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9 <title>D-BUS Specification</title>
10 <releaseinfo>Version 0.8</releaseinfo>
11 <date>06 September 2003</date>
14 <firstname>Havoc</firstname>
15 <surname>Pennington</surname>
17 <orgname>Red Hat, Inc.</orgname>
19 <email>hp@pobox.com</email>
24 <firstname>Anders</firstname>
25 <surname>Carlsson</surname>
27 <orgname>CodeFactory AB</orgname>
29 <email>andersca@codefactory.se</email>
34 <firstname>Alexander</firstname>
35 <surname>Larsson</surname>
37 <orgname>Red Hat, Inc.</orgname>
39 <email>alexl@redhat.com</email>
46 <sect1 id="introduction">
47 <title>Introduction</title>
49 D-BUS is a system for low-latency, low-overhead, easy to use
50 interprocess communication (IPC). In more detail:
54 D-BUS is <emphasis>low-latency</emphasis> because it is designed
55 to avoid round trips and allow asynchronous operation, much like
61 D-BUS is <emphasis>low-overhead</emphasis> because it uses a
62 binary protocol, and does not have to convert to and from a text
63 format such as XML. Because D-BUS is intended for potentially
64 high-resolution same-machine IPC, not primarily for Internet IPC,
65 this is an interesting optimization.
70 D-BUS is <emphasis>easy to use</emphasis> because it works in terms
71 of <firstterm>messages</firstterm> rather than byte streams, and
72 automatically handles a lot of the hard IPC issues. Also, the D-BUS
73 library is designed to be wrapped in a way that lets developers use
74 their framework's existing object/type system, rather than learning
75 a new one specifically for IPC.
81 The base D-BUS protocol is a peer-to-peer protocol, specified in <xref
82 linkend="message-protocol"/>. That is, it is a system for one application
83 to talk to a single other application. However, the primary intended
84 application of D-BUS is the D-BUS <firstterm>message bus</firstterm>,
85 specified in <xref linkend="message-bus"/>. The message bus is a special
86 application that accepts connections from multiple other applications, and
87 forwards messages among them.
90 Uses of D-BUS include notification of system changes (notification of when
91 a camera is plugged in to a computer, or a new version of some software
92 has been installed), or desktop interoperablity, for example a file
93 monitoring service or a configuration service.
97 <sect1 id="message-protocol">
98 <title>Message Protocol</title>
100 A <firstterm>message</firstterm> consists of a
101 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
102 think of a message as a package, the header is the address, and the body
103 contains the package contents. The message delivery system uses the header
104 information to figure out where to send the message and how to interpret
105 it; the recipient inteprets the body of the message.
109 The body of the message is made up of zero or more
110 <firstterm>arguments</firstterm>, which are typed
111 values, such as an integer or a byte array.
114 <sect2 id="message-protocol-header-encoding">
115 <title>Header Encoding</title>
117 Following the mandatory fields, there are zero or more named fields (see
118 <xref linkend="message-protocol-header-fields"/>), and then nul bytes
119 padding the header such that its total length in bytes is a multiple of
123 The header MUST begin with the following mandatory fields in the following
130 <entry>Description</entry>
135 <entry>1 byte</entry>
136 <entry>Endianness flag; ASCII 'l' for little-endian
137 or ASCII 'B' for big-endian.</entry>
140 <entry>1 byte</entry>
141 <entry>Type of message. Unknown types MUST be ignored.
142 Currently-defined types are described below.
146 <entry>1 byte</entry>
147 <entry>Bitwise OR of flags. Unknown flags
148 MUST be ignored. Currently-defined flags are described below.
152 <entry>1 byte</entry>
153 <entry>Major protocol version of the sending application. If
154 the major protocol version of the receiving application does not
155 match, the applications will not be able to communicate and the
156 D-BUS connection MUST be disconnected. The major protocol
157 version for this version of the specification is 0.
161 <entry>4 bytes</entry>
162 <entry>An unsigned 32-bit integer in the
163 message's byte order, indicating the total length in bytes of
164 the header including named fields and any alignment padding.
165 MUST be a multiple of 8.
169 <entry>4 bytes</entry>
170 <entry>An unsigned 32-bit integer in the
171 message's byte order, indicating the total length in bytes of
176 <entry>4 bytes</entry>
177 <entry>The message's serial number, an unsigned 32-bit integer in
178 the message's byte order. The serial number is a cookie used to
179 identify message replies; thus all outstanding unreplied-to messages
180 from the same connection MUST have a different serial number.
181 Zero is not a valid serial number, but all other numbers are
190 Types that can appear in the second byte of the header:
195 <entry>Conventional name</entry>
196 <entry>Decimal value</entry>
197 <entry>Description</entry>
202 <entry>INVALID</entry>
204 <entry>This is an invalid type, if seen in a message
205 the connection should be dropped immediately.</entry>
208 <entry>METHOD_CALL</entry>
210 <entry>Method call.</entry>
213 <entry>METHOD_RETURN</entry>
215 <entry>Method reply with returned data.</entry>
220 <entry>Error reply. If the first argument exists and is a
221 string, it is an error message.</entry>
224 <entry>SIGNAL</entry>
226 <entry>Signal emission.</entry>
233 Flags that can appear in the third byte of the header:
238 <entry>Conventional name</entry>
239 <entry>Hex value</entry>
240 <entry>Description</entry>
245 <entry>NO_REPLY_EXPECTED</entry>
247 <entry>This message does not expect method return replies or
248 error replies; the reply can be omitted as an
249 optimization. However, it is compliant with this specification
250 to return the reply despite this flag.</entry>
253 <entry>AUTO_ACTIVATION</entry>
255 <entry>This message automatically activates the
256 addressed service before the message is delivered.</entry>
264 <sect2 id="message-protocol-header-fields">
265 <title>Header Fields</title>
267 In addition to the required header information mentioned
268 in <xref linkend="message-protocol-header-encoding"/>,
269 the header must contain the required named header
270 fields and zero or more of the optional named
271 header fields. Future versions of this protocol
272 specification may add new fields. Implementations must
273 ignore fields they do not understand. Implementations
274 must not invent their own header fields; only changes to
275 this specification may introduce new header fields.
279 Header field names MUST consist of a single byte, possible values
280 of which are defined below. Following the name, the field MUST have
281 a type code represented as a single unsigned byte, and then a
282 properly-aligned value of that type. See <xref
283 linkend="message-protocol-arguments"/> for a description of how each
284 type is encoded. If an implementation sees a header field name that
285 it does not understand, it MUST ignore that field.
289 Here are the currently-defined named header fields:
294 <entry>Conventional Name</entry>
295 <entry>Decimal Value</entry>
297 <entry>Required</entry>
298 <entry>Description</entry>
303 <entry>INVALID</entry>
305 <entry>INVALID</entry>
307 <entry>Not a valid field name (error if it appears in a message)</entry>
312 <entry>OBJECT_PATH</entry>
314 <entry>The object to send the message to; objects are identified by
315 a path, "/foo/bar"</entry>
318 <entry>INTERFACE</entry>
320 <entry>STRING</entry>
322 <entry>The interface to invoke a method call on, or
323 that a signal is emitted from. e.g. "org.freedesktop.Introspectable"</entry>
326 <entry>MEMBER</entry>
328 <entry>STRING</entry>
330 <entry>The member, either the method name or signal name.
331 e.g. "Frobate"</entry>
334 <entry>ERROR_NAME</entry>
336 <entry>STRING</entry>
338 <entry>The name of the error that occurred, for errors</entry>
341 <entry>REPLY_SERIAL</entry>
343 <entry>UINT32</entry>
345 <entry>The serial number of the message this message is a reply
346 to. (The serial number is one of the mandatory header fields,
347 see <xref linkend="message-protocol-header-encoding"/>.)</entry>
350 <entry>DESTINATION</entry>
352 <entry>STRING</entry>
354 <entry>The name of the service this message should be routed to.
