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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.21</releaseinfo>
10 <date>2013-04-25</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>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>0.21</revnumber>
76 <date>2013-04-25</date>
77 <authorinitials>smcv</authorinitials>
78 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
79 Corrigendum #9)</revremark>
82 <revnumber>0.20</revnumber>
83 <date>22 February 2013</date>
84 <authorinitials>smcv, walters</authorinitials>
85 <revremark>reorganise for clarity, remove false claims about
86 basic types, mention /o/fd/DBus</revremark>
89 <revnumber>0.19</revnumber>
90 <date>20 February 2012</date>
91 <authorinitials>smcv/lp</authorinitials>
92 <revremark>formally define unique connection names and well-known
93 bus names; document best practices for interface, bus, member and
94 error names, and object paths; document the search path for session
95 and system services on Unix; document the systemd transport</revremark>
98 <revnumber>0.18</revnumber>
99 <date>29 July 2011</date>
100 <authorinitials>smcv</authorinitials>
101 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
102 match keyword; promote type system to a top-level section</revremark>
105 <revnumber>0.17</revnumber>
106 <date>1 June 2011</date>
107 <authorinitials>smcv/davidz</authorinitials>
108 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
109 by GVariant</revremark>
112 <revnumber>0.16</revnumber>
113 <date>11 April 2011</date>
114 <authorinitials></authorinitials>
115 <revremark>add path_namespace, arg0namespace; argNpath matches object
119 <revnumber>0.15</revnumber>
120 <date>3 November 2010</date>
121 <authorinitials></authorinitials>
122 <revremark></revremark>
125 <revnumber>0.14</revnumber>
126 <date>12 May 2010</date>
127 <authorinitials></authorinitials>
128 <revremark></revremark>
131 <revnumber>0.13</revnumber>
132 <date>23 Dezember 2009</date>
133 <authorinitials></authorinitials>
134 <revremark></revremark>
137 <revnumber>0.12</revnumber>
138 <date>7 November, 2006</date>
139 <authorinitials></authorinitials>
140 <revremark></revremark>
143 <revnumber>0.11</revnumber>
144 <date>6 February 2005</date>
145 <authorinitials></authorinitials>
146 <revremark></revremark>
149 <revnumber>0.10</revnumber>
150 <date>28 January 2005</date>
151 <authorinitials></authorinitials>
152 <revremark></revremark>
155 <revnumber>0.9</revnumber>
156 <date>7 Januar 2005</date>
157 <authorinitials></authorinitials>
158 <revremark></revremark>
161 <revnumber>0.8</revnumber>
162 <date>06 September 2003</date>
163 <authorinitials></authorinitials>
164 <revremark>First released document.</revremark>
169 <sect1 id="introduction">
170 <title>Introduction</title>
172 D-Bus is a system for low-latency, low-overhead, easy to use
173 interprocess communication (IPC). In more detail:
177 D-Bus is <emphasis>low-latency</emphasis> because it is designed
178 to avoid round trips and allow asynchronous operation, much like
184 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
185 binary protocol, and does not have to convert to and from a text
186 format such as XML. Because D-Bus is intended for potentially
187 high-resolution same-machine IPC, not primarily for Internet IPC,
188 this is an interesting optimization.
193 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
194 of <firstterm>messages</firstterm> rather than byte streams, and
195 automatically handles a lot of the hard IPC issues. Also, the D-Bus
196 library is designed to be wrapped in a way that lets developers use
197 their framework's existing object/type system, rather than learning
198 a new one specifically for IPC.
205 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
206 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
207 a system for one application to talk to a single other
208 application. However, the primary intended application of the protocol is the
209 D-Bus <firstterm>message bus</firstterm>, specified in <xref
210 linkend="message-bus"/>. The message bus is a special application that
211 accepts connections from multiple other applications, and forwards
216 Uses of D-Bus include notification of system changes (notification of when
217 a camera is plugged in to a computer, or a new version of some software
218 has been installed), or desktop interoperability, for example a file
219 monitoring service or a configuration service.
223 D-Bus is designed for two specific use cases:
227 A "system bus" for notifications from the system to user sessions,
228 and to allow the system to request input from user sessions.
233 A "session bus" used to implement desktop environments such as
238 D-Bus is not intended to be a generic IPC system for any possible
239 application, and intentionally omits many features found in other
240 IPC systems for this reason.
244 At the same time, the bus daemons offer a number of features not found in
245 other IPC systems, such as single-owner "bus names" (similar to X
246 selections), on-demand startup of services, and security policies.
247 In many ways, these features are the primary motivation for developing
248 D-Bus; other systems would have sufficed if IPC were the only goal.
252 D-Bus may turn out to be useful in unanticipated applications, but future
253 versions of this spec and the reference implementation probably will not
254 incorporate features that interfere with the core use cases.
258 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
259 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
260 document are to be interpreted as described in RFC 2119. However, the
261 document could use a serious audit to be sure it makes sense to do
262 so. Also, they are not capitalized.
265 <sect2 id="stability">
266 <title>Protocol and Specification Stability</title>
268 The D-Bus protocol is frozen (only compatible extensions are allowed) as
269 of November 8, 2006. However, this specification could still use a fair
270 bit of work to make interoperable reimplementation possible without
271 reference to the D-Bus reference implementation. Thus, this
272 specification is not marked 1.0. To mark it 1.0, we'd like to see
273 someone invest significant effort in clarifying the specification
274 language, and growing the specification to cover more aspects of the
275 reference implementation's behavior.
278 Until this work is complete, any attempt to reimplement D-Bus will
279 probably require looking at the reference implementation and/or asking
280 questions on the D-Bus mailing list about intended behavior.
281 Questions on the list are very welcome.
284 Nonetheless, this document should be a useful starting point and is
285 to our knowledge accurate, though incomplete.
291 <sect1 id="type-system">
292 <title>Type System</title>
295 D-Bus has a type system, in which values of various types can be
296 serialized into a sequence of bytes referred to as the
297 <firstterm>wire format</firstterm> in a standard way.
298 Converting a value from some other representation into the wire
299 format is called <firstterm>marshaling</firstterm> and converting
300 it back from the wire format is <firstterm>unmarshaling</firstterm>.
304 The D-Bus protocol does not include type tags in the marshaled data; a
305 block of marshaled values must have a known <firstterm>type
306 signature</firstterm>. The type signature is made up of zero or more
307 <firstterm id="term-single-complete-type">single complete
308 types</firstterm>, each made up of one or more
309 <firstterm>type codes</firstterm>.
313 A type code is an ASCII character representing the
314 type of a value. Because ASCII characters are used, the type signature
315 will always form a valid ASCII string. A simple string compare
316 determines whether two type signatures are equivalent.
320 A single complete type is a sequence of type codes that fully describes
321 one type: either a basic type, or a single fully-described container type.
322 A single complete type is a basic type code, a variant type code,
323 an array with its element type, or a struct with its fields (all of which
324 are defined below). So the following signatures are not single complete
335 And the following signatures contain multiple complete types:
345 Note however that a single complete type may <emphasis>contain</emphasis>
346 multiple other single complete types, by containing a struct or dict
350 <sect2 id="basic-types">
351 <title>Basic types</title>
354 The simplest type codes are the <firstterm id="term-basic-type">basic
355 types</firstterm>, which are the types whose structure is entirely
356 defined by their 1-character type code. Basic types consist of
357 fixed types and string-like types.
361 The <firstterm id="term-fixed-type">fixed types</firstterm>
362 are basic types whose values have a fixed length, namely BYTE,
363 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
368 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
369 the ASCII character 'i'. So the signature for a block of values
370 containing a single <literal>INT32</literal> would be:
374 A block of values containing two <literal>INT32</literal> would have this signature:
381 The characteristics of the fixed types are listed in this table.
387 <entry>Conventional name</entry>
388 <entry>ASCII type-code</entry>
389 <entry>Encoding</entry>
394 <entry><literal>BYTE</literal></entry>
395 <entry><literal>y</literal> (121)</entry>
396 <entry>Unsigned 8-bit integer</entry>
399 <entry><literal>BOOLEAN</literal></entry>
400 <entry><literal>b</literal> (98)</entry>
401 <entry>Boolean value: 0 is false, 1 is true, any other value
402 allowed by the marshalling format is invalid</entry>
405 <entry><literal>INT16</literal></entry>
406 <entry><literal>n</literal> (110)</entry>
407 <entry>Signed (two's complement) 16-bit integer</entry>
410 <entry><literal>UINT16</literal></entry>
411 <entry><literal>q</literal> (113)</entry>
412 <entry>Unsigned 16-bit integer</entry>
415 <entry><literal>INT32</literal></entry>
416 <entry><literal>i</literal> (105)</entry>
417 <entry>Signed (two's complement) 32-bit integer</entry>
420 <entry><literal>UINT32</literal></entry>
421 <entry><literal>u</literal> (117)</entry>
422 <entry>Unsigned 32-bit integer</entry>
425 <entry><literal>INT64</literal></entry>
426 <entry><literal>x</literal> (120)</entry>
427 <entry>Signed (two's complement) 64-bit integer
428 (mnemonic: x and t are the first characters in "sixty" not
429 already used for something more common)</entry>
432 <entry><literal>UINT64</literal></entry>
433 <entry><literal>t</literal> (116)</entry>
434 <entry>Unsigned 64-bit integer</entry>
437 <entry><literal>DOUBLE</literal></entry>
438 <entry><literal>d</literal> (100)</entry>
439 <entry>IEEE 754 double-precision floating point</entry>
442 <entry><literal>UNIX_FD</literal></entry>
443 <entry><literal>h</literal> (104)</entry>
444 <entry>Unsigned 32-bit integer representing an index into an
445 out-of-band array of file descriptors, transferred via some
446 platform-specific mechanism (mnemonic: h for handle)</entry>
454 The <firstterm id="term-string-like-type">string-like types</firstterm>
455 are basic types with a variable length. The value of any string-like
456 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
457 none of which may be U+0000. The UTF-8 text must be validated
458 strictly: in particular, it must not contain overlong sequences
459 or codepoints above U+10FFFF.
463 Since D-Bus Specification version 0.21, in accordance with Unicode
464 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
465 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
466 D-Bus rejected these noncharacters).
470 The marshalling formats for the string-like types all end with a
471 single zero (NUL) byte, but that byte is not considered to be part of
476 The characteristics of the string-like types are listed in this table.
482 <entry>Conventional name</entry>
483 <entry>ASCII type-code</entry>
484 <entry>Validity constraints</entry>
489 <entry><literal>STRING</literal></entry>
490 <entry><literal>s</literal> (115)</entry>
491 <entry>No extra constraints</entry>
494 <entry><literal>OBJECT_PATH</literal></entry>
495 <entry><literal>o</literal> (111)</entry>
497 <link linkend="message-protocol-marshaling-object-path">a
498 syntactically valid object path</link></entry>
501 <entry><literal>SIGNATURE</literal></entry>
502 <entry><literal>g</literal> (103)</entry>
504 <firstterm linkend="term-single-complete-type">single
505 complete types</firstterm></entry>
512 <sect3 id="message-protocol-marshaling-object-path">
513 <title>Valid Object Paths</title>
516 An object path is a name used to refer to an object instance.
517 Conceptually, each participant in a D-Bus message exchange may have
518 any number of object instances (think of C++ or Java objects) and each
519 such instance will have a path. Like a filesystem, the object
520 instances in an application form a hierarchical tree.
524 Object paths are often namespaced by starting with a reversed
525 domain name and containing an interface version number, in the
527 <link linkend="message-protocol-names-interface">interface
529 <link linkend="message-protocol-names-bus">well-known
531 This makes it possible to implement more than one service, or
532 more than one version of a service, in the same process,
533 even if the services share a connection but cannot otherwise
534 co-operate (for instance, if they are implemented by different
539 For instance, if the owner of <literal>example.com</literal> is
540 developing a D-Bus API for a music player, they might use the
541 hierarchy of object paths that start with
542 <literal>/com/example/MusicPlayer1</literal> for its objects.
546 The following rules define a valid object path. Implementations must
547 not send or accept messages with invalid object paths.
551 The path may be of any length.
556 The path must begin with an ASCII '/' (integer 47) character,
557 and must consist of elements separated by slash characters.
562 Each element must only contain the ASCII characters
568 No element may be the empty string.
573 Multiple '/' characters cannot occur in sequence.
578 A trailing '/' character is not allowed unless the
579 path is the root path (a single '/' character).
587 <sect3 id="message-protocol-marshaling-signature">
588 <title>Valid Signatures</title>
590 An implementation must not send or accept invalid signatures.
591 Valid signatures will conform to the following rules:
595 The signature is a list of single complete types.
596 Arrays must have element types, and structs must
597 have both open and close parentheses.
602 Only type codes, open and close parentheses, and open and
603 close curly brackets are allowed in the signature. The
604 <literal>STRUCT</literal> type code
605 is not allowed in signatures, because parentheses
606 are used instead. Similarly, the
607 <literal>DICT_ENTRY</literal> type code is not allowed in
608 signatures, because curly brackets are used instead.
613 The maximum depth of container type nesting is 32 array type
614 codes and 32 open parentheses. This implies that the maximum
615 total depth of recursion is 64, for an "array of array of array
616 of ... struct of struct of struct of ..." where there are 32
622 The maximum length of a signature is 255.
629 When signatures appear in messages, the marshalling format
630 guarantees that they will be followed by a nul byte (which can
631 be interpreted as either C-style string termination or the INVALID
632 type-code), but this is not conceptually part of the signature.
638 <sect2 id="container-types">
639 <title>Container types</title>
642 In addition to basic types, there are four <firstterm>container</firstterm>
643 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
644 and <literal>DICT_ENTRY</literal>.
648 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
649 code does not appear in signatures. Instead, ASCII characters
650 '(' and ')' are used to mark the beginning and end of the struct.
651 So for example, a struct containing two integers would have this
656 Structs can be nested, so for example a struct containing
657 an integer and another struct:
661 The value block storing that struct would contain three integers; the
662 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
667 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
668 but is useful in code that implements the protocol. This type code
669 is specified to allow such code to interoperate in non-protocol contexts.
673 Empty structures are not allowed; there must be at least one
674 type code between the parentheses.
678 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
679 followed by a <firstterm>single complete type</firstterm>. The single
680 complete type following the array is the type of each array element. So
681 the simple example is:
685 which is an array of 32-bit integers. But an array can be of any type,
686 such as this array-of-struct-with-two-int32-fields:
690 Or this array of array of integer:
697 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
698 type <literal>VARIANT</literal> will have the signature of a single complete type as part
699 of the <emphasis>value</emphasis>. This signature will be followed by a
700 marshaled value of that type.
704 Unlike a message signature, the variant signature can
705 contain only a single complete type. So "i", "ai"
706 or "(ii)" is OK, but "ii" is not. Use of variants may not
707 cause a total message depth to be larger than 64, including
708 other container types such as structures.
712 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
713 than parentheses it uses curly braces, and it has more restrictions.
714 The restrictions are: it occurs only as an array element type; it has
715 exactly two single complete types inside the curly braces; the first
716 single complete type (the "key") must be a basic type rather than a
717 container type. Implementations must not accept dict entries outside of
718 arrays, must not accept dict entries with zero, one, or more than two
719 fields, and must not accept dict entries with non-basic-typed keys. A
720 dict entry is always a key-value pair.
724 The first field in the <literal>DICT_ENTRY</literal> is always the key.
725 A message is considered corrupt if the same key occurs twice in the same
726 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
727 implementations are not required to reject dicts with duplicate keys.
731 In most languages, an array of dict entry would be represented as a
732 map, hash table, or dict object.
