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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>
258 <sect2 id="message-protocol-header-fields">
259 <title>Header Fields</title>
261 In addition to the required header information mentioned
262 in <xref linkend="message-protocol-header-encoding"/>,
263 the header may contain zero or more named
264 header fields. Future versions of this protocol
265 specification may add new fields. Implementations must
266 ignore fields they do not understand. Implementations
267 must not invent their own header fields; only changes to
268 this specification may introduce new header fields.
272 Header field names MUST consist of a single byte, possible values
273 of which are defined below. Following the name, the field MUST have
274 a type code represented as a single unsigned byte, and then a
275 properly-aligned value of that type. See <xref
276 linkend="message-protocol-arguments"/> for a description of how each
277 type is encoded. If an implementation sees a header field name that
278 it does not understand, it MUST ignore that field.
282 Here are the currently-defined named header fields:
287 <entry>Conventional Name</entry>
288 <entry>Decimal Value</entry>
290 <entry>Description</entry>
295 <entry>INVALID</entry>
297 <entry>INVALID</entry>
298 <entry>Not a valid field name (error if it appears in a message)</entry>
303 <entry>OBJECT_PATH</entry>
304 <entry>The object to send the message to; objects are identified by
305 a path, "/foo/bar"</entry>
308 <entry>INTERFACE</entry>
310 <entry>STRING</entry>
311 <entry>The interface to invoke a method call on, or
312 that a signal is emitted from. e.g. "org.freedesktop.Introspectable"</entry>
315 <entry>MEMBER</entry>
317 <entry>STRING</entry>
318 <entry>The member, either the method name or signal name.
319 e.g. "Frobate"</entry>
322 <entry>ERROR_NAME</entry>
324 <entry>STRING</entry>
325 <entry>The name of the error that occurred, for errors</entry>
328 <entry>REPLY_SERIAL</entry>
330 <entry>UINT32</entry>
331 <entry>The serial number of the message this message is a reply
332 to. (The serial number is one of the mandatory header fields,
333 see <xref linkend="message-protocol-header-encoding"/>.)</entry>
336 <entry>SERVICE</entry>
338 <entry>STRING</entry>
339 <entry>The name of the service this message should be routed to.
340 Only used in combination with the message bus, see
341 <xref linkend="message-bus"/>.</entry>
344 <entry>SENDER_SERVICE</entry>
346 <entry>STRING</entry>
347 <entry>Sender service. The name of the base service that sent
348 this message. The message bus fills in this field; the field is
349 only meaningful in combination with the message bus.</entry>
357 <sect2 id="message-protocol-header-padding">
358 <title>Header Alignment Padding</title>
360 To allow implementations to keep the header and the body in a single
361 buffer while keeping data types aligned, the total length of the header
362 must be a multiple of 8 bytes. To achieve this, the header MUST be padded
363 with nul bytes to align its total length on an 8-byte boundary.
364 The minimum number of padding bytes MUST be used. Because zero is an
365 invalid field name, implementations can distinguish padding (which must be
366 zero initialized) from additional named fields.
370 <sect2 id="message-protocol-arguments">
371 <title>Message Arguments</title>
373 The message body is made up of arguments. Each argument is a type code,
374 represented by a single unsigned byte, followed by the aligned value of
375 the argument in a type-dependent format. Alignment padding between the
376 typecode and the value is initialized to zero.
383 <entry>Type name</entry>
385 <entry>Description</entry>
390 <entry>INVALID</entry>
391 <entry>0 (ASCII NUL)</entry>
392 <entry>Not a valid type code (error if it appears in a message)</entry>
395 <entry>118 (ASCII 'v') </entry>
396 <entry>Marks a "void"/"unset"/"nonexistent"/"null" argument</entry>
399 <entry>121 (ASCII 'y')</entry>
400 <entry>8-bit unsigned integer</entry>
402 <entry>BOOLEAN</entry>
403 <entry>98 (ASCII 'b')</entry>
404 <entry>Boolean value, 0 is FALSE and 1 is TRUE. Everything else is invalid.</entry>
407 <entry>105 (ASCII 'i')</entry>
408 <entry>32-bit signed integer</entry>
410 <entry>UINT32</entry>
411 <entry>117 (ASCII 'u')</entry>
412 <entry>32-bit unsigned integer</entry>
415 <entry>120 (ASCII 'x')</entry>
416 <entry>64-bit signed integer</entry>
418 <entry>UINT64</entry>
419 <entry>116 (ASCII 't')</entry>
420 <entry>64-bit unsigned integer</entry>
422 <entry>DOUBLE</entry>
423 <entry>100 (ASCII 'd')</entry>
424 <entry>IEEE 754 double</entry>
426 <entry>STRING</entry>
427 <entry>115 (ASCII 's')</entry>
428 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be zero terminated. </entry>
430 <entry>CUSTOM</entry>
431 <entry>99 (ASCII 'c')</entry>
432 <entry>A named byte array, used for custom types</entry>
435 <entry>97 (ASCII 'a')</entry>
439 <entry>109 (ASCII 'm')</entry>
440 <entry>A dictionary of key/value pairs</entry>
442 <entry>OBJECT_PATH</entry>
443 <entry>111 (ASCII 'o')</entry>
444 <entry>Name of an object</entry>
451 The types are encoded as follows:
456 <entry>Type name</entry>
457 <entry>Encoding</entry>
462 <entry>INVALID</entry>
463 <entry>Not applicable; cannot be encoded.</entry>
466 <entry>No data is encoded; the type code is followed immediately
467 by the type code of the next argument.</entry>
470 <entry>A byte.</entry>
472 <entry>BOOLEAN</entry>
473 <entry>A byte, with valid values 0 and 1.</entry>
476 <entry>32-bit signed integer in the message's byte order, aligned to 4-byte boundary.</entry>
478 <entry>UINT32</entry>
479 <entry>32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary.</entry>
482 <entry>64-bit signed integer in the message's byte order, aligned to 8-byte boundary.</entry>
484 <entry>UINT64</entry>
485 <entry>64-bit unsigned integer in the message's byte order, aligned to 8-byte boundary.</entry>
487 <entry>DOUBLE</entry>
488 <entry>64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary.</entry>
490 <entry>STRING</entry>
491 <entry>UINT32 aligned to 4-byte boundary indicating the string's
492 length in bytes excluding its terminating nul, followed by
493 string data of the given length, followed by a terminating nul
497 <entry>CUSTOM</entry>
498 <entry>A string (encoded as the STRING type above) giving the
499 name of the type followed by an UINT32 aligned to 4-byte boundary
500 indicating the data length in bytes, followed by the data.
501 The string has some restrictions on its content, see
502 <xref linkend="message-protocol-names"/>.