355 Only used in combination with the message bus, see
356 <xref linkend="message-bus"/>.</entry>
359 <entry>SENDER</entry>
361 <entry>STRING</entry>
363 <entry>Sender service. The name of the base service that sent
364 this message. The message bus fills in this field; the field is
365 only meaningful in combination with the message bus.</entry>
373 <sect2 id="message-protocol-header-padding">
374 <title>Header Alignment Padding</title>
376 To allow implementations to keep the header and the body in a single
377 buffer while keeping data types aligned, the total length of the header
378 must be a multiple of 8 bytes. To achieve this, the header MUST be padded
379 with nul bytes to align its total length on an 8-byte boundary.
380 The minimum number of padding bytes MUST be used. Because zero is an
381 invalid field name, implementations can distinguish padding (which must be
382 zero initialized) from additional named fields.
386 <sect2 id="message-protocol-arguments">
387 <title>Message Arguments</title>
389 The message body is made up of arguments. Each argument is a type code,
390 represented by a single unsigned byte, followed by the aligned value of
391 the argument in a type-dependent format. Alignment padding between the
392 typecode and the value is initialized to zero.
399 <entry>Type name</entry>
401 <entry>Description</entry>
406 <entry>INVALID</entry>
407 <entry>0 (ASCII NUL)</entry>
408 <entry>Not a valid type code (error if it appears in a message)</entry>
411 <entry>118 (ASCII 'v') </entry>
412 <entry>Marks a "void"/"unset"/"nonexistent"/"null" argument</entry>
415 <entry>121 (ASCII 'y')</entry>
416 <entry>8-bit unsigned integer</entry>
418 <entry>BOOLEAN</entry>
419 <entry>98 (ASCII 'b')</entry>
420 <entry>Boolean value, 0 is FALSE and 1 is TRUE. Everything else is invalid.</entry>
423 <entry>105 (ASCII 'i')</entry>
424 <entry>32-bit signed integer</entry>
426 <entry>UINT32</entry>
427 <entry>117 (ASCII 'u')</entry>
428 <entry>32-bit unsigned integer</entry>
431 <entry>120 (ASCII 'x')</entry>
432 <entry>64-bit signed integer</entry>
434 <entry>UINT64</entry>
435 <entry>116 (ASCII 't')</entry>
436 <entry>64-bit unsigned integer</entry>
438 <entry>DOUBLE</entry>
439 <entry>100 (ASCII 'd')</entry>
440 <entry>IEEE 754 double</entry>
442 <entry>STRING</entry>
443 <entry>115 (ASCII 's')</entry>
444 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be zero terminated. </entry>
446 <entry>CUSTOM</entry>
447 <entry>99 (ASCII 'c')</entry>
448 <entry>A named byte array, used for custom types</entry>
451 <entry>97 (ASCII 'a')</entry>
455 <entry>109 (ASCII 'm')</entry>
456 <entry>A dictionary of key/value pairs</entry>
458 <entry>OBJECT_PATH</entry>
459 <entry>111 (ASCII 'o')</entry>
460 <entry>Name of an object</entry>
467 The types are encoded as follows:
472 <entry>Type name</entry>
473 <entry>Encoding</entry>
478 <entry>INVALID</entry>
479 <entry>Not applicable; cannot be encoded.</entry>
482 <entry>No data is encoded; the type code is followed immediately
483 by the type code of the next argument.</entry>
486 <entry>A byte.</entry>
488 <entry>BOOLEAN</entry>
489 <entry>A byte, with valid values 0 and 1.</entry>
492 <entry>32-bit signed integer in the message's byte order, aligned to 4-byte boundary.</entry>
494 <entry>UINT32</entry>
495 <entry>32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary.</entry>
498 <entry>64-bit signed integer in the message's byte order, aligned to 8-byte boundary.</entry>
500 <entry>UINT64</entry>
501 <entry>64-bit unsigned integer in the message's byte order, aligned to 8-byte boundary.</entry>
503 <entry>DOUBLE</entry>
504 <entry>64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary.</entry>
506 <entry>STRING</entry>
507 <entry>UINT32 aligned to 4-byte boundary indicating the string's
508 length in bytes excluding its terminating nul, followed by
509 string data of the given length, followed by a terminating nul
513 <entry>CUSTOM</entry>
514 <entry>A string (encoded as the STRING type above) giving the
515 name of the type followed by an UINT32 aligned to 4-byte boundary
516 indicating the data length in bytes, followed by the data.
517 The string has some restrictions on its content, see
518 <xref linkend="message-protocol-names"/>.
522 <entry>A sequence of bytes giving the element type of the array, terminated
523 by a type different from ARRAY (just one byte for one-dimensional arrays, but
524 larger for multi-dimensional arrays), followed by an UINT32 (aligned to 4 bytes)
525 giving the length of the array data in bytes. This is followed by each array entry
526 encoded the way it would normally be encoded, except arrays, which are encoded
527 without the type information, since that is already declared above. Arrays containing
532 <entry>UINT32 giving the length of the dictionary data in bytes.
533 This is followed by a number of keyname/value pairs, where the
534 keyname is encoded as a STRING above, and the value is encoded
535 as a byte with typecode and how that type normally would be encoded
539 <entry>OBJECT_PATH</entry>
540 <entry>Encoded as if it were a STRING.
549 <sect2 id="message-protocol-names">
550 <title>Valid names</title>
552 The various names in D-BUS messages have some restrictions.
554 <sect3 id="message-protocol-names-interface">
555 <title>Interface names</title>
557 Interfaces have names with type STRING, meaning that
558 they must be valid UTF-8. However, there are also some
559 additional restrictions that apply to interface names
562 <listitem><para>They are composed of 1 or more elements separated by
563 a period ('.') character. All elements must contain at least
567 <listitem><para>Each element must only contain the ASCII characters
568 "[A-Z][a-z][0-9]_" and must not begin with a digit.
572 <listitem><para>They must contain at least one '.' (period)
573 character (and thus at least two elements).
576 <listitem><para>They must not begin with a '.' (period) character.</para></listitem>
577 <listitem><para>They must not exceed 256 bytes in length.</para></listitem>
578 <listitem><para>They must be at least 1 byte in length.</para></listitem>
582 <sect3 id="message-protocol-names-service">
583 <title>Service names</title>
585 Service names have the same restrictions as interface names, with a
586 special exception for base services. A base service name's first
587 element must start with a colon (':') character. After the colon, any
588 characters in the range "[A-Z][a-z][0-9]_" may appear. Elements after
589 the first must follow the usual rules, except that they may start with
590 a digit. Service names not starting with a colon have none of these
591 exceptions and follow the same rules as interface names.
594 <sect3 id="message-protocol-names-method">
595 <title>Method names</title>
599 <listitem><para>Must only contain the ASCII characters
600 "[A-Z][a-z][0-9]_" and may not begin with a
601 digit.</para></listitem>
602 <listitem><para>Must not contain the '.' (period) character</para></listitem>
603 <listitem><para>Must not exceed 256 bytes in length</para></listitem>
604 <listitem><para>Must be at least 1 byte in length</para></listitem>
608 <sect3 id="message-protocol-names-path">
609 <title>Path names</title>
611 A path (type OBJECT_PATH) must begin with an ASCII '/' (slash)
612 character. Paths may not end with a slash character unless the path is
613 the one-byte string "/". Two slash characters may not appear adjacent
614 to one another (the empty string is not a valid "subdirectory"). Paths
615 may not exceed 256 bytes in length.
618 <sect3 id="message-protocol-names-error">
619 <title>Error names</title>
621 Error names have the same restrictions as interface names.
624 <sect3 id="message-protocol-names-custom">
625 <title>Custom types</title>
627 Custom type names for values of type CUSTOM follow the same
628 restrictions as interface names.
