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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
3 "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.25</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.25</revnumber>
75 <date>(not yet released)</date>
76 <authorinitials>n/a</authorinitials>
78 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
82 <revnumber>0.24</revnumber>
83 <date>2014-10-01</date>
84 <authorinitials>SMcV</authorinitials>
86 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
87 document how to quote match rules
91 <revnumber>0.23</revnumber>
92 <date>2014-01-06</date>
93 <authorinitials>SMcV, CY</authorinitials>
95 method call messages with no INTERFACE may be considered an error;
96 document tcp:bind=... and nonce-tcp:bind=...; define listenable
97 and connectable addresses
101 <revnumber>0.22</revnumber>
102 <date>2013-10-09</date>
103 <authorinitials></authorinitials>
104 <revremark>add GetConnectionCredentials, document
105 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
106 document and correct .service file syntax and naming
110 <revnumber>0.21</revnumber>
111 <date>2013-04-25</date>
112 <authorinitials>smcv</authorinitials>
113 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
114 Corrigendum #9)</revremark>
117 <revnumber>0.20</revnumber>
118 <date>22 February 2013</date>
119 <authorinitials>smcv, walters</authorinitials>
120 <revremark>reorganise for clarity, remove false claims about
121 basic types, mention /o/fd/DBus</revremark>
124 <revnumber>0.19</revnumber>
125 <date>20 February 2012</date>
126 <authorinitials>smcv/lp</authorinitials>
127 <revremark>formally define unique connection names and well-known
128 bus names; document best practices for interface, bus, member and
129 error names, and object paths; document the search path for session
130 and system services on Unix; document the systemd transport</revremark>
133 <revnumber>0.18</revnumber>
134 <date>29 July 2011</date>
135 <authorinitials>smcv</authorinitials>
136 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
137 match keyword; promote type system to a top-level section</revremark>
140 <revnumber>0.17</revnumber>
141 <date>1 June 2011</date>
142 <authorinitials>smcv/davidz</authorinitials>
143 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
144 by GVariant</revremark>
147 <revnumber>0.16</revnumber>
148 <date>11 April 2011</date>
149 <authorinitials></authorinitials>
150 <revremark>add path_namespace, arg0namespace; argNpath matches object
154 <revnumber>0.15</revnumber>
155 <date>3 November 2010</date>
156 <authorinitials></authorinitials>
157 <revremark></revremark>
160 <revnumber>0.14</revnumber>
161 <date>12 May 2010</date>
162 <authorinitials></authorinitials>
163 <revremark></revremark>
166 <revnumber>0.13</revnumber>
167 <date>23 Dezember 2009</date>
168 <authorinitials></authorinitials>
169 <revremark></revremark>
172 <revnumber>0.12</revnumber>
173 <date>7 November, 2006</date>
174 <authorinitials></authorinitials>
175 <revremark></revremark>
178 <revnumber>0.11</revnumber>
179 <date>6 February 2005</date>
180 <authorinitials></authorinitials>
181 <revremark></revremark>
184 <revnumber>0.10</revnumber>
185 <date>28 January 2005</date>
186 <authorinitials></authorinitials>
187 <revremark></revremark>
190 <revnumber>0.9</revnumber>
191 <date>7 Januar 2005</date>
192 <authorinitials></authorinitials>
193 <revremark></revremark>
196 <revnumber>0.8</revnumber>
197 <date>06 September 2003</date>
198 <authorinitials></authorinitials>
199 <revremark>First released document.</revremark>
204 <sect1 id="introduction">
205 <title>Introduction</title>
207 D-Bus is a system for low-overhead, easy to use
208 interprocess communication (IPC). In more detail:
212 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
213 binary protocol, and does not have to convert to and from a text
214 format such as XML. Because D-Bus is intended for potentially
215 high-resolution same-machine IPC, not primarily for Internet IPC,
216 this is an interesting optimization. D-Bus is also designed to
217 avoid round trips and allow asynchronous operation, much like
223 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
224 of <firstterm>messages</firstterm> rather than byte streams, and
225 automatically handles a lot of the hard IPC issues. Also, the D-Bus
226 library is designed to be wrapped in a way that lets developers use
227 their framework's existing object/type system, rather than learning
228 a new one specifically for IPC.
235 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
236 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
237 a system for one application to talk to a single other
238 application. However, the primary intended application of the protocol is the
239 D-Bus <firstterm>message bus</firstterm>, specified in <xref
240 linkend="message-bus"/>. The message bus is a special application that
241 accepts connections from multiple other applications, and forwards
246 Uses of D-Bus include notification of system changes (notification of when
247 a camera is plugged in to a computer, or a new version of some software
248 has been installed), or desktop interoperability, for example a file
249 monitoring service or a configuration service.
253 D-Bus is designed for two specific use cases:
257 A "system bus" for notifications from the system to user sessions,
258 and to allow the system to request input from user sessions.
263 A "session bus" used to implement desktop environments such as
268 D-Bus is not intended to be a generic IPC system for any possible
269 application, and intentionally omits many features found in other
270 IPC systems for this reason.
274 At the same time, the bus daemons offer a number of features not found in
275 other IPC systems, such as single-owner "bus names" (similar to X
276 selections), on-demand startup of services, and security policies.
277 In many ways, these features are the primary motivation for developing
278 D-Bus; other systems would have sufficed if IPC were the only goal.
282 D-Bus may turn out to be useful in unanticipated applications, but future
283 versions of this spec and the reference implementation probably will not
284 incorporate features that interfere with the core use cases.
288 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
289 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
290 document are to be interpreted as described in RFC 2119. However, the
291 document could use a serious audit to be sure it makes sense to do
292 so. Also, they are not capitalized.
295 <sect2 id="stability">
296 <title>Protocol and Specification Stability</title>
298 The D-Bus protocol is frozen (only compatible extensions are allowed) as
299 of November 8, 2006. However, this specification could still use a fair
300 bit of work to make interoperable reimplementation possible without
301 reference to the D-Bus reference implementation. Thus, this
302 specification is not marked 1.0. To mark it 1.0, we'd like to see
303 someone invest significant effort in clarifying the specification
304 language, and growing the specification to cover more aspects of the
305 reference implementation's behavior.
308 Until this work is complete, any attempt to reimplement D-Bus will
309 probably require looking at the reference implementation and/or asking
310 questions on the D-Bus mailing list about intended behavior.
311 Questions on the list are very welcome.
314 Nonetheless, this document should be a useful starting point and is
315 to our knowledge accurate, though incomplete.
321 <sect1 id="type-system">
322 <title>Type System</title>
325 D-Bus has a type system, in which values of various types can be
326 serialized into a sequence of bytes referred to as the
327 <firstterm>wire format</firstterm> in a standard way.
328 Converting a value from some other representation into the wire
329 format is called <firstterm>marshaling</firstterm> and converting
330 it back from the wire format is <firstterm>unmarshaling</firstterm>.
334 The D-Bus protocol does not include type tags in the marshaled data; a
335 block of marshaled values must have a known <firstterm>type
336 signature</firstterm>. The type signature is made up of zero or more
337 <firstterm id="term-single-complete-type">single complete
338 types</firstterm>, each made up of one or more
339 <firstterm>type codes</firstterm>.
343 A type code is an ASCII character representing the
344 type of a value. Because ASCII characters are used, the type signature
345 will always form a valid ASCII string. A simple string compare
346 determines whether two type signatures are equivalent.
350 A single complete type is a sequence of type codes that fully describes
351 one type: either a basic type, or a single fully-described container type.
352 A single complete type is a basic type code, a variant type code,
353 an array with its element type, or a struct with its fields (all of which
354 are defined below). So the following signatures are not single complete
365 And the following signatures contain multiple complete types:
375 Note however that a single complete type may <emphasis>contain</emphasis>
376 multiple other single complete types, by containing a struct or dict
380 <sect2 id="basic-types">
381 <title>Basic types</title>
384 The simplest type codes are the <firstterm id="term-basic-type">basic
385 types</firstterm>, which are the types whose structure is entirely
386 defined by their 1-character type code. Basic types consist of
387 fixed types and string-like types.
391 The <firstterm id="term-fixed-type">fixed types</firstterm>
392 are basic types whose values have a fixed length, namely BYTE,
393 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
398 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
399 the ASCII character 'i'. So the signature for a block of values
400 containing a single <literal>INT32</literal> would be:
404 A block of values containing two <literal>INT32</literal> would have this signature:
411 The characteristics of the fixed types are listed in this table.
417 <entry>Conventional name</entry>
418 <entry>ASCII type-code</entry>
419 <entry>Encoding</entry>
424 <entry><literal>BYTE</literal></entry>
425 <entry><literal>y</literal> (121)</entry>
426 <entry>Unsigned 8-bit integer</entry>
429 <entry><literal>BOOLEAN</literal></entry>
430 <entry><literal>b</literal> (98)</entry>
431 <entry>Boolean value: 0 is false, 1 is true, any other value
432 allowed by the marshalling format is invalid</entry>
435 <entry><literal>INT16</literal></entry>
436 <entry><literal>n</literal> (110)</entry>
437 <entry>Signed (two's complement) 16-bit integer</entry>
440 <entry><literal>UINT16</literal></entry>
441 <entry><literal>q</literal> (113)</entry>
442 <entry>Unsigned 16-bit integer</entry>
445 <entry><literal>INT32</literal></entry>
446 <entry><literal>i</literal> (105)</entry>
447 <entry>Signed (two's complement) 32-bit integer</entry>
450 <entry><literal>UINT32</literal></entry>
451 <entry><literal>u</literal> (117)</entry>
452 <entry>Unsigned 32-bit integer</entry>
455 <entry><literal>INT64</literal></entry>
456 <entry><literal>x</literal> (120)</entry>
457 <entry>Signed (two's complement) 64-bit integer
458 (mnemonic: x and t are the first characters in "sixty" not
459 already used for something more common)</entry>
462 <entry><literal>UINT64</literal></entry>
463 <entry><literal>t</literal> (116)</entry>
464 <entry>Unsigned 64-bit integer</entry>
467 <entry><literal>DOUBLE</literal></entry>
468 <entry><literal>d</literal> (100)</entry>
469 <entry>IEEE 754 double-precision floating point</entry>
472 <entry><literal>UNIX_FD</literal></entry>
473 <entry><literal>h</literal> (104)</entry>
474 <entry>Unsigned 32-bit integer representing an index into an
475 out-of-band array of file descriptors, transferred via some
476 platform-specific mechanism (mnemonic: h for handle)</entry>
484 The <firstterm id="term-string-like-type">string-like types</firstterm>
485 are basic types with a variable length. The value of any string-like
486 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
487 none of which may be U+0000. The UTF-8 text must be validated
488 strictly: in particular, it must not contain overlong sequences
489 or codepoints above U+10FFFF.