506 <entry>A sequence of bytes giving the element type of the array, terminated
507 by a type different from ARRAY (just one byte for one-dimensional arrays, but
508 larger for multi-dimensional arrays), followed by an UINT32 (aligned to 4 bytes)
509 giving the length of the array data in bytes. This is followed by each array entry
510 encoded the way it would normally be encoded, except arrays, which are encoded
511 without the type information, since that is already declared above. Arrays containing
516 <entry>UINT32 giving the length of the dictionary data in bytes.
517 This is followed by a number of keyname/value pairs, where the
518 keyname is encoded as a STRING above, and the value is encoded
519 as a byte with typecode and how that type normally would be encoded
523 <entry>OBJECT_PATH</entry>
524 <entry>Encoded as if it were a STRING.
533 <sect2 id="message-protocol-names">
534 <title>Valid names</title>
536 The various names in D-BUS messages have some restrictions.
538 <sect3 id="message-protocol-names-interface">
539 <title>Interface names</title>
541 Interfaces have names with type STRING, meaning that
542 they must be valid UTF-8. However, there are also some
543 additional restrictions that apply to interface names
546 <listitem><para>They are composed of 1 or more elements separated by
547 a period ('.') character. All elements must contain at least
551 <listitem><para>Each element must only contain the ASCII characters
552 "[A-Z][a-z][0-9]_" and must not begin with a digit.
556 <listitem><para>They must contain at least one '.' (period)
557 character (and thus at least two elements).
560 <listitem><para>They must not begin with a '.' (period) character.</para></listitem>
561 <listitem><para>They must not exceed 256 bytes in length.</para></listitem>
562 <listitem><para>They must be at least 1 byte in length.</para></listitem>
566 <sect3 id="message-protocol-names-service">
567 <title>Service names</title>
569 Service names have the same restrictions as interface names, with a
570 special exception for base services. A base service name's first
571 element must start with a colon (':') character. After the colon, any
572 characters in the range "[A-Z][a-z][0-9]_" may appear. Elements after
573 the first must follow the usual rules, except that they may start with
574 a digit. Service names not starting with a colon have none of these
575 exceptions and follow the same rules as interface names.
578 <sect3 id="message-protocol-names-method">
579 <title>Method names</title>
583 <listitem><para>Must only contain the ASCII characters
584 "[A-Z][a-z][0-9]_" and may not begin with a
585 digit.</para></listitem>
586 <listitem><para>Must not contain the '.' (period) character</para></listitem>
587 <listitem><para>Must not exceed 256 bytes in length</para></listitem>
588 <listitem><para>Must be at least 1 byte in length</para></listitem>
592 <sect3 id="message-protocol-names-path">
593 <title>Path names</title>
595 A path (type OBJECT_PATH) must begin with an ASCII '/' (slash)
596 character. Paths may not end with a slash character unless the path is
597 the one-byte string "/". Two slash characters may not appear adjacent
598 to one another (the empty string is not a valid "subdirectory"). Paths
599 may not exceed 256 bytes in length.
602 <sect3 id="message-protocol-names-error">
603 <title>Error names</title>
605 Error names have the same restrictions as interface names.
608 <sect3 id="message-protocol-names-custom">
609 <title>Custom types</title>
611 Custom type names for values of type CUSTOM follow the same
612 restrictions as interface names.
617 <sect2 id="message-protocol-types">
618 <title>Message types</title>
620 Each of the message types (METHOD_CALL, METHOD_RETURN, ERROR, and
621 SIGNAL) has its own expected usage conventions and header fields.
623 <sect3 id="message-protocol-types-method">
624 <title>Method Calls, Returns, and Errors</title>
626 Some messages invoke an operation on a remote object. These are
627 called method call messages and have the type tag METHOD_CALL. Such
628 messages map naturally to methods on objects in a typical program.
631 A method call message is expected to have a MEMBER header field
632 indicating the name of the method. Optionally, the message has an
633 INTERFACE field giving the interface the method is a part of. In the
634 absence of an INTERFACE field, if two interfaces on the same object have
635 a method with the same name, it is undefined which of the two methods
636 will be invoked. Implementations may also choose to return an error in
637 this ambiguous case. However, if a method name is unique
638 implementations should not require an interface field.
641 Method call messages also include a PATH field indicating the
642 object to invoke the method on. If the call is passing through
643 a message bus, the message will also have a SERVICE field giving
644 the service to receive the message.
647 When an application handles a method call message, it is expected to
648 return a reply. The reply is identified by a REPLY_SERIAL header field
649 indicating the serial number of the METHOD_CALL being replied to. The
650 reply can have one of two types; either METHOD_RETURN or ERROR.
653 If the reply has type METHOD_RETURN, the arguments to the reply message
654 are the return value(s) or "out parameters" of the method call.
655 If the reply has type ERROR, then an "exception" has been thrown,
656 and the call fails; no return value will be provided. It makes
657 no sense to send multiple replies to the same method call.
660 Even if a method call has no return values, a METHOD_RETURN
661 reply is expected, so the caller will know the method
662 was successfully processed.
665 The METHOD_RETURN or ERROR reply message MUST have the REPLY_SERIAL
666 header field. If this field is missing, it should be treated as
670 If a METHOD_CALL message has the flag NO_REPLY_EXPECTED,
671 then as an optimization the application receiving the method
672 call may choose to omit the reply message (regardless of
673 whether the reply would have been METHOD_RETURN or ERROR).
674 However, it is also acceptable to ignore the NO_REPLY_EXPECTED
675 flag and reply anyway.
677 <sect4 id="message-protocol-types-method-apis">
678 <title>Mapping method calls to native APIs</title>
680 APIs for D-BUS may map method calls to a method call in a specific
681 programming language, such as C++, or may map a method call written
682 in an IDL to a D-BUS message.
685 In APIs of this nature, arguments to a method are often termed "in"
686 (which implies sent in the METHOD_CALL), or "out" (which implies
687 returned in the METHOD_RETURN). Some APIs such as CORBA also have
688 "inout" arguments, which are both sent and received, i.e. the caller
689 passes in a value which is modified. Mapped to D-BUS, an "inout"
690 argument is equivalent to an "in" argument, followed by an "out"
691 argument. You can't pass things "by reference" over the wire, so
692 "inout" is purely an illusion of the in-process API.
695 Given a method with zero or one return values, followed by zero or more
696 arguments, where each argument may be "in", "out", or "inout", the
697 caller constructs a message by appending each "in" or "inout" argument,
698 in order. "out" arguments are not represented in the caller's message.
701 The recipient constructs a reply by appending first the return value
702 if any, then each "out" or "inout" argument, in order.
703 "in" arguments are not represented in the reply message.