633 <sect2 id="message-protocol-types">
634 <title>Message types</title>
636 Each of the message types (METHOD_CALL, METHOD_RETURN, ERROR, and
637 SIGNAL) has its own expected usage conventions and header fields.
639 <sect3 id="message-protocol-types-method">
640 <title>Method Calls, Returns, and Errors</title>
642 Some messages invoke an operation on a remote object. These are
643 called method call messages and have the type tag METHOD_CALL. Such
644 messages map naturally to methods on objects in a typical program.
647 A method call message is expected to have a MEMBER header field
648 indicating the name of the method. Optionally, the message has an
649 INTERFACE field giving the interface the method is a part of. In the
650 absence of an INTERFACE field, if two interfaces on the same object have
651 a method with the same name, it is undefined which of the two methods
652 will be invoked. Implementations may also choose to return an error in
653 this ambiguous case. However, if a method name is unique
654 implementations should not require an interface field.
657 Method call messages also include a PATH field indicating the
658 object to invoke the method on. If the call is passing through
659 a message bus, the message will also have a SERVICE field giving
660 the service to receive the message.
663 When an application handles a method call message, it is expected to
664 return a reply. The reply is identified by a REPLY_SERIAL header field
665 indicating the serial number of the METHOD_CALL being replied to. The
666 reply can have one of two types; either METHOD_RETURN or ERROR.
669 If the reply has type METHOD_RETURN, the arguments to the reply message
670 are the return value(s) or "out parameters" of the method call.
671 If the reply has type ERROR, then an "exception" has been thrown,
672 and the call fails; no return value will be provided. It makes
673 no sense to send multiple replies to the same method call.
676 Even if a method call has no return values, a METHOD_RETURN
677 reply is expected, so the caller will know the method
678 was successfully processed.
681 The METHOD_RETURN or ERROR reply message MUST have the REPLY_SERIAL
682 header field. If this field is missing, it should be treated as
686 If a METHOD_CALL message has the flag NO_REPLY_EXPECTED,
687 then as an optimization the application receiving the method
688 call may choose to omit the reply message (regardless of
689 whether the reply would have been METHOD_RETURN or ERROR).
690 However, it is also acceptable to ignore the NO_REPLY_EXPECTED
691 flag and reply anyway.
694 If a message has the flag AUTO_ACTIVATION, then the addressed
695 service will be activated before the message is delivered, if
696 not already active. The message will be held until the service
697 is successfully activated or has failed to activate; in case
698 of failure, an activation error will be returned.
700 <sect4 id="message-protocol-types-method-apis">
701 <title>Mapping method calls to native APIs</title>
703 APIs for D-BUS may map method calls to a method call in a specific
704 programming language, such as C++, or may map a method call written
705 in an IDL to a D-BUS message.
708 In APIs of this nature, arguments to a method are often termed "in"
709 (which implies sent in the METHOD_CALL), or "out" (which implies
710 returned in the METHOD_RETURN). Some APIs such as CORBA also have
711 "inout" arguments, which are both sent and received, i.e. the caller
712 passes in a value which is modified. Mapped to D-BUS, an "inout"
713 argument is equivalent to an "in" argument, followed by an "out"
714 argument. You can't pass things "by reference" over the wire, so
715 "inout" is purely an illusion of the in-process API.
718 Given a method with zero or one return values, followed by zero or more
719 arguments, where each argument may be "in", "out", or "inout", the
720 caller constructs a message by appending each "in" or "inout" argument,
721 in order. "out" arguments are not represented in the caller's message.
724 The recipient constructs a reply by appending first the return value
725 if any, then each "out" or "inout" argument, in order.
726 "in" arguments are not represented in the reply message.
732 <sect3 id="message-protocol-types-signal">
733 <title>Signal Emission</title>
735 Unlike method calls, signal emissions have no replies.
736 A signal emission is simply a single message of type SIGNAL.
737 It must have three header fields: PATH giving the object
738 the signal was emitted from, plus INTERFACE and MEMBER giving
739 the fully-qualified name of the signal.
743 <sect3 id="message-protocol-types-notation">
744 <title>Notation in this document</title>
746 This document uses a simple pseudo-IDL to describe particular method
747 calls and signals. Here is an example of a method call:
749 org.freedesktop.DBus.ActivateService (in STRING service_name, in UINT32 flags,
750 out UINT32 resultcode)
752 This means INTERFACE = org.freedesktop.DBus, MEMBER = ActivateService,
753 METHOD_CALL arguments are STRING and UINT32, METHOD_RETURN argument
754 is UINT32. Remember that the MEMBER field can't contain any '.' (period)
755 characters so it's known that the last part of the name in
756 the "IDL" is the member name.
759 In C++ that might end up looking like this:
761 unsigned int org::freedesktop::DBus::ActivateService (const char *service_name,
764 or equally valid, the return value could be done as an argument:
766 void org::freedesktop::DBus::ActivateService (const char *service_name,
768 unsigned int *resultcode);
770 It's really up to the API designer how they want to make
771 this look. You could design an API where the namespace wasn't used
772 in C++, using STL or Qt, using varargs, or whatever you wanted.
775 Signals are written as follows:
777 org.freedesktop.DBus.ServiceLost (STRING service_name)
779 Signals don't specify "in" vs. "out" because only
780 a single direction is possible.
783 In this ad hoc notation, the special type name ANY means any type
784 other than NIL, and the special type name ANY_OR_NIL means any valid
788 It isn't especially encouraged to use this lame pseudo-IDL in actual
789 API implementations; you might use the native notation for the
790 language you're using, or you might use COM or CORBA IDL, for example.
797 <sect1 id="auth-protocol">
798 <title>Authentication Protocol</title>
800 Before the flow of messages begins, two applications must
801 authenticate. A simple plain-text protocol is used for
802 authentication; this protocol is a SASL profile, and maps fairly
803 directly from the SASL specification. The message encoding is
804 NOT used here, only plain text messages.
807 In examples, "C:" and "S:" indicate lines sent by the client and
810 <sect2 id="auth-protocol-overview">
811 <title>Protocol Overview</title>
813 The protocol is a line-based protocol, where each line ends with
814 \r\n. Each line begins with an all-caps ASCII command name containing
815 only the character range [A-Z], a space, then any arguments for the
816 command, then the \r\n ending the line. The protocol is
817 case-sensitive. All bytes must be in the ASCII character set.
819 Commands from the client to the server are as follows:
822 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
823 <listitem><para>CANCEL</para></listitem>
824 <listitem><para>BEGIN</para></listitem>
825 <listitem><para>DATA <data in hex encoding></para></listitem>
826 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
829 From server to client are as follows:
832 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
833 <listitem><para>OK</para></listitem>
834 <listitem><para>DATA <data in hex encoding></para></listitem>
835 <listitem><para>ERROR</para></listitem>
839 <sect2 id="auth-nul-byte">
840 <title>Special credentials-passing nul byte</title>
842 Immediately after connecting to the server, the client must send a
843 single nul byte. This byte may be accompanied by credentials
844 information on some operating systems that use sendmsg() with
845 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
846 sockets. However, the nul byte MUST be sent even on other kinds of
847 socket, and even on operating systems that do not require a byte to be
848 sent in order to transmit credentials. The text protocol described in
849 this document begins after the single nul byte. If the first byte
850 received from the client is not a nul byte, the server may disconnect
854 A nul byte in any context other than the initial byte is an error;
855 the protocol is ASCII-only.
858 The credentials sent along with the nul byte may be used with the
859 SASL mechanism EXTERNAL.
862 <sect2 id="auth-command-auth">
863 <title>AUTH command</title>
865 If an AUTH command has no arguments, it is a request to list
866 available mechanisms. The server SHOULD respond with a REJECTED
867 command listing the mechanisms it understands.
