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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.21</releaseinfo>
10 <date>(not yet released)</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>not yet released (<ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>)</date>
77 <authorinitials></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.20</revnumber>
82 <date>22 February 2013</date>
83 <authorinitials>smcv, walters</authorinitials>
84 <revremark>reorganise for clarity, remove false claims about
85 basic types, mention /o/fd/DBus</revremark>
88 <revnumber>0.19</revnumber>
89 <date>20 February 2012</date>
90 <authorinitials>smcv/lp</authorinitials>
91 <revremark>formally define unique connection names and well-known
92 bus names; document best practices for interface, bus, member and
93 error names, and object paths; document the search path for session
94 and system services on Unix; document the systemd transport</revremark>
97 <revnumber>0.18</revnumber>
98 <date>29 July 2011</date>
99 <authorinitials>smcv</authorinitials>
100 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
101 match keyword; promote type system to a top-level section</revremark>
104 <revnumber>0.17</revnumber>
105 <date>1 June 2011</date>
106 <authorinitials>smcv/davidz</authorinitials>
107 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
108 by GVariant</revremark>
111 <revnumber>0.16</revnumber>
112 <date>11 April 2011</date>
113 <authorinitials></authorinitials>
114 <revremark>add path_namespace, arg0namespace; argNpath matches object
118 <revnumber>0.15</revnumber>
119 <date>3 November 2010</date>
120 <authorinitials></authorinitials>
121 <revremark></revremark>
124 <revnumber>0.14</revnumber>
125 <date>12 May 2010</date>
126 <authorinitials></authorinitials>
127 <revremark></revremark>
130 <revnumber>0.13</revnumber>
131 <date>23 Dezember 2009</date>
132 <authorinitials></authorinitials>
133 <revremark></revremark>
136 <revnumber>0.12</revnumber>
137 <date>7 November, 2006</date>
138 <authorinitials></authorinitials>
139 <revremark></revremark>
142 <revnumber>0.11</revnumber>
143 <date>6 February 2005</date>
144 <authorinitials></authorinitials>
145 <revremark></revremark>
148 <revnumber>0.10</revnumber>
149 <date>28 January 2005</date>
150 <authorinitials></authorinitials>
151 <revremark></revremark>
154 <revnumber>0.9</revnumber>
155 <date>7 Januar 2005</date>
156 <authorinitials></authorinitials>
157 <revremark></revremark>
160 <revnumber>0.8</revnumber>
161 <date>06 September 2003</date>
162 <authorinitials></authorinitials>
163 <revremark>First released document.</revremark>
168 <sect1 id="introduction">
169 <title>Introduction</title>
171 D-Bus is a system for low-latency, low-overhead, easy to use
172 interprocess communication (IPC). In more detail:
176 D-Bus is <emphasis>low-latency</emphasis> because it is designed
177 to avoid round trips and allow asynchronous operation, much like
183 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
184 binary protocol, and does not have to convert to and from a text
185 format such as XML. Because D-Bus is intended for potentially
186 high-resolution same-machine IPC, not primarily for Internet IPC,
187 this is an interesting optimization.
192 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
193 of <firstterm>messages</firstterm> rather than byte streams, and
194 automatically handles a lot of the hard IPC issues. Also, the D-Bus
195 library is designed to be wrapped in a way that lets developers use
196 their framework's existing object/type system, rather than learning
197 a new one specifically for IPC.
204 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
205 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
206 a system for one application to talk to a single other
207 application. However, the primary intended application of the protocol is the
208 D-Bus <firstterm>message bus</firstterm>, specified in <xref
209 linkend="message-bus"/>. The message bus is a special application that
210 accepts connections from multiple other applications, and forwards
215 Uses of D-Bus include notification of system changes (notification of when
216 a camera is plugged in to a computer, or a new version of some software
217 has been installed), or desktop interoperability, for example a file
218 monitoring service or a configuration service.
222 D-Bus is designed for two specific use cases:
226 A "system bus" for notifications from the system to user sessions,
227 and to allow the system to request input from user sessions.
232 A "session bus" used to implement desktop environments such as
237 D-Bus is not intended to be a generic IPC system for any possible
238 application, and intentionally omits many features found in other
239 IPC systems for this reason.
243 At the same time, the bus daemons offer a number of features not found in
244 other IPC systems, such as single-owner "bus names" (similar to X
245 selections), on-demand startup of services, and security policies.
246 In many ways, these features are the primary motivation for developing
247 D-Bus; other systems would have sufficed if IPC were the only goal.
251 D-Bus may turn out to be useful in unanticipated applications, but future
252 versions of this spec and the reference implementation probably will not
253 incorporate features that interfere with the core use cases.
257 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
258 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
259 document are to be interpreted as described in RFC 2119. However, the
260 document could use a serious audit to be sure it makes sense to do
261 so. Also, they are not capitalized.
264 <sect2 id="stability">
265 <title>Protocol and Specification Stability</title>
267 The D-Bus protocol is frozen (only compatible extensions are allowed) as
268 of November 8, 2006. However, this specification could still use a fair
269 bit of work to make interoperable reimplementation possible without
270 reference to the D-Bus reference implementation. Thus, this
271 specification is not marked 1.0. To mark it 1.0, we'd like to see
272 someone invest significant effort in clarifying the specification
273 language, and growing the specification to cover more aspects of the
274 reference implementation's behavior.
277 Until this work is complete, any attempt to reimplement D-Bus will
278 probably require looking at the reference implementation and/or asking
279 questions on the D-Bus mailing list about intended behavior.
280 Questions on the list are very welcome.
283 Nonetheless, this document should be a useful starting point and is
284 to our knowledge accurate, though incomplete.
290 <sect1 id="type-system">
291 <title>Type System</title>
294 D-Bus has a type system, in which values of various types can be
295 serialized into a sequence of bytes referred to as the
296 <firstterm>wire format</firstterm> in a standard way.
297 Converting a value from some other representation into the wire
298 format is called <firstterm>marshaling</firstterm> and converting
299 it back from the wire format is <firstterm>unmarshaling</firstterm>.
303 The D-Bus protocol does not include type tags in the marshaled data; a
304 block of marshaled values must have a known <firstterm>type
305 signature</firstterm>. The type signature is made up of zero or more
306 <firstterm id="term-single-complete-type">single complete
307 types</firstterm>, each made up of one or more
308 <firstterm>type codes</firstterm>.
312 A type code is an ASCII character representing the
313 type of a value. Because ASCII characters are used, the type signature
314 will always form a valid ASCII string. A simple string compare
315 determines whether two type signatures are equivalent.
319 A single complete type is a sequence of type codes that fully describes
320 one type: either a basic type, or a single fully-described container type.
321 A single complete type is a basic type code, a variant type code,
322 an array with its element type, or a struct with its fields (all of which
323 are defined below). So the following signatures are not single complete
334 And the following signatures contain multiple complete types:
344 Note however that a single complete type may <emphasis>contain</emphasis>
345 multiple other single complete types, by containing a struct or dict
349 <sect2 id="basic-types">
350 <title>Basic types</title>
353 The simplest type codes are the <firstterm id="term-basic-type">basic
354 types</firstterm>, which are the types whose structure is entirely
355 defined by their 1-character type code. Basic types consist of
356 fixed types and string-like types.
360 The <firstterm id="term-fixed-type">fixed types</firstterm>
361 are basic types whose values have a fixed length, namely BYTE,
362 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
367 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
368 the ASCII character 'i'. So the signature for a block of values
369 containing a single <literal>INT32</literal> would be:
373 A block of values containing two <literal>INT32</literal> would have this signature:
380 The characteristics of the fixed types are listed in this table.
386 <entry>Conventional name</entry>
387 <entry>ASCII type-code</entry>
388 <entry>Encoding</entry>
393 <entry><literal>BYTE</literal></entry>
394 <entry><literal>y</literal> (121)</entry>
395 <entry>Unsigned 8-bit integer</entry>
398 <entry><literal>BOOLEAN</literal></entry>
399 <entry><literal>b</literal> (98)</entry>
400 <entry>Boolean value: 0 is false, 1 is true, any other value
401 allowed by the marshalling format is invalid</entry>
404 <entry><literal>INT16</literal></entry>
405 <entry><literal>n</literal> (110)</entry>
406 <entry>Signed (two's complement) 16-bit integer</entry>
409 <entry><literal>UINT16</literal></entry>
410 <entry><literal>q</literal> (113)</entry>
411 <entry>Unsigned 16-bit integer</entry>
414 <entry><literal>INT32</literal></entry>
415 <entry><literal>i</literal> (105)</entry>
416 <entry>Signed (two's complement) 32-bit integer</entry>
419 <entry><literal>UINT32</literal></entry>
420 <entry><literal>u</literal> (117)</entry>
421 <entry>Unsigned 32-bit integer</entry>
424 <entry><literal>INT64</literal></entry>
425 <entry><literal>x</literal> (120)</entry>
426 <entry>Signed (two's complement) 64-bit integer
427 (mnemonic: x and t are the first characters in "sixty" not
428 already used for something more common)</entry>
431 <entry><literal>UINT64</literal></entry>
432 <entry><literal>t</literal> (116)</entry>
433 <entry>Unsigned 64-bit integer</entry>
436 <entry><literal>DOUBLE</literal></entry>
437 <entry><literal>d</literal> (100)</entry>
438 <entry>IEEE 754 double-precision floating point</entry>
441 <entry><literal>UNIX_FD</literal></entry>
442 <entry><literal>h</literal> (104)</entry>
443 <entry>Unsigned 32-bit integer representing an index into an
444 out-of-band array of file descriptors, transferred via some
445 platform-specific mechanism (mnemonic: h for handle)</entry>
453 The <firstterm id="term-string-like-type">string-like types</firstterm>
454 are basic types with a variable length. The value of any string-like
455 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
456 none of which may be U+0000. The UTF-8 text must be validated
457 strictly: in particular, it must not contain overlong sequences
458 or codepoints above U+10FFFF.
462 Since D-Bus Specification version 0.21, in accordance with Unicode
463 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
464 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
465 D-Bus rejected these noncharacters).
469 The marshalling formats for the string-like types all end with a
470 single zero (NUL) byte, but that byte is not considered to be part of
475 The characteristics of the string-like types are listed in this table.
481 <entry>Conventional name</entry>
482 <entry>ASCII type-code</entry>
483 <entry>Validity constraints</entry>
488 <entry><literal>STRING</literal></entry>
489 <entry><literal>s</literal> (115)</entry>
490 <entry>No extra constraints</entry>
493 <entry><literal>OBJECT_PATH</literal></entry>
494 <entry><literal>o</literal> (111)</entry>
496 <link linkend="message-protocol-marshaling-object-path">a
497 syntactically valid object path</link></entry>
500 <entry><literal>SIGNATURE</literal></entry>
501 <entry><literal>g</literal> (103)</entry>
503 <firstterm linkend="term-single-complete-type">single
504 complete types</firstterm></entry>
511 <sect3 id="message-protocol-marshaling-object-path">
512 <title>Valid Object Paths</title>
515 An object path is a name used to refer to an object instance.
516 Conceptually, each participant in a D-Bus message exchange may have
517 any number of object instances (think of C++ or Java objects) and each
518 such instance will have a path. Like a filesystem, the object
519 instances in an application form a hierarchical tree.
523 Object paths are often namespaced by starting with a reversed
524 domain name and containing an interface version number, in the
526 <link linkend="message-protocol-names-interface">interface
528 <link linkend="message-protocol-names-bus">well-known
530 This makes it possible to implement more than one service, or
531 more than one version of a service, in the same process,
532 even if the services share a connection but cannot otherwise
533 co-operate (for instance, if they are implemented by different
538 For instance, if the owner of <literal>example.com</literal> is
539 developing a D-Bus API for a music player, they might use the
540 hierarchy of object paths that start with
541 <literal>/com/example/MusicPlayer1</literal> for its objects.
545 The following rules define a valid object path. Implementations must
546 not send or accept messages with invalid object paths.
550 The path may be of any length.
555 The path must begin with an ASCII '/' (integer 47) character,
556 and must consist of elements separated by slash characters.
561 Each element must only contain the ASCII characters
567 No element may be the empty string.
572 Multiple '/' characters cannot occur in sequence.
577 A trailing '/' character is not allowed unless the
578 path is the root path (a single '/' character).
586 <sect3 id="message-protocol-marshaling-signature">
587 <title>Valid Signatures</title>
589 An implementation must not send or accept invalid signatures.
590 Valid signatures will conform to the following rules:
594 The signature is a list of single complete types.
595 Arrays must have element types, and structs must
596 have both open and close parentheses.
601 Only type codes, open and close parentheses, and open and
602 close curly brackets are allowed in the signature. The
603 <literal>STRUCT</literal> type code
604 is not allowed in signatures, because parentheses
605 are used instead. Similarly, the
606 <literal>DICT_ENTRY</literal> type code is not allowed in
607 signatures, because curly brackets are used instead.
612 The maximum depth of container type nesting is 32 array type
613 codes and 32 open parentheses. This implies that the maximum
614 total depth of recursion is 64, for an "array of array of array
615 of ... struct of struct of struct of ..." where there are 32
621 The maximum length of a signature is 255.
628 When signatures appear in messages, the marshalling format
629 guarantees that they will be followed by a nul byte (which can
630 be interpreted as either C-style string termination or the INVALID
631 type-code), but this is not conceptually part of the signature.
637 <sect2 id="container-types">
638 <title>Container types</title>
641 In addition to basic types, there are four <firstterm>container</firstterm>
642 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
643 and <literal>DICT_ENTRY</literal>.
647 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
648 code does not appear in signatures. Instead, ASCII characters
649 '(' and ')' are used to mark the beginning and end of the struct.
650 So for example, a struct containing two integers would have this
655 Structs can be nested, so for example a struct containing
656 an integer and another struct:
660 The value block storing that struct would contain three integers; the
661 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
666 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
667 but is useful in code that implements the protocol. This type code
668 is specified to allow such code to interoperate in non-protocol contexts.
672 Empty structures are not allowed; there must be at least one
673 type code between the parentheses.
677 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
678 followed by a <firstterm>single complete type</firstterm>. The single
679 complete type following the array is the type of each array element. So
680 the simple example is:
684 which is an array of 32-bit integers. But an array can be of any type,
685 such as this array-of-struct-with-two-int32-fields:
689 Or this array of array of integer:
696 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
697 type <literal>VARIANT</literal> will have the signature of a single complete type as part
698 of the <emphasis>value</emphasis>. This signature will be followed by a
699 marshaled value of that type.
703 Unlike a message signature, the variant signature can
704 contain only a single complete type. So "i", "ai"
705 or "(ii)" is OK, but "ii" is not. Use of variants may not
706 cause a total message depth to be larger than 64, including
707 other container types such as structures.
711 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
712 than parentheses it uses curly braces, and it has more restrictions.
713 The restrictions are: it occurs only as an array element type; it has
714 exactly two single complete types inside the curly braces; the first
715 single complete type (the "key") must be a basic type rather than a
716 container type. Implementations must not accept dict entries outside of
717 arrays, must not accept dict entries with zero, one, or more than two
718 fields, and must not accept dict entries with non-basic-typed keys. A
719 dict entry is always a key-value pair.
723 The first field in the <literal>DICT_ENTRY</literal> is always the key.
724 A message is considered corrupt if the same key occurs twice in the same
725 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
726 implementations are not required to reject dicts with duplicate keys.
730 In most languages, an array of dict entry would be represented as a
731 map, hash table, or dict object.
