
between the LLE intrinsic and the .LE.
operator.
LLt(String_A, String_B)
LLt: LOGICAL(KIND=1) function.
String_A: CHARACTER; scalar; INTENT(IN).
String_B: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns .TRUE. if String_A.LT.String_B, .FALSE. otherwise. String_A and String_B are interpreted as containing ASCII character codes. If either value contains a character not in the ASCII character set, the result is processor dependent.
If the String_A and String_B are not the same length, the shorter is compared as if spaces were appended to it to form a value that has the same length as the longer.
See LGe Intrinsic, for information on the distinction
between the LLT intrinsic and the .LT.
operator.
LnBlnk(String)
LnBlnk: INTEGER(KIND=1) function.
String: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the index of the last non-blank character in String.
LNBLNK and LEN_TRIM are equivalent.
Loc(Entity)
Loc: INTEGER(KIND=7) function.
Entity: Any type; cannot be a constant or expression.
Intrinsic groups: unix.
Description:
The LOC() intrinsic works the
same way as the %LOC() construct.
See The %LOC() Construct, for
more information.
Log(X)
Log: REAL or COMPLEX function, the exact type being that of argument X.
X: REAL or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the natural logarithm of X, which must
be greater than zero or, if type COMPLEX, must not
be zero.
See Exp Intrinsic, for the inverse of this function.
See Log10 Intrinsic, for the `common' (base-10) logarithm function.
Log10(X)
Log10: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the common logarithm (base 10) of X, which must be greater than zero.
The inverse of this function is 10. ** LOG10(X).
See Log Intrinsic, for the natural logarithm function.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Logical to use this name for an external procedure.
Long(A)
Long: INTEGER(KIND=1) function.
A: INTEGER(KIND=6); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Archaic form of INT() that is specific
to one type for A.
See Int Intrinsic.
The precise meaning of this intrinsic might change in a future version of the GNU Fortran language, as more is learned about how it is used.
LShift(I, Shift)
LShift: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Shift: INTEGER; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Returns I shifted to the left Shift bits.
Although similar to the expression I*(2**Shift), there are important differences. For example, the sign of the result is not necessarily the same as the sign of I.
Currently this intrinsic is defined assuming the underlying representation of I is as a two's-complement integer. It is unclear at this point whether that definition will apply when a different representation is involved.
See LShift Intrinsic, for the inverse of this function.
See IShft Intrinsic, for information on a more widely available left-shifting intrinsic that is also more precisely defined.
CALL LStat(File, SArray, Status)
File: CHARACTER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the given file File and places them in the array
SArray.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
If File is a symbolic link it returns data on the
link itself, so the routine is available only on systems that support
symbolic links.
The values in this array are extracted from the
stat structure as returned by fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return
(ENOSYS if the system does not provide lstat(2)).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See LStat Intrinsic (function).
LStat(File, SArray)
LStat: INTEGER(KIND=1) function.
File: CHARACTER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the given file File and places them in the array
SArray.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
If File is a symbolic link it returns data on the
link itself, so the routine is available only on systems that support
symbolic links.
The values in this array are extracted from the
stat structure as returned by fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
Returns 0 on success or a nonzero error code
(ENOSYS if the system does not provide lstat(2)).
For information on other intrinsics with the same name: See LStat Intrinsic (subroutine).
CALL LTime(STime, TArray)
STime: INTEGER(KIND=1); scalar; INTENT(IN).
TArray: INTEGER(KIND=1); DIMENSION(9); INTENT(OUT).
Intrinsic groups: unix.
Description:
Given a system time value STime, fills TArray with values
extracted from it appropriate to the GMT time zone using
localtime(3).
The array elements are as follows:
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MatMul to use this name for an external procedure.
Max(A-1, A-2, ..., A-n)
Max: INTEGER or REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
A: INTEGER or REAL; at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the argument with the largest value.
See Min Intrinsic, for the opposite function.
Max0(A-1, A-2, ..., A-n)
Max0: INTEGER(KIND=1) function.
A: INTEGER(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MAX() that is specific
to one type for A.
See Max Intrinsic.
Max1(A-1, A-2, ..., A-n)
Max1: INTEGER(KIND=1) function.
A: REAL(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MAX() that is specific
to one type for A and a different return type.
See Max Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MaxExponent to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MaxLoc to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MaxVal to use this name for an external procedure.
MClock()
MClock: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the number of clock ticks since the start of the process.
Supported on systems with clock(3) (q.v.).
This intrinsic is not fully portable, such as to systems
with 32-bit INTEGER types but supporting times
wider than 32 bits.
Therefore, the values returned by this intrinsic
might be, or become, negative,
or numerically less than previous values,
during a single run of the compiled program.
See MClock8 Intrinsic, for information on a similar intrinsic that might be portable to more GNU Fortran implementations, though to fewer Fortran compilers.
If the system does not support clock(3),
-1 is returned.
MClock8()
MClock8: INTEGER(KIND=2) function.
Intrinsic groups: unix.
Description:
Returns the number of clock ticks since the start of the process.
Supported on systems with clock(3) (q.v.).
Warning: this intrinsic does not increase the range
of the timing values over that returned by clock(3).
On a system with a 32-bit clock(3),
MCLOCK8 will return a 32-bit value,
even though converted to an INTEGER(KIND=2) value.
That means overflows of the 32-bit value can still occur.
Therefore, the values returned by this intrinsic
might be, or become, negative,
or numerically less than previous values,
during a single run of the compiled program.
No Fortran implementations other than GNU Fortran are known to support this intrinsic at the time of this writing. See MClock Intrinsic, for information on a similar intrinsic that might be portable to more Fortran compilers, though to fewer GNU Fortran implementations.
If the system does not support clock(3),
-1 is returned.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Merge to use this name for an external procedure.
Min(A-1, A-2, ..., A-n)
Min: INTEGER or REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
A: INTEGER or REAL; at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the argument with the smallest value.
See Max Intrinsic, for the opposite function.
Min0(A-1, A-2, ..., A-n)
Min0: INTEGER(KIND=1) function.
A: INTEGER(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MIN() that is specific
to one type for A.
See Min Intrinsic.
Min1(A-1, A-2, ..., A-n)
Min1: INTEGER(KIND=1) function.
A: REAL(KIND=1); at least two such arguments must be provided; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of MIN() that is specific
to one type for A and a different return type.
See Min Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MinExponent to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MinLoc to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL MinVal to use this name for an external procedure.
Mod(A, P)
Mod: INTEGER or REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
A: INTEGER or REAL; scalar; INTENT(IN).
P: INTEGER or REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns remainder calculated as:
A - (INT(A / P) * P)
P must not be zero.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Modulo to use this name for an external procedure.
CALL MvBits(From, FromPos, Len, TO, ToPos)
From: INTEGER; scalar; INTENT(IN).
FromPos: INTEGER; scalar; INTENT(IN).
Len: INTEGER; scalar; INTENT(IN).
TO: INTEGER with same KIND= value as for From; scalar; INTENT(INOUT).
ToPos: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Moves Len bits from positions FromPos through FromPos+Len-1 of From to positions ToPos through FromPos+Len-1 of TO. The portion of argument TO not affected by the movement of bits is unchanged. Arguments From and TO are permitted to be the same numeric storage unit. The values of FromPos+Len and ToPos+Len must be less than or equal to BIT_SIZE(From).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Nearest to use this name for an external procedure.
NInt(A)
NInt: INTEGER(KIND=1) function.
A: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns A with the fractional portion of its
magnitude eliminated by rounding to the nearest whole
number and with its sign preserved, converted
to type INTEGER(KIND=1).
If A is type COMPLEX, its real part is
rounded and converted.
A fractional portion exactly equal to .5 is rounded to the whole number that is larger in magnitude. (Also called “Fortran round”.)
See Int Intrinsic, for how to convert, truncate to whole number.
See ANInt Intrinsic, for how to round to nearest whole number without converting.
Not(I)
Not: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Intrinsic groups: mil, f90, vxt.
Description:
Returns value resulting from boolean NOT of each bit in I.
Or(I, J)
Or: INTEGER or LOGICAL function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER or LOGICAL; scalar; INTENT(IN).
J: INTEGER or LOGICAL; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Returns value resulting from boolean OR of pair of bits in each of I and J.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Pack to use this name for an external procedure.
CALL PError(String)
String: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Prints (on the C stderr stream) a newline-terminated error
message corresponding to the last system error.
This is prefixed by String, a colon and a space.
See perror(3).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Precision to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Present to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Product to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Radix to use this name for an external procedure.
Rand(Flag)
Rand: REAL(KIND=1) function.
Flag: INTEGER; OPTIONAL; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns a uniform quasi-random number between 0 and 1.
If Flag is 0, the next number in sequence is returned; if
Flag is 1, the generator is restarted by calling srand(0);
if Flag has any other value, it is used as a new seed with
srand.
See SRand Intrinsic.
Note: As typically implemented (by the routine of the same name in the C library), this random number generator is a very poor one, though the BSD and GNU libraries provide a much better implementation than the `traditional' one. On a different system you almost certainly want to use something better.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Random_Number to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Random_Seed to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Range to use this name for an external procedure.
Real(A)
Real: REAL function.
The exact type is REAL(KIND=1) when argument A is
any type other than COMPLEX, or when it is COMPLEX(KIND=1).
When A is any COMPLEX type other than COMPLEX(KIND=1),
this intrinsic is valid only when used as the argument to
REAL(), as explained below.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Converts A to REAL(KIND=1).
Use of REAL() with a COMPLEX argument
(other than COMPLEX(KIND=1)) is restricted to the following case:
REAL(REAL(A))
This expression converts the real part of A to
REAL(KIND=1).
See RealPart Intrinsic, for information on a GNU Fortran
intrinsic that extracts the real part of an arbitrary
COMPLEX value.
See REAL() and AIMAG() of Complex, for more information.
RealPart(Z)
RealPart: REAL function, the KIND= value of the type being that of argument Z.
Z: COMPLEX; scalar; INTENT(IN).
Intrinsic groups: gnu.
Description:
The real part of Z is returned, without conversion.
Note: The way to do this in standard Fortran 90
is REAL(Z).
However, when, for example, Z is COMPLEX(KIND=2),
REAL(Z) means something different for some compilers
that are not true Fortran 90 compilers but offer some
extensions standardized by Fortran 90 (such as the
DOUBLE COMPLEX type, also known as COMPLEX(KIND=2)).
The advantage of REALPART() is that, while not necessarily
more or less portable than REAL(), it is more likely to
cause a compiler that doesn't support it to produce a diagnostic
than generate incorrect code.
See REAL() and AIMAG() of Complex, for more information.
CALL Rename(Path1, Path2, Status)
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Renames the file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
See rename(2).
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See Rename Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Repeat to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Reshape to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL RRSpacing to use this name for an external procedure.
RShift(I, Shift)
RShift: INTEGER function, the KIND= value of the type being that of argument I.
I: INTEGER; scalar; INTENT(IN).
Shift: INTEGER; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Returns I shifted to the right Shift bits.
Although similar to the expression I/(2**Shift), there are important differences. For example, the sign of the result is undefined.
Currently this intrinsic is defined assuming the underlying representation of I is as a two's-complement integer. It is unclear at this point whether that definition will apply when a different representation is involved.
See RShift Intrinsic, for the inverse of this function.
See IShft Intrinsic, for information on a more widely available right-shifting intrinsic that is also more precisely defined.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Scale to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Scan to use this name for an external procedure.
Second()
Second: REAL(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the process's runtime in seconds—the same value as the
UNIX function etime returns.
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
For information on other intrinsics with the same name: See Second Intrinsic (subroutine).
CALL Second(Seconds)
Seconds: REAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Returns the process's runtime in seconds in Seconds—the same value
as the UNIX function etime returns.
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
This routine is known from Cray Fortran. See CPU_Time Intrinsic, for a standard equivalent.
For information on other intrinsics with the same name: See Second Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Selected_Int_Kind to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Selected_Real_Kind to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Set_Exponent to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Shape to use this name for an external procedure.
Short(A)
Short: INTEGER(KIND=6) function.
A: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns A with the fractional portion of its
magnitude truncated and its sign preserved, converted
to type INTEGER(KIND=6).
If A is type COMPLEX, its real part
is truncated and converted, and its imaginary part is disregarded.
See Int Intrinsic.
The precise meaning of this intrinsic might change in a future version of the GNU Fortran language, as more is learned about how it is used.
Sign(A, B)
Sign: INTEGER or REAL function, the exact type being the result of cross-promoting the
types of all the arguments.
A: INTEGER or REAL; scalar; INTENT(IN).
B: INTEGER or REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns ABS(A)*s, where s is +1 if B.GE.0, -1 otherwise.
See Abs Intrinsic, for the function that returns the magnitude of a value.
CALL Signal(Number, Handler, Status)
Number: INTEGER; scalar; INTENT(IN).
Handler: Signal handler (INTEGER FUNCTION or SUBROUTINE)
or dummy/global INTEGER(KIND=1) scalar.
Status: INTEGER(KIND=7); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
If Handler is a an EXTERNAL routine, arranges for it to be
invoked with a single integer argument (of system-dependent length)
when signal Number occurs.
If Handler is an integer, it can be
used to turn off handling of signal Number or revert to its default
action.
See signal(2).
Note that Handler will be called using C conventions,
so the value of its argument in Fortran terms
Fortran terms is obtained by applying %LOC() (or LOC()) to it.
The value returned by signal(2) is written to Status, if
that argument is supplied.
Otherwise the return value is ignored.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
Warning: Use of the libf2c run-time library function
signal_ directly
(such as via EXTERNAL SIGNAL)
requires use of the %VAL() construct
to pass an INTEGER value
(such as SIG_IGN or SIG_DFL)
for the Handler argument.
However, while CALL SIGNAL(signum, %VAL(SIG_IGN))
works when SIGNAL is treated as an external procedure
(and resolves, at link time, to libf2c's signal_ routine),
this construct is not valid when SIGNAL is recognized
as the intrinsic of that name.
Therefore, for maximum portability and reliability, code such references to the SIGNAL facility as follows:
INTRINSIC SIGNAL
...
CALL SIGNAL(signum, SIG_IGN)
g77 will compile such a call correctly,
while other compilers will generally either do so as well
or reject the INTRINSIC SIGNAL statement via a diagnostic,
allowing you to take appropriate action.
For information on other intrinsics with the same name: See Signal Intrinsic (function).
Sin(X)
Sin: REAL or COMPLEX function, the exact type being that of argument X.
X: REAL or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the sine of X, an angle measured in radians.
See ASin Intrinsic, for the inverse of this function.
SinH(X)
SinH: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the hyperbolic sine of X.
CALL Sleep(Seconds)
Seconds: INTEGER(KIND=1); scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Causes the process to pause for Seconds seconds.
See sleep(2).
Sngl(A)
Sngl: REAL(KIND=1) function.
A: REAL(KIND=2); scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Archaic form of REAL() that is specific
to one type for A.
See Real Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Spacing to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Spread to use this name for an external procedure.
SqRt(X)
SqRt: REAL or COMPLEX function, the exact type being that of argument X.
X: REAL or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the square root of X, which must not be negative.
To calculate and represent the square root of a negative number, complex arithmetic must be used. For example, SQRT(COMPLEX(X)).
The inverse of this function is SQRT(X) * SQRT(X).
CALL SRand(Seed)
Seed: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Reinitializes the generator with the seed in Seed. See IRand Intrinsic. See Rand Intrinsic.
CALL Stat(File, SArray, Status)
File: CHARACTER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the given file File and places them in the array
SArray.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
The values in this array are extracted from the
stat structure as returned by fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
If the Status argument is supplied, it contains 0 on success or a nonzero error code upon return.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See Stat Intrinsic (function).
Stat(File, SArray)
Stat: INTEGER(KIND=1) function.
File: CHARACTER; scalar; INTENT(IN).
SArray: INTEGER(KIND=1); DIMENSION(13); INTENT(OUT).
Intrinsic groups: unix.
Description:
Obtains data about the given file File and places them in the array
SArray.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
The values in this array are extracted from the
stat structure as returned by fstat(2) q.v., as follows:
Not all these elements are relevant on all systems. If an element is not relevant, it is returned as 0.
Returns 0 on success or a nonzero error code.
For information on other intrinsics with the same name: See Stat Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Sum to use this name for an external procedure.
CALL SymLnk(Path1, Path2, Status)
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Makes a symbolic link from file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return
(ENOSYS if the system does not provide symlink(2)).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See SymLnk Intrinsic (function).
CALL System(Command, Status)
Command: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Passes the command Command to a shell (see system(3)).
If argument Status is present, it contains the value returned by
system(3), presumably 0 if the shell command succeeded.
Note that which shell is used to invoke the command is system-dependent
and environment-dependent.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See System Intrinsic (function).
CALL System_Clock(Count, Rate, Max)
Count: INTEGER(KIND=1); scalar; INTENT(OUT).
Rate: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Max: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: f90.
Description:
Returns in Count the current value of the system clock; this is
the value returned by the UNIX function times(2)
in this implementation, but
isn't in general.
Rate is the number of clock ticks per second and
Max is the maximum value this can take, which isn't very useful
in this implementation since it's just the maximum C unsigned
int value.
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
Tan(X)
Tan: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the tangent of X, an angle measured in radians.
See ATan Intrinsic, for the inverse of this function.
TanH(X)
TanH: REAL function, the KIND= value of the type being that of argument X.
X: REAL; scalar; INTENT(IN).
Intrinsic groups: (standard FORTRAN 77).
Description:
Returns the hyperbolic tangent of X.
Time()
Time: INTEGER(KIND=1) function.
Intrinsic groups: unix.
Description:
Returns the current time encoded as an integer
(in the manner of the UNIX function time(3)).
This value is suitable for passing to CTIME,
GMTIME, and LTIME.
This intrinsic is not fully portable, such as to systems
with 32-bit INTEGER types but supporting times
wider than 32 bits.
Therefore, the values returned by this intrinsic
might be, or become, negative,
or numerically less than previous values,
during a single run of the compiled program.
See Time8 Intrinsic, for information on a similar intrinsic that might be portable to more GNU Fortran implementations, though to fewer Fortran compilers.
For information on other intrinsics with the same name: See Time Intrinsic (VXT).
Time8()
Time8: INTEGER(KIND=2) function.
Intrinsic groups: unix.
Description:
Returns the current time encoded as a long integer
(in the manner of the UNIX function time(3)).
This value is suitable for passing to CTIME,
GMTIME, and LTIME.
Warning: this intrinsic does not increase the range
of the timing values over that returned by time(3).
On a system with a 32-bit time(3),
TIME8 will return a 32-bit value,
even though converted to an INTEGER(KIND=2) value.
That means overflows of the 32-bit value can still occur.
Therefore, the values returned by this intrinsic
might be, or become, negative,
or numerically less than previous values,
during a single run of the compiled program.
No Fortran implementations other than GNU Fortran are known to support this intrinsic at the time of this writing. See Time Intrinsic (UNIX), for information on a similar intrinsic that might be portable to more Fortran compilers, though to fewer GNU Fortran implementations.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Tiny to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Transfer to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Transpose to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Trim to use this name for an external procedure.
CALL TtyNam(Unit, Name)
Unit: INTEGER; scalar; INTENT(IN).
Name: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets Name to the name of the terminal device open on logical unit Unit or to a blank string if Unit is not connected to a terminal.
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See TtyNam Intrinsic (function).
TtyNam(Unit)
TtyNam: CHARACTER*(*) function.
Unit: INTEGER; scalar; INTENT(IN).
Intrinsic groups: unix.
Description:
Returns the name of the terminal device open on logical unit Unit or a blank string if Unit is not connected to a terminal.
For information on other intrinsics with the same name: See TtyNam Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL UBound to use this name for an external procedure.
CALL UMask(Mask, Old)
Mask: INTEGER; scalar; INTENT(IN).
Old: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Sets the file creation mask to Mask and returns the old value in
argument Old if it is supplied.
See umask(2).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine.
For information on other intrinsics with the same name: See UMask Intrinsic (function).
CALL Unlink(File, Status)
File: CHARACTER; scalar; INTENT(IN).
Status: INTEGER(KIND=1); OPTIONAL; scalar; INTENT(OUT).
Intrinsic groups: unix.
Description:
Unlink the file File.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
If the Status argument is supplied, it contains
0 on success or a nonzero error code upon return.
See unlink(2).
Some non-GNU implementations of Fortran provide this intrinsic as only a function, not as a subroutine, or do not support the (optional) Status argument.
For information on other intrinsics with the same name: See Unlink Intrinsic (function).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Unpack to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL Verify to use this name for an external procedure.
XOr(I, J)
XOr: INTEGER or LOGICAL function, the exact type being the result of cross-promoting the
types of all the arguments.
I: INTEGER or LOGICAL; scalar; INTENT(IN).
J: INTEGER or LOGICAL; scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Returns value resulting from boolean exclusive-OR of pair of bits in each of I and J.
ZAbs(A)
ZAbs: REAL(KIND=2) function.
A: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of ABS() that is specific
to one type for A.
See Abs Intrinsic.
ZCos(X)
ZCos: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of COS() that is specific
to one type for X.
See Cos Intrinsic.
ZExp(X)
ZExp: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of EXP() that is specific
to one type for X.
See Exp Intrinsic.
ZLog(X)
ZLog: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of LOG() that is specific
to one type for X.
See Log Intrinsic.
ZSin(X)
ZSin: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of SIN() that is specific
to one type for X.
See Sin Intrinsic.
ZSqRt(X)
ZSqRt: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c.
Description:
Archaic form of SQRT() that is specific
to one type for X.
See SqRt Intrinsic.
(The following information augments or overrides the information in Chapter 18 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 18 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)
Underscores (_) are accepted in symbol names after the first character (which must be a letter).
A dollar sign at the end of an output format specification suppresses the newline at the end of the output.
Edit descriptors in FORMAT statements may contain compile-time
INTEGER constant expressions in angle brackets, such as
10 FORMAT (I<WIDTH>)
The OPEN specifier NAME= is equivalent to FILE=.
These Fortran 90 features are supported:
O and Z edit descriptors are supported for I/O of
integers in octal and hexadecimal formats, respectively.
FILE= specifier may be omitted in an OPEN statement if
STATUS='SCRATCH' is supplied. The STATUS='REPLACE'
specifier is supported.
For convenience this section collects a list (probably incomplete) of the Fortran 90 features supported by the GNU Fortran language, even if they are documented elsewhere. See Characters, Lines, and Execution Sequence, for information on additional fixed source form lexical issues. Further, the free source form is supported through the -ffree-form option. Other Fortran 90 features can be turned on by the -ff90 option; see Fortran 90. For information on the Fortran 90 intrinsics available, see Table of Intrinsic Functions.
") as well as single quotes. See Character Type.
CYCLE and EXITCYCLE and EXIT Statements.
DOUBLE COMPLEXDOUBLE COMPLEX Statement.
DO WHILEEND decorationEND DOKINDIMPLICIT NONEINCLUDE statementsNAMELISTOPEN specifiersSTATUS='REPLACE' is supported.
The FILE= specifier may be omitted in an OPEN statement if
STATUS='SCRATCH' is supplied.
FORMAT edit descriptorsZ edit descriptor is supported.
<, <=, ==, /=, > and
>= may be used instead of .LT., .LE., .EQ.,
.NE., .GT. and .GE. respectively.
SELECT CASESELECT CASE on CHARACTER Type.
KIND. See Kind Notation.
(KIND is of limited usefulness in the absence of the
KIND-related intrinsics, since these intrinsics permit writing
more widely portable code.) An example of supported KIND usage
is:
INTEGER (KIND=1) :: FOO=1, BAR=2
CHARACTER (LEN=3) FOO
PARAMETER and DIMENSION attributes aren't supported.
GNU Fortran supports a variety of features that are not considered part of the GNU Fortran language itself, but are representative of various dialects of Fortran that g77 supports in whole or in part.
Any of the features listed below might be disallowed by g77 unless some command-line option is specified. Currently, some of the features are accepted using the default invocation of g77, but that might change in the future.
Note: This portion of the documentation definitely needs a lot of work!
GNU Fortran accepts programs written in either fixed form or free form.
Fixed form corresponds to ANSI FORTRAN 77 (plus popular extensions, such as allowing tabs) and Fortran 90's fixed form.
Free form corresponds to Fortran 90's free form (though possibly not entirely up-to-date, and without complaining about some things that for which Fortran 90 requires diagnostics, such as the spaces in the constant in R = 3 . 1).
The way a Fortran compiler views source files depends entirely on the implementation choices made for the compiler, since those choices are explicitly left to the implementation by the published Fortran standards. GNU Fortran currently tries to be somewhat like a few popular compilers (f2c, Digital (“DEC”) Fortran, and so on).
This section describes how g77 interprets source lines.
Carriage returns (\r) in source lines are ignored. This is somewhat different from f2c, which seems to treat them as spaces outside character/Hollerith constants, and encodes them as \r inside such constants.
A source line with a <TAB> character anywhere in it is treated as entirely significant—however long it is—instead of ending in column 72 (for fixed-form source) or 132 (for free-form source). This also is different from f2c, which encodes tabs as \t (the ASCII <TAB> character) inside character and Hollerith constants, but nevertheless seems to treat the column position as if it had been affected by the canonical tab positioning.
g77 effectively translates tabs to the appropriate number of spaces (a la the default for the UNIX expand command) before doing any other processing, other than (currently) noting whether a tab was found on a line and using this information to decide how to interpret the length of the line and continued constants.
Source lines shorter than the applicable fixed-form length are treated as if they were padded with spaces to that length. (None of this is relevant to source files written in free form.)
This affects only continued character and Hollerith constants, and is a different interpretation than provided by some other popular compilers (although a bit more consistent with the traditional punched-card basis of Fortran and the way the Fortran standard expressed fixed source form).
g77 might someday offer an option to warn about cases where differences might be seen as a result of this treatment, and perhaps an option to specify the alternate behavior as well.
Note that this padding cannot apply to lines that are effectively of infinite length—such lines are specified using command-line options like -ffixed-line-length-none, for example.
Source lines longer than the applicable length are truncated to that length. Currently, g77 does not warn if the truncated characters are not spaces, to accommodate existing code written for systems that treated truncated text as commentary (especially in columns 73 through 80).
See Options Controlling Fortran Dialect, for information on the -ffixed-line-length-n option, which can be used to set the line length applicable to fixed-form source files.
A & in column 1 of fixed-form source denotes an arbitrary-length continuation line, imitating the behavior of f2c.
g77 supports use of /* to start a trailing comment. In the GNU Fortran language, ! is used for this purpose.
/* is not in the GNU Fortran language because the use of /* in a program might suggest to some readers that a block, not trailing, comment is started (and thus ended by */, not end of line), since that is the meaning of /* in C.
Also, such readers might think they can use // to start a trailing comment as an alternative to /*, but // already denotes concatenation, and such a “comment” might actually result in a program that compiles without error (though it would likely behave incorrectly).
Use of D or d as the first character (column 1) of a source line denotes a debug line.
In turn, a debug line is treated as either a comment line or a normal line, depending on whether debug lines are enabled.
When treated as a comment line, a line beginning with D or d is treated as if it the first character was C or c, respectively. When treated as a normal line, such a line is treated as if the first character was <SPC> (space).
(Currently, g77 provides no means for treating debug lines as normal lines.)
Dollar signs ($) are allowed in symbol names (after the first character) when the -fdollar-ok option is specified.
GNU Fortran offers the programmer way too much flexibility in deciding how source files are to be treated vis-a-vis uppercase and lowercase characters. There are 66 useful settings that affect case sensitivity, plus 10 settings that are nearly useless, with the remaining 116 settings being either redundant or useless.
None of these settings have any effect on the contents of comments (the text after a c or C in Column 1, for example) or of character or Hollerith constants. Note that things like the E in the statement CALL FOO(3.2E10) and the TO in ASSIGN 10 TO LAB are considered built-in keywords, and so are affected by these settings.
Low-level switches are identified in this section as follows:
Note 1: g77 eventually will support NAMELIST in a manner that is
consistent with these source switches—in the sense that input will be
expected to meet the same requirements as source code in terms
of matching symbol names and keywords (for the exponent letters).
Currently, however, NAMELIST is supported by libg2c,
which uppercases NAMELIST input and symbol names for matching.
This means not only that NAMELIST output currently shows symbol
(and keyword) names in uppercase even if lower-case source
conversion (option A2) is selected, but that NAMELIST cannot be
adequately supported when source case preservation (option A0)
is selected.
If A0 is selected, a warning message will be
output for each NAMELIST statement to this effect.
The behavior
of the program is undefined at run time if two or more symbol names
appear in a given NAMELIST such that the names are identical
when converted to upper case (e.g. NAMELIST /X/ VAR, Var, var).
For complete and total elegance, perhaps there should be a warning
when option A2 is selected, since the output of NAMELIST is currently
in uppercase but will someday be lowercase (when a libg77 is written),
but that seems to be overkill for a product in beta test.
Note 2: Rules for InitialCaps names are:
So A, Ab, ABc, AbC, and Abc are
valid InitialCaps names, but AB, A2, and ABC are
not.
Note that most, but not all, built-in names meet these
requirements—the exceptions are some of the two-letter format
specifiers, such as BN and BZ.
Here are the names of the corresponding command-line options:
A0: -fsource-case-preserve
A1: -fsource-case-upper
A2: -fsource-case-lower
B0: -fmatch-case-any
B1: -fmatch-case-upper
B2: -fmatch-case-lower
B3: -fmatch-case-initcap
C0: -fintrin-case-any
C1: -fintrin-case-upper
C2: -fintrin-case-lower
C3: -fintrin-case-initcap
D0: -fsymbol-case-any
D1: -fsymbol-case-upper
D2: -fsymbol-case-lower
D3: -fsymbol-case-initcap
Useful combinations of the above settings, along with abbreviated option names that set some of these combinations all at once:
1: A0-- B0--- C0--- D0--- -fcase-preserve
2: A0-- B0--- C0--- D-1--
3: A0-- B0--- C0--- D--2-
4: A0-- B0--- C0--- D---3
5: A0-- B0--- C-1-- D0---
6: A0-- B0--- C-1-- D-1--
7: A0-- B0--- C-1-- D--2-
8: A0-- B0--- C-1-- D---3
9: A0-- B0--- C--2- D0---
10: A0-- B0--- C--2- D-1--
11: A0-- B0--- C--2- D--2-
12: A0-- B0--- C--2- D---3
13: A0-- B0--- C---3 D0---
14: A0-- B0--- C---3 D-1--
15: A0-- B0--- C---3 D--2-
16: A0-- B0--- C---3 D---3
17: A0-- B-1-- C0--- D0---
18: A0-- B-1-- C0--- D-1--
19: A0-- B-1-- C0--- D--2-
20: A0-- B-1-- C0--- D---3
21: A0-- B-1-- C-1-- D0---
22: A0-- B-1-- C-1-- D-1-- -fcase-strict-upper
23: A0-- B-1-- C-1-- D--2-
24: A0-- B-1-- C-1-- D---3
25: A0-- B-1-- C--2- D0---
26: A0-- B-1-- C--2- D-1--
27: A0-- B-1-- C--2- D--2-
28: A0-- B-1-- C--2- D---3
29: A0-- B-1-- C---3 D0---
30: A0-- B-1-- C---3 D-1--
31: A0-- B-1-- C---3 D--2-
32: A0-- B-1-- C---3 D---3
33: A0-- B--2- C0--- D0---
34: A0-- B--2- C0--- D-1--
35: A0-- B--2- C0--- D--2-
36: A0-- B--2- C0--- D---3
37: A0-- B--2- C-1-- D0---
38: A0-- B--2- C-1-- D-1--
39: A0-- B--2- C-1-- D--2-
40: A0-- B--2- C-1-- D---3
41: A0-- B--2- C--2- D0---
42: A0-- B--2- C--2- D-1--
43: A0-- B--2- C--2- D--2- -fcase-strict-lower
44: A0-- B--2- C--2- D---3
45: A0-- B--2- C---3 D0---
46: A0-- B--2- C---3 D-1--
47: A0-- B--2- C---3 D--2-
48: A0-- B--2- C---3 D---3
49: A0-- B---3 C0--- D0---
50: A0-- B---3 C0--- D-1--
51: A0-- B---3 C0--- D--2-
52: A0-- B---3 C0--- D---3
53: A0-- B---3 C-1-- D0---
54: A0-- B---3 C-1-- D-1--
55: A0-- B---3 C-1-- D--2-
56: A0-- B---3 C-1-- D---3
57: A0-- B---3 C--2- D0---
58: A0-- B---3 C--2- D-1--
59: A0-- B---3 C--2- D--2-
60: A0-- B---3 C--2- D---3
61: A0-- B---3 C---3 D0---
62: A0-- B---3 C---3 D-1--
63: A0-- B---3 C---3 D--2-
64: A0-- B---3 C---3 D---3 -fcase-initcap
65: A-1- B01-- C01-- D01-- -fcase-upper
66: A--2 B0-2- C0-2- D0-2- -fcase-lower
Number 22 is the “strict” ANSI FORTRAN 77 model wherein all input (except comments, character constants, and Hollerith strings) must be entered in uppercase. Use -fcase-strict-upper to specify this combination.
Number 43 is like Number 22 except all input must be lowercase. Use -fcase-strict-lower to specify this combination.
Number 65 is the “classic” ANSI FORTRAN 77 model as implemented on many non-UNIX machines whereby all the source is translated to uppercase. Use -fcase-upper to specify this combination.
Number 66 is the “canonical” UNIX model whereby all the source is translated to lowercase. Use -fcase-lower to specify this combination.
