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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.26</releaseinfo>
10 <date>2015-02-19</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>0.26</revnumber>
75 <date>2015-02-19</date>
76 <authorinitials>smcv, rh</authorinitials>
78 GetConnectionCredentials can return LinuxSecurityLabel or
79 WindowsSID; add privileged BecomeMonitor method
83 <revnumber>0.25</revnumber>
84 <date>2014-11-10</date>
85 <authorinitials>smcv, lennart</authorinitials>
87 ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
91 <revnumber>0.24</revnumber>
92 <date>2014-10-01</date>
93 <authorinitials>SMcV</authorinitials>
95 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
96 document how to quote match rules
100 <revnumber>0.23</revnumber>
101 <date>2014-01-06</date>
102 <authorinitials>SMcV, CY</authorinitials>
104 method call messages with no INTERFACE may be considered an error;
105 document tcp:bind=... and nonce-tcp:bind=...; define listenable
106 and connectable addresses
110 <revnumber>0.22</revnumber>
111 <date>2013-10-09</date>
112 <authorinitials></authorinitials>
113 <revremark>add GetConnectionCredentials, document
114 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
115 document and correct .service file syntax and naming
119 <revnumber>0.21</revnumber>
120 <date>2013-04-25</date>
121 <authorinitials>smcv</authorinitials>
122 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
123 Corrigendum #9)</revremark>
126 <revnumber>0.20</revnumber>
127 <date>22 February 2013</date>
128 <authorinitials>smcv, walters</authorinitials>
129 <revremark>reorganise for clarity, remove false claims about
130 basic types, mention /o/fd/DBus</revremark>
133 <revnumber>0.19</revnumber>
134 <date>20 February 2012</date>
135 <authorinitials>smcv/lp</authorinitials>
136 <revremark>formally define unique connection names and well-known
137 bus names; document best practices for interface, bus, member and
138 error names, and object paths; document the search path for session
139 and system services on Unix; document the systemd transport</revremark>
142 <revnumber>0.18</revnumber>
143 <date>29 July 2011</date>
144 <authorinitials>smcv</authorinitials>
145 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
146 match keyword; promote type system to a top-level section</revremark>
149 <revnumber>0.17</revnumber>
150 <date>1 June 2011</date>
151 <authorinitials>smcv/davidz</authorinitials>
152 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
153 by GVariant</revremark>
156 <revnumber>0.16</revnumber>
157 <date>11 April 2011</date>
158 <authorinitials></authorinitials>
159 <revremark>add path_namespace, arg0namespace; argNpath matches object
163 <revnumber>0.15</revnumber>
164 <date>3 November 2010</date>
165 <authorinitials></authorinitials>
166 <revremark></revremark>
169 <revnumber>0.14</revnumber>
170 <date>12 May 2010</date>
171 <authorinitials></authorinitials>
172 <revremark></revremark>
175 <revnumber>0.13</revnumber>
176 <date>23 Dezember 2009</date>
177 <authorinitials></authorinitials>
178 <revremark></revremark>
181 <revnumber>0.12</revnumber>
182 <date>7 November, 2006</date>
183 <authorinitials></authorinitials>
184 <revremark></revremark>
187 <revnumber>0.11</revnumber>
188 <date>6 February 2005</date>
189 <authorinitials></authorinitials>
190 <revremark></revremark>
193 <revnumber>0.10</revnumber>
194 <date>28 January 2005</date>
195 <authorinitials></authorinitials>
196 <revremark></revremark>
199 <revnumber>0.9</revnumber>
200 <date>7 Januar 2005</date>
201 <authorinitials></authorinitials>
202 <revremark></revremark>
205 <revnumber>0.8</revnumber>
206 <date>06 September 2003</date>
207 <authorinitials></authorinitials>
208 <revremark>First released document.</revremark>
213 <sect1 id="introduction">
214 <title>Introduction</title>
216 D-Bus is a system for low-overhead, easy to use
217 interprocess communication (IPC). In more detail:
221 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
222 binary protocol, and does not have to convert to and from a text
223 format such as XML. Because D-Bus is intended for potentially
224 high-resolution same-machine IPC, not primarily for Internet IPC,
225 this is an interesting optimization. D-Bus is also designed to
226 avoid round trips and allow asynchronous operation, much like
232 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
233 of <firstterm>messages</firstterm> rather than byte streams, and
234 automatically handles a lot of the hard IPC issues. Also, the D-Bus
235 library is designed to be wrapped in a way that lets developers use
236 their framework's existing object/type system, rather than learning
237 a new one specifically for IPC.
244 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
245 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
246 a system for one application to talk to a single other
247 application. However, the primary intended application of the protocol is the
248 D-Bus <firstterm>message bus</firstterm>, specified in <xref
249 linkend="message-bus"/>. The message bus is a special application that
250 accepts connections from multiple other applications, and forwards
255 Uses of D-Bus include notification of system changes (notification of when
256 a camera is plugged in to a computer, or a new version of some software
257 has been installed), or desktop interoperability, for example a file
258 monitoring service or a configuration service.
262 D-Bus is designed for two specific use cases:
266 A "system bus" for notifications from the system to user sessions,
267 and to allow the system to request input from user sessions.
272 A "session bus" used to implement desktop environments such as
277 D-Bus is not intended to be a generic IPC system for any possible
278 application, and intentionally omits many features found in other
279 IPC systems for this reason.
283 At the same time, the bus daemons offer a number of features not found in
284 other IPC systems, such as single-owner "bus names" (similar to X
285 selections), on-demand startup of services, and security policies.
286 In many ways, these features are the primary motivation for developing
287 D-Bus; other systems would have sufficed if IPC were the only goal.
291 D-Bus may turn out to be useful in unanticipated applications, but future
292 versions of this spec and the reference implementation probably will not
293 incorporate features that interfere with the core use cases.
297 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
298 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
299 document are to be interpreted as described in RFC 2119. However, the
300 document could use a serious audit to be sure it makes sense to do
301 so. Also, they are not capitalized.
304 <sect2 id="stability">
305 <title>Protocol and Specification Stability</title>
307 The D-Bus protocol is frozen (only compatible extensions are allowed) as
308 of November 8, 2006. However, this specification could still use a fair
309 bit of work to make interoperable reimplementation possible without
310 reference to the D-Bus reference implementation. Thus, this
311 specification is not marked 1.0. To mark it 1.0, we'd like to see
312 someone invest significant effort in clarifying the specification
313 language, and growing the specification to cover more aspects of the
314 reference implementation's behavior.
317 Until this work is complete, any attempt to reimplement D-Bus will
318 probably require looking at the reference implementation and/or asking
319 questions on the D-Bus mailing list about intended behavior.
320 Questions on the list are very welcome.
323 Nonetheless, this document should be a useful starting point and is
324 to our knowledge accurate, though incomplete.
330 <sect1 id="type-system">
331 <title>Type System</title>
334 D-Bus has a type system, in which values of various types can be
335 serialized into a sequence of bytes referred to as the
336 <firstterm>wire format</firstterm> in a standard way.
337 Converting a value from some other representation into the wire
338 format is called <firstterm>marshaling</firstterm> and converting
339 it back from the wire format is <firstterm>unmarshaling</firstterm>.
343 The D-Bus protocol does not include type tags in the marshaled data; a
344 block of marshaled values must have a known <firstterm>type
345 signature</firstterm>. The type signature is made up of zero or more
346 <firstterm id="term-single-complete-type">single complete
347 types</firstterm>, each made up of one or more
348 <firstterm>type codes</firstterm>.
352 A type code is an ASCII character representing the
353 type of a value. Because ASCII characters are used, the type signature
354 will always form a valid ASCII string. A simple string compare
355 determines whether two type signatures are equivalent.
359 A single complete type is a sequence of type codes that fully describes
360 one type: either a basic type, or a single fully-described container type.
361 A single complete type is a basic type code, a variant type code,
362 an array with its element type, or a struct with its fields (all of which
363 are defined below). So the following signatures are not single complete
374 And the following signatures contain multiple complete types:
384 Note however that a single complete type may <emphasis>contain</emphasis>
385 multiple other single complete types, by containing a struct or dict
389 <sect2 id="basic-types">
390 <title>Basic types</title>
393 The simplest type codes are the <firstterm id="term-basic-type">basic
394 types</firstterm>, which are the types whose structure is entirely
395 defined by their 1-character type code. Basic types consist of
396 fixed types and string-like types.
400 The <firstterm id="term-fixed-type">fixed types</firstterm>
401 are basic types whose values have a fixed length, namely BYTE,
402 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
407 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
408 the ASCII character 'i'. So the signature for a block of values
409 containing a single <literal>INT32</literal> would be:
413 A block of values containing two <literal>INT32</literal> would have this signature:
420 The characteristics of the fixed types are listed in this table.
426 <entry>Conventional name</entry>
427 <entry>ASCII type-code</entry>
428 <entry>Encoding</entry>
433 <entry><literal>BYTE</literal></entry>
434 <entry><literal>y</literal> (121)</entry>
435 <entry>Unsigned 8-bit integer</entry>
438 <entry><literal>BOOLEAN</literal></entry>
439 <entry><literal>b</literal> (98)</entry>
440 <entry>Boolean value: 0 is false, 1 is true, any other value
441 allowed by the marshalling format is invalid</entry>
444 <entry><literal>INT16</literal></entry>
445 <entry><literal>n</literal> (110)</entry>
446 <entry>Signed (two's complement) 16-bit integer</entry>
449 <entry><literal>UINT16</literal></entry>
450 <entry><literal>q</literal> (113)</entry>
451 <entry>Unsigned 16-bit integer</entry>
454 <entry><literal>INT32</literal></entry>
455 <entry><literal>i</literal> (105)</entry>
456 <entry>Signed (two's complement) 32-bit integer</entry>
459 <entry><literal>UINT32</literal></entry>
460 <entry><literal>u</literal> (117)</entry>
461 <entry>Unsigned 32-bit integer</entry>
464 <entry><literal>INT64</literal></entry>
465 <entry><literal>x</literal> (120)</entry>
466 <entry>Signed (two's complement) 64-bit integer
467 (mnemonic: x and t are the first characters in "sixty" not
468 already used for something more common)</entry>
471 <entry><literal>UINT64</literal></entry>
472 <entry><literal>t</literal> (116)</entry>
473 <entry>Unsigned 64-bit integer</entry>
476 <entry><literal>DOUBLE</literal></entry>
477 <entry><literal>d</literal> (100)</entry>
478 <entry>IEEE 754 double-precision floating point</entry>
481 <entry><literal>UNIX_FD</literal></entry>
482 <entry><literal>h</literal> (104)</entry>
483 <entry>Unsigned 32-bit integer representing an index into an
484 out-of-band array of file descriptors, transferred via some
485 platform-specific mechanism (mnemonic: h for handle)</entry>
493 The <firstterm id="term-string-like-type">string-like types</firstterm>
494 are basic types with a variable length. The value of any string-like
495 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
496 none of which may be U+0000. The UTF-8 text must be validated
497 strictly: in particular, it must not contain overlong sequences
498 or codepoints above U+10FFFF.
502 Since D-Bus Specification version 0.21, in accordance with Unicode
503 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
504 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
505 D-Bus rejected these noncharacters).
509 The marshalling formats for the string-like types all end with a
510 single zero (NUL) byte, but that byte is not considered to be part of
515 The characteristics of the string-like types are listed in this table.
521 <entry>Conventional name</entry>
522 <entry>ASCII type-code</entry>
523 <entry>Validity constraints</entry>
528 <entry><literal>STRING</literal></entry>
529 <entry><literal>s</literal> (115)</entry>
530 <entry>No extra constraints</entry>
533 <entry><literal>OBJECT_PATH</literal></entry>
534 <entry><literal>o</literal> (111)</entry>
536 <link linkend="message-protocol-marshaling-object-path">a
537 syntactically valid object path</link></entry>
540 <entry><literal>SIGNATURE</literal></entry>
541 <entry><literal>g</literal> (103)</entry>
543 <firstterm linkend="term-single-complete-type">single
544 complete types</firstterm></entry>
551 <sect3 id="message-protocol-marshaling-object-path">
552 <title>Valid Object Paths</title>
555 An object path is a name used to refer to an object instance.
556 Conceptually, each participant in a D-Bus message exchange may have
557 any number of object instances (think of C++ or Java objects) and each
558 such instance will have a path. Like a filesystem, the object
559 instances in an application form a hierarchical tree.
563 Object paths are often namespaced by starting with a reversed
564 domain name and containing an interface version number, in the
566 <link linkend="message-protocol-names-interface">interface
568 <link linkend="message-protocol-names-bus">well-known
570 This makes it possible to implement more than one service, or
571 more than one version of a service, in the same process,
572 even if the services share a connection but cannot otherwise
573 co-operate (for instance, if they are implemented by different
578 For instance, if the owner of <literal>example.com</literal> is
579 developing a D-Bus API for a music player, they might use the
580 hierarchy of object paths that start with
581 <literal>/com/example/MusicPlayer1</literal> for its objects.
585 The following rules define a valid object path. Implementations must
586 not send or accept messages with invalid object paths.
590 The path may be of any length.
595 The path must begin with an ASCII '/' (integer 47) character,
596 and must consist of elements separated by slash characters.
601 Each element must only contain the ASCII characters
607 No element may be the empty string.
612 Multiple '/' characters cannot occur in sequence.
617 A trailing '/' character is not allowed unless the
618 path is the root path (a single '/' character).
626 <sect3 id="message-protocol-marshaling-signature">
627 <title>Valid Signatures</title>
629 An implementation must not send or accept invalid signatures.
630 Valid signatures will conform to the following rules:
634 The signature is a list of single complete types.
635 Arrays must have element types, and structs must
636 have both open and close parentheses.
641 Only type codes, open and close parentheses, and open and
642 close curly brackets are allowed in the signature. The
643 <literal>STRUCT</literal> type code
644 is not allowed in signatures, because parentheses
645 are used instead. Similarly, the
646 <literal>DICT_ENTRY</literal> type code is not allowed in
647 signatures, because curly brackets are used instead.
652 The maximum depth of container type nesting is 32 array type
653 codes and 32 open parentheses. This implies that the maximum
654 total depth of recursion is 64, for an "array of array of array
655 of ... struct of struct of struct of ..." where there are 32
661 The maximum length of a signature is 255.
668 When signatures appear in messages, the marshalling format
669 guarantees that they will be followed by a nul byte (which can
670 be interpreted as either C-style string termination or the INVALID
671 type-code), but this is not conceptually part of the signature.
677 <sect2 id="container-types">
678 <title>Container types</title>
681 In addition to basic types, there are four <firstterm>container</firstterm>
682 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
683 and <literal>DICT_ENTRY</literal>.
687 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
688 code does not appear in signatures. Instead, ASCII characters
689 '(' and ')' are used to mark the beginning and end of the struct.
690 So for example, a struct containing two integers would have this
695 Structs can be nested, so for example a struct containing
696 an integer and another struct:
700 The value block storing that struct would contain three integers; the
701 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
706 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
707 but is useful in code that implements the protocol. This type code
708 is specified to allow such code to interoperate in non-protocol contexts.
712 Empty structures are not allowed; there must be at least one
713 type code between the parentheses.
717 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
718 followed by a <firstterm>single complete type</firstterm>. The single
719 complete type following the array is the type of each array element. So
720 the simple example is:
724 which is an array of 32-bit integers. But an array can be of any type,
725 such as this array-of-struct-with-two-int32-fields:
729 Or this array of array of integer:
736 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
737 type <literal>VARIANT</literal> will have the signature of a single complete type as part
738 of the <emphasis>value</emphasis>. This signature will be followed by a
739 marshaled value of that type.
743 Unlike a message signature, the variant signature can
744 contain only a single complete type. So "i", "ai"
745 or "(ii)" is OK, but "ii" is not. Use of variants may not
746 cause a total message depth to be larger than 64, including
747 other container types such as structures.
751 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
752 than parentheses it uses curly braces, and it has more restrictions.
753 The restrictions are: it occurs only as an array element type; it has
754 exactly two single complete types inside the curly braces; the first
755 single complete type (the "key") must be a basic type rather than a
756 container type. Implementations must not accept dict entries outside of
757 arrays, must not accept dict entries with zero, one, or more than two
758 fields, and must not accept dict entries with non-basic-typed keys. A
759 dict entry is always a key-value pair.
763 The first field in the <literal>DICT_ENTRY</literal> is always the key.
764 A message is considered corrupt if the same key occurs twice in the same
765 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
766 implementations are not required to reject dicts with duplicate keys.
770 In most languages, an array of dict entry would be represented as a
771 map, hash table, or dict object.
776 <title>Summary of types</title>
779 The following table summarizes the D-Bus types.
784 <entry>Category</entry>
785 <entry>Conventional Name</entry>
787 <entry>Description</entry>
792 <entry>reserved</entry>
793 <entry><literal>INVALID</literal></entry>
794 <entry>0 (ASCII NUL)</entry>
795 <entry>Not a valid type code, used to terminate signatures</entry>
797 <entry>fixed, basic</entry>
798 <entry><literal>BYTE</literal></entry>
799 <entry>121 (ASCII 'y')</entry>
800 <entry>8-bit unsigned integer</entry>
802 <entry>fixed, basic</entry>
803 <entry><literal>BOOLEAN</literal></entry>
804 <entry>98 (ASCII 'b')</entry>
805 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
807 <entry>fixed, basic</entry>
808 <entry><literal>INT16</literal></entry>
809 <entry>110 (ASCII 'n')</entry>
810 <entry>16-bit signed integer</entry>
812 <entry>fixed, basic</entry>
813 <entry><literal>UINT16</literal></entry>
814 <entry>113 (ASCII 'q')</entry>
815 <entry>16-bit unsigned integer</entry>
817 <entry>fixed, basic</entry>
818 <entry><literal>INT32</literal></entry>
819 <entry>105 (ASCII 'i')</entry>
820 <entry>32-bit signed integer</entry>
822 <entry>fixed, basic</entry>
823 <entry><literal>UINT32</literal></entry>
824 <entry>117 (ASCII 'u')</entry>
825 <entry>32-bit unsigned integer</entry>
827 <entry>fixed, basic</entry>
828 <entry><literal>INT64</literal></entry>
829 <entry>120 (ASCII 'x')</entry>
830 <entry>64-bit signed integer</entry>
832 <entry>fixed, basic</entry>
833 <entry><literal>UINT64</literal></entry>
834 <entry>116 (ASCII 't')</entry>
835 <entry>64-bit unsigned integer</entry>
837 <entry>fixed, basic</entry>
838 <entry><literal>DOUBLE</literal></entry>
839 <entry>100 (ASCII 'd')</entry>
840 <entry>IEEE 754 double</entry>
842 <entry>string-like, basic</entry>
843 <entry><literal>STRING</literal></entry>
844 <entry>115 (ASCII 's')</entry>
845 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
847 <entry>string-like, basic</entry>
848 <entry><literal>OBJECT_PATH</literal></entry>
849 <entry>111 (ASCII 'o')</entry>
850 <entry>Name of an object instance</entry>
852 <entry>string-like, basic</entry>
853 <entry><literal>SIGNATURE</literal></entry>
854 <entry>103 (ASCII 'g')</entry>
855 <entry>A type signature</entry>
857 <entry>container</entry>
858 <entry><literal>ARRAY</literal></entry>
859 <entry>97 (ASCII 'a')</entry>
862 <entry>container</entry>
863 <entry><literal>STRUCT</literal></entry>
864 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
865 <entry>Struct; type code 114 'r' is reserved for use in
866 bindings and implementations to represent the general
867 concept of a struct, and must not appear in signatures
868 used on D-Bus.</entry>
870 <entry>container</entry>
871 <entry><literal>VARIANT</literal></entry>
872 <entry>118 (ASCII 'v') </entry>
873 <entry>Variant type (the type of the value is part of the value itself)</entry>
875 <entry>container</entry>
876 <entry><literal>DICT_ENTRY</literal></entry>
877 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
878 <entry>Entry in a dict or map (array of key-value pairs).