709 <sect3 id="message-protocol-types-signal">
710 <title>Signal Emission</title>
712 Unlike method calls, signal emissions have no replies.
713 A signal emission is simply a single message of type SIGNAL.
714 It must have three header fields: PATH giving the object
715 the signal was emitted from, plus INTERFACE and MEMBER giving
716 the fully-qualified name of the signal.
720 <sect3 id="message-protocol-types-notation">
721 <title>Notation in this document</title>
723 This document uses a simple pseudo-IDL to describe particular method
724 calls and signals. Here is an example of a method call:
726 org.freedesktop.DBus.ActivateService (in STRING service_name, in UINT32 flags,
727 out UINT32 resultcode)
729 This means INTERFACE = org.freedesktop.DBus, MEMBER = ActivateService,
730 METHOD_CALL arguments are STRING and UINT32, METHOD_RETURN argument
731 is UINT32. Remember that the MEMBER field can't contain any '.' (period)
732 characters so it's known that the last part of the name in
733 the "IDL" is the member name.
736 In C++ that might end up looking like this:
738 unsigned int org::freedesktop::DBus::ActivateService (const char *service_name,
741 or equally valid, the return value could be done as an argument:
743 void org::freedesktop::DBus::ActivateService (const char *service_name,
745 unsigned int *resultcode);
747 It's really up to the API designer how they want to make
748 this look. You could design an API where the namespace wasn't used
749 in C++, using STL or Qt, using varargs, or whatever you wanted.
752 Signals are written as follows:
754 org.freedesktop.DBus.ServiceLost (STRING service_name)
756 Signals don't specify "in" vs. "out" because only
757 a single direction is possible.
760 In this ad hoc notation, the special type name ANY means any type
761 other than NIL, and the special type name ANY_OR_NIL means any valid
765 It isn't especially encouraged to use this lame pseudo-IDL in actual
766 API implementations; you might use the native notation for the
767 language you're using, or you might use COM or CORBA IDL, for example.
774 <sect1 id="auth-protocol">
775 <title>Authentication Protocol</title>
777 Before the flow of messages begins, two applications must
778 authenticate. A simple plain-text protocol is used for
779 authentication; this protocol is a SASL profile, and maps fairly
780 directly from the SASL specification. The message encoding is
781 NOT used here, only plain text messages.
784 In examples, "C:" and "S:" indicate lines sent by the client and
787 <sect2 id="auth-protocol-overview">
788 <title>Protocol Overview</title>
790 The protocol is a line-based protocol, where each line ends with
791 \r\n. Each line begins with an all-caps ASCII command name containing
792 only the character range [A-Z], a space, then any arguments for the
793 command, then the \r\n ending the line. The protocol is
794 case-sensitive. All bytes must be in the ASCII character set.
796 Commands from the client to the server are as follows:
799 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
800 <listitem><para>CANCEL</para></listitem>
801 <listitem><para>BEGIN</para></listitem>
802 <listitem><para>DATA <data in base 64 encoding></para></listitem>
803 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
806 From server to client are as follows:
809 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
810 <listitem><para>OK</para></listitem>
811 <listitem><para>DATA <data in base 64 encoding></para></listitem>
812 <listitem><para>ERROR</para></listitem>
816 <sect2 id="auth-nul-byte">
817 <title>Special credentials-passing nul byte</title>
819 Immediately after connecting to the server, the client must send a
820 single nul byte. This byte may be accompanied by credentials
821 information on some operating systems that use sendmsg() with
822 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
823 sockets. However, the nul byte MUST be sent even on other kinds of
824 socket, and even on operating systems that do not require a byte to be
825 sent in order to transmit credentials. The text protocol described in
826 this document begins after the single nul byte. If the first byte
827 received from the client is not a nul byte, the server may disconnect
831 A nul byte in any context other than the initial byte is an error;
832 the protocol is ASCII-only.
835 The credentials sent along with the nul byte may be used with the
836 SASL mechanism EXTERNAL.
839 <sect2 id="auth-command-auth">
840 <title>AUTH command</title>
842 If an AUTH command has no arguments, it is a request to list
843 available mechanisms. The server SHOULD respond with a REJECTED
844 command listing the mechanisms it understands.
847 If an AUTH command specifies a mechanism, and the server supports
848 said mechanism, the server SHOULD begin exchanging SASL
849 challenge-response data with the client using DATA commands.
852 If the server does not support the mechanism given in the AUTH
853 command, it SHOULD send a REJECTED command listing the mechanisms
857 If the [initial-response] argument is provided, it is intended for
858 use with mechanisms that have no initial challenge (or an empty
859 initial challenge), as if it were the argument to an initial DATA
860 command. If the selected mechanism has an initial challenge, the
861 server should reject authentication by sending REJECTED.
864 If authentication succeeds after exchanging DATA commands,
865 an OK command should be sent to the client.
868 The first octet received by the client after the \r\n of the OK
869 command MUST be the first octet of the authenticated/encrypted
870 stream of D-BUS messages.
873 The first octet received by the server after the \r\n of the BEGIN
874 command from the client MUST be the first octet of the
875 authenticated/encrypted stream of D-BUS messages.
878 <sect2 id="auth-command-cancel">
879 <title>CANCEL Command</title>
881 At any time up to sending the BEGIN command, the client may send a
882 CANCEL command. On receiving the CANCEL command, the server MUST
883 send a REJECTED command and abort the current authentication
887 <sect2 id="auth-command-data">
888 <title>DATA Command</title>
890 The DATA command may come from either client or server, and simply
891 contains a base64-encoded block of data to be interpreted
892 according to the SASL mechanism in use.
895 Some SASL mechanisms support sending an "empty string";
896 FIXME we need some way to do this.
899 <sect2 id="auth-command-begin">
900 <title>BEGIN Command</title>
902 The BEGIN command acknowledges that the client has received an
903 OK command from the server, and that the stream of messages
907 The first octet received by the server after the \r\n of the BEGIN
908 command from the client MUST be the first octet of the
909 authenticated/encrypted stream of D-BUS messages.
912 <sect2 id="auth-command-rejected">
913 <title>REJECTED Command</title>
915 The REJECTED command indicates that the current authentication
916 exchange has failed, and further exchange of DATA is inappropriate.
917 The client would normally try another mechanism, or try providing
918 different responses to challenges.
920 Optionally, the REJECTED command has a space-separated list of
921 available auth mechanisms as arguments. If a server ever provides
922 a list of supported mechanisms, it MUST provide the same list
923 each time it sends a REJECTED message. Clients are free to
924 ignore all lists received after the first.
927 <sect2 id="auth-command-ok">
928 <title>OK Command</title>
930 The OK command indicates that the client has been authenticated,
931 and that further communication will be a stream of D-BUS messages
932 (optionally encrypted, as negotiated) rather than this protocol.