493 Since D-Bus Specification version 0.21, in accordance with Unicode
494 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
495 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
496 D-Bus rejected these noncharacters).
500 The marshalling formats for the string-like types all end with a
501 single zero (NUL) byte, but that byte is not considered to be part of
506 The characteristics of the string-like types are listed in this table.
512 <entry>Conventional name</entry>
513 <entry>ASCII type-code</entry>
514 <entry>Validity constraints</entry>
519 <entry><literal>STRING</literal></entry>
520 <entry><literal>s</literal> (115)</entry>
521 <entry>No extra constraints</entry>
524 <entry><literal>OBJECT_PATH</literal></entry>
525 <entry><literal>o</literal> (111)</entry>
527 <link linkend="message-protocol-marshaling-object-path">a
528 syntactically valid object path</link></entry>
531 <entry><literal>SIGNATURE</literal></entry>
532 <entry><literal>g</literal> (103)</entry>
534 <firstterm linkend="term-single-complete-type">single
535 complete types</firstterm></entry>
542 <sect3 id="message-protocol-marshaling-object-path">
543 <title>Valid Object Paths</title>
546 An object path is a name used to refer to an object instance.
547 Conceptually, each participant in a D-Bus message exchange may have
548 any number of object instances (think of C++ or Java objects) and each
549 such instance will have a path. Like a filesystem, the object
550 instances in an application form a hierarchical tree.
554 Object paths are often namespaced by starting with a reversed
555 domain name and containing an interface version number, in the
557 <link linkend="message-protocol-names-interface">interface
559 <link linkend="message-protocol-names-bus">well-known
561 This makes it possible to implement more than one service, or
562 more than one version of a service, in the same process,
563 even if the services share a connection but cannot otherwise
564 co-operate (for instance, if they are implemented by different
569 For instance, if the owner of <literal>example.com</literal> is
570 developing a D-Bus API for a music player, they might use the
571 hierarchy of object paths that start with
572 <literal>/com/example/MusicPlayer1</literal> for its objects.
576 The following rules define a valid object path. Implementations must
577 not send or accept messages with invalid object paths.
581 The path may be of any length.
586 The path must begin with an ASCII '/' (integer 47) character,
587 and must consist of elements separated by slash characters.
592 Each element must only contain the ASCII characters
598 No element may be the empty string.
603 Multiple '/' characters cannot occur in sequence.
608 A trailing '/' character is not allowed unless the
609 path is the root path (a single '/' character).
617 <sect3 id="message-protocol-marshaling-signature">
618 <title>Valid Signatures</title>
620 An implementation must not send or accept invalid signatures.
621 Valid signatures will conform to the following rules:
625 The signature is a list of single complete types.
626 Arrays must have element types, and structs must
627 have both open and close parentheses.
632 Only type codes, open and close parentheses, and open and
633 close curly brackets are allowed in the signature. The
634 <literal>STRUCT</literal> type code
635 is not allowed in signatures, because parentheses
636 are used instead. Similarly, the
637 <literal>DICT_ENTRY</literal> type code is not allowed in
638 signatures, because curly brackets are used instead.
643 The maximum depth of container type nesting is 32 array type
644 codes and 32 open parentheses. This implies that the maximum
645 total depth of recursion is 64, for an "array of array of array
646 of ... struct of struct of struct of ..." where there are 32
652 The maximum length of a signature is 255.
659 When signatures appear in messages, the marshalling format
660 guarantees that they will be followed by a nul byte (which can
661 be interpreted as either C-style string termination or the INVALID
662 type-code), but this is not conceptually part of the signature.
668 <sect2 id="container-types">
669 <title>Container types</title>
672 In addition to basic types, there are four <firstterm>container</firstterm>
673 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
674 and <literal>DICT_ENTRY</literal>.
678 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
679 code does not appear in signatures. Instead, ASCII characters
680 '(' and ')' are used to mark the beginning and end of the struct.
681 So for example, a struct containing two integers would have this
686 Structs can be nested, so for example a struct containing
687 an integer and another struct:
691 The value block storing that struct would contain three integers; the
692 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
697 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
698 but is useful in code that implements the protocol. This type code
699 is specified to allow such code to interoperate in non-protocol contexts.
703 Empty structures are not allowed; there must be at least one
704 type code between the parentheses.
708 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
709 followed by a <firstterm>single complete type</firstterm>. The single
710 complete type following the array is the type of each array element. So
711 the simple example is:
715 which is an array of 32-bit integers. But an array can be of any type,
716 such as this array-of-struct-with-two-int32-fields:
720 Or this array of array of integer:
727 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
728 type <literal>VARIANT</literal> will have the signature of a single complete type as part
729 of the <emphasis>value</emphasis>. This signature will be followed by a
730 marshaled value of that type.
734 Unlike a message signature, the variant signature can
735 contain only a single complete type. So "i", "ai"
736 or "(ii)" is OK, but "ii" is not. Use of variants may not
737 cause a total message depth to be larger than 64, including
738 other container types such as structures.
742 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
743 than parentheses it uses curly braces, and it has more restrictions.
744 The restrictions are: it occurs only as an array element type; it has
745 exactly two single complete types inside the curly braces; the first
746 single complete type (the "key") must be a basic type rather than a
747 container type. Implementations must not accept dict entries outside of
748 arrays, must not accept dict entries with zero, one, or more than two
749 fields, and must not accept dict entries with non-basic-typed keys. A
750 dict entry is always a key-value pair.
754 The first field in the <literal>DICT_ENTRY</literal> is always the key.
755 A message is considered corrupt if the same key occurs twice in the same
756 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
757 implementations are not required to reject dicts with duplicate keys.
761 In most languages, an array of dict entry would be represented as a
762 map, hash table, or dict object.
767 <title>Summary of types</title>
770 The following table summarizes the D-Bus types.
775 <entry>Category</entry>
776 <entry>Conventional Name</entry>
778 <entry>Description</entry>
783 <entry>reserved</entry>
784 <entry><literal>INVALID</literal></entry>
785 <entry>0 (ASCII NUL)</entry>
786 <entry>Not a valid type code, used to terminate signatures</entry>
788 <entry>fixed, basic</entry>
789 <entry><literal>BYTE</literal></entry>
790 <entry>121 (ASCII 'y')</entry>
791 <entry>8-bit unsigned integer</entry>
793 <entry>fixed, basic</entry>
794 <entry><literal>BOOLEAN</literal></entry>
795 <entry>98 (ASCII 'b')</entry>
796 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
798 <entry>fixed, basic</entry>
799 <entry><literal>INT16</literal></entry>
800 <entry>110 (ASCII 'n')</entry>
801 <entry>16-bit signed integer</entry>
803 <entry>fixed, basic</entry>
804 <entry><literal>UINT16</literal></entry>
805 <entry>113 (ASCII 'q')</entry>
806 <entry>16-bit unsigned integer</entry>
808 <entry>fixed, basic</entry>
809 <entry><literal>INT32</literal></entry>
810 <entry>105 (ASCII 'i')</entry>
811 <entry>32-bit signed integer</entry>
813 <entry>fixed, basic</entry>
814 <entry><literal>UINT32</literal></entry>
815 <entry>117 (ASCII 'u')</entry>
816 <entry>32-bit unsigned integer</entry>
818 <entry>fixed, basic</entry>
819 <entry><literal>INT64</literal></entry>
820 <entry>120 (ASCII 'x')</entry>
821 <entry>64-bit signed integer</entry>
823 <entry>fixed, basic</entry>
824 <entry><literal>UINT64</literal></entry>
825 <entry>116 (ASCII 't')</entry>
826 <entry>64-bit unsigned integer</entry>
828 <entry>fixed, basic</entry>
829 <entry><literal>DOUBLE</literal></entry>
830 <entry>100 (ASCII 'd')</entry>
831 <entry>IEEE 754 double</entry>
833 <entry>string-like, basic</entry>
834 <entry><literal>STRING</literal></entry>
835 <entry>115 (ASCII 's')</entry>
836 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
838 <entry>string-like, basic</entry>
839 <entry><literal>OBJECT_PATH</literal></entry>
840 <entry>111 (ASCII 'o')</entry>
841 <entry>Name of an object instance</entry>
843 <entry>string-like, basic</entry>
844 <entry><literal>SIGNATURE</literal></entry>
845 <entry>103 (ASCII 'g')</entry>
846 <entry>A type signature</entry>
848 <entry>container</entry>
849 <entry><literal>ARRAY</literal></entry>
850 <entry>97 (ASCII 'a')</entry>
853 <entry>container</entry>
854 <entry><literal>STRUCT</literal></entry>
855 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
856 <entry>Struct; type code 114 'r' is reserved for use in
857 bindings and implementations to represent the general
858 concept of a struct, and must not appear in signatures
859 used on D-Bus.</entry>
861 <entry>container</entry>
862 <entry><literal>VARIANT</literal></entry>
863 <entry>118 (ASCII 'v') </entry>
864 <entry>Variant type (the type of the value is part of the value itself)</entry>
866 <entry>container</entry>
867 <entry><literal>DICT_ENTRY</literal></entry>
868 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
869 <entry>Entry in a dict or map (array of key-value pairs).
870 Type code 101 'e' is reserved for use in bindings and
871 implementations to represent the general concept of a
872 dict or dict-entry, and must not appear in signatures
873 used on D-Bus.</entry>
875 <entry>fixed, basic</entry>
876 <entry><literal>UNIX_FD</literal></entry>
877 <entry>104 (ASCII 'h')</entry>
878 <entry>Unix file descriptor</entry>
881 <entry>reserved</entry>
882 <entry>(reserved)</entry>
883 <entry>109 (ASCII 'm')</entry>
884 <entry>Reserved for <ulink
885 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
886 'maybe' type compatible with the one in GVariant</ulink>,
887 and must not appear in signatures used on D-Bus until
888 specified here</entry>
891 <entry>reserved</entry>
892 <entry>(reserved)</entry>
893 <entry>42 (ASCII '*')</entry>
894 <entry>Reserved for use in bindings/implementations to
895 represent any <firstterm>single complete type</firstterm>,
896 and must not appear in signatures used on D-Bus.</entry>
899 <entry>reserved</entry>
900 <entry>(reserved)</entry>
901 <entry>63 (ASCII '?')</entry>
902 <entry>Reserved for use in bindings/implementations to
903 represent any <firstterm>basic type</firstterm>, and must
904 not appear in signatures used on D-Bus.</entry>
907 <entry>reserved</entry>
908 <entry>(reserved)</entry>
909 <entry>64 (ASCII '@'), 38 (ASCII '&'),
910 94 (ASCII '^')</entry>
911 <entry>Reserved for internal use by bindings/implementations,
912 and must not appear in signatures used on D-Bus.