There are a few nearly useless combinations:
67: A-1- B01-- C01-- D--2-
68: A-1- B01-- C01-- D---3
69: A-1- B01-- C--23 D01--
70: A-1- B01-- C--23 D--2-
71: A-1- B01-- C--23 D---3
72: A--2 B01-- C0-2- D-1--
73: A--2 B01-- C0-2- D---3
74: A--2 B01-- C-1-3 D0-2-
75: A--2 B01-- C-1-3 D-1--
76: A--2 B01-- C-1-3 D---3
The above allow some programs to be compiled but with restrictions that make most useful programs impossible: Numbers 67 and 72 warn about any user-defined symbol names (such as SUBROUTINE FOO); Numbers 68 and 73 warn about any user-defined symbol names longer than one character that don't have at least one non-alphabetic character after the first; Numbers 69 and 74 disallow any references to intrinsics; and Numbers 70, 71, 75, and 76 are combinations of the restrictions in 67+69, 68+69, 72+74, and 73+74, respectively.
All redundant combinations are shown in the above tables anyplace where more than one setting is shown for a low-level switch. For example, B0-2- means either setting 0 or 2 is valid for switch B. The “proper” setting in such a case is the one that copies the setting of switch A—any other setting might slightly reduce the speed of the compiler, though possibly to an unmeasurable extent.
All remaining combinations are useless in that they prevent successful compilation of non-null source files (source files with something other than comments).
g77 supports certain constructs that have different meanings in VXT Fortran than they do in the GNU Fortran language.
Generally, this manual uses the invented term VXT Fortran to refer VAX FORTRAN (circa v4). That compiler offered many popular features, though not necessarily those that are specific to the VAX processor architecture, the VMS operating system, or Digital Equipment Corporation's Fortran product line. (VAX and VMS probably are trademarks of Digital Equipment Corporation.)
An extension offered by a Digital Fortran product that also is offered by several other Fortran products for different kinds of systems is probably going to be considered for inclusion in g77 someday, and is considered a VXT Fortran feature.
The -fvxt option generally specifies that, where the meaning of a construct is ambiguous (means one thing in GNU Fortran and another in VXT Fortran), the VXT Fortran meaning is to be assumed.
g77 treats double-quote (")
as beginning an octal constant of INTEGER(KIND=1) type
when the -fvxt option is specified.
The form of this octal constant is
"octal-digits
where octal-digits is a nonempty string of characters in the set 01234567.
For example, the -fvxt option permits this:
PRINT *, "20
END
The above program would print the value 16.
See Integer Type, for information on the preferred construct for integer constants specified using GNU Fortran's octal notation.
(In the GNU Fortran language, the double-quote character (") delimits a character constant just as does apostrophe ('). There is no way to allow both constructs in the general case, since statements like PRINT *,"2000 !comment?" would be ambiguous.)
g77 treats an exclamation point (!) in column 6 of a fixed-form source file as a continuation character rather than as the beginning of a comment (as it does in any other column) when the -fvxt option is specified.
The following program, when run, prints a message indicating whether it is interpreted according to GNU Fortran (and Fortran 90) rules or VXT Fortran rules:
C234567 (This line begins in column 1.)
I = 0
!1
IF (I.EQ.0) PRINT *, ' I am a VXT Fortran program'
IF (I.EQ.1) PRINT *, ' I am a Fortran 90 program'
IF (I.LT.0 .OR. I.GT.1) PRINT *, ' I am a HAL 9000 computer'
END
(In the GNU Fortran and Fortran 90 languages, exclamation point is a valid character and, unlike space (<SPC>) or zero (0), marks a line as a continuation line when it appears in column 6.)
The GNU Fortran language includes a number of features that are part of Fortran 90, even when the -ff90 option is not specified. The features enabled by -ff90 are intended to be those that, when -ff90 is not specified, would have another meaning to g77—usually meaning something invalid in the GNU Fortran language.
So, the purpose of -ff90 is not to specify whether g77 is to gratuitously reject Fortran 90 constructs. The -pedantic option specified with -fno-f90 is intended to do that, although its implementation is certainly incomplete at this point.
When -ff90 is specified:
COMPLEX type,
is the same type as the real part of expr.
For example, assuming Z is type COMPLEX(KIND=2),
REAL(Z) would return a value of type REAL(KIND=2),
not of type REAL(KIND=1), since -ff90 is specified.
The -fpedantic command-line option specifies that g77 is to warn about code that is not standard-conforming. This is useful for finding some extensions g77 accepts that other compilers might not accept. (Note that the -pedantic and -pedantic-errors options always imply -fpedantic.)
With -fno-f90 in force, ANSI FORTRAN 77 is used as the standard for conforming code. With -ff90 in force, Fortran 90 is used.
The constructs for which g77 issues diagnostics when -fpedantic and -fno-f90 are in force are:
SUBROUTINE X(N)
REAL A(N)
...
where A is not listed in any ENTRY statement,
and thus is not a dummy argument.
These commas are disallowed by FORTRAN 77, but, while strictly superfluous, are syntactically elegant, especially given that commas are required in statements such as READ 99, I and PRINT *, J. Many compilers permit the superfluous commas for this reason.
DOUBLE COMPLEX, either explicitly or implicitly.
An explicit use of this type is via a DOUBLE COMPLEX or
IMPLICIT DOUBLE COMPLEX statement, for examples.
An example of an implicit use is the expression C*D,
where C is COMPLEX(KIND=1)
and D is DOUBLE PRECISION.
This expression is prohibited by ANSI FORTRAN 77
because the rules of promotion would suggest that it
produce a DOUBLE COMPLEX result—a type not
provided for by that standard.
INTEGER(KIND=1) in contexts such as:
GOTO.
FORMAT run-time expressions (not yet supported).
CHARACTER entities in specification statements.
DO
constructs in DATA statements.
LOGICAL expressions to INTEGER
in contexts such as arithmetic IF (where COMPLEX
expressions are disallowed anyway).
INTEGER I(10,20,4:2)
CHARACTER entities, as in:
PRINT *, ''
PRINT *, 'hello'(3:5)
PRINT *, FOO(,3)
COMMON
area is SAVEd (for targets where program units in a single source
file are “glued” together as they typically are for UNIX development
environments).
COMMON block.
DATA statement.
(In the GNU Fortran language, DATA I/1/ may be followed by INTEGER J, but not INTEGER I. The -fpedantic option disallows both of these.)
CALL FOO; CALL BAR
CHARACTER constants to initialize numeric entities, and vice
versa.
If -fpedantic is specified along with -ff90, the following constructs result in diagnostics:
INCLUDE directive.
The -fugly-* command-line options determine whether certain features supported by VAX FORTRAN and other such compilers, but considered too ugly to be in code that can be changed to use safer and/or more portable constructs, are accepted. These are humorously referred to as “distensions”, extensions that just plain look ugly in the harsh light of day.
The -fno-ugly-args option disables passing typeless and Hollerith constants as actual arguments in procedure invocations. For example:
CALL FOO(4HABCD)
CALL BAR('123'O)
These constructs can be too easily used to create non-portable code, but are not considered as “ugly” as others. Further, they are widely used in existing Fortran source code in ways that often are quite portable. Therefore, they are enabled by default.
The -fugly-assumed option enables the treatment of any array with a final dimension specified as 1 as an assumed-size array, as if * had been specified instead.
For example, DIMENSION X(1) is treated as if it
had read DIMENSION X(*) if X is listed as
a dummy argument in a preceding SUBROUTINE, FUNCTION,
or ENTRY statement in the same program unit.
Use an explicit lower bound to avoid this interpretation. For example, DIMENSION X(1:1) is never treated as if it had read DIMENSION X(*) or DIMENSION X(1:*). Nor is DIMENSION X(2-1) affected by this option, since that kind of expression is unlikely to have been intended to designate an assumed-size array.
This option is used to prevent warnings being issued about apparent out-of-bounds reference such as X(2) = 99.
It also prevents the array from being used in contexts that disallow assumed-size arrays, such as PRINT *,X. In such cases, a diagnostic is generated and the source file is not compiled.
The construct affected by this option is used only in old code that pre-exists the widespread acceptance of adjustable and assumed-size arrays in the Fortran community.
Note: This option does not affect how DIMENSION X(1) is
treated if X is listed as a dummy argument only
after the DIMENSION statement (presumably in
an ENTRY statement).
For example, -fugly-assumed has no effect on the
following program unit:
SUBROUTINE X
REAL A(1)
RETURN
ENTRY Y(A)
PRINT *, A
END
The -fugly-complex option enables
use of the REAL() and AIMAG()
intrinsics with arguments that are
COMPLEX types other than COMPLEX(KIND=1).
With -ff90 in effect, these intrinsics return the unconverted real and imaginary parts (respectively) of their argument.
With -fno-f90 in effect, these intrinsics convert
the real and imaginary parts to REAL(KIND=1), and return
the result of that conversion.
Due to this ambiguity, the GNU Fortran language defines
these constructs as invalid, except in the specific
case where they are entirely and solely passed as an
argument to an invocation of the REAL() intrinsic.
For example,
REAL(REAL(Z))
is permitted even when Z is COMPLEX(KIND=2)
and -fno-ugly-complex is in effect, because the
meaning is clear.
g77 enforces this restriction, unless -fugly-complex is specified, in which case the appropriate interpretation is chosen and no diagnostic is issued.
See CMPAMBIG, for information on how to cope with existing
code with unclear expectations of REAL() and AIMAG()
with COMPLEX(KIND=2) arguments.
See RealPart Intrinsic, for information on the REALPART()
intrinsic, used to extract the real part of a complex expression
without conversion.
See ImagPart Intrinsic, for information on the IMAGPART()
intrinsic, used to extract the imaginary part of a complex expression
without conversion.
The -fugly-comma option enables use of a single trailing comma to mean “pass an extra trailing null argument” in a list of actual arguments to an external procedure, and use of an empty list of arguments to such a procedure to mean “pass a single null argument”.
(Null arguments often are used in some procedure-calling schemes to indicate omitted arguments.)
For example, CALL FOO(,) means “pass two null arguments”, rather than “pass one null argument”. Also, CALL BAR() means “pass one null argument”.
This construct is considered “ugly” because it does not provide an elegant way to pass a single null argument that is syntactically distinct from passing no arguments. That is, this construct changes the meaning of code that makes no use of the construct.
So, with -fugly-comma in force, CALL FOO() and I = JFUNC() pass a single null argument, instead of passing no arguments as required by the Fortran 77 and 90 standards.
Note: Many systems gracefully allow the case where a procedure call passes one extra argument that the called procedure does not expect.
So, in practice, there might be no difference in the behavior of a program that does CALL FOO() or I = JFUNC() and is compiled with -fugly-comma in force as compared to its behavior when compiled with the default, -fno-ugly-comma, in force, assuming FOO and JFUNC do not expect any arguments to be passed.
The constructs disabled by -fno-ugly-init are:
DATA and PARAMETER statements, plus
type-declaration statements specifying initial values.
Here are some sample initializations that are disabled by the -fno-ugly-init option:
PARAMETER (VAL='9A304FFE'X)
REAL*8 STRING/8HOUTPUT00/
DATA VAR/4HABCD/
Here are more sample initializations that are disabled by the -fno-ugly-init option:
INTEGER IA
CHARACTER BELL
PARAMETER (IA = 'A')
PARAMETER (BELL = 7)
Here are sample statements that are disabled by the -fno-ugly-init option:
IVAR = 4HABCD
PRINT *, IMAX0(2HAB, 2HBA)
The above constructs, when used, can tend to result in non-portable code. But, they are widely used in existing Fortran code in ways that often are quite portable. Therefore, they are enabled by default.
The constructs enabled via -fugly-logint are:
INTEGER and LOGICAL as
dictated by
context (typically implies nonportable dependencies on how a
particular implementation encodes .TRUE. and .FALSE.).
LOGICAL variable in ASSIGN and assigned-GOTO
statements.
The above constructs are disabled by default because use of them tends to lead to non-portable code. Even existing Fortran code that uses that often turns out to be non-portable, if not outright buggy.
Some of this is due to differences among implementations as
far as how .TRUE. and .FALSE. are encoded as
INTEGER values—Fortran code that assumes a particular
coding is likely to use one of the above constructs, and is
also likely to not work correctly on implementations using
different encodings.
See Equivalence Versus Equality, for more information.
The -fugly-assign option forces g77 to use the same storage for assigned labels as it would for a normal assignment to the same variable.
For example, consider the following code fragment:
I = 3
ASSIGN 10 TO I
Normally, for portability and improved diagnostics, g77
reserves distinct storage for a “sibling” of I, used
only for ASSIGN statements to that variable (along with
the corresponding assigned-GOTO and assigned-FORMAT-I/O
statements that reference the variable).
However, some code (that violates the ANSI FORTRAN 77 standard)
attempts to copy assigned labels among variables involved with
ASSIGN statements, as in:
ASSIGN 10 TO I
ISTATE(5) = I
...
J = ISTATE(ICUR)
GOTO J
Such code doesn't work under g77 unless -fugly-assign
is specified on the command-line, ensuring that the value of I
referenced in the second line is whatever value g77 uses
to designate statement label 10, so the value may be
copied into the ISTATE array, later retrieved into a
variable of the appropriate type (J), and used as the target of
an assigned-GOTO statement.
Note: To avoid subtle program bugs,
when -fugly-assign is specified,
g77 requires the type of variables
specified in assigned-label contexts
must be the same type returned by %LOC().
On many systems, this type is effectively the same
as INTEGER(KIND=1), while, on others, it is
effectively the same as INTEGER(KIND=2).
Do not depend on g77 actually writing valid pointers to these variables, however. While g77 currently chooses that implementation, it might be changed in the future.
See Assigned Statement Labels (ASSIGN and GOTO), for implementation details on assigned-statement labels.
The GNU Fortran compiler, g77, supports programs written in the GNU Fortran language and in some other dialects of Fortran.
Some aspects of how g77 works are universal regardless of dialect, and yet are not properly part of the GNU Fortran language itself. These are described below.
Note: This portion of the documentation definitely needs a lot of work!
g77, as with GNU tools in general, imposes few arbitrary restrictions on lengths of identifiers, number of continuation lines, number of external symbols in a program, and so on.
For example, some other Fortran compiler have an option (such as -Nlx) to increase the limit on the number of continuation lines. Also, some Fortran compilation systems have an option (such as -Nxx) to increase the limit on the number of external symbols.
g77, gcc, and GNU ld (the GNU linker) have no equivalent options, since they do not impose arbitrary limits in these areas.
g77 does currently limit the number of dimensions in an array to the same degree as do the Fortran standards—seven (7). This restriction might be lifted in a future version.
As a portable Fortran implementation, g77 offers its users direct access to, and otherwise depends upon, the underlying facilities of the system used to build g77, the system on which g77 itself is used to compile programs, and the system on which the g77-compiled program is actually run. (For most users, the three systems are of the same type—combination of operating environment and hardware—often the same physical system.)
The run-time environment for a particular system inevitably imposes some limits on a program's use of various system facilities. These limits vary from system to system.
Even when such limits might be well beyond the possibility of being encountered on a particular system, the g77 run-time environment has certain built-in limits, usually, but not always, stemming from intrinsics with inherently limited interfaces.
Currently, the g77 run-time environment does not generally offer a less-limiting environment by augmenting the underlying system's own environment.
Therefore, code written in the GNU Fortran language, while syntactically and semantically portable, might nevertheless make non-portable assumptions about the run-time environment—assumptions that prove to be false for some particular environments.
The GNU Fortran language, the g77 compiler and run-time environment, and the g77 documentation do not yet offer comprehensive portable work-arounds for such limits, though programmers should be able to find their own in specific instances.
Not all of the limitations are described in this document. Some of the known limitations include:
Intrinsics that return values computed from system timers, whether elapsed (wall-clock) timers, process CPU timers, or other kinds of timers, are prone to experiencing wrap-around errors (or returning wrapped-around values from successive calls) due to insufficient ranges offered by the underlying system's timers.
Some of the symptoms of such behaviors include apparently negative time being computed for a duration, an extremely short amount of time being computed for a long duration, and an extremely long amount of time being computed for a short duration.
See the following for intrinsics known to have potential problems in these areas on at least some systems: CPU_Time Intrinsic, DTime Intrinsic (function), DTime Intrinsic (subroutine), ETime Intrinsic (function), ETime Intrinsic (subroutine), MClock Intrinsic, MClock8 Intrinsic, Secnds Intrinsic, Second Intrinsic (function), Second Intrinsic (subroutine), System_Clock Intrinsic, Time Intrinsic (UNIX), Time Intrinsic (VXT), Time8 Intrinsic.
While the g77 compiler itself is believed to be Year-2000 (Y2K) compliant, some intrinsics are not, and, potentially, some underlying systems are not, perhaps rendering some Y2K-compliant intrinsics non-compliant when used on those particular systems.
Fortran code that uses non-Y2K-compliant intrinsics (listed below) is, itself, almost certainly not compliant, and should be modified to use Y2K-compliant intrinsics instead.
Fortran code that uses no non-Y2K-compliant intrinsics, but which currently is running on a non-Y2K-compliant system, can be made more Y2K compliant by compiling and linking it for use on a new Y2K-compliant system, such as a new version of an old, non-Y2K-compliant, system.
Currently, information on Y2K and related issues is being maintained at http://www.gnu.org/software/year2000-list.html.
See the following for intrinsics known to have potential problems in these areas on at least some systems: Date Intrinsic, IDate Intrinsic (VXT).
The libg2c library
shipped with any g77 that warns
about invocation of a non-Y2K-compliant intrinsic
has renamed the EXTERNAL procedure names
of those intrinsics.
This is done so that
the libg2c implementations of these intrinsics
cannot be directly linked to
as EXTERNAL names
(which normally would avoid the non-Y2K-intrinsic warning).
The renamed forms of the EXTERNAL names
of these renamed procedures
may be linked to
by appending the string _y2kbug
to the name of the procedure
in the source code.
For example:
CHARACTER*20 STR
INTEGER YY, MM, DD
EXTERNAL DATE_Y2KBUG, VXTIDATE_Y2KBUG
CALL DATE_Y2KBUG (STR)
CALL VXTIDATE_Y2KBUG (MM, DD, YY)
(Note that the EXTERNAL statement
is not actually required,
since the modified names are not recognized as intrinsics
by the current version of g77.
But it is shown in this specific case,
for purposes of illustration.)
The renaming of EXTERNAL procedure names of these intrinsics
causes unresolved references at link time.
For example, EXTERNAL DATE; CALL DATE(STR)
is normally compiled by g77
as, in C, date_(&str, 20);.
This, in turn, links to the date_ procedure
in the libE77 portion of libg2c,
which purposely calls a nonexistent procedure
named G77_date_y2kbuggy_0.
The resulting link-time error is designed, via this name,
to encourage the programmer to look up the
index entries to this portion of the g77 documentation.
Generally, we recommend that the EXTERNAL method
of invoking procedures in libg2c
not be used.
When used, some of the correctness checking
normally performed by g77
is skipped.
In particular, it is probably better to use the
INTRINSIC method of invoking
non-Y2K-compliant procedures,
so anyone compiling the code
can quickly notice the potential Y2K problems
(via the warnings printing by g77)
without having to even look at the code itself.
If there are problems linking libg2c
to code compiled by g77
that involve the string y2kbug,
and these are not explained above,
that probably indicates
that a version of libg2c
older than g77
is being linked to,
or that the new library is being linked
to code compiled by an older version of g77.
That's because, as of the version that warns about
non-Y2K-compliant intrinsic invocation,
g77 references the libg2c implementations
of those intrinsics
using new names, containing the string y2kbug.
So, linking newly-compiled code
(invoking one of the intrinsics in question)
to an old library
might yield an unresolved reference
to G77_date_y2kbug_0.
(The old library calls it G77_date_0.)
Similarly, linking previously-compiled code
to a new library
might yield an unresolved reference
to G77_vxtidate_0.
(The new library calls it G77_vxtidate_y2kbug_0.)
The proper fix for the above problems
is to obtain the latest release of g77
and related products
(including libg2c)
and install them on all systems,
then recompile, relink, and install
(as appropriate)
all existing Fortran programs.
(Normally, this sort of renaming is steadfastly avoided.
In this case, however, it seems more important to highlight
potential Y2K problems
than to ease the transition
of potentially non-Y2K-compliant code
to new versions of g77 and libg2c.)
Currently, g77 uses the default INTEGER type
for array indexes,
which limits the sizes of single-dimension arrays
on systems offering a larger address space
than can be addressed by that type.
(That g77 puts all arrays in memory
could be considered another limitation—it
could use large temporary files—but that decision
is left to the programmer as an implementation choice
by most Fortran implementations.)
It is not yet clear whether this limitation never, sometimes, or always applies to the sizes of multiple-dimension arrays as a whole.
For example, on a system with 64-bit addresses
and 32-bit default INTEGER,
an array with a size greater than can be addressed
by a 32-bit offset
can be declared using multiple dimensions.
Such an array is therefore larger
than a single-dimension array can be,
on the same system.
Whether large multiple-dimension arrays are reliably supported depends mostly on the gcc back end (code generator) used by g77, and has not yet been fully investigated.
Currently, g77 uses the default INTEGER type
for the lengths of CHARACTER variables
and array elements.
This means that, for example,
a system with a 64-bit address space
and a 32-bit default INTEGER type
does not, under g77,
support a CHARACTER*n declaration
where n is greater than 2147483647.
Most intrinsics returning, or computing values based on, date information are prone to Year-10000 (Y10K) problems, due to supporting only 4 digits for the year.
See the following for examples: FDate Intrinsic (function), FDate Intrinsic (subroutine), IDate Intrinsic (UNIX), Time Intrinsic (VXT), Date_and_Time Intrinsic.
Fortran implementations have a fair amount of freedom given them by the
standard as far as how much storage space is used and how much precision
and range is offered by the various types such as LOGICAL(KIND=1),
INTEGER(KIND=1), REAL(KIND=1), REAL(KIND=2),
COMPLEX(KIND=1), and CHARACTER.
Further, many compilers offer so-called *n notation, but
the interpretation of n varies across compilers and target architectures.
The standard requires that LOGICAL(KIND=1), INTEGER(KIND=1),
and REAL(KIND=1)
occupy the same amount of storage space, and that COMPLEX(KIND=1)
and REAL(KIND=2) take twice as much storage space as REAL(KIND=1).
Further, it requires that COMPLEX(KIND=1)
entities be ordered such that when a COMPLEX(KIND=1) variable is
storage-associated (such as via EQUIVALENCE)
with a two-element REAL(KIND=1) array named R, R(1)
corresponds to the real element and R(2) to the imaginary
element of the COMPLEX(KIND=1) variable.
(Few requirements as to precision or ranges of any of these are
placed on the implementation, nor is the relationship of storage sizes of
these types to the CHARACTER type specified, by the standard.)
g77 follows the above requirements, warning when compiling
a program requires placement of items in memory that contradict the
requirements of the target architecture.
(For example, a program can require placement of a REAL(KIND=2)
on a boundary that is not an even multiple of its size, but still an
even multiple of the size of a REAL(KIND=1) variable.
On some target architectures, using the canonical
mapping of Fortran types to underlying architectural types, such
placement is prohibited by the machine definition or
the Application Binary Interface (ABI) in force for
the configuration defined for building gcc and g77.
g77 warns about such
situations when it encounters them.)
g77 follows consistent rules for configuring the mapping between Fortran types, including the *n notation, and the underlying architectural types as accessed by a similarly-configured applicable version of the gcc compiler. These rules offer a widely portable, consistent Fortran/C environment, although they might well conflict with the expectations of users of Fortran compilers designed and written for particular architectures.
These rules are based on the configuration that is in force for the
version of gcc built in the same release as g77 (and
which was therefore used to build both the g77 compiler
components and the libg2c run-time library):
REAL(KIND=1)float type.
REAL(KIND=2)float—usually, this is a double.
INTEGER(KIND=1)float—usually, this is either
an int or a long int.
LOGICAL(KIND=1)INTEGER(KIND=1).
INTEGER(KIND=2)INTEGER(KIND=1)—usually, this is either
a long int or a long long int.
LOGICAL(KIND=2)INTEGER(KIND=2).
INTEGER(KIND=3)char.
LOGICAL(KIND=3)INTEGER(KIND=3).
INTEGER(KIND=6)INTEGER(KIND=3)—usually, this is
a short.
LOGICAL(KIND=6)INTEGER(KIND=6).
COMPLEX(KIND=1)REAL(KIND=1) scalars (one for the real part followed by
one for the imaginary part).
COMPLEX(KIND=2)REAL(KIND=2) scalars.
*nCHARACTER.)
Same as whatever gcc type occupies n times the storage
space of a gcc char item.
DOUBLE PRECISIONREAL(KIND=2).
DOUBLE COMPLEXCOMPLEX(KIND=2).
Note that the above are proposed correspondences and might change in future versions of g77—avoid writing code depending on them.
Other types supported by g77
are derived from gcc types such as char, short,
int, long int, long long int, long double,
and so on.
That is, whatever types gcc already supports, g77 supports
now or probably will support in a future version.
The rules for the numeric-type*n notation
apply to these types,
and new values for numeric-type(KIND=n) will be
assigned in a way that encourages clarity, consistency, and portability.
g77 strictly assigns types to all constants not documented as “typeless” (typeless constants including '1'Z, for example). Many other Fortran compilers attempt to assign types to typed constants based on their context. This results in hard-to-find bugs, nonportable code, and is not in the spirit (though it strictly follows the letter) of the 77 and 90 standards.
g77 might offer, in a future release, explicit constructs by which a wider variety of typeless constants may be specified, and/or user-requested warnings indicating places where g77 might differ from how other compilers assign types to constants.
See Context-Sensitive Constants, for more information on this issue.
g77 offers an ever-widening set of intrinsics. Currently these all are procedures (functions and subroutines).
Some of these intrinsics are unimplemented, but their names reserved to reduce future problems with existing code as they are implemented. Others are implemented as part of the GNU Fortran language, while yet others are provided for compatibility with other dialects of Fortran but are not part of the GNU Fortran language.
To manage these distinctions, g77 provides intrinsic groups, a facility that is simply an extension of the intrinsic groups provided by the GNU Fortran language.
A given specific intrinsic belongs in one or more groups. Each group is deleted, disabled, hidden, or enabled by default or a command-line option. The meaning of each term follows.
INTRINSIC statement)
are disallowed through that group.
INTRINSIC statement.
The distinction between deleting and disabling a group is illustrated by the following example. Assume intrinsic FOO belongs only to group FGR. If group FGR is deleted, the following program unit will successfully compile, because FOO() will be seen as a reference to an external function named FOO:
PRINT *, FOO()
END
If group FGR is disabled, compiling the above program will produce diagnostics, either because the FOO intrinsic is improperly invoked or, if properly invoked, it is not enabled. To change the above program so it references an external function FOO instead of the disabled FOO intrinsic, add the following line to the top:
EXTERNAL FOO
So, deleting a group tells g77 to pretend as though the intrinsics in that group do not exist at all, whereas disabling it tells g77 to recognize them as (disabled) intrinsics in intrinsic-like contexts.
Hiding a group is like enabling it, but the intrinsic must be first
named in an INTRINSIC statement to be considered a reference to the
intrinsic rather than to an external procedure.
This might be the “safest” way to treat a new group of intrinsics
when compiling old
code, because it allows the old code to be generally written as if
those new intrinsics never existed, but to be changed to use them
by inserting INTRINSIC statements in the appropriate places.
However, it should be the goal of development to use EXTERNAL
for all names of external procedures that might be intrinsic names.
If an intrinsic is in more than one group, it is enabled if any of its
containing groups are enabled; if not so enabled, it is hidden if
any of its containing groups are hidden; if not so hidden, it is disabled
if any of its containing groups are disabled; if not so disabled, it is
deleted.
This extra complication is necessary because some intrinsics,
such as IBITS, belong to more than one group, and hence should be
enabled if any of the groups to which they belong are enabled, and so
on.
The groups are:
badu77gnuf2clibf2c.
f90milMVBITS, IAND, BTEST, and so on).
unixIARGC, EXIT, ERF, and so on).
vxtg77 supports intrinsics other than those in the GNU Fortran language proper. This set of intrinsics is described below.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL ACosD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AIMax0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AIMin0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AJMax0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL AJMin0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL ASinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL ATan2D to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL ATanD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL BITest to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL BJTest to use this name for an external procedure.
CDAbs(A)
CDAbs: REAL(KIND=2) function.
A: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of ABS() that is specific
to one type for A.
See Abs Intrinsic.
CDCos(X)
CDCos: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of COS() that is specific
to one type for X.
See Cos Intrinsic.
CDExp(X)
CDExp: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of EXP() that is specific
to one type for X.
See Exp Intrinsic.
CDLog(X)
CDLog: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of LOG() that is specific
to one type for X.
See Log Intrinsic.
CDSin(X)
CDSin: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of SIN() that is specific
to one type for X.
See Sin Intrinsic.
CDSqRt(X)
CDSqRt: COMPLEX(KIND=2) function.
X: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of SQRT() that is specific
to one type for X.
See SqRt Intrinsic.
ChDir(Dir)
ChDir: INTEGER(KIND=1) function.
Dir: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Sets the current working directory to be Dir.
Returns 0 on success or a nonzero error code.
See chdir(3).
Caution: Using this routine during I/O to a unit connected with a non-absolute file name can cause subsequent I/O on such a unit to fail because the I/O library might reopen files by name.
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See ChDir Intrinsic (subroutine).
ChMod(Name, Mode)
ChMod: INTEGER(KIND=1) function.
Name: CHARACTER; scalar; INTENT(IN).
Mode: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Changes the access mode of file Name according to the
specification Mode, which is given in the format of
chmod(1).
A null character (CHAR(0)) marks the end of
the name in Name—otherwise,
trailing blanks in Name are ignored.
Currently, Name must not contain the single quote
character.
Returns 0 on success or a nonzero error code otherwise.
Note that this currently works
by actually invoking /bin/chmod (or the chmod found when
the library was configured) and so might fail in some circumstances and
will, anyway, be slow.
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See ChMod Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL CosD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DACosD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DASinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DATan2D to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DATanD to use this name for an external procedure.
CALL Date(Date)
Date: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: vxt.
Description:
Returns Date in the form dd-mmm-yy, representing the numeric day of the month dd, a three-character abbreviation of the month name mmm and the last two digits of the year yy, e.g. 25-Nov-96.
This intrinsic is not recommended, due to the year 2000 approaching. Therefore, programs making use of this intrinsic might not be Year 2000 (Y2K) compliant. See CTime Intrinsic (subroutine), for information on obtaining more digits for the current (or any) date.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DbleQ to use this name for an external procedure.
DCmplx(X, Y)
DCmplx: COMPLEX(KIND=2) function.
X: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Y: INTEGER or REAL; OPTIONAL (must be omitted if X is COMPLEX); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
If X is not type COMPLEX,
constructs a value of type COMPLEX(KIND=2) from the
real and imaginary values specified by X and
Y, respectively.
If Y is omitted, 0D0 is assumed.
If X is type COMPLEX,
converts it to type COMPLEX(KIND=2).
Although this intrinsic is not standard Fortran,
it is a popular extension offered by many compilers
that support DOUBLE COMPLEX, since it offers
the easiest way to convert to DOUBLE COMPLEX
without using Fortran 90 features (such as the KIND=
argument to the CMPLX() intrinsic).
(CMPLX(0D0, 0D0) returns a single-precision
COMPLEX result, as required by standard FORTRAN 77.
That's why so many compilers provide DCMPLX(), since
DCMPLX(0D0, 0D0) returns a DOUBLE COMPLEX
result.
Still, DCMPLX() converts even REAL*16 arguments
to their REAL*8 equivalents in most dialects of
Fortran, so neither it nor CMPLX() allow easy
construction of arbitrary-precision values without
potentially forcing a conversion involving extending or
reducing precision.
GNU Fortran provides such an intrinsic, called COMPLEX().)
See Complex Intrinsic, for information on easily constructing
a COMPLEX value of arbitrary precision from REAL
arguments.
DConjg(Z)
DConjg: COMPLEX(KIND=2) function.
Z: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of CONJG() that is specific
to one type for Z.
See Conjg Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DCosD to use this name for an external procedure.
DFloat(A)
DFloat: REAL(KIND=2) function.
A: INTEGER; scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of REAL() that is specific
to one type for A.
See Real Intrinsic.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DFlotI to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DFlotJ to use this name for an external procedure.
DImag(Z)
DImag: REAL(KIND=2) function.
Z: COMPLEX(KIND=2); scalar; INTENT(IN).
Intrinsic groups: f2c, vxt.
Description:
Archaic form of AIMAG() that is specific
to one type for Z.
See AImag Intrinsic.
DReal(A)
DReal: REAL(KIND=2) function.
A: INTEGER, REAL, or COMPLEX; scalar; INTENT(IN).
Intrinsic groups: vxt.
Description:
Converts A to REAL(KIND=2).
If A is type COMPLEX, its real part
is converted (if necessary) to REAL(KIND=2),
and its imaginary part is disregarded.
Although this intrinsic is not standard Fortran,
it is a popular extension offered by many compilers
that support DOUBLE COMPLEX, since it offers
the easiest way to extract the real part of a DOUBLE COMPLEX
value without using the Fortran 90 REAL() intrinsic
in a way that produces a return value inconsistent with
the way many FORTRAN 77 compilers handle REAL() of
a DOUBLE COMPLEX value.
See RealPart Intrinsic, for information on a GNU Fortran intrinsic that avoids these areas of confusion.
See Dble Intrinsic, for information on the standard FORTRAN 77
replacement for DREAL().
See REAL() and AIMAG() of Complex, for more information on this issue.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DSinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL DTanD to use this name for an external procedure.
DTime(TArray)
DTime: REAL(KIND=1) function.
TArray: REAL(KIND=1); DIMENSION(2); INTENT(OUT).
Intrinsic groups: badu77.
Description:
Initially, return the number of seconds of runtime since the start of the process's execution as the function value, and the user and system components of this in TArray(1) and TArray(2) respectively. The functions' value is equal to TArray(1) + TArray(2).
Subsequent invocations of DTIME() return values accumulated since the previous invocation.
On some systems, the underlying timings are represented using types with sufficiently small limits that overflows (wraparounds) are possible, such as 32-bit types. Therefore, the values returned by this intrinsic might be, or become, negative, or numerically less than previous values, during a single run of the compiled program.
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See DTime Intrinsic (subroutine).
FGet(C)
FGet: INTEGER(KIND=1) function.
