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"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.17</releaseinfo>
10 <date>(not final)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
55 <revnumber>current</revnumber>
56 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
57 <authorinitials></authorinitials>
58 <revremark></revremark>
61 <revnumber>0.16</revnumber>
62 <date>11 April 2011</date>
63 <authorinitials></authorinitials>
64 <revremark>add path_namespace, arg0namespace; argNpath matches object
68 <revnumber>0.15</revnumber>
69 <date>3 November 2010</date>
70 <authorinitials></authorinitials>
71 <revremark></revremark>
74 <revnumber>0.14</revnumber>
75 <date>12 May 2010</date>
76 <authorinitials></authorinitials>
77 <revremark></revremark>
80 <revnumber>0.13</revnumber>
81 <date>23 Dezember 2009</date>
82 <authorinitials></authorinitials>
83 <revremark></revremark>
86 <revnumber>0.12</revnumber>
87 <date>7 November, 2006</date>
88 <authorinitials></authorinitials>
89 <revremark></revremark>
92 <revnumber>0.11</revnumber>
93 <date>6 February 2005</date>
94 <authorinitials></authorinitials>
95 <revremark></revremark>
98 <revnumber>0.10</revnumber>
99 <date>28 January 2005</date>
100 <authorinitials></authorinitials>
101 <revremark></revremark>
104 <revnumber>0.9</revnumber>
105 <date>7 Januar 2005</date>
106 <authorinitials></authorinitials>
107 <revremark></revremark>
110 <revnumber>0.8</revnumber>
111 <date>06 September 2003</date>
112 <authorinitials></authorinitials>
113 <revremark>First released document.</revremark>
118 <sect1 id="introduction">
119 <title>Introduction</title>
121 D-Bus is a system for low-latency, low-overhead, easy to use
122 interprocess communication (IPC). In more detail:
126 D-Bus is <emphasis>low-latency</emphasis> because it is designed
127 to avoid round trips and allow asynchronous operation, much like
133 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
134 binary protocol, and does not have to convert to and from a text
135 format such as XML. Because D-Bus is intended for potentially
136 high-resolution same-machine IPC, not primarily for Internet IPC,
137 this is an interesting optimization.
142 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
143 of <firstterm>messages</firstterm> rather than byte streams, and
144 automatically handles a lot of the hard IPC issues. Also, the D-Bus
145 library is designed to be wrapped in a way that lets developers use
146 their framework's existing object/type system, rather than learning
147 a new one specifically for IPC.
154 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
155 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
156 a system for one application to talk to a single other
157 application. However, the primary intended application of the protocol is the
158 D-Bus <firstterm>message bus</firstterm>, specified in <xref
159 linkend="message-bus"/>. The message bus is a special application that
160 accepts connections from multiple other applications, and forwards
165 Uses of D-Bus include notification of system changes (notification of when
166 a camera is plugged in to a computer, or a new version of some software
167 has been installed), or desktop interoperability, for example a file
168 monitoring service or a configuration service.
172 D-Bus is designed for two specific use cases:
176 A "system bus" for notifications from the system to user sessions,
177 and to allow the system to request input from user sessions.
182 A "session bus" used to implement desktop environments such as
187 D-Bus is not intended to be a generic IPC system for any possible
188 application, and intentionally omits many features found in other
189 IPC systems for this reason.
193 At the same time, the bus daemons offer a number of features not found in
194 other IPC systems, such as single-owner "bus names" (similar to X
195 selections), on-demand startup of services, and security policies.
196 In many ways, these features are the primary motivation for developing
197 D-Bus; other systems would have sufficed if IPC were the only goal.
201 D-Bus may turn out to be useful in unanticipated applications, but future
202 versions of this spec and the reference implementation probably will not
203 incorporate features that interfere with the core use cases.
207 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
208 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
209 document are to be interpreted as described in RFC 2119. However, the
210 document could use a serious audit to be sure it makes sense to do
211 so. Also, they are not capitalized.
214 <sect2 id="stability">
215 <title>Protocol and Specification Stability</title>
217 The D-Bus protocol is frozen (only compatible extensions are allowed) as
218 of November 8, 2006. However, this specification could still use a fair
219 bit of work to make interoperable reimplementation possible without
220 reference to the D-Bus reference implementation. Thus, this
221 specification is not marked 1.0. To mark it 1.0, we'd like to see
222 someone invest significant effort in clarifying the specification
223 language, and growing the specification to cover more aspects of the
224 reference implementation's behavior.
227 Until this work is complete, any attempt to reimplement D-Bus will
228 probably require looking at the reference implementation and/or asking
229 questions on the D-Bus mailing list about intended behavior.
230 Questions on the list are very welcome.
233 Nonetheless, this document should be a useful starting point and is
234 to our knowledge accurate, though incomplete.
240 <sect1 id="message-protocol">
241 <title>Message Protocol</title>
244 A <firstterm>message</firstterm> consists of a
245 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
246 think of a message as a package, the header is the address, and the body
247 contains the package contents. The message delivery system uses the header
248 information to figure out where to send the message and how to interpret
249 it; the recipient interprets the body of the message.
253 The body of the message is made up of zero or more
254 <firstterm>arguments</firstterm>, which are typed values, such as an
255 integer or a byte array.
259 Both header and body use the same type system and format for
260 serializing data. Each type of value has a wire format.
261 Converting a value from some other representation into the wire
262 format is called <firstterm>marshaling</firstterm> and converting
263 it back from the wire format is <firstterm>unmarshaling</firstterm>.
266 <sect2 id="message-protocol-signatures">
267 <title>Type Signatures</title>
270 The D-Bus protocol does not include type tags in the marshaled data; a
271 block of marshaled values must have a known <firstterm>type
272 signature</firstterm>. The type signature is made up of <firstterm>type
273 codes</firstterm>. A type code is an ASCII character representing the
274 type of a value. Because ASCII characters are used, the type signature
275 will always form a valid ASCII string. A simple string compare
276 determines whether two type signatures are equivalent.
280 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
281 the ASCII character 'i'. So the signature for a block of values
282 containing a single <literal>INT32</literal> would be:
286 A block of values containing two <literal>INT32</literal> would have this signature:
293 All <firstterm>basic</firstterm> types work like
294 <literal>INT32</literal> in this example. To marshal and unmarshal
295 basic types, you simply read one value from the data
296 block corresponding to each type code in the signature.
297 In addition to basic types, there are four <firstterm>container</firstterm>
298 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
299 and <literal>DICT_ENTRY</literal>.
303 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
304 code does not appear in signatures. Instead, ASCII characters
305 '(' and ')' are used to mark the beginning and end of the struct.
306 So for example, a struct containing two integers would have this
311 Structs can be nested, so for example a struct containing
312 an integer and another struct:
316 The value block storing that struct would contain three integers; the
317 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
322 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
323 but is useful in code that implements the protocol. This type code
324 is specified to allow such code to interoperate in non-protocol contexts.
328 Empty structures are not allowed; there must be at least one
329 type code between the parentheses.
333 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
334 followed by a <firstterm>single complete type</firstterm>. The single
335 complete type following the array is the type of each array element. So
336 the simple example is:
340 which is an array of 32-bit integers. But an array can be of any type,
341 such as this array-of-struct-with-two-int32-fields:
345 Or this array of array of integer:
352 The phrase <firstterm>single complete type</firstterm> deserves some
353 definition. A single complete type is a basic type code, a variant type code,
354 an array with its element type, or a struct with its fields.
355 So the following signatures are not single complete types:
365 And the following signatures contain multiple complete types:
375 Note however that a single complete type may <emphasis>contain</emphasis>
376 multiple other single complete types.
380 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
381 type <literal>VARIANT</literal> will have the signature of a single complete type as part
382 of the <emphasis>value</emphasis>. This signature will be followed by a
383 marshaled value of that type.
387 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
388 than parentheses it uses curly braces, and it has more restrictions.
389 The restrictions are: it occurs only as an array element type; it has
390 exactly two single complete types inside the curly braces; the first
391 single complete type (the "key") must be a basic type rather than a
392 container type. Implementations must not accept dict entries outside of
393 arrays, must not accept dict entries with zero, one, or more than two
394 fields, and must not accept dict entries with non-basic-typed keys. A
395 dict entry is always a key-value pair.
399 The first field in the <literal>DICT_ENTRY</literal> is always the key.
400 A message is considered corrupt if the same key occurs twice in the same
401 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
402 implementations are not required to reject dicts with duplicate keys.
406 In most languages, an array of dict entry would be represented as a
407 map, hash table, or dict object.
411 The following table summarizes the D-Bus types.
416 <entry>Conventional Name</entry>
418 <entry>Description</entry>
423 <entry><literal>INVALID</literal></entry>
424 <entry>0 (ASCII NUL)</entry>
425 <entry>Not a valid type code, used to terminate signatures</entry>
427 <entry><literal>BYTE</literal></entry>
428 <entry>121 (ASCII 'y')</entry>
429 <entry>8-bit unsigned integer</entry>
431 <entry><literal>BOOLEAN</literal></entry>
432 <entry>98 (ASCII 'b')</entry>
433 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
435 <entry><literal>INT16</literal></entry>
436 <entry>110 (ASCII 'n')</entry>
437 <entry>16-bit signed integer</entry>
439 <entry><literal>UINT16</literal></entry>
440 <entry>113 (ASCII 'q')</entry>
441 <entry>16-bit unsigned integer</entry>
443 <entry><literal>INT32</literal></entry>
444 <entry>105 (ASCII 'i')</entry>
445 <entry>32-bit signed integer</entry>
447 <entry><literal>UINT32</literal></entry>
448 <entry>117 (ASCII 'u')</entry>
449 <entry>32-bit unsigned integer</entry>
451 <entry><literal>INT64</literal></entry>
452 <entry>120 (ASCII 'x')</entry>
453 <entry>64-bit signed integer</entry>
455 <entry><literal>UINT64</literal></entry>
456 <entry>116 (ASCII 't')</entry>
457 <entry>64-bit unsigned integer</entry>
459 <entry><literal>DOUBLE</literal></entry>
460 <entry>100 (ASCII 'd')</entry>
461 <entry>IEEE 754 double</entry>
463 <entry><literal>STRING</literal></entry>
464 <entry>115 (ASCII 's')</entry>
465 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
467 <entry><literal>OBJECT_PATH</literal></entry>
468 <entry>111 (ASCII 'o')</entry>
469 <entry>Name of an object instance</entry>
471 <entry><literal>SIGNATURE</literal></entry>
472 <entry>103 (ASCII 'g')</entry>
473 <entry>A type signature</entry>
475 <entry><literal>ARRAY</literal></entry>
476 <entry>97 (ASCII 'a')</entry>
479 <entry><literal>STRUCT</literal></entry>
480 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
481 <entry>Struct; type code 114 'r' is reserved for use in
482 bindings and implementations to represent the general
483 concept of a struct, and must not appear in signatures
484 used on D-Bus.</entry>
486 <entry><literal>VARIANT</literal></entry>
487 <entry>118 (ASCII 'v') </entry>
488 <entry>Variant type (the type of the value is part of the value itself)</entry>
490 <entry><literal>DICT_ENTRY</literal></entry>
491 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
492 <entry>Entry in a dict or map (array of key-value pairs).
493 Type code 101 'e' is reserved for use in bindings and
494 implementations to represent the general concept of a
495 dict or dict-entry, and must not appear in signatures
496 used on D-Bus.</entry>
498 <entry><literal>UNIX_FD</literal></entry>
499 <entry>104 (ASCII 'h')</entry>
500 <entry>Unix file descriptor</entry>
503 <entry>(reserved)</entry>
504 <entry>109 (ASCII 'm')</entry>
505 <entry>Reserved for <ulink
506 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
507 'maybe' type compatible with the one in GVariant</ulink>,
508 and must not appear in signatures used on D-Bus until
509 specified here</entry>
512 <entry>(reserved)</entry>
513 <entry>42 (ASCII '*')</entry>
514 <entry>Reserved for use in bindings/implementations to
515 represent any <firstterm>single complete type</firstterm>,
516 and must not appear in signatures used on D-Bus.</entry>
519 <entry>(reserved)</entry>
520 <entry>63 (ASCII '?')</entry>
521 <entry>Reserved for use in bindings/implementations to
522 represent any <firstterm>basic type</firstterm>, and must
523 not appear in signatures used on D-Bus.</entry>
526 <entry>(reserved)</entry>
527 <entry>64 (ASCII '@'), 38 (ASCII '&'),
528 94 (ASCII '^')</entry>
529 <entry>Reserved for internal use by bindings/implementations,
530 and must not appear in signatures used on D-Bus.
531 GVariant uses these type-codes to encode calling
541 <sect2 id="message-protocol-marshaling">
542 <title>Marshaling (Wire Format)</title>
545 Given a type signature, a block of bytes can be converted into typed
546 values. This section describes the format of the block of bytes. Byte
547 order and alignment issues are handled uniformly for all D-Bus types.
551 A block of bytes has an associated byte order. The byte order
552 has to be discovered in some way; for D-Bus messages, the
553 byte order is part of the message header as described in
554 <xref linkend="message-protocol-messages"/>. For now, assume
555 that the byte order is known to be either little endian or big
560 Each value in a block of bytes is aligned "naturally," for example
561 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
562 8-byte boundary. To properly align a value, <firstterm>alignment
563 padding</firstterm> may be necessary. The alignment padding must always
564 be the minimum required padding to properly align the following value;
565 and it must always be made up of nul bytes. The alignment padding must
566 not be left uninitialized (it can't contain garbage), and more padding
567 than required must not be used.
