1 <?xml version="1.0" standalone="no"?>
2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
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>STRING</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>
392 <entry>Not a valid type code (error if it appears in a message)</entry>
396 <entry>Marks an "unset" or "nonexistent" argument</entry>
400 <entry>8-bit unsigned integer</entry>
402 <entry>BOOLEAN</entry>
404 <entry>Boolean value, 0 is FALSE and 1 is TRUE. Everything else is invalid.</entry>
408 <entry>32-bit signed integer</entry>
410 <entry>UINT32</entry>
412 <entry>32-bit unsigned integer</entry>
416 <entry>64-bit signed integer</entry>
418 <entry>UINT64</entry>
420 <entry>64-bit unsigned integer</entry>
422 <entry>DOUBLE</entry>
424 <entry>IEEE 754 double</entry>
426 <entry>STRING</entry>
428 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be zero terminated. </entry>
432 <entry>A named byte array, used for custom types</entry>
440 <entry>A dictionary of key/value pairs</entry>
447 The types are encoded as follows:
452 <entry>Type name</entry>
453 <entry>Encoding</entry>
458 <entry>INVALID</entry>
459 <entry>Not applicable; cannot be encoded.</entry>
462 <entry>No data is encoded; the type code is followed immediately
463 by the type code of the next argument.</entry>
466 <entry>A byte.</entry>
468 <entry>BOOLEAN</entry>
469 <entry>A byte, with valid values 0 and 1.</entry>
472 <entry>32-bit signed integer in the message's byte order, aligned to 4-byte boundary.</entry>
474 <entry>UINT32</entry>
475 <entry>32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary.</entry>
478 <entry>64-bit signed integer in the message's byte order, aligned to 8-byte boundary.</entry>
480 <entry>UINT64</entry>
481 <entry>64-bit unsigned integer in the message's byte order, aligned to 8-byte boundary.</entry>
483 <entry>DOUBLE</entry>
484 <entry>64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary.</entry>
486 <entry>STRING</entry>
487 <entry>UINT32 aligned to 4-byte boundary indicating the string's
488 length in bytes excluding its terminating nul, followed by
489 string data of the given length, followed by a terminating nul
494 <entry>A string (encoded as the STRING type above) giving the
495 name of the type followed by an UINT32 aligned to 4-byte boundary
496 indicating the data length in bytes, followed by the data.
500 <entry>A sequence of bytes giving the element type of the array, terminated
501 by a type different from ARRAY (just one byte for one-dimensional arrays, but
502 larger for multi-dimensional arrays), followed by an UINT32 (aligned to 4 bytes)
503 giving the length of the array data in bytes. This is followed by each array entry
504 encoded the way it would normally be encoded, except arrays, which are encoded
505 without the type information, since that is already declared above. Arrays containing
510 <entry>UINT32 giving the length of the dictionary data in bytes.
511 This is followed by a number of keyname/value pairs, where the
512 keyname is encoded as a STRING above, and the value is encoded
513 as a byte with typecode and how that type normally would be encoded
523 <sect2 id="message-protocol-names">
524 <title>Valid names</title>
526 The various header fields of type STRING have some restrictions
527 on the string's format.
529 <sect3 id="message-protocol-names-service">
530 <title>Service names</title>
532 Services have names with type STRING, meaning that
533 they must be valid UTF-8. However, there are also some
534 additional restrictions that apply to service names
537 <listitem><para>They must contain at least one '.' (period) character</para></listitem>
538 <listitem><para>They must not begin with a '.' (period) character</para></listitem>
539 <listitem><para>They must not exceed 256 bytes in length</para></listitem>
540 <listitem><para>They must be at least 1 byte in length</para></listitem>
543 As a special exception, base service names (those beginning with a colon
544 (':') character) need not contain a period.
547 FIXME really, shouldn't we ban basically everything non-alphanumeric
548 so the name will work in all programming languages?
551 <sect3 id="message-protocol-names-interface">
552 <title>Interface names</title>
554 Interface names have the same restrictions as service names,
555 but do not have the special exception for names beginning with
559 FIXME really, shouldn't we ban basically everything non-alphanumeric
560 so the name will work in all programming languages?
563 <sect3 id="message-protocol-names-method">
564 <title>Method names</title>
568 <listitem><para>May not contain the '.' (period) character</para></listitem>
569 <listitem><para>Must not exceed 256 bytes in length</para></listitem>
570 <listitem><para>Must be at least 1 byte in length</para></listitem>
574 FIXME really, shouldn't we ban basically everything non-alphanumeric
575 so the name will work in all programming languages?
578 <sect3 id="message-protocol-names-path">
579 <title>Path names</title>
581 A path must begin with an ASCII '/' (slash) character. Paths may not
582 end with a slash character unless the path is the one-byte string
583 "/". Two slash characters may not appear adjacent to one another (the
584 empty string is not a valid "subdirectory"). Paths may not exceed
588 <sect3 id="message-protocol-names-error">
589 <title>Error names</title>
591 Error names have the same restrictions as interface names.
594 FIXME really, shouldn't we ban basically everything non-alphanumeric
595 so the name will work in all programming languages?
600 <sect2 id="message-protocol-types">
601 <title>Message types</title>
603 Each of the message types (METHOD_CALL, METHOD_RETURN, ERROR, and
604 SIGNAL) has its own expected usage conventions and header fields.
606 <sect3 id="message-protocol-types-method">
607 <title>Method Calls, Returns, and Errors</title>
609 Some messages invoke an operation on a remote object. These are
610 called method call messages and have the type tag METHOD_CALL. Such
611 messages map naturally to methods on objects in a typical program.