736 <title>Summary of types</title>
739 The following table summarizes the D-Bus types.
744 <entry>Conventional Name</entry>
746 <entry>Description</entry>
751 <entry><literal>INVALID</literal></entry>
752 <entry>0 (ASCII NUL)</entry>
753 <entry>Not a valid type code, used to terminate signatures</entry>
755 <entry><literal>BYTE</literal></entry>
756 <entry>121 (ASCII 'y')</entry>
757 <entry>8-bit unsigned integer</entry>
759 <entry><literal>BOOLEAN</literal></entry>
760 <entry>98 (ASCII 'b')</entry>
761 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
763 <entry><literal>INT16</literal></entry>
764 <entry>110 (ASCII 'n')</entry>
765 <entry>16-bit signed integer</entry>
767 <entry><literal>UINT16</literal></entry>
768 <entry>113 (ASCII 'q')</entry>
769 <entry>16-bit unsigned integer</entry>
771 <entry><literal>INT32</literal></entry>
772 <entry>105 (ASCII 'i')</entry>
773 <entry>32-bit signed integer</entry>
775 <entry><literal>UINT32</literal></entry>
776 <entry>117 (ASCII 'u')</entry>
777 <entry>32-bit unsigned integer</entry>
779 <entry><literal>INT64</literal></entry>
780 <entry>120 (ASCII 'x')</entry>
781 <entry>64-bit signed integer</entry>
783 <entry><literal>UINT64</literal></entry>
784 <entry>116 (ASCII 't')</entry>
785 <entry>64-bit unsigned integer</entry>
787 <entry><literal>DOUBLE</literal></entry>
788 <entry>100 (ASCII 'd')</entry>
789 <entry>IEEE 754 double</entry>
791 <entry><literal>STRING</literal></entry>
792 <entry>115 (ASCII 's')</entry>
793 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
795 <entry><literal>OBJECT_PATH</literal></entry>
796 <entry>111 (ASCII 'o')</entry>
797 <entry>Name of an object instance</entry>
799 <entry><literal>SIGNATURE</literal></entry>
800 <entry>103 (ASCII 'g')</entry>
801 <entry>A type signature</entry>
803 <entry><literal>ARRAY</literal></entry>
804 <entry>97 (ASCII 'a')</entry>
807 <entry><literal>STRUCT</literal></entry>
808 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
809 <entry>Struct; type code 114 'r' is reserved for use in
810 bindings and implementations to represent the general
811 concept of a struct, and must not appear in signatures
812 used on D-Bus.</entry>
814 <entry><literal>VARIANT</literal></entry>
815 <entry>118 (ASCII 'v') </entry>
816 <entry>Variant type (the type of the value is part of the value itself)</entry>
818 <entry><literal>DICT_ENTRY</literal></entry>
819 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
820 <entry>Entry in a dict or map (array of key-value pairs).
821 Type code 101 'e' is reserved for use in bindings and
822 implementations to represent the general concept of a
823 dict or dict-entry, and must not appear in signatures
824 used on D-Bus.</entry>
826 <entry><literal>UNIX_FD</literal></entry>
827 <entry>104 (ASCII 'h')</entry>
828 <entry>Unix file descriptor</entry>
831 <entry>(reserved)</entry>
832 <entry>109 (ASCII 'm')</entry>
833 <entry>Reserved for <ulink
834 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
835 'maybe' type compatible with the one in GVariant</ulink>,
836 and must not appear in signatures used on D-Bus until
837 specified here</entry>
840 <entry>(reserved)</entry>
841 <entry>42 (ASCII '*')</entry>
842 <entry>Reserved for use in bindings/implementations to
843 represent any <firstterm>single complete type</firstterm>,
844 and must not appear in signatures used on D-Bus.</entry>
847 <entry>(reserved)</entry>
848 <entry>63 (ASCII '?')</entry>
849 <entry>Reserved for use in bindings/implementations to
850 represent any <firstterm>basic type</firstterm>, and must
851 not appear in signatures used on D-Bus.</entry>
854 <entry>(reserved)</entry>
855 <entry>64 (ASCII '@'), 38 (ASCII '&'),
856 94 (ASCII '^')</entry>
857 <entry>Reserved for internal use by bindings/implementations,
858 and must not appear in signatures used on D-Bus.
859 GVariant uses these type-codes to encode calling
870 <sect1 id="message-protocol-marshaling">
871 <title>Marshaling (Wire Format)</title>
874 D-Bus defines a marshalling format for its type system, which is
875 used in D-Bus messages. This is not the only possible marshalling
876 format for the type system: for instance, GVariant (part of GLib)
877 re-uses the D-Bus type system but implements an alternative marshalling
882 <title>Byte order and alignment</title>
885 Given a type signature, a block of bytes can be converted into typed
886 values. This section describes the format of the block of bytes. Byte
887 order and alignment issues are handled uniformly for all D-Bus types.
891 A block of bytes has an associated byte order. The byte order
892 has to be discovered in some way; for D-Bus messages, the
893 byte order is part of the message header as described in
894 <xref linkend="message-protocol-messages"/>. For now, assume
895 that the byte order is known to be either little endian or big
900 Each value in a block of bytes is aligned "naturally," for example
901 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
902 8-byte boundary. To properly align a value, <firstterm>alignment
903 padding</firstterm> may be necessary. The alignment padding must always
904 be the minimum required padding to properly align the following value;
905 and it must always be made up of nul bytes. The alignment padding must
906 not be left uninitialized (it can't contain garbage), and more padding
907 than required must not be used.
911 As an exception to natural alignment, <literal>STRUCT</literal> and
912 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
913 boundary, regardless of the alignments of their contents.
918 <title>Marshalling basic types</title>
921 To marshal and unmarshal fixed types, you simply read one value
922 from the data block corresponding to each type code in the signature.
923 All signed integer values are encoded in two's complement, DOUBLE
924 values are IEEE 754 double-precision floating-point, and BOOLEAN
925 values are encoded in 32 bits (of which only the least significant
930 The string-like types are all marshalled as a
931 fixed-length unsigned integer <varname>n</varname> giving the
932 length of the variable part, followed by <varname>n</varname>
933 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
934 which is not considered to be part of the text. The alignment
935 of the string-like type is the same as the alignment of
936 <varname>n</varname>.
940 For the STRING and OBJECT_PATH types, <varname>n</varname> is
941 encoded in 4 bytes, leading to 4-byte alignment.
942 For the SIGNATURE type, <varname>n</varname> is encoded as a single
943 byte. As a result, alignment padding is never required before a
949 <title>Marshalling containers</title>
952 Arrays are marshalled as a <literal>UINT32</literal>
953 <varname>n</varname> giving the length of the array data in bytes,
954 followed by alignment padding to the alignment boundary of the array
955 element type, followed by the <varname>n</varname> bytes of the
956 array elements marshalled in sequence. <varname>n</varname> does not
957 include the padding after the length, or any padding after the
962 For instance, if the current position in the message is a multiple
963 of 8 bytes and the byte-order is big-endian, an array containing only
964 the 64-bit integer 5 would be marshalled as:
967 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
968 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
969 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
974 Arrays have a maximum length defined to be 2 to the 26th power or
975 67108864. Implementations must not send or accept arrays exceeding this
980 Structs and dict entries are marshalled in the same way as their
981 contents, but their alignment is always to an 8-byte boundary,
982 even if their contents would normally be less strictly aligned.
986 Variants are marshalled as the <literal>SIGNATURE</literal> of
987 the contents (which must be a single complete type), followed by a
988 marshalled value with the type given by that signature. The
989 variant has the same 1-byte alignment as the signature, which means
990 that alignment padding before a variant is never needed.
991 Use of variants may not cause a total message depth to be larger
992 than 64, including other container types such as structures.
997 <title>Summary of D-Bus marshalling</title>
1000 Given all this, the types are marshaled on the wire as follows:
1005 <entry>Conventional Name</entry>
1006 <entry>Encoding</entry>
1007 <entry>Alignment</entry>
1012 <entry><literal>INVALID</literal></entry>
1013 <entry>Not applicable; cannot be marshaled.</entry>
1016 <entry><literal>BYTE</literal></entry>
1017 <entry>A single 8-bit byte.</entry>
1020 <entry><literal>BOOLEAN</literal></entry>
1021 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1024 <entry><literal>INT16</literal></entry>
1025 <entry>16-bit signed integer in the message's byte order.</entry>
1028 <entry><literal>UINT16</literal></entry>
1029 <entry>16-bit unsigned integer in the message's byte order.</entry>
1032 <entry><literal>INT32</literal></entry>
1033 <entry>32-bit signed integer in the message's byte order.</entry>
1036 <entry><literal>UINT32</literal></entry>
1037 <entry>32-bit unsigned integer in the message's byte order.</entry>
1040 <entry><literal>INT64</literal></entry>
1041 <entry>64-bit signed integer in the message's byte order.</entry>
1044 <entry><literal>UINT64</literal></entry>
1045 <entry>64-bit unsigned integer in the message's byte order.</entry>
1048 <entry><literal>DOUBLE</literal></entry>
1049 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1052 <entry><literal>STRING</literal></entry>
1053 <entry>A <literal>UINT32</literal> indicating the string's
1054 length in bytes excluding its terminating nul, followed by
1055 non-nul string data of the given length, followed by a terminating nul
1062 <entry><literal>OBJECT_PATH</literal></entry>
1063 <entry>Exactly the same as <literal>STRING</literal> except the
1064 content must be a valid object path (see above).
1070 <entry><literal>SIGNATURE</literal></entry>
1071 <entry>The same as <literal>STRING</literal> except the length is a single
1072 byte (thus signatures have a maximum length of 255)
1073 and the content must be a valid signature (see above).
1079 <entry><literal>ARRAY</literal></entry>
1081 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1082 alignment padding to the alignment boundary of the array element type,
1083 followed by each array element.
1089 <entry><literal>STRUCT</literal></entry>
1091 A struct must start on an 8-byte boundary regardless of the
1092 type of the struct fields. The struct value consists of each
1093 field marshaled in sequence starting from that 8-byte
1100 <entry><literal>VARIANT</literal></entry>
1102 The marshaled <literal>SIGNATURE</literal> of a single
1103 complete type, followed by a marshaled value with the type
1104 given in the signature.
1107 1 (alignment of the signature)
1110 <entry><literal>DICT_ENTRY</literal></entry>
1112 Identical to STRUCT.
1118 <entry><literal>UNIX_FD</literal></entry>
1119 <entry>32-bit unsigned integer in the message's byte
1120 order. The actual file descriptors need to be
1121 transferred out-of-band via some platform specific
1122 mechanism. On the wire, values of this type store the index to the
1123 file descriptor in the array of file descriptors that
1124 accompany the message.</entry>
1136 <sect1 id="message-protocol">
1137 <title>Message Protocol</title>
1140 A <firstterm>message</firstterm> consists of a
1141 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1142 think of a message as a package, the header is the address, and the body
1143 contains the package contents. The message delivery system uses the header
1144 information to figure out where to send the message and how to interpret
1145 it; the recipient interprets the body of the message.
1149 The body of the message is made up of zero or more
1150 <firstterm>arguments</firstterm>, which are typed values, such as an
1151 integer or a byte array.
1155 Both header and body use the D-Bus <link linkend="type-system">type
1156 system</link> and format for serializing data.
1159 <sect2 id="message-protocol-messages">
1160 <title>Message Format</title>
1163 A message consists of a header and a body. The header is a block of
1164 values with a fixed signature and meaning. The body is a separate block
1165 of values, with a signature specified in the header.
1169 The length of the header must be a multiple of 8, allowing the body to
1170 begin on an 8-byte boundary when storing the entire message in a single
1171 buffer. If the header does not naturally end on an 8-byte boundary
1172 up to 7 bytes of nul-initialized alignment padding must be added.
1176 The message body need not end on an 8-byte boundary.
1180 The maximum length of a message, including header, header alignment padding,
1181 and body is 2 to the 27th power or 134217728. Implementations must not
1182 send or accept messages exceeding this size.
1186 The signature of the header is:
1190 Written out more readably, this is:
1192 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1197 These values have the following meanings:
1202 <entry>Value</entry>
1203 <entry>Description</entry>
1208 <entry>1st <literal>BYTE</literal></entry>
1209 <entry>Endianness flag; ASCII 'l' for little-endian
1210 or ASCII 'B' for big-endian. Both header and body are
1211 in this endianness.</entry>
1214 <entry>2nd <literal>BYTE</literal></entry>
1215 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1216 Currently-defined types are described below.
1220 <entry>3rd <literal>BYTE</literal></entry>
1221 <entry>Bitwise OR of flags. Unknown flags
1222 must be ignored. Currently-defined flags are described below.
1226 <entry>4th <literal>BYTE</literal></entry>
1227 <entry>Major protocol version of the sending application. If
1228 the major protocol version of the receiving application does not
1229 match, the applications will not be able to communicate and the
1230 D-Bus connection must be disconnected. The major protocol
1231 version for this version of the specification is 1.
1235 <entry>1st <literal>UINT32</literal></entry>
1236 <entry>Length in bytes of the message body, starting
1237 from the end of the header. The header ends after
1238 its alignment padding to an 8-boundary.
1242 <entry>2nd <literal>UINT32</literal></entry>
1243 <entry>The serial of this message, used as a cookie
1244 by the sender to identify the reply corresponding
1245 to this request. This must not be zero.
1249 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1250 <entry>An array of zero or more <firstterm>header
1251 fields</firstterm> where the byte is the field code, and the
1252 variant is the field value. The message type determines
1253 which fields are required.
1261 <firstterm>Message types</firstterm> that can appear in the second byte
1267 <entry>Conventional name</entry>
1268 <entry>Decimal value</entry>
1269 <entry>Description</entry>
1274 <entry><literal>INVALID</literal></entry>
1276 <entry>This is an invalid type.</entry>
1279 <entry><literal>METHOD_CALL</literal></entry>
1281 <entry>Method call.</entry>
1284 <entry><literal>METHOD_RETURN</literal></entry>
1286 <entry>Method reply with returned data.</entry>
1289 <entry><literal>ERROR</literal></entry>
1291 <entry>Error reply. If the first argument exists and is a
1292 string, it is an error message.</entry>
1295 <entry><literal>SIGNAL</literal></entry>
1297 <entry>Signal emission.</entry>
1304 Flags that can appear in the third byte of the header:
1309 <entry>Conventional name</entry>
1310 <entry>Hex value</entry>
1311 <entry>Description</entry>
1316 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1318 <entry>This message does not expect method return replies or
1319 error replies; the reply can be omitted as an
1320 optimization. However, it is compliant with this specification
1321 to return the reply despite this flag and the only harm
1322 from doing so is extra network traffic.
1326 <entry><literal>NO_AUTO_START</literal></entry>
1328 <entry>The bus must not launch an owner
1329 for the destination name in response to this message.
1337 <sect3 id="message-protocol-header-fields">
1338 <title>Header Fields</title>
1341 The array at the end of the header contains <firstterm>header
1342 fields</firstterm>, where each field is a 1-byte field code followed
1343 by a field value. A header must contain the required header fields for
1344 its message type, and zero or more of any optional header
1345 fields. Future versions of this protocol specification may add new
1346 fields. Implementations must ignore fields they do not
1347 understand. Implementations must not invent their own header fields;
1348 only changes to this specification may introduce new header fields.
1352 Again, if an implementation sees a header field code that it does not
1353 expect, it must ignore that field, as it will be part of a new
1354 (but compatible) version of this specification. This also applies
1355 to known header fields appearing in unexpected messages, for
1356 example: if a signal has a reply serial it must be ignored
1357 even though it has no meaning as of this version of the spec.
1361 However, implementations must not send or accept known header fields
1362 with the wrong type stored in the field value. So for example a
1363 message with an <literal>INTERFACE</literal> field of type
1364 <literal>UINT32</literal> would be considered corrupt.
1368 Here are the currently-defined header fields:
1373 <entry>Conventional Name</entry>
1374 <entry>Decimal Code</entry>
1376 <entry>Required In</entry>
1377 <entry>Description</entry>
1382 <entry><literal>INVALID</literal></entry>
1385 <entry>not allowed</entry>
1386 <entry>Not a valid field name (error if it appears in a message)</entry>
1389 <entry><literal>PATH</literal></entry>
1391 <entry><literal>OBJECT_PATH</literal></entry>
1392 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1393 <entry>The object to send a call to,
1394 or the object a signal is emitted from.
1396 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1397 implementations should not send messages with this path,
1398 and the reference implementation of the bus daemon will
1399 disconnect any application that attempts to do so.
1403 <entry><literal>INTERFACE</literal></entry>
1405 <entry><literal>STRING</literal></entry>
1406 <entry><literal>SIGNAL</literal></entry>
1408 The interface to invoke a method call on, or
1409 that a signal is emitted from. Optional for
1410 method calls, required for signals.