571 Given all this, the types are marshaled on the wire as follows:
576 <entry>Conventional Name</entry>
577 <entry>Encoding</entry>
578 <entry>Alignment</entry>
583 <entry><literal>INVALID</literal></entry>
584 <entry>Not applicable; cannot be marshaled.</entry>
587 <entry><literal>BYTE</literal></entry>
588 <entry>A single 8-bit byte.</entry>
591 <entry><literal>BOOLEAN</literal></entry>
592 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
595 <entry><literal>INT16</literal></entry>
596 <entry>16-bit signed integer in the message's byte order.</entry>
599 <entry><literal>UINT16</literal></entry>
600 <entry>16-bit unsigned integer in the message's byte order.</entry>
603 <entry><literal>INT32</literal></entry>
604 <entry>32-bit signed integer in the message's byte order.</entry>
607 <entry><literal>UINT32</literal></entry>
608 <entry>32-bit unsigned integer in the message's byte order.</entry>
611 <entry><literal>INT64</literal></entry>
612 <entry>64-bit signed integer in the message's byte order.</entry>
615 <entry><literal>UINT64</literal></entry>
616 <entry>64-bit unsigned integer in the message's byte order.</entry>
619 <entry><literal>DOUBLE</literal></entry>
620 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
623 <entry><literal>STRING</literal></entry>
624 <entry>A <literal>UINT32</literal> indicating the string's
625 length in bytes excluding its terminating nul, followed by
626 non-nul string data of the given length, followed by a terminating nul
633 <entry><literal>OBJECT_PATH</literal></entry>
634 <entry>Exactly the same as <literal>STRING</literal> except the
635 content must be a valid object path (see below).
641 <entry><literal>SIGNATURE</literal></entry>
642 <entry>The same as <literal>STRING</literal> except the length is a single
643 byte (thus signatures have a maximum length of 255)
644 and the content must be a valid signature (see below).
650 <entry><literal>ARRAY</literal></entry>
652 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
653 alignment padding to the alignment boundary of the array element type,
654 followed by each array element. The array length is from the
655 end of the alignment padding to the end of the last element,
656 i.e. it does not include the padding after the length,
657 or any padding after the last element.
658 Arrays have a maximum length defined to be 2 to the 26th power or
659 67108864. Implementations must not send or accept arrays exceeding this
666 <entry><literal>STRUCT</literal></entry>
668 A struct must start on an 8-byte boundary regardless of the
669 type of the struct fields. The struct value consists of each
670 field marshaled in sequence starting from that 8-byte
677 <entry><literal>VARIANT</literal></entry>
679 A variant type has a marshaled
680 <literal>SIGNATURE</literal> followed by a marshaled
681 value with the type given in the signature. Unlike
682 a message signature, the variant signature can
683 contain only a single complete type. So "i", "ai"
684 or "(ii)" is OK, but "ii" is not. Use of variants may not
685 cause a total message depth to be larger than 64, including
686 other container types such as structures.
689 1 (alignment of the signature)
692 <entry><literal>DICT_ENTRY</literal></entry>
700 <entry><literal>UNIX_FD</literal></entry>
701 <entry>32-bit unsigned integer in the message's byte
702 order. The actual file descriptors need to be
703 transferred out-of-band via some platform specific
704 mechanism. On the wire, values of this type store the index to the
705 file descriptor in the array of file descriptors that
706 accompany the message.</entry>
714 <sect3 id="message-protocol-marshaling-object-path">
715 <title>Valid Object Paths</title>
718 An object path is a name used to refer to an object instance.
719 Conceptually, each participant in a D-Bus message exchange may have
720 any number of object instances (think of C++ or Java objects) and each
721 such instance will have a path. Like a filesystem, the object
722 instances in an application form a hierarchical tree.
726 The following rules define a valid object path. Implementations must
727 not send or accept messages with invalid object paths.
731 The path may be of any length.
736 The path must begin with an ASCII '/' (integer 47) character,
737 and must consist of elements separated by slash characters.
742 Each element must only contain the ASCII characters
748 No element may be the empty string.
753 Multiple '/' characters cannot occur in sequence.
758 A trailing '/' character is not allowed unless the
759 path is the root path (a single '/' character).
766 Object paths are often namespaced by starting with a reversed
767 domain name and containing an interface version number, in the
769 <link linkend="message-protocol-names-interface">interface
771 <link linkend="message-protocol-names-bus">well-known
773 This makes it possible to implement more than one service, or
774 more than one version of a service, in the same process,
775 even if the services share a connection but cannot otherwise
776 co-operate (for instance, if they are implemented by different
781 For instance, if the owner of <literal>example.com</literal> is
782 developing a D-Bus API for a music player, they might use the
783 hierarchy of object paths that start with
784 <literal>/com/example/MusicPlayer1</literal> for its objects.
788 <sect3 id="message-protocol-marshaling-signature">
789 <title>Valid Signatures</title>
791 An implementation must not send or accept invalid signatures.
792 Valid signatures will conform to the following rules:
796 The signature ends with a nul byte.
801 The signature is a list of single complete types.
802 Arrays must have element types, and structs must
803 have both open and close parentheses.
808 Only type codes and open and close parentheses are
809 allowed in the signature. The <literal>STRUCT</literal> type code
810 is not allowed in signatures, because parentheses
816 The maximum depth of container type nesting is 32 array type
817 codes and 32 open parentheses. This implies that the maximum
818 total depth of recursion is 64, for an "array of array of array
819 of ... struct of struct of struct of ..." where there are 32
825 The maximum length of a signature is 255.
830 Signatures must be nul-terminated.
839 <sect2 id="message-protocol-messages">
840 <title>Message Format</title>
843 A message consists of a header and a body. The header is a block of
844 values with a fixed signature and meaning. The body is a separate block
845 of values, with a signature specified in the header.
849 The length of the header must be a multiple of 8, allowing the body to
850 begin on an 8-byte boundary when storing the entire message in a single
851 buffer. If the header does not naturally end on an 8-byte boundary
852 up to 7 bytes of nul-initialized alignment padding must be added.
856 The message body need not end on an 8-byte boundary.
860 The maximum length of a message, including header, header alignment padding,
861 and body is 2 to the 27th power or 134217728. Implementations must not
862 send or accept messages exceeding this size.
866 The signature of the header is:
870 Written out more readably, this is:
872 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
877 These values have the following meanings:
883 <entry>Description</entry>
888 <entry>1st <literal>BYTE</literal></entry>
889 <entry>Endianness flag; ASCII 'l' for little-endian
890 or ASCII 'B' for big-endian. Both header and body are
891 in this endianness.</entry>
894 <entry>2nd <literal>BYTE</literal></entry>
895 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
896 Currently-defined types are described below.
900 <entry>3rd <literal>BYTE</literal></entry>
901 <entry>Bitwise OR of flags. Unknown flags
902 must be ignored. Currently-defined flags are described below.
906 <entry>4th <literal>BYTE</literal></entry>
907 <entry>Major protocol version of the sending application. If
908 the major protocol version of the receiving application does not
909 match, the applications will not be able to communicate and the
910 D-Bus connection must be disconnected. The major protocol
911 version for this version of the specification is 1.
915 <entry>1st <literal>UINT32</literal></entry>
916 <entry>Length in bytes of the message body, starting
917 from the end of the header. The header ends after
918 its alignment padding to an 8-boundary.
922 <entry>2nd <literal>UINT32</literal></entry>
923 <entry>The serial of this message, used as a cookie
924 by the sender to identify the reply corresponding
925 to this request. This must not be zero.
929 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
930 <entry>An array of zero or more <firstterm>header
931 fields</firstterm> where the byte is the field code, and the
932 variant is the field value. The message type determines
933 which fields are required.
941 <firstterm>Message types</firstterm> that can appear in the second byte
947 <entry>Conventional name</entry>
948 <entry>Decimal value</entry>
949 <entry>Description</entry>
954 <entry><literal>INVALID</literal></entry>
956 <entry>This is an invalid type.</entry>
959 <entry><literal>METHOD_CALL</literal></entry>
961 <entry>Method call.</entry>
964 <entry><literal>METHOD_RETURN</literal></entry>
966 <entry>Method reply with returned data.</entry>
969 <entry><literal>ERROR</literal></entry>
971 <entry>Error reply. If the first argument exists and is a
972 string, it is an error message.</entry>
975 <entry><literal>SIGNAL</literal></entry>
977 <entry>Signal emission.</entry>
984 Flags that can appear in the third byte of the header:
989 <entry>Conventional name</entry>
990 <entry>Hex value</entry>
991 <entry>Description</entry>
996 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
998 <entry>This message does not expect method return replies or
999 error replies; the reply can be omitted as an
1000 optimization. However, it is compliant with this specification
1001 to return the reply despite this flag and the only harm
1002 from doing so is extra network traffic.
1006 <entry><literal>NO_AUTO_START</literal></entry>
1008 <entry>The bus must not launch an owner
1009 for the destination name in response to this message.
1017 <sect3 id="message-protocol-header-fields">
1018 <title>Header Fields</title>
1021 The array at the end of the header contains <firstterm>header
1022 fields</firstterm>, where each field is a 1-byte field code followed
1023 by a field value. A header must contain the required header fields for
1024 its message type, and zero or more of any optional header
1025 fields. Future versions of this protocol specification may add new
1026 fields. Implementations must ignore fields they do not
1027 understand. Implementations must not invent their own header fields;
1028 only changes to this specification may introduce new header fields.
1032 Again, if an implementation sees a header field code that it does not
1033 expect, it must ignore that field, as it will be part of a new
1034 (but compatible) version of this specification. This also applies
1035 to known header fields appearing in unexpected messages, for
1036 example: if a signal has a reply serial it must be ignored
1037 even though it has no meaning as of this version of the spec.
1041 However, implementations must not send or accept known header fields
1042 with the wrong type stored in the field value. So for example a
1043 message with an <literal>INTERFACE</literal> field of type
1044 <literal>UINT32</literal> would be considered corrupt.
1048 Here are the currently-defined header fields:
1053 <entry>Conventional Name</entry>
1054 <entry>Decimal Code</entry>
1056 <entry>Required In</entry>
1057 <entry>Description</entry>
1062 <entry><literal>INVALID</literal></entry>
1065 <entry>not allowed</entry>
1066 <entry>Not a valid field name (error if it appears in a message)</entry>
1069 <entry><literal>PATH</literal></entry>
1071 <entry><literal>OBJECT_PATH</literal></entry>
1072 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1073 <entry>The object to send a call to,
1074 or the object a signal is emitted from.
1076 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1077 implementations should not send messages with this path,
1078 and the reference implementation of the bus daemon will
1079 disconnect any application that attempts to do so.
1083 <entry><literal>INTERFACE</literal></entry>
1085 <entry><literal>STRING</literal></entry>
1086 <entry><literal>SIGNAL</literal></entry>
1088 The interface to invoke a method call on, or
1089 that a signal is emitted from. Optional for
1090 method calls, required for signals.
1091 The special interface
1092 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1093 implementations should not send messages with this
1094 interface, and the reference implementation of the bus
1095 daemon will disconnect any application that attempts to
1100 <entry><literal>MEMBER</literal></entry>
1102 <entry><literal>STRING</literal></entry>
1103 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1104 <entry>The member, either the method name or signal name.</entry>
1107 <entry><literal>ERROR_NAME</literal></entry>
1109 <entry><literal>STRING</literal></entry>
1110 <entry><literal>ERROR</literal></entry>
1111 <entry>The name of the error that occurred, for errors</entry>
1114 <entry><literal>REPLY_SERIAL</literal></entry>
1116 <entry><literal>UINT32</literal></entry>
1117 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1118 <entry>The serial number of the message this message is a reply
1119 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1122 <entry><literal>DESTINATION</literal></entry>
1124 <entry><literal>STRING</literal></entry>
1125 <entry>optional</entry>
1126 <entry>The name of the connection this message is intended for.
1127 Only used in combination with the message bus, see
1128 <xref linkend="message-bus"/>.</entry>
1131 <entry><literal>SENDER</literal></entry>
1133 <entry><literal>STRING</literal></entry>
1134 <entry>optional</entry>
1135 <entry>Unique name of the sending connection.
1136 The message bus fills in this field so it is reliable; the field is
1137 only meaningful in combination with the message bus.</entry>
1140 <entry><literal>SIGNATURE</literal></entry>
1142 <entry><literal>SIGNATURE</literal></entry>
1143 <entry>optional</entry>
1144 <entry>The signature of the message body.
1145 If omitted, it is assumed to be the
1146 empty signature "" (i.e. the body must be 0-length).</entry>
1149 <entry><literal>UNIX_FDS</literal></entry>
1151 <entry><literal>UINT32</literal></entry>
1152 <entry>optional</entry>
1153 <entry>The number of Unix file descriptors that
1154 accompany the message. If omitted, it is assumed
1155 that no Unix file descriptors accompany the
1156 message. The actual file descriptors need to be
1157 transferred via platform specific mechanism
1158 out-of-band. They must be sent at the same time as
1159 part of the message itself. They may not be sent
1160 before the first byte of the message itself is
1161 transferred or after the last byte of the message
1171 <sect2 id="message-protocol-names">
1172 <title>Valid Names</title>
1174 The various names in D-Bus messages have some restrictions.
1177 There is a <firstterm>maximum name length</firstterm>
1178 of 255 which applies to bus names, interfaces, and members.
1180 <sect3 id="message-protocol-names-interface">
1181 <title>Interface names</title>
1183 Interfaces have names with type <literal>STRING</literal>, meaning that
1184 they must be valid UTF-8. However, there are also some
1185 additional restrictions that apply to interface names
1188 <listitem><para>Interface names are composed of 1 or more elements separated by
1189 a period ('.') character. All elements must contain at least
1193 <listitem><para>Each element must only contain the ASCII characters
1194 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1198 <listitem><para>Interface names must contain at least one '.' (period)
1199 character (and thus at least two elements).
1202 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1203 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1208 Interface names should start with the reversed DNS domain name of
1209 the author of the interface (in lower-case), like interface names
1210 in Java. It is conventional for the rest of the interface name
1211 to consist of words run together, with initial capital letters
1212 on all words ("CamelCase"). Several levels of hierarchy can be used.
1213 It is also a good idea to include the major version of the interface
1214 in the name, and increment it if incompatible changes are made;
1215 this way, a single object can implement several versions of an
1216 interface in parallel, if necessary.