935 The first octet received by the client after the \r\n of the OK
936 command MUST be the first octet of the authenticated/encrypted
937 stream of D-BUS messages.
940 The client MUST respond to the OK command by sending a BEGIN
941 command, followed by its stream of messages, or by disconnecting.
942 The server MUST NOT accept additional commands using this protocol
943 after the OK command has been sent.
946 <sect2 id="auth-command-error">
947 <title>ERROR Command</title>
949 The ERROR command indicates that either server or client did not
950 know a command, does not accept the given command in the current
951 context, or did not understand the arguments to the command. This
952 allows the protocol to be extended; a client or server can send a
953 command present or permitted only in new protocol versions, and if
954 an ERROR is received instead of an appropriate response, fall back
955 to using some other technique.
958 If an ERROR is sent, the server or client that sent the
959 error MUST continue as if the command causing the ERROR had never been
960 received. However, the the server or client receiving the error
961 should try something other than whatever caused the error;
962 if only canceling/rejecting the authentication.
965 <sect2 id="auth-examples">
966 <title>Authentication examples</title>
970 <title>Example of successful magic cookie authentication</title>
972 (MAGIC_COOKIE is a made up mechanism)
974 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
980 <title>Example of finding out mechanisms then picking one</title>
983 S: REJECTED KERBEROS_V4 SKEY
984 C: AUTH SKEY bW9yZ2Fu
985 S: DATA OTUgUWE1ODMwOA==
986 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
992 <title>Example of client sends unknown command then falls back to regular auth</title>
996 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1002 <title>Example of server doesn't support initial auth mechanism</title>
1004 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1005 S: REJECTED KERBEROS_V4 SKEY
1006 C: AUTH SKEY bW9yZ2Fu
1007 S: DATA OTUgUWE1ODMwOA==
1008 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1014 <title>Example of wrong password or the like followed by successful retry</title>
1016 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1017 S: REJECTED KERBEROS_V4 SKEY
1018 C: AUTH SKEY bW9yZ2Fu
1019 S: DATA OTUgUWE1ODMwOA==
1020 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1022 C: AUTH SKEY bW9yZ2Fu
1023 S: DATA OTUgUWE1ODMwOA==
1024 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1030 <title>Example of skey cancelled and restarted</title>
1032 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1033 S: REJECTED KERBEROS_V4 SKEY
1034 C: AUTH SKEY bW9yZ2Fu
1035 S: DATA OTUgUWE1ODMwOA==
1038 C: AUTH SKEY bW9yZ2Fu
1039 S: DATA OTUgUWE1ODMwOA==
1040 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1047 <sect2 id="auth-states">
1048 <title>Authentication state diagrams</title>
1051 This section documents the auth protocol in terms of
1052 a state machine for the client and the server. This is
1053 probably the most robust way to implement the protocol.
1056 <sect3 id="auth-states-client">
1057 <title>Client states</title>
1060 <title><emphasis>Start</emphasis></title>
1063 <listitem><para>send AUTH with initial data -> <emphasis>WaitingForData</emphasis></para></listitem>
1064 <listitem><para>send AUTH with no initial data -> <emphasis>WaitingForData</emphasis> or <emphasis>NeedSendData</emphasis> (depends on mechanism)</para></listitem>
1066 The <emphasis>Start</emphasis> state is stateful (it has a list of
1067 available mechanisms and those it has already attempted). This list
1068 is used to decide which AUTH command to send. When the list is
1069 exhausted, the client should give up and close the connection.
1074 <title><emphasis>WaitingForData</emphasis></title>
1078 <listitem><para>receive OK -> <emphasis>NeedSendBegin</emphasis></para></listitem>
1079 <listitem><para>receive REJECTED -> <emphasis>Start</emphasis></para></listitem>
1080 <listitem><para>receive ERROR -> <emphasis>Start</emphasis></para></listitem>
1081 <listitem><para>receive DATA -> <emphasis>NeedSendData</emphasis></para></listitem>
1082 <listitem><para>receive anything else -> <emphasis>NeedSendError</emphasis></para></listitem>
1084 When going back to <emphasis>Start</emphasis>, the mechanism in
1085 progress should be marked as failed and not retried (at least not
1086 with the same parameters). When receiving REJECTED with a list of
1087 mechanisms, the list should be recorded and used to select
1093 <title><emphasis>NeedSendData</emphasis></title>
1096 <listitem><para>send DATA -> <emphasis>WaitingForData</emphasis></para></listitem>
1097 <listitem><para>send CANCEL -> <emphasis>Start</emphasis></para></listitem>
1103 <title><emphasis>NeedSendError</emphasis></title>
1107 <listitem><para>send ERROR -> return to previous state</para></listitem>
1113 <title><emphasis>NeedSendBegin</emphasis></title>
1117 <listitem><para>send BEGIN -> Authorized</para></listitem>
1123 <title><emphasis>Authorized</emphasis></title>
1125 This is the end state, flow of messages begins.
1131 <sect3 id="auth-states-server">
1132 <title>Server states</title>
1135 <title><emphasis>WaitingForAuth</emphasis></title>
1138 <listitem><para>receive AUTH with initial response -> <emphasis>NeedSendData</emphasis></para></listitem>
1139 <listitem><para>receive AUTH without initial response -> <emphasis>NeedSendData</emphasis> or <emphasis>WaitingForData</emphasis> depending on mechanism</para></listitem>
1145 <title><emphasis>NeedSendData</emphasis></title>
1148 <listitem><para>send DATA -> <emphasis>WaitingForData</emphasis></para></listitem>
1149 <listitem><para>send ERROR -> <emphasis>WaitingForData</emphasis></para></listitem>
1150 <listitem><para>send REJECTED -> <emphasis>WaitingForAuth</emphasis></para></listitem>
1151 <listitem><para>send OK -> <emphasis>WaitingForBegin</emphasis></para></listitem>
1157 <title><emphasis>WaitingForData</emphasis></title>
1160 <listitem><para>receive DATA -> <emphasis>NeedSendData</emphasis></para></listitem>
1161 <listitem><para>receive CANCEL -> <emphasis>NeedSendRejected</emphasis></para></listitem>
1162 <listitem><para>receive ERROR -> <emphasis>NeedSendRejected</emphasis></para></listitem>
1163 <listitem><para>receive anything else -> <emphasis>NeedSendError</emphasis></para></listitem>
1169 <title><emphasis>NeedSendError</emphasis></title>
1173 <listitem><para>send ERROR -> return to previous state</para></listitem>
1179 <title><emphasis>NeedSendRejected</emphasis></title>
1183 <listitem><para>send REJECTED -> <emphasis>WaitingForAuth</emphasis></para></listitem>
1189 <title><emphasis>WaitingForBegin</emphasis></title>
1193 <listitem><para>receive BEGIN -> <emphasis>Authorized</emphasis></para></listitem>
1194 <listitem><para>receive anything else -> <emphasis>NeedSendError</emphasis></para></listitem>
1200 <title><emphasis>Authorized</emphasis></title>
1202 This is the end state, flow of messages begins.
1209 <sect2 id="auth-mechanisms">
1210 <title>Authentication mechanisms</title>
1212 This section describes some new authentication mechanisms.
1213 D-BUS also allows any standard SASL mechanism of course.
1215 <sect3 id="auth-mechanisms-sha">
1216 <title>DBUS_COOKIE_SHA1</title>
1218 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
1219 has the ability to read a private file owned by the user being
1220 authenticated. If the client can prove that it has access to a secret
1221 cookie stored in this file, then the client is authenticated.