614 A method call message is expected to have a MEMBER header field
615 indicating the name of the method. Optionally, the message has an
616 INTERFACE field giving the interface the method is a part of. In the
617 absence of an INTERFACE field, if two interfaces on the same object have
618 a method with the same name, it is undefined which of the two methods
619 will be invoked. Implementations may also choose to return an error in
620 this ambiguous case. However, if a method name is unique
621 implementations should not require an interface field.
624 Method call messages also include a PATH field indicating the
625 object to invoke the method on. If the call is passing through
626 a message bus, the message will also have a SERVICE field giving
627 the service to receive the message.
630 When an application handles a method call message, it is expected to
631 return a reply. The reply is identified by a REPLY_SERIAL header field
632 indicating the serial number of the METHOD_CALL being replied to. The
633 reply can have one of two types; either METHOD_RETURN or ERROR.
636 If the reply has type METHOD_RETURN, the arguments to the reply message
637 are the return value(s) or "out parameters" of the method call.
638 If the reply has type ERROR, then an "exception" has been thrown,
639 and the call fails; no return value will be provided. It makes
640 no sense to send multiple replies to the same method call.
643 Even if a method call has no return values, a METHOD_RETURN
644 reply is expected, so the caller will know the method
645 was successfully processed.
648 The METHOD_RETURN or ERROR reply message MUST have the REPLY_SERIAL
649 header field. If this field is missing, it should be treated as
653 If a METHOD_CALL message has the flag NO_REPLY_EXPECTED,
654 then as an optimization the application receiving the method
655 call may choose to omit the reply message (regardless of
656 whether the reply would have been METHOD_RETURN or ERROR).
657 However, it is also acceptable to ignore the NO_REPLY_EXPECTED
658 flag and reply anyway.
660 <sect4 id="message-protocol-types-method-apis">
661 <title>Mapping method calls to native APIs</title>
663 APIs for D-BUS may map method calls to a method call in a specific
664 programming language, such as C++, or may map a method call written
665 in an IDL to a D-BUS message.
668 In APIs of this nature, arguments to a method are often termed "in"
669 (which implies sent in the METHOD_CALL), or "out" (which implies
670 returned in the METHOD_RETURN). Some APIs such as CORBA also have
671 "inout" arguments, which are both sent and received, i.e. the caller
672 passes in a value which is modified. Mapped to D-BUS, an "inout"
673 argument is equivalent to an "in" argument, followed by an "out"
674 argument. You can't pass things "by reference" over the wire, so
675 "inout" is purely an illusion of the in-process API.
678 Given a method with zero or one return values, followed by zero or more
679 arguments, where each argument may be "in", "out", or "inout", the
680 caller constructs a message by appending each "in" or "inout" argument,
681 in order. "out" arguments are not represented in the caller's message.
684 The recipient constructs a reply by appending first the return value
685 if any, then each "out" or "inout" argument, in order.
686 "in" arguments are not represented in the reply message.
692 <sect3 id="message-protocol-types-signal">
693 <title>Signal Emission</title>
695 Unlike method calls, signal emissions have no replies.
696 A signal emission is simply a single message of type SIGNAL.
697 It must have three header fields: PATH giving the object
698 the signal was emitted from, plus INTERFACE and MEMBER giving
699 the fully-qualified name of the signal.
703 <sect3 id="message-protocol-types-notation">
704 <title>Notation in this document</title>
706 This document uses a simple pseudo-IDL to describe particular method
707 calls and signals. Here is an example of a method call:
709 org.freedesktop.DBus.ActivateService (in STRING service_name, in UINT32 flags,
710 out UINT32 resultcode)
712 This means INTERFACE = org.freedesktop.DBus, MEMBER = ActivateService,
713 METHOD_CALL arguments are STRING and UINT32, METHOD_RETURN argument
714 is UINT32. Remember that the MEMBER field can't contain any '.' (period)
715 characters so it's known that the last part of the name in
716 the "IDL" is the member name.
719 In C++ that might end up looking like this:
721 unsigned int org::freedesktop::DBus::ActivateService (const char *service_name,
724 or equally valid, the return value could be done as an argument:
726 void org::freedesktop::DBus::ActivateService (const char *service_name,
728 unsigned int *resultcode);
730 It's really up to the API designer how they want to make
731 this look. You could design an API where the namespace wasn't used
732 in C++, using STL or Qt, using varargs, or whatever you wanted.
735 Signals are written as follows:
737 org.freedesktop.DBus.ServiceLost (STRING service_name)
739 Signals don't specify "in" vs. "out" because only
740 a single direction is possible.
743 In this ad hoc notation, the special type name ANY means any type
744 other than NIL, and the special type name ANY_OR_NIL means any valid
748 It isn't especially encouraged to use this lame pseudo-IDL in actual
749 API implementations; you might use the native notation for the
750 language you're using, or you might use COM or CORBA IDL, for example.
757 <sect1 id="auth-protocol">
758 <title>Authentication Protocol</title>
760 Before the flow of messages begins, two applications must
761 authenticate. A simple plain-text protocol is used for
762 authentication; this protocol is a SASL profile, and maps fairly
763 directly from the SASL specification. The message encoding is
764 NOT used here, only plain text messages.
767 In examples, "C:" and "S:" indicate lines sent by the client and
770 <sect2 id="auth-protocol-overview">
771 <title>Protocol Overview</title>
773 The protocol is a line-based protocol, where each line ends with
774 \r\n. Each line begins with an all-caps ASCII command name containing
775 only the character range [A-Z], a space, then any arguments for the
776 command, then the \r\n ending the line. The protocol is
777 case-sensitive. All bytes must be in the ASCII character set.