1220 For instance, if the owner of <literal>example.com</literal> is
1221 developing a D-Bus API for a music player, they might define
1222 interfaces called <literal>com.example.MusicPlayer1</literal>,
1223 <literal>com.example.MusicPlayer1.Track</literal> and
1224 <literal>com.example.MusicPlayer1.Seekable</literal>.
1228 D-Bus does not distinguish between the concepts that would be
1229 called classes and interfaces in Java: either can be identified on
1230 D-Bus by an interface name.
1233 <sect3 id="message-protocol-names-bus">
1234 <title>Bus names</title>
1236 Connections have one or more bus names associated with them.
1237 A connection has exactly one bus name that is a <firstterm>unique
1238 connection name</firstterm>. The unique connection name remains
1239 with the connection for its entire lifetime.
1240 A bus name is of type <literal>STRING</literal>,
1241 meaning that it must be valid UTF-8. However, there are also
1242 some additional restrictions that apply to bus names
1245 <listitem><para>Bus names that start with a colon (':')
1246 character are unique connection names. Other bus names
1247 are called <firstterm>well-known bus names</firstterm>.
1250 <listitem><para>Bus names are composed of 1 or more elements separated by
1251 a period ('.') character. All elements must contain at least
1255 <listitem><para>Each element must only contain the ASCII characters
1256 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1257 connection name may begin with a digit, elements in
1258 other bus names must not begin with a digit.
1262 <listitem><para>Bus names must contain at least one '.' (period)
1263 character (and thus at least two elements).
1266 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1267 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1271 Note that the hyphen ('-') character is allowed in bus names but
1272 not in interface names.
1276 Like <link linkend="message-protocol-names-interface">interface
1277 names</link>, well-known bus names should start with the
1278 reversed DNS domain name of the author of the interface (in
1279 lower-case), and it is conventional for the rest of the well-known
1280 bus name to consist of words run together, with initial
1281 capital letters. As with interface names, including a version
1282 number in well-known bus names is a good idea; it's possible to
1283 have the well-known bus name for more than one version
1284 simultaneously if backwards compatibility is required.
1288 If a well-known bus name implies the presence of a "main" interface,
1289 that "main" interface is often given the same name as
1290 the well-known bus name, and situated at the corresponding object
1291 path. For instance, if the owner of <literal>example.com</literal>
1292 is developing a D-Bus API for a music player, they might define
1293 that any application that takes the well-known name
1294 <literal>com.example.MusicPlayer1</literal> should have an object
1295 at the object path <literal>/com/example/MusicPlayer1</literal>
1296 which implements the interface
1297 <literal>com.example.MusicPlayer1</literal>.
1300 <sect3 id="message-protocol-names-member">
1301 <title>Member names</title>
1303 Member (i.e. method or signal) names:
1305 <listitem><para>Must only contain the ASCII characters
1306 "[A-Z][a-z][0-9]_" and may not begin with a
1307 digit.</para></listitem>
1308 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1309 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1310 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1315 It is conventional for member names on D-Bus to consist of
1316 capitalized words with no punctuation ("camel-case").
1317 Method names should usually be verbs, such as
1318 <literal>GetItems</literal>, and signal names should usually be
1319 a description of an event, such as <literal>ItemsChanged</literal>.
1322 <sect3 id="message-protocol-names-error">
1323 <title>Error names</title>
1325 Error names have the same restrictions as interface names.
1329 Error names have the same naming conventions as interface
1330 names, and often contain <literal>.Error.</literal>; for instance,
1331 the owner of <literal>example.com</literal> might define the
1332 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1333 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1334 The errors defined by D-Bus itself, such as
1335 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1341 <sect2 id="message-protocol-types">
1342 <title>Message Types</title>
1344 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1345 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1346 This section describes these conventions.
1348 <sect3 id="message-protocol-types-method">
1349 <title>Method Calls</title>
1351 Some messages invoke an operation on a remote object. These are
1352 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1353 messages map naturally to methods on objects in a typical program.
1356 A method call message is required to have a <literal>MEMBER</literal> header field
1357 indicating the name of the method. Optionally, the message has an
1358 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1359 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1360 a method with the same name, it is undefined which of the two methods
1361 will be invoked. Implementations may also choose to return an error in
1362 this ambiguous case. However, if a method name is unique
1363 implementations must not require an interface field.
1366 Method call messages also include a <literal>PATH</literal> field
1367 indicating the object to invoke the method on. If the call is passing
1368 through a message bus, the message will also have a
1369 <literal>DESTINATION</literal> field giving the name of the connection
1370 to receive the message.
1373 When an application handles a method call message, it is required to
1374 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1375 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1376 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1379 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1380 are the return value(s) or "out parameters" of the method call.
1381 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1382 and the call fails; no return value will be provided. It makes
1383 no sense to send multiple replies to the same method call.
1386 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1387 reply is required, so the caller will know the method
1388 was successfully processed.
1391 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1395 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1396 then as an optimization the application receiving the method
1397 call may choose to omit the reply message (regardless of
1398 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1399 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1400 flag and reply anyway.
1403 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1404 destination name does not exist then a program to own the destination
1405 name will be started before the message is delivered. The message
1406 will be held until the new program is successfully started or has
1407 failed to start; in case of failure, an error will be returned. This
1408 flag is only relevant in the context of a message bus, it is ignored
1409 during one-to-one communication with no intermediate bus.
1411 <sect4 id="message-protocol-types-method-apis">
1412 <title>Mapping method calls to native APIs</title>
1414 APIs for D-Bus may map method calls to a method call in a specific
1415 programming language, such as C++, or may map a method call written
1416 in an IDL to a D-Bus message.
1419 In APIs of this nature, arguments to a method are often termed "in"
1420 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1421 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1422 "inout" arguments, which are both sent and received, i.e. the caller
1423 passes in a value which is modified. Mapped to D-Bus, an "inout"
1424 argument is equivalent to an "in" argument, followed by an "out"
1425 argument. You can't pass things "by reference" over the wire, so
1426 "inout" is purely an illusion of the in-process API.
1429 Given a method with zero or one return values, followed by zero or more
1430 arguments, where each argument may be "in", "out", or "inout", the
1431 caller constructs a message by appending each "in" or "inout" argument,
1432 in order. "out" arguments are not represented in the caller's message.
1435 The recipient constructs a reply by appending first the return value
1436 if any, then each "out" or "inout" argument, in order.
1437 "in" arguments are not represented in the reply message.
1440 Error replies are normally mapped to exceptions in languages that have
1444 In converting from native APIs to D-Bus, it is perhaps nice to
1445 map D-Bus naming conventions ("FooBar") to native conventions
1446 such as "fooBar" or "foo_bar" automatically. This is OK
1447 as long as you can say that the native API is one that
1448 was specifically written for D-Bus. It makes the most sense
1449 when writing object implementations that will be exported
1450 over the bus. Object proxies used to invoke remote D-Bus
1451 objects probably need the ability to call any D-Bus method,
1452 and thus a magic name mapping like this could be a problem.
1455 This specification doesn't require anything of native API bindings;
1456 the preceding is only a suggested convention for consistency
1462 <sect3 id="message-protocol-types-signal">
1463 <title>Signal Emission</title>
1465 Unlike method calls, signal emissions have no replies.
1466 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1467 It must have three header fields: <literal>PATH</literal> giving the object
1468 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1469 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1470 for signals, though it is optional for method calls.
1474 <sect3 id="message-protocol-types-errors">
1475 <title>Errors</title>
1477 Messages of type <literal>ERROR</literal> are most commonly replies
1478 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1479 to any kind of message. The message bus for example
1480 will return an <literal>ERROR</literal> in reply to a signal emission if
1481 the bus does not have enough memory to send the signal.
1484 An <literal>ERROR</literal> may have any arguments, but if the first
1485 argument is a <literal>STRING</literal>, it must be an error message.
1486 The error message may be logged or shown to the user
1491 <sect3 id="message-protocol-types-notation">
1492 <title>Notation in this document</title>
1494 This document uses a simple pseudo-IDL to describe particular method
1495 calls and signals. Here is an example of a method call:
1497 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1498 out UINT32 resultcode)
1500 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1501 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1502 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1503 characters so it's known that the last part of the name in
1504 the "IDL" is the member name.
1507 In C++ that might end up looking like this:
1509 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1510 unsigned int flags);
1512 or equally valid, the return value could be done as an argument:
1514 void org::freedesktop::DBus::StartServiceByName (const char *name,
1516 unsigned int *resultcode);
1518 It's really up to the API designer how they want to make
1519 this look. You could design an API where the namespace wasn't used
1520 in C++, using STL or Qt, using varargs, or whatever you wanted.
1523 Signals are written as follows:
1525 org.freedesktop.DBus.NameLost (STRING name)
1527 Signals don't specify "in" vs. "out" because only
1528 a single direction is possible.
1531 It isn't especially encouraged to use this lame pseudo-IDL in actual
1532 API implementations; you might use the native notation for the
1533 language you're using, or you might use COM or CORBA IDL, for example.
1538 <sect2 id="message-protocol-handling-invalid">
1539 <title>Invalid Protocol and Spec Extensions</title>
1542 For security reasons, the D-Bus protocol should be strictly parsed and
1543 validated, with the exception of defined extension points. Any invalid
1544 protocol or spec violations should result in immediately dropping the
1545 connection without notice to the other end. Exceptions should be
1546 carefully considered, e.g. an exception may be warranted for a
1547 well-understood idiosyncrasy of a widely-deployed implementation. In
1548 cases where the other end of a connection is 100% trusted and known to
1549 be friendly, skipping validation for performance reasons could also make
1550 sense in certain cases.
1554 Generally speaking violations of the "must" requirements in this spec
1555 should be considered possible attempts to exploit security, and violations
1556 of the "should" suggestions should be considered legitimate (though perhaps
1557 they should generate an error in some cases).
1561 The following extension points are built in to D-Bus on purpose and must
1562 not be treated as invalid protocol. The extension points are intended
1563 for use by future versions of this spec, they are not intended for third
1564 parties. At the moment, the only way a third party could extend D-Bus
1565 without breaking interoperability would be to introduce a way to negotiate new
1566 feature support as part of the auth protocol, using EXTENSION_-prefixed
1567 commands. There is not yet a standard way to negotiate features.
1571 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1572 commands result in an ERROR rather than a disconnect. This enables
1573 future extensions to the protocol. Commands starting with EXTENSION_ are
1574 reserved for third parties.
1579 The authentication protocol supports pluggable auth mechanisms.
1584 The address format (see <xref linkend="addresses"/>) supports new
1590 Messages with an unknown type (something other than
1591 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1592 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1593 Unknown-type messages must still be well-formed in the same way
1594 as the known messages, however. They still have the normal
1600 Header fields with an unknown or unexpected field code must be ignored,
1601 though again they must still be well-formed.
1606 New standard interfaces (with new methods and signals) can of course be added.
1616 <sect1 id="auth-protocol">
1617 <title>Authentication Protocol</title>
1619 Before the flow of messages begins, two applications must
1620 authenticate. A simple plain-text protocol is used for
1621 authentication; this protocol is a SASL profile, and maps fairly
1622 directly from the SASL specification. The message encoding is
1623 NOT used here, only plain text messages.
1626 In examples, "C:" and "S:" indicate lines sent by the client and
1627 server respectively.
1629 <sect2 id="auth-protocol-overview">
1630 <title>Protocol Overview</title>
1632 The protocol is a line-based protocol, where each line ends with
1633 \r\n. Each line begins with an all-caps ASCII command name containing
1634 only the character range [A-Z_], a space, then any arguments for the
1635 command, then the \r\n ending the line. The protocol is
1636 case-sensitive. All bytes must be in the ASCII character set.
1638 Commands from the client to the server are as follows:
1641 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1642 <listitem><para>CANCEL</para></listitem>
1643 <listitem><para>BEGIN</para></listitem>
1644 <listitem><para>DATA <data in hex encoding></para></listitem>
1645 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1646 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1649 From server to client are as follows:
1652 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1653 <listitem><para>OK <GUID in hex></para></listitem>
1654 <listitem><para>DATA <data in hex encoding></para></listitem>
1655 <listitem><para>ERROR</para></listitem>
1656 <listitem><para>AGREE_UNIX_FD</para></listitem>
1660 Unofficial extensions to the command set must begin with the letters
1661 "EXTENSION_", to avoid conflicts with future official commands.
1662 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1665 <sect2 id="auth-nul-byte">
1666 <title>Special credentials-passing nul byte</title>
1668 Immediately after connecting to the server, the client must send a
1669 single nul byte. This byte may be accompanied by credentials
1670 information on some operating systems that use sendmsg() with
1671 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1672 sockets. However, the nul byte must be sent even on other kinds of
1673 socket, and even on operating systems that do not require a byte to be
1674 sent in order to transmit credentials. The text protocol described in
1675 this document begins after the single nul byte. If the first byte
1676 received from the client is not a nul byte, the server may disconnect
1680 A nul byte in any context other than the initial byte is an error;
1681 the protocol is ASCII-only.
1684 The credentials sent along with the nul byte may be used with the
1685 SASL mechanism EXTERNAL.
1688 <sect2 id="auth-command-auth">
1689 <title>AUTH command</title>
1691 If an AUTH command has no arguments, it is a request to list
1692 available mechanisms. The server must respond with a REJECTED
1693 command listing the mechanisms it understands, or with an error.
1696 If an AUTH command specifies a mechanism, and the server supports
1697 said mechanism, the server should begin exchanging SASL
1698 challenge-response data with the client using DATA commands.
1701 If the server does not support the mechanism given in the AUTH
1702 command, it must send either a REJECTED command listing the mechanisms
1703 it does support, or an error.
1706 If the [initial-response] argument is provided, it is intended for use
1707 with mechanisms that have no initial challenge (or an empty initial
1708 challenge), as if it were the argument to an initial DATA command. If
1709 the selected mechanism has an initial challenge and [initial-response]
1710 was provided, the server should reject authentication by sending
1714 If authentication succeeds after exchanging DATA commands,
1715 an OK command must be sent to the client.
