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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.29</releaseinfo>
10 <date>2016-08-15</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>0.29</revnumber>
75 <date>2016-10-10</date>
76 <authorinitials>PW</authorinitials>
78 Introspection arguments may contain annotations; recommend against
79 using the object path '/'
83 <revnumber>0.28</revnumber>
84 <date>2016-08-15</date>
85 <authorinitials>PW</authorinitials>
86 <revremark>Clarify serialization</revremark>
89 <revnumber>0.27</revnumber>
90 <date>2015-12-02</date>
91 <authorinitials>LU</authorinitials>
92 <revremark>Services should not send unwanted replies</revremark>
95 <revnumber>0.26</revnumber>
96 <date>2015-02-19</date>
97 <authorinitials>smcv, rh</authorinitials>
99 GetConnectionCredentials can return LinuxSecurityLabel or
100 WindowsSID; add privileged BecomeMonitor method
104 <revnumber>0.25</revnumber>
105 <date>2014-11-10</date>
106 <authorinitials>smcv, lennart</authorinitials>
108 ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
112 <revnumber>0.24</revnumber>
113 <date>2014-10-01</date>
114 <authorinitials>SMcV</authorinitials>
116 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
117 document how to quote match rules
121 <revnumber>0.23</revnumber>
122 <date>2014-01-06</date>
123 <authorinitials>SMcV, CY</authorinitials>
125 method call messages with no INTERFACE may be considered an error;
126 document tcp:bind=... and nonce-tcp:bind=...; define listenable
127 and connectable addresses
131 <revnumber>0.22</revnumber>
132 <date>2013-10-09</date>
133 <authorinitials></authorinitials>
134 <revremark>add GetConnectionCredentials, document
135 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
136 document and correct .service file syntax and naming
140 <revnumber>0.21</revnumber>
141 <date>2013-04-25</date>
142 <authorinitials>smcv</authorinitials>
143 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
144 Corrigendum #9)</revremark>
147 <revnumber>0.20</revnumber>
148 <date>22 February 2013</date>
149 <authorinitials>smcv, walters</authorinitials>
150 <revremark>reorganise for clarity, remove false claims about
151 basic types, mention /o/fd/DBus</revremark>
154 <revnumber>0.19</revnumber>
155 <date>20 February 2012</date>
156 <authorinitials>smcv/lp</authorinitials>
157 <revremark>formally define unique connection names and well-known
158 bus names; document best practices for interface, bus, member and
159 error names, and object paths; document the search path for session
160 and system services on Unix; document the systemd transport</revremark>
163 <revnumber>0.18</revnumber>
164 <date>29 July 2011</date>
165 <authorinitials>smcv</authorinitials>
166 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
167 match keyword; promote type system to a top-level section</revremark>
170 <revnumber>0.17</revnumber>
171 <date>1 June 2011</date>
172 <authorinitials>smcv/davidz</authorinitials>
173 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
174 by GVariant</revremark>
177 <revnumber>0.16</revnumber>
178 <date>11 April 2011</date>
179 <authorinitials></authorinitials>
180 <revremark>add path_namespace, arg0namespace; argNpath matches object
184 <revnumber>0.15</revnumber>
185 <date>3 November 2010</date>
186 <authorinitials></authorinitials>
187 <revremark></revremark>
190 <revnumber>0.14</revnumber>
191 <date>12 May 2010</date>
192 <authorinitials></authorinitials>
193 <revremark></revremark>
196 <revnumber>0.13</revnumber>
197 <date>23 Dezember 2009</date>
198 <authorinitials></authorinitials>
199 <revremark></revremark>
202 <revnumber>0.12</revnumber>
203 <date>7 November, 2006</date>
204 <authorinitials></authorinitials>
205 <revremark></revremark>
208 <revnumber>0.11</revnumber>
209 <date>6 February 2005</date>
210 <authorinitials></authorinitials>
211 <revremark></revremark>
214 <revnumber>0.10</revnumber>
215 <date>28 January 2005</date>
216 <authorinitials></authorinitials>
217 <revremark></revremark>
220 <revnumber>0.9</revnumber>
221 <date>7 Januar 2005</date>
222 <authorinitials></authorinitials>
223 <revremark></revremark>
226 <revnumber>0.8</revnumber>
227 <date>06 September 2003</date>
228 <authorinitials></authorinitials>
229 <revremark>First released document.</revremark>
234 <sect1 id="introduction">
235 <title>Introduction</title>
237 D-Bus is a system for low-overhead, easy to use
238 interprocess communication (IPC). In more detail:
242 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
243 binary protocol, and does not have to convert to and from a text
244 format such as XML. Because D-Bus is intended for potentially
245 high-resolution same-machine IPC, not primarily for Internet IPC,
246 this is an interesting optimization. D-Bus is also designed to
247 avoid round trips and allow asynchronous operation, much like
253 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
254 of <firstterm>messages</firstterm> rather than byte streams, and
255 automatically handles a lot of the hard IPC issues. Also, the D-Bus
256 library is designed to be wrapped in a way that lets developers use
257 their framework's existing object/type system, rather than learning
258 a new one specifically for IPC.
265 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
266 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
267 a system for one application to talk to a single other
268 application. However, the primary intended application of the protocol is the
269 D-Bus <firstterm>message bus</firstterm>, specified in <xref
270 linkend="message-bus"/>. The message bus is a special application that
271 accepts connections from multiple other applications, and forwards
276 Uses of D-Bus include notification of system changes (notification of when
277 a camera is plugged in to a computer, or a new version of some software
278 has been installed), or desktop interoperability, for example a file
279 monitoring service or a configuration service.
283 D-Bus is designed for two specific use cases:
287 A "system bus" for notifications from the system to user sessions,
288 and to allow the system to request input from user sessions.
293 A "session bus" used to implement desktop environments such as
298 D-Bus is not intended to be a generic IPC system for any possible
299 application, and intentionally omits many features found in other
300 IPC systems for this reason.
304 At the same time, the bus daemons offer a number of features not found in
305 other IPC systems, such as single-owner "bus names" (similar to X
306 selections), on-demand startup of services, and security policies.
307 In many ways, these features are the primary motivation for developing
308 D-Bus; other systems would have sufficed if IPC were the only goal.
312 D-Bus may turn out to be useful in unanticipated applications, but future
313 versions of this spec and the reference implementation probably will not
314 incorporate features that interfere with the core use cases.
318 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
319 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
320 document are to be interpreted as described in RFC 2119. However, the
321 document could use a serious audit to be sure it makes sense to do
322 so. Also, they are not capitalized.
325 <sect2 id="stability">
326 <title>Protocol and Specification Stability</title>
328 The D-Bus protocol is frozen (only compatible extensions are allowed) as
329 of November 8, 2006. However, this specification could still use a fair
330 bit of work to make interoperable reimplementation possible without
331 reference to the D-Bus reference implementation. Thus, this
332 specification is not marked 1.0. To mark it 1.0, we'd like to see
333 someone invest significant effort in clarifying the specification
334 language, and growing the specification to cover more aspects of the
335 reference implementation's behavior.
338 Until this work is complete, any attempt to reimplement D-Bus will
339 probably require looking at the reference implementation and/or asking
340 questions on the D-Bus mailing list about intended behavior.
341 Questions on the list are very welcome.
344 Nonetheless, this document should be a useful starting point and is
345 to our knowledge accurate, though incomplete.
351 <sect1 id="type-system">
352 <title>Type System</title>
355 D-Bus has a type system, in which values of various types can be
356 serialized into a sequence of bytes referred to as the
357 <firstterm>wire format</firstterm> in a standard way.
358 Converting a value from some other representation into the wire
359 format is called <firstterm>marshaling</firstterm> and converting
360 it back from the wire format is <firstterm>unmarshaling</firstterm>.
364 The D-Bus protocol does not include type tags in the marshaled data; a
365 block of marshaled values must have a known <firstterm>type
366 signature</firstterm>. The type signature is made up of zero or more
367 <firstterm id="term-single-complete-type">single complete
368 types</firstterm>, each made up of one or more
369 <firstterm>type codes</firstterm>.
373 A type code is an ASCII character representing the
374 type of a value. Because ASCII characters are used, the type signature
375 will always form a valid ASCII string. A simple string compare
376 determines whether two type signatures are equivalent.
380 A single complete type is a sequence of type codes that fully describes
381 one type: either a basic type, or a single fully-described container type.
382 A single complete type is a basic type code, a variant type code,
383 an array with its element type, or a struct with its fields (all of which
384 are defined below). So the following signatures are not single complete
395 And the following signatures contain multiple complete types:
405 Note however that a single complete type may <emphasis>contain</emphasis>
406 multiple other single complete types, by containing a struct or dict
410 <sect2 id="basic-types">
411 <title>Basic types</title>
414 The simplest type codes are the <firstterm id="term-basic-type">basic
415 types</firstterm>, which are the types whose structure is entirely
416 defined by their 1-character type code. Basic types consist of
417 fixed types and string-like types.
421 The <firstterm id="term-fixed-type">fixed types</firstterm>
422 are basic types whose values have a fixed length, namely BYTE,
423 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
428 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
429 the ASCII character 'i'. So the signature for a block of values
430 containing a single <literal>INT32</literal> would be:
434 A block of values containing two <literal>INT32</literal> would have this signature:
441 The characteristics of the fixed types are listed in this table.
447 <entry>Conventional name</entry>
448 <entry>ASCII type-code</entry>
449 <entry>Encoding</entry>
454 <entry><literal>BYTE</literal></entry>
455 <entry><literal>y</literal> (121)</entry>
456 <entry>Unsigned 8-bit integer</entry>
459 <entry><literal>BOOLEAN</literal></entry>
460 <entry><literal>b</literal> (98)</entry>
461 <entry>Boolean value: 0 is false, 1 is true, any other value
462 allowed by the marshalling format is invalid</entry>
465 <entry><literal>INT16</literal></entry>
466 <entry><literal>n</literal> (110)</entry>
467 <entry>Signed (two's complement) 16-bit integer</entry>
470 <entry><literal>UINT16</literal></entry>
471 <entry><literal>q</literal> (113)</entry>
472 <entry>Unsigned 16-bit integer</entry>
475 <entry><literal>INT32</literal></entry>
476 <entry><literal>i</literal> (105)</entry>
477 <entry>Signed (two's complement) 32-bit integer</entry>
480 <entry><literal>UINT32</literal></entry>
481 <entry><literal>u</literal> (117)</entry>
482 <entry>Unsigned 32-bit integer</entry>
485 <entry><literal>INT64</literal></entry>
486 <entry><literal>x</literal> (120)</entry>
487 <entry>Signed (two's complement) 64-bit integer
488 (mnemonic: x and t are the first characters in "sixty" not
489 already used for something more common)</entry>
492 <entry><literal>UINT64</literal></entry>
493 <entry><literal>t</literal> (116)</entry>
494 <entry>Unsigned 64-bit integer</entry>
497 <entry><literal>DOUBLE</literal></entry>
498 <entry><literal>d</literal> (100)</entry>
499 <entry>IEEE 754 double-precision floating point</entry>
502 <entry><literal>UNIX_FD</literal></entry>
503 <entry><literal>h</literal> (104)</entry>
504 <entry>Unsigned 32-bit integer representing an index into an
505 out-of-band array of file descriptors, transferred via some
506 platform-specific mechanism (mnemonic: h for handle)</entry>
514 The <firstterm id="term-string-like-type">string-like types</firstterm>
515 are basic types with a variable length. The value of any string-like
516 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
517 none of which may be U+0000. The UTF-8 text must be validated
518 strictly: in particular, it must not contain overlong sequences
519 or codepoints above U+10FFFF.
523 Since D-Bus Specification version 0.21, in accordance with Unicode
524 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
525 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
526 D-Bus rejected these noncharacters).
530 The marshalling formats for the string-like types all end with a
531 single zero (NUL) byte, but that byte is not considered to be part of
536 The characteristics of the string-like types are listed in this table.
542 <entry>Conventional name</entry>
543 <entry>ASCII type-code</entry>
544 <entry>Validity constraints</entry>
549 <entry><literal>STRING</literal></entry>
550 <entry><literal>s</literal> (115)</entry>
551 <entry>No extra constraints</entry>
554 <entry><literal>OBJECT_PATH</literal></entry>
555 <entry><literal>o</literal> (111)</entry>
557 <link linkend="message-protocol-marshaling-object-path">a
558 syntactically valid object path</link></entry>
561 <entry><literal>SIGNATURE</literal></entry>
562 <entry><literal>g</literal> (103)</entry>
564 <firstterm linkend="term-single-complete-type">single
565 complete types</firstterm></entry>
572 <sect3 id="message-protocol-marshaling-object-path">
573 <title>Valid Object Paths</title>
576 An object path is a name used to refer to an object instance.
577 Conceptually, each participant in a D-Bus message exchange may have
578 any number of object instances (think of C++ or Java objects) and each
579 such instance will have a path. Like a filesystem, the object
580 instances in an application form a hierarchical tree.
584 Object paths are often namespaced by starting with a reversed
585 domain name and containing an interface version number, in the
587 <link linkend="message-protocol-names-interface">interface
589 <link linkend="message-protocol-names-bus">well-known
591 This makes it possible to implement more than one service, or
592 more than one version of a service, in the same process,
593 even if the services share a connection but cannot otherwise
594 co-operate (for instance, if they are implemented by different
599 Using an object path of <literal>/</literal> is allowed, but
600 recommended against, as it makes versioning of interfaces hard. Any
601 signals emitted from a D-Bus object have the service’s unique bus name
602 associated with them, rather than its well-known name. This means that
603 receipients of the signals must rely entirely on the signal name and
604 object path to work out which interface the signal originated from.
608 For instance, if the owner of <literal>example.com</literal> is
609 developing a D-Bus API for a music player, they might use the
610 hierarchy of object paths that start with
611 <literal>/com/example/MusicPlayer1</literal> for its objects.
615 The following rules define a valid object path. Implementations must
616 not send or accept messages with invalid object paths.
620 The path may be of any length.
625 The path must begin with an ASCII '/' (integer 47) character,
626 and must consist of elements separated by slash characters.
631 Each element must only contain the ASCII characters
637 No element may be the empty string.
642 Multiple '/' characters cannot occur in sequence.
647 A trailing '/' character is not allowed unless the
648 path is the root path (a single '/' character).
656 <sect3 id="message-protocol-marshaling-signature">
657 <title>Valid Signatures</title>
659 An implementation must not send or accept invalid signatures.
660 Valid signatures will conform to the following rules:
664 The signature is a list of single complete types.
665 Arrays must have element types, and structs must
666 have both open and close parentheses.
671 Only type codes, open and close parentheses, and open and
672 close curly brackets are allowed in the signature. The
673 <literal>STRUCT</literal> type code
674 is not allowed in signatures, because parentheses
675 are used instead. Similarly, the
676 <literal>DICT_ENTRY</literal> type code is not allowed in
677 signatures, because curly brackets are used instead.
682 The maximum depth of container type nesting is 32 array type
683 codes and 32 open parentheses. This implies that the maximum
684 total depth of recursion is 64, for an "array of array of array
685 of ... struct of struct of struct of ..." where there are 32
691 The maximum length of a signature is 255.
698 When signatures appear in messages, the marshalling format
699 guarantees that they will be followed by a nul byte (which can
700 be interpreted as either C-style string termination or the INVALID
701 type-code), but this is not conceptually part of the signature.
707 <sect2 id="container-types">
708 <title>Container types</title>
711 In addition to basic types, there are four <firstterm>container</firstterm>
712 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
713 and <literal>DICT_ENTRY</literal>.
717 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
718 code does not appear in signatures. Instead, ASCII characters
719 '(' and ')' are used to mark the beginning and end of the struct.
720 So for example, a struct containing two integers would have this
725 Structs can be nested, so for example a struct containing
726 an integer and another struct:
730 The value block storing that struct would contain three integers; the
731 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
736 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
737 but is useful in code that implements the protocol. This type code
738 is specified to allow such code to interoperate in non-protocol contexts.
742 Empty structures are not allowed; there must be at least one
743 type code between the parentheses.
747 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
748 followed by a <firstterm>single complete type</firstterm>. The single
749 complete type following the array is the type of each array element. So
750 the simple example is:
754 which is an array of 32-bit integers. But an array can be of any type,
755 such as this array-of-struct-with-two-int32-fields:
759 Or this array of array of integer:
766 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
767 type <literal>VARIANT</literal> will have the signature of a single complete type as part
768 of the <emphasis>value</emphasis>. This signature will be followed by a
769 marshaled value of that type.
773 Unlike a message signature, the variant signature can
774 contain only a single complete type. So "i", "ai"
775 or "(ii)" is OK, but "ii" is not. Use of variants may not
776 cause a total message depth to be larger than 64, including
777 other container types such as structures.
781 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
782 than parentheses it uses curly braces, and it has more restrictions.
783 The restrictions are: it occurs only as an array element type; it has
784 exactly two single complete types inside the curly braces; the first
785 single complete type (the "key") must be a basic type rather than a
786 container type. Implementations must not accept dict entries outside of
787 arrays, must not accept dict entries with zero, one, or more than two
788 fields, and must not accept dict entries with non-basic-typed keys. A
789 dict entry is always a key-value pair.
793 The first field in the <literal>DICT_ENTRY</literal> is always the key.
794 A message is considered corrupt if the same key occurs twice in the same
795 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
796 implementations are not required to reject dicts with duplicate keys.
800 In most languages, an array of dict entry would be represented as a
801 map, hash table, or dict object.