779 Commands from the client to the server are as follows:
782 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
783 <listitem><para>CANCEL</para></listitem>
784 <listitem><para>BEGIN</para></listitem>
785 <listitem><para>DATA <data in base 64 encoding></para></listitem>
786 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
789 From server to client are as follows:
792 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
793 <listitem><para>OK</para></listitem>
794 <listitem><para>DATA <data in base 64 encoding></para></listitem>
795 <listitem><para>ERROR</para></listitem>
799 <sect2 id="auth-nul-byte">
800 <title>Special credentials-passing nul byte</title>
802 Immediately after connecting to the server, the client must send a
803 single nul byte. This byte may be accompanied by credentials
804 information on some operating systems that use sendmsg() with
805 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
806 sockets. However, the nul byte MUST be sent even on other kinds of
807 socket, and even on operating systems that do not require a byte to be
808 sent in order to transmit credentials. The text protocol described in
809 this document begins after the single nul byte. If the first byte
810 received from the client is not a nul byte, the server may disconnect
814 A nul byte in any context other than the initial byte is an error;
815 the protocol is ASCII-only.
818 The credentials sent along with the nul byte may be used with the
819 SASL mechanism EXTERNAL.
822 <sect2 id="auth-command-auth">
823 <title>AUTH command</title>
825 If an AUTH command has no arguments, it is a request to list
826 available mechanisms. The server SHOULD respond with a REJECTED
827 command listing the mechanisms it understands.
830 If an AUTH command specifies a mechanism, and the server supports
831 said mechanism, the server SHOULD begin exchanging SASL
832 challenge-response data with the client using DATA commands.
835 If the server does not support the mechanism given in the AUTH
836 command, it SHOULD send a REJECTED command listing the mechanisms
840 If the [initial-response] argument is provided, it is intended for
841 use with mechanisms that have no initial challenge (or an empty
842 initial challenge), as if it were the argument to an initial DATA
843 command. If the selected mechanism has an initial challenge, the
844 server should reject authentication by sending REJECTED.
847 If authentication succeeds after exchanging DATA commands,
848 an OK command should be sent to the client.
851 The first octet received by the client after the \r\n of the OK
852 command MUST be the first octet of the authenticated/encrypted
853 stream of D-BUS messages.
856 The first octet received by the server after the \r\n of the BEGIN
857 command from the client MUST be the first octet of the
858 authenticated/encrypted stream of D-BUS messages.
861 <sect2 id="auth-command-cancel">
862 <title>CANCEL Command</title>
864 At any time up to sending the BEGIN command, the client may send a
865 CANCEL command. On receiving the CANCEL command, the server MUST
866 send a REJECTED command and abort the current authentication
870 <sect2 id="auth-command-data">
871 <title>DATA Command</title>
873 The DATA command may come from either client or server, and simply
874 contains a base64-encoded block of data to be interpreted
875 according to the SASL mechanism in use.
878 Some SASL mechanisms support sending an "empty string";
879 FIXME we need some way to do this.
882 <sect2 id="auth-command-begin">
883 <title>BEGIN Command</title>
885 The BEGIN command acknowledges that the client has received an
886 OK command from the server, and that the stream of messages
890 The first octet received by the server after the \r\n of the BEGIN
891 command from the client MUST be the first octet of the
892 authenticated/encrypted stream of D-BUS messages.
895 <sect2 id="auth-command-rejected">
896 <title>REJECTED Command</title>
898 The REJECTED command indicates that the current authentication
899 exchange has failed, and further exchange of DATA is inappropriate.
900 The client would normally try another mechanism, or try providing
901 different responses to challenges.
903 Optionally, the REJECTED command has a space-separated list of
904 available auth mechanisms as arguments. If a server ever provides
905 a list of supported mechanisms, it MUST provide the same list
906 each time it sends a REJECTED message. Clients are free to
907 ignore all lists received after the first.
910 <sect2 id="auth-command-ok">
911 <title>OK Command</title>
913 The OK command indicates that the client has been authenticated,
914 and that further communication will be a stream of D-BUS messages
915 (optionally encrypted, as negotiated) rather than this protocol.
918 The first octet received by the client after the \r\n of the OK
919 command MUST be the first octet of the authenticated/encrypted
920 stream of D-BUS messages.
923 The client MUST respond to the OK command by sending a BEGIN
924 command, followed by its stream of messages, or by disconnecting.
925 The server MUST NOT accept additional commands using this protocol
926 after the OK command has been sent.
929 <sect2 id="auth-command-error">
930 <title>ERROR Command</title>
932 The ERROR command indicates that either server or client did not
933 know a command, does not accept the given command in the current
934 context, or did not understand the arguments to the command. This
935 allows the protocol to be extended; a client or server can send a
936 command present or permitted only in new protocol versions, and if
937 an ERROR is received instead of an appropriate response, fall back
938 to using some other technique.
941 If an ERROR is sent, the server or client that sent the
942 error MUST continue as if the command causing the ERROR had never been
943 received. However, the the server or client receiving the error
944 should try something other than whatever caused the error;
945 if only canceling/rejecting the authentication.