C: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: badu77.
Description:
Reads a single character into C in stream mode from unit 5
(by-passing normal formatted input) using getc(3).
Returns 0 on
success, −1 on end-of-file, and the error code from
ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FGet Intrinsic (subroutine).
FGetC(Unit, C)
FGetC: INTEGER(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
C: CHARACTER; scalar; INTENT(OUT).
Intrinsic groups: badu77.
Description:
Reads a single character into C in stream mode from unit Unit
(by-passing normal formatted output) using getc(3).
Returns 0 on
success, −1 on end-of-file, and the error code from
ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FGetC Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL FloatI to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL FloatJ to use this name for an external procedure.
FPut(C)
FPut: INTEGER(KIND=1) function.
C: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Writes the single character C in stream mode to unit 6
(by-passing normal formatted output) using getc(3).
Returns 0 on
success, the error code from ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FPut Intrinsic (subroutine).
FPutC(Unit, C)
FPutC: INTEGER(KIND=1) function.
Unit: INTEGER; scalar; INTENT(IN).
C: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Writes the single character C in stream mode to unit Unit
(by-passing normal formatted output) using putc(3).
Returns 0 on
success, the error code from ferror(3) otherwise.
Stream I/O should not be mixed with normal record-oriented (formatted or unformatted) I/O on the same unit; the results are unpredictable.
For information on other intrinsics with the same name: See FPutC Intrinsic (subroutine).
CALL IDate(M, D, Y)
M: INTEGER(KIND=1); scalar; INTENT(OUT).
D: INTEGER(KIND=1); scalar; INTENT(OUT).
Y: INTEGER(KIND=1); scalar; INTENT(OUT).
Intrinsic groups: vxt.
Description:
Returns the numerical values of the current local time. The month (in the range 1–12) is returned in M, the day (in the range 1–31) in D, and the year in Y (in the range 0–99).
This intrinsic is not recommended, due to the fact that its return value for year wraps around century boundaries (change from a larger value to a smaller one). Therefore, programs making use of this intrinsic, for instance, might not be Year 2000 (Y2K) compliant. For example, the date might appear, to such programs, to wrap around as of the Year 2000.
See IDate Intrinsic (UNIX), for information on obtaining more digits for the current date.
For information on other intrinsics with the same name: See IDate Intrinsic (UNIX).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIAbs to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIAnd to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIBClr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIBits to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIBSet to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIDiM to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIDInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIDNnt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIEOr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIFix to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIOr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIQint to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIQNnt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IIShftC to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IISign to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IMax0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IMax1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IMin0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IMin1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IMod to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL INInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL INot to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL IZExt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIAbs to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIAnd to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIBClr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIBits to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIBSet to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIDiM to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIDInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIDNnt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIEOr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIFix to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIOr to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIQint to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIQNnt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIShft to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JIShftC to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JISign to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JMax0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JMax1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JMin0 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JMin1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JMod to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JNInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JNot to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL JZExt to use this name for an external procedure.
Kill(Pid, Signal)
Kill: INTEGER(KIND=1) function.
Pid: INTEGER; scalar; INTENT(IN).
Signal: INTEGER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Sends the signal specified by Signal to the process Pid.
Returns 0 on success or a nonzero error code.
See kill(2).
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See Kill Intrinsic (subroutine).
Link(Path1, Path2)
Link: INTEGER(KIND=1) function.
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Makes a (hard) link from file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
Returns 0 on success or a nonzero error code.
See link(2).
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See Link Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QAbs to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QACos to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QACosD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QASin to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QASinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QATan to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QATan2 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QATan2D to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QATanD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QCos to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QCosD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QCosH to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QDiM to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QExp to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QExt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QExtD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QFloat to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QLog to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QLog10 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QMax1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QMin1 to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QMod to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QNInt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QSin to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QSinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QSinH to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QSqRt to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QTan to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QTanD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL QTanH to use this name for an external procedure.
Rename(Path1, Path2)
Rename: INTEGER(KIND=1) function.
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Renames the file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
See rename(2).
Returns 0 on success or a nonzero error code.
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See Rename Intrinsic (subroutine).
Secnds(T)
Secnds: REAL(KIND=1) function.
T: REAL(KIND=1); scalar; INTENT(IN).
Intrinsic groups: vxt.
Description:
Returns the local time in seconds since midnight minus the value T.
This values returned by this intrinsic become numerically less than previous values (they wrap around) during a single run of the compiler program, under normal circumstances (such as running through the midnight hour).
Signal(Number, Handler)
Signal: INTEGER(KIND=7) function.
Number: INTEGER; scalar; INTENT(IN).
Handler: Signal handler (INTEGER FUNCTION or SUBROUTINE)
or dummy/global INTEGER(KIND=1) scalar.
Intrinsic groups: badu77.
Description:
If Handler is a an EXTERNAL routine, arranges for it to be
invoked with a single integer argument (of system-dependent length)
when signal Number occurs.
If Handler is an integer, it can be
used to turn off handling of signal Number or revert to its default
action.
See signal(2).
Note that Handler will be called using C conventions,
so the value of its argument in Fortran terms
is obtained by applying %LOC() (or LOC()) to it.
The value returned by signal(2) is returned.
Due to the side effects performed by this intrinsic, the function form is not recommended.
Warning: If the returned value is stored in
an INTEGER(KIND=1) (default INTEGER) argument,
truncation of the original return value occurs on some systems
(such as Alphas, which have 64-bit pointers but 32-bit default integers),
with no warning issued by g77 under normal circumstances.
Therefore, the following code fragment might silently fail on some systems:
INTEGER RTN
EXTERNAL MYHNDL
RTN = SIGNAL(signum, MYHNDL)
...
! Restore original handler:
RTN = SIGNAL(signum, RTN)
The reason for the failure is that RTN might not hold all the information on the original handler for the signal, thus restoring an invalid handler. This bug could manifest itself as a spurious run-time failure at an arbitrary point later during the program's execution, for example.
Warning: Use of the libf2c run-time library function
signal_ directly
(such as via EXTERNAL SIGNAL)
requires use of the %VAL() construct
to pass an INTEGER value
(such as SIG_IGN or SIG_DFL)
for the Handler argument.
However, while RTN = SIGNAL(signum, %VAL(SIG_IGN))
works when SIGNAL is treated as an external procedure
(and resolves, at link time, to libf2c's signal_ routine),
this construct is not valid when SIGNAL is recognized
as the intrinsic of that name.
Therefore, for maximum portability and reliability, code such references to the SIGNAL facility as follows:
INTRINSIC SIGNAL
...
RTN = SIGNAL(signum, SIG_IGN)
g77 will compile such a call correctly,
while other compilers will generally either do so as well
or reject the INTRINSIC SIGNAL statement via a diagnostic,
allowing you to take appropriate action.
For information on other intrinsics with the same name: See Signal Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL SinD to use this name for an external procedure.
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL SnglQ to use this name for an external procedure.
SymLnk(Path1, Path2)
SymLnk: INTEGER(KIND=1) function.
Path1: CHARACTER; scalar; INTENT(IN).
Path2: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Makes a symbolic link from file Path1 to Path2.
A null character (CHAR(0)) marks the end of
the names in Path1 and Path2—otherwise,
trailing blanks in Path1 and Path2 are ignored.
Returns 0 on success or a nonzero error code
(ENOSYS if the system does not provide symlink(2)).
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See SymLnk Intrinsic (subroutine).
System(Command)
System: INTEGER(KIND=1) function.
Command: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Passes the command Command to a shell (see system(3)).
Returns the value returned by
system(3), presumably 0 if the shell command succeeded.
Note that which shell is used to invoke the command is system-dependent
and environment-dependent.
Due to the side effects performed by this intrinsic, the function form is not recommended. However, the function form can be valid in cases where the actual side effects performed by the call are unimportant to the application.
For example, on a UNIX system, SAME = SYSTEM('cmp a b')
does not perform any side effects likely to be important to the
program, so the programmer would not care if the actual system
call (and invocation of cmp) was optimized away in a situation
where the return value could be determined otherwise, or was not
actually needed (SAME not actually referenced after the
sample assignment statement).
For information on other intrinsics with the same name: See System Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL TanD to use this name for an external procedure.
CALL Time(Time)
Time: CHARACTER*8; scalar; INTENT(OUT).
Intrinsic groups: vxt.
Description:
Returns in Time a character representation of the current time as
obtained from ctime(3).
Programs making use of this intrinsic might not be Year 10000 (Y10K) compliant. For example, the date might appear, to such programs, to wrap around (change from a larger value to a smaller one) as of the Year 10000.
See FDate Intrinsic (subroutine), for an equivalent routine.
For information on other intrinsics with the same name: See Time Intrinsic (UNIX).
UMask(Mask)
UMask: INTEGER(KIND=1) function.
Mask: INTEGER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Sets the file creation mask to Mask and returns the old value.
See umask(2).
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See UMask Intrinsic (subroutine).
Unlink(File)
Unlink: INTEGER(KIND=1) function.
File: CHARACTER; scalar; INTENT(IN).
Intrinsic groups: badu77.
Description:
Unlink the file File.
A null character (CHAR(0)) marks the end of
the name in File—otherwise,
trailing blanks in File are ignored.
Returns 0 on success or a nonzero error code.
See unlink(2).
Due to the side effects performed by this intrinsic, the function form is not recommended.
For information on other intrinsics with the same name: See Unlink Intrinsic (subroutine).
This intrinsic is not yet implemented. The name is, however, reserved as an intrinsic. Use EXTERNAL ZExt to use this name for an external procedure.
An individual Fortran source file can be compiled to an object (*.o) file instead of to the final program executable. This allows several portions of a program to be compiled at different times and linked together whenever a new version of the program is needed. However, it introduces the issue of object compatibility across the various object files (and libraries, or *.a files) that are linked together to produce any particular executable file.
Object compatibility is an issue when combining, in one
program, Fortran code compiled by more than one compiler
(or more than one configuration of a compiler).
If the compilers
disagree on how to transform the names of procedures, there
will normally be errors when linking such programs.
Worse, if the compilers agree on naming, but disagree on issues
like how to pass parameters, return arguments, and lay out
COMMON areas, the earliest detected errors might be the
incorrect results produced by the program (and that assumes
these errors are detected, which is not always the case).
Normally, g77 generates code that is object-compatible with code generated by a version of f2c configured (with, for example, f2c.h definitions) to be generally compatible with g77 as built by gcc. (Normally, f2c will, by default, conform to the appropriate configuration, but it is possible that older or perhaps even newer versions of f2c, or versions having certain configuration changes to f2c internals, will produce object files that are incompatible with g77.)
For example, a Fortran string subroutine
argument will become two arguments on the C side: a char *
and an int length.
Much of this compatibility results from the fact that
g77 uses the same run-time library,
libf2c, used by f2c,
though g77 gives its version the name libg2c
so as to avoid conflicts when linking,
installing them in the same directories,
and so on.
Other compilers might or might not generate code that
is object-compatible with libg2c and current g77,
and some might offer such compatibility only when explicitly
selected via a command-line option to the compiler.
Note: This portion of the documentation definitely needs a lot of work!
Specifying -fno-f2c allows g77 to generate, in some cases, faster code, by not needing to allow to the possibility of linking with code compiled by f2c.
For example, this affects how REAL(KIND=1),
COMPLEX(KIND=1), and COMPLEX(KIND=2) functions are called.
With -fno-f2c, they are
compiled as returning the appropriate gcc type
(float, __complex__ float, __complex__ double,
in many configurations).
With -ff2c in force, they
are compiled differently (with perhaps slower run-time performance)
to accommodate the restrictions inherent in f2c's use of K&R
C as an intermediate language—REAL(KIND=1) functions
return C's double type, while COMPLEX functions return
void and use an extra argument pointing to a place for the functions to
return their values.
It is possible that, in some cases, leaving -ff2c in force might produce faster code than using -fno-f2c. Feel free to experiment, but remember to experiment with changing the way entire programs and their Fortran libraries are compiled at a time, since this sort of experimentation affects the interface of code generated for a Fortran source file—that is, it affects object compatibility.
Note that f2c compatibility is a fairly static target to achieve, though not necessarily perfectly so, since, like g77, it is still being improved. However, specifying -fno-f2c causes g77 to generate code that will probably be incompatible with code generated by future versions of g77 when the same option is in force. You should make sure you are always able to recompile complete programs from source code when upgrading to new versions of g77 or f2c, especially when using options such as -fno-f2c.
Therefore, if you are using g77 to compile libraries and other
object files for possible future use and you don't want to require
recompilation for future use with subsequent versions of g77,
you might want to stick with f2c compatibility for now, and
carefully watch for any announcements about changes to the
f2c/libf2c interface that might affect existing programs
(thus requiring recompilation).
It is probable that a future version of g77 will not,
by default, generate object files compatible with f2c,
and that version probably would no longer use libf2c.
If you expect to depend on this compatibility in the
long term, use the options -ff2c -ff2c-library when compiling
all of the applicable code.
This should cause future versions of g77 either to produce
compatible code (at the expense of the availability of some features and
performance), or at the very least, to produce diagnostics.
(The library g77 produces will no longer be named libg2c
when it is no longer generally compatible with libf2c.
It will likely be referred to, and, if installed as a distinct
library, named libg77, or some other as-yet-unused name.)
On systems with Fortran compilers other than f2c and g77, code compiled by g77 is not expected to work well with code compiled by the native compiler. (This is true for f2c-compiled objects as well.) Libraries compiled with the native compiler probably will have to be recompiled with g77 to be used with g77-compiled code.
Reasons for such incompatibilities include:
This is why simply getting g77 to transform procedure names the same way a native compiler does is not usually a good idea—unless some effort has been made to ensure that, aside from the way the two compilers transform procedure names, everything else about the way they generate code for procedure interfaces is identical.
For example, on the Sun you would have to add -L/usr/lang/SCx.x -lF77 -lV77 to the link command.
Note: This portion of the documentation definitely needs a lot of work!
The following discussion assumes that you are running g77 in f2c compatibility mode, i.e. not using -fno-f2c. It provides some advice about quick and simple techniques for linking Fortran and C (or C++), the most common requirement. For the full story consult the description of code generation. See Debugging and Interfacing.
When linking Fortran and C, it's usually best to use g77 to do the linking so that the correct libraries are included (including the maths one). If you're linking with C++ you will want to add -lstdc++, -lg++ or whatever. If you need to use another driver program (or ld directly), you can find out what linkage options g77 passes by running g77 -v.
Even if you don't actually use it as a compiler, f2c from ftp://ftp.netlib.org/f2c/src, can be a useful tool when you're interfacing (linking) Fortran and C. See Generating Skeletons and Prototypes with f2c.
To use f2c for this purpose you only need retrieve and build the src directory from the distribution, consult the README instructions there for machine-specifics, and install the f2c program on your path.
Something else that might be useful is cfortran.h from ftp://zebra.desy.de/cfortran. This is a fairly general tool which can be used to generate interfaces for calling in both directions between Fortran and C. It can be used in f2c mode with g77—consult its documentation for details.
Generally, C code written to link with
g77 code—calling and/or being
called from Fortran—should #include <g2c.h> to define the C
versions of the Fortran types.
Don't assume Fortran INTEGER types
correspond to C ints, for instance; instead, declare them as
integer, a type defined by g2c.h.
g2c.h is installed where gcc will find it by
default, assuming you use a copy of gcc compatible with
g77, probably built at the same time as g77.
A simple and foolproof way to write g77-callable C routines—e.g. to
interface with an existing library—is to write a file (named, for
example, fred.f) of dummy Fortran
skeletons comprising just the declaration of the routine(s) and dummy
arguments plus END statements.
Then run f2c on file fred.f to produce fred.c
into which you can edit
useful code, confident the calling sequence is correct, at least.
(There are some errors otherwise commonly made in generating C
interfaces with f2c conventions,
such as not using doublereal
as the return type of a REAL FUNCTION.)
f2c also can help with calling Fortran from C, using its
-P option to generate C prototypes appropriate for calling the
Fortran.2
If the Fortran code containing any
routines to be called from C is in file joe.f, use the command
f2c -P joe.f to generate the file joe.P containing
prototype information.
#include this in the C which has to call
the Fortran routines to make sure you get it right.
See Arrays (DIMENSION), for information on the differences between the way Fortran (including compilers like g77) and C handle arrays.
f2c can be used to generate suitable code for compilation with a C++ system using the -C++ option. The important thing about linking g77-compiled code with C++ is that the prototypes for the g77 routines must specify C linkage to avoid name mangling. So, use an extern "C" declaration. f2c's -C++ option will not take care of this when generating skeletons or prototype files as above, however, it will avoid clashes with C++ reserved words in addition to those in C.
Unlike with some runtime systems, it shouldn't be necessary (unless there are bugs) to use a Fortran main program unit to ensure the runtime—specifically the I/O system—is initialized.
However, to use the g77 intrinsics GETARG and IARGC,
either the main routine from the libg2c library must be used,
or the f_setarg routine
(new as of egcs version 1.1 and g77 version 0.5.23)
must be called with the appropriate argc and argv arguments
prior to the program calling GETARG or IARGC.
To provide more flexibility for mixed-language programming
involving g77 while allowing for shared libraries,
as of egcs version 1.1 and g77 version 0.5.23,
g77's main routine in libg2c
does the following, in order:
f_setarg
with the incoming argc and argv arguments,
in the same order as for main itself.
This sets up the command-line environment
for GETARG and IARGC.
f_setsig (with no arguments).
This sets up the signaling and exception environment.
f_init (with no arguments).
This initializes the I/O environment,
though that should not be necessary,
as all I/O functions in libf2c
are believed to call f_init automatically,
if necessary.
(A future version of g77 might skip this explicit step, to speed up normal exit of a program.)
f_exit to be called (with no arguments)
when the program exits.
This ensures that the I/O environment is properly shut down before the program exits normally. Otherwise, output buffers might not be fully flushed, scratch files might not be deleted, and so on.
The simple way main does this is
to call f_exit itself after calling
MAIN__ (in the next step).
However, this does not catch the cases where the program
might call exit directly,
instead of using the EXIT intrinsic
(implemented as exit_ in libf2c).
So, main attempts to use
the operating environment's onexit or atexit
facility, if available,
to cause f_exit to be called automatically
upon any invocation of exit.
MAIN__ (with no arguments).
This starts executing the Fortran main program unit for
the application.
(Both g77 and f2c currently compile a main
program unit so that its global name is MAIN__.)
onexit or atexit is provided by the system,
calls f_exit.
exit with a zero argument,
to signal a successful program termination.
exit doesn't exit on the system.
All of the above names are C extern names,
i.e. not mangled.
When using the main procedure provided by g77
without a Fortran main program unit,
you need to provide MAIN__
as the entry point for your C code.
(Make sure you link the object file that defines that
entry point with the rest of your program.)
To provide your own main procedure
in place of g77's,
make sure you specify the object file defining that procedure
before -lg2c on the g77 command line.
Since the -lg2c option is implicitly provided,
this is usually straightforward.
(Use the --verbose option to see how and where
g77 implicitly adds -lg2c in a command line
that will link the program.
Feel free to specify -lg2c explicitly,
as appropriate.)
However, when providing your own main,
make sure you perform the appropriate tasks in the
appropriate order.
For example, if your main does not call f_setarg,
make sure the rest of your application does not call
GETARG or IARGC.
And, if your main fails to ensure that f_exit
is called upon program exit,
some files might end up incompletely written,
some scratch files might be left lying around,
and some existing files being written might be left
with old data not properly truncated at the end.
Note that, generally, the g77 operating environment
does not depend on a procedure named MAIN__ actually
being called prior to any other g77-compiled code.
That is, MAIN__ does not, itself,
set up any important operating-environment characteristics
upon which other code might depend.
This might change in future versions of g77,
with appropriate notification in the release notes.
For more information, consult the source code for the above routines. These are in gcc/libf2c/libF77/, named main.c, setarg.c, setsig.c, getarg_.c, and iargc_.c.
Also, the file gcc/gcc/f/com.c contains the code g77
uses to open-code (inline) references to IARGC.
GNU Fortran currently generates code that is object-compatible with the f2c converter. Also, it avoids limitations in the current GBE, such as the inability to generate a procedure with multiple entry points, by generating code that is structured differently (in terms of procedure names, scopes, arguments, and so on) than might be expected.
As a result, writing code in other languages that calls on, is called by, or shares in-memory data with g77-compiled code generally requires some understanding of the way g77 compiles code for various constructs.
Similarly, using a debugger to debug g77-compiled code, even if that debugger supports native Fortran debugging, generally requires this sort of information.
This section describes some of the basic information on how g77 compiles code for constructs involving interfaces to other languages and to debuggers.
Caution: Much or all of this information pertains to only the current release of g77, sometimes even to using certain compiler options with g77 (such as -fno-f2c). Do not write code that depends on this information without clearly marking said code as nonportable and subject to review for every new release of g77. This information is provided primarily to make debugging of code generated by this particular release of g77 easier for the user, and partly to make writing (generally nonportable) interface code easier. Both of these activities require tracking changes in new version of g77 as they are installed, because new versions can change the behaviors described in this section.
When g77 compiles a main program unit, it gives it the public
procedure name MAIN__.
The libg2c library has the actual main() procedure
as is typical of C-based environments, and
it is this procedure that performs some initial start-up
activity and then calls MAIN__.
Generally, g77 and libg2c are designed so that you need not
include a main program unit written in Fortran in your program—it
can be written in C or some other language.
Especially for I/O handling, this is the case, although g77 version 0.5.16
includes a bug fix for libg2c that solved a problem with using the
OPEN statement as the first Fortran I/O activity in a program
without a Fortran main program unit.
However, if you don't intend to use g77 (or f2c) to compile
your main program unit—that is, if you intend to compile a main()
procedure using some other language—you should carefully
examine the code for main() in libg2c, found in the source
file gcc/libf2c/libF77/main.c, to see what kinds of things
might need to be done by your main() in order to provide the
Fortran environment your Fortran code is expecting.
For example, libg2c's main() sets up the information used by
the IARGC and GETARG intrinsics.
Bypassing libg2c's main()
without providing a substitute for this activity would mean
that invoking IARGC and GETARG would produce undefined
results.
When debugging, one implication of the fact that main(), which
is the place where the debugged program “starts” from the
debugger's point of view, is in libg2c is that you won't be
starting your Fortran program at a point you recognize as your
Fortran code.
The standard way to get around this problem is to set a break
point (a one-time, or temporary, break point will do) at
the entrance to MAIN__, and then run the program.
A convenient way to do so is to add the gdb command
tbreak MAIN__
to the file .gdbinit in the directory in which you're debugging (using gdb).
After doing this, the debugger will see the current execution point of the program as at the beginning of the main program unit of your program.
Of course, if you really want to set a break point at some
other place in your program and just start the program
running, without first breaking at MAIN__,
that should work fine.
Currently, g77 passes arguments via reference—specifically, by passing a pointer to the location in memory of a variable, array, array element, a temporary location that holds the result of evaluating an expression, or a temporary or permanent location that holds the value of a constant.
Procedures that accept CHARACTER arguments are implemented by
g77 so that each CHARACTER argument has two actual arguments.
The first argument occupies the expected position in the argument list and has the user-specified name. This argument is a pointer to an array of characters, passed by the caller.
The second argument is appended to the end of the user-specified
calling sequence and is named __g77_length_x, where x
is the user-specified name.
This argument is of the C type ftnlen
(see gcc/libf2c/g2c.h.in for information on that type) and
is the number of characters the caller has allocated in the
array pointed to by the first argument.
A procedure will ignore the length argument if X is not declared
CHARACTER*(*), because for other declarations, it knows the
length.
Not all callers necessarily “know” this, however, which
is why they all pass the extra argument.
The contents of the CHARACTER argument are specified by the
address passed in the first argument (named after it).
The procedure can read or write these contents as appropriate.
When more than one CHARACTER argument is present in the argument
list, the length arguments are appended in the order
the original arguments appear.
So CALL FOO('HI','THERE') is implemented in
C as foo("hi","there",2,5);, ignoring the fact that g77
does not provide the trailing null bytes on the constant
strings (f2c does provide them, but they are unnecessary in
a Fortran environment, and you should not expect them to be
there).
Note that the above information applies to CHARACTER variables and
arrays only.
It does not apply to external CHARACTER
functions or to intrinsic CHARACTER functions.
That is, no second length argument is passed to FOO in this case:
CHARACTER X
EXTERNAL X
CALL FOO(X)
Nor does FOO expect such an argument in this case:
SUBROUTINE FOO(X)
CHARACTER X
EXTERNAL X
Because of this implementation detail, if a program has a bug
such that there is disagreement as to whether an argument is
a procedure, and the type of the argument is CHARACTER, subtle
symptoms might appear.
g77 handles in a special way functions that return the following types:
CHARACTER
COMPLEX
REAL(KIND=1)
For CHARACTER, g77 implements a subroutine (a C function
returning void)
with two arguments prepended: __g77_result, which the caller passes
as a pointer to a char array expected to hold the return value,
and __g77_length, which the caller passes as an ftnlen value
specifying the length of the return value as declared in the calling
program.
For CHARACTER*(*), the called function uses __g77_length
to determine the size of the array that __g77_result points to;
otherwise, it ignores that argument.
For COMPLEX, when -ff2c is in
force, g77 implements
a subroutine with one argument prepended: __g77_result, which the
caller passes as a pointer to a variable of the type of the function.
The called function writes the return value into this variable instead
of returning it as a function value.
When -fno-f2c is in force,
g77 implements a COMPLEX function as gcc's
__complex__ float or __complex__ double function
(or an emulation thereof, when -femulate-complex is in effect),
returning the result of the function in the same way as gcc would.
For REAL(KIND=1), when -ff2c is in force, g77 implements
a function that actually returns REAL(KIND=2) (typically
C's double type).
When -fno-f2c is in force, REAL(KIND=1)
functions return float.
Fortran permits each implementation to decide how to represent names as far as how they're seen in other contexts, such as debuggers and when interfacing to other languages, and especially as far as how casing is handled.
External names—names of entities that are public, or “accessible”, to all modules in a program—normally have an underscore (_) appended by g77, to generate code that is compatible with f2c. External names include names of Fortran things like common blocks, external procedures (subroutines and functions, but not including statement functions, which are internal procedures), and entry point names.
However, use of the -fno-underscoring option disables this kind of transformation of external names (though inhibiting the transformation certainly improves the chances of colliding with incompatible externals written in other languages—but that might be intentional.
When -funderscoring is in force, any name (external or local) that already has at least one underscore in it is implemented by g77 by appending two underscores. (This second underscore can be disabled via the -fno-second-underscore option.) External names are changed this way for f2c compatibility. Local names are changed this way to avoid collisions with external names that are different in the source code—f2c does the same thing, but there's no compatibility issue there except for user expectations while debugging.
For example:
Max_Cost = 0
Here, a user would, in the debugger, refer to this variable using the name max_cost__ (or MAX_COST__ or Max_Cost__, as described below). (We hope to improve g77 in this regard in the future—don't write scripts depending on this behavior! Also, consider experimenting with the -fno-underscoring option to try out debugging without having to massage names by hand like this.)
g77 provides a number of command-line options that allow the user to control how case mapping is handled for source files. The default is the traditional UNIX model for Fortran compilers—names are mapped to lower case. Other command-line options can be specified to map names to upper case, or to leave them exactly as written in the source file.
For example:
Foo = 9.436
Here, it is normally the case that the variable assigned will be named foo. This would be the name to enter when using a debugger to access the variable.
However, depending on the command-line options specified, the name implemented by g77 might instead be FOO or even Foo, thus affecting how debugging is done.
Also:
Call Foo
This would normally call a procedure that, if it were in a separate C program, be defined starting with the line:
void foo_()
However, g77 command-line options could be used to change the casing of names, resulting in the name FOO_ or Foo_ being given to the procedure instead of foo_, and the -fno-underscoring option could be used to inhibit the appending of the underscore to the name.
g77 names and lays out COMMON areas
the same way f2c does,
for compatibility with f2c.
g77 treats storage-associated areas involving a COMMON
block as explained in the section on common blocks.
A local EQUIVALENCE area is a collection of variables and arrays
connected to each other in any way via EQUIVALENCE, none of which are
listed in a COMMON statement.
(Note: g77 version 0.5.18 and earlier chose the name
for x using a different method when more than one name was
in the list of names of entities placed at the beginning of the
array.
Though the documentation specified that the first name listed in
the EQUIVALENCE statements was chosen for x, g77
in fact chose the name using a method that was so complicated,
it seemed easier to change it to an alphabetical sort than to describe the
previous method in the documentation.)
As of 0.5.20, g77 defaults to handling COMPLEX types
(and related intrinsics, constants, functions, and so on)
in a manner that
makes direct debugging involving these types in Fortran
language mode difficult.
Essentially, g77 implements these types using an
internal construct similar to C's struct, at least
as seen by the gcc back end.
Currently, the back end, when outputting debugging info with
the compiled code for the assembler to digest, does not detect
these struct types as being substitutes for Fortran
complex.
As a result, the Fortran language modes of debuggers such as
gdb see these types as C struct types, which
they might or might not support.
Until this is fixed, switch to C language mode to work with
entities of COMPLEX type and then switch back to Fortran language
mode afterward.
(In gdb, this is accomplished via set lang c and
either set lang fortran or set lang auto.)
Fortran uses “column-major ordering” in its arrays. This differs from other languages, such as C, which use “row-major ordering”. The difference is that, with Fortran, array elements adjacent to each other in memory differ in the first subscript instead of the last; A(5,10,20) immediately follows A(4,10,20), whereas with row-major ordering it would follow A(5,10,19).
This consideration affects not only interfacing with and debugging Fortran code, it can greatly affect how code is designed and written, especially when code speed and size is a concern.
Fortran also differs from C, a popular language for interfacing and to support directly in debuggers, in the way arrays are treated. In C, arrays are single-dimensional and have interesting relationships to pointers, neither of which is true for Fortran. As a result, dealing with Fortran arrays from within an environment limited to C concepts can be challenging.
For example, accessing the array element A(5,10,20) is easy enough in Fortran (use A(5,10,20)), but in C some difficult machinations are needed. First, C would treat the A array as a single-dimension array. Second, C does not understand low bounds for arrays as does Fortran. Third, C assumes a low bound of zero (0), while Fortran defaults to a low bound of one (1) and can supports an arbitrary low bound. Therefore, calculations must be done to determine what the C equivalent of A(5,10,20) would be, and these calculations require knowing the dimensions of A.
For DIMENSION A(2:11,21,0:29), the calculation of the offset of A(5,10,20) would be:
(5-2)
+ (10-1)*(11-2+1)
+ (20-0)*(11-2+1)*(21-1+1)
= 4293
So the C equivalent in this case would be a[4293].
When using a debugger directly on Fortran code, the C equivalent might not work, because some debuggers cannot understand the notion of low bounds other than zero. However, unlike f2c, g77 does inform the GBE that a multi-dimensional array (like A in the above example) is really multi-dimensional, rather than a single-dimensional array, so at least the dimensionality of the array is preserved.
Debuggers that understand Fortran should have no trouble with nonzero low bounds, but for non-Fortran debuggers, especially C debuggers, the above example might have a C equivalent of a[4305]. This calculation is arrived at by eliminating the subtraction of the lower bound in the first parenthesized expression on each line—that is, for (5-2) substitute (5), for (10-1) substitute (10), and for (20-0) substitute (20). Actually, the implication of this can be that the expression *(&a[2][1][0] + 4293) works fine, but that a[20][10][5] produces the equivalent of *(&a[0][0][0] + 4305) because of the missing lower bounds.
Come to think of it, perhaps the behavior is due to the debugger internally compensating for the lower bounds by offsetting the base address of a, leaving &a set lower, in this case, than &a[2][1][0] (the address of its first element as identified by subscripts equal to the corresponding lower bounds).
You know, maybe nobody really needs to use arrays.
Adjustable and automatic arrays in Fortran require the implementation (in this case, the g77 compiler) to “memorize” the expressions that dimension the arrays each time the procedure is invoked. This is so that subsequent changes to variables used in those expressions, made during execution of the procedure, do not have any effect on the dimensions of those arrays.
For example:
REAL ARRAY(5)
DATA ARRAY/5*2/
CALL X(ARRAY, 5)
END
SUBROUTINE X(A, N)
DIMENSION A(N)
N = 20
PRINT *, N, A
END
Here, the implementation should, when running the program, print something like:
20 2. 2. 2. 2. 2.
Note that this shows that while the value of N was successfully changed, the size of the A array remained at 5 elements.
To support this, g77 generates code that executes before any user
code (and before the internally generated computed GOTO to handle
alternate entry points, as described below) that evaluates each
(nonconstant) expression in the list of subscripts for an
array, and saves the result of each such evaluation to be used when
determining the size of the array (instead of re-evaluating the
expressions).
So, in the above example, when X is first invoked, code is executed that copies the value of N to a temporary. And that same temporary serves as the actual high bound for the single dimension of the A array (the low bound being the constant 1). Since the user program cannot (legitimately) change the value of the temporary during execution of the procedure, the size of the array remains constant during each invocation.
For alternate entry points, the code g77 generates takes into
account the possibility that a dummy adjustable array is not actually
passed to the actual entry point being invoked at that time.
In that case, the public procedure implementing the entry point
passes to the master private procedure implementing all the
code for the entry points a NULL pointer where a pointer to that
adjustable array would be expected.
The g77-generated code
doesn't attempt to evaluate any of the expressions in the subscripts
for an array if the pointer to that array is NULL at run time in
such cases.
(Don't depend on this particular implementation
by writing code that purposely passes NULL pointers where the
callee expects adjustable arrays, even if you know the callee
won't reference the arrays—nor should you pass NULL pointers
for any dummy arguments used in calculating the bounds of such
arrays or leave undefined any values used for that purpose in
COMMON—because the way g77 implements these things might
change in the future!)
The GBE does not understand the general concept of
alternate entry points as Fortran provides via the ENTRY statement.
g77 gets around this by using an approach to compiling procedures
having at least one ENTRY statement that is almost identical to the
approach used by f2c.