913 GVariant uses these type-codes to encode calling
924 <sect1 id="message-protocol-marshaling">
925 <title>Marshaling (Wire Format)</title>
928 D-Bus defines a marshalling format for its type system, which is
929 used in D-Bus messages. This is not the only possible marshalling
930 format for the type system: for instance, GVariant (part of GLib)
931 re-uses the D-Bus type system but implements an alternative marshalling
936 <title>Byte order and alignment</title>
939 Given a type signature, a block of bytes can be converted into typed
940 values. This section describes the format of the block of bytes. Byte
941 order and alignment issues are handled uniformly for all D-Bus types.
945 A block of bytes has an associated byte order. The byte order
946 has to be discovered in some way; for D-Bus messages, the
947 byte order is part of the message header as described in
948 <xref linkend="message-protocol-messages"/>. For now, assume
949 that the byte order is known to be either little endian or big
954 Each value in a block of bytes is aligned "naturally," for example
955 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
956 8-byte boundary. To properly align a value, <firstterm>alignment
957 padding</firstterm> may be necessary. The alignment padding must always
958 be the minimum required padding to properly align the following value;
959 and it must always be made up of nul bytes. The alignment padding must
960 not be left uninitialized (it can't contain garbage), and more padding
961 than required must not be used.
965 As an exception to natural alignment, <literal>STRUCT</literal> and
966 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
967 boundary, regardless of the alignments of their contents.
972 <title>Marshalling basic types</title>
975 To marshal and unmarshal fixed types, you simply read one value
976 from the data block corresponding to each type code in the signature.
977 All signed integer values are encoded in two's complement, DOUBLE
978 values are IEEE 754 double-precision floating-point, and BOOLEAN
979 values are encoded in 32 bits (of which only the least significant
984 The string-like types are all marshalled as a
985 fixed-length unsigned integer <varname>n</varname> giving the
986 length of the variable part, followed by <varname>n</varname>
987 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
988 which is not considered to be part of the text. The alignment
989 of the string-like type is the same as the alignment of
990 <varname>n</varname>.
994 For the STRING and OBJECT_PATH types, <varname>n</varname> is
995 encoded in 4 bytes, leading to 4-byte alignment.
996 For the SIGNATURE type, <varname>n</varname> is encoded as a single
997 byte. As a result, alignment padding is never required before a
1003 <title>Marshalling containers</title>
1006 Arrays are marshalled as a <literal>UINT32</literal>
1007 <varname>n</varname> giving the length of the array data in bytes,
1008 followed by alignment padding to the alignment boundary of the array
1009 element type, followed by the <varname>n</varname> bytes of the
1010 array elements marshalled in sequence. <varname>n</varname> does not
1011 include the padding after the length, or any padding after the
1016 For instance, if the current position in the message is a multiple
1017 of 8 bytes and the byte-order is big-endian, an array containing only
1018 the 64-bit integer 5 would be marshalled as:
1021 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
1022 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1023 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1028 Arrays have a maximum length defined to be 2 to the 26th power or
1029 67108864 (64 MiB). Implementations must not send or accept arrays
1030 exceeding this length.
1034 Structs and dict entries are marshalled in the same way as their
1035 contents, but their alignment is always to an 8-byte boundary,
1036 even if their contents would normally be less strictly aligned.
1040 Variants are marshalled as the <literal>SIGNATURE</literal> of
1041 the contents (which must be a single complete type), followed by a
1042 marshalled value with the type given by that signature. The
1043 variant has the same 1-byte alignment as the signature, which means
1044 that alignment padding before a variant is never needed.
1045 Use of variants may not cause a total message depth to be larger
1046 than 64, including other container types such as structures.
1051 <title>Summary of D-Bus marshalling</title>
1054 Given all this, the types are marshaled on the wire as follows:
1059 <entry>Conventional Name</entry>
1060 <entry>Encoding</entry>
1061 <entry>Alignment</entry>
1066 <entry><literal>INVALID</literal></entry>
1067 <entry>Not applicable; cannot be marshaled.</entry>
1070 <entry><literal>BYTE</literal></entry>
1071 <entry>A single 8-bit byte.</entry>
1074 <entry><literal>BOOLEAN</literal></entry>
1075 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1078 <entry><literal>INT16</literal></entry>
1079 <entry>16-bit signed integer in the message's byte order.</entry>
1082 <entry><literal>UINT16</literal></entry>
1083 <entry>16-bit unsigned integer in the message's byte order.</entry>
1086 <entry><literal>INT32</literal></entry>
1087 <entry>32-bit signed integer in the message's byte order.</entry>
1090 <entry><literal>UINT32</literal></entry>
1091 <entry>32-bit unsigned integer in the message's byte order.</entry>
1094 <entry><literal>INT64</literal></entry>
1095 <entry>64-bit signed integer in the message's byte order.</entry>
1098 <entry><literal>UINT64</literal></entry>
1099 <entry>64-bit unsigned integer in the message's byte order.</entry>
1102 <entry><literal>DOUBLE</literal></entry>
1103 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1106 <entry><literal>STRING</literal></entry>
1107 <entry>A <literal>UINT32</literal> indicating the string's
1108 length in bytes excluding its terminating nul, followed by
1109 non-nul string data of the given length, followed by a terminating nul
1116 <entry><literal>OBJECT_PATH</literal></entry>
1117 <entry>Exactly the same as <literal>STRING</literal> except the
1118 content must be a valid object path (see above).
1124 <entry><literal>SIGNATURE</literal></entry>
1125 <entry>The same as <literal>STRING</literal> except the length is a single
1126 byte (thus signatures have a maximum length of 255)
1127 and the content must be a valid signature (see above).
1133 <entry><literal>ARRAY</literal></entry>
1135 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1136 alignment padding to the alignment boundary of the array element type,
1137 followed by each array element.
1143 <entry><literal>STRUCT</literal></entry>
1145 A struct must start on an 8-byte boundary regardless of the
1146 type of the struct fields. The struct value consists of each
1147 field marshaled in sequence starting from that 8-byte
1154 <entry><literal>VARIANT</literal></entry>
1156 The marshaled <literal>SIGNATURE</literal> of a single
1157 complete type, followed by a marshaled value with the type
1158 given in the signature.
1161 1 (alignment of the signature)
1164 <entry><literal>DICT_ENTRY</literal></entry>
1166 Identical to STRUCT.
1172 <entry><literal>UNIX_FD</literal></entry>
1173 <entry>32-bit unsigned integer in the message's byte
1174 order. The actual file descriptors need to be
1175 transferred out-of-band via some platform specific
1176 mechanism. On the wire, values of this type store the index to the
1177 file descriptor in the array of file descriptors that
1178 accompany the message.</entry>
1190 <sect1 id="message-protocol">
1191 <title>Message Protocol</title>
1194 A <firstterm>message</firstterm> consists of a
1195 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1196 think of a message as a package, the header is the address, and the body
1197 contains the package contents. The message delivery system uses the header
1198 information to figure out where to send the message and how to interpret
1199 it; the recipient interprets the body of the message.
1203 The body of the message is made up of zero or more
1204 <firstterm>arguments</firstterm>, which are typed values, such as an
1205 integer or a byte array.
1209 Both header and body use the D-Bus <link linkend="type-system">type
1210 system</link> and format for serializing data.
1213 <sect2 id="message-protocol-messages">
1214 <title>Message Format</title>
1217 A message consists of a header and a body. The header is a block of
1218 values with a fixed signature and meaning. The body is a separate block
1219 of values, with a signature specified in the header.
1223 The length of the header must be a multiple of 8, allowing the body to
1224 begin on an 8-byte boundary when storing the entire message in a single
1225 buffer. If the header does not naturally end on an 8-byte boundary
1226 up to 7 bytes of nul-initialized alignment padding must be added.
1230 The message body need not end on an 8-byte boundary.
1234 The maximum length of a message, including header, header alignment padding,
1235 and body is 2 to the 27th power or 134217728 (128 MiB).
1236 Implementations must not send or accept messages exceeding this size.
1240 The signature of the header is:
1244 Written out more readably, this is:
1246 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1251 These values have the following meanings:
1256 <entry>Value</entry>
1257 <entry>Description</entry>
1262 <entry>1st <literal>BYTE</literal></entry>
1263 <entry>Endianness flag; ASCII 'l' for little-endian
1264 or ASCII 'B' for big-endian. Both header and body are
1265 in this endianness.</entry>
1268 <entry>2nd <literal>BYTE</literal></entry>
1269 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1270 Currently-defined types are described below.
1274 <entry>3rd <literal>BYTE</literal></entry>
1275 <entry>Bitwise OR of flags. Unknown flags
1276 must be ignored. Currently-defined flags are described below.
1280 <entry>4th <literal>BYTE</literal></entry>
1281 <entry>Major protocol version of the sending application. If
1282 the major protocol version of the receiving application does not
1283 match, the applications will not be able to communicate and the
1284 D-Bus connection must be disconnected. The major protocol
1285 version for this version of the specification is 1.
1289 <entry>1st <literal>UINT32</literal></entry>
1290 <entry>Length in bytes of the message body, starting
1291 from the end of the header. The header ends after
1292 its alignment padding to an 8-boundary.
1296 <entry>2nd <literal>UINT32</literal></entry>
1297 <entry>The serial of this message, used as a cookie
1298 by the sender to identify the reply corresponding
1299 to this request. This must not be zero.
1303 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1304 <entry>An array of zero or more <firstterm>header
1305 fields</firstterm> where the byte is the field code, and the
1306 variant is the field value. The message type determines
1307 which fields are required.
1315 <firstterm>Message types</firstterm> that can appear in the second byte
1321 <entry>Conventional name</entry>
1322 <entry>Decimal value</entry>
1323 <entry>Description</entry>
1328 <entry><literal>INVALID</literal></entry>
1330 <entry>This is an invalid type.</entry>
1333 <entry><literal>METHOD_CALL</literal></entry>
1335 <entry>Method call. This message type may prompt a
1339 <entry><literal>METHOD_RETURN</literal></entry>
1341 <entry>Method reply with returned data.</entry>
1344 <entry><literal>ERROR</literal></entry>
1346 <entry>Error reply. If the first argument exists and is a
1347 string, it is an error message.</entry>
1350 <entry><literal>SIGNAL</literal></entry>
1352 <entry>Signal emission.</entry>
1359 Flags that can appear in the third byte of the header:
1364 <entry>Conventional name</entry>
1365 <entry>Hex value</entry>
1366 <entry>Description</entry>
1371 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1375 This message does not expect method return replies or
1376 error replies, even if it is of a type that can
1377 have a reply; the reply can be omitted as an
1378 optimization. It is compliant with this specification
1379 to return the reply despite this flag, although doing
1380 so on a bus with a non-trivial security policy
1381 (such as the well-known system bus) may result in
1382 access denial messages being logged for the reply.
