D-BUS is a system for low-latency, low-overhead, easy to use interprocess communication (IPC). In more detail:
D-BUS is low-latency because it is designed to avoid round trips and allow asynchronous operation, much like the X protocol.
D-BUS is low-overhead because it uses a binary protocol, and does not have to convert to and from a text format such as XML. Because D-BUS is intended for potentially high-resolution same-machine IPC, not primarily for Internet IPC, this is an interesting optimization.
D-BUS is easy to use because it works in terms of messages rather than byte streams, and does not require users to understand any complex concepts such as a new type system or elaborate APIs. Libraries implementing D-BUS may choose to abstract messages as "method calls" (see the Section called Method Call Mapping).
The base D-BUS protocol is a peer-to-peer protocol, specified in the Section called Message Protocol. That is, it is a system for one application to talk to a single other application. However, the primary intended application of D-BUS is the D-BUS message bus, specified in the Section called Message Bus Specification. The message bus is a special application that accepts connections from multiple other applications, and forwards messages among them.
Things that D-BUS can be used for is for example notification of system changes (notification of when a camera is plugged in to a computer, or a new version of some software has been installed), or desktop interoperablity, for example a file monitoring service or a configuration service.
A message consists of a header and a body. If you think of a message as a package, the header is the address, and the body contains the package contents. The message delivery system uses the header information to figure out where to send the message and how to interpret it; the recipient inteprets the body of the message.
The body of the message is made up of zero or more arguments, which are typed values, such as an integer or a byte array.
Following the mandatory fields, there are zero or more named fields (see the Section called Header Fields), and then nul bytes padding the header such that its total length in bytes is a multiple of 8.
The header MUST begin with the following mandatory fields in the following order:
| Size | Description |
|---|---|
| 1 byte | Endianness flag; ASCII 'l' for little-endian or ASCII 'B' for big-endian. |
| 1 byte | Bitwise OR of flags. Unknown flags MUST be ignored. Currently-defined flags are described below. |
| 1 byte | Major protocol version of the sending application. If the major protocol version of the receiving application does not match, the applications will not be able to communicate and the D-BUS connection MUST be disconnected. The major protocol version for this version of the specification is 0. |
| 1 byte | A nul byte, reserved for future use. Any value for this byte MUST be accepted. |
| 4 bytes | An unsigned 32-bit integer in the message's byte order, indicating the total length in bytes of the header including named fields and any alignment padding. MUST be a multiple of 8. |
| 4 bytes | An unsigned 32-bit integer in the message's byte order, indicating the total length in bytes of the message body. |
| 4 bytes | The message's serial number, an unsigned 32-bit integer in the message's byte order. Applications MUST NOT reuse the same serial number for different messages more often than 32-bit unsigned integer wraparound. Zero is not a valid serial number. |
Flags that can appear in the second byte of the header:
In addition to the required header information mentioned in the Section called Header Encoding, the header may contain zero or more named header fields. These fields are named to allow future versions of this protocol specification to add new fields; implementations must ignore fields they do not understand. Implementations must not invent their own header fields; only changes to this specification may introduce new header fields.
Header field names MUST consist of 4 non-nul bytes. The field name is NOT nul terminated; it occupies exactly 4 bytes. Following the name, the field MUST have a type code represented as a single unsigned byte, and then a properly-aligned value of that type. See the Section called Message Arguments for a description of how each type is encoded. If an implementation sees a header field name that it does not understand, it MUST ignore that field.
Here are the currently-defined named header fields:
| Name | Type | Description |
|---|---|---|
| name | STRING | The name of the message, such as org.freedesktop.Peer.Ping |
| rply | UINT32 | The serial number of the message this message is a reply to. (The serial number is one of the mandatory header fields, see the Section called Header Encoding.) |
| srvc | STRING | The name of the service this message should be routed to. Only used in combination with the message bus, see the Section called Message Bus Specification. |
| sndr | STRING | The name of the base service that sent this message. The message bus fills in this field; the field is only meaningful in combination with the message bus. |
To allow implementations to keep the header and the body in a single buffer while keeping data types aligned, the total length of the header must be a multiple of 8 bytes. To achieve this, the header MUST be padded with nul bytes to align its total length on an 8-byte boundary. The minimum number of padding bytes MUST be used. Because all possible named fields use at least 8 bytes, implementations can distinguish padding (which must be less than 8 bytes) from additional named fields (which must be at least 8 bytes).