948 <sect2 id="auth-examples">
949 <title>Authentication examples</title>
953 <title>Example of successful magic cookie authentication</title>
955 (MAGIC_COOKIE is a made up mechanism)
957 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
963 <title>Example of finding out mechanisms then picking one</title>
966 S: REJECTED KERBEROS_V4 SKEY
967 C: AUTH SKEY bW9yZ2Fu
968 S: DATA OTUgUWE1ODMwOA==
969 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
975 <title>Example of client sends unknown command then falls back to regular auth</title>
979 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
985 <title>Example of server doesn't support initial auth mechanism</title>
987 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
988 S: REJECTED KERBEROS_V4 SKEY
989 C: AUTH SKEY bW9yZ2Fu
990 S: DATA OTUgUWE1ODMwOA==
991 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
997 <title>Example of wrong password or the like followed by successful retry</title>
999 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1000 S: REJECTED KERBEROS_V4 SKEY
1001 C: AUTH SKEY bW9yZ2Fu
1002 S: DATA OTUgUWE1ODMwOA==
1003 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1005 C: AUTH SKEY bW9yZ2Fu
1006 S: DATA OTUgUWE1ODMwOA==
1007 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1013 <title>Example of skey cancelled and restarted</title>
1015 C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
1016 S: REJECTED KERBEROS_V4 SKEY
1017 C: AUTH SKEY bW9yZ2Fu
1018 S: DATA OTUgUWE1ODMwOA==
1021 C: AUTH SKEY bW9yZ2Fu
1022 S: DATA OTUgUWE1ODMwOA==
1023 C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
1030 <sect2 id="auth-states">
1031 <title>Authentication state diagrams</title>
1037 <sect2 id="auth-mechanisms">
1038 <title>Authentication mechanisms</title>
1040 This section describes some new authentication mechanisms.
1041 D-BUS also allows any standard SASL mechanism of course.
1043 <sect3 id="auth-mechanisms-sha">
1044 <title>DBUS_COOKIE_SHA1</title>
1046 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
1047 has the ability to read a private file owned by the user being
1048 authenticated. If the client can prove that it has access to a secret
1049 cookie stored in this file, then the client is authenticated.
1050 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
1054 Authentication proceeds as follows:
1058 The client sends the username it would like to authenticate
1064 The server sends the name of its "cookie context" (see below); a
1065 space character; the integer ID of the secret cookie the client
1066 must demonstrate knowledge of; a space character; then a
1067 hex-encoded randomly-generated challenge string.
1072 The client locates the cookie, and generates its own hex-encoded
1073 randomly-generated challenge string. The client then
1074 concatentates the server's hex-encoded challenge, a ":"
1075 character, its own hex-encoded challenge, another ":" character,
1076 and the hex-encoded cookie. It computes the SHA-1 hash of this
1077 composite string. It sends back to the server the client's
1078 hex-encoded challenge string, a space character, and the SHA-1
1084 The server generates the same concatenated string used by the
1085 client and computes its SHA-1 hash. It compares the hash with
1086 the hash received from the client; if the two hashes match, the
1087 client is authenticated.
1093 Each server has a "cookie context," which is a name that identifies a
1094 set of cookies that apply to that server. A sample context might be
1095 "org_freedesktop_session_bus". Context names must be valid ASCII,
1096 nonzero length, and may not contain the characters slash ("/"),
1097 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
1098 tab ("\t"), or period ("."). There is a default context,
1099 "org_freedesktop_global" that's used by servers that do not specify
1103 Cookies are stored in a user's home directory, in the directory
1104 <filename>~/.dbus-keyrings/</filename>. This directory must
1105 not be readable or writable by other users. If it is,
1106 clients and servers must ignore it. The directory
1107 contains cookie files named after the cookie context.
1110 A cookie file contains one cookie per line. Each line
1111 has three space-separated fields:
1115 The cookie ID number, which must be a non-negative integer and
1116 may not be used twice in the same file.
1121 The cookie's creation time, in UNIX seconds-since-the-epoch
1127 The cookie itself, a hex-encoded random block of bytes.
1133 Only server processes modify the cookie file.
1134 They must do so with this procedure:
1138 Create a lockfile name by appending ".lock" to the name of the
1139 cookie file. The server should attempt to create this file
1140 using <literal>O_CREAT | O_EXCL</literal>. If file creation
1141 fails, the lock fails. Servers should retry for a reasonable
1142 period of time, then they may choose to delete an existing lock
1143 to keep users from having to manually delete a stale
1144 lock. <footnote><para>Lockfiles are used instead of real file
1145 locking <literal>fcntl()</literal> because real locking
1146 implementations are still flaky on network
1147 filesystems.</para></footnote>
1152 Once the lockfile has been created, the server loads the cookie
1153 file. It should then delete any cookies that are old (the
1154 timeout can be fairly short), or more than a reasonable
1155 time in the future (so that cookies never accidentally
1156 become permanent, if the clock was set far into the future
1157 at some point). If no recent keys remain, the
1158 server may generate a new key.
1163 The pruned and possibly added-to cookie file
1164 must be resaved atomically (using a temporary
1165 file which is rename()'d).
1170 The lock must be dropped by deleting the lockfile.
1176 Clients need not lock the file in order to load it,
1177 because servers are required to save the file atomically.
1182 <sect1 id="addresses">
1183 <title>Server Addresses</title>
1185 Server addresses consist of a transport name followed by a colon, and
1186 then an optional, comma-separated list of keys and values in the form key=value.
1187 [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
1191 <programlisting>unix:path=/tmp/dbus-test</programlisting>
1192 Which is the address to a unix socket with the path /tmp/dbus-test.
1195 [FIXME clarify if attempting to connect to each is a requirement
1196 or just a suggestion]
1197 When connecting to a server, multiple server addresses can be
1198 separated by a semi-colon. The library will then try to connect
1199 to the first address and if that fails, it'll try to connect to
1200 the next one specified, and so forth. For example
1201 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
1204 [FIXME we need to specify in detail each transport and its possible arguments]
1205 Current transports include: unix domain sockets (including
1206 abstract namespace on linux), TCP/IP, and a debug/testing transport using
1207 in-process pipes. Future possible transports include one that
1208 tunnels over X11 protocol.