1385 Note that METHOD_CALL is the only message type currently
1386 defined in this specification that can expect a reply,
1387 so the presence or absence of this flag in the other
1388 three message types that are currently
1389 documented is meaningless: replies to those message
1390 types should not be sent, whether this flag is present
1396 <entry><literal>NO_AUTO_START</literal></entry>
1398 <entry>The bus must not launch an owner
1399 for the destination name in response to this message.
1403 <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
1407 This flag may be set on a method call message to
1408 inform the receiving side that the caller is prepared
1409 to wait for interactive authorization, which might
1410 take a considerable time to complete. For instance,
1411 if this flag is set, it would be appropriate to
1412 query the user for passwords or confirmation via
1413 Polkit or a similar framework.
1416 This flag is only useful when
1417 unprivileged code calls a more privileged method call,
1418 and an authorization framework is deployed that allows
1419 possibly interactive authorization. If no such framework
1420 is deployed it has no effect. This flag should not
1421 be set by default by client implementations. If it is
1422 set, the caller should also set a suitably long timeout
1423 on the method call to make sure the user interaction
1424 may complete. This flag is only valid for method call
1425 messages, and shall be ignored otherwise.
1428 Interaction that takes place as a part of the
1429 effect of the method being called is outside the scope
1430 of this flag, even if it could also be characterized
1431 as authentication or authorization. For instance, in
1432 a method call that directs a network management service
1433 to attempt to connect to a virtual private network,
1434 this flag should control how the network management
1435 service makes the decision "is this user allowed to
1436 change system network configuration?", but it should
1437 not affect how or whether the network management
1438 service interacts with the user to obtain the credentials
1439 that are required for access to the VPN.
1442 If a this flag is not set on a method call, and a
1443 service determines that the requested operation is
1444 not allowed without interactive authorization, but
1445 could be allowed after successful interactive
1446 authorization, it may return the
1447 <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
1451 The absence of this flag does not guarantee that
1452 interactive authorization will not be applied, since
1453 existing services that pre-date this flag might
1454 already use interactive authorization. However,
1455 existing D-Bus APIs that will use interactive
1456 authorization should document that the call may take
1457 longer than usual, and new D-Bus APIs should avoid
1458 interactive authorization in the absence of this flag.
1467 <sect3 id="message-protocol-header-fields">
1468 <title>Header Fields</title>
1471 The array at the end of the header contains <firstterm>header
1472 fields</firstterm>, where each field is a 1-byte field code followed
1473 by a field value. A header must contain the required header fields for
1474 its message type, and zero or more of any optional header
1475 fields. Future versions of this protocol specification may add new
1476 fields. Implementations must ignore fields they do not
1477 understand. Implementations must not invent their own header fields;
1478 only changes to this specification may introduce new header fields.
1482 Again, if an implementation sees a header field code that it does not
1483 expect, it must ignore that field, as it will be part of a new
1484 (but compatible) version of this specification. This also applies
1485 to known header fields appearing in unexpected messages, for
1486 example: if a signal has a reply serial it must be ignored
1487 even though it has no meaning as of this version of the spec.
1491 However, implementations must not send or accept known header fields
1492 with the wrong type stored in the field value. So for example a
1493 message with an <literal>INTERFACE</literal> field of type
1494 <literal>UINT32</literal> would be considered corrupt.
1498 Here are the currently-defined header fields:
1503 <entry>Conventional Name</entry>
1504 <entry>Decimal Code</entry>
1506 <entry>Required In</entry>
1507 <entry>Description</entry>
1512 <entry><literal>INVALID</literal></entry>
1515 <entry>not allowed</entry>
1516 <entry>Not a valid field name (error if it appears in a message)</entry>
1519 <entry><literal>PATH</literal></entry>
1521 <entry><literal>OBJECT_PATH</literal></entry>
1522 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1523 <entry>The object to send a call to,
1524 or the object a signal is emitted from.
1526 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1527 implementations should not send messages with this path,
1528 and the reference implementation of the bus daemon will
1529 disconnect any application that attempts to do so.
1533 <entry><literal>INTERFACE</literal></entry>
1535 <entry><literal>STRING</literal></entry>
1536 <entry><literal>SIGNAL</literal></entry>
1538 The interface to invoke a method call on, or
1539 that a signal is emitted from. Optional for
1540 method calls, required for signals.
1541 The special interface
1542 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1543 implementations should not send messages with this
1544 interface, and the reference implementation of the bus
1545 daemon will disconnect any application that attempts to
1550 <entry><literal>MEMBER</literal></entry>
1552 <entry><literal>STRING</literal></entry>
1553 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1554 <entry>The member, either the method name or signal name.</entry>
1557 <entry><literal>ERROR_NAME</literal></entry>
1559 <entry><literal>STRING</literal></entry>
1560 <entry><literal>ERROR</literal></entry>
1561 <entry>The name of the error that occurred, for errors</entry>
1564 <entry><literal>REPLY_SERIAL</literal></entry>
1566 <entry><literal>UINT32</literal></entry>
1567 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1568 <entry>The serial number of the message this message is a reply
1569 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1572 <entry><literal>DESTINATION</literal></entry>
1574 <entry><literal>STRING</literal></entry>
1575 <entry>optional</entry>
1576 <entry>The name of the connection this message is intended for.
1577 Only used in combination with the message bus, see
1578 <xref linkend="message-bus"/>.</entry>
1581 <entry><literal>SENDER</literal></entry>
1583 <entry><literal>STRING</literal></entry>
1584 <entry>optional</entry>
1585 <entry>Unique name of the sending connection.
1586 The message bus fills in this field so it is reliable; the field is
1587 only meaningful in combination with the message bus.</entry>
1590 <entry><literal>SIGNATURE</literal></entry>
1592 <entry><literal>SIGNATURE</literal></entry>
1593 <entry>optional</entry>
1594 <entry>The signature of the message body.
1595 If omitted, it is assumed to be the
1596 empty signature "" (i.e. the body must be 0-length).</entry>
1599 <entry><literal>UNIX_FDS</literal></entry>
1601 <entry><literal>UINT32</literal></entry>
1602 <entry>optional</entry>
1603 <entry>The number of Unix file descriptors that
1604 accompany the message. If omitted, it is assumed
1605 that no Unix file descriptors accompany the
1606 message. The actual file descriptors need to be
1607 transferred via platform specific mechanism
1608 out-of-band. They must be sent at the same time as
1609 part of the message itself. They may not be sent
1610 before the first byte of the message itself is
1611 transferred or after the last byte of the message
1621 <sect2 id="message-protocol-names">
1622 <title>Valid Names</title>
1624 The various names in D-Bus messages have some restrictions.
1627 There is a <firstterm>maximum name length</firstterm>
1628 of 255 which applies to bus names, interfaces, and members.
1630 <sect3 id="message-protocol-names-interface">
1631 <title>Interface names</title>
1633 Interfaces have names with type <literal>STRING</literal>, meaning that
1634 they must be valid UTF-8. However, there are also some
1635 additional restrictions that apply to interface names
1638 <listitem><para>Interface names are composed of 1 or more elements separated by
1639 a period ('.') character. All elements must contain at least
1643 <listitem><para>Each element must only contain the ASCII characters
1644 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1648 <listitem><para>Interface names must contain at least one '.' (period)
1649 character (and thus at least two elements).
1652 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1653 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1658 Interface names should start with the reversed DNS domain name of
1659 the author of the interface (in lower-case), like interface names
1660 in Java. It is conventional for the rest of the interface name
1661 to consist of words run together, with initial capital letters
1662 on all words ("CamelCase"). Several levels of hierarchy can be used.
1663 It is also a good idea to include the major version of the interface
1664 in the name, and increment it if incompatible changes are made;
1665 this way, a single object can implement several versions of an
1666 interface in parallel, if necessary.
1670 For instance, if the owner of <literal>example.com</literal> is
1671 developing a D-Bus API for a music player, they might define
1672 interfaces called <literal>com.example.MusicPlayer1</literal>,
1673 <literal>com.example.MusicPlayer1.Track</literal> and
1674 <literal>com.example.MusicPlayer1.Seekable</literal>.
1678 D-Bus does not distinguish between the concepts that would be
1679 called classes and interfaces in Java: either can be identified on
1680 D-Bus by an interface name.
1683 <sect3 id="message-protocol-names-bus">
1684 <title>Bus names</title>
1686 Connections have one or more bus names associated with them.
1687 A connection has exactly one bus name that is a <firstterm>unique
1688 connection name</firstterm>. The unique connection name remains
1689 with the connection for its entire lifetime.
1690 A bus name is of type <literal>STRING</literal>,
1691 meaning that it must be valid UTF-8. However, there are also
1692 some additional restrictions that apply to bus names
1695 <listitem><para>Bus names that start with a colon (':')
1696 character are unique connection names. Other bus names
1697 are called <firstterm>well-known bus names</firstterm>.
1700 <listitem><para>Bus names are composed of 1 or more elements separated by
1701 a period ('.') character. All elements must contain at least
1705 <listitem><para>Each element must only contain the ASCII characters
1706 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1707 connection name may begin with a digit, elements in
1708 other bus names must not begin with a digit.
1712 <listitem><para>Bus names must contain at least one '.' (period)
1713 character (and thus at least two elements).
1716 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1717 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1721 Note that the hyphen ('-') character is allowed in bus names but
1722 not in interface names.
1726 Like <link linkend="message-protocol-names-interface">interface
1727 names</link>, well-known bus names should start with the
1728 reversed DNS domain name of the author of the interface (in
1729 lower-case), and it is conventional for the rest of the well-known
1730 bus name to consist of words run together, with initial
1731 capital letters. As with interface names, including a version
1732 number in well-known bus names is a good idea; it's possible to
1733 have the well-known bus name for more than one version
1734 simultaneously if backwards compatibility is required.
1738 If a well-known bus name implies the presence of a "main" interface,
1739 that "main" interface is often given the same name as
1740 the well-known bus name, and situated at the corresponding object
1741 path. For instance, if the owner of <literal>example.com</literal>
1742 is developing a D-Bus API for a music player, they might define
1743 that any application that takes the well-known name
1744 <literal>com.example.MusicPlayer1</literal> should have an object
1745 at the object path <literal>/com/example/MusicPlayer1</literal>
1746 which implements the interface
1747 <literal>com.example.MusicPlayer1</literal>.