737 <title>Summary of types</title>
740 The following table summarizes the D-Bus types.
745 <entry>Conventional Name</entry>
747 <entry>Description</entry>
752 <entry><literal>INVALID</literal></entry>
753 <entry>0 (ASCII NUL)</entry>
754 <entry>Not a valid type code, used to terminate signatures</entry>
756 <entry><literal>BYTE</literal></entry>
757 <entry>121 (ASCII 'y')</entry>
758 <entry>8-bit unsigned integer</entry>
760 <entry><literal>BOOLEAN</literal></entry>
761 <entry>98 (ASCII 'b')</entry>
762 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
764 <entry><literal>INT16</literal></entry>
765 <entry>110 (ASCII 'n')</entry>
766 <entry>16-bit signed integer</entry>
768 <entry><literal>UINT16</literal></entry>
769 <entry>113 (ASCII 'q')</entry>
770 <entry>16-bit unsigned integer</entry>
772 <entry><literal>INT32</literal></entry>
773 <entry>105 (ASCII 'i')</entry>
774 <entry>32-bit signed integer</entry>
776 <entry><literal>UINT32</literal></entry>
777 <entry>117 (ASCII 'u')</entry>
778 <entry>32-bit unsigned integer</entry>
780 <entry><literal>INT64</literal></entry>
781 <entry>120 (ASCII 'x')</entry>
782 <entry>64-bit signed integer</entry>
784 <entry><literal>UINT64</literal></entry>
785 <entry>116 (ASCII 't')</entry>
786 <entry>64-bit unsigned integer</entry>
788 <entry><literal>DOUBLE</literal></entry>
789 <entry>100 (ASCII 'd')</entry>
790 <entry>IEEE 754 double</entry>
792 <entry><literal>STRING</literal></entry>
793 <entry>115 (ASCII 's')</entry>
794 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
796 <entry><literal>OBJECT_PATH</literal></entry>
797 <entry>111 (ASCII 'o')</entry>
798 <entry>Name of an object instance</entry>
800 <entry><literal>SIGNATURE</literal></entry>
801 <entry>103 (ASCII 'g')</entry>
802 <entry>A type signature</entry>
804 <entry><literal>ARRAY</literal></entry>
805 <entry>97 (ASCII 'a')</entry>
808 <entry><literal>STRUCT</literal></entry>
809 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
810 <entry>Struct; type code 114 'r' is reserved for use in
811 bindings and implementations to represent the general
812 concept of a struct, and must not appear in signatures
813 used on D-Bus.</entry>
815 <entry><literal>VARIANT</literal></entry>
816 <entry>118 (ASCII 'v') </entry>
817 <entry>Variant type (the type of the value is part of the value itself)</entry>
819 <entry><literal>DICT_ENTRY</literal></entry>
820 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
821 <entry>Entry in a dict or map (array of key-value pairs).
822 Type code 101 'e' is reserved for use in bindings and
823 implementations to represent the general concept of a
824 dict or dict-entry, and must not appear in signatures
825 used on D-Bus.</entry>
827 <entry><literal>UNIX_FD</literal></entry>
828 <entry>104 (ASCII 'h')</entry>
829 <entry>Unix file descriptor</entry>
832 <entry>(reserved)</entry>
833 <entry>109 (ASCII 'm')</entry>
834 <entry>Reserved for <ulink
835 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
836 'maybe' type compatible with the one in GVariant</ulink>,
837 and must not appear in signatures used on D-Bus until
838 specified here</entry>
841 <entry>(reserved)</entry>
842 <entry>42 (ASCII '*')</entry>
843 <entry>Reserved for use in bindings/implementations to
844 represent any <firstterm>single complete type</firstterm>,
845 and must not appear in signatures used on D-Bus.</entry>
848 <entry>(reserved)</entry>
849 <entry>63 (ASCII '?')</entry>
850 <entry>Reserved for use in bindings/implementations to
851 represent any <firstterm>basic type</firstterm>, and must
852 not appear in signatures used on D-Bus.</entry>
855 <entry>(reserved)</entry>
856 <entry>64 (ASCII '@'), 38 (ASCII '&'),
857 94 (ASCII '^')</entry>
858 <entry>Reserved for internal use by bindings/implementations,
859 and must not appear in signatures used on D-Bus.
860 GVariant uses these type-codes to encode calling
871 <sect1 id="message-protocol-marshaling">
872 <title>Marshaling (Wire Format)</title>
875 D-Bus defines a marshalling format for its type system, which is
876 used in D-Bus messages. This is not the only possible marshalling
877 format for the type system: for instance, GVariant (part of GLib)
878 re-uses the D-Bus type system but implements an alternative marshalling
883 <title>Byte order and alignment</title>
886 Given a type signature, a block of bytes can be converted into typed
887 values. This section describes the format of the block of bytes. Byte
888 order and alignment issues are handled uniformly for all D-Bus types.
892 A block of bytes has an associated byte order. The byte order
893 has to be discovered in some way; for D-Bus messages, the
894 byte order is part of the message header as described in
895 <xref linkend="message-protocol-messages"/>. For now, assume
896 that the byte order is known to be either little endian or big
901 Each value in a block of bytes is aligned "naturally," for example
902 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
903 8-byte boundary. To properly align a value, <firstterm>alignment
904 padding</firstterm> may be necessary. The alignment padding must always
905 be the minimum required padding to properly align the following value;
906 and it must always be made up of nul bytes. The alignment padding must
907 not be left uninitialized (it can't contain garbage), and more padding
908 than required must not be used.
912 As an exception to natural alignment, <literal>STRUCT</literal> and
913 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
914 boundary, regardless of the alignments of their contents.
919 <title>Marshalling basic types</title>
922 To marshal and unmarshal fixed types, you simply read one value
923 from the data block corresponding to each type code in the signature.
924 All signed integer values are encoded in two's complement, DOUBLE
925 values are IEEE 754 double-precision floating-point, and BOOLEAN
926 values are encoded in 32 bits (of which only the least significant
931 The string-like types are all marshalled as a
932 fixed-length unsigned integer <varname>n</varname> giving the
933 length of the variable part, followed by <varname>n</varname>
934 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
935 which is not considered to be part of the text. The alignment
936 of the string-like type is the same as the alignment of
937 <varname>n</varname>.
941 For the STRING and OBJECT_PATH types, <varname>n</varname> is
942 encoded in 4 bytes, leading to 4-byte alignment.
943 For the SIGNATURE type, <varname>n</varname> is encoded as a single
944 byte. As a result, alignment padding is never required before a
950 <title>Marshalling containers</title>
953 Arrays are marshalled as a <literal>UINT32</literal>
954 <varname>n</varname> giving the length of the array data in bytes,
955 followed by alignment padding to the alignment boundary of the array
956 element type, followed by the <varname>n</varname> bytes of the
957 array elements marshalled in sequence. <varname>n</varname> does not
958 include the padding after the length, or any padding after the
963 For instance, if the current position in the message is a multiple
964 of 8 bytes and the byte-order is big-endian, an array containing only
965 the 64-bit integer 5 would be marshalled as:
968 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
969 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
970 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
975 Arrays have a maximum length defined to be 2 to the 26th power or
976 67108864. Implementations must not send or accept arrays exceeding this
981 Structs and dict entries are marshalled in the same way as their
982 contents, but their alignment is always to an 8-byte boundary,
983 even if their contents would normally be less strictly aligned.
987 Variants are marshalled as the <literal>SIGNATURE</literal> of
988 the contents (which must be a single complete type), followed by a
989 marshalled value with the type given by that signature. The
990 variant has the same 1-byte alignment as the signature, which means
991 that alignment padding before a variant is never needed.
992 Use of variants may not cause a total message depth to be larger
993 than 64, including other container types such as structures.
998 <title>Summary of D-Bus marshalling</title>
1001 Given all this, the types are marshaled on the wire as follows:
1006 <entry>Conventional Name</entry>
1007 <entry>Encoding</entry>
1008 <entry>Alignment</entry>
1013 <entry><literal>INVALID</literal></entry>
1014 <entry>Not applicable; cannot be marshaled.</entry>
1017 <entry><literal>BYTE</literal></entry>
1018 <entry>A single 8-bit byte.</entry>
1021 <entry><literal>BOOLEAN</literal></entry>
1022 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1025 <entry><literal>INT16</literal></entry>
1026 <entry>16-bit signed integer in the message's byte order.</entry>
1029 <entry><literal>UINT16</literal></entry>
1030 <entry>16-bit unsigned integer in the message's byte order.</entry>
1033 <entry><literal>INT32</literal></entry>
1034 <entry>32-bit signed integer in the message's byte order.</entry>
1037 <entry><literal>UINT32</literal></entry>
1038 <entry>32-bit unsigned integer in the message's byte order.</entry>
1041 <entry><literal>INT64</literal></entry>
1042 <entry>64-bit signed integer in the message's byte order.</entry>
1045 <entry><literal>UINT64</literal></entry>
1046 <entry>64-bit unsigned integer in the message's byte order.</entry>
1049 <entry><literal>DOUBLE</literal></entry>
1050 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1053 <entry><literal>STRING</literal></entry>
1054 <entry>A <literal>UINT32</literal> indicating the string's
1055 length in bytes excluding its terminating nul, followed by
1056 non-nul string data of the given length, followed by a terminating nul
1063 <entry><literal>OBJECT_PATH</literal></entry>
1064 <entry>Exactly the same as <literal>STRING</literal> except the
1065 content must be a valid object path (see above).
1071 <entry><literal>SIGNATURE</literal></entry>
1072 <entry>The same as <literal>STRING</literal> except the length is a single
1073 byte (thus signatures have a maximum length of 255)
1074 and the content must be a valid signature (see above).
1080 <entry><literal>ARRAY</literal></entry>
1082 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1083 alignment padding to the alignment boundary of the array element type,
1084 followed by each array element.
1090 <entry><literal>STRUCT</literal></entry>
1092 A struct must start on an 8-byte boundary regardless of the
1093 type of the struct fields. The struct value consists of each
1094 field marshaled in sequence starting from that 8-byte
1101 <entry><literal>VARIANT</literal></entry>
1103 The marshaled <literal>SIGNATURE</literal> of a single
1104 complete type, followed by a marshaled value with the type
1105 given in the signature.
1108 1 (alignment of the signature)
1111 <entry><literal>DICT_ENTRY</literal></entry>
1113 Identical to STRUCT.
1119 <entry><literal>UNIX_FD</literal></entry>
1120 <entry>32-bit unsigned integer in the message's byte
1121 order. The actual file descriptors need to be
1122 transferred out-of-band via some platform specific
1123 mechanism. On the wire, values of this type store the index to the
1124 file descriptor in the array of file descriptors that
1125 accompany the message.</entry>
1137 <sect1 id="message-protocol">
1138 <title>Message Protocol</title>
1141 A <firstterm>message</firstterm> consists of a
1142 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1143 think of a message as a package, the header is the address, and the body
1144 contains the package contents. The message delivery system uses the header
1145 information to figure out where to send the message and how to interpret
1146 it; the recipient interprets the body of the message.
1150 The body of the message is made up of zero or more
1151 <firstterm>arguments</firstterm>, which are typed values, such as an
1152 integer or a byte array.
1156 Both header and body use the D-Bus <link linkend="type-system">type
1157 system</link> and format for serializing data.
1160 <sect2 id="message-protocol-messages">
1161 <title>Message Format</title>
1164 A message consists of a header and a body. The header is a block of
1165 values with a fixed signature and meaning. The body is a separate block
1166 of values, with a signature specified in the header.
1170 The length of the header must be a multiple of 8, allowing the body to
1171 begin on an 8-byte boundary when storing the entire message in a single
1172 buffer. If the header does not naturally end on an 8-byte boundary
1173 up to 7 bytes of nul-initialized alignment padding must be added.
1177 The message body need not end on an 8-byte boundary.
1181 The maximum length of a message, including header, header alignment padding,
1182 and body is 2 to the 27th power or 134217728. Implementations must not
1183 send or accept messages exceeding this size.
1187 The signature of the header is:
1191 Written out more readably, this is:
1193 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1198 These values have the following meanings:
1203 <entry>Value</entry>
1204 <entry>Description</entry>
1209 <entry>1st <literal>BYTE</literal></entry>
1210 <entry>Endianness flag; ASCII 'l' for little-endian
1211 or ASCII 'B' for big-endian. Both header and body are
1212 in this endianness.</entry>
1215 <entry>2nd <literal>BYTE</literal></entry>
1216 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1217 Currently-defined types are described below.
1221 <entry>3rd <literal>BYTE</literal></entry>
1222 <entry>Bitwise OR of flags. Unknown flags
1223 must be ignored. Currently-defined flags are described below.
1227 <entry>4th <literal>BYTE</literal></entry>
1228 <entry>Major protocol version of the sending application. If
1229 the major protocol version of the receiving application does not
1230 match, the applications will not be able to communicate and the
1231 D-Bus connection must be disconnected. The major protocol
1232 version for this version of the specification is 1.
1236 <entry>1st <literal>UINT32</literal></entry>
1237 <entry>Length in bytes of the message body, starting
1238 from the end of the header. The header ends after
1239 its alignment padding to an 8-boundary.
1243 <entry>2nd <literal>UINT32</literal></entry>
1244 <entry>The serial of this message, used as a cookie
1245 by the sender to identify the reply corresponding
1246 to this request. This must not be zero.
1250 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1251 <entry>An array of zero or more <firstterm>header
1252 fields</firstterm> where the byte is the field code, and the
1253 variant is the field value. The message type determines
1254 which fields are required.
1262 <firstterm>Message types</firstterm> that can appear in the second byte
1268 <entry>Conventional name</entry>
1269 <entry>Decimal value</entry>
1270 <entry>Description</entry>
1275 <entry><literal>INVALID</literal></entry>
1277 <entry>This is an invalid type.</entry>
1280 <entry><literal>METHOD_CALL</literal></entry>
1282 <entry>Method call.</entry>
1285 <entry><literal>METHOD_RETURN</literal></entry>
1287 <entry>Method reply with returned data.</entry>
1290 <entry><literal>ERROR</literal></entry>
1292 <entry>Error reply. If the first argument exists and is a
1293 string, it is an error message.</entry>
1296 <entry><literal>SIGNAL</literal></entry>
1298 <entry>Signal emission.</entry>
1305 Flags that can appear in the third byte of the header:
1310 <entry>Conventional name</entry>
1311 <entry>Hex value</entry>
1312 <entry>Description</entry>
1317 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1319 <entry>This message does not expect method return replies or
1320 error replies; the reply can be omitted as an
1321 optimization. However, it is compliant with this specification
1322 to return the reply despite this flag and the only harm
1323 from doing so is extra network traffic.
1327 <entry><literal>NO_AUTO_START</literal></entry>
1329 <entry>The bus must not launch an owner
1330 for the destination name in response to this message.
1338 <sect3 id="message-protocol-header-fields">
1339 <title>Header Fields</title>
1342 The array at the end of the header contains <firstterm>header
1343 fields</firstterm>, where each field is a 1-byte field code followed
1344 by a field value. A header must contain the required header fields for
1345 its message type, and zero or more of any optional header
1346 fields. Future versions of this protocol specification may add new
1347 fields. Implementations must ignore fields they do not
1348 understand. Implementations must not invent their own header fields;
1349 only changes to this specification may introduce new header fields.
1353 Again, if an implementation sees a header field code that it does not
1354 expect, it must ignore that field, as it will be part of a new
1355 (but compatible) version of this specification. This also applies
1356 to known header fields appearing in unexpected messages, for
1357 example: if a signal has a reply serial it must be ignored
1358 even though it has no meaning as of this version of the spec.
1362 However, implementations must not send or accept known header fields
1363 with the wrong type stored in the field value. So for example a
1364 message with an <literal>INTERFACE</literal> field of type
1365 <literal>UINT32</literal> would be considered corrupt.
1369 Here are the currently-defined header fields:
1374 <entry>Conventional Name</entry>
1375 <entry>Decimal Code</entry>
1377 <entry>Required In</entry>
1378 <entry>Description</entry>
1383 <entry><literal>INVALID</literal></entry>
1386 <entry>not allowed</entry>
1387 <entry>Not a valid field name (error if it appears in a message)</entry>
1390 <entry><literal>PATH</literal></entry>
1392 <entry><literal>OBJECT_PATH</literal></entry>
1393 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1394 <entry>The object to send a call to,
1395 or the object a signal is emitted from.
1397 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1398 implementations should not send messages with this path,
1399 and the reference implementation of the bus daemon will
1400 disconnect any application that attempts to do so.
1404 <entry><literal>INTERFACE</literal></entry>
1406 <entry><literal>STRING</literal></entry>
1407 <entry><literal>SIGNAL</literal></entry>
1409 The interface to invoke a method call on, or
1410 that a signal is emitted from. Optional for
1411 method calls, required for signals.