(An alternate approach could be used that
would probably generate faster, but larger, code that would also
be a bit easier to debug.)
Information on how g77 implements ENTRY is provided for those
trying to debug such code.
The choice of implementation seems
unlikely to affect code (compiled in other languages) that interfaces
to such code.
g77 compiles exactly one public procedure for the primary entry
point of a procedure plus each ENTRY point it specifies, as usual.
That is, in terms of the public interface, there is no difference
between
SUBROUTINE X
END
SUBROUTINE Y
END
and:
SUBROUTINE X
ENTRY Y
END
The difference between the above two cases lies in the code compiled for the X and Y procedures themselves, plus the fact that, for the second case, an extra internal procedure is compiled.
For every Fortran procedure with at least one ENTRY
statement, g77 compiles an extra procedure
named __g77_masterfun_x, where x is
the name of the primary entry point (which, in the above case,
using the standard compiler options, would be x_ in C).
This extra procedure is compiled as a private procedure—that is, a procedure not accessible by name to separately compiled modules. It contains all the code in the program unit, including the code for the primary entry point plus for every entry point. (The code for each public procedure is quite short, and explained later.)
The extra procedure has some other interesting characteristics.
The argument list for this procedure is invented by g77.
It contains
a single integer argument named __g77_which_entrypoint,
passed by value (as in Fortran's %VAL() intrinsic), specifying the
entry point index—0 for the primary entry point, 1 for the
first entry point (the first ENTRY statement encountered), 2 for
the second entry point, and so on.
It also contains, for functions returning CHARACTER and
(when -ff2c is in effect) COMPLEX functions,
and for functions returning different types among the
ENTRY statements (e.g. REAL FUNCTION R()
containing ENTRY I()), an argument named __g77_result that
is expected at run time to contain a pointer to where to store
the result of the entry point.
For CHARACTER functions, this
storage area is an array of the appropriate number of characters;
for COMPLEX functions, it is the appropriate area for the return
type; for multiple-return-type functions, it is a union of all the supported return
types (which cannot include CHARACTER, since combining CHARACTER
and non-CHARACTER return types via ENTRY in a single function
is not supported by g77).
For CHARACTER functions, the __g77_result argument is followed
by yet another argument named __g77_length that, at run time,
specifies the caller's expected length of the returned value.
Note that only CHARACTER*(*) functions and entry points actually
make use of this argument, even though it is always passed by
all callers of public CHARACTER functions (since the caller does not
generally know whether such a function is CHARACTER*(*) or whether
there are any other callers that don't have that information).
The rest of the argument list is the union of all the arguments
specified for all the entry points (in their usual forms, e.g.
CHARACTER arguments have extra length arguments, all appended at
the end of this list).
This is considered the “master list” of
arguments.
The code for this procedure has, before the code for the first executable statement, code much like that for the following Fortran statement:
GOTO (100000,100001,100002), __g77_which_entrypoint
100000 ...code for primary entry point...
100001 ...code immediately following first ENTRY statement...
100002 ...code immediately following second ENTRY statement...
(Note that invalid Fortran statement labels and variable names are used in the above example to highlight the fact that it represents code generated by the g77 internals, not code to be written by the user.)
It is this code that, when the procedure is called, picks which entry point to start executing.
Getting back to the public procedures (x and Y in the original
example), those procedures are fairly simple.
Their interfaces
are just like they would be if they were self-contained procedures
(without ENTRY), of course, since that is what the callers
expect.
Their code consists of simply calling the private
procedure, described above, with the appropriate extra arguments
(the entry point index, and perhaps a pointer to a multiple-type-
return variable, local to the public procedure, that contains
all the supported returnable non-character types).
For arguments
that are not listed for a given entry point that are listed for
other entry points, and therefore that are in the “master list”
for the private procedure, null pointers (in C, the NULL macro)
are passed.
Also, for entry points that are part of a multiple-type-
returning function, code is compiled after the call of the private
procedure to extract from the multi-type union the appropriate result,
depending on the type of the entry point in question, returning
that result to the original caller.
When debugging a procedure containing alternate entry points, you can either set a break point on the public procedure itself (e.g. a break point on X or Y) or on the private procedure that contains most of the pertinent code (e.g. __g77_masterfun_x). If you do the former, you should use the debugger's command to “step into” the called procedure to get to the actual code; with the latter approach, the break point leaves you right at the actual code, skipping over the public entry point and its call to the private procedure (unless you have set a break point there as well, of course).
Further, the list of dummy arguments that is visible when the private procedure is active is going to be the expanded version of the list for whichever particular entry point is active, as explained above, and the way in which return values are handled might well be different from how they would be handled for an equivalent single-entry function.
Subroutines with alternate returns (e.g. SUBROUTINE X(*) and
CALL X(*50)) are implemented by g77 as functions returning
the C int type.
The actual alternate-return arguments are omitted from the calling sequence.
Instead, the caller uses
the return value to do a rough equivalent of the Fortran
computed-GOTO statement, as in GOTO (50), X() in the
example above (where X is quietly declared as an INTEGER(KIND=1)
function), and the callee just returns whatever integer
is specified in the RETURN statement for the subroutine
For example, RETURN 1 is implemented as X = 1 followed
by RETURN
in C, and RETURN by itself is X = 0 and RETURN).
For portability to machines where a pointer (such as to a label,
which is how g77 implements ASSIGN and its relatives,
the assigned-GOTO and assigned-FORMAT-I/O statements)
is wider (bitwise) than an INTEGER(KIND=1), g77
uses a different memory location to hold the ASSIGNed value of a variable
than it does the numerical value in that variable, unless the
variable is wide enough (can hold enough bits).
In particular, while g77 implements
I = 10
as, in C notation, i = 10;, it implements
ASSIGN 10 TO I
as, in GNU's extended C notation (for the label syntax), __g77_ASSIGN_I = &&L10; (where L10 is just a massaging of the Fortran label 10 to make the syntax C-like; g77 doesn't actually generate the name L10 or any other name like that, since debuggers cannot access labels anyway).
While this currently means that an ASSIGN statement does not
overwrite the numeric contents of its target variable, do not
write any code depending on this feature.
g77 has already changed this implementation across
versions and might do so in the future.
This information is provided only to make debugging Fortran programs
compiled with the current version of g77 somewhat easier.
If there's no debugger-visible variable named __g77_ASSIGN_I
in a program unit that does ASSIGN 10 TO I, that probably
means g77 has decided it can store the pointer to the label directly
into I itself.
See Ugly Assigned Labels, for information on a command-line option to force g77 to use the same storage for both normal and assigned-label uses of a variable.
The libg2c library currently has the following table to relate
error code numbers, returned in IOSTAT= variables, to messages.
This information should, in future versions of this document, be
expanded upon to include detailed descriptions of each message.
In line with good coding practices, any of the numbers in the
list below should not be directly written into Fortran
code you write.
Instead, make a separate INCLUDE file that defines
PARAMETER names for them, and use those in your code,
so you can more easily change the actual numbers in the future.
The information below is culled from the definition
of F_err in f/runtime/libI77/err.c in the
g77 source tree.
100: "error in format"
101: "illegal unit number"
102: "formatted io not allowed"
103: "unformatted io not allowed"
104: "direct io not allowed"
105: "sequential io not allowed"
106: "can't backspace file"
107: "null file name"
108: "can't stat file"
109: "unit not connected"
110: "off end of record"
111: "truncation failed in endfile"
112: "incomprehensible list input"
113: "out of free space"
114: "unit not connected"
115: "read unexpected character"
116: "bad logical input field"
117: "bad variable type"
118: "bad namelist name"
119: "variable not in namelist"
120: "no end record"
121: "variable count incorrect"
122: "subscript for scalar variable"
123: "invalid array section"
124: "substring out of bounds"
125: "subscript out of bounds"
126: "can't read file"
127: "can't write file"
128: "'new' file exists"
129: "can't append to file"
130: "non-positive record number"
131: "I/O started while already doing I/O"
Most users of g77 can be divided into two camps:
Users writing new code generally understand most of the necessary
aspects of Fortran to write “mainstream” code, but often need
help deciding how to handle problems, such as the construction
of libraries containing BLOCK DATA.
Users dealing with “legacy” code sometimes don't have much experience with Fortran, but believe that the code they're compiling already works when compiled by other compilers (and might not understand why, as is sometimes the case, it doesn't work when compiled by g77).
The following information is designed to help users do a better job coping with existing, “legacy” Fortran code, and with writing new code as well.
Without f2c, g77 would have taken much longer to do and probably not been as good for quite a while. Sometimes people who notice how much g77 depends on, and documents encouragement to use, f2c ask why g77 was created if f2c already existed.
This section gives some basic answers to these questions, though it is not intended to be comprehensive.
g77 offers several extensions to FORTRAN 77 language that f2c doesn't:
CYCLE and EXIT
SELECT CASE
KIND= and LEN= notation
FORMAT statements
(such as FORMAT(I<J>),
where J is a PARAMETER named constant)
MvBits intrinsic
libU77 (Unix-compatibility) library,
with routines known to compiler as intrinsics
(so they work even when compiler options are used
to change the interfaces used by Fortran routines)
g77 also implements iterative DO loops
so that they work even in the presence of certain “extreme” inputs,
unlike f2c.
See Loops.
However, f2c offers a few that g77 doesn't, such as:
PARAMETER statements
AUTOMATIC statement
It is expected that g77 will offer some or all of these missing features at some time in the future.
g77 offers better diagnosis of problems in FORMAT statements.
f2c doesn't, for example, emit any diagnostic for
FORMAT(XZFAJG10324),
leaving that to be diagnosed, at run time, by
the libf2c run-time library.
g77 offers compiler options that f2c doesn't, most of which are designed to more easily accommodate legacy code:
However, f2c offers a few that g77 doesn't,
like an option to have REAL default to REAL*8.
It is expected that g77 will offer all of the
missing options pertinent to being a Fortran compiler
at some time in the future.
Saving the steps of writing and then rereading C code is a big reason why g77 should be able to compile code much faster than using f2c in conjunction with the equivalent invocation of gcc.
However, due to g77's youth, lots of self-checking is still being performed. As a result, this improvement is as yet unrealized (though the potential seems to be there for quite a big speedup in the future). It is possible that, as of version 0.5.18, g77 is noticeably faster compiling many Fortran source files than using f2c in conjunction with gcc.
g77 has the potential to better optimize code than f2c, even when gcc is used to compile the output of f2c, because f2c must necessarily translate Fortran into a somewhat lower-level language (C) that cannot preserve all the information that is potentially useful for optimization, while g77 can gather, preserve, and transmit that information directly to the GBE.
For example, g77 implements ASSIGN and assigned
GOTO using direct assignment of pointers to labels and direct
jumps to labels, whereas f2c maps the assigned labels to
integer values and then uses a C switch statement to encode
the assigned GOTO statements.
However, as is typical, theory and reality don't quite match, at least not in all cases, so it is still the case that f2c plus gcc can generate code that is faster than g77.
Version 0.5.18 of g77 offered default settings and options, via patches to the gcc back end, that allow for better program speed, though some of these improvements also affected the performance of programs translated by f2c and then compiled by g77's version of gcc.
Version 0.5.20 of g77 offers further performance improvements, at least one of which (alias analysis) is not generally applicable to f2c (though f2c could presumably be changed to also take advantage of this new capability of the gcc back end, assuming this is made available in an upcoming release of gcc).
Because g77 compiles directly to assembler code like gcc, instead of translating to an intermediate language (C) as does f2c, support for debugging can be better for g77 than f2c.
However, although g77 might be somewhat more “native” in terms of debugging support than f2c plus gcc, there still are a lot of things “not quite right”. Many of the important ones should be resolved in the near future.
For example, g77 doesn't have to worry about reserved names like f2c does. Given FOR = WHILE, f2c must necessarily translate this to something other than for = while;, because C reserves those words.
However, g77 does still uses things like an extra level of indirection
for ENTRY-laden procedures—in this case, because the back end doesn't
yet support multiple entry points.
Another example is that, given
COMMON A, B
EQUIVALENCE (B, C)
the g77 user should be able to access the variables directly, by name, without having to traverse C-like structures and unions, while f2c is unlikely to ever offer this ability (due to limitations in the C language).
However, due to apparent bugs in the back end, g77 currently doesn't
take advantage of this facility at all—it doesn't emit any debugging
information for COMMON and EQUIVALENCE areas,
other than information
on the array of char it creates (and, in the case
of local EQUIVALENCE, names) for each such area.
Yet another example is arrays. g77 represents them to the debugger using the same “dimensionality” as in the source code, while f2c must necessarily convert them all to one-dimensional arrays to fit into the confines of the C language. However, the level of support offered by debuggers for interactive Fortran-style access to arrays as compiled by g77 can vary widely. In some cases, it can actually be an advantage that f2c converts everything to widely supported C semantics.
In fairness, g77 could do many of the things f2c does to get things working at least as well as f2c—for now, the developers prefer making g77 work the way they think it is supposed to, and finding help improving the other products (the back end of gcc; gdb; and so on) to get things working properly.
To avoid the extensive hassle that would be needed to avoid this, f2c uses C character constants to encode character and Hollerith constants. That means a constant like 'HELLO' is translated to "hello" in C, which further means that an extra null byte is present at the end of the constant. This null byte is superfluous.
g77 does not generate such null bytes. This represents significant savings of resources, such as on systems where /dev/null or /dev/zero represent bottlenecks in the systems' performance, because g77 simply asks for fewer zeros from the operating system than f2c. (Avoiding spurious use of zero bytes, each byte typically have eight zero bits, also reduces the liabilities in case Microsoft's rumored patent on the digits 0 and 1 is upheld.)
To ensure that block data program units are linked, especially a concern
when they are put into libraries, give each one a name (as in
BLOCK DATA FOO) and make sure there is an EXTERNAL FOO
statement in every program unit that uses any common block
initialized by the corresponding BLOCK DATA.
g77 currently compiles a BLOCK DATA as if it were a
SUBROUTINE,
that is, it generates an actual procedure having the appropriate name.
The procedure does nothing but return immediately if it happens to be
called.
For EXTERNAL FOO, where FOO is not otherwise referenced in the
same program unit, g77 assumes there exists a BLOCK DATA FOO
in the program and ensures that by generating a
reference to it so the linker will make sure it is present.
(Specifically, g77 outputs in the data section a static pointer to the
external name FOO.)
The implementation g77 currently uses to make this work is one of the few things not compatible with f2c as currently shipped. f2c currently does nothing with EXTERNAL FOO except issue a warning that FOO is not otherwise referenced, and, for BLOCK DATA FOO, f2c doesn't generate a dummy procedure with the name FOO. The upshot is that you shouldn't mix f2c and g77 in this particular case. If you use f2c to compile BLOCK DATA FOO, then any g77-compiled program unit that says EXTERNAL FOO will result in an unresolved reference when linked. If you do the opposite, then FOO might not be linked in under various circumstances (such as when FOO is in a library, or you're using a “clever” linker—so clever, it produces a broken program with little or no warning by omitting initializations of global data because they are contained in unreferenced procedures).
The changes you make to your code to make g77 handle this situation, however, appear to be a widely portable way to handle it. That is, many systems permit it (as they should, since the FORTRAN 77 standard permits EXTERNAL FOO when FOO is a block data program unit), and of the ones that might not link BLOCK DATA FOO under some circumstances, most of them appear to do so once EXTERNAL FOO is present in the appropriate program units.
Here is the recommended approach to modifying a program containing a program unit such as the following:
BLOCK DATA FOO
COMMON /VARS/ X, Y, Z
DATA X, Y, Z / 3., 4., 5. /
END
If the above program unit might be placed in a library module, then
ensure that every program unit in every program that references that
particular COMMON area uses the EXTERNAL statement
to force the area to be initialized.
For example, change a program unit that starts with
INTEGER FUNCTION CURX()
COMMON /VARS/ X, Y, Z
CURX = X
END
so that it uses the EXTERNAL statement, as in:
INTEGER FUNCTION CURX()
COMMON /VARS/ X, Y, Z
EXTERNAL FOO
CURX = X
END
That way, CURX is compiled by g77 (and many other
compilers) so that the linker knows it must include FOO,
the BLOCK DATA program unit that sets the initial values
for the variables in VAR, in the executable program.
The meaning of a DO loop in Fortran is precisely specified
in the Fortran standard...and is quite different from what
many programmers might expect.
In particular, Fortran iterative DO loops are implemented as if
the number of trips through the loop is calculated before
the loop is entered.
The number of trips for a loop is calculated from the start, end, and increment values specified in a statement such as:
DO iter = start, end, increment
The trip count is evaluated using a fairly simple formula based on the three values following the = in the statement, and it is that trip count that is effectively decremented during each iteration of the loop. If, at the beginning of an iteration of the loop, the trip count is zero or negative, the loop terminates. The per-loop-iteration modifications to iter are not related to determining whether to terminate the loop.
There are two important things to remember about the trip count:
INTEGER(KIND=1).
These two items mean that there are loops that cannot
be written in straightforward fashion using the Fortran DO.
For example, on a system with the canonical 32-bit two's-complement
implementation of INTEGER(KIND=1), the following loop will not work:
DO I = -2000000000, 2000000000
Although the start and end values are well within
the range of INTEGER(KIND=1), the trip count is not.
The expected trip count is 40000000001, which is outside
the range of INTEGER(KIND=1) on many systems.
Instead, the above loop should be constructed this way:
I = -2000000000
DO
IF (I .GT. 2000000000) EXIT
...
I = I + 1
END DO
The simple DO construct and the EXIT statement
(used to leave the innermost loop)
are F90 features that g77 supports.
Some Fortran compilers have buggy implementations of DO,
in that they don't follow the standard.
They implement DO as a straightforward translation
to what, in C, would be a for statement.
Instead of creating a temporary variable to hold the trip count
as calculated at run time, these compilers
use the iteration variable iter to control
whether the loop continues at each iteration.
The bug in such an implementation shows up when the trip count is within the range of the type of iter, but the magnitude of ABS(end) + ABS(incr) exceeds that range. For example:
DO I = 2147483600, 2147483647
A loop started by the above statement will work as implemented by g77, but the use, by some compilers, of a more C-like implementation akin to
for (i = 2147483600; i <= 2147483647; ++i)
produces a loop that does not terminate, because i can never be greater than 2147483647, since incrementing it beyond that value overflows i, setting it to -2147483648. This is a large, negative number that still is less than 2147483647.
Another example of unexpected behavior of DO involves
using a nonintegral iteration variable iter, that is,
a REAL variable.
Consider the following program:
DATA BEGIN, END, STEP /.1, .31, .007/
DO 10 R = BEGIN, END, STEP
IF (R .GT. END) PRINT *, R, ' .GT. ', END, '!!'
PRINT *,R
10 CONTINUE
PRINT *,'LAST = ',R
IF (R .LE. END) PRINT *, R, ' .LE. ', END, '!!'
END
A C-like view of DO would hold that the two “exclamatory”
PRINT statements are never executed.
However, this is the output of running the above program
as compiled by g77 on a GNU/Linux ix86 system:
.100000001
.107000001
.114
.120999999
...
.289000005
.296000004
.303000003
LAST = .310000002
.310000002 .LE. .310000002!!
Note that one of the two checks in the program turned up
an apparent violation of the programmer's expectation—yet,
the loop is correctly implemented by g77, in that
it has 30 iterations.
This trip count of 30 is correct when evaluated using
the floating-point representations for the begin,
end, and incr values (.1, .31, .007) on GNU/Linux
ix86 are used.
On other systems, an apparently more accurate trip count
of 31 might result, but, nevertheless, g77 is
faithfully following the Fortran standard, and the result
is not what the author of the sample program above
apparently expected.
(Such other systems might, for different values in the DATA
statement, violate the other programmer's expectation,
for example.)
Due to this combination of imprecise representation
of floating-point values and the often-misunderstood
interpretation of DO by standard-conforming
compilers such as g77, use of DO loops
with REAL iteration
variables is not recommended.
Such use can be caught by specifying -Wsurprising.
See Warning Options, for more information on this
option.
Getting Fortran programs to work in the first place can be quite a challenge—even when the programs already work on other systems, or when using other compilers.
g77 offers some facilities that might be useful for tracking down bugs in such programs.
A fruitful source of bugs in Fortran source code is use, or mis-use, of Fortran's implicit-typing feature, whereby the type of a variable, array, or function is determined by the first character of its name.
Simple cases of this include statements like LOGX=9.227,
without a statement such as REAL LOGX.
In this case, LOGX is implicitly given INTEGER(KIND=1)
type, with the result of the assignment being that it is given
the value 9.
More involved cases include a function that is defined starting
with a statement like DOUBLE PRECISION FUNCTION IPS(...).
Any caller of this function that does not also declare IPS
as type DOUBLE PRECISION (or, in GNU Fortran, REAL(KIND=2))
is likely to assume it returns
INTEGER, or some other type, leading to invalid results
or even program crashes.
The -Wimplicit option might catch failures to properly specify the types of variables, arrays, and functions in the code.
However, in code that makes heavy use of Fortran's
implicit-typing facility, this option might produce so
many warnings about cases that are working, it would be
hard to find the one or two that represent bugs.
This is why so many experienced Fortran programmers strongly
recommend widespread use of the IMPLICIT NONE statement,
despite it not being standard FORTRAN 77, to completely turn
off implicit typing.
(g77 supports IMPLICIT NONE, as do almost all
FORTRAN 77 compilers.)
Note that -Wimplicit catches only implicit typing of names. It does not catch implicit typing of expressions such as X**(2/3). Such expressions can be buggy as well—in fact, X**(2/3) is equivalent to X**0, due to the way Fortran expressions are given types and then evaluated. (In this particular case, the programmer probably wanted X**(2./3.).)
Many Fortran programs were developed on systems that provided automatic initialization of all, or some, variables and arrays to zero. As a result, many of these programs depend, sometimes inadvertently, on this behavior, though to do so violates the Fortran standards.
You can ask g77 for this behavior by specifying the -finit-local-zero option when compiling Fortran code. (You might want to specify -fno-automatic as well, to avoid code-size inflation for non-optimized compilations.)
Note that a program that works better when compiled with the -finit-local-zero option is almost certainly depending on a particular system's, or compiler's, tendency to initialize some variables to zero. It might be worthwhile finding such cases and fixing them, using techniques such as compiling with the -O -Wuninitialized options using g77.
Many Fortran programs were developed on systems that
saved the values of all, or some, variables and arrays
across procedure calls.
As a result, many of these programs depend, sometimes
inadvertently, on being able to assign a value to a
variable, perform a RETURN to a calling procedure,
and, upon subsequent invocation, reference the previously
assigned variable to obtain the value.
They expect this despite not using the SAVE statement
to specify that the value in a variable is expected to survive
procedure returns and calls.
Depending on variables and arrays to retain values across
procedure calls without using SAVE to require it violates
the Fortran standards.
You can ask g77 to assume SAVE is specified for all
relevant (local) variables and arrays by using the
-fno-automatic option.
Note that a program that works better when compiled with the
-fno-automatic option
is almost certainly depending on not having to use
the SAVE statement as required by the Fortran standard.
It might be worthwhile finding such cases and fixing them,
using techniques such as compiling with the -O -Wuninitialized
options using g77.
The -Wunused option can find bugs involving implicit typing, sometimes more easily than using -Wimplicit in code that makes heavy use of implicit typing. An unused variable or array might indicate that the spelling for its declaration is different from that of its intended uses.
Other than cases involving typos, unused variables rarely indicate actual bugs in a program. However, investigating such cases thoroughly has, on occasion, led to the discovery of code that had not been completely written—where the programmer wrote declarations as needed for the whole algorithm, wrote some or even most of the code for that algorithm, then got distracted and forgot that the job was not complete.
As with unused variables, It is possible that unused arguments to a procedure might indicate a bug. Compile with -W -Wunused option to catch cases of unused arguments.
Note that -W also enables warnings regarding overflow of floating-point constants under certain circumstances.
The -Wsurprising option can help find bugs involving
expression evaluation or in
the way DO loops with non-integral iteration variables
are handled.
Cases found by this option might indicate a difference of
interpretation between the author of the code involved, and
a standard-conforming compiler such as g77.
Such a difference might produce actual bugs.
In any case, changing the code to explicitly do what the programmer might have expected it to do, so g77 and other compilers are more likely to follow the programmer's expectations, might be worthwhile, especially if such changes make the program work better.
The -falias-check, -fargument-alias, -fargument-noalias, and -fno-argument-noalias-global options, introduced in version 0.5.20 and g77's version 2.7.2.2.f.2 of gcc, were withdrawn as of g77 version 0.5.23 due to their not being supported by gcc version 2.8.
These options control the assumptions regarding aliasing (overlapping) of writes and reads to main memory (core) made by the gcc back end.
The information below still is useful, but applies to only those versions of g77 that support the alias analysis implied by support for these options.
These options are effective only when compiling with -O (specifying any level other than -O0) or with -falias-check.
The default for Fortran code is -fargument-noalias-global. (The default for C code and code written in other C-based languages is -fargument-alias. These defaults apply regardless of whether you use g77 or gcc to compile your code.)
Note that, on some systems, compiling with -fforce-addr in effect can produce more optimal code when the default aliasing options are in effect (and when optimization is enabled).
If your program is not working when compiled with optimization,
it is possible it is violating the Fortran standards (77 and 90)
by relying on the ability to “safely” modify variables and
arrays that are aliased, via procedure calls, to other variables
and arrays, without using EQUIVALENCE to explicitly
set up this kind of aliasing.
(The FORTRAN 77 standard's prohibition of this sort of
overlap, generally referred to therein as “storage
association”, appears in Sections 15.9.3.6.
This prohibition allows implementations, such as g77,
to, for example, implement the passing of procedures and
even values in COMMON via copy operations into local,
perhaps more efficiently accessed temporaries at entry to a
procedure, and, where appropriate, via copy operations back
out to their original locations in memory at exit from that
procedure, without having to take into consideration the
order in which the local copies are updated by the code,
among other things.)
To test this hypothesis, try compiling your program with the -fargument-alias option, which causes the compiler to revert to assumptions essentially the same as made by versions of g77 prior to 0.5.20.
If the program works using this option, that strongly suggests that the bug is in your program. Finding and fixing the bug(s) should result in a program that is more standard-conforming and that can be compiled by g77 in a way that results in a faster executable.
(You might want to try compiling with -fargument-noalias,
a kind of half-way point, to see if the problem is limited to
aliasing between dummy arguments and COMMON variables—this
option assumes that such aliasing is not done, while still allowing
aliasing among dummy arguments.)
An example of aliasing that is invalid according to the standards is shown in the following program, which might not produce the expected results when executed:
I = 1
CALL FOO(I, I)
PRINT *, I
END
SUBROUTINE FOO(J, K)
J = J + K
K = J * K
PRINT *, J, K
END
The above program attempts to use the temporary aliasing of the J and K arguments in FOO to effect a pathological behavior—the simultaneous changing of the values of both J and K when either one of them is written.
The programmer likely expects the program to print these values:
2 4
4
However, since the program is not standard-conforming, an implementation's behavior when running it is undefined, because subroutine FOO modifies at least one of the arguments, and they are aliased with each other. (Even if one of the assignment statements was deleted, the program would still violate these rules. This kind of on-the-fly aliasing is permitted by the standard only when none of the aliased items are defined, or written, while the aliasing is in effect.)
As a practical example, an optimizing compiler might schedule the J = part of the second line of FOO after the reading of J and K for the J * K expression, resulting in the following output:
2 2
2
Essentially, compilers are promised (by the standard and, therefore,
by programmers who write code they claim to be standard-conforming)
that if they cannot detect aliasing via static analysis of a single
program unit's EQUIVALENCE and COMMON statements, no
such aliasing exists.
In such cases, compilers are free to assume that an assignment to
one variable will not change the value of another variable, allowing
it to avoid generating code to re-read the value of the other
variable, to re-schedule reads and writes, and so on, to produce
a faster executable.
The same promise holds true for arrays (as seen by the called
procedure)—an element of one dummy array cannot be aliased
with, or overlap, any element of another dummy array or be
in a COMMON area known to the procedure.
(These restrictions apply only when the procedure defines, or writes to, one of the aliased variables or arrays.)
Unfortunately, there is no way to find all possible cases of violations of the prohibitions against aliasing in Fortran code. Static analysis is certainly imperfect, as is run-time analysis, since neither can catch all violations. (Static analysis can catch all likely violations, and some that might never actually happen, while run-time analysis can catch only those violations that actually happen during a particular run. Neither approach can cope with programs mixing Fortran code with routines written in other languages, however.)
Currently, g77 provides neither static nor run-time facilities to detect any cases of this problem, although other products might. Run-time facilities are more likely to be offered by future versions of g77, though patches improving g77 so that it provides either form of detection are welcome.
For several versions prior to 0.5.20, g77 configured its
version of the libf2c run-time library so that one of
its configuration macros, ALWAYS_FLUSH, was defined.
This was done as a result of a belief that many programs expected
output to be flushed to the operating system (under UNIX, via
the fflush() library call) with the result that errors,
such as disk full, would be immediately flagged via the
relevant ERR= and IOSTAT= mechanism.
Because of the adverse effects this approach had on the performance
of many programs, g77 no longer configures libf2c
(now named libg2c in its g77 incarnation)
to always flush output.
If your program depends on this behavior, either insert the
appropriate CALL FLUSH statements, or modify the sources
to the libg2c, rebuild and reinstall g77, and
relink your programs with the modified library.
(Ideally, libg2c would offer the choice at run-time, so
that a compile-time option to g77 or f2c could
result in generating the appropriate calls to flushing or
non-flushing library routines.)
Some Fortran programs require output
(writes) to be flushed to the operating system (under UNIX,
via the fflush() library call) so that errors,
such as disk full, are immediately flagged via the relevant
ERR= and IOSTAT= mechanism, instead of such
errors being flagged later as subsequent writes occur, forcing
the previously written data to disk, or when the file is
closed.
Essentially, the difference can be viewed as synchronous error reporting (immediate flagging of errors during writes) versus asynchronous, or, more precisely, buffered error reporting (detection of errors might be delayed).
libg2c supports flagging write errors immediately when
it is built with the ALWAYS_FLUSH macro defined.
This results in a libg2c that runs slower, sometimes
quite a bit slower, under certain circumstances—for example,
accessing files via the networked file system NFS—but the
effect can be more reliable, robust file I/O.
If you know that Fortran programs requiring this level of precision
of error reporting are to be compiled using the
version of g77 you are building, you might wish to
modify the g77 source tree so that the version of
libg2c is built with the ALWAYS_FLUSH macro
defined, enabling this behavior.
To do this, find this line in gcc/libf2c/f2c.h in your g77 source tree:
/* #define ALWAYS_FLUSH */
Remove the leading /* , so the line begins with #define, and the trailing */.
Then build or rebuild g77 as appropriate.
If your program crashes at run time with a message including
the text illegal unit number, that probably is
a message from the run-time library, libg2c.
The message means that your program has attempted to use a
file unit number that is out of the range accepted by
libg2c.
Normally, this range is 0 through 99, and the high end
of the range is controlled by a libg2c source-file
macro named MXUNIT.
If you can easily change your program to use unit numbers in the range 0 through 99, you should do so.
As distributed, whether as part of f2c or g77,
libf2c accepts file unit numbers only in the range
0 through 99.
For example, a statement such as WRITE (UNIT=100) causes
a run-time crash in libf2c, because the unit number,
100, is out of range.
If you know that Fortran programs at your installation require
the use of unit numbers higher than 99, you can change the
value of the MXUNIT macro, which represents the maximum unit
number, to an appropriately higher value.
To do this, edit the file gcc/libf2c/libI77/fio.h in your g77 source tree, changing the following line:
#define MXUNIT 100
Change the line so that the value of MXUNIT is defined to be
at least one greater than the maximum unit number used by
the Fortran programs on your system.
(For example, a program that does WRITE (UNIT=255) would require
MXUNIT set to at least 256 to avoid crashing.)
Then build or rebuild g77 as appropriate.
Note: Changing this macro has no effect on other limits
your system might place on the number of files open at the same time.
That is, the macro might allow a program to do WRITE (UNIT=100),
but the library and operating system underlying libf2c might
disallow it if many other files have already been opened (via OPEN or
implicitly via READ, WRITE, and so on).
Information on how to increase these other limits should be found
in your system's documentation.
If your program depends on exact IEEE 754 floating-point handling it may help on some systems—specifically x86 or m68k hardware—to use the -ffloat-store option or to reset the precision flag on the floating-point unit. See Optimize Options.
However, it might be better simply to put the FPU into double precision mode and not take the performance hit of -ffloat-store. On x86 and m68k GNU systems you can do this with a technique similar to that for turning on floating-point exceptions (see Floating-point Exception Handling). The control word could be set to double precision by some code like this one:
#include <fpu_control.h>
{
fpu_control_t cw = (_FPU_DEFAULT & ~_FPU_EXTENDED) | _FPU_DOUBLE;
_FPU_SETCW(cw);
}
(It is not clear whether this has any effect on the operation of the GNU maths library, but we have no evidence of it causing trouble.)
Some targets (such as the Alpha) may need special options for full IEEE conformance. See Hardware Models and Configurations (Using the GNU Compiler Collection (GCC)).
Code containing inconsistent calling sequences in the same file is normally rejected—see GLOBALS. (Use, say, ftnchek to ensure consistency across source files. See Generating Skeletons and Prototypes with f2c.)
Mysterious errors, which may appear to be code generation problems, can
appear specifically on the x86 architecture with some such
inconsistencies. On x86 hardware, floating-point return values of
functions are placed on the floating-point unit's register stack, not
the normal stack. Thus calling a REAL or DOUBLE PRECISION
FUNCTION as some other sort of procedure, or vice versa,
scrambles the floating-point stack. This may break unrelated code
executed later. Similarly if, say, external C routines are written
incorrectly.
These options should be used only as a quick-and-dirty way to determine how well your program will run under different compilation models without having to change the source. Some are more problematic than others, depending on how portable and maintainable you want the program to be (and, of course, whether you are allowed to change it at all is crucial).