806 <title>Summary of types</title>
809 The following table summarizes the D-Bus types.
814 <entry>Category</entry>
815 <entry>Conventional Name</entry>
817 <entry>Description</entry>
822 <entry>reserved</entry>
823 <entry><literal>INVALID</literal></entry>
824 <entry>0 (ASCII NUL)</entry>
825 <entry>Not a valid type code, used to terminate signatures</entry>
827 <entry>fixed, basic</entry>
828 <entry><literal>BYTE</literal></entry>
829 <entry>121 (ASCII 'y')</entry>
830 <entry>8-bit unsigned integer</entry>
832 <entry>fixed, basic</entry>
833 <entry><literal>BOOLEAN</literal></entry>
834 <entry>98 (ASCII 'b')</entry>
835 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
837 <entry>fixed, basic</entry>
838 <entry><literal>INT16</literal></entry>
839 <entry>110 (ASCII 'n')</entry>
840 <entry>16-bit signed integer</entry>
842 <entry>fixed, basic</entry>
843 <entry><literal>UINT16</literal></entry>
844 <entry>113 (ASCII 'q')</entry>
845 <entry>16-bit unsigned integer</entry>
847 <entry>fixed, basic</entry>
848 <entry><literal>INT32</literal></entry>
849 <entry>105 (ASCII 'i')</entry>
850 <entry>32-bit signed integer</entry>
852 <entry>fixed, basic</entry>
853 <entry><literal>UINT32</literal></entry>
854 <entry>117 (ASCII 'u')</entry>
855 <entry>32-bit unsigned integer</entry>
857 <entry>fixed, basic</entry>
858 <entry><literal>INT64</literal></entry>
859 <entry>120 (ASCII 'x')</entry>
860 <entry>64-bit signed integer</entry>
862 <entry>fixed, basic</entry>
863 <entry><literal>UINT64</literal></entry>
864 <entry>116 (ASCII 't')</entry>
865 <entry>64-bit unsigned integer</entry>
867 <entry>fixed, basic</entry>
868 <entry><literal>DOUBLE</literal></entry>
869 <entry>100 (ASCII 'd')</entry>
870 <entry>IEEE 754 double</entry>
872 <entry>string-like, basic</entry>
873 <entry><literal>STRING</literal></entry>
874 <entry>115 (ASCII 's')</entry>
875 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
877 <entry>string-like, basic</entry>
878 <entry><literal>OBJECT_PATH</literal></entry>
879 <entry>111 (ASCII 'o')</entry>
880 <entry>Name of an object instance</entry>
882 <entry>string-like, basic</entry>
883 <entry><literal>SIGNATURE</literal></entry>
884 <entry>103 (ASCII 'g')</entry>
885 <entry>A type signature</entry>
887 <entry>container</entry>
888 <entry><literal>ARRAY</literal></entry>
889 <entry>97 (ASCII 'a')</entry>
892 <entry>container</entry>
893 <entry><literal>STRUCT</literal></entry>
894 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
895 <entry>Struct; type code 114 'r' is reserved for use in
896 bindings and implementations to represent the general
897 concept of a struct, and must not appear in signatures
898 used on D-Bus.</entry>
900 <entry>container</entry>
901 <entry><literal>VARIANT</literal></entry>
902 <entry>118 (ASCII 'v') </entry>
903 <entry>Variant type (the type of the value is part of the value itself)</entry>
905 <entry>container</entry>
906 <entry><literal>DICT_ENTRY</literal></entry>
907 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
908 <entry>Entry in a dict or map (array of key-value pairs).
909 Type code 101 'e' is reserved for use in bindings and
910 implementations to represent the general concept of a
911 dict or dict-entry, and must not appear in signatures
912 used on D-Bus.</entry>
914 <entry>fixed, basic</entry>
915 <entry><literal>UNIX_FD</literal></entry>
916 <entry>104 (ASCII 'h')</entry>
917 <entry>Unix file descriptor</entry>
920 <entry>reserved</entry>
921 <entry>(reserved)</entry>
922 <entry>109 (ASCII 'm')</entry>
923 <entry>Reserved for <ulink
924 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
925 'maybe' type compatible with the one in GVariant</ulink>,
926 and must not appear in signatures used on D-Bus until
927 specified here</entry>
930 <entry>reserved</entry>
931 <entry>(reserved)</entry>
932 <entry>42 (ASCII '*')</entry>
933 <entry>Reserved for use in bindings/implementations to
934 represent any <firstterm>single complete type</firstterm>,
935 and must not appear in signatures used on D-Bus.</entry>
938 <entry>reserved</entry>
939 <entry>(reserved)</entry>
940 <entry>63 (ASCII '?')</entry>
941 <entry>Reserved for use in bindings/implementations to
942 represent any <firstterm>basic type</firstterm>, and must
943 not appear in signatures used on D-Bus.</entry>
946 <entry>reserved</entry>
947 <entry>(reserved)</entry>
948 <entry>64 (ASCII '@'), 38 (ASCII '&'),
949 94 (ASCII '^')</entry>
950 <entry>Reserved for internal use by bindings/implementations,
951 and must not appear in signatures used on D-Bus.
952 GVariant uses these type-codes to encode calling
963 <sect1 id="message-protocol-marshaling">
964 <title>Marshaling (Wire Format)</title>
967 D-Bus defines a marshalling format for its type system, which is
968 used in D-Bus messages. This is not the only possible marshalling
969 format for the type system: for instance, GVariant (part of GLib)
970 re-uses the D-Bus type system but implements an alternative marshalling
975 <title>Byte order and alignment</title>
978 Given a type signature, a block of bytes can be converted into typed
979 values. This section describes the format of the block of bytes. Byte
980 order and alignment issues are handled uniformly for all D-Bus types.
984 A block of bytes has an associated byte order. The byte order
985 has to be discovered in some way; for D-Bus messages, the
986 byte order is part of the message header as described in
987 <xref linkend="message-protocol-messages"/>. For now, assume
988 that the byte order is known to be either little endian or big
993 Each value in a block of bytes is aligned "naturally," for example
994 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
995 8-byte boundary. Boundaries are calculated globally, with respect to
996 the first byte in the message. To properly align a value,
997 <firstterm>alignment padding</firstterm> may be necessary before the
998 value. The alignment padding must always
999 be the minimum required padding to properly align the following value;
1000 and it must always be made up of nul bytes. The alignment padding must
1001 not be left uninitialized (it can't contain garbage), and more padding
1002 than required must not be used.
1006 As an exception to natural alignment, <literal>STRUCT</literal> and
1007 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
1008 boundary, regardless of the alignments of their contents.
1013 <title>Marshalling basic types</title>
1016 To marshal and unmarshal fixed types, you simply read one value
1017 from the data block corresponding to each type code in the signature.
1018 All signed integer values are encoded in two's complement, DOUBLE
1019 values are IEEE 754 double-precision floating-point, and BOOLEAN
1020 values are encoded in 32 bits (of which only the least significant
1025 The string-like types (STRING, OBJECT_PATH and SIGNATURE) are all
1027 fixed-length unsigned integer <varname>n</varname> giving the
1028 length of the variable part, followed by <varname>n</varname>
1029 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
1030 which is not considered to be part of the text. The alignment
1031 of the string-like type is the same as the alignment of
1032 <varname>n</varname>: any padding required for <varname>n</varname>
1033 appears immediately before <varname>n</varname> itself. There is never
1034 any alignment padding between <varname>n</varname> and the string text,
1035 or between the string text and the trailing nul. The alignment padding
1036 for the next value in the message (if there is one) starts after the
1041 For the STRING and OBJECT_PATH types, <varname>n</varname> is
1042 encoded in 4 bytes (a <literal>UINT32</literal>), leading to 4-byte
1043 alignment. For the SIGNATURE type, <varname>n</varname> is encoded as a
1044 single byte (a <literal>UINT8</literal>). As a result, alignment
1045 padding is never required before a SIGNATURE.
1049 For example, if the current position is a multiple of 8 bytes from the
1050 beginning of a little-endian message, strings ‘foo’, ‘+’ and ‘bar’
1051 would be serialized in sequence as follows:
1054 <lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
1055 0x03 0x00 0x00 0x00 <lineannotation>length of ‘foo’ = 3</lineannotation>
1056 0x66 0x6f 0x6f <lineannotation>‘foo’</lineannotation>
1057 0x00 <lineannotation>trailing nul</lineannotation>
1059 <lineannotation>no padding required, we are already at a multiple of 4</lineannotation>
1060 0x01 0x00 0x00 0x00 <lineannotation>length of ‘+’ = 1</lineannotation>
1061 0x2b <lineannotation>‘+’</lineannotation>
1062 0x00 <lineannotation>trailing nul</lineannotation>
1064 0x00 0x00 <lineannotation>2 bytes of padding to reach next multiple of 4</lineannotation>
1065 0x03 0x00 0x00 0x00 <lineannotation>length of ‘bar’ = 1</lineannotation>
1066 0x62 0x61 0x72 <lineannotation>‘bar’</lineannotation>
1067 0x00 <lineannotation>trailing nul</lineannotation>
1073 <title>Marshalling containers</title>
1076 Arrays are marshalled as a <literal>UINT32</literal>
1077 <varname>n</varname> giving the length of the array data in bytes,
1078 followed by alignment padding to the alignment boundary of the array
1079 element type, followed by the <varname>n</varname> bytes of the
1080 array elements marshalled in sequence. <varname>n</varname> does not
1081 include the padding after the length, or any padding after the
1082 last element. i.e. <varname>n</varname> should be divisible by the
1083 number of elements in the array.
1087 For instance, if the current position in the message is a multiple
1088 of 8 bytes and the byte-order is big-endian, an array containing only
1089 the 64-bit integer 5 would be marshalled as:
1092 00 00 00 08 <lineannotation><varname>n</varname> = 8 bytes of data</lineannotation>
1093 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1094 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1099 Arrays have a maximum length defined to be 2 to the 26th power or
1100 67108864 (64 MiB). Implementations must not send or accept arrays
1101 exceeding this length.
1105 Structs and dict entries are marshalled in the same way as their
1106 contents, but their alignment is always to an 8-byte boundary,
1107 even if their contents would normally be less strictly aligned.
1111 Variants are marshalled as the <literal>SIGNATURE</literal> of
1112 the contents (which must be a single complete type), followed by a
1113 marshalled value with the type given by that signature. The
1114 variant has the same 1-byte alignment as the signature, which means
1115 that alignment padding before a variant is never needed.
1116 Use of variants must not cause a total message depth to be larger
1117 than 64, including other container types such as structures.
1118 (See <link linkend="message-protocol-marshaling-signature">Valid
1124 <title>Summary of D-Bus marshalling</title>
1127 Given all this, the types are marshaled on the wire as follows:
1132 <entry>Conventional Name</entry>
1133 <entry>Encoding</entry>
1134 <entry>Alignment</entry>
1139 <entry><literal>INVALID</literal></entry>
1140 <entry>Not applicable; cannot be marshaled.</entry>
1143 <entry><literal>BYTE</literal></entry>
1144 <entry>A single 8-bit byte.</entry>
1147 <entry><literal>BOOLEAN</literal></entry>
1148 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1151 <entry><literal>INT16</literal></entry>
1152 <entry>16-bit signed integer in the message's byte order.</entry>
1155 <entry><literal>UINT16</literal></entry>
1156 <entry>16-bit unsigned integer in the message's byte order.</entry>
1159 <entry><literal>INT32</literal></entry>
1160 <entry>32-bit signed integer in the message's byte order.</entry>
1163 <entry><literal>UINT32</literal></entry>
1164 <entry>32-bit unsigned integer in the message's byte order.</entry>
1167 <entry><literal>INT64</literal></entry>
1168 <entry>64-bit signed integer in the message's byte order.</entry>
1171 <entry><literal>UINT64</literal></entry>
1172 <entry>64-bit unsigned integer in the message's byte order.</entry>
1175 <entry><literal>DOUBLE</literal></entry>
1176 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1179 <entry><literal>STRING</literal></entry>
1180 <entry>A <literal>UINT32</literal> indicating the string's
1181 length in bytes excluding its terminating nul, followed by
1182 non-nul string data of the given length, followed by a terminating nul
1189 <entry><literal>OBJECT_PATH</literal></entry>
1190 <entry>Exactly the same as <literal>STRING</literal> except the
1191 content must be a valid object path (see above).
1197 <entry><literal>SIGNATURE</literal></entry>
1198 <entry>The same as <literal>STRING</literal> except the length is a single
1199 byte (thus signatures have a maximum length of 255)
1200 and the content must be a valid signature (see above).
1206 <entry><literal>ARRAY</literal></entry>
1208 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1209 alignment padding to the alignment boundary of the array element type,
1210 followed by each array element.
1216 <entry><literal>STRUCT</literal></entry>
1218 A struct must start on an 8-byte boundary regardless of the
1219 type of the struct fields. The struct value consists of each
1220 field marshaled in sequence starting from that 8-byte
1227 <entry><literal>VARIANT</literal></entry>
1229 The marshaled <literal>SIGNATURE</literal> of a single
1230 complete type, followed by a marshaled value with the type
1231 given in the signature.
1234 1 (alignment of the signature)
1237 <entry><literal>DICT_ENTRY</literal></entry>
1239 Identical to STRUCT.
1245 <entry><literal>UNIX_FD</literal></entry>
1246 <entry>32-bit unsigned integer in the message's byte
1247 order. The actual file descriptors need to be
1248 transferred out-of-band via some platform specific
1249 mechanism. On the wire, values of this type store the index to the
1250 file descriptor in the array of file descriptors that
1251 accompany the message.</entry>
1263 <sect1 id="message-protocol">
1264 <title>Message Protocol</title>
1267 A <firstterm>message</firstterm> consists of a
1268 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1269 think of a message as a package, the header is the address, and the body
1270 contains the package contents. The message delivery system uses the header
1271 information to figure out where to send the message and how to interpret
1272 it; the recipient interprets the body of the message.
1276 The body of the message is made up of zero or more
1277 <firstterm>arguments</firstterm>, which are typed values, such as an
1278 integer or a byte array.
1282 Both header and body use the D-Bus <link linkend="type-system">type
1283 system</link> and format for serializing data.
1286 <sect2 id="message-protocol-messages">
1287 <title>Message Format</title>
1290 A message consists of a header and a body. The header is a block of
1291 values with a fixed signature and meaning. The body is a separate block
1292 of values, with a signature specified in the header.
1296 The length of the header must be a multiple of 8, allowing the body to
1297 begin on an 8-byte boundary when storing the entire message in a single
1298 buffer. If the header does not naturally end on an 8-byte boundary
1299 up to 7 bytes of nul-initialized alignment padding must be added.
1303 The message body need not end on an 8-byte boundary.
1307 The maximum length of a message, including header, header alignment padding,
1308 and body is 2 to the 27th power or 134217728 (128 MiB).
1309 Implementations must not send or accept messages exceeding this size.
1313 The signature of the header is:
1317 Written out more readably, this is:
1319 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1324 These values have the following meanings:
1329 <entry>Value</entry>
1330 <entry>Description</entry>
1335 <entry>1st <literal>BYTE</literal></entry>
1336 <entry>Endianness flag; ASCII 'l' for little-endian
1337 or ASCII 'B' for big-endian. Both header and body are
1338 in this endianness.</entry>
1341 <entry>2nd <literal>BYTE</literal></entry>
1342 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1343 Currently-defined types are described below.
1347 <entry>3rd <literal>BYTE</literal></entry>
1348 <entry>Bitwise OR of flags. Unknown flags
1349 must be ignored. Currently-defined flags are described below.
1353 <entry>4th <literal>BYTE</literal></entry>
1354 <entry>Major protocol version of the sending application. If
1355 the major protocol version of the receiving application does not
1356 match, the applications will not be able to communicate and the
1357 D-Bus connection must be disconnected. The major protocol
1358 version for this version of the specification is 1.
1362 <entry>1st <literal>UINT32</literal></entry>
1363 <entry>Length in bytes of the message body, starting
1364 from the end of the header. The header ends after
1365 its alignment padding to an 8-boundary.
1369 <entry>2nd <literal>UINT32</literal></entry>
1370 <entry>The serial of this message, used as a cookie
1371 by the sender to identify the reply corresponding
1372 to this request. This must not be zero.
1376 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1377 <entry>An array of zero or more <firstterm>header
1378 fields</firstterm> where the byte is the field code, and the
1379 variant is the field value. The message type determines
1380 which fields are required.
1388 <firstterm>Message types</firstterm> that can appear in the second byte
1394 <entry>Conventional name</entry>
1395 <entry>Decimal value</entry>
1396 <entry>Description</entry>
1401 <entry><literal>INVALID</literal></entry>
1403 <entry>This is an invalid type.</entry>
1406 <entry><literal>METHOD_CALL</literal></entry>
1408 <entry>Method call. This message type may prompt a
1412 <entry><literal>METHOD_RETURN</literal></entry>
1414 <entry>Method reply with returned data.</entry>
1417 <entry><literal>ERROR</literal></entry>
1419 <entry>Error reply. If the first argument exists and is a
1420 string, it is an error message.</entry>
1423 <entry><literal>SIGNAL</literal></entry>
1425 <entry>Signal emission.</entry>
1432 Flags that can appear in the third byte of the header:
1437 <entry>Conventional name</entry>
1438 <entry>Hex value</entry>
1439 <entry>Description</entry>
1444 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1448 This message does not expect method return replies or
1449 error replies, even if it is of a type that can
1450 have a reply; the reply should be omitted.
1453 Note that METHOD_CALL is the only message type currently
1454 defined in this specification that can expect a reply,
1455 so the presence or absence of this flag in the other
1456 three message types that are currently
1457 documented is meaningless: replies to those message
1458 types should not be sent, whether this flag is present
1464 <entry><literal>NO_AUTO_START</literal></entry>
1466 <entry>The bus must not launch an owner
1467 for the destination name in response to this message.
1471 <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
1475 This flag may be set on a method call message to
1476 inform the receiving side that the caller is prepared
1477 to wait for interactive authorization, which might
1478 take a considerable time to complete. For instance,
1479 if this flag is set, it would be appropriate to
1480 query the user for passwords or confirmation via
1481 Polkit or a similar framework.
1484 This flag is only useful when
1485 unprivileged code calls a more privileged method call,
1486 and an authorization framework is deployed that allows
1487 possibly interactive authorization. If no such framework
1488 is deployed it has no effect. This flag should not
1489 be set by default by client implementations. If it is
1490 set, the caller should also set a suitably long timeout
1491 on the method call to make sure the user interaction
1492 may complete. This flag is only valid for method call
1493 messages, and shall be ignored otherwise.
1496 Interaction that takes place as a part of the
1497 effect of the method being called is outside the scope
1498 of this flag, even if it could also be characterized
1499 as authentication or authorization. For instance, in
1500 a method call that directs a network management service
1501 to attempt to connect to a virtual private network,
1502 this flag should control how the network management
1503 service makes the decision "is this user allowed to
1504 change system network configuration?", but it should
1505 not affect how or whether the network management
1506 service interacts with the user to obtain the credentials
1507 that are required for access to the VPN.
1510 If a this flag is not set on a method call, and a
1511 service determines that the requested operation is
1512 not allowed without interactive authorization, but
1513 could be allowed after successful interactive
1514 authorization, it may return the
1515 <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
1519 The absence of this flag does not guarantee that
1520 interactive authorization will not be applied, since
1521 existing services that pre-date this flag might
1522 already use interactive authorization. However,
1523 existing D-Bus APIs that will use interactive
1524 authorization should document that the call may take
1525 longer than usual, and new D-Bus APIs should avoid
1526 interactive authorization in the absence of this flag.
1535 <sect3 id="message-protocol-header-fields">
1536 <title>Header Fields</title>
1539 The array at the end of the header contains <firstterm>header
1540 fields</firstterm>, where each field is a 1-byte field code followed
1541 by a field value. A header must contain the required header fields for
1542 its message type, and zero or more of any optional header
1543 fields. Future versions of this protocol specification may add new
1544 fields. Implementations must ignore fields they do not
1545 understand. Implementations must not invent their own header fields;
1546 only changes to this specification may introduce new header fields.
1550 Again, if an implementation sees a header field code that it does not
1551 expect, it must ignore that field, as it will be part of a new
1552 (but compatible) version of this specification. This also applies
1553 to known header fields appearing in unexpected messages, for
1554 example: if a signal has a reply serial it must be ignored
1555 even though it has no meaning as of this version of the spec.
1559 However, implementations must not send or accept known header fields
1560 with the wrong type stored in the field value. So for example a
1561 message with an <literal>INTERFACE</literal> field of type
1562 <literal>UINT32</literal> would be considered corrupt.