879 Type code 101 'e' is reserved for use in bindings and
880 implementations to represent the general concept of a
881 dict or dict-entry, and must not appear in signatures
882 used on D-Bus.</entry>
884 <entry>fixed, basic</entry>
885 <entry><literal>UNIX_FD</literal></entry>
886 <entry>104 (ASCII 'h')</entry>
887 <entry>Unix file descriptor</entry>
890 <entry>reserved</entry>
891 <entry>(reserved)</entry>
892 <entry>109 (ASCII 'm')</entry>
893 <entry>Reserved for <ulink
894 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
895 'maybe' type compatible with the one in GVariant</ulink>,
896 and must not appear in signatures used on D-Bus until
897 specified here</entry>
900 <entry>reserved</entry>
901 <entry>(reserved)</entry>
902 <entry>42 (ASCII '*')</entry>
903 <entry>Reserved for use in bindings/implementations to
904 represent any <firstterm>single complete type</firstterm>,
905 and must not appear in signatures used on D-Bus.</entry>
908 <entry>reserved</entry>
909 <entry>(reserved)</entry>
910 <entry>63 (ASCII '?')</entry>
911 <entry>Reserved for use in bindings/implementations to
912 represent any <firstterm>basic type</firstterm>, and must
913 not appear in signatures used on D-Bus.</entry>
916 <entry>reserved</entry>
917 <entry>(reserved)</entry>
918 <entry>64 (ASCII '@'), 38 (ASCII '&'),
919 94 (ASCII '^')</entry>
920 <entry>Reserved for internal use by bindings/implementations,
921 and must not appear in signatures used on D-Bus.
922 GVariant uses these type-codes to encode calling
933 <sect1 id="message-protocol-marshaling">
934 <title>Marshaling (Wire Format)</title>
937 D-Bus defines a marshalling format for its type system, which is
938 used in D-Bus messages. This is not the only possible marshalling
939 format for the type system: for instance, GVariant (part of GLib)
940 re-uses the D-Bus type system but implements an alternative marshalling
945 <title>Byte order and alignment</title>
948 Given a type signature, a block of bytes can be converted into typed
949 values. This section describes the format of the block of bytes. Byte
950 order and alignment issues are handled uniformly for all D-Bus types.
954 A block of bytes has an associated byte order. The byte order
955 has to be discovered in some way; for D-Bus messages, the
956 byte order is part of the message header as described in
957 <xref linkend="message-protocol-messages"/>. For now, assume
958 that the byte order is known to be either little endian or big
963 Each value in a block of bytes is aligned "naturally," for example
964 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
965 8-byte boundary. To properly align a value, <firstterm>alignment
966 padding</firstterm> may be necessary. The alignment padding must always
967 be the minimum required padding to properly align the following value;
968 and it must always be made up of nul bytes. The alignment padding must
969 not be left uninitialized (it can't contain garbage), and more padding
970 than required must not be used.
974 As an exception to natural alignment, <literal>STRUCT</literal> and
975 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
976 boundary, regardless of the alignments of their contents.
981 <title>Marshalling basic types</title>
984 To marshal and unmarshal fixed types, you simply read one value
985 from the data block corresponding to each type code in the signature.
986 All signed integer values are encoded in two's complement, DOUBLE
987 values are IEEE 754 double-precision floating-point, and BOOLEAN
988 values are encoded in 32 bits (of which only the least significant
993 The string-like types are all marshalled as a
994 fixed-length unsigned integer <varname>n</varname> giving the
995 length of the variable part, followed by <varname>n</varname>
996 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
997 which is not considered to be part of the text. The alignment
998 of the string-like type is the same as the alignment of
999 <varname>n</varname>.
1003 For the STRING and OBJECT_PATH types, <varname>n</varname> is
1004 encoded in 4 bytes, leading to 4-byte alignment.
1005 For the SIGNATURE type, <varname>n</varname> is encoded as a single
1006 byte. As a result, alignment padding is never required before a
1012 <title>Marshalling containers</title>
1015 Arrays are marshalled as a <literal>UINT32</literal>
1016 <varname>n</varname> giving the length of the array data in bytes,
1017 followed by alignment padding to the alignment boundary of the array
1018 element type, followed by the <varname>n</varname> bytes of the
1019 array elements marshalled in sequence. <varname>n</varname> does not
1020 include the padding after the length, or any padding after the
1025 For instance, if the current position in the message is a multiple
1026 of 8 bytes and the byte-order is big-endian, an array containing only
1027 the 64-bit integer 5 would be marshalled as:
1030 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
1031 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1032 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1037 Arrays have a maximum length defined to be 2 to the 26th power or
1038 67108864 (64 MiB). Implementations must not send or accept arrays
1039 exceeding this length.
1043 Structs and dict entries are marshalled in the same way as their
1044 contents, but their alignment is always to an 8-byte boundary,
1045 even if their contents would normally be less strictly aligned.
1049 Variants are marshalled as the <literal>SIGNATURE</literal> of
1050 the contents (which must be a single complete type), followed by a
1051 marshalled value with the type given by that signature. The
1052 variant has the same 1-byte alignment as the signature, which means
1053 that alignment padding before a variant is never needed.
1054 Use of variants may not cause a total message depth to be larger
1055 than 64, including other container types such as structures.
1060 <title>Summary of D-Bus marshalling</title>
1063 Given all this, the types are marshaled on the wire as follows:
1068 <entry>Conventional Name</entry>
1069 <entry>Encoding</entry>
1070 <entry>Alignment</entry>
1075 <entry><literal>INVALID</literal></entry>
1076 <entry>Not applicable; cannot be marshaled.</entry>
1079 <entry><literal>BYTE</literal></entry>
1080 <entry>A single 8-bit byte.</entry>
1083 <entry><literal>BOOLEAN</literal></entry>
1084 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1087 <entry><literal>INT16</literal></entry>
1088 <entry>16-bit signed integer in the message's byte order.</entry>
1091 <entry><literal>UINT16</literal></entry>
1092 <entry>16-bit unsigned integer in the message's byte order.</entry>
1095 <entry><literal>INT32</literal></entry>
1096 <entry>32-bit signed integer in the message's byte order.</entry>
1099 <entry><literal>UINT32</literal></entry>
1100 <entry>32-bit unsigned integer in the message's byte order.</entry>
1103 <entry><literal>INT64</literal></entry>
1104 <entry>64-bit signed integer in the message's byte order.</entry>
1107 <entry><literal>UINT64</literal></entry>
1108 <entry>64-bit unsigned integer in the message's byte order.</entry>
1111 <entry><literal>DOUBLE</literal></entry>
1112 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1115 <entry><literal>STRING</literal></entry>
1116 <entry>A <literal>UINT32</literal> indicating the string's
1117 length in bytes excluding its terminating nul, followed by
1118 non-nul string data of the given length, followed by a terminating nul
1125 <entry><literal>OBJECT_PATH</literal></entry>
1126 <entry>Exactly the same as <literal>STRING</literal> except the
1127 content must be a valid object path (see above).
1133 <entry><literal>SIGNATURE</literal></entry>
1134 <entry>The same as <literal>STRING</literal> except the length is a single
1135 byte (thus signatures have a maximum length of 255)
1136 and the content must be a valid signature (see above).
1142 <entry><literal>ARRAY</literal></entry>
1144 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1145 alignment padding to the alignment boundary of the array element type,
1146 followed by each array element.
1152 <entry><literal>STRUCT</literal></entry>
1154 A struct must start on an 8-byte boundary regardless of the
1155 type of the struct fields. The struct value consists of each
1156 field marshaled in sequence starting from that 8-byte
1163 <entry><literal>VARIANT</literal></entry>
1165 The marshaled <literal>SIGNATURE</literal> of a single
1166 complete type, followed by a marshaled value with the type
1167 given in the signature.
1170 1 (alignment of the signature)
1173 <entry><literal>DICT_ENTRY</literal></entry>
1175 Identical to STRUCT.
1181 <entry><literal>UNIX_FD</literal></entry>
1182 <entry>32-bit unsigned integer in the message's byte
1183 order. The actual file descriptors need to be
1184 transferred out-of-band via some platform specific
1185 mechanism. On the wire, values of this type store the index to the
1186 file descriptor in the array of file descriptors that
1187 accompany the message.</entry>
1199 <sect1 id="message-protocol">
1200 <title>Message Protocol</title>
1203 A <firstterm>message</firstterm> consists of a
1204 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1205 think of a message as a package, the header is the address, and the body
1206 contains the package contents. The message delivery system uses the header
1207 information to figure out where to send the message and how to interpret
1208 it; the recipient interprets the body of the message.
1212 The body of the message is made up of zero or more
1213 <firstterm>arguments</firstterm>, which are typed values, such as an
1214 integer or a byte array.
1218 Both header and body use the D-Bus <link linkend="type-system">type
1219 system</link> and format for serializing data.
1222 <sect2 id="message-protocol-messages">
1223 <title>Message Format</title>
1226 A message consists of a header and a body. The header is a block of
1227 values with a fixed signature and meaning. The body is a separate block
1228 of values, with a signature specified in the header.
1232 The length of the header must be a multiple of 8, allowing the body to
1233 begin on an 8-byte boundary when storing the entire message in a single
1234 buffer. If the header does not naturally end on an 8-byte boundary
1235 up to 7 bytes of nul-initialized alignment padding must be added.
1239 The message body need not end on an 8-byte boundary.
1243 The maximum length of a message, including header, header alignment padding,
1244 and body is 2 to the 27th power or 134217728 (128 MiB).
1245 Implementations must not send or accept messages exceeding this size.
1249 The signature of the header is:
1253 Written out more readably, this is:
1255 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1260 These values have the following meanings:
1265 <entry>Value</entry>
1266 <entry>Description</entry>
1271 <entry>1st <literal>BYTE</literal></entry>
1272 <entry>Endianness flag; ASCII 'l' for little-endian
1273 or ASCII 'B' for big-endian. Both header and body are
1274 in this endianness.</entry>
1277 <entry>2nd <literal>BYTE</literal></entry>
1278 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1279 Currently-defined types are described below.
1283 <entry>3rd <literal>BYTE</literal></entry>
1284 <entry>Bitwise OR of flags. Unknown flags
1285 must be ignored. Currently-defined flags are described below.
1289 <entry>4th <literal>BYTE</literal></entry>
1290 <entry>Major protocol version of the sending application. If
1291 the major protocol version of the receiving application does not
1292 match, the applications will not be able to communicate and the
1293 D-Bus connection must be disconnected. The major protocol
1294 version for this version of the specification is 1.
1298 <entry>1st <literal>UINT32</literal></entry>
1299 <entry>Length in bytes of the message body, starting
1300 from the end of the header. The header ends after
1301 its alignment padding to an 8-boundary.
1305 <entry>2nd <literal>UINT32</literal></entry>
1306 <entry>The serial of this message, used as a cookie
1307 by the sender to identify the reply corresponding
1308 to this request. This must not be zero.
1312 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1313 <entry>An array of zero or more <firstterm>header
1314 fields</firstterm> where the byte is the field code, and the
1315 variant is the field value. The message type determines
1316 which fields are required.
1324 <firstterm>Message types</firstterm> that can appear in the second byte
1330 <entry>Conventional name</entry>
1331 <entry>Decimal value</entry>
1332 <entry>Description</entry>
1337 <entry><literal>INVALID</literal></entry>
1339 <entry>This is an invalid type.</entry>
1342 <entry><literal>METHOD_CALL</literal></entry>
1344 <entry>Method call. This message type may prompt a
1348 <entry><literal>METHOD_RETURN</literal></entry>
1350 <entry>Method reply with returned data.</entry>
1353 <entry><literal>ERROR</literal></entry>
1355 <entry>Error reply. If the first argument exists and is a
1356 string, it is an error message.</entry>
1359 <entry><literal>SIGNAL</literal></entry>
1361 <entry>Signal emission.</entry>
1368 Flags that can appear in the third byte of the header:
1373 <entry>Conventional name</entry>
1374 <entry>Hex value</entry>
1375 <entry>Description</entry>
1380 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1384 This message does not expect method return replies or
1385 error replies, even if it is of a type that can
1386 have a reply; the reply should be omitted.
1389 Note that METHOD_CALL is the only message type currently
1390 defined in this specification that can expect a reply,
1391 so the presence or absence of this flag in the other
1392 three message types that are currently
1393 documented is meaningless: replies to those message
1394 types should not be sent, whether this flag is present
1400 <entry><literal>NO_AUTO_START</literal></entry>
1402 <entry>The bus must not launch an owner
1403 for the destination name in response to this message.
1407 <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
1411 This flag may be set on a method call message to
1412 inform the receiving side that the caller is prepared
1413 to wait for interactive authorization, which might
1414 take a considerable time to complete. For instance,
1415 if this flag is set, it would be appropriate to
1416 query the user for passwords or confirmation via
1417 Polkit or a similar framework.
1420 This flag is only useful when
1421 unprivileged code calls a more privileged method call,
1422 and an authorization framework is deployed that allows
1423 possibly interactive authorization. If no such framework
1424 is deployed it has no effect. This flag should not
1425 be set by default by client implementations. If it is
1426 set, the caller should also set a suitably long timeout
1427 on the method call to make sure the user interaction
1428 may complete. This flag is only valid for method call
1429 messages, and shall be ignored otherwise.
1432 Interaction that takes place as a part of the
1433 effect of the method being called is outside the scope
1434 of this flag, even if it could also be characterized
1435 as authentication or authorization. For instance, in
1436 a method call that directs a network management service
1437 to attempt to connect to a virtual private network,
1438 this flag should control how the network management
1439 service makes the decision "is this user allowed to
1440 change system network configuration?", but it should
1441 not affect how or whether the network management
1442 service interacts with the user to obtain the credentials
1443 that are required for access to the VPN.
1446 If a this flag is not set on a method call, and a
1447 service determines that the requested operation is
1448 not allowed without interactive authorization, but
1449 could be allowed after successful interactive
1450 authorization, it may return the
1451 <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
1455 The absence of this flag does not guarantee that
1456 interactive authorization will not be applied, since
1457 existing services that pre-date this flag might
1458 already use interactive authorization. However,
1459 existing D-Bus APIs that will use interactive
1460 authorization should document that the call may take
1461 longer than usual, and new D-Bus APIs should avoid
1462 interactive authorization in the absence of this flag.
1471 <sect3 id="message-protocol-header-fields">
1472 <title>Header Fields</title>
1475 The array at the end of the header contains <firstterm>header
1476 fields</firstterm>, where each field is a 1-byte field code followed
1477 by a field value. A header must contain the required header fields for
1478 its message type, and zero or more of any optional header
1479 fields. Future versions of this protocol specification may add new
1480 fields. Implementations must ignore fields they do not
1481 understand. Implementations must not invent their own header fields;
1482 only changes to this specification may introduce new header fields.
1486 Again, if an implementation sees a header field code that it does not
1487 expect, it must ignore that field, as it will be part of a new
1488 (but compatible) version of this specification. This also applies
1489 to known header fields appearing in unexpected messages, for
1490 example: if a signal has a reply serial it must be ignored
1491 even though it has no meaning as of this version of the spec.
1495 However, implementations must not send or accept known header fields
1496 with the wrong type stored in the field value. So for example a
1497 message with an <literal>INTERFACE</literal> field of type
1498 <literal>UINT32</literal> would be considered corrupt.
1502 Here are the currently-defined header fields:
1507 <entry>Conventional Name</entry>
1508 <entry>Decimal Code</entry>
1510 <entry>Required In</entry>
1511 <entry>Description</entry>
1516 <entry><literal>INVALID</literal></entry>
1519 <entry>not allowed</entry>
1520 <entry>Not a valid field name (error if it appears in a message)</entry>
1523 <entry><literal>PATH</literal></entry>
1525 <entry><literal>OBJECT_PATH</literal></entry>
1526 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1527 <entry>The object to send a call to,
1528 or the object a signal is emitted from.
1530 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1531 implementations should not send messages with this path,
1532 and the reference implementation of the bus daemon will
1533 disconnect any application that attempts to do so.
1537 <entry><literal>INTERFACE</literal></entry>
1539 <entry><literal>STRING</literal></entry>
1540 <entry><literal>SIGNAL</literal></entry>
1542 The interface to invoke a method call on, or
1543 that a signal is emitted from. Optional for
1544 method calls, required for signals.
1545 The special interface
1546 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1547 implementations should not send messages with this
1548 interface, and the reference implementation of the bus
1549 daemon will disconnect any application that attempts to
1554 <entry><literal>MEMBER</literal></entry>
1556 <entry><literal>STRING</literal></entry>
1557 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1558 <entry>The member, either the method name or signal name.</entry>
1561 <entry><literal>ERROR_NAME</literal></entry>
1563 <entry><literal>STRING</literal></entry>
1564 <entry><literal>ERROR</literal></entry>
1565 <entry>The name of the error that occurred, for errors</entry>
1568 <entry><literal>REPLY_SERIAL</literal></entry>
1570 <entry><literal>UINT32</literal></entry>
1571 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1572 <entry>The serial number of the message this message is a reply
1573 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1576 <entry><literal>DESTINATION</literal></entry>
1578 <entry><literal>STRING</literal></entry>
1579 <entry>optional</entry>
1580 <entry>The name of the connection this message is intended for.