1411 The special interface
1412 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1413 implementations should not send messages with this
1414 interface, and the reference implementation of the bus
1415 daemon will disconnect any application that attempts to
1420 <entry><literal>MEMBER</literal></entry>
1422 <entry><literal>STRING</literal></entry>
1423 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1424 <entry>The member, either the method name or signal name.</entry>
1427 <entry><literal>ERROR_NAME</literal></entry>
1429 <entry><literal>STRING</literal></entry>
1430 <entry><literal>ERROR</literal></entry>
1431 <entry>The name of the error that occurred, for errors</entry>
1434 <entry><literal>REPLY_SERIAL</literal></entry>
1436 <entry><literal>UINT32</literal></entry>
1437 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1438 <entry>The serial number of the message this message is a reply
1439 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1442 <entry><literal>DESTINATION</literal></entry>
1444 <entry><literal>STRING</literal></entry>
1445 <entry>optional</entry>
1446 <entry>The name of the connection this message is intended for.
1447 Only used in combination with the message bus, see
1448 <xref linkend="message-bus"/>.</entry>
1451 <entry><literal>SENDER</literal></entry>
1453 <entry><literal>STRING</literal></entry>
1454 <entry>optional</entry>
1455 <entry>Unique name of the sending connection.
1456 The message bus fills in this field so it is reliable; the field is
1457 only meaningful in combination with the message bus.</entry>
1460 <entry><literal>SIGNATURE</literal></entry>
1462 <entry><literal>SIGNATURE</literal></entry>
1463 <entry>optional</entry>
1464 <entry>The signature of the message body.
1465 If omitted, it is assumed to be the
1466 empty signature "" (i.e. the body must be 0-length).</entry>
1469 <entry><literal>UNIX_FDS</literal></entry>
1471 <entry><literal>UINT32</literal></entry>
1472 <entry>optional</entry>
1473 <entry>The number of Unix file descriptors that
1474 accompany the message. If omitted, it is assumed
1475 that no Unix file descriptors accompany the
1476 message. The actual file descriptors need to be
1477 transferred via platform specific mechanism
1478 out-of-band. They must be sent at the same time as
1479 part of the message itself. They may not be sent
1480 before the first byte of the message itself is
1481 transferred or after the last byte of the message
1491 <sect2 id="message-protocol-names">
1492 <title>Valid Names</title>
1494 The various names in D-Bus messages have some restrictions.
1497 There is a <firstterm>maximum name length</firstterm>
1498 of 255 which applies to bus names, interfaces, and members.
1500 <sect3 id="message-protocol-names-interface">
1501 <title>Interface names</title>
1503 Interfaces have names with type <literal>STRING</literal>, meaning that
1504 they must be valid UTF-8. However, there are also some
1505 additional restrictions that apply to interface names
1508 <listitem><para>Interface names are composed of 1 or more elements separated by
1509 a period ('.') character. All elements must contain at least
1513 <listitem><para>Each element must only contain the ASCII characters
1514 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1518 <listitem><para>Interface names must contain at least one '.' (period)
1519 character (and thus at least two elements).
1522 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1523 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1528 Interface names should start with the reversed DNS domain name of
1529 the author of the interface (in lower-case), like interface names
1530 in Java. It is conventional for the rest of the interface name
1531 to consist of words run together, with initial capital letters
1532 on all words ("CamelCase"). Several levels of hierarchy can be used.
1533 It is also a good idea to include the major version of the interface
1534 in the name, and increment it if incompatible changes are made;
1535 this way, a single object can implement several versions of an
1536 interface in parallel, if necessary.
1540 For instance, if the owner of <literal>example.com</literal> is
1541 developing a D-Bus API for a music player, they might define
1542 interfaces called <literal>com.example.MusicPlayer1</literal>,
1543 <literal>com.example.MusicPlayer1.Track</literal> and
1544 <literal>com.example.MusicPlayer1.Seekable</literal>.
1548 D-Bus does not distinguish between the concepts that would be
1549 called classes and interfaces in Java: either can be identified on
1550 D-Bus by an interface name.
1553 <sect3 id="message-protocol-names-bus">
1554 <title>Bus names</title>
1556 Connections have one or more bus names associated with them.
1557 A connection has exactly one bus name that is a <firstterm>unique
1558 connection name</firstterm>. The unique connection name remains
1559 with the connection for its entire lifetime.
1560 A bus name is of type <literal>STRING</literal>,
1561 meaning that it must be valid UTF-8. However, there are also
1562 some additional restrictions that apply to bus names
1565 <listitem><para>Bus names that start with a colon (':')
1566 character are unique connection names. Other bus names
1567 are called <firstterm>well-known bus names</firstterm>.
1570 <listitem><para>Bus names are composed of 1 or more elements separated by
1571 a period ('.') character. All elements must contain at least
1575 <listitem><para>Each element must only contain the ASCII characters
1576 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1577 connection name may begin with a digit, elements in
1578 other bus names must not begin with a digit.
1582 <listitem><para>Bus names must contain at least one '.' (period)
1583 character (and thus at least two elements).
1586 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1587 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1591 Note that the hyphen ('-') character is allowed in bus names but
1592 not in interface names.
1596 Like <link linkend="message-protocol-names-interface">interface
1597 names</link>, well-known bus names should start with the
1598 reversed DNS domain name of the author of the interface (in
1599 lower-case), and it is conventional for the rest of the well-known
1600 bus name to consist of words run together, with initial
1601 capital letters. As with interface names, including a version
1602 number in well-known bus names is a good idea; it's possible to
1603 have the well-known bus name for more than one version
1604 simultaneously if backwards compatibility is required.
1608 If a well-known bus name implies the presence of a "main" interface,
1609 that "main" interface is often given the same name as
1610 the well-known bus name, and situated at the corresponding object
1611 path. For instance, if the owner of <literal>example.com</literal>
1612 is developing a D-Bus API for a music player, they might define
1613 that any application that takes the well-known name
1614 <literal>com.example.MusicPlayer1</literal> should have an object
1615 at the object path <literal>/com/example/MusicPlayer1</literal>
1616 which implements the interface
1617 <literal>com.example.MusicPlayer1</literal>.
1620 <sect3 id="message-protocol-names-member">
1621 <title>Member names</title>
1623 Member (i.e. method or signal) names:
1625 <listitem><para>Must only contain the ASCII characters
1626 "[A-Z][a-z][0-9]_" and may not begin with a
1627 digit.</para></listitem>
1628 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1629 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1630 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1635 It is conventional for member names on D-Bus to consist of
1636 capitalized words with no punctuation ("camel-case").
1637 Method names should usually be verbs, such as
1638 <literal>GetItems</literal>, and signal names should usually be
1639 a description of an event, such as <literal>ItemsChanged</literal>.
1642 <sect3 id="message-protocol-names-error">
1643 <title>Error names</title>
1645 Error names have the same restrictions as interface names.
1649 Error names have the same naming conventions as interface
1650 names, and often contain <literal>.Error.</literal>; for instance,
1651 the owner of <literal>example.com</literal> might define the
1652 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1653 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1654 The errors defined by D-Bus itself, such as
1655 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1661 <sect2 id="message-protocol-types">
1662 <title>Message Types</title>
1664 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1665 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1666 This section describes these conventions.
1668 <sect3 id="message-protocol-types-method">
1669 <title>Method Calls</title>
1671 Some messages invoke an operation on a remote object. These are
1672 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1673 messages map naturally to methods on objects in a typical program.
1676 A method call message is required to have a <literal>MEMBER</literal> header field
1677 indicating the name of the method. Optionally, the message has an
1678 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1679 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1680 a method with the same name, it is undefined which of the two methods
1681 will be invoked. Implementations may also choose to return an error in
1682 this ambiguous case. However, if a method name is unique
1683 implementations must not require an interface field.
1686 Method call messages also include a <literal>PATH</literal> field
1687 indicating the object to invoke the method on. If the call is passing
1688 through a message bus, the message will also have a
1689 <literal>DESTINATION</literal> field giving the name of the connection
1690 to receive the message.
1693 When an application handles a method call message, it is required to
1694 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1695 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1696 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1699 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1700 are the return value(s) or "out parameters" of the method call.
1701 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1702 and the call fails; no return value will be provided. It makes
1703 no sense to send multiple replies to the same method call.
1706 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1707 reply is required, so the caller will know the method
1708 was successfully processed.
1711 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1715 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1716 then as an optimization the application receiving the method
1717 call may choose to omit the reply message (regardless of
1718 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1719 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1720 flag and reply anyway.
1723 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1724 destination name does not exist then a program to own the destination
1725 name will be started before the message is delivered. The message
1726 will be held until the new program is successfully started or has
1727 failed to start; in case of failure, an error will be returned. This
1728 flag is only relevant in the context of a message bus, it is ignored
1729 during one-to-one communication with no intermediate bus.
1731 <sect4 id="message-protocol-types-method-apis">
1732 <title>Mapping method calls to native APIs</title>
1734 APIs for D-Bus may map method calls to a method call in a specific
1735 programming language, such as C++, or may map a method call written
1736 in an IDL to a D-Bus message.
1739 In APIs of this nature, arguments to a method are often termed "in"
1740 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1741 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1742 "inout" arguments, which are both sent and received, i.e. the caller
1743 passes in a value which is modified. Mapped to D-Bus, an "inout"
1744 argument is equivalent to an "in" argument, followed by an "out"
1745 argument. You can't pass things "by reference" over the wire, so
1746 "inout" is purely an illusion of the in-process API.
1749 Given a method with zero or one return values, followed by zero or more
1750 arguments, where each argument may be "in", "out", or "inout", the
1751 caller constructs a message by appending each "in" or "inout" argument,
1752 in order. "out" arguments are not represented in the caller's message.
1755 The recipient constructs a reply by appending first the return value
1756 if any, then each "out" or "inout" argument, in order.
1757 "in" arguments are not represented in the reply message.
1760 Error replies are normally mapped to exceptions in languages that have
1764 In converting from native APIs to D-Bus, it is perhaps nice to
1765 map D-Bus naming conventions ("FooBar") to native conventions
1766 such as "fooBar" or "foo_bar" automatically. This is OK
1767 as long as you can say that the native API is one that
1768 was specifically written for D-Bus. It makes the most sense
1769 when writing object implementations that will be exported
1770 over the bus. Object proxies used to invoke remote D-Bus
1771 objects probably need the ability to call any D-Bus method,
1772 and thus a magic name mapping like this could be a problem.
1775 This specification doesn't require anything of native API bindings;
1776 the preceding is only a suggested convention for consistency
1782 <sect3 id="message-protocol-types-signal">
1783 <title>Signal Emission</title>
1785 Unlike method calls, signal emissions have no replies.
1786 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1787 It must have three header fields: <literal>PATH</literal> giving the object
1788 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1789 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1790 for signals, though it is optional for method calls.
1794 <sect3 id="message-protocol-types-errors">
1795 <title>Errors</title>
1797 Messages of type <literal>ERROR</literal> are most commonly replies
1798 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1799 to any kind of message. The message bus for example
1800 will return an <literal>ERROR</literal> in reply to a signal emission if
1801 the bus does not have enough memory to send the signal.
1804 An <literal>ERROR</literal> may have any arguments, but if the first
1805 argument is a <literal>STRING</literal>, it must be an error message.
1806 The error message may be logged or shown to the user
1811 <sect3 id="message-protocol-types-notation">
1812 <title>Notation in this document</title>
1814 This document uses a simple pseudo-IDL to describe particular method
1815 calls and signals. Here is an example of a method call:
1817 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1818 out UINT32 resultcode)
1820 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1821 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1822 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1823 characters so it's known that the last part of the name in
1824 the "IDL" is the member name.
1827 In C++ that might end up looking like this:
1829 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1830 unsigned int flags);
1832 or equally valid, the return value could be done as an argument:
1834 void org::freedesktop::DBus::StartServiceByName (const char *name,
1836 unsigned int *resultcode);
1838 It's really up to the API designer how they want to make
1839 this look. You could design an API where the namespace wasn't used
1840 in C++, using STL or Qt, using varargs, or whatever you wanted.
1843 Signals are written as follows:
1845 org.freedesktop.DBus.NameLost (STRING name)
1847 Signals don't specify "in" vs. "out" because only
1848 a single direction is possible.
1851 It isn't especially encouraged to use this lame pseudo-IDL in actual
1852 API implementations; you might use the native notation for the
1853 language you're using, or you might use COM or CORBA IDL, for example.
1858 <sect2 id="message-protocol-handling-invalid">
1859 <title>Invalid Protocol and Spec Extensions</title>
1862 For security reasons, the D-Bus protocol should be strictly parsed and
1863 validated, with the exception of defined extension points. Any invalid
1864 protocol or spec violations should result in immediately dropping the
1865 connection without notice to the other end. Exceptions should be
1866 carefully considered, e.g. an exception may be warranted for a
1867 well-understood idiosyncrasy of a widely-deployed implementation. In
1868 cases where the other end of a connection is 100% trusted and known to
1869 be friendly, skipping validation for performance reasons could also make
1870 sense in certain cases.
1874 Generally speaking violations of the "must" requirements in this spec
1875 should be considered possible attempts to exploit security, and violations
1876 of the "should" suggestions should be considered legitimate (though perhaps
1877 they should generate an error in some cases).
1881 The following extension points are built in to D-Bus on purpose and must
1882 not be treated as invalid protocol. The extension points are intended
1883 for use by future versions of this spec, they are not intended for third
1884 parties. At the moment, the only way a third party could extend D-Bus
1885 without breaking interoperability would be to introduce a way to negotiate new
1886 feature support as part of the auth protocol, using EXTENSION_-prefixed
1887 commands. There is not yet a standard way to negotiate features.
1891 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1892 commands result in an ERROR rather than a disconnect. This enables
1893 future extensions to the protocol. Commands starting with EXTENSION_ are
1894 reserved for third parties.
1899 The authentication protocol supports pluggable auth mechanisms.
1904 The address format (see <xref linkend="addresses"/>) supports new
1910 Messages with an unknown type (something other than
1911 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1912 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1913 Unknown-type messages must still be well-formed in the same way
1914 as the known messages, however. They still have the normal
1920 Header fields with an unknown or unexpected field code must be ignored,
1921 though again they must still be well-formed.
1926 New standard interfaces (with new methods and signals) can of course be added.
1936 <sect1 id="auth-protocol">
1937 <title>Authentication Protocol</title>
1939 Before the flow of messages begins, two applications must
1940 authenticate. A simple plain-text protocol is used for
1941 authentication; this protocol is a SASL profile, and maps fairly
1942 directly from the SASL specification. The message encoding is
1943 NOT used here, only plain text messages.
1946 In examples, "C:" and "S:" indicate lines sent by the client and
1947 server respectively.
1949 <sect2 id="auth-protocol-overview">
1950 <title>Protocol Overview</title>
1952 The protocol is a line-based protocol, where each line ends with
1953 \r\n. Each line begins with an all-caps ASCII command name containing
1954 only the character range [A-Z_], a space, then any arguments for the
1955 command, then the \r\n ending the line. The protocol is
1956 case-sensitive. All bytes must be in the ASCII character set.
1958 Commands from the client to the server are as follows:
1961 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1962 <listitem><para>CANCEL</para></listitem>
1963 <listitem><para>BEGIN</para></listitem>
1964 <listitem><para>DATA <data in hex encoding></para></listitem>
1965 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1966 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1969 From server to client are as follows:
1972 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1973 <listitem><para>OK <GUID in hex></para></listitem>
1974 <listitem><para>DATA <data in hex encoding></para></listitem>
1975 <listitem><para>ERROR</para></listitem>
1976 <listitem><para>AGREE_UNIX_FD</para></listitem>
1980 Unofficial extensions to the command set must begin with the letters
1981 "EXTENSION_", to avoid conflicts with future official commands.
1982 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1985 <sect2 id="auth-nul-byte">
1986 <title>Special credentials-passing nul byte</title>
1988 Immediately after connecting to the server, the client must send a
1989 single nul byte. This byte may be accompanied by credentials
1990 information on some operating systems that use sendmsg() with
1991 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1992 sockets. However, the nul byte must be sent even on other kinds of
1993 socket, and even on operating systems that do not require a byte to be
1994 sent in order to transmit credentials. The text protocol described in
1995 this document begins after the single nul byte. If the first byte
1996 received from the client is not a nul byte, the server may disconnect
2000 A nul byte in any context other than the initial byte is an error;
2001 the protocol is ASCII-only.
