# D-Bus Specification

Version 0.12
7 November 2006
Havoc Pennington
Red Hat, Inc.

    	    hp@pobox.com

Anders Carlsson
CodeFactory AB

                andersca@codefactory.se

Alexander Larsson
Red Hat, Inc.

                alexl@redhat.com

## Introduction

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 automatically handles a lot of the hard IPC issues. Also, the D-Bus library is designed to be wrapped in a way that lets developers use their framework's existing object/type system, rather than learning a new one specifically for IPC.

The base D-Bus protocol is a one-to-one (peer-to-peer or client-server) protocol, specified in . That is, it is a system for one application to talk to a single other application. However, the primary intended application of the protocol is the D-Bus message bus, specified in . The message bus is a special application that accepts connections from multiple other applications, and forwards messages among them.

Uses of D-Bus include 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 interoperability, for example a file monitoring service or a configuration service.

D-Bus is designed for two specific use cases:

- A "system bus" for notifications from the system to user sessions, and to allow the system to request input from user sessions.

- A "session bus" used to implement desktop environments such as GNOME and KDE.

D-Bus is not intended to be a generic IPC system for any possible application, and intentionally omits many features found in other IPC systems for this reason.

At the same time, the bus daemons offer a number of features not found in other IPC systems, such as single-owner "bus names" (similar to X selections), on-demand startup of services, and security policies. In many ways, these features are the primary motivation for developing D-Bus; other systems would have sufficed if IPC were the only goal.

D-Bus may turn out to be useful in unanticipated applications, but future versions of this spec and the reference implementation probably will not incorporate features that interfere with the core use cases.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. However, the document could use a serious audit to be sure it makes sense to do so. Also, they are not capitalized.

### Protocol and Specification Stability

The D-Bus protocol is frozen (only compatible extensions are allowed) as of November 8, 2006. However, this specification could still use a fair bit of work to make interoperable reimplementation possible without reference to the D-Bus reference implementation. Thus, this specification is not marked 1.0. To mark it 1.0, we'd like to see someone invest significant effort in clarifying the specification language, and growing the specification to cover more aspects of the reference implementation's behavior.

Until this work is complete, any attempt to reimplement D-Bus will probably require looking at the reference implementation and/or asking questions on the D-Bus mailing list about intended behavior. Questions on the list are very welcome.

Nonetheless, this document should be a useful starting point and is to our knowledge accurate, though incomplete.

## Message Protocol

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 interprets 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.

Both header and body use the same type system and format for serializing data. Each type of value has a wire format. Converting a value from some other representation into the wire format is called marshaling and converting it back from the wire format is unmarshaling.

### Type Signatures

The D-Bus protocol does not include type tags in the marshaled data; a block of marshaled values must have a known type signature. The type signature is made up of type codes. A type code is an ASCII character representing the type of a value. Because ASCII characters are used, the type signature will always form a valid ASCII string. A simple string compare determines whether two type signatures are equivalent.

As a simple example, the type code for 32-bit integer (`INT32`) is the ASCII character 'i'. So the signature for a block of values containing a single `INT32` would be:

              "i"

A block of values containing two `INT32` would have this signature:

              "ii"

All basic types work like `INT32` in this example. To marshal and unmarshal basic types, you simply read one value from the data block corresponding to each type code in the signature. In addition to basic types, there are four container types: `STRUCT`, `ARRAY`, `VARIANT`, and `DICT_ENTRY`.

`STRUCT` has a type code, ASCII character 'r', but this type code does not appear in signatures. Instead, ASCII characters '(' and ')' are used to mark the beginning and end of the struct. So for example, a struct containing two integers would have this signature:

              "(ii)"

Structs can be nested, so for example a struct containing an integer and another struct:

              "(i(ii))"

The value block storing that struct would contain three integers; the type signature allows you to distinguish "(i(ii))" from "((ii)i)" or "(iii)" or "iii".

The `STRUCT` type code 'r' is not currently used in the D-Bus protocol, but is useful in code that implements the protocol. This type code is specified to allow such code to interoperate in non-protocol contexts.

Empty structures are not allowed; there must be at least one type code between the parentheses.

`ARRAY` has ASCII character 'a' as type code. The array type code must be followed by a single complete type. The single complete type following the array is the type of each array element. So the simple example is:

              "ai"

which is an array of 32-bit integers. But an array can be of any type, such as this array-of-struct-with-two-int32-fields:

              "a(ii)"

Or this array of array of integer:

              "aai"

The phrase single complete type deserves some definition. A single complete type is a basic type code, a variant type code, an array with its element type, or a struct with its fields. So the following signatures are not single complete types:

              "aa"

              "(ii"

              "ii)"

And the following signatures contain multiple complete types:

              "ii"

              "aiai"

              "(ii)(ii)"

Note however that a single complete type may contain multiple other single complete types.

`VARIANT` has ASCII character 'v' as its type code. A marshaled value of type `VARIANT` will have the signature of a single complete type as part of the value. This signature will be followed by a marshaled value of that type.

A `DICT_ENTRY` works exactly like a struct, but rather than parentheses it uses curly braces, and it has more restrictions. The restrictions are: it occurs only as an array element type; it has exactly two single complete types inside the curly braces; the first single complete type (the "key") must be a basic type rather than a container type. Implementations must not accept dict entries outside of arrays, must not accept dict entries with zero, one, or more than two fields, and must not accept dict entries with non-basic-typed keys. A dict entry is always a key-value pair.

The first field in the `DICT_ENTRY` is always the key. A message is considered corrupt if the same key occurs twice in the same array of `DICT_ENTRY`. However, for performance reasons implementations are not required to reject dicts with duplicate keys.

In most languages, an array of dict entry would be represented as a map, hash table, or dict object.

The following table summarizes the D-Bus types.

Conventional Name | Code | Description
--- | --- | ---
`INVALID` | 0 (ASCII NUL) | Not a valid type code, used to terminate signatures
`BYTE` | 121 (ASCII 'y') | 8-bit unsigned integer
`BOOLEAN` | 98 (ASCII 'b') | Boolean value, 0 is `FALSE` and 1 is `TRUE`. Everything else is invalid.
`INT16` | 110 (ASCII 'n') | 16-bit signed integer
`UINT16` | 113 (ASCII 'q') | 16-bit unsigned integer
`INT32` | 105 (ASCII 'i') | 32-bit signed integer
`UINT32` | 117 (ASCII 'u') | 32-bit unsigned integer
`INT64` | 120 (ASCII 'x') | 64-bit signed integer
`UINT64` | 116 (ASCII 't') | 64-bit unsigned integer
`DOUBLE` | 100 (ASCII 'd') | IEEE 754 double
`STRING` | 115 (ASCII 's') | UTF-8 string (must be valid UTF-8). Must be nul terminated and contain no other nul bytes.
`OBJECT_PATH` | 111 (ASCII 'o') | Name of an object instance
`SIGNATURE` | 103 (ASCII 'g') | A type signature
`ARRAY` | 97 (ASCII 'a') | Array
`STRUCT` | 114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')') | Struct
`VARIANT` | 118 (ASCII 'v') | Variant type (the type of the value is part of the value itself)
`DICT_ENTRY` | 101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') | Entry in a dict or map (array of key-value pairs)

### Marshaling (Wire Format)

Given a type signature, a block of bytes can be converted into typed values. This section describes the format of the block of bytes. Byte order and alignment issues are handled uniformly for all D-Bus types.

A block of bytes has an associated byte order. The byte order has to be discovered in some way; for D-Bus messages, the byte order is part of the message header as described in . For now, assume that the byte order is known to be either little endian or big endian.