1581 Only used in combination with the message bus, see
1582 <xref linkend="message-bus"/>.</entry>
1585 <entry><literal>SENDER</literal></entry>
1587 <entry><literal>STRING</literal></entry>
1588 <entry>optional</entry>
1589 <entry>Unique name of the sending connection.
1590 The message bus fills in this field so it is reliable; the field is
1591 only meaningful in combination with the message bus.</entry>
1594 <entry><literal>SIGNATURE</literal></entry>
1596 <entry><literal>SIGNATURE</literal></entry>
1597 <entry>optional</entry>
1598 <entry>The signature of the message body.
1599 If omitted, it is assumed to be the
1600 empty signature "" (i.e. the body must be 0-length).</entry>
1603 <entry><literal>UNIX_FDS</literal></entry>
1605 <entry><literal>UINT32</literal></entry>
1606 <entry>optional</entry>
1607 <entry>The number of Unix file descriptors that
1608 accompany the message. If omitted, it is assumed
1609 that no Unix file descriptors accompany the
1610 message. The actual file descriptors need to be
1611 transferred via platform specific mechanism
1612 out-of-band. They must be sent at the same time as
1613 part of the message itself. They may not be sent
1614 before the first byte of the message itself is
1615 transferred or after the last byte of the message
1625 <sect2 id="message-protocol-names">
1626 <title>Valid Names</title>
1628 The various names in D-Bus messages have some restrictions.
1631 There is a <firstterm>maximum name length</firstterm>
1632 of 255 which applies to bus names, interfaces, and members.
1634 <sect3 id="message-protocol-names-interface">
1635 <title>Interface names</title>
1637 Interfaces have names with type <literal>STRING</literal>, meaning that
1638 they must be valid UTF-8. However, there are also some
1639 additional restrictions that apply to interface names
1642 <listitem><para>Interface names are composed of 1 or more elements separated by
1643 a period ('.') character. All elements must contain at least
1647 <listitem><para>Each element must only contain the ASCII characters
1648 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1652 <listitem><para>Interface names must contain at least one '.' (period)
1653 character (and thus at least two elements).
1656 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1657 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1662 Interface names should start with the reversed DNS domain name of
1663 the author of the interface (in lower-case), like interface names
1664 in Java. It is conventional for the rest of the interface name
1665 to consist of words run together, with initial capital letters
1666 on all words ("CamelCase"). Several levels of hierarchy can be used.
1667 It is also a good idea to include the major version of the interface
1668 in the name, and increment it if incompatible changes are made;
1669 this way, a single object can implement several versions of an
1670 interface in parallel, if necessary.
1674 For instance, if the owner of <literal>example.com</literal> is
1675 developing a D-Bus API for a music player, they might define
1676 interfaces called <literal>com.example.MusicPlayer1</literal>,
1677 <literal>com.example.MusicPlayer1.Track</literal> and
1678 <literal>com.example.MusicPlayer1.Seekable</literal>.
1682 D-Bus does not distinguish between the concepts that would be
1683 called classes and interfaces in Java: either can be identified on
1684 D-Bus by an interface name.
1687 <sect3 id="message-protocol-names-bus">
1688 <title>Bus names</title>
1690 Connections have one or more bus names associated with them.
1691 A connection has exactly one bus name that is a <firstterm>unique
1692 connection name</firstterm>. The unique connection name remains
1693 with the connection for its entire lifetime.
1694 A bus name is of type <literal>STRING</literal>,
1695 meaning that it must be valid UTF-8. However, there are also
1696 some additional restrictions that apply to bus names
1699 <listitem><para>Bus names that start with a colon (':')
1700 character are unique connection names. Other bus names
1701 are called <firstterm>well-known bus names</firstterm>.
1704 <listitem><para>Bus names are composed of 1 or more elements separated by
1705 a period ('.') character. All elements must contain at least
1709 <listitem><para>Each element must only contain the ASCII characters
1710 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1711 connection name may begin with a digit, elements in
1712 other bus names must not begin with a digit.
1716 <listitem><para>Bus names must contain at least one '.' (period)
1717 character (and thus at least two elements).
1720 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1721 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1725 Note that the hyphen ('-') character is allowed in bus names but
1726 not in interface names.
1730 Like <link linkend="message-protocol-names-interface">interface
1731 names</link>, well-known bus names should start with the
1732 reversed DNS domain name of the author of the interface (in
1733 lower-case), and it is conventional for the rest of the well-known
1734 bus name to consist of words run together, with initial
1735 capital letters. As with interface names, including a version
1736 number in well-known bus names is a good idea; it's possible to
1737 have the well-known bus name for more than one version
1738 simultaneously if backwards compatibility is required.
1742 If a well-known bus name implies the presence of a "main" interface,
1743 that "main" interface is often given the same name as
1744 the well-known bus name, and situated at the corresponding object
1745 path. For instance, if the owner of <literal>example.com</literal>
1746 is developing a D-Bus API for a music player, they might define
1747 that any application that takes the well-known name
1748 <literal>com.example.MusicPlayer1</literal> should have an object
1749 at the object path <literal>/com/example/MusicPlayer1</literal>
1750 which implements the interface
1751 <literal>com.example.MusicPlayer1</literal>.
1754 <sect3 id="message-protocol-names-member">
1755 <title>Member names</title>
1757 Member (i.e. method or signal) names:
1759 <listitem><para>Must only contain the ASCII characters
1760 "[A-Z][a-z][0-9]_" and may not begin with a
1761 digit.</para></listitem>
1762 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1763 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1764 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1769 It is conventional for member names on D-Bus to consist of
1770 capitalized words with no punctuation ("camel-case").
1771 Method names should usually be verbs, such as
1772 <literal>GetItems</literal>, and signal names should usually be
1773 a description of an event, such as <literal>ItemsChanged</literal>.
1776 <sect3 id="message-protocol-names-error">
1777 <title>Error names</title>
1779 Error names have the same restrictions as interface names.
1783 Error names have the same naming conventions as interface
1784 names, and often contain <literal>.Error.</literal>; for instance,
1785 the owner of <literal>example.com</literal> might define the
1786 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1787 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1788 The errors defined by D-Bus itself, such as
1789 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1795 <sect2 id="message-protocol-types">
1796 <title>Message Types</title>
1798 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1799 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1800 This section describes these conventions.
1802 <sect3 id="message-protocol-types-method">
1803 <title>Method Calls</title>
1805 Some messages invoke an operation on a remote object. These are
1806 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1807 messages map naturally to methods on objects in a typical program.
1810 A method call message is required to have a <literal>MEMBER</literal> header field
1811 indicating the name of the method. Optionally, the message has an
1812 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1813 Including the <literal>INTERFACE</literal> in all method call
1814 messages is strongly recommended.
1817 In the absence of an <literal>INTERFACE</literal> field, if two
1818 or more interfaces on the same object have a method with the same
1819 name, it is undefined which of those methods will be invoked.
1820 Implementations may choose to either return an error, or deliver the
1821 message as though it had an arbitrary one of those interfaces.
1824 In some situations (such as the well-known system bus), messages
1825 are filtered through an access-control list external to the
1826 remote object implementation. If that filter rejects certain
1827 messages by matching their interface, or accepts only messages
1828 to specific interfaces, it must also reject messages that have no
1829 <literal>INTERFACE</literal>: otherwise, malicious
1830 applications could use this to bypass the filter.
1833 Method call messages also include a <literal>PATH</literal> field
1834 indicating the object to invoke the method on. If the call is passing
1835 through a message bus, the message will also have a
1836 <literal>DESTINATION</literal> field giving the name of the connection
1837 to receive the message.
1840 When an application handles a method call message, it is required to
1841 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1842 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1843 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1846 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1847 are the return value(s) or "out parameters" of the method call.
1848 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1849 and the call fails; no return value will be provided. It makes
1850 no sense to send multiple replies to the same method call.
1853 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1854 reply is required, so the caller will know the method
1855 was successfully processed.
1858 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1862 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1863 then the application receiving the method should not send the reply message (regardless of
1864 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1867 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1868 destination name does not exist then a program to own the destination
1869 name will be started before the message is delivered. The message
1870 will be held until the new program is successfully started or has
1871 failed to start; in case of failure, an error will be returned. This
1872 flag is only relevant in the context of a message bus, it is ignored
1873 during one-to-one communication with no intermediate bus.
1875 <sect4 id="message-protocol-types-method-apis">
1876 <title>Mapping method calls to native APIs</title>
1878 APIs for D-Bus may map method calls to a method call in a specific
1879 programming language, such as C++, or may map a method call written
1880 in an IDL to a D-Bus message.
1883 In APIs of this nature, arguments to a method are often termed "in"
1884 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1885 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1886 "inout" arguments, which are both sent and received, i.e. the caller
1887 passes in a value which is modified. Mapped to D-Bus, an "inout"
1888 argument is equivalent to an "in" argument, followed by an "out"
1889 argument. You can't pass things "by reference" over the wire, so
1890 "inout" is purely an illusion of the in-process API.
1893 Given a method with zero or one return values, followed by zero or more
1894 arguments, where each argument may be "in", "out", or "inout", the
1895 caller constructs a message by appending each "in" or "inout" argument,
1896 in order. "out" arguments are not represented in the caller's message.
1899 The recipient constructs a reply by appending first the return value
1900 if any, then each "out" or "inout" argument, in order.
1901 "in" arguments are not represented in the reply message.
1904 Error replies are normally mapped to exceptions in languages that have
1908 In converting from native APIs to D-Bus, it is perhaps nice to
1909 map D-Bus naming conventions ("FooBar") to native conventions
1910 such as "fooBar" or "foo_bar" automatically. This is OK
1911 as long as you can say that the native API is one that
1912 was specifically written for D-Bus. It makes the most sense
1913 when writing object implementations that will be exported
1914 over the bus. Object proxies used to invoke remote D-Bus
1915 objects probably need the ability to call any D-Bus method,
1916 and thus a magic name mapping like this could be a problem.
1919 This specification doesn't require anything of native API bindings;
1920 the preceding is only a suggested convention for consistency
1926 <sect3 id="message-protocol-types-signal">
1927 <title>Signal Emission</title>
1929 Unlike method calls, signal emissions have no replies.
1930 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1931 It must have three header fields: <literal>PATH</literal> giving the object
1932 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1933 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1934 for signals, though it is optional for method calls.
1938 <sect3 id="message-protocol-types-errors">
1939 <title>Errors</title>
1941 Messages of type <literal>ERROR</literal> are most commonly replies
1942 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1943 to any kind of message. The message bus for example
1944 will return an <literal>ERROR</literal> in reply to a signal emission if
1945 the bus does not have enough memory to send the signal.
1948 An <literal>ERROR</literal> may have any arguments, but if the first
1949 argument is a <literal>STRING</literal>, it must be an error message.
1950 The error message may be logged or shown to the user
1955 <sect3 id="message-protocol-types-notation">
1956 <title>Notation in this document</title>
1958 This document uses a simple pseudo-IDL to describe particular method
1959 calls and signals. Here is an example of a method call:
1961 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1962 out UINT32 resultcode)
1964 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1965 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1966 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1967 characters so it's known that the last part of the name in
1968 the "IDL" is the member name.
1971 In C++ that might end up looking like this:
1973 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1974 unsigned int flags);
1976 or equally valid, the return value could be done as an argument:
1978 void org::freedesktop::DBus::StartServiceByName (const char *name,
1980 unsigned int *resultcode);
1982 It's really up to the API designer how they want to make
1983 this look. You could design an API where the namespace wasn't used
1984 in C++, using STL or Qt, using varargs, or whatever you wanted.
1987 Signals are written as follows:
1989 org.freedesktop.DBus.NameLost (STRING name)
1991 Signals don't specify "in" vs. "out" because only
1992 a single direction is possible.
1995 It isn't especially encouraged to use this lame pseudo-IDL in actual
1996 API implementations; you might use the native notation for the
1997 language you're using, or you might use COM or CORBA IDL, for example.
2002 <sect2 id="message-protocol-handling-invalid">
2003 <title>Invalid Protocol and Spec Extensions</title>
2006 For security reasons, the D-Bus protocol should be strictly parsed and
2007 validated, with the exception of defined extension points. Any invalid
2008 protocol or spec violations should result in immediately dropping the
2009 connection without notice to the other end. Exceptions should be
2010 carefully considered, e.g. an exception may be warranted for a
2011 well-understood idiosyncrasy of a widely-deployed implementation. In
2012 cases where the other end of a connection is 100% trusted and known to
2013 be friendly, skipping validation for performance reasons could also make
2014 sense in certain cases.
2018 Generally speaking violations of the "must" requirements in this spec
2019 should be considered possible attempts to exploit security, and violations
2020 of the "should" suggestions should be considered legitimate (though perhaps
2021 they should generate an error in some cases).
2025 The following extension points are built in to D-Bus on purpose and must
2026 not be treated as invalid protocol. The extension points are intended
2027 for use by future versions of this spec, they are not intended for third
2028 parties. At the moment, the only way a third party could extend D-Bus
2029 without breaking interoperability would be to introduce a way to negotiate new
2030 feature support as part of the auth protocol, using EXTENSION_-prefixed
2031 commands. There is not yet a standard way to negotiate features.
2035 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
2036 commands result in an ERROR rather than a disconnect. This enables
2037 future extensions to the protocol. Commands starting with EXTENSION_ are
2038 reserved for third parties.
2043 The authentication protocol supports pluggable auth mechanisms.
2048 The address format (see <xref linkend="addresses"/>) supports new
2054 Messages with an unknown type (something other than
2055 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
2056 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
2057 Unknown-type messages must still be well-formed in the same way
2058 as the known messages, however. They still have the normal
2064 Header fields with an unknown or unexpected field code must be ignored,
2065 though again they must still be well-formed.
2070 New standard interfaces (with new methods and signals) can of course be added.
2080 <sect1 id="auth-protocol">
2081 <title>Authentication Protocol</title>
2083 Before the flow of messages begins, two applications must
2084 authenticate. A simple plain-text protocol is used for
2085 authentication; this protocol is a SASL profile, and maps fairly
2086 directly from the SASL specification. The message encoding is
2087 NOT used here, only plain text messages.
2090 In examples, "C:" and "S:" indicate lines sent by the client and
2091 server respectively.
2093 <sect2 id="auth-protocol-overview">
2094 <title>Protocol Overview</title>
2096 The protocol is a line-based protocol, where each line ends with
2097 \r\n. Each line begins with an all-caps ASCII command name containing
2098 only the character range [A-Z_], a space, then any arguments for the
2099 command, then the \r\n ending the line. The protocol is
2100 case-sensitive. All bytes must be in the ASCII character set.
2102 Commands from the client to the server are as follows:
2105 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2106 <listitem><para>CANCEL</para></listitem>
2107 <listitem><para>BEGIN</para></listitem>
2108 <listitem><para>DATA <data in hex encoding></para></listitem>
2109 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2110 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2113 From server to client are as follows:
2116 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2117 <listitem><para>OK <GUID in hex></para></listitem>
2118 <listitem><para>DATA <data in hex encoding></para></listitem>
2119 <listitem><para>ERROR</para></listitem>
2120 <listitem><para>AGREE_UNIX_FD</para></listitem>
2124 Unofficial extensions to the command set must begin with the letters
2125 "EXTENSION_", to avoid conflicts with future official commands.
2126 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2129 <sect2 id="auth-nul-byte">
2130 <title>Special credentials-passing nul byte</title>
2132 Immediately after connecting to the server, the client must send a
2133 single nul byte. This byte may be accompanied by credentials
2134 information on some operating systems that use sendmsg() with
2135 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2136 sockets. However, the nul byte must be sent even on other kinds of
2137 socket, and even on operating systems that do not require a byte to be
2138 sent in order to transmit credentials. The text protocol described in
2139 this document begins after the single nul byte. If the first byte
2140 received from the client is not a nul byte, the server may disconnect
2144 A nul byte in any context other than the initial byte is an error;
2145 the protocol is ASCII-only.
2148 The credentials sent along with the nul byte may be used with the
2149 SASL mechanism EXTERNAL.
2152 <sect2 id="auth-command-auth">
2153 <title>AUTH command</title>
2155 If an AUTH command has no arguments, it is a request to list
2156 available mechanisms. The server must respond with a REJECTED
2157 command listing the mechanisms it understands, or with an error.
2160 If an AUTH command specifies a mechanism, and the server supports
2161 said mechanism, the server should begin exchanging SASL
2162 challenge-response data with the client using DATA commands.
2165 If the server does not support the mechanism given in the AUTH
2166 command, it must send either a REJECTED command listing the mechanisms
2167 it does support, or an error.
2170 If the [initial-response] argument is provided, it is intended for use
2171 with mechanisms that have no initial challenge (or an empty initial
2172 challenge), as if it were the argument to an initial DATA command. If
2173 the selected mechanism has an initial challenge and [initial-response]
2174 was provided, the server should reject authentication by sending
2178 If authentication succeeds after exchanging DATA commands,
2179 an OK command must be sent to the client.
2182 The first octet received by the server after the \r\n of the BEGIN
2183 command from the client must be the first octet of the
2184 authenticated/encrypted stream of D-Bus messages.
2187 If BEGIN is received by the server, the first octet received
2188 by the client after the \r\n of the OK command must be the
2189 first octet of the authenticated/encrypted stream of D-Bus
2193 <sect2 id="auth-command-cancel">
2194 <title>CANCEL Command</title>
2196 At any time up to sending the BEGIN command, the client may send a
2197 CANCEL command. On receiving the CANCEL command, the server must
2198 send a REJECTED command and abort the current authentication
2202 <sect2 id="auth-command-data">
2203 <title>DATA Command</title>
2205 The DATA command may come from either client or server, and simply
2206 contains a hex-encoded block of data to be interpreted
2207 according to the SASL mechanism in use.
2210 Some SASL mechanisms support sending an "empty string";
2211 FIXME we need some way to do this.
2214 <sect2 id="auth-command-begin">
2215 <title>BEGIN Command</title>
2217 The BEGIN command acknowledges that the client has received an
2218 OK command from the server, and that the stream of messages
2222 The first octet received by the server after the \r\n of the BEGIN
2223 command from the client must be the first octet of the
2224 authenticated/encrypted stream of D-Bus messages.
2227 <sect2 id="auth-command-rejected">
2228 <title>REJECTED Command</title>
2230 The REJECTED command indicates that the current authentication
2231 exchange has failed, and further exchange of DATA is inappropriate.