2004 The credentials sent along with the nul byte may be used with the
2005 SASL mechanism EXTERNAL.
2008 <sect2 id="auth-command-auth">
2009 <title>AUTH command</title>
2011 If an AUTH command has no arguments, it is a request to list
2012 available mechanisms. The server must respond with a REJECTED
2013 command listing the mechanisms it understands, or with an error.
2016 If an AUTH command specifies a mechanism, and the server supports
2017 said mechanism, the server should begin exchanging SASL
2018 challenge-response data with the client using DATA commands.
2021 If the server does not support the mechanism given in the AUTH
2022 command, it must send either a REJECTED command listing the mechanisms
2023 it does support, or an error.
2026 If the [initial-response] argument is provided, it is intended for use
2027 with mechanisms that have no initial challenge (or an empty initial
2028 challenge), as if it were the argument to an initial DATA command. If
2029 the selected mechanism has an initial challenge and [initial-response]
2030 was provided, the server should reject authentication by sending
2034 If authentication succeeds after exchanging DATA commands,
2035 an OK command must be sent to the client.
2038 The first octet received by the server after the \r\n of the BEGIN
2039 command from the client must be the first octet of the
2040 authenticated/encrypted stream of D-Bus messages.
2043 If BEGIN is received by the server, the first octet received
2044 by the client after the \r\n of the OK command must be the
2045 first octet of the authenticated/encrypted stream of D-Bus
2049 <sect2 id="auth-command-cancel">
2050 <title>CANCEL Command</title>
2052 At any time up to sending the BEGIN command, the client may send a
2053 CANCEL command. On receiving the CANCEL command, the server must
2054 send a REJECTED command and abort the current authentication
2058 <sect2 id="auth-command-data">
2059 <title>DATA Command</title>
2061 The DATA command may come from either client or server, and simply
2062 contains a hex-encoded block of data to be interpreted
2063 according to the SASL mechanism in use.
2066 Some SASL mechanisms support sending an "empty string";
2067 FIXME we need some way to do this.
2070 <sect2 id="auth-command-begin">
2071 <title>BEGIN Command</title>
2073 The BEGIN command acknowledges that the client has received an
2074 OK command from the server, and that the stream of messages
2078 The first octet received by the server after the \r\n of the BEGIN
2079 command from the client must be the first octet of the
2080 authenticated/encrypted stream of D-Bus messages.
2083 <sect2 id="auth-command-rejected">
2084 <title>REJECTED Command</title>
2086 The REJECTED command indicates that the current authentication
2087 exchange has failed, and further exchange of DATA is inappropriate.
2088 The client would normally try another mechanism, or try providing
2089 different responses to challenges.
2091 Optionally, the REJECTED command has a space-separated list of
2092 available auth mechanisms as arguments. If a server ever provides
2093 a list of supported mechanisms, it must provide the same list
2094 each time it sends a REJECTED message. Clients are free to
2095 ignore all lists received after the first.
2098 <sect2 id="auth-command-ok">
2099 <title>OK Command</title>
2101 The OK command indicates that the client has been
2102 authenticated. The client may now proceed with negotiating
2103 Unix file descriptor passing. To do that it shall send
2104 NEGOTIATE_UNIX_FD to the server.
2107 Otherwise, the client must respond to the OK command by
2108 sending a BEGIN command, followed by its stream of messages,
2109 or by disconnecting. The server must not accept additional
2110 commands using this protocol after the BEGIN command has been
2111 received. Further communication will be a stream of D-Bus
2112 messages (optionally encrypted, as negotiated) rather than
2116 If a client sends BEGIN the first octet received by the client
2117 after the \r\n of the OK command must be the first octet of
2118 the authenticated/encrypted stream of D-Bus messages.
2121 The OK command has one argument, which is the GUID of the server.
2122 See <xref linkend="addresses"/> for more on server GUIDs.
2125 <sect2 id="auth-command-error">
2126 <title>ERROR Command</title>
2128 The ERROR command indicates that either server or client did not
2129 know a command, does not accept the given command in the current
2130 context, or did not understand the arguments to the command. This
2131 allows the protocol to be extended; a client or server can send a
2132 command present or permitted only in new protocol versions, and if
2133 an ERROR is received instead of an appropriate response, fall back
2134 to using some other technique.
2137 If an ERROR is sent, the server or client that sent the
2138 error must continue as if the command causing the ERROR had never been
2139 received. However, the the server or client receiving the error
2140 should try something other than whatever caused the error;
2141 if only canceling/rejecting the authentication.
2144 If the D-Bus protocol changes incompatibly at some future time,
2145 applications implementing the new protocol would probably be able to
2146 check for support of the new protocol by sending a new command and
2147 receiving an ERROR from applications that don't understand it. Thus the
2148 ERROR feature of the auth protocol is an escape hatch that lets us
2149 negotiate extensions or changes to the D-Bus protocol in the future.
2152 <sect2 id="auth-command-negotiate-unix-fd">
2153 <title>NEGOTIATE_UNIX_FD Command</title>
2155 The NEGOTIATE_UNIX_FD command indicates that the client
2156 supports Unix file descriptor passing. This command may only
2157 be sent after the connection is authenticated, i.e. after OK
2158 was received by the client. This command may only be sent on
2159 transports that support Unix file descriptor passing.
2162 On receiving NEGOTIATE_UNIX_FD the server must respond with
2163 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2164 the transport chosen supports Unix file descriptor passing and
2165 the server supports this feature. It shall respond the latter
2166 if the transport does not support Unix file descriptor
2167 passing, the server does not support this feature, or the
2168 server decides not to enable file descriptor passing due to
2169 security or other reasons.
2172 <sect2 id="auth-command-agree-unix-fd">
2173 <title>AGREE_UNIX_FD Command</title>
2175 The AGREE_UNIX_FD command indicates that the server supports
2176 Unix file descriptor passing. This command may only be sent
2177 after the connection is authenticated, and the client sent
2178 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2179 command may only be sent on transports that support Unix file
2183 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2184 followed by its stream of messages, or by disconnecting. The
2185 server must not accept additional commands using this protocol
2186 after the BEGIN command has been received. Further
2187 communication will be a stream of D-Bus messages (optionally
2188 encrypted, as negotiated) rather than this protocol.
2191 <sect2 id="auth-command-future">
2192 <title>Future Extensions</title>
2194 Future extensions to the authentication and negotiation
2195 protocol are possible. For that new commands may be
2196 introduced. If a client or server receives an unknown command
2197 it shall respond with ERROR and not consider this fatal. New
2198 commands may be introduced both before, and after
2199 authentication, i.e. both before and after the OK command.
2202 <sect2 id="auth-examples">
2203 <title>Authentication examples</title>
2207 <title>Example of successful magic cookie authentication</title>
2209 (MAGIC_COOKIE is a made up mechanism)
2211 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2217 <title>Example of finding out mechanisms then picking one</title>
2220 S: REJECTED KERBEROS_V4 SKEY
2221 C: AUTH SKEY 7ab83f32ee
2222 S: DATA 8799cabb2ea93e
2223 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2229 <title>Example of client sends unknown command then falls back to regular auth</title>
2233 C: AUTH MAGIC_COOKIE 3736343435313230333039
2239 <title>Example of server doesn't support initial auth mechanism</title>
2241 C: AUTH MAGIC_COOKIE 3736343435313230333039
2242 S: REJECTED KERBEROS_V4 SKEY
2243 C: AUTH SKEY 7ab83f32ee
2244 S: DATA 8799cabb2ea93e
2245 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2251 <title>Example of wrong password or the like followed by successful retry</title>
2253 C: AUTH MAGIC_COOKIE 3736343435313230333039
2254 S: REJECTED KERBEROS_V4 SKEY
2255 C: AUTH SKEY 7ab83f32ee
2256 S: DATA 8799cabb2ea93e
2257 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2259 C: AUTH SKEY 7ab83f32ee
2260 S: DATA 8799cabb2ea93e
2261 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2267 <title>Example of skey cancelled and restarted</title>
2269 C: AUTH MAGIC_COOKIE 3736343435313230333039
2270 S: REJECTED KERBEROS_V4 SKEY
2271 C: AUTH SKEY 7ab83f32ee
2272 S: DATA 8799cabb2ea93e
2275 C: AUTH SKEY 7ab83f32ee
2276 S: DATA 8799cabb2ea93e
2277 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2283 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2285 (MAGIC_COOKIE is a made up mechanism)
2287 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2289 C: NEGOTIATE_UNIX_FD
2295 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2297 (MAGIC_COOKIE is a made up mechanism)
2299 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2301 C: NEGOTIATE_UNIX_FD
2308 <sect2 id="auth-states">
2309 <title>Authentication state diagrams</title>
2312 This section documents the auth protocol in terms of
2313 a state machine for the client and the server. This is
2314 probably the most robust way to implement the protocol.
2317 <sect3 id="auth-states-client">
2318 <title>Client states</title>
2321 To more precisely describe the interaction between the
2322 protocol state machine and the authentication mechanisms the
2323 following notation is used: MECH(CHALL) means that the
2324 server challenge CHALL was fed to the mechanism MECH, which
2330 CONTINUE(RESP) means continue the auth conversation
2331 and send RESP as the response to the server;
2337 OK(RESP) means that after sending RESP to the server
2338 the client side of the auth conversation is finished
2339 and the server should return "OK";
2345 ERROR means that CHALL was invalid and could not be
2351 Both RESP and CHALL may be empty.
2355 The Client starts by getting an initial response from the
2356 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2357 the mechanism did not provide an initial response. If the
2358 mechanism returns CONTINUE, the client starts in state
2359 <emphasis>WaitingForData</emphasis>, if the mechanism
2360 returns OK the client starts in state
2361 <emphasis>WaitingForOK</emphasis>.
2365 The client should keep track of available mechanisms and
2366 which it mechanisms it has already attempted. This list is
2367 used to decide which AUTH command to send. When the list is
2368 exhausted, the client should give up and close the
2373 <title><emphasis>WaitingForData</emphasis></title>
2381 MECH(CHALL) returns CONTINUE(RESP) → send
2383 <emphasis>WaitingForData</emphasis>
2387 MECH(CHALL) returns OK(RESP) → send DATA
2388 RESP, goto <emphasis>WaitingForOK</emphasis>
2392 MECH(CHALL) returns ERROR → send ERROR
2393 [msg], goto <emphasis>WaitingForData</emphasis>
2401 Receive REJECTED [mechs] →
2402 send AUTH [next mech], goto
2403 WaitingForData or <emphasis>WaitingForOK</emphasis>
2408 Receive ERROR → send
2410 <emphasis>WaitingForReject</emphasis>
2415 Receive OK → send
2416 BEGIN, terminate auth
2417 conversation, authenticated
2422 Receive anything else → send
2424 <emphasis>WaitingForData</emphasis>
2432 <title><emphasis>WaitingForOK</emphasis></title>
2437 Receive OK → send BEGIN, terminate auth
2438 conversation, <emphasis>authenticated</emphasis>
2443 Receive REJECTED [mechs] → send AUTH [next mech],
2444 goto <emphasis>WaitingForData</emphasis> or
2445 <emphasis>WaitingForOK</emphasis>
2451 Receive DATA → send CANCEL, goto
2452 <emphasis>WaitingForReject</emphasis>
2458 Receive ERROR → send CANCEL, goto
2459 <emphasis>WaitingForReject</emphasis>
2465 Receive anything else → send ERROR, goto
2466 <emphasis>WaitingForOK</emphasis>
2474 <title><emphasis>WaitingForReject</emphasis></title>
2479 Receive REJECTED [mechs] → send AUTH [next mech],
2480 goto <emphasis>WaitingForData</emphasis> or
2481 <emphasis>WaitingForOK</emphasis>
2487 Receive anything else → terminate auth
2488 conversation, disconnect
2497 <sect3 id="auth-states-server">
2498 <title>Server states</title>
2501 For the server MECH(RESP) means that the client response
2502 RESP was fed to the the mechanism MECH, which returns one of
2507 CONTINUE(CHALL) means continue the auth conversation and
2508 send CHALL as the challenge to the client;
2514 OK means that the client has been successfully
2521 REJECTED means that the client failed to authenticate or
2522 there was an error in RESP.
2527 The server starts out in state
2528 <emphasis>WaitingForAuth</emphasis>. If the client is
2529 rejected too many times the server must disconnect the
2534 <title><emphasis>WaitingForAuth</emphasis></title>
2540 Receive AUTH → send REJECTED [mechs], goto
2541 <emphasis>WaitingForAuth</emphasis>
2547 Receive AUTH MECH RESP
2551 MECH not valid mechanism → send REJECTED
2553 <emphasis>WaitingForAuth</emphasis>
2557 MECH(RESP) returns CONTINUE(CHALL) → send
2559 <emphasis>WaitingForData</emphasis>
2563 MECH(RESP) returns OK → send OK, goto
2564 <emphasis>WaitingForBegin</emphasis>
2568 MECH(RESP) returns REJECTED → send REJECTED
2570 <emphasis>WaitingForAuth</emphasis>
2578 Receive BEGIN → terminate
2579 auth conversation, disconnect
2585 Receive ERROR → send REJECTED [mechs], goto
2586 <emphasis>WaitingForAuth</emphasis>
2592 Receive anything else → send
2594 <emphasis>WaitingForAuth</emphasis>
2603 <title><emphasis>WaitingForData</emphasis></title>
2611 MECH(RESP) returns CONTINUE(CHALL) → send
2613 <emphasis>WaitingForData</emphasis>
2617 MECH(RESP) returns OK → send OK, goto
2618 <emphasis>WaitingForBegin</emphasis>
2622 MECH(RESP) returns REJECTED → send REJECTED
2624 <emphasis>WaitingForAuth</emphasis>
2632 Receive BEGIN → terminate auth conversation,
2639 Receive CANCEL → send REJECTED [mechs], goto
2640 <emphasis>WaitingForAuth</emphasis>
2646 Receive ERROR → send REJECTED [mechs], goto
2647 <emphasis>WaitingForAuth</emphasis>
2653 Receive anything else → send ERROR, goto
2654 <emphasis>WaitingForData</emphasis>
2662 <title><emphasis>WaitingForBegin</emphasis></title>
2667 Receive BEGIN → terminate auth conversation,
2668 client authenticated
2674 Receive CANCEL → send REJECTED [mechs], goto
2675 <emphasis>WaitingForAuth</emphasis>
2681 Receive ERROR → send REJECTED [mechs], goto
2682 <emphasis>WaitingForAuth</emphasis>
2688 Receive anything else → send ERROR, goto
2689 <emphasis>WaitingForBegin</emphasis>
2699 <sect2 id="auth-mechanisms">
2700 <title>Authentication mechanisms</title>
2702 This section describes some new authentication mechanisms.
2703 D-Bus also allows any standard SASL mechanism of course.
2705 <sect3 id="auth-mechanisms-sha">
2706 <title>DBUS_COOKIE_SHA1</title>
2708 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2709 has the ability to read a private file owned by the user being
2710 authenticated. If the client can prove that it has access to a secret
2711 cookie stored in this file, then the client is authenticated.
2712 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2716 Throughout this description, "hex encoding" must output the digits
2717 from a to f in lower-case; the digits A to F must not be used
2718 in the DBUS_COOKIE_SHA1 mechanism.
2721 Authentication proceeds as follows:
2725 The client sends the username it would like to authenticate
2731 The server sends the name of its "cookie context" (see below); a
2732 space character; the integer ID of the secret cookie the client
2733 must demonstrate knowledge of; a space character; then a
2734 randomly-generated challenge string, all of this hex-encoded into
2740 The client locates the cookie and generates its own
2741 randomly-generated challenge string. The client then concatenates
2742 the server's decoded challenge, a ":" character, its own challenge,
2743 another ":" character, and the cookie. It computes the SHA-1 hash
2744 of this composite string as a hex digest. It concatenates the
2745 client's challenge string, a space character, and the SHA-1 hex
2746 digest, hex-encodes the result and sends it back to the server.
2751 The server generates the same concatenated string used by the
2752 client and computes its SHA-1 hash. It compares the hash with
2753 the hash received from the client; if the two hashes match, the
2754 client is authenticated.
