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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.27</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>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.27</revnumber>
75 <date>(not yet released)</date>
76 <authorinitials>n/a</authorinitials>
78 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
82 <revnumber>0.26</revnumber>
83 <date>2015-02-19</date>
84 <authorinitials>smcv, rh</authorinitials>
86 GetConnectionCredentials can return LinuxSecurityLabel or
87 WindowsSID; add privileged BecomeMonitor method
91 <revnumber>0.25</revnumber>
92 <date>2014-11-10</date>
93 <authorinitials>smcv, lennart</authorinitials>
95 ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
99 <revnumber>0.24</revnumber>
100 <date>2014-10-01</date>
101 <authorinitials>SMcV</authorinitials>
103 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
104 document how to quote match rules
108 <revnumber>0.23</revnumber>
109 <date>2014-01-06</date>
110 <authorinitials>SMcV, CY</authorinitials>
112 method call messages with no INTERFACE may be considered an error;
113 document tcp:bind=... and nonce-tcp:bind=...; define listenable
114 and connectable addresses
118 <revnumber>0.22</revnumber>
119 <date>2013-10-09</date>
120 <authorinitials></authorinitials>
121 <revremark>add GetConnectionCredentials, document
122 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
123 document and correct .service file syntax and naming
127 <revnumber>0.21</revnumber>
128 <date>2013-04-25</date>
129 <authorinitials>smcv</authorinitials>
130 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
131 Corrigendum #9)</revremark>
134 <revnumber>0.20</revnumber>
135 <date>22 February 2013</date>
136 <authorinitials>smcv, walters</authorinitials>
137 <revremark>reorganise for clarity, remove false claims about
138 basic types, mention /o/fd/DBus</revremark>
141 <revnumber>0.19</revnumber>
142 <date>20 February 2012</date>
143 <authorinitials>smcv/lp</authorinitials>
144 <revremark>formally define unique connection names and well-known
145 bus names; document best practices for interface, bus, member and
146 error names, and object paths; document the search path for session
147 and system services on Unix; document the systemd transport</revremark>
150 <revnumber>0.18</revnumber>
151 <date>29 July 2011</date>
152 <authorinitials>smcv</authorinitials>
153 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
154 match keyword; promote type system to a top-level section</revremark>
157 <revnumber>0.17</revnumber>
158 <date>1 June 2011</date>
159 <authorinitials>smcv/davidz</authorinitials>
160 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
161 by GVariant</revremark>
164 <revnumber>0.16</revnumber>
165 <date>11 April 2011</date>
166 <authorinitials></authorinitials>
167 <revremark>add path_namespace, arg0namespace; argNpath matches object
171 <revnumber>0.15</revnumber>
172 <date>3 November 2010</date>
173 <authorinitials></authorinitials>
174 <revremark></revremark>
177 <revnumber>0.14</revnumber>
178 <date>12 May 2010</date>
179 <authorinitials></authorinitials>
180 <revremark></revremark>
183 <revnumber>0.13</revnumber>
184 <date>23 Dezember 2009</date>
185 <authorinitials></authorinitials>
186 <revremark></revremark>
189 <revnumber>0.12</revnumber>
190 <date>7 November, 2006</date>
191 <authorinitials></authorinitials>
192 <revremark></revremark>
195 <revnumber>0.11</revnumber>
196 <date>6 February 2005</date>
197 <authorinitials></authorinitials>
198 <revremark></revremark>
201 <revnumber>0.10</revnumber>
202 <date>28 January 2005</date>
203 <authorinitials></authorinitials>
204 <revremark></revremark>
207 <revnumber>0.9</revnumber>
208 <date>7 Januar 2005</date>
209 <authorinitials></authorinitials>
210 <revremark></revremark>
213 <revnumber>0.8</revnumber>
214 <date>06 September 2003</date>
215 <authorinitials></authorinitials>
216 <revremark>First released document.</revremark>
221 <sect1 id="introduction">
222 <title>Introduction</title>
224 D-Bus is a system for low-overhead, easy to use
225 interprocess communication (IPC). In more detail:
229 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
230 binary protocol, and does not have to convert to and from a text
231 format such as XML. Because D-Bus is intended for potentially
232 high-resolution same-machine IPC, not primarily for Internet IPC,
233 this is an interesting optimization. D-Bus is also designed to
234 avoid round trips and allow asynchronous operation, much like
240 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
241 of <firstterm>messages</firstterm> rather than byte streams, and
242 automatically handles a lot of the hard IPC issues. Also, the D-Bus
243 library is designed to be wrapped in a way that lets developers use
244 their framework's existing object/type system, rather than learning
245 a new one specifically for IPC.
252 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
253 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
254 a system for one application to talk to a single other
255 application. However, the primary intended application of the protocol is the
256 D-Bus <firstterm>message bus</firstterm>, specified in <xref
257 linkend="message-bus"/>. The message bus is a special application that
258 accepts connections from multiple other applications, and forwards
263 Uses of D-Bus include notification of system changes (notification of when
264 a camera is plugged in to a computer, or a new version of some software
265 has been installed), or desktop interoperability, for example a file
266 monitoring service or a configuration service.
270 D-Bus is designed for two specific use cases:
274 A "system bus" for notifications from the system to user sessions,
275 and to allow the system to request input from user sessions.
280 A "session bus" used to implement desktop environments such as
285 D-Bus is not intended to be a generic IPC system for any possible
286 application, and intentionally omits many features found in other
287 IPC systems for this reason.
291 At the same time, the bus daemons offer a number of features not found in
292 other IPC systems, such as single-owner "bus names" (similar to X
293 selections), on-demand startup of services, and security policies.
294 In many ways, these features are the primary motivation for developing
295 D-Bus; other systems would have sufficed if IPC were the only goal.
299 D-Bus may turn out to be useful in unanticipated applications, but future
300 versions of this spec and the reference implementation probably will not
301 incorporate features that interfere with the core use cases.
305 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
306 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
307 document are to be interpreted as described in RFC 2119. However, the
308 document could use a serious audit to be sure it makes sense to do
309 so. Also, they are not capitalized.
312 <sect2 id="stability">
313 <title>Protocol and Specification Stability</title>
315 The D-Bus protocol is frozen (only compatible extensions are allowed) as
316 of November 8, 2006. However, this specification could still use a fair
317 bit of work to make interoperable reimplementation possible without
318 reference to the D-Bus reference implementation. Thus, this
319 specification is not marked 1.0. To mark it 1.0, we'd like to see
320 someone invest significant effort in clarifying the specification
321 language, and growing the specification to cover more aspects of the
322 reference implementation's behavior.
325 Until this work is complete, any attempt to reimplement D-Bus will
326 probably require looking at the reference implementation and/or asking
327 questions on the D-Bus mailing list about intended behavior.
328 Questions on the list are very welcome.
331 Nonetheless, this document should be a useful starting point and is
332 to our knowledge accurate, though incomplete.
338 <sect1 id="type-system">
339 <title>Type System</title>
342 D-Bus has a type system, in which values of various types can be
343 serialized into a sequence of bytes referred to as the
344 <firstterm>wire format</firstterm> in a standard way.
345 Converting a value from some other representation into the wire
346 format is called <firstterm>marshaling</firstterm> and converting
347 it back from the wire format is <firstterm>unmarshaling</firstterm>.
351 The D-Bus protocol does not include type tags in the marshaled data; a
352 block of marshaled values must have a known <firstterm>type
353 signature</firstterm>. The type signature is made up of zero or more
354 <firstterm id="term-single-complete-type">single complete
355 types</firstterm>, each made up of one or more
356 <firstterm>type codes</firstterm>.
360 A type code is an ASCII character representing the
361 type of a value. Because ASCII characters are used, the type signature
362 will always form a valid ASCII string. A simple string compare
363 determines whether two type signatures are equivalent.
367 A single complete type is a sequence of type codes that fully describes
368 one type: either a basic type, or a single fully-described container type.
369 A single complete type is a basic type code, a variant type code,
370 an array with its element type, or a struct with its fields (all of which
371 are defined below). So the following signatures are not single complete
382 And the following signatures contain multiple complete types:
392 Note however that a single complete type may <emphasis>contain</emphasis>
393 multiple other single complete types, by containing a struct or dict
397 <sect2 id="basic-types">
398 <title>Basic types</title>
401 The simplest type codes are the <firstterm id="term-basic-type">basic
402 types</firstterm>, which are the types whose structure is entirely
403 defined by their 1-character type code. Basic types consist of
404 fixed types and string-like types.
408 The <firstterm id="term-fixed-type">fixed types</firstterm>
409 are basic types whose values have a fixed length, namely BYTE,
410 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
415 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
416 the ASCII character 'i'. So the signature for a block of values
417 containing a single <literal>INT32</literal> would be:
421 A block of values containing two <literal>INT32</literal> would have this signature:
428 The characteristics of the fixed types are listed in this table.
434 <entry>Conventional name</entry>
435 <entry>ASCII type-code</entry>
436 <entry>Encoding</entry>
441 <entry><literal>BYTE</literal></entry>
442 <entry><literal>y</literal> (121)</entry>
443 <entry>Unsigned 8-bit integer</entry>
446 <entry><literal>BOOLEAN</literal></entry>
447 <entry><literal>b</literal> (98)</entry>
448 <entry>Boolean value: 0 is false, 1 is true, any other value
449 allowed by the marshalling format is invalid</entry>
452 <entry><literal>INT16</literal></entry>
453 <entry><literal>n</literal> (110)</entry>
454 <entry>Signed (two's complement) 16-bit integer</entry>
457 <entry><literal>UINT16</literal></entry>
458 <entry><literal>q</literal> (113)</entry>
459 <entry>Unsigned 16-bit integer</entry>
462 <entry><literal>INT32</literal></entry>
463 <entry><literal>i</literal> (105)</entry>
464 <entry>Signed (two's complement) 32-bit integer</entry>
467 <entry><literal>UINT32</literal></entry>
468 <entry><literal>u</literal> (117)</entry>
469 <entry>Unsigned 32-bit integer</entry>
472 <entry><literal>INT64</literal></entry>
473 <entry><literal>x</literal> (120)</entry>
474 <entry>Signed (two's complement) 64-bit integer
475 (mnemonic: x and t are the first characters in "sixty" not
476 already used for something more common)</entry>
479 <entry><literal>UINT64</literal></entry>
480 <entry><literal>t</literal> (116)</entry>
481 <entry>Unsigned 64-bit integer</entry>
484 <entry><literal>DOUBLE</literal></entry>
485 <entry><literal>d</literal> (100)</entry>
486 <entry>IEEE 754 double-precision floating point</entry>
489 <entry><literal>UNIX_FD</literal></entry>
490 <entry><literal>h</literal> (104)</entry>
491 <entry>Unsigned 32-bit integer representing an index into an
492 out-of-band array of file descriptors, transferred via some
493 platform-specific mechanism (mnemonic: h for handle)</entry>
501 The <firstterm id="term-string-like-type">string-like types</firstterm>
502 are basic types with a variable length. The value of any string-like
503 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
504 none of which may be U+0000. The UTF-8 text must be validated
505 strictly: in particular, it must not contain overlong sequences
506 or codepoints above U+10FFFF.
510 Since D-Bus Specification version 0.21, in accordance with Unicode
511 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
512 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
513 D-Bus rejected these noncharacters).
517 The marshalling formats for the string-like types all end with a
518 single zero (NUL) byte, but that byte is not considered to be part of
523 The characteristics of the string-like types are listed in this table.
529 <entry>Conventional name</entry>
530 <entry>ASCII type-code</entry>
531 <entry>Validity constraints</entry>
536 <entry><literal>STRING</literal></entry>
537 <entry><literal>s</literal> (115)</entry>
538 <entry>No extra constraints</entry>
541 <entry><literal>OBJECT_PATH</literal></entry>
542 <entry><literal>o</literal> (111)</entry>
544 <link linkend="message-protocol-marshaling-object-path">a
545 syntactically valid object path</link></entry>
548 <entry><literal>SIGNATURE</literal></entry>
549 <entry><literal>g</literal> (103)</entry>
551 <firstterm linkend="term-single-complete-type">single
552 complete types</firstterm></entry>
559 <sect3 id="message-protocol-marshaling-object-path">
560 <title>Valid Object Paths</title>
563 An object path is a name used to refer to an object instance.
564 Conceptually, each participant in a D-Bus message exchange may have
565 any number of object instances (think of C++ or Java objects) and each
566 such instance will have a path. Like a filesystem, the object
567 instances in an application form a hierarchical tree.
571 Object paths are often namespaced by starting with a reversed
572 domain name and containing an interface version number, in the
574 <link linkend="message-protocol-names-interface">interface
576 <link linkend="message-protocol-names-bus">well-known
578 This makes it possible to implement more than one service, or
579 more than one version of a service, in the same process,
580 even if the services share a connection but cannot otherwise
581 co-operate (for instance, if they are implemented by different
586 For instance, if the owner of <literal>example.com</literal> is
587 developing a D-Bus API for a music player, they might use the
588 hierarchy of object paths that start with
589 <literal>/com/example/MusicPlayer1</literal> for its objects.
593 The following rules define a valid object path. Implementations must
594 not send or accept messages with invalid object paths.
598 The path may be of any length.
603 The path must begin with an ASCII '/' (integer 47) character,
604 and must consist of elements separated by slash characters.
609 Each element must only contain the ASCII characters
615 No element may be the empty string.
620 Multiple '/' characters cannot occur in sequence.
625 A trailing '/' character is not allowed unless the
626 path is the root path (a single '/' character).
634 <sect3 id="message-protocol-marshaling-signature">
635 <title>Valid Signatures</title>
637 An implementation must not send or accept invalid signatures.
638 Valid signatures will conform to the following rules:
642 The signature is a list of single complete types.
643 Arrays must have element types, and structs must
644 have both open and close parentheses.
649 Only type codes, open and close parentheses, and open and
650 close curly brackets are allowed in the signature. The
651 <literal>STRUCT</literal> type code
652 is not allowed in signatures, because parentheses
653 are used instead. Similarly, the
654 <literal>DICT_ENTRY</literal> type code is not allowed in
655 signatures, because curly brackets are used instead.
660 The maximum depth of container type nesting is 32 array type
661 codes and 32 open parentheses. This implies that the maximum
662 total depth of recursion is 64, for an "array of array of array
663 of ... struct of struct of struct of ..." where there are 32
669 The maximum length of a signature is 255.
676 When signatures appear in messages, the marshalling format
677 guarantees that they will be followed by a nul byte (which can
678 be interpreted as either C-style string termination or the INVALID
679 type-code), but this is not conceptually part of the signature.
685 <sect2 id="container-types">
686 <title>Container types</title>
689 In addition to basic types, there are four <firstterm>container</firstterm>
690 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
691 and <literal>DICT_ENTRY</literal>.
695 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
696 code does not appear in signatures. Instead, ASCII characters
697 '(' and ')' are used to mark the beginning and end of the struct.
698 So for example, a struct containing two integers would have this
703 Structs can be nested, so for example a struct containing
704 an integer and another struct:
708 The value block storing that struct would contain three integers; the
709 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
714 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
715 but is useful in code that implements the protocol. This type code
716 is specified to allow such code to interoperate in non-protocol contexts.
720 Empty structures are not allowed; there must be at least one
721 type code between the parentheses.
725 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
726 followed by a <firstterm>single complete type</firstterm>. The single
727 complete type following the array is the type of each array element. So
728 the simple example is:
732 which is an array of 32-bit integers. But an array can be of any type,
733 such as this array-of-struct-with-two-int32-fields:
737 Or this array of array of integer:
744 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
745 type <literal>VARIANT</literal> will have the signature of a single complete type as part
746 of the <emphasis>value</emphasis>. This signature will be followed by a
747 marshaled value of that type.
751 Unlike a message signature, the variant signature can
752 contain only a single complete type. So "i", "ai"
753 or "(ii)" is OK, but "ii" is not. Use of variants may not
754 cause a total message depth to be larger than 64, including
755 other container types such as structures.
759 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
760 than parentheses it uses curly braces, and it has more restrictions.
761 The restrictions are: it occurs only as an array element type; it has
762 exactly two single complete types inside the curly braces; the first
763 single complete type (the "key") must be a basic type rather than a
764 container type. Implementations must not accept dict entries outside of
765 arrays, must not accept dict entries with zero, one, or more than two
766 fields, and must not accept dict entries with non-basic-typed keys. A
767 dict entry is always a key-value pair.
771 The first field in the <literal>DICT_ENTRY</literal> is always the key.
772 A message is considered corrupt if the same key occurs twice in the same
773 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
774 implementations are not required to reject dicts with duplicate keys.
778 In most languages, an array of dict entry would be represented as a
779 map, hash table, or dict object.
784 <title>Summary of types</title>
787 The following table summarizes the D-Bus types.
792 <entry>Category</entry>
793 <entry>Conventional Name</entry>
795 <entry>Description</entry>
800 <entry>reserved</entry>
801 <entry><literal>INVALID</literal></entry>
802 <entry>0 (ASCII NUL)</entry>
803 <entry>Not a valid type code, used to terminate signatures</entry>
805 <entry>fixed, basic</entry>
806 <entry><literal>BYTE</literal></entry>
807 <entry>121 (ASCII 'y')</entry>
808 <entry>8-bit unsigned integer</entry>
810 <entry>fixed, basic</entry>
811 <entry><literal>BOOLEAN</literal></entry>
812 <entry>98 (ASCII 'b')</entry>
813 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
815 <entry>fixed, basic</entry>
816 <entry><literal>INT16</literal></entry>
817 <entry>110 (ASCII 'n')</entry>
818 <entry>16-bit signed integer</entry>
820 <entry>fixed, basic</entry>
821 <entry><literal>UINT16</literal></entry>
822 <entry>113 (ASCII 'q')</entry>
823 <entry>16-bit unsigned integer</entry>
825 <entry>fixed, basic</entry>
826 <entry><literal>INT32</literal></entry>
827 <entry>105 (ASCII 'i')</entry>
828 <entry>32-bit signed integer</entry>
830 <entry>fixed, basic</entry>
831 <entry><literal>UINT32</literal></entry>
832 <entry>117 (ASCII 'u')</entry>
833 <entry>32-bit unsigned integer</entry>
835 <entry>fixed, basic</entry>
836 <entry><literal>INT64</literal></entry>
837 <entry>120 (ASCII 'x')</entry>
838 <entry>64-bit signed integer</entry>
840 <entry>fixed, basic</entry>
841 <entry><literal>UINT64</literal></entry>
842 <entry>116 (ASCII 't')</entry>
843 <entry>64-bit unsigned integer</entry>
845 <entry>fixed, basic</entry>
846 <entry><literal>DOUBLE</literal></entry>
847 <entry>100 (ASCII 'd')</entry>
848 <entry>IEEE 754 double</entry>
850 <entry>string-like, basic</entry>
851 <entry><literal>STRING</literal></entry>
852 <entry>115 (ASCII 's')</entry>
853 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
855 <entry>string-like, basic</entry>
856 <entry><literal>OBJECT_PATH</literal></entry>
857 <entry>111 (ASCII 'o')</entry>
858 <entry>Name of an object instance</entry>
860 <entry>string-like, basic</entry>
861 <entry><literal>SIGNATURE</literal></entry>
862 <entry>103 (ASCII 'g')</entry>
863 <entry>A type signature</entry>
865 <entry>container</entry>
866 <entry><literal>ARRAY</literal></entry>
867 <entry>97 (ASCII 'a')</entry>
870 <entry>container</entry>
871 <entry><literal>STRUCT</literal></entry>
872 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
873 <entry>Struct; type code 114 'r' is reserved for use in
874 bindings and implementations to represent the general
875 concept of a struct, and must not appear in signatures
876 used on D-Bus.</entry>
878 <entry>container</entry>
879 <entry><literal>VARIANT</literal></entry>
880 <entry>118 (ASCII 'v') </entry>
881 <entry>Variant type (the type of the value is part of the value itself)</entry>
883 <entry>container</entry>
884 <entry><literal>DICT_ENTRY</literal></entry>
885 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
886 <entry>Entry in a dict or map (array of key-value pairs).