2232 The client would normally try another mechanism, or try providing
2233 different responses to challenges.
2235 Optionally, the REJECTED command has a space-separated list of
2236 available auth mechanisms as arguments. If a server ever provides
2237 a list of supported mechanisms, it must provide the same list
2238 each time it sends a REJECTED message. Clients are free to
2239 ignore all lists received after the first.
2242 <sect2 id="auth-command-ok">
2243 <title>OK Command</title>
2245 The OK command indicates that the client has been
2246 authenticated. The client may now proceed with negotiating
2247 Unix file descriptor passing. To do that it shall send
2248 NEGOTIATE_UNIX_FD to the server.
2251 Otherwise, the client must respond to the OK command by
2252 sending a BEGIN command, followed by its stream of messages,
2253 or by disconnecting. The server must not accept additional
2254 commands using this protocol after the BEGIN command has been
2255 received. Further communication will be a stream of D-Bus
2256 messages (optionally encrypted, as negotiated) rather than
2260 If a client sends BEGIN the first octet received by the client
2261 after the \r\n of the OK command must be the first octet of
2262 the authenticated/encrypted stream of D-Bus messages.
2265 The OK command has one argument, which is the GUID of the server.
2266 See <xref linkend="addresses"/> for more on server GUIDs.
2269 <sect2 id="auth-command-error">
2270 <title>ERROR Command</title>
2272 The ERROR command indicates that either server or client did not
2273 know a command, does not accept the given command in the current
2274 context, or did not understand the arguments to the command. This
2275 allows the protocol to be extended; a client or server can send a
2276 command present or permitted only in new protocol versions, and if
2277 an ERROR is received instead of an appropriate response, fall back
2278 to using some other technique.
2281 If an ERROR is sent, the server or client that sent the
2282 error must continue as if the command causing the ERROR had never been
2283 received. However, the the server or client receiving the error
2284 should try something other than whatever caused the error;
2285 if only canceling/rejecting the authentication.
2288 If the D-Bus protocol changes incompatibly at some future time,
2289 applications implementing the new protocol would probably be able to
2290 check for support of the new protocol by sending a new command and
2291 receiving an ERROR from applications that don't understand it. Thus the
2292 ERROR feature of the auth protocol is an escape hatch that lets us
2293 negotiate extensions or changes to the D-Bus protocol in the future.
2296 <sect2 id="auth-command-negotiate-unix-fd">
2297 <title>NEGOTIATE_UNIX_FD Command</title>
2299 The NEGOTIATE_UNIX_FD command indicates that the client
2300 supports Unix file descriptor passing. This command may only
2301 be sent after the connection is authenticated, i.e. after OK
2302 was received by the client. This command may only be sent on
2303 transports that support Unix file descriptor passing.
2306 On receiving NEGOTIATE_UNIX_FD the server must respond with
2307 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2308 the transport chosen supports Unix file descriptor passing and
2309 the server supports this feature. It shall respond the latter
2310 if the transport does not support Unix file descriptor
2311 passing, the server does not support this feature, or the
2312 server decides not to enable file descriptor passing due to
2313 security or other reasons.
2316 <sect2 id="auth-command-agree-unix-fd">
2317 <title>AGREE_UNIX_FD Command</title>
2319 The AGREE_UNIX_FD command indicates that the server supports
2320 Unix file descriptor passing. This command may only be sent
2321 after the connection is authenticated, and the client sent
2322 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2323 command may only be sent on transports that support Unix file
2327 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2328 followed by its stream of messages, or by disconnecting. The
2329 server must not accept additional commands using this protocol
2330 after the BEGIN command has been received. Further
2331 communication will be a stream of D-Bus messages (optionally
2332 encrypted, as negotiated) rather than this protocol.
2335 <sect2 id="auth-command-future">
2336 <title>Future Extensions</title>
2338 Future extensions to the authentication and negotiation
2339 protocol are possible. For that new commands may be
2340 introduced. If a client or server receives an unknown command
2341 it shall respond with ERROR and not consider this fatal. New
2342 commands may be introduced both before, and after
2343 authentication, i.e. both before and after the OK command.
2346 <sect2 id="auth-examples">
2347 <title>Authentication examples</title>
2351 <title>Example of successful magic cookie authentication</title>
2353 (MAGIC_COOKIE is a made up mechanism)
2355 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2361 <title>Example of finding out mechanisms then picking one</title>
2364 S: REJECTED KERBEROS_V4 SKEY
2365 C: AUTH SKEY 7ab83f32ee
2366 S: DATA 8799cabb2ea93e
2367 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2373 <title>Example of client sends unknown command then falls back to regular auth</title>
2377 C: AUTH MAGIC_COOKIE 3736343435313230333039
2383 <title>Example of server doesn't support initial auth mechanism</title>
2385 C: AUTH MAGIC_COOKIE 3736343435313230333039
2386 S: REJECTED KERBEROS_V4 SKEY
2387 C: AUTH SKEY 7ab83f32ee
2388 S: DATA 8799cabb2ea93e
2389 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2395 <title>Example of wrong password or the like followed by successful retry</title>
2397 C: AUTH MAGIC_COOKIE 3736343435313230333039
2398 S: REJECTED KERBEROS_V4 SKEY
2399 C: AUTH SKEY 7ab83f32ee
2400 S: DATA 8799cabb2ea93e
2401 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2403 C: AUTH SKEY 7ab83f32ee
2404 S: DATA 8799cabb2ea93e
2405 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2411 <title>Example of skey cancelled and restarted</title>
2413 C: AUTH MAGIC_COOKIE 3736343435313230333039
2414 S: REJECTED KERBEROS_V4 SKEY
2415 C: AUTH SKEY 7ab83f32ee
2416 S: DATA 8799cabb2ea93e
2419 C: AUTH SKEY 7ab83f32ee
2420 S: DATA 8799cabb2ea93e
2421 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2427 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2429 (MAGIC_COOKIE is a made up mechanism)
2431 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2433 C: NEGOTIATE_UNIX_FD
2439 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2441 (MAGIC_COOKIE is a made up mechanism)
2443 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2445 C: NEGOTIATE_UNIX_FD
2452 <sect2 id="auth-states">
2453 <title>Authentication state diagrams</title>
2456 This section documents the auth protocol in terms of
2457 a state machine for the client and the server. This is
2458 probably the most robust way to implement the protocol.
2461 <sect3 id="auth-states-client">
2462 <title>Client states</title>
2465 To more precisely describe the interaction between the
2466 protocol state machine and the authentication mechanisms the
2467 following notation is used: MECH(CHALL) means that the
2468 server challenge CHALL was fed to the mechanism MECH, which
2474 CONTINUE(RESP) means continue the auth conversation
2475 and send RESP as the response to the server;
2481 OK(RESP) means that after sending RESP to the server
2482 the client side of the auth conversation is finished
2483 and the server should return "OK";
2489 ERROR means that CHALL was invalid and could not be
2495 Both RESP and CHALL may be empty.
2499 The Client starts by getting an initial response from the
2500 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2501 the mechanism did not provide an initial response. If the
2502 mechanism returns CONTINUE, the client starts in state
2503 <emphasis>WaitingForData</emphasis>, if the mechanism
2504 returns OK the client starts in state
2505 <emphasis>WaitingForOK</emphasis>.
2509 The client should keep track of available mechanisms and
2510 which it mechanisms it has already attempted. This list is
2511 used to decide which AUTH command to send. When the list is
2512 exhausted, the client should give up and close the
2517 <title><emphasis>WaitingForData</emphasis></title>
2525 MECH(CHALL) returns CONTINUE(RESP) → send
2527 <emphasis>WaitingForData</emphasis>
2531 MECH(CHALL) returns OK(RESP) → send DATA
2532 RESP, goto <emphasis>WaitingForOK</emphasis>
2536 MECH(CHALL) returns ERROR → send ERROR
2537 [msg], goto <emphasis>WaitingForData</emphasis>
2545 Receive REJECTED [mechs] →
2546 send AUTH [next mech], goto
2547 WaitingForData or <emphasis>WaitingForOK</emphasis>
2552 Receive ERROR → send
2554 <emphasis>WaitingForReject</emphasis>
2559 Receive OK → send
2560 BEGIN, terminate auth
2561 conversation, authenticated
2566 Receive anything else → send
2568 <emphasis>WaitingForData</emphasis>
2576 <title><emphasis>WaitingForOK</emphasis></title>
2581 Receive OK → send BEGIN, terminate auth
2582 conversation, <emphasis>authenticated</emphasis>
2587 Receive REJECTED [mechs] → send AUTH [next mech],
2588 goto <emphasis>WaitingForData</emphasis> or
2589 <emphasis>WaitingForOK</emphasis>
2595 Receive DATA → send CANCEL, goto
2596 <emphasis>WaitingForReject</emphasis>
2602 Receive ERROR → send CANCEL, goto
2603 <emphasis>WaitingForReject</emphasis>
2609 Receive anything else → send ERROR, goto
2610 <emphasis>WaitingForOK</emphasis>
2618 <title><emphasis>WaitingForReject</emphasis></title>
2623 Receive REJECTED [mechs] → send AUTH [next mech],
2624 goto <emphasis>WaitingForData</emphasis> or
2625 <emphasis>WaitingForOK</emphasis>
2631 Receive anything else → terminate auth
2632 conversation, disconnect
2641 <sect3 id="auth-states-server">
2642 <title>Server states</title>
2645 For the server MECH(RESP) means that the client response
2646 RESP was fed to the the mechanism MECH, which returns one of
2651 CONTINUE(CHALL) means continue the auth conversation and
2652 send CHALL as the challenge to the client;
2658 OK means that the client has been successfully
2665 REJECTED means that the client failed to authenticate or
2666 there was an error in RESP.
2671 The server starts out in state
2672 <emphasis>WaitingForAuth</emphasis>. If the client is
2673 rejected too many times the server must disconnect the
2678 <title><emphasis>WaitingForAuth</emphasis></title>
2684 Receive AUTH → send REJECTED [mechs], goto
2685 <emphasis>WaitingForAuth</emphasis>
2691 Receive AUTH MECH RESP
2695 MECH not valid mechanism → send REJECTED
2697 <emphasis>WaitingForAuth</emphasis>
2701 MECH(RESP) returns CONTINUE(CHALL) → send
2703 <emphasis>WaitingForData</emphasis>
2707 MECH(RESP) returns OK → send OK, goto
2708 <emphasis>WaitingForBegin</emphasis>
2712 MECH(RESP) returns REJECTED → send REJECTED
2714 <emphasis>WaitingForAuth</emphasis>
2722 Receive BEGIN → terminate
2723 auth conversation, disconnect
2729 Receive ERROR → send REJECTED [mechs], goto
2730 <emphasis>WaitingForAuth</emphasis>
2736 Receive anything else → send
2738 <emphasis>WaitingForAuth</emphasis>
2747 <title><emphasis>WaitingForData</emphasis></title>
2755 MECH(RESP) returns CONTINUE(CHALL) → send
2757 <emphasis>WaitingForData</emphasis>
2761 MECH(RESP) returns OK → send OK, goto
2762 <emphasis>WaitingForBegin</emphasis>
2766 MECH(RESP) returns REJECTED → send REJECTED
2768 <emphasis>WaitingForAuth</emphasis>
2776 Receive BEGIN → terminate auth conversation,
2783 Receive CANCEL → send REJECTED [mechs], goto
2784 <emphasis>WaitingForAuth</emphasis>
2790 Receive ERROR → send REJECTED [mechs], goto
2791 <emphasis>WaitingForAuth</emphasis>
2797 Receive anything else → send ERROR, goto
2798 <emphasis>WaitingForData</emphasis>
2806 <title><emphasis>WaitingForBegin</emphasis></title>
2811 Receive BEGIN → terminate auth conversation,
2812 client authenticated
2818 Receive CANCEL → send REJECTED [mechs], goto
2819 <emphasis>WaitingForAuth</emphasis>
2825 Receive ERROR → send REJECTED [mechs], goto
2826 <emphasis>WaitingForAuth</emphasis>
2832 Receive anything else → send ERROR, goto
2833 <emphasis>WaitingForBegin</emphasis>
2843 <sect2 id="auth-mechanisms">
2844 <title>Authentication mechanisms</title>
2846 This section describes some new authentication mechanisms.
2847 D-Bus also allows any standard SASL mechanism of course.
2849 <sect3 id="auth-mechanisms-sha">
2850 <title>DBUS_COOKIE_SHA1</title>
2852 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2853 has the ability to read a private file owned by the user being
2854 authenticated. If the client can prove that it has access to a secret
2855 cookie stored in this file, then the client is authenticated.
2856 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2860 Throughout this description, "hex encoding" must output the digits
2861 from a to f in lower-case; the digits A to F must not be used
2862 in the DBUS_COOKIE_SHA1 mechanism.
2865 Authentication proceeds as follows:
2869 The client sends the username it would like to authenticate
2875 The server sends the name of its "cookie context" (see below); a
2876 space character; the integer ID of the secret cookie the client
2877 must demonstrate knowledge of; a space character; then a
2878 randomly-generated challenge string, all of this hex-encoded into
2884 The client locates the cookie and generates its own
2885 randomly-generated challenge string. The client then concatenates
2886 the server's decoded challenge, a ":" character, its own challenge,
2887 another ":" character, and the cookie. It computes the SHA-1 hash
2888 of this composite string as a hex digest. It concatenates the
2889 client's challenge string, a space character, and the SHA-1 hex
2890 digest, hex-encodes the result and sends it back to the server.
2895 The server generates the same concatenated string used by the
2896 client and computes its SHA-1 hash. It compares the hash with
2897 the hash received from the client; if the two hashes match, the
2898 client is authenticated.
2904 Each server has a "cookie context," which is a name that identifies a
2905 set of cookies that apply to that server. A sample context might be
2906 "org_freedesktop_session_bus". Context names must be valid ASCII,
2907 nonzero length, and may not contain the characters slash ("/"),
2908 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2909 tab ("\t"), or period ("."). There is a default context,
2910 "org_freedesktop_general" that's used by servers that do not specify
2914 Cookies are stored in a user's home directory, in the directory
2915 <filename>~/.dbus-keyrings/</filename>. This directory must
2916 not be readable or writable by other users. If it is,
2917 clients and servers must ignore it. The directory
2918 contains cookie files named after the cookie context.
2921 A cookie file contains one cookie per line. Each line
2922 has three space-separated fields:
2926 The cookie ID number, which must be a non-negative integer and
2927 may not be used twice in the same file.
2932 The cookie's creation time, in UNIX seconds-since-the-epoch
2938 The cookie itself, a hex-encoded random block of bytes. The cookie
2939 may be of any length, though obviously security increases
2940 as the length increases.
2946 Only server processes modify the cookie file.
2947 They must do so with this procedure:
2951 Create a lockfile name by appending ".lock" to the name of the
2952 cookie file. The server should attempt to create this file
2953 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2954 fails, the lock fails. Servers should retry for a reasonable
2955 period of time, then they may choose to delete an existing lock
2956 to keep users from having to manually delete a stale
2957 lock. <footnote><para>Lockfiles are used instead of real file
2958 locking <literal>fcntl()</literal> because real locking
2959 implementations are still flaky on network
2960 filesystems.</para></footnote>
2965 Once the lockfile has been created, the server loads the cookie
2966 file. It should then delete any cookies that are old (the
2967 timeout can be fairly short), or more than a reasonable
2968 time in the future (so that cookies never accidentally
2969 become permanent, if the clock was set far into the future
2970 at some point). If no recent keys remain, the
2971 server may generate a new key.
2976 The pruned and possibly added-to cookie file
2977 must be resaved atomically (using a temporary
2978 file which is rename()'d).
2983 The lock must be dropped by deleting the lockfile.
2989 Clients need not lock the file in order to load it,
2990 because servers are required to save the file atomically.
2995 <sect1 id="addresses">
2996 <title>Server Addresses</title>
2998 Server addresses consist of a transport name followed by a colon, and
2999 then an optional, comma-separated list of keys and values in the form key=value.
3000 Each value is escaped.
3004 <programlisting>unix:path=/tmp/dbus-test</programlisting>
3005 Which is the address to a unix socket with the path /tmp/dbus-test.
3008 Value escaping is similar to URI escaping but simpler.
3012 The set of optionally-escaped bytes is:
3013 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
3014 <emphasis>byte</emphasis> (note, not character) which is not in the
3015 set of optionally-escaped bytes must be replaced with an ASCII
3016 percent (<literal>%</literal>) and the value of the byte in hex.
3017 The hex value must always be two digits, even if the first digit is
3018 zero. The optionally-escaped bytes may be escaped if desired.
3023 To unescape, append each byte in the value; if a byte is an ASCII
3024 percent (<literal>%</literal>) character then append the following
3025 hex value instead. It is an error if a <literal>%</literal> byte
3026 does not have two hex digits following. It is an error if a
3027 non-optionally-escaped byte is seen unescaped.
3031 The set of optionally-escaped bytes is intended to preserve address
3032 readability and convenience.
3036 A server may specify a key-value pair with the key <literal>guid</literal>
3037 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
3038 describes the format of the <literal>guid</literal> field. If present,
3039 this UUID may be used to distinguish one server address from another. A
3040 server should use a different UUID for each address it listens on. For
3041 example, if a message bus daemon offers both UNIX domain socket and TCP
3042 connections, but treats clients the same regardless of how they connect,
3043 those two connections are equivalent post-connection but should have
3044 distinct UUIDs to distinguish the kinds of connection.
3048 The intent of the address UUID feature is to allow a client to avoid
3049 opening multiple identical connections to the same server, by allowing the
3050 client to check whether an address corresponds to an already-existing
3051 connection. Comparing two addresses is insufficient, because addresses
3052 can be recycled by distinct servers, and equivalent addresses may look
3053 different if simply compared as strings (for example, the host in a TCP
3054 address can be given as an IP address or as a hostname).
3058 Note that the address key is <literal>guid</literal> even though the
3059 rest of the API and documentation says "UUID," for historical reasons.
3063 [FIXME clarify if attempting to connect to each is a requirement
3064 or just a suggestion]
3065 When connecting to a server, multiple server addresses can be
3066 separated by a semi-colon. The library will then try to connect
3067 to the first address and if that fails, it'll try to connect to
3068 the next one specified, and so forth. For example
3069 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3073 Some addresses are <firstterm>connectable</firstterm>. A connectable
3074 address is one containing enough information for a client to connect
3075 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3076 is a connectable address. It is not necessarily possible to listen
3077 on every connectable address: for instance, it is not possible to
3078 listen on a <literal>unixexec:</literal> address.