1222 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
1226 Authentication proceeds as follows:
1230 The client sends the username it would like to authenticate
1236 The server sends the name of its "cookie context" (see below); a
1237 space character; the integer ID of the secret cookie the client
1238 must demonstrate knowledge of; a space character; then a
1239 hex-encoded randomly-generated challenge string.
1244 The client locates the cookie, and generates its own hex-encoded
1245 randomly-generated challenge string. The client then
1246 concatentates the server's hex-encoded challenge, a ":"
1247 character, its own hex-encoded challenge, another ":" character,
1248 and the hex-encoded cookie. It computes the SHA-1 hash of this
1249 composite string. It sends back to the server the client's
1250 hex-encoded challenge string, a space character, and the SHA-1
1256 The server generates the same concatenated string used by the
1257 client and computes its SHA-1 hash. It compares the hash with
1258 the hash received from the client; if the two hashes match, the
1259 client is authenticated.
1265 Each server has a "cookie context," which is a name that identifies a
1266 set of cookies that apply to that server. A sample context might be
1267 "org_freedesktop_session_bus". Context names must be valid ASCII,
1268 nonzero length, and may not contain the characters slash ("/"),
1269 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
1270 tab ("\t"), or period ("."). There is a default context,
1271 "org_freedesktop_global" that's used by servers that do not specify
1275 Cookies are stored in a user's home directory, in the directory
1276 <filename>~/.dbus-keyrings/</filename>. This directory must
1277 not be readable or writable by other users. If it is,
1278 clients and servers must ignore it. The directory
1279 contains cookie files named after the cookie context.
1282 A cookie file contains one cookie per line. Each line
1283 has three space-separated fields:
1287 The cookie ID number, which must be a non-negative integer and
1288 may not be used twice in the same file.
1293 The cookie's creation time, in UNIX seconds-since-the-epoch
1299 The cookie itself, a hex-encoded random block of bytes.
1305 Only server processes modify the cookie file.
1306 They must do so with this procedure:
1310 Create a lockfile name by appending ".lock" to the name of the
1311 cookie file. The server should attempt to create this file
1312 using <literal>O_CREAT | O_EXCL</literal>. If file creation
1313 fails, the lock fails. Servers should retry for a reasonable
1314 period of time, then they may choose to delete an existing lock
1315 to keep users from having to manually delete a stale
1316 lock. <footnote><para>Lockfiles are used instead of real file
1317 locking <literal>fcntl()</literal> because real locking
1318 implementations are still flaky on network
1319 filesystems.</para></footnote>
1324 Once the lockfile has been created, the server loads the cookie
1325 file. It should then delete any cookies that are old (the
1326 timeout can be fairly short), or more than a reasonable
1327 time in the future (so that cookies never accidentally
1328 become permanent, if the clock was set far into the future
1329 at some point). If no recent keys remain, the
1330 server may generate a new key.
1335 The pruned and possibly added-to cookie file
1336 must be resaved atomically (using a temporary
1337 file which is rename()'d).
1342 The lock must be dropped by deleting the lockfile.
1348 Clients need not lock the file in order to load it,
1349 because servers are required to save the file atomically.
1354 <sect1 id="addresses">
1355 <title>Server Addresses</title>
1357 Server addresses consist of a transport name followed by a colon, and
1358 then an optional, comma-separated list of keys and values in the form key=value.
1359 [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
1363 <programlisting>unix:path=/tmp/dbus-test</programlisting>
1364 Which is the address to a unix socket with the path /tmp/dbus-test.
1367 [FIXME clarify if attempting to connect to each is a requirement
1368 or just a suggestion]
1369 When connecting to a server, multiple server addresses can be
1370 separated by a semi-colon. The library will then try to connect
1371 to the first address and if that fails, it'll try to connect to
1372 the next one specified, and so forth. For example
1373 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
1376 [FIXME we need to specify in detail each transport and its possible arguments]
1377 Current transports include: unix domain sockets (including
1378 abstract namespace on linux), TCP/IP, and a debug/testing transport using
1379 in-process pipes. Future possible transports include one that
1380 tunnels over X11 protocol.
1384 <sect1 id="standard-messages">
1385 <title>Standard Peer-to-Peer Messages</title>
1387 See <xref linkend="message-protocol-types-notation"/> for details on
1388 the notation used in this section.
1390 <sect2 id="standard-messages-ping">
1391 <title><literal>org.freedesktop.Peer.Ping</literal></title>
1394 org.freedesktop.Peer.Ping ()
1398 On receipt of the METHOD_CALL
1399 message <literal>org.freedesktop.Peer.Ping</literal>, an application
1400 should do nothing other than reply with a METHOD_RETURN as usual.
1404 <sect2 id="standard-messages-get-props">
1405 <title><literal>org.freedesktop.Props.Get</literal></title>
1407 [FIXME this is just a bogus made-up method that isn't implemented
1408 or thought through, to save an example of table formatting for the
1409 argument descriptions]
1411 org.freedesktop.Props.Get (in STRING property_name,
1412 out ANY_OR_NIL property_value)
1419 <entry>Argument</entry>
1421 <entry>Description</entry>
1427 <entry>in STRING</entry>
1428 <entry>Name of the property to get</entry>
1432 <entry>out ANY_OR_NIL</entry>
1433 <entry>The value of the property. The type depends on the property.</entry>
1442 <sect1 id="message-bus">
1443 <title>Message Bus Specification</title>
1444 <sect2 id="message-bus-overview">
1445 <title>Message Bus Overview</title>
1447 The message bus accepts connections from one or more applications.
1448 Once connected, applications can send and receive messages from
1449 the message bus, as in the peer-to-peer case.
1452 The message bus keeps track of a set of
1453 <firstterm>services</firstterm>. A service is simply a name, such as
1454 <literal>com.yoyodyne.Screensaver</literal>, which can be
1455 <firstterm>owned</firstterm> by one or more of the connected
1456 applications. The message bus itself always owns the special service
1457 <literal>org.freedesktop.DBus</literal>.