Each value in a block of bytes is aligned "naturally," for example 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an 8-byte boundary. To properly align a value, alignment padding may be necessary. The alignment padding must always be the minimum required padding to properly align the following value; and it must always be made up of nul bytes. The alignment padding must not be left uninitialized (it can't contain garbage), and more padding than required must not be used.

Given all this, the types are marshaled on the wire as follows:

Conventional Name | Encoding | Alignment
--- | --- | ---
`INVALID` | Not applicable; cannot be marshaled. | N/A
`BYTE` | A single 8-bit byte. | 1
`BOOLEAN` | As for `UINT32`, but only 0 and 1 are valid values. | 4
`INT16` | 16-bit signed integer in the message's byte order. | 2
`UINT16` | 16-bit unsigned integer in the message's byte order. | 2
`INT32` | 32-bit signed integer in the message's byte order. | 4
`UINT32` | 32-bit unsigned integer in the message's byte order. | 4
`INT64` | 64-bit signed integer in the message's byte order. | 8
`UINT64` | 64-bit unsigned integer in the message's byte order. | 8
`DOUBLE` | 64-bit IEEE 754 double in the message's byte order. | 8
`STRING` | A `UINT32` indicating the string's length in bytes excluding its terminating nul, followed by non-nul string data of the given length, followed by a terminating nul byte. | 4 (for the length)
`OBJECT_PATH` | Exactly the same as `STRING` except the content must be a valid object path (see below). | 4 (for the length)
`SIGNATURE` | The same as `STRING` except the length is a single byte (thus signatures have a maximum length of 255) and the content must be a valid signature (see below). | 1
`ARRAY` | A `UINT32` giving the length of the array data in bytes, followed by alignment padding to the alignment boundary of the array element type, followed by each array element. The array length is from the end of the alignment padding to the end of the last element, i.e. it does not include the padding after the length, or any padding after the last element. Arrays have a maximum length defined to be 2 to the 26th power or 67108864. Implementations must not send or accept arrays exceeding this length. | 4 (for the length)
`STRUCT` | A struct must start on an 8-byte boundary regardless of the type of the struct fields. The struct value consists of each field marshaled in sequence starting from that 8-byte alignment boundary. | 8
`VARIANT` | A variant type has a marshaled `SIGNATURE` followed by a marshaled value with the type given in the signature. Unlike a message signature, the variant signature can contain only a single complete type. So "i", "ai" or "(ii)" is OK, but "ii" is not. Use of variants may not cause a total message depth to be larger than 64, including other container types such as structures. | 1 (alignment of the signature)
`DICT_ENTRY` | Identical to STRUCT. | 8

#### Valid Object Paths

An object path is a name used to refer to an object instance. Conceptually, each participant in a D-Bus message exchange may have any number of object instances (think of C++ or Java objects) and each such instance will have a path. Like a filesystem, the object instances in an application form a hierarchical tree.

The following rules define a valid object path. Implementations must not send or accept messages with invalid object paths.

- The path may be of any length.

- The path must begin with an ASCII '/' (integer 47) character, and must consist of elements separated by slash characters.

- Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_"

- No element may be the empty string.

- Multiple '/' characters cannot occur in sequence.

- A trailing '/' character is not allowed unless the path is the root path (a single '/' character).

#### Valid Signatures

An implementation must not send or accept invalid signatures. Valid signatures will conform to the following rules:

- The signature ends with a nul byte.

- The signature is a list of single complete types. Arrays must have element types, and structs must have both open and close parentheses.

- Only type codes and open and close parentheses are allowed in the signature. The `STRUCT` type code is not allowed in signatures, because parentheses are used instead.

- The maximum depth of container type nesting is 32 array type codes and 32 open parentheses. This implies that the maximum total depth of recursion is 64, for an "array of array of array of ... struct of struct of struct of ..." where there are 32 array and 32 struct.

- The maximum length of a signature is 255.

- Signatures must be nul-terminated.

### Message Format

A message consists of a header and a body. The header is a block of values with a fixed signature and meaning. The body is a separate block of values, with a signature specified in the header.

The length of the header must be a multiple of 8, allowing the body to begin on an 8-byte boundary when storing the entire message in a single buffer. If the header does not naturally end on an 8-byte boundary up to 7 bytes of nul-initialized alignment padding must be added.

The message body need not end on an 8-byte boundary.

The maximum length of a message, including header, header alignment padding, and body is 2 to the 27th power or 134217728. Implementations must not send or accept messages exceeding this size.

The signature of the header is:

              "yyyyuua(yv)"

Written out more readably, this is:

              BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)

These values have the following meanings:

Value | Description
--- | ---
1st `BYTE` | Endianness flag; ASCII 'l' for little-endian or ASCII 'B' for big-endian. Both header and body are in this endianness.
2nd `BYTE` | Message type. Unknown types must be ignored. Currently-defined types are described below.
3rd `BYTE` | Bitwise OR of flags. Unknown flags must be ignored. Currently-defined flags are described below.
4th `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 1.
1st `UINT32` | Length in bytes of the message body, starting from the end of the header. The header ends after its alignment padding to an 8-boundary.
2nd `UINT32` | The serial of this message, used as a cookie by the sender to identify the reply corresponding to this request. This must not be zero.
`ARRAY` of `STRUCT` of (`BYTE`,`VARIANT`) | An array of zero or more header fields where the byte is the field code, and the variant is the field value. The message type determines which fields are required.

Message types that can appear in the second byte of the header are:

Conventional name | Decimal value | Description
--- | --- | ---
`INVALID` | 0 | This is an invalid type.
`METHOD_CALL` | 1 | Method call.
`METHOD_RETURN` | 2 | Method reply with returned data.
`ERROR` | 3 | Error reply. If the first argument exists and is a string, it is an error message.
`SIGNAL` | 4 | Signal emission.

Flags that can appear in the third byte of the header:

Conventional name | Hex value | Description
--- | --- | ---
`NO_REPLY_EXPECTED` | 0x1 | This message does not expect method return replies or error replies; the reply can be omitted as an optimization. However, it is compliant with this specification to return the reply despite this flag and the only harm from doing so is extra network traffic.
`NO_AUTO_START` | 0x2 | The bus must not launch an owner for the destination name in response to this message.

#### Header Fields

The array at the end of the header contains header fields, where each field is a 1-byte field code followed by a field value. A header must contain the required header fields for its message type, and zero or more of any optional header fields. Future versions of this protocol specification may 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.

Again, if an implementation sees a header field code that it does not expect, it must ignore that field, as it will be part of a new (but compatible) version of this specification. This also applies to known header fields appearing in unexpected messages, for example: if a signal has a reply serial it must be ignored even though it has no meaning as of this version of the spec.

However, implementations must not send or accept known header fields with the wrong type stored in the field value. So for example a message with an `INTERFACE` field of type `UINT32` would be considered corrupt.