1750 <sect3 id="message-protocol-names-member">
1751 <title>Member names</title>
1753 Member (i.e. method or signal) names:
1755 <listitem><para>Must only contain the ASCII characters
1756 "[A-Z][a-z][0-9]_" and may not begin with a
1757 digit.</para></listitem>
1758 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1759 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1760 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1765 It is conventional for member names on D-Bus to consist of
1766 capitalized words with no punctuation ("camel-case").
1767 Method names should usually be verbs, such as
1768 <literal>GetItems</literal>, and signal names should usually be
1769 a description of an event, such as <literal>ItemsChanged</literal>.
1772 <sect3 id="message-protocol-names-error">
1773 <title>Error names</title>
1775 Error names have the same restrictions as interface names.
1779 Error names have the same naming conventions as interface
1780 names, and often contain <literal>.Error.</literal>; for instance,
1781 the owner of <literal>example.com</literal> might define the
1782 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1783 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1784 The errors defined by D-Bus itself, such as
1785 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1791 <sect2 id="message-protocol-types">
1792 <title>Message Types</title>
1794 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1795 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1796 This section describes these conventions.
1798 <sect3 id="message-protocol-types-method">
1799 <title>Method Calls</title>
1801 Some messages invoke an operation on a remote object. These are
1802 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1803 messages map naturally to methods on objects in a typical program.
1806 A method call message is required to have a <literal>MEMBER</literal> header field
1807 indicating the name of the method. Optionally, the message has an
1808 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1809 Including the <literal>INTERFACE</literal> in all method call
1810 messages is strongly recommended.
1813 In the absence of an <literal>INTERFACE</literal> field, if two
1814 or more interfaces on the same object have a method with the same
1815 name, it is undefined which of those methods will be invoked.
1816 Implementations may choose to either return an error, or deliver the
1817 message as though it had an arbitrary one of those interfaces.
1820 In some situations (such as the well-known system bus), messages
1821 are filtered through an access-control list external to the
1822 remote object implementation. If that filter rejects certain
1823 messages by matching their interface, or accepts only messages
1824 to specific interfaces, it must also reject messages that have no
1825 <literal>INTERFACE</literal>: otherwise, malicious
1826 applications could use this to bypass the filter.
1829 Method call messages also include a <literal>PATH</literal> field
1830 indicating the object to invoke the method on. If the call is passing
1831 through a message bus, the message will also have a
1832 <literal>DESTINATION</literal> field giving the name of the connection
1833 to receive the message.
1836 When an application handles a method call message, it is required to
1837 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1838 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1839 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1842 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1843 are the return value(s) or "out parameters" of the method call.
1844 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1845 and the call fails; no return value will be provided. It makes
1846 no sense to send multiple replies to the same method call.
1849 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1850 reply is required, so the caller will know the method
1851 was successfully processed.
1854 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1858 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1859 then as an optimization the application receiving the method
1860 call may choose to omit the reply message (regardless of
1861 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1862 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1863 flag and reply anyway.
1866 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1867 destination name does not exist then a program to own the destination
1868 name will be started before the message is delivered. The message
1869 will be held until the new program is successfully started or has
1870 failed to start; in case of failure, an error will be returned. This
1871 flag is only relevant in the context of a message bus, it is ignored
1872 during one-to-one communication with no intermediate bus.
1874 <sect4 id="message-protocol-types-method-apis">
1875 <title>Mapping method calls to native APIs</title>
1877 APIs for D-Bus may map method calls to a method call in a specific
1878 programming language, such as C++, or may map a method call written
1879 in an IDL to a D-Bus message.
1882 In APIs of this nature, arguments to a method are often termed "in"
1883 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1884 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1885 "inout" arguments, which are both sent and received, i.e. the caller
1886 passes in a value which is modified. Mapped to D-Bus, an "inout"
1887 argument is equivalent to an "in" argument, followed by an "out"
1888 argument. You can't pass things "by reference" over the wire, so
1889 "inout" is purely an illusion of the in-process API.
1892 Given a method with zero or one return values, followed by zero or more
1893 arguments, where each argument may be "in", "out", or "inout", the
1894 caller constructs a message by appending each "in" or "inout" argument,
1895 in order. "out" arguments are not represented in the caller's message.
1898 The recipient constructs a reply by appending first the return value
1899 if any, then each "out" or "inout" argument, in order.
1900 "in" arguments are not represented in the reply message.
1903 Error replies are normally mapped to exceptions in languages that have
1907 In converting from native APIs to D-Bus, it is perhaps nice to
1908 map D-Bus naming conventions ("FooBar") to native conventions
1909 such as "fooBar" or "foo_bar" automatically. This is OK
1910 as long as you can say that the native API is one that
1911 was specifically written for D-Bus. It makes the most sense
1912 when writing object implementations that will be exported
1913 over the bus. Object proxies used to invoke remote D-Bus
1914 objects probably need the ability to call any D-Bus method,
1915 and thus a magic name mapping like this could be a problem.
1918 This specification doesn't require anything of native API bindings;
1919 the preceding is only a suggested convention for consistency
1925 <sect3 id="message-protocol-types-signal">
1926 <title>Signal Emission</title>
1928 Unlike method calls, signal emissions have no replies.
1929 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1930 It must have three header fields: <literal>PATH</literal> giving the object
1931 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1932 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1933 for signals, though it is optional for method calls.
1937 <sect3 id="message-protocol-types-errors">
1938 <title>Errors</title>
1940 Messages of type <literal>ERROR</literal> are most commonly replies
1941 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1942 to any kind of message. The message bus for example
1943 will return an <literal>ERROR</literal> in reply to a signal emission if
1944 the bus does not have enough memory to send the signal.
1947 An <literal>ERROR</literal> may have any arguments, but if the first
1948 argument is a <literal>STRING</literal>, it must be an error message.
1949 The error message may be logged or shown to the user
1954 <sect3 id="message-protocol-types-notation">
1955 <title>Notation in this document</title>
1957 This document uses a simple pseudo-IDL to describe particular method
1958 calls and signals. Here is an example of a method call:
1960 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1961 out UINT32 resultcode)
1963 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1964 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1965 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1966 characters so it's known that the last part of the name in
1967 the "IDL" is the member name.
1970 In C++ that might end up looking like this:
1972 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1973 unsigned int flags);
1975 or equally valid, the return value could be done as an argument:
1977 void org::freedesktop::DBus::StartServiceByName (const char *name,
1979 unsigned int *resultcode);
1981 It's really up to the API designer how they want to make
1982 this look. You could design an API where the namespace wasn't used
1983 in C++, using STL or Qt, using varargs, or whatever you wanted.
1986 Signals are written as follows:
1988 org.freedesktop.DBus.NameLost (STRING name)
1990 Signals don't specify "in" vs. "out" because only
1991 a single direction is possible.
1994 It isn't especially encouraged to use this lame pseudo-IDL in actual
1995 API implementations; you might use the native notation for the
1996 language you're using, or you might use COM or CORBA IDL, for example.
2001 <sect2 id="message-protocol-handling-invalid">
2002 <title>Invalid Protocol and Spec Extensions</title>
2005 For security reasons, the D-Bus protocol should be strictly parsed and
2006 validated, with the exception of defined extension points. Any invalid
2007 protocol or spec violations should result in immediately dropping the
2008 connection without notice to the other end. Exceptions should be
2009 carefully considered, e.g. an exception may be warranted for a
2010 well-understood idiosyncrasy of a widely-deployed implementation. In
2011 cases where the other end of a connection is 100% trusted and known to
2012 be friendly, skipping validation for performance reasons could also make
2013 sense in certain cases.
2017 Generally speaking violations of the "must" requirements in this spec
2018 should be considered possible attempts to exploit security, and violations
2019 of the "should" suggestions should be considered legitimate (though perhaps
2020 they should generate an error in some cases).
2024 The following extension points are built in to D-Bus on purpose and must
2025 not be treated as invalid protocol. The extension points are intended
2026 for use by future versions of this spec, they are not intended for third
2027 parties. At the moment, the only way a third party could extend D-Bus
2028 without breaking interoperability would be to introduce a way to negotiate new
2029 feature support as part of the auth protocol, using EXTENSION_-prefixed
2030 commands. There is not yet a standard way to negotiate features.
2034 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
2035 commands result in an ERROR rather than a disconnect. This enables
2036 future extensions to the protocol. Commands starting with EXTENSION_ are
2037 reserved for third parties.
2042 The authentication protocol supports pluggable auth mechanisms.
2047 The address format (see <xref linkend="addresses"/>) supports new
2053 Messages with an unknown type (something other than
2054 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
2055 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
2056 Unknown-type messages must still be well-formed in the same way
2057 as the known messages, however. They still have the normal
2063 Header fields with an unknown or unexpected field code must be ignored,
2064 though again they must still be well-formed.
2069 New standard interfaces (with new methods and signals) can of course be added.
2079 <sect1 id="auth-protocol">
2080 <title>Authentication Protocol</title>
2082 Before the flow of messages begins, two applications must
2083 authenticate. A simple plain-text protocol is used for
2084 authentication; this protocol is a SASL profile, and maps fairly
2085 directly from the SASL specification. The message encoding is
2086 NOT used here, only plain text messages.
2089 In examples, "C:" and "S:" indicate lines sent by the client and
2090 server respectively.
2092 <sect2 id="auth-protocol-overview">
2093 <title>Protocol Overview</title>
2095 The protocol is a line-based protocol, where each line ends with
2096 \r\n. Each line begins with an all-caps ASCII command name containing
2097 only the character range [A-Z_], a space, then any arguments for the
2098 command, then the \r\n ending the line. The protocol is
2099 case-sensitive. All bytes must be in the ASCII character set.
2101 Commands from the client to the server are as follows:
2104 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2105 <listitem><para>CANCEL</para></listitem>
2106 <listitem><para>BEGIN</para></listitem>
2107 <listitem><para>DATA <data in hex encoding></para></listitem>
2108 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2109 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2112 From server to client are as follows:
2115 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2116 <listitem><para>OK <GUID in hex></para></listitem>
2117 <listitem><para>DATA <data in hex encoding></para></listitem>
2118 <listitem><para>ERROR</para></listitem>
2119 <listitem><para>AGREE_UNIX_FD</para></listitem>
2123 Unofficial extensions to the command set must begin with the letters
2124 "EXTENSION_", to avoid conflicts with future official commands.
2125 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2128 <sect2 id="auth-nul-byte">
2129 <title>Special credentials-passing nul byte</title>
2131 Immediately after connecting to the server, the client must send a
2132 single nul byte. This byte may be accompanied by credentials
2133 information on some operating systems that use sendmsg() with
2134 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2135 sockets. However, the nul byte must be sent even on other kinds of
2136 socket, and even on operating systems that do not require a byte to be
2137 sent in order to transmit credentials. The text protocol described in
2138 this document begins after the single nul byte. If the first byte
2139 received from the client is not a nul byte, the server may disconnect
2143 A nul byte in any context other than the initial byte is an error;
2144 the protocol is ASCII-only.