1212 <sect1 id="standard-messages">
1213 <title>Standard Peer-to-Peer Messages</title>
1215 See <xref linkend="message-protocol-types-notation"/> for details on
1216 the notation used in this section.
1218 <sect2 id="standard-messages-ping">
1219 <title><literal>org.freedesktop.Peer.Ping</literal></title>
1222 org.freedesktop.Peer.Ping ()
1226 On receipt of the METHOD_CALL
1227 message <literal>org.freedesktop.Peer.Ping</literal>, an application
1228 should do nothing other than reply with a METHOD_RETURN as usual.
1232 <sect2 id="standard-messages-get-props">
1233 <title><literal>org.freedesktop.Props.Get</literal></title>
1235 [FIXME this is just a bogus made-up method that isn't implemented
1236 or thought through, to save an example of table formatting for the
1237 argument descriptions]
1239 org.freedesktop.Props.Get (in STRING property_name,
1240 out ANY_OR_NIL property_value)
1247 <entry>Argument</entry>
1249 <entry>Description</entry>
1255 <entry>in STRING</entry>
1256 <entry>Name of the property to get</entry>
1260 <entry>out ANY_OR_NIL</entry>
1261 <entry>The value of the property. The type depends on the property.</entry>
1270 <sect1 id="message-bus">
1271 <title>Message Bus Specification</title>
1272 <sect2 id="message-bus-overview">
1273 <title>Message Bus Overview</title>
1275 The message bus accepts connections from one or more applications.
1276 Once connected, applications can send and receive messages from
1277 the message bus, as in the peer-to-peer case.
1280 The message bus keeps track of a set of
1281 <firstterm>services</firstterm>. A service is simply a name, such as
1282 <literal>com.yoyodyne.Screensaver</literal>, which can be
1283 <firstterm>owned</firstterm> by one or more of the connected
1284 applications. The message bus itself always owns the special service
1285 <literal>org.freedesktop.DBus</literal>.
1288 Services may have <firstterm>secondary owners</firstterm>. Secondary owners
1289 of a service are kept in a queue; if the primary owner of a service
1290 disconnects, or releases the service, the next secondary owner becomes
1291 the new owner of the service.
1294 Messages may have a <literal>SERVICE</literal> field (see <xref
1295 linkend="message-protocol-header-fields"/>). When the message bus
1296 receives a message, if the <literal>SERVICE</literal> field is absent, the
1297 message is taken to be a standard peer-to-peer message and interpreted
1298 by the message bus itself. For example, sending
1299 an <literal>org.freedesktop.Peer.Ping</literal> message with no
1300 <literal>SERVICE</literal> will cause the message bus itself to reply
1301 to the ping immediately; the message bus would never make
1302 this message visible to other applications.
1305 If the <literal>SERVICE</literal> field is present, then it indicates a
1306 request for the message bus to route the message. In the usual case,
1307 messages are routed to the owner of the named service.
1308 Messages may also be <firstterm>broadcast</firstterm>
1309 by sending them to the special service
1310 <literal>org.freedesktop.DBus.Broadcast</literal>. Broadcast messages are
1311 sent to all applications with <firstterm>message matching
1312 rules</firstterm> that match the message.
1315 Continuing the <literal>org.freedesktop.Peer.Ping</literal> example, if
1316 the ping message were sent with a <literal>SERVICE</literal> name of
1317 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
1318 forwarded, and the Yoyodyne Corporation screensaver application would be
1319 expected to reply to the ping. If
1320 <literal>org.freedesktop.Peer.Ping</literal> were sent to
1321 <literal>org.freedesktop.DBus.Broadcast</literal>, then multiple applications
1322 might receive the ping, and all would normally reply to it.
1326 <sect2 id="message-bus-services">
1327 <title>Message Bus Services</title>
1329 A service is a name that identifies a certain application. Each
1330 application connected to the message bus has at least one service name
1331 assigned at connection time and returned in response to the
1332 <literal>org.freedesktop.DBus.Hello</literal> message.
1333 This automatically-assigned service name is called
1334 the application's <firstterm>base service</firstterm>.
1335 Base service names are unique and MUST never be reused for two different
1339 Ownership of the base service is a prerequisite for interaction with
1340 the message bus. It logically follows that the base service is always
1341 the first service that an application comes to own, and the last
1342 service that it loses ownership of.
1345 Base service names must begin with the character ':' (ASCII colon
1346 character); service names that are not base service names must not begin
1347 with this character. (The bus must reject any attempt by an application
1348 to manually create a service name beginning with ':'.) This restriction
1349 categorically prevents "spoofing"; messages sent to a base service name
1350 will always go to a single application instance and that instance only.
1353 An application can request additional service names to be associated
1355 <literal>org.freedesktop.DBus.AcquireService</literal>
1356 message. [FIXME what service names are allowed; ASCII or unicode;
1360 [FIXME this needs more detail, and should move the service-related message
1361 descriptions up into this section perhaps]
1362 Service ownership handling can be specified in the flags part
1363 of the <literal>org.freedesktop.DBus.AcquireService</literal>
1364 message. If an application specifies the
1365 DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications
1366 trying to acquire the service will be put in a queue. When the
1367 primary owner disconnects from the bus or removes ownership
1368 from the service, the next application in the queue will be the
1369 primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT
1370 flag is not specified, then the primary owner will lose
1371 ownership whenever another application requests ownership of the
1375 When a client disconnects from the bus, all the services that
1376 the clients own are deleted, or in the case of a service that
1377 prohibits replacement, ownership is transferred to the next
1378 client in the queue, if any.