1412 The special interface
1413 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1414 implementations should not send messages with this
1415 interface, and the reference implementation of the bus
1416 daemon will disconnect any application that attempts to
1421 <entry><literal>MEMBER</literal></entry>
1423 <entry><literal>STRING</literal></entry>
1424 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1425 <entry>The member, either the method name or signal name.</entry>
1428 <entry><literal>ERROR_NAME</literal></entry>
1430 <entry><literal>STRING</literal></entry>
1431 <entry><literal>ERROR</literal></entry>
1432 <entry>The name of the error that occurred, for errors</entry>
1435 <entry><literal>REPLY_SERIAL</literal></entry>
1437 <entry><literal>UINT32</literal></entry>
1438 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1439 <entry>The serial number of the message this message is a reply
1440 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1443 <entry><literal>DESTINATION</literal></entry>
1445 <entry><literal>STRING</literal></entry>
1446 <entry>optional</entry>
1447 <entry>The name of the connection this message is intended for.
1448 Only used in combination with the message bus, see
1449 <xref linkend="message-bus"/>.</entry>
1452 <entry><literal>SENDER</literal></entry>
1454 <entry><literal>STRING</literal></entry>
1455 <entry>optional</entry>
1456 <entry>Unique name of the sending connection.
1457 The message bus fills in this field so it is reliable; the field is
1458 only meaningful in combination with the message bus.</entry>
1461 <entry><literal>SIGNATURE</literal></entry>
1463 <entry><literal>SIGNATURE</literal></entry>
1464 <entry>optional</entry>
1465 <entry>The signature of the message body.
1466 If omitted, it is assumed to be the
1467 empty signature "" (i.e. the body must be 0-length).</entry>
1470 <entry><literal>UNIX_FDS</literal></entry>
1472 <entry><literal>UINT32</literal></entry>
1473 <entry>optional</entry>
1474 <entry>The number of Unix file descriptors that
1475 accompany the message. If omitted, it is assumed
1476 that no Unix file descriptors accompany the
1477 message. The actual file descriptors need to be
1478 transferred via platform specific mechanism
1479 out-of-band. They must be sent at the same time as
1480 part of the message itself. They may not be sent
1481 before the first byte of the message itself is
1482 transferred or after the last byte of the message
1492 <sect2 id="message-protocol-names">
1493 <title>Valid Names</title>
1495 The various names in D-Bus messages have some restrictions.
1498 There is a <firstterm>maximum name length</firstterm>
1499 of 255 which applies to bus names, interfaces, and members.
1501 <sect3 id="message-protocol-names-interface">
1502 <title>Interface names</title>
1504 Interfaces have names with type <literal>STRING</literal>, meaning that
1505 they must be valid UTF-8. However, there are also some
1506 additional restrictions that apply to interface names
1509 <listitem><para>Interface names are composed of 1 or more elements separated by
1510 a period ('.') character. All elements must contain at least
1514 <listitem><para>Each element must only contain the ASCII characters
1515 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1519 <listitem><para>Interface names must contain at least one '.' (period)
1520 character (and thus at least two elements).
1523 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1524 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1529 Interface names should start with the reversed DNS domain name of
1530 the author of the interface (in lower-case), like interface names
1531 in Java. It is conventional for the rest of the interface name
1532 to consist of words run together, with initial capital letters
1533 on all words ("CamelCase"). Several levels of hierarchy can be used.
1534 It is also a good idea to include the major version of the interface
1535 in the name, and increment it if incompatible changes are made;
1536 this way, a single object can implement several versions of an
1537 interface in parallel, if necessary.
1541 For instance, if the owner of <literal>example.com</literal> is
1542 developing a D-Bus API for a music player, they might define
1543 interfaces called <literal>com.example.MusicPlayer1</literal>,
1544 <literal>com.example.MusicPlayer1.Track</literal> and
1545 <literal>com.example.MusicPlayer1.Seekable</literal>.
1549 D-Bus does not distinguish between the concepts that would be
1550 called classes and interfaces in Java: either can be identified on
1551 D-Bus by an interface name.
1554 <sect3 id="message-protocol-names-bus">
1555 <title>Bus names</title>
1557 Connections have one or more bus names associated with them.
1558 A connection has exactly one bus name that is a <firstterm>unique
1559 connection name</firstterm>. The unique connection name remains
1560 with the connection for its entire lifetime.
1561 A bus name is of type <literal>STRING</literal>,
1562 meaning that it must be valid UTF-8. However, there are also
1563 some additional restrictions that apply to bus names
1566 <listitem><para>Bus names that start with a colon (':')
1567 character are unique connection names. Other bus names
1568 are called <firstterm>well-known bus names</firstterm>.
1571 <listitem><para>Bus names are composed of 1 or more elements separated by
1572 a period ('.') character. All elements must contain at least
1576 <listitem><para>Each element must only contain the ASCII characters
1577 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1578 connection name may begin with a digit, elements in
1579 other bus names must not begin with a digit.
1583 <listitem><para>Bus names must contain at least one '.' (period)
1584 character (and thus at least two elements).
1587 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1588 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1592 Note that the hyphen ('-') character is allowed in bus names but
1593 not in interface names.
1597 Like <link linkend="message-protocol-names-interface">interface
1598 names</link>, well-known bus names should start with the
1599 reversed DNS domain name of the author of the interface (in
1600 lower-case), and it is conventional for the rest of the well-known
1601 bus name to consist of words run together, with initial
1602 capital letters. As with interface names, including a version
1603 number in well-known bus names is a good idea; it's possible to
1604 have the well-known bus name for more than one version
1605 simultaneously if backwards compatibility is required.
1609 If a well-known bus name implies the presence of a "main" interface,
1610 that "main" interface is often given the same name as
1611 the well-known bus name, and situated at the corresponding object
1612 path. For instance, if the owner of <literal>example.com</literal>
1613 is developing a D-Bus API for a music player, they might define
1614 that any application that takes the well-known name
1615 <literal>com.example.MusicPlayer1</literal> should have an object
1616 at the object path <literal>/com/example/MusicPlayer1</literal>
1617 which implements the interface
1618 <literal>com.example.MusicPlayer1</literal>.
1621 <sect3 id="message-protocol-names-member">
1622 <title>Member names</title>
1624 Member (i.e. method or signal) names:
1626 <listitem><para>Must only contain the ASCII characters
1627 "[A-Z][a-z][0-9]_" and may not begin with a
1628 digit.</para></listitem>
1629 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1630 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1631 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1636 It is conventional for member names on D-Bus to consist of
1637 capitalized words with no punctuation ("camel-case").
1638 Method names should usually be verbs, such as
1639 <literal>GetItems</literal>, and signal names should usually be
1640 a description of an event, such as <literal>ItemsChanged</literal>.
1643 <sect3 id="message-protocol-names-error">
1644 <title>Error names</title>
1646 Error names have the same restrictions as interface names.
1650 Error names have the same naming conventions as interface
1651 names, and often contain <literal>.Error.</literal>; for instance,
1652 the owner of <literal>example.com</literal> might define the
1653 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1654 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1655 The errors defined by D-Bus itself, such as
1656 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1662 <sect2 id="message-protocol-types">
1663 <title>Message Types</title>
1665 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1666 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1667 This section describes these conventions.
1669 <sect3 id="message-protocol-types-method">
1670 <title>Method Calls</title>
1672 Some messages invoke an operation on a remote object. These are
1673 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1674 messages map naturally to methods on objects in a typical program.
1677 A method call message is required to have a <literal>MEMBER</literal> header field
1678 indicating the name of the method. Optionally, the message has an
1679 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1680 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1681 a method with the same name, it is undefined which of the two methods
1682 will be invoked. Implementations may also choose to return an error in
1683 this ambiguous case. However, if a method name is unique
1684 implementations must not require an interface field.
1687 Method call messages also include a <literal>PATH</literal> field
1688 indicating the object to invoke the method on. If the call is passing
1689 through a message bus, the message will also have a
1690 <literal>DESTINATION</literal> field giving the name of the connection
1691 to receive the message.
1694 When an application handles a method call message, it is required to
1695 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1696 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1697 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1700 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1701 are the return value(s) or "out parameters" of the method call.
1702 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1703 and the call fails; no return value will be provided. It makes
1704 no sense to send multiple replies to the same method call.
1707 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1708 reply is required, so the caller will know the method
1709 was successfully processed.
1712 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1716 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1717 then as an optimization the application receiving the method
1718 call may choose to omit the reply message (regardless of
1719 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1720 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1721 flag and reply anyway.
1724 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1725 destination name does not exist then a program to own the destination
1726 name will be started before the message is delivered. The message
1727 will be held until the new program is successfully started or has
1728 failed to start; in case of failure, an error will be returned. This
1729 flag is only relevant in the context of a message bus, it is ignored
1730 during one-to-one communication with no intermediate bus.
1732 <sect4 id="message-protocol-types-method-apis">
1733 <title>Mapping method calls to native APIs</title>
1735 APIs for D-Bus may map method calls to a method call in a specific
1736 programming language, such as C++, or may map a method call written
1737 in an IDL to a D-Bus message.
1740 In APIs of this nature, arguments to a method are often termed "in"
1741 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1742 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1743 "inout" arguments, which are both sent and received, i.e. the caller
1744 passes in a value which is modified. Mapped to D-Bus, an "inout"
1745 argument is equivalent to an "in" argument, followed by an "out"
1746 argument. You can't pass things "by reference" over the wire, so
1747 "inout" is purely an illusion of the in-process API.
1750 Given a method with zero or one return values, followed by zero or more
1751 arguments, where each argument may be "in", "out", or "inout", the
1752 caller constructs a message by appending each "in" or "inout" argument,
1753 in order. "out" arguments are not represented in the caller's message.
1756 The recipient constructs a reply by appending first the return value
1757 if any, then each "out" or "inout" argument, in order.
1758 "in" arguments are not represented in the reply message.
1761 Error replies are normally mapped to exceptions in languages that have
1765 In converting from native APIs to D-Bus, it is perhaps nice to
1766 map D-Bus naming conventions ("FooBar") to native conventions
1767 such as "fooBar" or "foo_bar" automatically. This is OK
1768 as long as you can say that the native API is one that
1769 was specifically written for D-Bus. It makes the most sense
1770 when writing object implementations that will be exported
1771 over the bus. Object proxies used to invoke remote D-Bus
1772 objects probably need the ability to call any D-Bus method,
1773 and thus a magic name mapping like this could be a problem.
1776 This specification doesn't require anything of native API bindings;
1777 the preceding is only a suggested convention for consistency
1783 <sect3 id="message-protocol-types-signal">
1784 <title>Signal Emission</title>
1786 Unlike method calls, signal emissions have no replies.
1787 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1788 It must have three header fields: <literal>PATH</literal> giving the object
1789 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1790 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1791 for signals, though it is optional for method calls.
1795 <sect3 id="message-protocol-types-errors">
1796 <title>Errors</title>
1798 Messages of type <literal>ERROR</literal> are most commonly replies
1799 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1800 to any kind of message. The message bus for example
1801 will return an <literal>ERROR</literal> in reply to a signal emission if
1802 the bus does not have enough memory to send the signal.
1805 An <literal>ERROR</literal> may have any arguments, but if the first
1806 argument is a <literal>STRING</literal>, it must be an error message.
1807 The error message may be logged or shown to the user
1812 <sect3 id="message-protocol-types-notation">
1813 <title>Notation in this document</title>
1815 This document uses a simple pseudo-IDL to describe particular method
1816 calls and signals. Here is an example of a method call:
1818 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1819 out UINT32 resultcode)
1821 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1822 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1823 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1824 characters so it's known that the last part of the name in
1825 the "IDL" is the member name.
1828 In C++ that might end up looking like this:
1830 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1831 unsigned int flags);
1833 or equally valid, the return value could be done as an argument:
1835 void org::freedesktop::DBus::StartServiceByName (const char *name,
1837 unsigned int *resultcode);
1839 It's really up to the API designer how they want to make
1840 this look. You could design an API where the namespace wasn't used
1841 in C++, using STL or Qt, using varargs, or whatever you wanted.
1844 Signals are written as follows:
1846 org.freedesktop.DBus.NameLost (STRING name)
1848 Signals don't specify "in" vs. "out" because only
1849 a single direction is possible.
1852 It isn't especially encouraged to use this lame pseudo-IDL in actual
1853 API implementations; you might use the native notation for the
1854 language you're using, or you might use COM or CORBA IDL, for example.
1859 <sect2 id="message-protocol-handling-invalid">
1860 <title>Invalid Protocol and Spec Extensions</title>
1863 For security reasons, the D-Bus protocol should be strictly parsed and
1864 validated, with the exception of defined extension points. Any invalid
1865 protocol or spec violations should result in immediately dropping the
1866 connection without notice to the other end. Exceptions should be
1867 carefully considered, e.g. an exception may be warranted for a
1868 well-understood idiosyncrasy of a widely-deployed implementation. In
1869 cases where the other end of a connection is 100% trusted and known to
1870 be friendly, skipping validation for performance reasons could also make
1871 sense in certain cases.
1875 Generally speaking violations of the "must" requirements in this spec
1876 should be considered possible attempts to exploit security, and violations
1877 of the "should" suggestions should be considered legitimate (though perhaps
1878 they should generate an error in some cases).
1882 The following extension points are built in to D-Bus on purpose and must
1883 not be treated as invalid protocol. The extension points are intended
1884 for use by future versions of this spec, they are not intended for third
1885 parties. At the moment, the only way a third party could extend D-Bus
1886 without breaking interoperability would be to introduce a way to negotiate new
1887 feature support as part of the auth protocol, using EXTENSION_-prefixed
1888 commands. There is not yet a standard way to negotiate features.
1892 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1893 commands result in an ERROR rather than a disconnect. This enables
1894 future extensions to the protocol. Commands starting with EXTENSION_ are
1895 reserved for third parties.
1900 The authentication protocol supports pluggable auth mechanisms.
1905 The address format (see <xref linkend="addresses"/>) supports new
1911 Messages with an unknown type (something other than
1912 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1913 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1914 Unknown-type messages must still be well-formed in the same way
1915 as the known messages, however. They still have the normal
1921 Header fields with an unknown or unexpected field code must be ignored,
1922 though again they must still be well-formed.
1927 New standard interfaces (with new methods and signals) can of course be added.
1937 <sect1 id="auth-protocol">
1938 <title>Authentication Protocol</title>
1940 Before the flow of messages begins, two applications must
1941 authenticate. A simple plain-text protocol is used for
1942 authentication; this protocol is a SASL profile, and maps fairly
1943 directly from the SASL specification. The message encoding is
1944 NOT used here, only plain text messages.
1947 In examples, "C:" and "S:" indicate lines sent by the client and
1948 server respectively.
1950 <sect2 id="auth-protocol-overview">
1951 <title>Protocol Overview</title>
1953 The protocol is a line-based protocol, where each line ends with
1954 \r\n. Each line begins with an all-caps ASCII command name containing
1955 only the character range [A-Z_], a space, then any arguments for the
1956 command, then the \r\n ending the line. The protocol is
1957 case-sensitive. All bytes must be in the ASCII character set.
1959 Commands from the client to the server are as follows:
1962 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1963 <listitem><para>CANCEL</para></listitem>
1964 <listitem><para>BEGIN</para></listitem>
1965 <listitem><para>DATA <data in hex encoding></para></listitem>
1966 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1967 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1970 From server to client are as follows:
1973 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1974 <listitem><para>OK <GUID in hex></para></listitem>
1975 <listitem><para>DATA <data in hex encoding></para></listitem>
1976 <listitem><para>ERROR</para></listitem>
1977 <listitem><para>AGREE_UNIX_FD</para></listitem>
1981 Unofficial extensions to the command set must begin with the letters
1982 "EXTENSION_", to avoid conflicts with future official commands.
1983 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1986 <sect2 id="auth-nul-byte">
1987 <title>Special credentials-passing nul byte</title>
1989 Immediately after connecting to the server, the client must send a
1990 single nul byte. This byte may be accompanied by credentials
1991 information on some operating systems that use sendmsg() with
1992 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1993 sockets. However, the nul byte must be sent even on other kinds of
1994 socket, and even on operating systems that do not require a byte to be
1995 sent in order to transmit credentials. The text protocol described in
1996 this document begins after the single nul byte. If the first byte
1997 received from the client is not a nul byte, the server may disconnect
2001 A nul byte in any context other than the initial byte is an error;
2002 the protocol is ASCII-only.