2147 The credentials sent along with the nul byte may be used with the
2148 SASL mechanism EXTERNAL.
2151 <sect2 id="auth-command-auth">
2152 <title>AUTH command</title>
2154 If an AUTH command has no arguments, it is a request to list
2155 available mechanisms. The server must respond with a REJECTED
2156 command listing the mechanisms it understands, or with an error.
2159 If an AUTH command specifies a mechanism, and the server supports
2160 said mechanism, the server should begin exchanging SASL
2161 challenge-response data with the client using DATA commands.
2164 If the server does not support the mechanism given in the AUTH
2165 command, it must send either a REJECTED command listing the mechanisms
2166 it does support, or an error.
2169 If the [initial-response] argument is provided, it is intended for use
2170 with mechanisms that have no initial challenge (or an empty initial
2171 challenge), as if it were the argument to an initial DATA command. If
2172 the selected mechanism has an initial challenge and [initial-response]
2173 was provided, the server should reject authentication by sending
2177 If authentication succeeds after exchanging DATA commands,
2178 an OK command must be sent to the client.
2181 The first octet received by the server after the \r\n of the BEGIN
2182 command from the client must be the first octet of the
2183 authenticated/encrypted stream of D-Bus messages.
2186 If BEGIN is received by the server, the first octet received
2187 by the client after the \r\n of the OK command must be the
2188 first octet of the authenticated/encrypted stream of D-Bus
2192 <sect2 id="auth-command-cancel">
2193 <title>CANCEL Command</title>
2195 At any time up to sending the BEGIN command, the client may send a
2196 CANCEL command. On receiving the CANCEL command, the server must
2197 send a REJECTED command and abort the current authentication
2201 <sect2 id="auth-command-data">
2202 <title>DATA Command</title>
2204 The DATA command may come from either client or server, and simply
2205 contains a hex-encoded block of data to be interpreted
2206 according to the SASL mechanism in use.
2209 Some SASL mechanisms support sending an "empty string";
2210 FIXME we need some way to do this.
2213 <sect2 id="auth-command-begin">
2214 <title>BEGIN Command</title>
2216 The BEGIN command acknowledges that the client has received an
2217 OK command from the server, and that the stream of messages
2221 The first octet received by the server after the \r\n of the BEGIN
2222 command from the client must be the first octet of the
2223 authenticated/encrypted stream of D-Bus messages.
2226 <sect2 id="auth-command-rejected">
2227 <title>REJECTED Command</title>
2229 The REJECTED command indicates that the current authentication
2230 exchange has failed, and further exchange of DATA is inappropriate.
2231 The client would normally try another mechanism, or try providing
2232 different responses to challenges.
2234 Optionally, the REJECTED command has a space-separated list of
2235 available auth mechanisms as arguments. If a server ever provides
2236 a list of supported mechanisms, it must provide the same list
2237 each time it sends a REJECTED message. Clients are free to
2238 ignore all lists received after the first.
2241 <sect2 id="auth-command-ok">
2242 <title>OK Command</title>
2244 The OK command indicates that the client has been
2245 authenticated. The client may now proceed with negotiating
2246 Unix file descriptor passing. To do that it shall send
2247 NEGOTIATE_UNIX_FD to the server.
2250 Otherwise, the client must respond to the OK command by
2251 sending a BEGIN command, followed by its stream of messages,
2252 or by disconnecting. The server must not accept additional
2253 commands using this protocol after the BEGIN command has been
2254 received. Further communication will be a stream of D-Bus
2255 messages (optionally encrypted, as negotiated) rather than
2259 If a client sends BEGIN the first octet received by the client
2260 after the \r\n of the OK command must be the first octet of
2261 the authenticated/encrypted stream of D-Bus messages.
2264 The OK command has one argument, which is the GUID of the server.
2265 See <xref linkend="addresses"/> for more on server GUIDs.
2268 <sect2 id="auth-command-error">
2269 <title>ERROR Command</title>
2271 The ERROR command indicates that either server or client did not
2272 know a command, does not accept the given command in the current
2273 context, or did not understand the arguments to the command. This
2274 allows the protocol to be extended; a client or server can send a
2275 command present or permitted only in new protocol versions, and if
2276 an ERROR is received instead of an appropriate response, fall back
2277 to using some other technique.
2280 If an ERROR is sent, the server or client that sent the
2281 error must continue as if the command causing the ERROR had never been
2282 received. However, the the server or client receiving the error
2283 should try something other than whatever caused the error;
2284 if only canceling/rejecting the authentication.
2287 If the D-Bus protocol changes incompatibly at some future time,
2288 applications implementing the new protocol would probably be able to
2289 check for support of the new protocol by sending a new command and
2290 receiving an ERROR from applications that don't understand it. Thus the
2291 ERROR feature of the auth protocol is an escape hatch that lets us
2292 negotiate extensions or changes to the D-Bus protocol in the future.
2295 <sect2 id="auth-command-negotiate-unix-fd">
2296 <title>NEGOTIATE_UNIX_FD Command</title>
2298 The NEGOTIATE_UNIX_FD command indicates that the client
2299 supports Unix file descriptor passing. This command may only
2300 be sent after the connection is authenticated, i.e. after OK
2301 was received by the client. This command may only be sent on
2302 transports that support Unix file descriptor passing.
2305 On receiving NEGOTIATE_UNIX_FD the server must respond with
2306 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2307 the transport chosen supports Unix file descriptor passing and
2308 the server supports this feature. It shall respond the latter
2309 if the transport does not support Unix file descriptor
2310 passing, the server does not support this feature, or the
2311 server decides not to enable file descriptor passing due to
2312 security or other reasons.
2315 <sect2 id="auth-command-agree-unix-fd">
2316 <title>AGREE_UNIX_FD Command</title>
2318 The AGREE_UNIX_FD command indicates that the server supports
2319 Unix file descriptor passing. This command may only be sent
2320 after the connection is authenticated, and the client sent
2321 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2322 command may only be sent on transports that support Unix file
2326 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2327 followed by its stream of messages, or by disconnecting. The
2328 server must not accept additional commands using this protocol
2329 after the BEGIN command has been received. Further
2330 communication will be a stream of D-Bus messages (optionally
2331 encrypted, as negotiated) rather than this protocol.
2334 <sect2 id="auth-command-future">
2335 <title>Future Extensions</title>
2337 Future extensions to the authentication and negotiation
2338 protocol are possible. For that new commands may be
2339 introduced. If a client or server receives an unknown command
2340 it shall respond with ERROR and not consider this fatal. New
2341 commands may be introduced both before, and after
2342 authentication, i.e. both before and after the OK command.
2345 <sect2 id="auth-examples">
2346 <title>Authentication examples</title>
2350 <title>Example of successful magic cookie authentication</title>
2352 (MAGIC_COOKIE is a made up mechanism)
2354 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2360 <title>Example of finding out mechanisms then picking one</title>
2363 S: REJECTED KERBEROS_V4 SKEY
2364 C: AUTH SKEY 7ab83f32ee
2365 S: DATA 8799cabb2ea93e
2366 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2372 <title>Example of client sends unknown command then falls back to regular auth</title>
2376 C: AUTH MAGIC_COOKIE 3736343435313230333039
2382 <title>Example of server doesn't support initial auth mechanism</title>
2384 C: AUTH MAGIC_COOKIE 3736343435313230333039
2385 S: REJECTED KERBEROS_V4 SKEY
2386 C: AUTH SKEY 7ab83f32ee
2387 S: DATA 8799cabb2ea93e
2388 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2394 <title>Example of wrong password or the like followed by successful retry</title>
2396 C: AUTH MAGIC_COOKIE 3736343435313230333039
2397 S: REJECTED KERBEROS_V4 SKEY
2398 C: AUTH SKEY 7ab83f32ee
2399 S: DATA 8799cabb2ea93e
2400 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2402 C: AUTH SKEY 7ab83f32ee
2403 S: DATA 8799cabb2ea93e
2404 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2410 <title>Example of skey cancelled and restarted</title>
2412 C: AUTH MAGIC_COOKIE 3736343435313230333039
2413 S: REJECTED KERBEROS_V4 SKEY
2414 C: AUTH SKEY 7ab83f32ee
2415 S: DATA 8799cabb2ea93e
2418 C: AUTH SKEY 7ab83f32ee
2419 S: DATA 8799cabb2ea93e
2420 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2426 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2428 (MAGIC_COOKIE is a made up mechanism)
2430 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2432 C: NEGOTIATE_UNIX_FD
2438 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2440 (MAGIC_COOKIE is a made up mechanism)
2442 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2444 C: NEGOTIATE_UNIX_FD
2451 <sect2 id="auth-states">
2452 <title>Authentication state diagrams</title>
2455 This section documents the auth protocol in terms of
2456 a state machine for the client and the server. This is
2457 probably the most robust way to implement the protocol.
2460 <sect3 id="auth-states-client">
2461 <title>Client states</title>
2464 To more precisely describe the interaction between the
2465 protocol state machine and the authentication mechanisms the
2466 following notation is used: MECH(CHALL) means that the
2467 server challenge CHALL was fed to the mechanism MECH, which
2473 CONTINUE(RESP) means continue the auth conversation
2474 and send RESP as the response to the server;
2480 OK(RESP) means that after sending RESP to the server
2481 the client side of the auth conversation is finished
2482 and the server should return "OK";
2488 ERROR means that CHALL was invalid and could not be
2494 Both RESP and CHALL may be empty.
2498 The Client starts by getting an initial response from the
2499 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2500 the mechanism did not provide an initial response. If the
2501 mechanism returns CONTINUE, the client starts in state
2502 <emphasis>WaitingForData</emphasis>, if the mechanism
2503 returns OK the client starts in state
2504 <emphasis>WaitingForOK</emphasis>.