The message body is made up of arguments. Each argument is a type code, represented by a single unsigned byte, followed by the aligned value of the argument in a type-dependent format. Alignment padding between the typecode and the value is initialized to zero.
| Type name | Code | Description |
|---|---|---|
| INVALID | 0 | Not a valid type code (error if it appears in a message) |
| NIL | 1 | Marks an "unset" or "nonexistent" argument |
| BYTE | 2 | 8-bit unsigned integer |
| BOOLEAN | 3 | Boolean value, 0 is FALSE and 1 is TRUE. Everything else is invalid. |
| INT32 | 4 | 32-bit signed integer |
| UINT32 | 5 | 32-bit unsigned integer |
| INT64 | 6 | 64-bit signed integer |
| UINT64 | 7 | 64-bit unsigned integer |
| DOUBLE | 8 | IEEE 754 double |
| STRING | 9 | UTF-8 string (must be valid UTF-8). Must be zero terminated. |
| NAMED | 10 | A named byte array, used for custom types |
| ARRAY | 11 | Array |
| DICT | 12 | A dictionary of key/value pairs |
The types are encoded as follows:
| Type name | Encoding |
|---|---|
| INVALID | Not applicable; cannot be encoded. |
| NIL | No data is encoded; the type code is followed immediately by the type code of the next argument. |
| BYTE | A byte. |
| BOOLEAN | A byte, with valid values 0 and 1. |
| INT32 | 32-bit signed integer in the message's byte order, aligned to 4-byte boundary. |
| UINT32 | 32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary. |
| INT64 | 64-bit signed integer in the message's byte order, aligned to 8-byte boundary. |
| UINT64 | 64-bit unsigned integer in the message's byte order, aligned to 8-byte boundary. |
| DOUBLE | 64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary. |
| STRING | UINT32 aligned to 4-byte boundary indicating the string's length in bytes excluding its terminating nul, followed by string data of the given length, followed by a terminating nul byte. |
| NAMED | A string (encoded as the STRING type above) giving the name of the type followed by an UINT32 aligned to 4-byte boundary indicating the data length in bytes, followed by the data. |
| ARRAY | A sequence of bytes giving the element type of the array, terminated by a type different from ARRAY (just one byte for one-dimensional arrays, but larger for multi-dimensional arrays), followed by an UINT32 (aligned to 4 bytes) giving the length of the array data in bytes. This is followed by each array entry encoded the way it would normally be encoded, except arrays, which are encoded without the type information, since that is already declared above. Arrays containing NIL are not allowed. |
| DICT | UINT32 giving the length of the dictionary data in bytes. This is followed by a number of keyname/value pairs, where the keyname is encoded as a STRING above, and the value is encoded as a byte with typecode and how that type normally would be encoded alone. |
Messages and services have names with type STRING, meaning that they must be valid UTF-8. However, there are also some additional restrictions that apply to message and service names specifically:
They must contain at least one '.' (period) character
They must not begin with a '.' (period) character
They must not exceed 256 bytes in length
They must be at least 1 byte in length
Before the flow of messages begins, two applications must authenticate. A simple plain-text protocol is used for authentication; this protocol is a SASL profile, and maps fairly directly from the SASL specification. The message encoding is NOT used here, only plain text messages.
In examples, "C:" and "S:" indicate lines sent by the client and server respectively.
The protocol is a line-based protocol, where each line ends with \r\n. Each line begins with an all-caps ASCII command name containing only the character range [A-Z], a space, then any arguments for the command, then the \r\n ending the line. The protocol is case-sensitive. All bytes must be in the ASCII character set. Commands from the client to the server are as follows:
AUTH [mechanism] [initial-response]
CANCEL
BEGIN
DATA <data in base 64 encoding>
ERROR [human-readable error explanation]
REJECTED <space-separated list of mechanism names>
OK
DATA <data in base 64 encoding>
ERROR
Immediately after connecting to the server, the client must send a single nul byte. This byte may be accompanied by credentials information on some operating systems that use sendmsg() with SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain sockets. However, the nul byte MUST be sent even on other kinds of socket, and even on operating systems that do not require a byte to be sent in order to transmit credentials. The text protocol described in this document begins after the single nul byte. If the first byte received from the client is not a nul byte, the server may disconnect that client.