2760 Each server has a "cookie context," which is a name that identifies a
2761 set of cookies that apply to that server. A sample context might be
2762 "org_freedesktop_session_bus". Context names must be valid ASCII,
2763 nonzero length, and may not contain the characters slash ("/"),
2764 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2765 tab ("\t"), or period ("."). There is a default context,
2766 "org_freedesktop_general" that's used by servers that do not specify
2770 Cookies are stored in a user's home directory, in the directory
2771 <filename>~/.dbus-keyrings/</filename>. This directory must
2772 not be readable or writable by other users. If it is,
2773 clients and servers must ignore it. The directory
2774 contains cookie files named after the cookie context.
2777 A cookie file contains one cookie per line. Each line
2778 has three space-separated fields:
2782 The cookie ID number, which must be a non-negative integer and
2783 may not be used twice in the same file.
2788 The cookie's creation time, in UNIX seconds-since-the-epoch
2794 The cookie itself, a hex-encoded random block of bytes. The cookie
2795 may be of any length, though obviously security increases
2796 as the length increases.
2802 Only server processes modify the cookie file.
2803 They must do so with this procedure:
2807 Create a lockfile name by appending ".lock" to the name of the
2808 cookie file. The server should attempt to create this file
2809 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2810 fails, the lock fails. Servers should retry for a reasonable
2811 period of time, then they may choose to delete an existing lock
2812 to keep users from having to manually delete a stale
2813 lock. <footnote><para>Lockfiles are used instead of real file
2814 locking <literal>fcntl()</literal> because real locking
2815 implementations are still flaky on network
2816 filesystems.</para></footnote>
2821 Once the lockfile has been created, the server loads the cookie
2822 file. It should then delete any cookies that are old (the
2823 timeout can be fairly short), or more than a reasonable
2824 time in the future (so that cookies never accidentally
2825 become permanent, if the clock was set far into the future
2826 at some point). If no recent keys remain, the
2827 server may generate a new key.
2832 The pruned and possibly added-to cookie file
2833 must be resaved atomically (using a temporary
2834 file which is rename()'d).
2839 The lock must be dropped by deleting the lockfile.
2845 Clients need not lock the file in order to load it,
2846 because servers are required to save the file atomically.
2851 <sect1 id="addresses">
2852 <title>Server Addresses</title>
2854 Server addresses consist of a transport name followed by a colon, and
2855 then an optional, comma-separated list of keys and values in the form key=value.
2856 Each value is escaped.
2860 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2861 Which is the address to a unix socket with the path /tmp/dbus-test.
2864 Value escaping is similar to URI escaping but simpler.
2868 The set of optionally-escaped bytes is:
2869 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2870 <emphasis>byte</emphasis> (note, not character) which is not in the
2871 set of optionally-escaped bytes must be replaced with an ASCII
2872 percent (<literal>%</literal>) and the value of the byte in hex.
2873 The hex value must always be two digits, even if the first digit is
2874 zero. The optionally-escaped bytes may be escaped if desired.
2879 To unescape, append each byte in the value; if a byte is an ASCII
2880 percent (<literal>%</literal>) character then append the following
2881 hex value instead. It is an error if a <literal>%</literal> byte
2882 does not have two hex digits following. It is an error if a
2883 non-optionally-escaped byte is seen unescaped.
2887 The set of optionally-escaped bytes is intended to preserve address
2888 readability and convenience.
2892 A server may specify a key-value pair with the key <literal>guid</literal>
2893 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2894 describes the format of the <literal>guid</literal> field. If present,
2895 this UUID may be used to distinguish one server address from another. A
2896 server should use a different UUID for each address it listens on. For
2897 example, if a message bus daemon offers both UNIX domain socket and TCP
2898 connections, but treats clients the same regardless of how they connect,
2899 those two connections are equivalent post-connection but should have
2900 distinct UUIDs to distinguish the kinds of connection.
2904 The intent of the address UUID feature is to allow a client to avoid
2905 opening multiple identical connections to the same server, by allowing the
2906 client to check whether an address corresponds to an already-existing
2907 connection. Comparing two addresses is insufficient, because addresses
2908 can be recycled by distinct servers, and equivalent addresses may look
2909 different if simply compared as strings (for example, the host in a TCP
2910 address can be given as an IP address or as a hostname).
2914 Note that the address key is <literal>guid</literal> even though the
2915 rest of the API and documentation says "UUID," for historical reasons.
2919 [FIXME clarify if attempting to connect to each is a requirement
2920 or just a suggestion]
2921 When connecting to a server, multiple server addresses can be
2922 separated by a semi-colon. The library will then try to connect
2923 to the first address and if that fails, it'll try to connect to
2924 the next one specified, and so forth. For example
2925 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2930 <sect1 id="transports">
2931 <title>Transports</title>
2933 [FIXME we need to specify in detail each transport and its possible arguments]
2935 Current transports include: unix domain sockets (including
2936 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2937 using in-process pipes. Future possible transports include one that
2938 tunnels over X11 protocol.
2941 <sect2 id="transports-unix-domain-sockets">
2942 <title>Unix Domain Sockets</title>
2944 Unix domain sockets can be either paths in the file system or on Linux
2945 kernels, they can be abstract which are similar to paths but
2946 do not show up in the file system.
2950 When a socket is opened by the D-Bus library it truncates the path
2951 name right before the first trailing Nul byte. This is true for both
2952 normal paths and abstract paths. Note that this is a departure from
2953 previous versions of D-Bus that would create sockets with a fixed
2954 length path name. Names which were shorter than the fixed length
2955 would be padded by Nul bytes.
2958 Unix domain sockets are not available on Windows.
2960 <sect3 id="transports-unix-domain-sockets-addresses">
2961 <title>Server Address Format</title>
2963 Unix domain socket addresses are identified by the "unix:" prefix
2964 and support the following key/value pairs:
2971 <entry>Values</entry>
2972 <entry>Description</entry>
2978 <entry>(path)</entry>
2979 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2982 <entry>tmpdir</entry>
2983 <entry>(path)</entry>
2984 <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>
2987 <entry>abstract</entry>
2988 <entry>(string)</entry>
2989 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2996 <sect2 id="transports-launchd">
2997 <title>launchd</title>
2999 launchd is an open-source server management system that replaces init, inetd
3000 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3001 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3005 launchd allocates a socket and provides it with the unix path through the
3006 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3007 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3008 it through its environment.
3009 Other processes can query for the launchd socket by executing:
3010 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3011 This is normally done by the D-Bus client library so doesn't have to be done
3015 launchd is not available on Microsoft Windows.
3017 <sect3 id="transports-launchd-addresses">
3018 <title>Server Address Format</title>
3020 launchd addresses are identified by the "launchd:" prefix
3021 and support the following key/value pairs:
3028 <entry>Values</entry>
3029 <entry>Description</entry>
3035 <entry>(environment variable)</entry>
3036 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3043 <sect2 id="transports-systemd">
3044 <title>systemd</title>
3046 systemd is an open-source server management system that
3047 replaces init and inetd on newer Linux systems. It supports
3048 socket activation. The D-Bus systemd transport is used to acquire
3049 socket activation file descriptors from systemd and use them
3050 as D-Bus transport when the current process is spawned by
3051 socket activation from it.
3054 The systemd transport accepts only one or more Unix domain or
3055 TCP streams sockets passed in via socket activation.
3058 The systemd transport is not available on non-Linux operating systems.
3061 The systemd transport defines no parameter keys.
3064 <sect2 id="transports-tcp-sockets">
3065 <title>TCP Sockets</title>
3067 The tcp transport provides TCP/IP based connections between clients
3068 located on the same or different hosts.
3071 Using tcp transport without any additional secure authentification mechanismus
3072 over a network is unsecure.
3075 Windows notes: Because of the tcp stack on Windows does not provide sending
3076 credentials over a tcp connection, the EXTERNAL authentification
3077 mechanismus does not work.
3079 <sect3 id="transports-tcp-sockets-addresses">
3080 <title>Server Address Format</title>
3082 TCP/IP socket addresses are identified by the "tcp:" prefix
3083 and support the following key/value pairs:
3090 <entry>Values</entry>
3091 <entry>Description</entry>
3097 <entry>(string)</entry>
3098 <entry>dns name or ip address</entry>
3102 <entry>(number)</entry>
3103 <entry>The tcp port the server will open. A zero value let the server
3104 choose a free port provided from the underlaying operating system.
3105 libdbus is able to retrieve the real used port from the server.
3109 <entry>family</entry>
3110 <entry>(string)</entry>
3111 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3118 <sect2 id="transports-nonce-tcp-sockets">
3119 <title>Nonce-secured TCP Sockets</title>
3121 The nonce-tcp transport provides a secured TCP transport, using a
3122 simple authentication mechanism to ensure that only clients with read
3123 access to a certain location in the filesystem can connect to the server.
3124 The server writes a secret, the nonce, to a file and an incoming client
3125 connection is only accepted if the client sends the nonce right after
3126 the connect. The nonce mechanism requires no setup and is orthogonal to
3127 the higher-level authentication mechanisms described in the
3128 Authentication section.
3132 On start, the server generates a random 16 byte nonce and writes it
3133 to a file in the user's temporary directory. The nonce file location
3134 is published as part of the server's D-Bus address using the
3135 "noncefile" key-value pair.
3137 After an accept, the server reads 16 bytes from the socket. If the
3138 read bytes do not match the nonce stored in the nonce file, the
3139 server MUST immediately drop the connection.
3140 If the nonce match the received byte sequence, the client is accepted
3141 and the transport behaves like an unsecured tcp transport.
3144 After a successful connect to the server socket, the client MUST read
3145 the nonce from the file published by the server via the noncefile=
3146 key-value pair and send it over the socket. After that, the
3147 transport behaves like an unsecured tcp transport.
3149 <sect3 id="transports-nonce-tcp-sockets-addresses">
3150 <title>Server Address Format</title>
3152 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3153 and support the following key/value pairs:
3160 <entry>Values</entry>
3161 <entry>Description</entry>
3167 <entry>(string)</entry>
3168 <entry>dns name or ip address</entry>
3172 <entry>(number)</entry>
3173 <entry>The tcp port the server will open. A zero value let the server
3174 choose a free port provided from the underlaying operating system.
3175 libdbus is able to retrieve the real used port from the server.
3179 <entry>family</entry>
3180 <entry>(string)</entry>
3181 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3184 <entry>noncefile</entry>
3185 <entry>(path)</entry>
3186 <entry>file location containing the secret</entry>
3193 <sect2 id="transports-exec">
3194 <title>Executed Subprocesses on Unix</title>
3196 This transport forks off a process and connects its standard
3197 input and standard output with an anonymous Unix domain
3198 socket. This socket is then used for communication by the
3199 transport. This transport may be used to use out-of-process
3200 forwarder programs as basis for the D-Bus protocol.
3203 The forked process will inherit the standard error output and
3204 process group from the parent process.
3207 Executed subprocesses are not available on Windows.
3209 <sect3 id="transports-exec-addresses">
3210 <title>Server Address Format</title>
3212 Executed subprocess addresses are identified by the "unixexec:" prefix
3213 and support the following key/value pairs:
3220 <entry>Values</entry>
3221 <entry>Description</entry>
3227 <entry>(path)</entry>
3228 <entry>Path of the binary to execute, either an absolute
3229 path or a binary name that is searched for in the default
3230 search path of the OS. This corresponds to the first
3231 argument of execlp(). This key is mandatory.</entry>
3234 <entry>argv0</entry>
3235 <entry>(string)</entry>
3236 <entry>The program name to use when executing the
3237 binary. If omitted the same value as specified for path=
3238 will be used. This corresponds to the second argument of
3242 <entry>argv1, argv2, ...</entry>
3243 <entry>(string)</entry>
3244 <entry>Arguments to pass to the binary. This corresponds
3245 to the third and later arguments of execlp(). If a
3246 specific argvX is not specified no further argvY for Y > X
3247 are taken into account.</entry>
3255 <sect1 id="meta-transports">
3256 <title>Meta Transports</title>
3258 Meta transports are a kind of transport with special enhancements or
3259 behavior. Currently available meta transports include: autolaunch
3262 <sect2 id="meta-transports-autolaunch">
3263 <title>Autolaunch</title>
3264 <para>The autolaunch transport provides a way for dbus clients to autodetect
3265 a running dbus session bus and to autolaunch a session bus if not present.
3267 <sect3 id="meta-transports-autolaunch-addresses">
3268 <title>Server Address Format</title>
3270 Autolaunch addresses uses the "autolaunch:" prefix and support the
3271 following key/value pairs:
3278 <entry>Values</entry>
3279 <entry>Description</entry>
3284 <entry>scope</entry>
3285 <entry>(string)</entry>
3286 <entry>scope of autolaunch (Windows only)
3290 "*install-path" - limit session bus to dbus installation path.
3291 The dbus installation path is determined from the location of
3292 the shared dbus library. If the library is located in a 'bin'
3293 subdirectory the installation root is the directory above,
3294 otherwise the directory where the library lives is taken as
3297 <install-root>/bin/[lib]dbus-1.dll
3298 <install-root>/[lib]dbus-1.dll
3304 "*user" - limit session bus to the recent user.
3309 other values - specify dedicated session bus like "release",
3321 <sect3 id="meta-transports-autolaunch-windows-implementation">
3322 <title>Windows implementation</title>
3324 On start, the server opens a platform specific transport, creates a mutex
3325 and a shared memory section containing the related session bus address.
3326 This mutex will be inspected by the dbus client library to detect a
3327 running dbus session bus. The access to the mutex and the shared memory
3328 section are protected by global locks.
3331 In the recent implementation the autolaunch transport uses a tcp transport
3332 on localhost with a port choosen from the operating system. This detail may
3333 change in the future.
3336 Disclaimer: The recent implementation is in an early state and may not
3337 work in all cirumstances and/or may have security issues. Because of this
3338 the implementation is not documentated yet.
3345 <title>UUIDs</title>
3347 A working D-Bus implementation uses universally-unique IDs in two places.
3348 First, each server address has a UUID identifying the address,
3349 as described in <xref linkend="addresses"/>. Second, each operating
3350 system kernel instance running a D-Bus client or server has a UUID
3351 identifying that kernel, retrieved by invoking the method
3352 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3353 linkend="standard-interfaces-peer"/>).
3356 The term "UUID" in this document is intended literally, i.e. an
3357 identifier that is universally unique. It is not intended to refer to
3358 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3361 The UUID must contain 128 bits of data and be hex-encoded. The
3362 hex-encoded string may not contain hyphens or other non-hex-digit
3363 characters, and it must be exactly 32 characters long. To generate a
3364 UUID, the current reference implementation concatenates 96 bits of random
3365 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3369 It would also be acceptable and probably better to simply generate 128
3370 bits of random data, as long as the random number generator is of high
3371 quality. The timestamp could conceivably help if the random bits are not
3372 very random. With a quality random number generator, collisions are
3373 extremely unlikely even with only 96 bits, so it's somewhat academic.
3376 Implementations should, however, stick to random data for the first 96 bits
3381 <sect1 id="standard-interfaces">
3382 <title>Standard Interfaces</title>
3384 See <xref linkend="message-protocol-types-notation"/> for details on
3385 the notation used in this section. There are some standard interfaces
3386 that may be useful across various D-Bus applications.
3388 <sect2 id="standard-interfaces-peer">
3389 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3391 The <literal>org.freedesktop.DBus.Peer</literal> interface
3394 org.freedesktop.DBus.Peer.Ping ()
3395 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3399 On receipt of the <literal>METHOD_CALL</literal> message
3400 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3401 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3402 usual. It does not matter which object path a ping is sent to. The
3403 reference implementation handles this method automatically.
3406 On receipt of the <literal>METHOD_CALL</literal> message
3407 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3408 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3409 UUID representing the identity of the machine the process is running on.
3410 This UUID must be the same for all processes on a single system at least
3411 until that system next reboots. It should be the same across reboots
3412 if possible, but this is not always possible to implement and is not
3414 It does not matter which object path a GetMachineId is sent to. The
3415 reference implementation handles this method automatically.
3418 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3419 a virtual machine running on a hypervisor, rather than a physical machine.
3420 Basically if two processes see the same UUID, they should also see the same
3421 shared memory, UNIX domain sockets, process IDs, and other features that require
3422 a running OS kernel in common between the processes.