1381 <sect2 id="message-bus-routing">
1382 <title>Message Bus Message Routing</title>
1384 When a message is received by the message bus, the message's
1385 <literal>sndr</literal> header field MUST be set to the base service of
1386 the application which sent the message. If the service already has
1387 a <literal>sndr</literal> field, the pre-existing field is replaced.
1388 This rule means that a replies are always sent to the base service name,
1389 i.e. to the same application that sent the message being replied to.
1392 [FIXME go into detail about broadcast, multicast, unicast, etc.]
1395 <sect2 id="message-bus-activation">
1396 <title>Message Bus Service Activation</title>
1398 <firstterm>Activation</firstterm> means to locate a service
1399 owner for a service that is currently unowned. For now, it
1400 means to launch an executable that will take ownership of
1401 a particular service.
1404 To find an executable corresponding to a particular service, the bus
1405 daemon looks for <firstterm>service description files</firstterm>.
1406 Service description files define a mapping from service names to
1407 executables. Different kinds of message bus will look for these files
1408 in different places, see <xref linkend="message-bus-types"/>.
1411 [FIXME the file format should be much better specified than
1412 "similar to .desktop entries" esp. since desktop entries are
1413 already badly-specified. ;-)] Service description files have
1414 the ".service" file extension. The message bus will only load
1415 service description files ending with .service; all other
1416 files will be ignored. The file format is similar to that of
1418 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
1419 entries</ulink>. All service description files must be in
1420 UTF-8 encoding. To ensure that there will be no name
1421 collisions, service files must be namespaced using the same
1422 mechanism as messages and service names.
1425 <title>Example service description file</title>
1427 # Sample service description file
1429 Name=org.gnome.ConfigurationDatabase
1430 Exec=/usr/libexec/gconfd-2
1435 When an application requests a service to be activated, the
1436 bus daemon tries to find it in the list of activation
1437 entries. It then tries to spawn the executable associated with
1438 it. If this fails, it will report an error. [FIXME what
1439 happens if two .service files offer the same service; what
1440 kind of error is reported, should we have a way for the client
1444 The executable launched will have the environment variable
1445 <literal>DBUS_ACTIVATION_ADDRESS</literal> set to the address of the
1446 message bus so it can connect and register the appropriate services.
1449 The executable being launched may want to know whether the message bus
1450 activating it is one of the well-known message buses (see <xref
1451 linkend="message-bus-types"/>). To facilitate this, the bus MUST also set
1452 the <literal>DBUS_ACTIVATION_BUS_TYPE</literal> environment variable if it is one
1453 of the well-known buses. The currently-defined values for this variable
1454 are <literal>system</literal> for the systemwide message bus,
1455 and <literal>session</literal> for the per-login-session message
1456 bus. The activated executable must still connect to the address given
1457 in <literal>DBUS_ACTIVATION_ADDRESS</literal>, but may assume that the
1458 resulting connection is to the well-known bus.
1461 [FIXME there should be a timeout somewhere, either specified
1462 in the .service file, by the client, or just a global value
1463 and if the client being activated fails to connect within that
1464 timeout, an error should be sent back.]
1468 <sect2 id="message-bus-types">
1469 <title>Well-known Message Bus Instances</title>
1471 Two standard message bus instances are defined here, along with how
1472 to locate them and where their service files live.
1474 <sect3 id="message-bus-types-login">
1475 <title>Login session message bus</title>
1477 Each time a user logs in, a <firstterm>login session message
1478 bus</firstterm> may be started. All applications in the user's login
1479 session may interact with one another using this message bus.
1482 The address of the login session message bus is given
1483 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
1484 variable. If that variable is not set, applications may
1485 also try to read the address from the X Window System root
1486 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
1487 The root window property must have type <literal>STRING</literal>.
1488 The environment variable should have precedence over the
1489 root window property.
1492 [FIXME specify location of .service files, probably using
1493 DESKTOP_DIRS etc. from basedir specification, though login session
1494 bus is not really desktop-specific]
1497 <sect3 id="message-bus-types-system">
1498 <title>System message bus</title>
1500 A computer may have a <firstterm>system message bus</firstterm>,
1501 accessible to all applications on the system. This message bus may be
1502 used to broadcast system events, such as adding new hardware devices,
1503 changes in the printer queue, and so forth.
1506 The address of the login session message bus is given
1507 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
1508 variable. If that variable is not set, applications should try
1509 to connect to the well-known address
1510 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
1513 The D-BUS reference implementation actually honors the
1514 <literal>$(localstatedir)</literal> configure option
1515 for this address, on both client and server side.
1520 [FIXME specify location of system bus .service files]
1525 <sect2 id="message-bus-messages">
1526 <title>Message Bus Messages</title>
1528 The special message bus service <literal>org.freedesktop.DBus</literal>
1529 responds to a number of messages, allowing applications to
1530 interact with the message bus.
1533 <sect3 id="bus-messages-hello">
1534 <title><literal>org.freedesktop.DBus.Hello</literal></title>
1545 <entry>Argument</entry>
1547 <entry>Description</entry>
1553 <entry>STRING</entry>
1554 <entry>Name of the service assigned to the application</entry>
1561 Before an application is able to send messages to other
1562 applications it must send the
1563 <literal>org.freedesktop.DBus.Hello</literal> message to the
1564 message bus service. If an application tries to send a
1565 message to another application, or a message to the message
1566 bus service that isn't the
1567 <literal>org.freedesktop.DBus.Hello</literal> message, it
1568 will be disconnected from the bus. If a client wishes to
1569 disconnect from the bus, it just has to disconnect from the
1570 transport used. No de-registration message is necessary.