887 Type code 101 'e' is reserved for use in bindings and
888 implementations to represent the general concept of a
889 dict or dict-entry, and must not appear in signatures
890 used on D-Bus.</entry>
892 <entry>fixed, basic</entry>
893 <entry><literal>UNIX_FD</literal></entry>
894 <entry>104 (ASCII 'h')</entry>
895 <entry>Unix file descriptor</entry>
898 <entry>reserved</entry>
899 <entry>(reserved)</entry>
900 <entry>109 (ASCII 'm')</entry>
901 <entry>Reserved for <ulink
902 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
903 'maybe' type compatible with the one in GVariant</ulink>,
904 and must not appear in signatures used on D-Bus until
905 specified here</entry>
908 <entry>reserved</entry>
909 <entry>(reserved)</entry>
910 <entry>42 (ASCII '*')</entry>
911 <entry>Reserved for use in bindings/implementations to
912 represent any <firstterm>single complete type</firstterm>,
913 and must not appear in signatures used on D-Bus.</entry>
916 <entry>reserved</entry>
917 <entry>(reserved)</entry>
918 <entry>63 (ASCII '?')</entry>
919 <entry>Reserved for use in bindings/implementations to
920 represent any <firstterm>basic type</firstterm>, and must
921 not appear in signatures used on D-Bus.</entry>
924 <entry>reserved</entry>
925 <entry>(reserved)</entry>
926 <entry>64 (ASCII '@'), 38 (ASCII '&'),
927 94 (ASCII '^')</entry>
928 <entry>Reserved for internal use by bindings/implementations,
929 and must not appear in signatures used on D-Bus.
930 GVariant uses these type-codes to encode calling
941 <sect1 id="message-protocol-marshaling">
942 <title>Marshaling (Wire Format)</title>
945 D-Bus defines a marshalling format for its type system, which is
946 used in D-Bus messages. This is not the only possible marshalling
947 format for the type system: for instance, GVariant (part of GLib)
948 re-uses the D-Bus type system but implements an alternative marshalling
953 <title>Byte order and alignment</title>
956 Given a type signature, a block of bytes can be converted into typed
957 values. This section describes the format of the block of bytes. Byte
958 order and alignment issues are handled uniformly for all D-Bus types.
962 A block of bytes has an associated byte order. The byte order
963 has to be discovered in some way; for D-Bus messages, the
964 byte order is part of the message header as described in
965 <xref linkend="message-protocol-messages"/>. For now, assume
966 that the byte order is known to be either little endian or big
971 Each value in a block of bytes is aligned "naturally," for example
972 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
973 8-byte boundary. To properly align a value, <firstterm>alignment
974 padding</firstterm> may be necessary. The alignment padding must always
975 be the minimum required padding to properly align the following value;
976 and it must always be made up of nul bytes. The alignment padding must
977 not be left uninitialized (it can't contain garbage), and more padding
978 than required must not be used.
982 As an exception to natural alignment, <literal>STRUCT</literal> and
983 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
984 boundary, regardless of the alignments of their contents.
989 <title>Marshalling basic types</title>
992 To marshal and unmarshal fixed types, you simply read one value
993 from the data block corresponding to each type code in the signature.
994 All signed integer values are encoded in two's complement, DOUBLE
995 values are IEEE 754 double-precision floating-point, and BOOLEAN
996 values are encoded in 32 bits (of which only the least significant
1001 The string-like types are all marshalled as a
1002 fixed-length unsigned integer <varname>n</varname> giving the
1003 length of the variable part, followed by <varname>n</varname>
1004 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
1005 which is not considered to be part of the text. The alignment
1006 of the string-like type is the same as the alignment of
1007 <varname>n</varname>.
1011 For the STRING and OBJECT_PATH types, <varname>n</varname> is
1012 encoded in 4 bytes, leading to 4-byte alignment.
1013 For the SIGNATURE type, <varname>n</varname> is encoded as a single
1014 byte. As a result, alignment padding is never required before a
1020 <title>Marshalling containers</title>
1023 Arrays are marshalled as a <literal>UINT32</literal>
1024 <varname>n</varname> giving the length of the array data in bytes,
1025 followed by alignment padding to the alignment boundary of the array
1026 element type, followed by the <varname>n</varname> bytes of the
1027 array elements marshalled in sequence. <varname>n</varname> does not
1028 include the padding after the length, or any padding after the
1033 For instance, if the current position in the message is a multiple
1034 of 8 bytes and the byte-order is big-endian, an array containing only
1035 the 64-bit integer 5 would be marshalled as:
1038 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
1039 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1040 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1045 Arrays have a maximum length defined to be 2 to the 26th power or
1046 67108864 (64 MiB). Implementations must not send or accept arrays
1047 exceeding this length.
1051 Structs and dict entries are marshalled in the same way as their
1052 contents, but their alignment is always to an 8-byte boundary,
1053 even if their contents would normally be less strictly aligned.
1057 Variants are marshalled as the <literal>SIGNATURE</literal> of
1058 the contents (which must be a single complete type), followed by a
1059 marshalled value with the type given by that signature. The
1060 variant has the same 1-byte alignment as the signature, which means
1061 that alignment padding before a variant is never needed.
1062 Use of variants may not cause a total message depth to be larger
1063 than 64, including other container types such as structures.
1068 <title>Summary of D-Bus marshalling</title>
1071 Given all this, the types are marshaled on the wire as follows:
1076 <entry>Conventional Name</entry>
1077 <entry>Encoding</entry>
1078 <entry>Alignment</entry>
1083 <entry><literal>INVALID</literal></entry>
1084 <entry>Not applicable; cannot be marshaled.</entry>
1087 <entry><literal>BYTE</literal></entry>
1088 <entry>A single 8-bit byte.</entry>
1091 <entry><literal>BOOLEAN</literal></entry>
1092 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1095 <entry><literal>INT16</literal></entry>
1096 <entry>16-bit signed integer in the message's byte order.</entry>
1099 <entry><literal>UINT16</literal></entry>
1100 <entry>16-bit unsigned integer in the message's byte order.</entry>
1103 <entry><literal>INT32</literal></entry>
1104 <entry>32-bit signed integer in the message's byte order.</entry>
1107 <entry><literal>UINT32</literal></entry>
1108 <entry>32-bit unsigned integer in the message's byte order.</entry>
1111 <entry><literal>INT64</literal></entry>
1112 <entry>64-bit signed integer in the message's byte order.</entry>
1115 <entry><literal>UINT64</literal></entry>
1116 <entry>64-bit unsigned integer in the message's byte order.</entry>
1119 <entry><literal>DOUBLE</literal></entry>
1120 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1123 <entry><literal>STRING</literal></entry>
1124 <entry>A <literal>UINT32</literal> indicating the string's
1125 length in bytes excluding its terminating nul, followed by
1126 non-nul string data of the given length, followed by a terminating nul
1133 <entry><literal>OBJECT_PATH</literal></entry>
1134 <entry>Exactly the same as <literal>STRING</literal> except the
1135 content must be a valid object path (see above).
1141 <entry><literal>SIGNATURE</literal></entry>
1142 <entry>The same as <literal>STRING</literal> except the length is a single
1143 byte (thus signatures have a maximum length of 255)
1144 and the content must be a valid signature (see above).
1150 <entry><literal>ARRAY</literal></entry>
1152 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1153 alignment padding to the alignment boundary of the array element type,
1154 followed by each array element.
1160 <entry><literal>STRUCT</literal></entry>
1162 A struct must start on an 8-byte boundary regardless of the
1163 type of the struct fields. The struct value consists of each
1164 field marshaled in sequence starting from that 8-byte
1171 <entry><literal>VARIANT</literal></entry>
1173 The marshaled <literal>SIGNATURE</literal> of a single
1174 complete type, followed by a marshaled value with the type
1175 given in the signature.
1178 1 (alignment of the signature)
1181 <entry><literal>DICT_ENTRY</literal></entry>
1183 Identical to STRUCT.
1189 <entry><literal>UNIX_FD</literal></entry>
1190 <entry>32-bit unsigned integer in the message's byte
1191 order. The actual file descriptors need to be
1192 transferred out-of-band via some platform specific
1193 mechanism. On the wire, values of this type store the index to the
1194 file descriptor in the array of file descriptors that
1195 accompany the message.</entry>
1207 <sect1 id="message-protocol">
1208 <title>Message Protocol</title>
1211 A <firstterm>message</firstterm> consists of a
1212 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1213 think of a message as a package, the header is the address, and the body
1214 contains the package contents. The message delivery system uses the header
1215 information to figure out where to send the message and how to interpret
1216 it; the recipient interprets the body of the message.
1220 The body of the message is made up of zero or more
1221 <firstterm>arguments</firstterm>, which are typed values, such as an
1222 integer or a byte array.
1226 Both header and body use the D-Bus <link linkend="type-system">type
1227 system</link> and format for serializing data.
1230 <sect2 id="message-protocol-messages">
1231 <title>Message Format</title>
1234 A message consists of a header and a body. The header is a block of
1235 values with a fixed signature and meaning. The body is a separate block
1236 of values, with a signature specified in the header.
1240 The length of the header must be a multiple of 8, allowing the body to
1241 begin on an 8-byte boundary when storing the entire message in a single
1242 buffer. If the header does not naturally end on an 8-byte boundary
1243 up to 7 bytes of nul-initialized alignment padding must be added.
1247 The message body need not end on an 8-byte boundary.
1251 The maximum length of a message, including header, header alignment padding,
1252 and body is 2 to the 27th power or 134217728 (128 MiB).
1253 Implementations must not send or accept messages exceeding this size.
1257 The signature of the header is:
1261 Written out more readably, this is:
1263 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1268 These values have the following meanings:
1273 <entry>Value</entry>
1274 <entry>Description</entry>
1279 <entry>1st <literal>BYTE</literal></entry>
1280 <entry>Endianness flag; ASCII 'l' for little-endian
1281 or ASCII 'B' for big-endian. Both header and body are
1282 in this endianness.</entry>
1285 <entry>2nd <literal>BYTE</literal></entry>
1286 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1287 Currently-defined types are described below.
1291 <entry>3rd <literal>BYTE</literal></entry>
1292 <entry>Bitwise OR of flags. Unknown flags
1293 must be ignored. Currently-defined flags are described below.
1297 <entry>4th <literal>BYTE</literal></entry>
1298 <entry>Major protocol version of the sending application. If
1299 the major protocol version of the receiving application does not
1300 match, the applications will not be able to communicate and the
1301 D-Bus connection must be disconnected. The major protocol
1302 version for this version of the specification is 1.
1306 <entry>1st <literal>UINT32</literal></entry>
1307 <entry>Length in bytes of the message body, starting
1308 from the end of the header. The header ends after
1309 its alignment padding to an 8-boundary.
1313 <entry>2nd <literal>UINT32</literal></entry>
1314 <entry>The serial of this message, used as a cookie
1315 by the sender to identify the reply corresponding
1316 to this request. This must not be zero.
1320 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1321 <entry>An array of zero or more <firstterm>header
1322 fields</firstterm> where the byte is the field code, and the
1323 variant is the field value. The message type determines
1324 which fields are required.
1332 <firstterm>Message types</firstterm> that can appear in the second byte
1338 <entry>Conventional name</entry>
1339 <entry>Decimal value</entry>
1340 <entry>Description</entry>
1345 <entry><literal>INVALID</literal></entry>
1347 <entry>This is an invalid type.</entry>
1350 <entry><literal>METHOD_CALL</literal></entry>
1352 <entry>Method call. This message type may prompt a
1356 <entry><literal>METHOD_RETURN</literal></entry>
1358 <entry>Method reply with returned data.</entry>
1361 <entry><literal>ERROR</literal></entry>
1363 <entry>Error reply. If the first argument exists and is a
1364 string, it is an error message.</entry>
1367 <entry><literal>SIGNAL</literal></entry>
1369 <entry>Signal emission.</entry>
1376 Flags that can appear in the third byte of the header:
1381 <entry>Conventional name</entry>
1382 <entry>Hex value</entry>
1383 <entry>Description</entry>
1388 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1392 This message does not expect method return replies or
1393 error replies, even if it is of a type that can
1394 have a reply; the reply should be omitted.
1397 Note that METHOD_CALL is the only message type currently
1398 defined in this specification that can expect a reply,
1399 so the presence or absence of this flag in the other
1400 three message types that are currently
1401 documented is meaningless: replies to those message
1402 types should not be sent, whether this flag is present
1408 <entry><literal>NO_AUTO_START</literal></entry>
1410 <entry>The bus must not launch an owner
1411 for the destination name in response to this message.
1415 <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
1419 This flag may be set on a method call message to
1420 inform the receiving side that the caller is prepared
1421 to wait for interactive authorization, which might
1422 take a considerable time to complete. For instance,
1423 if this flag is set, it would be appropriate to
1424 query the user for passwords or confirmation via
1425 Polkit or a similar framework.
1428 This flag is only useful when
1429 unprivileged code calls a more privileged method call,
1430 and an authorization framework is deployed that allows
1431 possibly interactive authorization. If no such framework
1432 is deployed it has no effect. This flag should not
1433 be set by default by client implementations. If it is
1434 set, the caller should also set a suitably long timeout
1435 on the method call to make sure the user interaction
1436 may complete. This flag is only valid for method call
1437 messages, and shall be ignored otherwise.
1440 Interaction that takes place as a part of the
1441 effect of the method being called is outside the scope
1442 of this flag, even if it could also be characterized
1443 as authentication or authorization. For instance, in
1444 a method call that directs a network management service
1445 to attempt to connect to a virtual private network,
1446 this flag should control how the network management
1447 service makes the decision "is this user allowed to
1448 change system network configuration?", but it should
1449 not affect how or whether the network management
1450 service interacts with the user to obtain the credentials
1451 that are required for access to the VPN.
1454 If a this flag is not set on a method call, and a
1455 service determines that the requested operation is
1456 not allowed without interactive authorization, but
1457 could be allowed after successful interactive
1458 authorization, it may return the
1459 <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
1463 The absence of this flag does not guarantee that
1464 interactive authorization will not be applied, since
1465 existing services that pre-date this flag might
1466 already use interactive authorization. However,
1467 existing D-Bus APIs that will use interactive
1468 authorization should document that the call may take
1469 longer than usual, and new D-Bus APIs should avoid
1470 interactive authorization in the absence of this flag.
1479 <sect3 id="message-protocol-header-fields">
1480 <title>Header Fields</title>
1483 The array at the end of the header contains <firstterm>header
1484 fields</firstterm>, where each field is a 1-byte field code followed
1485 by a field value. A header must contain the required header fields for
1486 its message type, and zero or more of any optional header
1487 fields. Future versions of this protocol specification may add new
1488 fields. Implementations must ignore fields they do not
1489 understand. Implementations must not invent their own header fields;
1490 only changes to this specification may introduce new header fields.
1494 Again, if an implementation sees a header field code that it does not
1495 expect, it must ignore that field, as it will be part of a new
1496 (but compatible) version of this specification. This also applies
1497 to known header fields appearing in unexpected messages, for
1498 example: if a signal has a reply serial it must be ignored
1499 even though it has no meaning as of this version of the spec.
1503 However, implementations must not send or accept known header fields
1504 with the wrong type stored in the field value. So for example a
1505 message with an <literal>INTERFACE</literal> field of type
1506 <literal>UINT32</literal> would be considered corrupt.
1510 Here are the currently-defined header fields:
1515 <entry>Conventional Name</entry>
1516 <entry>Decimal Code</entry>
1518 <entry>Required In</entry>
1519 <entry>Description</entry>
1524 <entry><literal>INVALID</literal></entry>
1527 <entry>not allowed</entry>
1528 <entry>Not a valid field name (error if it appears in a message)</entry>
1531 <entry><literal>PATH</literal></entry>
1533 <entry><literal>OBJECT_PATH</literal></entry>
1534 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1535 <entry>The object to send a call to,
1536 or the object a signal is emitted from.
1538 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1539 implementations should not send messages with this path,
1540 and the reference implementation of the bus daemon will
1541 disconnect any application that attempts to do so.
1545 <entry><literal>INTERFACE</literal></entry>
1547 <entry><literal>STRING</literal></entry>
1548 <entry><literal>SIGNAL</literal></entry>
1550 The interface to invoke a method call on, or
1551 that a signal is emitted from. Optional for
1552 method calls, required for signals.
1553 The special interface
1554 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1555 implementations should not send messages with this
1556 interface, and the reference implementation of the bus
1557 daemon will disconnect any application that attempts to
1562 <entry><literal>MEMBER</literal></entry>
1564 <entry><literal>STRING</literal></entry>
1565 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1566 <entry>The member, either the method name or signal name.</entry>
1569 <entry><literal>ERROR_NAME</literal></entry>
1571 <entry><literal>STRING</literal></entry>
1572 <entry><literal>ERROR</literal></entry>
1573 <entry>The name of the error that occurred, for errors</entry>
1576 <entry><literal>REPLY_SERIAL</literal></entry>
1578 <entry><literal>UINT32</literal></entry>
1579 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1580 <entry>The serial number of the message this message is a reply
1581 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1584 <entry><literal>DESTINATION</literal></entry>
1586 <entry><literal>STRING</literal></entry>
1587 <entry>optional</entry>
1588 <entry>The name of the connection this message is intended for.