3425 The UUID is often used where other programs might use a hostname. Hostnames
3426 can change without rebooting, however, or just be "localhost" - so the UUID
3430 <xref linkend="uuids"/> explains the format of the UUID.
3434 <sect2 id="standard-interfaces-introspectable">
3435 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3437 This interface has one method:
3439 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3443 Objects instances may implement
3444 <literal>Introspect</literal> which returns an XML description of
3445 the object, including its interfaces (with signals and methods), objects
3446 below it in the object path tree, and its properties.
3449 <xref linkend="introspection-format"/> describes the format of this XML string.
3452 <sect2 id="standard-interfaces-properties">
3453 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3455 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3456 or <firstterm>attributes</firstterm>. These can be exposed via the
3457 <literal>org.freedesktop.DBus.Properties</literal> interface.
3461 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3462 in STRING property_name,
3464 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3465 in STRING property_name,
3467 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3468 out DICT<STRING,VARIANT> props);
3472 It is conventional to give D-Bus properties names consisting of
3473 capitalized words without punctuation ("CamelCase"), like
3474 <link linkend="message-protocol-names-member">member names</link>.
3475 For instance, the GObject property
3476 <literal>connection-status</literal> or the Qt property
3477 <literal>connectionStatus</literal> could be represented on D-Bus
3478 as <literal>ConnectionStatus</literal>.
3481 Strictly speaking, D-Bus property names are not required to follow
3482 the same naming restrictions as member names, but D-Bus property
3483 names that would not be valid member names (in particular,
3484 GObject-style dash-separated property names) can cause interoperability
3485 problems and should be avoided.
3488 The available properties and whether they are writable can be determined
3489 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3490 see <xref linkend="standard-interfaces-introspectable"/>.
3493 An empty string may be provided for the interface name; in this case,
3494 if there are multiple properties on an object with the same name,
3495 the results are undefined (picking one by according to an arbitrary
3496 deterministic rule, or returning an error, are the reasonable
3500 If one or more properties change on an object, the
3501 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3502 signal may be emitted (this signal was added in 0.14):
3506 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3507 DICT<STRING,VARIANT> changed_properties,
3508 ARRAY<STRING> invalidated_properties);
3512 where <literal>changed_properties</literal> is a dictionary
3513 containing the changed properties with the new values and
3514 <literal>invalidated_properties</literal> is an array of
3515 properties that changed but the value is not conveyed.
3518 Whether the <literal>PropertiesChanged</literal> signal is
3519 supported can be determined by calling
3520 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3521 that the signal may be supported for an object but it may
3522 differ how whether and how it is used on a per-property basis
3523 (for e.g. performance or security reasons). Each property (or
3524 the parent interface) must be annotated with the
3525 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3526 annotation to convey this (usually the default value
3527 <literal>true</literal> is sufficient meaning that the
3528 annotation does not need to be used). See <xref
3529 linkend="introspection-format"/> for details on this
3534 <sect2 id="standard-interfaces-objectmanager">
3535 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3537 An API can optionally make use of this interface for one or
3538 more sub-trees of objects. The root of each sub-tree implements
3539 this interface so other applications can get all objects,
3540 interfaces and properties in a single method call. It is
3541 appropriate to use this interface if users of the tree of
3542 objects are expected to be interested in all interfaces of all
3543 objects in the tree; a more granular API should be used if
3544 users of the objects are expected to be interested in a small
3545 subset of the objects, a small subset of their interfaces, or
3549 The method that applications can use to get all objects and
3550 properties is <literal>GetManagedObjects</literal>:
3554 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3558 The return value of this method is a dict whose keys are
3559 object paths. All returned object paths are children of the
3560 object path implementing this interface, i.e. their object
3561 paths start with the ObjectManager's object path plus '/'.
3564 Each value is a dict whose keys are interfaces names. Each
3565 value in this inner dict is the same dict that would be
3566 returned by the <link
3567 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3568 method for that combination of object path and interface. If
3569 an interface has no properties, the empty dict is returned.
3572 Changes are emitted using the following two signals:
3576 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3577 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3578 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3579 ARRAY<STRING> interfaces);
3583 The <literal>InterfacesAdded</literal> signal is emitted when
3584 either a new object is added or when an existing object gains
3585 one or more interfaces. The
3586 <literal>InterfacesRemoved</literal> signal is emitted
3587 whenever an object is removed or it loses one or more
3588 interfaces. The second parameter of the
3589 <literal>InterfacesAdded</literal> signal contains a dict with
3590 the interfaces and properties (if any) that have been added to
3591 the given object path. Similarly, the second parameter of the
3592 <literal>InterfacesRemoved</literal> signal contains an array
3593 of the interfaces that were removed. Note that changes on
3594 properties on existing interfaces are not reported using this
3595 interface - an application should also monitor the existing <link
3596 linkend="standard-interfaces-properties">PropertiesChanged</link>
3597 signal on each object.
3600 Applications SHOULD NOT export objects that are children of an
3601 object (directly or otherwise) implementing this interface but
3602 which are not returned in the reply from the
3603 <literal>GetManagedObjects()</literal> method of this
3604 interface on the given object.
3607 The intent of the <literal>ObjectManager</literal> interface
3608 is to make it easy to write a robust client
3609 implementation. The trivial client implementation only needs
3610 to make two method calls:
3614 org.freedesktop.DBus.AddMatch (bus_proxy,
3615 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3616 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3620 on the message bus and the remote application's
3621 <literal>ObjectManager</literal>, respectively. Whenever a new
3622 remote object is created (or an existing object gains a new
3623 interface), the <literal>InterfacesAdded</literal> signal is
3624 emitted, and since this signal contains all properties for the
3625 interfaces, no calls to the
3626 <literal>org.freedesktop.Properties</literal> interface on the
3627 remote object are needed. Additionally, since the initial
3628 <literal>AddMatch()</literal> rule already includes signal
3629 messages from the newly created child object, no new
3630 <literal>AddMatch()</literal> call is needed.
3635 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3636 interface was added in version 0.17 of the D-Bus
3643 <sect1 id="introspection-format">
3644 <title>Introspection Data Format</title>
3646 As described in <xref linkend="standard-interfaces-introspectable"/>,
3647 objects may be introspected at runtime, returning an XML string
3648 that describes the object. The same XML format may be used in
3649 other contexts as well, for example as an "IDL" for generating
3650 static language bindings.
3653 Here is an example of introspection data:
3655 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3656 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3657 <node name="/org/freedesktop/sample_object">
3658 <interface name="org.freedesktop.SampleInterface">
3659 <method name="Frobate">
3660 <arg name="foo" type="i" direction="in"/>
3661 <arg name="bar" type="s" direction="out"/>
3662 <arg name="baz" type="a{us}" direction="out"/>
3663 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3665 <method name="Bazify">
3666 <arg name="bar" type="(iiu)" direction="in"/>
3667 <arg name="bar" type="v" direction="out"/>
3669 <method name="Mogrify">
3670 <arg name="bar" type="(iiav)" direction="in"/>
3672 <signal name="Changed">
3673 <arg name="new_value" type="b"/>
3675 <property name="Bar" type="y" access="readwrite"/>
3677 <node name="child_of_sample_object"/>
3678 <node name="another_child_of_sample_object"/>
3683 A more formal DTD and spec needs writing, but here are some quick notes.
3687 Only the root <node> element can omit the node name, as it's
3688 known to be the object that was introspected. If the root
3689 <node> does have a name attribute, it must be an absolute
3690 object path. If child <node> have object paths, they must be
3696 If a child <node> has any sub-elements, then they
3697 must represent a complete introspection of the child.
3698 If a child <node> is empty, then it may or may
3699 not have sub-elements; the child must be introspected
3700 in order to find out. The intent is that if an object
3701 knows that its children are "fast" to introspect
3702 it can go ahead and return their information, but
3703 otherwise it can omit it.
3708 The direction element on <arg> may be omitted,
3709 in which case it defaults to "in" for method calls
3710 and "out" for signals. Signals only allow "out"
3711 so while direction may be specified, it's pointless.
3716 The possible directions are "in" and "out",
3717 unlike CORBA there is no "inout"
3722 The possible property access flags are
3723 "readwrite", "read", and "write"
3728 Multiple interfaces can of course be listed for
3734 The "name" attribute on arguments is optional.
3740 Method, interface, property, and signal elements may have
3741 "annotations", which are generic key/value pairs of metadata.
3742 They are similar conceptually to Java's annotations and C# attributes.
3743 Well-known annotations:
3750 <entry>Values (separated by ,)</entry>
3751 <entry>Description</entry>
3756 <entry>org.freedesktop.DBus.Deprecated</entry>
3757 <entry>true,false</entry>
3758 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3761 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3762 <entry>(string)</entry>
3763 <entry>The C symbol; may be used for methods and interfaces</entry>
3766 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3767 <entry>true,false</entry>
3768 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3771 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3772 <entry>true,invalidates,false</entry>
3775 If set to <literal>false</literal>, the
3776 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3778 linkend="standard-interfaces-properties"/> is not
3779 guaranteed to be emitted if the property changes.
3782 If set to <literal>invalidates</literal> the signal
3783 is emitted but the value is not included in the
3787 If set to <literal>true</literal> the signal is
3788 emitted with the value included.
3791 The value for the annotation defaults to
3792 <literal>true</literal> if the enclosing interface
3793 element does not specify the annotation. Otherwise it
3794 defaults to the value specified in the enclosing
3803 <sect1 id="message-bus">
3804 <title>Message Bus Specification</title>
3805 <sect2 id="message-bus-overview">
3806 <title>Message Bus Overview</title>
3808 The message bus accepts connections from one or more applications.
3809 Once connected, applications can exchange messages with other
3810 applications that are also connected to the bus.
3813 In order to route messages among connections, the message bus keeps a
3814 mapping from names to connections. Each connection has one
3815 unique-for-the-lifetime-of-the-bus name automatically assigned.
3816 Applications may request additional names for a connection. Additional
3817 names are usually "well-known names" such as
3818 "org.freedesktop.TextEditor". When a name is bound to a connection,
3819 that connection is said to <firstterm>own</firstterm> the name.
3822 The bus itself owns a special name,
3823 <literal>org.freedesktop.DBus</literal>, with an object
3824 located at <literal>/org/freedesktop/DBus</literal> that
3825 implements the <literal>org.freedesktop.DBus</literal>
3826 interface. This service allows applications to make
3827 administrative requests of the bus itself. For example,
3828 applications can ask the bus to assign a name to a connection.
3831 Each name may have <firstterm>queued owners</firstterm>. When an
3832 application requests a name for a connection and the name is already in
3833 use, the bus will optionally add the connection to a queue waiting for
3834 the name. If the current owner of the name disconnects or releases
3835 the name, the next connection in the queue will become the new owner.
3839 This feature causes the right thing to happen if you start two text
3840 editors for example; the first one may request "org.freedesktop.TextEditor",
3841 and the second will be queued as a possible owner of that name. When
3842 the first exits, the second will take over.
3846 Applications may send <firstterm>unicast messages</firstterm> to
3847 a specific recipient or to the message bus itself, or
3848 <firstterm>broadcast messages</firstterm> to all interested recipients.
3849 See <xref linkend="message-bus-routing"/> for details.
3853 <sect2 id="message-bus-names">
3854 <title>Message Bus Names</title>
3856 Each connection has at least one name, assigned at connection time and
3857 returned in response to the
3858 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3859 automatically-assigned name is called the connection's <firstterm>unique
3860 name</firstterm>. Unique names are never reused for two different
3861 connections to the same bus.
3864 Ownership of a unique name is a prerequisite for interaction with
3865 the message bus. It logically follows that the unique name is always
3866 the first name that an application comes to own, and the last
3867 one that it loses ownership of.
3870 Unique connection names must begin with the character ':' (ASCII colon
3871 character); bus names that are not unique names must not begin
3872 with this character. (The bus must reject any attempt by an application
3873 to manually request a name beginning with ':'.) This restriction
3874 categorically prevents "spoofing"; messages sent to a unique name
3875 will always go to the expected connection.
3878 When a connection is closed, all the names that it owns are deleted (or
3879 transferred to the next connection in the queue if any).
3882 A connection can request additional names to be associated with it using
3883 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3884 linkend="message-protocol-names-bus"/> describes the format of a valid
3885 name. These names can be released again using the
3886 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3889 <sect3 id="bus-messages-request-name">
3890 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3894 UINT32 RequestName (in STRING name, in UINT32 flags)
3901 <entry>Argument</entry>
3903 <entry>Description</entry>
3909 <entry>STRING</entry>
3910 <entry>Name to request</entry>
3914 <entry>UINT32</entry>
3915 <entry>Flags</entry>
3925 <entry>Argument</entry>
3927 <entry>Description</entry>
3933 <entry>UINT32</entry>
3934 <entry>Return value</entry>
3941 This method call should be sent to
3942 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3943 assign the given name to the method caller. Each name maintains a
3944 queue of possible owners, where the head of the queue is the primary
3945 or current owner of the name. Each potential owner in the queue
3946 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3947 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3948 call. When RequestName is invoked the following occurs:
3952 If the method caller is currently the primary owner of the name,
3953 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3954 values are updated with the values from the new RequestName call,
3955 and nothing further happens.
3961 If the current primary owner (head of the queue) has
3962 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3963 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3964 the caller of RequestName replaces the current primary owner at
3965 the head of the queue and the current primary owner moves to the
3966 second position in the queue. If the caller of RequestName was
3967 in the queue previously its flags are updated with the values from
3968 the new RequestName in addition to moving it to the head of the queue.
3974 If replacement is not possible, and the method caller is
3975 currently in the queue but not the primary owner, its flags are
3976 updated with the values from the new RequestName call.
3982 If replacement is not possible, and the method caller is
3983 currently not in the queue, the method caller is appended to the
3990 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3991 set and is not the primary owner, it is removed from the
3992 queue. This can apply to the previous primary owner (if it
3993 was replaced) or the method caller (if it updated the
3994 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3995 queue, or if it was just added to the queue with that flag set).
4001 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4002 queue," even if another application already in the queue had specified
4003 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4004 that does not allow replacement goes away, and the next primary owner
4005 does allow replacement. In this case, queued items that specified
4006 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4007 automatically replace the new primary owner. In other words,
4008 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4009 time RequestName is called. This is deliberate to avoid an infinite loop
4010 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4011 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4014 The flags argument contains any of the following values logically ORed
4021 <entry>Conventional Name</entry>
4022 <entry>Value</entry>
4023 <entry>Description</entry>
4028 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4032 If an application A specifies this flag and succeeds in
4033 becoming the owner of the name, and another application B
4034 later calls RequestName with the
4035 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4036 will lose ownership and receive a
4037 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4038 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4039 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4040 is not specified by application B, then application B will not replace
4041 application A as the owner.
4046 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4050 Try to replace the current owner if there is one. If this
4051 flag is not set the application will only become the owner of
4052 the name if there is no current owner. If this flag is set,
4053 the application will replace the current owner if
4054 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4059 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4063 Without this flag, if an application requests a name that is
4064 already owned, the application will be placed in a queue to
4065 own the name when the current owner gives it up. If this
4066 flag is given, the application will not be placed in the
4067 queue, the request for the name will simply fail. This flag
4068 also affects behavior when an application is replaced as
4069 name owner; by default the application moves back into the
4070 waiting queue, unless this flag was provided when the application
4071 became the name owner.
4079 The return code can be one of the following values:
4085 <entry>Conventional Name</entry>
4086 <entry>Value</entry>
4087 <entry>Description</entry>
4092 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4093 <entry>1</entry> <entry>The caller is now the primary owner of
4094 the name, replacing any previous owner. Either the name had no
4095 owner before, or the caller specified
4096 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4097 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4100 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4103 <entry>The name already had an owner,
4104 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4105 the current owner did not specify
4106 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4107 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4111 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4112 <entry>The name already has an owner,
4113 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4114 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4115 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4116 specified by the requesting application.</entry>
4119 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4121 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4129 <sect3 id="bus-messages-release-name">
4130 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4134 UINT32 ReleaseName (in STRING name)
4141 <entry>Argument</entry>
4143 <entry>Description</entry>
4149 <entry>STRING</entry>
4150 <entry>Name to release</entry>
4160 <entry>Argument</entry>
4162 <entry>Description</entry>
4168 <entry>UINT32</entry>
4169 <entry>Return value</entry>
4176 This method call should be sent to
4177 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4178 release the method caller's claim to the given name. If the caller is
4179 the primary owner, a new primary owner will be selected from the
4180 queue if any other owners are waiting. If the caller is waiting in
4181 the queue for the name, the caller will removed from the queue and
4182 will not be made an owner of the name if it later becomes available.