Here are the currently-defined header fields:

Conventional Name | Decimal Code | Type | Required In | Description
--- | --- | --- | --- | ---
`INVALID` | 0 | N/A | not allowed | Not a valid field name (error if it appears in a message)
`PATH` | 1 | `OBJECT_PATH` | `METHOD_CALL`, `SIGNAL` | The object to send a call to, or the object a signal is emitted from. The special path `/org/freedesktop/DBus/Local` is reserved; implementations should not send messages with this path, and the reference implementation of the bus daemon will disconnect any application that attempts to do so.
`INTERFACE` | 2 | `STRING` | `SIGNAL` | The interface to invoke a method call on, or that a signal is emitted from. Optional for method calls, required for signals. The special interface `org.freedesktop.DBus.Local` is reserved; implementations should not send messages with this interface, and the reference implementation of the bus daemon will disconnect any application that attempts to do so.
`MEMBER` | 3 | `STRING` | `METHOD_CALL`, `SIGNAL` | The member, either the method name or signal name.
`ERROR_NAME` | 4 | `STRING` | `ERROR` | The name of the error that occurred, for errors
`REPLY_SERIAL` | 5 | `UINT32` | `ERROR`, `METHOD_RETURN` | The serial number of the message this message is a reply to. (The serial number is the second `UINT32` in the header.)
`DESTINATION` | 6 | `STRING` | optional | The name of the connection this message is intended for. Only used in combination with the message bus, see .
`SENDER` | 7 | `STRING` | optional | Unique name of the sending connection. The message bus fills in this field so it is reliable; the field is only meaningful in combination with the message bus.
`SIGNATURE` | 8 | `SIGNATURE` | optional | The signature of the message body. If omitted, it is assumed to be the empty signature "" (i.e. the body must be 0-length).

### Valid Names

The various names in D-Bus messages have some restrictions.

There is a maximum name length of 255 which applies to bus names, interfaces, and members.

#### Interface names

Interfaces have names with type `STRING`, meaning that they must be valid UTF-8. However, there are also some additional restrictions that apply to interface names specifically:

- Interface names are composed of 1 or more elements separated by a period ('.') character. All elements must contain at least one character.

- Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_" and must not begin with a digit.

- Interface names must contain at least one '.' (period) character (and thus at least two elements).

- Interface names must not begin with a '.' (period) character.

- Interface names must not exceed the maximum name length.

#### Bus names

Connections have one or more bus names associated with them. A connection has exactly one bus name that is a unique connection name. The unique connection name remains with the connection for its entire lifetime. A bus name is of type `STRING`, meaning that it must be valid UTF-8. However, there are also some additional restrictions that apply to bus names specifically:

- Bus names that start with a colon (':') character are unique connection names.

- Bus names are composed of 1 or more elements separated by a period ('.') character. All elements must contain at least one character.

- Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_-". Only elements that are part of a unique connection name may begin with a digit, elements in other bus names must not begin with a digit.

- Bus names must contain at least one '.' (period) character (and thus at least two elements).

- Bus names must not begin with a '.' (period) character.

- Bus names must not exceed the maximum name length.

Note that the hyphen ('-') character is allowed in bus names but not in interface names.

#### Member names

Member (i.e. method or signal) names:

- Must only contain the ASCII characters "[A-Z][a-z][0-9]_" and may not begin with a digit.

- Must not contain the '.' (period) character.

- Must not exceed the maximum name length.

- Must be at least 1 byte in length.

#### Error names

Error names have the same restrictions as interface names.

### Message Types

Each of the message types (`METHOD_CALL`, `METHOD_RETURN`, `ERROR`, and `SIGNAL`) has its own expected usage conventions and header fields. This section describes these conventions.

#### Method Calls

Some messages invoke an operation on a remote object. These are called method call messages and have the type tag `METHOD_CALL`. Such messages map naturally to methods on objects in a typical program.

A method call message is required to have a `MEMBER` header field indicating the name of the method. Optionally, the message has an `INTERFACE` field giving the interface the method is a part of. In the absence of an `INTERFACE` field, if two interfaces on the same object have a method with the same name, it is undefined which of the two methods will be invoked. Implementations may also choose to return an error in this ambiguous case. However, if a method name is unique implementations must not require an interface field.

Method call messages also include a `PATH` field indicating the object to invoke the method on. If the call is passing through a message bus, the message will also have a `DESTINATION` field giving the name of the connection to receive the message.

When an application handles a method call message, it is required to return a reply. The reply is identified by a `REPLY_SERIAL` header field indicating the serial number of the `METHOD_CALL` being replied to. The reply can have one of two types; either `METHOD_RETURN` or `ERROR`.

If the reply has type `METHOD_RETURN`, the arguments to the reply message are the return value(s) or "out parameters" of the method call. If the reply has type `ERROR`, then an "exception" has been thrown, and the call fails; no return value will be provided. It makes no sense to send multiple replies to the same method call.

Even if a method call has no return values, a `METHOD_RETURN` reply is required, so the caller will know the method was successfully processed.

The `METHOD_RETURN` or `ERROR` reply message must have the `REPLY_SERIAL` header field.

If a `METHOD_CALL` message has the flag `NO_REPLY_EXPECTED`, then as an optimization the application receiving the method call may choose to omit the reply message (regardless of whether the reply would have been `METHOD_RETURN` or `ERROR`). However, it is also acceptable to ignore the `NO_REPLY_EXPECTED` flag and reply anyway.

Unless a message has the flag `NO_AUTO_START`, if the destination name does not exist then a program to own the destination name will be started before the message is delivered. The message will be held until the new program is successfully started or has failed to start; in case of failure, an error will be returned. This flag is only relevant in the context of a message bus, it is ignored during one-to-one communication with no intermediate bus.

##### Mapping method calls to native APIs

APIs for D-Bus may map method calls to a method call in a specific programming language, such as C++, or may map a method call written in an IDL to a D-Bus message.

In APIs of this nature, arguments to a method are often termed "in" (which implies sent in the `METHOD_CALL`), or "out" (which implies returned in the `METHOD_RETURN`). Some APIs such as CORBA also have "inout" arguments, which are both sent and received, i.e. the caller passes in a value which is modified. Mapped to D-Bus, an "inout" argument is equivalent to an "in" argument, followed by an "out" argument. You can't pass things "by reference" over the wire, so "inout" is purely an illusion of the in-process API.

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.

Error replies are normally mapped to exceptions in languages that have exceptions.

In converting from native APIs to D-Bus, it is perhaps nice to map D-Bus naming conventions ("FooBar") to native conventions such as "fooBar" or "foo_bar" automatically. This is OK as long as you can say that the native API is one that was specifically written for D-Bus. It makes the most sense when writing object implementations that will be exported over the bus. Object proxies used to invoke remote D-Bus objects probably need the ability to call any D-Bus method, and thus a magic name mapping like this could be a problem.

This specification doesn't require anything of native API bindings; the preceding is only a suggested convention for consistency among bindings.

#### Signal Emission

Unlike method calls, signal emissions have no replies. A signal emission is simply a single message of type `SIGNAL`. It must have three header fields: `PATH` giving the object the signal was emitted from, plus `INTERFACE` and `MEMBER` giving the fully-qualified name of the signal. The `INTERFACE` header is required for signals, though it is optional for method calls.

#### Errors

Messages of type `ERROR` are most commonly replies to a `METHOD_CALL`, but may be returned in reply to any kind of message. The message bus for example will return an `ERROR` in reply to a signal emission if the bus does not have enough memory to send the signal.

An `ERROR` may have any arguments, but if the first argument is a `STRING`, it must be an error message. The error message may be logged or shown to the user in some way.

#### Notation in this document

This document uses a simple pseudo-IDL to describe particular method calls and signals. Here is an example of a method call:

                org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
                                                         out UINT32 resultcode)

This means `INTERFACE` = org.freedesktop.DBus, `MEMBER` = StartServiceByName, `METHOD_CALL` arguments are `STRING` and `UINT32`, `METHOD_RETURN` argument is `UINT32`. Remember that the `MEMBER` field can't contain any '.' (period) characters so it's known that the last part of the name in the "IDL" is the member name.