2005 The credentials sent along with the nul byte may be used with the
2006 SASL mechanism EXTERNAL.
2009 <sect2 id="auth-command-auth">
2010 <title>AUTH command</title>
2012 If an AUTH command has no arguments, it is a request to list
2013 available mechanisms. The server must respond with a REJECTED
2014 command listing the mechanisms it understands, or with an error.
2017 If an AUTH command specifies a mechanism, and the server supports
2018 said mechanism, the server should begin exchanging SASL
2019 challenge-response data with the client using DATA commands.
2022 If the server does not support the mechanism given in the AUTH
2023 command, it must send either a REJECTED command listing the mechanisms
2024 it does support, or an error.
2027 If the [initial-response] argument is provided, it is intended for use
2028 with mechanisms that have no initial challenge (or an empty initial
2029 challenge), as if it were the argument to an initial DATA command. If
2030 the selected mechanism has an initial challenge and [initial-response]
2031 was provided, the server should reject authentication by sending
2035 If authentication succeeds after exchanging DATA commands,
2036 an OK command must be sent to the client.
2039 The first octet received by the server after the \r\n of the BEGIN
2040 command from the client must be the first octet of the
2041 authenticated/encrypted stream of D-Bus messages.
2044 If BEGIN is received by the server, the first octet received
2045 by the client after the \r\n of the OK command must be the
2046 first octet of the authenticated/encrypted stream of D-Bus
2050 <sect2 id="auth-command-cancel">
2051 <title>CANCEL Command</title>
2053 At any time up to sending the BEGIN command, the client may send a
2054 CANCEL command. On receiving the CANCEL command, the server must
2055 send a REJECTED command and abort the current authentication
2059 <sect2 id="auth-command-data">
2060 <title>DATA Command</title>
2062 The DATA command may come from either client or server, and simply
2063 contains a hex-encoded block of data to be interpreted
2064 according to the SASL mechanism in use.
2067 Some SASL mechanisms support sending an "empty string";
2068 FIXME we need some way to do this.
2071 <sect2 id="auth-command-begin">
2072 <title>BEGIN Command</title>
2074 The BEGIN command acknowledges that the client has received an
2075 OK command from the server, and that the stream of messages
2079 The first octet received by the server after the \r\n of the BEGIN
2080 command from the client must be the first octet of the
2081 authenticated/encrypted stream of D-Bus messages.
2084 <sect2 id="auth-command-rejected">
2085 <title>REJECTED Command</title>
2087 The REJECTED command indicates that the current authentication
2088 exchange has failed, and further exchange of DATA is inappropriate.
2089 The client would normally try another mechanism, or try providing
2090 different responses to challenges.
2092 Optionally, the REJECTED command has a space-separated list of
2093 available auth mechanisms as arguments. If a server ever provides
2094 a list of supported mechanisms, it must provide the same list
2095 each time it sends a REJECTED message. Clients are free to
2096 ignore all lists received after the first.
2099 <sect2 id="auth-command-ok">
2100 <title>OK Command</title>
2102 The OK command indicates that the client has been
2103 authenticated. The client may now proceed with negotiating
2104 Unix file descriptor passing. To do that it shall send
2105 NEGOTIATE_UNIX_FD to the server.
2108 Otherwise, the client must respond to the OK command by
2109 sending a BEGIN command, followed by its stream of messages,
2110 or by disconnecting. The server must not accept additional
2111 commands using this protocol after the BEGIN command has been
2112 received. Further communication will be a stream of D-Bus
2113 messages (optionally encrypted, as negotiated) rather than
2117 If a client sends BEGIN the first octet received by the client
2118 after the \r\n of the OK command must be the first octet of
2119 the authenticated/encrypted stream of D-Bus messages.
2122 The OK command has one argument, which is the GUID of the server.
2123 See <xref linkend="addresses"/> for more on server GUIDs.
2126 <sect2 id="auth-command-error">
2127 <title>ERROR Command</title>
2129 The ERROR command indicates that either server or client did not
2130 know a command, does not accept the given command in the current
2131 context, or did not understand the arguments to the command. This
2132 allows the protocol to be extended; a client or server can send a
2133 command present or permitted only in new protocol versions, and if
2134 an ERROR is received instead of an appropriate response, fall back
2135 to using some other technique.
2138 If an ERROR is sent, the server or client that sent the
2139 error must continue as if the command causing the ERROR had never been
2140 received. However, the the server or client receiving the error
2141 should try something other than whatever caused the error;
2142 if only canceling/rejecting the authentication.
2145 If the D-Bus protocol changes incompatibly at some future time,
2146 applications implementing the new protocol would probably be able to
2147 check for support of the new protocol by sending a new command and
2148 receiving an ERROR from applications that don't understand it. Thus the
2149 ERROR feature of the auth protocol is an escape hatch that lets us
2150 negotiate extensions or changes to the D-Bus protocol in the future.
2153 <sect2 id="auth-command-negotiate-unix-fd">
2154 <title>NEGOTIATE_UNIX_FD Command</title>
2156 The NEGOTIATE_UNIX_FD command indicates that the client
2157 supports Unix file descriptor passing. This command may only
2158 be sent after the connection is authenticated, i.e. after OK
2159 was received by the client. This command may only be sent on
2160 transports that support Unix file descriptor passing.
2163 On receiving NEGOTIATE_UNIX_FD the server must respond with
2164 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2165 the transport chosen supports Unix file descriptor passing and
2166 the server supports this feature. It shall respond the latter
2167 if the transport does not support Unix file descriptor
2168 passing, the server does not support this feature, or the
2169 server decides not to enable file descriptor passing due to
2170 security or other reasons.
2173 <sect2 id="auth-command-agree-unix-fd">
2174 <title>AGREE_UNIX_FD Command</title>
2176 The AGREE_UNIX_FD command indicates that the server supports
2177 Unix file descriptor passing. This command may only be sent
2178 after the connection is authenticated, and the client sent
2179 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2180 command may only be sent on transports that support Unix file
2184 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2185 followed by its stream of messages, or by disconnecting. The
2186 server must not accept additional commands using this protocol
2187 after the BEGIN command has been received. Further
2188 communication will be a stream of D-Bus messages (optionally
2189 encrypted, as negotiated) rather than this protocol.
2192 <sect2 id="auth-command-future">
2193 <title>Future Extensions</title>
2195 Future extensions to the authentication and negotiation
2196 protocol are possible. For that new commands may be
2197 introduced. If a client or server receives an unknown command
2198 it shall respond with ERROR and not consider this fatal. New
2199 commands may be introduced both before, and after
2200 authentication, i.e. both before and after the OK command.
2203 <sect2 id="auth-examples">
2204 <title>Authentication examples</title>
2208 <title>Example of successful magic cookie authentication</title>
2210 (MAGIC_COOKIE is a made up mechanism)
2212 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2218 <title>Example of finding out mechanisms then picking one</title>
2221 S: REJECTED KERBEROS_V4 SKEY
2222 C: AUTH SKEY 7ab83f32ee
2223 S: DATA 8799cabb2ea93e
2224 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2230 <title>Example of client sends unknown command then falls back to regular auth</title>
2234 C: AUTH MAGIC_COOKIE 3736343435313230333039
2240 <title>Example of server doesn't support initial auth mechanism</title>
2242 C: AUTH MAGIC_COOKIE 3736343435313230333039
2243 S: REJECTED KERBEROS_V4 SKEY
2244 C: AUTH SKEY 7ab83f32ee
2245 S: DATA 8799cabb2ea93e
2246 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2252 <title>Example of wrong password or the like followed by successful retry</title>
2254 C: AUTH MAGIC_COOKIE 3736343435313230333039
2255 S: REJECTED KERBEROS_V4 SKEY
2256 C: AUTH SKEY 7ab83f32ee
2257 S: DATA 8799cabb2ea93e
2258 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2260 C: AUTH SKEY 7ab83f32ee
2261 S: DATA 8799cabb2ea93e
2262 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2268 <title>Example of skey cancelled and restarted</title>
2270 C: AUTH MAGIC_COOKIE 3736343435313230333039
2271 S: REJECTED KERBEROS_V4 SKEY
2272 C: AUTH SKEY 7ab83f32ee
2273 S: DATA 8799cabb2ea93e
2276 C: AUTH SKEY 7ab83f32ee
2277 S: DATA 8799cabb2ea93e
2278 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2284 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2286 (MAGIC_COOKIE is a made up mechanism)
2288 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2290 C: NEGOTIATE_UNIX_FD
2296 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2298 (MAGIC_COOKIE is a made up mechanism)
2300 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2302 C: NEGOTIATE_UNIX_FD
2309 <sect2 id="auth-states">
2310 <title>Authentication state diagrams</title>
2313 This section documents the auth protocol in terms of
2314 a state machine for the client and the server. This is
2315 probably the most robust way to implement the protocol.
2318 <sect3 id="auth-states-client">
2319 <title>Client states</title>
2322 To more precisely describe the interaction between the
2323 protocol state machine and the authentication mechanisms the
2324 following notation is used: MECH(CHALL) means that the
2325 server challenge CHALL was fed to the mechanism MECH, which
2331 CONTINUE(RESP) means continue the auth conversation
2332 and send RESP as the response to the server;
2338 OK(RESP) means that after sending RESP to the server
2339 the client side of the auth conversation is finished
2340 and the server should return "OK";
2346 ERROR means that CHALL was invalid and could not be
2352 Both RESP and CHALL may be empty.
2356 The Client starts by getting an initial response from the
2357 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2358 the mechanism did not provide an initial response. If the
2359 mechanism returns CONTINUE, the client starts in state
2360 <emphasis>WaitingForData</emphasis>, if the mechanism
2361 returns OK the client starts in state
2362 <emphasis>WaitingForOK</emphasis>.
2366 The client should keep track of available mechanisms and
2367 which it mechanisms it has already attempted. This list is
2368 used to decide which AUTH command to send. When the list is
2369 exhausted, the client should give up and close the
2374 <title><emphasis>WaitingForData</emphasis></title>
2382 MECH(CHALL) returns CONTINUE(RESP) → send
2384 <emphasis>WaitingForData</emphasis>
2388 MECH(CHALL) returns OK(RESP) → send DATA
2389 RESP, goto <emphasis>WaitingForOK</emphasis>
2393 MECH(CHALL) returns ERROR → send ERROR
2394 [msg], goto <emphasis>WaitingForData</emphasis>
2402 Receive REJECTED [mechs] →
2403 send AUTH [next mech], goto
2404 WaitingForData or <emphasis>WaitingForOK</emphasis>
2409 Receive ERROR → send
2411 <emphasis>WaitingForReject</emphasis>
2416 Receive OK → send
2417 BEGIN, terminate auth
2418 conversation, authenticated
2423 Receive anything else → send
2425 <emphasis>WaitingForData</emphasis>
2433 <title><emphasis>WaitingForOK</emphasis></title>
2438 Receive OK → send BEGIN, terminate auth
2439 conversation, <emphasis>authenticated</emphasis>
2444 Receive REJECTED [mechs] → send AUTH [next mech],
2445 goto <emphasis>WaitingForData</emphasis> or
2446 <emphasis>WaitingForOK</emphasis>
2452 Receive DATA → send CANCEL, goto
2453 <emphasis>WaitingForReject</emphasis>
2459 Receive ERROR → send CANCEL, goto
2460 <emphasis>WaitingForReject</emphasis>
2466 Receive anything else → send ERROR, goto
2467 <emphasis>WaitingForOK</emphasis>
2475 <title><emphasis>WaitingForReject</emphasis></title>
2480 Receive REJECTED [mechs] → send AUTH [next mech],
2481 goto <emphasis>WaitingForData</emphasis> or
2482 <emphasis>WaitingForOK</emphasis>
2488 Receive anything else → terminate auth
2489 conversation, disconnect
2498 <sect3 id="auth-states-server">
2499 <title>Server states</title>
2502 For the server MECH(RESP) means that the client response
2503 RESP was fed to the the mechanism MECH, which returns one of
2508 CONTINUE(CHALL) means continue the auth conversation and
2509 send CHALL as the challenge to the client;
2515 OK means that the client has been successfully
2522 REJECTED means that the client failed to authenticate or
2523 there was an error in RESP.
2528 The server starts out in state
2529 <emphasis>WaitingForAuth</emphasis>. If the client is
2530 rejected too many times the server must disconnect the
2535 <title><emphasis>WaitingForAuth</emphasis></title>
2541 Receive AUTH → send REJECTED [mechs], goto
2542 <emphasis>WaitingForAuth</emphasis>
2548 Receive AUTH MECH RESP
2552 MECH not valid mechanism → send REJECTED
2554 <emphasis>WaitingForAuth</emphasis>
2558 MECH(RESP) returns CONTINUE(CHALL) → send
2560 <emphasis>WaitingForData</emphasis>
2564 MECH(RESP) returns OK → send OK, goto
2565 <emphasis>WaitingForBegin</emphasis>
2569 MECH(RESP) returns REJECTED → send REJECTED
2571 <emphasis>WaitingForAuth</emphasis>
2579 Receive BEGIN → terminate
2580 auth conversation, disconnect
2586 Receive ERROR → send REJECTED [mechs], goto
2587 <emphasis>WaitingForAuth</emphasis>
2593 Receive anything else → send
2595 <emphasis>WaitingForAuth</emphasis>
2604 <title><emphasis>WaitingForData</emphasis></title>
2612 MECH(RESP) returns CONTINUE(CHALL) → send
2614 <emphasis>WaitingForData</emphasis>
2618 MECH(RESP) returns OK → send OK, goto
2619 <emphasis>WaitingForBegin</emphasis>
2623 MECH(RESP) returns REJECTED → send REJECTED
2625 <emphasis>WaitingForAuth</emphasis>
2633 Receive BEGIN → terminate auth conversation,
2640 Receive CANCEL → send REJECTED [mechs], goto
2641 <emphasis>WaitingForAuth</emphasis>
2647 Receive ERROR → send REJECTED [mechs], goto
2648 <emphasis>WaitingForAuth</emphasis>
2654 Receive anything else → send ERROR, goto
2655 <emphasis>WaitingForData</emphasis>
2663 <title><emphasis>WaitingForBegin</emphasis></title>
2668 Receive BEGIN → terminate auth conversation,
2669 client authenticated
2675 Receive CANCEL → send REJECTED [mechs], goto
2676 <emphasis>WaitingForAuth</emphasis>
2682 Receive ERROR → send REJECTED [mechs], goto
2683 <emphasis>WaitingForAuth</emphasis>
2689 Receive anything else → send ERROR, goto
2690 <emphasis>WaitingForBegin</emphasis>
2700 <sect2 id="auth-mechanisms">
2701 <title>Authentication mechanisms</title>
2703 This section describes some new authentication mechanisms.
2704 D-Bus also allows any standard SASL mechanism of course.
2706 <sect3 id="auth-mechanisms-sha">
2707 <title>DBUS_COOKIE_SHA1</title>
2709 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2710 has the ability to read a private file owned by the user being
2711 authenticated. If the client can prove that it has access to a secret
2712 cookie stored in this file, then the client is authenticated.
2713 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2717 Throughout this description, "hex encoding" must output the digits
2718 from a to f in lower-case; the digits A to F must not be used
2719 in the DBUS_COOKIE_SHA1 mechanism.
2722 Authentication proceeds as follows:
2726 The client sends the username it would like to authenticate
2732 The server sends the name of its "cookie context" (see below); a
2733 space character; the integer ID of the secret cookie the client
2734 must demonstrate knowledge of; a space character; then a
2735 randomly-generated challenge string, all of this hex-encoded into
2741 The client locates the cookie and generates its own
2742 randomly-generated challenge string. The client then concatenates
2743 the server's decoded challenge, a ":" character, its own challenge,
2744 another ":" character, and the cookie. It computes the SHA-1 hash
2745 of this composite string as a hex digest. It concatenates the
2746 client's challenge string, a space character, and the SHA-1 hex
2747 digest, hex-encodes the result and sends it back to the server.
2752 The server generates the same concatenated string used by the
2753 client and computes its SHA-1 hash. It compares the hash with
2754 the hash received from the client; if the two hashes match, the
2755 client is authenticated.