2508 The client should keep track of available mechanisms and
2509 which it mechanisms it has already attempted. This list is
2510 used to decide which AUTH command to send. When the list is
2511 exhausted, the client should give up and close the
2516 <title><emphasis>WaitingForData</emphasis></title>
2524 MECH(CHALL) returns CONTINUE(RESP) → send
2526 <emphasis>WaitingForData</emphasis>
2530 MECH(CHALL) returns OK(RESP) → send DATA
2531 RESP, goto <emphasis>WaitingForOK</emphasis>
2535 MECH(CHALL) returns ERROR → send ERROR
2536 [msg], goto <emphasis>WaitingForData</emphasis>
2544 Receive REJECTED [mechs] →
2545 send AUTH [next mech], goto
2546 WaitingForData or <emphasis>WaitingForOK</emphasis>
2551 Receive ERROR → send
2553 <emphasis>WaitingForReject</emphasis>
2558 Receive OK → send
2559 BEGIN, terminate auth
2560 conversation, authenticated
2565 Receive anything else → send
2567 <emphasis>WaitingForData</emphasis>
2575 <title><emphasis>WaitingForOK</emphasis></title>
2580 Receive OK → send BEGIN, terminate auth
2581 conversation, <emphasis>authenticated</emphasis>
2586 Receive REJECTED [mechs] → send AUTH [next mech],
2587 goto <emphasis>WaitingForData</emphasis> or
2588 <emphasis>WaitingForOK</emphasis>
2594 Receive DATA → send CANCEL, goto
2595 <emphasis>WaitingForReject</emphasis>
2601 Receive ERROR → send CANCEL, goto
2602 <emphasis>WaitingForReject</emphasis>
2608 Receive anything else → send ERROR, goto
2609 <emphasis>WaitingForOK</emphasis>
2617 <title><emphasis>WaitingForReject</emphasis></title>
2622 Receive REJECTED [mechs] → send AUTH [next mech],
2623 goto <emphasis>WaitingForData</emphasis> or
2624 <emphasis>WaitingForOK</emphasis>
2630 Receive anything else → terminate auth
2631 conversation, disconnect
2640 <sect3 id="auth-states-server">
2641 <title>Server states</title>
2644 For the server MECH(RESP) means that the client response
2645 RESP was fed to the the mechanism MECH, which returns one of
2650 CONTINUE(CHALL) means continue the auth conversation and
2651 send CHALL as the challenge to the client;
2657 OK means that the client has been successfully
2664 REJECTED means that the client failed to authenticate or
2665 there was an error in RESP.
2670 The server starts out in state
2671 <emphasis>WaitingForAuth</emphasis>. If the client is
2672 rejected too many times the server must disconnect the
2677 <title><emphasis>WaitingForAuth</emphasis></title>
2683 Receive AUTH → send REJECTED [mechs], goto
2684 <emphasis>WaitingForAuth</emphasis>
2690 Receive AUTH MECH RESP
2694 MECH not valid mechanism → send REJECTED
2696 <emphasis>WaitingForAuth</emphasis>
2700 MECH(RESP) returns CONTINUE(CHALL) → send
2702 <emphasis>WaitingForData</emphasis>
2706 MECH(RESP) returns OK → send OK, goto
2707 <emphasis>WaitingForBegin</emphasis>
2711 MECH(RESP) returns REJECTED → send REJECTED
2713 <emphasis>WaitingForAuth</emphasis>
2721 Receive BEGIN → terminate
2722 auth conversation, disconnect
2728 Receive ERROR → send REJECTED [mechs], goto
2729 <emphasis>WaitingForAuth</emphasis>
2735 Receive anything else → send
2737 <emphasis>WaitingForAuth</emphasis>
2746 <title><emphasis>WaitingForData</emphasis></title>
2754 MECH(RESP) returns CONTINUE(CHALL) → send
2756 <emphasis>WaitingForData</emphasis>
2760 MECH(RESP) returns OK → send OK, goto
2761 <emphasis>WaitingForBegin</emphasis>
2765 MECH(RESP) returns REJECTED → send REJECTED
2767 <emphasis>WaitingForAuth</emphasis>
2775 Receive BEGIN → terminate auth conversation,
2782 Receive CANCEL → send REJECTED [mechs], goto
2783 <emphasis>WaitingForAuth</emphasis>
2789 Receive ERROR → send REJECTED [mechs], goto
2790 <emphasis>WaitingForAuth</emphasis>
2796 Receive anything else → send ERROR, goto
2797 <emphasis>WaitingForData</emphasis>
2805 <title><emphasis>WaitingForBegin</emphasis></title>
2810 Receive BEGIN → terminate auth conversation,
2811 client authenticated
2817 Receive CANCEL → send REJECTED [mechs], goto
2818 <emphasis>WaitingForAuth</emphasis>
2824 Receive ERROR → send REJECTED [mechs], goto
2825 <emphasis>WaitingForAuth</emphasis>
2831 Receive anything else → send ERROR, goto
2832 <emphasis>WaitingForBegin</emphasis>
2842 <sect2 id="auth-mechanisms">
2843 <title>Authentication mechanisms</title>
2845 This section describes some new authentication mechanisms.
2846 D-Bus also allows any standard SASL mechanism of course.
2848 <sect3 id="auth-mechanisms-sha">
2849 <title>DBUS_COOKIE_SHA1</title>
2851 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2852 has the ability to read a private file owned by the user being
2853 authenticated. If the client can prove that it has access to a secret
2854 cookie stored in this file, then the client is authenticated.
2855 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2859 Throughout this description, "hex encoding" must output the digits
2860 from a to f in lower-case; the digits A to F must not be used
2861 in the DBUS_COOKIE_SHA1 mechanism.
2864 Authentication proceeds as follows:
2868 The client sends the username it would like to authenticate
2874 The server sends the name of its "cookie context" (see below); a
2875 space character; the integer ID of the secret cookie the client
2876 must demonstrate knowledge of; a space character; then a
2877 randomly-generated challenge string, all of this hex-encoded into
2883 The client locates the cookie and generates its own
2884 randomly-generated challenge string. The client then concatenates
2885 the server's decoded challenge, a ":" character, its own challenge,
2886 another ":" character, and the cookie. It computes the SHA-1 hash
2887 of this composite string as a hex digest. It concatenates the
2888 client's challenge string, a space character, and the SHA-1 hex
2889 digest, hex-encodes the result and sends it back to the server.
2894 The server generates the same concatenated string used by the
2895 client and computes its SHA-1 hash. It compares the hash with
2896 the hash received from the client; if the two hashes match, the
2897 client is authenticated.
2903 Each server has a "cookie context," which is a name that identifies a
2904 set of cookies that apply to that server. A sample context might be
2905 "org_freedesktop_session_bus". Context names must be valid ASCII,
2906 nonzero length, and may not contain the characters slash ("/"),
2907 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2908 tab ("\t"), or period ("."). There is a default context,
2909 "org_freedesktop_general" that's used by servers that do not specify
2913 Cookies are stored in a user's home directory, in the directory
2914 <filename>~/.dbus-keyrings/</filename>. This directory must
2915 not be readable or writable by other users. If it is,
2916 clients and servers must ignore it. The directory
2917 contains cookie files named after the cookie context.
2920 A cookie file contains one cookie per line. Each line
2921 has three space-separated fields:
2925 The cookie ID number, which must be a non-negative integer and
2926 may not be used twice in the same file.
2931 The cookie's creation time, in UNIX seconds-since-the-epoch
2937 The cookie itself, a hex-encoded random block of bytes. The cookie
2938 may be of any length, though obviously security increases
2939 as the length increases.
2945 Only server processes modify the cookie file.
2946 They must do so with this procedure:
2950 Create a lockfile name by appending ".lock" to the name of the
2951 cookie file. The server should attempt to create this file
2952 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2953 fails, the lock fails. Servers should retry for a reasonable
2954 period of time, then they may choose to delete an existing lock
2955 to keep users from having to manually delete a stale
2956 lock. <footnote><para>Lockfiles are used instead of real file
2957 locking <literal>fcntl()</literal> because real locking
2958 implementations are still flaky on network
2959 filesystems.</para></footnote>
2964 Once the lockfile has been created, the server loads the cookie
2965 file. It should then delete any cookies that are old (the
2966 timeout can be fairly short), or more than a reasonable
2967 time in the future (so that cookies never accidentally
2968 become permanent, if the clock was set far into the future
2969 at some point). If no recent keys remain, the
2970 server may generate a new key.
2975 The pruned and possibly added-to cookie file
2976 must be resaved atomically (using a temporary
2977 file which is rename()'d).
2982 The lock must be dropped by deleting the lockfile.
2988 Clients need not lock the file in order to load it,
2989 because servers are required to save the file atomically.
2994 <sect1 id="addresses">
2995 <title>Server Addresses</title>
2997 Server addresses consist of a transport name followed by a colon, and
2998 then an optional, comma-separated list of keys and values in the form key=value.
2999 Each value is escaped.
3003 <programlisting>unix:path=/tmp/dbus-test</programlisting>
3004 Which is the address to a unix socket with the path /tmp/dbus-test.
3007 Value escaping is similar to URI escaping but simpler.
3011 The set of optionally-escaped bytes is:
3012 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
3013 <emphasis>byte</emphasis> (note, not character) which is not in the
3014 set of optionally-escaped bytes must be replaced with an ASCII
3015 percent (<literal>%</literal>) and the value of the byte in hex.
3016 The hex value must always be two digits, even if the first digit is
3017 zero. The optionally-escaped bytes may be escaped if desired.
3022 To unescape, append each byte in the value; if a byte is an ASCII
3023 percent (<literal>%</literal>) character then append the following
3024 hex value instead. It is an error if a <literal>%</literal> byte
3025 does not have two hex digits following. It is an error if a
3026 non-optionally-escaped byte is seen unescaped.
3030 The set of optionally-escaped bytes is intended to preserve address
3031 readability and convenience.
3035 A server may specify a key-value pair with the key <literal>guid</literal>
3036 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
3037 describes the format of the <literal>guid</literal> field. If present,
3038 this UUID may be used to distinguish one server address from another. A
3039 server should use a different UUID for each address it listens on. For
3040 example, if a message bus daemon offers both UNIX domain socket and TCP
3041 connections, but treats clients the same regardless of how they connect,
3042 those two connections are equivalent post-connection but should have
3043 distinct UUIDs to distinguish the kinds of connection.
3047 The intent of the address UUID feature is to allow a client to avoid
3048 opening multiple identical connections to the same server, by allowing the
3049 client to check whether an address corresponds to an already-existing
3050 connection. Comparing two addresses is insufficient, because addresses
3051 can be recycled by distinct servers, and equivalent addresses may look
3052 different if simply compared as strings (for example, the host in a TCP
3053 address can be given as an IP address or as a hostname).
3057 Note that the address key is <literal>guid</literal> even though the
3058 rest of the API and documentation says "UUID," for historical reasons.
3062 [FIXME clarify if attempting to connect to each is a requirement
3063 or just a suggestion]
3064 When connecting to a server, multiple server addresses can be
3065 separated by a semi-colon. The library will then try to connect
3066 to the first address and if that fails, it'll try to connect to
3067 the next one specified, and so forth. For example
3068 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3072 Some addresses are <firstterm>connectable</firstterm>. A connectable
3073 address is one containing enough information for a client to connect
3074 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3075 is a connectable address. It is not necessarily possible to listen
3076 on every connectable address: for instance, it is not possible to
3077 listen on a <literal>unixexec:</literal> address.