870 If an AUTH command specifies a mechanism, and the server supports
871 said mechanism, the server SHOULD begin exchanging SASL
872 challenge-response data with the client using DATA commands.
875 If the server does not support the mechanism given in the AUTH
876 command, it SHOULD send a REJECTED command listing the mechanisms
880 If the [initial-response] argument is provided, it is intended for
881 use with mechanisms that have no initial challenge (or an empty
882 initial challenge), as if it were the argument to an initial DATA
883 command. If the selected mechanism has an initial challenge, the
884 server should reject authentication by sending REJECTED.
887 If authentication succeeds after exchanging DATA commands,
888 an OK command should be sent to the client.
891 The first octet received by the client after the \r\n of the OK
892 command MUST be the first octet of the authenticated/encrypted
893 stream of D-BUS messages.
896 The first octet received by the server after the \r\n of the BEGIN
897 command from the client MUST be the first octet of the
898 authenticated/encrypted stream of D-BUS messages.
901 <sect2 id="auth-command-cancel">
902 <title>CANCEL Command</title>
904 At any time up to sending the BEGIN command, the client may send a
905 CANCEL command. On receiving the CANCEL command, the server MUST
906 send a REJECTED command and abort the current authentication
910 <sect2 id="auth-command-data">
911 <title>DATA Command</title>
913 The DATA command may come from either client or server, and simply
914 contains a hex-encoded block of data to be interpreted
915 according to the SASL mechanism in use.
918 Some SASL mechanisms support sending an "empty string";
919 FIXME we need some way to do this.
922 <sect2 id="auth-command-begin">
923 <title>BEGIN Command</title>
925 The BEGIN command acknowledges that the client has received an
926 OK command from the server, and that the stream of messages
930 The first octet received by the server after the \r\n of the BEGIN
931 command from the client MUST be the first octet of the
932 authenticated/encrypted stream of D-BUS messages.
935 <sect2 id="auth-command-rejected">
936 <title>REJECTED Command</title>
938 The REJECTED command indicates that the current authentication
939 exchange has failed, and further exchange of DATA is inappropriate.
940 The client would normally try another mechanism, or try providing
941 different responses to challenges.
943 Optionally, the REJECTED command has a space-separated list of
944 available auth mechanisms as arguments. If a server ever provides
945 a list of supported mechanisms, it MUST provide the same list
946 each time it sends a REJECTED message. Clients are free to
947 ignore all lists received after the first.
950 <sect2 id="auth-command-ok">
951 <title>OK Command</title>
953 The OK command indicates that the client has been authenticated,
954 and that further communication will be a stream of D-BUS messages
955 (optionally encrypted, as negotiated) rather than this protocol.
958 The first octet received by the client after the \r\n of the OK
959 command MUST be the first octet of the authenticated/encrypted
960 stream of D-BUS messages.
963 The client MUST respond to the OK command by sending a BEGIN
964 command, followed by its stream of messages, or by disconnecting.
965 The server MUST NOT accept additional commands using this protocol
966 after the OK command has been sent.
969 <sect2 id="auth-command-error">
970 <title>ERROR Command</title>
972 The ERROR command indicates that either server or client did not
973 know a command, does not accept the given command in the current
974 context, or did not understand the arguments to the command. This
975 allows the protocol to be extended; a client or server can send a
976 command present or permitted only in new protocol versions, and if
977 an ERROR is received instead of an appropriate response, fall back
978 to using some other technique.
981 If an ERROR is sent, the server or client that sent the
982 error MUST continue as if the command causing the ERROR had never been
983 received. However, the the server or client receiving the error
984 should try something other than whatever caused the error;
985 if only canceling/rejecting the authentication.
988 <sect2 id="auth-examples">
989 <title>Authentication examples</title>
993 <title>Example of successful magic cookie authentication</title>
995 (MAGIC_COOKIE is a made up mechanism)
997 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1003 <title>Example of finding out mechanisms then picking one</title>
1006 S: REJECTED KERBEROS_V4 SKEY
1007 C: AUTH SKEY 7ab83f32ee
1008 S: DATA 8799cabb2ea93e
1009 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1015 <title>Example of client sends unknown command then falls back to regular auth</title>
1019 C: AUTH MAGIC_COOKIE 3736343435313230333039
1025 <title>Example of server doesn't support initial auth mechanism</title>
1027 C: AUTH MAGIC_COOKIE 3736343435313230333039
1028 S: REJECTED KERBEROS_V4 SKEY
1029 C: AUTH SKEY 7ab83f32ee
1030 S: DATA 8799cabb2ea93e
1031 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1037 <title>Example of wrong password or the like followed by successful retry</title>
1039 C: AUTH MAGIC_COOKIE 3736343435313230333039
1040 S: REJECTED KERBEROS_V4 SKEY
1041 C: AUTH SKEY 7ab83f32ee
1042 S: DATA 8799cabb2ea93e
1043 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1045 C: AUTH SKEY 7ab83f32ee
1046 S: DATA 8799cabb2ea93e
1047 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1053 <title>Example of skey cancelled and restarted</title>
1055 C: AUTH MAGIC_COOKIE 3736343435313230333039
1056 S: REJECTED KERBEROS_V4 SKEY
1057 C: AUTH SKEY 7ab83f32ee
1058 S: DATA 8799cabb2ea93e
1061 C: AUTH SKEY 7ab83f32ee
1062 S: DATA 8799cabb2ea93e
1063 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1070 <sect2 id="auth-states">
1071 <title>Authentication state diagrams</title>
1074 This section documents the auth protocol in terms of
1075 a state machine for the client and the server. This is
1076 probably the most robust way to implement the protocol.
1079 <sect3 id="auth-states-client">
1080 <title>Client states</title>
1083 To more precisely describe the interaction between the
1084 protocol state machine and the authentication mechanisms the
1085 following notation is used: MECH(CHALL) means that the
1086 server challenge CHALL was fed to the mechanism MECH, which
1092 CONTINUE(RESP) means continue the auth conversation
1093 and send RESP as the response to the server;
1099 OK(RESP) means that after sending RESP to the server
1100 the client side of the auth conversation is finished
1101 and the server should return "OK";
1107 ERROR means that CHALL was invalid and could not be
1113 Both RESP and CHALL may be empty.
1117 The Client starts by getting an initial response from the
1118 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1119 the mechanism did not provide an initial response. If the
1120 mechanism returns CONTINUE, the client starts in state
1121 <emphasis>WaitingForData</emphasis>, if the mechanism
1122 returns OK the client starts in state
1123 <emphasis>WaitingForOK</emphasis>.