2761 Each server has a "cookie context," which is a name that identifies a
2762 set of cookies that apply to that server. A sample context might be
2763 "org_freedesktop_session_bus". Context names must be valid ASCII,
2764 nonzero length, and may not contain the characters slash ("/"),
2765 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2766 tab ("\t"), or period ("."). There is a default context,
2767 "org_freedesktop_general" that's used by servers that do not specify
2771 Cookies are stored in a user's home directory, in the directory
2772 <filename>~/.dbus-keyrings/</filename>. This directory must
2773 not be readable or writable by other users. If it is,
2774 clients and servers must ignore it. The directory
2775 contains cookie files named after the cookie context.
2778 A cookie file contains one cookie per line. Each line
2779 has three space-separated fields:
2783 The cookie ID number, which must be a non-negative integer and
2784 may not be used twice in the same file.
2789 The cookie's creation time, in UNIX seconds-since-the-epoch
2795 The cookie itself, a hex-encoded random block of bytes. The cookie
2796 may be of any length, though obviously security increases
2797 as the length increases.
2803 Only server processes modify the cookie file.
2804 They must do so with this procedure:
2808 Create a lockfile name by appending ".lock" to the name of the
2809 cookie file. The server should attempt to create this file
2810 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2811 fails, the lock fails. Servers should retry for a reasonable
2812 period of time, then they may choose to delete an existing lock
2813 to keep users from having to manually delete a stale
2814 lock. <footnote><para>Lockfiles are used instead of real file
2815 locking <literal>fcntl()</literal> because real locking
2816 implementations are still flaky on network
2817 filesystems.</para></footnote>
2822 Once the lockfile has been created, the server loads the cookie
2823 file. It should then delete any cookies that are old (the
2824 timeout can be fairly short), or more than a reasonable
2825 time in the future (so that cookies never accidentally
2826 become permanent, if the clock was set far into the future
2827 at some point). If no recent keys remain, the
2828 server may generate a new key.
2833 The pruned and possibly added-to cookie file
2834 must be resaved atomically (using a temporary
2835 file which is rename()'d).
2840 The lock must be dropped by deleting the lockfile.
2846 Clients need not lock the file in order to load it,
2847 because servers are required to save the file atomically.
2852 <sect1 id="addresses">
2853 <title>Server Addresses</title>
2855 Server addresses consist of a transport name followed by a colon, and
2856 then an optional, comma-separated list of keys and values in the form key=value.
2857 Each value is escaped.
2861 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2862 Which is the address to a unix socket with the path /tmp/dbus-test.
2865 Value escaping is similar to URI escaping but simpler.
2869 The set of optionally-escaped bytes is:
2870 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2871 <emphasis>byte</emphasis> (note, not character) which is not in the
2872 set of optionally-escaped bytes must be replaced with an ASCII
2873 percent (<literal>%</literal>) and the value of the byte in hex.
2874 The hex value must always be two digits, even if the first digit is
2875 zero. The optionally-escaped bytes may be escaped if desired.
2880 To unescape, append each byte in the value; if a byte is an ASCII
2881 percent (<literal>%</literal>) character then append the following
2882 hex value instead. It is an error if a <literal>%</literal> byte
2883 does not have two hex digits following. It is an error if a
2884 non-optionally-escaped byte is seen unescaped.
2888 The set of optionally-escaped bytes is intended to preserve address
2889 readability and convenience.
2893 A server may specify a key-value pair with the key <literal>guid</literal>
2894 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2895 describes the format of the <literal>guid</literal> field. If present,
2896 this UUID may be used to distinguish one server address from another. A
2897 server should use a different UUID for each address it listens on. For
2898 example, if a message bus daemon offers both UNIX domain socket and TCP
2899 connections, but treats clients the same regardless of how they connect,
2900 those two connections are equivalent post-connection but should have
2901 distinct UUIDs to distinguish the kinds of connection.
2905 The intent of the address UUID feature is to allow a client to avoid
2906 opening multiple identical connections to the same server, by allowing the
2907 client to check whether an address corresponds to an already-existing
2908 connection. Comparing two addresses is insufficient, because addresses
2909 can be recycled by distinct servers, and equivalent addresses may look
2910 different if simply compared as strings (for example, the host in a TCP
2911 address can be given as an IP address or as a hostname).
2915 Note that the address key is <literal>guid</literal> even though the
2916 rest of the API and documentation says "UUID," for historical reasons.
2920 [FIXME clarify if attempting to connect to each is a requirement
2921 or just a suggestion]
2922 When connecting to a server, multiple server addresses can be
2923 separated by a semi-colon. The library will then try to connect
2924 to the first address and if that fails, it'll try to connect to
2925 the next one specified, and so forth. For example
2926 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2931 <sect1 id="transports">
2932 <title>Transports</title>
2934 [FIXME we need to specify in detail each transport and its possible arguments]
2936 Current transports include: unix domain sockets (including
2937 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2938 using in-process pipes. Future possible transports include one that
2939 tunnels over X11 protocol.
2942 <sect2 id="transports-unix-domain-sockets">
2943 <title>Unix Domain Sockets</title>
2945 Unix domain sockets can be either paths in the file system or on Linux
2946 kernels, they can be abstract which are similar to paths but
2947 do not show up in the file system.
2951 When a socket is opened by the D-Bus library it truncates the path
2952 name right before the first trailing Nul byte. This is true for both
2953 normal paths and abstract paths. Note that this is a departure from
2954 previous versions of D-Bus that would create sockets with a fixed
2955 length path name. Names which were shorter than the fixed length
2956 would be padded by Nul bytes.
2959 Unix domain sockets are not available on Windows.
2961 <sect3 id="transports-unix-domain-sockets-addresses">
2962 <title>Server Address Format</title>
2964 Unix domain socket addresses are identified by the "unix:" prefix
2965 and support the following key/value pairs:
2972 <entry>Values</entry>
2973 <entry>Description</entry>
2979 <entry>(path)</entry>
2980 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2983 <entry>tmpdir</entry>
2984 <entry>(path)</entry>
2985 <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>
2988 <entry>abstract</entry>
2989 <entry>(string)</entry>
2990 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2997 <sect2 id="transports-launchd">
2998 <title>launchd</title>
3000 launchd is an open-source server management system that replaces init, inetd
3001 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3002 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3006 launchd allocates a socket and provides it with the unix path through the
3007 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3008 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3009 it through its environment.
3010 Other processes can query for the launchd socket by executing:
3011 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3012 This is normally done by the D-Bus client library so doesn't have to be done
3016 launchd is not available on Microsoft Windows.
3018 <sect3 id="transports-launchd-addresses">
3019 <title>Server Address Format</title>
3021 launchd addresses are identified by the "launchd:" prefix
3022 and support the following key/value pairs:
3029 <entry>Values</entry>
3030 <entry>Description</entry>
3036 <entry>(environment variable)</entry>
3037 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3044 <sect2 id="transports-systemd">
3045 <title>systemd</title>
3047 systemd is an open-source server management system that
3048 replaces init and inetd on newer Linux systems. It supports
3049 socket activation. The D-Bus systemd transport is used to acquire
3050 socket activation file descriptors from systemd and use them
3051 as D-Bus transport when the current process is spawned by
3052 socket activation from it.
3055 The systemd transport accepts only one or more Unix domain or
3056 TCP streams sockets passed in via socket activation.
3059 The systemd transport is not available on non-Linux operating systems.
3062 The systemd transport defines no parameter keys.
3065 <sect2 id="transports-tcp-sockets">
3066 <title>TCP Sockets</title>
3068 The tcp transport provides TCP/IP based connections between clients
3069 located on the same or different hosts.
3072 Using tcp transport without any additional secure authentification mechanismus
3073 over a network is unsecure.
3076 Windows notes: Because of the tcp stack on Windows does not provide sending
3077 credentials over a tcp connection, the EXTERNAL authentification
3078 mechanismus does not work.
3080 <sect3 id="transports-tcp-sockets-addresses">
3081 <title>Server Address Format</title>
3083 TCP/IP socket addresses are identified by the "tcp:" prefix
3084 and support the following key/value pairs:
3091 <entry>Values</entry>
3092 <entry>Description</entry>
3098 <entry>(string)</entry>
3099 <entry>dns name or ip address</entry>
3103 <entry>(number)</entry>
3104 <entry>The tcp port the server will open. A zero value let the server
3105 choose a free port provided from the underlaying operating system.
3106 libdbus is able to retrieve the real used port from the server.
3110 <entry>family</entry>
3111 <entry>(string)</entry>
3112 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3119 <sect2 id="transports-nonce-tcp-sockets">
3120 <title>Nonce-secured TCP Sockets</title>
3122 The nonce-tcp transport provides a secured TCP transport, using a
3123 simple authentication mechanism to ensure that only clients with read
3124 access to a certain location in the filesystem can connect to the server.
3125 The server writes a secret, the nonce, to a file and an incoming client
3126 connection is only accepted if the client sends the nonce right after
3127 the connect. The nonce mechanism requires no setup and is orthogonal to
3128 the higher-level authentication mechanisms described in the
3129 Authentication section.
3133 On start, the server generates a random 16 byte nonce and writes it
3134 to a file in the user's temporary directory. The nonce file location
3135 is published as part of the server's D-Bus address using the
3136 "noncefile" key-value pair.
3138 After an accept, the server reads 16 bytes from the socket. If the
3139 read bytes do not match the nonce stored in the nonce file, the
3140 server MUST immediately drop the connection.
3141 If the nonce match the received byte sequence, the client is accepted
3142 and the transport behaves like an unsecured tcp transport.
3145 After a successful connect to the server socket, the client MUST read
3146 the nonce from the file published by the server via the noncefile=
3147 key-value pair and send it over the socket. After that, the
3148 transport behaves like an unsecured tcp transport.
3150 <sect3 id="transports-nonce-tcp-sockets-addresses">
3151 <title>Server Address Format</title>
3153 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3154 and support the following key/value pairs:
3161 <entry>Values</entry>
3162 <entry>Description</entry>
3168 <entry>(string)</entry>
3169 <entry>dns name or ip address</entry>
3173 <entry>(number)</entry>
3174 <entry>The tcp port the server will open. A zero value let the server
3175 choose a free port provided from the underlaying operating system.
3176 libdbus is able to retrieve the real used port from the server.
3180 <entry>family</entry>
3181 <entry>(string)</entry>
3182 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3185 <entry>noncefile</entry>
3186 <entry>(path)</entry>
3187 <entry>file location containing the secret</entry>
3194 <sect2 id="transports-exec">
3195 <title>Executed Subprocesses on Unix</title>
3197 This transport forks off a process and connects its standard
3198 input and standard output with an anonymous Unix domain
3199 socket. This socket is then used for communication by the
3200 transport. This transport may be used to use out-of-process
3201 forwarder programs as basis for the D-Bus protocol.
3204 The forked process will inherit the standard error output and
3205 process group from the parent process.
3208 Executed subprocesses are not available on Windows.
3210 <sect3 id="transports-exec-addresses">
3211 <title>Server Address Format</title>
3213 Executed subprocess addresses are identified by the "unixexec:" prefix
3214 and support the following key/value pairs:
3221 <entry>Values</entry>
3222 <entry>Description</entry>
3228 <entry>(path)</entry>
3229 <entry>Path of the binary to execute, either an absolute
3230 path or a binary name that is searched for in the default
3231 search path of the OS. This corresponds to the first
3232 argument of execlp(). This key is mandatory.</entry>
3235 <entry>argv0</entry>
3236 <entry>(string)</entry>
3237 <entry>The program name to use when executing the
3238 binary. If omitted the same value as specified for path=
3239 will be used. This corresponds to the second argument of
3243 <entry>argv1, argv2, ...</entry>
3244 <entry>(string)</entry>
3245 <entry>Arguments to pass to the binary. This corresponds
3246 to the third and later arguments of execlp(). If a
3247 specific argvX is not specified no further argvY for Y > X
3248 are taken into account.</entry>
3256 <sect1 id="meta-transports">
3257 <title>Meta Transports</title>
3259 Meta transports are a kind of transport with special enhancements or
3260 behavior. Currently available meta transports include: autolaunch
3263 <sect2 id="meta-transports-autolaunch">
3264 <title>Autolaunch</title>
3265 <para>The autolaunch transport provides a way for dbus clients to autodetect
3266 a running dbus session bus and to autolaunch a session bus if not present.
3268 <sect3 id="meta-transports-autolaunch-addresses">
3269 <title>Server Address Format</title>
3271 Autolaunch addresses uses the "autolaunch:" prefix and support the
3272 following key/value pairs:
3279 <entry>Values</entry>
3280 <entry>Description</entry>
3285 <entry>scope</entry>
3286 <entry>(string)</entry>
3287 <entry>scope of autolaunch (Windows only)
3291 "*install-path" - limit session bus to dbus installation path.
3292 The dbus installation path is determined from the location of
3293 the shared dbus library. If the library is located in a 'bin'
3294 subdirectory the installation root is the directory above,
3295 otherwise the directory where the library lives is taken as
3298 <install-root>/bin/[lib]dbus-1.dll
3299 <install-root>/[lib]dbus-1.dll
3305 "*user" - limit session bus to the recent user.
3310 other values - specify dedicated session bus like "release",
3322 <sect3 id="meta-transports-autolaunch-windows-implementation">
3323 <title>Windows implementation</title>
3325 On start, the server opens a platform specific transport, creates a mutex
3326 and a shared memory section containing the related session bus address.
3327 This mutex will be inspected by the dbus client library to detect a
3328 running dbus session bus. The access to the mutex and the shared memory
3329 section are protected by global locks.
3332 In the recent implementation the autolaunch transport uses a tcp transport
3333 on localhost with a port choosen from the operating system. This detail may
3334 change in the future.
3337 Disclaimer: The recent implementation is in an early state and may not
3338 work in all cirumstances and/or may have security issues. Because of this
3339 the implementation is not documentated yet.
3346 <title>UUIDs</title>
3348 A working D-Bus implementation uses universally-unique IDs in two places.
3349 First, each server address has a UUID identifying the address,
3350 as described in <xref linkend="addresses"/>. Second, each operating
3351 system kernel instance running a D-Bus client or server has a UUID
3352 identifying that kernel, retrieved by invoking the method
3353 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3354 linkend="standard-interfaces-peer"/>).
3357 The term "UUID" in this document is intended literally, i.e. an
3358 identifier that is universally unique. It is not intended to refer to
3359 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3362 The UUID must contain 128 bits of data and be hex-encoded. The
3363 hex-encoded string may not contain hyphens or other non-hex-digit
3364 characters, and it must be exactly 32 characters long. To generate a
3365 UUID, the current reference implementation concatenates 96 bits of random
3366 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3370 It would also be acceptable and probably better to simply generate 128
3371 bits of random data, as long as the random number generator is of high
3372 quality. The timestamp could conceivably help if the random bits are not
3373 very random. With a quality random number generator, collisions are
3374 extremely unlikely even with only 96 bits, so it's somewhat academic.
3377 Implementations should, however, stick to random data for the first 96 bits
3382 <sect1 id="standard-interfaces">
3383 <title>Standard Interfaces</title>
3385 See <xref linkend="message-protocol-types-notation"/> for details on
3386 the notation used in this section. There are some standard interfaces
3387 that may be useful across various D-Bus applications.
3389 <sect2 id="standard-interfaces-peer">
3390 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3392 The <literal>org.freedesktop.DBus.Peer</literal> interface
3395 org.freedesktop.DBus.Peer.Ping ()
3396 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3400 On receipt of the <literal>METHOD_CALL</literal> message
3401 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3402 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3403 usual. It does not matter which object path a ping is sent to. The
3404 reference implementation handles this method automatically.
3407 On receipt of the <literal>METHOD_CALL</literal> message
3408 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3409 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3410 UUID representing the identity of the machine the process is running on.
3411 This UUID must be the same for all processes on a single system at least
3412 until that system next reboots. It should be the same across reboots
3413 if possible, but this is not always possible to implement and is not
3415 It does not matter which object path a GetMachineId is sent to. The
3416 reference implementation handles this method automatically.
3419 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3420 a virtual machine running on a hypervisor, rather than a physical machine.
3421 Basically if two processes see the same UUID, they should also see the same
3422 shared memory, UNIX domain sockets, process IDs, and other features that require
3423 a running OS kernel in common between the processes.