A nul byte in any context other than the initial byte is an error; the protocol is ASCII-only.
The credentials sent along with the nul byte may be used with the SASL mechanism EXTERNAL.
If an AUTH command has no arguments, it is a request to list available mechanisms. The server SHOULD respond with a REJECTED command listing the mechanisms it understands.
If an AUTH command specifies a mechanism, and the server supports said mechanism, the server SHOULD begin exchanging SASL challenge-response data with the client using DATA commands.
If the server does not support the mechanism given in the AUTH command, it SHOULD send a REJECTED command listing the mechanisms it does support.
If the [initial-response] argument is provided, it is intended for use with mechanisms that have no initial challenge (or an empty initial challenge), as if it were the argument to an initial DATA command. If the selected mechanism has an initial challenge, the server should reject authentication by sending REJECTED.
If authentication succeeds after exchanging DATA commands, an OK command should be sent to the client.
The first octet received by the client after the \r\n of the OK command MUST be the first octet of the authenticated/encrypted stream of D-BUS messages.
The first octet received by the server after the \r\n of the BEGIN command from the client MUST be the first octet of the authenticated/encrypted stream of D-BUS messages.
At any time up to sending the BEGIN command, the client may send a CANCEL command. On receiving the CANCEL command, the server MUST send a REJECTED command and abort the current authentication exchange.
The DATA command may come from either client or server, and simply contains a base64-encoded block of data to be interpreted according to the SASL mechanism in use.
Some SASL mechanisms support sending an "empty string"; FIXME we need some way to do this.
The BEGIN command acknowledges that the client has received an OK command from the server, and that the stream of messages is about to begin.
The first octet received by the server after the \r\n of the BEGIN command from the client MUST be the first octet of the authenticated/encrypted stream of D-BUS messages.
The REJECTED command indicates that the current authentication exchange has failed, and further exchange of DATA is inappropriate. The client would normally try another mechanism, or try providing different responses to challenges.
Optionally, the REJECTED command has a space-separated list of available auth mechanisms as arguments. If a server ever provides a list of supported mechanisms, it MUST provide the same list each time it sends a REJECTED message. Clients are free to ignore all lists received after the first.
The OK command indicates that the client has been authenticated, and that further communication will be a stream of D-BUS messages (optionally encrypted, as negotiated) rather than this protocol.
The first octet received by the client after the \r\n of the OK command MUST be the first octet of the authenticated/encrypted stream of D-BUS messages.
The client MUST respond to the OK command by sending a BEGIN command, followed by its stream of messages, or by disconnecting. The server MUST NOT accept additional commands using this protocol after the OK command has been sent.
The ERROR command indicates that either server or client did not know a command, does not accept the given command in the current context, or did not understand the arguments to the command. This allows the protocol to be extended; a client or server can send a command present or permitted only in new protocol versions, and if an ERROR is received instead of an appropriate response, fall back to using some other technique.
If an ERROR is sent, the server or client that sent the error MUST continue as if the command causing the ERROR had never been received. However, the the server or client receiving the error should try something other than whatever caused the error; if only canceling/rejecting the authentication.
(MAGIC_COOKIE is a made up mechanism)
C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
S: OK
C: BEGIN
|
Figure 1. Example of successful magic cookie authentication
C: AUTH
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY bW9yZ2Fu
S: DATA OTUgUWE1ODMwOA==
C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: OK
C: BEGIN
|
Figure 2. Example of finding out mechanisms then picking one
C: FOOBAR
S: ERROR
C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
S: OK
C: BEGIN
|
Figure 3. Example of client sends unknown command then falls back to regular auth
C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY bW9yZ2Fu
S: DATA OTUgUWE1ODMwOA==
C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: OK
C: BEGIN
|
Figure 4. Example of server doesn't support initial auth mechanism
C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY bW9yZ2Fu
S: DATA OTUgUWE1ODMwOA==
C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: REJECTED
C: AUTH SKEY bW9yZ2Fu
S: DATA OTUgUWE1ODMwOA==
C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
S: OK
C: BEGIN
|
Figure 5. Example of wrong password or the like followed by successful retry
This section describes some new authentication mechanisms. D-BUS also allows any standard SASL mechanism of course.