1127 The client should keep track of available mechanisms and
1128 which it mechanisms it has already attempted. This list is
1129 used to decide which AUTH command to send. When the list is
1130 exhausted, the client should give up and close the
1135 <title><emphasis>WaitingForData</emphasis></title>
1143 MECH(CHALL) returns CONTINUE(RESP) → send
1145 <emphasis>WaitingForData</emphasis>
1149 MECH(CHALL) returns OK(RESP) → send DATA
1150 RESP, goto <emphasis>WaitingForOK</emphasis>
1154 MECH(CHALL) returns ERROR → send ERROR
1155 [msg], goto <emphasis>WaitingForData</emphasis>
1163 Receive REJECTED [mechs] →
1164 send AUTH [next mech], goto
1165 WaitingForData or <emphasis>WaitingForOK</emphasis>
1170 Receive ERROR → send
1172 <emphasis>WaitingForReject</emphasis>
1177 Receive OK → send
1178 BEGIN, terminate auth
1179 conversation, authenticated
1184 Receive anything else → send
1186 <emphasis>WaitingForData</emphasis>
1194 <title><emphasis>WaitingForOK</emphasis></title>
1199 Receive OK → send BEGIN, terminate auth
1200 conversation, <emphasis>authenticated</emphasis>
1205 Receive REJECT [mechs] → send AUTH [next mech],
1206 goto <emphasis>WaitingForData</emphasis> or
1207 <emphasis>WaitingForOK</emphasis>
1213 Receive DATA → send CANCEL, goto
1214 <emphasis>WaitingForReject</emphasis>
1220 Receive ERROR → send CANCEL, goto
1221 <emphasis>WaitingForReject</emphasis>
1227 Receive anything else → send ERROR, goto
1228 <emphasis>WaitingForOK</emphasis>
1236 <title><emphasis>WaitingForReject</emphasis></title>
1241 Receive REJECT [mechs] → send AUTH [next mech],
1242 goto <emphasis>WaitingForData</emphasis> or
1243 <emphasis>WaitingForOK</emphasis>
1249 Receive anything else → terminate auth
1250 conversation, disconnect
1259 <sect3 id="auth-states-server">
1260 <title>Server states</title>
1263 For the server MECH(RESP) means that the client response
1264 RESP was fed to the the mechanism MECH, which returns one of
1269 CONTINUE(CHALL) means continue the auth conversation and
1270 send CHALL as the challenge to the client;
1276 OK means that the client has been successfully
1283 REJECT means that the client failed to authenticate or
1284 there was an error in RESP.
1289 The server starts out in state
1290 <emphasis>WaitingForAuth</emphasis>. If the client is
1291 rejected too many times the server must disconnect the
1296 <title><emphasis>WaitingForAuth</emphasis></title>
1302 Receive AUTH → send REJECTED [mechs], goto
1303 <emphasis>WaitingForAuth</emphasis>
1309 Receive AUTH MECH RESP
1313 MECH not valid mechanism → send REJECTED
1315 <emphasis>WaitingForAuth</emphasis>
1319 MECH(RESP) returns CONTINUE(CHALL) → send
1321 <emphasis>WaitingForData</emphasis>
1325 MECH(RESP) returns OK → send OK, goto
1326 <emphasis>WaitingForBegin</emphasis>
1330 MECH(RESP) returns REJECT → send REJECTED
1332 <emphasis>WaitingForAuth</emphasis>
1340 Receive BEGIN → terminate
1341 auth conversation, disconnect
1347 Receive ERROR → send REJECTED [mechs], goto
1348 <emphasis>WaitingForAuth</emphasis>
1354 Receive anything else → send
1356 <emphasis>WaitingForAuth</emphasis>
1365 <title><emphasis>WaitingForData</emphasis></title>
1373 MECH(RESP) returns CONTINUE(CHALL) → send
1375 <emphasis>WaitingForData</emphasis>
1379 MECH(RESP) returns OK → send OK, goto
1380 <emphasis>WaitingForBegin</emphasis>
1384 MECH(RESP) returns REJECT → send REJECTED
1386 <emphasis>WaitingForAuth</emphasis>
1394 Receive BEGIN → terminate auth conversation,
1401 Receive CANCEL → send REJECTED [mechs], goto
1402 <emphasis>WaitingForAuth</emphasis>
1408 Receive ERROR → send REJECTED [mechs], goto
1409 <emphasis>WaitingForAuth</emphasis>
1415 Receive anything else → send ERROR, goto
1416 <emphasis>WaitingForData</emphasis>
1424 <title><emphasis>WaitingForBegin</emphasis></title>
1429 Receive BEGIN → terminate auth conversation,
1430 client authenticated
1436 Receive CANCEL → send REJECTED [mechs], goto
1437 <emphasis>WaitingForAuth</emphasis>
1443 Receive ERROR → send REJECTED [mechs], goto
1444 <emphasis>WaitingForAuth</emphasis>
1450 Receive anything else → send ERROR, goto
1451 <emphasis>WaitingForBegin</emphasis>
1461 <sect2 id="auth-mechanisms">
1462 <title>Authentication mechanisms</title>
1464 This section describes some new authentication mechanisms.
1465 D-BUS also allows any standard SASL mechanism of course.
1467 <sect3 id="auth-mechanisms-sha">
1468 <title>DBUS_COOKIE_SHA1</title>
1470 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
1471 has the ability to read a private file owned by the user being
1472 authenticated. If the client can prove that it has access to a secret
1473 cookie stored in this file, then the client is authenticated.
1474 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
1478 Authentication proceeds as follows:
1482 The client sends the username it would like to authenticate
1488 The server sends the name of its "cookie context" (see below); a
1489 space character; the integer ID of the secret cookie the client
1490 must demonstrate knowledge of; a space character; then a
1491 hex-encoded randomly-generated challenge string.
1496 The client locates the cookie, and generates its own hex-encoded
1497 randomly-generated challenge string. The client then
1498 concatentates the server's hex-encoded challenge, a ":"
1499 character, its own hex-encoded challenge, another ":" character,
1500 and the hex-encoded cookie. It computes the SHA-1 hash of this
1501 composite string. It sends back to the server the client's
1502 hex-encoded challenge string, a space character, and the SHA-1
1508 The server generates the same concatenated string used by the
1509 client and computes its SHA-1 hash. It compares the hash with
1510 the hash received from the client; if the two hashes match, the
1511 client is authenticated.
1517 Each server has a "cookie context," which is a name that identifies a
1518 set of cookies that apply to that server. A sample context might be
1519 "org_freedesktop_session_bus". Context names must be valid ASCII,
1520 nonzero length, and may not contain the characters slash ("/"),
1521 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
1522 tab ("\t"), or period ("."). There is a default context,
1523 "org_freedesktop_global" that's used by servers that do not specify
1527 Cookies are stored in a user's home directory, in the directory
1528 <filename>~/.dbus-keyrings/</filename>. This directory must
1529 not be readable or writable by other users. If it is,
1530 clients and servers must ignore it. The directory
1531 contains cookie files named after the cookie context.
1534 A cookie file contains one cookie per line. Each line
1535 has three space-separated fields:
1539 The cookie ID number, which must be a non-negative integer and
1540 may not be used twice in the same file.
1545 The cookie's creation time, in UNIX seconds-since-the-epoch
1551 The cookie itself, a hex-encoded random block of bytes.
1557 Only server processes modify the cookie file.
1558 They must do so with this procedure:
1562 Create a lockfile name by appending ".lock" to the name of the
1563 cookie file. The server should attempt to create this file
1564 using <literal>O_CREAT | O_EXCL</literal>. If file creation
1565 fails, the lock fails. Servers should retry for a reasonable
1566 period of time, then they may choose to delete an existing lock
1567 to keep users from having to manually delete a stale
1568 lock. <footnote><para>Lockfiles are used instead of real file
1569 locking <literal>fcntl()</literal> because real locking
1570 implementations are still flaky on network
1571 filesystems.</para></footnote>
1576 Once the lockfile has been created, the server loads the cookie
1577 file. It should then delete any cookies that are old (the
1578 timeout can be fairly short), or more than a reasonable
1579 time in the future (so that cookies never accidentally
1580 become permanent, if the clock was set far into the future
1581 at some point). If no recent keys remain, the
1582 server may generate a new key.
1587 The pruned and possibly added-to cookie file
1588 must be resaved atomically (using a temporary
1589 file which is rename()'d).
1594 The lock must be dropped by deleting the lockfile.
1600 Clients need not lock the file in order to load it,
1601 because servers are required to save the file atomically.
1606 <sect1 id="addresses">
1607 <title>Server Addresses</title>
1609 Server addresses consist of a transport name followed by a colon, and
1610 then an optional, comma-separated list of keys and values in the form key=value.