3426 The UUID is often used where other programs might use a hostname. Hostnames
3427 can change without rebooting, however, or just be "localhost" - so the UUID
3431 <xref linkend="uuids"/> explains the format of the UUID.
3435 <sect2 id="standard-interfaces-introspectable">
3436 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3438 This interface has one method:
3440 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3444 Objects instances may implement
3445 <literal>Introspect</literal> which returns an XML description of
3446 the object, including its interfaces (with signals and methods), objects
3447 below it in the object path tree, and its properties.
3450 <xref linkend="introspection-format"/> describes the format of this XML string.
3453 <sect2 id="standard-interfaces-properties">
3454 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3456 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3457 or <firstterm>attributes</firstterm>. These can be exposed via the
3458 <literal>org.freedesktop.DBus.Properties</literal> interface.
3462 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3463 in STRING property_name,
3465 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3466 in STRING property_name,
3468 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3469 out DICT<STRING,VARIANT> props);
3473 It is conventional to give D-Bus properties names consisting of
3474 capitalized words without punctuation ("CamelCase"), like
3475 <link linkend="message-protocol-names-member">member names</link>.
3476 For instance, the GObject property
3477 <literal>connection-status</literal> or the Qt property
3478 <literal>connectionStatus</literal> could be represented on D-Bus
3479 as <literal>ConnectionStatus</literal>.
3482 Strictly speaking, D-Bus property names are not required to follow
3483 the same naming restrictions as member names, but D-Bus property
3484 names that would not be valid member names (in particular,
3485 GObject-style dash-separated property names) can cause interoperability
3486 problems and should be avoided.
3489 The available properties and whether they are writable can be determined
3490 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3491 see <xref linkend="standard-interfaces-introspectable"/>.
3494 An empty string may be provided for the interface name; in this case,
3495 if there are multiple properties on an object with the same name,
3496 the results are undefined (picking one by according to an arbitrary
3497 deterministic rule, or returning an error, are the reasonable
3501 If one or more properties change on an object, the
3502 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3503 signal may be emitted (this signal was added in 0.14):
3507 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3508 DICT<STRING,VARIANT> changed_properties,
3509 ARRAY<STRING> invalidated_properties);
3513 where <literal>changed_properties</literal> is a dictionary
3514 containing the changed properties with the new values and
3515 <literal>invalidated_properties</literal> is an array of
3516 properties that changed but the value is not conveyed.
3519 Whether the <literal>PropertiesChanged</literal> signal is
3520 supported can be determined by calling
3521 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3522 that the signal may be supported for an object but it may
3523 differ how whether and how it is used on a per-property basis
3524 (for e.g. performance or security reasons). Each property (or
3525 the parent interface) must be annotated with the
3526 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3527 annotation to convey this (usually the default value
3528 <literal>true</literal> is sufficient meaning that the
3529 annotation does not need to be used). See <xref
3530 linkend="introspection-format"/> for details on this
3535 <sect2 id="standard-interfaces-objectmanager">
3536 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3538 An API can optionally make use of this interface for one or
3539 more sub-trees of objects. The root of each sub-tree implements
3540 this interface so other applications can get all objects,
3541 interfaces and properties in a single method call. It is
3542 appropriate to use this interface if users of the tree of
3543 objects are expected to be interested in all interfaces of all
3544 objects in the tree; a more granular API should be used if
3545 users of the objects are expected to be interested in a small
3546 subset of the objects, a small subset of their interfaces, or
3550 The method that applications can use to get all objects and
3551 properties is <literal>GetManagedObjects</literal>:
3555 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3559 The return value of this method is a dict whose keys are
3560 object paths. All returned object paths are children of the
3561 object path implementing this interface, i.e. their object
3562 paths start with the ObjectManager's object path plus '/'.
3565 Each value is a dict whose keys are interfaces names. Each
3566 value in this inner dict is the same dict that would be
3567 returned by the <link
3568 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3569 method for that combination of object path and interface. If
3570 an interface has no properties, the empty dict is returned.
3573 Changes are emitted using the following two signals:
3577 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3578 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3579 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3580 ARRAY<STRING> interfaces);
3584 The <literal>InterfacesAdded</literal> signal is emitted when
3585 either a new object is added or when an existing object gains
3586 one or more interfaces. The
3587 <literal>InterfacesRemoved</literal> signal is emitted
3588 whenever an object is removed or it loses one or more
3589 interfaces. The second parameter of the
3590 <literal>InterfacesAdded</literal> signal contains a dict with
3591 the interfaces and properties (if any) that have been added to
3592 the given object path. Similarly, the second parameter of the
3593 <literal>InterfacesRemoved</literal> signal contains an array
3594 of the interfaces that were removed. Note that changes on
3595 properties on existing interfaces are not reported using this
3596 interface - an application should also monitor the existing <link
3597 linkend="standard-interfaces-properties">PropertiesChanged</link>
3598 signal on each object.
3601 Applications SHOULD NOT export objects that are children of an
3602 object (directly or otherwise) implementing this interface but
3603 which are not returned in the reply from the
3604 <literal>GetManagedObjects()</literal> method of this
3605 interface on the given object.
3608 The intent of the <literal>ObjectManager</literal> interface
3609 is to make it easy to write a robust client
3610 implementation. The trivial client implementation only needs
3611 to make two method calls:
3615 org.freedesktop.DBus.AddMatch (bus_proxy,
3616 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3617 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3621 on the message bus and the remote application's
3622 <literal>ObjectManager</literal>, respectively. Whenever a new
3623 remote object is created (or an existing object gains a new
3624 interface), the <literal>InterfacesAdded</literal> signal is
3625 emitted, and since this signal contains all properties for the
3626 interfaces, no calls to the
3627 <literal>org.freedesktop.Properties</literal> interface on the
3628 remote object are needed. Additionally, since the initial
3629 <literal>AddMatch()</literal> rule already includes signal
3630 messages from the newly created child object, no new
3631 <literal>AddMatch()</literal> call is needed.
3636 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3637 interface was added in version 0.17 of the D-Bus
3644 <sect1 id="introspection-format">
3645 <title>Introspection Data Format</title>
3647 As described in <xref linkend="standard-interfaces-introspectable"/>,
3648 objects may be introspected at runtime, returning an XML string
3649 that describes the object. The same XML format may be used in
3650 other contexts as well, for example as an "IDL" for generating
3651 static language bindings.
3654 Here is an example of introspection data:
3656 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3657 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3658 <node name="/org/freedesktop/sample_object">
3659 <interface name="org.freedesktop.SampleInterface">
3660 <method name="Frobate">
3661 <arg name="foo" type="i" direction="in"/>
3662 <arg name="bar" type="s" direction="out"/>
3663 <arg name="baz" type="a{us}" direction="out"/>
3664 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3666 <method name="Bazify">
3667 <arg name="bar" type="(iiu)" direction="in"/>
3668 <arg name="bar" type="v" direction="out"/>
3670 <method name="Mogrify">
3671 <arg name="bar" type="(iiav)" direction="in"/>
3673 <signal name="Changed">
3674 <arg name="new_value" type="b"/>
3676 <property name="Bar" type="y" access="readwrite"/>
3678 <node name="child_of_sample_object"/>
3679 <node name="another_child_of_sample_object"/>
3684 A more formal DTD and spec needs writing, but here are some quick notes.
3688 Only the root <node> element can omit the node name, as it's
3689 known to be the object that was introspected. If the root
3690 <node> does have a name attribute, it must be an absolute
3691 object path. If child <node> have object paths, they must be
3697 If a child <node> has any sub-elements, then they
3698 must represent a complete introspection of the child.
3699 If a child <node> is empty, then it may or may
3700 not have sub-elements; the child must be introspected
3701 in order to find out. The intent is that if an object
3702 knows that its children are "fast" to introspect
3703 it can go ahead and return their information, but
3704 otherwise it can omit it.
3709 The direction element on <arg> may be omitted,
3710 in which case it defaults to "in" for method calls
3711 and "out" for signals. Signals only allow "out"
3712 so while direction may be specified, it's pointless.
3717 The possible directions are "in" and "out",
3718 unlike CORBA there is no "inout"
3723 The possible property access flags are
3724 "readwrite", "read", and "write"
3729 Multiple interfaces can of course be listed for
3735 The "name" attribute on arguments is optional.
3741 Method, interface, property, and signal elements may have
3742 "annotations", which are generic key/value pairs of metadata.
3743 They are similar conceptually to Java's annotations and C# attributes.
3744 Well-known annotations:
3751 <entry>Values (separated by ,)</entry>
3752 <entry>Description</entry>
3757 <entry>org.freedesktop.DBus.Deprecated</entry>
3758 <entry>true,false</entry>
3759 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3762 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3763 <entry>(string)</entry>
3764 <entry>The C symbol; may be used for methods and interfaces</entry>
3767 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3768 <entry>true,false</entry>
3769 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3772 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3773 <entry>true,invalidates,false</entry>
3776 If set to <literal>false</literal>, the
3777 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3779 linkend="standard-interfaces-properties"/> is not
3780 guaranteed to be emitted if the property changes.
3783 If set to <literal>invalidates</literal> the signal
3784 is emitted but the value is not included in the
3788 If set to <literal>true</literal> the signal is
3789 emitted with the value included.
3792 The value for the annotation defaults to
3793 <literal>true</literal> if the enclosing interface
3794 element does not specify the annotation. Otherwise it
3795 defaults to the value specified in the enclosing
3804 <sect1 id="message-bus">
3805 <title>Message Bus Specification</title>
3806 <sect2 id="message-bus-overview">
3807 <title>Message Bus Overview</title>
3809 The message bus accepts connections from one or more applications.
3810 Once connected, applications can exchange messages with other
3811 applications that are also connected to the bus.
3814 In order to route messages among connections, the message bus keeps a
3815 mapping from names to connections. Each connection has one
3816 unique-for-the-lifetime-of-the-bus name automatically assigned.
3817 Applications may request additional names for a connection. Additional
3818 names are usually "well-known names" such as
3819 "org.freedesktop.TextEditor". When a name is bound to a connection,
3820 that connection is said to <firstterm>own</firstterm> the name.
3823 The bus itself owns a special name,
3824 <literal>org.freedesktop.DBus</literal>, with an object
3825 located at <literal>/org/freedesktop/DBus</literal> that
3826 implements the <literal>org.freedesktop.DBus</literal>
3827 interface. This service allows applications to make
3828 administrative requests of the bus itself. For example,
3829 applications can ask the bus to assign a name to a connection.
3832 Each name may have <firstterm>queued owners</firstterm>. When an
3833 application requests a name for a connection and the name is already in
3834 use, the bus will optionally add the connection to a queue waiting for
3835 the name. If the current owner of the name disconnects or releases
3836 the name, the next connection in the queue will become the new owner.
3840 This feature causes the right thing to happen if you start two text
3841 editors for example; the first one may request "org.freedesktop.TextEditor",
3842 and the second will be queued as a possible owner of that name. When
3843 the first exits, the second will take over.
3847 Applications may send <firstterm>unicast messages</firstterm> to
3848 a specific recipient or to the message bus itself, or
3849 <firstterm>broadcast messages</firstterm> to all interested recipients.
3850 See <xref linkend="message-bus-routing"/> for details.
3854 <sect2 id="message-bus-names">
3855 <title>Message Bus Names</title>
3857 Each connection has at least one name, assigned at connection time and
3858 returned in response to the
3859 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3860 automatically-assigned name is called the connection's <firstterm>unique
3861 name</firstterm>. Unique names are never reused for two different
3862 connections to the same bus.
3865 Ownership of a unique name is a prerequisite for interaction with
3866 the message bus. It logically follows that the unique name is always
3867 the first name that an application comes to own, and the last
3868 one that it loses ownership of.
3871 Unique connection names must begin with the character ':' (ASCII colon
3872 character); bus names that are not unique names must not begin
3873 with this character. (The bus must reject any attempt by an application
3874 to manually request a name beginning with ':'.) This restriction
3875 categorically prevents "spoofing"; messages sent to a unique name
3876 will always go to the expected connection.
3879 When a connection is closed, all the names that it owns are deleted (or
3880 transferred to the next connection in the queue if any).
3883 A connection can request additional names to be associated with it using
3884 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3885 linkend="message-protocol-names-bus"/> describes the format of a valid
3886 name. These names can be released again using the
3887 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3890 <sect3 id="bus-messages-request-name">
3891 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3895 UINT32 RequestName (in STRING name, in UINT32 flags)
3902 <entry>Argument</entry>
3904 <entry>Description</entry>
3910 <entry>STRING</entry>
3911 <entry>Name to request</entry>
3915 <entry>UINT32</entry>
3916 <entry>Flags</entry>
3926 <entry>Argument</entry>
3928 <entry>Description</entry>
3934 <entry>UINT32</entry>
3935 <entry>Return value</entry>
3942 This method call should be sent to
3943 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3944 assign the given name to the method caller. Each name maintains a
3945 queue of possible owners, where the head of the queue is the primary
3946 or current owner of the name. Each potential owner in the queue
3947 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3948 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3949 call. When RequestName is invoked the following occurs:
3953 If the method caller is currently the primary owner of the name,
3954 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3955 values are updated with the values from the new RequestName call,
3956 and nothing further happens.
3962 If the current primary owner (head of the queue) has
3963 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3964 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3965 the caller of RequestName replaces the current primary owner at
3966 the head of the queue and the current primary owner moves to the
3967 second position in the queue. If the caller of RequestName was
3968 in the queue previously its flags are updated with the values from
3969 the new RequestName in addition to moving it to the head of the queue.
3975 If replacement is not possible, and the method caller is
3976 currently in the queue but not the primary owner, its flags are
3977 updated with the values from the new RequestName call.
3983 If replacement is not possible, and the method caller is
3984 currently not in the queue, the method caller is appended to the
3991 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3992 set and is not the primary owner, it is removed from the
3993 queue. This can apply to the previous primary owner (if it
3994 was replaced) or the method caller (if it updated the
3995 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3996 queue, or if it was just added to the queue with that flag set).
4002 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4003 queue," even if another application already in the queue had specified
4004 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4005 that does not allow replacement goes away, and the next primary owner
4006 does allow replacement. In this case, queued items that specified
4007 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4008 automatically replace the new primary owner. In other words,
4009 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4010 time RequestName is called. This is deliberate to avoid an infinite loop
4011 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4012 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4015 The flags argument contains any of the following values logically ORed
4022 <entry>Conventional Name</entry>
4023 <entry>Value</entry>
4024 <entry>Description</entry>
4029 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4033 If an application A specifies this flag and succeeds in
4034 becoming the owner of the name, and another application B
4035 later calls RequestName with the
4036 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4037 will lose ownership and receive a
4038 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4039 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4040 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4041 is not specified by application B, then application B will not replace
4042 application A as the owner.
4047 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4051 Try to replace the current owner if there is one. If this
4052 flag is not set the application will only become the owner of
4053 the name if there is no current owner. If this flag is set,
4054 the application will replace the current owner if
4055 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4060 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4064 Without this flag, if an application requests a name that is
4065 already owned, the application will be placed in a queue to
4066 own the name when the current owner gives it up. If this
4067 flag is given, the application will not be placed in the
4068 queue, the request for the name will simply fail. This flag
4069 also affects behavior when an application is replaced as
4070 name owner; by default the application moves back into the
4071 waiting queue, unless this flag was provided when the application
4072 became the name owner.
4080 The return code can be one of the following values:
4086 <entry>Conventional Name</entry>
4087 <entry>Value</entry>
4088 <entry>Description</entry>
4093 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4094 <entry>1</entry> <entry>The caller is now the primary owner of
4095 the name, replacing any previous owner. Either the name had no
4096 owner before, or the caller specified
4097 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4098 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4101 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4104 <entry>The name already had an owner,
4105 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4106 the current owner did not specify
4107 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4108 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4112 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4113 <entry>The name already has an owner,
4114 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4115 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4116 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4117 specified by the requesting application.</entry>
4120 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4122 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4130 <sect3 id="bus-messages-release-name">
4131 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4135 UINT32 ReleaseName (in STRING name)
4142 <entry>Argument</entry>
4144 <entry>Description</entry>
4150 <entry>STRING</entry>
4151 <entry>Name to release</entry>
4161 <entry>Argument</entry>
4163 <entry>Description</entry>
4169 <entry>UINT32</entry>
4170 <entry>Return value</entry>
4177 This method call should be sent to
4178 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4179 release the method caller's claim to the given name. If the caller is
4180 the primary owner, a new primary owner will be selected from the
4181 queue if any other owners are waiting. If the caller is waiting in
4182 the queue for the name, the caller will removed from the queue and
4183 will not be made an owner of the name if it later becomes available.