The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client has the ability to read a private file owned by the user being authenticated. If the client can prove that it has access to a secret cookie stored in this file, then the client is authenticated. Thus the security of DBUS_COOKIE_SHA1 depends on a secure home directory.
Authentication proceeds as follows:
The client sends the username it would like to authenticate as.
The server sends the name of its "cookie context" (see below); a space character; the integer ID of the secret cookie the client must demonstrate knowledge of; a space character; then a hex-encoded randomly-generated challenge string.
The client locates the cookie, and generates its own hex-encoded randomly-generated challenge string. The client then concatentates the server's hex-encoded challenge, a ":" character, its own hex-encoded challenge, another ":" character, and the hex-encoded cookie. It computes the SHA-1 hash of this composite string. It sends back to the server the client's hex-encoded challenge string, a space character, and the SHA-1 hash.
The server generates the same concatenated string used by the client and computes its SHA-1 hash. It compares the hash with the hash received from the client; if the two hashes match, the client is authenticated.
Each server has a "cookie context," which is a name that identifies a set of cookies that apply to that server. A sample context might be "org_freedesktop_session_bus". Context names must be valid ASCII, nonzero length, and may not contain the characters slash ("/"), backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"), tab ("\t"), or period ("."). There is a default context, "org_freedesktop_global" that's used by servers that do not specify otherwise.
Cookies are stored in a user's home directory, in the directory ~/.dbus-keyrings/. This directory must not be readable or writable by other users. If it is, clients and servers must ignore it. The directory contains cookie files named after the cookie context.
A cookie file contains one cookie per line. Each line has three space-separated fields:
The cookie ID number, which must be a non-negative integer and may not be used twice in the same file.
The cookie's creation time, in UNIX seconds-since-the-epoch format.
The cookie itself, a hex-encoded random block of bytes.
Only server processes modify the cookie file. They must do so with this procedure:
Create a lockfile name by appending ".lock" to the name of the cookie file. The server should attempt to create this file using O_CREAT | O_EXCL. If file creation fails, the lock fails. Servers should retry for a reasonable period of time, then they may choose to delete an existing lock to keep users from having to manually delete a stale lock. [1]
Once the lockfile has been created, the server loads the cookie file. It should then delete any cookies that are old (the timeout can be fairly short), or more than a reasonable time in the future (so that cookies never accidentally become permanent, if the clock was set far into the future at some point). If no recent keys remain, the server may generate a new key.
The pruned and possibly added-to cookie file must be resaved atomically (using a temporary file which is rename()'d).
The lock must be dropped by deleting the lockfile.
Clients need not lock the file in order to load it, because servers are required to save the file atomically.
Server addresses consist of a transport name followed by a colon, and then an optional, comma-separated list of keys and values in the form key=value. [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
For example:
unix:path=/tmp/dbus-test |
[FIXME clarify if attempting to connect to each is a requirement or just a suggestion] When connecting to a server, multiple server addresses can be separated by a semi-colon. The library will then try to connect to the first address and if that fails, it'll try to connect to the next one specified, and so forth. For example
unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2 |
[FIXME we need to specify in detail each transport and its possible arguments] Current transports include: unix domain sockets (including abstract namespace on linux), TCP/IP, and a debug/testing transport using in-process pipes. Future possible transports include one that tunnels over X11 protocol.
This section documents conventions that are not essential to D-BUS functionality, but should generally be followed in order to simplify programmer's lives.
Messages are normally named in the form "org.freedesktop.Peer.Ping", which has three distinct components:
Message names have a Java-style namespace: a reversed domain name. The components of the domain are normally lowercase.
The next part of the message name can be thought of as the name of a singleton object, or as the name of a package of related messages. More than one dot-separated component might be used here. (Note that D-BUS does not define any idea of object instances or object references.) The package or object name is capitalized LikeThis.
The final part of the message name is the most specific, and should be a verb indicating an operation to be performed on the object. The method or operation name is capitalized LikeThis.
A reply to a message conventionally has the same name as the message being replied to. When following method call conventions (see the Section called Method Call Mapping), this convention is mandatory, because a message with multiple possible replies can't be mapped to method call semantics without special-case code.
Some implementations of D-BUS may present an API that translates object method calls into D-BUS messages. This document does not specify in detail how such an API should look or work. However, it does specify how message-based protocols should be designed to be friendly to such an API.