1460 Services may have <firstterm>secondary owners</firstterm>. Secondary owners
1461 of a service are kept in a queue; if the primary owner of a service
1462 disconnects, or releases the service, the next secondary owner becomes
1463 the new owner of the service.
1466 Messages may have a <literal>SERVICE</literal> field (see <xref
1467 linkend="message-protocol-header-fields"/>). When the message bus
1468 receives a message, if the <literal>SERVICE</literal> field is absent, the
1469 message is taken to be a standard peer-to-peer message and interpreted
1470 by the message bus itself. For example, sending
1471 an <literal>org.freedesktop.Peer.Ping</literal> message with no
1472 <literal>SERVICE</literal> will cause the message bus itself to reply
1473 to the ping immediately; the message bus would never make
1474 this message visible to other applications.
1477 If the <literal>SERVICE</literal> field is present, then it indicates a
1478 request for the message bus to route the message. In the usual case,
1479 messages are routed to the owner of the named service.
1480 Messages may also be <firstterm>broadcast</firstterm>
1481 by sending them to the special service
1482 <literal>org.freedesktop.DBus.Broadcast</literal>. Broadcast messages are
1483 sent to all applications with <firstterm>message matching
1484 rules</firstterm> that match the message.
1487 Continuing the <literal>org.freedesktop.Peer.Ping</literal> example, if
1488 the ping message were sent with a <literal>SERVICE</literal> name of
1489 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
1490 forwarded, and the Yoyodyne Corporation screensaver application would be
1491 expected to reply to the ping. If
1492 <literal>org.freedesktop.Peer.Ping</literal> were sent to
1493 <literal>org.freedesktop.DBus.Broadcast</literal>, then multiple applications
1494 might receive the ping, and all would normally reply to it.
1498 <sect2 id="message-bus-services">
1499 <title>Message Bus Services</title>
1501 A service is a name that identifies a certain application. Each
1502 application connected to the message bus has at least one service name
1503 assigned at connection time and returned in response to the
1504 <literal>org.freedesktop.DBus.Hello</literal> message.
1505 This automatically-assigned service name is called
1506 the application's <firstterm>base service</firstterm>.
1507 Base service names are unique and MUST never be reused for two different
1511 Ownership of the base service is a prerequisite for interaction with
1512 the message bus. It logically follows that the base service is always
1513 the first service that an application comes to own, and the last
1514 service that it loses ownership of.
1517 Base service names must begin with the character ':' (ASCII colon
1518 character); service names that are not base service names must not begin
1519 with this character. (The bus must reject any attempt by an application
1520 to manually create a service name beginning with ':'.) This restriction
1521 categorically prevents "spoofing"; messages sent to a base service name
1522 will always go to a single application instance and that instance only.
1525 An application can request additional service names to be associated
1527 <literal>org.freedesktop.DBus.AcquireService</literal>
1528 message. [FIXME what service names are allowed; ASCII or unicode;
1532 [FIXME this needs more detail, and should move the service-related message
1533 descriptions up into this section perhaps]
1534 Service ownership handling can be specified in the flags part
1535 of the <literal>org.freedesktop.DBus.AcquireService</literal>
1536 message. If an application specifies the
1537 DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications
1538 trying to acquire the service will be put in a queue. When the
1539 primary owner disconnects from the bus or removes ownership
1540 from the service, the next application in the queue will be the
1541 primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT
1542 flag is not specified, then the primary owner will lose
1543 ownership whenever another application requests ownership of the
1547 When a client disconnects from the bus, all the services that
1548 the clients own are deleted, or in the case of a service that
1549 prohibits replacement, ownership is transferred to the next
1550 client in the queue, if any.
1553 <sect2 id="message-bus-routing">
1554 <title>Message Bus Message Routing</title>
1556 When a message is received by the message bus, the message's
1557 <literal>sndr</literal> header field MUST be set to the base service of
1558 the application which sent the message. If the service already has
1559 a <literal>sndr</literal> field, the pre-existing field is replaced.
1560 This rule means that a replies are always sent to the base service name,
1561 i.e. to the same application that sent the message being replied to.
1564 [FIXME go into detail about broadcast, multicast, unicast, etc.]
1567 <sect2 id="message-bus-activation">
1568 <title>Message Bus Service Activation</title>
1570 <firstterm>Activation</firstterm> means to locate a service
1571 owner for a service that is currently unowned. For now, it
1572 means to launch an executable that will take ownership of
1573 a particular service.
1576 To find an executable corresponding to a particular service, the bus
1577 daemon looks for <firstterm>service description files</firstterm>.
1578 Service description files define a mapping from service names to
1579 executables. Different kinds of message bus will look for these files
1580 in different places, see <xref linkend="message-bus-types"/>.
1583 [FIXME the file format should be much better specified than
1584 "similar to .desktop entries" esp. since desktop entries are
1585 already badly-specified. ;-)] Service description files have
1586 the ".service" file extension. The message bus will only load
1587 service description files ending with .service; all other
1588 files will be ignored. The file format is similar to that of
1590 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
1591 entries</ulink>. All service description files must be in
1592 UTF-8 encoding. To ensure that there will be no name
1593 collisions, service files must be namespaced using the same
1594 mechanism as messages and service names.
1597 <title>Example service description file</title>
1599 # Sample service description file
1601 Name=org.gnome.ConfigurationDatabase
1602 Exec=/usr/libexec/gconfd-2
1607 When an application requests a service to be activated, the
1608 bus daemon tries to find it in the list of activation
1609 entries. It then tries to spawn the executable associated with
1610 it. If this fails, it will report an error. [FIXME what
1611 happens if two .service files offer the same service; what
1612 kind of error is reported, should we have a way for the client
1616 The executable launched will have the environment variable
1617 <literal>DBUS_ACTIVATION_ADDRESS</literal> set to the address of the
1618 message bus so it can connect and register the appropriate services.
1621 The executable being launched may want to know whether the message bus
1622 activating it is one of the well-known message buses (see <xref
1623 linkend="message-bus-types"/>). To facilitate this, the bus MUST also set
1624 the <literal>DBUS_ACTIVATION_BUS_TYPE</literal> environment variable if it is one
1625 of the well-known buses. The currently-defined values for this variable
1626 are <literal>system</literal> for the systemwide message bus,
1627 and <literal>session</literal> for the per-login-session message
1628 bus. The activated executable must still connect to the address given
1629 in <literal>DBUS_ACTIVATION_ADDRESS</literal>, but may assume that the
1630 resulting connection is to the well-known bus.
1633 [FIXME there should be a timeout somewhere, either specified
1634 in the .service file, by the client, or just a global value
1635 and if the client being activated fails to connect within that
1636 timeout, an error should be sent back.]