4184 If there are no other owners in the queue for the name, it will be
4185 removed from the bus entirely.
4187 The return code can be one of the following values:
4193 <entry>Conventional Name</entry>
4194 <entry>Value</entry>
4195 <entry>Description</entry>
4200 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4201 <entry>1</entry> <entry>The caller has released his claim on
4202 the given name. Either the caller was the primary owner of
4203 the name, and the name is now unused or taken by somebody
4204 waiting in the queue for the name, or the caller was waiting
4205 in the queue for the name and has now been removed from the
4209 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4211 <entry>The given name does not exist on this bus.</entry>
4214 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4216 <entry>The caller was not the primary owner of this name,
4217 and was also not waiting in the queue to own this name.</entry>
4225 <sect3 id="bus-messages-list-queued-owners">
4226 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4230 ARRAY of STRING ListQueuedOwners (in STRING name)
4237 <entry>Argument</entry>
4239 <entry>Description</entry>
4245 <entry>STRING</entry>
4246 <entry>The well-known bus name to query, such as
4247 <literal>com.example.cappuccino</literal></entry>
4257 <entry>Argument</entry>
4259 <entry>Description</entry>
4265 <entry>ARRAY of STRING</entry>
4266 <entry>The unique bus names of connections currently queued
4267 for the name</entry>
4274 This method call should be sent to
4275 <literal>org.freedesktop.DBus</literal> and lists the connections
4276 currently queued for a bus name (see
4277 <xref linkend="term-queued-owner"/>).
4282 <sect2 id="message-bus-routing">
4283 <title>Message Bus Message Routing</title>
4286 Messages may have a <literal>DESTINATION</literal> field (see <xref
4287 linkend="message-protocol-header-fields"/>), resulting in a
4288 <firstterm>unicast message</firstterm>. If the
4289 <literal>DESTINATION</literal> field is present, it specifies a message
4290 recipient by name. Method calls and replies normally specify this field.
4291 The message bus must send messages (of any type) with the
4292 <literal>DESTINATION</literal> field set to the specified recipient,
4293 regardless of whether the recipient has set up a match rule matching
4298 When the message bus receives a signal, if the
4299 <literal>DESTINATION</literal> field is absent, it is considered to
4300 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4301 applications with <firstterm>message matching rules</firstterm> that
4302 match the message. Most signal messages are broadcasts.
4306 Unicast signal messages (those with a <literal>DESTINATION</literal>
4307 field) are not commonly used, but they are treated like any unicast
4308 message: they are delivered to the specified receipient,
4309 regardless of its match rules. One use for unicast signals is to
4310 avoid a race condition in which a signal is emitted before the intended
4311 recipient can call <xref linkend="bus-messages-add-match"/> to
4312 receive that signal: if the signal is sent directly to that recipient
4313 using a unicast message, it does not need to add a match rule at all,
4314 and there is no race condition. Another use for unicast signals,
4315 on message buses whose security policy prevents eavesdropping, is to
4316 send sensitive information which should only be visible to one
4321 When the message bus receives a method call, if the
4322 <literal>DESTINATION</literal> field is absent, the call is taken to be
4323 a standard one-to-one message and interpreted by the message bus
4324 itself. For example, sending an
4325 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4326 <literal>DESTINATION</literal> will cause the message bus itself to
4327 reply to the ping immediately; the message bus will not make this
4328 message visible to other applications.
4332 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4333 the ping message were sent with a <literal>DESTINATION</literal> name of
4334 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4335 forwarded, and the Yoyodyne Corporation screensaver application would be
4336 expected to reply to the ping.
4340 Message bus implementations may impose a security policy which
4341 prevents certain messages from being sent or received.
4342 When a message cannot be sent or received due to a security
4343 policy, the message bus should send an error reply, unless the
4344 original message had the <literal>NO_REPLY</literal> flag.
4347 <sect3 id="message-bus-routing-eavesdropping">
4348 <title>Eavesdropping</title>
4350 Receiving a unicast message whose <literal>DESTINATION</literal>
4351 indicates a different recipient is called
4352 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4353 a security boundary (like the standard system bus), the security
4354 policy should usually prevent eavesdropping, since unicast messages
4355 are normally kept private and may contain security-sensitive
4360 Eavesdropping is mainly useful for debugging tools, such as
4361 the <literal>dbus-monitor</literal> tool in the reference
4362 implementation of D-Bus. Tools which eavesdrop on the message bus
4363 should be careful to avoid sending a reply or error in response to
4364 messages intended for a different client.
4368 Clients may attempt to eavesdrop by adding match rules
4369 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4370 the <literal>eavesdrop='true'</literal> match. If the message bus'
4371 security policy does not allow eavesdropping, the match rule can
4372 still be added, but will not have any practical effect. For
4373 compatibility with older message bus implementations, if adding such
4374 a match rule results in an error reply, the client may fall back to
4375 adding the same rule with the <literal>eavesdrop</literal> match
4380 <sect3 id="message-bus-routing-match-rules">
4381 <title>Match Rules</title>
4383 An important part of the message bus routing protocol is match
4384 rules. Match rules describe the messages that should be sent to a
4385 client, based on the contents of the message. Broadcast signals
4386 are only sent to clients which have a suitable match rule: this
4387 avoids waking up client processes to deal with signals that are
4388 not relevant to that client.
4391 Messages that list a client as their <literal>DESTINATION</literal>
4392 do not need to match the client's match rules, and are sent to that
4393 client regardless. As a result, match rules are mainly used to
4394 receive a subset of broadcast signals.
4397 Match rules can also be used for eavesdropping
4398 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4399 if the security policy of the message bus allows it.
4402 Match rules are added using the AddMatch bus method
4403 (see <xref linkend="bus-messages-add-match"/>). Rules are
4404 specified as a string of comma separated key/value pairs.
4405 Excluding a key from the rule indicates a wildcard match.
4406 For instance excluding the the member from a match rule but
4407 adding a sender would let all messages from that sender through.
4408 An example of a complete rule would be
4409 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4412 The following table describes the keys that can be used to create
4414 The following table summarizes the D-Bus types.
4420 <entry>Possible Values</entry>
4421 <entry>Description</entry>
4426 <entry><literal>type</literal></entry>
4427 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4428 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4431 <entry><literal>sender</literal></entry>
4432 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4433 and <xref linkend="term-unique-name"/> respectively)
4435 <entry>Match messages sent by a particular sender. An example of a sender match
4436 is sender='org.freedesktop.Hal'</entry>
4439 <entry><literal>interface</literal></entry>
4440 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4441 <entry>Match messages sent over or to a particular interface. An example of an
4442 interface match is interface='org.freedesktop.Hal.Manager'.
4443 If a message omits the interface header, it must not match any rule
4444 that specifies this key.</entry>
4447 <entry><literal>member</literal></entry>
4448 <entry>Any valid method or signal name</entry>
4449 <entry>Matches messages which have the give method or signal name. An example of
4450 a member match is member='NameOwnerChanged'</entry>
4453 <entry><literal>path</literal></entry>
4454 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4455 <entry>Matches messages which are sent from or to the given object. An example of a
4456 path match is path='/org/freedesktop/Hal/Manager'</entry>
4459 <entry><literal>path_namespace</literal></entry>
4460 <entry>An object path</entry>
4463 Matches messages which are sent from or to an
4464 object for which the object path is either the
4465 given value, or that value followed by one or
4466 more path components.
4471 <literal>path_namespace='/com/example/foo'</literal>
4472 would match signals sent by
4473 <literal>/com/example/foo</literal>
4475 <literal>/com/example/foo/bar</literal>,
4477 <literal>/com/example/foobar</literal>.
4481 Using both <literal>path</literal> and
4482 <literal>path_namespace</literal> in the same match
4483 rule is not allowed.
4488 This match key was added in version 0.16 of the
4489 D-Bus specification and implemented by the bus
4490 daemon in dbus 1.5.0 and later.
4496 <entry><literal>destination</literal></entry>
4497 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4498 <entry>Matches messages which are being sent to the given unique name. An
4499 example of a destination match is destination=':1.0'</entry>
4502 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4503 <entry>Any string</entry>
4504 <entry>Arg matches are special and are used for further restricting the
4505 match based on the arguments in the body of a message. Only arguments of type
4506 STRING can be matched in this way. An example of an argument match
4507 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4511 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4512 <entry>Any string</entry>
4514 <para>Argument path matches provide a specialised form of wildcard matching for
4515 path-like namespaces. They can match arguments whose type is either STRING or
4516 OBJECT_PATH. As with normal argument matches,
4517 if the argument is exactly equal to the string given in the match
4518 rule then the rule is satisfied. Additionally, there is also a
4519 match when either the string given in the match rule or the
4520 appropriate message argument ends with '/' and is a prefix of the
4521 other. An example argument path match is arg0path='/aa/bb/'. This
4522 would match messages with first arguments of '/', '/aa/',
4523 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4524 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4526 <para>This is intended for monitoring “directories” in file system-like
4527 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4528 system. An application interested in all nodes in a particular hierarchy would
4529 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4530 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4531 represent a modification to the “bar” property, or a signal with zeroth
4532 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4533 many properties within that directory, and the interested application would be
4534 notified in both cases.</para>
4537 This match key was added in version 0.12 of the
4538 D-Bus specification, implemented for STRING
4539 arguments by the bus daemon in dbus 1.2.0 and later,
4540 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4547 <entry><literal>arg0namespace</literal></entry>
4548 <entry>Like a bus name, except that the string is not
4549 required to contain a '.' (period)</entry>
4551 <para>Match messages whose first argument is of type STRING, and is a bus name
4552 or interface name within the specified namespace. This is primarily intended
4553 for watching name owner changes for a group of related bus names, rather than
4554 for a single name or all name changes.</para>
4556 <para>Because every valid interface name is also a valid
4557 bus name, this can also be used for messages whose
4558 first argument is an interface name.</para>
4560 <para>For example, the match rule
4561 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4562 matches name owner changes for bus names such as
4563 <literal>com.example.backend.foo</literal>,
4564 <literal>com.example.backend.foo.bar</literal>, and
4565 <literal>com.example.backend</literal> itself.</para>
4567 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4570 This match key was added in version 0.16 of the
4571 D-Bus specification and implemented by the bus
4572 daemon in dbus 1.5.0 and later.
4578 <entry><literal>eavesdrop</literal></entry>
4579 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4580 <entry>Since D-Bus 1.5.6, match rules do not
4581 match messages which have a <literal>DESTINATION</literal>
4582 field unless the match rule specifically
4584 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4585 by specifying <literal>eavesdrop='true'</literal>
4586 in the match rule. <literal>eavesdrop='false'</literal>
4587 restores the default behaviour. Messages are
4588 delivered to their <literal>DESTINATION</literal>
4589 regardless of match rules, so this match does not
4590 affect normal delivery of unicast messages.
4591 If the message bus has a security policy which forbids
4592 eavesdropping, this match may still be used without error,
4593 but will not have any practical effect.
4594 In older versions of D-Bus, this match was not allowed
4595 in match rules, and all match rules behaved as if
4596 <literal>eavesdrop='true'</literal> had been used.
4605 <sect2 id="message-bus-starting-services">
4606 <title>Message Bus Starting Services</title>
4608 The message bus can start applications on behalf of other applications.
4609 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4610 An application that can be started in this way is called a
4611 <firstterm>service</firstterm>.
4614 With D-Bus, starting a service is normally done by name. That is,
4615 applications ask the message bus to start some program that will own a
4616 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4617 This implies a contract documented along with the name
4618 <literal>org.freedesktop.TextEditor</literal> for which objects
4619 the owner of that name will provide, and what interfaces those
4623 To find an executable corresponding to a particular name, the bus daemon
4624 looks for <firstterm>service description files</firstterm>. Service
4625 description files define a mapping from names to executables. Different
4626 kinds of message bus will look for these files in different places, see
4627 <xref linkend="message-bus-types"/>.
4630 Service description files have the ".service" file
4631 extension. The message bus will only load service description files
4632 ending with .service; all other files will be ignored. The file format
4633 is similar to that of <ulink
4634 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4635 entries</ulink>. All service description files must be in UTF-8
4636 encoding. To ensure that there will be no name collisions, service files
4637 must be namespaced using the same mechanism as messages and service
4642 [FIXME the file format should be much better specified than "similar to
4643 .desktop entries" esp. since desktop entries are already
4644 badly-specified. ;-)]
4645 These sections from the specification apply to service files as well:
4648 <listitem><para>General syntax</para></listitem>
4649 <listitem><para>Comment format</para></listitem>
4653 <title>Example service description file</title>
4655 # Sample service description file
4657 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4658 Exec=/usr/libexec/gconfd-2
4663 When an application asks to start a service by name, the bus daemon tries to
4664 find a service that will own that name. It then tries to spawn the
4665 executable associated with it. If this fails, it will report an
4666 error. [FIXME what happens if two .service files offer the same service;
4667 what kind of error is reported, should we have a way for the client to
4671 The executable launched will have the environment variable
4672 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4673 message bus so it can connect and request the appropriate names.
4676 The executable being launched may want to know whether the message bus
4677 starting it is one of the well-known message buses (see <xref
4678 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4679 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4680 of the well-known buses. The currently-defined values for this variable
4681 are <literal>system</literal> for the systemwide message bus,
4682 and <literal>session</literal> for the per-login-session message
4683 bus. The new executable must still connect to the address given
4684 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4685 resulting connection is to the well-known bus.
4688 [FIXME there should be a timeout somewhere, either specified
4689 in the .service file, by the client, or just a global value
4690 and if the client being activated fails to connect within that
4691 timeout, an error should be sent back.]
4694 <sect3 id="message-bus-starting-services-scope">
4695 <title>Message Bus Service Scope</title>
4697 The "scope" of a service is its "per-", such as per-session,
4698 per-machine, per-home-directory, or per-display. The reference
4699 implementation doesn't yet support starting services in a different
4700 scope from the message bus itself. So e.g. if you start a service
4701 on the session bus its scope is per-session.
4704 We could add an optional scope to a bus name. For example, for
4705 per-(display,session pair), we could have a unique ID for each display
4706 generated automatically at login and set on screen 0 by executing a
4707 special "set display ID" binary. The ID would be stored in a
4708 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4709 random bytes. This ID would then be used to scope names.
4710 Starting/locating a service could be done by ID-name pair rather than
4714 Contrast this with a per-display scope. To achieve that, we would
4715 want a single bus spanning all sessions using a given display.
4716 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4717 property on screen 0 of the display, pointing to this bus.
4722 <sect2 id="message-bus-types">
4723 <title>Well-known Message Bus Instances</title>
4725 Two standard message bus instances are defined here, along with how
4726 to locate them and where their service files live.
4728 <sect3 id="message-bus-types-login">
4729 <title>Login session message bus</title>
4731 Each time a user logs in, a <firstterm>login session message
4732 bus</firstterm> may be started. All applications in the user's login
4733 session may interact with one another using this message bus.
4736 The address of the login session message bus is given
4737 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4738 variable. If that variable is not set, applications may
4739 also try to read the address from the X Window System root
4740 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4741 The root window property must have type <literal>STRING</literal>.
4742 The environment variable should have precedence over the
4743 root window property.
4745 <para>The address of the login session message bus is given in the
4746 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4747 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4748 "autolaunch:", the system should use platform-specific methods of
4749 locating a running D-Bus session server, or starting one if a running
4750 instance cannot be found. Note that this mechanism is not recommended
4751 for attempting to determine if a daemon is running. It is inherently
4752 racy to attempt to make this determination, since the bus daemon may
4753 be started just before or just after the determination is made.