1566 Here are the currently-defined header fields:
1571 <entry>Conventional Name</entry>
1572 <entry>Decimal Code</entry>
1574 <entry>Required In</entry>
1575 <entry>Description</entry>
1580 <entry><literal>INVALID</literal></entry>
1583 <entry>not allowed</entry>
1584 <entry>Not a valid field name (error if it appears in a message)</entry>
1587 <entry><literal>PATH</literal></entry>
1589 <entry><literal>OBJECT_PATH</literal></entry>
1590 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1591 <entry>The object to send a call to,
1592 or the object a signal is emitted from.
1594 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1595 implementations should not send messages with this path,
1596 and the reference implementation of the bus daemon will
1597 disconnect any application that attempts to do so.
1601 <entry><literal>INTERFACE</literal></entry>
1603 <entry><literal>STRING</literal></entry>
1604 <entry><literal>SIGNAL</literal></entry>
1606 The interface to invoke a method call on, or
1607 that a signal is emitted from. Optional for
1608 method calls, required for signals.
1609 The special interface
1610 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1611 implementations should not send messages with this
1612 interface, and the reference implementation of the bus
1613 daemon will disconnect any application that attempts to
1618 <entry><literal>MEMBER</literal></entry>
1620 <entry><literal>STRING</literal></entry>
1621 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1622 <entry>The member, either the method name or signal name.</entry>
1625 <entry><literal>ERROR_NAME</literal></entry>
1627 <entry><literal>STRING</literal></entry>
1628 <entry><literal>ERROR</literal></entry>
1629 <entry>The name of the error that occurred, for errors</entry>
1632 <entry><literal>REPLY_SERIAL</literal></entry>
1634 <entry><literal>UINT32</literal></entry>
1635 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1636 <entry>The serial number of the message this message is a reply
1637 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1640 <entry><literal>DESTINATION</literal></entry>
1642 <entry><literal>STRING</literal></entry>
1643 <entry>optional</entry>
1644 <entry>The name of the connection this message is intended for.
1645 Only used in combination with the message bus, see
1646 <xref linkend="message-bus"/>.</entry>
1649 <entry><literal>SENDER</literal></entry>
1651 <entry><literal>STRING</literal></entry>
1652 <entry>optional</entry>
1653 <entry>Unique name of the sending connection.
1654 The message bus fills in this field so it is reliable; the field is
1655 only meaningful in combination with the message bus.</entry>
1658 <entry><literal>SIGNATURE</literal></entry>
1660 <entry><literal>SIGNATURE</literal></entry>
1661 <entry>optional</entry>
1662 <entry>The signature of the message body.
1663 If omitted, it is assumed to be the
1664 empty signature "" (i.e. the body must be 0-length).</entry>
1667 <entry><literal>UNIX_FDS</literal></entry>
1669 <entry><literal>UINT32</literal></entry>
1670 <entry>optional</entry>
1671 <entry>The number of Unix file descriptors that
1672 accompany the message. If omitted, it is assumed
1673 that no Unix file descriptors accompany the
1674 message. The actual file descriptors need to be
1675 transferred via platform specific mechanism
1676 out-of-band. They must be sent at the same time as
1677 part of the message itself. They may not be sent
1678 before the first byte of the message itself is
1679 transferred or after the last byte of the message
1689 <sect2 id="message-protocol-names">
1690 <title>Valid Names</title>
1692 The various names in D-Bus messages have some restrictions.
1695 There is a <firstterm>maximum name length</firstterm>
1696 of 255 which applies to bus names, interfaces, and members.
1698 <sect3 id="message-protocol-names-interface">
1699 <title>Interface names</title>
1701 Interfaces have names with type <literal>STRING</literal>, meaning that
1702 they must be valid UTF-8. However, there are also some
1703 additional restrictions that apply to interface names
1706 <listitem><para>Interface names are composed of 1 or more elements separated by
1707 a period ('.') character. All elements must contain at least
1711 <listitem><para>Each element must only contain the ASCII characters
1712 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1716 <listitem><para>Interface names must contain at least one '.' (period)
1717 character (and thus at least two elements).
1720 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1721 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1726 Interface names should start with the reversed DNS domain name of
1727 the author of the interface (in lower-case), like interface names
1728 in Java. It is conventional for the rest of the interface name
1729 to consist of words run together, with initial capital letters
1730 on all words ("CamelCase"). Several levels of hierarchy can be used.
1731 It is also a good idea to include the major version of the interface
1732 in the name, and increment it if incompatible changes are made;
1733 this way, a single object can implement several versions of an
1734 interface in parallel, if necessary.
1738 For instance, if the owner of <literal>example.com</literal> is
1739 developing a D-Bus API for a music player, they might define
1740 interfaces called <literal>com.example.MusicPlayer1</literal>,
1741 <literal>com.example.MusicPlayer1.Track</literal> and
1742 <literal>com.example.MusicPlayer1.Seekable</literal>.
1746 D-Bus does not distinguish between the concepts that would be
1747 called classes and interfaces in Java: either can be identified on
1748 D-Bus by an interface name.
1751 <sect3 id="message-protocol-names-bus">
1752 <title>Bus names</title>
1754 Connections have one or more bus names associated with them.
1755 A connection has exactly one bus name that is a <firstterm>unique
1756 connection name</firstterm>. The unique connection name remains
1757 with the connection for its entire lifetime.
1758 A bus name is of type <literal>STRING</literal>,
1759 meaning that it must be valid UTF-8. However, there are also
1760 some additional restrictions that apply to bus names
1763 <listitem><para>Bus names that start with a colon (':')
1764 character are unique connection names. Other bus names
1765 are called <firstterm>well-known bus names</firstterm>.
1768 <listitem><para>Bus names are composed of 1 or more elements separated by
1769 a period ('.') character. All elements must contain at least
1773 <listitem><para>Each element must only contain the ASCII characters
1774 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1775 connection name may begin with a digit, elements in
1776 other bus names must not begin with a digit.
1780 <listitem><para>Bus names must contain at least one '.' (period)
1781 character (and thus at least two elements).
1784 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1785 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1789 Note that the hyphen ('-') character is allowed in bus names but
1790 not in interface names.
1794 Like <link linkend="message-protocol-names-interface">interface
1795 names</link>, well-known bus names should start with the
1796 reversed DNS domain name of the author of the interface (in
1797 lower-case), and it is conventional for the rest of the well-known
1798 bus name to consist of words run together, with initial
1799 capital letters. As with interface names, including a version
1800 number in well-known bus names is a good idea; it's possible to
1801 have the well-known bus name for more than one version
1802 simultaneously if backwards compatibility is required.
1806 If a well-known bus name implies the presence of a "main" interface,
1807 that "main" interface is often given the same name as
1808 the well-known bus name, and situated at the corresponding object
1809 path. For instance, if the owner of <literal>example.com</literal>
1810 is developing a D-Bus API for a music player, they might define
1811 that any application that takes the well-known name
1812 <literal>com.example.MusicPlayer1</literal> should have an object
1813 at the object path <literal>/com/example/MusicPlayer1</literal>
1814 which implements the interface
1815 <literal>com.example.MusicPlayer1</literal>.
1818 <sect3 id="message-protocol-names-member">
1819 <title>Member names</title>
1821 Member (i.e. method or signal) names:
1823 <listitem><para>Must only contain the ASCII characters
1824 "[A-Z][a-z][0-9]_" and may not begin with a
1825 digit.</para></listitem>
1826 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1827 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1828 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1833 It is conventional for member names on D-Bus to consist of
1834 capitalized words with no punctuation ("camel-case").
1835 Method names should usually be verbs, such as
1836 <literal>GetItems</literal>, and signal names should usually be
1837 a description of an event, such as <literal>ItemsChanged</literal>.
1840 <sect3 id="message-protocol-names-error">
1841 <title>Error names</title>
1843 Error names have the same restrictions as interface names.
1847 Error names have the same naming conventions as interface
1848 names, and often contain <literal>.Error.</literal>; for instance,
1849 the owner of <literal>example.com</literal> might define the
1850 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1851 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1852 The errors defined by D-Bus itself, such as
1853 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1859 <sect2 id="message-protocol-types">
1860 <title>Message Types</title>
1862 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1863 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1864 This section describes these conventions.
1866 <sect3 id="message-protocol-types-method">
1867 <title>Method Calls</title>
1869 Some messages invoke an operation on a remote object. These are
1870 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1871 messages map naturally to methods on objects in a typical program.
1874 A method call message is required to have a <literal>MEMBER</literal> header field
1875 indicating the name of the method. Optionally, the message has an
1876 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1877 Including the <literal>INTERFACE</literal> in all method call
1878 messages is strongly recommended.
1881 In the absence of an <literal>INTERFACE</literal> field, if two
1882 or more interfaces on the same object have a method with the same
1883 name, it is undefined which of those methods will be invoked.
1884 Implementations may choose to either return an error, or deliver the
1885 message as though it had an arbitrary one of those interfaces.
1888 In some situations (such as the well-known system bus), messages
1889 are filtered through an access-control list external to the
1890 remote object implementation. If that filter rejects certain
1891 messages by matching their interface, or accepts only messages
1892 to specific interfaces, it must also reject messages that have no
1893 <literal>INTERFACE</literal>: otherwise, malicious
1894 applications could use this to bypass the filter.
1897 Method call messages also include a <literal>PATH</literal> field
1898 indicating the object to invoke the method on. If the call is passing
1899 through a message bus, the message will also have a
1900 <literal>DESTINATION</literal> field giving the name of the connection
1901 to receive the message.
1904 When an application handles a method call message, it is required to
1905 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1906 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1907 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1910 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1911 are the return value(s) or "out parameters" of the method call.
1912 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1913 and the call fails; no return value will be provided. It makes
1914 no sense to send multiple replies to the same method call.
1917 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1918 reply is required, so the caller will know the method
1919 was successfully processed.
1922 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1926 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1927 then the application receiving the method should not send the reply message (regardless of
1928 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1931 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1932 destination name does not exist then a program to own the destination
1933 name will be started before the message is delivered. See
1934 <xref linkend="message-bus-starting-services"/>.
1936 will be held until the new program is successfully started or has
1937 failed to start; in case of failure, an error will be returned. This
1938 flag is only relevant in the context of a message bus, it is ignored
1939 during one-to-one communication with no intermediate bus.
1941 <sect4 id="message-protocol-types-method-apis">
1942 <title>Mapping method calls to native APIs</title>
1944 APIs for D-Bus may map method calls to a method call in a specific
1945 programming language, such as C++, or may map a method call written
1946 in an IDL to a D-Bus message.
1949 In APIs of this nature, arguments to a method are often termed "in"
1950 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1951 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1952 "inout" arguments, which are both sent and received, i.e. the caller
1953 passes in a value which is modified. Mapped to D-Bus, an "inout"
1954 argument is equivalent to an "in" argument, followed by an "out"
1955 argument. You can't pass things "by reference" over the wire, so
1956 "inout" is purely an illusion of the in-process API.
1959 Given a method with zero or one return values, followed by zero or more
1960 arguments, where each argument may be "in", "out", or "inout", the
1961 caller constructs a message by appending each "in" or "inout" argument,
1962 in order. "out" arguments are not represented in the caller's message.
1965 The recipient constructs a reply by appending first the return value
1966 if any, then each "out" or "inout" argument, in order.
1967 "in" arguments are not represented in the reply message.
1970 Error replies are normally mapped to exceptions in languages that have
1974 In converting from native APIs to D-Bus, it is perhaps nice to
1975 map D-Bus naming conventions ("FooBar") to native conventions
1976 such as "fooBar" or "foo_bar" automatically. This is OK
1977 as long as you can say that the native API is one that
1978 was specifically written for D-Bus. It makes the most sense
1979 when writing object implementations that will be exported
1980 over the bus. Object proxies used to invoke remote D-Bus
1981 objects probably need the ability to call any D-Bus method,
1982 and thus a magic name mapping like this could be a problem.
1985 This specification doesn't require anything of native API bindings;
1986 the preceding is only a suggested convention for consistency
1992 <sect3 id="message-protocol-types-signal">
1993 <title>Signal Emission</title>
1995 Unlike method calls, signal emissions have no replies.
1996 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1997 It must have three header fields: <literal>PATH</literal> giving the object
1998 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1999 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
2000 for signals, though it is optional for method calls.
2004 <sect3 id="message-protocol-types-errors">
2005 <title>Errors</title>
2007 Messages of type <literal>ERROR</literal> are most commonly replies
2008 to a <literal>METHOD_CALL</literal>, but may be returned in reply
2009 to any kind of message. The message bus for example
2010 will return an <literal>ERROR</literal> in reply to a signal emission if
2011 the bus does not have enough memory to send the signal.
2014 An <literal>ERROR</literal> may have any arguments, but if the first
2015 argument is a <literal>STRING</literal>, it must be an error message.
2016 The error message may be logged or shown to the user
2021 <sect3 id="message-protocol-types-notation">
2022 <title>Notation in this document</title>
2024 This document uses a simple pseudo-IDL to describe particular method
2025 calls and signals. Here is an example of a method call:
2027 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
2028 out UINT32 resultcode)
2030 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
2031 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
2032 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
2033 characters so it's known that the last part of the name in
2034 the "IDL" is the member name.
2037 In C++ that might end up looking like this:
2039 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
2040 unsigned int flags);
2042 or equally valid, the return value could be done as an argument:
2044 void org::freedesktop::DBus::StartServiceByName (const char *name,
2046 unsigned int *resultcode);
2048 It's really up to the API designer how they want to make
2049 this look. You could design an API where the namespace wasn't used
2050 in C++, using STL or Qt, using varargs, or whatever you wanted.
2053 Signals are written as follows:
2055 org.freedesktop.DBus.NameLost (STRING name)
2057 Signals don't specify "in" vs. "out" because only
2058 a single direction is possible.
2061 It isn't especially encouraged to use this lame pseudo-IDL in actual
2062 API implementations; you might use the native notation for the
2063 language you're using, or you might use COM or CORBA IDL, for example.
2068 <sect2 id="message-protocol-handling-invalid">
2069 <title>Invalid Protocol and Spec Extensions</title>
2072 For security reasons, the D-Bus protocol should be strictly parsed and
2073 validated, with the exception of defined extension points. Any invalid
2074 protocol or spec violations should result in immediately dropping the
2075 connection without notice to the other end. Exceptions should be
2076 carefully considered, e.g. an exception may be warranted for a
2077 well-understood idiosyncrasy of a widely-deployed implementation. In
2078 cases where the other end of a connection is 100% trusted and known to
2079 be friendly, skipping validation for performance reasons could also make
2080 sense in certain cases.
2084 Generally speaking violations of the "must" requirements in this spec
2085 should be considered possible attempts to exploit security, and violations
2086 of the "should" suggestions should be considered legitimate (though perhaps
2087 they should generate an error in some cases).
2091 The following extension points are built in to D-Bus on purpose and must
2092 not be treated as invalid protocol. The extension points are intended
2093 for use by future versions of this spec, they are not intended for third
2094 parties. At the moment, the only way a third party could extend D-Bus
2095 without breaking interoperability would be to introduce a way to negotiate new
2096 feature support as part of the auth protocol, using EXTENSION_-prefixed
2097 commands. There is not yet a standard way to negotiate features.
2101 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
2102 commands result in an ERROR rather than a disconnect. This enables
2103 future extensions to the protocol. Commands starting with EXTENSION_ are
2104 reserved for third parties.
2109 The authentication protocol supports pluggable auth mechanisms.
2114 The address format (see <xref linkend="addresses"/>) supports new
2120 Messages with an unknown type (something other than
2121 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
2122 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
2123 Unknown-type messages must still be well-formed in the same way
2124 as the known messages, however. They still have the normal
2130 Header fields with an unknown or unexpected field code must be ignored,
2131 though again they must still be well-formed.
2136 New standard interfaces (with new methods and signals) can of course be added.
2146 <sect1 id="auth-protocol">
2147 <title>Authentication Protocol</title>
2149 Before the flow of messages begins, two applications must
2150 authenticate. A simple plain-text protocol is used for
2151 authentication; this protocol is a SASL profile, and maps fairly
2152 directly from the SASL specification. The message encoding is
2153 NOT used here, only plain text messages.
2156 In examples, "C:" and "S:" indicate lines sent by the client and
2157 server respectively.
2159 <sect2 id="auth-protocol-overview">
2160 <title>Protocol Overview</title>
2162 The protocol is a line-based protocol, where each line ends with
2163 \r\n. Each line begins with an all-caps ASCII command name containing
2164 only the character range [A-Z_], a space, then any arguments for the
2165 command, then the \r\n ending the line. The protocol is
2166 case-sensitive. All bytes must be in the ASCII character set.
2168 Commands from the client to the server are as follows:
2171 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2172 <listitem><para>CANCEL</para></listitem>
2173 <listitem><para>BEGIN</para></listitem>
2174 <listitem><para>DATA <data in hex encoding></para></listitem>
2175 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2176 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2179 From server to client are as follows:
2182 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2183 <listitem><para>OK <GUID in hex></para></listitem>
2184 <listitem><para>DATA <data in hex encoding></para></listitem>
2185 <listitem><para>ERROR</para></listitem>
2186 <listitem><para>AGREE_UNIX_FD</para></listitem>
2190 Unofficial extensions to the command set must begin with the letters
2191 "EXTENSION_", to avoid conflicts with future official commands.
2192 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2195 <sect2 id="auth-nul-byte">
2196 <title>Special credentials-passing nul byte</title>
2198 Immediately after connecting to the server, the client must send a
2199 single nul byte. This byte may be accompanied by credentials
2200 information on some operating systems that use sendmsg() with
2201 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2202 sockets. However, the nul byte must be sent even on other kinds of
2203 socket, and even on operating systems that do not require a byte to be
2204 sent in order to transmit credentials. The text protocol described in
2205 this document begins after the single nul byte. If the first byte
2206 received from the client is not a nul byte, the server may disconnect
2210 A nul byte in any context other than the initial byte is an error;
2211 the protocol is ASCII-only.
2214 The credentials sent along with the nul byte may be used with the
2215 SASL mechanism EXTERNAL.
2218 <sect2 id="auth-command-auth">
2219 <title>AUTH command</title>
2221 If an AUTH command has no arguments, it is a request to list
2222 available mechanisms. The server must respond with a REJECTED
2223 command listing the mechanisms it understands, or with an error.
2226 If an AUTH command specifies a mechanism, and the server supports
2227 said mechanism, the server should begin exchanging SASL
2228 challenge-response data with the client using DATA commands.
2231 If the server does not support the mechanism given in the AUTH
2232 command, it must send either a REJECTED command listing the mechanisms
2233 it does support, or an error.
2236 If the [initial-response] argument is provided, it is intended for use
2237 with mechanisms that have no initial challenge (or an empty initial
2238 challenge), as if it were the argument to an initial DATA command. If
2239 the selected mechanism has an initial challenge and [initial-response]
2240 was provided, the server should reject authentication by sending
2244 If authentication succeeds after exchanging DATA commands,
2245 an OK command must be sent to the client.
2248 The first octet received by the server after the \r\n of the BEGIN
2249 command from the client must be the first octet of the
2250 authenticated/encrypted stream of D-Bus messages.
