Associated with each program are a collection of variables which can be used to modify how that program is built. There is a similar list of such variables for each library. The canonical name of the program (or library) is used as a base for naming these variables.
In the list below, we use the name "maude" to refer to the program or library. In your Makefile.am you would replace this with the canonical name of your program. This list also refers to "maude" as a program, but in general the same rules apply for both static and dynamic libraries; the documentation below notes situations where programs and libraries differ.
This variable, if it exists, lists all the source files which are compiled to build the program. These files are added to the distribution by default. When building the program, Automake will cause each source file to be compiled to a single .o file (or .lo when using libtool). Normally these object files are named after the source file, but other factors can change this. If a file in the _SOURCES variable has an unrecognized extension, Automake will do one of two things with it. If a suffix rule exists for turning files with the unrecognized extension into .o files, then automake will treat this file as it will any other source file (the section called “Support for Other Languages”). Otherwise, the file will be ignored as though it were a header file.
The prefixes dist_ and nodist_ can be used to control whether files listed in a _SOURCES variable are distributed. dist_ is redundant, as sources are distributed by default, but it can be specified for clarity if desired.
It is possible to have both dist_ and nodist_ variants of a given _SOURCES variable at once; this lets you easily distribute some files and not others, for instance:
nodist_maude_SOURCES = nodist.c dist_maude_SOURCES = dist-me.c
By default the output file (on Unix systems, the .o file) will be put into the current build directory. However, if the option subdir-objects is in effect in the current directory then the .o file will be put into the subdirectory named after the source file. For instance, with subdir-objects enabled, sub/dir/file.c will be compiled to sub/dir/file.o. Some people prefer this mode of operation. You can specify subdir-objects in AUTOMAKE_OPTIONS (Chapter 18).
Automake needs to know the list of files you intend to compile statically. For one thing, this is the only way Automake has of knowing what sort of language support a given Makefile.in requires. [3] This means that, for example, you can't put a configure substitution like @my_sources@ into a _SOURCES variable. If you intend to conditionally compile source files and use configure to substitute the appropriate object names into, e.g., _LDADD (see below), then you should list the corresponding source files in the EXTRA_ variable.
This variable also supports dist_ and nodist_ prefixes, e.g., nodist_EXTRA_maude_SOURCES.
A static library is created by default by invoking $(AR) cru followed by the name of the library and then the objects being put into the library. You can override this by setting the _AR variable. This is usually used with C++; some C++ compilers require a special invocation in order to instantiate all the templates which should go into a library. For instance, the SGI C++ compiler likes this macro set like so:
libmaude_a_AR = $(CXX) -ar -o
Extra objects can be added to a static library using the _LIBADD variable. This should be used for objects determined by configure. Note that _LIBADD is not used for shared libraries; there you must use _LDADD.
Extra objects can be added to a shared library or a program by listing them in the _LDADD variable. This should be used for objects determined by configure.
_LDADD is inappropriate for passing program-specific linker flags (except for -l, -L, -dlopen and -dlpreopen). Use the _LDFLAGS variable for this purpose.
For instance, if your configure.in uses AC_PATH_XTRA, you could link your program against the X libraries like so:
maude_LDADD = $(X_PRE_LIBS) $(X_LIBS) $(X_EXTRA_LIBS)
This variable is used to pass extra flags to the link step of a program or a shared library.
You can override the linker on a per-program basis. By default the linker is chosen according to the languages used by the program. For instance, a program that includes C++ source code would use the C++ compiler to link. The _LINK variable must hold the name of a command which can be passed all the .o file names as arguments. Note that the name of the underlying program is not passed to _LINK; typically one uses $@:
maude_LINK = $(CCLD) -magic -o $@
Automake allows you to set compilation flags on a per-program (or per-library) basis. A single source file can be included in several programs, and it will potentially be compiled with different flags for each program. This works for any language directly supported by Automake. The flags are _CFLAGS, _CXXFLAGS, _OBJCFLAGS, _YFLAGS, _ASFLAGS, _FFLAGS, _RFLAGS, and _GCJFLAGS.
When using a per-program compilation flag, Automake will choose a different name for the intermediate object files. Ordinarily a file like sample.c will be compiled to produce sample.o. However, if the program's _CFLAGS variable is set, then the object file will be named, for instance, maude-sample.o.
In compilations with per-program flags, the ordinary AM_ form of the flags variable is not automatically included in the compilation (however, the user form of the variable is included). So for instance, if you want the hypothetical maude compilations to also use the value of AM_CFLAGS, you would need to write:
maude_CFLAGS = ... your flags ... $(AM_CFLAGS)
It is also occasionally useful to have a program depend on some other target which is not actually part of that program. This can be done using the _DEPENDENCIES variable. Each program depends on the contents of such a variable, but no further interpretation is done.
If _DEPENDENCIES is not supplied, it is computed by Automake. The automatically-assigned value is the contents of _LDADD, with most configure substitutions, -l, -L, -dlopen and -dlpreopen options removed. The configure substitutions that are left in are only @LIBOBJS@ and @ALLOCA@; these are left because it is known that they will not cause an invalid value for _DEPENDENCIES to be generated.
On some platforms the allowable file names are very short. In order to support these systems and per-program compilation flags at the same time, Automake allows you to set a "short name" which will influence how intermediate object files are named. For instance, if you set maude_SHORTNAME to m, then in the above per-program compilation flag example the object file would be named m-sample.o rather than maude-sample.o. This facility is rarely needed in practice, and we recommend avoiding it until you find it is required.