1718 The first octet received by the server after the \r\n of the BEGIN
1719 command from the client must be the first octet of the
1720 authenticated/encrypted stream of D-Bus messages.
1723 If BEGIN is received by the server, the first octet received
1724 by the client after the \r\n of the OK command must be the
1725 first octet of the authenticated/encrypted stream of D-Bus
1729 <sect2 id="auth-command-cancel">
1730 <title>CANCEL Command</title>
1732 At any time up to sending the BEGIN command, the client may send a
1733 CANCEL command. On receiving the CANCEL command, the server must
1734 send a REJECTED command and abort the current authentication
1738 <sect2 id="auth-command-data">
1739 <title>DATA Command</title>
1741 The DATA command may come from either client or server, and simply
1742 contains a hex-encoded block of data to be interpreted
1743 according to the SASL mechanism in use.
1746 Some SASL mechanisms support sending an "empty string";
1747 FIXME we need some way to do this.
1750 <sect2 id="auth-command-begin">
1751 <title>BEGIN Command</title>
1753 The BEGIN command acknowledges that the client has received an
1754 OK command from the server, and that the stream of messages
1758 The first octet received by the server after the \r\n of the BEGIN
1759 command from the client must be the first octet of the
1760 authenticated/encrypted stream of D-Bus messages.
1763 <sect2 id="auth-command-rejected">
1764 <title>REJECTED Command</title>
1766 The REJECTED command indicates that the current authentication
1767 exchange has failed, and further exchange of DATA is inappropriate.
1768 The client would normally try another mechanism, or try providing
1769 different responses to challenges.
1771 Optionally, the REJECTED command has a space-separated list of
1772 available auth mechanisms as arguments. If a server ever provides
1773 a list of supported mechanisms, it must provide the same list
1774 each time it sends a REJECTED message. Clients are free to
1775 ignore all lists received after the first.
1778 <sect2 id="auth-command-ok">
1779 <title>OK Command</title>
1781 The OK command indicates that the client has been
1782 authenticated. The client may now proceed with negotiating
1783 Unix file descriptor passing. To do that it shall send
1784 NEGOTIATE_UNIX_FD to the server.
1787 Otherwise, the client must respond to the OK command by
1788 sending a BEGIN command, followed by its stream of messages,
1789 or by disconnecting. The server must not accept additional
1790 commands using this protocol after the BEGIN command has been
1791 received. Further communication will be a stream of D-Bus
1792 messages (optionally encrypted, as negotiated) rather than
1796 If a client sends BEGIN the first octet received by the client
1797 after the \r\n of the OK command must be the first octet of
1798 the authenticated/encrypted stream of D-Bus messages.
1801 The OK command has one argument, which is the GUID of the server.
1802 See <xref linkend="addresses"/> for more on server GUIDs.
1805 <sect2 id="auth-command-error">
1806 <title>ERROR Command</title>
1808 The ERROR command indicates that either server or client did not
1809 know a command, does not accept the given command in the current
1810 context, or did not understand the arguments to the command. This
1811 allows the protocol to be extended; a client or server can send a
1812 command present or permitted only in new protocol versions, and if
1813 an ERROR is received instead of an appropriate response, fall back
1814 to using some other technique.
1817 If an ERROR is sent, the server or client that sent the
1818 error must continue as if the command causing the ERROR had never been
1819 received. However, the the server or client receiving the error
1820 should try something other than whatever caused the error;
1821 if only canceling/rejecting the authentication.
1824 If the D-Bus protocol changes incompatibly at some future time,
1825 applications implementing the new protocol would probably be able to
1826 check for support of the new protocol by sending a new command and
1827 receiving an ERROR from applications that don't understand it. Thus the
1828 ERROR feature of the auth protocol is an escape hatch that lets us
1829 negotiate extensions or changes to the D-Bus protocol in the future.
1832 <sect2 id="auth-command-negotiate-unix-fd">
1833 <title>NEGOTIATE_UNIX_FD Command</title>
1835 The NEGOTIATE_UNIX_FD command indicates that the client
1836 supports Unix file descriptor passing. This command may only
1837 be sent after the connection is authenticated, i.e. after OK
1838 was received by the client. This command may only be sent on
1839 transports that support Unix file descriptor passing.
1842 On receiving NEGOTIATE_UNIX_FD the server must respond with
1843 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1844 the transport chosen supports Unix file descriptor passing and
1845 the server supports this feature. It shall respond the latter
1846 if the transport does not support Unix file descriptor
1847 passing, the server does not support this feature, or the
1848 server decides not to enable file descriptor passing due to
1849 security or other reasons.
1852 <sect2 id="auth-command-agree-unix-fd">
1853 <title>AGREE_UNIX_FD Command</title>
1855 The AGREE_UNIX_FD command indicates that the server supports
1856 Unix file descriptor passing. This command may only be sent
1857 after the connection is authenticated, and the client sent
1858 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1859 command may only be sent on transports that support Unix file
1863 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1864 followed by its stream of messages, or by disconnecting. The
1865 server must not accept additional commands using this protocol
1866 after the BEGIN command has been received. Further
1867 communication will be a stream of D-Bus messages (optionally
1868 encrypted, as negotiated) rather than this protocol.
1871 <sect2 id="auth-command-future">
1872 <title>Future Extensions</title>
1874 Future extensions to the authentication and negotiation
1875 protocol are possible. For that new commands may be
1876 introduced. If a client or server receives an unknown command
1877 it shall respond with ERROR and not consider this fatal. New
1878 commands may be introduced both before, and after
1879 authentication, i.e. both before and after the OK command.
1882 <sect2 id="auth-examples">
1883 <title>Authentication examples</title>
1887 <title>Example of successful magic cookie authentication</title>
1889 (MAGIC_COOKIE is a made up mechanism)
1891 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1897 <title>Example of finding out mechanisms then picking one</title>
1900 S: REJECTED KERBEROS_V4 SKEY
1901 C: AUTH SKEY 7ab83f32ee
1902 S: DATA 8799cabb2ea93e
1903 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1909 <title>Example of client sends unknown command then falls back to regular auth</title>
1913 C: AUTH MAGIC_COOKIE 3736343435313230333039
1919 <title>Example of server doesn't support initial auth mechanism</title>
1921 C: AUTH MAGIC_COOKIE 3736343435313230333039
1922 S: REJECTED KERBEROS_V4 SKEY
1923 C: AUTH SKEY 7ab83f32ee
1924 S: DATA 8799cabb2ea93e
1925 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1931 <title>Example of wrong password or the like followed by successful retry</title>
1933 C: AUTH MAGIC_COOKIE 3736343435313230333039
1934 S: REJECTED KERBEROS_V4 SKEY
1935 C: AUTH SKEY 7ab83f32ee
1936 S: DATA 8799cabb2ea93e
1937 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1939 C: AUTH SKEY 7ab83f32ee
1940 S: DATA 8799cabb2ea93e
1941 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1947 <title>Example of skey cancelled and restarted</title>
1949 C: AUTH MAGIC_COOKIE 3736343435313230333039
1950 S: REJECTED KERBEROS_V4 SKEY
1951 C: AUTH SKEY 7ab83f32ee
1952 S: DATA 8799cabb2ea93e
1955 C: AUTH SKEY 7ab83f32ee
1956 S: DATA 8799cabb2ea93e
1957 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1963 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1965 (MAGIC_COOKIE is a made up mechanism)
1967 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1969 C: NEGOTIATE_UNIX_FD
1975 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1977 (MAGIC_COOKIE is a made up mechanism)
1979 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1981 C: NEGOTIATE_UNIX_FD
1988 <sect2 id="auth-states">
1989 <title>Authentication state diagrams</title>
1992 This section documents the auth protocol in terms of
1993 a state machine for the client and the server. This is
1994 probably the most robust way to implement the protocol.
1997 <sect3 id="auth-states-client">
1998 <title>Client states</title>
2001 To more precisely describe the interaction between the
2002 protocol state machine and the authentication mechanisms the
2003 following notation is used: MECH(CHALL) means that the
2004 server challenge CHALL was fed to the mechanism MECH, which
2010 CONTINUE(RESP) means continue the auth conversation
2011 and send RESP as the response to the server;
2017 OK(RESP) means that after sending RESP to the server
2018 the client side of the auth conversation is finished
2019 and the server should return "OK";
2025 ERROR means that CHALL was invalid and could not be
2031 Both RESP and CHALL may be empty.
2035 The Client starts by getting an initial response from the
2036 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2037 the mechanism did not provide an initial response. If the
2038 mechanism returns CONTINUE, the client starts in state
2039 <emphasis>WaitingForData</emphasis>, if the mechanism
2040 returns OK the client starts in state
2041 <emphasis>WaitingForOK</emphasis>.
2045 The client should keep track of available mechanisms and
2046 which it mechanisms it has already attempted. This list is
2047 used to decide which AUTH command to send. When the list is
2048 exhausted, the client should give up and close the
2053 <title><emphasis>WaitingForData</emphasis></title>
2061 MECH(CHALL) returns CONTINUE(RESP) → send
2063 <emphasis>WaitingForData</emphasis>
2067 MECH(CHALL) returns OK(RESP) → send DATA
2068 RESP, goto <emphasis>WaitingForOK</emphasis>
2072 MECH(CHALL) returns ERROR → send ERROR
2073 [msg], goto <emphasis>WaitingForData</emphasis>
2081 Receive REJECTED [mechs] →
2082 send AUTH [next mech], goto
2083 WaitingForData or <emphasis>WaitingForOK</emphasis>
2088 Receive ERROR → send
2090 <emphasis>WaitingForReject</emphasis>
2095 Receive OK → send
2096 BEGIN, terminate auth
2097 conversation, authenticated
2102 Receive anything else → send
2104 <emphasis>WaitingForData</emphasis>
2112 <title><emphasis>WaitingForOK</emphasis></title>
2117 Receive OK → send BEGIN, terminate auth
2118 conversation, <emphasis>authenticated</emphasis>
2123 Receive REJECT [mechs] → send AUTH [next mech],
2124 goto <emphasis>WaitingForData</emphasis> or
2125 <emphasis>WaitingForOK</emphasis>
2131 Receive DATA → send CANCEL, goto
2132 <emphasis>WaitingForReject</emphasis>
2138 Receive ERROR → send CANCEL, goto
2139 <emphasis>WaitingForReject</emphasis>
2145 Receive anything else → send ERROR, goto
2146 <emphasis>WaitingForOK</emphasis>
2154 <title><emphasis>WaitingForReject</emphasis></title>
2159 Receive REJECT [mechs] → send AUTH [next mech],
2160 goto <emphasis>WaitingForData</emphasis> or
2161 <emphasis>WaitingForOK</emphasis>
2167 Receive anything else → terminate auth
2168 conversation, disconnect
2177 <sect3 id="auth-states-server">
2178 <title>Server states</title>
2181 For the server MECH(RESP) means that the client response
2182 RESP was fed to the the mechanism MECH, which returns one of
2187 CONTINUE(CHALL) means continue the auth conversation and
2188 send CHALL as the challenge to the client;
2194 OK means that the client has been successfully
2201 REJECT means that the client failed to authenticate or
2202 there was an error in RESP.
2207 The server starts out in state
2208 <emphasis>WaitingForAuth</emphasis>. If the client is
2209 rejected too many times the server must disconnect the
2214 <title><emphasis>WaitingForAuth</emphasis></title>
2220 Receive AUTH → send REJECTED [mechs], goto
2221 <emphasis>WaitingForAuth</emphasis>
2227 Receive AUTH MECH RESP
2231 MECH not valid mechanism → send REJECTED
2233 <emphasis>WaitingForAuth</emphasis>
2237 MECH(RESP) returns CONTINUE(CHALL) → send
2239 <emphasis>WaitingForData</emphasis>
2243 MECH(RESP) returns OK → send OK, goto
2244 <emphasis>WaitingForBegin</emphasis>
2248 MECH(RESP) returns REJECT → send REJECTED
2250 <emphasis>WaitingForAuth</emphasis>
2258 Receive BEGIN → terminate
2259 auth conversation, disconnect
2265 Receive ERROR → send REJECTED [mechs], goto
2266 <emphasis>WaitingForAuth</emphasis>
2272 Receive anything else → send
2274 <emphasis>WaitingForAuth</emphasis>
2283 <title><emphasis>WaitingForData</emphasis></title>
2291 MECH(RESP) returns CONTINUE(CHALL) → send
2293 <emphasis>WaitingForData</emphasis>
2297 MECH(RESP) returns OK → send OK, goto
2298 <emphasis>WaitingForBegin</emphasis>
2302 MECH(RESP) returns REJECT → send REJECTED
2304 <emphasis>WaitingForAuth</emphasis>
2312 Receive BEGIN → terminate auth conversation,
2319 Receive CANCEL → send REJECTED [mechs], goto
2320 <emphasis>WaitingForAuth</emphasis>
2326 Receive ERROR → send REJECTED [mechs], goto
2327 <emphasis>WaitingForAuth</emphasis>
2333 Receive anything else → send ERROR, goto
2334 <emphasis>WaitingForData</emphasis>
2342 <title><emphasis>WaitingForBegin</emphasis></title>
2347 Receive BEGIN → terminate auth conversation,
2348 client authenticated
2354 Receive CANCEL → send REJECTED [mechs], goto
2355 <emphasis>WaitingForAuth</emphasis>
2361 Receive ERROR → send REJECTED [mechs], goto
2362 <emphasis>WaitingForAuth</emphasis>
2368 Receive anything else → send ERROR, goto
2369 <emphasis>WaitingForBegin</emphasis>
2379 <sect2 id="auth-mechanisms">
2380 <title>Authentication mechanisms</title>
2382 This section describes some new authentication mechanisms.
2383 D-Bus also allows any standard SASL mechanism of course.
2385 <sect3 id="auth-mechanisms-sha">
2386 <title>DBUS_COOKIE_SHA1</title>
2388 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2389 has the ability to read a private file owned by the user being
2390 authenticated. If the client can prove that it has access to a secret
2391 cookie stored in this file, then the client is authenticated.