2253 If BEGIN is received by the server, the first octet received
2254 by the client after the \r\n of the OK command must be the
2255 first octet of the authenticated/encrypted stream of D-Bus
2259 <sect2 id="auth-command-cancel">
2260 <title>CANCEL Command</title>
2262 At any time up to sending the BEGIN command, the client may send a
2263 CANCEL command. On receiving the CANCEL command, the server must
2264 send a REJECTED command and abort the current authentication
2268 <sect2 id="auth-command-data">
2269 <title>DATA Command</title>
2271 The DATA command may come from either client or server, and simply
2272 contains a hex-encoded block of data to be interpreted
2273 according to the SASL mechanism in use.
2276 Some SASL mechanisms support sending an "empty string";
2277 FIXME we need some way to do this.
2280 <sect2 id="auth-command-begin">
2281 <title>BEGIN Command</title>
2283 The BEGIN command acknowledges that the client has received an
2284 OK command from the server, and that the stream of messages
2288 The first octet received by the server after the \r\n of the BEGIN
2289 command from the client must be the first octet of the
2290 authenticated/encrypted stream of D-Bus messages.
2293 <sect2 id="auth-command-rejected">
2294 <title>REJECTED Command</title>
2296 The REJECTED command indicates that the current authentication
2297 exchange has failed, and further exchange of DATA is inappropriate.
2298 The client would normally try another mechanism, or try providing
2299 different responses to challenges.
2301 Optionally, the REJECTED command has a space-separated list of
2302 available auth mechanisms as arguments. If a server ever provides
2303 a list of supported mechanisms, it must provide the same list
2304 each time it sends a REJECTED message. Clients are free to
2305 ignore all lists received after the first.
2308 <sect2 id="auth-command-ok">
2309 <title>OK Command</title>
2311 The OK command indicates that the client has been
2312 authenticated. The client may now proceed with negotiating
2313 Unix file descriptor passing. To do that it shall send
2314 NEGOTIATE_UNIX_FD to the server.
2317 Otherwise, the client must respond to the OK command by
2318 sending a BEGIN command, followed by its stream of messages,
2319 or by disconnecting. The server must not accept additional
2320 commands using this protocol after the BEGIN command has been
2321 received. Further communication will be a stream of D-Bus
2322 messages (optionally encrypted, as negotiated) rather than
2326 If a client sends BEGIN the first octet received by the client
2327 after the \r\n of the OK command must be the first octet of
2328 the authenticated/encrypted stream of D-Bus messages.
2331 The OK command has one argument, which is the GUID of the server.
2332 See <xref linkend="addresses"/> for more on server GUIDs.
2335 <sect2 id="auth-command-error">
2336 <title>ERROR Command</title>
2338 The ERROR command indicates that either server or client did not
2339 know a command, does not accept the given command in the current
2340 context, or did not understand the arguments to the command. This
2341 allows the protocol to be extended; a client or server can send a
2342 command present or permitted only in new protocol versions, and if
2343 an ERROR is received instead of an appropriate response, fall back
2344 to using some other technique.
2347 If an ERROR is sent, the server or client that sent the
2348 error must continue as if the command causing the ERROR had never been
2349 received. However, the the server or client receiving the error
2350 should try something other than whatever caused the error;
2351 if only canceling/rejecting the authentication.
2354 If the D-Bus protocol changes incompatibly at some future time,
2355 applications implementing the new protocol would probably be able to
2356 check for support of the new protocol by sending a new command and
2357 receiving an ERROR from applications that don't understand it. Thus the
2358 ERROR feature of the auth protocol is an escape hatch that lets us
2359 negotiate extensions or changes to the D-Bus protocol in the future.
2362 <sect2 id="auth-command-negotiate-unix-fd">
2363 <title>NEGOTIATE_UNIX_FD Command</title>
2365 The NEGOTIATE_UNIX_FD command indicates that the client
2366 supports Unix file descriptor passing. This command may only
2367 be sent after the connection is authenticated, i.e. after OK
2368 was received by the client. This command may only be sent on
2369 transports that support Unix file descriptor passing.
2372 On receiving NEGOTIATE_UNIX_FD the server must respond with
2373 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2374 the transport chosen supports Unix file descriptor passing and
2375 the server supports this feature. It shall respond the latter
2376 if the transport does not support Unix file descriptor
2377 passing, the server does not support this feature, or the
2378 server decides not to enable file descriptor passing due to
2379 security or other reasons.
2382 <sect2 id="auth-command-agree-unix-fd">
2383 <title>AGREE_UNIX_FD Command</title>
2385 The AGREE_UNIX_FD command indicates that the server supports
2386 Unix file descriptor passing. This command may only be sent
2387 after the connection is authenticated, and the client sent
2388 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2389 command may only be sent on transports that support Unix file
2393 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2394 followed by its stream of messages, or by disconnecting. The
2395 server must not accept additional commands using this protocol
2396 after the BEGIN command has been received. Further
2397 communication will be a stream of D-Bus messages (optionally
2398 encrypted, as negotiated) rather than this protocol.
2401 <sect2 id="auth-command-future">
2402 <title>Future Extensions</title>
2404 Future extensions to the authentication and negotiation
2405 protocol are possible. For that new commands may be
2406 introduced. If a client or server receives an unknown command
2407 it shall respond with ERROR and not consider this fatal. New
2408 commands may be introduced both before, and after
2409 authentication, i.e. both before and after the OK command.
2412 <sect2 id="auth-examples">
2413 <title>Authentication examples</title>
2417 <title>Example of successful magic cookie authentication</title>
2419 (MAGIC_COOKIE is a made up mechanism)
2421 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2427 <title>Example of finding out mechanisms then picking one</title>
2430 S: REJECTED KERBEROS_V4 SKEY
2431 C: AUTH SKEY 7ab83f32ee
2432 S: DATA 8799cabb2ea93e
2433 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2439 <title>Example of client sends unknown command then falls back to regular auth</title>
2443 C: AUTH MAGIC_COOKIE 3736343435313230333039
2449 <title>Example of server doesn't support initial auth mechanism</title>
2451 C: AUTH MAGIC_COOKIE 3736343435313230333039
2452 S: REJECTED KERBEROS_V4 SKEY
2453 C: AUTH SKEY 7ab83f32ee
2454 S: DATA 8799cabb2ea93e
2455 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2461 <title>Example of wrong password or the like followed by successful retry</title>
2463 C: AUTH MAGIC_COOKIE 3736343435313230333039
2464 S: REJECTED KERBEROS_V4 SKEY
2465 C: AUTH SKEY 7ab83f32ee
2466 S: DATA 8799cabb2ea93e
2467 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2469 C: AUTH SKEY 7ab83f32ee
2470 S: DATA 8799cabb2ea93e
2471 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2477 <title>Example of skey cancelled and restarted</title>
2479 C: AUTH MAGIC_COOKIE 3736343435313230333039
2480 S: REJECTED KERBEROS_V4 SKEY
2481 C: AUTH SKEY 7ab83f32ee
2482 S: DATA 8799cabb2ea93e
2485 C: AUTH SKEY 7ab83f32ee
2486 S: DATA 8799cabb2ea93e
2487 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2493 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2495 (MAGIC_COOKIE is a made up mechanism)
2497 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2499 C: NEGOTIATE_UNIX_FD
2505 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2507 (MAGIC_COOKIE is a made up mechanism)
2509 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2511 C: NEGOTIATE_UNIX_FD
2518 <sect2 id="auth-states">
2519 <title>Authentication state diagrams</title>
2522 This section documents the auth protocol in terms of
2523 a state machine for the client and the server. This is
2524 probably the most robust way to implement the protocol.
2527 <sect3 id="auth-states-client">
2528 <title>Client states</title>
2531 To more precisely describe the interaction between the
2532 protocol state machine and the authentication mechanisms the
2533 following notation is used: MECH(CHALL) means that the
2534 server challenge CHALL was fed to the mechanism MECH, which
2540 CONTINUE(RESP) means continue the auth conversation
2541 and send RESP as the response to the server;
2547 OK(RESP) means that after sending RESP to the server
2548 the client side of the auth conversation is finished
2549 and the server should return "OK";
2555 ERROR means that CHALL was invalid and could not be
2561 Both RESP and CHALL may be empty.
2565 The Client starts by getting an initial response from the
2566 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2567 the mechanism did not provide an initial response. If the
2568 mechanism returns CONTINUE, the client starts in state
2569 <emphasis>WaitingForData</emphasis>, if the mechanism
2570 returns OK the client starts in state
2571 <emphasis>WaitingForOK</emphasis>.
2575 The client should keep track of available mechanisms and
2576 which it mechanisms it has already attempted. This list is
2577 used to decide which AUTH command to send. When the list is
2578 exhausted, the client should give up and close the
2583 <title><emphasis>WaitingForData</emphasis></title>
2591 MECH(CHALL) returns CONTINUE(RESP) → send
2593 <emphasis>WaitingForData</emphasis>
2597 MECH(CHALL) returns OK(RESP) → send DATA
2598 RESP, goto <emphasis>WaitingForOK</emphasis>
2602 MECH(CHALL) returns ERROR → send ERROR
2603 [msg], goto <emphasis>WaitingForData</emphasis>
2611 Receive REJECTED [mechs] →
2612 send AUTH [next mech], goto
2613 WaitingForData or <emphasis>WaitingForOK</emphasis>
2618 Receive ERROR → send
2620 <emphasis>WaitingForReject</emphasis>
2625 Receive OK → send
2626 BEGIN, terminate auth
2627 conversation, authenticated
2632 Receive anything else → send
2634 <emphasis>WaitingForData</emphasis>
2642 <title><emphasis>WaitingForOK</emphasis></title>
2647 Receive OK → send BEGIN, terminate auth
2648 conversation, <emphasis>authenticated</emphasis>
2653 Receive REJECTED [mechs] → send AUTH [next mech],
2654 goto <emphasis>WaitingForData</emphasis> or
2655 <emphasis>WaitingForOK</emphasis>
2661 Receive DATA → send CANCEL, goto
2662 <emphasis>WaitingForReject</emphasis>
2668 Receive ERROR → send CANCEL, goto
2669 <emphasis>WaitingForReject</emphasis>
2675 Receive anything else → send ERROR, goto
2676 <emphasis>WaitingForOK</emphasis>
2684 <title><emphasis>WaitingForReject</emphasis></title>
2689 Receive REJECTED [mechs] → send AUTH [next mech],
2690 goto <emphasis>WaitingForData</emphasis> or
2691 <emphasis>WaitingForOK</emphasis>
2697 Receive anything else → terminate auth
2698 conversation, disconnect
2707 <sect3 id="auth-states-server">
2708 <title>Server states</title>
2711 For the server MECH(RESP) means that the client response
2712 RESP was fed to the the mechanism MECH, which returns one of
2717 CONTINUE(CHALL) means continue the auth conversation and
2718 send CHALL as the challenge to the client;
2724 OK means that the client has been successfully
2731 REJECTED means that the client failed to authenticate or
2732 there was an error in RESP.
2737 The server starts out in state
2738 <emphasis>WaitingForAuth</emphasis>. If the client is
2739 rejected too many times the server must disconnect the
2744 <title><emphasis>WaitingForAuth</emphasis></title>
2750 Receive AUTH → send REJECTED [mechs], goto
2751 <emphasis>WaitingForAuth</emphasis>
2757 Receive AUTH MECH RESP
2761 MECH not valid mechanism → send REJECTED
2763 <emphasis>WaitingForAuth</emphasis>
2767 MECH(RESP) returns CONTINUE(CHALL) → send
2769 <emphasis>WaitingForData</emphasis>
2773 MECH(RESP) returns OK → send OK, goto
2774 <emphasis>WaitingForBegin</emphasis>
2778 MECH(RESP) returns REJECTED → send REJECTED
2780 <emphasis>WaitingForAuth</emphasis>
2788 Receive BEGIN → terminate
2789 auth conversation, disconnect
2795 Receive ERROR → send REJECTED [mechs], goto
2796 <emphasis>WaitingForAuth</emphasis>
2802 Receive anything else → send
2804 <emphasis>WaitingForAuth</emphasis>
2813 <title><emphasis>WaitingForData</emphasis></title>
2821 MECH(RESP) returns CONTINUE(CHALL) → send
2823 <emphasis>WaitingForData</emphasis>
2827 MECH(RESP) returns OK → send OK, goto
2828 <emphasis>WaitingForBegin</emphasis>
2832 MECH(RESP) returns REJECTED → send REJECTED
2834 <emphasis>WaitingForAuth</emphasis>
2842 Receive BEGIN → terminate auth conversation,
2849 Receive CANCEL → send REJECTED [mechs], goto
2850 <emphasis>WaitingForAuth</emphasis>
2856 Receive ERROR → send REJECTED [mechs], goto
2857 <emphasis>WaitingForAuth</emphasis>
2863 Receive anything else → send ERROR, goto
2864 <emphasis>WaitingForData</emphasis>
2872 <title><emphasis>WaitingForBegin</emphasis></title>
2877 Receive BEGIN → terminate auth conversation,
2878 client authenticated
2884 Receive CANCEL → send REJECTED [mechs], goto
2885 <emphasis>WaitingForAuth</emphasis>
2891 Receive ERROR → send REJECTED [mechs], goto
2892 <emphasis>WaitingForAuth</emphasis>
2898 Receive anything else → send ERROR, goto
2899 <emphasis>WaitingForBegin</emphasis>
2909 <sect2 id="auth-mechanisms">
2910 <title>Authentication mechanisms</title>
2912 This section describes some new authentication mechanisms.
2913 D-Bus also allows any standard SASL mechanism of course.
2915 <sect3 id="auth-mechanisms-sha">
2916 <title>DBUS_COOKIE_SHA1</title>
2918 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2919 has the ability to read a private file owned by the user being
2920 authenticated. If the client can prove that it has access to a secret
2921 cookie stored in this file, then the client is authenticated.
2922 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2926 Throughout this description, "hex encoding" must output the digits
2927 from a to f in lower-case; the digits A to F must not be used
2928 in the DBUS_COOKIE_SHA1 mechanism.
2931 Authentication proceeds as follows:
2935 The client sends the username it would like to authenticate
2941 The server sends the name of its "cookie context" (see below); a
2942 space character; the integer ID of the secret cookie the client
2943 must demonstrate knowledge of; a space character; then a
2944 randomly-generated challenge string, all of this hex-encoded into
2950 The client locates the cookie and generates its own
2951 randomly-generated challenge string. The client then concatenates
2952 the server's decoded challenge, a ":" character, its own challenge,
2953 another ":" character, and the cookie. It computes the SHA-1 hash
2954 of this composite string as a hex digest. It concatenates the
2955 client's challenge string, a space character, and the SHA-1 hex
2956 digest, hex-encodes the result and sends it back to the server.
2961 The server generates the same concatenated string used by the
2962 client and computes its SHA-1 hash. It compares the hash with
2963 the hash received from the client; if the two hashes match, the
2964 client is authenticated.
2970 Each server has a "cookie context," which is a name that identifies a
2971 set of cookies that apply to that server. A sample context might be
2972 "org_freedesktop_session_bus". Context names must be valid ASCII,
2973 nonzero length, and may not contain the characters slash ("/"),
2974 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2975 tab ("\t"), or period ("."). There is a default context,
2976 "org_freedesktop_general" that's used by servers that do not specify
2980 Cookies are stored in a user's home directory, in the directory
2981 <filename>~/.dbus-keyrings/</filename>. This directory must
2982 not be readable or writable by other users. If it is,
2983 clients and servers must ignore it. The directory
2984 contains cookie files named after the cookie context.
2987 A cookie file contains one cookie per line. Each line
2988 has three space-separated fields:
2992 The cookie ID number, which must be a non-negative integer and
2993 may not be used twice in the same file.
2998 The cookie's creation time, in UNIX seconds-since-the-epoch
3004 The cookie itself, a hex-encoded random block of bytes. The cookie
3005 may be of any length, though obviously security increases
3006 as the length increases.
3012 Only server processes modify the cookie file.
3013 They must do so with this procedure:
3017 Create a lockfile name by appending ".lock" to the name of the
3018 cookie file. The server should attempt to create this file
3019 using <literal>O_CREAT | O_EXCL</literal>. If file creation
3020 fails, the lock fails. Servers should retry for a reasonable
3021 period of time, then they may choose to delete an existing lock
3022 to keep users from having to manually delete a stale
3023 lock. <footnote><para>Lockfiles are used instead of real file
3024 locking <literal>fcntl()</literal> because real locking
3025 implementations are still flaky on network
3026 filesystems.</para></footnote>
3031 Once the lockfile has been created, the server loads the cookie
3032 file. It should then delete any cookies that are old (the
3033 timeout can be fairly short), or more than a reasonable
3034 time in the future (so that cookies never accidentally
3035 become permanent, if the clock was set far into the future
3036 at some point). If no recent keys remain, the
3037 server may generate a new key.
3042 The pruned and possibly added-to cookie file
3043 must be resaved atomically (using a temporary
3044 file which is rename()'d).
3049 The lock must be dropped by deleting the lockfile.
3055 Clients need not lock the file in order to load it,
3056 because servers are required to save the file atomically.
3061 <sect1 id="addresses">
3062 <title>Server Addresses</title>
3064 Server addresses consist of a transport name followed by a colon, and
3065 then an optional, comma-separated list of keys and values in the form key=value.
3066 Each value is escaped.
3070 <programlisting>unix:path=/tmp/dbus-test</programlisting>
3071 Which is the address to a unix socket with the path /tmp/dbus-test.
3074 Value escaping is similar to URI escaping but simpler.
3078 The set of optionally-escaped bytes is:
3079 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
3080 <emphasis>byte</emphasis> (note, not character) which is not in the
3081 set of optionally-escaped bytes must be replaced with an ASCII
3082 percent (<literal>%</literal>) and the value of the byte in hex.
3083 The hex value must always be two digits, even if the first digit is
3084 zero. The optionally-escaped bytes may be escaped if desired.
3089 To unescape, append each byte in the value; if a byte is an ASCII
3090 percent (<literal>%</literal>) character then append the following
3091 hex value instead. It is an error if a <literal>%</literal> byte
3092 does not have two hex digits following. It is an error if a
3093 non-optionally-escaped byte is seen unescaped.
3097 The set of optionally-escaped bytes is intended to preserve address
3098 readability and convenience.
3102 A server may specify a key-value pair with the key <literal>guid</literal>
3103 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
3104 describes the format of the <literal>guid</literal> field. If present,
3105 this UUID may be used to distinguish one server address from another. A
3106 server should use a different UUID for each address it listens on. For
3107 example, if a message bus daemon offers both UNIX domain socket and TCP
3108 connections, but treats clients the same regardless of how they connect,
3109 those two connections are equivalent post-connection but should have
3110 distinct UUIDs to distinguish the kinds of connection.
3114 The intent of the address UUID feature is to allow a client to avoid
3115 opening multiple identical connections to the same server, by allowing the
3116 client to check whether an address corresponds to an already-existing
3117 connection. Comparing two addresses is insufficient, because addresses
3118 can be recycled by distinct servers, and equivalent addresses may look
3119 different if simply compared as strings (for example, the host in a TCP
3120 address can be given as an IP address or as a hostname).
3124 Note that the address key is <literal>guid</literal> even though the
3125 rest of the API and documentation says "UUID," for historical reasons.