You should not continue to use these command-line options to compile a given program, but rather should make changes to the source code:
-finit-local-zeroMany other compilers do this automatically, which means lots of Fortran code developed with those compilers depends on it.
It is safer (and probably
would produce a faster program) to find the variables and arrays that
need such initialization and provide it explicitly via DATA, so that
-finit-local-zero is not needed.
Consider using -Wuninitialized (which requires -O) to find likely candidates, but do not specify -finit-local-zero or -fno-automatic, or this technique won't work.
-fno-automaticSAVE statements.)
Many other compilers do this automatically, which means lots of Fortran code developed with those compilers depends on it.
The effect of this is that all non-automatic variables and arrays
are made static, that is, not placed on the stack or in heap storage.
This might cause a buggy program to appear to work better.
If so, rather than relying on this command-line option (and hoping all
compilers provide the equivalent one), add SAVE
statements to some or all program unit sources, as appropriate.
Consider using -Wuninitialized (which requires -O)
to find likely candidates, but
do not specify -finit-local-zero or -fno-automatic,
or this technique won't work.
The default is -fautomatic, which tells g77 to try and put variables and arrays on the stack (or in fast registers) where possible and reasonable. This tends to make programs faster.
Note: Automatic variables and arrays are not affected
by this option.
These are variables and arrays that are necessarily automatic,
either due to explicit statements, or due to the way they are
declared.
Examples include local variables and arrays not given the
SAVE attribute in procedures declared RECURSIVE,
and local arrays declared with non-constant bounds (automatic
arrays).
Currently, g77 supports only automatic arrays, not
RECURSIVE procedures or other means of explicitly
specifying that variables or arrays are automatic.
-fgroup-intrinsics-hideEXTERNAL for any external procedure
that might be the name of an intrinsic.
It is easy to find these using -fgroup-intrinsics-disable.
Aside from the usual gcc options, such as -O, -ffast-math, and so on, consider trying some of the following approaches to speed up your program (once you get it working).
On some systems, such as those with Pentium Pro CPUs, programs
that make heavy use of REAL(KIND=2) (DOUBLE PRECISION)
might run much slower
than possible due to the compiler not aligning these 64-bit
values to 64-bit boundaries in memory.
(The effect also is present, though
to a lesser extent, on the 586 (Pentium) architecture.)
The Intel x86 architecture generally ensures that these programs will work on all its implementations, but particular implementations (such as Pentium Pro) perform better with more strict alignment. (Such behavior isn't unique to the Intel x86 architecture.) Other architectures might demand 64-bit alignment of 64-bit data.
There are a variety of approaches to use to address this problem:
COMMON and EQUIVALENCE areas such
that the variables and arrays with the widest alignment
guidelines come first.
For example, on most systems, this would mean placing
COMPLEX(KIND=2), REAL(KIND=2), and
INTEGER(KIND=2) entities first, followed by REAL(KIND=1),
INTEGER(KIND=1), and LOGICAL(KIND=1) entities, then
INTEGER(KIND=6) entities, and finally CHARACTER
and INTEGER(KIND=3) entities.
The reason to use such placement is it makes it more likely
that your data will be aligned properly, without requiring
you to do detailed analysis of each aggregate (COMMON
and EQUIVALENCE) area.
Specifically, on systems where the above guidelines are
appropriate, placing CHARACTER entities before
REAL(KIND=2) entities can work just as well,
but only if the number of bytes occupied by the CHARACTER
entities is divisible by the recommended alignment for
REAL(KIND=2).
By ordering the placement of entities in aggregate areas according to the simple guidelines above, you avoid having to carefully count the number of bytes occupied by each entity to determine whether the actual alignment of each subsequent entity meets the alignment guidelines for the type of that entity.
If you don't ensure correct alignment of COMMON elements, the
compiler may be forced by some systems to violate the Fortran semantics by
adding padding to get DOUBLE PRECISION data properly aligned.
If the unfortunate practice is employed of overlaying different types of
data in the COMMON block, the different variants
of this block may become misaligned with respect to each other.
Even if your platform doesn't require strict alignment,
COMMON should be laid out as above for portability.
(Unfortunately the FORTRAN 77 standard didn't anticipate this
possible requirement, which is compiler-independent on a given platform.)
COMMON to be padded if necessary to align
DOUBLE PRECISION data.
When DOUBLE PRECISION data is forcibly aligned
in COMMON by g77 due to specifying -malign-double,
g77 issues a warning about the need to
insert padding.
In this case, each and every program unit that uses
the same COMMON area
must specify the same layout of variables and their types
for that area
and be compiled with -malign-double as well.
g77 will issue warnings in each case,
but as long as every program unit using that area
is compiled with the same warnings,
the resulting object files should work when linked together
unless the program makes additional assumptions about
COMMON area layouts that are outside the scope
of the FORTRAN 77 standard,
or uses EQUIVALENCE or different layouts
in ways that assume no padding is ever inserted by the compiler.
main().
The recent one from GNU (glibc2) will do this on x86 systems,
but we don't know of any other x86 setups where it will be right.
Read your system's documentation to determine if
it is appropriate to upgrade to a more recent version
to obtain the optimal alignment.
Progress is being made on making this work “out of the box” on future versions of g77, gcc, and some of the relevant operating systems (such as GNU/Linux).
If you're using -fno-automatic already, you probably should change your code to allow compilation with -fautomatic (the default), to allow the program to run faster.
Similarly, you should be able to use -fno-init-local-zero (the default) instead of -finit-local-zero. This is because it is rare that every variable affected by these options in a given program actually needs to be so affected.
For example, -fno-automatic, which effectively SAVEs
every local non-automatic variable and array, affects even things like
DO iteration
variables, which rarely need to be SAVEd, and this often reduces
run-time performances.
Similarly, -fno-init-local-zero forces such
variables to be initialized to zero—when SAVEd (such as when
-fno-automatic), this by itself generally affects only
startup time for a program, but when not SAVEd,
it can slow down the procedure every time it is called.
See Overly Convenient Command-Line Options, for information on the -fno-automatic and -finit-local-zero options and how to convert their use into selective changes in your own code.
If you aren't linking with any code compiled using
f2c, try using the -fno-f2c option when
compiling all the code in your program.
(Note that libf2c is not an example of code
that is compiled using f2c—it is compiled by a C
compiler, typically gcc.)
Using an appropriate -m option to generate specific code for your CPU may be worthwhile, though it may mean the executable won't run on other versions of the CPU that don't support the same instruction set. See Hardware Models and Configurations (Using the GNU Compiler Collection (GCC)). For instance on an x86 system the compiler might have been built—as shown by g77 -v—for the target i386-pc-linux-gnu, i.e. an i386 CPU. In that case to generate code best optimized for a Pentium you could use the option -march=pentium.
For recent CPUs that don't have explicit support in the released version of gcc, it might still be possible to get improvements with certain -m options.
-fomit-frame-pointer can help performance on x86 systems and others. It will, however, inhibit debugging on the systems on which it is not turned on anyway by -O.
This section describes known problems that affect users of GNU Fortran. Most of these are not GNU Fortran bugs per se—if they were, we would fix them. But the result for a user might be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.
(Note that some of this portion of the manual is lifted directly from the gcc manual, with minor modifications to tailor it to users of g77. Anytime a bug seems to have more to do with the gcc portion of g77, see Known Causes of Trouble with GCC (Using the GNU Compiler Collection (GCC)).)
These are bugs to which the maintainers often have to reply, “but that isn't a bug in g77...”. Some of these already are fixed in new versions of other software; some still need to be fixed; some are problems with how g77 is installed or is being used; some are the result of bad hardware that causes software to misbehave in sometimes bizarre ways; some just cannot be addressed at this time until more is known about the problem.
Please don't re-report these bugs to the g77 maintainers—if you must remind someone how important it is to you that the problem be fixed, talk to the people responsible for the other products identified below, but preferably only after you've tried the latest versions of those products. The g77 maintainers have their hands full working on just fixing and improving g77, without serving as a clearinghouse for all bugs that happen to affect g77 users.
See Collected Fortran Wisdom, for information on behavior of Fortran programs, and the programs that compile them, that might be thought to indicate bugs.
A whole variety of strange behaviors can occur when the software, or the way you are using the software, stresses the hardware in a way that triggers hardware bugs. This might seem hard to believe, but it happens frequently enough that there exist documents explaining in detail what the various causes of the problems are, what typical symptoms look like, and so on.
Generally these problems are referred to in this document as “signal 11” crashes, because the Linux kernel, running on the most popular hardware (the Intel x86 line), often stresses the hardware more than other popular operating systems. When hardware problems do occur under GNU/Linux on x86 systems, these often manifest themselves as “signal 11” problems, as illustrated by the following diagnostic:
sh# g77 myprog.f
gcc: Internal compiler error: program f771 got fatal signal 11
sh#
It is very important to remember that the above message is not the only one that indicates a hardware problem, nor does it always indicate a hardware problem.
In particular, on systems other than those running the Linux kernel, the message might appear somewhat or very different, as it will if the error manifests itself while running a program other than the g77 compiler. For example, it will appear somewhat different when running your program, when running Emacs, and so on.
How to cope with such problems is well beyond the scope of this manual.
However, users of Linux-based systems (such as GNU/Linux) should review http://www.bitwizard.nl/sig11/, a source of detailed information on diagnosing hardware problems, by recognizing their common symptoms.
Users of other operating systems and hardware might find this reference useful as well. If you know of similar material for another hardware/software combination, please let us know so we can consider including a reference to it in future versions of this manual.
On some systems, perhaps just those with out-of-date (shared?) libraries, unresolved-reference errors happen when linking g77-compiled programs (which should be done using g77).
If this happens to you, try appending -lc to the command you use to link the program, e.g. g77 foo.f -lc. g77 already specifies -lg2c -lm when it calls the linker, but it cannot also specify -lc because not all systems have a file named libc.a.
It is unclear at this point whether there are legitimately installed systems where -lg2c -lm is insufficient to resolve code produced by g77.
If your program doesn't link due to unresolved references to names like _main, make sure you're using the g77 command to do the link, since this command ensures that the necessary libraries are loaded by specifying -lg2c -lm when it invokes the gcc command to do the actual link. (Use the -v option to discover more about what actually happens when you use the g77 and gcc commands.)
Also, try specifying -lc as the last item on the g77 command line, in case that helps.
On some older GNU/Linux systems, programs with common blocks larger than 16MB cannot be linked without some kind of error message being produced.
This is a bug in older versions of ld, fixed in
more recent versions of binutils, such as version 2.6.
There are some known problems when using gdb on code
compiled by g77.
Inadequate investigation as of the release of 0.5.16 results in not
knowing which products are the culprit, but gdb-4.14 definitely
crashes when, for example, an attempt is made to print the contents
of a COMPLEX(KIND=2) dummy array, on at least some GNU/Linux
machines, plus some others.
Attempts to access assumed-size arrays are
also known to crash recent versions of gdb.
(gdb's Fortran support was done for a different compiler
and isn't properly compatible with g77.)
Developers of Fortran code on NeXTStep (all architectures) have to watch out for the following problem when writing programs with large, statically allocated (i.e. non-stack based) data structures (common blocks, saved arrays).
Due to the way the native loader (/bin/ld) lays out data structures in virtual memory, it is very easy to create an executable wherein the __DATA segment overlaps (has addresses in common) with the UNIX STACK segment.
This leads to all sorts of trouble, from the executable simply not executing, to bus errors. The NeXTStep command line tool ebadexec points to the problem as follows:
% /bin/ebadexec a.out
/bin/ebadexec: __LINKEDIT segment (truncated address = 0x3de000
rounded size = 0x2a000) of executable file: a.out overlaps with UNIX
STACK segment (truncated address = 0x400000 rounded size =
0x3c00000) of executable file: a.out
(In the above case, it is the __LINKEDIT segment that overlaps the stack segment.)
This can be cured by assigning the __DATA segment (virtual) addresses beyond the stack segment. A conservative estimate for this is from address 6000000 (hexadecimal) onwards—this has always worked for me [Toon Moene]:
% g77 -segaddr __DATA 6000000 test.f
% ebadexec a.out
ebadexec: file: a.out appears to be executable
%
Browsing through gcc/gcc/f/Makefile.in,
you will find that the f771 program itself also has to be
linked with these flags—it has large statically allocated
data structures.
(Version 0.5.18 reduces this somewhat, but probably
not enough.)
(The above item was contributed by Toon Moene (toon@moene.indiv.nluug.nl).)
g77 code might fail at runtime (probably with a “segmentation violation”) due to overflowing the stack. This happens most often on systems with an environment that provides substantially more heap space (for use when arbitrarily allocating and freeing memory) than stack space.
Often this can be cured by increasing or removing your shell's limit on stack usage, typically using limit stacksize (in csh and derivatives) or ulimit -s (in sh and derivatives).
Increasing the allowed stack size might, however, require changing some operating system or system configuration parameters.
You might be able to work around the problem by compiling with the -fno-automatic option to reduce stack usage, probably at the expense of speed.
g77, on most machines, puts many variables and arrays on the stack
where possible, and can be configured (by changing
FFECOM_sizeMAXSTACKITEM in gcc/gcc/f/com.c) to force
smaller-sized entities into static storage (saving
on stack space) or permit larger-sized entities to be put on the
stack (which can improve run-time performance, as it presents
more opportunities for the GBE to optimize the generated code).
Note: Putting more variables and arrays on the stack
might cause problems due to system-dependent limits on stack size.
Also, the value of FFECOM_sizeMAXSTACKITEM has no
effect on automatic variables and arrays.
See But-bugs, for more information.
Note: While libg2c places a limit on the range
of Fortran file-unit numbers, the underlying library and operating
system might impose different kinds of limits.
For example, some systems limit the number of files simultaneously
open by a running program.
Information on how to increase these limits should be found
in your system's documentation.
However, if your program uses large automatic arrays
(for example, has declarations like REAL A(N) where
A is a local array and N is a dummy or
COMMON variable that can have a large value),
neither use of -fno-automatic,
nor changing the cut-off point for g77 for using the stack,
will solve the problem by changing the placement of these
large arrays, as they are necessarily automatic.
g77 currently provides no means to specify that automatic arrays are to be allocated on the heap instead of the stack. So, other than increasing the stack size, your best bet is to change your source code to avoid large automatic arrays. Methods for doing this currently are outside the scope of this document.
(Note: If your system puts stack and heap space in the same memory area, such that they are effectively combined, then a stack overflow probably indicates a program that is either simply too large for the system, or buggy.)
It is occasionally reported that a “simple” program,
such as a “Hello, World!” program, does nothing when
it is run, even though the compiler reported no errors,
despite the program containing nothing other than a
simple PRINT statement.
This most often happens because the program has been compiled and linked on a UNIX system and named test, though other names can lead to similarly unexpected run-time behavior on various systems.
Essentially this problem boils down to giving your program a name that is already known to the shell you are using to identify some other program, which the shell continues to execute instead of your program when you invoke it via, for example:
sh# test
sh#
Under UNIX and many other system, a simple command name invokes a searching mechanism that might well not choose the program located in the current working directory if there is another alternative (such as the test command commonly installed on UNIX systems).
The reliable way to invoke a program you just linked in the current directory under UNIX is to specify it using an explicit pathname, as in:
sh# ./test
Hello, World!
sh#
Users who encounter this problem should take the time to read up on how their shell searches for commands, how to set their search path, and so on. The relevant UNIX commands to learn about include man, info (on GNU systems), setenv (or set and env), which, and find.
g77 code might fail at runtime with “segmentation violation”, “bus error”, or even something as subtle as a procedure call overwriting a variable or array element that it is not supposed to touch.
These can be symptoms of a wide variety of actual bugs that occurred earlier during the program's run, but manifested themselves as visible problems some time later.
Overflowing the bounds of an array—usually by writing beyond the end of it—is one of two kinds of bug that often occurs in Fortran code. (Compile your code with the -fbounds-check option to catch many of these kinds of errors at program run time.)
The other kind of bug is a mismatch between the actual arguments passed to a procedure and the dummy arguments as declared by that procedure.
Both of these kinds of bugs, and some others as well, can be difficult to track down, because the bug can change its behavior, or even appear to not occur, when using a debugger.
That is, these bugs can be quite sensitive to data, including data representing the placement of other data in memory (that is, pointers, such as the placement of stack frames in memory).
g77 now offers the ability to catch and report some of these problems at compile, link, or run time, such as by generating code to detect references to beyond the bounds of most arrays (except assumed-size arrays), and checking for agreement between calling and called procedures. Future improvements are likely to be made in the procedure-mismatch area, at least.
In the meantime, finding and fixing the programming bugs that lead to these behaviors is, ultimately, the user's responsibility, as difficult as that task can sometimes be.
One runtime problem that has been observed might have a simple solution.
If a formatted WRITE produces an endless stream of spaces, check
that your program is linked against the correct version of the C library.
The configuration process takes care to account for your
system's normal libc not being ANSI-standard, which will
otherwise cause this behavior.
If your system's default library is
ANSI-standard and you subsequently link against a non-ANSI one, there
might be problems such as this one.
Specifically, on Solaris2 systems,
avoid picking up the BSD library from /usr/ucblib.
Some programs appear to produce inconsistent floating-point results compiled by g77 versus by other compilers.
Often the reason for this behavior is the fact that floating-point values are represented on almost all Fortran systems by approximations, and these approximations are inexact even for apparently simple values like 0.1, 0.2, 0.3, 0.4, 0.6, 0.7, 0.8, 0.9, 1.1, and so on. Most Fortran systems, including all current ports of g77, use binary arithmetic to represent these approximations.
Therefore, the exact value of any floating-point approximation
as manipulated by g77-compiled code is representable by
adding some combination of the values 1.0, 0.5, 0.25, 0.125, and
so on (just keep dividing by two) through the precision of the
fraction (typically around 23 bits for REAL(KIND=1), 52 for
REAL(KIND=2)), then multiplying the sum by a integral
power of two (in Fortran, by 2**N) that typically is between
-127 and +128 for REAL(KIND=1) and -1023 and +1024 for
REAL(KIND=2), then multiplying by -1 if the number
is negative.
So, a value like 0.2 is exactly represented in decimal—since it is a fraction, 2/10, with a denominator that is compatible with the base of the number system (base 10). However, 2/10 cannot be represented by any finite number of sums of any of 1.0, 0.5, 0.25, and so on, so 0.2 cannot be exactly represented in binary notation.
(On the other hand, decimal notation can represent any binary number in a finite number of digits. Decimal notation cannot do so with ternary, or base-3, notation, which would represent floating-point numbers as sums of any of 1/1, 1/3, 1/9, and so on. After all, no finite number of decimal digits can exactly represent 1/3. Fortunately, few systems use ternary notation.)
Moreover, differences in the way run-time I/O libraries convert between these approximations and the decimal representation often used by programmers and the programs they write can result in apparent differences between results that do not actually exist, or exist to such a small degree that they usually are not worth worrying about.
For example, consider the following program:
PRINT *, 0.2
END
When compiled by g77, the above program might output 0.20000003, while another compiler might produce a executable that outputs 0.2.
This particular difference is due to the fact that, currently,
conversion of floating-point values by the libg2c library,
used by g77, handles only double-precision values.
Since 0.2 in the program is a single-precision value, it is converted to double precision (still in binary notation) before being converted back to decimal. The conversion to binary appends binary zero digits to the original value—which, again, is an inexact approximation of 0.2—resulting in an approximation that is much less exact than is connoted by the use of double precision.
(The appending of binary zero digits has essentially the same effect as taking a particular decimal approximation of 1/3, such as 0.3333333, and appending decimal zeros to it, producing 0.33333330000000000. Treating the resulting decimal approximation as if it really had 18 or so digits of valid precision would make it seem a very poor approximation of 1/3.)
As a result of converting the single-precision approximation to double precision by appending binary zeros, the conversion of the resulting double-precision value to decimal produces what looks like an incorrect result, when in fact the result is inexact, and is probably no less inaccurate or imprecise an approximation of 0.2 than is produced by other compilers that happen to output the converted value as “exactly” 0.2. (Some compilers behave in a way that can make them appear to retain more accuracy across a conversion of a single-precision constant to double precision. See Context-Sensitive Constants, to see why this practice is illusory and even dangerous.)
Note that a more exact approximation of the constant is computed when the program is changed to specify a double-precision constant:
PRINT *, 0.2D0
END
Future versions of g77 and/or libg2c might convert
single-precision values directly to decimal,
instead of converting them to double precision first.
This would tend to result in output that is more consistent
with that produced by some other Fortran implementations.
A useful source of information on floating-point computation is David Goldberg, `What Every Computer Scientist Should Know About Floating-Point Arithmetic', Computing Surveys, 23, March 1991, pp. 5-48. An online version is available at http://docs.sun.com/.
Information related to the IEEE 754 floating-point standard by a leading light can be found at http://http.cs.berkeley.edu/%7Ewkahan/ieee754status/; see also slides from the short course referenced from http://http.cs.berkeley.edu/%7Efateman/.
The supplement to the PostScript-formatted Goldberg document, referenced above, is available in HTML format. See `Differences Among IEEE 754 Implementations' by Doug Priest. This document explores some of the issues surrounding computing of extended (80-bit) results on processors such as the x86, especially when those results are arbitrarily truncated to 32-bit or 64-bit values by the compiler as “spills”.
(Note: g77 specifically, and gcc generally, does arbitrarily truncate 80-bit results during spills as of this writing. It is not yet clear whether a future version of the GNU compiler suite will offer 80-bit spills as an option, or perhaps even as the default behavior.)
The GNU C library provides routines for controlling the FPU, and other documentation about this.
See Floating-point precision, regarding IEEE 754 conformance.
This section identifies bugs that g77 users
might run into in the GCC-3.3.4 version
of g77.
This includes bugs that are actually in the gcc
back end (GBE) or in libf2c, because those
sets of code are at least somewhat under the control
of (and necessarily intertwined with) g77,
so it isn't worth separating them out.
For information on bugs in other versions of g77,
see News About GNU Fortran.
There, lists of bugs fixed in various versions of g77
can help determine what bugs existed in prior versions.
An online, “live” version of this document
(derived directly from the mainline, development version
of g77 within gcc)
is available via
http://gcc.gnu.org/onlinedocs/g77/Trouble.html.
Follow the “Known Bugs” link.
The following information was last updated on 2002-02-01:
g77 fails to warn about
use of a “live” iterative-DO variable
as an implied-DO variable
in a WRITE or PRINT statement
(although it does warn about this in a READ statement).
g77's straightforward handling of
label references and definitions sometimes prevents the GBE
from unrolling loops.
Until this is solved, try inserting or removing CONTINUE
statements as the terminal statement, using the END DO
form instead, and so on.
INCLUDE
statements from within INCLUDE'd or #include'd files.
g77 assumes that INTEGER(KIND=1) constants range
from -2**31 to 2**31-1 (the range for
two's-complement 32-bit values),
instead of determining their range from the actual range of the
type for the configuration (and, someday, for the constant).
Further, it generally doesn't implement the handling of constants very well in that it makes assumptions about the configuration that it no longer makes regarding variables (types).
Included with this item is the fact that g77 doesn't recognize
that, on IEEE-754/854-compliant systems, 0./0. should produce a NaN
and no warning instead of the value 0. and a warning.
g77 uses way too much memory and CPU time to process large aggregate
areas having any initialized elements.
For example, REAL A(1000000) followed by DATA A(1)/1/ takes up way too much time and space, including the size of the generated assembler file.
Version 0.5.18 improves cases like this—specifically, cases of sparse initialization that leave large, contiguous areas uninitialized—significantly. However, even with the improvements, these cases still require too much memory and CPU time.
(Version 0.5.18 also improves cases where the initial values are zero to a much greater degree, so if the above example ends with DATA A(1)/0/, the compile-time performance will be about as good as it will ever get, aside from unrelated improvements to the compiler.)
Note that g77 does display a warning message to
notify the user before the compiler appears to hang.
A warning message is issued when g77 sees code that provides
initial values (e.g. via DATA) to an aggregate area (COMMON
or EQUIVALENCE, or even a large enough array or CHARACTER
variable)
that is large enough to increase g77's compile time by roughly
a factor of 10.
This size currently is quite small, since g77
currently has a known bug requiring too much memory
and time to handle such cases.
In gcc/gcc/f/data.c, the macro
FFEDATA_sizeTOO_BIG_INIT_ is defined
to the minimum size for the warning to appear.
The size is specified in storage units,
which can be bytes, words, or whatever, on a case-by-case basis.
After changing this macro definition, you must
(of course) rebuild and reinstall g77 for
the change to take effect.
Note that, as of version 0.5.18, improvements have
reduced the scope of the problem for sparse
initialization of large arrays, especially those
with large, contiguous uninitialized areas.
However, the warning is issued at a point prior to
when g77 knows whether the initialization is sparse,
and delaying the warning could mean it is produced
too late to be helpful.
Therefore, the macro definition should not be adjusted to reflect sparse cases. Instead, adjust it to generate the warning when densely initialized arrays begin to cause responses noticeably slower than linear performance would suggest.
MAIN__ (or MAIN___ or MAIN_ if
MAIN__ doesn't exist)
and run the program until it hits the breakpoint.
At that point, the
main program unit is activated and about to execute its first
executable statement, but that's the state in which the debugger should
start up, as is the case for languages like C.
g77-compiled code using debuggers other than
gdb is likely not to work.
Getting g77 and gdb to work together is a known
problem—getting g77 to work properly with other
debuggers, for which source code often is unavailable to g77
developers, seems like a much larger, unknown problem,
and is a lower priority than making g77 and gdb
work together properly.
On the other hand, information about problems other debuggers
have with g77 output might make it easier to properly
fix g77, and perhaps even improve gdb, so it
is definitely welcome.
Such information might even lead to all relevant products
working together properly sooner.
g77 doesn't work perfectly on 64-bit configurations
such as the Digital Semiconductor (“DEC”) Alpha.
This problem is largely resolved as of version 0.5.23.
g77 currently inserts needless padding for things like
COMMON A,IPAD where A is CHARACTER*1 and IPAD
is INTEGER(KIND=1) on machines like x86,
because the back end insists that IPAD
be aligned to a 4-byte boundary,
but the processor has no such requirement
(though it is usually good for performance).
The gcc back end needs to provide a wider array
of specifications of alignment requirements and preferences for targets,
and front ends like g77 should take advantage of this
when it becomes available.
libf2c routines that perform some run-time
arithmetic on COMPLEX operands
were modified circa version 0.5.20 of g77
to work properly even in the presence of aliased operands.
While the g77 and netlib versions of libf2c
differ on how this is accomplished,
the main differences are that we believe
the g77 version works properly
even in the presence of partially aliased operands.
However, these modifications have reduced performance on targets such as x86, due to the extra copies of operands involved.
This section lists features we know are missing from g77, and which we want to add someday. (There is no priority implied in the ordering below.)
GNU Fortran language:
GNU Fortran dialects:
New facilities:
Better diagnostics:
Run-time facilities:
Debugging:
g77 needs to provide, as the default source-line model, a “pure visual” mode, where the interpretation of a source program in this mode can be accurately determined by a user looking at a traditionally displayed rendition of the program (assuming the user knows whether the program is fixed or free form).
The design should assume the user cannot tell tabs from spaces and cannot see trailing spaces on lines, but has canonical tab stops and, for fixed-form source, has the ability to always know exactly where column 72 is (since the Fortran standard itself requires this for fixed-form source).
This would change the default treatment of fixed-form source to not treat lines with tabs as if they were infinitely long—instead, they would end at column 72 just as if the tabs were replaced by spaces in the canonical way.
As part of this, provide common alternate models (Digital, f2c, and so on) via command-line options. This includes allowing arbitrarily long lines for free-form source as well as fixed-form source and providing various limits and diagnostics as appropriate.
Also, g77 should offer, perhaps even default to, warnings when characters beyond the last valid column are anything other than spaces. This would mean code with “sequence numbers” in columns 73 through 80 would be rejected, and there's a lot of that kind of code around, but one of the most frequent bugs encountered by new users is accidentally writing fixed-form source code into and beyond column 73. So, maybe the users of old code would be able to more easily handle having to specify, say, a -Wno-col73to80 option.
g77 does not support many of the features that distinguish Fortran 90 (and, now, Fortran 95) from ANSI FORTRAN 77.
Some Fortran 90 features are supported, because they make sense to offer even to die-hard users of F77. For example, many of them codify various ways F77 has been extended to meet users' needs during its tenure, so g77 might as well offer them as the primary way to meet those same needs, even if it offers compatibility with one or more of the ways those needs were met by other F77 compilers in the industry.
Still, many important F90 features are not supported, because no attempt has been made to research each and every feature and assess its viability in g77. In the meantime, users who need those features must use Fortran 90 compilers anyway, and the best approach to adding some F90 features to GNU Fortran might well be to fund a comprehensive project to create GNU Fortran 95.
PARAMETER Statements
g77 doesn't allow intrinsics in PARAMETER statements.
Related to this, g77 doesn't allow non-integral
exponentiation in PARAMETER statements, such as
PARAMETER (R=2**.25).
It is unlikely g77 will ever support this feature,
as doing it properly requires complete emulation of
a target computer's floating-point facilities when
building g77 as a cross-compiler.
But, if the gcc back end is enhanced to provide
such a facility, g77 will likely use that facility
in implementing this feature soon afterwards.
g77 doesn't support arbitrary operands for concatenation in contexts where run-time allocation is required. For example:
SUBROUTINE X(A)
CHARACTER*(*) A
CALL FOO(A // 'suffix')
SELECT CASE on CHARACTER TypeCharacter-type selector/cases for SELECT CASE currently
are not supported.
RECURSIVE Keyword
g77 doesn't support the RECURSIVE keyword that
F90 compilers do.
Nor does it provide any means for compiling procedures
designed to do recursion.
All recursive code can be rewritten to not use recursion, but the result is not pretty.
Some compilers, such as f2c, have an option (-r8,
-qrealsize=8 or
similar) that provides automatic treatment of REAL
entities such that they have twice the storage size, and
a corresponding increase in the range and precision, of what
would normally be the REAL(KIND=1) (default REAL) type.
(This affects COMPLEX the same way.)
They also typically offer another option (-i8) to increase
INTEGER entities so they are twice as large
(with roughly twice as much range).
(There are potential pitfalls in using these options.)
g77 does not yet offer any option that performs these kinds of transformations. Part of the problem is the lack of detailed specifications regarding exactly how these options affect the interpretation of constants, intrinsics, and so on.
Until g77 addresses this need, programmers could improve the portability of their code by modifying it to not require compile-time options to produce correct results. Some free tools are available which may help, specifically in Toolpack (which one would expect to be sound) and the fortran section of the Netlib repository.
Use of preprocessors can provide a fairly portable means to work around the lack of widely portable methods in the Fortran language itself (though increasing acceptance of Fortran 90 would alleviate this problem).
g77 doesn't fully support INTEGER*2, LOGICAL*1,
and similar.
In the meantime, version 0.5.18 provides rudimentary support
for them.
g77 doesn't support INTEGER, REAL, and COMPLEX equivalents
for all applicable back-end-supported types (char, short int,
int, long int, long long int, and long double).
This means providing intrinsic support, and maybe constant
support (using F90 syntax) as well, and, for most
machines will result in automatic support of INTEGER*1,
INTEGER*2, INTEGER*8, maybe even REAL*16,
and so on.
g77 doesn't support more general expressions to dimension arrays, such as array element references, function references, etc.
For example, g77 currently does not accept the following:
SUBROUTINE X(M, N)
INTEGER N(10), M(N(2), N(1))
g77 doesn't support pointers or allocatable objects
(other than automatic arrays).
This set of features is
probably considered just behind intrinsics
in PARAMETER statements on the list of large,
important things to add to g77.
In the meantime, consider using the INTEGER(KIND=7)
declaration to specify that a variable must be
able to hold a pointer.
This construct is not portable to other non-GNU compilers,
but it is portable to all machines GNU Fortran supports
when g77 is used.
See Functions and Subroutines, for information on
%VAL(), %REF(), and %DESCR()
constructs, which are useful for passing pointers to
procedures written in languages other than Fortran.
g77 rejects things other compilers accept, like INTRINSIC SQRT,SQRT. As time permits in the future, some of these things that are easy for humans to read and write and unlikely to be intended to mean something else will be accepted by g77 (though -fpedantic should trigger warnings about such non-standard constructs).
Until g77 no longer gratuitously rejects sensible code, you might as well fix your code to be more standard-conforming and portable.
The kind of case that is important to except from the recommendation to change your code is one where following good coding rules would force you to write non-standard code that nevertheless has a clear meaning.
For example, when writing an INCLUDE file that
defines a common block, it might be appropriate to
include a SAVE statement for the common block
(such as SAVE /CBLOCK/), so that variables
defined in the common block retain their values even
when all procedures declaring the common block become
inactive (return to their callers).
However, putting SAVE statements in an INCLUDE
file would prevent otherwise standard-conforming code
from also specifying the SAVE statement, by itself,
to indicate that all local variables and arrays are to
have the SAVE attribute.
For this reason, g77 already has been changed to allow this combination, because although the general problem of gratuitously rejecting unambiguous and “safe” constructs still exists in g77, this particular construct was deemed useful enough that it was worth fixing g77 for just this case.
So, while there is no need to change your code to avoid using this particular construct, there might be other, equally appropriate but non-standard constructs, that you shouldn't have to stop using just because g77 (or any other compiler) gratuitously rejects it.
Until the general problem is solved, if you have any such construct you believe is worthwhile using (e.g. not just an arbitrary, redundant specification of an attribute), please submit a bug report with an explanation, so we can consider fixing g77 just for cases like yours.
READONLY Keyword
Support for READONLY, in OPEN statements,
requires libg2c support,
to make sure that CLOSE(...,STATUS='DELETE')
does not delete a file opened on a unit
with the READONLY keyword,
and perhaps to trigger a fatal diagnostic
if a WRITE or PRINT
to such a unit is attempted.
Note: It is not sufficient for g77 and libg2c
(its version of libf2c)
to assume that READONLY does not need some kind of explicit support
at run time,
due to UNIX systems not (generally) needing it.
g77 is not just a UNIX-based compiler!