1611 [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
1615 <programlisting>unix:path=/tmp/dbus-test</programlisting>
1616 Which is the address to a unix socket with the path /tmp/dbus-test.
1619 [FIXME clarify if attempting to connect to each is a requirement
1620 or just a suggestion]
1621 When connecting to a server, multiple server addresses can be
1622 separated by a semi-colon. The library will then try to connect
1623 to the first address and if that fails, it'll try to connect to
1624 the next one specified, and so forth. For example
1625 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
1628 [FIXME we need to specify in detail each transport and its possible arguments]
1629 Current transports include: unix domain sockets (including
1630 abstract namespace on linux), TCP/IP, and a debug/testing transport using
1631 in-process pipes. Future possible transports include one that
1632 tunnels over X11 protocol.
1636 <sect1 id="standard-messages">
1637 <title>Standard Peer-to-Peer Messages</title>
1639 See <xref linkend="message-protocol-types-notation"/> for details on
1640 the notation used in this section.
1642 <sect2 id="standard-messages-ping">
1643 <title><literal>org.freedesktop.Peer.Ping</literal></title>
1646 org.freedesktop.Peer.Ping ()
1650 On receipt of the METHOD_CALL
1651 message <literal>org.freedesktop.Peer.Ping</literal>, an application
1652 should do nothing other than reply with a METHOD_RETURN as usual.
1656 <sect2 id="standard-messages-get-props">
1657 <title><literal>org.freedesktop.Props.Get</literal></title>
1659 [FIXME this is just a bogus made-up method that isn't implemented
1660 or thought through, to save an example of table formatting for the
1661 argument descriptions]
1663 org.freedesktop.Props.Get (in STRING property_name,
1664 out ANY_OR_NIL property_value)
1671 <entry>Argument</entry>
1673 <entry>Description</entry>
1679 <entry>in STRING</entry>
1680 <entry>Name of the property to get</entry>
1684 <entry>out ANY_OR_NIL</entry>
1685 <entry>The value of the property. The type depends on the property.</entry>
1694 <sect1 id="message-bus">
1695 <title>Message Bus Specification</title>
1696 <sect2 id="message-bus-overview">
1697 <title>Message Bus Overview</title>
1699 The message bus accepts connections from one or more applications.
1700 Once connected, applications can send and receive messages from
1701 the message bus, as in the peer-to-peer case.
1704 The message bus keeps track of a set of
1705 <firstterm>services</firstterm>. A service is simply a name, such as
1706 <literal>com.yoyodyne.Screensaver</literal>, which can be
1707 <firstterm>owned</firstterm> by one or more of the connected
1708 applications. The message bus itself always owns the special service
1709 <literal>org.freedesktop.DBus</literal>.
1712 Services may have <firstterm>secondary owners</firstterm>. Secondary owners
1713 of a service are kept in a queue; if the primary owner of a service
1714 disconnects, or releases the service, the next secondary owner becomes
1715 the new owner of the service.
1718 Messages may have a <literal>SERVICE</literal> field (see <xref
1719 linkend="message-protocol-header-fields"/>). When the message bus
1720 receives a message, if the <literal>SERVICE</literal> field is absent, the
1721 message is taken to be a standard peer-to-peer message and interpreted
1722 by the message bus itself. For example, sending
1723 an <literal>org.freedesktop.Peer.Ping</literal> message with no
1724 <literal>SERVICE</literal> will cause the message bus itself to reply
1725 to the ping immediately; the message bus would never make
1726 this message visible to other applications.
1729 If the <literal>SERVICE</literal> field is present, then it indicates a
1730 request for the message bus to route the message. In the usual case,
1731 messages are routed to the owner of the named service.
1732 Messages may also be <firstterm>broadcast</firstterm>
1733 by sending them to the special service
1734 <literal>org.freedesktop.DBus.Broadcast</literal>. Broadcast messages are
1735 sent to all applications with <firstterm>message matching
1736 rules</firstterm> that match the message.
1739 Continuing the <literal>org.freedesktop.Peer.Ping</literal> example, if
1740 the ping message were sent with a <literal>SERVICE</literal> name of
1741 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
1742 forwarded, and the Yoyodyne Corporation screensaver application would be
1743 expected to reply to the ping. If
1744 <literal>org.freedesktop.Peer.Ping</literal> were sent to
1745 <literal>org.freedesktop.DBus.Broadcast</literal>, then multiple applications
1746 might receive the ping, and all would normally reply to it.
1750 <sect2 id="message-bus-services">
1751 <title>Message Bus Services</title>
1753 A service is a name that identifies a certain application. Each
1754 application connected to the message bus has at least one service name
1755 assigned at connection time and returned in response to the
1756 <literal>org.freedesktop.DBus.Hello</literal> message.
1757 This automatically-assigned service name is called
1758 the application's <firstterm>base service</firstterm>.
1759 Base service names are unique and MUST never be reused for two different
1763 Ownership of the base service is a prerequisite for interaction with
1764 the message bus. It logically follows that the base service is always
1765 the first service that an application comes to own, and the last
1766 service that it loses ownership of.
1769 Base service names must begin with the character ':' (ASCII colon
1770 character); service names that are not base service names must not begin
1771 with this character. (The bus must reject any attempt by an application
1772 to manually create a service name beginning with ':'.) This restriction
1773 categorically prevents "spoofing"; messages sent to a base service name
1774 will always go to a single application instance and that instance only.
1777 An application can request additional service names to be associated
1779 <literal>org.freedesktop.DBus.AcquireService</literal>
1780 message. [FIXME what service names are allowed; ASCII or unicode;
1784 [FIXME this needs more detail, and should move the service-related message
1785 descriptions up into this section perhaps]
1786 Service ownership handling can be specified in the flags part
1787 of the <literal>org.freedesktop.DBus.AcquireService</literal>
1788 message. If an application specifies the
1789 DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications
1790 trying to acquire the service will be put in a queue. When the
1791 primary owner disconnects from the bus or removes ownership
1792 from the service, the next application in the queue will be the
1793 primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT
1794 flag is not specified, then the primary owner will lose
1795 ownership whenever another application requests ownership of the
1799 When a client disconnects from the bus, all the services that
1800 the clients own are deleted, or in the case of a service that
1801 prohibits replacement, ownership is transferred to the next
1802 client in the queue, if any.
1805 <sect2 id="message-bus-routing">
1806 <title>Message Bus Message Routing</title>
1808 When a message is received by the message bus, the message's
1809 <literal>sndr</literal> header field MUST be set to the base service of
1810 the application which sent the message. If the service already has
1811 a <literal>sndr</literal> field, the pre-existing field is replaced.
1812 This rule means that a replies are always sent to the base service name,
1813 i.e. to the same application that sent the message being replied to.
1816 [FIXME go into detail about broadcast, multicast, unicast, etc.]
1819 <sect2 id="message-bus-activation">
1820 <title>Message Bus Service Activation</title>
1822 <firstterm>Activation</firstterm> means to locate a service
1823 owner for a service that is currently unowned. For now, it
1824 means to launch an executable that will take ownership of
1825 a particular service.
1828 To find an executable corresponding to a particular service, the bus
1829 daemon looks for <firstterm>service description files</firstterm>.
1830 Service description files define a mapping from service names to
1831 executables. Different kinds of message bus will look for these files
1832 in different places, see <xref linkend="message-bus-types"/>.
1835 [FIXME the file format should be much better specified than
1836 "similar to .desktop entries" esp. since desktop entries are
1837 already badly-specified. ;-)] Service description files have
1838 the ".service" file extension. The message bus will only load
1839 service description files ending with .service; all other
1840 files will be ignored. The file format is similar to that of
1842 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
1843 entries</ulink>. All service description files must be in
1844 UTF-8 encoding. To ensure that there will be no name
1845 collisions, service files must be namespaced using the same
1846 mechanism as messages and service names.