2392 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2396 Throughout this description, "hex encoding" must output the digits
2397 from a to f in lower-case; the digits A to F must not be used
2398 in the DBUS_COOKIE_SHA1 mechanism.
2401 Authentication proceeds as follows:
2405 The client sends the username it would like to authenticate
2411 The server sends the name of its "cookie context" (see below); a
2412 space character; the integer ID of the secret cookie the client
2413 must demonstrate knowledge of; a space character; then a
2414 randomly-generated challenge string, all of this hex-encoded into
2420 The client locates the cookie and generates its own
2421 randomly-generated challenge string. The client then concatenates
2422 the server's decoded challenge, a ":" character, its own challenge,
2423 another ":" character, and the cookie. It computes the SHA-1 hash
2424 of this composite string as a hex digest. It concatenates the
2425 client's challenge string, a space character, and the SHA-1 hex
2426 digest, hex-encodes the result and sends it back to the server.
2431 The server generates the same concatenated string used by the
2432 client and computes its SHA-1 hash. It compares the hash with
2433 the hash received from the client; if the two hashes match, the
2434 client is authenticated.
2440 Each server has a "cookie context," which is a name that identifies a
2441 set of cookies that apply to that server. A sample context might be
2442 "org_freedesktop_session_bus". Context names must be valid ASCII,
2443 nonzero length, and may not contain the characters slash ("/"),
2444 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2445 tab ("\t"), or period ("."). There is a default context,
2446 "org_freedesktop_general" that's used by servers that do not specify
2450 Cookies are stored in a user's home directory, in the directory
2451 <filename>~/.dbus-keyrings/</filename>. This directory must
2452 not be readable or writable by other users. If it is,
2453 clients and servers must ignore it. The directory
2454 contains cookie files named after the cookie context.
2457 A cookie file contains one cookie per line. Each line
2458 has three space-separated fields:
2462 The cookie ID number, which must be a non-negative integer and
2463 may not be used twice in the same file.
2468 The cookie's creation time, in UNIX seconds-since-the-epoch
2474 The cookie itself, a hex-encoded random block of bytes. The cookie
2475 may be of any length, though obviously security increases
2476 as the length increases.
2482 Only server processes modify the cookie file.
2483 They must do so with this procedure:
2487 Create a lockfile name by appending ".lock" to the name of the
2488 cookie file. The server should attempt to create this file
2489 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2490 fails, the lock fails. Servers should retry for a reasonable
2491 period of time, then they may choose to delete an existing lock
2492 to keep users from having to manually delete a stale
2493 lock. <footnote><para>Lockfiles are used instead of real file
2494 locking <literal>fcntl()</literal> because real locking
2495 implementations are still flaky on network
2496 filesystems.</para></footnote>
2501 Once the lockfile has been created, the server loads the cookie
2502 file. It should then delete any cookies that are old (the
2503 timeout can be fairly short), or more than a reasonable
2504 time in the future (so that cookies never accidentally
2505 become permanent, if the clock was set far into the future
2506 at some point). If no recent keys remain, the
2507 server may generate a new key.
2512 The pruned and possibly added-to cookie file
2513 must be resaved atomically (using a temporary
2514 file which is rename()'d).
2519 The lock must be dropped by deleting the lockfile.
2525 Clients need not lock the file in order to load it,
2526 because servers are required to save the file atomically.
2531 <sect1 id="addresses">
2532 <title>Server Addresses</title>
2534 Server addresses consist of a transport name followed by a colon, and
2535 then an optional, comma-separated list of keys and values in the form key=value.
2536 Each value is escaped.
2540 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2541 Which is the address to a unix socket with the path /tmp/dbus-test.
2544 Value escaping is similar to URI escaping but simpler.
2548 The set of optionally-escaped bytes is:
2549 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2550 <emphasis>byte</emphasis> (note, not character) which is not in the
2551 set of optionally-escaped bytes must be replaced with an ASCII
2552 percent (<literal>%</literal>) and the value of the byte in hex.
2553 The hex value must always be two digits, even if the first digit is
2554 zero. The optionally-escaped bytes may be escaped if desired.
2559 To unescape, append each byte in the value; if a byte is an ASCII
2560 percent (<literal>%</literal>) character then append the following
2561 hex value instead. It is an error if a <literal>%</literal> byte
2562 does not have two hex digits following. It is an error if a
2563 non-optionally-escaped byte is seen unescaped.
2567 The set of optionally-escaped bytes is intended to preserve address
2568 readability and convenience.
2572 A server may specify a key-value pair with the key <literal>guid</literal>
2573 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2574 describes the format of the <literal>guid</literal> field. If present,
2575 this UUID may be used to distinguish one server address from another. A
2576 server should use a different UUID for each address it listens on. For
2577 example, if a message bus daemon offers both UNIX domain socket and TCP
2578 connections, but treats clients the same regardless of how they connect,
2579 those two connections are equivalent post-connection but should have
2580 distinct UUIDs to distinguish the kinds of connection.
2584 The intent of the address UUID feature is to allow a client to avoid
2585 opening multiple identical connections to the same server, by allowing the
2586 client to check whether an address corresponds to an already-existing
2587 connection. Comparing two addresses is insufficient, because addresses
2588 can be recycled by distinct servers, and equivalent addresses may look
2589 different if simply compared as strings (for example, the host in a TCP
2590 address can be given as an IP address or as a hostname).
2594 Note that the address key is <literal>guid</literal> even though the
2595 rest of the API and documentation says "UUID," for historical reasons.
2599 [FIXME clarify if attempting to connect to each is a requirement
2600 or just a suggestion]
2601 When connecting to a server, multiple server addresses can be
2602 separated by a semi-colon. The library will then try to connect
2603 to the first address and if that fails, it'll try to connect to
2604 the next one specified, and so forth. For example
2605 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2610 <sect1 id="transports">
2611 <title>Transports</title>
2613 [FIXME we need to specify in detail each transport and its possible arguments]
2615 Current transports include: unix domain sockets (including
2616 abstract namespace on linux), launchd, TCP/IP, and a debug/testing transport
2617 using in-process pipes. Future possible transports include one that
2618 tunnels over X11 protocol.
2621 <sect2 id="transports-unix-domain-sockets">
2622 <title>Unix Domain Sockets</title>
2624 Unix domain sockets can be either paths in the file system or on Linux
2625 kernels, they can be abstract which are similar to paths but
2626 do not show up in the file system.
2630 When a socket is opened by the D-Bus library it truncates the path
2631 name right before the first trailing Nul byte. This is true for both
2632 normal paths and abstract paths. Note that this is a departure from
2633 previous versions of D-Bus that would create sockets with a fixed
2634 length path name. Names which were shorter than the fixed length
2635 would be padded by Nul bytes.
2638 Unix domain sockets are not available on windows.
2640 <sect3 id="transports-unix-domain-sockets-addresses">
2641 <title>Server Address Format</title>
2643 Unix domain socket addresses are identified by the "unix:" prefix
2644 and support the following key/value pairs:
2651 <entry>Values</entry>
2652 <entry>Description</entry>
2658 <entry>(path)</entry>
2659 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2662 <entry>tmpdir</entry>
2663 <entry>(path)</entry>
2664 <entry>temporary directory in which a socket file with a random file name starting with 'dbus-' will be created by the server. This key can only be used in server addresses, not in client addresses. If set, the "path" and "abstract" key must not be set.</entry>
2667 <entry>abstract</entry>
2668 <entry>(string)</entry>
2669 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2676 <sect2 id="transports-launchd">
2677 <title>launchd</title>
2679 launchd is a open-source server management system that replaces init, inetd
2680 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2681 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2685 launchd allocates a socket and provides it with the unix path through the
2686 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2687 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2688 it through its environment.
2689 Other processes can query for the launchd socket by executing:
2690 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2691 This is normally done by the D-Bus client library so doesn't have to be done
2695 launchd is not available on Microsoft Windows.
2697 <sect3 id="transports-launchd-addresses">
2698 <title>Server Address Format</title>
2700 launchd addresses are identified by the "launchd:" prefix
2701 and support the following key/value pairs:
2708 <entry>Values</entry>
2709 <entry>Description</entry>
2715 <entry>(environment variable)</entry>
2716 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2723 <sect2 id="transports-tcp-sockets">
2724 <title>TCP Sockets</title>
2726 The tcp transport provides TCP/IP based connections between clients
2727 located on the same or different hosts.
2730 Using tcp transport without any additional secure authentification mechanismus
2731 over a network is unsecure.
2734 Windows notes: Because of the tcp stack on windows does not provide sending
2735 credentials over a tcp connection, the EXTERNAL authentification
2736 mechanismus does not work.
2738 <sect3 id="transports-tcp-sockets-addresses">
2739 <title>Server Address Format</title>
2741 TCP/IP socket addresses are identified by the "tcp:" prefix
2742 and support the following key/value pairs:
2749 <entry>Values</entry>
2750 <entry>Description</entry>
2756 <entry>(string)</entry>
2757 <entry>dns name or ip address</entry>
2761 <entry>(number)</entry>
2762 <entry>The tcp port the server will open. A zero value let the server
2763 choose a free port provided from the underlaying operating system.
2764 libdbus is able to retrieve the real used port from the server.
2768 <entry>family</entry>
2769 <entry>(string)</entry>
2770 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2777 <sect2 id="transports-nonce-tcp-sockets">
2778 <title>Nonce-secured TCP Sockets</title>
2780 The nonce-tcp transport provides a secured TCP transport, using a
2781 simple authentication mechanism to ensure that only clients with read
2782 access to a certain location in the filesystem can connect to the server.
2783 The server writes a secret, the nonce, to a file and an incoming client
2784 connection is only accepted if the client sends the nonce right after
2785 the connect. The nonce mechanism requires no setup and is orthogonal to
2786 the higher-level authentication mechanisms described in the
2787 Authentication section.
2791 On start, the server generates a random 16 byte nonce and writes it
2792 to a file in the user's temporary directory. The nonce file location
2793 is published as part of the server's D-Bus address using the
2794 "noncefile" key-value pair.
2796 After an accept, the server reads 16 bytes from the socket. If the
2797 read bytes do not match the nonce stored in the nonce file, the
2798 server MUST immediately drop the connection.
2799 If the nonce match the received byte sequence, the client is accepted
2800 and the transport behaves like an unsecured tcp transport.
2803 After a successful connect to the server socket, the client MUST read
2804 the nonce from the file published by the server via the noncefile=
2805 key-value pair and send it over the socket. After that, the
2806 transport behaves like an unsecured tcp transport.
2808 <sect3 id="transports-nonce-tcp-sockets-addresses">
2809 <title>Server Address Format</title>
2811 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2812 and support the following key/value pairs:
2819 <entry>Values</entry>
2820 <entry>Description</entry>
2826 <entry>(string)</entry>
2827 <entry>dns name or ip address</entry>
2831 <entry>(number)</entry>
2832 <entry>The tcp port the server will open. A zero value let the server
2833 choose a free port provided from the underlaying operating system.
2834 libdbus is able to retrieve the real used port from the server.
2838 <entry>family</entry>
2839 <entry>(string)</entry>
2840 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2843 <entry>noncefile</entry>
2844 <entry>(path)</entry>
2845 <entry>file location containing the secret</entry>
2853 <sect1 id="meta-transports">
2854 <title>Meta Transports</title>
2856 Meta transports are a kind of transport with special enhancements or
2857 behavior. Currently available meta transports include: autolaunch
2860 <sect2 id="meta-transports-autolaunch">
2861 <title>Autolaunch</title>
2862 <para>The autolaunch transport provides a way for dbus clients to autodetect
2863 a running dbus session bus and to autolaunch a session bus if not present.
2865 <sect3 id="meta-transports-autolaunch-addresses">
2866 <title>Server Address Format</title>
2868 Autolaunch addresses uses the "autolaunch:" prefix and support the
2869 following key/value pairs:
2876 <entry>Values</entry>
2877 <entry>Description</entry>
2882 <entry>scope</entry>
2883 <entry>(string)</entry>
2884 <entry>scope of autolaunch (Windows only)
2888 "*install-path" - limit session bus to dbus installation path.
2889 The dbus installation path is determined from the location of
2890 the shared dbus library. If the library is located in a 'bin'
2891 subdirectory the installation root is the directory above,
2892 otherwise the directory where the library lives is taken as
2895 <install-root>/bin/[lib]dbus-1.dll
2896 <install-root>/[lib]dbus-1.dll
2902 "*user" - limit session bus to the recent user.
2907 other values - specify dedicated session bus like "release",
2919 <sect3 id="meta-transports-autolaunch-windows-implementation">
2920 <title>Windows implementation</title>
2922 On start, the server opens a platform specific transport, creates a mutex
2923 and a shared memory section containing the related session bus address.
2924 This mutex will be inspected by the dbus client library to detect a
2925 running dbus session bus. The access to the mutex and the shared memory
2926 section are protected by global locks.
2929 In the recent implementation the autolaunch transport uses a tcp transport
2930 on localhost with a port choosen from the operating system. This detail may
2931 change in the future.
2934 Disclaimer: The recent implementation is in an early state and may not
2935 work in all cirumstances and/or may have security issues. Because of this
2936 the implementation is not documentated yet.
2943 <title>UUIDs</title>
2945 A working D-Bus implementation uses universally-unique IDs in two places.
2946 First, each server address has a UUID identifying the address,
2947 as described in <xref linkend="addresses"/>. Second, each operating
2948 system kernel instance running a D-Bus client or server has a UUID
2949 identifying that kernel, retrieved by invoking the method
2950 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2951 linkend="standard-interfaces-peer"/>).