Remember that D-BUS does not have object references or object instances. So when one application sends the message org.freedesktop.Peer.Ping, it sends it to another application, not to any kind of sub-portion of that application. However, a convenience API used within the recipient application may route all messages that start with org.freedesktop.Peer to a particular object instance, and may invoke the Ping() method on said instance in order to handle the message. This is a convenience API based on method calls.
A "method call" consists of a message and, optionally, a reply to that message. The name of the "method" is the last component of the message, for example, org.freedesktop.Peer.Ping would map to the method Ping() on some object.
Arguments to a method may be considered "in" (processed by the recipient of the message), or "out" (returned to the sender of the message in the reply). "inout" arguments are both sent and received, i.e. the caller passes in a value which is modified. An "inout" argument is equivalent to an "in" argument, followed by an "out" argument.
Given a method with zero or one return values, followed by zero or more arguments, where each argument may be "in", "out", or "inout", the caller constructs a message by appending each "in" or "inout" argument, in order. "out" arguments are not represented in the caller's message.
The recipient constructs a reply by appending first the return value if any, then each "out" or "inout" argument, in order. "in" arguments are not represented in the reply message.
The standard reply message MUST have the same name as the message being replied to, and MUST set the "rply" header field to the serial number of the message being replied to.
If an error occurs, an error reply may be sent in place of the standard reply. Error replies can be identified by a special header flag, see the Section called Header Encoding. Error replies have a name which reflects the type of error that occurred. Error replies would generally be mapped to exceptions in a programming language. If an error reply has a first argument, and that argument has type STRING, then the argument must be an error message.
[FIXME discuss mapping of broadcast messages + matching rules to signals and slots]
In the following message definitions, "method call notation" is presented in addition to simply listing the message names and arguments. The special type name ANY means any type other than NIL, and the special type name ANY_OR_NIL means any valid type. [FIXME the messages here are just made up to illustrate the format for defining them]
As a method:
void Ping ()
|
On receipt of the message org.freedesktop.Peer.Ping, an application should reply with org.freedesktop.Peer.Ping. Neither the message nor its reply have any arguments. [FIXME the messages here are just made up to illustrate the format for defining them]
The message bus accepts connections from one or more applications. Once connected, applications can send and receive messages from the message bus, as in the peer-to-peer case.
The message bus keeps track of a set of services. A service is simply a name, such as com.yoyodyne.Screensaver, which can be owned by one or more of the connected applications. The message bus itself always owns the special service org.freedesktop.DBus.
Services may have secondary owners. Secondary owners of a service are kept in a queue; if the primary owner of a service disconnects, or releases the service, the next secondary owner becomes the new owner of the service.
Messages may have a srvc field (see the Section called Header Fields). When the message bus receives a message, if the srvc field is absent, the message is taken to be a standard peer-to-peer message and interpreted by the message bus itself. For example, sending an org.freedesktop.Peer.Ping message with no srvc will cause the message bus itself to reply to the ping immediately; the message bus would never make this message visible to other applications.
If the srvc field is present, then it indicates a request for the message bus to route the message. In the usual case, messages are routed to the owner of the named service. Messages may also be broadcast by sending them to the special service org.freedesktop.DBus.Broadcast. Broadcast messages are sent to all applications with message matching rules that match the message.
Continuing the org.freedesktop.Peer.Ping example, if the ping message were sent with a srvc name of com.yoyodyne.Screensaver, then the ping would be forwarded, and the Yoyodyne Corporation screensaver application would be expected to reply to the ping. If org.freedesktop.Peer.Ping were sent to org.freedesktop.DBus.Broadcast, then multiple applications might receive the ping, and all would normally reply to it.
A service is a name that identifies a certain application. Each application connected to the message bus has at least one service name assigned at connection time and returned in response to the org.freedesktop.DBus.Hello message. This automatically-assigned service name is called the application's base service. Base service names are unique and MUST never be reused for two different applications.
Ownership of the base service is a prerequisite for interaction with the message bus. It logically follows that the base service is always the first service that an application comes to own, and the last service that it loses ownership of.
Base service names must begin with the character ':' (ASCII colon character); service names that are not base service names must not begin with this character. (The bus must reject any attempt by an application to manually create a service name beginning with ':'.) This restriction categorically prevents "spoofing"; messages sent to a base service name will always go to a single application instance and that instance only.