Further, mounting of non-UNIX filesystems on UNIX systems
(such as via NFS)
might require proper READONLY support.
(Similar issues might be involved with supporting the SHARED
keyword.)
FLUSH Statementg77 could perhaps use a FLUSH statement that
does what CALL FLUSH does,
but that supports * as the unit designator (same unit as for
PRINT) and accepts ERR= and/or IOSTAT=
specifiers.
FORMAT Statements
g77 doesn't support FORMAT(I<J>) and the like.
Supporting this requires a significant redesign or replacement
of libg2c.
However, g77 does support this construct when the expression is constant (as of version 0.5.22). For example:
PARAMETER (IWIDTH = 12)
10 FORMAT (I<IWIDTH>)
Otherwise, at least for output (PRINT and
WRITE), Fortran code making use of this feature can
be rewritten to avoid it by constructing the FORMAT
string in a CHARACTER variable or array, then
using that variable or array in place of the FORMAT
statement label to do the original PRINT or WRITE.
Many uses of this feature on input can be rewritten this way as well, but not all can. For example, this can be rewritten:
READ 20, I
20 FORMAT (I<J>)
However, this cannot, in general, be rewritten, especially
when ERR= and END= constructs are employed:
READ 30, J, I
30 FORMAT (I<J>)
g77 needs to provide some way, a la gcc, for g77 code to specify explicit assembler code.
The Q edit descriptor in FORMATs isn't supported.
(This is meant to get the number of characters remaining in an input record.)
Supporting this requires a significant redesign or replacement
of libg2c.
A workaround might be using internal I/O or the stream-based intrinsics. See FGetC Intrinsic (subroutine).
g77 doesn't accept PARAMETER I=1.
Supporting this obsolete form of
the PARAMETER statement would not be particularly hard, as most of the
parsing code is already in place and working.
Until time/money is
spent implementing it, you might as well fix your code to use the
standard form, PARAMETER (I=1) (possibly needing
INTEGER I preceding the PARAMETER statement as well,
otherwise, in the obsolete form of PARAMETER, the
type of the variable is set from the type of the constant being
assigned to it).
TYPE and ACCEPT I/O Statements
g77 doesn't support the I/O statements TYPE and
ACCEPT.
These are common extensions that should be easy to support,
but also are fairly easy to work around in user code.
Generally, any TYPE fmt,list I/O statement can be replaced by PRINT fmt,list. And, any ACCEPT fmt,list statement can be replaced by READ fmt,list.
STRUCTURE, UNION, RECORD, MAP
g77 doesn't support STRUCTURE, UNION, RECORD,
MAP.
This set of extensions is quite a bit
lower on the list of large, important things to add to g77, partly
because it requires a great deal of work either upgrading or
replacing libg2c.
OPEN, CLOSE, and INQUIRE Keywords
g77 doesn't have support for keywords such as DISP='DELETE' in
the OPEN, CLOSE, and INQUIRE statements.
These extensions are easy to add to g77 itself, but
require much more work on libg2c.
g77 doesn't support FORM='PRINT' or an equivalent to
translate the traditional `carriage control' characters in column 1 of
output to use backspaces, carriage returns and the like. However
programs exist to translate them in output files (or standard output).
These are typically called either fpr or asa. You can get
a version of asa from
ftp://sunsite.unc.edu/pub/Linux/devel/lang/fortran for GNU
systems which will probably build easily on other systems.
Alternatively, fpr is in BSD distributions in various archive
sites.
ENCODE and DECODE
g77 doesn't support ENCODE or DECODE.
These statements are best replaced by READ and WRITE statements involving internal files (CHARACTER variables and arrays).
For example, replace a code fragment like
INTEGER*1 LINE(80)
...
DECODE (80, 9000, LINE) A, B, C
...
9000 FORMAT (1X, 3(F10.5))
with:
CHARACTER*80 LINE
...
READ (UNIT=LINE, FMT=9000) A, B, C
...
9000 FORMAT (1X, 3(F10.5))
Similarly, replace a code fragment like
INTEGER*1 LINE(80)
...
ENCODE (80, 9000, LINE) A, B, C
...
9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
with:
CHARACTER*80 LINE
...
WRITE (UNIT=LINE, FMT=9000) A, B, C
...
9000 FORMAT (1X, 'OUTPUT IS ', 3(F10.5))
It is entirely possible that ENCODE and DECODE will
be supported by a future version of g77.
AUTOMATIC Statement
g77 doesn't support the AUTOMATIC statement that
f2c does.
AUTOMATIC would identify a variable or array
as not being SAVE'd, which is normally the default,
but which would be especially useful for code that, generally,
needed to be compiled with the -fno-automatic option.
AUTOMATIC also would serve as a hint to the compiler that placing
the variable or array—even a very large array–on the stack is acceptable.
AUTOMATIC would not, by itself, designate the containing procedure
as recursive.
AUTOMATIC should work syntactically like SAVE,
in that AUTOMATIC with no variables listed should apply to
all pertinent variables and arrays
(which would not include common blocks or their members).
Variables and arrays denoted as AUTOMATIC
would not be permitted to be initialized via DATA
or other specification of any initial values,
requiring explicit initialization,
such as via assignment statements.
Perhaps UNSAVE and STATIC,
as strict semantic opposites to SAVE and AUTOMATIC,
should be provided as well.
g77 should offer VXT-Fortran-style suppression of virtual spaces at the end of a source line if an appropriate command-line option is specified.
This affects cases where a character constant is continued onto the next line in a fixed-form source file, as in the following example:
10 PRINT *,'HOW MANY
1 SPACES?'
g77, and many other compilers, virtually extend the continued line through column 72 with spaces that become part of the character constant, but Digital Fortran normally didn't, leaving only one space between MANY and SPACES? in the output of the above statement.
Fairly recently, at least one version of Digital Fortran was enhanced to provide the other behavior when a command-line option is specified, apparently due to demand from readers of the USENET group comp.lang.fortran to offer conformance to this widespread practice in the industry. g77 should return the favor by offering conformance to Digital's approach to handling the above example.
g77 should offer a preprocessor designed specifically for Fortran to replace cpp -traditional. There are several out there worth evaluating, at least.
Such a preprocessor would recognize Hollerith constants,
properly parse comments and character constants, and so on.
It might also recognize, process, and thus preprocess
files included via the INCLUDE directive.
g77 does not allow REAL and other non-integral types for
arguments to intrinsics like And, Or, and Shift.
For example, this program is rejected by g77, because
the intrinsic Iand does not accept REAL arguments:
DATA A/7.54/, B/9.112/
PRINT *, IAND(A, B)
END
An option such as -fugly-char should be provided to allow
REAL*8 A1
DATA A1 / '12345678' /
and:
REAL*8 A1
A1 = 'ABCDEFGH'
POSIX Standardg77 should support the POSIX standard for Fortran.
The gcc backend and, consequently, g77, currently provides no general control over whether or not floating-point exceptions are trapped or ignored. (Ignoring them typically results in NaN values being propagated in systems that conform to IEEE 754.) The behavior is normally inherited from the system-dependent startup code, though some targets, such as the Alpha, have code generation options which change the behavior.
Most systems provide some C-callable mechanism to change this; this can
be invoked at startup using gcc's constructor attribute.
For example, just compiling and linking the following C code with your
program will turn on exception trapping for the “common” exceptions
on a GNU system using glibc 2.2 or newer:
#define _GNU_SOURCE 1
#include <fenv.h>
static void __attribute__ ((constructor))
trapfpe ()
{
/* Enable some exceptions. At startup all exceptions are masked. */
feenableexcept (FE_INVALID|FE_DIVBYZERO|FE_OVERFLOW);
}
A convenient trick is to compile this something like:
gcc -o libtrapfpe.a trapfpe.c
and then use it by adding -trapfpe to the g77 command line when linking.
g77 doesn't accept some particularly nonportable,
silent data-type conversions such as LOGICAL
to REAL (as in A=.FALSE., where A
is type REAL), that other compilers might
quietly accept.
Some of these conversions are accepted by g77 when the -fugly-logint option is specified. Perhaps it should accept more or all of them.
Currently, automatic arrays always are allocated on the stack. For situations where the stack cannot be made large enough, g77 should offer a compiler option that specifies allocation of automatic arrays in heap storage.
Neither the code produced by g77 nor the libg2c library
are thread-safe, nor does g77 have support for parallel processing
(other than the instruction-level parallelism available on some
processors).
A package such as PVM might help here.
An option such as -fdebug-lines should be provided to turn fixed-form lines beginning with D to be treated as if they began with a space, instead of as if they began with a C (as comment lines).
Because of how g77 generates code via the back end, it doesn't always provide warnings the user wants. Consider:
PROGRAM X
PRINT *, A
END
Currently, the above is not flagged as a case of using an uninitialized variable, because g77 generates a run-time library call that looks, to the GBE, like it might actually modify A at run time. (And, in fact, depending on the previous run-time library call, it would!)
Fixing this requires one of the following:
libg77, that provides
a more “clean” interface,
vis-a-vis input, output, and modified arguments,
so the GBE can tell what's going on.
This would provide a pretty big performance improvement, at least theoretically, and, ultimately, in practice, for some types of code.
This might also provide a performance boost for some code, because A might then end up living in a register, which could help with inner loops.
ADDR_EXPR
but with extra information on the fact that the
item pointed to won't be modified
(a la const in C).
Probably the best solution for now, but not quite trivial to implement in the general case.
g77 generally should continue processing for warnings and recoverable (user) errors whenever possible—that is, it shouldn't gratuitously make bad or useless code.
For example:
INTRINSIC ZABS
CALL FOO(ZABS)
END
When compiling the above with -ff2c-intrinsics-disable,
g77 should indeed complain about passing ZABS,
but it still should compile, instead of rejecting
the entire CALL statement.
(Some of this is related to improving
the compiler internals to improve how statements are analyzed.)
-Wconversion and related should flag places where non-standard conversions are found. Perhaps much of this would be part of -Wugly*.
g77 needs a new option, like -Wintrinsics, to warn about use of
non-standard intrinsics without explicit INTRINSIC statements for them.
This would help find code that might fail silently when ported to another
compiler.
DO Variableg77 should warn about modifying DO variables
via EQUIVALENCE.
(The internal information gathered to produce this warning
might also be useful in setting the
internal “doiter” flag for a variable or even array
reference within a loop, since that might produce faster code someday.)
For example, this code is invalid, so g77 should warn about the invalid assignment to NOTHER:
EQUIVALENCE (I, NOTHER)
DO I = 1, 100
IF (I.EQ. 10) NOTHER = 20
END DO
g77 needs to support -fpedantic more thoroughly,
and use it only to generate
warnings instead of rejecting constructs outright.
Have it warn:
if a variable that dimensions an array is not a dummy or placed
explicitly in COMMON (F77 does not allow it to be
placed in COMMON via EQUIVALENCE); if specification statements
follow statement-function-definition statements; about all sorts of
syntactic extensions.
g77 needs a -Wpromotions option to warn if source code appears
to expect automatic, silent, and
somewhat dangerous compiler-assisted conversion of REAL(KIND=1)
constants to REAL(KIND=2) based on context.
For example, it would warn about cases like this:
DOUBLE PRECISION FOO
PARAMETER (TZPHI = 9.435784839284958)
FOO = TZPHI * 3D0
g77 should disallow statements like RETURN 2HAB, which are invalid in both source forms (unlike RETURN (2HAB), which probably still makes no sense but at least can be reliably parsed). Fixed-form processing rejects it, but not free-form, except in a way that is a bit difficult to understand.
g77 should complain when a list of dummy arguments containing an adjustable dummy array does not also contain every variable listed in the dimension list of the adjustable array.
Currently, g77 does complain about a variable that
dimensions an array but doesn't appear in any dummy list or COMMON
area, but this needs to be extended to catch cases where it doesn't appear in
every dummy list that also lists any arrays it dimensions.
For example, g77 should warn about the entry point ALT below, since it includes ARRAY but not ISIZE in its list of arguments:
SUBROUTINE PRIMARY(ARRAY, ISIZE)
REAL ARRAY(ISIZE)
ENTRY ALT(ARRAY)
g77 should check FORMAT specifiers for validity
as it does FORMAT statements.
For example, a diagnostic would be produced for:
PRINT 'HI THERE!' !User meant PRINT *, 'HI THERE!'
g77 needs a set of options such as -Wugly*, -Wautomatic, -Wvxt, -Wf90, and so on. These would warn about places in the user's source where ambiguities are found, helpful in resolving ambiguities in the program's dialect or dialects.
g77 should warn about unused labels when -Wunused is in effect.
g77 needs an option to suppress information messages (notes). -w does this but also suppresses warnings. The default should be to suppress info messages.
Perhaps info messages should simply be eliminated.
g77 needs an option to initialize everything (not otherwise
explicitly initialized) to “weird”
(machine-dependent) values, e.g. NaNs, bad (non-NULL) pointers, and
largest-magnitude integers, would help track down references to
some kinds of uninitialized variables at run time.
Note that use of the options -O -Wuninitialized can catch many such bugs at compile time.
g77 has no facility for exchanging unformatted files with systems using different number formats—even differing only in endianness (byte order)—or written by other compilers. Some compilers provide facilities at least for doing byte-swapping during unformatted I/O.
It is unrealistic to expect to cope with exchanging unformatted files with arbitrary other compiler runtimes, but the g77 runtime should at least be able to read files written by g77 on systems with different number formats, particularly if they differ only in byte order.
In case you do need to write a program to translate to or from
g77 (libf2c) unformatted files, they are written as
follows:
The record length is of C type
long; this means that it is 8 bytes on 64-bit systems such as
Alpha GNU/Linux and 4 bytes on other systems, such as x86 GNU/Linux.
Consequently such files cannot be exchanged between 64-bit and 32-bit
systems, even with the same basic number format.
REC=records) written and recl is the
record length in bytes specified in the OPEN statement
(RECL=recl). Data appear in the records as determined by
the relevant WRITE statement. Dummy records with arbitrary
contents appear in the file in place of records which haven't been
written.
Thus for exchanging a sequential or direct access unformatted file
between big- and little-endian 32-bit systems using IEEE 754 floating
point it would be sufficient to reverse the bytes in consecutive words
in the file if, and only if, only REAL*4, COMPLEX,
INTEGER*4 and/or LOGICAL*4 data have been written to it by
g77.
If necessary, it is possible to do byte-oriented i/o with g77's
FGETC and FPUTC intrinsics. Byte-swapping can be done in
Fortran by equivalencing larger sized variables to an INTEGER*1
array or a set of scalars.
If you need to exchange binary data between arbitrary system and compiler variations, we recommend using a portable binary format with Fortran bindings, such as NCSA's HDF (http://hdf.ncsa.uiuc.edu/) or PACT's PDB3 (http://www.llnl.gov/def_sci/pact/pact_homepage.html). (Unlike, say, CDF or XDR, HDF-like systems write in the native number formats and only incur overhead when they are read on a system with a different format.) A future g77 runtime library should use such techniques.
Values output using list-directed I/O (PRINT *, R, D) should be written with a field width, precision, and so on appropriate for the type (precision) of each value.
(Currently, no distinction is made between single-precision
and double-precision values
by libf2c.)
It is likely this item will require the libg77 project
to be undertaken.
In the meantime, use of formatted I/O is recommended.
While it might be of little consolation,
g77 does support FORMAT(F<WIDTH>.4), for example,
as long as WIDTH is defined as a named constant
(via PARAMETER).
That at least allows some compile-time specification
of the precision of a data type,
perhaps controlled by preprocessing directives.
The default I/O units,
specified by READ fmt,
READ (UNIT=*),
WRITE (UNIT=*), and
PRINT fmt,
should not be units 5 (input) and 6 (output),
but, rather, unit numbers not normally available
for use in statements such as OPEN and CLOSE.
Changing this would allow a program to connect units 5 and 6
to files via OPEN,
but still use READ (UNIT=*) and PRINT
to do I/O to the “console”.
This change probably requires the libg77 project.
g77 should output debugging information for statements labels, for use by debuggers that know how to support them. Same with weirder things like construct names. It is not yet known if any debug formats or debuggers support these.
These problems are perhaps regrettable, but we don't know any practical way around them for now.
The current external-interface design, which includes naming of external procedures, COMMON blocks, and the library interface, has various usability problems, including things like adding underscores where not really necessary (and preventing easier inter-language operability) and yet not providing complete namespace freedom for user C code linked with Fortran apps (due to the naming of functions in the library, among other things).
Project GNU should at least get all this “right” for systems it fully controls, such as the Hurd, and provide defaults and options for compatibility with existing systems and interoperability with popular existing compilers.
g77 doesn't allow a common block and an external procedure or
BLOCK DATA to have the same name.
Some systems allow this, but g77 does not,
to be compatible with f2c.
g77 could special-case the way it handles
BLOCK DATA, since it is not compatible with f2c in this
particular area (necessarily, since g77 offers an
important feature here), but
it is likely that such special-casing would be very annoying to people
with programs that use EXTERNAL FOO, with no other mention of
FOO in the same program unit, to refer to external procedures, since
the result would be that g77 would treat these references as requests to
force-load BLOCK DATA program units.
In that case, if g77 modified
names of BLOCK DATA so they could have the same names as
COMMON, users
would find that their programs wouldn't link because the FOO procedure
didn't have its name translated the same way.
(Strictly speaking,
g77 could emit a null-but-externally-satisfying definition of
FOO with its name transformed as if it had been a
BLOCK DATA, but that probably invites more trouble than it's
worth.)
g77 disallows IMPLICIT CHARACTER*(*).
This is not standard-conforming.
This section lists changes that people frequently request, but which we do not make because we think GNU Fortran is better without them.
In the opinion of many experienced Fortran users, -fno-backslash should be the default, not -fbackslash, as currently set by g77.
First of all, you can always specify -fno-backslash to turn off this processing.
Despite not being within the spirit (though apparently within the letter) of the ANSI FORTRAN 77 standard, g77 defaults to -fbackslash because that is what most UNIX f77 commands default to, and apparently lots of code depends on this feature.
This is a particularly troubling issue. The use of a C construct in the midst of Fortran code is bad enough, worse when it makes existing Fortran programs stop working (as happens when programs written for non-UNIX systems are ported to UNIX systems with compilers that provide the -fbackslash feature as the default—sometimes with no option to turn it off).
The author of GNU Fortran wished, for reasons of linguistic purity, to make -fno-backslash the default for GNU Fortran and thus require users of UNIX f77 and f2c to specify -fbackslash to get the UNIX behavior.
However, the realization that g77 is intended as a replacement for UNIX f77, caused the author to choose to make g77 as compatible with f77 as feasible, which meant making -fbackslash the default.
The primary focus on compatibility is at the source-code level, and the question became “What will users expect a replacement for f77 to do, by default?” Although at least one UNIX f77 does not provide -fbackslash as a default, it appears that the majority of them do, which suggests that the majority of code that is compiled by UNIX f77 compilers expects -fbackslash to be the default.
It is probably the case that more code exists that would not work with -fbackslash in force than code that requires it be in force.
However, most of that code is not being compiled with f77, and when it is, new build procedures (shell scripts, makefiles, and so on) must be set up anyway so that they work under UNIX. That makes a much more natural and safe opportunity for non-UNIX users to adapt their build procedures for g77's default of -fbackslash than would exist for the majority of UNIX f77 users who would have to modify existing, working build procedures to explicitly specify -fbackslash if that was not the default.
One suggestion has been to configure the default for -fbackslash (and perhaps other options as well) based on the configuration of g77.
This is technically quite straightforward, but will be avoided even in cases where not configuring defaults to be dependent on a particular configuration greatly inconveniences some users of legacy code.
Many users appreciate the GNU compilers because they provide an environment that is uniform across machines. These users would be inconvenienced if the compiler treated things like the format of the source code differently on certain machines.
Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU Fortran compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. (This is consistent with the design goals for gcc. To change them for g77, you must first change them for gcc. Do not ask the maintainers of g77 to do this for you, or to disassociate g77 from the widely understood, if not widely agreed-upon, goals for GNU compilers in general.)
This is why GNU Fortran does and will treat backslashes in the same fashion on all types of machines (by default). See Direction of Language Development, for more information on this overall philosophy guiding the development of the GNU Fortran language.
Of course, users strongly concerned about portability should indicate explicitly in their build procedures which options are expected by their source code, or write source code that has as few such expectations as possible.
For example, avoid writing code that depends on backslash (\) being interpreted either way in particular, such as by starting a program unit with:
CHARACTER BACKSL
PARAMETER (BACKSL = '\\')
Then, use concatenation of BACKSL anyplace a backslash is desired. In this way, users can write programs which have the same meaning in many Fortran dialects.
(However, this technique does not work for Hollerith constants—which is just as well, since the only generally portable uses for Hollerith constants are in places where character constants can and should be used instead, for readability.)
g77 does not allow DATA VAR/1/ to appear in the source code before COMMON VAR, DIMENSION VAR(10), INTEGER VAR, and so on. In general, g77 requires initialization of a variable or array to be specified after all other specifications of attributes (type, size, placement, and so on) of that variable or array are specified (though confirmation of data type is permitted).
It is possible g77 will someday allow all of this, even though it is not allowed by the FORTRAN 77 standard.
Then again, maybe it is better to have
g77 always require placement of DATA
so that it can possibly immediately write constants
to the output file, thus saving time and space.
That is, DATA A/1000000*1/ should perhaps always
be immediately writable to canonical assembler, unless it's already known
to be in a COMMON area following as-yet-uninitialized stuff,
and to do this it cannot be followed by COMMON A.
g77 treats procedure references to possible intrinsic names as always enabling their intrinsic nature, regardless of whether the form of the reference is valid for that intrinsic.
For example, CALL SQRT is interpreted by g77 as
an invalid reference to the SQRT intrinsic function,
because the reference is a subroutine invocation.
First, g77 recognizes the statement CALL SQRT as a reference to a procedure named SQRT, not to a variable with that name (as it would for a statement such as V = SQRT).
Next, g77 establishes that, in the program unit being compiled,
SQRT is an intrinsic—not a subroutine that
happens to have the same name as an intrinsic (as would be
the case if, for example, EXTERNAL SQRT was present).
Finally, g77 recognizes that the form of the
reference is invalid for that particular intrinsic.
That is, it recognizes that it is invalid for an intrinsic
function, such as SQRT, to be invoked as
a subroutine.
At that point, g77 issues a diagnostic.
Some users claim that it is “obvious” that CALL SQRT references an external subroutine of their own, not an intrinsic function.
However, g77 knows about intrinsic subroutines, not just functions, and is able to support both having the same names, for example.
As a result of this, g77 rejects calls to intrinsics that are not subroutines, and function invocations of intrinsics that are not functions, just as it (and most compilers) rejects invocations of intrinsics with the wrong number (or types) of arguments.
So, use the EXTERNAL SQRT statement in a program unit that calls a user-written subroutine named SQRT.
g77 does not use context to determine the types of
constants or named constants (PARAMETER), except
for (non-standard) typeless constants such as '123'O.
For example, consider the following statement:
PRINT *, 9.435784839284958 * 2D0
g77 will interpret the (truncated) constant
9.435784839284958 as a REAL(KIND=1), not REAL(KIND=2),
constant, because the suffix D0 is not specified.
As a result, the output of the above statement when compiled by g77 will appear to have “less precision” than when compiled by other compilers.
In these and other cases, some compilers detect the fact that a single-precision constant is used in a double-precision context and therefore interpret the single-precision constant as if it was explicitly specified as a double-precision constant. (This has the effect of appending decimal, not binary, zeros to the fractional part of the number—producing different computational results.)
The reason this misfeature is dangerous is that a slight, apparently innocuous change to the source code can change the computational results. Consider:
REAL ALMOST, CLOSE
DOUBLE PRECISION FIVE
PARAMETER (ALMOST = 5.000000000001)
FIVE = 5
CLOSE = 5.000000000001
PRINT *, 5.000000000001 - FIVE
PRINT *, ALMOST - FIVE
PRINT *, CLOSE - FIVE
END
Running the above program should result in the same value being printed three times. With g77 as the compiler, it does.
However, compiled by many other compilers, running the above program would print two or three distinct values, because in two or three of the statements, the constant 5.000000000001, which on most systems is exactly equal to 5. when interpreted as a single-precision constant, is instead interpreted as a double-precision constant, preserving the represented precision. However, this “clever” promotion of type does not extend to variables or, in some compilers, to named constants.
Since programmers often are encouraged to replace manifest
constants or permanently-assigned variables with named
constants (PARAMETER in Fortran), and might need
to replace some constants with variables having the same
values for pertinent portions of code,
it is important that compilers treat code so modified in the
same way so that the results of such programs are the same.
g77 helps in this regard by treating constants just
the same as variables in terms of determining their types
in a context-independent way.
Still, there is a lot of existing Fortran code that has been written to depend on the way other compilers freely interpret constants' types based on context, so anything g77 can do to help flag cases of this in such code could be very helpful.
Use of .EQ. and .NE. on LOGICAL operands
is not supported, except via -fugly-logint, which is not
recommended except for legacy code (where the behavior expected
by the code is assumed).
Legacy code should be changed, as resources permit, to use .EQV.
and .NEQV. instead, as these are permitted by the various
Fortran standards.
New code should never be written expecting .EQ. or .NE.
to work if either of its operands is LOGICAL.
The problem with supporting this “feature” is that there is unlikely to be consensus on how it works, as illustrated by the following sample program:
LOGICAL L,M,N
DATA L,M,N /3*.FALSE./
IF (L.AND.M.EQ.N) PRINT *,'L.AND.M.EQ.N'
END
The issue raised by the above sample program is: what is the
precedence of .EQ. (and .NE.) when applied to
LOGICAL operands?
Some programmers will argue that it is the same as the precedence
for .EQ. when applied to numeric (such as INTEGER)
operands.
By this interpretation, the subexpression M.EQ.N must be
evaluated first in the above program, resulting in a program that,
when run, does not execute the PRINT statement.
Other programmers will argue that the precedence is the same as
the precedence for .EQV., which is restricted by the standards
to LOGICAL operands.
By this interpretation, the subexpression L.AND.M must be
evaluated first, resulting in a program that does execute
the PRINT statement.
Assigning arbitrary semantic interpretations to syntactic expressions that might legitimately have more than one “obvious” interpretation is generally unwise.
The creators of the various Fortran standards have done a good job
in this case, requiring a distinct set of operators (which have their
own distinct precedence) to compare LOGICAL operands.
This requirement results in expression syntax with more certain
precedence (without requiring substantial context), making it easier
for programmers to read existing code.
g77 will avoid muddying up elements of the Fortran language
that were well-designed in the first place.
(Ask C programmers about the precedence of expressions such as (a) & (b) and (a) - (b)—they cannot even tell you, without knowing more context, whether the & and - operators are infix (binary) or unary!)
Most dangerous of all is the fact that, even assuming consensus on its meaning, an expression like L.AND.M.EQ.N, if it is the result of a typographical error, doesn't look like it has such a typo. Even experienced Fortran programmers would not likely notice that L.AND.M.EQV.N was, in fact, intended.
So, this is a prime example of a circumstance in which a quality compiler diagnoses the code, instead of leaving it up to someone debugging it to know to turn on special compiler options that might diagnose it.
g77 does not necessarily produce code that, when run, performs side effects (such as those performed by function invocations) in the same order as in some other compiler—or even in the same order as another version, port, or invocation (using different command-line options) of g77.
It is never safe to depend on the order of evaluation of side effects. For example, an expression like this may very well behave differently from one compiler to another:
J = IFUNC() - IFUNC()
There is no guarantee that IFUNC will be evaluated in any particular order. Either invocation might happen first. If IFUNC returns 5 the first time it is invoked, and returns 12 the second time, J might end up with the value 7, or it might end up with -7.
Generally, in Fortran, procedures with side-effects intended to be visible to the caller are best designed as subroutines, not functions. Examples of such side-effects include:
An example of a side-effect that is not intended to be visible to the caller is a function that maintains a cache of recently calculated results, intended solely to speed repeated invocations of the function with identical arguments. Such a function can be safely used in expressions, because if the compiler optimizes away one or more calls to the function, operation of the program is unaffected (aside from being speeded up).
The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings might indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU Fortran. Many warnings are issued only if you ask for them, with one of the -W options (for instance, -Wall requests a variety of useful warnings).
Note: Currently, the text of the line and a pointer to the column is printed in most g77 diagnostics.
See Options to Request or Suppress Warnings, for more detail on these and related command-line options.
Please consider offering useful answers to these questions!
LOC() and other intrinsics are probably somewhat misclassified.
Is the a need for more precise classification of intrinsics, and if so,
what are the appropriate groupings?
Is there a need to individually
enable/disable/delete/hide intrinsics from the command line?
Your bug reports play an essential role in making GNU Fortran reliable.
When you encounter a problem, the first thing to do is to see if it is already known. See Trouble. If it isn't known, then you should report the problem.
See Known Causes of Trouble with GNU Fortran, for information on problems we already know about.
See How To Get Help with GNU Fortran, for information on where to ask for help.
If you are not sure whether you have found a bug, here are some guidelines:
However, you must double-check to make sure, because you might have run into an incompatibility between GNU Fortran and traditional Fortran. These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features.
Or you might have a program whose behavior is undefined, which happened by chance to give the desired results with another Fortran compiler. It is best to check the relevant Fortran standard thoroughly if it is possible that the program indeed does something undefined.
After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.
It might help if, in your submission, you identified the specific language in the relevant Fortran standard that specifies the desired behavior, if it isn't likely to be obvious and agreed-upon by all Fortran users.
Many, perhaps most, bug reports against g77 turn out to be bugs in the user's code. While we find such bug reports educational, they sometimes take a considerable amount of time to track down or at least respond to—time we could be spending making g77, not some user's code, better.
Some steps you can take to verify that the bug is not certainly in the code you're compiling with g77:
If you investigate the warnings and find evidence of possible bugs in your code, fix them first and retry g77.
If your code works with any of these combinations, that is not proof that the bug isn't in g77—a g77 bug exposed by your code might simply be avoided, or have a different, more subtle effect, when different options are used—but it can be a strong indicator that your code is making unwarranted assumptions about the Fortran dialect and/or underlying machine it is being compiled and run on.
See Overly Convenient Command-Line Options, for information on the -fno-automatic and -finit-local-zero options and how to convert their use into selective changes in your own code.
Here are some sample Makefile rules using ftnchek “project” files to do cross-file checking and sfmakedepend (from ftp://ahab.rutgers.edu/pub/perl/sfmakedepend) to maintain dependencies automatically. These assume the use of GNU make.
# Dummy suffix for ftnchek targets:
.SUFFIXES: .chek
.PHONY: chekall
# How to compile .f files (for implicit rule):
FC = g77
# Assume `include' directory:
FFLAGS = -Iinclude -g -O -Wall
# Flags for ftnchek:
CHEK1 = -array=0 -include=includes -noarray
CHEK2 = -nonovice -usage=1 -notruncation
CHEKFLAGS = $(CHEK1) $(CHEK2)
# Run ftnchek with all the .prj files except the one corresponding
# to the target's root:
%.chek : %.f ; \
ftnchek $(filter-out $*.prj,$(PRJS)) $(CHEKFLAGS) \
-noextern -library $<
# Derive a project file from a source file:
%.prj : %.f ; \
ftnchek $(CHEKFLAGS) -noextern -project -library $<
# The list of objects is assumed to be in variable OBJS.
# Sources corresponding to the objects:
SRCS = $(OBJS:%.o=%.f)
# ftnchek project files:
PRJS = $(OBJS:%.o=%.prj)
# Build the program
prog: $(OBJS) ; \
$(FC) -o $ $(OBJS)
chekall: $(PRJS) ; \
ftnchek $(CHEKFLAGS) $(PRJS)
prjs: $(PRJS)
# For Emacs M-x find-tag:
TAGS: $(SRCS) ; \
etags $(SRCS)
# Rebuild dependencies:
depend: ; \
sfmakedepend -I $(PLTLIBDIR) -I includes -a prj $(SRCS1)
However, even if your code works on many compilers except g77, that does not mean the bug is in g77. It might mean the bug is in your code, and that g77 simply exposes it more readily than other compilers.
Bugs should be reported to our bug database. Please refer to http://gcc.gnu.org/bugs.html for up-to-date instructions how to submit bug reports. Copies of this file in HTML (bugs.html) and plain text (BUGS) are also part of GCC releases.
If you need help installing, using or changing GNU Fortran, there are two ways to find it:
To add a new command-line option to g77, first decide what kind of option you wish to add. Search the g77 and gcc documentation for one or more options that is most closely like the one you want to add (in terms of what kind of effect it has, and so on) to help clarify its nature.
Fortran options are listed in the file
gcc/gcc/f/lang-options.h,
which is used during the build of gcc to
build a list of all options that are accepted by
at least one language's compiler.
This list goes into the documented_lang_options array
in gcc/toplev.c, which uses this array to
determine whether a particular option should be
offered to the linked-in front end for processing
by calling lang_option_decode, which, for
g77, is in gcc/gcc/f/com.c and just
calls ffe_decode_option.
If the linked-in front end “rejects” a particular option passed to it, toplev.c just ignores the option, because some language's compiler is willing to accept it.
This allows commands like gcc -fno-asm foo.c bar.f
to work, even though Fortran compilation does
not currently support the -fno-asm option;
even though the f771 version of lang_decode_option
rejects -fno-asm, toplev.c doesn't
produce a diagnostic because some other language (C)
does accept it.
This also means that commands like g77 -fno-asm foo.f yield no diagnostics, despite the fact that no phase of the command was able to recognize and process -fno-asm—perhaps a warning about this would be helpful if it were possible.
Code that processes Fortran options is found in
gcc/gcc/f/top.c, function ffe_decode_option.
This code needs to check positive and negative forms
of each option.
The defaults for Fortran options are set in their global definitions, also found in gcc/gcc/f/top.c. Many of these defaults are actually macros defined in gcc/gcc/f/target.h, since they might be machine-specific. However, since, in practice, GNU compilers should behave the same way on all configurations (especially when it comes to language constructs), the practice of setting defaults in target.h is likely to be deprecated and, ultimately, stopped in future versions of g77.