In C++ that might end up looking like this:

                unsigned int org::freedesktop::DBus::StartServiceByName (const char  *name,
                                                                         unsigned int flags);

or equally valid, the return value could be done as an argument:

                void org::freedesktop::DBus::StartServiceByName (const char   *name,
                                                                 unsigned int  flags,
                                                                 unsigned int *resultcode);

It's really up to the API designer how they want to make this look. You could design an API where the namespace wasn't used in C++, using STL or Qt, using varargs, or whatever you wanted.

Signals are written as follows:

                org.freedesktop.DBus.NameLost (STRING name)

Signals don't specify "in" vs. "out" because only a single direction is possible.

It isn't especially encouraged to use this lame pseudo-IDL in actual API implementations; you might use the native notation for the language you're using, or you might use COM or CORBA IDL, for example.

### Invalid Protocol and Spec Extensions

For security reasons, the D-Bus protocol should be strictly parsed and validated, with the exception of defined extension points. Any invalid protocol or spec violations should result in immediately dropping the connection without notice to the other end. Exceptions should be carefully considered, e.g. an exception may be warranted for a well-understood idiosyncrasy of a widely-deployed implementation. In cases where the other end of a connection is 100% trusted and known to be friendly, skipping validation for performance reasons could also make sense in certain cases.

Generally speaking violations of the "must" requirements in this spec should be considered possible attempts to exploit security, and violations of the "should" suggestions should be considered legitimate (though perhaps they should generate an error in some cases).

The following extension points are built in to D-Bus on purpose and must not be treated as invalid protocol. The extension points are intended for use by future versions of this spec, they are not intended for third parties. At the moment, the only way a third party could extend D-Bus without breaking interoperability would be to introduce a way to negotiate new feature support as part of the auth protocol, using EXTENSION_-prefixed commands. There is not yet a standard way to negotiate features.

- In the authentication protocol (see ) unknown commands result in an ERROR rather than a disconnect. This enables future extensions to the protocol. Commands starting with EXTENSION_ are reserved for third parties.

- The authentication protocol supports pluggable auth mechanisms.

- The address format (see ) supports new kinds of transport.

- Messages with an unknown type (something other than `METHOD_CALL`, `METHOD_RETURN`, `ERROR`, `SIGNAL`) are ignored. Unknown-type messages must still be well-formed in the same way as the known messages, however. They still have the normal header and body.

- Header fields with an unknown or unexpected field code must be ignored, though again they must still be well-formed.

- New standard interfaces (with new methods and signals) can of course be added.

## Authentication Protocol

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.

### Protocol Overview

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 hex encoding>

- ERROR [human-readable error explanation]

From server to client are as follows:

- REJECTED <space-separated list of mechanism names>

- OK <GUID in hex>

- DATA <data in hex encoding>

- ERROR

Unofficial extensions to the command set must begin with the letters "EXTENSION_", to avoid conflicts with future official commands. For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".

### Special credentials-passing nul byte

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.

### AUTH command

If an AUTH command has no arguments, it is a request to list available mechanisms. The server must respond with a REJECTED command listing the mechanisms it understands, or with an error.

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 must send either a REJECTED command listing the mechanisms it does support, or an error.

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 and [initial-response] was provided, the server should reject authentication by sending REJECTED.

If authentication succeeds after exchanging DATA commands, an OK command must 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.

### CANCEL Command

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.

### DATA Command

The DATA command may come from either client or server, and simply contains a hex-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.

### BEGIN Command

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.

### REJECTED Command

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.

### OK Command

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 OK command has one argument, which is the GUID of the server. See for more on server GUIDs.

### ERROR Command

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.

If the D-Bus protocol changes incompatibly at some future time, applications implementing the new protocol would probably be able to check for support of the new protocol by sending a new command and receiving an ERROR from applications that don't understand it. Thus the ERROR feature of the auth protocol is an escape hatch that lets us negotiate extensions or changes to the D-Bus protocol in the future.

### Authentication examples

Example of successful magic cookie authentication

                (MAGIC_COOKIE is a made up mechanism)

                C: AUTH MAGIC_COOKIE 3138363935333137393635383634
                S: OK 1234deadbeef
                C: BEGIN

Example of finding out mechanisms then picking one

                C: AUTH
                S: REJECTED KERBEROS_V4 SKEY
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
                S: OK 1234deadbeef
                C: BEGIN

Example of client sends unknown command then falls back to regular auth

                C: FOOBAR
                S: ERROR
                C: AUTH MAGIC_COOKIE 3736343435313230333039
                S: OK 1234deadbeef
                C: BEGIN

Example of server doesn't support initial auth mechanism

                C: AUTH MAGIC_COOKIE 3736343435313230333039
                S: REJECTED KERBEROS_V4 SKEY
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
                S: OK 1234deadbeef
                C: BEGIN

Example of wrong password or the like followed by successful retry

                C: AUTH MAGIC_COOKIE 3736343435313230333039
                S: REJECTED KERBEROS_V4 SKEY
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
                S: REJECTED
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
                S: OK 1234deadbeef
                C: BEGIN

Example of skey cancelled and restarted

                C: AUTH MAGIC_COOKIE 3736343435313230333039
                S: REJECTED KERBEROS_V4 SKEY
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: CANCEL
                S: REJECTED
                C: AUTH SKEY 7ab83f32ee
                S: DATA 8799cabb2ea93e
                C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
                S: OK 1234deadbeef
                C: BEGIN

### Authentication state diagrams

This section documents the auth protocol in terms of a state machine for the client and the server. This is probably the most robust way to implement the protocol.

#### Client states

To more precisely describe the interaction between the protocol state machine and the authentication mechanisms the following notation is used: MECH(CHALL) means that the server challenge CHALL was fed to the mechanism MECH, which returns one of

- CONTINUE(RESP) means continue the auth conversation and send RESP as the response to the server;

- OK(RESP) means that after sending RESP to the server the client side of the auth conversation is finished and the server should return "OK";

- ERROR means that CHALL was invalid and could not be processed.

Both RESP and CHALL may be empty.

The Client starts by getting an initial response from the default mechanism and sends AUTH MECH RESP, or AUTH MECH if the mechanism did not provide an initial response. If the mechanism returns CONTINUE, the client starts in state WaitingForData, if the mechanism returns OK the client starts in state WaitingForOK.

The client should keep track of available mechanisms and which it mechanisms it has already attempted. This list is used to decide which AUTH command to send. When the list is exhausted, the client should give up and close the connection.

WaitingForData

- Receive DATA CHALL

  - MECH(CHALL) returns CONTINUE(RESP) → send DATA RESP, goto WaitingForData
  - MECH(CHALL) returns OK(RESP) → send DATA RESP, goto WaitingForOK
  - MECH(CHALL) returns ERROR → send ERROR [msg], goto WaitingForData

- Receive REJECTED [mechs] → send AUTH [next mech], goto WaitingForData or WaitingForOK

- Receive ERROR → send CANCEL, goto WaitingForReject

- Receive OK → send BEGIN, terminate auth conversation, authenticated

- Receive anything else → send ERROR, goto WaitingForData

WaitingForOK

- Receive OK → send BEGIN, terminate auth conversation, authenticated

- Receive REJECT [mechs] → send AUTH [next mech], goto WaitingForData or WaitingForOK

- Receive DATA → send CANCEL, goto WaitingForReject

- Receive ERROR → send CANCEL, goto WaitingForReject

- Receive anything else → send ERROR, goto WaitingForOK

WaitingForReject

- Receive REJECT [mechs] → send AUTH [next mech], goto WaitingForData or WaitingForOK

- Receive anything else → terminate auth conversation, disconnect

#### Server states

For the server MECH(RESP) means that the client response RESP was fed to the the mechanism MECH, which returns one of

- CONTINUE(CHALL) means continue the auth conversation and send CHALL as the challenge to the client;

- OK means that the client has been successfully authenticated;

- REJECT means that the client failed to authenticate or there was an error in RESP.