1589 Only used in combination with the message bus, see
1590 <xref linkend="message-bus"/>.</entry>
1593 <entry><literal>SENDER</literal></entry>
1595 <entry><literal>STRING</literal></entry>
1596 <entry>optional</entry>
1597 <entry>Unique name of the sending connection.
1598 The message bus fills in this field so it is reliable; the field is
1599 only meaningful in combination with the message bus.</entry>
1602 <entry><literal>SIGNATURE</literal></entry>
1604 <entry><literal>SIGNATURE</literal></entry>
1605 <entry>optional</entry>
1606 <entry>The signature of the message body.
1607 If omitted, it is assumed to be the
1608 empty signature "" (i.e. the body must be 0-length).</entry>
1611 <entry><literal>UNIX_FDS</literal></entry>
1613 <entry><literal>UINT32</literal></entry>
1614 <entry>optional</entry>
1615 <entry>The number of Unix file descriptors that
1616 accompany the message. If omitted, it is assumed
1617 that no Unix file descriptors accompany the
1618 message. The actual file descriptors need to be
1619 transferred via platform specific mechanism
1620 out-of-band. They must be sent at the same time as
1621 part of the message itself. They may not be sent
1622 before the first byte of the message itself is
1623 transferred or after the last byte of the message
1633 <sect2 id="message-protocol-names">
1634 <title>Valid Names</title>
1636 The various names in D-Bus messages have some restrictions.
1639 There is a <firstterm>maximum name length</firstterm>
1640 of 255 which applies to bus names, interfaces, and members.
1642 <sect3 id="message-protocol-names-interface">
1643 <title>Interface names</title>
1645 Interfaces have names with type <literal>STRING</literal>, meaning that
1646 they must be valid UTF-8. However, there are also some
1647 additional restrictions that apply to interface names
1650 <listitem><para>Interface names are composed of 1 or more elements separated by
1651 a period ('.') character. All elements must contain at least
1655 <listitem><para>Each element must only contain the ASCII characters
1656 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1660 <listitem><para>Interface names must contain at least one '.' (period)
1661 character (and thus at least two elements).
1664 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1665 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1670 Interface names should start with the reversed DNS domain name of
1671 the author of the interface (in lower-case), like interface names
1672 in Java. It is conventional for the rest of the interface name
1673 to consist of words run together, with initial capital letters
1674 on all words ("CamelCase"). Several levels of hierarchy can be used.
1675 It is also a good idea to include the major version of the interface
1676 in the name, and increment it if incompatible changes are made;
1677 this way, a single object can implement several versions of an
1678 interface in parallel, if necessary.
1682 For instance, if the owner of <literal>example.com</literal> is
1683 developing a D-Bus API for a music player, they might define
1684 interfaces called <literal>com.example.MusicPlayer1</literal>,
1685 <literal>com.example.MusicPlayer1.Track</literal> and
1686 <literal>com.example.MusicPlayer1.Seekable</literal>.
1690 D-Bus does not distinguish between the concepts that would be
1691 called classes and interfaces in Java: either can be identified on
1692 D-Bus by an interface name.
1695 <sect3 id="message-protocol-names-bus">
1696 <title>Bus names</title>
1698 Connections have one or more bus names associated with them.
1699 A connection has exactly one bus name that is a <firstterm>unique
1700 connection name</firstterm>. The unique connection name remains
1701 with the connection for its entire lifetime.
1702 A bus name is of type <literal>STRING</literal>,
1703 meaning that it must be valid UTF-8. However, there are also
1704 some additional restrictions that apply to bus names
1707 <listitem><para>Bus names that start with a colon (':')
1708 character are unique connection names. Other bus names
1709 are called <firstterm>well-known bus names</firstterm>.
1712 <listitem><para>Bus names are composed of 1 or more elements separated by
1713 a period ('.') character. All elements must contain at least
1717 <listitem><para>Each element must only contain the ASCII characters
1718 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1719 connection name may begin with a digit, elements in
1720 other bus names must not begin with a digit.
1724 <listitem><para>Bus names must contain at least one '.' (period)
1725 character (and thus at least two elements).
1728 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1729 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1733 Note that the hyphen ('-') character is allowed in bus names but
1734 not in interface names.
1738 Like <link linkend="message-protocol-names-interface">interface
1739 names</link>, well-known bus names should start with the
1740 reversed DNS domain name of the author of the interface (in
1741 lower-case), and it is conventional for the rest of the well-known
1742 bus name to consist of words run together, with initial
1743 capital letters. As with interface names, including a version
1744 number in well-known bus names is a good idea; it's possible to
1745 have the well-known bus name for more than one version
1746 simultaneously if backwards compatibility is required.
1750 If a well-known bus name implies the presence of a "main" interface,
1751 that "main" interface is often given the same name as
1752 the well-known bus name, and situated at the corresponding object
1753 path. For instance, if the owner of <literal>example.com</literal>
1754 is developing a D-Bus API for a music player, they might define
1755 that any application that takes the well-known name
1756 <literal>com.example.MusicPlayer1</literal> should have an object
1757 at the object path <literal>/com/example/MusicPlayer1</literal>
1758 which implements the interface
1759 <literal>com.example.MusicPlayer1</literal>.
1762 <sect3 id="message-protocol-names-member">
1763 <title>Member names</title>
1765 Member (i.e. method or signal) names:
1767 <listitem><para>Must only contain the ASCII characters
1768 "[A-Z][a-z][0-9]_" and may not begin with a
1769 digit.</para></listitem>
1770 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1771 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1772 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1777 It is conventional for member names on D-Bus to consist of
1778 capitalized words with no punctuation ("camel-case").
1779 Method names should usually be verbs, such as
1780 <literal>GetItems</literal>, and signal names should usually be
1781 a description of an event, such as <literal>ItemsChanged</literal>.
1784 <sect3 id="message-protocol-names-error">
1785 <title>Error names</title>
1787 Error names have the same restrictions as interface names.
1791 Error names have the same naming conventions as interface
1792 names, and often contain <literal>.Error.</literal>; for instance,
1793 the owner of <literal>example.com</literal> might define the
1794 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1795 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1796 The errors defined by D-Bus itself, such as
1797 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1803 <sect2 id="message-protocol-types">
1804 <title>Message Types</title>
1806 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1807 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1808 This section describes these conventions.
1810 <sect3 id="message-protocol-types-method">
1811 <title>Method Calls</title>
1813 Some messages invoke an operation on a remote object. These are
1814 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1815 messages map naturally to methods on objects in a typical program.
1818 A method call message is required to have a <literal>MEMBER</literal> header field
1819 indicating the name of the method. Optionally, the message has an
1820 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1821 Including the <literal>INTERFACE</literal> in all method call
1822 messages is strongly recommended.
1825 In the absence of an <literal>INTERFACE</literal> field, if two
1826 or more interfaces on the same object have a method with the same
1827 name, it is undefined which of those methods will be invoked.
1828 Implementations may choose to either return an error, or deliver the
1829 message as though it had an arbitrary one of those interfaces.
1832 In some situations (such as the well-known system bus), messages
1833 are filtered through an access-control list external to the
1834 remote object implementation. If that filter rejects certain
1835 messages by matching their interface, or accepts only messages
1836 to specific interfaces, it must also reject messages that have no
1837 <literal>INTERFACE</literal>: otherwise, malicious
1838 applications could use this to bypass the filter.
1841 Method call messages also include a <literal>PATH</literal> field
1842 indicating the object to invoke the method on. If the call is passing
1843 through a message bus, the message will also have a
1844 <literal>DESTINATION</literal> field giving the name of the connection
1845 to receive the message.
1848 When an application handles a method call message, it is required to
1849 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1850 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1851 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1854 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1855 are the return value(s) or "out parameters" of the method call.
1856 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1857 and the call fails; no return value will be provided. It makes
1858 no sense to send multiple replies to the same method call.
1861 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1862 reply is required, so the caller will know the method
1863 was successfully processed.
1866 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1870 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1871 then the application receiving the method should not send the reply message (regardless of
1872 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1875 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1876 destination name does not exist then a program to own the destination
1877 name will be started before the message is delivered. The message
1878 will be held until the new program is successfully started or has
1879 failed to start; in case of failure, an error will be returned. This
1880 flag is only relevant in the context of a message bus, it is ignored
1881 during one-to-one communication with no intermediate bus.
1883 <sect4 id="message-protocol-types-method-apis">
1884 <title>Mapping method calls to native APIs</title>
1886 APIs for D-Bus may map method calls to a method call in a specific
1887 programming language, such as C++, or may map a method call written
1888 in an IDL to a D-Bus message.
1891 In APIs of this nature, arguments to a method are often termed "in"
1892 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1893 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1894 "inout" arguments, which are both sent and received, i.e. the caller
1895 passes in a value which is modified. Mapped to D-Bus, an "inout"
1896 argument is equivalent to an "in" argument, followed by an "out"
1897 argument. You can't pass things "by reference" over the wire, so
1898 "inout" is purely an illusion of the in-process API.
1901 Given a method with zero or one return values, followed by zero or more
1902 arguments, where each argument may be "in", "out", or "inout", the
1903 caller constructs a message by appending each "in" or "inout" argument,
1904 in order. "out" arguments are not represented in the caller's message.
1907 The recipient constructs a reply by appending first the return value
1908 if any, then each "out" or "inout" argument, in order.
1909 "in" arguments are not represented in the reply message.
1912 Error replies are normally mapped to exceptions in languages that have
1916 In converting from native APIs to D-Bus, it is perhaps nice to
1917 map D-Bus naming conventions ("FooBar") to native conventions
1918 such as "fooBar" or "foo_bar" automatically. This is OK
1919 as long as you can say that the native API is one that
1920 was specifically written for D-Bus. It makes the most sense
1921 when writing object implementations that will be exported
1922 over the bus. Object proxies used to invoke remote D-Bus
1923 objects probably need the ability to call any D-Bus method,
1924 and thus a magic name mapping like this could be a problem.
1927 This specification doesn't require anything of native API bindings;
1928 the preceding is only a suggested convention for consistency
1934 <sect3 id="message-protocol-types-signal">
1935 <title>Signal Emission</title>
1937 Unlike method calls, signal emissions have no replies.
1938 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1939 It must have three header fields: <literal>PATH</literal> giving the object
1940 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1941 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1942 for signals, though it is optional for method calls.
1946 <sect3 id="message-protocol-types-errors">
1947 <title>Errors</title>
1949 Messages of type <literal>ERROR</literal> are most commonly replies
1950 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1951 to any kind of message. The message bus for example
1952 will return an <literal>ERROR</literal> in reply to a signal emission if
1953 the bus does not have enough memory to send the signal.
1956 An <literal>ERROR</literal> may have any arguments, but if the first
1957 argument is a <literal>STRING</literal>, it must be an error message.
1958 The error message may be logged or shown to the user
1963 <sect3 id="message-protocol-types-notation">
1964 <title>Notation in this document</title>
1966 This document uses a simple pseudo-IDL to describe particular method
1967 calls and signals. Here is an example of a method call:
1969 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1970 out UINT32 resultcode)
1972 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1973 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1974 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1975 characters so it's known that the last part of the name in
1976 the "IDL" is the member name.
1979 In C++ that might end up looking like this:
1981 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1982 unsigned int flags);
1984 or equally valid, the return value could be done as an argument:
1986 void org::freedesktop::DBus::StartServiceByName (const char *name,
1988 unsigned int *resultcode);
1990 It's really up to the API designer how they want to make
1991 this look. You could design an API where the namespace wasn't used
1992 in C++, using STL or Qt, using varargs, or whatever you wanted.
1995 Signals are written as follows:
1997 org.freedesktop.DBus.NameLost (STRING name)
1999 Signals don't specify "in" vs. "out" because only
2000 a single direction is possible.
2003 It isn't especially encouraged to use this lame pseudo-IDL in actual
2004 API implementations; you might use the native notation for the
2005 language you're using, or you might use COM or CORBA IDL, for example.
2010 <sect2 id="message-protocol-handling-invalid">
2011 <title>Invalid Protocol and Spec Extensions</title>
2014 For security reasons, the D-Bus protocol should be strictly parsed and
2015 validated, with the exception of defined extension points. Any invalid
2016 protocol or spec violations should result in immediately dropping the
2017 connection without notice to the other end. Exceptions should be
2018 carefully considered, e.g. an exception may be warranted for a
2019 well-understood idiosyncrasy of a widely-deployed implementation. In
2020 cases where the other end of a connection is 100% trusted and known to
2021 be friendly, skipping validation for performance reasons could also make
2022 sense in certain cases.
2026 Generally speaking violations of the "must" requirements in this spec
2027 should be considered possible attempts to exploit security, and violations
2028 of the "should" suggestions should be considered legitimate (though perhaps
2029 they should generate an error in some cases).
2033 The following extension points are built in to D-Bus on purpose and must
2034 not be treated as invalid protocol. The extension points are intended
2035 for use by future versions of this spec, they are not intended for third
2036 parties. At the moment, the only way a third party could extend D-Bus
2037 without breaking interoperability would be to introduce a way to negotiate new
2038 feature support as part of the auth protocol, using EXTENSION_-prefixed
2039 commands. There is not yet a standard way to negotiate features.
2043 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
2044 commands result in an ERROR rather than a disconnect. This enables
2045 future extensions to the protocol. Commands starting with EXTENSION_ are
2046 reserved for third parties.
2051 The authentication protocol supports pluggable auth mechanisms.
2056 The address format (see <xref linkend="addresses"/>) supports new
2062 Messages with an unknown type (something other than
2063 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
2064 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
2065 Unknown-type messages must still be well-formed in the same way
2066 as the known messages, however. They still have the normal
2072 Header fields with an unknown or unexpected field code must be ignored,
2073 though again they must still be well-formed.
2078 New standard interfaces (with new methods and signals) can of course be added.
2088 <sect1 id="auth-protocol">
2089 <title>Authentication Protocol</title>
2091 Before the flow of messages begins, two applications must
2092 authenticate. A simple plain-text protocol is used for
2093 authentication; this protocol is a SASL profile, and maps fairly
2094 directly from the SASL specification. The message encoding is
2095 NOT used here, only plain text messages.
2098 In examples, "C:" and "S:" indicate lines sent by the client and
2099 server respectively.
2101 <sect2 id="auth-protocol-overview">
2102 <title>Protocol Overview</title>
2104 The protocol is a line-based protocol, where each line ends with
2105 \r\n. Each line begins with an all-caps ASCII command name containing
2106 only the character range [A-Z_], a space, then any arguments for the
2107 command, then the \r\n ending the line. The protocol is
2108 case-sensitive. All bytes must be in the ASCII character set.
2110 Commands from the client to the server are as follows:
2113 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2114 <listitem><para>CANCEL</para></listitem>
2115 <listitem><para>BEGIN</para></listitem>
2116 <listitem><para>DATA <data in hex encoding></para></listitem>
2117 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2118 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2121 From server to client are as follows:
2124 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2125 <listitem><para>OK <GUID in hex></para></listitem>
2126 <listitem><para>DATA <data in hex encoding></para></listitem>
2127 <listitem><para>ERROR</para></listitem>
2128 <listitem><para>AGREE_UNIX_FD</para></listitem>
2132 Unofficial extensions to the command set must begin with the letters
2133 "EXTENSION_", to avoid conflicts with future official commands.
2134 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2137 <sect2 id="auth-nul-byte">
2138 <title>Special credentials-passing nul byte</title>
2140 Immediately after connecting to the server, the client must send a
2141 single nul byte. This byte may be accompanied by credentials
2142 information on some operating systems that use sendmsg() with
2143 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2144 sockets. However, the nul byte must be sent even on other kinds of
2145 socket, and even on operating systems that do not require a byte to be
2146 sent in order to transmit credentials. The text protocol described in
2147 this document begins after the single nul byte. If the first byte
2148 received from the client is not a nul byte, the server may disconnect
2152 A nul byte in any context other than the initial byte is an error;
2153 the protocol is ASCII-only.
2156 The credentials sent along with the nul byte may be used with the
2157 SASL mechanism EXTERNAL.
2160 <sect2 id="auth-command-auth">
2161 <title>AUTH command</title>
2163 If an AUTH command has no arguments, it is a request to list
2164 available mechanisms. The server must respond with a REJECTED
2165 command listing the mechanisms it understands, or with an error.
2168 If an AUTH command specifies a mechanism, and the server supports
2169 said mechanism, the server should begin exchanging SASL
2170 challenge-response data with the client using DATA commands.
2173 If the server does not support the mechanism given in the AUTH
2174 command, it must send either a REJECTED command listing the mechanisms
2175 it does support, or an error.
2178 If the [initial-response] argument is provided, it is intended for use
2179 with mechanisms that have no initial challenge (or an empty initial
2180 challenge), as if it were the argument to an initial DATA command. If
2181 the selected mechanism has an initial challenge and [initial-response]
2182 was provided, the server should reject authentication by sending
2186 If authentication succeeds after exchanging DATA commands,
2187 an OK command must be sent to the client.
2190 The first octet received by the server after the \r\n of the BEGIN
2191 command from the client must be the first octet of the
2192 authenticated/encrypted stream of D-Bus messages.
2195 If BEGIN is received by the server, the first octet received
2196 by the client after the \r\n of the OK command must be the
2197 first octet of the authenticated/encrypted stream of D-Bus
2201 <sect2 id="auth-command-cancel">
2202 <title>CANCEL Command</title>
2204 At any time up to sending the BEGIN command, the client may send a
2205 CANCEL command. On receiving the CANCEL command, the server must
2206 send a REJECTED command and abort the current authentication
2210 <sect2 id="auth-command-data">
2211 <title>DATA Command</title>
2213 The DATA command may come from either client or server, and simply
2214 contains a hex-encoded block of data to be interpreted
2215 according to the SASL mechanism in use.
2218 Some SASL mechanisms support sending an "empty string";
2219 FIXME we need some way to do this.
2222 <sect2 id="auth-command-begin">
2223 <title>BEGIN Command</title>
2225 The BEGIN command acknowledges that the client has received an
2226 OK command from the server, and that the stream of messages
2230 The first octet received by the server after the \r\n of the BEGIN
2231 command from the client must be the first octet of the
2232 authenticated/encrypted stream of D-Bus messages.
2235 <sect2 id="auth-command-rejected">
2236 <title>REJECTED Command</title>
2238 The REJECTED command indicates that the current authentication
2239 exchange has failed, and further exchange of DATA is inappropriate.
2240 The client would normally try another mechanism, or try providing
2241 different responses to challenges.