4183 If there are no other owners in the queue for the name, it will be
4184 removed from the bus entirely.
4186 The return code can be one of the following values:
4192 <entry>Conventional Name</entry>
4193 <entry>Value</entry>
4194 <entry>Description</entry>
4199 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4200 <entry>1</entry> <entry>The caller has released his claim on
4201 the given name. Either the caller was the primary owner of
4202 the name, and the name is now unused or taken by somebody
4203 waiting in the queue for the name, or the caller was waiting
4204 in the queue for the name and has now been removed from the
4208 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4210 <entry>The given name does not exist on this bus.</entry>
4213 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4215 <entry>The caller was not the primary owner of this name,
4216 and was also not waiting in the queue to own this name.</entry>
4224 <sect3 id="bus-messages-list-queued-owners">
4225 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4229 ARRAY of STRING ListQueuedOwners (in STRING name)
4236 <entry>Argument</entry>
4238 <entry>Description</entry>
4244 <entry>STRING</entry>
4245 <entry>The well-known bus name to query, such as
4246 <literal>com.example.cappuccino</literal></entry>
4256 <entry>Argument</entry>
4258 <entry>Description</entry>
4264 <entry>ARRAY of STRING</entry>
4265 <entry>The unique bus names of connections currently queued
4266 for the name</entry>
4273 This method call should be sent to
4274 <literal>org.freedesktop.DBus</literal> and lists the connections
4275 currently queued for a bus name (see
4276 <xref linkend="term-queued-owner"/>).
4281 <sect2 id="message-bus-routing">
4282 <title>Message Bus Message Routing</title>
4285 Messages may have a <literal>DESTINATION</literal> field (see <xref
4286 linkend="message-protocol-header-fields"/>), resulting in a
4287 <firstterm>unicast message</firstterm>. If the
4288 <literal>DESTINATION</literal> field is present, it specifies a message
4289 recipient by name. Method calls and replies normally specify this field.
4290 The message bus must send messages (of any type) with the
4291 <literal>DESTINATION</literal> field set to the specified recipient,
4292 regardless of whether the recipient has set up a match rule matching
4297 When the message bus receives a signal, if the
4298 <literal>DESTINATION</literal> field is absent, it is considered to
4299 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4300 applications with <firstterm>message matching rules</firstterm> that
4301 match the message. Most signal messages are broadcasts.
4305 Unicast signal messages (those with a <literal>DESTINATION</literal>
4306 field) are not commonly used, but they are treated like any unicast
4307 message: they are delivered to the specified receipient,
4308 regardless of its match rules. One use for unicast signals is to
4309 avoid a race condition in which a signal is emitted before the intended
4310 recipient can call <xref linkend="bus-messages-add-match"/> to
4311 receive that signal: if the signal is sent directly to that recipient
4312 using a unicast message, it does not need to add a match rule at all,
4313 and there is no race condition. Another use for unicast signals,
4314 on message buses whose security policy prevents eavesdropping, is to
4315 send sensitive information which should only be visible to one
4320 When the message bus receives a method call, if the
4321 <literal>DESTINATION</literal> field is absent, the call is taken to be
4322 a standard one-to-one message and interpreted by the message bus
4323 itself. For example, sending an
4324 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4325 <literal>DESTINATION</literal> will cause the message bus itself to
4326 reply to the ping immediately; the message bus will not make this
4327 message visible to other applications.
4331 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4332 the ping message were sent with a <literal>DESTINATION</literal> name of
4333 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4334 forwarded, and the Yoyodyne Corporation screensaver application would be
4335 expected to reply to the ping.
4339 Message bus implementations may impose a security policy which
4340 prevents certain messages from being sent or received.
4341 When a message cannot be sent or received due to a security
4342 policy, the message bus should send an error reply, unless the
4343 original message had the <literal>NO_REPLY</literal> flag.
4346 <sect3 id="message-bus-routing-eavesdropping">
4347 <title>Eavesdropping</title>
4349 Receiving a unicast message whose <literal>DESTINATION</literal>
4350 indicates a different recipient is called
4351 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4352 a security boundary (like the standard system bus), the security
4353 policy should usually prevent eavesdropping, since unicast messages
4354 are normally kept private and may contain security-sensitive
4359 Eavesdropping is mainly useful for debugging tools, such as
4360 the <literal>dbus-monitor</literal> tool in the reference
4361 implementation of D-Bus. Tools which eavesdrop on the message bus
4362 should be careful to avoid sending a reply or error in response to
4363 messages intended for a different client.
4367 Clients may attempt to eavesdrop by adding match rules
4368 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4369 the <literal>eavesdrop='true'</literal> match. If the message bus'
4370 security policy does not allow eavesdropping, the match rule can
4371 still be added, but will not have any practical effect. For
4372 compatibility with older message bus implementations, if adding such
4373 a match rule results in an error reply, the client may fall back to
4374 adding the same rule with the <literal>eavesdrop</literal> match
4379 <sect3 id="message-bus-routing-match-rules">
4380 <title>Match Rules</title>
4382 An important part of the message bus routing protocol is match
4383 rules. Match rules describe the messages that should be sent to a
4384 client, based on the contents of the message. Broadcast signals
4385 are only sent to clients which have a suitable match rule: this
4386 avoids waking up client processes to deal with signals that are
4387 not relevant to that client.
4390 Messages that list a client as their <literal>DESTINATION</literal>
4391 do not need to match the client's match rules, and are sent to that
4392 client regardless. As a result, match rules are mainly used to
4393 receive a subset of broadcast signals.
4396 Match rules can also be used for eavesdropping
4397 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4398 if the security policy of the message bus allows it.
4401 Match rules are added using the AddMatch bus method
4402 (see <xref linkend="bus-messages-add-match"/>). Rules are
4403 specified as a string of comma separated key/value pairs.
4404 Excluding a key from the rule indicates a wildcard match.
4405 For instance excluding the the member from a match rule but
4406 adding a sender would let all messages from that sender through.
4407 An example of a complete rule would be
4408 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4411 The following table describes the keys that can be used to create
4413 The following table summarizes the D-Bus types.
4419 <entry>Possible Values</entry>
4420 <entry>Description</entry>
4425 <entry><literal>type</literal></entry>
4426 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4427 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4430 <entry><literal>sender</literal></entry>
4431 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4432 and <xref linkend="term-unique-name"/> respectively)
4434 <entry>Match messages sent by a particular sender. An example of a sender match
4435 is sender='org.freedesktop.Hal'</entry>
4438 <entry><literal>interface</literal></entry>
4439 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4440 <entry>Match messages sent over or to a particular interface. An example of an
4441 interface match is interface='org.freedesktop.Hal.Manager'.
4442 If a message omits the interface header, it must not match any rule
4443 that specifies this key.</entry>
4446 <entry><literal>member</literal></entry>
4447 <entry>Any valid method or signal name</entry>
4448 <entry>Matches messages which have the give method or signal name. An example of
4449 a member match is member='NameOwnerChanged'</entry>
4452 <entry><literal>path</literal></entry>
4453 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4454 <entry>Matches messages which are sent from or to the given object. An example of a
4455 path match is path='/org/freedesktop/Hal/Manager'</entry>
4458 <entry><literal>path_namespace</literal></entry>
4459 <entry>An object path</entry>
4462 Matches messages which are sent from or to an
4463 object for which the object path is either the
4464 given value, or that value followed by one or
4465 more path components.
4470 <literal>path_namespace='/com/example/foo'</literal>
4471 would match signals sent by
4472 <literal>/com/example/foo</literal>
4474 <literal>/com/example/foo/bar</literal>,
4476 <literal>/com/example/foobar</literal>.
4480 Using both <literal>path</literal> and
4481 <literal>path_namespace</literal> in the same match
4482 rule is not allowed.
4487 This match key was added in version 0.16 of the
4488 D-Bus specification and implemented by the bus
4489 daemon in dbus 1.5.0 and later.
4495 <entry><literal>destination</literal></entry>
4496 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4497 <entry>Matches messages which are being sent to the given unique name. An
4498 example of a destination match is destination=':1.0'</entry>
4501 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4502 <entry>Any string</entry>
4503 <entry>Arg matches are special and are used for further restricting the
4504 match based on the arguments in the body of a message. Only arguments of type
4505 STRING can be matched in this way. An example of an argument match
4506 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4510 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4511 <entry>Any string</entry>
4513 <para>Argument path matches provide a specialised form of wildcard matching for
4514 path-like namespaces. They can match arguments whose type is either STRING or
4515 OBJECT_PATH. As with normal argument matches,
4516 if the argument is exactly equal to the string given in the match
4517 rule then the rule is satisfied. Additionally, there is also a
4518 match when either the string given in the match rule or the
4519 appropriate message argument ends with '/' and is a prefix of the
4520 other. An example argument path match is arg0path='/aa/bb/'. This
4521 would match messages with first arguments of '/', '/aa/',
4522 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4523 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4525 <para>This is intended for monitoring “directories” in file system-like
4526 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4527 system. An application interested in all nodes in a particular hierarchy would
4528 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4529 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4530 represent a modification to the “bar” property, or a signal with zeroth
4531 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4532 many properties within that directory, and the interested application would be
4533 notified in both cases.</para>
4536 This match key was added in version 0.12 of the
4537 D-Bus specification, implemented for STRING
4538 arguments by the bus daemon in dbus 1.2.0 and later,
4539 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4546 <entry><literal>arg0namespace</literal></entry>
4547 <entry>Like a bus name, except that the string is not
4548 required to contain a '.' (period)</entry>
4550 <para>Match messages whose first argument is of type STRING, and is a bus name
4551 or interface name within the specified namespace. This is primarily intended
4552 for watching name owner changes for a group of related bus names, rather than
4553 for a single name or all name changes.</para>
4555 <para>Because every valid interface name is also a valid
4556 bus name, this can also be used for messages whose
4557 first argument is an interface name.</para>
4559 <para>For example, the match rule
4560 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4561 matches name owner changes for bus names such as
4562 <literal>com.example.backend.foo</literal>,
4563 <literal>com.example.backend.foo.bar</literal>, and
4564 <literal>com.example.backend</literal> itself.</para>
4566 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4569 This match key was added in version 0.16 of the
4570 D-Bus specification and implemented by the bus
4571 daemon in dbus 1.5.0 and later.
4577 <entry><literal>eavesdrop</literal></entry>
4578 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4579 <entry>Since D-Bus 1.5.6, match rules do not
4580 match messages which have a <literal>DESTINATION</literal>
4581 field unless the match rule specifically
4583 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4584 by specifying <literal>eavesdrop='true'</literal>
4585 in the match rule. <literal>eavesdrop='false'</literal>
4586 restores the default behaviour. Messages are
4587 delivered to their <literal>DESTINATION</literal>
4588 regardless of match rules, so this match does not
4589 affect normal delivery of unicast messages.
4590 If the message bus has a security policy which forbids
4591 eavesdropping, this match may still be used without error,
4592 but will not have any practical effect.
4593 In older versions of D-Bus, this match was not allowed
4594 in match rules, and all match rules behaved as if
4595 <literal>eavesdrop='true'</literal> had been used.
4604 <sect2 id="message-bus-starting-services">
4605 <title>Message Bus Starting Services</title>
4607 The message bus can start applications on behalf of other applications.
4608 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4609 An application that can be started in this way is called a
4610 <firstterm>service</firstterm>.
4613 With D-Bus, starting a service is normally done by name. That is,
4614 applications ask the message bus to start some program that will own a
4615 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4616 This implies a contract documented along with the name
4617 <literal>org.freedesktop.TextEditor</literal> for which objects
4618 the owner of that name will provide, and what interfaces those
4622 To find an executable corresponding to a particular name, the bus daemon
4623 looks for <firstterm>service description files</firstterm>. Service
4624 description files define a mapping from names to executables. Different
4625 kinds of message bus will look for these files in different places, see
4626 <xref linkend="message-bus-types"/>.
4629 Service description files have the ".service" file
4630 extension. The message bus will only load service description files
4631 ending with .service; all other files will be ignored. The file format
4632 is similar to that of <ulink
4633 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4634 entries</ulink>. All service description files must be in UTF-8
4635 encoding. To ensure that there will be no name collisions, service files
4636 must be namespaced using the same mechanism as messages and service
4641 [FIXME the file format should be much better specified than "similar to
4642 .desktop entries" esp. since desktop entries are already
4643 badly-specified. ;-)]
4644 These sections from the specification apply to service files as well:
4647 <listitem><para>General syntax</para></listitem>
4648 <listitem><para>Comment format</para></listitem>
4652 <title>Example service description file</title>
4654 # Sample service description file
4656 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4657 Exec=/usr/libexec/gconfd-2
4662 When an application asks to start a service by name, the bus daemon tries to
4663 find a service that will own that name. It then tries to spawn the
4664 executable associated with it. If this fails, it will report an
4665 error. [FIXME what happens if two .service files offer the same service;
4666 what kind of error is reported, should we have a way for the client to
4670 The executable launched will have the environment variable
4671 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4672 message bus so it can connect and request the appropriate names.
4675 The executable being launched may want to know whether the message bus
4676 starting it is one of the well-known message buses (see <xref
4677 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4678 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4679 of the well-known buses. The currently-defined values for this variable
4680 are <literal>system</literal> for the systemwide message bus,
4681 and <literal>session</literal> for the per-login-session message
4682 bus. The new executable must still connect to the address given
4683 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4684 resulting connection is to the well-known bus.
4687 [FIXME there should be a timeout somewhere, either specified
4688 in the .service file, by the client, or just a global value
4689 and if the client being activated fails to connect within that
4690 timeout, an error should be sent back.]
4693 <sect3 id="message-bus-starting-services-scope">
4694 <title>Message Bus Service Scope</title>
4696 The "scope" of a service is its "per-", such as per-session,
4697 per-machine, per-home-directory, or per-display. The reference
4698 implementation doesn't yet support starting services in a different
4699 scope from the message bus itself. So e.g. if you start a service
4700 on the session bus its scope is per-session.
4703 We could add an optional scope to a bus name. For example, for
4704 per-(display,session pair), we could have a unique ID for each display
4705 generated automatically at login and set on screen 0 by executing a
4706 special "set display ID" binary. The ID would be stored in a
4707 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4708 random bytes. This ID would then be used to scope names.
4709 Starting/locating a service could be done by ID-name pair rather than
4713 Contrast this with a per-display scope. To achieve that, we would
4714 want a single bus spanning all sessions using a given display.
4715 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4716 property on screen 0 of the display, pointing to this bus.
4721 <sect2 id="message-bus-types">
4722 <title>Well-known Message Bus Instances</title>
4724 Two standard message bus instances are defined here, along with how
4725 to locate them and where their service files live.
4727 <sect3 id="message-bus-types-login">
4728 <title>Login session message bus</title>
4730 Each time a user logs in, a <firstterm>login session message
4731 bus</firstterm> may be started. All applications in the user's login
4732 session may interact with one another using this message bus.
4735 The address of the login session message bus is given
4736 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4737 variable. If that variable is not set, applications may
4738 also try to read the address from the X Window System root
4739 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4740 The root window property must have type <literal>STRING</literal>.
4741 The environment variable should have precedence over the
4742 root window property.
4744 <para>The address of the login session message bus is given in the
4745 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4746 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4747 "autolaunch:", the system should use platform-specific methods of
4748 locating a running D-Bus session server, or starting one if a running
4749 instance cannot be found. Note that this mechanism is not recommended
4750 for attempting to determine if a daemon is running. It is inherently
4751 racy to attempt to make this determination, since the bus daemon may
4752 be started just before or just after the determination is made.