Accessor macros for Fortran options, used by code in the g77 FFE, are defined in gcc/gcc/f/top.h.
Compiler options are listed in gcc/toplev.c
in the array f_options.
An option not listed in lang_options is
looked up in f_options and handled from there.
The defaults for compiler options are set in the global definitions for the corresponding variables, some of which are in gcc/toplev.c.
You can set different defaults for Fortran-oriented or Fortran-reticent compiler options by changing the source code of g77 and rebuilding. How to do this depends on the version of g77:
G77 0.5.24 (EGCS 1.1)G77 0.5.25 (EGCS 1.2 - which became GCC 2.95)lang_init_options routine in gcc/gcc/f/com.c.
(Note that these versions of g77
perform internal consistency checking automatically
when the -fversion option is specified.)
G77 0.5.23G77 0.5.24 (EGCS 1.0)f771 handles the -fset-g77-defaults
option, which is always provided as the first option when
called by g77 or gcc.
This code is in ffe_decode_options in gcc/gcc/f/top.c.
Have it change just the variables that you want to default
to a different setting for Fortran compiles compared to
compiles of other languages.
The -fset-g77-defaults option is passed to f771
automatically because of the specification information
kept in gcc/gcc/f/lang-specs.h.
This file tells the gcc command how to recognize,
in this case, Fortran source files (those to be preprocessed,
and those that are not), and further, how to invoke the
appropriate programs (including f771) to process
those source files.
It is in gcc/gcc/f/lang-specs.h that -fset-g77-defaults, -fversion, and other options are passed, as appropriate, even when the user has not explicitly specified them. Other “internal” options such as -quiet also are passed via this mechanism.
If you want to contribute to g77 by doing research, design, specification, documentation, coding, or testing, the following information should give you some ideas.
Don't bother doing any performance analysis until most of the following items are taken care of, because there's no question they represent serious space/time problems, although some of them show up only given certain kinds of (popular) input.
malloc package and its uses to specify more info about
memory pools and, where feasible, use obstacks to implement them.
COMMON areas, EQUIVALENCE areas) so zeros need not be output.
This would reduce memory usage for large initialized aggregate
areas, even ones with only one initialized element.
As of version 0.5.18, a portion of this item has already been accomplished.
ffewhere type, which points to the error as much as is
desired by the configuration, will do, and don't pass ffelexToken types
where a simple ffewhere type will do.
Then, allow new default
configuration of ffewhere such that the source line text is not
preserved, and leave it to things like Emacs' next-error function
to point to them (now that next-error supports column,
or, perhaps, character-offset, numbers).
The change in calling sequences should improve performance somewhat,
as should not having to save source lines.
(Whether this whole
item will improve performance is questionable, but it should
improve maintainability.)
Much of this work should be put off until after g77 has all the features necessary for its widespread acceptance as a useful F77 compiler. However, perhaps this work can be done in parallel during the feature-adding work.
libg2c and #include'ing the resulting
file in f2c+gcc—that is, inline all run-time-library functions
that are at all worth inlining.
(Some of this has already been done, such as for integral exponentiation.)
VAR_DECL,
make CHAR_VAR, not a
temporary, be the receiver for CHAR_FUNC.
(This is now done for COMPLEX variables.)
libgcc so no special linking is required to
link Fortran programs using standard language features.
This library
would speed up lots of things, from I/O (using precompiled formats,
doing just one, or, at most, very few, calls for arrays or array sections,
and so on) to general computing (array/section implementations of
various intrinsics, implementation of commonly performed loops that
aren't likely to be optimally compiled otherwise, etc.).
Among the important things the library would do are:
libg2c would be moved at least to the
g77 compile phase, if not to finer grains (such as choosing how
list-directed I/O formatting is done by default at OPEN time, for
preconnected units via options or even statements in the main program
unit, maybe even on a per-I/O basis with appropriate pragma-like
devices).
COMPLEX functions return their values in the way
gcc would if they were declared __complex__ float,
rather than using
the mechanism currently used by CHARACTER functions (whereby the
functions are compiled as returning void and their first arg is
a pointer to where to store the result).
(Don't append underscores to
external names for COMPLEX functions in some cases once g77 uses
gcc rather than f2c calling conventions.)
doiter references where possible.
For example, CALL FOO(I) cannot modify I if within
a DO loop that uses I as the
iteration variable, and the back end might find that info useful
in determining whether it needs to read I back into a register after
the call.
(It normally has to do that, unless it knows FOO never
modifies its passed-by-reference argument, which is rarely the case
for Fortran-77 code.)
Making g77 easier to configure, port, build, and install, either as a single-system compiler or as a cross-compiler, would be very useful.
libg2c) should improve portability as well as
produce more optimal code.
Further, g77 and the new library should
conspire to simplify naming of externals, such as by removing unnecessarily
added underscores, and to reduce/eliminate the possibility of naming
conflicts, while making debugger more straightforward.
Also, it should
make multi-language applications more feasible, such as by providing
Fortran intrinsics that get Fortran unit numbers given C FILE *
descriptors.
main().
This would do many useful things such as provide more flexibility in terms of setting up exception handling, not requiring programmers to start their debugging sessions with breakpoint MAIN__ followed by run, and so on.
These extensions are not the sort of things users ask for “by name”, but they might improve the usability of g77, and Fortran in general, in the long run. Some of these items really pertain to improving g77 internals so that some popular extensions can be more easily supported.
NUMERIC type to designate typeless numeric constants,
named and unnamed.
The idea is to provide a forward-looking, effective
replacement for things like the old-style PARAMETER statement
when people
really need typelessness in a maintainable, portable, clearly documented
way.
Maybe TYPELESS would include CHARACTER, POINTER,
and whatever else might come along.
(This is not really a call for polymorphism per se, just
an ability to express limited, syntactic polymorphism.)
libg2c issue.)
UNIT= in the first example is invalid.
Make sure this is what users of this feature would expect.
STRUCTURE, UNION, MAP, and RECORD
fully.
Currently there is no support at all
for %FILL in STRUCTURE and related syntax,
whereas the rest of the
stuff has at least some parsing support.
This requires either major
changes to libg2c or its replacement.
INTERFACE and END INTERFACE, and their contained
procedure interface bodies (blocks?).
ENTRY doesn't support F90 RESULT() yet,
since that was added after S8.112.
OPENed,is positioned at the beginning, the end, or wherever—it
might be nice to offer an option of opening to “undefined” status, requiring
an explicit absolute-positioning operation to be performed before any
other (besides CLOSE) to assist in making applications port to systems
(some IBM?) that OPEN to the end of a file or some such thing.
This items pertain to generalizing g77's view of the machine model to more fully accept whatever the GBE provides it via its configuration.
REAL_VALUE_TYPE to represent floating-point constants
exclusively so the target float format need not be required.
This
means changing the way g77 handles initialization of aggregate areas
having more than one type, such as REAL and INTEGER,
because currently
it initializes them as if they were arrays of char and uses the
bit patterns of the constants of the various types in them to determine
what to stuff in elements of the arrays.
Better info on how g77 works and how to port it is needed.
See Front End, which contains some information on g77 internals.
Some more items that would make g77 more reliable and easier to maintain:
PARAMETER—if
it seems
important to preserve the left-to-right-in-source order of production
of diagnostics.)
ffewheres in ffeglobal_terminate_1.
outpooldisp mechanism with malloc_pool_use.
opANY in more places in com.c, std.c,
and ste.c, and get rid of the opCONVERT(opANY) kludge
(after determining if there is indeed no real need for it).
const and other such stuff.
ffebld_new calls (those outside of ffeexpr.c or
inside but invoked via paths not involving ffeexpr_lhs or
ffeexpr_rhs) might be creating things
in improper pools, leading to such things staying around too long or
(doubtful, but possible and dangerous) not long enough.
ffebld_list_new (or whatever) calls might not be matched by
ffebld_list_bottom (or whatever) calls, which might someday matter.
(It definitely is not a problem just yet.)
EQUIVALENCE something
due to alignment/mismatch or other problems—they end up without
ffestorag objects, so maybe the backend (and other parts of the front
end) can notice that and handle like an opANY (do what it wants, just
don't complain or crash).
Most of this seems to have been addressed
by now, but a code review wouldn't hurt.
These are things users might not ask about, or that need to be looked into, before worrying about. Also here are items that involve reducing unnecessary diagnostic clutter.
FUNCTION and ENTRY point types disagree (CHARACTER
lengths, type classes, and so on),
ANY-ize the offending ENTRY point and any new dummies
it specifies.
INTEGER X(20)
CONTINUE
DATA (X(I), J= 1, 20) /20*5/
END
(The CONTINUE statement ensures the DATA statement
is processed in the context of executable, not specification,
statements.)
This chapter describes some aspects of the design and implementation
of the g77 front end.
To find about things that are “To Be Determined” or “To Be Done”, search for the string TBD. If you want to help by working on one or more of these items, email gcc@gcc.gnu.org. If you're planning to do more than just research issues and offer comments, see http://gcc.gnu.org/contribute.html for steps you might need to take first.
The current directory layout includes the following:
libg2c configuration and g2c.h file generation
libg2c
libg2c
libc for libg2c
Components of note in g77 are described below.
f/ as a whole contains the source for g77,
while libf2c/ contains a portion of the separate program
f2c.
Note that the libf2c code is not part of the program g77,
just distributed with it.
f/ contains text files that document the Fortran compiler, source
files for the GNU Fortran Front End (FFE), and some other stuff.
The g77 compiler code is placed in f/ because it,
along with its contents,
is designed to be a subdirectory of a gcc source directory,
gcc/,
which is structured so that language-specific front ends can be “dropped
in” as subdirectories.
The C++ front end (g++), is an example of this—it resides in
the cp/ subdirectory.
Note that the C front end (also referred to as gcc)
is an exception to this, as its source files reside
in the gcc/ directory itself.
libf2c/ contains the run-time libraries for the f2c program,
also used by g77.
These libraries normally referred to collectively as libf2c.
When built as part of g77,
libf2c is installed under the name libg2c to avoid
conflict with any existing version of libf2c,
and thus is often referred to as libg2c when the
g77 version is specifically being referred to.
The netlib version of libf2c/
contains two distinct libraries,
libF77 and libI77,
each in their own subdirectories.
In g77, this distinction is not made,
beyond maintaining the subdirectory structure in the source-code tree.
libf2c/ is not part of the program g77,
just distributed with it.
It contains files not present
in the official (netlib) version of libf2c,
and also contains some minor changes made from libf2c,
to fix some bugs,
and to facilitate automatic configuration, building, and installation of
libf2c (as libg2c) for use by g77 users.
See libf2c/README for more information,
including licensing conditions
governing distribution of programs containing code from libg2c.
libg2c, g77's version of libf2c,
adds Dave Love's implementation of libU77,
in the libf2c/libU77/ directory.
This library is distributed under the
GNU Library General Public License (LGPL)—see the
file libf2c/libU77/COPYING.LIB
for more information,
as this license
governs distribution conditions for programs containing code
from this portion of the library.
Files of note in f/ and libf2c/ are described below:
g77 documentation:
info -f f/g77.info -n "Actual Bugs"
g77 internals.
libg2c internals.
g77 documentation, plus internal
changes of import.
Or use:
info -f f/g77.info -n News
g77 documentation, in Info format,
produced by building g77.
All users of g77 (not just installers) should read this,
using the more command if neither the info command,
nor GNU Emacs (with its Info mode), are available, or if users
aren't yet accustomed to using these tools.
All of these files are readable as “plain text” files,
though they're easier to navigate using Info readers
such as info and GNU Emacs Info mode.
If you want to explore the FFE code, which lives entirely in f/,
here are a few clues.
The file g77spec.c contains the g77-specific source code
for the g77 command only—this just forms a variant of the
gcc command, so,
just as the gcc command itself does not contain the C front end,
the g77 command does not contain the Fortran front end (FFE).
The FFE code ends up in an executable named f771,
which does the actual compiling,
so it contains the FFE plus the gcc back end (GBE),
the latter to do most of the optimization, and the code generation.
The file parse.c is the source file for yyparse(),
which is invoked by the GBE to start the compilation process,
for f771.
The file top.c contains the top-level FFE function ffe_file
and it (along with top.h) define all ffe_[a-z].*, ffe[A-Z].*,
and FFE_[A-Za-z].* symbols.
The file fini.c is a main() program that is used when building
the FFE to generate C header and source files for recognizing keywords.
The files malloc.c and malloc.h comprise a memory manager
that defines all malloc_[a-z].*, malloc[A-Z].*, and
MALLOC_[A-Za-z].* symbols.
All other modules named xyz are comprised of all files named xyz*.ext and define all ffexyz_[a-z].*, ffexyz[A-Z].*, and FFEXYZ_[A-Za-z].* symbols. If you understand all this, congratulations—it's easier for me to remember how it works than to type in these regular expressions. But it does make it easy to find where a symbol is defined. For example, the symbol ffexyz_set_something would be defined in xyz.h and implemented there (if it's a macro) or in xyz.c.
The “porting” files of note currently are:
ARRAY_SIZE and such.
INTEGER*8 map, for example),
how to convert between them,
and so on.
Over time, versions of g77 rely less on this file
and more on run-time configuration based on GBE info
in com.c.
If you want to debug the f771 executable,
for example if it crashes,
note that the global variables lineno and input_filename
are usually set to reflect the current line being read by the lexer
during the first-pass analysis of a program unit and to reflect
the current line being processed during the second-pass compilation
of a program unit.
If an invocation of the function ffestd_exec_end is on the stack,
the compiler is in the second pass, otherwise it is in the first.
(This information might help you reduce a test case and/or work around
a bug in g77 until a fix is available.)
The order of phases translating source code to the form accepted by the GBE is:
To get a rough idea of how a particularly twisted Fortran statement gets treated by the passes, consider:
FORMAT(I2 4H)=(J/
& I3)
The job of lex.c is to know enough about Fortran syntax rules to break the statement up into distinct lexemes without requiring any feedback from subsequent phases:
`FORMAT'
`('
`I24H'
`)'
`='
`('
`J'
`/'
`I3'
`)'
The job of sta.c is to figure out the kind of statement, or, at least, statement form, that sequence of lexemes represent.
The sooner it can do this (in terms of using the smallest number of lexemes, starting with the first for each statement), the better, because that leaves diagnostics for problems beyond the recognition of the statement form to subsequent phases, which can usually better describe the nature of the problem.
In this case, the = at “level zero”
(not nested within parentheses)
tells sta.c that this is an assignment-form,
not FORMAT, statement.
An assignment-form statement might be a statement-function definition or an executable assignment statement.
To make that determination, sta.c looks at the first two lexemes.
Since the second lexeme is (, the first must represent an array for this to be an assignment statement, else it's a statement function.
Either way, sta.c hands off the statement to stq.c (via sti.c, which expands INCLUDE files). stq.c figures out what a statement that is, on its own, ambiguous, must actually be based on the context established by previous statements.
So, stq.c watches the statement stream for executable statements, END statements, and so on, so it knows whether A(B)=C is (intended as) a statement-function definition or an assignment statement.
After establishing the context-aware statement info, stq.c passes the original sample statement on to stb.c (either its statement-function parser or its assignment-statement parser).
stb.c forms a statement-specific record containing the pertinent information. That information includes a source expression and, for an assignment statement, a destination expression. Expressions are parsed by expr.c.
This record is passed to stc.c, which copes with the implications of the statement within the context established by previous statements.
For example, if it's the first statement in the file
or after an END statement,
stc.c recognizes that, first of all,
a main program unit is now being lexed
(and tells that to std.c
before telling it about the current statement).
stc.c attaches whatever information it can, usually derived from the context established by the preceding statements, and passes the information to std.c.
std.c saves this information away, since the GBE cannot cope with information that might be incomplete at this stage.
For example, I3 might later be determined
to be an argument to an alternate ENTRY point.
When std.c is told about the end of an external (top-level) program unit, it passes all the information it has saved away on statements in that program unit to ste.c.
ste.c “expands” each statement, in sequence, by constructing the appropriate GBE information and calling the appropriate GBE routines.
Details on the transformational phases follow. Keep in mind that Fortran numbering is used, so the first character on a line is column 1, decimal numbering is used, and so on.
The g77stripcard program handles removing content beyond
column 72 (adjustable via a command-line option),
optionally warning about that content being something other
than trailing whitespace or Fortran commentary.
This program is needed because lex.c doesn't pay attention
to maximum line lengths at all, to make it easier to maintain,
as well as faster (for sources that don't depend on the maximum
column length vis-a-vis trailing non-blank non-commentary content).
Just how this program will be run—whether automatically for old source (perhaps as the default for .f files?)—is not yet determined.
In the meantime, it might as well be implemented as a typical UNIX pipe.
It should accept a -fline-length-n option, with the default line length set to 72.
When the text it strips off the end of a line is not blank
(not spaces and tabs),
it should insert an additional comment line
(beginning with !,
so it works for both fixed-form and free-form files)
containing the text,
following the stripped line.
The inserted comment should have a prefix of some kind,
TBD, that distinguishes the comment as representing stripped text.
Users could use that to sed out such lines, if they wished—it
seems silly to provide a command-line option to delete information
when it can be so easily filtered out by another program.
(This inserted comment should be designed to “fit in” well
with whatever the Fortran community is using these days for
preprocessor, translator, and other such products, like OpenMP.
What that's all about, and how g77 can elegantly fit its
special comment conventions into it all, is TBD as well.
We don't want to reinvent the wheel here, but if there turn out
to be too many conflicting conventions, we might have to invent
one that looks nothing like the others, but which offers their
host products a better infrastructure in which to fit and coexist
peacefully.)
g77stripcard probably shouldn't do any tab expansion or other
fancy stuff.
People can use expand or other pre-filtering if they like.
The idea here is to keep each stage quite simple, while providing
excellent performance for “normal” code.
(Code with junk beyond column 73 is not really “normal”, as it comes from a card-punch heritage, and will be increasingly hard for tomorrow's Fortran programmers to read.)
To help make the lexer simple, fast, and easy to maintain,
while also having g77 generally encourage Fortran programmers
to write simple, maintainable, portable code by maximizing the
performance of compiling that kind of code:
Some other distinctions will be handled by subsequent phases, so at least one of them will have to know which form is involved.
For example, I = 2 . 4 is acceptable in fixed form,
and works in free form as well given the implementation g77
presently uses.
But the standard requires a diagnostic for it in free form,
so the parser has to be able to recognize that
the lexemes aren't contiguous
(information the lexer does have to provide)
and that free-form source is being parsed,
so it can provide the diagnostic.
The g77 lexer doesn't try to gather 2 . 4 into a single lexeme.
Otherwise, it'd have to know a whole lot more about how to parse Fortran,
or subsequent phases (mainly parsing) would have two paths through
lots of critical code—one to handle the lexeme 2, .,
and 4 in sequence, another to handle the lexeme 2.4.
That is, once it starts parsing the “statement” part of a line (column 7 for fixed-form, column 1 for free-form), it'll keep going until it finds a newline, rather than ignoring everything past a particular column (72 or 132).
The implication here is that there shouldn't be anything past that last column, other than whitespace or commentary, because users using typical editors (or viewing output as typically printed) won't necessarily know just where the last column is.
Code that has “garbage” beyond the last column
(almost certainly only fixed-form code with a punched-card legacy,
such as code using columns 73-80 for “sequence numbers”)
will have to be run through g77stripcard first.
Also, keeping track of the maximum column position while also watching out for the end of a line and while reading from a file just makes things slower. Since a file must be read, and watching for the end of the line is necessary (unless the typical input file was preprocessed to include the necessary number of trailing spaces), dropping the tracking of the maximum column position is the only way to reduce the complexity of the pertinent code while maintaining high performance.
Code written in other character sets will have to be converted first.
Specifically, a tab is converted to between one and eight spaces as necessary to reach column n, where dividing (n - 1) by eight results in a remainder of zero.
That saves having to pass most source files through expand.
Otherwise, it is rejected (with a diagnostic).
This includes backspaces, form feeds, and the like.
(It might make sense to allow a form feed in column 1 as long as that's the only character on a line. It certainly wouldn't seem to cost much in terms of performance.)
It will be up to subsequent phases to decide to fold case.
Current plans are to permit any casing for Fortran (reserved) keywords while preserving casing for user-defined names. (This might not be made the default for .f files, though.)
Preserving case seems necessary to provide more direct access
to facilities outside of g77, such as to C or Pascal code.
Names of intrinsics will probably be matchable in any case,
(How external SiN; r = sin(x) would be handled is TBD.
I think old g77 might already handle that pretty elegantly,
but whether we can cope with allowing the same fragment to reference
a different procedure, even with the same interface,
via s = SiN(r), needs to be determined.
If it can't, we need to make sure that when code introduces
a user-defined name, any intrinsic matching that name
using a case-insensitive comparison
is “turned off”.)
CHARACTER and Hollerith constants
are not allowed.
This avoids the confusion introduced by some Fortran compiler vendors providing C-like interpretation of backslashes, while others provide straight-through interpretation.
Some kind of lexical construct (TBD) will be provided to allow
flagging of a CHARACTER
(but probably not a Hollerith)
constant that permits backslashes.
It'll necessarily be a prefix, such as:
PRINT *, C'This line has a backspace \b here.'
PRINT *, F'This line has a straight backslash \ here.'
Further, command-line options might be provided to specify that
one prefix or the other is to be assumed as the default
for CHARACTER constants.
However, it seems more helpful for g77 to provide a program
that converts prefix all constants
(or just those containing backslashes)
with the desired designation,
so printouts of code can be read
without knowing the compile-time options used when compiling it.
If such a program is provided
(let's name it g77slash for now),
then a command-line option to g77 should not be provided.
(Though, given that it'll be easy to implement, it might be hard
to resist user requests for it “to compile faster than if we
have to invoke another filter”.)
This program would take a command-line option to specify the default interpretation of slashes, affecting which prefix it uses for constants.
g77slash probably should automatically convert Hollerith
constants that contain slashes
to the appropriate CHARACTER constants.
Then g77 wouldn't have to define a prefix syntax for Hollerith
constants specifying whether they want C-style or straight-through
backslashes.
The above implements nearly exactly what is specified by Character Set, and Lines, except it also provides automatic conversion of tabs and ignoring of newline-related carriage returns, as well as accommodating form-neutral INCLUDE files.
It also implements the “pure visual” model,
by which is meant that a user viewing his code
in a typical text editor
(assuming it's not preprocessed via g77stripcard or similar)
doesn't need any special knowledge
of whether spaces on the screen are really tabs,
whether lines end immediately after the last visible non-space character
or after a number of spaces and tabs that follow it,
or whether the last line in the file is ended by a newline.
Most editors don't make these distinctions, the ANSI FORTRAN 77 standard doesn't require them to, and it permits a standard-conforming compiler to define a method for transforming source code to “standard form” however it wants.
So, GNU Fortran defines it such that users have the best chance of having the code be interpreted the way it looks on the screen of the typical editor.
(Fancy editors should never be required to correctly read code written in classic two-dimensional-plaintext form. By correct reading I mean ability to read it, book-like, without mistaking text ignored by the compiler for program code and vice versa, and without having to count beyond the first several columns. The vague meaning of ASCII TAB, among other things, complicates this somewhat, but as long as “everyone”, including the editor, other tools, and printer, agrees about the every-eighth-column convention, the GNU Fortran “pure visual” model meets these requirements. Any language or user-visible source form requiring special tagging of tabs, the ends of lines after spaces/tabs, and so on, fails to meet this fairly straightforward specification. Fortunately, Fortran itself does not mandate such a failure, though most vendor-supplied defaults for their Fortran compilers do fail to meet this specification for readability.)
Further, this model provides a clean interface
to whatever preprocessors or code-generators are used
to produce input to this phase of g77.
Mainly, they need not worry about long lines.
This section is not about transforming “gotchas” into something else. It is about the weirder aspects of transforming Fortran, however that's defined, into a more modern, canonical form.
Each lexeme carries with it a pointer to where it appears in the source.
To provide the ability for diagnostics to point to column numbers, in addition to line numbers and names, lexemes that represent more than one (significant) character in the source code need, generally, to provide pointers to where each character appears in the source.
This provides the ability to properly identify the precise location of the problem in code like
SUBROUTINE X
END
BLOCK DATA X
END
which, in fixed-form source, would result in single lexemes consisting of the strings SUBROUTINEX and BLOCKDATAX. (The problem is that X is defined twice, so a pointer to the X in the second definition, as well as a follow-up pointer to the corresponding pointer in the first, would be preferable to pointing to the beginnings of the statements.)
This need also arises when parsing (and diagnosing) FORMAT
statements.
Further, it arises when diagnosing
FMT= specifiers that contain constants
(or partial constants, or even propagated constants!)
in I/O statements, as in:
PRINT '(I2, 3HAB)', J
(A pointer to the beginning of the prematurely-terminated Hollerith constant, and/or to the close parenthese, is preferable to a pointer to the open-parenthese or the apostrophe that precedes it.)
Multi-character lexemes, which would seem to naturally include
at least digit strings, alphanumeric strings, CHARACTER
constants, and Hollerith constants, therefore need to provide
location information on each character.
(Maybe Hollerith constants don't, but it's unnecessary to except them.)
The question then arises, what about other multi-character lexemes, such as ** and //, and Fortran 90's (/, /), ::, and so on?
Turns out there's a need to identify the location of the second character of these two-character lexemes. For example, in I(/J) = K, the slash needs to be diagnosed as the problem, not the open parenthese. Similarly, it is preferable to diagnose the second slash in I = J // K rather than the first, given the implicit typing rules, which would result in the compiler disallowing the attempted concatenation of two integers. (Though, since that's more of a semantic issue, it's not that much preferable.)
Even sequences that could be parsed as digit strings could use location info, for example, to diagnose the 9 in the octal constant O'129'. (This probably will be parsed as a character string, to be consistent with the parsing of Z'129A'.)
To avoid the hassle of recording the location of the second character, while also preserving the general rule that each significant character is distinctly pointed to by the lexeme that contains it, it's best to simply not have any fixed-size lexemes larger than one character.
This new design is expected to make checking for two * lexemes in a row much easier than the old design, so this is not much of a sacrifice. It probably makes the lexer much easier to implement than it makes the parser harder.
Certain lexemes need to be padded with virtual spaces when the end of the line (or file) is encountered.
This is necessary in fixed form, to handle lines that don't extend to column 72, assuming that's the line length in effect.
Last I checked, the Fortran 90 standard actually required the compiler to silently accept something like
FORMAT ( 1 2 Htwelve chars )
as a valid FORMAT statement specifying a twelve-character
Hollerith constant.
The implication here is that, since the new lexer is a zero-feedback one,
it won't know that the special case of a FORMAT statement being parsed
requires apparently distinct lexemes 1 and 2 to be treated as
a single lexeme.
(This is a horrible misfeature of the Fortran 90 language. It's one of many such misfeatures that almost make me want to not support them, and forge ahead with designing a new “GNU Fortran” language that has the features, but not the misfeatures, of Fortran 90, and provide utility programs to do the conversion automatically.)
So, the lexer must gather distinct chunks of decimal strings into a single lexeme in contexts where a single decimal lexeme might start a Hollerith constant.
(Which probably means it might as well do that all the time for all multi-character lexemes, even in free-form mode, leaving it to subsequent phases to pull them apart as they see fit.)
Compare the treatment of this to how
CHARACTER * 4 5 HEY
and
CHARACTER * 12 HEY
must be treated—the former must be diagnosed, due to the separation between lexemes, the latter must be accepted as a proper declaration.
Recognizing a Hollerith constant—specifically, that an H or h after a digit string begins such a constant—requires some knowledge of context.
Hollerith constants (such as 2HAB) can appear after:
Hollerith constants don't appear after:
While
REAL FUNCTION FOO ()
must be a FUNCTION statement and
REAL FUNCTION FOO (5)
must be a type-definition statement,
REAL FUNCTION FOO (names)
where names is a comma-separated list of names, can be one or the other.
The only way to disambiguate that statement (short of mandating free-form source or a short maximum length for name for external procedures) is based on the context of the statement.
In particular, the statement is known to be within an
already-started program unit
(but not at the outer level of the CONTAINS block),
it is a type-declaration statement.
Otherwise, the statement is a FUNCTION statement,
in that it begins a function program unit
(external, or, within CONTAINS, nested).
The statement
READ (N)
is equivalent to either
READ (UNIT=(N))
or
READ (FMT=(N))
depending on which would be valid in context.
Specifically, if N is type INTEGER,
READ (FMT=(N)) would not be valid,
because parentheses may not be used around N,
whereas they may around it in READ (UNIT=(N)).
Further, if N is type CHARACTER,
the opposite is true—READ (UNIT=(N)) is not valid,
but READ (FMT=(N)) is.
Strictly speaking, if anything follows
READ (N)
in the statement, whether the first lexeme after the close parenthese is a comma could be used to disambiguate the two cases, without looking at the type of N, because the comma is required for the READ (FMT=(N)) interpretation and disallowed for the READ (UNIT=(N)) interpretation.
However, in practice, many Fortran compilers allow the comma for the READ (UNIT=(N)) interpretation anyway (in that they generally allow a leading comma before an I/O list in an I/O statement), and much code takes advantage of this allowance.
(This is quite a reasonable allowance, since the juxtaposition of a comma-separated list immediately after an I/O control-specification list, which is also comma-separated, without an intervening comma, looks sufficiently “wrong” to programmers that they can't resist the itch to insert the comma. READ (I, J), K, L simply looks cleaner than READ (I, J) K, L.)
So, type-based disambiguation is needed unless strict adherence to the standard is always assumed, and we're not going to assume that.
Continue researching gotchas, designing the transformational process, and implementing it.
Specific issues to resolve:
USE processing take place?
This gets into the whole issue of how g77 should handle the concept
of modules.
I think GNAT already takes on this issue, but don't know more than that.
Jim Giles has written extensively on comp.lang.fortran
about his opinions on module handling, as have others.
Jim's views should be taken into account.
Actually, Richard M. Stallman (RMS) also has written up some guidelines for implementing such things, but I'm not sure where I read them. Perhaps the old gcc2@cygnus.com list.
If someone could dig references to these up and get them to me, that would be much appreciated! Even though modules are not on the short-term list for implementation, it'd be helpful to know now how to avoid making them harder to implement them later.
g77 command become just a script that invokes
all the various preprocessing that might be needed,
thus making it seem slower than necessary for legacy code
that people are unwilling to convert,
or should we provide a separate script for that,
thus encouraging people to convert their code once and for all?
At least, a separate script to behave as old g77 did,
perhaps named g77old, might ease the transition,
as might a corresponding one that converts source codes
named g77oldnew.
These scripts would take all the pertinent options g77 used
to take and run the appropriate filters,
passing the results to g77 or just making new sources out of them
(in a subdirectory, leaving the user to do the dirty deed of
moving or copying them over the old sources).
CHARACTER
(or Hollerith) constants?
Knowing what other compilers provide would help.
I've asked info-gnu-fortran@gnu.org for input on this. Not having to support these makes it easier to write the new front end, and might also avoid complicated its design.
The consensus to date (1999-11-17) has been to drop this support. Can't recall anybody saying they're using it, in fact.
Don't poke the bear.
The g77 front end generates code
via the gcc back end.
The gcc back end (GBE) is a large, complex
labyrinth of intricate code
written in a combination of the C language
and specialized languages internal to gcc.
While the code that implements the GBE is written in a combination of languages, the GBE itself is, to the front end for a language like Fortran, best viewed as a compiler that compiles its own, unique, language.
The GBE's “source”, then, is written in this language, which consists primarily of a combination of calls to GBE functions and tree nodes (which are, themselves, created by calling GBE functions).
So, the g77 generates code by, in effect,
translating the Fortran code it reads
into a form “written” in the “language”
of the gcc back end.
This language will heretofore be referred to as GBEL, for GNU Back End Language.
GBEL is an evolving language,
not fully specified in any published form
as of this writing.
It offers many facilities,
but its “core” facilities
are those that corresponding most directly
to those needed to support gcc
(compiling code written in GNU C).
The g77 Fortran Front End (FFE)
is designed and implemented
to navigate the currents and eddies
of ongoing GBEL and gcc development
while also delivering on the potential
of an integrated FFE
(as compared to using a converter like f2c
and feeding the output into gcc).
Goals of the FFE's code-generation strategy include:
g77-specific) constructs,
such as command-line options.
The strategies historically, and currently, used by the FFE to achieve these goals include:
“Don't poke the bear” somewhat summarizes the above strategies.
The GBE is the bear.
The FFE is designed and implemented to avoid poking it
in ways that are likely to just annoy it.
The FFE usually either tackles it head-on,
or avoids treating it in ways dissimilar to how
the gcc front end treats it.
For example, the FFE uses the native array facility in the back end
instead of the lower-level pointer-arithmetic facility
used by gcc when compiling f2c output).
Theoretically, this presents more opportunities for optimization,
faster compile times,
and the production of more faithful debugging information.
These benefits were not, however, immediately realized,
mainly because gcc itself makes little or no use
of the native array facility.
Complex arithmetic is a case study of the evolution of this strategy. When originally implemented, the GBEL had just evolved its own native complex-arithmetic facility, so the FFE took advantage of that.
When porting g77 to 64-bit systems,
it was discovered that the GBE didn't really
implement its native complex-arithmetic facility properly.
The short-term solution was to rewrite the FFE
to instead use the lower-level facilities
that'd be used by gcc-compiled code
(assuming that code, itself, didn't use the native complex type
provided, as an extension, by gcc),
since these were known to work,
and, in any case, if shown to not work,
would likely be rapidly fixed
(since they'd likely not work for vanilla C code in similar circumstances).
However, the rewrite accommodated the original, native approach as well
by offering a command-line option to select it over the emulated approach.
This allowed users, and especially GBE maintainers, to try out
fixes to complex-arithmetic support in the GBE
while g77 continued to default to compiling more code correctly,
albeit producing (typically) slower executables.