2243 Optionally, the REJECTED command has a space-separated list of
2244 available auth mechanisms as arguments. If a server ever provides
2245 a list of supported mechanisms, it must provide the same list
2246 each time it sends a REJECTED message. Clients are free to
2247 ignore all lists received after the first.
2250 <sect2 id="auth-command-ok">
2251 <title>OK Command</title>
2253 The OK command indicates that the client has been
2254 authenticated. The client may now proceed with negotiating
2255 Unix file descriptor passing. To do that it shall send
2256 NEGOTIATE_UNIX_FD to the server.
2259 Otherwise, the client must respond to the OK command by
2260 sending a BEGIN command, followed by its stream of messages,
2261 or by disconnecting. The server must not accept additional
2262 commands using this protocol after the BEGIN command has been
2263 received. Further communication will be a stream of D-Bus
2264 messages (optionally encrypted, as negotiated) rather than
2268 If a client sends BEGIN the first octet received by the client
2269 after the \r\n of the OK command must be the first octet of
2270 the authenticated/encrypted stream of D-Bus messages.
2273 The OK command has one argument, which is the GUID of the server.
2274 See <xref linkend="addresses"/> for more on server GUIDs.
2277 <sect2 id="auth-command-error">
2278 <title>ERROR Command</title>
2280 The ERROR command indicates that either server or client did not
2281 know a command, does not accept the given command in the current
2282 context, or did not understand the arguments to the command. This
2283 allows the protocol to be extended; a client or server can send a
2284 command present or permitted only in new protocol versions, and if
2285 an ERROR is received instead of an appropriate response, fall back
2286 to using some other technique.
2289 If an ERROR is sent, the server or client that sent the
2290 error must continue as if the command causing the ERROR had never been
2291 received. However, the the server or client receiving the error
2292 should try something other than whatever caused the error;
2293 if only canceling/rejecting the authentication.
2296 If the D-Bus protocol changes incompatibly at some future time,
2297 applications implementing the new protocol would probably be able to
2298 check for support of the new protocol by sending a new command and
2299 receiving an ERROR from applications that don't understand it. Thus the
2300 ERROR feature of the auth protocol is an escape hatch that lets us
2301 negotiate extensions or changes to the D-Bus protocol in the future.
2304 <sect2 id="auth-command-negotiate-unix-fd">
2305 <title>NEGOTIATE_UNIX_FD Command</title>
2307 The NEGOTIATE_UNIX_FD command indicates that the client
2308 supports Unix file descriptor passing. This command may only
2309 be sent after the connection is authenticated, i.e. after OK
2310 was received by the client. This command may only be sent on
2311 transports that support Unix file descriptor passing.
2314 On receiving NEGOTIATE_UNIX_FD the server must respond with
2315 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2316 the transport chosen supports Unix file descriptor passing and
2317 the server supports this feature. It shall respond the latter
2318 if the transport does not support Unix file descriptor
2319 passing, the server does not support this feature, or the
2320 server decides not to enable file descriptor passing due to
2321 security or other reasons.
2324 <sect2 id="auth-command-agree-unix-fd">
2325 <title>AGREE_UNIX_FD Command</title>
2327 The AGREE_UNIX_FD command indicates that the server supports
2328 Unix file descriptor passing. This command may only be sent
2329 after the connection is authenticated, and the client sent
2330 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2331 command may only be sent on transports that support Unix file
2335 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2336 followed by its stream of messages, or by disconnecting. The
2337 server must not accept additional commands using this protocol
2338 after the BEGIN command has been received. Further
2339 communication will be a stream of D-Bus messages (optionally
2340 encrypted, as negotiated) rather than this protocol.
2343 <sect2 id="auth-command-future">
2344 <title>Future Extensions</title>
2346 Future extensions to the authentication and negotiation
2347 protocol are possible. For that new commands may be
2348 introduced. If a client or server receives an unknown command
2349 it shall respond with ERROR and not consider this fatal. New
2350 commands may be introduced both before, and after
2351 authentication, i.e. both before and after the OK command.
2354 <sect2 id="auth-examples">
2355 <title>Authentication examples</title>
2359 <title>Example of successful magic cookie authentication</title>
2361 (MAGIC_COOKIE is a made up mechanism)
2363 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2369 <title>Example of finding out mechanisms then picking one</title>
2372 S: REJECTED KERBEROS_V4 SKEY
2373 C: AUTH SKEY 7ab83f32ee
2374 S: DATA 8799cabb2ea93e
2375 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2381 <title>Example of client sends unknown command then falls back to regular auth</title>
2385 C: AUTH MAGIC_COOKIE 3736343435313230333039
2391 <title>Example of server doesn't support initial auth mechanism</title>
2393 C: AUTH MAGIC_COOKIE 3736343435313230333039
2394 S: REJECTED KERBEROS_V4 SKEY
2395 C: AUTH SKEY 7ab83f32ee
2396 S: DATA 8799cabb2ea93e
2397 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2403 <title>Example of wrong password or the like followed by successful retry</title>
2405 C: AUTH MAGIC_COOKIE 3736343435313230333039
2406 S: REJECTED KERBEROS_V4 SKEY
2407 C: AUTH SKEY 7ab83f32ee
2408 S: DATA 8799cabb2ea93e
2409 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2411 C: AUTH SKEY 7ab83f32ee
2412 S: DATA 8799cabb2ea93e
2413 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2419 <title>Example of skey cancelled and restarted</title>
2421 C: AUTH MAGIC_COOKIE 3736343435313230333039
2422 S: REJECTED KERBEROS_V4 SKEY
2423 C: AUTH SKEY 7ab83f32ee
2424 S: DATA 8799cabb2ea93e
2427 C: AUTH SKEY 7ab83f32ee
2428 S: DATA 8799cabb2ea93e
2429 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2435 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2437 (MAGIC_COOKIE is a made up mechanism)
2439 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2441 C: NEGOTIATE_UNIX_FD
2447 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2449 (MAGIC_COOKIE is a made up mechanism)
2451 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2453 C: NEGOTIATE_UNIX_FD
2460 <sect2 id="auth-states">
2461 <title>Authentication state diagrams</title>
2464 This section documents the auth protocol in terms of
2465 a state machine for the client and the server. This is
2466 probably the most robust way to implement the protocol.
2469 <sect3 id="auth-states-client">
2470 <title>Client states</title>
2473 To more precisely describe the interaction between the
2474 protocol state machine and the authentication mechanisms the
2475 following notation is used: MECH(CHALL) means that the
2476 server challenge CHALL was fed to the mechanism MECH, which
2482 CONTINUE(RESP) means continue the auth conversation
2483 and send RESP as the response to the server;
2489 OK(RESP) means that after sending RESP to the server
2490 the client side of the auth conversation is finished
2491 and the server should return "OK";
2497 ERROR means that CHALL was invalid and could not be
2503 Both RESP and CHALL may be empty.
2507 The Client starts by getting an initial response from the
2508 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2509 the mechanism did not provide an initial response. If the
2510 mechanism returns CONTINUE, the client starts in state
2511 <emphasis>WaitingForData</emphasis>, if the mechanism
2512 returns OK the client starts in state
2513 <emphasis>WaitingForOK</emphasis>.
2517 The client should keep track of available mechanisms and
2518 which it mechanisms it has already attempted. This list is
2519 used to decide which AUTH command to send. When the list is
2520 exhausted, the client should give up and close the
2525 <title><emphasis>WaitingForData</emphasis></title>
2533 MECH(CHALL) returns CONTINUE(RESP) → send
2535 <emphasis>WaitingForData</emphasis>
2539 MECH(CHALL) returns OK(RESP) → send DATA
2540 RESP, goto <emphasis>WaitingForOK</emphasis>
2544 MECH(CHALL) returns ERROR → send ERROR
2545 [msg], goto <emphasis>WaitingForData</emphasis>
2553 Receive REJECTED [mechs] →
2554 send AUTH [next mech], goto
2555 WaitingForData or <emphasis>WaitingForOK</emphasis>
2560 Receive ERROR → send
2562 <emphasis>WaitingForReject</emphasis>
2567 Receive OK → send
2568 BEGIN, terminate auth
2569 conversation, authenticated
2574 Receive anything else → send
2576 <emphasis>WaitingForData</emphasis>
2584 <title><emphasis>WaitingForOK</emphasis></title>
2589 Receive OK → send BEGIN, terminate auth
2590 conversation, <emphasis>authenticated</emphasis>
2595 Receive REJECTED [mechs] → send AUTH [next mech],
2596 goto <emphasis>WaitingForData</emphasis> or
2597 <emphasis>WaitingForOK</emphasis>
2603 Receive DATA → send CANCEL, goto
2604 <emphasis>WaitingForReject</emphasis>
2610 Receive ERROR → send CANCEL, goto
2611 <emphasis>WaitingForReject</emphasis>
2617 Receive anything else → send ERROR, goto
2618 <emphasis>WaitingForOK</emphasis>
2626 <title><emphasis>WaitingForReject</emphasis></title>
2631 Receive REJECTED [mechs] → send AUTH [next mech],
2632 goto <emphasis>WaitingForData</emphasis> or
2633 <emphasis>WaitingForOK</emphasis>
2639 Receive anything else → terminate auth
2640 conversation, disconnect
2649 <sect3 id="auth-states-server">
2650 <title>Server states</title>
2653 For the server MECH(RESP) means that the client response
2654 RESP was fed to the the mechanism MECH, which returns one of
2659 CONTINUE(CHALL) means continue the auth conversation and
2660 send CHALL as the challenge to the client;
2666 OK means that the client has been successfully
2673 REJECTED means that the client failed to authenticate or
2674 there was an error in RESP.
2679 The server starts out in state
2680 <emphasis>WaitingForAuth</emphasis>. If the client is
2681 rejected too many times the server must disconnect the
2686 <title><emphasis>WaitingForAuth</emphasis></title>
2692 Receive AUTH → send REJECTED [mechs], goto
2693 <emphasis>WaitingForAuth</emphasis>
2699 Receive AUTH MECH RESP
2703 MECH not valid mechanism → send REJECTED
2705 <emphasis>WaitingForAuth</emphasis>
2709 MECH(RESP) returns CONTINUE(CHALL) → send
2711 <emphasis>WaitingForData</emphasis>
2715 MECH(RESP) returns OK → send OK, goto
2716 <emphasis>WaitingForBegin</emphasis>
2720 MECH(RESP) returns REJECTED → send REJECTED
2722 <emphasis>WaitingForAuth</emphasis>
2730 Receive BEGIN → terminate
2731 auth conversation, disconnect
2737 Receive ERROR → send REJECTED [mechs], goto
2738 <emphasis>WaitingForAuth</emphasis>
2744 Receive anything else → send
2746 <emphasis>WaitingForAuth</emphasis>
2755 <title><emphasis>WaitingForData</emphasis></title>
2763 MECH(RESP) returns CONTINUE(CHALL) → send
2765 <emphasis>WaitingForData</emphasis>
2769 MECH(RESP) returns OK → send OK, goto
2770 <emphasis>WaitingForBegin</emphasis>
2774 MECH(RESP) returns REJECTED → send REJECTED
2776 <emphasis>WaitingForAuth</emphasis>
2784 Receive BEGIN → terminate auth conversation,
2791 Receive CANCEL → send REJECTED [mechs], goto
2792 <emphasis>WaitingForAuth</emphasis>
2798 Receive ERROR → send REJECTED [mechs], goto
2799 <emphasis>WaitingForAuth</emphasis>
2805 Receive anything else → send ERROR, goto
2806 <emphasis>WaitingForData</emphasis>
2814 <title><emphasis>WaitingForBegin</emphasis></title>
2819 Receive BEGIN → terminate auth conversation,
2820 client authenticated
2826 Receive CANCEL → send REJECTED [mechs], goto
2827 <emphasis>WaitingForAuth</emphasis>
2833 Receive ERROR → send REJECTED [mechs], goto
2834 <emphasis>WaitingForAuth</emphasis>
2840 Receive anything else → send ERROR, goto
2841 <emphasis>WaitingForBegin</emphasis>
2851 <sect2 id="auth-mechanisms">
2852 <title>Authentication mechanisms</title>
2854 This section describes some new authentication mechanisms.
2855 D-Bus also allows any standard SASL mechanism of course.
2857 <sect3 id="auth-mechanisms-sha">
2858 <title>DBUS_COOKIE_SHA1</title>
2860 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2861 has the ability to read a private file owned by the user being
2862 authenticated. If the client can prove that it has access to a secret
2863 cookie stored in this file, then the client is authenticated.
2864 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2868 Throughout this description, "hex encoding" must output the digits
2869 from a to f in lower-case; the digits A to F must not be used
2870 in the DBUS_COOKIE_SHA1 mechanism.
2873 Authentication proceeds as follows:
2877 The client sends the username it would like to authenticate
2883 The server sends the name of its "cookie context" (see below); a
2884 space character; the integer ID of the secret cookie the client
2885 must demonstrate knowledge of; a space character; then a
2886 randomly-generated challenge string, all of this hex-encoded into
2892 The client locates the cookie and generates its own
2893 randomly-generated challenge string. The client then concatenates
2894 the server's decoded challenge, a ":" character, its own challenge,
2895 another ":" character, and the cookie. It computes the SHA-1 hash
2896 of this composite string as a hex digest. It concatenates the
2897 client's challenge string, a space character, and the SHA-1 hex
2898 digest, hex-encodes the result and sends it back to the server.
2903 The server generates the same concatenated string used by the
2904 client and computes its SHA-1 hash. It compares the hash with
2905 the hash received from the client; if the two hashes match, the
2906 client is authenticated.
2912 Each server has a "cookie context," which is a name that identifies a
2913 set of cookies that apply to that server. A sample context might be
2914 "org_freedesktop_session_bus". Context names must be valid ASCII,
2915 nonzero length, and may not contain the characters slash ("/"),
2916 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2917 tab ("\t"), or period ("."). There is a default context,
2918 "org_freedesktop_general" that's used by servers that do not specify
2922 Cookies are stored in a user's home directory, in the directory
2923 <filename>~/.dbus-keyrings/</filename>. This directory must
2924 not be readable or writable by other users. If it is,
2925 clients and servers must ignore it. The directory
2926 contains cookie files named after the cookie context.
2929 A cookie file contains one cookie per line. Each line
2930 has three space-separated fields:
2934 The cookie ID number, which must be a non-negative integer and
2935 may not be used twice in the same file.
2940 The cookie's creation time, in UNIX seconds-since-the-epoch
2946 The cookie itself, a hex-encoded random block of bytes. The cookie
2947 may be of any length, though obviously security increases
2948 as the length increases.
2954 Only server processes modify the cookie file.
2955 They must do so with this procedure:
2959 Create a lockfile name by appending ".lock" to the name of the
2960 cookie file. The server should attempt to create this file
2961 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2962 fails, the lock fails. Servers should retry for a reasonable
2963 period of time, then they may choose to delete an existing lock
2964 to keep users from having to manually delete a stale
2965 lock. <footnote><para>Lockfiles are used instead of real file
2966 locking <literal>fcntl()</literal> because real locking
2967 implementations are still flaky on network
2968 filesystems.</para></footnote>
2973 Once the lockfile has been created, the server loads the cookie
2974 file. It should then delete any cookies that are old (the
2975 timeout can be fairly short), or more than a reasonable
2976 time in the future (so that cookies never accidentally
2977 become permanent, if the clock was set far into the future
2978 at some point). If no recent keys remain, the
2979 server may generate a new key.
2984 The pruned and possibly added-to cookie file
2985 must be resaved atomically (using a temporary
2986 file which is rename()'d).
2991 The lock must be dropped by deleting the lockfile.
2997 Clients need not lock the file in order to load it,
2998 because servers are required to save the file atomically.
3003 <sect1 id="addresses">
3004 <title>Server Addresses</title>
3006 Server addresses consist of a transport name followed by a colon, and
3007 then an optional, comma-separated list of keys and values in the form key=value.
3008 Each value is escaped.
3012 <programlisting>unix:path=/tmp/dbus-test</programlisting>
3013 Which is the address to a unix socket with the path /tmp/dbus-test.
3016 Value escaping is similar to URI escaping but simpler.
3020 The set of optionally-escaped bytes is:
3021 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
3022 <emphasis>byte</emphasis> (note, not character) which is not in the
3023 set of optionally-escaped bytes must be replaced with an ASCII
3024 percent (<literal>%</literal>) and the value of the byte in hex.
3025 The hex value must always be two digits, even if the first digit is
3026 zero. The optionally-escaped bytes may be escaped if desired.
3031 To unescape, append each byte in the value; if a byte is an ASCII
3032 percent (<literal>%</literal>) character then append the following
3033 hex value instead. It is an error if a <literal>%</literal> byte
3034 does not have two hex digits following. It is an error if a
3035 non-optionally-escaped byte is seen unescaped.
3039 The set of optionally-escaped bytes is intended to preserve address
3040 readability and convenience.
3044 A server may specify a key-value pair with the key <literal>guid</literal>
3045 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
3046 describes the format of the <literal>guid</literal> field. If present,
3047 this UUID may be used to distinguish one server address from another. A
3048 server should use a different UUID for each address it listens on. For
3049 example, if a message bus daemon offers both UNIX domain socket and TCP
3050 connections, but treats clients the same regardless of how they connect,
3051 those two connections are equivalent post-connection but should have
3052 distinct UUIDs to distinguish the kinds of connection.
3056 The intent of the address UUID feature is to allow a client to avoid
3057 opening multiple identical connections to the same server, by allowing the
3058 client to check whether an address corresponds to an already-existing
3059 connection. Comparing two addresses is insufficient, because addresses
3060 can be recycled by distinct servers, and equivalent addresses may look
3061 different if simply compared as strings (for example, the host in a TCP
3062 address can be given as an IP address or as a hostname).
3066 Note that the address key is <literal>guid</literal> even though the
3067 rest of the API and documentation says "UUID," for historical reasons.
3071 [FIXME clarify if attempting to connect to each is a requirement
3072 or just a suggestion]
3073 When connecting to a server, multiple server addresses can be
3074 separated by a semi-colon. The library will then try to connect
3075 to the first address and if that fails, it'll try to connect to
3076 the next one specified, and so forth. For example
3077 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3081 Some addresses are <firstterm>connectable</firstterm>. A connectable
3082 address is one containing enough information for a client to connect
3083 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3084 is a connectable address. It is not necessarily possible to listen
3085 on every connectable address: for instance, it is not possible to
3086 listen on a <literal>unixexec:</literal> address.
3090 Some addresses are <firstterm>listenable</firstterm>. A listenable
3091 address is one containing enough information for a server to listen on
3092 it, producing a connectable address (which may differ from the
3093 original address). Many listenable addresses are not connectable:
3094 for instance, <literal>tcp:host=127.0.0.1</literal>
3095 is listenable, but not connectable (because it does not specify
3100 Listening on an address that is not connectable will result in a
3101 connectable address that is not the same as the listenable address.