3082 Some addresses are <firstterm>listenable</firstterm>. A listenable
3083 address is one containing enough information for a server to listen on
3084 it, producing a connectable address (which may differ from the
3085 original address). Many listenable addresses are not connectable:
3086 for instance, <literal>tcp:host=127.0.0.1</literal>
3087 is listenable, but not connectable (because it does not specify
3092 Listening on an address that is not connectable will result in a
3093 connectable address that is not the same as the listenable address.
3094 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3095 might result in the connectable address
3096 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3097 listening on <literal>unix:tmpdir=/tmp</literal>
3098 might result in the connectable address
3099 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>, or
3100 listening on <literal>unix:runtime=yes</literal>
3101 might result in the connectable address
3102 <literal>unix:path=/run/user/1234/bus</literal>.
3106 <sect1 id="transports">
3107 <title>Transports</title>
3109 [FIXME we need to specify in detail each transport and its possible arguments]
3111 Current transports include: unix domain sockets (including
3112 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3113 using in-process pipes. Future possible transports include one that
3114 tunnels over X11 protocol.
3117 <sect2 id="transports-unix-domain-sockets">
3118 <title>Unix Domain Sockets</title>
3120 Unix domain sockets can be either paths in the file system or on Linux
3121 kernels, they can be abstract which are similar to paths but
3122 do not show up in the file system.
3126 When a socket is opened by the D-Bus library it truncates the path
3127 name right before the first trailing Nul byte. This is true for both
3128 normal paths and abstract paths. Note that this is a departure from
3129 previous versions of D-Bus that would create sockets with a fixed
3130 length path name. Names which were shorter than the fixed length
3131 would be padded by Nul bytes.
3134 Unix domain sockets are not available on Windows.
3137 Unix addresses that specify <literal>path</literal> or
3138 <literal>abstract</literal> are both listenable and connectable.
3139 Unix addresses that specify <literal>tmpdir</literal> are only
3140 listenable: the corresponding connectable address will specify
3141 either <literal>path</literal> or <literal>abstract</literal>.
3142 Similarly, Unix addresses that specify <literal>runtime</literal>
3143 are only listenable, and the corresponding connectable address
3144 will specify <literal>path</literal>.
3146 <sect3 id="transports-unix-domain-sockets-addresses">
3147 <title>Server Address Format</title>
3149 Unix domain socket addresses are identified by the "unix:" prefix
3150 and support the following key/value pairs:
3157 <entry>Values</entry>
3158 <entry>Description</entry>
3164 <entry>(path)</entry>
3165 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3168 <entry>tmpdir</entry>
3169 <entry>(path)</entry>
3170 <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>
3173 <entry>abstract</entry>
3174 <entry>(string)</entry>
3175 <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>
3178 <entry>runtime</entry>
3179 <entry><literal>yes</literal></entry>
3180 <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>
3186 Exactly one of the keys <literal>path</literal>,
3187 <literal>abstract</literal>, <literal>runtime</literal> or
3188 <literal>tmpdir</literal> must be provided.
3192 <sect2 id="transports-launchd">
3193 <title>launchd</title>
3195 launchd is an open-source server management system that replaces init, inetd
3196 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3197 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3201 launchd allocates a socket and provides it with the unix path through the
3202 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3203 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3204 it through its environment.
3205 Other processes can query for the launchd socket by executing:
3206 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3207 This is normally done by the D-Bus client library so doesn't have to be done
3211 launchd is not available on Microsoft Windows.
3214 launchd addresses are listenable and connectable.
3216 <sect3 id="transports-launchd-addresses">
3217 <title>Server Address Format</title>
3219 launchd addresses are identified by the "launchd:" prefix
3220 and support the following key/value pairs:
3227 <entry>Values</entry>
3228 <entry>Description</entry>
3234 <entry>(environment variable)</entry>
3235 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3241 The <literal>env</literal> key is required.
3245 <sect2 id="transports-systemd">
3246 <title>systemd</title>
3248 systemd is an open-source server management system that
3249 replaces init and inetd on newer Linux systems. It supports
3250 socket activation. The D-Bus systemd transport is used to acquire
3251 socket activation file descriptors from systemd and use them
3252 as D-Bus transport when the current process is spawned by
3253 socket activation from it.
3256 The systemd transport accepts only one or more Unix domain or
3257 TCP streams sockets passed in via socket activation.
3260 The systemd transport is not available on non-Linux operating systems.
3263 The systemd transport defines no parameter keys.
3266 systemd addresses are listenable, but not connectable. The
3267 corresponding connectable address is the <literal>unix</literal>
3268 or <literal>tcp</literal> address of the socket.
3271 <sect2 id="transports-tcp-sockets">
3272 <title>TCP Sockets</title>
3274 The tcp transport provides TCP/IP based connections between clients
3275 located on the same or different hosts.
3278 Using tcp transport without any additional secure authentification mechanismus
3279 over a network is unsecure.
3282 On Windows and most Unix platforms, the TCP stack is unable to transfer
3283 credentials over a TCP connection, so the EXTERNAL authentication
3284 mechanism does not work for this transport.
3287 All <literal>tcp</literal> addresses are listenable.
3288 <literal>tcp</literal> addresses in which both
3289 <literal>host</literal> and <literal>port</literal> are
3290 specified, and <literal>port</literal> is non-zero,
3291 are also connectable.
3293 <sect3 id="transports-tcp-sockets-addresses">
3294 <title>Server Address Format</title>
3296 TCP/IP socket addresses are identified by the "tcp:" prefix
3297 and support the following key/value pairs:
3304 <entry>Values</entry>
3305 <entry>Description</entry>
3311 <entry>(string)</entry>
3312 <entry>DNS name or IP address</entry>
3316 <entry>(string)</entry>
3317 <entry>Used in a listenable address to configure the interface
3318 on which the server will listen: either the IP address of one of
3319 the local machine's interfaces (most commonly <literal>127.0.0.1
3320 </literal>), or a DNS name that resolves to one of those IP
3321 addresses, or '*' to listen on all interfaces simultaneously.
3322 If not specified, the default is the same value as "host".
3327 <entry>(number)</entry>
3328 <entry>The tcp port the server will open. A zero value let the server
3329 choose a free port provided from the underlaying operating system.
3330 libdbus is able to retrieve the real used port from the server.
3334 <entry>family</entry>
3335 <entry>(string)</entry>
3336 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3343 <sect2 id="transports-nonce-tcp-sockets">
3344 <title>Nonce-secured TCP Sockets</title>
3346 The nonce-tcp transport provides a secured TCP transport, using a
3347 simple authentication mechanism to ensure that only clients with read
3348 access to a certain location in the filesystem can connect to the server.
3349 The server writes a secret, the nonce, to a file and an incoming client
3350 connection is only accepted if the client sends the nonce right after
3351 the connect. The nonce mechanism requires no setup and is orthogonal to
3352 the higher-level authentication mechanisms described in the
3353 Authentication section.
3357 On start, the server generates a random 16 byte nonce and writes it
3358 to a file in the user's temporary directory. The nonce file location
3359 is published as part of the server's D-Bus address using the
3360 "noncefile" key-value pair.
3362 After an accept, the server reads 16 bytes from the socket. If the
3363 read bytes do not match the nonce stored in the nonce file, the
3364 server MUST immediately drop the connection.
3365 If the nonce match the received byte sequence, the client is accepted
3366 and the transport behaves like an unsecured tcp transport.
3369 After a successful connect to the server socket, the client MUST read
3370 the nonce from the file published by the server via the noncefile=
3371 key-value pair and send it over the socket. After that, the
3372 transport behaves like an unsecured tcp transport.
3375 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3376 <literal>host</literal>, <literal>port</literal> and
3377 <literal>noncefile</literal> are all specified,
3378 and <literal>port</literal> is nonzero, are also connectable.
3380 <sect3 id="transports-nonce-tcp-sockets-addresses">
3381 <title>Server Address Format</title>
3383 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3384 and support the following key/value pairs:
3391 <entry>Values</entry>
3392 <entry>Description</entry>
3398 <entry>(string)</entry>
3399 <entry>DNS name or IP address</entry>
3403 <entry>(string)</entry>
3404 <entry>The same as for tcp: addresses
3409 <entry>(number)</entry>
3410 <entry>The tcp port the server will open. A zero value let the server
3411 choose a free port provided from the underlaying operating system.
3412 libdbus is able to retrieve the real used port from the server.
3416 <entry>family</entry>
3417 <entry>(string)</entry>
3418 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3421 <entry>noncefile</entry>
3422 <entry>(path)</entry>
3423 <entry>File location containing the secret.
3424 This is only meaningful in connectable addresses:
3425 a listening D-Bus server that offers this transport
3426 will always create a new nonce file.</entry>
3433 <sect2 id="transports-exec">
3434 <title>Executed Subprocesses on Unix</title>
3436 This transport forks off a process and connects its standard
3437 input and standard output with an anonymous Unix domain
3438 socket. This socket is then used for communication by the
3439 transport. This transport may be used to use out-of-process
3440 forwarder programs as basis for the D-Bus protocol.
3443 The forked process will inherit the standard error output and
3444 process group from the parent process.
3447 Executed subprocesses are not available on Windows.
3450 <literal>unixexec</literal> addresses are connectable, but are not
3453 <sect3 id="transports-exec-addresses">
3454 <title>Server Address Format</title>
3456 Executed subprocess addresses are identified by the "unixexec:" prefix
3457 and support the following key/value pairs:
3464 <entry>Values</entry>
3465 <entry>Description</entry>
3471 <entry>(path)</entry>
3472 <entry>Path of the binary to execute, either an absolute
3473 path or a binary name that is searched for in the default
3474 search path of the OS. This corresponds to the first
3475 argument of execlp(). This key is mandatory.</entry>
3478 <entry>argv0</entry>
3479 <entry>(string)</entry>
3480 <entry>The program name to use when executing the
3481 binary. If omitted the same value as specified for path=
3482 will be used. This corresponds to the second argument of
3486 <entry>argv1, argv2, ...</entry>
3487 <entry>(string)</entry>
3488 <entry>Arguments to pass to the binary. This corresponds
3489 to the third and later arguments of execlp(). If a
3490 specific argvX is not specified no further argvY for Y > X
3491 are taken into account.</entry>
3499 <sect1 id="meta-transports">
3500 <title>Meta Transports</title>
3502 Meta transports are a kind of transport with special enhancements or
3503 behavior. Currently available meta transports include: autolaunch
3506 <sect2 id="meta-transports-autolaunch">
3507 <title>Autolaunch</title>
3508 <para>The autolaunch transport provides a way for dbus clients to autodetect
3509 a running dbus session bus and to autolaunch a session bus if not present.
3512 On Unix, <literal>autolaunch</literal> addresses are connectable,
3516 On Windows, <literal>autolaunch</literal> addresses are both
3517 connectable and listenable.
3520 <sect3 id="meta-transports-autolaunch-addresses">
3521 <title>Server Address Format</title>
3523 Autolaunch addresses uses the "autolaunch:" prefix and support the
3524 following key/value pairs:
3531 <entry>Values</entry>
3532 <entry>Description</entry>
3537 <entry>scope</entry>
3538 <entry>(string)</entry>
3539 <entry>scope of autolaunch (Windows only)
3543 "*install-path" - limit session bus to dbus installation path.
3544 The dbus installation path is determined from the location of
3545 the shared dbus library. If the library is located in a 'bin'
3546 subdirectory the installation root is the directory above,
3547 otherwise the directory where the library lives is taken as
3550 <install-root>/bin/[lib]dbus-1.dll
3551 <install-root>/[lib]dbus-1.dll
3557 "*user" - limit session bus to the recent user.
3562 other values - specify dedicated session bus like "release",
3574 <sect3 id="meta-transports-autolaunch-windows-implementation">
3575 <title>Windows implementation</title>
3577 On start, the server opens a platform specific transport, creates a mutex
3578 and a shared memory section containing the related session bus address.
3579 This mutex will be inspected by the dbus client library to detect a
3580 running dbus session bus. The access to the mutex and the shared memory
3581 section are protected by global locks.
3584 In the recent implementation the autolaunch transport uses a tcp transport
3585 on localhost with a port choosen from the operating system. This detail may
3586 change in the future.
3589 Disclaimer: The recent implementation is in an early state and may not
3590 work in all cirumstances and/or may have security issues. Because of this
3591 the implementation is not documentated yet.
3598 <title>UUIDs</title>
3600 A working D-Bus implementation uses universally-unique IDs in two places.
3601 First, each server address has a UUID identifying the address,
3602 as described in <xref linkend="addresses"/>. Second, each operating
3603 system kernel instance running a D-Bus client or server has a UUID
3604 identifying that kernel, retrieved by invoking the method
3605 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3606 linkend="standard-interfaces-peer"/>).
3609 The term "UUID" in this document is intended literally, i.e. an
3610 identifier that is universally unique. It is not intended to refer to
3611 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3614 The UUID must contain 128 bits of data and be hex-encoded. The
3615 hex-encoded string may not contain hyphens or other non-hex-digit
3616 characters, and it must be exactly 32 characters long. To generate a
3617 UUID, the current reference implementation concatenates 96 bits of random
3618 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3622 It would also be acceptable and probably better to simply generate 128
3623 bits of random data, as long as the random number generator is of high
3624 quality. The timestamp could conceivably help if the random bits are not
3625 very random. With a quality random number generator, collisions are
3626 extremely unlikely even with only 96 bits, so it's somewhat academic.
3629 Implementations should, however, stick to random data for the first 96 bits
3634 <sect1 id="standard-interfaces">
3635 <title>Standard Interfaces</title>
3637 See <xref linkend="message-protocol-types-notation"/> for details on
3638 the notation used in this section. There are some standard interfaces
3639 that may be useful across various D-Bus applications.
3641 <sect2 id="standard-interfaces-peer">
3642 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3644 The <literal>org.freedesktop.DBus.Peer</literal> interface
3647 org.freedesktop.DBus.Peer.Ping ()
3648 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3652 On receipt of the <literal>METHOD_CALL</literal> message
3653 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3654 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3655 usual. It does not matter which object path a ping is sent to. The
3656 reference implementation handles this method automatically.
3659 On receipt of the <literal>METHOD_CALL</literal> message
3660 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3661 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3662 UUID representing the identity of the machine the process is running on.
3663 This UUID must be the same for all processes on a single system at least
3664 until that system next reboots. It should be the same across reboots
3665 if possible, but this is not always possible to implement and is not
3667 It does not matter which object path a GetMachineId is sent to. The
3668 reference implementation handles this method automatically.
3671 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3672 a virtual machine running on a hypervisor, rather than a physical machine.
3673 Basically if two processes see the same UUID, they should also see the same
3674 shared memory, UNIX domain sockets, process IDs, and other features that require
3675 a running OS kernel in common between the processes.
3678 The UUID is often used where other programs might use a hostname. Hostnames
3679 can change without rebooting, however, or just be "localhost" - so the UUID
3683 <xref linkend="uuids"/> explains the format of the UUID.
3687 <sect2 id="standard-interfaces-introspectable">
3688 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3690 This interface has one method:
3692 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3696 Objects instances may implement
3697 <literal>Introspect</literal> which returns an XML description of
3698 the object, including its interfaces (with signals and methods), objects
3699 below it in the object path tree, and its properties.
3702 <xref linkend="introspection-format"/> describes the format of this XML string.
3705 <sect2 id="standard-interfaces-properties">
3706 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3708 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3709 or <firstterm>attributes</firstterm>. These can be exposed via the
3710 <literal>org.freedesktop.DBus.Properties</literal> interface.
3714 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3715 in STRING property_name,
3717 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3718 in STRING property_name,
3720 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3721 out DICT<STRING,VARIANT> props);
3725 It is conventional to give D-Bus properties names consisting of
3726 capitalized words without punctuation ("CamelCase"), like
3727 <link linkend="message-protocol-names-member">member names</link>.
3728 For instance, the GObject property
3729 <literal>connection-status</literal> or the Qt property
3730 <literal>connectionStatus</literal> could be represented on D-Bus
3731 as <literal>ConnectionStatus</literal>.
3734 Strictly speaking, D-Bus property names are not required to follow
3735 the same naming restrictions as member names, but D-Bus property
3736 names that would not be valid member names (in particular,
3737 GObject-style dash-separated property names) can cause interoperability
3738 problems and should be avoided.
3741 The available properties and whether they are writable can be determined
3742 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3743 see <xref linkend="standard-interfaces-introspectable"/>.
3746 An empty string may be provided for the interface name; in this case,
3747 if there are multiple properties on an object with the same name,
3748 the results are undefined (picking one by according to an arbitrary
3749 deterministic rule, or returning an error, are the reasonable
3753 If one or more properties change on an object, the
3754 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3755 signal may be emitted (this signal was added in 0.14):
3759 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3760 DICT<STRING,VARIANT> changed_properties,
3761 ARRAY<STRING> invalidated_properties);
3765 where <literal>changed_properties</literal> is a dictionary
3766 containing the changed properties with the new values and
3767 <literal>invalidated_properties</literal> is an array of
3768 properties that changed but the value is not conveyed.
3771 Whether the <literal>PropertiesChanged</literal> signal is
3772 supported can be determined by calling
3773 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3774 that the signal may be supported for an object but it may
3775 differ how whether and how it is used on a per-property basis
3776 (for e.g. performance or security reasons). Each property (or
3777 the parent interface) must be annotated with the
3778 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3779 annotation to convey this (usually the default value
3780 <literal>true</literal> is sufficient meaning that the
3781 annotation does not need to be used). See <xref
3782 linkend="introspection-format"/> for details on this
3787 <sect2 id="standard-interfaces-objectmanager">
3788 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3790 An API can optionally make use of this interface for one or
3791 more sub-trees of objects. The root of each sub-tree implements
3792 this interface so other applications can get all objects,
3793 interfaces and properties in a single method call. It is
3794 appropriate to use this interface if users of the tree of
3795 objects are expected to be interested in all interfaces of all
3796 objects in the tree; a more granular API should be used if
3797 users of the objects are expected to be interested in a small
3798 subset of the objects, a small subset of their interfaces, or
3802 The method that applications can use to get all objects and
3803 properties is <literal>GetManagedObjects</literal>:
3807 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3811 The return value of this method is a dict whose keys are
3812 object paths. All returned object paths are children of the
3813 object path implementing this interface, i.e. their object
3814 paths start with the ObjectManager's object path plus '/'.
3817 Each value is a dict whose keys are interfaces names. Each
3818 value in this inner dict is the same dict that would be
3819 returned by the <link
3820 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3821 method for that combination of object path and interface. If
3822 an interface has no properties, the empty dict is returned.