4754 Therefore, it is recommended that applications do not try to make this
4755 determination for their functionality purposes, and instead they
4756 should attempt to start the server.</para>
4758 <sect4 id="message-bus-types-login-x-windows">
4759 <title>X Windowing System</title>
4761 For the X Windowing System, the application must locate the
4762 window owner of the selection represented by the atom formed by
4766 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4770 <para>the current user's username</para>
4774 <para>the literal character '_' (underscore)</para>
4778 <para>the machine's ID</para>
4784 The following properties are defined for the window that owns
4786 <informaltable frame="all">
4795 <para>meaning</para>
4801 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4805 <para>the actual address of the server socket</para>
4811 <para>_DBUS_SESSION_BUS_PID</para>
4815 <para>the PID of the server process</para>
4824 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4825 present in this window.
4829 If the X selection cannot be located or if reading the
4830 properties from the window fails, the implementation MUST conclude
4831 that there is no D-Bus server running and proceed to start a new
4832 server. (See below on concurrency issues)
4836 Failure to connect to the D-Bus server address thus obtained
4837 MUST be treated as a fatal connection error and should be reported
4842 As an alternative, an implementation MAY find the information
4843 in the following file located in the current user's home directory,
4844 in subdirectory .dbus/session-bus/:
4847 <para>the machine's ID</para>
4851 <para>the literal character '-' (dash)</para>
4855 <para>the X display without the screen number, with the
4856 following prefixes removed, if present: ":", "localhost:"
4857 ."localhost.localdomain:". That is, a display of
4858 "localhost:10.0" produces just the number "10"</para>
4864 The contents of this file NAME=value assignment pairs and
4865 lines starting with # are comments (no comments are allowed
4866 otherwise). The following variable names are defined:
4873 <para>Variable</para>
4877 <para>meaning</para>
4883 <para>DBUS_SESSION_BUS_ADDRESS</para>
4887 <para>the actual address of the server socket</para>
4893 <para>DBUS_SESSION_BUS_PID</para>
4897 <para>the PID of the server process</para>
4903 <para>DBUS_SESSION_BUS_WINDOWID</para>
4907 <para>the window ID</para>
4916 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4921 Failure to open this file MUST be interpreted as absence of a
4922 running server. Therefore, the implementation MUST proceed to
4923 attempting to launch a new bus server if the file cannot be
4928 However, success in opening this file MUST NOT lead to the
4929 conclusion that the server is running. Thus, a failure to connect to
4930 the bus address obtained by the alternative method MUST NOT be
4931 considered a fatal error. If the connection cannot be established,
4932 the implementation MUST proceed to check the X selection settings or
4933 to start the server on its own.
4937 If the implementation concludes that the D-Bus server is not
4938 running it MUST attempt to start a new server and it MUST also
4939 ensure that the daemon started as an effect of the "autolaunch"
4940 mechanism provides the lookup mechanisms described above, so
4941 subsequent calls can locate the newly started server. The
4942 implementation MUST also ensure that if two or more concurrent
4943 initiations happen, only one server remains running and all other
4944 initiations are able to obtain the address of this server and
4945 connect to it. In other words, the implementation MUST ensure that
4946 the X selection is not present when it attempts to set it, without
4947 allowing another process to set the selection between the
4948 verification and the setting (e.g., by using XGrabServer /
4955 On Unix systems, the session bus should search for .service files
4956 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4958 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4959 Implementations may also search additional locations, which
4960 should be searched with lower priority than anything in
4961 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4962 for example, the reference implementation also
4963 looks in <literal>${datadir}/dbus-1/services</literal> as
4964 set at compile time.
4967 As described in the XDG Base Directory Specification, software
4968 packages should install their session .service files to their
4969 configured <literal>${datadir}/dbus-1/services</literal>,
4970 where <literal>${datadir}</literal> is as defined by the GNU
4971 coding standards. System administrators or users can arrange
4972 for these service files to be read by setting XDG_DATA_DIRS or by
4973 symlinking them into the default locations.
4977 <sect3 id="message-bus-types-system">
4978 <title>System message bus</title>
4980 A computer may have a <firstterm>system message bus</firstterm>,
4981 accessible to all applications on the system. This message bus may be
4982 used to broadcast system events, such as adding new hardware devices,
4983 changes in the printer queue, and so forth.
4986 The address of the system message bus is given
4987 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4988 variable. If that variable is not set, applications should try
4989 to connect to the well-known address
4990 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4993 The D-Bus reference implementation actually honors the
4994 <literal>$(localstatedir)</literal> configure option
4995 for this address, on both client and server side.
5000 On Unix systems, the system bus should default to searching
5001 for .service files in
5002 <literal>/usr/local/share/dbus-1/system-services</literal>,
5003 <literal>/usr/share/dbus-1/system-services</literal> and
5004 <literal>/lib/dbus-1/system-services</literal>, with that order
5005 of precedence. It may also search other implementation-specific
5006 locations, but should not vary these locations based on environment
5010 The system bus is security-sensitive and is typically executed
5011 by an init system with a clean environment. Its launch helper
5012 process is particularly security-sensitive, and specifically
5013 clears its own environment.
5018 Software packages should install their system .service
5019 files to their configured
5020 <literal>${datadir}/dbus-1/system-services</literal>,
5021 where <literal>${datadir}</literal> is as defined by the GNU
5022 coding standards. System administrators can arrange
5023 for these service files to be read by editing the system bus'
5024 configuration file or by symlinking them into the default
5030 <sect2 id="message-bus-messages">
5031 <title>Message Bus Messages</title>
5033 The special message bus name <literal>org.freedesktop.DBus</literal>
5034 responds to a number of additional messages.
5037 <sect3 id="bus-messages-hello">
5038 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5049 <entry>Argument</entry>
5051 <entry>Description</entry>
5057 <entry>STRING</entry>
5058 <entry>Unique name assigned to the connection</entry>
5065 Before an application is able to send messages to other applications
5066 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5067 to the message bus to obtain a unique name. If an application without
5068 a unique name tries to send a message to another application, or a
5069 message to the message bus itself that isn't the
5070 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5071 disconnected from the bus.
5074 There is no corresponding "disconnect" request; if a client wishes to
5075 disconnect from the bus, it simply closes the socket (or other
5076 communication channel).
5079 <sect3 id="bus-messages-list-names">
5080 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5084 ARRAY of STRING ListNames ()
5091 <entry>Argument</entry>
5093 <entry>Description</entry>
5099 <entry>ARRAY of STRING</entry>
5100 <entry>Array of strings where each string is a bus name</entry>
5107 Returns a list of all currently-owned names on the bus.
5110 <sect3 id="bus-messages-list-activatable-names">
5111 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5115 ARRAY of STRING ListActivatableNames ()
5122 <entry>Argument</entry>
5124 <entry>Description</entry>
5130 <entry>ARRAY of STRING</entry>
5131 <entry>Array of strings where each string is a bus name</entry>
5138 Returns a list of all names that can be activated on the bus.
5141 <sect3 id="bus-messages-name-exists">
5142 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5146 BOOLEAN NameHasOwner (in STRING name)
5153 <entry>Argument</entry>
5155 <entry>Description</entry>
5161 <entry>STRING</entry>
5162 <entry>Name to check</entry>
5172 <entry>Argument</entry>
5174 <entry>Description</entry>
5180 <entry>BOOLEAN</entry>
5181 <entry>Return value, true if the name exists</entry>
5188 Checks if the specified name exists (currently has an owner).
5192 <sect3 id="bus-messages-name-owner-changed">
5193 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5197 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5204 <entry>Argument</entry>
5206 <entry>Description</entry>
5212 <entry>STRING</entry>
5213 <entry>Name with a new owner</entry>
5217 <entry>STRING</entry>
5218 <entry>Old owner or empty string if none</entry>
5222 <entry>STRING</entry>
5223 <entry>New owner or empty string if none</entry>
5230 This signal indicates that the owner of a name has changed.
5231 It's also the signal to use to detect the appearance of
5232 new names on the bus.
5235 <sect3 id="bus-messages-name-lost">
5236 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5240 NameLost (STRING name)
5247 <entry>Argument</entry>
5249 <entry>Description</entry>
5255 <entry>STRING</entry>
5256 <entry>Name which was lost</entry>
5263 This signal is sent to a specific application when it loses
5264 ownership of a name.
5268 <sect3 id="bus-messages-name-acquired">
5269 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5273 NameAcquired (STRING name)
5280 <entry>Argument</entry>
5282 <entry>Description</entry>
5288 <entry>STRING</entry>
5289 <entry>Name which was acquired</entry>
5296 This signal is sent to a specific application when it gains
5297 ownership of a name.
5301 <sect3 id="bus-messages-start-service-by-name">
5302 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5306 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5313 <entry>Argument</entry>
5315 <entry>Description</entry>
5321 <entry>STRING</entry>
5322 <entry>Name of the service to start</entry>
5326 <entry>UINT32</entry>
5327 <entry>Flags (currently not used)</entry>
5337 <entry>Argument</entry>
5339 <entry>Description</entry>
5345 <entry>UINT32</entry>
5346 <entry>Return value</entry>
5351 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5355 The return value can be one of the following values:
5360 <entry>Identifier</entry>
5361 <entry>Value</entry>
5362 <entry>Description</entry>
5367 <entry>DBUS_START_REPLY_SUCCESS</entry>
5369 <entry>The service was successfully started.</entry>
5372 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5374 <entry>A connection already owns the given name.</entry>
5383 <sect3 id="bus-messages-update-activation-environment">
5384 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5388 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5395 <entry>Argument</entry>
5397 <entry>Description</entry>
5403 <entry>ARRAY of DICT<STRING,STRING></entry>
5404 <entry>Environment to add or update</entry>
5409 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5412 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5415 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.
5420 <sect3 id="bus-messages-get-name-owner">
5421 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5425 STRING GetNameOwner (in STRING name)
5432 <entry>Argument</entry>
5434 <entry>Description</entry>
5440 <entry>STRING</entry>
5441 <entry>Name to get the owner of</entry>
5451 <entry>Argument</entry>
5453 <entry>Description</entry>
5459 <entry>STRING</entry>
5460 <entry>Return value, a unique connection name</entry>
5465 Returns the unique connection name of the primary owner of the name
5466 given. If the requested name doesn't have an owner, returns a
5467 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5471 <sect3 id="bus-messages-get-connection-unix-user">
5472 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5476 UINT32 GetConnectionUnixUser (in STRING bus_name)
5483 <entry>Argument</entry>
5485 <entry>Description</entry>
5491 <entry>STRING</entry>
5492 <entry>Unique or well-known bus name of the connection to
5493 query, such as <literal>:12.34</literal> or
5494 <literal>com.example.tea</literal></entry>
5504 <entry>Argument</entry>
5506 <entry>Description</entry>
5512 <entry>UINT32</entry>
5513 <entry>Unix user ID</entry>
5518 Returns the Unix user ID of the process connected to the server. If
5519 unable to determine it (for instance, because the process is not on the
5520 same machine as the bus daemon), an error is returned.
5524 <sect3 id="bus-messages-get-connection-unix-process-id">
5525 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5529 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5536 <entry>Argument</entry>
5538 <entry>Description</entry>
5544 <entry>STRING</entry>
5545 <entry>Unique or well-known bus name of the connection to
5546 query, such as <literal>:12.34</literal> or
5547 <literal>com.example.tea</literal></entry>
5557 <entry>Argument</entry>
5559 <entry>Description</entry>
5565 <entry>UINT32</entry>
5566 <entry>Unix process id</entry>
5571 Returns the Unix process ID of the process connected to the server. If
5572 unable to determine it (for instance, because the process is not on the
5573 same machine as the bus daemon), an error is returned.
5577 <sect3 id="bus-messages-add-match">
5578 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5582 AddMatch (in STRING rule)
5589 <entry>Argument</entry>
5591 <entry>Description</entry>
5597 <entry>STRING</entry>
5598 <entry>Match rule to add to the connection</entry>
5603 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5604 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5608 <sect3 id="bus-messages-remove-match">
5609 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5613 RemoveMatch (in STRING rule)
5620 <entry>Argument</entry>
5622 <entry>Description</entry>
5628 <entry>STRING</entry>
5629 <entry>Match rule to remove from the connection</entry>
5634 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5635 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5640 <sect3 id="bus-messages-get-id">
5641 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5645 GetId (out STRING id)
5652 <entry>Argument</entry>
5654 <entry>Description</entry>
5660 <entry>STRING</entry>
5661 <entry>Unique ID identifying the bus daemon</entry>
5666 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5667 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5668 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5669 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5670 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5671 by org.freedesktop.DBus.Peer.GetMachineId().
5672 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5680 <appendix id="implementation-notes">
5681 <title>Implementation notes</title>
5682 <sect1 id="implementation-notes-subsection">
5690 <glossary><title>Glossary</title>
5692 This glossary defines some of the terms used in this specification.
5695 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5698 The message bus maintains an association between names and
5699 connections. (Normally, there's one connection per application.) A
5700 bus name is simply an identifier used to locate connections. For
5701 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5702 name might be used to send a message to a screensaver from Yoyodyne
5703 Corporation. An application is said to <firstterm>own</firstterm> a
5704 name if the message bus has associated the application's connection
5705 with the name. Names may also have <firstterm>queued
5706 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5707 The bus assigns a unique name to each connection,
5708 see <xref linkend="term-unique-name"/>. Other names
5709 can be thought of as "well-known names" and are
5710 used to find applications that offer specific functionality.
5714 See <xref linkend="message-protocol-names-bus"/> for details of
5715 the syntax and naming conventions for bus names.
5720 <glossentry id="term-message"><glossterm>Message</glossterm>
5723 A message is the atomic unit of communication via the D-Bus
5724 protocol. It consists of a <firstterm>header</firstterm> and a
5725 <firstterm>body</firstterm>; the body is made up of
5726 <firstterm>arguments</firstterm>.
5731 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5734 The message bus is a special application that forwards
5735 or routes messages between a group of applications
5736 connected to the message bus. It also manages
5737 <firstterm>names</firstterm> used for routing
5743 <glossentry id="term-name"><glossterm>Name</glossterm>
5746 See <xref linkend="term-bus-name"/>. "Name" may
5747 also be used to refer to some of the other names
5748 in D-Bus, such as interface names.
5753 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5756 Used to prevent collisions when defining new interfaces, bus names
5757 etc. The convention used is the same one Java uses for defining
5758 classes: a reversed domain name.
5759 See <xref linkend="message-protocol-names-bus"/>,
5760 <xref linkend="message-protocol-names-interface"/>,
5761 <xref linkend="message-protocol-names-error"/>,
5762 <xref linkend="message-protocol-marshaling-object-path"/>.
5767 <glossentry id="term-object"><glossterm>Object</glossterm>
5770 Each application contains <firstterm>objects</firstterm>, which have
5771 <firstterm>interfaces</firstterm> and
5772 <firstterm>methods</firstterm>. Objects are referred to by a name,
5773 called a <firstterm>path</firstterm>.
5778 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5781 An application talking directly to another application, without going
5782 through a message bus. One-to-one connections may be "peer to peer" or
5783 "client to server." The D-Bus protocol has no concept of client
5784 vs. server after a connection has authenticated; the flow of messages
5785 is symmetrical (full duplex).
5790 <glossentry id="term-path"><glossterm>Path</glossterm>
5793 Object references (object names) in D-Bus are organized into a
5794 filesystem-style hierarchy, so each object is named by a path. As in
5795 LDAP, there's no difference between "files" and "directories"; a path
5796 can refer to an object, while still having child objects below it.
5801 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5804 Each bus name has a primary owner; messages sent to the name go to the
5805 primary owner. However, certain names also maintain a queue of
5806 secondary owners "waiting in the wings." If the primary owner releases
5807 the name, then the first secondary owner in the queue automatically
5808 becomes the new owner of the name.
5813 <glossentry id="term-service"><glossterm>Service</glossterm>
5816 A service is an executable that can be launched by the bus daemon.
5817 Services normally guarantee some particular features, for example they
5818 may guarantee that they will request a specific name such as
5819 "org.freedesktop.Screensaver", have a singleton object
5820 "/org/freedesktop/Application", and that object will implement the
5821 interface "org.freedesktop.ScreensaverControl".
5826 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5829 ".service files" tell the bus about service applications that can be
5830 launched (see <xref linkend="term-service"/>). Most importantly they
5831 provide a mapping from bus names to services that will request those
5832 names when they start up.
5837 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5840 The special name automatically assigned to each connection by the
5841 message bus. This name will never change owner, and will be unique
5842 (never reused during the lifetime of the message bus).
5843 It will begin with a ':' character.