2954 The term "UUID" in this document is intended literally, i.e. an
2955 identifier that is universally unique. It is not intended to refer to
2956 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2959 The UUID must contain 128 bits of data and be hex-encoded. The
2960 hex-encoded string may not contain hyphens or other non-hex-digit
2961 characters, and it must be exactly 32 characters long. To generate a
2962 UUID, the current reference implementation concatenates 96 bits of random
2963 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2967 It would also be acceptable and probably better to simply generate 128
2968 bits of random data, as long as the random number generator is of high
2969 quality. The timestamp could conceivably help if the random bits are not
2970 very random. With a quality random number generator, collisions are
2971 extremely unlikely even with only 96 bits, so it's somewhat academic.
2974 Implementations should, however, stick to random data for the first 96 bits
2979 <sect1 id="standard-interfaces">
2980 <title>Standard Interfaces</title>
2982 See <xref linkend="message-protocol-types-notation"/> for details on
2983 the notation used in this section. There are some standard interfaces
2984 that may be useful across various D-Bus applications.
2986 <sect2 id="standard-interfaces-peer">
2987 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2989 The <literal>org.freedesktop.DBus.Peer</literal> interface
2992 org.freedesktop.DBus.Peer.Ping ()
2993 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2997 On receipt of the <literal>METHOD_CALL</literal> message
2998 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2999 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3000 usual. It does not matter which object path a ping is sent to. The
3001 reference implementation handles this method automatically.
3004 On receipt of the <literal>METHOD_CALL</literal> message
3005 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3006 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3007 UUID representing the identity of the machine the process is running on.
3008 This UUID must be the same for all processes on a single system at least
3009 until that system next reboots. It should be the same across reboots
3010 if possible, but this is not always possible to implement and is not
3012 It does not matter which object path a GetMachineId is sent to. The
3013 reference implementation handles this method automatically.
3016 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3017 a virtual machine running on a hypervisor, rather than a physical machine.
3018 Basically if two processes see the same UUID, they should also see the same
3019 shared memory, UNIX domain sockets, process IDs, and other features that require
3020 a running OS kernel in common between the processes.
3023 The UUID is often used where other programs might use a hostname. Hostnames
3024 can change without rebooting, however, or just be "localhost" - so the UUID
3028 <xref linkend="uuids"/> explains the format of the UUID.
3032 <sect2 id="standard-interfaces-introspectable">
3033 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3035 This interface has one method:
3037 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3041 Objects instances may implement
3042 <literal>Introspect</literal> which returns an XML description of
3043 the object, including its interfaces (with signals and methods), objects
3044 below it in the object path tree, and its properties.
3047 <xref linkend="introspection-format"/> describes the format of this XML string.
3050 <sect2 id="standard-interfaces-properties">
3051 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3053 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3054 or <firstterm>attributes</firstterm>. These can be exposed via the
3055 <literal>org.freedesktop.DBus.Properties</literal> interface.
3059 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3060 in STRING property_name,
3062 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3063 in STRING property_name,
3065 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3066 out DICT<STRING,VARIANT> props);
3070 The available properties and whether they are writable can be determined
3071 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3072 see <xref linkend="standard-interfaces-introspectable"/>.
3075 An empty string may be provided for the interface name; in this case,
3076 if there are multiple properties on an object with the same name,
3077 the results are undefined (picking one by according to an arbitrary
3078 deterministic rule, or returning an error, are the reasonable
3082 If one or more properties change on an object, the
3083 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3084 signal may be emitted (this signal was added in 0.14):
3088 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3089 DICT<STRING,VARIANT> changed_properties,
3090 ARRAY<STRING> invalidated_properties);
3094 where <literal>changed_properties</literal> is a dictionary
3095 containing the changed properties with the new values and
3096 <literal>invalidated_properties</literal> is an array of
3097 properties that changed but the value is not conveyed.
3100 Whether the <literal>PropertiesChanged</literal> signal is
3101 supported can be determined by calling
3102 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3103 that the signal may be supported for an object but it may
3104 differ how whether and how it is used on a per-property basis
3105 (for e.g. performance or security reasons). Each property (or
3106 the parent interface) must be annotated with the
3107 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3108 annotation to convey this (usually the default value
3109 <literal>true</literal> is sufficient meaning that the
3110 annotation does not need to be used). See <xref
3111 linkend="introspection-format"/> for details on this
3117 <sect1 id="introspection-format">
3118 <title>Introspection Data Format</title>
3120 As described in <xref linkend="standard-interfaces-introspectable"/>,
3121 objects may be introspected at runtime, returning an XML string
3122 that describes the object. The same XML format may be used in
3123 other contexts as well, for example as an "IDL" for generating
3124 static language bindings.
3127 Here is an example of introspection data:
3129 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3130 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3131 <node name="/org/freedesktop/sample_object">
3132 <interface name="org.freedesktop.SampleInterface">
3133 <method name="Frobate">
3134 <arg name="foo" type="i" direction="in"/>
3135 <arg name="bar" type="s" direction="out"/>
3136 <arg name="baz" type="a{us}" direction="out"/>
3137 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3139 <method name="Bazify">
3140 <arg name="bar" type="(iiu)" direction="in"/>
3141 <arg name="bar" type="v" direction="out"/>
3143 <method name="Mogrify">
3144 <arg name="bar" type="(iiav)" direction="in"/>
3146 <signal name="Changed">
3147 <arg name="new_value" type="b"/>
3149 <property name="Bar" type="y" access="readwrite"/>
3151 <node name="child_of_sample_object"/>
3152 <node name="another_child_of_sample_object"/>
3157 A more formal DTD and spec needs writing, but here are some quick notes.
3161 Only the root <node> element can omit the node name, as it's
3162 known to be the object that was introspected. If the root
3163 <node> does have a name attribute, it must be an absolute
3164 object path. If child <node> have object paths, they must be
3170 If a child <node> has any sub-elements, then they
3171 must represent a complete introspection of the child.
3172 If a child <node> is empty, then it may or may
3173 not have sub-elements; the child must be introspected
3174 in order to find out. The intent is that if an object
3175 knows that its children are "fast" to introspect
3176 it can go ahead and return their information, but
3177 otherwise it can omit it.
3182 The direction element on <arg> may be omitted,
3183 in which case it defaults to "in" for method calls
3184 and "out" for signals. Signals only allow "out"
3185 so while direction may be specified, it's pointless.
3190 The possible directions are "in" and "out",
3191 unlike CORBA there is no "inout"
3196 The possible property access flags are
3197 "readwrite", "read", and "write"
3202 Multiple interfaces can of course be listed for
3208 The "name" attribute on arguments is optional.
3214 Method, interface, property, and signal elements may have
3215 "annotations", which are generic key/value pairs of metadata.
3216 They are similar conceptually to Java's annotations and C# attributes.
3217 Well-known annotations:
3224 <entry>Values (separated by ,)</entry>
3225 <entry>Description</entry>
3230 <entry>org.freedesktop.DBus.Deprecated</entry>
3231 <entry>true,false</entry>
3232 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3235 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3236 <entry>(string)</entry>
3237 <entry>The C symbol; may be used for methods and interfaces</entry>
3240 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3241 <entry>true,false</entry>
3242 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3245 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3246 <entry>true,invalidates,false</entry>
3249 If set to <literal>false</literal>, the
3250 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3252 linkend="standard-interfaces-properties"/> is not
3253 guaranteed to be emitted if the property changes.
3256 If set to <literal>invalidates</literal> the signal
3257 is emitted but the value is not included in the
3261 If set to <literal>true</literal> the signal is
3262 emitted with the value included.
3265 The value for the annotation defaults to
3266 <literal>true</literal> if the enclosing interface
3267 element does not specify the annotation. Otherwise it
3268 defaults to the value specified in the enclosing
3277 <sect1 id="message-bus">
3278 <title>Message Bus Specification</title>
3279 <sect2 id="message-bus-overview">
3280 <title>Message Bus Overview</title>
3282 The message bus accepts connections from one or more applications.
3283 Once connected, applications can exchange messages with other
3284 applications that are also connected to the bus.
3287 In order to route messages among connections, the message bus keeps a
3288 mapping from names to connections. Each connection has one
3289 unique-for-the-lifetime-of-the-bus name automatically assigned.
3290 Applications may request additional names for a connection. Additional
3291 names are usually "well-known names" such as
3292 "org.freedesktop.TextEditor". When a name is bound to a connection,
3293 that connection is said to <firstterm>own</firstterm> the name.
3296 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3297 This name routes messages to the bus, allowing applications to make
3298 administrative requests. For example, applications can ask the bus
3299 to assign a name to a connection.
3302 Each name may have <firstterm>queued owners</firstterm>. When an
3303 application requests a name for a connection and the name is already in
3304 use, the bus will optionally add the connection to a queue waiting for
3305 the name. If the current owner of the name disconnects or releases
3306 the name, the next connection in the queue will become the new owner.
3310 This feature causes the right thing to happen if you start two text
3311 editors for example; the first one may request "org.freedesktop.TextEditor",
3312 and the second will be queued as a possible owner of that name. When
3313 the first exits, the second will take over.
3317 Messages may have a <literal>DESTINATION</literal> field (see <xref
3318 linkend="message-protocol-header-fields"/>). If the
3319 <literal>DESTINATION</literal> field is present, it specifies a message
3320 recipient by name. Method calls and replies normally specify this field.
3321 The message bus must send messages (of any type) with the
3322 <literal>DESTINATION</literal> field set to the specified recipient,
3323 regardless of whether the recipient has set up a match rule matching
3328 Signals normally do not specify a destination; they are sent to all
3329 applications with <firstterm>message matching rules</firstterm> that
3334 When the message bus receives a method call, if the
3335 <literal>DESTINATION</literal> field is absent, the call is taken to be
3336 a standard one-to-one message and interpreted by the message bus
3337 itself. For example, sending an
3338 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3339 <literal>DESTINATION</literal> will cause the message bus itself to
3340 reply to the ping immediately; the message bus will not make this
3341 message visible to other applications.
3345 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3346 the ping message were sent with a <literal>DESTINATION</literal> name of
3347 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3348 forwarded, and the Yoyodyne Corporation screensaver application would be
3349 expected to reply to the ping.
3353 <sect2 id="message-bus-names">
3354 <title>Message Bus Names</title>
3356 Each connection has at least one name, assigned at connection time and
3357 returned in response to the
3358 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3359 automatically-assigned name is called the connection's <firstterm>unique
3360 name</firstterm>. Unique names are never reused for two different
3361 connections to the same bus.
3364 Ownership of a unique name is a prerequisite for interaction with
3365 the message bus. It logically follows that the unique name is always
3366 the first name that an application comes to own, and the last
3367 one that it loses ownership of.
3370 Unique connection names must begin with the character ':' (ASCII colon
3371 character); bus names that are not unique names must not begin
3372 with this character. (The bus must reject any attempt by an application
3373 to manually request a name beginning with ':'.) This restriction
3374 categorically prevents "spoofing"; messages sent to a unique name
3375 will always go to the expected connection.
3378 When a connection is closed, all the names that it owns are deleted (or
3379 transferred to the next connection in the queue if any).
3382 A connection can request additional names to be associated with it using
3383 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3384 linkend="message-protocol-names-bus"/> describes the format of a valid
3385 name. These names can be released again using the
3386 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3389 <sect3 id="bus-messages-request-name">
3390 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3394 UINT32 RequestName (in STRING name, in UINT32 flags)
3401 <entry>Argument</entry>
3403 <entry>Description</entry>
3409 <entry>STRING</entry>
3410 <entry>Name to request</entry>
3414 <entry>UINT32</entry>
3415 <entry>Flags</entry>
3425 <entry>Argument</entry>
3427 <entry>Description</entry>
3433 <entry>UINT32</entry>
3434 <entry>Return value</entry>
3441 This method call should be sent to
3442 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3443 assign the given name to the method caller. Each name maintains a
3444 queue of possible owners, where the head of the queue is the primary
3445 or current owner of the name. Each potential owner in the queue
3446 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3447 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3448 call. When RequestName is invoked the following occurs:
3452 If the method caller is currently the primary owner of the name,
3453 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3454 values are updated with the values from the new RequestName call,
3455 and nothing further happens.
3461 If the current primary owner (head of the queue) has
3462 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3463 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3464 the caller of RequestName replaces the current primary owner at
3465 the head of the queue and the current primary owner moves to the
3466 second position in the queue. If the caller of RequestName was
3467 in the queue previously its flags are updated with the values from
3468 the new RequestName in addition to moving it to the head of the queue.
3474 If replacement is not possible, and the method caller is
3475 currently in the queue but not the primary owner, its flags are
3476 updated with the values from the new RequestName call.
3482 If replacement is not possible, and the method caller is
3483 currently not in the queue, the method caller is appended to the
3490 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3491 set and is not the primary owner, it is removed from the
3492 queue. This can apply to the previous primary owner (if it
3493 was replaced) or the method caller (if it updated the
3494 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3495 queue, or if it was just added to the queue with that flag set).
3501 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3502 queue," even if another application already in the queue had specified
3503 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3504 that does not allow replacement goes away, and the next primary owner
3505 does allow replacement. In this case, queued items that specified
3506 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3507 automatically replace the new primary owner. In other words,
3508 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3509 time RequestName is called. This is deliberate to avoid an infinite loop
3510 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3511 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3514 The flags argument contains any of the following values logically ORed
3521 <entry>Conventional Name</entry>
3522 <entry>Value</entry>
3523 <entry>Description</entry>
3528 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3532 If an application A specifies this flag and succeeds in
3533 becoming the owner of the name, and another application B
3534 later calls RequestName with the
3535 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3536 will lose ownership and receive a
3537 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3538 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3539 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3540 is not specified by application B, then application B will not replace
3541 application A as the owner.
3546 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3550 Try to replace the current owner if there is one. If this
3551 flag is not set the application will only become the owner of
3552 the name if there is no current owner. If this flag is set,
3553 the application will replace the current owner if
3554 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3559 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3563 Without this flag, if an application requests a name that is
3564 already owned, the application will be placed in a queue to
3565 own the name when the current owner gives it up. If this
3566 flag is given, the application will not be placed in the
3567 queue, the request for the name will simply fail. This flag
3568 also affects behavior when an application is replaced as
3569 name owner; by default the application moves back into the
3570 waiting queue, unless this flag was provided when the application
3571 became the name owner.