1849 <title>Example service description file</title>
1851 # Sample service description file
1853 Name=org.gnome.ConfigurationDatabase
1854 Exec=/usr/libexec/gconfd-2
1859 When an application requests a service to be activated, the
1860 bus daemon tries to find it in the list of activation
1861 entries. It then tries to spawn the executable associated with
1862 it. If this fails, it will report an error. [FIXME what
1863 happens if two .service files offer the same service; what
1864 kind of error is reported, should we have a way for the client
1868 The executable launched will have the environment variable
1869 <literal>DBUS_ACTIVATION_ADDRESS</literal> set to the address of the
1870 message bus so it can connect and register the appropriate services.
1873 The executable being launched may want to know whether the message bus
1874 activating it is one of the well-known message buses (see <xref
1875 linkend="message-bus-types"/>). To facilitate this, the bus MUST also set
1876 the <literal>DBUS_ACTIVATION_BUS_TYPE</literal> environment variable if it is one
1877 of the well-known buses. The currently-defined values for this variable
1878 are <literal>system</literal> for the systemwide message bus,
1879 and <literal>session</literal> for the per-login-session message
1880 bus. The activated executable must still connect to the address given
1881 in <literal>DBUS_ACTIVATION_ADDRESS</literal>, but may assume that the
1882 resulting connection is to the well-known bus.
1885 [FIXME there should be a timeout somewhere, either specified
1886 in the .service file, by the client, or just a global value
1887 and if the client being activated fails to connect within that
1888 timeout, an error should be sent back.]
1892 <sect2 id="message-bus-types">
1893 <title>Well-known Message Bus Instances</title>
1895 Two standard message bus instances are defined here, along with how
1896 to locate them and where their service files live.
1898 <sect3 id="message-bus-types-login">
1899 <title>Login session message bus</title>
1901 Each time a user logs in, a <firstterm>login session message
1902 bus</firstterm> may be started. All applications in the user's login
1903 session may interact with one another using this message bus.
1906 The address of the login session message bus is given
1907 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
1908 variable. If that variable is not set, applications may
1909 also try to read the address from the X Window System root
1910 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
1911 The root window property must have type <literal>STRING</literal>.
1912 The environment variable should have precedence over the
1913 root window property.
1916 [FIXME specify location of .service files, probably using
1917 DESKTOP_DIRS etc. from basedir specification, though login session
1918 bus is not really desktop-specific]
1921 <sect3 id="message-bus-types-system">
1922 <title>System message bus</title>
1924 A computer may have a <firstterm>system message bus</firstterm>,
1925 accessible to all applications on the system. This message bus may be
1926 used to broadcast system events, such as adding new hardware devices,
1927 changes in the printer queue, and so forth.
1930 The address of the login session message bus is given
1931 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
1932 variable. If that variable is not set, applications should try
1933 to connect to the well-known address
1934 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
1937 The D-BUS reference implementation actually honors the
1938 <literal>$(localstatedir)</literal> configure option
1939 for this address, on both client and server side.
1944 [FIXME specify location of system bus .service files]
1949 <sect2 id="message-bus-messages">
1950 <title>Message Bus Messages</title>
1952 The special message bus service <literal>org.freedesktop.DBus</literal>
1953 responds to a number of messages, allowing applications to
1954 interact with the message bus.
1957 <sect3 id="bus-messages-hello">
1958 <title><literal>org.freedesktop.DBus.Hello</literal></title>
1969 <entry>Argument</entry>
1971 <entry>Description</entry>
1977 <entry>STRING</entry>
1978 <entry>Name of the service assigned to the application</entry>
1985 Before an application is able to send messages to other
1986 applications it must send the
1987 <literal>org.freedesktop.DBus.Hello</literal> message to the
1988 message bus service. If an application tries to send a
1989 message to another application, or a message to the message
1990 bus service that isn't the
1991 <literal>org.freedesktop.DBus.Hello</literal> message, it
1992 will be disconnected from the bus. If a client wishes to
1993 disconnect from the bus, it just has to disconnect from the
1994 transport used. No de-registration message is necessary.
1997 The reply message contains the name of the application's base service.
2000 <sect3 id="bus-messages-list-services">
2001 <title><literal>org.freedesktop.DBus.ListServices</literal></title>
2005 STRING_ARRAY ListServices ()
2012 <entry>Argument</entry>
2014 <entry>Description</entry>
2020 <entry>STRING_ARRAY</entry>
2021 <entry>Array of strings where each string is the name of a service</entry>
2028 Returns a list of all existing services registered with the message bus.
2031 <sect3 id="bus-messages-service-exists">
2032 <title><literal>org.freedesktop.DBus.ServiceExists</literal></title>
2036 BOOLEAN ServiceExists (in STRING service_name)
2043 <entry>Argument</entry>
2045 <entry>Description</entry>
2051 <entry>STRING</entry>
2052 <entry>Name of the service</entry>
2062 <entry>Argument</entry>
2064 <entry>Description</entry>
2070 <entry>BOOLEAN</entry>
2071 <entry>Return value, true if the service exists</entry>
2078 Checks if a service with a specified name exists.
2082 <sect3 id="bus-messages-acquire-service">
2083 <title><literal>org.freedesktop.DBus.AcquireService</literal></title>
2087 UINT32 AcquireService (in STRING service_name)
2094 <entry>Argument</entry>
2096 <entry>Description</entry>
2102 <entry>STRING</entry>
2103 <entry>Name of the service</entry>
2107 <entry>UINT32</entry>
2108 <entry>Flags</entry>
2118 <entry>Argument</entry>
2120 <entry>Description</entry>
2126 <entry>UINT32</entry>
2127 <entry>Return value</entry>
2134 Tries to become owner of a specific service. The flags
2135 specified can be the following values logically ORed together:
2141 <entry>Identifier</entry>
2142 <entry>Value</entry>
2143 <entry>Description</entry>
2148 <entry>DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT</entry>
2151 If the application succeeds in being the owner of the specified service,
2152 then ownership of the service can't be transferred until the service
2153 disconnects. If this flag is not set, then any application trying to become
2154 the owner of the service will succeed and the previous owner will be
2155 sent a <literal>org.freedesktop.DBus.ServiceLost</literal> message.
2159 <entry>DBUS_SERVICE_FLAGS_REPLACE_EXISTING</entry>
2161 <entry>Try to replace the current owner if there is one. If this flag
2162 is not set the application will only become the owner of the service if
2163 there is no current owner.</entry>
2169 [FIXME if it's one of the following values, why are the values
2170 done as flags instead of just 0, 1, 2, 3, 4]
2171 The return value can be one of the following values:
2177 <entry>Identifier</entry>
2178 <entry>Value</entry>
2179 <entry>Description</entry>
2184 <entry>DBUS_SERVICE_REPLY_PRIMARY_OWNER</entry>
2186 <entry>The application is now the primary owner of the service.</entry>
2189 <entry>DBUS_SERVICE_REPLY_IN_QUEUE</entry>
2191 <entry>The service already has an owner which do not want to give up ownership and therefore the application has been put in a queue.</entry>
2194 <entry>DBUS_SERVICE_REPLY_SERVICE_EXISTS</entry>
2196 <entry>The service does already have a primary owner, and DBUS_SERVICE_FLAG_REPLACE_EXISTING was not specified when trying to acquire the service.</entry>
2199 <entry>DBUS_SERVICE_REPLY_ALREADY_OWNER</entry>
2201 <entry>The application trying to request ownership of the service is already the owner of it.</entry>
2208 <sect3 id="bus-messages-service-acquired">
2209 <title><literal>org.freedesktop.DBus.ServiceAcquired</literal></title>
2213 ServiceAcquired (in STRING service_name)
2220 <entry>Argument</entry>
2222 <entry>Description</entry>
2228 <entry>STRING</entry>
2229 <entry>Name of the service</entry>
2233 <entry>UINT32</entry>
2234 <entry>Flags</entry>
2241 This message is sent to a specific application when it becomes the
2242 primary owner of a service.