3129 [FIXME clarify if attempting to connect to each is a requirement
3130 or just a suggestion]
3131 When connecting to a server, multiple server addresses can be
3132 separated by a semi-colon. The library will then try to connect
3133 to the first address and if that fails, it'll try to connect to
3134 the next one specified, and so forth. For example
3135 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3139 Some addresses are <firstterm>connectable</firstterm>. A connectable
3140 address is one containing enough information for a client to connect
3141 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3142 is a connectable address. It is not necessarily possible to listen
3143 on every connectable address: for instance, it is not possible to
3144 listen on a <literal>unixexec:</literal> address.
3148 Some addresses are <firstterm>listenable</firstterm>. A listenable
3149 address is one containing enough information for a server to listen on
3150 it, producing a connectable address (which may differ from the
3151 original address). Many listenable addresses are not connectable:
3152 for instance, <literal>tcp:host=127.0.0.1</literal>
3153 is listenable, but not connectable (because it does not specify
3158 Listening on an address that is not connectable will result in a
3159 connectable address that is not the same as the listenable address.
3160 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3161 might result in the connectable address
3162 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3163 listening on <literal>unix:tmpdir=/tmp</literal>
3164 might result in the connectable address
3165 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>, or
3166 listening on <literal>unix:runtime=yes</literal>
3167 might result in the connectable address
3168 <literal>unix:path=/run/user/1234/bus</literal>.
3172 <sect1 id="transports">
3173 <title>Transports</title>
3175 [FIXME we need to specify in detail each transport and its possible arguments]
3177 Current transports include: unix domain sockets (including
3178 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3179 using in-process pipes. Future possible transports include one that
3180 tunnels over X11 protocol.
3183 <sect2 id="transports-unix-domain-sockets">
3184 <title>Unix Domain Sockets</title>
3186 Unix domain sockets can be either paths in the file system or on Linux
3187 kernels, they can be abstract which are similar to paths but
3188 do not show up in the file system.
3192 When a socket is opened by the D-Bus library it truncates the path
3193 name right before the first trailing Nul byte. This is true for both
3194 normal paths and abstract paths. Note that this is a departure from
3195 previous versions of D-Bus that would create sockets with a fixed
3196 length path name. Names which were shorter than the fixed length
3197 would be padded by Nul bytes.
3200 Unix domain sockets are not available on Windows.
3203 Unix addresses that specify <literal>path</literal> or
3204 <literal>abstract</literal> are both listenable and connectable.
3205 Unix addresses that specify <literal>tmpdir</literal> are only
3206 listenable: the corresponding connectable address will specify
3207 either <literal>path</literal> or <literal>abstract</literal>.
3208 Similarly, Unix addresses that specify <literal>runtime</literal>
3209 are only listenable, and the corresponding connectable address
3210 will specify <literal>path</literal>.
3212 <sect3 id="transports-unix-domain-sockets-addresses">
3213 <title>Server Address Format</title>
3215 Unix domain socket addresses are identified by the "unix:" prefix
3216 and support the following key/value pairs:
3223 <entry>Values</entry>
3224 <entry>Description</entry>
3230 <entry>(path)</entry>
3231 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3234 <entry>tmpdir</entry>
3235 <entry>(path)</entry>
3236 <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>
3239 <entry>abstract</entry>
3240 <entry>(string)</entry>
3241 <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>
3244 <entry>runtime</entry>
3245 <entry><literal>yes</literal></entry>
3246 <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>
3252 Exactly one of the keys <literal>path</literal>,
3253 <literal>abstract</literal>, <literal>runtime</literal> or
3254 <literal>tmpdir</literal> must be provided.
3258 <sect2 id="transports-launchd">
3259 <title>launchd</title>
3261 launchd is an open-source server management system that replaces init, inetd
3262 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3263 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3267 launchd allocates a socket and provides it with the unix path through the
3268 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3269 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3270 it through its environment.
3271 Other processes can query for the launchd socket by executing:
3272 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3273 This is normally done by the D-Bus client library so doesn't have to be done
3277 launchd is not available on Microsoft Windows.
3280 launchd addresses are listenable and connectable.
3282 <sect3 id="transports-launchd-addresses">
3283 <title>Server Address Format</title>
3285 launchd addresses are identified by the "launchd:" prefix
3286 and support the following key/value pairs:
3293 <entry>Values</entry>
3294 <entry>Description</entry>
3300 <entry>(environment variable)</entry>
3301 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3307 The <literal>env</literal> key is required.
3311 <sect2 id="transports-systemd">
3312 <title>systemd</title>
3314 systemd is an open-source server management system that
3315 replaces init and inetd on newer Linux systems. It supports
3316 socket activation. The D-Bus systemd transport is used to acquire
3317 socket activation file descriptors from systemd and use them
3318 as D-Bus transport when the current process is spawned by
3319 socket activation from it.
3322 The systemd transport accepts only one or more Unix domain or
3323 TCP streams sockets passed in via socket activation.
3326 The systemd transport is not available on non-Linux operating systems.
3329 The systemd transport defines no parameter keys.
3332 systemd addresses are listenable, but not connectable. The
3333 corresponding connectable address is the <literal>unix</literal>
3334 or <literal>tcp</literal> address of the socket.
3337 <sect2 id="transports-tcp-sockets">
3338 <title>TCP Sockets</title>
3340 The tcp transport provides TCP/IP based connections between clients
3341 located on the same or different hosts.
3344 Using tcp transport without any additional secure authentification mechanismus
3345 over a network is unsecure.
3348 On Windows and most Unix platforms, the TCP stack is unable to transfer
3349 credentials over a TCP connection, so the EXTERNAL authentication
3350 mechanism does not work for this transport.
3353 All <literal>tcp</literal> addresses are listenable.
3354 <literal>tcp</literal> addresses in which both
3355 <literal>host</literal> and <literal>port</literal> are
3356 specified, and <literal>port</literal> is non-zero,
3357 are also connectable.
3359 <sect3 id="transports-tcp-sockets-addresses">
3360 <title>Server Address Format</title>
3362 TCP/IP socket addresses are identified by the "tcp:" prefix
3363 and support the following key/value pairs:
3370 <entry>Values</entry>
3371 <entry>Description</entry>
3377 <entry>(string)</entry>
3378 <entry>DNS name or IP address</entry>
3382 <entry>(string)</entry>
3383 <entry>Used in a listenable address to configure the interface
3384 on which the server will listen: either the IP address of one of
3385 the local machine's interfaces (most commonly <literal>127.0.0.1
3386 </literal>), or a DNS name that resolves to one of those IP
3387 addresses, or '*' to listen on all interfaces simultaneously.
3388 If not specified, the default is the same value as "host".
3393 <entry>(number)</entry>
3394 <entry>The tcp port the server will open. A zero value let the server
3395 choose a free port provided from the underlaying operating system.
3396 libdbus is able to retrieve the real used port from the server.
3400 <entry>family</entry>
3401 <entry>(string)</entry>
3402 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3409 <sect2 id="transports-nonce-tcp-sockets">
3410 <title>Nonce-secured TCP Sockets</title>
3412 The nonce-tcp transport provides a secured TCP transport, using a
3413 simple authentication mechanism to ensure that only clients with read
3414 access to a certain location in the filesystem can connect to the server.
3415 The server writes a secret, the nonce, to a file and an incoming client
3416 connection is only accepted if the client sends the nonce right after
3417 the connect. The nonce mechanism requires no setup and is orthogonal to
3418 the higher-level authentication mechanisms described in the
3419 Authentication section.
3423 On start, the server generates a random 16 byte nonce and writes it
3424 to a file in the user's temporary directory. The nonce file location
3425 is published as part of the server's D-Bus address using the
3426 "noncefile" key-value pair.
3428 After an accept, the server reads 16 bytes from the socket. If the
3429 read bytes do not match the nonce stored in the nonce file, the
3430 server MUST immediately drop the connection.
3431 If the nonce match the received byte sequence, the client is accepted
3432 and the transport behaves like an unsecured tcp transport.
3435 After a successful connect to the server socket, the client MUST read
3436 the nonce from the file published by the server via the noncefile=
3437 key-value pair and send it over the socket. After that, the
3438 transport behaves like an unsecured tcp transport.
3441 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3442 <literal>host</literal>, <literal>port</literal> and
3443 <literal>noncefile</literal> are all specified,
3444 and <literal>port</literal> is nonzero, are also connectable.
3446 <sect3 id="transports-nonce-tcp-sockets-addresses">
3447 <title>Server Address Format</title>
3449 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3450 and support the following key/value pairs:
3457 <entry>Values</entry>
3458 <entry>Description</entry>
3464 <entry>(string)</entry>
3465 <entry>DNS name or IP address</entry>
3469 <entry>(string)</entry>
3470 <entry>The same as for tcp: addresses
3475 <entry>(number)</entry>
3476 <entry>The tcp port the server will open. A zero value let the server
3477 choose a free port provided from the underlaying operating system.
3478 libdbus is able to retrieve the real used port from the server.
3482 <entry>family</entry>
3483 <entry>(string)</entry>
3484 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3487 <entry>noncefile</entry>
3488 <entry>(path)</entry>
3489 <entry>File location containing the secret.
3490 This is only meaningful in connectable addresses:
3491 a listening D-Bus server that offers this transport
3492 will always create a new nonce file.</entry>
3499 <sect2 id="transports-exec">
3500 <title>Executed Subprocesses on Unix</title>
3502 This transport forks off a process and connects its standard
3503 input and standard output with an anonymous Unix domain
3504 socket. This socket is then used for communication by the
3505 transport. This transport may be used to use out-of-process
3506 forwarder programs as basis for the D-Bus protocol.
3509 The forked process will inherit the standard error output and
3510 process group from the parent process.
3513 Executed subprocesses are not available on Windows.
3516 <literal>unixexec</literal> addresses are connectable, but are not
3519 <sect3 id="transports-exec-addresses">
3520 <title>Server Address Format</title>
3522 Executed subprocess addresses are identified by the "unixexec:" prefix
3523 and support the following key/value pairs:
3530 <entry>Values</entry>
3531 <entry>Description</entry>
3537 <entry>(path)</entry>
3538 <entry>Path of the binary to execute, either an absolute
3539 path or a binary name that is searched for in the default
3540 search path of the OS. This corresponds to the first
3541 argument of execlp(). This key is mandatory.</entry>
3544 <entry>argv0</entry>
3545 <entry>(string)</entry>
3546 <entry>The program name to use when executing the
3547 binary. If omitted the same value as specified for path=
3548 will be used. This corresponds to the second argument of
3552 <entry>argv1, argv2, ...</entry>
3553 <entry>(string)</entry>
3554 <entry>Arguments to pass to the binary. This corresponds
3555 to the third and later arguments of execlp(). If a
3556 specific argvX is not specified no further argvY for Y > X
3557 are taken into account.</entry>
3565 <sect1 id="meta-transports">
3566 <title>Meta Transports</title>
3568 Meta transports are a kind of transport with special enhancements or
3569 behavior. Currently available meta transports include: autolaunch
3572 <sect2 id="meta-transports-autolaunch">
3573 <title>Autolaunch</title>
3574 <para>The autolaunch transport provides a way for dbus clients to autodetect
3575 a running dbus session bus and to autolaunch a session bus if not present.
3578 On Unix, <literal>autolaunch</literal> addresses are connectable,
3582 On Windows, <literal>autolaunch</literal> addresses are both
3583 connectable and listenable.
3586 <sect3 id="meta-transports-autolaunch-addresses">
3587 <title>Server Address Format</title>
3589 Autolaunch addresses uses the "autolaunch:" prefix and support the
3590 following key/value pairs:
3597 <entry>Values</entry>
3598 <entry>Description</entry>
3603 <entry>scope</entry>
3604 <entry>(string)</entry>
3605 <entry>scope of autolaunch (Windows only)
3609 "*install-path" - limit session bus to dbus installation path.
3610 The dbus installation path is determined from the location of
3611 the shared dbus library. If the library is located in a 'bin'
3612 subdirectory the installation root is the directory above,
3613 otherwise the directory where the library lives is taken as
3616 <install-root>/bin/[lib]dbus-1.dll
3617 <install-root>/[lib]dbus-1.dll
3623 "*user" - limit session bus to the recent user.
3628 other values - specify dedicated session bus like "release",
3640 <sect3 id="meta-transports-autolaunch-windows-implementation">
3641 <title>Windows implementation</title>
3643 On start, the server opens a platform specific transport, creates a mutex
3644 and a shared memory section containing the related session bus address.
3645 This mutex will be inspected by the dbus client library to detect a
3646 running dbus session bus. The access to the mutex and the shared memory
3647 section are protected by global locks.
3650 In the recent implementation the autolaunch transport uses a tcp transport
3651 on localhost with a port choosen from the operating system. This detail may
3652 change in the future.
3655 Disclaimer: The recent implementation is in an early state and may not
3656 work in all cirumstances and/or may have security issues. Because of this
3657 the implementation is not documentated yet.
3664 <title>UUIDs</title>
3666 A working D-Bus implementation uses universally-unique IDs in two places.
3667 First, each server address has a UUID identifying the address,
3668 as described in <xref linkend="addresses"/>. Second, each operating
3669 system kernel instance running a D-Bus client or server has a UUID
3670 identifying that kernel, retrieved by invoking the method
3671 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3672 linkend="standard-interfaces-peer"/>).
3675 The term "UUID" in this document is intended literally, i.e. an
3676 identifier that is universally unique. It is not intended to refer to
3677 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3680 The UUID must contain 128 bits of data and be hex-encoded. The
3681 hex-encoded string may not contain hyphens or other non-hex-digit
3682 characters, and it must be exactly 32 characters long. To generate a
3683 UUID, the current reference implementation concatenates 96 bits of random
3684 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3688 It would also be acceptable and probably better to simply generate 128
3689 bits of random data, as long as the random number generator is of high
3690 quality. The timestamp could conceivably help if the random bits are not
3691 very random. With a quality random number generator, collisions are
3692 extremely unlikely even with only 96 bits, so it's somewhat academic.
3695 Implementations should, however, stick to random data for the first 96 bits
3700 <sect1 id="standard-interfaces">
3701 <title>Standard Interfaces</title>
3703 See <xref linkend="message-protocol-types-notation"/> for details on
3704 the notation used in this section. There are some standard interfaces
3705 that may be useful across various D-Bus applications.
3707 <sect2 id="standard-interfaces-peer">
3708 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3710 The <literal>org.freedesktop.DBus.Peer</literal> interface
3713 org.freedesktop.DBus.Peer.Ping ()
3714 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3718 On receipt of the <literal>METHOD_CALL</literal> message
3719 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3720 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3721 usual. It does not matter which object path a ping is sent to. The
3722 reference implementation handles this method automatically.
3725 On receipt of the <literal>METHOD_CALL</literal> message
3726 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3727 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3728 UUID representing the identity of the machine the process is running on.
3729 This UUID must be the same for all processes on a single system at least
3730 until that system next reboots. It should be the same across reboots
3731 if possible, but this is not always possible to implement and is not
3733 It does not matter which object path a GetMachineId is sent to. The
3734 reference implementation handles this method automatically.
3737 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3738 a virtual machine running on a hypervisor, rather than a physical machine.
3739 Basically if two processes see the same UUID, they should also see the same
3740 shared memory, UNIX domain sockets, process IDs, and other features that require
3741 a running OS kernel in common between the processes.
3744 The UUID is often used where other programs might use a hostname. Hostnames
3745 can change without rebooting, however, or just be "localhost" - so the UUID
3749 <xref linkend="uuids"/> explains the format of the UUID.
3753 <sect2 id="standard-interfaces-introspectable">
3754 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3756 This interface has one method:
3758 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3762 Objects instances may implement
3763 <literal>Introspect</literal> which returns an XML description of
3764 the object, including its interfaces (with signals and methods), objects
3765 below it in the object path tree, and its properties.
3768 <xref linkend="introspection-format"/> describes the format of this XML string.
3771 <sect2 id="standard-interfaces-properties">
3772 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3774 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3775 or <firstterm>attributes</firstterm>. These can be exposed via the
3776 <literal>org.freedesktop.DBus.Properties</literal> interface.
3780 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3781 in STRING property_name,
3783 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3784 in STRING property_name,
3786 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3787 out DICT<STRING,VARIANT> props);
3791 It is conventional to give D-Bus properties names consisting of
3792 capitalized words without punctuation ("CamelCase"), like
3793 <link linkend="message-protocol-names-member">member names</link>.
3794 For instance, the GObject property
3795 <literal>connection-status</literal> or the Qt property
3796 <literal>connectionStatus</literal> could be represented on D-Bus
3797 as <literal>ConnectionStatus</literal>.
3800 Strictly speaking, D-Bus property names are not required to follow
3801 the same naming restrictions as member names, but D-Bus property
3802 names that would not be valid member names (in particular,
3803 GObject-style dash-separated property names) can cause interoperability
3804 problems and should be avoided.
3807 The available properties and whether they are writable can be determined
3808 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3809 see <xref linkend="standard-interfaces-introspectable"/>.
3812 An empty string may be provided for the interface name; in this case,
3813 if there are multiple properties on an object with the same name,
3814 the results are undefined (picking one by according to an arbitrary
3815 deterministic rule, or returning an error, are the reasonable
3819 If one or more properties change on an object, the
3820 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3821 signal may be emitted (this signal was added in 0.14):
3825 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3826 DICT<STRING,VARIANT> changed_properties,
3827 ARRAY<STRING> invalidated_properties);
3831 where <literal>changed_properties</literal> is a dictionary
3832 containing the changed properties with the new values and
3833 <literal>invalidated_properties</literal> is an array of
3834 properties that changed but the value is not conveyed.
3837 Whether the <literal>PropertiesChanged</literal> signal is
3838 supported can be determined by calling
3839 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3840 that the signal may be supported for an object but it may
3841 differ how whether and how it is used on a per-property basis
3842 (for e.g. performance or security reasons). Each property (or
3843 the parent interface) must be annotated with the
3844 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3845 annotation to convey this (usually the default value
3846 <literal>true</literal> is sufficient meaning that the
3847 annotation does not need to be used). See <xref
3848 linkend="introspection-format"/> for details on this
3853 <sect2 id="standard-interfaces-objectmanager">
3854 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3856 An API can optionally make use of this interface for one or
3857 more sub-trees of objects. The root of each sub-tree implements
3858 this interface so other applications can get all objects,
3859 interfaces and properties in a single method call. It is
3860 appropriate to use this interface if users of the tree of
3861 objects are expected to be interested in all interfaces of all
3862 objects in the tree; a more granular API should be used if
3863 users of the objects are expected to be interested in a small
3864 subset of the objects, a small subset of their interfaces, or
3868 The method that applications can use to get all objects and
3869 properties is <literal>GetManagedObjects</literal>:
3873 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3877 The return value of this method is a dict whose keys are
3878 object paths. All returned object paths are children of the
3879 object path implementing this interface, i.e. their object
3880 paths start with the ObjectManager's object path plus '/'.
3883 Each value is a dict whose keys are interfaces names. Each
3884 value in this inner dict is the same dict that would be
3885 returned by the <link
3886 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3887 method for that combination of object path and interface. If
3888 an interface has no properties, the empty dict is returned.