1640 <sect2 id="message-bus-types">
1641 <title>Well-known Message Bus Instances</title>
1643 Two standard message bus instances are defined here, along with how
1644 to locate them and where their service files live.
1646 <sect3 id="message-bus-types-login">
1647 <title>Login session message bus</title>
1649 Each time a user logs in, a <firstterm>login session message
1650 bus</firstterm> may be started. All applications in the user's login
1651 session may interact with one another using this message bus.
1654 The address of the login session message bus is given
1655 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
1656 variable. If that variable is not set, applications may
1657 also try to read the address from the X Window System root
1658 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
1659 The root window property must have type <literal>STRING</literal>.
1660 The environment variable should have precedence over the
1661 root window property.
1664 [FIXME specify location of .service files, probably using
1665 DESKTOP_DIRS etc. from basedir specification, though login session
1666 bus is not really desktop-specific]
1669 <sect3 id="message-bus-types-system">
1670 <title>System message bus</title>
1672 A computer may have a <firstterm>system message bus</firstterm>,
1673 accessible to all applications on the system. This message bus may be
1674 used to broadcast system events, such as adding new hardware devices,
1675 changes in the printer queue, and so forth.
1678 The address of the login session message bus is given
1679 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
1680 variable. If that variable is not set, applications should try
1681 to connect to the well-known address
1682 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
1685 The D-BUS reference implementation actually honors the
1686 <literal>$(localstatedir)</literal> configure option
1687 for this address, on both client and server side.
1692 [FIXME specify location of system bus .service files]
1697 <sect2 id="message-bus-messages">
1698 <title>Message Bus Messages</title>
1700 The special message bus service <literal>org.freedesktop.DBus</literal>
1701 responds to a number of messages, allowing applications to
1702 interact with the message bus.
1705 <sect3 id="bus-messages-hello">
1706 <title><literal>org.freedesktop.DBus.Hello</literal></title>
1717 <entry>Argument</entry>
1719 <entry>Description</entry>
1725 <entry>STRING</entry>
1726 <entry>Name of the service assigned to the application</entry>
1733 Before an application is able to send messages to other
1734 applications it must send the
1735 <literal>org.freedesktop.DBus.Hello</literal> message to the
1736 message bus service. If an application tries to send a
1737 message to another application, or a message to the message
1738 bus service that isn't the
1739 <literal>org.freedesktop.DBus.Hello</literal> message, it
1740 will be disconnected from the bus. If a client wishes to
1741 disconnect from the bus, it just has to disconnect from the
1742 transport used. No de-registration message is necessary.
1745 The reply message contains the name of the application's base service.
1748 <sect3 id="bus-messages-list-services">
1749 <title><literal>org.freedesktop.DBus.ListServices</literal></title>
1753 STRING_ARRAY ListServices ()
1760 <entry>Argument</entry>
1762 <entry>Description</entry>
1768 <entry>STRING_ARRAY</entry>
1769 <entry>Array of strings where each string is the name of a service</entry>
1776 Returns a list of all existing services registered with the message bus.
1779 <sect3 id="bus-messages-service-exists">
1780 <title><literal>org.freedesktop.DBus.ServiceExists</literal></title>
1784 BOOLEAN ServiceExists (in STRING service_name)
1791 <entry>Argument</entry>
1793 <entry>Description</entry>
1799 <entry>STRING</entry>
1800 <entry>Name of the service</entry>
1810 <entry>Argument</entry>
1812 <entry>Description</entry>
1818 <entry>BOOLEAN</entry>
1819 <entry>Return value, true if the service exists</entry>
1826 Checks if a service with a specified name exists.
1830 <sect3 id="bus-messages-acquire-service">
1831 <title><literal>org.freedesktop.DBus.AcquireService</literal></title>
1835 UINT32 AcquireService (in STRING service_name)
1842 <entry>Argument</entry>
1844 <entry>Description</entry>
1850 <entry>STRING</entry>
1851 <entry>Name of the service</entry>
1855 <entry>UINT32</entry>
1856 <entry>Flags</entry>
1866 <entry>Argument</entry>
1868 <entry>Description</entry>
1874 <entry>UINT32</entry>
1875 <entry>Return value</entry>
1882 Tries to become owner of a specific service. The flags
1883 specified can be the following values logically ORed together:
1889 <entry>Identifier</entry>
1890 <entry>Value</entry>
1891 <entry>Description</entry>
1896 <entry>DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT</entry>
1899 If the application succeeds in being the owner of the specified service,
1900 then ownership of the service can't be transferred until the service
1901 disconnects. If this flag is not set, then any application trying to become
1902 the owner of the service will succeed and the previous owner will be
1903 sent a <literal>org.freedesktop.DBus.ServiceLost</literal> message.
1907 <entry>DBUS_SERVICE_FLAGS_REPLACE_EXISTING</entry>
1909 <entry>Try to replace the current owner if there is one. If this flag
1910 is not set the application will only become the owner of the service if
1911 there is no current owner.</entry>
1917 [FIXME if it's one of the following values, why are the values
1918 done as flags instead of just 0, 1, 2, 3, 4]
1919 The return value can be one of the following values:
1925 <entry>Identifier</entry>
1926 <entry>Value</entry>
1927 <entry>Description</entry>
1932 <entry>DBUS_SERVICE_REPLY_PRIMARY_OWNER</entry>
1934 <entry>The application is now the primary owner of the service.</entry>
1937 <entry>DBUS_SERVICE_REPLY_IN_QUEUE</entry>
1939 <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>
1942 <entry>DBUS_SERVICE_REPLY_SERVICE_EXISTS</entry>
1944 <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>
1947 <entry>DBUS_SERVICE_REPLY_ALREADY_OWNER</entry>
1949 <entry>The application trying to request ownership of the service is already the owner of it.</entry>
1956 <sect3 id="bus-messages-service-acquired">
1957 <title><literal>org.freedesktop.DBus.ServiceAcquired</literal></title>
1961 ServiceAcquired (in STRING service_name)
1968 <entry>Argument</entry>
1970 <entry>Description</entry>
1976 <entry>STRING</entry>
1977 <entry>Name of the service</entry>
1981 <entry>UINT32</entry>
1982 <entry>Flags</entry>
1989 This message is sent to a specific application when it becomes the
1990 primary owner of a service.
1993 <sect3 id="bus-messages-service-lost">
1994 <title><literal>org.freedesktop.DBus.ServiceLost</literal></title>
1998 ServiceLost (in STRING service_name)
2005 <entry>Argument</entry>
2007 <entry>Description</entry>
2013 <entry>STRING</entry>
2014 <entry>Name of the service</entry>
2018 <entry>UINT32</entry>
2019 <entry>Flags</entry>
2026 This message is sent to a specific application when it loses primary
2027 ownership of a service.