3579 The return code can be one of the following values:
3585 <entry>Conventional Name</entry>
3586 <entry>Value</entry>
3587 <entry>Description</entry>
3592 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3593 <entry>1</entry> <entry>The caller is now the primary owner of
3594 the name, replacing any previous owner. Either the name had no
3595 owner before, or the caller specified
3596 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3597 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3600 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3603 <entry>The name already had an owner,
3604 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3605 the current owner did not specify
3606 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3607 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3611 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3612 <entry>The name already has an owner,
3613 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3614 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3615 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3616 specified by the requesting application.</entry>
3619 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3621 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3629 <sect3 id="bus-messages-release-name">
3630 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3634 UINT32 ReleaseName (in STRING name)
3641 <entry>Argument</entry>
3643 <entry>Description</entry>
3649 <entry>STRING</entry>
3650 <entry>Name to release</entry>
3660 <entry>Argument</entry>
3662 <entry>Description</entry>
3668 <entry>UINT32</entry>
3669 <entry>Return value</entry>
3676 This method call should be sent to
3677 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3678 release the method caller's claim to the given name. If the caller is
3679 the primary owner, a new primary owner will be selected from the
3680 queue if any other owners are waiting. If the caller is waiting in
3681 the queue for the name, the caller will removed from the queue and
3682 will not be made an owner of the name if it later becomes available.
3683 If there are no other owners in the queue for the name, it will be
3684 removed from the bus entirely.
3686 The return code can be one of the following values:
3692 <entry>Conventional Name</entry>
3693 <entry>Value</entry>
3694 <entry>Description</entry>
3699 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3700 <entry>1</entry> <entry>The caller has released his claim on
3701 the given name. Either the caller was the primary owner of
3702 the name, and the name is now unused or taken by somebody
3703 waiting in the queue for the name, or the caller was waiting
3704 in the queue for the name and has now been removed from the
3708 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3710 <entry>The given name does not exist on this bus.</entry>
3713 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3715 <entry>The caller was not the primary owner of this name,
3716 and was also not waiting in the queue to own this name.</entry>
3724 <sect3 id="bus-messages-list-queued-owners">
3725 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3729 ARRAY of STRING ListQueuedOwners (in STRING name)
3736 <entry>Argument</entry>
3738 <entry>Description</entry>
3744 <entry>STRING</entry>
3745 <entry>The well-known bus name to query, such as
3746 <literal>com.example.cappuccino</literal></entry>
3756 <entry>Argument</entry>
3758 <entry>Description</entry>
3764 <entry>ARRAY of STRING</entry>
3765 <entry>The unique bus names of connections currently queued
3766 for the name</entry>
3773 This method call should be sent to
3774 <literal>org.freedesktop.DBus</literal> and lists the connections
3775 currently queued for a bus name (see
3776 <xref linkend="term-queued-owner"/>).
3781 <sect2 id="message-bus-routing">
3782 <title>Message Bus Message Routing</title>
3786 <sect3 id="message-bus-routing-match-rules">
3787 <title>Match Rules</title>
3789 An important part of the message bus routing protocol is match
3790 rules. Match rules describe what messages can be sent to a client
3791 based on the contents of the message. When a message is routed
3792 through the bus it is compared to clients' match rules. If any
3793 of the rules match, the message is dispatched to the client.
3794 If none of the rules match the message never leaves the bus. This
3795 is an effective way to control traffic over the bus and to make sure
3796 only relevant message need to be processed by the client.
3799 Match rules are added using the AddMatch bus method
3800 (see <xref linkend="bus-messages-add-match"/>). Rules are
3801 specified as a string of comma separated key/value pairs.
3802 Excluding a key from the rule indicates a wildcard match.
3803 For instance excluding the the member from a match rule but
3804 adding a sender would let all messages from that sender through.
3805 An example of a complete rule would be
3806 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3809 The following table describes the keys that can be used to create
3811 The following table summarizes the D-Bus types.
3817 <entry>Possible Values</entry>
3818 <entry>Description</entry>
3823 <entry><literal>type</literal></entry>
3824 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3825 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3828 <entry><literal>sender</literal></entry>
3829 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3830 and <xref linkend="term-unique-name"/> respectively)
3832 <entry>Match messages sent by a particular sender. An example of a sender match
3833 is sender='org.freedesktop.Hal'</entry>
3836 <entry><literal>interface</literal></entry>
3837 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3838 <entry>Match messages sent over or to a particular interface. An example of an
3839 interface match is interface='org.freedesktop.Hal.Manager'.
3840 If a message omits the interface header, it must not match any rule
3841 that specifies this key.</entry>
3844 <entry><literal>member</literal></entry>
3845 <entry>Any valid method or signal name</entry>
3846 <entry>Matches messages which have the give method or signal name. An example of
3847 a member match is member='NameOwnerChanged'</entry>
3850 <entry><literal>path</literal></entry>
3851 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3852 <entry>Matches messages which are sent from or to the given object. An example of a
3853 path match is path='/org/freedesktop/Hal/Manager'</entry>
3856 <entry><literal>path_namespace</literal></entry>
3857 <entry>An object path</entry>
3860 Matches messages which are sent from or to an
3861 object for which the object path is either the
3862 given value, or that value followed by one or
3863 more path components.
3868 <literal>path_namespace='/com/example/foo'</literal>
3869 would match signals sent by
3870 <literal>/com/example/foo</literal>
3872 <literal>/com/example/foo/bar</literal>,
3874 <literal>/com/example/foobar</literal>.
3878 Using both <literal>path</literal> and
3879 <literal>path_namespace</literal> in the same match
3880 rule is not allowed.
3885 This match key was added in version 0.16 of the
3886 D-Bus specification and implemented by the bus
3887 daemon in dbus 1.5.0 and later.
3893 <entry><literal>destination</literal></entry>
3894 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3895 <entry>Matches messages which are being sent to the given unique name. An
3896 example of a destination match is destination=':1.0'</entry>
3899 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3900 <entry>Any string</entry>
3901 <entry>Arg matches are special and are used for further restricting the
3902 match based on the arguments in the body of a message. Only arguments of type
3903 STRING can be matched in this way. An example of an argument match
3904 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3908 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
3909 <entry>Any string</entry>
3911 <para>Argument path matches provide a specialised form of wildcard matching for
3912 path-like namespaces. They can match arguments whose type is either STRING or
3913 OBJECT_PATH. As with normal argument matches,
3914 if the argument is exactly equal to the string given in the match
3915 rule then the rule is satisfied. Additionally, there is also a
3916 match when either the string given in the match rule or the
3917 appropriate message argument ends with '/' and is a prefix of the
3918 other. An example argument path match is arg0path='/aa/bb/'. This
3919 would match messages with first arguments of '/', '/aa/',
3920 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
3921 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
3923 <para>This is intended for monitoring “directories” in file system-like
3924 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
3925 system. An application interested in all nodes in a particular hierarchy would
3926 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
3927 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
3928 represent a modification to the “bar” property, or a signal with zeroth
3929 argument <literal>"/ca/example/"</literal> to represent atomic modification of
3930 many properties within that directory, and the interested application would be
3931 notified in both cases.</para>
3934 This match key was added in version 0.12 of the
3935 D-Bus specification, implemented for STRING
3936 arguments by the bus daemon in dbus 1.2.0 and later,
3937 and implemented for OBJECT_PATH arguments in dbus 1.5.0
3944 <entry><literal>arg0namespace</literal></entry>
3945 <entry>Like a bus name, except that the string is not
3946 required to contain a '.' (period)</entry>
3948 <para>Match messages whose first argument is of type STRING, and is a bus name
3949 or interface name within the specified namespace. This is primarily intended
3950 for watching name owner changes for a group of related bus names, rather than
3951 for a single name or all name changes.</para>
3953 <para>Because every valid interface name is also a valid
3954 bus name, this can also be used for messages whose
3955 first argument is an interface name.</para>
3957 <para>For example, the match rule
3958 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
3959 matches name owner changes for bus names such as
3960 <literal>com.example.backend.foo</literal>,
3961 <literal>com.example.backend.foo.bar</literal>, and
3962 <literal>com.example.backend</literal> itself.</para>
3964 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
3967 This match key was added in version 0.16 of the
3968 D-Bus specification and implemented by the bus
3969 daemon in dbus 1.5.0 and later.
3980 <sect2 id="message-bus-starting-services">
3981 <title>Message Bus Starting Services</title>
3983 The message bus can start applications on behalf of other applications.
3984 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3985 An application that can be started in this way is called a
3986 <firstterm>service</firstterm>.
3989 With D-Bus, starting a service is normally done by name. That is,
3990 applications ask the message bus to start some program that will own a
3991 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3992 This implies a contract documented along with the name
3993 <literal>org.freedesktop.TextEditor</literal> for which objects
3994 the owner of that name will provide, and what interfaces those
3998 To find an executable corresponding to a particular name, the bus daemon
3999 looks for <firstterm>service description files</firstterm>. Service
4000 description files define a mapping from names to executables. Different
4001 kinds of message bus will look for these files in different places, see
4002 <xref linkend="message-bus-types"/>.
4005 Service description files have the ".service" file
4006 extension. The message bus will only load service description files
4007 ending with .service; all other files will be ignored. The file format
4008 is similar to that of <ulink
4009 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4010 entries</ulink>. All service description files must be in UTF-8
4011 encoding. To ensure that there will be no name collisions, service files
4012 must be namespaced using the same mechanism as messages and service
4017 [FIXME the file format should be much better specified than "similar to
4018 .desktop entries" esp. since desktop entries are already
4019 badly-specified. ;-)]
4020 These sections from the specification apply to service files as well:
4023 <listitem><para>General syntax</para></listitem>
4024 <listitem><para>Comment format</para></listitem>
4028 <title>Example service description file</title>
4030 # Sample service description file
4032 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4033 Exec=/usr/libexec/gconfd-2
4038 When an application asks to start a service by name, the bus daemon tries to
4039 find a service that will own that name. It then tries to spawn the
4040 executable associated with it. If this fails, it will report an
4041 error. [FIXME what happens if two .service files offer the same service;
4042 what kind of error is reported, should we have a way for the client to
4046 The executable launched will have the environment variable
4047 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4048 message bus so it can connect and request the appropriate names.
4051 The executable being launched may want to know whether the message bus
4052 starting it is one of the well-known message buses (see <xref
4053 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4054 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4055 of the well-known buses. The currently-defined values for this variable
4056 are <literal>system</literal> for the systemwide message bus,
4057 and <literal>session</literal> for the per-login-session message
4058 bus. The new executable must still connect to the address given
4059 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4060 resulting connection is to the well-known bus.
4063 [FIXME there should be a timeout somewhere, either specified
4064 in the .service file, by the client, or just a global value
4065 and if the client being activated fails to connect within that
4066 timeout, an error should be sent back.]
4069 <sect3 id="message-bus-starting-services-scope">
4070 <title>Message Bus Service Scope</title>
4072 The "scope" of a service is its "per-", such as per-session,
4073 per-machine, per-home-directory, or per-display. The reference
4074 implementation doesn't yet support starting services in a different
4075 scope from the message bus itself. So e.g. if you start a service
4076 on the session bus its scope is per-session.
4079 We could add an optional scope to a bus name. For example, for
4080 per-(display,session pair), we could have a unique ID for each display
4081 generated automatically at login and set on screen 0 by executing a
4082 special "set display ID" binary. The ID would be stored in a
4083 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4084 random bytes. This ID would then be used to scope names.
4085 Starting/locating a service could be done by ID-name pair rather than
4089 Contrast this with a per-display scope. To achieve that, we would
4090 want a single bus spanning all sessions using a given display.
4091 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4092 property on screen 0 of the display, pointing to this bus.
4097 <sect2 id="message-bus-types">
4098 <title>Well-known Message Bus Instances</title>
4100 Two standard message bus instances are defined here, along with how
4101 to locate them and where their service files live.
4103 <sect3 id="message-bus-types-login">
4104 <title>Login session message bus</title>
4106 Each time a user logs in, a <firstterm>login session message
4107 bus</firstterm> may be started. All applications in the user's login
4108 session may interact with one another using this message bus.
4111 The address of the login session message bus is given
4112 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4113 variable. If that variable is not set, applications may
4114 also try to read the address from the X Window System root
4115 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4116 The root window property must have type <literal>STRING</literal>.
4117 The environment variable should have precedence over the
4118 root window property.
4120 <para>The address of the login session message bus is given in the
4121 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4122 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4123 "autolaunch:", the system should use platform-specific methods of
4124 locating a running D-Bus session server, or starting one if a running
4125 instance cannot be found. Note that this mechanism is not recommended
4126 for attempting to determine if a daemon is running. It is inherently
4127 racy to attempt to make this determination, since the bus daemon may
4128 be started just before or just after the determination is made.
4129 Therefore, it is recommended that applications do not try to make this
4130 determination for their functionality purposes, and instead they
4131 should attempt to start the server.</para>
4133 <sect4 id="message-bus-types-login-x-windows">
4134 <title>X Windowing System</title>
4136 For the X Windowing System, the application must locate the
4137 window owner of the selection represented by the atom formed by
4141 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4145 <para>the current user's username</para>
4149 <para>the literal character '_' (underscore)</para>
4153 <para>the machine's ID</para>
4159 The following properties are defined for the window that owns
4161 <informaltable frame="all">
4170 <para>meaning</para>
4176 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4180 <para>the actual address of the server socket</para>
4186 <para>_DBUS_SESSION_BUS_PID</para>
4190 <para>the PID of the server process</para>
4199 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4200 present in this window.