An application can request additional service names to be associated with it using the org.freedesktop.DBus.AcquireService message. [FIXME what service names are allowed; ASCII or unicode; length limit; etc.]
[FIXME this needs more detail, and should move the service-related message descriptions up into this section perhaps] Service ownership handling can be specified in the flags part of the org.freedesktop.DBus.AcquireService message. If an application specifies the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications trying to acquire the service will be put in a queue. When the primary owner disconnects from the bus or removes ownership from the service, the next application in the queue will be the primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag is not specified, then the primary owner will lose ownership whenever another application requests ownership of the service.
When a client disconnects from the bus, all the services that the clients own are deleted, or in the case of a service that prohibits replacement, ownership is transferred to the next client in the queue, if any.
When a message is received by the message bus, the message's sndr header field MUST be set to the base service of the application which sent the message. If the service already has a sndr field, the pre-existing field is replaced. This rule means that a replies are always sent to the base service name, i.e. to the same application that sent the message being replied to.
[FIXME go into detail about broadcast, multicast, unicast, etc.]
Activation means to locate a service owner for a service that is currently unowned. For now, it means to launch an executable that will take ownership of a particular service.
To find an executable corresponding to a particular service, the bus daemon looks for service description files. Service description files define a mapping from service names to executables. Different kinds of message bus will look for these files in different places, see the Section called Well-known Message Bus Instances.
[FIXME the file format should be much better specified than "similar to .desktop entries" esp. since desktop entries are already badly-specified. ;-)] Service description files have the ".service" file extension. The message bus will only load service description files ending with .service; all other files will be ignored. The file format is similar to that of desktop entries. All service description files must be in UTF-8 encoding. To ensure that there will be no name collisions, service files must be namespaced using the same mechanism as messages and service names.
# Sample service description file
[D-BUS Service]
Name=org.gnome.ConfigurationDatabase
Exec=/usr/libexec/gconfd-2
|
Figure 7. Example service description file
When an application requests a service to be activated, the bus daemon tries to find it in the list of activation entries. It then tries to spawn the executable associated with it. If this fails, it will report an error. [FIXME what happens if two .service files offer the same service; what kind of error is reported, should we have a way for the client to choose one?]
The executable launched will have the environment variable DBUS_ACTIVATION_ADDRESS set to the address of the message bus so it can connect and register the appropriate services.
The executable being launched may want to know whether the message bus activating it is one of the well-known message buses (see the Section called Well-known Message Bus Instances). To facilitate this, the bus MUST also set the DBUS_ACTIVATION_BUS_TYPE environment variable if it is one of the well-known buses. The currently-defined values for this variable are system for the systemwide message bus, and session for the per-login-session message bus. The activated executable must still connect to the address given in DBUS_ACTIVATION_ADDRESS, but may assume that the resulting connection is to the well-known bus.
[FIXME there should be a timeout somewhere, either specified in the .service file, by the client, or just a global value and if the client being activated fails to connect within that timeout, an error should be sent back.]
Two standard message bus instances are defined here, along with how to locate them and where their service files live.
Each time a user logs in, a login session message bus may be started. All applications in the user's login session may interact with one another using this message bus.
The address of the login session message bus is given in the DBUS_SESSION_BUS_ADDRESS environment variable. If that variable is not set, applications may also try to read the address from the X Window System root window property _DBUS_SESSION_BUS_ADDRESS. The root window property must have type STRING. The environment variable should have precedence over the root window property.
[FIXME specify location of .service files, probably using DESKTOP_DIRS etc. from basedir specification, though login session bus is not really desktop-specific]
A computer may have a system message bus, accessible to all applications on the system. This message bus may be used to broadcast system events, such as adding new hardware devices, changes in the printer queue, and so forth.
The address of the login session message bus is given in the DBUS_SYSTEM_BUS_ADDRESS environment variable. If that variable is not set, applications should try to connect to the well-known address unix:path=/var/run/dbus/system_bus_socket. [2]
[FIXME specify location of system bus .service files]
The special message bus service org.freedesktop.DBus responds to a number of messages, allowing applications to interact with the message bus.