3891 Changes are emitted using the following two signals:
3895 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3896 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3897 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3898 ARRAY<STRING> interfaces);
3902 The <literal>InterfacesAdded</literal> signal is emitted when
3903 either a new object is added or when an existing object gains
3904 one or more interfaces. The
3905 <literal>InterfacesRemoved</literal> signal is emitted
3906 whenever an object is removed or it loses one or more
3907 interfaces. The second parameter of the
3908 <literal>InterfacesAdded</literal> signal contains a dict with
3909 the interfaces and properties (if any) that have been added to
3910 the given object path. Similarly, the second parameter of the
3911 <literal>InterfacesRemoved</literal> signal contains an array
3912 of the interfaces that were removed. Note that changes on
3913 properties on existing interfaces are not reported using this
3914 interface - an application should also monitor the existing <link
3915 linkend="standard-interfaces-properties">PropertiesChanged</link>
3916 signal on each object.
3919 Applications SHOULD NOT export objects that are children of an
3920 object (directly or otherwise) implementing this interface but
3921 which are not returned in the reply from the
3922 <literal>GetManagedObjects()</literal> method of this
3923 interface on the given object.
3926 The intent of the <literal>ObjectManager</literal> interface
3927 is to make it easy to write a robust client
3928 implementation. The trivial client implementation only needs
3929 to make two method calls:
3933 org.freedesktop.DBus.AddMatch (bus_proxy,
3934 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3935 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3939 on the message bus and the remote application's
3940 <literal>ObjectManager</literal>, respectively. Whenever a new
3941 remote object is created (or an existing object gains a new
3942 interface), the <literal>InterfacesAdded</literal> signal is
3943 emitted, and since this signal contains all properties for the
3944 interfaces, no calls to the
3945 <literal>org.freedesktop.Properties</literal> interface on the
3946 remote object are needed. Additionally, since the initial
3947 <literal>AddMatch()</literal> rule already includes signal
3948 messages from the newly created child object, no new
3949 <literal>AddMatch()</literal> call is needed.
3954 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3955 interface was added in version 0.17 of the D-Bus
3962 <sect1 id="introspection-format">
3963 <title>Introspection Data Format</title>
3965 As described in <xref linkend="standard-interfaces-introspectable"/>,
3966 objects may be introspected at runtime, returning an XML string
3967 that describes the object. The same XML format may be used in
3968 other contexts as well, for example as an "IDL" for generating
3969 static language bindings.
3972 Here is an example of introspection data:
3974 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3975 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3976 <node name="/com/example/sample_object">
3977 <interface name="com.example.SampleInterface">
3978 <method name="Frobate">
3979 <arg name="foo" type="i" direction="in"/>
3980 <arg name="bar" type="s" direction="out"/>
3981 <arg name="baz" type="a{us}" direction="out"/>
3982 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3984 <method name="Bazify">
3985 <arg name="bar" type="(iiu)" direction="in"/>
3986 <arg name="bar" type="v" direction="out"/>
3988 <method name="Mogrify">
3989 <arg name="bar" type="(iiav)" direction="in"/>
3991 <signal name="Changed">
3992 <arg name="new_value" type="b"/>
3994 <property name="Bar" type="y" access="readwrite"/>
3996 <node name="child_of_sample_object"/>
3997 <node name="another_child_of_sample_object"/>
4002 A more formal DTD and spec needs writing, but here are some quick notes.
4006 Only the root <node> element can omit the node name, as it's
4007 known to be the object that was introspected. If the root
4008 <node> does have a name attribute, it must be an absolute
4009 object path. If child <node> have object paths, they must be
4015 If a child <node> has any sub-elements, then they
4016 must represent a complete introspection of the child.
4017 If a child <node> is empty, then it may or may
4018 not have sub-elements; the child must be introspected
4019 in order to find out. The intent is that if an object
4020 knows that its children are "fast" to introspect
4021 it can go ahead and return their information, but
4022 otherwise it can omit it.
4027 The direction element on <arg> may be omitted,
4028 in which case it defaults to "in" for method calls
4029 and "out" for signals. Signals only allow "out"
4030 so while direction may be specified, it's pointless.
4035 The possible directions are "in" and "out",
4036 unlike CORBA there is no "inout"
4041 The possible property access flags are
4042 "readwrite", "read", and "write"
4047 Multiple interfaces can of course be listed for
4053 The "name" attribute on arguments is optional.
4059 Method, interface, property, signal, and argument elements may have
4060 "annotations", which are generic key/value pairs of metadata.
4061 They are similar conceptually to Java's annotations and C# attributes.
4062 Well-known annotations:
4069 <entry>Values (separated by ,)</entry>
4070 <entry>Description</entry>
4075 <entry>org.freedesktop.DBus.Deprecated</entry>
4076 <entry>true,false</entry>
4077 <entry>Whether or not the entity is deprecated; defaults to false</entry>
4080 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
4081 <entry>(string)</entry>
4082 <entry>The C symbol; may be used for methods and interfaces</entry>
4085 <entry>org.freedesktop.DBus.Method.NoReply</entry>
4086 <entry>true,false</entry>
4087 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
4090 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
4091 <entry>true,invalidates,const,false</entry>
4094 If set to <literal>false</literal>, the
4095 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
4097 linkend="standard-interfaces-properties"/> is not
4098 guaranteed to be emitted if the property changes.
4101 If set to <literal>const</literal> the property never
4102 changes value during the lifetime of the object it
4103 belongs to, and hence the signal is never emitted for
4107 If set to <literal>invalidates</literal> the signal
4108 is emitted but the value is not included in the
4112 If set to <literal>true</literal> the signal is
4113 emitted with the value included.
4116 The value for the annotation defaults to
4117 <literal>true</literal> if the enclosing interface
4118 element does not specify the annotation. Otherwise it
4119 defaults to the value specified in the enclosing
4123 This annotation is intended to be used by code
4124 generators to implement client-side caching of
4125 property values. For all properties for which the
4126 annotation is set to <literal>const</literal>,
4127 <literal>invalidates</literal> or
4128 <literal>true</literal> the client may
4129 unconditionally cache the values as the properties
4130 don't change or notifications are generated for them
4139 <sect1 id="message-bus">
4140 <title>Message Bus Specification</title>
4141 <sect2 id="message-bus-overview">
4142 <title>Message Bus Overview</title>
4144 The message bus accepts connections from one or more applications.
4145 Once connected, applications can exchange messages with other
4146 applications that are also connected to the bus.
4149 In order to route messages among connections, the message bus keeps a
4150 mapping from names to connections. Each connection has one
4151 unique-for-the-lifetime-of-the-bus name automatically assigned.
4152 Applications may request additional names for a connection. Additional
4153 names are usually "well-known names" such as
4154 "com.example.TextEditor". When a name is bound to a connection,
4155 that connection is said to <firstterm>own</firstterm> the name.
4158 The bus itself owns a special name,
4159 <literal>org.freedesktop.DBus</literal>, with an object
4160 located at <literal>/org/freedesktop/DBus</literal> that
4161 implements the <literal>org.freedesktop.DBus</literal>
4162 interface. This service allows applications to make
4163 administrative requests of the bus itself. For example,
4164 applications can ask the bus to assign a name to a connection.
4167 Each name may have <firstterm>queued owners</firstterm>. When an
4168 application requests a name for a connection and the name is already in
4169 use, the bus will optionally add the connection to a queue waiting for
4170 the name. If the current owner of the name disconnects or releases
4171 the name, the next connection in the queue will become the new owner.
4175 This feature causes the right thing to happen if you start two text
4176 editors for example; the first one may request "com.example.TextEditor",
4177 and the second will be queued as a possible owner of that name. When
4178 the first exits, the second will take over.
4182 Applications may send <firstterm>unicast messages</firstterm> to
4183 a specific recipient or to the message bus itself, or
4184 <firstterm>broadcast messages</firstterm> to all interested recipients.
4185 See <xref linkend="message-bus-routing"/> for details.
4189 <sect2 id="message-bus-names">
4190 <title>Message Bus Names</title>
4192 Each connection has at least one name, assigned at connection time and
4193 returned in response to the
4194 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4195 automatically-assigned name is called the connection's <firstterm>unique
4196 name</firstterm>. Unique names are never reused for two different
4197 connections to the same bus.
4200 Ownership of a unique name is a prerequisite for interaction with
4201 the message bus. It logically follows that the unique name is always
4202 the first name that an application comes to own, and the last
4203 one that it loses ownership of.
4206 Unique connection names must begin with the character ':' (ASCII colon
4207 character); bus names that are not unique names must not begin
4208 with this character. (The bus must reject any attempt by an application
4209 to manually request a name beginning with ':'.) This restriction
4210 categorically prevents "spoofing"; messages sent to a unique name
4211 will always go to the expected connection.
4214 When a connection is closed, all the names that it owns are deleted (or
4215 transferred to the next connection in the queue if any).
4218 A connection can request additional names to be associated with it using
4219 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4220 linkend="message-protocol-names-bus"/> describes the format of a valid
4221 name. These names can be released again using the
4222 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4225 <sect3 id="bus-messages-request-name">
4226 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4230 UINT32 RequestName (in STRING name, in UINT32 flags)
4237 <entry>Argument</entry>
4239 <entry>Description</entry>
4245 <entry>STRING</entry>
4246 <entry>Name to request</entry>
4250 <entry>UINT32</entry>
4251 <entry>Flags</entry>
4261 <entry>Argument</entry>
4263 <entry>Description</entry>
4269 <entry>UINT32</entry>
4270 <entry>Return value</entry>
4277 This method call should be sent to
4278 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4279 assign the given name to the method caller. Each name maintains a
4280 queue of possible owners, where the head of the queue is the primary
4281 or current owner of the name. Each potential owner in the queue
4282 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4283 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4284 call. When RequestName is invoked the following occurs:
4288 If the method caller is currently the primary owner of the name,
4289 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4290 values are updated with the values from the new RequestName call,
4291 and nothing further happens.
4297 If the current primary owner (head of the queue) has
4298 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4299 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4300 the caller of RequestName replaces the current primary owner at
4301 the head of the queue and the current primary owner moves to the
4302 second position in the queue. If the caller of RequestName was
4303 in the queue previously its flags are updated with the values from
4304 the new RequestName in addition to moving it to the head of the queue.
4310 If replacement is not possible, and the method caller is
4311 currently in the queue but not the primary owner, its flags are
4312 updated with the values from the new RequestName call.
4318 If replacement is not possible, and the method caller is
4319 currently not in the queue, the method caller is appended to the
4326 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4327 set and is not the primary owner, it is removed from the
4328 queue. This can apply to the previous primary owner (if it
4329 was replaced) or the method caller (if it updated the
4330 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4331 queue, or if it was just added to the queue with that flag set).
4337 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4338 queue," even if another application already in the queue had specified
4339 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4340 that does not allow replacement goes away, and the next primary owner
4341 does allow replacement. In this case, queued items that specified
4342 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4343 automatically replace the new primary owner. In other words,
4344 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4345 time RequestName is called. This is deliberate to avoid an infinite loop
4346 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4347 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4350 The flags argument contains any of the following values logically ORed
4357 <entry>Conventional Name</entry>
4358 <entry>Value</entry>
4359 <entry>Description</entry>
4364 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4368 If an application A specifies this flag and succeeds in
4369 becoming the owner of the name, and another application B
4370 later calls RequestName with the
4371 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4372 will lose ownership and receive a
4373 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4374 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4375 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4376 is not specified by application B, then application B will not replace
4377 application A as the owner.
4382 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4386 Try to replace the current owner if there is one. If this
4387 flag is not set the application will only become the owner of
4388 the name if there is no current owner. If this flag is set,
4389 the application will replace the current owner if
4390 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4395 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4399 Without this flag, if an application requests a name that is
4400 already owned, the application will be placed in a queue to
4401 own the name when the current owner gives it up. If this
4402 flag is given, the application will not be placed in the
4403 queue, the request for the name will simply fail. This flag
4404 also affects behavior when an application is replaced as
4405 name owner; by default the application moves back into the
4406 waiting queue, unless this flag was provided when the application
4407 became the name owner.
4415 The return code can be one of the following values:
4421 <entry>Conventional Name</entry>
4422 <entry>Value</entry>
4423 <entry>Description</entry>
4428 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4429 <entry>1</entry> <entry>The caller is now the primary owner of
4430 the name, replacing any previous owner. Either the name had no
4431 owner before, or the caller specified
4432 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4433 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4436 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4439 <entry>The name already had an owner,
4440 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4441 the current owner did not specify
4442 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4443 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4447 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4448 <entry>The name already has an owner,
4449 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4450 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4451 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4452 specified by the requesting application.</entry>
4455 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4457 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4465 <sect3 id="bus-messages-release-name">
4466 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4470 UINT32 ReleaseName (in STRING name)
4477 <entry>Argument</entry>
4479 <entry>Description</entry>
4485 <entry>STRING</entry>
4486 <entry>Name to release</entry>
4496 <entry>Argument</entry>
4498 <entry>Description</entry>
4504 <entry>UINT32</entry>
4505 <entry>Return value</entry>
4512 This method call should be sent to
4513 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4514 release the method caller's claim to the given name. If the caller is
4515 the primary owner, a new primary owner will be selected from the
4516 queue if any other owners are waiting. If the caller is waiting in
4517 the queue for the name, the caller will removed from the queue and
4518 will not be made an owner of the name if it later becomes available.
4519 If there are no other owners in the queue for the name, it will be
4520 removed from the bus entirely.
4522 The return code can be one of the following values:
4528 <entry>Conventional Name</entry>
4529 <entry>Value</entry>
4530 <entry>Description</entry>
4535 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4536 <entry>1</entry> <entry>The caller has released his claim on
4537 the given name. Either the caller was the primary owner of
4538 the name, and the name is now unused or taken by somebody
4539 waiting in the queue for the name, or the caller was waiting
4540 in the queue for the name and has now been removed from the
4544 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4546 <entry>The given name does not exist on this bus.</entry>
4549 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4551 <entry>The caller was not the primary owner of this name,
4552 and was also not waiting in the queue to own this name.</entry>
4560 <sect3 id="bus-messages-list-queued-owners">
4561 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4565 ARRAY of STRING ListQueuedOwners (in STRING name)
4572 <entry>Argument</entry>
4574 <entry>Description</entry>
4580 <entry>STRING</entry>
4581 <entry>The well-known bus name to query, such as
4582 <literal>com.example.cappuccino</literal></entry>
4592 <entry>Argument</entry>
4594 <entry>Description</entry>
4600 <entry>ARRAY of STRING</entry>
4601 <entry>The unique bus names of connections currently queued
4602 for the name</entry>
4609 This method call should be sent to
4610 <literal>org.freedesktop.DBus</literal> and lists the connections
4611 currently queued for a bus name (see
4612 <xref linkend="term-queued-owner"/>).
4617 <sect2 id="message-bus-routing">
4618 <title>Message Bus Message Routing</title>
4621 Messages may have a <literal>DESTINATION</literal> field (see <xref
4622 linkend="message-protocol-header-fields"/>), resulting in a
4623 <firstterm>unicast message</firstterm>. If the
4624 <literal>DESTINATION</literal> field is present, it specifies a message
4625 recipient by name. Method calls and replies normally specify this field.
4626 The message bus must send messages (of any type) with the
4627 <literal>DESTINATION</literal> field set to the specified recipient,
4628 regardless of whether the recipient has set up a match rule matching
4633 When the message bus receives a signal, if the
4634 <literal>DESTINATION</literal> field is absent, it is considered to
4635 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4636 applications with <firstterm>message matching rules</firstterm> that
4637 match the message. Most signal messages are broadcasts, and
4638 no other message types currently defined in this specification
4643 Unicast signal messages (those with a <literal>DESTINATION</literal>
4644 field) are not commonly used, but they are treated like any unicast
4645 message: they are delivered to the specified receipient,
4646 regardless of its match rules. One use for unicast signals is to
4647 avoid a race condition in which a signal is emitted before the intended
4648 recipient can call <xref linkend="bus-messages-add-match"/> to
4649 receive that signal: if the signal is sent directly to that recipient
4650 using a unicast message, it does not need to add a match rule at all,
4651 and there is no race condition. Another use for unicast signals,
4652 on message buses whose security policy prevents eavesdropping, is to
4653 send sensitive information which should only be visible to one
4658 When the message bus receives a method call, if the
4659 <literal>DESTINATION</literal> field is absent, the call is taken to be
4660 a standard one-to-one message and interpreted by the message bus
4661 itself. For example, sending an
4662 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4663 <literal>DESTINATION</literal> will cause the message bus itself to
4664 reply to the ping immediately; the message bus will not make this
4665 message visible to other applications.
4669 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4670 the ping message were sent with a <literal>DESTINATION</literal> name of
4671 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4672 forwarded, and the Yoyodyne Corporation screensaver application would be
4673 expected to reply to the ping.
4677 Message bus implementations may impose a security policy which
4678 prevents certain messages from being sent or received.
4679 When a method call message cannot be sent or received due to a security
4680 policy, the message bus should send an error reply, unless the
4681 original message had the <literal>NO_REPLY</literal> flag.
4684 <sect3 id="message-bus-routing-eavesdropping">
4685 <title>Eavesdropping</title>
4687 Receiving a unicast message whose <literal>DESTINATION</literal>
4688 indicates a different recipient is called
4689 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4690 a security boundary (like the standard system bus), the security
4691 policy should usually prevent eavesdropping, since unicast messages
4692 are normally kept private and may contain security-sensitive
4697 Eavesdropping is mainly useful for debugging tools, such as
4698 the <literal>dbus-monitor</literal> tool in the reference
4699 implementation of D-Bus. Tools which eavesdrop on the message bus
4700 should be careful to avoid sending a reply or error in response to
4701 messages intended for a different client.
4705 Clients may attempt to eavesdrop by adding match rules
4706 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4707 the <literal>eavesdrop='true'</literal> match. If the message bus'
4708 security policy does not allow eavesdropping, the match rule can
4709 still be added, but will not have any practical effect. For
4710 compatibility with older message bus implementations, if adding such
4711 a match rule results in an error reply, the client may fall back to
4712 adding the same rule with the <literal>eavesdrop</literal> match
4717 Eavesdropping interacts poorly with buses with non-trivial
4718 access control restrictions. The
4719 <xref linkend="bus-messages-become-monitor"/> method provides
4720 an alternative way to monitor buses.
4724 <sect3 id="message-bus-routing-match-rules">
4725 <title>Match Rules</title>
4727 An important part of the message bus routing protocol is match
4728 rules. Match rules describe the messages that should be sent to a
4729 client, based on the contents of the message. Broadcast signals
4730 are only sent to clients which have a suitable match rule: this
4731 avoids waking up client processes to deal with signals that are
4732 not relevant to that client.
4735 Messages that list a client as their <literal>DESTINATION</literal>
4736 do not need to match the client's match rules, and are sent to that
4737 client regardless. As a result, match rules are mainly used to
4738 receive a subset of broadcast signals.
4741 Match rules can also be used for eavesdropping
4742 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4743 if the security policy of the message bus allows it.
4746 Match rules are added using the AddMatch bus method
4747 (see <xref linkend="bus-messages-add-match"/>). Rules are
4748 specified as a string of comma separated key/value pairs.