1573 The reply message contains the name of the application's base service.
1576 <sect3 id="bus-messages-list-services">
1577 <title><literal>org.freedesktop.DBus.ListServices</literal></title>
1581 STRING_ARRAY ListServices ()
1588 <entry>Argument</entry>
1590 <entry>Description</entry>
1596 <entry>STRING_ARRAY</entry>
1597 <entry>Array of strings where each string is the name of a service</entry>
1604 Returns a list of all existing services registered with the message bus.
1607 <sect3 id="bus-messages-service-exists">
1608 <title><literal>org.freedesktop.DBus.ServiceExists</literal></title>
1612 BOOLEAN ServiceExists (in STRING service_name)
1619 <entry>Argument</entry>
1621 <entry>Description</entry>
1627 <entry>STRING</entry>
1628 <entry>Name of the service</entry>
1638 <entry>Argument</entry>
1640 <entry>Description</entry>
1646 <entry>BOOLEAN</entry>
1647 <entry>Return value, true if the service exists</entry>
1654 Checks if a service with a specified name exists.
1658 <sect3 id="bus-messages-acquire-service">
1659 <title><literal>org.freedesktop.DBus.AcquireService</literal></title>
1663 UINT32 AcquireService (in STRING service_name)
1670 <entry>Argument</entry>
1672 <entry>Description</entry>
1678 <entry>STRING</entry>
1679 <entry>Name of the service</entry>
1683 <entry>UINT32</entry>
1684 <entry>Flags</entry>
1694 <entry>Argument</entry>
1696 <entry>Description</entry>
1702 <entry>UINT32</entry>
1703 <entry>Return value</entry>
1710 Tries to become owner of a specific service. The flags
1711 specified can be the following values logically ORed together:
1717 <entry>Identifier</entry>
1718 <entry>Value</entry>
1719 <entry>Description</entry>
1724 <entry>DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT</entry>
1727 If the application succeeds in being the owner of the specified service,
1728 then ownership of the service can't be transferred until the service
1729 disconnects. If this flag is not set, then any application trying to become
1730 the owner of the service will succeed and the previous owner will be
1731 sent a <literal>org.freedesktop.DBus.ServiceLost</literal> message.
1735 <entry>DBUS_SERVICE_FLAGS_REPLACE_EXISTING</entry>
1737 <entry>Try to replace the current owner if there is one. If this flag
1738 is not set the application will only become the owner of the service if
1739 there is no current owner.</entry>
1745 [FIXME if it's one of the following values, why are the values
1746 done as flags instead of just 0, 1, 2, 3, 4]
1747 The return value can be one of the following values:
1753 <entry>Identifier</entry>
1754 <entry>Value</entry>
1755 <entry>Description</entry>
1760 <entry>DBUS_SERVICE_REPLY_PRIMARY_OWNER</entry>
1762 <entry>The application is now the primary owner of the service.</entry>
1765 <entry>DBUS_SERVICE_REPLY_IN_QUEUE</entry>
1767 <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>
1770 <entry>DBUS_SERVICE_REPLY_SERVICE_EXISTS</entry>
1772 <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>
1775 <entry>DBUS_SERVICE_REPLY_ALREADY_OWNER</entry>
1777 <entry>The application trying to request ownership of the service is already the owner of it.</entry>
1784 <sect3 id="bus-messages-service-acquired">
1785 <title><literal>org.freedesktop.DBus.ServiceAcquired</literal></title>
1789 ServiceAcquired (in STRING service_name)
1796 <entry>Argument</entry>
1798 <entry>Description</entry>
1804 <entry>STRING</entry>
1805 <entry>Name of the service</entry>
1809 <entry>UINT32</entry>
1810 <entry>Flags</entry>
1817 This message is sent to a specific application when it becomes the
1818 primary owner of a service.
1821 <sect3 id="bus-messages-service-lost">
1822 <title><literal>org.freedesktop.DBus.ServiceLost</literal></title>
1826 ServiceLost (in STRING service_name)
1833 <entry>Argument</entry>
1835 <entry>Description</entry>
1841 <entry>STRING</entry>
1842 <entry>Name of the service</entry>
1846 <entry>UINT32</entry>
1847 <entry>Flags</entry>
1854 This message is sent to a specific application when it loses primary
1855 ownership of a service.
1857 [FIXME instead of ServiceLost/ServiceCreated going only to
1858 a specific app, why not just OwnerChanged that covers both
1859 lost and created and changed owner and deleted]
1863 <sect3 id="bus-messages-service-created">
1864 <title><literal>org.freedesktop.DBus.ServiceCreated</literal></title>
1868 ServiceCreated (in STRING service_name)
1875 <entry>Argument</entry>
1877 <entry>Description</entry>
1883 <entry>STRING</entry>
1884 <entry>Name of the service</entry>
1888 <entry>UINT32</entry>
1889 <entry>Flags</entry>
1896 This message is broadcast to all applications when a service has been
1897 successfully registered on the message bus.
1901 <sect3 id="bus-messages-service-deleted">
1902 <title><literal>org.freedesktop.DBus.ServiceDeleted</literal></title>
1906 ServiceDeleted (in STRING service_name)
1913 <entry>Argument</entry>
1915 <entry>Description</entry>
1921 <entry>STRING</entry>
1922 <entry>Name of the service</entry>
1926 <entry>UINT32</entry>
1927 <entry>Flags</entry>
1934 This message is broadcast to all applications when a service has been
1935 deleted from the message bus.