2029 [FIXME instead of ServiceLost/ServiceCreated going only to
2030 a specific app, why not just OwnerChanged that covers both
2031 lost and created and changed owner and deleted]
2035 <sect3 id="bus-messages-service-created">
2036 <title><literal>org.freedesktop.DBus.ServiceCreated</literal></title>
2040 ServiceCreated (in STRING service_name)
2047 <entry>Argument</entry>
2049 <entry>Description</entry>
2055 <entry>STRING</entry>
2056 <entry>Name of the service</entry>
2060 <entry>UINT32</entry>
2061 <entry>Flags</entry>
2068 This message is broadcast to all applications when a service has been
2069 successfully registered on the message bus.
2073 <sect3 id="bus-messages-service-deleted">
2074 <title><literal>org.freedesktop.DBus.ServiceDeleted</literal></title>
2078 ServiceDeleted (in STRING service_name)
2085 <entry>Argument</entry>
2087 <entry>Description</entry>
2093 <entry>STRING</entry>
2094 <entry>Name of the service</entry>
2098 <entry>UINT32</entry>
2099 <entry>Flags</entry>
2106 This message is broadcast to all applications when a service has been
2107 deleted from the message bus.
2111 <sect3 id="bus-messages-activate-service">
2112 <title><literal>org.freedesktop.DBus.ActivateService</literal></title>
2116 UINT32 ActivateService (in STRING service_name, in UINT32 flags)
2123 <entry>Argument</entry>
2125 <entry>Description</entry>
2131 <entry>STRING</entry>
2132 <entry>Name of the service to activate</entry>
2136 <entry>UINT32</entry>
2137 <entry>Flags (currently not used)</entry>
2147 <entry>Argument</entry>
2149 <entry>Description</entry>
2155 <entry>UINT32</entry>
2156 <entry>Return value</entry>
2161 Tries to launch the executable associated with a service. For more information, see <xref linkend="message-bus-activation"/>.
2163 [FIXME need semantics in much more detail here; for example,
2164 if I activate a service then send it a message, is the message
2165 queued for the new service or is there a race]
2168 The return value can be one of the following values:
2173 <entry>Identifier</entry>
2174 <entry>Value</entry>
2175 <entry>Description</entry>
2180 <entry>DBUS_ACTIVATION_REPLY_ACTIVATED</entry>
2182 <entry>The service was activated successfully.</entry>
2185 <entry>DBUS_ACTIVATION_REPLY_ALREADY_ACTIVE</entry>
2187 <entry>The service is already active.</entry>
2196 <sect3 id="bus-messages-out-of-memory">
2197 <title><literal>org.freedesktop.DBus.Error.NoMemory</literal></title>
2205 Sent by the message bus when it can't process a message due to an out of memory failure.
2209 <sect3 id="bus-messages-service-does-not-exist">
2210 <title><literal>org.freedesktop.DBus.Error.ServiceDoesNotExist</literal></title>
2214 void ServiceDoesNotExist (in STRING error)
2218 Sent by the message bus as a reply to a client that tried to send a message to a service that doesn't exist.
2225 <appendix id="implementation-notes">
2226 <title>Implementation notes</title>
2227 <sect1 id="implementation-notes-subsection">
2235 <glossary><title>Glossary</title>
2237 This glossary defines some of the terms used in this specification.
2240 <glossentry id="term-activation"><glossterm>Activation</glossterm>
2243 The process of creating an owner for a particular service,
2244 typically by launching an executable.
2249 <glossentry id="term-base-service"><glossterm>Base Service</glossterm>
2252 The special service automatically assigned to an application by the
2253 message bus. This service may never change owner, and the service
2254 name will be unique (never reused during the lifetime of the
2260 <glossentry id="term-broadcast"><glossterm>Broadcast</glossterm>
2263 A message sent to the special <literal>org.freedesktop.DBus.Broadcast</literal>
2264 service; the message bus will forward the broadcast message
2265 to all applications that have expressed interest in it.
2270 <glossentry id="term-message"><glossterm>Message</glossterm>
2273 A message is the atomic unit of communication via the D-BUS
2274 protocol. It consists of a <firstterm>header</firstterm> and a
2275 <firstterm>body</firstterm>; the body is made up of
2276 <firstterm>arguments</firstterm>.
2281 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
2284 The message bus is a special application that forwards
2285 or broadcasts messages between a group of applications
2286 connected to the message bus. It also manages
2287 <firstterm>services</firstterm>.
2292 <glossentry id="namespace"><glossterm>Namespace</glossterm>
2295 Used to prevent collisions when defining message and service
2296 names. The convention used is the same as Java uses for
2297 defining classes: a reversed domain name.
2302 <glossentry id="term-object"><glossterm>Object</glossterm>
2305 Each application contains <firstterm>objects</firstterm>,
2306 which have <firstterm>interfaces</firstterm> and
2307 <firstterm>methods</firstterm>. Objects are referred to
2308 by a name, called a <firstterm>path</firstterm> or
2309 <firstterm>object reference</firstterm>.
2314 <glossentry id="term-path"><glossterm>Path</glossterm>
2317 Object references (object names) in D-BUS are
2318 organized into a filesystem-style hierarchy, so
2319 each object is named by a path. As in LDAP,
2320 there's no difference between "files" and "directories";
2321 a path can refer to an object, while still having
2322 child objects below it.
2327 <glossentry id="peer-to-peer"><glossterm>Peer-to-peer</glossterm>
2330 An application talking directly to another application, without going through a message bus.
2334 <glossentry id="term-secondary-owner"><glossterm>Secondary service owner</glossterm>
2337 Each service has a primary owner; messages sent to the service name
2338 go to the primary owner. However, certain services also maintain
2339 a queue of secondary owners "waiting in the wings." If
2340 the primary owner releases the service, then the first secondary
2341 owner in the queue automatically becomes the primary owner.
2345 <glossentry id="term-service"><glossterm>Service</glossterm>
2348 A service is simply a named list of applications. For example, the
2349 hypothetical <literal>com.yoyodyne.Screensaver</literal> service might
2350 accept messages that affect a screensaver from Yoyodyne Corporation.
2351 An application is said to <firstterm>own</firstterm> a service if the
2352 message bus has associated the application with the service name.
2353 Services may also have <firstterm>secondary owners</firstterm> (see
2354 <xref linkend="term-secondary-owner"/>).
2358 <glossentry id="term-service-name"><glossterm>Service name</glossterm>
2361 The name used when referring to a service. If the service is
2362 a base service it has a unique service name, for example
2363 ":1-20", and otherwise it should be namespaced.
2367 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
2370 ".service files" tell the bus how to activate a particular service.
2371 See <xref linkend="term-activation"/>