3102 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3103 might result in the connectable address
3104 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3105 listening on <literal>unix:tmpdir=/tmp</literal>
3106 might result in the connectable address
3107 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>, or
3108 listening on <literal>unix:runtime=yes</literal>
3109 might result in the connectable address
3110 <literal>unix:path=/run/user/1234/bus</literal>.
3114 <sect1 id="transports">
3115 <title>Transports</title>
3117 [FIXME we need to specify in detail each transport and its possible arguments]
3119 Current transports include: unix domain sockets (including
3120 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3121 using in-process pipes. Future possible transports include one that
3122 tunnels over X11 protocol.
3125 <sect2 id="transports-unix-domain-sockets">
3126 <title>Unix Domain Sockets</title>
3128 Unix domain sockets can be either paths in the file system or on Linux
3129 kernels, they can be abstract which are similar to paths but
3130 do not show up in the file system.
3134 When a socket is opened by the D-Bus library it truncates the path
3135 name right before the first trailing Nul byte. This is true for both
3136 normal paths and abstract paths. Note that this is a departure from
3137 previous versions of D-Bus that would create sockets with a fixed
3138 length path name. Names which were shorter than the fixed length
3139 would be padded by Nul bytes.
3142 Unix domain sockets are not available on Windows.
3145 Unix addresses that specify <literal>path</literal> or
3146 <literal>abstract</literal> are both listenable and connectable.
3147 Unix addresses that specify <literal>tmpdir</literal> are only
3148 listenable: the corresponding connectable address will specify
3149 either <literal>path</literal> or <literal>abstract</literal>.
3150 Similarly, Unix addresses that specify <literal>runtime</literal>
3151 are only listenable, and the corresponding connectable address
3152 will specify <literal>path</literal>.
3154 <sect3 id="transports-unix-domain-sockets-addresses">
3155 <title>Server Address Format</title>
3157 Unix domain socket addresses are identified by the "unix:" prefix
3158 and support the following key/value pairs:
3165 <entry>Values</entry>
3166 <entry>Description</entry>
3172 <entry>(path)</entry>
3173 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3176 <entry>tmpdir</entry>
3177 <entry>(path)</entry>
3178 <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>
3181 <entry>abstract</entry>
3182 <entry>(string)</entry>
3183 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tmpdir" key must not be set. This key is only supported on platforms with "abstract Unix sockets", of which Linux is the only known example.</entry>
3186 <entry>runtime</entry>
3187 <entry><literal>yes</literal></entry>
3188 <entry>If given, This key can only be used in server addresses, not in client addresses. If set, its value must be <literal>yes</literal>. This is typically used in an address string like <literal>unix:runtime=yes;unix:tmpdir=/tmp</literal> so that there can be a fallback if <literal>XDG_RUNTIME_DIR</literal> is not set.</entry>
3194 Exactly one of the keys <literal>path</literal>,
3195 <literal>abstract</literal>, <literal>runtime</literal> or
3196 <literal>tmpdir</literal> must be provided.
3200 <sect2 id="transports-launchd">
3201 <title>launchd</title>
3203 launchd is an open-source server management system that replaces init, inetd
3204 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3205 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3209 launchd allocates a socket and provides it with the unix path through the
3210 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3211 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3212 it through its environment.
3213 Other processes can query for the launchd socket by executing:
3214 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3215 This is normally done by the D-Bus client library so doesn't have to be done
3219 launchd is not available on Microsoft Windows.
3222 launchd addresses are listenable and connectable.
3224 <sect3 id="transports-launchd-addresses">
3225 <title>Server Address Format</title>
3227 launchd addresses are identified by the "launchd:" prefix
3228 and support the following key/value pairs:
3235 <entry>Values</entry>
3236 <entry>Description</entry>
3242 <entry>(environment variable)</entry>
3243 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3249 The <literal>env</literal> key is required.
3253 <sect2 id="transports-systemd">
3254 <title>systemd</title>
3256 systemd is an open-source server management system that
3257 replaces init and inetd on newer Linux systems. It supports
3258 socket activation. The D-Bus systemd transport is used to acquire
3259 socket activation file descriptors from systemd and use them
3260 as D-Bus transport when the current process is spawned by
3261 socket activation from it.
3264 The systemd transport accepts only one or more Unix domain or
3265 TCP streams sockets passed in via socket activation.
3268 The systemd transport is not available on non-Linux operating systems.
3271 The systemd transport defines no parameter keys.
3274 systemd addresses are listenable, but not connectable. The
3275 corresponding connectable address is the <literal>unix</literal>
3276 or <literal>tcp</literal> address of the socket.
3279 <sect2 id="transports-tcp-sockets">
3280 <title>TCP Sockets</title>
3282 The tcp transport provides TCP/IP based connections between clients
3283 located on the same or different hosts.
3286 Using tcp transport without any additional secure authentification mechanismus
3287 over a network is unsecure.
3290 On Windows and most Unix platforms, the TCP stack is unable to transfer
3291 credentials over a TCP connection, so the EXTERNAL authentication
3292 mechanism does not work for this transport.
3295 All <literal>tcp</literal> addresses are listenable.
3296 <literal>tcp</literal> addresses in which both
3297 <literal>host</literal> and <literal>port</literal> are
3298 specified, and <literal>port</literal> is non-zero,
3299 are also connectable.
3301 <sect3 id="transports-tcp-sockets-addresses">
3302 <title>Server Address Format</title>
3304 TCP/IP socket addresses are identified by the "tcp:" prefix
3305 and support the following key/value pairs:
3312 <entry>Values</entry>
3313 <entry>Description</entry>
3319 <entry>(string)</entry>
3320 <entry>DNS name or IP address</entry>
3324 <entry>(string)</entry>
3325 <entry>Used in a listenable address to configure the interface
3326 on which the server will listen: either the IP address of one of
3327 the local machine's interfaces (most commonly <literal>127.0.0.1
3328 </literal>), or a DNS name that resolves to one of those IP
3329 addresses, or '*' to listen on all interfaces simultaneously.
3330 If not specified, the default is the same value as "host".
3335 <entry>(number)</entry>
3336 <entry>The tcp port the server will open. A zero value let the server
3337 choose a free port provided from the underlaying operating system.
3338 libdbus is able to retrieve the real used port from the server.
3342 <entry>family</entry>
3343 <entry>(string)</entry>
3344 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3351 <sect2 id="transports-nonce-tcp-sockets">
3352 <title>Nonce-secured TCP Sockets</title>
3354 The nonce-tcp transport provides a secured TCP transport, using a
3355 simple authentication mechanism to ensure that only clients with read
3356 access to a certain location in the filesystem can connect to the server.
3357 The server writes a secret, the nonce, to a file and an incoming client
3358 connection is only accepted if the client sends the nonce right after
3359 the connect. The nonce mechanism requires no setup and is orthogonal to
3360 the higher-level authentication mechanisms described in the
3361 Authentication section.
3365 On start, the server generates a random 16 byte nonce and writes it
3366 to a file in the user's temporary directory. The nonce file location
3367 is published as part of the server's D-Bus address using the
3368 "noncefile" key-value pair.
3370 After an accept, the server reads 16 bytes from the socket. If the
3371 read bytes do not match the nonce stored in the nonce file, the
3372 server MUST immediately drop the connection.
3373 If the nonce match the received byte sequence, the client is accepted
3374 and the transport behaves like an unsecured tcp transport.
3377 After a successful connect to the server socket, the client MUST read
3378 the nonce from the file published by the server via the noncefile=
3379 key-value pair and send it over the socket. After that, the
3380 transport behaves like an unsecured tcp transport.
3383 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3384 <literal>host</literal>, <literal>port</literal> and
3385 <literal>noncefile</literal> are all specified,
3386 and <literal>port</literal> is nonzero, are also connectable.
3388 <sect3 id="transports-nonce-tcp-sockets-addresses">
3389 <title>Server Address Format</title>
3391 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3392 and support the following key/value pairs:
3399 <entry>Values</entry>
3400 <entry>Description</entry>
3406 <entry>(string)</entry>
3407 <entry>DNS name or IP address</entry>
3411 <entry>(string)</entry>
3412 <entry>The same as for tcp: addresses
3417 <entry>(number)</entry>
3418 <entry>The tcp port the server will open. A zero value let the server
3419 choose a free port provided from the underlaying operating system.
3420 libdbus is able to retrieve the real used port from the server.
3424 <entry>family</entry>
3425 <entry>(string)</entry>
3426 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3429 <entry>noncefile</entry>
3430 <entry>(path)</entry>
3431 <entry>File location containing the secret.
3432 This is only meaningful in connectable addresses:
3433 a listening D-Bus server that offers this transport
3434 will always create a new nonce file.</entry>
3441 <sect2 id="transports-exec">
3442 <title>Executed Subprocesses on Unix</title>
3444 This transport forks off a process and connects its standard
3445 input and standard output with an anonymous Unix domain
3446 socket. This socket is then used for communication by the
3447 transport. This transport may be used to use out-of-process
3448 forwarder programs as basis for the D-Bus protocol.
3451 The forked process will inherit the standard error output and
3452 process group from the parent process.
3455 Executed subprocesses are not available on Windows.
3458 <literal>unixexec</literal> addresses are connectable, but are not
3461 <sect3 id="transports-exec-addresses">
3462 <title>Server Address Format</title>
3464 Executed subprocess addresses are identified by the "unixexec:" prefix
3465 and support the following key/value pairs:
3472 <entry>Values</entry>
3473 <entry>Description</entry>
3479 <entry>(path)</entry>
3480 <entry>Path of the binary to execute, either an absolute
3481 path or a binary name that is searched for in the default
3482 search path of the OS. This corresponds to the first
3483 argument of execlp(). This key is mandatory.</entry>
3486 <entry>argv0</entry>
3487 <entry>(string)</entry>
3488 <entry>The program name to use when executing the
3489 binary. If omitted the same value as specified for path=
3490 will be used. This corresponds to the second argument of
3494 <entry>argv1, argv2, ...</entry>
3495 <entry>(string)</entry>
3496 <entry>Arguments to pass to the binary. This corresponds
3497 to the third and later arguments of execlp(). If a
3498 specific argvX is not specified no further argvY for Y > X
3499 are taken into account.</entry>
3507 <sect1 id="meta-transports">
3508 <title>Meta Transports</title>
3510 Meta transports are a kind of transport with special enhancements or
3511 behavior. Currently available meta transports include: autolaunch
3514 <sect2 id="meta-transports-autolaunch">
3515 <title>Autolaunch</title>
3516 <para>The autolaunch transport provides a way for dbus clients to autodetect
3517 a running dbus session bus and to autolaunch a session bus if not present.
3520 On Unix, <literal>autolaunch</literal> addresses are connectable,
3524 On Windows, <literal>autolaunch</literal> addresses are both
3525 connectable and listenable.
3528 <sect3 id="meta-transports-autolaunch-addresses">
3529 <title>Server Address Format</title>
3531 Autolaunch addresses uses the "autolaunch:" prefix and support the
3532 following key/value pairs:
3539 <entry>Values</entry>
3540 <entry>Description</entry>
3545 <entry>scope</entry>
3546 <entry>(string)</entry>
3547 <entry>scope of autolaunch (Windows only)
3551 "*install-path" - limit session bus to dbus installation path.
3552 The dbus installation path is determined from the location of
3553 the shared dbus library. If the library is located in a 'bin'
3554 subdirectory the installation root is the directory above,
3555 otherwise the directory where the library lives is taken as
3558 <install-root>/bin/[lib]dbus-1.dll
3559 <install-root>/[lib]dbus-1.dll
3565 "*user" - limit session bus to the recent user.
3570 other values - specify dedicated session bus like "release",
3582 <sect3 id="meta-transports-autolaunch-windows-implementation">
3583 <title>Windows implementation</title>
3585 On start, the server opens a platform specific transport, creates a mutex
3586 and a shared memory section containing the related session bus address.
3587 This mutex will be inspected by the dbus client library to detect a
3588 running dbus session bus. The access to the mutex and the shared memory
3589 section are protected by global locks.
3592 In the recent implementation the autolaunch transport uses a tcp transport
3593 on localhost with a port choosen from the operating system. This detail may
3594 change in the future.
3597 Disclaimer: The recent implementation is in an early state and may not
3598 work in all cirumstances and/or may have security issues. Because of this
3599 the implementation is not documentated yet.
3606 <title>UUIDs</title>
3608 A working D-Bus implementation uses universally-unique IDs in two places.
3609 First, each server address has a UUID identifying the address,
3610 as described in <xref linkend="addresses"/>. Second, each operating
3611 system kernel instance running a D-Bus client or server has a UUID
3612 identifying that kernel, retrieved by invoking the method
3613 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3614 linkend="standard-interfaces-peer"/>).
3617 The term "UUID" in this document is intended literally, i.e. an
3618 identifier that is universally unique. It is not intended to refer to
3619 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3622 The UUID must contain 128 bits of data and be hex-encoded. The
3623 hex-encoded string may not contain hyphens or other non-hex-digit
3624 characters, and it must be exactly 32 characters long. To generate a
3625 UUID, the current reference implementation concatenates 96 bits of random
3626 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3630 It would also be acceptable and probably better to simply generate 128
3631 bits of random data, as long as the random number generator is of high
3632 quality. The timestamp could conceivably help if the random bits are not
3633 very random. With a quality random number generator, collisions are
3634 extremely unlikely even with only 96 bits, so it's somewhat academic.
3637 Implementations should, however, stick to random data for the first 96 bits
3642 <sect1 id="standard-interfaces">
3643 <title>Standard Interfaces</title>
3645 See <xref linkend="message-protocol-types-notation"/> for details on
3646 the notation used in this section. There are some standard interfaces
3647 that may be useful across various D-Bus applications.
3649 <sect2 id="standard-interfaces-peer">
3650 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3652 The <literal>org.freedesktop.DBus.Peer</literal> interface
3655 org.freedesktop.DBus.Peer.Ping ()
3656 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3660 On receipt of the <literal>METHOD_CALL</literal> message
3661 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3662 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3663 usual. It does not matter which object path a ping is sent to. The
3664 reference implementation handles this method automatically.
3667 On receipt of the <literal>METHOD_CALL</literal> message
3668 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3669 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3670 UUID representing the identity of the machine the process is running on.
3671 This UUID must be the same for all processes on a single system at least
3672 until that system next reboots. It should be the same across reboots
3673 if possible, but this is not always possible to implement and is not
3675 It does not matter which object path a GetMachineId is sent to. The
3676 reference implementation handles this method automatically.
3679 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3680 a virtual machine running on a hypervisor, rather than a physical machine.
3681 Basically if two processes see the same UUID, they should also see the same
3682 shared memory, UNIX domain sockets, process IDs, and other features that require
3683 a running OS kernel in common between the processes.
3686 The UUID is often used where other programs might use a hostname. Hostnames
3687 can change without rebooting, however, or just be "localhost" - so the UUID
3691 <xref linkend="uuids"/> explains the format of the UUID.
3695 <sect2 id="standard-interfaces-introspectable">
3696 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3698 This interface has one method:
3700 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3704 Objects instances may implement
3705 <literal>Introspect</literal> which returns an XML description of
3706 the object, including its interfaces (with signals and methods), objects
3707 below it in the object path tree, and its properties.
3710 <xref linkend="introspection-format"/> describes the format of this XML string.
3713 <sect2 id="standard-interfaces-properties">
3714 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3716 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3717 or <firstterm>attributes</firstterm>. These can be exposed via the
3718 <literal>org.freedesktop.DBus.Properties</literal> interface.
3722 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3723 in STRING property_name,
3725 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3726 in STRING property_name,
3728 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3729 out DICT<STRING,VARIANT> props);
3733 It is conventional to give D-Bus properties names consisting of
3734 capitalized words without punctuation ("CamelCase"), like
3735 <link linkend="message-protocol-names-member">member names</link>.
3736 For instance, the GObject property
3737 <literal>connection-status</literal> or the Qt property
3738 <literal>connectionStatus</literal> could be represented on D-Bus
3739 as <literal>ConnectionStatus</literal>.
3742 Strictly speaking, D-Bus property names are not required to follow
3743 the same naming restrictions as member names, but D-Bus property
3744 names that would not be valid member names (in particular,
3745 GObject-style dash-separated property names) can cause interoperability
3746 problems and should be avoided.
3749 The available properties and whether they are writable can be determined
3750 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3751 see <xref linkend="standard-interfaces-introspectable"/>.
3754 An empty string may be provided for the interface name; in this case,
3755 if there are multiple properties on an object with the same name,
3756 the results are undefined (picking one by according to an arbitrary
3757 deterministic rule, or returning an error, are the reasonable
3761 If one or more properties change on an object, the
3762 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3763 signal may be emitted (this signal was added in 0.14):
3767 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3768 DICT<STRING,VARIANT> changed_properties,
3769 ARRAY<STRING> invalidated_properties);
3773 where <literal>changed_properties</literal> is a dictionary
3774 containing the changed properties with the new values and
3775 <literal>invalidated_properties</literal> is an array of
3776 properties that changed but the value is not conveyed.
3779 Whether the <literal>PropertiesChanged</literal> signal is
3780 supported can be determined by calling
3781 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3782 that the signal may be supported for an object but it may
3783 differ how whether and how it is used on a per-property basis
3784 (for e.g. performance or security reasons). Each property (or
3785 the parent interface) must be annotated with the
3786 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3787 annotation to convey this (usually the default value
3788 <literal>true</literal> is sufficient meaning that the
3789 annotation does not need to be used). See <xref
3790 linkend="introspection-format"/> for details on this
3795 <sect2 id="standard-interfaces-objectmanager">
3796 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3798 An API can optionally make use of this interface for one or
3799 more sub-trees of objects. The root of each sub-tree implements
3800 this interface so other applications can get all objects,
3801 interfaces and properties in a single method call. It is
3802 appropriate to use this interface if users of the tree of
3803 objects are expected to be interested in all interfaces of all
3804 objects in the tree; a more granular API should be used if
3805 users of the objects are expected to be interested in a small
3806 subset of the objects, a small subset of their interfaces, or
3810 The method that applications can use to get all objects and
3811 properties is <literal>GetManagedObjects</literal>:
3815 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3819 The return value of this method is a dict whose keys are
3820 object paths. All returned object paths are children of the
3821 object path implementing this interface, i.e. their object
3822 paths start with the ObjectManager's object path plus '/'.
3825 Each value is a dict whose keys are interfaces names. Each
3826 value in this inner dict is the same dict that would be
3827 returned by the <link
3828 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3829 method for that combination of object path and interface. If
3830 an interface has no properties, the empty dict is returned.