As a method:
STRING Hello ()
|
Before an application is able to send messages to other applications it must send the org.freedesktop.DBus.Hello message to the message bus service. If an application tries to send a message to another application, or a message to the message bus service that isn't the org.freedesktop.DBus.Hello message, it will be disconnected from the bus. If a client wishes to disconnect from the bus, it just has to disconnect from the transport used. No de-registration message is necessary.
The reply message contains the name of the application's base service.
As a method:
STRING_ARRAY ListServices ()
|
Returns a list of all existing services registered with the message bus.
As a method:
BOOLEAN ServiceExists (in STRING service_name)
|
Checks if a service with a specified name exists.
As a method:
UINT32 AcquireService (in STRING service_name)
|
Tries to become owner of a specific service. The flags specified can be the following values logically ORed together:
| Identifier | Value | Description |
|---|---|---|
| DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT | 0x1 | If the application succeeds in being the owner of the specified service, then ownership of the service can't be transferred until the service disconnects. If this flag is not set, then any application trying to become the owner of the service will succeed and the previous owner will be sent a org.freedesktop.DBus.ServiceLost message. |
| DBUS_SERVICE_FLAGS_REPLACE_EXISTING | 0x2 | Try to replace the current owner if there is one. If this flag is not set the application will only become the owner of the service if there is no current owner. |
| Identifier | Value | Description |
|---|---|---|
| DBUS_SERVICE_REPLY_PRIMARY_OWNER | 0x1 | The application is now the primary owner of the service. |
| DBUS_SERVICE_REPLY_IN_QUEUE | 0x2 | The service already has an owner which do not want to give up ownership and therefore the application has been put in a queue. |
| DBUS_SERVICE_REPLY_SERVICE_EXISTS | 0x4 | The service does already have a primary owner, and DBUS_SERVICE_FLAG_REPLACE_EXISTING was not specified when trying to acquire the service. |
| DBUS_SERVICE_REPLY_ALREADY_OWNER | 0x8 | The application trying to request ownership of the service is already the owner of it. |
As a method:
ServiceAcquired (in STRING service_name)
|
This message is sent to a specific application when it becomes the primary owner of a service.
As a method:
ServiceLost (in STRING service_name)
|
This message is sent to a specific application when it loses primary ownership of a service. [FIXME instead of ServiceLost/ServiceCreated going only to a specific app, why not just OwnerChanged that covers both lost and created and changed owner and deleted]
As a method:
ServiceCreated (in STRING service_name)
|
This message is broadcast to all applications when a service has been successfully registered on the message bus.
As a method:
ServiceDeleted (in STRING service_name)
|
This message is broadcast to all applications when a service has been deleted from the message bus.
As a method:
UINT32 ActivateService (in STRING service_name, in UINT32 flags)
|
The return value can be one of the following values:
This glossary defines some of the terms used in this specification.
The process of creating an owner for a particular service, typically by launching an executable.
The special service automatically assigned to an application by the message bus. This service may never change owner, and the service name will be unique (never reused during the lifetime of the message bus).
A message sent to the special org.freedesktop.DBus.Broadcast service; the message bus will forward the broadcast message to all applications that have expressed interest in it.
A message is the atomic unit of communication via the D-BUS protocol. It consists of a header and a body; the body is made up of arguments.
The message bus is a special application that forwards or broadcasts messages between a group of applications connected to the message bus. It also manages services.
Used to prevent collisions when defining message and service names. The convention used is the same as Java uses for defining classes: a reversed domain name.
An application talking directly to another application, without going through a message bus.
Each service has a primary owner; messages sent to the service name go to the primary owner. However, certain services also maintain a queue of secondary owners "waiting in the wings." If the primary owner releases the service, then the first secondary owner in the queue automatically becomes the primary owner.
A service is simply a named list of applications. For example, the hypothetical com.yoyodyne.Screensaver service might accept messages that affect a screensaver from Yoyodyne Corporation. An application is said to own a service if the message bus has associated the application with the service name. Services may also have secondary owners (see Secondary service owner).
The name used when referring to a service. If the service is a base service it has a unique service name, for example ":1-20", and otherwise it should be namespaced.
".service files" tell the bus how to activate a particular service. See Activation
| [1] | Lockfiles are used instead of real file locking fcntl() because real locking implementations are still flaky on network filesystems. |
| [2] | The D-BUS reference implementation actually honors the $(localstatedir) configure option for this address, on both client and server side. |