4204 If the X selection cannot be located or if reading the
4205 properties from the window fails, the implementation MUST conclude
4206 that there is no D-Bus server running and proceed to start a new
4207 server. (See below on concurrency issues)
4211 Failure to connect to the D-Bus server address thus obtained
4212 MUST be treated as a fatal connection error and should be reported
4217 As an alternative, an implementation MAY find the information
4218 in the following file located in the current user's home directory,
4219 in subdirectory .dbus/session-bus/:
4222 <para>the machine's ID</para>
4226 <para>the literal character '-' (dash)</para>
4230 <para>the X display without the screen number, with the
4231 following prefixes removed, if present: ":", "localhost:"
4232 ."localhost.localdomain:". That is, a display of
4233 "localhost:10.0" produces just the number "10"</para>
4239 The contents of this file NAME=value assignment pairs and
4240 lines starting with # are comments (no comments are allowed
4241 otherwise). The following variable names are defined:
4248 <para>Variable</para>
4252 <para>meaning</para>
4258 <para>DBUS_SESSION_BUS_ADDRESS</para>
4262 <para>the actual address of the server socket</para>
4268 <para>DBUS_SESSION_BUS_PID</para>
4272 <para>the PID of the server process</para>
4278 <para>DBUS_SESSION_BUS_WINDOWID</para>
4282 <para>the window ID</para>
4291 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4296 Failure to open this file MUST be interpreted as absence of a
4297 running server. Therefore, the implementation MUST proceed to
4298 attempting to launch a new bus server if the file cannot be
4303 However, success in opening this file MUST NOT lead to the
4304 conclusion that the server is running. Thus, a failure to connect to
4305 the bus address obtained by the alternative method MUST NOT be
4306 considered a fatal error. If the connection cannot be established,
4307 the implementation MUST proceed to check the X selection settings or
4308 to start the server on its own.
4312 If the implementation concludes that the D-Bus server is not
4313 running it MUST attempt to start a new server and it MUST also
4314 ensure that the daemon started as an effect of the "autolaunch"
4315 mechanism provides the lookup mechanisms described above, so
4316 subsequent calls can locate the newly started server. The
4317 implementation MUST also ensure that if two or more concurrent
4318 initiations happen, only one server remains running and all other
4319 initiations are able to obtain the address of this server and
4320 connect to it. In other words, the implementation MUST ensure that
4321 the X selection is not present when it attempts to set it, without
4322 allowing another process to set the selection between the
4323 verification and the setting (e.g., by using XGrabServer /
4330 [FIXME specify location of .service files, probably using
4331 DESKTOP_DIRS etc. from basedir specification, though login session
4332 bus is not really desktop-specific]
4336 <sect3 id="message-bus-types-system">
4337 <title>System message bus</title>
4339 A computer may have a <firstterm>system message bus</firstterm>,
4340 accessible to all applications on the system. This message bus may be
4341 used to broadcast system events, such as adding new hardware devices,
4342 changes in the printer queue, and so forth.
4345 The address of the system message bus is given
4346 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4347 variable. If that variable is not set, applications should try
4348 to connect to the well-known address
4349 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4352 The D-Bus reference implementation actually honors the
4353 <literal>$(localstatedir)</literal> configure option
4354 for this address, on both client and server side.
4359 [FIXME specify location of system bus .service files]
4364 <sect2 id="message-bus-messages">
4365 <title>Message Bus Messages</title>
4367 The special message bus name <literal>org.freedesktop.DBus</literal>
4368 responds to a number of additional messages.
4371 <sect3 id="bus-messages-hello">
4372 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4383 <entry>Argument</entry>
4385 <entry>Description</entry>
4391 <entry>STRING</entry>
4392 <entry>Unique name assigned to the connection</entry>
4399 Before an application is able to send messages to other applications
4400 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4401 to the message bus to obtain a unique name. If an application without
4402 a unique name tries to send a message to another application, or a
4403 message to the message bus itself that isn't the
4404 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4405 disconnected from the bus.
4408 There is no corresponding "disconnect" request; if a client wishes to
4409 disconnect from the bus, it simply closes the socket (or other
4410 communication channel).
4413 <sect3 id="bus-messages-list-names">
4414 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4418 ARRAY of STRING ListNames ()
4425 <entry>Argument</entry>
4427 <entry>Description</entry>
4433 <entry>ARRAY of STRING</entry>
4434 <entry>Array of strings where each string is a bus name</entry>
4441 Returns a list of all currently-owned names on the bus.
4444 <sect3 id="bus-messages-list-activatable-names">
4445 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4449 ARRAY of STRING ListActivatableNames ()
4456 <entry>Argument</entry>
4458 <entry>Description</entry>
4464 <entry>ARRAY of STRING</entry>
4465 <entry>Array of strings where each string is a bus name</entry>
4472 Returns a list of all names that can be activated on the bus.
4475 <sect3 id="bus-messages-name-exists">
4476 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4480 BOOLEAN NameHasOwner (in STRING name)
4487 <entry>Argument</entry>
4489 <entry>Description</entry>
4495 <entry>STRING</entry>
4496 <entry>Name to check</entry>
4506 <entry>Argument</entry>
4508 <entry>Description</entry>
4514 <entry>BOOLEAN</entry>
4515 <entry>Return value, true if the name exists</entry>
4522 Checks if the specified name exists (currently has an owner).
4526 <sect3 id="bus-messages-name-owner-changed">
4527 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4531 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4538 <entry>Argument</entry>
4540 <entry>Description</entry>
4546 <entry>STRING</entry>
4547 <entry>Name with a new owner</entry>
4551 <entry>STRING</entry>
4552 <entry>Old owner or empty string if none</entry>
4556 <entry>STRING</entry>
4557 <entry>New owner or empty string if none</entry>
4564 This signal indicates that the owner of a name has changed.
4565 It's also the signal to use to detect the appearance of
4566 new names on the bus.
4569 <sect3 id="bus-messages-name-lost">
4570 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4574 NameLost (STRING name)
4581 <entry>Argument</entry>
4583 <entry>Description</entry>
4589 <entry>STRING</entry>
4590 <entry>Name which was lost</entry>
4597 This signal is sent to a specific application when it loses
4598 ownership of a name.
4602 <sect3 id="bus-messages-name-acquired">
4603 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4607 NameAcquired (STRING name)
4614 <entry>Argument</entry>
4616 <entry>Description</entry>
4622 <entry>STRING</entry>
4623 <entry>Name which was acquired</entry>
4630 This signal is sent to a specific application when it gains
4631 ownership of a name.
4635 <sect3 id="bus-messages-start-service-by-name">
4636 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4640 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4647 <entry>Argument</entry>
4649 <entry>Description</entry>
4655 <entry>STRING</entry>
4656 <entry>Name of the service to start</entry>
4660 <entry>UINT32</entry>
4661 <entry>Flags (currently not used)</entry>
4671 <entry>Argument</entry>
4673 <entry>Description</entry>
4679 <entry>UINT32</entry>
4680 <entry>Return value</entry>
4685 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4689 The return value can be one of the following values:
4694 <entry>Identifier</entry>
4695 <entry>Value</entry>
4696 <entry>Description</entry>
4701 <entry>DBUS_START_REPLY_SUCCESS</entry>
4703 <entry>The service was successfully started.</entry>
4706 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4708 <entry>A connection already owns the given name.</entry>
4717 <sect3 id="bus-messages-update-activation-environment">
4718 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4722 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4729 <entry>Argument</entry>
4731 <entry>Description</entry>
4737 <entry>ARRAY of DICT<STRING,STRING></entry>
4738 <entry>Environment to add or update</entry>
4743 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4746 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4749 Note, both the environment variable names and values must be valid UTF-8. There's no way to update the activation environment with data that is invalid UTF-8.
4754 <sect3 id="bus-messages-get-name-owner">
4755 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4759 STRING GetNameOwner (in STRING name)
4766 <entry>Argument</entry>
4768 <entry>Description</entry>
4774 <entry>STRING</entry>
4775 <entry>Name to get the owner of</entry>
4785 <entry>Argument</entry>
4787 <entry>Description</entry>
4793 <entry>STRING</entry>
4794 <entry>Return value, a unique connection name</entry>
4799 Returns the unique connection name of the primary owner of the name
4800 given. If the requested name doesn't have an owner, returns a
4801 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4805 <sect3 id="bus-messages-get-connection-unix-user">
4806 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4810 UINT32 GetConnectionUnixUser (in STRING bus_name)
4817 <entry>Argument</entry>
4819 <entry>Description</entry>
4825 <entry>STRING</entry>
4826 <entry>Unique or well-known bus name of the connection to
4827 query, such as <literal>:12.34</literal> or
4828 <literal>com.example.tea</literal></entry>
4838 <entry>Argument</entry>
4840 <entry>Description</entry>
4846 <entry>UINT32</entry>
4847 <entry>Unix user ID</entry>
4852 Returns the Unix user ID of the process connected to the server. If
4853 unable to determine it (for instance, because the process is not on the
4854 same machine as the bus daemon), an error is returned.
4858 <sect3 id="bus-messages-get-connection-unix-process-id">
4859 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
4863 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
4870 <entry>Argument</entry>
4872 <entry>Description</entry>
4878 <entry>STRING</entry>
4879 <entry>Unique or well-known bus name of the connection to
4880 query, such as <literal>:12.34</literal> or
4881 <literal>com.example.tea</literal></entry>
4891 <entry>Argument</entry>
4893 <entry>Description</entry>
4899 <entry>UINT32</entry>
4900 <entry>Unix process id</entry>
4905 Returns the Unix process ID of the process connected to the server. If
4906 unable to determine it (for instance, because the process is not on the
4907 same machine as the bus daemon), an error is returned.
4911 <sect3 id="bus-messages-add-match">
4912 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
4916 AddMatch (in STRING rule)
4923 <entry>Argument</entry>
4925 <entry>Description</entry>
4931 <entry>STRING</entry>
4932 <entry>Match rule to add to the connection</entry>
4937 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
4938 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
4942 <sect3 id="bus-messages-remove-match">
4943 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
4947 RemoveMatch (in STRING rule)
4954 <entry>Argument</entry>
4956 <entry>Description</entry>
4962 <entry>STRING</entry>
4963 <entry>Match rule to remove from the connection</entry>
4968 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
4969 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
4974 <sect3 id="bus-messages-get-id">
4975 <title><literal>org.freedesktop.DBus.GetId</literal></title>
4979 GetId (out STRING id)
4986 <entry>Argument</entry>
4988 <entry>Description</entry>
4994 <entry>STRING</entry>
4995 <entry>Unique ID identifying the bus daemon</entry>
5000 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5001 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5002 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5003 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5004 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5005 by org.freedesktop.DBus.Peer.GetMachineId().
5006 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5014 <appendix id="implementation-notes">
5015 <title>Implementation notes</title>
5016 <sect1 id="implementation-notes-subsection">
5024 <glossary><title>Glossary</title>
5026 This glossary defines some of the terms used in this specification.
5029 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5032 The message bus maintains an association between names and
5033 connections. (Normally, there's one connection per application.) A
5034 bus name is simply an identifier used to locate connections. For
5035 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5036 name might be used to send a message to a screensaver from Yoyodyne
5037 Corporation. An application is said to <firstterm>own</firstterm> a
5038 name if the message bus has associated the application's connection
5039 with the name. Names may also have <firstterm>queued
5040 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5041 The bus assigns a unique name to each connection,
5042 see <xref linkend="term-unique-name"/>. Other names
5043 can be thought of as "well-known names" and are
5044 used to find applications that offer specific functionality.
5049 <glossentry id="term-message"><glossterm>Message</glossterm>
5052 A message is the atomic unit of communication via the D-Bus
5053 protocol. It consists of a <firstterm>header</firstterm> and a
5054 <firstterm>body</firstterm>; the body is made up of
5055 <firstterm>arguments</firstterm>.
5060 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5063 The message bus is a special application that forwards
5064 or routes messages between a group of applications
5065 connected to the message bus. It also manages
5066 <firstterm>names</firstterm> used for routing
5072 <glossentry id="term-name"><glossterm>Name</glossterm>
5075 See <xref linkend="term-bus-name"/>. "Name" may
5076 also be used to refer to some of the other names
5077 in D-Bus, such as interface names.
5082 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5085 Used to prevent collisions when defining new interfaces or bus
5086 names. The convention used is the same one Java uses for defining
5087 classes: a reversed domain name.
5092 <glossentry id="term-object"><glossterm>Object</glossterm>
5095 Each application contains <firstterm>objects</firstterm>, which have
5096 <firstterm>interfaces</firstterm> and
5097 <firstterm>methods</firstterm>. Objects are referred to by a name,
5098 called a <firstterm>path</firstterm>.
5103 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5106 An application talking directly to another application, without going
5107 through a message bus. One-to-one connections may be "peer to peer" or
5108 "client to server." The D-Bus protocol has no concept of client
5109 vs. server after a connection has authenticated; the flow of messages
5110 is symmetrical (full duplex).
5115 <glossentry id="term-path"><glossterm>Path</glossterm>
5118 Object references (object names) in D-Bus are organized into a
5119 filesystem-style hierarchy, so each object is named by a path. As in
5120 LDAP, there's no difference between "files" and "directories"; a path
5121 can refer to an object, while still having child objects below it.
5126 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5129 Each bus name has a primary owner; messages sent to the name go to the
5130 primary owner. However, certain names also maintain a queue of
5131 secondary owners "waiting in the wings." If the primary owner releases
5132 the name, then the first secondary owner in the queue automatically
5133 becomes the new owner of the name.
5138 <glossentry id="term-service"><glossterm>Service</glossterm>
5141 A service is an executable that can be launched by the bus daemon.
5142 Services normally guarantee some particular features, for example they
5143 may guarantee that they will request a specific name such as
5144 "org.freedesktop.Screensaver", have a singleton object
5145 "/org/freedesktop/Application", and that object will implement the
5146 interface "org.freedesktop.ScreensaverControl".
5151 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5154 ".service files" tell the bus about service applications that can be
5155 launched (see <xref linkend="term-service"/>). Most importantly they
5156 provide a mapping from bus names to services that will request those
5157 names when they start up.
5162 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5165 The special name automatically assigned to each connection by the
5166 message bus. This name will never change owner, and will be unique
5167 (never reused during the lifetime of the message bus).
5168 It will begin with a ':' character.