3833 Changes are emitted using the following two signals:
3837 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3838 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3839 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3840 ARRAY<STRING> interfaces);
3844 The <literal>InterfacesAdded</literal> signal is emitted when
3845 either a new object is added or when an existing object gains
3846 one or more interfaces. The
3847 <literal>InterfacesRemoved</literal> signal is emitted
3848 whenever an object is removed or it loses one or more
3849 interfaces. The second parameter of the
3850 <literal>InterfacesAdded</literal> signal contains a dict with
3851 the interfaces and properties (if any) that have been added to
3852 the given object path. Similarly, the second parameter of the
3853 <literal>InterfacesRemoved</literal> signal contains an array
3854 of the interfaces that were removed. Note that changes on
3855 properties on existing interfaces are not reported using this
3856 interface - an application should also monitor the existing <link
3857 linkend="standard-interfaces-properties">PropertiesChanged</link>
3858 signal on each object.
3861 Applications SHOULD NOT export objects that are children of an
3862 object (directly or otherwise) implementing this interface but
3863 which are not returned in the reply from the
3864 <literal>GetManagedObjects()</literal> method of this
3865 interface on the given object.
3868 The intent of the <literal>ObjectManager</literal> interface
3869 is to make it easy to write a robust client
3870 implementation. The trivial client implementation only needs
3871 to make two method calls:
3875 org.freedesktop.DBus.AddMatch (bus_proxy,
3876 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3877 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3881 on the message bus and the remote application's
3882 <literal>ObjectManager</literal>, respectively. Whenever a new
3883 remote object is created (or an existing object gains a new
3884 interface), the <literal>InterfacesAdded</literal> signal is
3885 emitted, and since this signal contains all properties for the
3886 interfaces, no calls to the
3887 <literal>org.freedesktop.Properties</literal> interface on the
3888 remote object are needed. Additionally, since the initial
3889 <literal>AddMatch()</literal> rule already includes signal
3890 messages from the newly created child object, no new
3891 <literal>AddMatch()</literal> call is needed.
3896 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3897 interface was added in version 0.17 of the D-Bus
3904 <sect1 id="introspection-format">
3905 <title>Introspection Data Format</title>
3907 As described in <xref linkend="standard-interfaces-introspectable"/>,
3908 objects may be introspected at runtime, returning an XML string
3909 that describes the object. The same XML format may be used in
3910 other contexts as well, for example as an "IDL" for generating
3911 static language bindings.
3914 Here is an example of introspection data:
3916 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3917 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3918 <node name="/com/example/sample_object">
3919 <interface name="com.example.SampleInterface">
3920 <method name="Frobate">
3921 <arg name="foo" type="i" direction="in"/>
3922 <arg name="bar" type="s" direction="out"/>
3923 <arg name="baz" type="a{us}" direction="out"/>
3924 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3926 <method name="Bazify">
3927 <arg name="bar" type="(iiu)" direction="in"/>
3928 <arg name="bar" type="v" direction="out"/>
3930 <method name="Mogrify">
3931 <arg name="bar" type="(iiav)" direction="in"/>
3933 <signal name="Changed">
3934 <arg name="new_value" type="b"/>
3936 <property name="Bar" type="y" access="readwrite"/>
3938 <node name="child_of_sample_object"/>
3939 <node name="another_child_of_sample_object"/>
3944 A more formal DTD and spec needs writing, but here are some quick notes.
3948 Only the root <node> element can omit the node name, as it's
3949 known to be the object that was introspected. If the root
3950 <node> does have a name attribute, it must be an absolute
3951 object path. If child <node> have object paths, they must be
3957 If a child <node> has any sub-elements, then they
3958 must represent a complete introspection of the child.
3959 If a child <node> is empty, then it may or may
3960 not have sub-elements; the child must be introspected
3961 in order to find out. The intent is that if an object
3962 knows that its children are "fast" to introspect
3963 it can go ahead and return their information, but
3964 otherwise it can omit it.
3969 The direction element on <arg> may be omitted,
3970 in which case it defaults to "in" for method calls
3971 and "out" for signals. Signals only allow "out"
3972 so while direction may be specified, it's pointless.
3977 The possible directions are "in" and "out",
3978 unlike CORBA there is no "inout"
3983 The possible property access flags are
3984 "readwrite", "read", and "write"
3989 Multiple interfaces can of course be listed for
3995 The "name" attribute on arguments is optional.
4001 Method, interface, property, and signal elements may have
4002 "annotations", which are generic key/value pairs of metadata.
4003 They are similar conceptually to Java's annotations and C# attributes.
4004 Well-known annotations:
4011 <entry>Values (separated by ,)</entry>
4012 <entry>Description</entry>
4017 <entry>org.freedesktop.DBus.Deprecated</entry>
4018 <entry>true,false</entry>
4019 <entry>Whether or not the entity is deprecated; defaults to false</entry>
4022 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
4023 <entry>(string)</entry>
4024 <entry>The C symbol; may be used for methods and interfaces</entry>
4027 <entry>org.freedesktop.DBus.Method.NoReply</entry>
4028 <entry>true,false</entry>
4029 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
4032 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
4033 <entry>true,invalidates,const,false</entry>
4036 If set to <literal>false</literal>, the
4037 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
4039 linkend="standard-interfaces-properties"/> is not
4040 guaranteed to be emitted if the property changes.
4043 If set to <literal>const</literal> the property never
4044 changes value during the lifetime of the object it
4045 belongs to, and hence the signal is never emitted for
4049 If set to <literal>invalidates</literal> the signal
4050 is emitted but the value is not included in the
4054 If set to <literal>true</literal> the signal is
4055 emitted with the value included.
4058 The value for the annotation defaults to
4059 <literal>true</literal> if the enclosing interface
4060 element does not specify the annotation. Otherwise it
4061 defaults to the value specified in the enclosing
4065 This annotation is intended to be used by code
4066 generators to implement client-side caching of
4067 property values. For all properties for which the
4068 annotation is set to <literal>const</literal>,
4069 <literal>invalidates</literal> or
4070 <literal>true</literal> the client may
4071 unconditionally cache the values as the properties
4072 don't change or notifications are generated for them
4081 <sect1 id="message-bus">
4082 <title>Message Bus Specification</title>
4083 <sect2 id="message-bus-overview">
4084 <title>Message Bus Overview</title>
4086 The message bus accepts connections from one or more applications.
4087 Once connected, applications can exchange messages with other
4088 applications that are also connected to the bus.
4091 In order to route messages among connections, the message bus keeps a
4092 mapping from names to connections. Each connection has one
4093 unique-for-the-lifetime-of-the-bus name automatically assigned.
4094 Applications may request additional names for a connection. Additional
4095 names are usually "well-known names" such as
4096 "com.example.TextEditor". When a name is bound to a connection,
4097 that connection is said to <firstterm>own</firstterm> the name.
4100 The bus itself owns a special name,
4101 <literal>org.freedesktop.DBus</literal>, with an object
4102 located at <literal>/org/freedesktop/DBus</literal> that
4103 implements the <literal>org.freedesktop.DBus</literal>
4104 interface. This service allows applications to make
4105 administrative requests of the bus itself. For example,
4106 applications can ask the bus to assign a name to a connection.
4109 Each name may have <firstterm>queued owners</firstterm>. When an
4110 application requests a name for a connection and the name is already in
4111 use, the bus will optionally add the connection to a queue waiting for
4112 the name. If the current owner of the name disconnects or releases
4113 the name, the next connection in the queue will become the new owner.
4117 This feature causes the right thing to happen if you start two text
4118 editors for example; the first one may request "com.example.TextEditor",
4119 and the second will be queued as a possible owner of that name. When
4120 the first exits, the second will take over.
4124 Applications may send <firstterm>unicast messages</firstterm> to
4125 a specific recipient or to the message bus itself, or
4126 <firstterm>broadcast messages</firstterm> to all interested recipients.
4127 See <xref linkend="message-bus-routing"/> for details.
4131 <sect2 id="message-bus-names">
4132 <title>Message Bus Names</title>
4134 Each connection has at least one name, assigned at connection time and
4135 returned in response to the
4136 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4137 automatically-assigned name is called the connection's <firstterm>unique
4138 name</firstterm>. Unique names are never reused for two different
4139 connections to the same bus.
4142 Ownership of a unique name is a prerequisite for interaction with
4143 the message bus. It logically follows that the unique name is always
4144 the first name that an application comes to own, and the last
4145 one that it loses ownership of.
4148 Unique connection names must begin with the character ':' (ASCII colon
4149 character); bus names that are not unique names must not begin
4150 with this character. (The bus must reject any attempt by an application
4151 to manually request a name beginning with ':'.) This restriction
4152 categorically prevents "spoofing"; messages sent to a unique name
4153 will always go to the expected connection.
4156 When a connection is closed, all the names that it owns are deleted (or
4157 transferred to the next connection in the queue if any).
4160 A connection can request additional names to be associated with it using
4161 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4162 linkend="message-protocol-names-bus"/> describes the format of a valid
4163 name. These names can be released again using the
4164 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4167 <sect3 id="bus-messages-request-name">
4168 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4172 UINT32 RequestName (in STRING name, in UINT32 flags)
4179 <entry>Argument</entry>
4181 <entry>Description</entry>
4187 <entry>STRING</entry>
4188 <entry>Name to request</entry>
4192 <entry>UINT32</entry>
4193 <entry>Flags</entry>
4203 <entry>Argument</entry>
4205 <entry>Description</entry>
4211 <entry>UINT32</entry>
4212 <entry>Return value</entry>
4219 This method call should be sent to
4220 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4221 assign the given name to the method caller. Each name maintains a
4222 queue of possible owners, where the head of the queue is the primary
4223 or current owner of the name. Each potential owner in the queue
4224 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4225 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4226 call. When RequestName is invoked the following occurs:
4230 If the method caller is currently the primary owner of the name,
4231 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4232 values are updated with the values from the new RequestName call,
4233 and nothing further happens.
4239 If the current primary owner (head of the queue) has
4240 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4241 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4242 the caller of RequestName replaces the current primary owner at
4243 the head of the queue and the current primary owner moves to the
4244 second position in the queue. If the caller of RequestName was
4245 in the queue previously its flags are updated with the values from
4246 the new RequestName in addition to moving it to the head of the queue.
4252 If replacement is not possible, and the method caller is
4253 currently in the queue but not the primary owner, its flags are
4254 updated with the values from the new RequestName call.
4260 If replacement is not possible, and the method caller is
4261 currently not in the queue, the method caller is appended to the
4268 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4269 set and is not the primary owner, it is removed from the
4270 queue. This can apply to the previous primary owner (if it
4271 was replaced) or the method caller (if it updated the
4272 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4273 queue, or if it was just added to the queue with that flag set).
4279 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4280 queue," even if another application already in the queue had specified
4281 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4282 that does not allow replacement goes away, and the next primary owner
4283 does allow replacement. In this case, queued items that specified
4284 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4285 automatically replace the new primary owner. In other words,
4286 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4287 time RequestName is called. This is deliberate to avoid an infinite loop
4288 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4289 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4292 The flags argument contains any of the following values logically ORed
4299 <entry>Conventional Name</entry>
4300 <entry>Value</entry>
4301 <entry>Description</entry>
4306 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4310 If an application A specifies this flag and succeeds in
4311 becoming the owner of the name, and another application B
4312 later calls RequestName with the
4313 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4314 will lose ownership and receive a
4315 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4316 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4317 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4318 is not specified by application B, then application B will not replace
4319 application A as the owner.
4324 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4328 Try to replace the current owner if there is one. If this
4329 flag is not set the application will only become the owner of
4330 the name if there is no current owner. If this flag is set,
4331 the application will replace the current owner if
4332 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4337 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4341 Without this flag, if an application requests a name that is
4342 already owned, the application will be placed in a queue to
4343 own the name when the current owner gives it up. If this
4344 flag is given, the application will not be placed in the
4345 queue, the request for the name will simply fail. This flag
4346 also affects behavior when an application is replaced as
4347 name owner; by default the application moves back into the
4348 waiting queue, unless this flag was provided when the application
4349 became the name owner.
4357 The return code can be one of the following values:
4363 <entry>Conventional Name</entry>
4364 <entry>Value</entry>
4365 <entry>Description</entry>
4370 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4371 <entry>1</entry> <entry>The caller is now the primary owner of
4372 the name, replacing any previous owner. Either the name had no
4373 owner before, or the caller specified
4374 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4375 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4378 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4381 <entry>The name already had an owner,
4382 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4383 the current owner did not specify
4384 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4385 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4389 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4390 <entry>The name already has an owner,
4391 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4392 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4393 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4394 specified by the requesting application.</entry>
4397 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4399 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4407 <sect3 id="bus-messages-release-name">
4408 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4412 UINT32 ReleaseName (in STRING name)
4419 <entry>Argument</entry>
4421 <entry>Description</entry>
4427 <entry>STRING</entry>
4428 <entry>Name to release</entry>
4438 <entry>Argument</entry>
4440 <entry>Description</entry>
4446 <entry>UINT32</entry>
4447 <entry>Return value</entry>
4454 This method call should be sent to
4455 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4456 release the method caller's claim to the given name. If the caller is
4457 the primary owner, a new primary owner will be selected from the
4458 queue if any other owners are waiting. If the caller is waiting in
4459 the queue for the name, the caller will removed from the queue and
4460 will not be made an owner of the name if it later becomes available.
4461 If there are no other owners in the queue for the name, it will be
4462 removed from the bus entirely.
4464 The return code can be one of the following values:
4470 <entry>Conventional Name</entry>
4471 <entry>Value</entry>
4472 <entry>Description</entry>
4477 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4478 <entry>1</entry> <entry>The caller has released his claim on
4479 the given name. Either the caller was the primary owner of
4480 the name, and the name is now unused or taken by somebody
4481 waiting in the queue for the name, or the caller was waiting
4482 in the queue for the name and has now been removed from the
4486 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4488 <entry>The given name does not exist on this bus.</entry>
4491 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4493 <entry>The caller was not the primary owner of this name,
4494 and was also not waiting in the queue to own this name.</entry>
4502 <sect3 id="bus-messages-list-queued-owners">
4503 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4507 ARRAY of STRING ListQueuedOwners (in STRING name)
4514 <entry>Argument</entry>
4516 <entry>Description</entry>
4522 <entry>STRING</entry>
4523 <entry>The well-known bus name to query, such as
4524 <literal>com.example.cappuccino</literal></entry>
4534 <entry>Argument</entry>
4536 <entry>Description</entry>
4542 <entry>ARRAY of STRING</entry>
4543 <entry>The unique bus names of connections currently queued
4544 for the name</entry>
4551 This method call should be sent to
4552 <literal>org.freedesktop.DBus</literal> and lists the connections
4553 currently queued for a bus name (see
4554 <xref linkend="term-queued-owner"/>).
4559 <sect2 id="message-bus-routing">
4560 <title>Message Bus Message Routing</title>
4563 Messages may have a <literal>DESTINATION</literal> field (see <xref
4564 linkend="message-protocol-header-fields"/>), resulting in a
4565 <firstterm>unicast message</firstterm>. If the
4566 <literal>DESTINATION</literal> field is present, it specifies a message
4567 recipient by name. Method calls and replies normally specify this field.
4568 The message bus must send messages (of any type) with the
4569 <literal>DESTINATION</literal> field set to the specified recipient,
4570 regardless of whether the recipient has set up a match rule matching
4575 When the message bus receives a signal, if the
4576 <literal>DESTINATION</literal> field is absent, it is considered to
4577 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4578 applications with <firstterm>message matching rules</firstterm> that
4579 match the message. Most signal messages are broadcasts, and
4580 no other message types currently defined in this specification
4585 Unicast signal messages (those with a <literal>DESTINATION</literal>
4586 field) are not commonly used, but they are treated like any unicast
4587 message: they are delivered to the specified receipient,
4588 regardless of its match rules. One use for unicast signals is to
4589 avoid a race condition in which a signal is emitted before the intended
4590 recipient can call <xref linkend="bus-messages-add-match"/> to
4591 receive that signal: if the signal is sent directly to that recipient
4592 using a unicast message, it does not need to add a match rule at all,
4593 and there is no race condition. Another use for unicast signals,
4594 on message buses whose security policy prevents eavesdropping, is to
4595 send sensitive information which should only be visible to one
4600 When the message bus receives a method call, if the
4601 <literal>DESTINATION</literal> field is absent, the call is taken to be
4602 a standard one-to-one message and interpreted by the message bus
4603 itself. For example, sending an
4604 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4605 <literal>DESTINATION</literal> will cause the message bus itself to
4606 reply to the ping immediately; the message bus will not make this
4607 message visible to other applications.
4611 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4612 the ping message were sent with a <literal>DESTINATION</literal> name of
4613 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4614 forwarded, and the Yoyodyne Corporation screensaver application would be
4615 expected to reply to the ping.
4619 Message bus implementations may impose a security policy which
4620 prevents certain messages from being sent or received.
4621 When a method call message cannot be sent or received due to a security
4622 policy, the message bus should send an error reply, unless the
4623 original message had the <literal>NO_REPLY</literal> flag.
4626 <sect3 id="message-bus-routing-eavesdropping">
4627 <title>Eavesdropping</title>
4629 Receiving a unicast message whose <literal>DESTINATION</literal>
4630 indicates a different recipient is called
4631 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4632 a security boundary (like the standard system bus), the security
4633 policy should usually prevent eavesdropping, since unicast messages
4634 are normally kept private and may contain security-sensitive
4639 Eavesdropping is mainly useful for debugging tools, such as
4640 the <literal>dbus-monitor</literal> tool in the reference
4641 implementation of D-Bus. Tools which eavesdrop on the message bus
4642 should be careful to avoid sending a reply or error in response to
4643 messages intended for a different client.
4647 Clients may attempt to eavesdrop by adding match rules
4648 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4649 the <literal>eavesdrop='true'</literal> match. If the message bus'
4650 security policy does not allow eavesdropping, the match rule can
4651 still be added, but will not have any practical effect. For
4652 compatibility with older message bus implementations, if adding such
4653 a match rule results in an error reply, the client may fall back to
4654 adding the same rule with the <literal>eavesdrop</literal> match
4659 Eavesdropping interacts poorly with buses with non-trivial
4660 access control restrictions. The
4661 <xref linkend="bus-messages-become-monitor"/> method provides
4662 an alternative way to monitor buses.
4666 <sect3 id="message-bus-routing-match-rules">
4667 <title>Match Rules</title>
4669 An important part of the message bus routing protocol is match
4670 rules. Match rules describe the messages that should be sent to a
4671 client, based on the contents of the message. Broadcast signals
4672 are only sent to clients which have a suitable match rule: this
4673 avoids waking up client processes to deal with signals that are
4674 not relevant to that client.
