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 is 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.
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, a signed 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 integer wraparound. Serial numbers must be greater than zero. |
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, 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 | INT32 | 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 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, followed by the value of the argument in a type-dependent format.
The type codes are as follows:
| 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 |
| INT32 | 2 | 32-bit signed integer |
| UINT32 | 3 | 32-bit unsigned integer |
| DOUBLE | 4 | IEEE 754 double |
| STRING | 5 | UTF-8 string (must be valid UTF-8) |
| INT32_ARRAY | 6 | Array of INT32 |
| UINT32_ARRAY | 7 | Array of UINT32 |
| DOUBLE_ARRAY | 8 | Array of DOUBLE |
| BYTE_ARRAY | 9 | Array of bytes |
| STRING_ARRAY | 10 | Array of STRING |
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. |
| 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. |
| 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. |
| INT32_ARRAY | UINT32 giving the number of values in the array, followed by the given number of INT32 values. |
| UINT32_ARRAY | UINT32 giving the number of values in the array, followed by the given number of UINT32 values. |
| DOUBLE_ARRAY | UINT32 giving the number of values in the array, followed by the given number of DOUBLE values aligned to 8-byte boundary. |
| BYTE_ARRAY | UINT32 giving the number of values in the array, followed by the given number of one-byte values. |
| STRING_ARRAY | UINT32 giving the number of values in the array, followed by the given number of STRING values. |
Before the flow of messages begins, two applications must authenticate. A simple text protocol is used for authentication; this protocol is a SASL profile, and maps fairly directly from the SASL specification.
[move the dbus-sasl-profile.txt stuff into here and clean it up]
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.
For example:
unix:path=/tmp/dbus-test |
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 |
Currently, a transport over local UNIX sockets exists, a debug transport that only works in-process and therefore can be used for for unit testing also exists. It is possible that other transports are added, such as a TCP/IP transport, and a transport that works over X11.
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.
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 of the connected applications. The message bus itself always owns the special service org.freedesktop.DBus.
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.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.Broadcast, then multiple applications might receive the ping, and all would normally reply to it.
The special message bus service org.freedesktop.DBus responds to a number of messages, allowing applications to interact with the message bus.
[document the messages here]
Two standard message bus instances are defined here, along with how to locate them.
Each time a user logs in, a desktop session message bus may be started. All applications in the user's login session may interact with one another using this message bus. [specify how to find the address of the desktop session message bus via environment variable and/or X property]
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. [specify how to find the address of the system message bus]
This glossary defines some of the terms used in this specification.
A message sent to the special org.freedesktop.Broadcast service; the message bus will forward the broadcast message to all clients 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.
A service is simply a named application that other applications can refer to. 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.