1939 <sect3 id="bus-messages-activate-service">
1940 <title><literal>org.freedesktop.DBus.ActivateService</literal></title>
1944 UINT32 ActivateService (in STRING service_name, in UINT32 flags)
1951 <entry>Argument</entry>
1953 <entry>Description</entry>
1959 <entry>STRING</entry>
1960 <entry>Name of the service to activate</entry>
1964 <entry>UINT32</entry>
1965 <entry>Flags (currently not used)</entry>
1975 <entry>Argument</entry>
1977 <entry>Description</entry>
1983 <entry>UINT32</entry>
1984 <entry>Return value</entry>
1989 Tries to launch the executable associated with a service. For more information, see <xref linkend="message-bus-activation"/>.
1991 [FIXME need semantics in much more detail here; for example,
1992 if I activate a service then send it a message, is the message
1993 queued for the new service or is there a race]
1996 The return value can be one of the following values:
2001 <entry>Identifier</entry>
2002 <entry>Value</entry>
2003 <entry>Description</entry>
2008 <entry>DBUS_ACTIVATION_REPLY_ACTIVATED</entry>
2010 <entry>The service was activated successfully.</entry>
2013 <entry>DBUS_ACTIVATION_REPLY_ALREADY_ACTIVE</entry>
2015 <entry>The service is already active.</entry>
2024 <sect3 id="bus-messages-out-of-memory">
2025 <title><literal>org.freedesktop.DBus.Error.NoMemory</literal></title>
2033 Sent by the message bus when it can't process a message due to an out of memory failure.
2037 <sect3 id="bus-messages-service-does-not-exist">
2038 <title><literal>org.freedesktop.DBus.Error.ServiceDoesNotExist</literal></title>
2042 void ServiceDoesNotExist (in STRING error)
2046 Sent by the message bus as a reply to a client that tried to send a message to a service that doesn't exist.
2053 <appendix id="implementation-notes">
2054 <title>Implementation notes</title>
2055 <sect1 id="implementation-notes-subsection">
2063 <glossary><title>Glossary</title>
2065 This glossary defines some of the terms used in this specification.
2068 <glossentry id="term-activation"><glossterm>Activation</glossterm>
2071 The process of creating an owner for a particular service,
2072 typically by launching an executable.
2077 <glossentry id="term-base-service"><glossterm>Base Service</glossterm>
2080 The special service automatically assigned to an application by the
2081 message bus. This service may never change owner, and the service
2082 name will be unique (never reused during the lifetime of the
2088 <glossentry id="term-broadcast"><glossterm>Broadcast</glossterm>
2091 A message sent to the special <literal>org.freedesktop.DBus.Broadcast</literal>
2092 service; the message bus will forward the broadcast message
2093 to all applications that have expressed interest in it.
2098 <glossentry id="term-message"><glossterm>Message</glossterm>
2101 A message is the atomic unit of communication via the D-BUS
2102 protocol. It consists of a <firstterm>header</firstterm> and a
2103 <firstterm>body</firstterm>; the body is made up of
2104 <firstterm>arguments</firstterm>.
2109 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
2112 The message bus is a special application that forwards
2113 or broadcasts messages between a group of applications
2114 connected to the message bus. It also manages
2115 <firstterm>services</firstterm>.
2120 <glossentry id="namespace"><glossterm>Namespace</glossterm>
2123 Used to prevent collisions when defining message and service
2124 names. The convention used is the same as Java uses for
2125 defining classes: a reversed domain name.
2130 <glossentry id="term-object"><glossterm>Object</glossterm>
2133 Each application contains <firstterm>objects</firstterm>,
2134 which have <firstterm>interfaces</firstterm> and
2135 <firstterm>methods</firstterm>. Objects are referred to
2136 by a name, called a <firstterm>path</firstterm> or
2137 <firstterm>object reference</firstterm>.
2142 <glossentry id="term-path"><glossterm>Path</glossterm>
2145 Object references (object names) in D-BUS are
2146 organized into a filesystem-style hierarchy, so
2147 each object is named by a path. As in LDAP,
2148 there's no difference between "files" and "directories";
2149 a path can refer to an object, while still having
2150 child objects below it.
2155 <glossentry id="peer-to-peer"><glossterm>Peer-to-peer</glossterm>
2158 An application talking directly to another application, without going through a message bus.
2162 <glossentry id="term-secondary-owner"><glossterm>Secondary service owner</glossterm>
2165 Each service has a primary owner; messages sent to the service name
2166 go to the primary owner. However, certain services also maintain
2167 a queue of secondary owners "waiting in the wings." If
2168 the primary owner releases the service, then the first secondary
2169 owner in the queue automatically becomes the primary owner.
2173 <glossentry id="term-service"><glossterm>Service</glossterm>
2176 A service is simply a named list of applications. For example, the
2177 hypothetical <literal>com.yoyodyne.Screensaver</literal> service might
2178 accept messages that affect a screensaver from Yoyodyne Corporation.
2179 An application is said to <firstterm>own</firstterm> a service if the
2180 message bus has associated the application with the service name.
2181 Services may also have <firstterm>secondary owners</firstterm> (see
2182 <xref linkend="term-secondary-owner"/>).
2186 <glossentry id="term-service-name"><glossterm>Service name</glossterm>
2189 The name used when referring to a service. If the service is
2190 a base service it has a unique service name, for example
2191 ":1-20", and otherwise it should be namespaced.
2195 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
2198 ".service files" tell the bus how to activate a particular service.
2199 See <xref linkend="term-activation"/>