4677 Messages that list a client as their <literal>DESTINATION</literal>
4678 do not need to match the client's match rules, and are sent to that
4679 client regardless. As a result, match rules are mainly used to
4680 receive a subset of broadcast signals.
4683 Match rules can also be used for eavesdropping
4684 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4685 if the security policy of the message bus allows it.
4688 Match rules are added using the AddMatch bus method
4689 (see <xref linkend="bus-messages-add-match"/>). Rules are
4690 specified as a string of comma separated key/value pairs.
4691 Excluding a key from the rule indicates a wildcard match.
4692 For instance excluding the the member from a match rule but
4693 adding a sender would let all messages from that sender through.
4694 An example of a complete rule would be
4695 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4698 Within single quotes (ASCII apostrophe, U+0027), a backslash
4699 (U+005C) represents itself, and an apostrophe ends the quoted
4700 section. Outside single quotes, \' (backslash, apostrophe)
4701 represents an apostrophe, and any backslash not followed by
4702 an apostrophe represents itself. For instance, the match rules
4703 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4704 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4705 both match messages where the arguments are a 1-character string
4706 containing an apostrophe, a 1-character string containing a
4707 backslash, a 1-character string containing a comma, and a
4708 2-character string containing two backslashes<footnote>
4710 This idiosyncratic quoting style is based on the rules for
4711 escaping items to appear inside single-quoted strings
4712 in POSIX <literal>/bin/sh</literal>, but please
4713 note that backslashes that are not inside single quotes have
4714 different behaviour. This syntax does not offer any way to
4715 represent an apostrophe inside single quotes (it is necessary
4716 to leave the single-quoted section, backslash-escape the
4717 apostrophe and re-enter single quotes), or to represent a
4718 comma outside single quotes (it is necessary to wrap it in
4719 a single-quoted section).
4724 The following table describes the keys that can be used to create
4731 <entry>Possible Values</entry>
4732 <entry>Description</entry>
4737 <entry><literal>type</literal></entry>
4738 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4739 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4742 <entry><literal>sender</literal></entry>
4743 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4744 and <xref linkend="term-unique-name"/> respectively)
4746 <entry>Match messages sent by a particular sender. An example of a sender match
4747 is sender='org.freedesktop.Hal'</entry>
4750 <entry><literal>interface</literal></entry>
4751 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4752 <entry>Match messages sent over or to a particular interface. An example of an
4753 interface match is interface='org.freedesktop.Hal.Manager'.
4754 If a message omits the interface header, it must not match any rule
4755 that specifies this key.</entry>
4758 <entry><literal>member</literal></entry>
4759 <entry>Any valid method or signal name</entry>
4760 <entry>Matches messages which have the give method or signal name. An example of
4761 a member match is member='NameOwnerChanged'</entry>
4764 <entry><literal>path</literal></entry>
4765 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4766 <entry>Matches messages which are sent from or to the given object. An example of a
4767 path match is path='/org/freedesktop/Hal/Manager'</entry>
4770 <entry><literal>path_namespace</literal></entry>
4771 <entry>An object path</entry>
4774 Matches messages which are sent from or to an
4775 object for which the object path is either the
4776 given value, or that value followed by one or
4777 more path components.
4782 <literal>path_namespace='/com/example/foo'</literal>
4783 would match signals sent by
4784 <literal>/com/example/foo</literal>
4786 <literal>/com/example/foo/bar</literal>,
4788 <literal>/com/example/foobar</literal>.
4792 Using both <literal>path</literal> and
4793 <literal>path_namespace</literal> in the same match
4794 rule is not allowed.
4799 This match key was added in version 0.16 of the
4800 D-Bus specification and implemented by the bus
4801 daemon in dbus 1.5.0 and later.
4807 <entry><literal>destination</literal></entry>
4808 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4809 <entry>Matches messages which are being sent to the given unique name. An
4810 example of a destination match is destination=':1.0'</entry>
4813 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4814 <entry>Any string</entry>
4815 <entry>Arg matches are special and are used for further restricting the
4816 match based on the arguments in the body of a message. Only arguments of type
4817 STRING can be matched in this way. An example of an argument match
4818 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4822 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4823 <entry>Any string</entry>
4825 <para>Argument path matches provide a specialised form of wildcard matching for
4826 path-like namespaces. They can match arguments whose type is either STRING or
4827 OBJECT_PATH. As with normal argument matches,
4828 if the argument is exactly equal to the string given in the match
4829 rule then the rule is satisfied. Additionally, there is also a
4830 match when either the string given in the match rule or the
4831 appropriate message argument ends with '/' and is a prefix of the
4832 other. An example argument path match is arg0path='/aa/bb/'. This
4833 would match messages with first arguments of '/', '/aa/',
4834 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4835 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4837 <para>This is intended for monitoring “directories” in file system-like
4838 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4839 system. An application interested in all nodes in a particular hierarchy would
4840 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4841 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4842 represent a modification to the “bar” property, or a signal with zeroth
4843 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4844 many properties within that directory, and the interested application would be
4845 notified in both cases.</para>
4848 This match key was added in version 0.12 of the
4849 D-Bus specification, implemented for STRING
4850 arguments by the bus daemon in dbus 1.2.0 and later,
4851 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4858 <entry><literal>arg0namespace</literal></entry>
4859 <entry>Like a bus name, except that the string is not
4860 required to contain a '.' (period)</entry>
4862 <para>Match messages whose first argument is of type STRING, and is a bus name
4863 or interface name within the specified namespace. This is primarily intended
4864 for watching name owner changes for a group of related bus names, rather than
4865 for a single name or all name changes.</para>
4867 <para>Because every valid interface name is also a valid
4868 bus name, this can also be used for messages whose
4869 first argument is an interface name.</para>
4871 <para>For example, the match rule
4872 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4873 matches name owner changes for bus names such as
4874 <literal>com.example.backend.foo</literal>,
4875 <literal>com.example.backend.foo.bar</literal>, and
4876 <literal>com.example.backend</literal> itself.</para>
4878 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4881 This match key was added in version 0.16 of the
4882 D-Bus specification and implemented by the bus
4883 daemon in dbus 1.5.0 and later.
4889 <entry><literal>eavesdrop</literal></entry>
4890 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4891 <entry>Since D-Bus 1.5.6, match rules do not
4892 match messages which have a <literal>DESTINATION</literal>
4893 field unless the match rule specifically
4895 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4896 by specifying <literal>eavesdrop='true'</literal>
4897 in the match rule. <literal>eavesdrop='false'</literal>
4898 restores the default behaviour. Messages are
4899 delivered to their <literal>DESTINATION</literal>
4900 regardless of match rules, so this match does not
4901 affect normal delivery of unicast messages.
4902 If the message bus has a security policy which forbids
4903 eavesdropping, this match may still be used without error,
4904 but will not have any practical effect.
4905 In older versions of D-Bus, this match was not allowed
4906 in match rules, and all match rules behaved as if
4907 <literal>eavesdrop='true'</literal> had been used.
4916 <sect2 id="message-bus-starting-services">
4917 <title>Message Bus Starting Services</title>
4919 The message bus can start applications on behalf of other applications.
4920 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4921 An application that can be started in this way is called a
4922 <firstterm>service</firstterm>.
4925 With D-Bus, starting a service is normally done by name. That is,
4926 applications ask the message bus to start some program that will own a
4927 well-known name, such as <literal>com.example.TextEditor</literal>.
4928 This implies a contract documented along with the name
4929 <literal>com.example.TextEditor</literal> for which object
4930 the owner of that name will provide, and what interfaces those
4934 To find an executable corresponding to a particular name, the bus daemon
4935 looks for <firstterm>service description files</firstterm>. Service
4936 description files define a mapping from names to executables. Different
4937 kinds of message bus will look for these files in different places, see
4938 <xref linkend="message-bus-types"/>.
4941 Service description files have the ".service" file
4942 extension. The message bus will only load service description files
4943 ending with .service; all other files will be ignored. The file format
4944 is similar to that of <ulink
4945 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4946 entries</ulink>. All service description files must be in UTF-8
4947 encoding. To ensure that there will be no name collisions, service files
4948 must be namespaced using the same mechanism as messages and service
4953 On the well-known system bus, the name of a service description file
4954 must be its well-known name plus <literal>.service</literal>,
4956 <literal>com.example.ConfigurationDatabase.service</literal>.
4960 On the well-known session bus, services should follow the same
4961 service description file naming convention as on the system bus,
4962 but for backwards compatibility they are not required to do so.
4966 [FIXME the file format should be much better specified than "similar to
4967 .desktop entries" esp. since desktop entries are already
4968 badly-specified. ;-)]
4969 These sections from the specification apply to service files as well:
4972 <listitem><para>General syntax</para></listitem>
4973 <listitem><para>Comment format</para></listitem>
4976 Service description files must contain a
4977 <literal>D-BUS Service</literal> group with at least the keys
4978 <literal>Name</literal> (the well-known name of the service)
4979 and <literal>Exec</literal> (the command to be executed).
4982 <title>Example service description file</title>
4984 # Sample service description file
4986 Name=com.example.ConfigurationDatabase
4987 Exec=/usr/bin/sample-configd
4993 Additionally, service description files for the well-known system
4994 bus on Unix must contain a <literal>User</literal> key, whose value
4995 is the name of a user account (e.g. <literal>root</literal>).
4996 The system service will be run as that user.
5000 When an application asks to start a service by name, the bus daemon tries to
5001 find a service that will own that name. It then tries to spawn the
5002 executable associated with it. If this fails, it will report an
5007 On the well-known system bus, it is not possible for two .service files
5008 in the same directory to offer the same service, because they are
5009 constrained to have names that match the service name.
5013 On the well-known session bus, if two .service files in the same
5014 directory offer the same service name, the result is undefined.
5015 Distributors should avoid this situation, for instance by naming
5016 session services' .service files according to their service name.
5020 If two .service files in different directories offer the same
5021 service name, the one in the higher-priority directory is used:
5022 for instance, on the system bus, .service files in
5023 /usr/local/share/dbus-1/system-services take precedence over those
5024 in /usr/share/dbus-1/system-services.
5027 The executable launched will have the environment variable
5028 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
5029 message bus so it can connect and request the appropriate names.
5032 The executable being launched may want to know whether the message bus
5033 starting it is one of the well-known message buses (see <xref
5034 linkend="message-bus-types"/>). To facilitate this, the bus must also set
5035 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
5036 of the well-known buses. The currently-defined values for this variable
5037 are <literal>system</literal> for the systemwide message bus,
5038 and <literal>session</literal> for the per-login-session message
5039 bus. The new executable must still connect to the address given
5040 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
5041 resulting connection is to the well-known bus.
5044 [FIXME there should be a timeout somewhere, either specified
5045 in the .service file, by the client, or just a global value
5046 and if the client being activated fails to connect within that
5047 timeout, an error should be sent back.]
5050 <sect3 id="message-bus-starting-services-scope">
5051 <title>Message Bus Service Scope</title>
5053 The "scope" of a service is its "per-", such as per-session,
5054 per-machine, per-home-directory, or per-display. The reference
5055 implementation doesn't yet support starting services in a different
5056 scope from the message bus itself. So e.g. if you start a service
5057 on the session bus its scope is per-session.
5060 We could add an optional scope to a bus name. For example, for
5061 per-(display,session pair), we could have a unique ID for each display
5062 generated automatically at login and set on screen 0 by executing a
5063 special "set display ID" binary. The ID would be stored in a
5064 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
5065 random bytes. This ID would then be used to scope names.
5066 Starting/locating a service could be done by ID-name pair rather than
5070 Contrast this with a per-display scope. To achieve that, we would
5071 want a single bus spanning all sessions using a given display.
5072 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
5073 property on screen 0 of the display, pointing to this bus.
5078 <sect2 id="message-bus-types">
5079 <title>Well-known Message Bus Instances</title>
5081 Two standard message bus instances are defined here, along with how
5082 to locate them and where their service files live.
5084 <sect3 id="message-bus-types-login">
5085 <title>Login session message bus</title>
5087 Each time a user logs in, a <firstterm>login session message
5088 bus</firstterm> may be started. All applications in the user's login
5089 session may interact with one another using this message bus.
5092 The address of the login session message bus is given
5093 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5094 variable. If that variable is not set, applications may
5095 also try to read the address from the X Window System root
5096 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5097 The root window property must have type <literal>STRING</literal>.
5098 The environment variable should have precedence over the
5099 root window property.
5101 <para>The address of the login session message bus is given in the
5102 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5103 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5104 "autolaunch:", the system should use platform-specific methods of
5105 locating a running D-Bus session server, or starting one if a running
5106 instance cannot be found. Note that this mechanism is not recommended
5107 for attempting to determine if a daemon is running. It is inherently
5108 racy to attempt to make this determination, since the bus daemon may
5109 be started just before or just after the determination is made.
5110 Therefore, it is recommended that applications do not try to make this
5111 determination for their functionality purposes, and instead they
5112 should attempt to start the server.</para>
5114 <sect4 id="message-bus-types-login-x-windows">
5115 <title>X Windowing System</title>
5117 For the X Windowing System, the application must locate the
5118 window owner of the selection represented by the atom formed by
5122 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5126 <para>the current user's username</para>
5130 <para>the literal character '_' (underscore)</para>
5134 <para>the machine's ID</para>
5140 The following properties are defined for the window that owns
5142 <informaltable frame="all">
5151 <para>meaning</para>
5157 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5161 <para>the actual address of the server socket</para>
5167 <para>_DBUS_SESSION_BUS_PID</para>
5171 <para>the PID of the server process</para>
5180 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5181 present in this window.
5185 If the X selection cannot be located or if reading the
5186 properties from the window fails, the implementation MUST conclude
5187 that there is no D-Bus server running and proceed to start a new
5188 server. (See below on concurrency issues)
5192 Failure to connect to the D-Bus server address thus obtained
5193 MUST be treated as a fatal connection error and should be reported
5198 As an alternative, an implementation MAY find the information
5199 in the following file located in the current user's home directory,
5200 in subdirectory .dbus/session-bus/:
5203 <para>the machine's ID</para>
5207 <para>the literal character '-' (dash)</para>
5211 <para>the X display without the screen number, with the
5212 following prefixes removed, if present: ":", "localhost:"
5213 ."localhost.localdomain:". That is, a display of
5214 "localhost:10.0" produces just the number "10"</para>
5220 The contents of this file NAME=value assignment pairs and
5221 lines starting with # are comments (no comments are allowed
5222 otherwise). The following variable names are defined:
5229 <para>Variable</para>
5233 <para>meaning</para>
5239 <para>DBUS_SESSION_BUS_ADDRESS</para>
5243 <para>the actual address of the server socket</para>
5249 <para>DBUS_SESSION_BUS_PID</para>
5253 <para>the PID of the server process</para>
5259 <para>DBUS_SESSION_BUS_WINDOWID</para>
5263 <para>the window ID</para>
5272 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5277 Failure to open this file MUST be interpreted as absence of a
5278 running server. Therefore, the implementation MUST proceed to
5279 attempting to launch a new bus server if the file cannot be
5284 However, success in opening this file MUST NOT lead to the
5285 conclusion that the server is running. Thus, a failure to connect to
5286 the bus address obtained by the alternative method MUST NOT be
5287 considered a fatal error. If the connection cannot be established,
5288 the implementation MUST proceed to check the X selection settings or
5289 to start the server on its own.
5293 If the implementation concludes that the D-Bus server is not
5294 running it MUST attempt to start a new server and it MUST also
5295 ensure that the daemon started as an effect of the "autolaunch"
5296 mechanism provides the lookup mechanisms described above, so
5297 subsequent calls can locate the newly started server. The
5298 implementation MUST also ensure that if two or more concurrent
5299 initiations happen, only one server remains running and all other
5300 initiations are able to obtain the address of this server and
5301 connect to it. In other words, the implementation MUST ensure that
5302 the X selection is not present when it attempts to set it, without
5303 allowing another process to set the selection between the
5304 verification and the setting (e.g., by using XGrabServer /
5311 On Unix systems, the session bus should search for .service files
5312 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5314 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5315 Implementations may also search additional locations, which
5316 should be searched with lower priority than anything in
5317 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5318 for example, the reference implementation also
5319 looks in <literal>${datadir}/dbus-1/services</literal> as
5320 set at compile time.
5323 As described in the XDG Base Directory Specification, software
5324 packages should install their session .service files to their
5325 configured <literal>${datadir}/dbus-1/services</literal>,
5326 where <literal>${datadir}</literal> is as defined by the GNU
5327 coding standards. System administrators or users can arrange
5328 for these service files to be read by setting XDG_DATA_DIRS or by
5329 symlinking them into the default locations.
5333 <sect3 id="message-bus-types-system">
5334 <title>System message bus</title>
5336 A computer may have a <firstterm>system message bus</firstterm>,
5337 accessible to all applications on the system. This message bus may be
5338 used to broadcast system events, such as adding new hardware devices,
5339 changes in the printer queue, and so forth.
5342 The address of the system message bus is given
5343 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5344 variable. If that variable is not set, applications should try
5345 to connect to the well-known address
5346 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5349 The D-Bus reference implementation actually honors the
5350 <literal>$(localstatedir)</literal> configure option
5351 for this address, on both client and server side.
5356 On Unix systems, the system bus should default to searching
5357 for .service files in
5358 <literal>/usr/local/share/dbus-1/system-services</literal>,
5359 <literal>/usr/share/dbus-1/system-services</literal> and
5360 <literal>/lib/dbus-1/system-services</literal>, with that order
5361 of precedence. It may also search other implementation-specific
5362 locations, but should not vary these locations based on environment
5366 The system bus is security-sensitive and is typically executed
5367 by an init system with a clean environment. Its launch helper
5368 process is particularly security-sensitive, and specifically
5369 clears its own environment.
5374 Software packages should install their system .service
5375 files to their configured
5376 <literal>${datadir}/dbus-1/system-services</literal>,
5377 where <literal>${datadir}</literal> is as defined by the GNU
5378 coding standards. System administrators can arrange
5379 for these service files to be read by editing the system bus'
5380 configuration file or by symlinking them into the default
5386 <sect2 id="message-bus-messages">
5387 <title>Message Bus Messages</title>
5389 The special message bus name <literal>org.freedesktop.DBus</literal>
5390 responds to a number of additional messages.
5393 <sect3 id="bus-messages-hello">
5394 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5405 <entry>Argument</entry>
5407 <entry>Description</entry>
5413 <entry>STRING</entry>
5414 <entry>Unique name assigned to the connection</entry>
5421 Before an application is able to send messages to other applications
5422 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5423 to the message bus to obtain a unique name. If an application without
5424 a unique name tries to send a message to another application, or a
5425 message to the message bus itself that isn't the
5426 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5427 disconnected from the bus.