4749 Excluding a key from the rule indicates a wildcard match.
4750 For instance excluding the the member from a match rule but
4751 adding a sender would let all messages from that sender through.
4752 An example of a complete rule would be
4753 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4756 Within single quotes (ASCII apostrophe, U+0027), a backslash
4757 (U+005C) represents itself, and an apostrophe ends the quoted
4758 section. Outside single quotes, \' (backslash, apostrophe)
4759 represents an apostrophe, and any backslash not followed by
4760 an apostrophe represents itself. For instance, the match rules
4761 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4762 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4763 both match messages where the arguments are a 1-character string
4764 containing an apostrophe, a 1-character string containing a
4765 backslash, a 1-character string containing a comma, and a
4766 2-character string containing two backslashes<footnote>
4768 This idiosyncratic quoting style is based on the rules for
4769 escaping items to appear inside single-quoted strings
4770 in POSIX <literal>/bin/sh</literal>, but please
4771 note that backslashes that are not inside single quotes have
4772 different behaviour. This syntax does not offer any way to
4773 represent an apostrophe inside single quotes (it is necessary
4774 to leave the single-quoted section, backslash-escape the
4775 apostrophe and re-enter single quotes), or to represent a
4776 comma outside single quotes (it is necessary to wrap it in
4777 a single-quoted section).
4782 The following table describes the keys that can be used to create
4789 <entry>Possible Values</entry>
4790 <entry>Description</entry>
4795 <entry><literal>type</literal></entry>
4796 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4797 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4800 <entry><literal>sender</literal></entry>
4801 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4802 and <xref linkend="term-unique-name"/> respectively)
4804 <entry>Match messages sent by a particular sender. An example of a sender match
4805 is sender='org.freedesktop.Hal'</entry>
4808 <entry><literal>interface</literal></entry>
4809 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4810 <entry>Match messages sent over or to a particular interface. An example of an
4811 interface match is interface='org.freedesktop.Hal.Manager'.
4812 If a message omits the interface header, it must not match any rule
4813 that specifies this key.</entry>
4816 <entry><literal>member</literal></entry>
4817 <entry>Any valid method or signal name</entry>
4818 <entry>Matches messages which have the give method or signal name. An example of
4819 a member match is member='NameOwnerChanged'</entry>
4822 <entry><literal>path</literal></entry>
4823 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4824 <entry>Matches messages which are sent from or to the given object. An example of a
4825 path match is path='/org/freedesktop/Hal/Manager'</entry>
4828 <entry><literal>path_namespace</literal></entry>
4829 <entry>An object path</entry>
4832 Matches messages which are sent from or to an
4833 object for which the object path is either the
4834 given value, or that value followed by one or
4835 more path components.
4840 <literal>path_namespace='/com/example/foo'</literal>
4841 would match signals sent by
4842 <literal>/com/example/foo</literal>
4844 <literal>/com/example/foo/bar</literal>,
4846 <literal>/com/example/foobar</literal>.
4850 Using both <literal>path</literal> and
4851 <literal>path_namespace</literal> in the same match
4852 rule is not allowed.
4857 This match key was added in version 0.16 of the
4858 D-Bus specification and implemented by the bus
4859 daemon in dbus 1.5.0 and later.
4865 <entry><literal>destination</literal></entry>
4866 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4867 <entry>Matches messages which are being sent to the given unique name. An
4868 example of a destination match is destination=':1.0'</entry>
4871 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4872 <entry>Any string</entry>
4873 <entry>Arg matches are special and are used for further restricting the
4874 match based on the arguments in the body of a message. Only arguments of type
4875 STRING can be matched in this way. An example of an argument match
4876 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4880 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4881 <entry>Any string</entry>
4883 <para>Argument path matches provide a specialised form of wildcard matching for
4884 path-like namespaces. They can match arguments whose type is either STRING or
4885 OBJECT_PATH. As with normal argument matches,
4886 if the argument is exactly equal to the string given in the match
4887 rule then the rule is satisfied. Additionally, there is also a
4888 match when either the string given in the match rule or the
4889 appropriate message argument ends with '/' and is a prefix of the
4890 other. An example argument path match is arg0path='/aa/bb/'. This
4891 would match messages with first arguments of '/', '/aa/',
4892 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4893 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4895 <para>This is intended for monitoring “directories” in file system-like
4896 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4897 system. An application interested in all nodes in a particular hierarchy would
4898 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4899 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4900 represent a modification to the “bar” property, or a signal with zeroth
4901 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4902 many properties within that directory, and the interested application would be
4903 notified in both cases.</para>
4906 This match key was added in version 0.12 of the
4907 D-Bus specification, implemented for STRING
4908 arguments by the bus daemon in dbus 1.2.0 and later,
4909 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4916 <entry><literal>arg0namespace</literal></entry>
4917 <entry>Like a bus name, except that the string is not
4918 required to contain a '.' (period)</entry>
4920 <para>Match messages whose first argument is of type STRING, and is a bus name
4921 or interface name within the specified namespace. This is primarily intended
4922 for watching name owner changes for a group of related bus names, rather than
4923 for a single name or all name changes.</para>
4925 <para>Because every valid interface name is also a valid
4926 bus name, this can also be used for messages whose
4927 first argument is an interface name.</para>
4929 <para>For example, the match rule
4930 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4931 matches name owner changes for bus names such as
4932 <literal>com.example.backend.foo</literal>,
4933 <literal>com.example.backend.foo.bar</literal>, and
4934 <literal>com.example.backend</literal> itself.</para>
4936 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4939 This match key was added in version 0.16 of the
4940 D-Bus specification and implemented by the bus
4941 daemon in dbus 1.5.0 and later.
4947 <entry><literal>eavesdrop</literal></entry>
4948 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4949 <entry>Since D-Bus 1.5.6, match rules do not
4950 match messages which have a <literal>DESTINATION</literal>
4951 field unless the match rule specifically
4953 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4954 by specifying <literal>eavesdrop='true'</literal>
4955 in the match rule. <literal>eavesdrop='false'</literal>
4956 restores the default behaviour. Messages are
4957 delivered to their <literal>DESTINATION</literal>
4958 regardless of match rules, so this match does not
4959 affect normal delivery of unicast messages.
4960 If the message bus has a security policy which forbids
4961 eavesdropping, this match may still be used without error,
4962 but will not have any practical effect.
4963 In older versions of D-Bus, this match was not allowed
4964 in match rules, and all match rules behaved as if
4965 <literal>eavesdrop='true'</literal> had been used.
4974 <sect2 id="message-bus-starting-services">
4975 <title>Message Bus Starting Services (Activation)</title>
4977 The message bus can start applications on behalf of other applications.
4978 This is referred to as <firstterm>service activation</firstterm> or
4979 <firstterm>activation</firstterm>.
4980 An application that can be started in this way is called a
4981 <firstterm>service</firstterm> or an
4982 <firstterm>activatable service</firstterm>.
4986 In D-Bus, starting a service is normally done by
4987 <firstterm>auto-starting</firstterm>, which is one form of activation.
4988 In auto-starting, applications send a
4989 message to a particular well-known name, such as
4990 <literal>com.example.TextEditor</literal>, without specifying the
4991 <literal>NO_AUTO_START</literal> flag in the message header.
4992 If no application on the bus owns the requested name, but the bus
4993 daemon does know how to start an activatable service for that name,
4994 then the bus daemon will start that service, wait for it to request
4995 that name, and deliver the message to it.
4999 It is also possible for applications to send an explicit request to
5000 start a service: this is another form of activation, distinct from
5002 <xref linkend="bus-messages-start-service-by-name"/> for details.
5006 In either case, this implies a contract documented along with the name
5007 <literal>com.example.TextEditor</literal> for which object
5008 the owner of that name will provide, and what interfaces those
5013 To find an executable corresponding to a particular name, the bus daemon
5014 looks for <firstterm>service description files</firstterm>. Service
5015 description files define a mapping from names to executables. Different
5016 kinds of message bus will look for these files in different places, see
5017 <xref linkend="message-bus-types"/>.
5020 Service description files have the ".service" file
5021 extension. The message bus will only load service description files
5022 ending with .service; all other files will be ignored. The file format
5023 is similar to that of <ulink
5024 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
5025 entries</ulink>. All service description files must be in UTF-8
5026 encoding. To ensure that there will be no name collisions, service files
5027 must be namespaced using the same mechanism as messages and service
5032 On the well-known system bus, the name of a service description file
5033 must be its well-known name plus <literal>.service</literal>,
5035 <literal>com.example.ConfigurationDatabase.service</literal>.
5039 On the well-known session bus, services should follow the same
5040 service description file naming convention as on the system bus,
5041 but for backwards compatibility they are not required to do so.
5045 [FIXME the file format should be much better specified than "similar to
5046 .desktop entries" esp. since desktop entries are already
5047 badly-specified. ;-)]
5048 These sections from the specification apply to service files as well:
5051 <listitem><para>General syntax</para></listitem>
5052 <listitem><para>Comment format</para></listitem>
5055 Service description files must contain a
5056 <literal>D-BUS Service</literal> group with at least the keys
5057 <literal>Name</literal> (the well-known name of the service)
5058 and <literal>Exec</literal> (the command to be executed).
5061 <title>Example service description file</title>
5063 # Sample service description file
5065 Name=com.example.ConfigurationDatabase
5066 Exec=/usr/bin/sample-configd
5072 Additionally, service description files for the well-known system
5073 bus on Unix must contain a <literal>User</literal> key, whose value
5074 is the name of a user account (e.g. <literal>root</literal>).
5075 The system service will be run as that user.
5079 When an application asks to start a service by name, the bus daemon tries to
5080 find a service that will own that name. It then tries to spawn the
5081 executable associated with it. If this fails, it will report an
5086 On the well-known system bus, it is not possible for two .service files
5087 in the same directory to offer the same service, because they are
5088 constrained to have names that match the service name.
5092 On the well-known session bus, if two .service files in the same
5093 directory offer the same service name, the result is undefined.
5094 Distributors should avoid this situation, for instance by naming
5095 session services' .service files according to their service name.
5099 If two .service files in different directories offer the same
5100 service name, the one in the higher-priority directory is used:
5101 for instance, on the system bus, .service files in
5102 /usr/local/share/dbus-1/system-services take precedence over those
5103 in /usr/share/dbus-1/system-services.
5106 The executable launched will have the environment variable
5107 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
5108 message bus so it can connect and request the appropriate names.
5111 The executable being launched may want to know whether the message bus
5112 starting it is one of the well-known message buses (see <xref
5113 linkend="message-bus-types"/>). To facilitate this, the bus must also set
5114 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
5115 of the well-known buses. The currently-defined values for this variable
5116 are <literal>system</literal> for the systemwide message bus,
5117 and <literal>session</literal> for the per-login-session message
5118 bus. The new executable must still connect to the address given
5119 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
5120 resulting connection is to the well-known bus.
5123 [FIXME there should be a timeout somewhere, either specified
5124 in the .service file, by the client, or just a global value
5125 and if the client being activated fails to connect within that
5126 timeout, an error should be sent back.]
5129 <sect3 id="message-bus-starting-services-scope">
5130 <title>Message Bus Service Scope</title>
5132 The "scope" of a service is its "per-", such as per-session,
5133 per-machine, per-home-directory, or per-display. The reference
5134 implementation doesn't yet support starting services in a different
5135 scope from the message bus itself. So e.g. if you start a service
5136 on the session bus its scope is per-session.
5139 We could add an optional scope to a bus name. For example, for
5140 per-(display,session pair), we could have a unique ID for each display
5141 generated automatically at login and set on screen 0 by executing a
5142 special "set display ID" binary. The ID would be stored in a
5143 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
5144 random bytes. This ID would then be used to scope names.
5145 Starting/locating a service could be done by ID-name pair rather than
5149 Contrast this with a per-display scope. To achieve that, we would
5150 want a single bus spanning all sessions using a given display.
5151 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
5152 property on screen 0 of the display, pointing to this bus.
5157 <sect2 id="message-bus-types">
5158 <title>Well-known Message Bus Instances</title>
5160 Two standard message bus instances are defined here, along with how
5161 to locate them and where their service files live.
5163 <sect3 id="message-bus-types-login">
5164 <title>Login session message bus</title>
5166 Each time a user logs in, a <firstterm>login session message
5167 bus</firstterm> may be started. All applications in the user's login
5168 session may interact with one another using this message bus.
5171 The address of the login session message bus is given
5172 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5173 variable. If that variable is not set, applications may
5174 also try to read the address from the X Window System root
5175 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5176 The root window property must have type <literal>STRING</literal>.
5177 The environment variable should have precedence over the
5178 root window property.
5180 <para>The address of the login session message bus is given in the
5181 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5182 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5183 "autolaunch:", the system should use platform-specific methods of
5184 locating a running D-Bus session server, or starting one if a running
5185 instance cannot be found. Note that this mechanism is not recommended
5186 for attempting to determine if a daemon is running. It is inherently
5187 racy to attempt to make this determination, since the bus daemon may
5188 be started just before or just after the determination is made.
5189 Therefore, it is recommended that applications do not try to make this
5190 determination for their functionality purposes, and instead they
5191 should attempt to start the server.</para>
5193 <sect4 id="message-bus-types-login-x-windows">
5194 <title>X Windowing System</title>
5196 For the X Windowing System, the application must locate the
5197 window owner of the selection represented by the atom formed by
5201 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5205 <para>the current user's username</para>
5209 <para>the literal character '_' (underscore)</para>
5213 <para>the machine's ID</para>
5219 The following properties are defined for the window that owns
5221 <informaltable frame="all">
5230 <para>meaning</para>
5236 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5240 <para>the actual address of the server socket</para>
5246 <para>_DBUS_SESSION_BUS_PID</para>
5250 <para>the PID of the server process</para>
5259 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5260 present in this window.
5264 If the X selection cannot be located or if reading the
5265 properties from the window fails, the implementation MUST conclude
5266 that there is no D-Bus server running and proceed to start a new
5267 server. (See below on concurrency issues)
5271 Failure to connect to the D-Bus server address thus obtained
5272 MUST be treated as a fatal connection error and should be reported
5277 As an alternative, an implementation MAY find the information
5278 in the following file located in the current user's home directory,
5279 in subdirectory .dbus/session-bus/:
5282 <para>the machine's ID</para>
5286 <para>the literal character '-' (dash)</para>
5290 <para>the X display without the screen number, with the
5291 following prefixes removed, if present: ":", "localhost:"
5292 ."localhost.localdomain:". That is, a display of
5293 "localhost:10.0" produces just the number "10"</para>
5299 The contents of this file NAME=value assignment pairs and
5300 lines starting with # are comments (no comments are allowed
5301 otherwise). The following variable names are defined:
5308 <para>Variable</para>
5312 <para>meaning</para>
5318 <para>DBUS_SESSION_BUS_ADDRESS</para>
5322 <para>the actual address of the server socket</para>
5328 <para>DBUS_SESSION_BUS_PID</para>
5332 <para>the PID of the server process</para>
5338 <para>DBUS_SESSION_BUS_WINDOWID</para>
5342 <para>the window ID</para>
5351 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5356 Failure to open this file MUST be interpreted as absence of a
5357 running server. Therefore, the implementation MUST proceed to
5358 attempting to launch a new bus server if the file cannot be
5363 However, success in opening this file MUST NOT lead to the
5364 conclusion that the server is running. Thus, a failure to connect to
5365 the bus address obtained by the alternative method MUST NOT be
5366 considered a fatal error. If the connection cannot be established,
5367 the implementation MUST proceed to check the X selection settings or
5368 to start the server on its own.
5372 If the implementation concludes that the D-Bus server is not
5373 running it MUST attempt to start a new server and it MUST also
5374 ensure that the daemon started as an effect of the "autolaunch"
5375 mechanism provides the lookup mechanisms described above, so
5376 subsequent calls can locate the newly started server. The
5377 implementation MUST also ensure that if two or more concurrent
5378 initiations happen, only one server remains running and all other
5379 initiations are able to obtain the address of this server and
5380 connect to it. In other words, the implementation MUST ensure that
5381 the X selection is not present when it attempts to set it, without
5382 allowing another process to set the selection between the
5383 verification and the setting (e.g., by using XGrabServer /
5390 On Unix systems, the session bus should search for .service files
5391 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5393 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5394 Implementations may also search additional locations, which
5395 should be searched with lower priority than anything in
5396 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5397 for example, the reference implementation also
5398 looks in <literal>${datadir}/dbus-1/services</literal> as
5399 set at compile time.
5402 As described in the XDG Base Directory Specification, software
5403 packages should install their session .service files to their
5404 configured <literal>${datadir}/dbus-1/services</literal>,
5405 where <literal>${datadir}</literal> is as defined by the GNU
5406 coding standards. System administrators or users can arrange
5407 for these service files to be read by setting XDG_DATA_DIRS or by
5408 symlinking them into the default locations.
5412 <sect3 id="message-bus-types-system">
5413 <title>System message bus</title>
5415 A computer may have a <firstterm>system message bus</firstterm>,
5416 accessible to all applications on the system. This message bus may be
5417 used to broadcast system events, such as adding new hardware devices,
5418 changes in the printer queue, and so forth.
5421 The address of the system message bus is given
5422 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5423 variable. If that variable is not set, applications should try
5424 to connect to the well-known address
5425 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5428 The D-Bus reference implementation actually honors the
5429 <literal>$(localstatedir)</literal> configure option
5430 for this address, on both client and server side.
5435 On Unix systems, the system bus should default to searching
5436 for .service files in
5437 <literal>/usr/local/share/dbus-1/system-services</literal>,
5438 <literal>/usr/share/dbus-1/system-services</literal> and
5439 <literal>/lib/dbus-1/system-services</literal>, with that order
5440 of precedence. It may also search other implementation-specific
5441 locations, but should not vary these locations based on environment
5445 The system bus is security-sensitive and is typically executed
5446 by an init system with a clean environment. Its launch helper
5447 process is particularly security-sensitive, and specifically
5448 clears its own environment.
5453 Software packages should install their system .service
5454 files to their configured
5455 <literal>${datadir}/dbus-1/system-services</literal>,
5456 where <literal>${datadir}</literal> is as defined by the GNU
5457 coding standards. System administrators can arrange
5458 for these service files to be read by editing the system bus'
5459 configuration file or by symlinking them into the default
5465 <sect2 id="message-bus-messages">
5466 <title>Message Bus Messages</title>
5468 The special message bus name <literal>org.freedesktop.DBus</literal>
5469 responds to a number of additional messages.
5472 <sect3 id="bus-messages-hello">
5473 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5484 <entry>Argument</entry>
5486 <entry>Description</entry>
5492 <entry>STRING</entry>
5493 <entry>Unique name assigned to the connection</entry>
5500 Before an application is able to send messages to other applications
5501 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5502 to the message bus to obtain a unique name. If an application without
5503 a unique name tries to send a message to another application, or a
5504 message to the message bus itself that isn't the
5505 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5506 disconnected from the bus.
5509 There is no corresponding "disconnect" request; if a client wishes to
5510 disconnect from the bus, it simply closes the socket (or other
5511 communication channel).