4753 Therefore, it is recommended that applications do not try to make this
4754 determination for their functionality purposes, and instead they
4755 should attempt to start the server.</para>
4757 <sect4 id="message-bus-types-login-x-windows">
4758 <title>X Windowing System</title>
4760 For the X Windowing System, the application must locate the
4761 window owner of the selection represented by the atom formed by
4765 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4769 <para>the current user's username</para>
4773 <para>the literal character '_' (underscore)</para>
4777 <para>the machine's ID</para>
4783 The following properties are defined for the window that owns
4785 <informaltable frame="all">
4794 <para>meaning</para>
4800 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4804 <para>the actual address of the server socket</para>
4810 <para>_DBUS_SESSION_BUS_PID</para>
4814 <para>the PID of the server process</para>
4823 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4824 present in this window.
4828 If the X selection cannot be located or if reading the
4829 properties from the window fails, the implementation MUST conclude
4830 that there is no D-Bus server running and proceed to start a new
4831 server. (See below on concurrency issues)
4835 Failure to connect to the D-Bus server address thus obtained
4836 MUST be treated as a fatal connection error and should be reported
4841 As an alternative, an implementation MAY find the information
4842 in the following file located in the current user's home directory,
4843 in subdirectory .dbus/session-bus/:
4846 <para>the machine's ID</para>
4850 <para>the literal character '-' (dash)</para>
4854 <para>the X display without the screen number, with the
4855 following prefixes removed, if present: ":", "localhost:"
4856 ."localhost.localdomain:". That is, a display of
4857 "localhost:10.0" produces just the number "10"</para>
4863 The contents of this file NAME=value assignment pairs and
4864 lines starting with # are comments (no comments are allowed
4865 otherwise). The following variable names are defined:
4872 <para>Variable</para>
4876 <para>meaning</para>
4882 <para>DBUS_SESSION_BUS_ADDRESS</para>
4886 <para>the actual address of the server socket</para>
4892 <para>DBUS_SESSION_BUS_PID</para>
4896 <para>the PID of the server process</para>
4902 <para>DBUS_SESSION_BUS_WINDOWID</para>
4906 <para>the window ID</para>
4915 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4920 Failure to open this file MUST be interpreted as absence of a
4921 running server. Therefore, the implementation MUST proceed to
4922 attempting to launch a new bus server if the file cannot be
4927 However, success in opening this file MUST NOT lead to the
4928 conclusion that the server is running. Thus, a failure to connect to
4929 the bus address obtained by the alternative method MUST NOT be
4930 considered a fatal error. If the connection cannot be established,
4931 the implementation MUST proceed to check the X selection settings or
4932 to start the server on its own.
4936 If the implementation concludes that the D-Bus server is not
4937 running it MUST attempt to start a new server and it MUST also
4938 ensure that the daemon started as an effect of the "autolaunch"
4939 mechanism provides the lookup mechanisms described above, so
4940 subsequent calls can locate the newly started server. The
4941 implementation MUST also ensure that if two or more concurrent
4942 initiations happen, only one server remains running and all other
4943 initiations are able to obtain the address of this server and
4944 connect to it. In other words, the implementation MUST ensure that
4945 the X selection is not present when it attempts to set it, without
4946 allowing another process to set the selection between the
4947 verification and the setting (e.g., by using XGrabServer /
4954 On Unix systems, the session bus should search for .service files
4955 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4957 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4958 Implementations may also search additional locations, which
4959 should be searched with lower priority than anything in
4960 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4961 for example, the reference implementation also
4962 looks in <literal>${datadir}/dbus-1/services</literal> as
4963 set at compile time.
4966 As described in the XDG Base Directory Specification, software
4967 packages should install their session .service files to their
4968 configured <literal>${datadir}/dbus-1/services</literal>,
4969 where <literal>${datadir}</literal> is as defined by the GNU
4970 coding standards. System administrators or users can arrange
4971 for these service files to be read by setting XDG_DATA_DIRS or by
4972 symlinking them into the default locations.
4976 <sect3 id="message-bus-types-system">
4977 <title>System message bus</title>
4979 A computer may have a <firstterm>system message bus</firstterm>,
4980 accessible to all applications on the system. This message bus may be
4981 used to broadcast system events, such as adding new hardware devices,
4982 changes in the printer queue, and so forth.
4985 The address of the system message bus is given
4986 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4987 variable. If that variable is not set, applications should try
4988 to connect to the well-known address
4989 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4992 The D-Bus reference implementation actually honors the
4993 <literal>$(localstatedir)</literal> configure option
4994 for this address, on both client and server side.
4999 On Unix systems, the system bus should default to searching
5000 for .service files in
5001 <literal>/usr/local/share/dbus-1/system-services</literal>,
5002 <literal>/usr/share/dbus-1/system-services</literal> and
5003 <literal>/lib/dbus-1/system-services</literal>, with that order
5004 of precedence. It may also search other implementation-specific
5005 locations, but should not vary these locations based on environment
5009 The system bus is security-sensitive and is typically executed
5010 by an init system with a clean environment. Its launch helper
5011 process is particularly security-sensitive, and specifically
5012 clears its own environment.
5017 Software packages should install their system .service
5018 files to their configured
5019 <literal>${datadir}/dbus-1/system-services</literal>,
5020 where <literal>${datadir}</literal> is as defined by the GNU
5021 coding standards. System administrators can arrange
5022 for these service files to be read by editing the system bus'
5023 configuration file or by symlinking them into the default
5029 <sect2 id="message-bus-messages">
5030 <title>Message Bus Messages</title>
5032 The special message bus name <literal>org.freedesktop.DBus</literal>
5033 responds to a number of additional messages.
5036 <sect3 id="bus-messages-hello">
5037 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5048 <entry>Argument</entry>
5050 <entry>Description</entry>
5056 <entry>STRING</entry>
5057 <entry>Unique name assigned to the connection</entry>
5064 Before an application is able to send messages to other applications
5065 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5066 to the message bus to obtain a unique name. If an application without
5067 a unique name tries to send a message to another application, or a
5068 message to the message bus itself that isn't the
5069 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5070 disconnected from the bus.
5073 There is no corresponding "disconnect" request; if a client wishes to
5074 disconnect from the bus, it simply closes the socket (or other
5075 communication channel).
5078 <sect3 id="bus-messages-list-names">
5079 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5083 ARRAY of STRING ListNames ()
5090 <entry>Argument</entry>
5092 <entry>Description</entry>
5098 <entry>ARRAY of STRING</entry>
5099 <entry>Array of strings where each string is a bus name</entry>
5106 Returns a list of all currently-owned names on the bus.
5109 <sect3 id="bus-messages-list-activatable-names">
5110 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5114 ARRAY of STRING ListActivatableNames ()
5121 <entry>Argument</entry>
5123 <entry>Description</entry>
5129 <entry>ARRAY of STRING</entry>
5130 <entry>Array of strings where each string is a bus name</entry>
5137 Returns a list of all names that can be activated on the bus.
5140 <sect3 id="bus-messages-name-exists">
5141 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5145 BOOLEAN NameHasOwner (in STRING name)
5152 <entry>Argument</entry>
5154 <entry>Description</entry>
5160 <entry>STRING</entry>
5161 <entry>Name to check</entry>
5171 <entry>Argument</entry>
5173 <entry>Description</entry>
5179 <entry>BOOLEAN</entry>
5180 <entry>Return value, true if the name exists</entry>
5187 Checks if the specified name exists (currently has an owner).
5191 <sect3 id="bus-messages-name-owner-changed">
5192 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5196 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5203 <entry>Argument</entry>
5205 <entry>Description</entry>
5211 <entry>STRING</entry>
5212 <entry>Name with a new owner</entry>
5216 <entry>STRING</entry>
5217 <entry>Old owner or empty string if none</entry>
5221 <entry>STRING</entry>
5222 <entry>New owner or empty string if none</entry>
5229 This signal indicates that the owner of a name has changed.
5230 It's also the signal to use to detect the appearance of
5231 new names on the bus.
5234 <sect3 id="bus-messages-name-lost">
5235 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5239 NameLost (STRING name)
5246 <entry>Argument</entry>
5248 <entry>Description</entry>
5254 <entry>STRING</entry>
5255 <entry>Name which was lost</entry>
5262 This signal is sent to a specific application when it loses
5263 ownership of a name.
5267 <sect3 id="bus-messages-name-acquired">
5268 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5272 NameAcquired (STRING name)
5279 <entry>Argument</entry>
5281 <entry>Description</entry>
5287 <entry>STRING</entry>
5288 <entry>Name which was acquired</entry>
5295 This signal is sent to a specific application when it gains
5296 ownership of a name.
5300 <sect3 id="bus-messages-start-service-by-name">
5301 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5305 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5312 <entry>Argument</entry>
5314 <entry>Description</entry>
5320 <entry>STRING</entry>
5321 <entry>Name of the service to start</entry>
5325 <entry>UINT32</entry>
5326 <entry>Flags (currently not used)</entry>
5336 <entry>Argument</entry>
5338 <entry>Description</entry>
5344 <entry>UINT32</entry>
5345 <entry>Return value</entry>
5350 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5354 The return value can be one of the following values:
5359 <entry>Identifier</entry>
5360 <entry>Value</entry>
5361 <entry>Description</entry>
5366 <entry>DBUS_START_REPLY_SUCCESS</entry>
5368 <entry>The service was successfully started.</entry>
5371 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5373 <entry>A connection already owns the given name.</entry>
5382 <sect3 id="bus-messages-update-activation-environment">
5383 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5387 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5394 <entry>Argument</entry>
5396 <entry>Description</entry>
5402 <entry>ARRAY of DICT<STRING,STRING></entry>
5403 <entry>Environment to add or update</entry>
5408 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5411 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5414 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.
5419 <sect3 id="bus-messages-get-name-owner">
5420 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5424 STRING GetNameOwner (in STRING name)
5431 <entry>Argument</entry>
5433 <entry>Description</entry>
5439 <entry>STRING</entry>
5440 <entry>Name to get the owner of</entry>
5450 <entry>Argument</entry>
5452 <entry>Description</entry>
5458 <entry>STRING</entry>
5459 <entry>Return value, a unique connection name</entry>
5464 Returns the unique connection name of the primary owner of the name
5465 given. If the requested name doesn't have an owner, returns a
5466 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5470 <sect3 id="bus-messages-get-connection-unix-user">
5471 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5475 UINT32 GetConnectionUnixUser (in STRING bus_name)
5482 <entry>Argument</entry>
5484 <entry>Description</entry>
5490 <entry>STRING</entry>
5491 <entry>Unique or well-known bus name of the connection to
5492 query, such as <literal>:12.34</literal> or
5493 <literal>com.example.tea</literal></entry>
5503 <entry>Argument</entry>
5505 <entry>Description</entry>
5511 <entry>UINT32</entry>
5512 <entry>Unix user ID</entry>
5517 Returns the Unix user ID of the process connected to the server. If
5518 unable to determine it (for instance, because the process is not on the
5519 same machine as the bus daemon), an error is returned.
5523 <sect3 id="bus-messages-get-connection-unix-process-id">
5524 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5528 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5535 <entry>Argument</entry>
5537 <entry>Description</entry>
5543 <entry>STRING</entry>
5544 <entry>Unique or well-known bus name of the connection to
5545 query, such as <literal>:12.34</literal> or
5546 <literal>com.example.tea</literal></entry>
5556 <entry>Argument</entry>
5558 <entry>Description</entry>
5564 <entry>UINT32</entry>
5565 <entry>Unix process id</entry>
5570 Returns the Unix process ID of the process connected to the server. If
5571 unable to determine it (for instance, because the process is not on the
5572 same machine as the bus daemon), an error is returned.
5576 <sect3 id="bus-messages-add-match">
5577 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5581 AddMatch (in STRING rule)
5588 <entry>Argument</entry>
5590 <entry>Description</entry>
5596 <entry>STRING</entry>
5597 <entry>Match rule to add to the connection</entry>
5602 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5603 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5607 <sect3 id="bus-messages-remove-match">
5608 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5612 RemoveMatch (in STRING rule)
5619 <entry>Argument</entry>
5621 <entry>Description</entry>
5627 <entry>STRING</entry>
5628 <entry>Match rule to remove from the connection</entry>
5633 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5634 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5639 <sect3 id="bus-messages-get-id">
5640 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5644 GetId (out STRING id)
5651 <entry>Argument</entry>
5653 <entry>Description</entry>
5659 <entry>STRING</entry>
5660 <entry>Unique ID identifying the bus daemon</entry>
5665 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5666 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5667 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5668 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5669 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5670 by org.freedesktop.DBus.Peer.GetMachineId().
5671 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5679 <appendix id="implementation-notes">
5680 <title>Implementation notes</title>
5681 <sect1 id="implementation-notes-subsection">
5689 <glossary><title>Glossary</title>
5691 This glossary defines some of the terms used in this specification.
5694 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5697 The message bus maintains an association between names and
5698 connections. (Normally, there's one connection per application.) A
5699 bus name is simply an identifier used to locate connections. For
5700 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5701 name might be used to send a message to a screensaver from Yoyodyne
5702 Corporation. An application is said to <firstterm>own</firstterm> a
5703 name if the message bus has associated the application's connection
5704 with the name. Names may also have <firstterm>queued
5705 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5706 The bus assigns a unique name to each connection,
5707 see <xref linkend="term-unique-name"/>. Other names
5708 can be thought of as "well-known names" and are
5709 used to find applications that offer specific functionality.
5713 See <xref linkend="message-protocol-names-bus"/> for details of
5714 the syntax and naming conventions for bus names.
5719 <glossentry id="term-message"><glossterm>Message</glossterm>
5722 A message is the atomic unit of communication via the D-Bus
5723 protocol. It consists of a <firstterm>header</firstterm> and a
5724 <firstterm>body</firstterm>; the body is made up of
5725 <firstterm>arguments</firstterm>.
5730 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5733 The message bus is a special application that forwards
5734 or routes messages between a group of applications
5735 connected to the message bus. It also manages
5736 <firstterm>names</firstterm> used for routing
5742 <glossentry id="term-name"><glossterm>Name</glossterm>
5745 See <xref linkend="term-bus-name"/>. "Name" may
5746 also be used to refer to some of the other names
5747 in D-Bus, such as interface names.
5752 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5755 Used to prevent collisions when defining new interfaces, bus names
5756 etc. The convention used is the same one Java uses for defining
5757 classes: a reversed domain name.
5758 See <xref linkend="message-protocol-names-bus"/>,
5759 <xref linkend="message-protocol-names-interface"/>,
5760 <xref linkend="message-protocol-names-error"/>,
5761 <xref linkend="message-protocol-marshaling-object-path"/>.
5766 <glossentry id="term-object"><glossterm>Object</glossterm>
5769 Each application contains <firstterm>objects</firstterm>, which have
5770 <firstterm>interfaces</firstterm> and
5771 <firstterm>methods</firstterm>. Objects are referred to by a name,
5772 called a <firstterm>path</firstterm>.
5777 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5780 An application talking directly to another application, without going
5781 through a message bus. One-to-one connections may be "peer to peer" or
5782 "client to server." The D-Bus protocol has no concept of client
5783 vs. server after a connection has authenticated; the flow of messages
5784 is symmetrical (full duplex).
5789 <glossentry id="term-path"><glossterm>Path</glossterm>
5792 Object references (object names) in D-Bus are organized into a
5793 filesystem-style hierarchy, so each object is named by a path. As in
5794 LDAP, there's no difference between "files" and "directories"; a path
5795 can refer to an object, while still having child objects below it.
5800 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5803 Each bus name has a primary owner; messages sent to the name go to the
5804 primary owner. However, certain names also maintain a queue of
5805 secondary owners "waiting in the wings." If the primary owner releases
5806 the name, then the first secondary owner in the queue automatically
5807 becomes the new owner of the name.
5812 <glossentry id="term-service"><glossterm>Service</glossterm>
5815 A service is an executable that can be launched by the bus daemon.
5816 Services normally guarantee some particular features, for example they
5817 may guarantee that they will request a specific name such as
5818 "org.freedesktop.Screensaver", have a singleton object
5819 "/org/freedesktop/Application", and that object will implement the
5820 interface "org.freedesktop.ScreensaverControl".
5825 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5828 ".service files" tell the bus about service applications that can be
5829 launched (see <xref linkend="term-service"/>). Most importantly they
5830 provide a mapping from bus names to services that will request those
5831 names when they start up.
5836 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5839 The special name automatically assigned to each connection by the
5840 message bus. This name will never change owner, and will be unique
5841 (never reused during the lifetime of the message bus).
5842 It will begin with a ':' character.