2245 <sect3 id="bus-messages-service-lost">
2246 <title><literal>org.freedesktop.DBus.ServiceLost</literal></title>
2250 ServiceLost (in STRING service_name)
2257 <entry>Argument</entry>
2259 <entry>Description</entry>
2265 <entry>STRING</entry>
2266 <entry>Name of the service</entry>
2270 <entry>UINT32</entry>
2271 <entry>Flags</entry>
2278 This message is sent to a specific application when it loses primary
2279 ownership of a service.
2281 [FIXME instead of ServiceLost/ServiceCreated going only to
2282 a specific app, why not just OwnerChanged that covers both
2283 lost and created and changed owner and deleted]
2287 <sect3 id="bus-messages-service-created">
2288 <title><literal>org.freedesktop.DBus.ServiceCreated</literal></title>
2292 ServiceCreated (in STRING service_name)
2299 <entry>Argument</entry>
2301 <entry>Description</entry>
2307 <entry>STRING</entry>
2308 <entry>Name of the service</entry>
2312 <entry>UINT32</entry>
2313 <entry>Flags</entry>
2320 This message is broadcast to all applications when a service has been
2321 successfully registered on the message bus.
2325 <sect3 id="bus-messages-service-deleted">
2326 <title><literal>org.freedesktop.DBus.ServiceDeleted</literal></title>
2330 ServiceDeleted (in STRING service_name)
2337 <entry>Argument</entry>
2339 <entry>Description</entry>
2345 <entry>STRING</entry>
2346 <entry>Name of the service</entry>
2350 <entry>UINT32</entry>
2351 <entry>Flags</entry>
2358 This message is broadcast to all applications when a service has been
2359 deleted from the message bus.
2363 <sect3 id="bus-messages-activate-service">
2364 <title><literal>org.freedesktop.DBus.ActivateService</literal></title>
2368 UINT32 ActivateService (in STRING service_name, in UINT32 flags)
2375 <entry>Argument</entry>
2377 <entry>Description</entry>
2383 <entry>STRING</entry>
2384 <entry>Name of the service to activate</entry>
2388 <entry>UINT32</entry>
2389 <entry>Flags (currently not used)</entry>
2399 <entry>Argument</entry>
2401 <entry>Description</entry>
2407 <entry>UINT32</entry>
2408 <entry>Return value</entry>
2413 Tries to launch the executable associated with a service. For more information, see <xref linkend="message-bus-activation"/>.
2415 [FIXME need semantics in much more detail here; for example,
2416 if I activate a service then send it a message, is the message
2417 queued for the new service or is there a race]
2420 The return value can be one of the following values:
2425 <entry>Identifier</entry>
2426 <entry>Value</entry>
2427 <entry>Description</entry>
2432 <entry>DBUS_ACTIVATION_REPLY_ACTIVATED</entry>
2434 <entry>The service was activated successfully.</entry>
2437 <entry>DBUS_ACTIVATION_REPLY_ALREADY_ACTIVE</entry>
2439 <entry>The service is already active.</entry>
2448 <sect3 id="bus-messages-get-service-owner">
2449 <title><literal>org.freedesktop.DBus.GetServiceOwner</literal></title>
2453 STRING GetServiceOwner (in STRING service_name)
2460 <entry>Argument</entry>
2462 <entry>Description</entry>
2468 <entry>STRING</entry>
2469 <entry>Name of the service to query</entry>
2479 <entry>Argument</entry>
2481 <entry>Description</entry>
2487 <entry>STRING</entry>
2488 <entry>Return value, a base service name</entry>
2493 Returns the base service name of the primary owner of the
2494 service in argument. If the requested service isn't active,
2496 <literal>org.freedesktop.DBus.Error.ServiceHasNoOwner</literal> error.
2500 <sect3 id="bus-messages-out-of-memory">
2501 <title><literal>org.freedesktop.DBus.Error.NoMemory</literal></title>
2509 Sent by the message bus when it can't process a message due to an out of memory failure.
2513 <sect3 id="bus-messages-service-does-not-exist">
2514 <title><literal>org.freedesktop.DBus.Error.ServiceDoesNotExist</literal></title>
2518 void ServiceDoesNotExist (in STRING error)
2522 Sent by the message bus as a reply to a client that tried to send a message to a service that doesn't exist.
2529 <appendix id="implementation-notes">
2530 <title>Implementation notes</title>
2531 <sect1 id="implementation-notes-subsection">
2539 <glossary><title>Glossary</title>
2541 This glossary defines some of the terms used in this specification.
2544 <glossentry id="term-activation"><glossterm>Activation</glossterm>
2547 The process of creating an owner for a particular service,
2548 typically by launching an executable.
2553 <glossentry id="term-base-service"><glossterm>Base Service</glossterm>
2556 The special service automatically assigned to an application by the
2557 message bus. This service may never change owner, and the service
2558 name will be unique (never reused during the lifetime of the
2564 <glossentry id="term-broadcast"><glossterm>Broadcast</glossterm>
2567 A message sent to the special <literal>org.freedesktop.DBus.Broadcast</literal>
2568 service; the message bus will forward the broadcast message
2569 to all applications that have expressed interest in it.
2574 <glossentry id="term-message"><glossterm>Message</glossterm>
2577 A message is the atomic unit of communication via the D-BUS
2578 protocol. It consists of a <firstterm>header</firstterm> and a
2579 <firstterm>body</firstterm>; the body is made up of
2580 <firstterm>arguments</firstterm>.
2585 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
2588 The message bus is a special application that forwards
2589 or broadcasts messages between a group of applications
2590 connected to the message bus. It also manages
2591 <firstterm>services</firstterm>.
2596 <glossentry id="namespace"><glossterm>Namespace</glossterm>
2599 Used to prevent collisions when defining message and service
2600 names. The convention used is the same as Java uses for
2601 defining classes: a reversed domain name.
2606 <glossentry id="term-object"><glossterm>Object</glossterm>
2609 Each application contains <firstterm>objects</firstterm>,
2610 which have <firstterm>interfaces</firstterm> and
2611 <firstterm>methods</firstterm>. Objects are referred to
2612 by a name, called a <firstterm>path</firstterm> or
2613 <firstterm>object reference</firstterm>.
2618 <glossentry id="term-path"><glossterm>Path</glossterm>
2621 Object references (object names) in D-BUS are
2622 organized into a filesystem-style hierarchy, so
2623 each object is named by a path. As in LDAP,
2624 there's no difference between "files" and "directories";
2625 a path can refer to an object, while still having
2626 child objects below it.
2631 <glossentry id="peer-to-peer"><glossterm>Peer-to-peer</glossterm>
2634 An application talking directly to another application, without going through a message bus.
2638 <glossentry id="term-secondary-owner"><glossterm>Secondary service owner</glossterm>
2641 Each service has a primary owner; messages sent to the service name
2642 go to the primary owner. However, certain services also maintain
2643 a queue of secondary owners "waiting in the wings." If
2644 the primary owner releases the service, then the first secondary
2645 owner in the queue automatically becomes the primary owner.
2649 <glossentry id="term-service"><glossterm>Service</glossterm>
2652 A service is simply a named list of applications. For example, the
2653 hypothetical <literal>com.yoyodyne.Screensaver</literal> service might
2654 accept messages that affect a screensaver from Yoyodyne Corporation.
2655 An application is said to <firstterm>own</firstterm> a service if the
2656 message bus has associated the application with the service name.
2657 Services may also have <firstterm>secondary owners</firstterm> (see
2658 <xref linkend="term-secondary-owner"/>).
2662 <glossentry id="term-service-name"><glossterm>Service name</glossterm>
2665 The name used when referring to a service. If the service is
2666 a base service it has a unique service name, for example
2667 ":1-20", and otherwise it should be namespaced.
2671 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
2674 ".service files" tell the bus how to activate a particular service.
2675 See <xref linkend="term-activation"/>