5514 <sect3 id="bus-messages-list-names">
5515 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5519 ARRAY of STRING ListNames ()
5526 <entry>Argument</entry>
5528 <entry>Description</entry>
5534 <entry>ARRAY of STRING</entry>
5535 <entry>Array of strings where each string is a bus name</entry>
5542 Returns a list of all currently-owned names on the bus.
5545 <sect3 id="bus-messages-list-activatable-names">
5546 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5550 ARRAY of STRING ListActivatableNames ()
5557 <entry>Argument</entry>
5559 <entry>Description</entry>
5565 <entry>ARRAY of STRING</entry>
5566 <entry>Array of strings where each string is a bus name</entry>
5573 Returns a list of all names that can be activated on the bus.
5576 <sect3 id="bus-messages-name-exists">
5577 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5581 BOOLEAN NameHasOwner (in STRING name)
5588 <entry>Argument</entry>
5590 <entry>Description</entry>
5596 <entry>STRING</entry>
5597 <entry>Name to check</entry>
5607 <entry>Argument</entry>
5609 <entry>Description</entry>
5615 <entry>BOOLEAN</entry>
5616 <entry>Return value, true if the name exists</entry>
5623 Checks if the specified name exists (currently has an owner).
5627 <sect3 id="bus-messages-name-owner-changed">
5628 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5632 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5639 <entry>Argument</entry>
5641 <entry>Description</entry>
5647 <entry>STRING</entry>
5648 <entry>Name with a new owner</entry>
5652 <entry>STRING</entry>
5653 <entry>Old owner or empty string if none</entry>
5657 <entry>STRING</entry>
5658 <entry>New owner or empty string if none</entry>
5665 This signal indicates that the owner of a name has changed.
5666 It's also the signal to use to detect the appearance of
5667 new names on the bus.
5670 <sect3 id="bus-messages-name-lost">
5671 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5675 NameLost (STRING name)
5682 <entry>Argument</entry>
5684 <entry>Description</entry>
5690 <entry>STRING</entry>
5691 <entry>Name which was lost</entry>
5698 This signal is sent to a specific application when it loses
5699 ownership of a name.
5703 <sect3 id="bus-messages-name-acquired">
5704 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5708 NameAcquired (STRING name)
5715 <entry>Argument</entry>
5717 <entry>Description</entry>
5723 <entry>STRING</entry>
5724 <entry>Name which was acquired</entry>
5731 This signal is sent to a specific application when it gains
5732 ownership of a name.
5736 <sect3 id="bus-messages-start-service-by-name">
5737 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5741 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5748 <entry>Argument</entry>
5750 <entry>Description</entry>
5756 <entry>STRING</entry>
5757 <entry>Name of the service to start</entry>
5761 <entry>UINT32</entry>
5762 <entry>Flags (currently not used)</entry>
5772 <entry>Argument</entry>
5774 <entry>Description</entry>
5780 <entry>UINT32</entry>
5781 <entry>Return value</entry>
5786 Tries to launch the executable associated with a name (service
5787 activation), as an explicit request. This is an alternative to
5788 relying on auto-starting. For more information on how services
5789 are activated and the difference between auto-starting and explicit
5791 <xref linkend="message-bus-starting-services"/>.
5794 It is often preferable to carry out auto-starting
5795 instead of calling this method. This is because calling this method
5797 <ulink url="https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use">time-of-check/time-of-use</ulink>
5798 issue: if a caller asks the message bus to start a service so that
5799 the same caller can make follow-up method calls to that service,
5800 the fact that the message bus was able to start the required
5801 service is no guarantee that it will not have crashed or otherwise
5802 exited by the time the caller makes those follow-up method calls.
5803 As a result, calling this method does not remove the need for
5804 the caller to handle errors from method calls. Given that fact,
5805 it is usually simpler to rely on auto-starting, in which the
5806 required service starts as a side-effect of the first method call.
5809 The return value can be one of the following values:
5814 <entry>Identifier</entry>
5815 <entry>Value</entry>
5816 <entry>Description</entry>
5821 <entry>DBUS_START_REPLY_SUCCESS</entry>
5823 <entry>The service was successfully started.</entry>
5826 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5828 <entry>A connection already owns the given name.</entry>
5837 <sect3 id="bus-messages-update-activation-environment">
5838 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5842 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5849 <entry>Argument</entry>
5851 <entry>Description</entry>
5857 <entry>ARRAY of DICT<STRING,STRING></entry>
5858 <entry>Environment to add or update</entry>
5863 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5866 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5869 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.
5874 <sect3 id="bus-messages-get-name-owner">
5875 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5879 STRING GetNameOwner (in STRING name)
5886 <entry>Argument</entry>
5888 <entry>Description</entry>
5894 <entry>STRING</entry>
5895 <entry>Name to get the owner of</entry>
5905 <entry>Argument</entry>
5907 <entry>Description</entry>
5913 <entry>STRING</entry>
5914 <entry>Return value, a unique connection name</entry>
5919 Returns the unique connection name of the primary owner of the name
5920 given. If the requested name doesn't have an owner, returns a
5921 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5925 <sect3 id="bus-messages-get-connection-unix-user">
5926 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5930 UINT32 GetConnectionUnixUser (in STRING bus_name)
5937 <entry>Argument</entry>
5939 <entry>Description</entry>
5945 <entry>STRING</entry>
5946 <entry>Unique or well-known bus name of the connection to
5947 query, such as <literal>:12.34</literal> or
5948 <literal>com.example.tea</literal></entry>
5958 <entry>Argument</entry>
5960 <entry>Description</entry>
5966 <entry>UINT32</entry>
5967 <entry>Unix user ID</entry>
5972 Returns the Unix user ID of the process connected to the server. If
5973 unable to determine it (for instance, because the process is not on the
5974 same machine as the bus daemon), an error is returned.
5978 <sect3 id="bus-messages-get-connection-unix-process-id">
5979 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5983 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5990 <entry>Argument</entry>
5992 <entry>Description</entry>
5998 <entry>STRING</entry>
5999 <entry>Unique or well-known bus name of the connection to
6000 query, such as <literal>:12.34</literal> or
6001 <literal>com.example.tea</literal></entry>
6011 <entry>Argument</entry>
6013 <entry>Description</entry>
6019 <entry>UINT32</entry>
6020 <entry>Unix process id</entry>
6025 Returns the Unix process ID of the process connected to the server. If
6026 unable to determine it (for instance, because the process is not on the
6027 same machine as the bus daemon), an error is returned.
6031 <sect3 id="bus-messages-get-connection-credentials">
6032 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
6036 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
6043 <entry>Argument</entry>
6045 <entry>Description</entry>
6051 <entry>STRING</entry>
6052 <entry>Unique or well-known bus name of the connection to
6053 query, such as <literal>:12.34</literal> or
6054 <literal>com.example.tea</literal></entry>
6064 <entry>Argument</entry>
6066 <entry>Description</entry>
6072 <entry>DICT<STRING,VARIANT></entry>
6073 <entry>Credentials</entry>
6081 Returns as many credentials as possible for the process connected to
6082 the server. If unable to determine certain credentials (for instance,
6083 because the process is not on the same machine as the bus daemon,
6084 or because this version of the bus daemon does not support a
6085 particular security framework), or if the values of those credentials
6086 cannot be represented as documented here, then those credentials
6091 Keys in the returned dictionary not containing "." are defined
6092 by this specification. Bus daemon implementors supporting
6093 credentials frameworks not mentioned in this document should either
6094 contribute patches to this specification, or use keys containing
6095 "." and starting with a reversed domain name.
6101 <entry>Value type</entry>
6102 <entry>Value</entry>
6107 <entry>UnixUserID</entry>
6108 <entry>UINT32</entry>
6109 <entry>The numeric Unix user ID, as defined by POSIX</entry>
6112 <entry>ProcessID</entry>
6113 <entry>UINT32</entry>
6114 <entry>The numeric process ID, on platforms that have
6115 this concept. On Unix, this is the process ID defined by
6119 <entry>WindowsSID</entry>
6120 <entry>STRING</entry>
6121 <entry>The Windows security identifier in its string form,
6122 e.g. "S-1-5-21-3623811015-3361044348-30300820-1013" for
6123 a domain or local computer user or "S-1-5-18" for the
6124 LOCAL_SYSTEM user</entry>
6128 <entry>LinuxSecurityLabel</entry>
6129 <entry>ARRAY of BYTE</entry>
6131 <para>On Linux systems, the security label that would result
6132 from the SO_PEERSEC getsockopt call. The array contains
6133 the non-zero bytes of the security label in an unspecified
6134 ASCII-compatible encoding<footnote>
6135 <para>It could be ASCII or UTF-8, but could also be
6136 ISO Latin-1 or any other encoding.</para>
6137 </footnote>, followed by a single zero byte.</para>
6139 For example, the SELinux context
6140 <literal>system_u:system_r:init_t:s0</literal>
6141 (a string of length 27) would be encoded as 28 bytes
6142 ending with ':', 's', '0', '\x00'.<footnote>
6143 <para>Note that this is not the same as the older
6144 GetConnectionSELinuxContext method, which does
6145 not append the zero byte. Always appending the
6146 zero byte allows callers to read the string
6147 from the message payload without copying.</para>
6151 On SELinux systems this is the SELinux context, as output
6152 by <literal>ps -Z</literal> or <literal>ls -Z</literal>.
6153 Typical values might include
6154 <literal>system_u:system_r:init_t:s0</literal>,
6155 <literal>unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023</literal>,
6157 <literal>unconfined_u:unconfined_r:chrome_sandbox_t:s0-s0:c0.c1023</literal>.
6160 On Smack systems, this is the Smack label.
6161 Typical values might include
6162 <literal>_</literal>, <literal>*</literal>,
6163 <literal>User</literal>, <literal>System</literal>
6164 or <literal>System::Shared</literal>.
6167 On AppArmor systems, this is the AppArmor context,
6168 a composite string encoding the AppArmor label (one or more
6169 profiles) and the enforcement mode.
6170 Typical values might include <literal>unconfined</literal>,
6171 <literal>/usr/bin/firefox (enforce)</literal> or
6172 <literal>user1 (complain)</literal>.
6183 This method was added in D-Bus 1.7 to reduce the round-trips
6184 required to list a process's credentials. In older versions, calling
6185 this method will fail: applications should recover by using the
6186 separate methods such as
6187 <xref linkend="bus-messages-get-connection-unix-user"/>
6192 <sect3 id="bus-messages-get-adt-audit-session-data">
6193 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
6197 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
6204 <entry>Argument</entry>
6206 <entry>Description</entry>
6212 <entry>STRING</entry>
6213 <entry>Unique or well-known bus name of the connection to
6214 query, such as <literal>:12.34</literal> or
6215 <literal>com.example.tea</literal></entry>
6225 <entry>Argument</entry>
6227 <entry>Description</entry>
6233 <entry>ARRAY of BYTE</entry>
6234 <entry>auditing data as returned by
6235 adt_export_session_data()</entry>
6240 Returns auditing data used by Solaris ADT, in an unspecified
6241 binary format. If you know what this means, please contribute
6242 documentation via the D-Bus bug tracking system.
6243 This method is on the core DBus interface for historical reasons;
6244 the same information should be made available via
6245 <xref linkend="bus-messages-get-connection-credentials"/>
6250 <sect3 id="bus-messages-get-connection-selinux-security-context">
6251 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6255 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6262 <entry>Argument</entry>
6264 <entry>Description</entry>
6270 <entry>STRING</entry>
6271 <entry>Unique or well-known bus name of the connection to
6272 query, such as <literal>:12.34</literal> or
6273 <literal>com.example.tea</literal></entry>
6283 <entry>Argument</entry>
6285 <entry>Description</entry>
6291 <entry>ARRAY of BYTE</entry>
6292 <entry>some sort of string of bytes, not necessarily UTF-8,
6293 not including '\0'</entry>
6298 Returns the security context used by SELinux, in an unspecified
6299 format. If you know what this means, please contribute
6300 documentation via the D-Bus bug tracking system.
6301 This method is on the core DBus interface for historical reasons;
6302 the same information should be made available via
6303 <xref linkend="bus-messages-get-connection-credentials"/>
6309 <sect3 id="bus-messages-add-match">
6310 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6314 AddMatch (in STRING rule)
6321 <entry>Argument</entry>
6323 <entry>Description</entry>
6329 <entry>STRING</entry>
6330 <entry>Match rule to add to the connection</entry>
6335 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6336 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6340 <sect3 id="bus-messages-remove-match">
6341 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6345 RemoveMatch (in STRING rule)
6352 <entry>Argument</entry>
6354 <entry>Description</entry>
6360 <entry>STRING</entry>
6361 <entry>Match rule to remove from the connection</entry>
6366 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6367 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6372 <sect3 id="bus-messages-get-id">
6373 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6377 GetId (out STRING id)
6384 <entry>Argument</entry>
6386 <entry>Description</entry>
6392 <entry>STRING</entry>
6393 <entry>Unique ID identifying the bus daemon</entry>
6398 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6399 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6400 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6401 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6402 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6403 by org.freedesktop.DBus.Peer.GetMachineId().
6404 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6408 <sect3 id="bus-messages-become-monitor">
6409 <title><literal>org.freedesktop.DBus.Monitoring.BecomeMonitor</literal></title>
6413 BecomeMonitor (in ARRAY of STRING rule, in UINT32 flags)
6420 <entry>Argument</entry>
6422 <entry>Description</entry>
6428 <entry>ARRAY of STRING</entry>
6429 <entry>Match rules to add to the connection</entry>
6433 <entry>UINT32</entry>
6434 <entry>Not used, must be 0</entry>
6442 Converts the connection into a <emphasis>monitor
6443 connection</emphasis> which can be used as a debugging/monitoring
6444 tool. Only a user who is privileged on this
6445 bus (by some implementation-specific definition) may create
6446 monitor connections<footnote>
6448 In the reference implementation,
6449 the default configuration is that each user (identified by
6450 numeric user ID) may monitor their own session bus,
6451 and the root user (user ID zero) may monitor the
6458 Monitor connections lose all their bus names, including the unique
6459 connection name, and all their match rules. Sending messages on a
6460 monitor connection is not allowed: applications should use a private
6461 connection for monitoring.
6465 Monitor connections may receive all messages, even messages that
6466 should only have gone to some other connection ("eavesdropping").
6467 The first argument is a list of match rules, which replace any
6468 match rules that were previously active for this connection.
6469 These match rules are always treated as if they contained the
6470 special <literal>eavesdrop='true'</literal> member.
6474 As a special case, an empty list of match rules (which would
6475 otherwise match nothing, making the monitor useless) is treated
6476 as a shorthand for matching all messages.
6480 The second argument might be used for flags to influence the
6481 behaviour of the monitor connection in future D-Bus versions.
6485 Message bus implementations should attempt to minimize the
6486 side-effects of monitoring — in particular, unlike ordinary
6487 eavesdropping, monitoring the system bus does not require the
6488 access control rules to be relaxed, which would change the set
6489 of messages that can be delivered to their (non-monitor)
6490 destinations. However, it is unavoidable that monitoring
6491 will increase the message bus's resource consumption. In
6492 edge cases where there was barely enough time or memory without
6493 monitoring, this might result in message deliveries failing
6494 when they would otherwise have succeeded.
6502 <appendix id="implementation-notes">
6503 <title>Implementation notes</title>
6504 <sect1 id="implementation-notes-subsection">
6512 <glossary><title>Glossary</title>
6514 This glossary defines some of the terms used in this specification.
6517 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6520 The message bus maintains an association between names and
6521 connections. (Normally, there's one connection per application.) A
6522 bus name is simply an identifier used to locate connections. For
6523 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6524 name might be used to send a message to a screensaver from Yoyodyne
6525 Corporation. An application is said to <firstterm>own</firstterm> a
6526 name if the message bus has associated the application's connection
6527 with the name. Names may also have <firstterm>queued
6528 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6529 The bus assigns a unique name to each connection,
6530 see <xref linkend="term-unique-name"/>. Other names
6531 can be thought of as "well-known names" and are
6532 used to find applications that offer specific functionality.
6536 See <xref linkend="message-protocol-names-bus"/> for details of
6537 the syntax and naming conventions for bus names.
6542 <glossentry id="term-message"><glossterm>Message</glossterm>
6545 A message is the atomic unit of communication via the D-Bus
6546 protocol. It consists of a <firstterm>header</firstterm> and a
6547 <firstterm>body</firstterm>; the body is made up of
6548 <firstterm>arguments</firstterm>.
6553 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6556 The message bus is a special application that forwards
6557 or routes messages between a group of applications
6558 connected to the message bus. It also manages
6559 <firstterm>names</firstterm> used for routing
6565 <glossentry id="term-name"><glossterm>Name</glossterm>
6568 See <xref linkend="term-bus-name"/>. "Name" may
6569 also be used to refer to some of the other names
6570 in D-Bus, such as interface names.
6575 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6578 Used to prevent collisions when defining new interfaces, bus names
6579 etc. The convention used is the same one Java uses for defining
6580 classes: a reversed domain name.
6581 See <xref linkend="message-protocol-names-bus"/>,
6582 <xref linkend="message-protocol-names-interface"/>,
6583 <xref linkend="message-protocol-names-error"/>,
6584 <xref linkend="message-protocol-marshaling-object-path"/>.
6589 <glossentry id="term-object"><glossterm>Object</glossterm>
6592 Each application contains <firstterm>objects</firstterm>, which have
6593 <firstterm>interfaces</firstterm> and
6594 <firstterm>methods</firstterm>. Objects are referred to by a name,
6595 called a <firstterm>path</firstterm>.
6600 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6603 An application talking directly to another application, without going
6604 through a message bus. One-to-one connections may be "peer to peer" or
6605 "client to server." The D-Bus protocol has no concept of client
6606 vs. server after a connection has authenticated; the flow of messages
6607 is symmetrical (full duplex).
6612 <glossentry id="term-path"><glossterm>Path</glossterm>
6615 Object references (object names) in D-Bus are organized into a
6616 filesystem-style hierarchy, so each object is named by a path. As in
6617 LDAP, there's no difference between "files" and "directories"; a path
6618 can refer to an object, while still having child objects below it.
6623 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6626 Each bus name has a primary owner; messages sent to the name go to the
6627 primary owner. However, certain names also maintain a queue of
6628 secondary owners "waiting in the wings." If the primary owner releases
6629 the name, then the first secondary owner in the queue automatically
6630 becomes the new owner of the name.
6635 <glossentry id="term-service"><glossterm>Service</glossterm>
6638 A service is an executable that can be launched by the bus daemon.
6639 Services normally guarantee some particular features, for example they
6640 may guarantee that they will request a specific name such as
6641 "com.example.Screensaver", have a singleton object
6642 "/com/example/Application", and that object will implement the
6643 interface "com.example.Screensaver.Control".
6648 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6651 ".service files" tell the bus about service applications that can be
6652 launched (see <xref linkend="term-service"/>). Most importantly they
6653 provide a mapping from bus names to services that will request those
6654 names when they start up.
6659 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6662 The special name automatically assigned to each connection by the
6663 message bus. This name will never change owner, and will be unique
6664 (never reused during the lifetime of the message bus).
6665 It will begin with a ':' character.
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