3825 Changes are emitted using the following two signals:
3829 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3830 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3831 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3832 ARRAY<STRING> interfaces);
3836 The <literal>InterfacesAdded</literal> signal is emitted when
3837 either a new object is added or when an existing object gains
3838 one or more interfaces. The
3839 <literal>InterfacesRemoved</literal> signal is emitted
3840 whenever an object is removed or it loses one or more
3841 interfaces. The second parameter of the
3842 <literal>InterfacesAdded</literal> signal contains a dict with
3843 the interfaces and properties (if any) that have been added to
3844 the given object path. Similarly, the second parameter of the
3845 <literal>InterfacesRemoved</literal> signal contains an array
3846 of the interfaces that were removed. Note that changes on
3847 properties on existing interfaces are not reported using this
3848 interface - an application should also monitor the existing <link
3849 linkend="standard-interfaces-properties">PropertiesChanged</link>
3850 signal on each object.
3853 Applications SHOULD NOT export objects that are children of an
3854 object (directly or otherwise) implementing this interface but
3855 which are not returned in the reply from the
3856 <literal>GetManagedObjects()</literal> method of this
3857 interface on the given object.
3860 The intent of the <literal>ObjectManager</literal> interface
3861 is to make it easy to write a robust client
3862 implementation. The trivial client implementation only needs
3863 to make two method calls:
3867 org.freedesktop.DBus.AddMatch (bus_proxy,
3868 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3869 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3873 on the message bus and the remote application's
3874 <literal>ObjectManager</literal>, respectively. Whenever a new
3875 remote object is created (or an existing object gains a new
3876 interface), the <literal>InterfacesAdded</literal> signal is
3877 emitted, and since this signal contains all properties for the
3878 interfaces, no calls to the
3879 <literal>org.freedesktop.Properties</literal> interface on the
3880 remote object are needed. Additionally, since the initial
3881 <literal>AddMatch()</literal> rule already includes signal
3882 messages from the newly created child object, no new
3883 <literal>AddMatch()</literal> call is needed.
3888 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3889 interface was added in version 0.17 of the D-Bus
3896 <sect1 id="introspection-format">
3897 <title>Introspection Data Format</title>
3899 As described in <xref linkend="standard-interfaces-introspectable"/>,
3900 objects may be introspected at runtime, returning an XML string
3901 that describes the object. The same XML format may be used in
3902 other contexts as well, for example as an "IDL" for generating
3903 static language bindings.
3906 Here is an example of introspection data:
3908 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3909 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3910 <node name="/com/example/sample_object">
3911 <interface name="com.example.SampleInterface">
3912 <method name="Frobate">
3913 <arg name="foo" type="i" direction="in"/>
3914 <arg name="bar" type="s" direction="out"/>
3915 <arg name="baz" type="a{us}" direction="out"/>
3916 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3918 <method name="Bazify">
3919 <arg name="bar" type="(iiu)" direction="in"/>
3920 <arg name="bar" type="v" direction="out"/>
3922 <method name="Mogrify">
3923 <arg name="bar" type="(iiav)" direction="in"/>
3925 <signal name="Changed">
3926 <arg name="new_value" type="b"/>
3928 <property name="Bar" type="y" access="readwrite"/>
3930 <node name="child_of_sample_object"/>
3931 <node name="another_child_of_sample_object"/>
3936 A more formal DTD and spec needs writing, but here are some quick notes.
3940 Only the root <node> element can omit the node name, as it's
3941 known to be the object that was introspected. If the root
3942 <node> does have a name attribute, it must be an absolute
3943 object path. If child <node> have object paths, they must be
3949 If a child <node> has any sub-elements, then they
3950 must represent a complete introspection of the child.
3951 If a child <node> is empty, then it may or may
3952 not have sub-elements; the child must be introspected
3953 in order to find out. The intent is that if an object
3954 knows that its children are "fast" to introspect
3955 it can go ahead and return their information, but
3956 otherwise it can omit it.
3961 The direction element on <arg> may be omitted,
3962 in which case it defaults to "in" for method calls
3963 and "out" for signals. Signals only allow "out"
3964 so while direction may be specified, it's pointless.
3969 The possible directions are "in" and "out",
3970 unlike CORBA there is no "inout"
3975 The possible property access flags are
3976 "readwrite", "read", and "write"
3981 Multiple interfaces can of course be listed for
3987 The "name" attribute on arguments is optional.
3993 Method, interface, property, and signal elements may have
3994 "annotations", which are generic key/value pairs of metadata.
3995 They are similar conceptually to Java's annotations and C# attributes.
3996 Well-known annotations:
4003 <entry>Values (separated by ,)</entry>
4004 <entry>Description</entry>
4009 <entry>org.freedesktop.DBus.Deprecated</entry>
4010 <entry>true,false</entry>
4011 <entry>Whether or not the entity is deprecated; defaults to false</entry>
4014 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
4015 <entry>(string)</entry>
4016 <entry>The C symbol; may be used for methods and interfaces</entry>
4019 <entry>org.freedesktop.DBus.Method.NoReply</entry>
4020 <entry>true,false</entry>
4021 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
4024 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
4025 <entry>true,invalidates,const,false</entry>
4028 If set to <literal>false</literal>, the
4029 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
4031 linkend="standard-interfaces-properties"/> is not
4032 guaranteed to be emitted if the property changes.
4035 If set to <literal>const</literal> the property never
4036 changes value during the lifetime of the object it
4037 belongs to, and hence the signal is never emitted for
4041 If set to <literal>invalidates</literal> the signal
4042 is emitted but the value is not included in the
4046 If set to <literal>true</literal> the signal is
4047 emitted with the value included.
4050 The value for the annotation defaults to
4051 <literal>true</literal> if the enclosing interface
4052 element does not specify the annotation. Otherwise it
4053 defaults to the value specified in the enclosing
4057 This annotation is intended to be used by code
4058 generators to implement client-side caching of
4059 property values. For all properties for which the
4060 annotation is set to <literal>const</literal>,
4061 <literal>invalidates</literal> or
4062 <literal>true</literal> the client may
4063 unconditionally cache the values as the properties
4064 don't change or notifications are generated for them
4073 <sect1 id="message-bus">
4074 <title>Message Bus Specification</title>
4075 <sect2 id="message-bus-overview">
4076 <title>Message Bus Overview</title>
4078 The message bus accepts connections from one or more applications.
4079 Once connected, applications can exchange messages with other
4080 applications that are also connected to the bus.
4083 In order to route messages among connections, the message bus keeps a
4084 mapping from names to connections. Each connection has one
4085 unique-for-the-lifetime-of-the-bus name automatically assigned.
4086 Applications may request additional names for a connection. Additional
4087 names are usually "well-known names" such as
4088 "com.example.TextEditor". When a name is bound to a connection,
4089 that connection is said to <firstterm>own</firstterm> the name.
4092 The bus itself owns a special name,
4093 <literal>org.freedesktop.DBus</literal>, with an object
4094 located at <literal>/org/freedesktop/DBus</literal> that
4095 implements the <literal>org.freedesktop.DBus</literal>
4096 interface. This service allows applications to make
4097 administrative requests of the bus itself. For example,
4098 applications can ask the bus to assign a name to a connection.
4101 Each name may have <firstterm>queued owners</firstterm>. When an
4102 application requests a name for a connection and the name is already in
4103 use, the bus will optionally add the connection to a queue waiting for
4104 the name. If the current owner of the name disconnects or releases
4105 the name, the next connection in the queue will become the new owner.
4109 This feature causes the right thing to happen if you start two text
4110 editors for example; the first one may request "com.example.TextEditor",
4111 and the second will be queued as a possible owner of that name. When
4112 the first exits, the second will take over.
4116 Applications may send <firstterm>unicast messages</firstterm> to
4117 a specific recipient or to the message bus itself, or
4118 <firstterm>broadcast messages</firstterm> to all interested recipients.
4119 See <xref linkend="message-bus-routing"/> for details.
4123 <sect2 id="message-bus-names">
4124 <title>Message Bus Names</title>
4126 Each connection has at least one name, assigned at connection time and
4127 returned in response to the
4128 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4129 automatically-assigned name is called the connection's <firstterm>unique
4130 name</firstterm>. Unique names are never reused for two different
4131 connections to the same bus.
4134 Ownership of a unique name is a prerequisite for interaction with
4135 the message bus. It logically follows that the unique name is always
4136 the first name that an application comes to own, and the last
4137 one that it loses ownership of.
4140 Unique connection names must begin with the character ':' (ASCII colon
4141 character); bus names that are not unique names must not begin
4142 with this character. (The bus must reject any attempt by an application
4143 to manually request a name beginning with ':'.) This restriction
4144 categorically prevents "spoofing"; messages sent to a unique name
4145 will always go to the expected connection.
4148 When a connection is closed, all the names that it owns are deleted (or
4149 transferred to the next connection in the queue if any).
4152 A connection can request additional names to be associated with it using
4153 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4154 linkend="message-protocol-names-bus"/> describes the format of a valid
4155 name. These names can be released again using the
4156 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4159 <sect3 id="bus-messages-request-name">
4160 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4164 UINT32 RequestName (in STRING name, in UINT32 flags)
4171 <entry>Argument</entry>
4173 <entry>Description</entry>
4179 <entry>STRING</entry>
4180 <entry>Name to request</entry>
4184 <entry>UINT32</entry>
4185 <entry>Flags</entry>
4195 <entry>Argument</entry>
4197 <entry>Description</entry>
4203 <entry>UINT32</entry>
4204 <entry>Return value</entry>
4211 This method call should be sent to
4212 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4213 assign the given name to the method caller. Each name maintains a
4214 queue of possible owners, where the head of the queue is the primary
4215 or current owner of the name. Each potential owner in the queue
4216 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4217 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4218 call. When RequestName is invoked the following occurs:
4222 If the method caller is currently the primary owner of the name,
4223 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4224 values are updated with the values from the new RequestName call,
4225 and nothing further happens.
4231 If the current primary owner (head of the queue) has
4232 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4233 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4234 the caller of RequestName replaces the current primary owner at
4235 the head of the queue and the current primary owner moves to the
4236 second position in the queue. If the caller of RequestName was
4237 in the queue previously its flags are updated with the values from
4238 the new RequestName in addition to moving it to the head of the queue.
4244 If replacement is not possible, and the method caller is
4245 currently in the queue but not the primary owner, its flags are
4246 updated with the values from the new RequestName call.
4252 If replacement is not possible, and the method caller is
4253 currently not in the queue, the method caller is appended to the
4260 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4261 set and is not the primary owner, it is removed from the
4262 queue. This can apply to the previous primary owner (if it
4263 was replaced) or the method caller (if it updated the
4264 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4265 queue, or if it was just added to the queue with that flag set).
4271 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4272 queue," even if another application already in the queue had specified
4273 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4274 that does not allow replacement goes away, and the next primary owner
4275 does allow replacement. In this case, queued items that specified
4276 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4277 automatically replace the new primary owner. In other words,
4278 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4279 time RequestName is called. This is deliberate to avoid an infinite loop
4280 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4281 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4284 The flags argument contains any of the following values logically ORed
4291 <entry>Conventional Name</entry>
4292 <entry>Value</entry>
4293 <entry>Description</entry>
4298 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4302 If an application A specifies this flag and succeeds in
4303 becoming the owner of the name, and another application B
4304 later calls RequestName with the
4305 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4306 will lose ownership and receive a
4307 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4308 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4309 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4310 is not specified by application B, then application B will not replace
4311 application A as the owner.
4316 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4320 Try to replace the current owner if there is one. If this
4321 flag is not set the application will only become the owner of
4322 the name if there is no current owner. If this flag is set,
4323 the application will replace the current owner if
4324 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4329 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4333 Without this flag, if an application requests a name that is
4334 already owned, the application will be placed in a queue to
4335 own the name when the current owner gives it up. If this
4336 flag is given, the application will not be placed in the
4337 queue, the request for the name will simply fail. This flag
4338 also affects behavior when an application is replaced as
4339 name owner; by default the application moves back into the
4340 waiting queue, unless this flag was provided when the application
4341 became the name owner.
4349 The return code can be one of the following values:
4355 <entry>Conventional Name</entry>
4356 <entry>Value</entry>
4357 <entry>Description</entry>
4362 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4363 <entry>1</entry> <entry>The caller is now the primary owner of
4364 the name, replacing any previous owner. Either the name had no
4365 owner before, or the caller specified
4366 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4367 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4370 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4373 <entry>The name already had an owner,
4374 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4375 the current owner did not specify
4376 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4377 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4381 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4382 <entry>The name already has an owner,
4383 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4384 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4385 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4386 specified by the requesting application.</entry>
4389 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4391 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4399 <sect3 id="bus-messages-release-name">
4400 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4404 UINT32 ReleaseName (in STRING name)
4411 <entry>Argument</entry>
4413 <entry>Description</entry>
4419 <entry>STRING</entry>
4420 <entry>Name to release</entry>
4430 <entry>Argument</entry>
4432 <entry>Description</entry>
4438 <entry>UINT32</entry>
4439 <entry>Return value</entry>
4446 This method call should be sent to
4447 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4448 release the method caller's claim to the given name. If the caller is
4449 the primary owner, a new primary owner will be selected from the
4450 queue if any other owners are waiting. If the caller is waiting in
4451 the queue for the name, the caller will removed from the queue and
4452 will not be made an owner of the name if it later becomes available.
4453 If there are no other owners in the queue for the name, it will be
4454 removed from the bus entirely.
4456 The return code can be one of the following values:
4462 <entry>Conventional Name</entry>
4463 <entry>Value</entry>
4464 <entry>Description</entry>
4469 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4470 <entry>1</entry> <entry>The caller has released his claim on
4471 the given name. Either the caller was the primary owner of
4472 the name, and the name is now unused or taken by somebody
4473 waiting in the queue for the name, or the caller was waiting
4474 in the queue for the name and has now been removed from the
4478 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4480 <entry>The given name does not exist on this bus.</entry>
4483 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4485 <entry>The caller was not the primary owner of this name,
4486 and was also not waiting in the queue to own this name.</entry>
4494 <sect3 id="bus-messages-list-queued-owners">
4495 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4499 ARRAY of STRING ListQueuedOwners (in STRING name)
4506 <entry>Argument</entry>
4508 <entry>Description</entry>
4514 <entry>STRING</entry>
4515 <entry>The well-known bus name to query, such as
4516 <literal>com.example.cappuccino</literal></entry>
4526 <entry>Argument</entry>
4528 <entry>Description</entry>
4534 <entry>ARRAY of STRING</entry>
4535 <entry>The unique bus names of connections currently queued
4536 for the name</entry>
4543 This method call should be sent to
4544 <literal>org.freedesktop.DBus</literal> and lists the connections
4545 currently queued for a bus name (see
4546 <xref linkend="term-queued-owner"/>).
4551 <sect2 id="message-bus-routing">
4552 <title>Message Bus Message Routing</title>
4555 Messages may have a <literal>DESTINATION</literal> field (see <xref
4556 linkend="message-protocol-header-fields"/>), resulting in a
4557 <firstterm>unicast message</firstterm>. If the
4558 <literal>DESTINATION</literal> field is present, it specifies a message
4559 recipient by name. Method calls and replies normally specify this field.
4560 The message bus must send messages (of any type) with the
4561 <literal>DESTINATION</literal> field set to the specified recipient,
4562 regardless of whether the recipient has set up a match rule matching
4567 When the message bus receives a signal, if the
4568 <literal>DESTINATION</literal> field is absent, it is considered to
4569 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4570 applications with <firstterm>message matching rules</firstterm> that
4571 match the message. Most signal messages are broadcasts, and
4572 no other message types currently defined in this specification
4577 Unicast signal messages (those with a <literal>DESTINATION</literal>
4578 field) are not commonly used, but they are treated like any unicast
4579 message: they are delivered to the specified receipient,
4580 regardless of its match rules. One use for unicast signals is to
4581 avoid a race condition in which a signal is emitted before the intended
4582 recipient can call <xref linkend="bus-messages-add-match"/> to
4583 receive that signal: if the signal is sent directly to that recipient
4584 using a unicast message, it does not need to add a match rule at all,
4585 and there is no race condition. Another use for unicast signals,
4586 on message buses whose security policy prevents eavesdropping, is to
4587 send sensitive information which should only be visible to one
4592 When the message bus receives a method call, if the
4593 <literal>DESTINATION</literal> field is absent, the call is taken to be
4594 a standard one-to-one message and interpreted by the message bus
4595 itself. For example, sending an
4596 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4597 <literal>DESTINATION</literal> will cause the message bus itself to
4598 reply to the ping immediately; the message bus will not make this
4599 message visible to other applications.
4603 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4604 the ping message were sent with a <literal>DESTINATION</literal> name of
4605 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4606 forwarded, and the Yoyodyne Corporation screensaver application would be
4607 expected to reply to the ping.
4611 Message bus implementations may impose a security policy which
4612 prevents certain messages from being sent or received.
4613 When a method call message cannot be sent or received due to a security
4614 policy, the message bus should send an error reply, unless the
4615 original message had the <literal>NO_REPLY</literal> flag.
4618 <sect3 id="message-bus-routing-eavesdropping">
4619 <title>Eavesdropping</title>
4621 Receiving a unicast message whose <literal>DESTINATION</literal>
4622 indicates a different recipient is called
4623 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4624 a security boundary (like the standard system bus), the security
4625 policy should usually prevent eavesdropping, since unicast messages
4626 are normally kept private and may contain security-sensitive
4631 Eavesdropping is mainly useful for debugging tools, such as
4632 the <literal>dbus-monitor</literal> tool in the reference
4633 implementation of D-Bus. Tools which eavesdrop on the message bus
4634 should be careful to avoid sending a reply or error in response to
4635 messages intended for a different client.
4639 Clients may attempt to eavesdrop by adding match rules
4640 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4641 the <literal>eavesdrop='true'</literal> match. If the message bus'
4642 security policy does not allow eavesdropping, the match rule can
4643 still be added, but will not have any practical effect. For
4644 compatibility with older message bus implementations, if adding such
4645 a match rule results in an error reply, the client may fall back to
4646 adding the same rule with the <literal>eavesdrop</literal> match
4651 Eavesdropping interacts poorly with buses with non-trivial
4652 access control restrictions. The
4653 <xref linkend="bus-messages-become-monitor"/> method provides
4654 an alternative way to monitor buses.