5430 There is no corresponding "disconnect" request; if a client wishes to
5431 disconnect from the bus, it simply closes the socket (or other
5432 communication channel).
5435 <sect3 id="bus-messages-list-names">
5436 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5440 ARRAY of STRING ListNames ()
5447 <entry>Argument</entry>
5449 <entry>Description</entry>
5455 <entry>ARRAY of STRING</entry>
5456 <entry>Array of strings where each string is a bus name</entry>
5463 Returns a list of all currently-owned names on the bus.
5466 <sect3 id="bus-messages-list-activatable-names">
5467 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5471 ARRAY of STRING ListActivatableNames ()
5478 <entry>Argument</entry>
5480 <entry>Description</entry>
5486 <entry>ARRAY of STRING</entry>
5487 <entry>Array of strings where each string is a bus name</entry>
5494 Returns a list of all names that can be activated on the bus.
5497 <sect3 id="bus-messages-name-exists">
5498 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5502 BOOLEAN NameHasOwner (in STRING name)
5509 <entry>Argument</entry>
5511 <entry>Description</entry>
5517 <entry>STRING</entry>
5518 <entry>Name to check</entry>
5528 <entry>Argument</entry>
5530 <entry>Description</entry>
5536 <entry>BOOLEAN</entry>
5537 <entry>Return value, true if the name exists</entry>
5544 Checks if the specified name exists (currently has an owner).
5548 <sect3 id="bus-messages-name-owner-changed">
5549 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5553 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5560 <entry>Argument</entry>
5562 <entry>Description</entry>
5568 <entry>STRING</entry>
5569 <entry>Name with a new owner</entry>
5573 <entry>STRING</entry>
5574 <entry>Old owner or empty string if none</entry>
5578 <entry>STRING</entry>
5579 <entry>New owner or empty string if none</entry>
5586 This signal indicates that the owner of a name has changed.
5587 It's also the signal to use to detect the appearance of
5588 new names on the bus.
5591 <sect3 id="bus-messages-name-lost">
5592 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5596 NameLost (STRING name)
5603 <entry>Argument</entry>
5605 <entry>Description</entry>
5611 <entry>STRING</entry>
5612 <entry>Name which was lost</entry>
5619 This signal is sent to a specific application when it loses
5620 ownership of a name.
5624 <sect3 id="bus-messages-name-acquired">
5625 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5629 NameAcquired (STRING name)
5636 <entry>Argument</entry>
5638 <entry>Description</entry>
5644 <entry>STRING</entry>
5645 <entry>Name which was acquired</entry>
5652 This signal is sent to a specific application when it gains
5653 ownership of a name.
5657 <sect3 id="bus-messages-start-service-by-name">
5658 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5662 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5669 <entry>Argument</entry>
5671 <entry>Description</entry>
5677 <entry>STRING</entry>
5678 <entry>Name of the service to start</entry>
5682 <entry>UINT32</entry>
5683 <entry>Flags (currently not used)</entry>
5693 <entry>Argument</entry>
5695 <entry>Description</entry>
5701 <entry>UINT32</entry>
5702 <entry>Return value</entry>
5707 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5711 The return value can be one of the following values:
5716 <entry>Identifier</entry>
5717 <entry>Value</entry>
5718 <entry>Description</entry>
5723 <entry>DBUS_START_REPLY_SUCCESS</entry>
5725 <entry>The service was successfully started.</entry>
5728 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5730 <entry>A connection already owns the given name.</entry>
5739 <sect3 id="bus-messages-update-activation-environment">
5740 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5744 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5751 <entry>Argument</entry>
5753 <entry>Description</entry>
5759 <entry>ARRAY of DICT<STRING,STRING></entry>
5760 <entry>Environment to add or update</entry>
5765 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5768 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5771 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.
5776 <sect3 id="bus-messages-get-name-owner">
5777 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5781 STRING GetNameOwner (in STRING name)
5788 <entry>Argument</entry>
5790 <entry>Description</entry>
5796 <entry>STRING</entry>
5797 <entry>Name to get the owner of</entry>
5807 <entry>Argument</entry>
5809 <entry>Description</entry>
5815 <entry>STRING</entry>
5816 <entry>Return value, a unique connection name</entry>
5821 Returns the unique connection name of the primary owner of the name
5822 given. If the requested name doesn't have an owner, returns a
5823 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5827 <sect3 id="bus-messages-get-connection-unix-user">
5828 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5832 UINT32 GetConnectionUnixUser (in STRING bus_name)
5839 <entry>Argument</entry>
5841 <entry>Description</entry>
5847 <entry>STRING</entry>
5848 <entry>Unique or well-known bus name of the connection to
5849 query, such as <literal>:12.34</literal> or
5850 <literal>com.example.tea</literal></entry>
5860 <entry>Argument</entry>
5862 <entry>Description</entry>
5868 <entry>UINT32</entry>
5869 <entry>Unix user ID</entry>
5874 Returns the Unix user ID of the process connected to the server. If
5875 unable to determine it (for instance, because the process is not on the
5876 same machine as the bus daemon), an error is returned.
5880 <sect3 id="bus-messages-get-connection-unix-process-id">
5881 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5885 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5892 <entry>Argument</entry>
5894 <entry>Description</entry>
5900 <entry>STRING</entry>
5901 <entry>Unique or well-known bus name of the connection to
5902 query, such as <literal>:12.34</literal> or
5903 <literal>com.example.tea</literal></entry>
5913 <entry>Argument</entry>
5915 <entry>Description</entry>
5921 <entry>UINT32</entry>
5922 <entry>Unix process id</entry>
5927 Returns the Unix process ID of the process connected to the server. If
5928 unable to determine it (for instance, because the process is not on the
5929 same machine as the bus daemon), an error is returned.
5933 <sect3 id="bus-messages-get-connection-credentials">
5934 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5938 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5945 <entry>Argument</entry>
5947 <entry>Description</entry>
5953 <entry>STRING</entry>
5954 <entry>Unique or well-known bus name of the connection to
5955 query, such as <literal>:12.34</literal> or
5956 <literal>com.example.tea</literal></entry>
5966 <entry>Argument</entry>
5968 <entry>Description</entry>
5974 <entry>DICT<STRING,VARIANT></entry>
5975 <entry>Credentials</entry>
5983 Returns as many credentials as possible for the process connected to
5984 the server. If unable to determine certain credentials (for instance,
5985 because the process is not on the same machine as the bus daemon,
5986 or because this version of the bus daemon does not support a
5987 particular security framework), or if the values of those credentials
5988 cannot be represented as documented here, then those credentials
5993 Keys in the returned dictionary not containing "." are defined
5994 by this specification. Bus daemon implementors supporting
5995 credentials frameworks not mentioned in this document should either
5996 contribute patches to this specification, or use keys containing
5997 "." and starting with a reversed domain name.
6003 <entry>Value type</entry>
6004 <entry>Value</entry>
6009 <entry>UnixUserID</entry>
6010 <entry>UINT32</entry>
6011 <entry>The numeric Unix user ID, as defined by POSIX</entry>
6014 <entry>ProcessID</entry>
6015 <entry>UINT32</entry>
6016 <entry>The numeric process ID, on platforms that have
6017 this concept. On Unix, this is the process ID defined by
6021 <entry>WindowsSID</entry>
6022 <entry>STRING</entry>
6023 <entry>The Windows security identifier in its string form,
6024 e.g. "S-1-5-21-3623811015-3361044348-30300820-1013" for
6025 a domain or local computer user or "S-1-5-18" for the
6026 LOCAL_SYSTEM user</entry>
6030 <entry>LinuxSecurityLabel</entry>
6031 <entry>ARRAY of BYTE</entry>
6033 <para>On Linux systems, the security label that would result
6034 from the SO_PEERSEC getsockopt call. The array contains
6035 the non-zero bytes of the security label in an unspecified
6036 ASCII-compatible encoding<footnote>
6037 <para>It could be ASCII or UTF-8, but could also be
6038 ISO Latin-1 or any other encoding.</para>
6039 </footnote>, followed by a single zero byte.</para>
6041 For example, the SELinux context
6042 <literal>system_u:system_r:init_t:s0</literal>
6043 (a string of length 27) would be encoded as 28 bytes
6044 ending with ':', 's', '0', '\x00'.<footnote>
6045 <para>Note that this is not the same as the older
6046 GetConnectionSELinuxContext method, which does
6047 not append the zero byte. Always appending the
6048 zero byte allows callers to read the string
6049 from the message payload without copying.</para>
6053 On SELinux systems this is the SELinux context, as output
6054 by <literal>ps -Z</literal> or <literal>ls -Z</literal>.
6055 Typical values might include
6056 <literal>system_u:system_r:init_t:s0</literal>,
6057 <literal>unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023</literal>,
6059 <literal>unconfined_u:unconfined_r:chrome_sandbox_t:s0-s0:c0.c1023</literal>.
6062 On Smack systems, this is the Smack label.
6063 Typical values might include
6064 <literal>_</literal>, <literal>*</literal>,
6065 <literal>User</literal>, <literal>System</literal>
6066 or <literal>System::Shared</literal>.
6069 On AppArmor systems, this is the AppArmor context,
6070 a composite string encoding the AppArmor label (one or more
6071 profiles) and the enforcement mode.
6072 Typical values might include <literal>unconfined</literal>,
6073 <literal>/usr/bin/firefox (enforce)</literal> or
6074 <literal>user1 (complain)</literal>.
6085 This method was added in D-Bus 1.7 to reduce the round-trips
6086 required to list a process's credentials. In older versions, calling
6087 this method will fail: applications should recover by using the
6088 separate methods such as
6089 <xref linkend="bus-messages-get-connection-unix-user"/>
6094 <sect3 id="bus-messages-get-adt-audit-session-data">
6095 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
6099 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
6106 <entry>Argument</entry>
6108 <entry>Description</entry>
6114 <entry>STRING</entry>
6115 <entry>Unique or well-known bus name of the connection to
6116 query, such as <literal>:12.34</literal> or
6117 <literal>com.example.tea</literal></entry>
6127 <entry>Argument</entry>
6129 <entry>Description</entry>
6135 <entry>ARRAY of BYTE</entry>
6136 <entry>auditing data as returned by
6137 adt_export_session_data()</entry>
6142 Returns auditing data used by Solaris ADT, in an unspecified
6143 binary format. If you know what this means, please contribute
6144 documentation via the D-Bus bug tracking system.
6145 This method is on the core DBus interface for historical reasons;
6146 the same information should be made available via
6147 <xref linkend="bus-messages-get-connection-credentials"/>
6152 <sect3 id="bus-messages-get-connection-selinux-security-context">
6153 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6157 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6164 <entry>Argument</entry>
6166 <entry>Description</entry>
6172 <entry>STRING</entry>
6173 <entry>Unique or well-known bus name of the connection to
6174 query, such as <literal>:12.34</literal> or
6175 <literal>com.example.tea</literal></entry>
6185 <entry>Argument</entry>
6187 <entry>Description</entry>
6193 <entry>ARRAY of BYTE</entry>
6194 <entry>some sort of string of bytes, not necessarily UTF-8,
6195 not including '\0'</entry>
6200 Returns the security context used by SELinux, in an unspecified
6201 format. If you know what this means, please contribute
6202 documentation via the D-Bus bug tracking system.
6203 This method is on the core DBus interface for historical reasons;
6204 the same information should be made available via
6205 <xref linkend="bus-messages-get-connection-credentials"/>
6211 <sect3 id="bus-messages-add-match">
6212 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6216 AddMatch (in STRING rule)
6223 <entry>Argument</entry>
6225 <entry>Description</entry>
6231 <entry>STRING</entry>
6232 <entry>Match rule to add to the connection</entry>
6237 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6238 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6242 <sect3 id="bus-messages-remove-match">
6243 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6247 RemoveMatch (in STRING rule)
6254 <entry>Argument</entry>
6256 <entry>Description</entry>
6262 <entry>STRING</entry>
6263 <entry>Match rule to remove from the connection</entry>
6268 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6269 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6274 <sect3 id="bus-messages-get-id">
6275 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6279 GetId (out STRING id)
6286 <entry>Argument</entry>
6288 <entry>Description</entry>
6294 <entry>STRING</entry>
6295 <entry>Unique ID identifying the bus daemon</entry>
6300 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6301 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6302 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6303 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6304 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6305 by org.freedesktop.DBus.Peer.GetMachineId().
6306 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6310 <sect3 id="bus-messages-become-monitor">
6311 <title><literal>org.freedesktop.DBus.Monitoring.BecomeMonitor</literal></title>
6315 BecomeMonitor (in ARRAY of STRING rule, in UINT32 flags)
6322 <entry>Argument</entry>
6324 <entry>Description</entry>
6330 <entry>ARRAY of STRING</entry>
6331 <entry>Match rules to add to the connection</entry>
6335 <entry>UINT32</entry>
6336 <entry>Not used, must be 0</entry>
6344 Converts the connection into a <emphasis>monitor
6345 connection</emphasis> which can be used as a debugging/monitoring
6346 tool. Only a user who is privileged on this
6347 bus (by some implementation-specific definition) may create
6348 monitor connections<footnote>
6350 In the reference implementation,
6351 the default configuration is that each user (identified by
6352 numeric user ID) may monitor their own session bus,
6353 and the root user (user ID zero) may monitor the
6360 Monitor connections lose all their bus names, including the unique
6361 connection name, and all their match rules. Sending messages on a
6362 monitor connection is not allowed: applications should use a private
6363 connection for monitoring.
6367 Monitor connections may receive all messages, even messages that
6368 should only have gone to some other connection ("eavesdropping").
6369 The first argument is a list of match rules, which replace any
6370 match rules that were previously active for this connection.
6371 These match rules are always treated as if they contained the
6372 special <literal>eavesdrop='true'</literal> member.
6376 As a special case, an empty list of match rules (which would
6377 otherwise match nothing, making the monitor useless) is treated
6378 as a shorthand for matching all messages.
6382 The second argument might be used for flags to influence the
6383 behaviour of the monitor connection in future D-Bus versions.
6387 Message bus implementations should attempt to minimize the
6388 side-effects of monitoring — in particular, unlike ordinary
6389 eavesdropping, monitoring the system bus does not require the
6390 access control rules to be relaxed, which would change the set
6391 of messages that can be delivered to their (non-monitor)
6392 destinations. However, it is unavoidable that monitoring
6393 will increase the message bus's resource consumption. In
6394 edge cases where there was barely enough time or memory without
6395 monitoring, this might result in message deliveries failing
6396 when they would otherwise have succeeded.
6404 <appendix id="implementation-notes">
6405 <title>Implementation notes</title>
6406 <sect1 id="implementation-notes-subsection">
6414 <glossary><title>Glossary</title>
6416 This glossary defines some of the terms used in this specification.
6419 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6422 The message bus maintains an association between names and
6423 connections. (Normally, there's one connection per application.) A
6424 bus name is simply an identifier used to locate connections. For
6425 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6426 name might be used to send a message to a screensaver from Yoyodyne
6427 Corporation. An application is said to <firstterm>own</firstterm> a
6428 name if the message bus has associated the application's connection
6429 with the name. Names may also have <firstterm>queued
6430 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6431 The bus assigns a unique name to each connection,
6432 see <xref linkend="term-unique-name"/>. Other names
6433 can be thought of as "well-known names" and are
6434 used to find applications that offer specific functionality.
6438 See <xref linkend="message-protocol-names-bus"/> for details of
6439 the syntax and naming conventions for bus names.
6444 <glossentry id="term-message"><glossterm>Message</glossterm>
6447 A message is the atomic unit of communication via the D-Bus
6448 protocol. It consists of a <firstterm>header</firstterm> and a
6449 <firstterm>body</firstterm>; the body is made up of
6450 <firstterm>arguments</firstterm>.
6455 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6458 The message bus is a special application that forwards
6459 or routes messages between a group of applications
6460 connected to the message bus. It also manages
6461 <firstterm>names</firstterm> used for routing
6467 <glossentry id="term-name"><glossterm>Name</glossterm>
6470 See <xref linkend="term-bus-name"/>. "Name" may
6471 also be used to refer to some of the other names
6472 in D-Bus, such as interface names.
6477 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6480 Used to prevent collisions when defining new interfaces, bus names
6481 etc. The convention used is the same one Java uses for defining
6482 classes: a reversed domain name.
6483 See <xref linkend="message-protocol-names-bus"/>,
6484 <xref linkend="message-protocol-names-interface"/>,
6485 <xref linkend="message-protocol-names-error"/>,
6486 <xref linkend="message-protocol-marshaling-object-path"/>.
6491 <glossentry id="term-object"><glossterm>Object</glossterm>
6494 Each application contains <firstterm>objects</firstterm>, which have
6495 <firstterm>interfaces</firstterm> and
6496 <firstterm>methods</firstterm>. Objects are referred to by a name,
6497 called a <firstterm>path</firstterm>.
6502 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6505 An application talking directly to another application, without going
6506 through a message bus. One-to-one connections may be "peer to peer" or
6507 "client to server." The D-Bus protocol has no concept of client
6508 vs. server after a connection has authenticated; the flow of messages
6509 is symmetrical (full duplex).
6514 <glossentry id="term-path"><glossterm>Path</glossterm>
6517 Object references (object names) in D-Bus are organized into a
6518 filesystem-style hierarchy, so each object is named by a path. As in
6519 LDAP, there's no difference between "files" and "directories"; a path
6520 can refer to an object, while still having child objects below it.
6525 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6528 Each bus name has a primary owner; messages sent to the name go to the
6529 primary owner. However, certain names also maintain a queue of
6530 secondary owners "waiting in the wings." If the primary owner releases
6531 the name, then the first secondary owner in the queue automatically
6532 becomes the new owner of the name.
6537 <glossentry id="term-service"><glossterm>Service</glossterm>
6540 A service is an executable that can be launched by the bus daemon.
6541 Services normally guarantee some particular features, for example they
6542 may guarantee that they will request a specific name such as
6543 "com.example.Screensaver", have a singleton object
6544 "/com/example/Application", and that object will implement the
6545 interface "com.example.Screensaver.Control".
6550 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6553 ".service files" tell the bus about service applications that can be
6554 launched (see <xref linkend="term-service"/>). Most importantly they
6555 provide a mapping from bus names to services that will request those
6556 names when they start up.
6561 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6564 The special name automatically assigned to each connection by the
6565 message bus. This name will never change owner, and will be unique
6566 (never reused during the lifetime of the message bus).
6567 It will begin with a ':' character.