The server starts out in state WaitingForAuth. If the client is rejected too many times the server must disconnect the client.

WaitingForAuth

- Receive AUTH → send REJECTED [mechs], goto WaitingForAuth

- Receive AUTH MECH RESP

  - MECH not valid mechanism → send REJECTED [mechs], goto WaitingForAuth
  - MECH(RESP) returns CONTINUE(CHALL) → send DATA CHALL, goto WaitingForData
  - MECH(RESP) returns OK → send OK, goto WaitingForBegin
  - MECH(RESP) returns REJECT → send REJECTED [mechs], goto WaitingForAuth

- Receive BEGIN → terminate auth conversation, disconnect

- Receive ERROR → send REJECTED [mechs], goto WaitingForAuth

- Receive anything else → send ERROR, goto WaitingForAuth

WaitingForData

- Receive DATA RESP

  - MECH(RESP) returns CONTINUE(CHALL) → send DATA CHALL, goto WaitingForData
  - MECH(RESP) returns OK → send OK, goto WaitingForBegin
  - MECH(RESP) returns REJECT → send REJECTED [mechs], goto WaitingForAuth

- Receive BEGIN → terminate auth conversation, disconnect

- Receive CANCEL → send REJECTED [mechs], goto WaitingForAuth

- Receive ERROR → send REJECTED [mechs], goto WaitingForAuth

- Receive anything else → send ERROR, goto WaitingForData

WaitingForBegin

- Receive BEGIN → terminate auth conversation, client authenticated

- Receive CANCEL → send REJECTED [mechs], goto WaitingForAuth

- Receive ERROR → send REJECTED [mechs], goto WaitingForAuth

- Receive anything else → send ERROR, goto WaitingForBegin

### Authentication mechanisms

This section describes some new authentication mechanisms. D-Bus also allows any standard SASL mechanism of course.

#### DBUS_COOKIE_SHA1

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.

Throughout this description, "hex encoding" must output the digits from a to f in lower-case; the digits A to F must not be used in the DBUS_COOKIE_SHA1 mechanism.

Authentication proceeds as follows:

- The client sends the username it would like to authenticate as, hex-encoded.

- 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 randomly-generated challenge string, all of this hex-encoded into one, single string.

- The client locates the cookie and generates its own randomly-generated challenge string. The client then concatenates the server's decoded challenge, a ":" character, its own challenge, another ":" character, and the cookie. It computes the SHA-1 hash of this composite string as a hex digest. It concatenates the client's challenge string, a space character, and the SHA-1 hex digest, hex-encodes the result and sends it back to the server.

- 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_general" 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. The cookie may be of any length, though obviously security increases as the length increases.

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.

  Lockfiles are used instead of real file locking `fcntl()` because real locking implementations are still flaky on network filesystems.

- 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

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. Each value is escaped.

For example:

    unix:path=/tmp/dbus-test

Which is the address to a unix socket with the path /tmp/dbus-test.

Value escaping is similar to URI escaping but simpler.

- The set of optionally-escaped bytes is: `[0-9A-Za-z_-/.\]`. To escape, each byte (note, not character) which is not in the set of optionally-escaped bytes must be replaced with an ASCII percent (`%`) and the value of the byte in hex. The hex value must always be two digits, even if the first digit is zero. The optionally-escaped bytes may be escaped if desired.

- To unescape, append each byte in the value; if a byte is an ASCII percent (`%`) character then append the following hex value instead. It is an error if a `%` byte does not have two hex digits following. It is an error if a non-optionally-escaped byte is seen unescaped.

The set of optionally-escaped bytes is intended to preserve address readability and convenience.

A server may specify a key-value pair with the key `guid` and the value a hex-encoded 16-byte sequence. describes the format of the `guid` field. If present, this UUID may be used to distinguish one server address from another. A server should use a different UUID for each address it listens on. For example, if a message bus daemon offers both UNIX domain socket and TCP connections, but treats clients the same regardless of how they connect, those two connections are equivalent post-connection but should have distinct UUIDs to distinguish the kinds of connection.

The intent of the address UUID feature is to allow a client to avoid opening multiple identical connections to the same server, by allowing the client to check whether an address corresponds to an already-existing connection. Comparing two addresses is insufficient, because addresses can be recycled by distinct servers, and equivalent addresses may look different if simply compared as strings (for example, the host in a TCP address can be given as an IP address or as a hostname).

Note that the address key is `guid` even though the rest of the API and documentation says "UUID," for historical reasons.

[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

## Transports

[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.

### Unix Domain Sockets

Unix domain sockets can be either paths in the file system or on Linux kernels, they can be abstract which are similar to paths but do not show up in the file system.

When a socket is opened by the D-Bus library it truncates the path name right before the first trailing Nul byte. This is true for both normal paths and abstract paths. Note that this is a departure from previous versions of D-Bus that would create sockets with a fixed length path name. Names which were shorter than the fixed length would be padded by Nul bytes.

## Naming Conventions

D-Bus namespaces are all lowercase and correspond to reversed domain names, as with Java. e.g. "org.freedesktop"

Interface, signal, method, and property names are "WindowsStyleCaps", note that the first letter is capitalized, unlike Java.

Object paths are normally all lowercase with underscores used rather than hyphens.

## UUIDs

A working D-Bus implementation uses universally-unique IDs in two places. First, each server address has a UUID identifying the address, as described in . Second, each operating system kernel instance running a D-Bus client or server has a UUID identifying that kernel, retrieved by invoking the method org.freedesktop.DBus.Peer.GetMachineId() (see ).

The term "UUID" in this document is intended literally, i.e. an identifier that is universally unique. It is not intended to refer to RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.

The UUID must contain 128 bits of data and be hex-encoded. The hex-encoded string may not contain hyphens or other non-hex-digit characters, and it must be exactly 32 characters long. To generate a UUID, the current reference implementation concatenates 96 bits of random data followed by the 32-bit time in seconds since the UNIX epoch (in big endian byte order).

It would also be acceptable and probably better to simply generate 128 bits of random data, as long as the random number generator is of high quality. The timestamp could conceivably help if the random bits are not very random. With a quality random number generator, collisions are extremely unlikely even with only 96 bits, so it's somewhat academic.

Implementations should, however, stick to random data for the first 96 bits of the UUID.

## Standard Interfaces

See for details on the notation used in this section. There are some standard interfaces that may be useful across various D-Bus applications.

### `org.freedesktop.DBus.Peer`

The `org.freedesktop.DBus.Peer` interface has two methods:

              org.freedesktop.DBus.Peer.Ping ()
              org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)

On receipt of the `METHOD_CALL` message `org.freedesktop.DBus.Peer.Ping`, an application should do nothing other than reply with a `METHOD_RETURN` as usual. It does not matter which object path a ping is sent to. The reference implementation handles this method automatically.

On receipt of the `METHOD_CALL` message `org.freedesktop.DBus.Peer.GetMachineId`, an application should reply with a `METHOD_RETURN` containing a hex-encoded UUID representing the identity of the machine the process is running on. This UUID must be the same for all processes on a single system at least until that system next reboots. It should be the same across reboots if possible, but this is not always possible to implement and is not guaranteed. It does not matter which object path a GetMachineId is sent to. The reference implementation handles this method automatically.