4658 <sect3 id="message-bus-routing-match-rules">
4659 <title>Match Rules</title>
4661 An important part of the message bus routing protocol is match
4662 rules. Match rules describe the messages that should be sent to a
4663 client, based on the contents of the message. Broadcast signals
4664 are only sent to clients which have a suitable match rule: this
4665 avoids waking up client processes to deal with signals that are
4666 not relevant to that client.
4669 Messages that list a client as their <literal>DESTINATION</literal>
4670 do not need to match the client's match rules, and are sent to that
4671 client regardless. As a result, match rules are mainly used to
4672 receive a subset of broadcast signals.
4675 Match rules can also be used for eavesdropping
4676 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4677 if the security policy of the message bus allows it.
4680 Match rules are added using the AddMatch bus method
4681 (see <xref linkend="bus-messages-add-match"/>). Rules are
4682 specified as a string of comma separated key/value pairs.
4683 Excluding a key from the rule indicates a wildcard match.
4684 For instance excluding the the member from a match rule but
4685 adding a sender would let all messages from that sender through.
4686 An example of a complete rule would be
4687 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4690 Within single quotes (ASCII apostrophe, U+0027), a backslash
4691 (U+005C) represents itself, and an apostrophe ends the quoted
4692 section. Outside single quotes, \' (backslash, apostrophe)
4693 represents an apostrophe, and any backslash not followed by
4694 an apostrophe represents itself. For instance, the match rules
4695 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4696 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4697 both match messages where the arguments are a 1-character string
4698 containing an apostrophe, a 1-character string containing a
4699 backslash, a 1-character string containing a comma, and a
4700 2-character string containing two backslashes<footnote>
4702 This idiosyncratic quoting style is based on the rules for
4703 escaping items to appear inside single-quoted strings
4704 in POSIX <literal>/bin/sh</literal>, but please
4705 note that backslashes that are not inside single quotes have
4706 different behaviour. This syntax does not offer any way to
4707 represent an apostrophe inside single quotes (it is necessary
4708 to leave the single-quoted section, backslash-escape the
4709 apostrophe and re-enter single quotes), or to represent a
4710 comma outside single quotes (it is necessary to wrap it in
4711 a single-quoted section).
4716 The following table describes the keys that can be used to create
4723 <entry>Possible Values</entry>
4724 <entry>Description</entry>
4729 <entry><literal>type</literal></entry>
4730 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4731 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4734 <entry><literal>sender</literal></entry>
4735 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4736 and <xref linkend="term-unique-name"/> respectively)
4738 <entry>Match messages sent by a particular sender. An example of a sender match
4739 is sender='org.freedesktop.Hal'</entry>
4742 <entry><literal>interface</literal></entry>
4743 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4744 <entry>Match messages sent over or to a particular interface. An example of an
4745 interface match is interface='org.freedesktop.Hal.Manager'.
4746 If a message omits the interface header, it must not match any rule
4747 that specifies this key.</entry>
4750 <entry><literal>member</literal></entry>
4751 <entry>Any valid method or signal name</entry>
4752 <entry>Matches messages which have the give method or signal name. An example of
4753 a member match is member='NameOwnerChanged'</entry>
4756 <entry><literal>path</literal></entry>
4757 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4758 <entry>Matches messages which are sent from or to the given object. An example of a
4759 path match is path='/org/freedesktop/Hal/Manager'</entry>
4762 <entry><literal>path_namespace</literal></entry>
4763 <entry>An object path</entry>
4766 Matches messages which are sent from or to an
4767 object for which the object path is either the
4768 given value, or that value followed by one or
4769 more path components.
4774 <literal>path_namespace='/com/example/foo'</literal>
4775 would match signals sent by
4776 <literal>/com/example/foo</literal>
4778 <literal>/com/example/foo/bar</literal>,
4780 <literal>/com/example/foobar</literal>.
4784 Using both <literal>path</literal> and
4785 <literal>path_namespace</literal> in the same match
4786 rule is not allowed.
4791 This match key was added in version 0.16 of the
4792 D-Bus specification and implemented by the bus
4793 daemon in dbus 1.5.0 and later.
4799 <entry><literal>destination</literal></entry>
4800 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4801 <entry>Matches messages which are being sent to the given unique name. An
4802 example of a destination match is destination=':1.0'</entry>
4805 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4806 <entry>Any string</entry>
4807 <entry>Arg matches are special and are used for further restricting the
4808 match based on the arguments in the body of a message. Only arguments of type
4809 STRING can be matched in this way. An example of an argument match
4810 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4814 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4815 <entry>Any string</entry>
4817 <para>Argument path matches provide a specialised form of wildcard matching for
4818 path-like namespaces. They can match arguments whose type is either STRING or
4819 OBJECT_PATH. As with normal argument matches,
4820 if the argument is exactly equal to the string given in the match
4821 rule then the rule is satisfied. Additionally, there is also a
4822 match when either the string given in the match rule or the
4823 appropriate message argument ends with '/' and is a prefix of the
4824 other. An example argument path match is arg0path='/aa/bb/'. This
4825 would match messages with first arguments of '/', '/aa/',
4826 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4827 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4829 <para>This is intended for monitoring “directories” in file system-like
4830 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4831 system. An application interested in all nodes in a particular hierarchy would
4832 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4833 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4834 represent a modification to the “bar” property, or a signal with zeroth
4835 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4836 many properties within that directory, and the interested application would be
4837 notified in both cases.</para>
4840 This match key was added in version 0.12 of the
4841 D-Bus specification, implemented for STRING
4842 arguments by the bus daemon in dbus 1.2.0 and later,
4843 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4850 <entry><literal>arg0namespace</literal></entry>
4851 <entry>Like a bus name, except that the string is not
4852 required to contain a '.' (period)</entry>
4854 <para>Match messages whose first argument is of type STRING, and is a bus name
4855 or interface name within the specified namespace. This is primarily intended
4856 for watching name owner changes for a group of related bus names, rather than
4857 for a single name or all name changes.</para>
4859 <para>Because every valid interface name is also a valid
4860 bus name, this can also be used for messages whose
4861 first argument is an interface name.</para>
4863 <para>For example, the match rule
4864 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4865 matches name owner changes for bus names such as
4866 <literal>com.example.backend.foo</literal>,
4867 <literal>com.example.backend.foo.bar</literal>, and
4868 <literal>com.example.backend</literal> itself.</para>
4870 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4873 This match key was added in version 0.16 of the
4874 D-Bus specification and implemented by the bus
4875 daemon in dbus 1.5.0 and later.
4881 <entry><literal>eavesdrop</literal></entry>
4882 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4883 <entry>Since D-Bus 1.5.6, match rules do not
4884 match messages which have a <literal>DESTINATION</literal>
4885 field unless the match rule specifically
4887 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4888 by specifying <literal>eavesdrop='true'</literal>
4889 in the match rule. <literal>eavesdrop='false'</literal>
4890 restores the default behaviour. Messages are
4891 delivered to their <literal>DESTINATION</literal>
4892 regardless of match rules, so this match does not
4893 affect normal delivery of unicast messages.
4894 If the message bus has a security policy which forbids
4895 eavesdropping, this match may still be used without error,
4896 but will not have any practical effect.
4897 In older versions of D-Bus, this match was not allowed
4898 in match rules, and all match rules behaved as if
4899 <literal>eavesdrop='true'</literal> had been used.
4908 <sect2 id="message-bus-starting-services">
4909 <title>Message Bus Starting Services</title>
4911 The message bus can start applications on behalf of other applications.
4912 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4913 An application that can be started in this way is called a
4914 <firstterm>service</firstterm>.
4917 With D-Bus, starting a service is normally done by name. That is,
4918 applications ask the message bus to start some program that will own a
4919 well-known name, such as <literal>com.example.TextEditor</literal>.
4920 This implies a contract documented along with the name
4921 <literal>com.example.TextEditor</literal> for which object
4922 the owner of that name will provide, and what interfaces those
4926 To find an executable corresponding to a particular name, the bus daemon
4927 looks for <firstterm>service description files</firstterm>. Service
4928 description files define a mapping from names to executables. Different
4929 kinds of message bus will look for these files in different places, see
4930 <xref linkend="message-bus-types"/>.
4933 Service description files have the ".service" file
4934 extension. The message bus will only load service description files
4935 ending with .service; all other files will be ignored. The file format
4936 is similar to that of <ulink
4937 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4938 entries</ulink>. All service description files must be in UTF-8
4939 encoding. To ensure that there will be no name collisions, service files
4940 must be namespaced using the same mechanism as messages and service
4945 On the well-known system bus, the name of a service description file
4946 must be its well-known name plus <literal>.service</literal>,
4948 <literal>com.example.ConfigurationDatabase.service</literal>.
4952 On the well-known session bus, services should follow the same
4953 service description file naming convention as on the system bus,
4954 but for backwards compatibility they are not required to do so.
4958 [FIXME the file format should be much better specified than "similar to
4959 .desktop entries" esp. since desktop entries are already
4960 badly-specified. ;-)]
4961 These sections from the specification apply to service files as well:
4964 <listitem><para>General syntax</para></listitem>
4965 <listitem><para>Comment format</para></listitem>
4968 Service description files must contain a
4969 <literal>D-BUS Service</literal> group with at least the keys
4970 <literal>Name</literal> (the well-known name of the service)
4971 and <literal>Exec</literal> (the command to be executed).
4974 <title>Example service description file</title>
4976 # Sample service description file
4978 Name=com.example.ConfigurationDatabase
4979 Exec=/usr/bin/sample-configd
4985 Additionally, service description files for the well-known system
4986 bus on Unix must contain a <literal>User</literal> key, whose value
4987 is the name of a user account (e.g. <literal>root</literal>).
4988 The system service will be run as that user.
4992 When an application asks to start a service by name, the bus daemon tries to
4993 find a service that will own that name. It then tries to spawn the
4994 executable associated with it. If this fails, it will report an
4999 On the well-known system bus, it is not possible for two .service files
5000 in the same directory to offer the same service, because they are
5001 constrained to have names that match the service name.
5005 On the well-known session bus, if two .service files in the same
5006 directory offer the same service name, the result is undefined.
5007 Distributors should avoid this situation, for instance by naming
5008 session services' .service files according to their service name.
5012 If two .service files in different directories offer the same
5013 service name, the one in the higher-priority directory is used:
5014 for instance, on the system bus, .service files in
5015 /usr/local/share/dbus-1/system-services take precedence over those
5016 in /usr/share/dbus-1/system-services.
5019 The executable launched will have the environment variable
5020 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
5021 message bus so it can connect and request the appropriate names.
5024 The executable being launched may want to know whether the message bus
5025 starting it is one of the well-known message buses (see <xref
5026 linkend="message-bus-types"/>). To facilitate this, the bus must also set
5027 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
5028 of the well-known buses. The currently-defined values for this variable
5029 are <literal>system</literal> for the systemwide message bus,
5030 and <literal>session</literal> for the per-login-session message
5031 bus. The new executable must still connect to the address given
5032 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
5033 resulting connection is to the well-known bus.
5036 [FIXME there should be a timeout somewhere, either specified
5037 in the .service file, by the client, or just a global value
5038 and if the client being activated fails to connect within that
5039 timeout, an error should be sent back.]
5042 <sect3 id="message-bus-starting-services-scope">
5043 <title>Message Bus Service Scope</title>
5045 The "scope" of a service is its "per-", such as per-session,
5046 per-machine, per-home-directory, or per-display. The reference
5047 implementation doesn't yet support starting services in a different
5048 scope from the message bus itself. So e.g. if you start a service
5049 on the session bus its scope is per-session.
5052 We could add an optional scope to a bus name. For example, for
5053 per-(display,session pair), we could have a unique ID for each display
5054 generated automatically at login and set on screen 0 by executing a
5055 special "set display ID" binary. The ID would be stored in a
5056 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
5057 random bytes. This ID would then be used to scope names.
5058 Starting/locating a service could be done by ID-name pair rather than
5062 Contrast this with a per-display scope. To achieve that, we would
5063 want a single bus spanning all sessions using a given display.
5064 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
5065 property on screen 0 of the display, pointing to this bus.
5070 <sect2 id="message-bus-types">
5071 <title>Well-known Message Bus Instances</title>
5073 Two standard message bus instances are defined here, along with how
5074 to locate them and where their service files live.
5076 <sect3 id="message-bus-types-login">
5077 <title>Login session message bus</title>
5079 Each time a user logs in, a <firstterm>login session message
5080 bus</firstterm> may be started. All applications in the user's login
5081 session may interact with one another using this message bus.
5084 The address of the login session message bus is given
5085 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5086 variable. If that variable is not set, applications may
5087 also try to read the address from the X Window System root
5088 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5089 The root window property must have type <literal>STRING</literal>.
5090 The environment variable should have precedence over the
5091 root window property.
5093 <para>The address of the login session message bus is given in the
5094 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5095 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5096 "autolaunch:", the system should use platform-specific methods of
5097 locating a running D-Bus session server, or starting one if a running
5098 instance cannot be found. Note that this mechanism is not recommended
5099 for attempting to determine if a daemon is running. It is inherently
5100 racy to attempt to make this determination, since the bus daemon may
5101 be started just before or just after the determination is made.
5102 Therefore, it is recommended that applications do not try to make this
5103 determination for their functionality purposes, and instead they
5104 should attempt to start the server.</para>
5106 <sect4 id="message-bus-types-login-x-windows">
5107 <title>X Windowing System</title>
5109 For the X Windowing System, the application must locate the
5110 window owner of the selection represented by the atom formed by
5114 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5118 <para>the current user's username</para>
5122 <para>the literal character '_' (underscore)</para>
5126 <para>the machine's ID</para>
5132 The following properties are defined for the window that owns
5134 <informaltable frame="all">
5143 <para>meaning</para>
5149 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5153 <para>the actual address of the server socket</para>
5159 <para>_DBUS_SESSION_BUS_PID</para>
5163 <para>the PID of the server process</para>
5172 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5173 present in this window.
5177 If the X selection cannot be located or if reading the
5178 properties from the window fails, the implementation MUST conclude
5179 that there is no D-Bus server running and proceed to start a new
5180 server. (See below on concurrency issues)
5184 Failure to connect to the D-Bus server address thus obtained
5185 MUST be treated as a fatal connection error and should be reported
5190 As an alternative, an implementation MAY find the information
5191 in the following file located in the current user's home directory,
5192 in subdirectory .dbus/session-bus/:
5195 <para>the machine's ID</para>
5199 <para>the literal character '-' (dash)</para>
5203 <para>the X display without the screen number, with the
5204 following prefixes removed, if present: ":", "localhost:"
5205 ."localhost.localdomain:". That is, a display of
5206 "localhost:10.0" produces just the number "10"</para>
5212 The contents of this file NAME=value assignment pairs and
5213 lines starting with # are comments (no comments are allowed
5214 otherwise). The following variable names are defined:
5221 <para>Variable</para>
5225 <para>meaning</para>
5231 <para>DBUS_SESSION_BUS_ADDRESS</para>
5235 <para>the actual address of the server socket</para>
5241 <para>DBUS_SESSION_BUS_PID</para>
5245 <para>the PID of the server process</para>
5251 <para>DBUS_SESSION_BUS_WINDOWID</para>
5255 <para>the window ID</para>
5264 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5269 Failure to open this file MUST be interpreted as absence of a
5270 running server. Therefore, the implementation MUST proceed to
5271 attempting to launch a new bus server if the file cannot be
5276 However, success in opening this file MUST NOT lead to the
5277 conclusion that the server is running. Thus, a failure to connect to
5278 the bus address obtained by the alternative method MUST NOT be
5279 considered a fatal error. If the connection cannot be established,
5280 the implementation MUST proceed to check the X selection settings or
5281 to start the server on its own.
5285 If the implementation concludes that the D-Bus server is not
5286 running it MUST attempt to start a new server and it MUST also
5287 ensure that the daemon started as an effect of the "autolaunch"
5288 mechanism provides the lookup mechanisms described above, so
5289 subsequent calls can locate the newly started server. The
5290 implementation MUST also ensure that if two or more concurrent
5291 initiations happen, only one server remains running and all other
5292 initiations are able to obtain the address of this server and
5293 connect to it. In other words, the implementation MUST ensure that
5294 the X selection is not present when it attempts to set it, without
5295 allowing another process to set the selection between the
5296 verification and the setting (e.g., by using XGrabServer /
5303 On Unix systems, the session bus should search for .service files
5304 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5306 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5307 Implementations may also search additional locations, which
5308 should be searched with lower priority than anything in
5309 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5310 for example, the reference implementation also
5311 looks in <literal>${datadir}/dbus-1/services</literal> as
5312 set at compile time.
5315 As described in the XDG Base Directory Specification, software
5316 packages should install their session .service files to their
5317 configured <literal>${datadir}/dbus-1/services</literal>,
5318 where <literal>${datadir}</literal> is as defined by the GNU
5319 coding standards. System administrators or users can arrange
5320 for these service files to be read by setting XDG_DATA_DIRS or by
5321 symlinking them into the default locations.
5325 <sect3 id="message-bus-types-system">
5326 <title>System message bus</title>
5328 A computer may have a <firstterm>system message bus</firstterm>,
5329 accessible to all applications on the system. This message bus may be
5330 used to broadcast system events, such as adding new hardware devices,
5331 changes in the printer queue, and so forth.
5334 The address of the system message bus is given
5335 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5336 variable. If that variable is not set, applications should try
5337 to connect to the well-known address
5338 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5341 The D-Bus reference implementation actually honors the
5342 <literal>$(localstatedir)</literal> configure option
5343 for this address, on both client and server side.
5348 On Unix systems, the system bus should default to searching
5349 for .service files in
5350 <literal>/usr/local/share/dbus-1/system-services</literal>,
5351 <literal>/usr/share/dbus-1/system-services</literal> and
5352 <literal>/lib/dbus-1/system-services</literal>, with that order
5353 of precedence. It may also search other implementation-specific
5354 locations, but should not vary these locations based on environment
5358 The system bus is security-sensitive and is typically executed
5359 by an init system with a clean environment. Its launch helper
5360 process is particularly security-sensitive, and specifically
5361 clears its own environment.
5366 Software packages should install their system .service
5367 files to their configured
5368 <literal>${datadir}/dbus-1/system-services</literal>,
5369 where <literal>${datadir}</literal> is as defined by the GNU
5370 coding standards. System administrators can arrange
5371 for these service files to be read by editing the system bus'
5372 configuration file or by symlinking them into the default
5378 <sect2 id="message-bus-messages">
5379 <title>Message Bus Messages</title>
5381 The special message bus name <literal>org.freedesktop.DBus</literal>
5382 responds to a number of additional messages.