As of April 1999, it appeared that the last few bugs in the GBE's support of its native complex-arithmetic facility were worked out. The FFE was changed back to default to using that native facility, leaving emulation as an option.
Later during the release cycle (which was called EGCS 1.2, but soon became GCC 2.95), bugs in the native facility were found. Reactions among various people included “the last thing we should do is change the default back”, “we must change the default back”, and “let's figure out whether we can narrow down the bugs to few enough cases to allow the now-months-long-tested default to remain the same”. The latter viewpoint won that particular time. The bugs exposed other concerns regarding ABI compliance when the ABI specified treatment of complex data as different from treatment of what Fortran and GNU C consider the equivalent aggregation (structure) of real (or float) pairs.
Other Fortran constructs—arrays, character strings,
complex division, COMMON and EQUIVALENCE aggregates,
and so on—involve issues similar to those pertaining to complex arithmetic.
So, it is possible that the history of how the FFE handled complex arithmetic will be repeated, probably in modified form (and hopefully over shorter timeframes), for some of these other facilities.
The FFE does not tell the GBE anything about a program unit until after the last statement in that unit has been parsed. (A program unit is a Fortran concept that corresponds, in the C world, mostly closely to functions definitions in ISO C. That is, a program unit in Fortran is like a top-level function in C. Nested functions, found among the extensions offered by GNU C, correspond roughly to Fortran's statement functions.)
So, while parsing the code in a program unit, the FFE saves up all the information on statements, expressions, names, and so on, until it has seen the last statement.
At that point, the FFE revisits the saved information (in what amounts to a second pass over the program unit) to perform the actual translation of the program unit into GBEL, ultimating in the generation of assembly code for it.
Some lookahead is performed during this second pass, so the FFE could be viewed as a “two-plus-pass” design.
Most of the code that turns the first pass (parsing) into a second pass for code generation is in gcc/gcc/f/std.c.
It has external functions, called mainly by siblings in gcc/gcc/f/stc.c, that record the information on statements and expressions in the order they are seen in the source code. These functions save that information.
It also has an external function that revisits that information, calling the siblings in gcc/gcc/f/ste.c, which handles the actual code generation (by generating GBEL code, that is, by calling GBE routines to represent and specify expressions, statements, and so on).
The need for two passes was not immediately evident
during the design and implementation of the code in the FFE
that was to produce GBEL.
Only after a few kludges,
to handle things like incorrectly-guessed ASSIGN label nature,
had been implemented,
did enough evidence pile up to make it clear
that std.c had to be introduced to intercept,
save, then revisit as part of a second pass,
the digested contents of a program unit.
Other such missteps have occurred during the evolution of the FFE, because of the different goals of the FFE and the GBE.
Because the GBE's original, and still primary, goal
was to directly support the GNU C language,
the GBEL, and the GBE itself,
requires more complexity
on the part of most front ends
than it requires of gcc's.
For example,
the GBEL offers an interface that permits the gcc front end
to implement most, or all, of the language features it supports,
without the front end having to
make use of non-user-defined variables.
(It's almost certainly the case that all of K&R C,
and probably ANSI C as well,
is handled by the gcc front end
without declaring such variables.)
The FFE, on the other hand, must resort to a variety of “tricks” to achieve its goals.
Consider the following C code:
int
foo (int a, int b)
{
int c = 0;
if ((c = bar (c)) == 0)
goto done;
quux (c << 1);
done:
return c;
}
Note what kinds of objects are declared, or defined, before their use, and before any actual code generation involving them would normally take place:
Whereas, the following items can, and do, suddenly appear “out of the blue” in C:
Not surprisingly, the GBE faithfully permits the latter set of items to be “discovered” partway through GBEL “programs”, just as they are permitted to in C.
Yet, the GBE has tended, at least in the past, to be reticent to fully support similar “late” discovery of items in the former set.
This makes Fortran a poor fit for the “safe” subset of GBEL. Consider:
FUNCTION X (A, ARRAY, ID1)
CHARACTER*(*) A
DOUBLE PRECISION X, Y, Z, TMP, EE, PI
REAL ARRAY(ID1*ID2)
COMMON ID2
EXTERNAL FRED
ASSIGN 100 TO J
CALL FOO (I)
IF (I .EQ. 0) PRINT *, A(0)
GOTO 200
ENTRY Y (Z)
ASSIGN 101 TO J
200 PRINT *, A(1)
READ *, TMP
GOTO J
100 X = TMP * EE
RETURN
101 Y = TMP * PI
CALL FRED
DATA EE, PI /2.71D0, 3.14D0/
END
Here are some observations about the above code, which, while somewhat contrived, conforms to the FORTRAN 77 and Fortran 90 standards:
ENTRY statement is parsed.
DATA statement is parsed.
REAL
or a subroutine
(which can be thought of as returning type void
or, to support alternate returns in a simple way,
type int)
is not known
until the CALL FRED statement is parsed.
FORMAT label
or the label of an executable statement
is not known
until the X = statement is parsed.
(These two types of labels get very different treatment,
especially when ASSIGN'ed.)
ASSIGN statement is parsed.
(This happens after executable code has been seen.)
Very few of these “discoveries” can be accommodated by the GBE as it has evolved over the years. The GBEL doesn't support several of them, and those it might appear to support don't always work properly, especially in combination with other GBEL and GBE features, as implemented in the GBE.
(Had the GBE and its GBEL originally evolved to support g77,
the shoe would be on the other foot, so to speak—most, if not all,
of the above would be directly supported by the GBEL,
and a few C constructs would probably not, as they are in reality,
be supported.
Both this mythical, and today's real, GBE caters to its GBEL
by, sometimes, scrambling around, cleaning up after itself—after
discovering that assumptions it made earlier during code generation
are incorrect.
That's not a great design, since it indicates significant code
paths that might be rarely tested but used in some key production
environments.)
So, the FFE handles these discrepancies—between the order in which it discovers facts about the code it is compiling, and the order in which the GBEL and GBE support such discoveries—by performing what amounts to two passes over each program unit.
(A few ambiguities can remain at that point,
such as whether, given EXTERNAL BAZ
and no other reference to BAZ in the program unit,
it is a subroutine, a function, or a block-data—which, in C-speak,
governs its declared return type.
Fortunately, these distinctions are easily finessed
for the procedure, library, and object-file interfaces
supported by g77.)
Consider the following Fortran code, which uses various extensions (including some to Fortran 90):
SUBROUTINE X(A)
CHARACTER*(*) A
COMPLEX CFUNC
INTEGER*2 CLOCKS(200)
INTEGER IFUNC
CALL SYSTEM_CLOCK (CLOCKS (IFUNC (CFUNC ('('//A//')'))))
The above poses the following challenges to any Fortran compiler
that uses run-time interfaces, and a run-time library, roughly similar
to those used by g77:
SYSTEM_CLOCK
expects to set an INTEGER*4 variable via its COUNT argument,
the compiler must make available to it a temporary variable of that type.
SYSTEM_CLOCK library routine returns,
the compiler must ensure that the temporary variable it wrote
is copied into the appropriate element of the CLOCKS array.
(This assumes the compiler doesn't just reject the code,
which it should if it is compiling under some kind of a “strict” option.)
SYSTEM_CLOCK library routine returns),
the compiler must ensure that the IFUNC function is called.
That requires evaluating its argument,
which requires, for g77
(assuming -ff2c is in force),
reserving a temporary variable of type COMPLEX
for use as a repository for the return value
being computed by CFUNC.
CFUNC
should, ideally, be deallocated
(or, at least, left to the GBE to dispose of, as it sees fit)
as soon as CFUNC returns,
which means before IFUNC is called
(as it might need a lot of dynamically allocated memory).
g77 currently doesn't support all of the above,
but, so that it might someday, it has evolved to handle
at least some of the above requirements.
Meeting the above requirements is made more challenging by conforming to the requirements of the GBEL/GBE combination.
Most Fortran statements are given their own block, and, for temporary variables they might need, their own scope. (A block is what distinguishes { foo (); } from just foo (); in C. A scope is included with every such block, providing a distinct name space for local variables.)
Label definitions for the statement precede this block, so 10 PRINT *, I is handled more like fl10: { ... } than { fl10: ... } (where fl10 is just a notation meaning “Fortran Label 10” for the purposes of this document).
Any temporaries needed during, but not beyond, execution of a Fortran statement, are made local to the scope of that statement's block.
This allows the GBE to share storage for these temporaries among the various statements without the FFE having to manage that itself.
(The GBE could, of course, decide to optimize management of these temporaries. For example, it could, theoretically, schedule some of the computations involving these temporaries to occur in parallel. More practically, it might leave the storage for some temporaries “live” beyond their scopes, to reduce the number of manipulations of the stack pointer at run time.)
Temporaries needed across distinct statement boundaries usually
are associated with Fortran blocks (such as DO/END DO).
(Also, there might be temporaries not associated with blocks at all—these
would be in the scope of the entire program unit.)
Each Fortran block should get its own block/scope in the GBE.
This is best, because it allows temporaries to be more naturally handled.
However, it might pose problems when handling labels
(in particular, when they're the targets of GOTOs outside the Fortran
block), and generally just hassling with replicating
parts of the gcc front end
(because the FFE needs to support
an arbitrary number of nested back-end blocks
if each Fortran block gets one).
So, there might still be a need for top-level temporaries, whose “owning” scope is that of the containing procedure.
Also, there seems to be problems declaring new variables after generating code (within a block) in the back end, leading to, e.g., label not defined before binding contour or similar messages, when compiling with -fstack-check or when compiling for certain targets.
Because of that, and because sometimes these temporaries are not discovered until in the middle of of generating code for an expression statement (as in the case of the optimization for X**I), it seems best to always pre-scan all the expressions that'll be expanded for a block before generating any of the code for that block.
This pre-scan then handles discovering and declaring, to the back end, the temporaries needed for that block.
It's also important to treat distinct items in an I/O list as distinct statements deserving their own blocks. That's because there's a requirement that each I/O item be fully processed before the next one, which matters in cases like READ (*,*), I, A(I)—the element of A read in the second item must be determined from the value of I read in the first item.
DO WHILE(expr) must be implemented so that temporaries needed to evaluate expr are generated just for the test, each time.
Consider how DO WHILE (A//B .NE. 'END'); ...; END DO is transformed:
for (;;)
{
int temp0;
{
char temp1[large];
libg77_catenate (temp1, a, b);
temp0 = libg77_ne (temp1, 'END');
}
if (! temp0)
break;
...
}
In this case, it seems like a time/space tradeoff between allocating and deallocating temp1 for each iteration and allocating it just once for the entire loop.
However, if temp1 is allocated just once for the entire loop, it could be the wrong size for subsequent iterations of that loop in cases like DO WHILE (A(I:J)//B .NE. 'END'), because the body of the loop might modify I or J.
So, the above implementation is used, though a more optimal one can be used in specific circumstances.
An iterative DO loop
(one that specifies an iteration variable)
is required by the Fortran standards
to be implemented as though an iteration count
is computed before entering the loop body,
and that iteration count used to determine
the number of times the loop body is to be performed
(assuming the loop isn't cut short via GOTO or EXIT).
The FFE handles this by allocating a temporary variable to contain the computed number of iterations. Since this variable must be in a scope that includes the entire loop, a GBEL block is created for that loop, and the variable declared as belonging to the scope of that block.
Consider:
SUBROUTINE X(A,B,C)
CHARACTER*(*) A, B, C
LOGICAL LFUNC
IF (LFUNC (A//B)) THEN
CALL SUBR1
ELSE IF (LFUNC (A//C)) THEN
CALL SUBR2
ELSE
CALL SUBR3
END
The arguments to the two calls to LFUNC
require dynamic allocation (at run time),
but are not required during execution of the CALL statements.
So, the scopes of those temporaries must be within blocks inside
the block corresponding to the Fortran IF block.
This cannot be represented “naturally”
in vanilla C, nor in GBEL.
The if, elseif, else,
and endif constructs
provided by both languages must,
for a given if block,
share the same C/GBE block.
Therefore, any temporaries needed during evaluation of expr
while executing ELSE IF(expr)
must either have been predeclared
at the top of the corresponding IF block,
or declared within a new block for that ELSE IF—a block that,
since it cannot contain the else or else if itself
(due to the above requirement),
actually implements the rest of the IF block's
ELSE IF and ELSE statements
within an inner block.
The FFE takes the latter approach.
SELECT CASE poses a few interesting problems for code generation,
if efficiency and frugal stack management are important.
Consider SELECT CASE (I('PREFIX'//A)),
where A is CHARACTER*(*).
In a case like this—basically,
in any case where largish temporaries are needed
to evaluate the expression—those temporaries should
not be “live” during execution of any of the CASE blocks.
So, evaluation of the expression is best done within its own block,
which in turn is within the SELECT CASE block itself
(which contains the code for the CASE blocks as well,
though each within their own block).
Otherwise, we'd have the rough equivalent of this pseudo-code:
{
char temp[large];
libg77_catenate (temp, 'prefix', a);
switch (i (temp))
{
case 0:
...
}
}
And that would leave temp[large] in scope during the CASE blocks (although a clever back end *could* see that it isn't referenced in them, and thus free that temp before executing the blocks).
So this approach is used instead:
{
int temp0;
{
char temp1[large];
libg77_catenate (temp1, 'prefix', a);
temp0 = i (temp1);
}
switch (temp0)
{
case 0:
...
}
}
Note how temp1 goes out of scope before starting the switch, thus making it easy for a back end to free it.
The problem that solution has, however, is with SELECT CASE('prefix'//A) (which is currently not supported).
Unless the GBEL is extended to support arbitrarily long character strings
in its case facility,
the FFE has to implement SELECT CASE on CHARACTER
(probably excepting CHARACTER*1)
using a cascade of
if, elseif, else, and endif constructs
in GBEL.
To prevent the (potentially large) temporary,
needed to hold the selected expression itself ('prefix'//A),
from being in scope during execution of the CASE blocks,
two approaches are available:
CASE tests,
producing an integer ordinal that is used,
a la temp0 in the earlier example,
as if SELECT CASE(temp0) had been written.
Each corresponding CASE is replaced with CASE(i),
where i is the ordinal for that case,
determined while, or before,
generating the cascade of if-related constructs
to cope with CHARACTER selection.
CASE string
that'll actually be compared against the expression
(in this case, 'prefix'//A).
Since that length must be constant
(because CASE expressions are all constant),
it won't be so large,
and, further, temp1 need not be dynamically allocated,
since normal CHARACTER assignment can be used
into the fixed-length temp0.
Both of these solutions require SELECT CASE implementation
to be changed so all the corresponding CASE statements
are seen during the actual code generation for SELECT CASE.
The interactions between statements, expressions, and subexpressions at program run time can be viewed as:
action(expr)
Here, action is the series of steps performed to effect the statement, and expr is the expression whose value is used by action.
Expanding the above shows a typical order of events at run time:
Evaluate expr
Perform action, using result of evaluation of expr
Clean up after evaluating expr
So, if evaluating expr requires allocating memory, that memory can be freed before performing action only if it is not needed to hold the result of evaluating expr. Otherwise, it must be freed no sooner than after action has been performed.
The above are recursive definitions, in the sense that they apply to subexpressions of expr.
That is, evaluating expr involves evaluating all of its subexpressions, performing the action that computes the result value of expr, then cleaning up after evaluating those subexpressions.
The recursive nature of this evaluation is implemented via recursive-descent transformation of the top-level statements, their expressions, their subexpressions, and so on.
However, that recursive-descent transformation is, due to the nature of the GBEL, focused primarily on generating a single stream of code to be executed at run time.
Yet, from the above, it's clear that multiple streams of code must effectively be simultaneously generated during the recursive-descent analysis of statements.
The primary stream implements the primary action items, while at least two other streams implement the evaluation and clean-up items.
Requirements imposed by expressions include:
Names exported by FFE modules have the following (regular-expression) forms.
Note that all names beginning ffemod or FFEmod,
where mod is lowercase or uppercase alphanumerics, respectively,
are exported by the module ffemod,
with the source code doing the exporting in mod.h.
(Usually, the source code for the implementation is in mod.c.)
Identifiers that don't fit the following forms are not considered exported, even if they are according to the C language. (For example, they might be made available to other modules solely for use within expansions of exported macros, not for use within any source code in those other modules.)
ffemodFFEumod_[A-Z][A-Z0-9_]*A #define or enum constant of the type ffemod.
ffemod[A-Z][A-Z][a-z0-9]*The portion of the identifier after ffemod is
referred to as ctype, a capitalized (mixed-case) form
of type.
FFEumod_type[A-Z][A-Z0-9_]*[A-Z0-9]?A #define or enum constant of the type
ffemodtype,
where type is the lowercase form of ctype
in an exported typedef.
ffemod_valueffemod_value_inputBelow are names used for value and input, along with their definitions.
colfilefindinitializeintint.
islenlinelookupnametext that points to a name of something.
newfind without crashing.
ptrunterminatetextchar * that points to generic text.
Some diagnostics produced by g77 require sufficient explanation that the explanations are given below, and the diagnostics themselves identify the appropriate explanation.
Identification uses the GNU Info format—specifically, the info command that displays the explanation is given within square brackets in the diagnostic. For example:
foo.f:5: Invalid statement [info -f g77 M FOOEY]
More details about the above diagnostic is found in the g77 Info documentation, menu item M, submenu item FOOEY, which is displayed by typing the UNIX command info -f g77 M FOOEY.
Other Info readers, such as EMACS, may be just as easily used to display the pertinent node. In the above example, g77 is the Info document name, M is the top-level menu item to select, and, in that node (named Diagnostics, the name of this chapter, which is the very text you're reading now), FOOEY is the menu item to select.
CMPAMBIG
Ambiguous use of intrinsic intrinsic ...
The type of the argument to the invocation of the intrinsic
intrinsic is a COMPLEX type other than COMPLEX(KIND=1).
Typically, it is COMPLEX(KIND=2), also known as
DOUBLE COMPLEX.
The interpretation of this invocation depends on the particular
dialect of Fortran for which the code was written.
Some dialects convert the real part of the argument to
REAL(KIND=1), thus losing precision; other dialects,
and Fortran 90, do no such conversion.
So, GNU Fortran rejects such invocations except under certain circumstances, to avoid making an incorrect assumption that results in generating the wrong code.
To determine the dialect of the program unit, perhaps even whether that particular invocation is properly coded, determine how the result of the intrinsic is used.
The result of intrinsic is expected (by the original programmer)
to be REAL(KIND=1) (the non-Fortran-90 interpretation) if:
REAL(KIND=1).
For example,
a procedure with no DOUBLE PRECISION or IMPLICIT DOUBLE PRECISION
statement specifying the dummy argument corresponding to an
actual argument of REAL(Z), where Z is declared
DOUBLE COMPLEX, strongly suggests that the programmer
expected REAL(Z) to return REAL(KIND=1) instead
of REAL(KIND=2).
REAL(KIND=2) but where treating the intrinsic
invocation as REAL(KIND=2) would result in unnecessary
promotions and (typically) more expensive operations on the
wider type.
For example:
DOUBLE COMPLEX Z
...
R(1) = T * REAL(Z)
The above example suggests the programmer expected the real part
of Z to be converted to REAL(KIND=1) before being
multiplied by T (presumed, along with R above, to
be type REAL(KIND=1)).
Otherwise, the conversion would have to be delayed until after
the multiplication, requiring not only an extra conversion
(of T to REAL(KIND=2)), but a (typically) more
expensive multiplication (a double-precision multiplication instead
of a single-precision one).
The result of intrinsic is expected (by the original programmer)
to be REAL(KIND=2) (the Fortran 90 interpretation) if:
REAL(KIND=2).
For example, a procedure specifying a DOUBLE PRECISION
dummy argument corresponding to an
actual argument of REAL(Z), where Z is declared
DOUBLE COMPLEX, strongly suggests that the programmer
expected REAL(Z) to return REAL(KIND=2) instead
of REAL(KIND=1).
REAL(KIND=2) operands,
or is assigned to a REAL(KIND=2) variable or array element.
For example:
DOUBLE COMPLEX Z
DOUBLE PRECISION R, T
...
R(1) = T * REAL(Z)
The above example suggests the programmer expected the real part
of Z to not be converted to REAL(KIND=1)
by the REAL() intrinsic.
Otherwise, the conversion would have to be immediately followed
by a conversion back to REAL(KIND=2), losing
the original, full precision of the real part of Z,
before being multiplied by T.
Once you have determined whether a particular invocation of intrinsic expects the Fortran 90 interpretation, you can:
REAL) or DIMAG(expr) (if intrinsic
is AIMAG)
if it expected the Fortran 90 interpretation.
This assumes expr is COMPLEX(KIND=2)—if it is
some other type, such as COMPLEX*32, you should use the
appropriate intrinsic, such as the one to convert to REAL*16
(perhaps DBLEQ() in place of DBLE(), and
QIMAG() in place of DIMAG()).
REAL(KIND=1) in all working
Fortran compilers.
If you don't want to change the code, and you are certain that all ambiguous invocations of intrinsic in the source file have the same expectation regarding interpretation, you can:
See REAL() and AIMAG() of Complex, for more information on this issue.
Note: If the above suggestions don't produce enough evidence as to whether a particular program expects the Fortran 90 interpretation of this ambiguous invocation of intrinsic, there is one more thing you can try.
If you have access to most or all the compilers used on the program to create successfully tested and deployed executables, read the documentation for, and also test out, each compiler to determine how it treats the intrinsic intrinsic in this case. (If all the compilers don't agree on an interpretation, there might be lurking bugs in the deployed versions of the program.)
The following sample program might help:
PROGRAM JCB003
C
C Written by James Craig Burley 1997-02-23.
C
C Determine how compilers handle non-standard REAL
C and AIMAG on DOUBLE COMPLEX operands.
C
DOUBLE COMPLEX Z
REAL R
Z = (3.3D0, 4.4D0)
R = Z
CALL DUMDUM(Z, R)
R = REAL(Z) - R
IF (R .NE. 0.) PRINT *, 'REAL() is Fortran 90'
IF (R .EQ. 0.) PRINT *, 'REAL() is not Fortran 90'
R = 4.4D0
CALL DUMDUM(Z, R)
R = AIMAG(Z) - R
IF (R .NE. 0.) PRINT *, 'AIMAG() is Fortran 90'
IF (R .EQ. 0.) PRINT *, 'AIMAG() is not Fortran 90'
END
C
C Just to make sure compiler doesn't use naive flow
C analysis to optimize away careful work above,
C which might invalidate results....
C
SUBROUTINE DUMDUM(Z, R)
DOUBLE COMPLEX Z
REAL R
END
If the above program prints contradictory results on a particular compiler, run away!
EXPIMP
Intrinsic intrinsic referenced ...
The intrinsic is explicitly declared in one program unit in the source file and implicitly used as an intrinsic in another program unit in the same source file.
This diagnostic is designed to catch cases where a program might depend on using the name intrinsic as an intrinsic in one program unit and as a global name (such as the name of a subroutine or function) in another, but g77 recognizes the name as an intrinsic in both cases.
After verifying that the program unit making implicit use of the intrinsic is indeed written expecting the intrinsic, add an INTRINSIC intrinsic statement to that program unit to prevent this warning.
This and related warnings are disabled by using the -Wno-globals option when compiling.
Note that this warning is not issued for standard intrinsics. Standard intrinsics include those described in the FORTRAN 77 standard and, if -ff90 is specified, those described in the Fortran 90 standard. Such intrinsics are not as likely to be confused with user procedures as intrinsics provided as extensions to the standard by g77.
INTGLOB
Same name `intrinsic' given ...
The name intrinsic is used for a global entity (a common block or a program unit) in one program unit and implicitly used as an intrinsic in another program unit.
This diagnostic is designed to catch cases where a program intends to use a name entirely as a global name, but g77 recognizes the name as an intrinsic in the program unit that references the name, a situation that would likely produce incorrect code.
For example:
INTEGER FUNCTION TIME()
...
END
...
PROGRAM SAMP
INTEGER TIME
PRINT *, 'Time is ', TIME()
END
The above example defines a program unit named TIME, but
the reference to TIME in the main program unit SAMP
is normally treated by g77 as a reference to the intrinsic
TIME() (unless a command-line option that prevents such
treatment has been specified).
As a result, the program SAMP will not invoke the TIME function in the same source file.
Since g77 recognizes libU77 procedures as
intrinsics, and since some existing code uses the same names
for its own procedures as used by some libU77
procedures, this situation is expected to arise often enough
to make this sort of warning worth issuing.
After verifying that the program unit making implicit use of the intrinsic is indeed written expecting the intrinsic, add an INTRINSIC intrinsic statement to that program unit to prevent this warning.
Or, if you believe the program unit is designed to invoke the program-defined procedure instead of the intrinsic (as recognized by g77), add an EXTERNAL intrinsic statement to the program unit that references the name to prevent this warning.
This and related warnings are disabled by using the -Wno-globals option when compiling.
Note that this warning is not issued for standard intrinsics. Standard intrinsics include those described in the FORTRAN 77 standard and, if -ff90 is specified, those described in the Fortran 90 standard. Such intrinsics are not as likely to be confused with user procedures as intrinsics provided as extensions to the standard by g77.
LEX
Unrecognized character ...
Invalid first character ...
Line too long ...
Non-numeric character ...
Continuation indicator ...
Label at ... invalid with continuation line indicator ...
Character constant ...
Continuation line ...
Statement at ... begins with invalid token
Although the diagnostics identify specific problems, they can be produced when general problems such as the following occur:
If the code in the file does not look like many of the examples elsewhere in this document, it might not be Fortran code. (Note that Fortran code often is written in lower case letters, while the examples in this document use upper case letters, for stylistic reasons.)
For example, if the file contains lots of strange-looking characters, it might be APL source code; if it contains lots of parentheses, it might be Lisp source code; if it contains lots of bugs, it might be C++ source code.
Free form is a newer form for Fortran code. The older, classic form is called fixed form.
Fixed-form code is visually fairly distinctive, because numerical labels and comments are all that appear in the first five columns of a line, the sixth column is reserved to denote continuation lines, and actual statements start at or beyond column 7. Spaces generally are not significant, so if you see statements such as REALX,Y and DO10I=1,100, you are looking at fixed-form code. Comment lines are indicated by the letter C or the symbol * in column 1. (Some code uses ! or /* to begin in-line comments, which many compilers support.)
Free-form code is distinguished from fixed-form source primarily by the fact that statements may start anywhere. (If lots of statements start in columns 1 through 6, that's a strong indicator of free-form source.) Consecutive keywords must be separated by spaces, so REALX,Y is not valid, while REAL X,Y is. There are no comment lines per se, but ! starts a comment anywhere in a line (other than within a character or Hollerith constant).
See Source Form, for more information.
Statements in fixed-form code must be entirely contained within columns 7 through 72 on a given line. Starting them “early” is more likely to result in diagnostics than finishing them “late”, though both kinds of errors are often caught at compile time.
For example, if the following code fragment is edited by following the commented instructions literally, the result, shown afterward, would produce a diagnostic when compiled:
C On XYZZY systems, remove "C" on next line:
C CALL XYZZY_RESET
The result of editing the above line might be:
C On XYZZY systems, remove "C" on next line:
CALL XYZZY_RESET
However, that leaves the first C in the CALL
statement in column 6, making it a comment line, which is
not really what the author intended, and which is likely
to result in one of the above-listed diagnostics.
Replacing the C in column 1 with a space
is the proper change to make, to ensure the CALL
keyword starts in or after column 7.
Another common mistake like this is to forget that fixed-form source lines are significant through only column 72, and that, normally, any text beyond column 72 is ignored or is diagnosed at compile time.
See Source Form, for more information.
A source file containing lines beginning with #define,
#include, #if, and so on is likely one that
requires preprocessing.
If the file's suffix is .f, .for, or .FOR, the file normally will be compiled without preprocessing by g77.
Change the file's suffix from .f to .F (or, on systems with case-insensitive file names, to .fpp or .FPP), from .for to .fpp, or from .FOR to .FPP. g77 compiles files with such names with preprocessing.
Or, learn how to use gcc's -x option to specify the language f77-cpp-input for Fortran files that require preprocessing. See Options Controlling the Kind of Output (Using the GNU Compiler Collection (GCC)).
Examples of errors resulting from preprocessor macro expansion include exceeding the line-length limit, improperly starting, terminating, or incorporating the apostrophe or double-quote in a character constant, improperly forming a Hollerith constant, and so on.
See Options Controlling the Kind of Output, for suggestions about how to use, and not use, preprocessing for Fortran code.
GLOBALS
Global name name defined at ... already defined...
Global name name at ... has different type...
Too many arguments passed to name at ...
Too few arguments passed to name at ...
Argument #n of name is ...
These messages all identify disagreements about the global procedure named name among different program units (usually including name itself).
Whether a particular disagreement is reported as a warning or an error can depend on the relative order of the disagreeing portions of the source file.
Disagreements between a procedure invocation and the subsequent procedure itself are, usually, diagnosed as errors when the procedure itself precedes the invocation. Other disagreements are diagnosed via warnings.
This distinction, between warnings and errors, is due primarily to the present tendency of the gcc back end to inline only those procedure invocations that are preceded by the corresponding procedure definitions. If the gcc back end is changed to inline “forward references”, in which invocations precede definitions, the g77 front end will be changed to treat both orderings as errors, accordingly.
The sorts of disagreements that are diagnosed by g77 include whether a procedure is a subroutine or function; if it is a function, the type of the return value of the procedure; the number of arguments the procedure accepts; and the type of each argument.
Disagreements regarding global names among program units in a Fortran program should be fixed in the code itself. However, if that is not immediately practical, and the code has been working for some time, it is possible it will work when compiled with the -fno-globals option.
The -fno-globals option causes these diagnostics to all be warnings and disables all inlining of references to global procedures (to avoid subsequent compiler crashes and bad-code generation). Use of the -Wno-globals option as well as -fno-globals suppresses all of these diagnostics. (-Wno-globals by itself disables only the warnings, not the errors.)
After using -fno-globals to work around these problems, it is wise to stop using that option and address them by fixing the Fortran code, because such problems, while they might not actually result in bugs on some systems, indicate that the code is not as portable as it could be. In particular, the code might appear to work on a particular system, but have bugs that affect the reliability of the data without exhibiting any other outward manifestations of the bugs.
LINKFAILOn AIX 4.1, g77 might not build with the native (non-GNU) tools due to a linker bug in coping with the -bbigtoc option which leads to a Relocation overflow error. The GNU linker is not recommended on current AIX versions, though; it was developed under a now-unsupported version. This bug is said to be fixed by `update PTF U455193 for APAR IX75823'.
Compiling with -mminimal-toc might solve this problem, e.g. by adding
BOOT_CFLAGS='-mminimal-toc -O2 -g'
to the make bootstrap command line.
Y2KBADIntrinsic `name', invoked at (^), known to be non-Y2K-compliant...
This diagnostic indicates that the specific intrinsic invoked by the name name is known to have an interface that is not Year-2000 (Y2K) compliant.
AImag intrinsic: REAL() and AIMAG() of ComplexAnd intrinsic: Bit Operations on Floating-point DataAUTOMATIC statement: AUTOMATIC Statementbadu77 intrinsics: Fortran Dialect Optionsbadu77 intrinsics group: Intrinsic GroupsCmplx intrinsic: CMPLX() of DOUBLE PRECISIONCOMMON layout: Aligned DataCOMMON statement: Multiple Definitions of External NamesCOMMON statement: Common BlocksCOMPLEX intrinsics: Fortran Dialect OptionsDIMENSION statement: Array Bounds ExpressionsDO loops, one-trip: Fortran Dialect OptionsDO loops, zero-trip: Fortran Dialect Optionsf771, program: What is GNU Fortran?f90 intrinsics group: Intrinsic Groupsg77: Front Endg77, front end: Front Endgcc, back end: Philosophy of Code GenerationGetArg intrinsic: Main Program Unitgnu intrinsics group: Intrinsic GroupsIArgC intrinsic: Main Program UnitDO: Optimize OptionsINTEGER*2 support: Popular Non-standard TypesINTEGER*8 support: Full Support for Compiler TypesAImag: REAL() and AIMAG() of ComplexAnd: Bit Operations on Floating-point Databadu77: Fortran Dialect OptionsCmplx: CMPLX() of DOUBLE PRECISIONCOMPLEX: Fortran Dialect OptionsGetArg: Main Program UnitIArgC: Main Program UnitOr: Bit Operations on Floating-point DataReal: REAL() and AIMAG() of ComplexShift: Bit Operations on Floating-point DataDO: Optimize OptionsCOMMON blocks: Aligned DataLOGICAL*1 support: Popular Non-standard Typesmil intrinsics group: Intrinsic GroupsDO loops: Fortran Dialect OptionsOr intrinsic: Bit Operations on Floating-point Dataf771: What is GNU Fortran?Real intrinsic: REAL() and AIMAG() of ComplexREAL*16 support: Full Support for Compiler TypesShift intrinsic: Bit Operations on Floating-point DataAUTOMATIC: AUTOMATIC StatementCOMMON: Multiple Definitions of External NamesCOMMON: Common BlocksDIMENSION: Array Bounds ExpressionsINTEGER*2: Popular Non-standard TypesINTEGER*8: Full Support for Compiler TypesLOGICAL*1: Popular Non-standard TypesREAL*16: Full Support for Compiler TypesDO loops: Fortran Dialect Options[1] loop discovery refers to the
process by which a compiler, or indeed any reader of a program,
determines which portions of the program are more likely to be executed
repeatedly as it is being run. Such discovery typically is done early
when compiling using optimization techniques, so the “discovered”
loops get more attention—and more run-time resources, such as
registers—from the compiler. It is easy to “discover” loops that are
constructed out of looping constructs in the language
(such as Fortran's DO). For some programs, “discovering” loops
constructed out of lower-level constructs (such as IF and
GOTO) can lead to generation of more optimal code
than otherwise.
[2] The files generated like this can also be used for inter-unit consistency checking of dummy and actual arguments, although the ftnchek tool from ftp://ftp.netlib.org/fortran or ftp://ftp.dsm.fordham.edu is probably better for this purpose.
[3] No, not that one.