The UUID is intended to be per-instance-of-the-operating-system, so may represent a virtual machine running on a hypervisor, rather than a physical machine. Basically if two processes see the same UUID, they should also see the same shared memory, UNIX domain sockets, process IDs, and other features that require a running OS kernel in common between the processes.

The UUID is often used where other programs might use a hostname. Hostnames can change without rebooting, however, or just be "localhost" - so the UUID is more robust.

explains the format of the UUID.

### `org.freedesktop.DBus.Introspectable`

This interface has one method:

              org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)

Objects instances may implement `Introspect` which returns an XML description of the object, including its interfaces (with signals and methods), objects below it in the object path tree, and its properties.

describes the format of this XML string.

### `org.freedesktop.DBus.Properties`

Many native APIs will have a concept of object properties or attributes. These can be exposed via the `org.freedesktop.DBus.Properties` interface.

                  org.freedesktop.DBus.Properties.Get (in STRING interface_name,
                                                       in STRING property_name,
                                                       out VARIANT value);
                  org.freedesktop.DBus.Properties.Set (in STRING interface_name,
                                                       in STRING property_name,
                                                       in VARIANT value);
                  org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
                                                          out DICT<STRING,VARIANT> props);

The available properties and whether they are writable can be determined by calling `org.freedesktop.DBus.Introspectable.Introspect`, see .

An empty string may be provided for the interface name; in this case, if there are multiple properties on an object with the same name, the results are undefined (picking one by according to an arbitrary deterministic rule, or returning an error, are the reasonable possibilities).

## Introspection Data Format

As described in , objects may be introspected at runtime, returning an XML string that describes the object. The same XML format may be used in other contexts as well, for example as an "IDL" for generating static language bindings.

Here is an example of introspection data:

            <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
             "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
            <node name="/org/freedesktop/sample_object">
              <interface name="org.freedesktop.SampleInterface">
                <method name="Frobate">
                  <arg name="foo" type="i" direction="in"/>
                  <arg name="bar" type="s" direction="out"/>
                  <arg name="baz" type="a{us}" direction="out"/>
                  <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
                </method>
                <method name="Bazify">
                  <arg name="bar" type="(iiu)" direction="in"/>
                  <arg name="bar" type="v" direction="out"/>
                </method>
                <method name="Mogrify">
                  <arg name="bar" type="(iiav)" direction="in"/>
                </method>
                <signal name="Changed">
                  <arg name="new_value" type="b"/>
                </signal>
                <property name="Bar" type="y" access="readwrite"/>
              </interface>
              <node name="child_of_sample_object"/>
              <node name="another_child_of_sample_object"/>
           </node>

A more formal DTD and spec needs writing, but here are some quick notes.

- Only the root <node> element can omit the node name, as it's known to be the object that was introspected. If the root <node> does have a name attribute, it must be an absolute object path. If child <node> have object paths, they must be relative.

- If a child <node> has any sub-elements, then they must represent a complete introspection of the child. If a child <node> is empty, then it may or may not have sub-elements; the child must be introspected in order to find out. The intent is that if an object knows that its children are "fast" to introspect it can go ahead and return their information, but otherwise it can omit it.

- The direction element on <arg> may be omitted, in which case it defaults to "in" for method calls and "out" for signals. Signals only allow "out" so while direction may be specified, it's pointless.

- The possible directions are "in" and "out", unlike CORBA there is no "inout"

- The possible property access flags are "readwrite", "read", and "write"

- Multiple interfaces can of course be listed for one <node>.

- The "name" attribute on arguments is optional.

Method, interface, property, and signal elements may have "annotations", which are generic key/value pairs of metadata. They are similar conceptually to Java's annotations and C# attributes. Well-known annotations:

Name | Values (separated by ,) | Description
--- | --- | ---
org.freedesktop.DBus.Deprecated | true,false | Whether or not the entity is deprecated; defaults to false
org.freedesktop.DBus.GLib.CSymbol | (string) | The C symbol; may be used for methods and interfaces
org.freedesktop.DBus.Method.NoReply | true,false | If set, don't expect a reply to the method call; defaults to false.

## Message Bus Specification

### Message Bus Overview

The message bus accepts connections from one or more applications. Once connected, applications can exchange messages with other applications that are also connected to the bus.

In order to route messages among connections, the message bus keeps a mapping from names to connections. Each connection has one unique-for-the-lifetime-of-the-bus name automatically assigned. Applications may request additional names for a connection. Additional names are usually "well-known names" such as "org.freedesktop.TextEditor". When a name is bound to a connection, that connection is said to own the name.

The bus itself owns a special name, `org.freedesktop.DBus`. This name routes messages to the bus, allowing applications to make administrative requests. For example, applications can ask the bus to assign a name to a connection.

Each name may have queued owners. When an application requests a name for a connection and the name is already in use, the bus will optionally add the connection to a queue waiting for the name. If the current owner of the name disconnects or releases the name, the next connection in the queue will become the new owner.

This feature causes the right thing to happen if you start two text editors for example; the first one may request "org.freedesktop.TextEditor", and the second will be queued as a possible owner of that name. When the first exits, the second will take over.

Messages may have a `DESTINATION` field (see ). If the `DESTINATION` field is present, it specifies a message recipient by name. Method calls and replies normally specify this field.

Signals normally do not specify a destination; they are sent to all applications with message matching rules that match the message.

When the message bus receives a method call, if the `DESTINATION` field is absent, the call is taken to be a standard one-to-one message and interpreted by the message bus itself. For example, sending an `org.freedesktop.DBus.Peer.Ping` message with no `DESTINATION` will cause the message bus itself to reply to the ping immediately; the message bus will not make this message visible to other applications.

Continuing the `org.freedesktop.DBus.Peer.Ping` example, if the ping message were sent with a `DESTINATION` 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.

### Message Bus Names

Each connection has at least one name, assigned at connection time and returned in response to the `org.freedesktop.DBus.Hello` method call. This automatically-assigned name is called the connection's unique name. Unique names are never reused for two different connections to the same bus.

Ownership of a unique name is a prerequisite for interaction with the message bus. It logically follows that the unique name is always the first name that an application comes to own, and the last one that it loses ownership of.

Unique connection names must begin with the character ':' (ASCII colon character); bus names that are not unique names must not begin with this character. (The bus must reject any attempt by an application to manually request a name beginning with ':'.) This restriction categorically prevents "spoofing"; messages sent to a unique name will always go to the expected connection.

When a connection is closed, all the names that it owns are deleted (or transferred to the next connection in the queue if any).

A connection can request additional names to be associated with it using the `org.freedesktop.DBus.RequestName` message. describes the format of a valid name. These names can be released again using the `org.freedesktop.DBus.ReleaseName` message.

#### `org.freedesktop.DBus.RequestName`

As a method:

                UINT32 RequestName (in STRING name, in UINT32 flags)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name to request
1 | UINT32 | Flags

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | UINT32 | Return value

This method call should be sent to `org.freedesktop.DBus` and asks the message bus to assign the given name to the method caller. Each name maintains a queue of possible owners, where the head of the queue is the primary or current owner of the name. Each potential owner in the queue maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName call. When RequestName is invoked the following occurs:

- If the method caller is currently the primary owner of the name, the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE values are updated with the values from the new RequestName call, and nothing further happens.