5385 <sect3 id="bus-messages-hello">
5386 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5397 <entry>Argument</entry>
5399 <entry>Description</entry>
5405 <entry>STRING</entry>
5406 <entry>Unique name assigned to the connection</entry>
5413 Before an application is able to send messages to other applications
5414 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5415 to the message bus to obtain a unique name. If an application without
5416 a unique name tries to send a message to another application, or a
5417 message to the message bus itself that isn't the
5418 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5419 disconnected from the bus.
5422 There is no corresponding "disconnect" request; if a client wishes to
5423 disconnect from the bus, it simply closes the socket (or other
5424 communication channel).
5427 <sect3 id="bus-messages-list-names">
5428 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5432 ARRAY of STRING ListNames ()
5439 <entry>Argument</entry>
5441 <entry>Description</entry>
5447 <entry>ARRAY of STRING</entry>
5448 <entry>Array of strings where each string is a bus name</entry>
5455 Returns a list of all currently-owned names on the bus.
5458 <sect3 id="bus-messages-list-activatable-names">
5459 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5463 ARRAY of STRING ListActivatableNames ()
5470 <entry>Argument</entry>
5472 <entry>Description</entry>
5478 <entry>ARRAY of STRING</entry>
5479 <entry>Array of strings where each string is a bus name</entry>
5486 Returns a list of all names that can be activated on the bus.
5489 <sect3 id="bus-messages-name-exists">
5490 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5494 BOOLEAN NameHasOwner (in STRING name)
5501 <entry>Argument</entry>
5503 <entry>Description</entry>
5509 <entry>STRING</entry>
5510 <entry>Name to check</entry>
5520 <entry>Argument</entry>
5522 <entry>Description</entry>
5528 <entry>BOOLEAN</entry>
5529 <entry>Return value, true if the name exists</entry>
5536 Checks if the specified name exists (currently has an owner).
5540 <sect3 id="bus-messages-name-owner-changed">
5541 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5545 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5552 <entry>Argument</entry>
5554 <entry>Description</entry>
5560 <entry>STRING</entry>
5561 <entry>Name with a new owner</entry>
5565 <entry>STRING</entry>
5566 <entry>Old owner or empty string if none</entry>
5570 <entry>STRING</entry>
5571 <entry>New owner or empty string if none</entry>
5578 This signal indicates that the owner of a name has changed.
5579 It's also the signal to use to detect the appearance of
5580 new names on the bus.
5583 <sect3 id="bus-messages-name-lost">
5584 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5588 NameLost (STRING name)
5595 <entry>Argument</entry>
5597 <entry>Description</entry>
5603 <entry>STRING</entry>
5604 <entry>Name which was lost</entry>
5611 This signal is sent to a specific application when it loses
5612 ownership of a name.
5616 <sect3 id="bus-messages-name-acquired">
5617 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5621 NameAcquired (STRING name)
5628 <entry>Argument</entry>
5630 <entry>Description</entry>
5636 <entry>STRING</entry>
5637 <entry>Name which was acquired</entry>
5644 This signal is sent to a specific application when it gains
5645 ownership of a name.
5649 <sect3 id="bus-messages-start-service-by-name">
5650 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5654 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5661 <entry>Argument</entry>
5663 <entry>Description</entry>
5669 <entry>STRING</entry>
5670 <entry>Name of the service to start</entry>
5674 <entry>UINT32</entry>
5675 <entry>Flags (currently not used)</entry>
5685 <entry>Argument</entry>
5687 <entry>Description</entry>
5693 <entry>UINT32</entry>
5694 <entry>Return value</entry>
5699 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5703 The return value can be one of the following values:
5708 <entry>Identifier</entry>
5709 <entry>Value</entry>
5710 <entry>Description</entry>
5715 <entry>DBUS_START_REPLY_SUCCESS</entry>
5717 <entry>The service was successfully started.</entry>
5720 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5722 <entry>A connection already owns the given name.</entry>
5731 <sect3 id="bus-messages-update-activation-environment">
5732 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5736 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5743 <entry>Argument</entry>
5745 <entry>Description</entry>
5751 <entry>ARRAY of DICT<STRING,STRING></entry>
5752 <entry>Environment to add or update</entry>
5757 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5760 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5763 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.
5768 <sect3 id="bus-messages-get-name-owner">
5769 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5773 STRING GetNameOwner (in STRING name)
5780 <entry>Argument</entry>
5782 <entry>Description</entry>
5788 <entry>STRING</entry>
5789 <entry>Name to get the owner of</entry>
5799 <entry>Argument</entry>
5801 <entry>Description</entry>
5807 <entry>STRING</entry>
5808 <entry>Return value, a unique connection name</entry>
5813 Returns the unique connection name of the primary owner of the name
5814 given. If the requested name doesn't have an owner, returns a
5815 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5819 <sect3 id="bus-messages-get-connection-unix-user">
5820 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5824 UINT32 GetConnectionUnixUser (in STRING bus_name)
5831 <entry>Argument</entry>
5833 <entry>Description</entry>
5839 <entry>STRING</entry>
5840 <entry>Unique or well-known bus name of the connection to
5841 query, such as <literal>:12.34</literal> or
5842 <literal>com.example.tea</literal></entry>
5852 <entry>Argument</entry>
5854 <entry>Description</entry>
5860 <entry>UINT32</entry>
5861 <entry>Unix user ID</entry>
5866 Returns the Unix user ID of the process connected to the server. If
5867 unable to determine it (for instance, because the process is not on the
5868 same machine as the bus daemon), an error is returned.
5872 <sect3 id="bus-messages-get-connection-unix-process-id">
5873 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5877 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5884 <entry>Argument</entry>
5886 <entry>Description</entry>
5892 <entry>STRING</entry>
5893 <entry>Unique or well-known bus name of the connection to
5894 query, such as <literal>:12.34</literal> or
5895 <literal>com.example.tea</literal></entry>
5905 <entry>Argument</entry>
5907 <entry>Description</entry>
5913 <entry>UINT32</entry>
5914 <entry>Unix process id</entry>
5919 Returns the Unix process ID of the process connected to the server. If
5920 unable to determine it (for instance, because the process is not on the
5921 same machine as the bus daemon), an error is returned.
5925 <sect3 id="bus-messages-get-connection-credentials">
5926 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5930 DICT<STRING,VARIANT> GetConnectionCredentials (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>DICT<STRING,VARIANT></entry>
5967 <entry>Credentials</entry>
5975 Returns as many credentials as possible for the process connected to
5976 the server. If unable to determine certain credentials (for instance,
5977 because the process is not on the same machine as the bus daemon,
5978 or because this version of the bus daemon does not support a
5979 particular security framework), or if the values of those credentials
5980 cannot be represented as documented here, then those credentials
5985 Keys in the returned dictionary not containing "." are defined
5986 by this specification. Bus daemon implementors supporting
5987 credentials frameworks not mentioned in this document should either
5988 contribute patches to this specification, or use keys containing
5989 "." and starting with a reversed domain name.
5995 <entry>Value type</entry>
5996 <entry>Value</entry>
6001 <entry>UnixUserID</entry>
6002 <entry>UINT32</entry>
6003 <entry>The numeric Unix user ID, as defined by POSIX</entry>
6006 <entry>ProcessID</entry>
6007 <entry>UINT32</entry>
6008 <entry>The numeric process ID, on platforms that have
6009 this concept. On Unix, this is the process ID defined by
6013 <entry>WindowsSID</entry>
6014 <entry>STRING</entry>
6015 <entry>The Windows security identifier in its string form,
6016 e.g. "S-1-5-21-3623811015-3361044348-30300820-1013" for
6017 a domain or local computer user or "S-1-5-18" for the
6018 LOCAL_SYSTEM user</entry>
6022 <entry>LinuxSecurityLabel</entry>
6023 <entry>ARRAY of BYTE</entry>
6025 <para>On Linux systems, the security label that would result
6026 from the SO_PEERSEC getsockopt call. The array contains
6027 the non-zero bytes of the security label in an unspecified
6028 ASCII-compatible encoding<footnote>
6029 <para>It could be ASCII or UTF-8, but could also be
6030 ISO Latin-1 or any other encoding.</para>
6031 </footnote>, followed by a single zero byte.</para>
6033 For example, the SELinux context
6034 <literal>system_u:system_r:init_t:s0</literal>
6035 (a string of length 27) would be encoded as 28 bytes
6036 ending with ':', 's', '0', '\x00'.<footnote>
6037 <para>Note that this is not the same as the older
6038 GetConnectionSELinuxContext method, which does
6039 not append the zero byte. Always appending the
6040 zero byte allows callers to read the string
6041 from the message payload without copying.</para>
6045 On SELinux systems this is the SELinux context, as output
6046 by <literal>ps -Z</literal> or <literal>ls -Z</literal>.
6047 Typical values might include
6048 <literal>system_u:system_r:init_t:s0</literal>,
6049 <literal>unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023</literal>,
6051 <literal>unconfined_u:unconfined_r:chrome_sandbox_t:s0-s0:c0.c1023</literal>.
6054 On Smack systems, this is the Smack label.
6055 Typical values might include
6056 <literal>_</literal>, <literal>*</literal>,
6057 <literal>User</literal>, <literal>System</literal>
6058 or <literal>System::Shared</literal>.
6061 On AppArmor systems, this is the AppArmor context,
6062 a composite string encoding the AppArmor label (one or more
6063 profiles) and the enforcement mode.
6064 Typical values might include <literal>unconfined</literal>,
6065 <literal>/usr/bin/firefox (enforce)</literal> or
6066 <literal>user1 (complain)</literal>.
6077 This method was added in D-Bus 1.7 to reduce the round-trips
6078 required to list a process's credentials. In older versions, calling
6079 this method will fail: applications should recover by using the
6080 separate methods such as
6081 <xref linkend="bus-messages-get-connection-unix-user"/>
6086 <sect3 id="bus-messages-get-adt-audit-session-data">
6087 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
6091 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
6098 <entry>Argument</entry>
6100 <entry>Description</entry>
6106 <entry>STRING</entry>
6107 <entry>Unique or well-known bus name of the connection to
6108 query, such as <literal>:12.34</literal> or
6109 <literal>com.example.tea</literal></entry>
6119 <entry>Argument</entry>
6121 <entry>Description</entry>
6127 <entry>ARRAY of BYTE</entry>
6128 <entry>auditing data as returned by
6129 adt_export_session_data()</entry>
6134 Returns auditing data used by Solaris ADT, in an unspecified
6135 binary format. If you know what this means, please contribute
6136 documentation via the D-Bus bug tracking system.
6137 This method is on the core DBus interface for historical reasons;
6138 the same information should be made available via
6139 <xref linkend="bus-messages-get-connection-credentials"/>
6144 <sect3 id="bus-messages-get-connection-selinux-security-context">
6145 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6149 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6156 <entry>Argument</entry>
6158 <entry>Description</entry>
6164 <entry>STRING</entry>
6165 <entry>Unique or well-known bus name of the connection to
6166 query, such as <literal>:12.34</literal> or
6167 <literal>com.example.tea</literal></entry>
6177 <entry>Argument</entry>
6179 <entry>Description</entry>
6185 <entry>ARRAY of BYTE</entry>
6186 <entry>some sort of string of bytes, not necessarily UTF-8,
6187 not including '\0'</entry>
6192 Returns the security context used by SELinux, in an unspecified
6193 format. If you know what this means, please contribute
6194 documentation via the D-Bus bug tracking system.
6195 This method is on the core DBus interface for historical reasons;
6196 the same information should be made available via
6197 <xref linkend="bus-messages-get-connection-credentials"/>
6203 <sect3 id="bus-messages-add-match">
6204 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6208 AddMatch (in STRING rule)
6215 <entry>Argument</entry>
6217 <entry>Description</entry>
6223 <entry>STRING</entry>
6224 <entry>Match rule to add to the connection</entry>
6229 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6230 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6234 <sect3 id="bus-messages-remove-match">
6235 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6239 RemoveMatch (in STRING rule)
6246 <entry>Argument</entry>
6248 <entry>Description</entry>
6254 <entry>STRING</entry>
6255 <entry>Match rule to remove from the connection</entry>
6260 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6261 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6266 <sect3 id="bus-messages-get-id">
6267 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6271 GetId (out STRING id)
6278 <entry>Argument</entry>
6280 <entry>Description</entry>
6286 <entry>STRING</entry>
6287 <entry>Unique ID identifying the bus daemon</entry>
6292 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6293 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6294 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6295 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6296 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6297 by org.freedesktop.DBus.Peer.GetMachineId().
6298 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6302 <sect3 id="bus-messages-become-monitor">
6303 <title><literal>org.freedesktop.DBus.Monitoring.BecomeMonitor</literal></title>
6307 BecomeMonitor (in ARRAY of STRING rule, in UINT32 flags)
6314 <entry>Argument</entry>
6316 <entry>Description</entry>
6322 <entry>ARRAY of STRING</entry>
6323 <entry>Match rules to add to the connection</entry>
6327 <entry>UINT32</entry>
6328 <entry>Not used, must be 0</entry>
6336 Converts the connection into a <emphasis>monitor
6337 connection</emphasis> which can be used as a debugging/monitoring
6338 tool. Only a user who is privileged on this
6339 bus (by some implementation-specific definition) may create
6340 monitor connections<footnote>
6342 In the reference implementation,
6343 the default configuration is that each user (identified by
6344 numeric user ID) may monitor their own session bus,
6345 and the root user (user ID zero) may monitor the
6352 Monitor connections lose all their bus names, including the unique
6353 connection name, and all their match rules. Sending messages on a
6354 monitor connection is not allowed: applications should use a private
6355 connection for monitoring.
6359 Monitor connections may receive all messages, even messages that
6360 should only have gone to some other connection ("eavesdropping").
6361 The first argument is a list of match rules, which replace any
6362 match rules that were previously active for this connection.
6363 These match rules are always treated as if they contained the
6364 special <literal>eavesdrop='true'</literal> member.
6368 As a special case, an empty list of match rules (which would
6369 otherwise match nothing, making the monitor useless) is treated
6370 as a shorthand for matching all messages.
6374 The second argument might be used for flags to influence the
6375 behaviour of the monitor connection in future D-Bus versions.
6379 Message bus implementations should attempt to minimize the
6380 side-effects of monitoring — in particular, unlike ordinary
6381 eavesdropping, monitoring the system bus does not require the
6382 access control rules to be relaxed, which would change the set
6383 of messages that can be delivered to their (non-monitor)
6384 destinations. However, it is unavoidable that monitoring
6385 will increase the message bus's resource consumption. In
6386 edge cases where there was barely enough time or memory without
6387 monitoring, this might result in message deliveries failing
6388 when they would otherwise have succeeded.
6396 <appendix id="implementation-notes">
6397 <title>Implementation notes</title>
6398 <sect1 id="implementation-notes-subsection">
6406 <glossary><title>Glossary</title>
6408 This glossary defines some of the terms used in this specification.
6411 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6414 The message bus maintains an association between names and
6415 connections. (Normally, there's one connection per application.) A
6416 bus name is simply an identifier used to locate connections. For
6417 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6418 name might be used to send a message to a screensaver from Yoyodyne
6419 Corporation. An application is said to <firstterm>own</firstterm> a
6420 name if the message bus has associated the application's connection
6421 with the name. Names may also have <firstterm>queued
6422 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6423 The bus assigns a unique name to each connection,
6424 see <xref linkend="term-unique-name"/>. Other names
6425 can be thought of as "well-known names" and are
6426 used to find applications that offer specific functionality.
6430 See <xref linkend="message-protocol-names-bus"/> for details of
6431 the syntax and naming conventions for bus names.
6436 <glossentry id="term-message"><glossterm>Message</glossterm>
6439 A message is the atomic unit of communication via the D-Bus
6440 protocol. It consists of a <firstterm>header</firstterm> and a
6441 <firstterm>body</firstterm>; the body is made up of
6442 <firstterm>arguments</firstterm>.
6447 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6450 The message bus is a special application that forwards
6451 or routes messages between a group of applications
6452 connected to the message bus. It also manages
6453 <firstterm>names</firstterm> used for routing
6459 <glossentry id="term-name"><glossterm>Name</glossterm>
6462 See <xref linkend="term-bus-name"/>. "Name" may
6463 also be used to refer to some of the other names
6464 in D-Bus, such as interface names.
6469 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6472 Used to prevent collisions when defining new interfaces, bus names
6473 etc. The convention used is the same one Java uses for defining
6474 classes: a reversed domain name.
6475 See <xref linkend="message-protocol-names-bus"/>,
6476 <xref linkend="message-protocol-names-interface"/>,
6477 <xref linkend="message-protocol-names-error"/>,
6478 <xref linkend="message-protocol-marshaling-object-path"/>.
6483 <glossentry id="term-object"><glossterm>Object</glossterm>
6486 Each application contains <firstterm>objects</firstterm>, which have
6487 <firstterm>interfaces</firstterm> and
6488 <firstterm>methods</firstterm>. Objects are referred to by a name,
6489 called a <firstterm>path</firstterm>.
6494 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6497 An application talking directly to another application, without going
6498 through a message bus. One-to-one connections may be "peer to peer" or
6499 "client to server." The D-Bus protocol has no concept of client
6500 vs. server after a connection has authenticated; the flow of messages
6501 is symmetrical (full duplex).
6506 <glossentry id="term-path"><glossterm>Path</glossterm>
6509 Object references (object names) in D-Bus are organized into a
6510 filesystem-style hierarchy, so each object is named by a path. As in
6511 LDAP, there's no difference between "files" and "directories"; a path
6512 can refer to an object, while still having child objects below it.
6517 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6520 Each bus name has a primary owner; messages sent to the name go to the
6521 primary owner. However, certain names also maintain a queue of
6522 secondary owners "waiting in the wings." If the primary owner releases
6523 the name, then the first secondary owner in the queue automatically
6524 becomes the new owner of the name.
6529 <glossentry id="term-service"><glossterm>Service</glossterm>
6532 A service is an executable that can be launched by the bus daemon.
6533 Services normally guarantee some particular features, for example they
6534 may guarantee that they will request a specific name such as
6535 "com.example.Screensaver", have a singleton object
6536 "/com/example/Application", and that object will implement the
6537 interface "com.example.Screensaver.Control".
6542 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6545 ".service files" tell the bus about service applications that can be
6546 launched (see <xref linkend="term-service"/>). Most importantly they
6547 provide a mapping from bus names to services that will request those
6548 names when they start up.
6553 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6556 The special name automatically assigned to each connection by the
6557 message bus. This name will never change owner, and will be unique
6558 (never reused during the lifetime of the message bus).
6559 It will begin with a ':' character.