- If the current primary owner (head of the queue) has DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then the caller of RequestName replaces the current primary owner at the head of the queue and the current primary owner moves to the second position in the queue. If the caller of RequestName was in the queue previously its flags are updated with the values from the new RequestName in addition to moving it to the head of the queue.

- If replacement is not possible, and the method caller is currently in the queue but not the primary owner, its flags are updated with the values from the new RequestName call.

- If replacement is not possible, and the method caller is currently not in the queue, the method caller is appended to the queue.

- If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE set and is not the primary owner, it is removed from the queue. This can apply to the previous primary owner (if it was replaced) or the method caller (if it updated the DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the queue, or if it was just added to the queue with that flag set).

Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the queue," even if another application already in the queue had specified DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner that does not allow replacement goes away, and the next primary owner does allow replacement. In this case, queued items that specified DBUS_NAME_FLAG_REPLACE_EXISTING do not automatically replace the new primary owner. In other words, DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the time RequestName is called. This is deliberate to avoid an infinite loop anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_REPLACE_EXISTING.

The flags argument contains any of the following values logically ORed together:

Conventional Name | Value | Description
--- | --- | ---
DBUS_NAME_FLAG_ALLOW_REPLACEMENT | 0x1 | If an application A specifies this flag and succeeds in becoming the owner of the name, and another application B later calls RequestName with the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A will lose ownership and receive a `org.freedesktop.DBus.NameLost` signal, and application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING is not specified by application B, then application B will not replace application A as the owner.
DBUS_NAME_FLAG_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 name if there is no current owner. If this flag is set, the application will replace the current owner if the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
DBUS_NAME_FLAG_DO_NOT_QUEUE | 0x4 | Without this flag, if an application requests a name that is already owned, the application will be placed in a queue to own the name when the current owner gives it up. If this flag is given, the application will not be placed in the queue, the request for the name will simply fail. This flag also affects behavior when an application is replaced as name owner; by default the application moves back into the waiting queue, unless this flag was provided when the application became the name owner.

The return code can be one of the following values:

Conventional Name | Value | Description
--- | --- | ---
DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER | 1 | The caller is now the primary owner of the name, replacing any previous owner. Either the name had no owner before, or the caller specified DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
DBUS_REQUEST_NAME_REPLY_IN_QUEUE | 2 | The name already had an owner, DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either the current owner did not specify DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
DBUS_REQUEST_NAME_REPLY_EXISTS | 3 | The name already has an owner, DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not specified by the requesting application.
DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER | 4 | The application trying to request ownership of a name is already the owner of it.

#### `org.freedesktop.DBus.ReleaseName`

As a method:

                UINT32 ReleaseName (in STRING name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name to release

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | UINT32 | Return value

This method call should be sent to `org.freedesktop.DBus` and asks the message bus to release the method caller's claim to the given name. If the caller is the primary owner, a new primary owner will be selected from the queue if any other owners are waiting. If the caller is waiting in the queue for the name, the caller will removed from the queue and will not be made an owner of the name if it later becomes available. If there are no other owners in the queue for the name, it will be removed from the bus entirely. The return code can be one of the following values:

Conventional Name | Value | Description
--- | --- | ---
DBUS_RELEASE_NAME_REPLY_RELEASED | 1 | The caller has released his claim on the given name. Either the caller was the primary owner of the name, and the name is now unused or taken by somebody waiting in the queue for the name, or the caller was waiting in the queue for the name and has now been removed from the queue.
DBUS_RELEASE_NAME_REPLY_NON_EXISTENT | 2 | The given name does not exist on this bus.
DBUS_RELEASE_NAME_REPLY_NOT_OWNER | 3 | The caller was not the primary owner of this name, and was also not waiting in the queue to own this name.

### Message Bus Message Routing

FIXME

#### Match Rules

An important part of the message bus routing protocol is match rules. Match rules describe what messages can be sent to a client based on the contents of the message. When a message is routed through the bus it is compared to clients' match rules. If any of the rules match, the message is dispatched to the client. If none of the rules match the message never leaves the bus. This is an effective way to control traffic over the bus and to make sure only relevant message need to be processed by the client.

Match rules are added using the AddMatch bus method (see xref linkend="bus-messages-add-match"/>). Rules are specified as a string of comma separated key/value pairs. Excluding a key from the rule indicates a wildcard match. For instance excluding the the member from a match rule but adding a sender would let all messages from that sender through. An example of a complete rule would be "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"

The following table describes the keys that can be used to create a match rule: The following table summarizes the D-Bus types.

Key | Possible Values | Description
--- | --- | ---
`type` | 'signal', 'method_call', 'method_return', 'error' | Match on the message type. An example of a type match is type='signal'
`sender` | A bus or unique name (see and respectively) | Match messages sent by a particular sender. An example of a sender match is sender='org.freedesktop.Hal'
`interface` | An interface name (see ) | Match messages sent over or to a particular interface. An example of an interface match is interface='org.freedesktop.Hal.Manager'. If a message omits the interface header, it must not match any rule that specifies this key.
`member` | Any valid method or signal name | Matches messages which have the give method or signal name. An example of a member match is member='NameOwnerChanged'
`path` | An object path (see ) | Matches messages which are sent from or to the given object. An example of a path match is path='/org/freedesktop/Hal/Manager'
`destination` | A unique name (see ) | Matches messages which are being sent to the given unique name. An example of a destination match is destination=':1.0'
`arg[0, 1, 2, 3, ...]` | Any string | Arg matches are special and are used for further restricting the match based on the arguments in the body of a message. As of this time only string arguments can be matched. An example of an argument match would be arg3='Foo'. Only argument indexes from 0 to 63 should be accepted.
`arg[0, 1, 2, 3, ...]path` | Any string | Argument path matches provide a specialised form of wildcard matching for path-like namespaces. As with normal argument matches, if the argument is exactly equal to the string given in the match rule then the rule is satisfied. Additionally, there is also a match when either the string given in the match rule or the appropriate message argument ends with '/' and is a prefix of the other. An example argument path match is arg0path='/aa/bb/'. This would match messages with first arguments of '/', '/aa/', '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.

### Message Bus Starting Services

The message bus can start applications on behalf of other applications. In CORBA terms, this would be called activation. An application that can be started in this way is called a service.

With D-Bus, starting a service is normally done by name. That is, applications ask the message bus to start some program that will own a well-known name, such as `org.freedesktop.TextEditor`. This implies a contract documented along with the name `org.freedesktop.TextEditor` for which objects the owner of that name will provide, and what interfaces those objects will have.

To find an executable corresponding to a particular name, the bus daemon looks for service description files. Service description files define a mapping from names to executables. Different kinds of message bus will look for these files in different places, see .

[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.

Example service description file

                # Sample service description file
                [D-BUS Service]
                Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
                Exec=/usr/libexec/gconfd-2

When an application asks to start a service by name, the bus daemon tries to find a service that will own that name. 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_STARTER_ADDRESS` set to the address of the message bus so it can connect and request the appropriate names.

The executable being launched may want to know whether the message bus starting it is one of the well-known message buses (see ). To facilitate this, the bus must also set the `DBUS_STARTER_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 new executable must still connect to the address given in `DBUS_STARTER_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.]

#### Message Bus Service Scope

The "scope" of a service is its "per-", such as per-session, per-machine, per-home-directory, or per-display. The reference implementation doesn't yet support starting services in a different scope from the message bus itself. So e.g. if you start a service on the session bus its scope is per-session.

We could add an optional scope to a bus name. For example, for per-(display,session pair), we could have a unique ID for each display generated automatically at login and set on screen 0 by executing a special "set display ID" binary. The ID would be stored in a `_DBUS_DISPLAY_ID` property and would be a string of random bytes. This ID would then be used to scope names. Starting/locating a service could be done by ID-name pair rather than only by name.

Contrast this with a per-display scope. To achieve that, we would want a single bus spanning all sessions using a given display. So we might set a `_DBUS_DISPLAY_BUS_ADDRESS` property on screen 0 of the display, pointing to this bus.

### Well-known Message Bus Instances

Two standard message bus instances are defined here, along with how to locate them and where their service files live.

#### Login session message bus

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]

#### System message bus

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 system 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`.

The D-Bus reference implementation actually honors the `$(localstatedir)` configure option for this address, on both client and server side.

[FIXME specify location of system bus .service files]

### Message Bus Messages

The special message bus name `org.freedesktop.DBus` responds to a number of additional messages.

#### `org.freedesktop.DBus.Hello`

As a method:

                STRING Hello ()

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Unique name assigned to the connection

Before an application is able to send messages to other applications it must send the `org.freedesktop.DBus.Hello` message to the message bus to obtain a unique name. If an application without a unique name tries to send a message to another application, or a message to the message bus itself that isn't the `org.freedesktop.DBus.Hello` message, it will be disconnected from the bus.

There is no corresponding "disconnect" request; if a client wishes to disconnect from the bus, it simply closes the socket (or other communication channel).

#### `org.freedesktop.DBus.ListNames`

As a method:

                ARRAY of STRING ListNames ()

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | ARRAY of STRING | Array of strings where each string is a bus name

Returns a list of all currently-owned names on the bus.

#### `org.freedesktop.DBus.ListActivatableNames`

As a method:

                ARRAY of STRING ListActivatableNames ()

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | ARRAY of STRING | Array of strings where each string is a bus name

Returns a list of all names that can be activated on the bus.

#### `org.freedesktop.DBus.NameHasOwner`

As a method:

                BOOLEAN NameHasOwner (in STRING name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name to check

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | BOOLEAN | Return value, true if the name exists

Checks if the specified name exists (currently has an owner).

#### `org.freedesktop.DBus.NameOwnerChanged`

This is a signal:

                NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name with a new owner
1 | STRING | Old owner or empty string if none
2 | STRING | New owner or empty string if none

This signal indicates that the owner of a name has changed. It's also the signal to use to detect the appearance of new names on the bus.

#### `org.freedesktop.DBus.NameLost`

This is a signal:

                NameLost (STRING name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name which was lost

This signal is sent to a specific application when it loses ownership of a name.

#### `org.freedesktop.DBus.NameAcquired`

This is a signal:

                NameAcquired (STRING name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name which was acquired

This signal is sent to a specific application when it gains ownership of a name.

#### `org.freedesktop.DBus.StartServiceByName`

As a method:

                UINT32 StartServiceByName (in STRING name, in UINT32 flags)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name of the service to start
1 | UINT32 | Flags (currently not used)

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | UINT32 | Return value

Tries to launch the executable associated with a name. For more information, see .

The return value can be one of the following values:

Identifier | Value | Description
--- | --- | ---
DBUS_START_REPLY_SUCCESS | 1 | The service was successfully started.
DBUS_START_REPLY_ALREADY_RUNNING | 2 | A connection already owns the given name.

#### `org.freedesktop.DBus.UpdateActivationEnvironment`

As a method:

                UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | ARRAY of DICT<STRING,STRING> | Environment to add or update

Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.

Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.

#### `org.freedesktop.DBus.GetNameOwner`

As a method:

                STRING GetNameOwner (in STRING name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name to get the owner of

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Return value, a unique connection name

Returns the unique connection name of the primary owner of the name given. If the requested name doesn't have an owner, returns a `org.freedesktop.DBus.Error.NameHasNoOwner` error.

#### `org.freedesktop.DBus.GetConnectionUnixUser`

As a method:

                UINT32 GetConnectionUnixUser (in STRING connection_name)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Name of the connection to query

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | UINT32 | unix user id

Returns the unix uid of the process connected to the server. If unable to determine it, a `org.freedesktop.DBus.Error.Failed` error is returned.

#### `org.freedesktop.DBus.AddMatch`

As a method:

                AddMatch (in STRING rule)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Match rule to add to the connection

Adds a match rule to match messages going through the message bus (see ). If the bus does not have enough resources the `org.freedesktop.DBus.Error.OOM` error is returned.

#### `org.freedesktop.DBus.RemoveMatch`

As a method:

                RemoveMatch (in STRING rule)

Message arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Match rule to remove from the connection

Removes the first rule that matches (see ). If the rule is not found the `org.freedesktop.DBus.Error.MatchRuleNotFound` error is returned.

#### `org.freedesktop.DBus.GetId`

As a method:

                GetId (out STRING id)

Reply arguments:

Argument | Type | Description
--- | --- | ---
0 | STRING | Unique ID identifying the bus daemon

Gets the unique ID of the bus. The unique ID here is shared among all addresses the bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in . Each address the bus is listening on also has its own unique ID, as described in . The per-bus and per-address IDs are not related. There is also a per-machine ID, described in and returned by org.freedesktop.DBus.Peer.GetMachineId(). For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.

## Glossary

This glossary defines some of the terms used in this specification.

Bus Name

  The message bus maintains an association between names and connections. (Normally, there's one connection per application.) A bus name is simply an identifier used to locate connections. For example, the hypothetical `com.yoyodyne.Screensaver` name might be used to send a message to a screensaver from Yoyodyne Corporation. An application is said to own a name if the message bus has associated the application's connection with the name. Names may also have queued owners (see ). The bus assigns a unique name to each connection, see . Other names can be thought of as "well-known names" and are used to find applications that offer specific functionality.

Message

  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.

Message Bus

  The message bus is a special application that forwards or routes messages between a group of applications connected to the message bus. It also manages names used for routing messages.

Name

  See . "Name" may also be used to refer to some of the other names in D-Bus, such as interface names.

Namespace

  Used to prevent collisions when defining new interfaces or bus names. The convention used is the same one Java uses for defining classes: a reversed domain name.

Object

  Each application contains objects, which have interfaces and methods. Objects are referred to by a name, called a path.

One-to-One

  An application talking directly to another application, without going through a message bus. One-to-one connections may be "peer to peer" or "client to server." The D-Bus protocol has no concept of client vs. server after a connection has authenticated; the flow of messages is symmetrical (full duplex).

Path

  Object references (object names) in D-Bus are organized into a filesystem-style hierarchy, so each object is named by a path. As in LDAP, there's no difference between "files" and "directories"; a path can refer to an object, while still having child objects below it.

Queued Name Owner

  Each bus name has a primary owner; messages sent to the name go to the primary owner. However, certain names also maintain a queue of secondary owners "waiting in the wings." If the primary owner releases the name, then the first secondary owner in the queue automatically becomes the new owner of the name.

Service

  A service is an executable that can be launched by the bus daemon. Services normally guarantee some particular features, for example they may guarantee that they will request a specific name such as "org.freedesktop.Screensaver", have a singleton object "/org/freedesktop/Application", and that object will implement the interface "org.freedesktop.ScreensaverControl".

Service Description Files

  ".service files" tell the bus about service applications that can be launched (see ). Most importantly they provide a mapping from bus names to services that will request those names when they start up.

Unique Connection Name

  The special name automatically assigned to each connection by the message bus. This name will never change owner, and will be unique (never reused during the lifetime of the message bus). It will begin with a ':' character.