3081 Some addresses are <firstterm>listenable</firstterm>. A listenable
3082 address is one containing enough information for a server to listen on
3083 it, producing a connectable address (which may differ from the
3084 original address). Many listenable addresses are not connectable:
3085 for instance, <literal>tcp:host=127.0.0.1</literal>
3086 is listenable, but not connectable (because it does not specify
3091 Listening on an address that is not connectable will result in a
3092 connectable address that is not the same as the listenable address.
3093 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3094 might result in the connectable address
3095 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3096 or listening on <literal>unix:tmpdir=/tmp</literal>
3097 might result in the connectable address
3098 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3102 <sect1 id="transports">
3103 <title>Transports</title>
3105 [FIXME we need to specify in detail each transport and its possible arguments]
3107 Current transports include: unix domain sockets (including
3108 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3109 using in-process pipes. Future possible transports include one that
3110 tunnels over X11 protocol.
3113 <sect2 id="transports-unix-domain-sockets">
3114 <title>Unix Domain Sockets</title>
3116 Unix domain sockets can be either paths in the file system or on Linux
3117 kernels, they can be abstract which are similar to paths but
3118 do not show up in the file system.
3122 When a socket is opened by the D-Bus library it truncates the path
3123 name right before the first trailing Nul byte. This is true for both
3124 normal paths and abstract paths. Note that this is a departure from
3125 previous versions of D-Bus that would create sockets with a fixed
3126 length path name. Names which were shorter than the fixed length
3127 would be padded by Nul bytes.
3130 Unix domain sockets are not available on Windows.
3133 Unix addresses that specify <literal>path</literal> or
3134 <literal>abstract</literal> are both listenable and connectable.
3135 Unix addresses that specify <literal>tmpdir</literal> are only
3136 listenable: the corresponding connectable address will specify
3137 either <literal>path</literal> or <literal>abstract</literal>.
3139 <sect3 id="transports-unix-domain-sockets-addresses">
3140 <title>Server Address Format</title>
3142 Unix domain socket addresses are identified by the "unix:" prefix
3143 and support the following key/value pairs:
3150 <entry>Values</entry>
3151 <entry>Description</entry>
3157 <entry>(path)</entry>
3158 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3161 <entry>tmpdir</entry>
3162 <entry>(path)</entry>
3163 <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>
3166 <entry>abstract</entry>
3167 <entry>(string)</entry>
3168 <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>
3174 Exactly one of the keys <literal>path</literal>,
3175 <literal>abstract</literal> or
3176 <literal>tmpdir</literal> must be provided.
3180 <sect2 id="transports-launchd">
3181 <title>launchd</title>
3183 launchd is an open-source server management system that replaces init, inetd
3184 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3185 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3189 launchd allocates a socket and provides it with the unix path through the
3190 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3191 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3192 it through its environment.
3193 Other processes can query for the launchd socket by executing:
3194 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3195 This is normally done by the D-Bus client library so doesn't have to be done
3199 launchd is not available on Microsoft Windows.
3202 launchd addresses are listenable and connectable.
3204 <sect3 id="transports-launchd-addresses">
3205 <title>Server Address Format</title>
3207 launchd addresses are identified by the "launchd:" prefix
3208 and support the following key/value pairs:
3215 <entry>Values</entry>
3216 <entry>Description</entry>
3222 <entry>(environment variable)</entry>
3223 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3229 The <literal>env</literal> key is required.
3233 <sect2 id="transports-systemd">
3234 <title>systemd</title>
3236 systemd is an open-source server management system that
3237 replaces init and inetd on newer Linux systems. It supports
3238 socket activation. The D-Bus systemd transport is used to acquire
3239 socket activation file descriptors from systemd and use them
3240 as D-Bus transport when the current process is spawned by
3241 socket activation from it.
3244 The systemd transport accepts only one or more Unix domain or
3245 TCP streams sockets passed in via socket activation.
3248 The systemd transport is not available on non-Linux operating systems.
3251 The systemd transport defines no parameter keys.
3254 systemd addresses are listenable, but not connectable. The
3255 corresponding connectable address is the <literal>unix</literal>
3256 or <literal>tcp</literal> address of the socket.
3259 <sect2 id="transports-tcp-sockets">
3260 <title>TCP Sockets</title>
3262 The tcp transport provides TCP/IP based connections between clients
3263 located on the same or different hosts.
3266 Using tcp transport without any additional secure authentification mechanismus
3267 over a network is unsecure.
3270 On Windows and most Unix platforms, the TCP stack is unable to transfer
3271 credentials over a TCP connection, so the EXTERNAL authentication
3272 mechanism does not work for this transport.
3275 All <literal>tcp</literal> addresses are listenable.
3276 <literal>tcp</literal> addresses in which both
3277 <literal>host</literal> and <literal>port</literal> are
3278 specified, and <literal>port</literal> is non-zero,
3279 are also connectable.
3281 <sect3 id="transports-tcp-sockets-addresses">
3282 <title>Server Address Format</title>
3284 TCP/IP socket addresses are identified by the "tcp:" prefix
3285 and support the following key/value pairs:
3292 <entry>Values</entry>
3293 <entry>Description</entry>
3299 <entry>(string)</entry>
3300 <entry>DNS name or IP address</entry>
3304 <entry>(string)</entry>
3305 <entry>Used in a listenable address to configure the interface
3306 on which the server will listen: either the IP address of one of
3307 the local machine's interfaces (most commonly <literal>127.0.0.1
3308 </literal>), or a DNS name that resolves to one of those IP
3309 addresses, or '*' to listen on all interfaces simultaneously.
3310 If not specified, the default is the same value as "host".
3315 <entry>(number)</entry>
3316 <entry>The tcp port the server will open. A zero value let the server
3317 choose a free port provided from the underlaying operating system.
3318 libdbus is able to retrieve the real used port from the server.
3322 <entry>family</entry>
3323 <entry>(string)</entry>
3324 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3331 <sect2 id="transports-nonce-tcp-sockets">
3332 <title>Nonce-secured TCP Sockets</title>
3334 The nonce-tcp transport provides a secured TCP transport, using a
3335 simple authentication mechanism to ensure that only clients with read
3336 access to a certain location in the filesystem can connect to the server.
3337 The server writes a secret, the nonce, to a file and an incoming client
3338 connection is only accepted if the client sends the nonce right after
3339 the connect. The nonce mechanism requires no setup and is orthogonal to
3340 the higher-level authentication mechanisms described in the
3341 Authentication section.
3345 On start, the server generates a random 16 byte nonce and writes it
3346 to a file in the user's temporary directory. The nonce file location
3347 is published as part of the server's D-Bus address using the
3348 "noncefile" key-value pair.
3350 After an accept, the server reads 16 bytes from the socket. If the
3351 read bytes do not match the nonce stored in the nonce file, the
3352 server MUST immediately drop the connection.
3353 If the nonce match the received byte sequence, the client is accepted
3354 and the transport behaves like an unsecured tcp transport.
3357 After a successful connect to the server socket, the client MUST read
3358 the nonce from the file published by the server via the noncefile=
3359 key-value pair and send it over the socket. After that, the
3360 transport behaves like an unsecured tcp transport.
3363 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3364 <literal>host</literal>, <literal>port</literal> and
3365 <literal>noncefile</literal> are all specified,
3366 and <literal>port</literal> is nonzero, are also connectable.
3368 <sect3 id="transports-nonce-tcp-sockets-addresses">
3369 <title>Server Address Format</title>
3371 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3372 and support the following key/value pairs:
3379 <entry>Values</entry>
3380 <entry>Description</entry>
3386 <entry>(string)</entry>
3387 <entry>DNS name or IP address</entry>
3391 <entry>(string)</entry>
3392 <entry>The same as for tcp: addresses
3397 <entry>(number)</entry>
3398 <entry>The tcp port the server will open. A zero value let the server
3399 choose a free port provided from the underlaying operating system.
3400 libdbus is able to retrieve the real used port from the server.
3404 <entry>family</entry>
3405 <entry>(string)</entry>
3406 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3409 <entry>noncefile</entry>
3410 <entry>(path)</entry>
3411 <entry>File location containing the secret.
3412 This is only meaningful in connectable addresses:
3413 a listening D-Bus server that offers this transport
3414 will always create a new nonce file.</entry>
3421 <sect2 id="transports-exec">
3422 <title>Executed Subprocesses on Unix</title>
3424 This transport forks off a process and connects its standard
3425 input and standard output with an anonymous Unix domain
3426 socket. This socket is then used for communication by the
3427 transport. This transport may be used to use out-of-process
3428 forwarder programs as basis for the D-Bus protocol.
3431 The forked process will inherit the standard error output and
3432 process group from the parent process.
3435 Executed subprocesses are not available on Windows.
3438 <literal>unixexec</literal> addresses are connectable, but are not
3441 <sect3 id="transports-exec-addresses">
3442 <title>Server Address Format</title>
3444 Executed subprocess addresses are identified by the "unixexec:" prefix
3445 and support the following key/value pairs:
3452 <entry>Values</entry>
3453 <entry>Description</entry>
3459 <entry>(path)</entry>
3460 <entry>Path of the binary to execute, either an absolute
3461 path or a binary name that is searched for in the default
3462 search path of the OS. This corresponds to the first
3463 argument of execlp(). This key is mandatory.</entry>
3466 <entry>argv0</entry>
3467 <entry>(string)</entry>
3468 <entry>The program name to use when executing the
3469 binary. If omitted the same value as specified for path=
3470 will be used. This corresponds to the second argument of
3474 <entry>argv1, argv2, ...</entry>
3475 <entry>(string)</entry>
3476 <entry>Arguments to pass to the binary. This corresponds
3477 to the third and later arguments of execlp(). If a
3478 specific argvX is not specified no further argvY for Y > X
3479 are taken into account.</entry>
3487 <sect1 id="meta-transports">
3488 <title>Meta Transports</title>
3490 Meta transports are a kind of transport with special enhancements or
3491 behavior. Currently available meta transports include: autolaunch
3494 <sect2 id="meta-transports-autolaunch">
3495 <title>Autolaunch</title>
3496 <para>The autolaunch transport provides a way for dbus clients to autodetect
3497 a running dbus session bus and to autolaunch a session bus if not present.
3500 On Unix, <literal>autolaunch</literal> addresses are connectable,
3504 On Windows, <literal>autolaunch</literal> addresses are both
3505 connectable and listenable.
3508 <sect3 id="meta-transports-autolaunch-addresses">
3509 <title>Server Address Format</title>
3511 Autolaunch addresses uses the "autolaunch:" prefix and support the
3512 following key/value pairs:
3519 <entry>Values</entry>
3520 <entry>Description</entry>
3525 <entry>scope</entry>
3526 <entry>(string)</entry>
3527 <entry>scope of autolaunch (Windows only)
3531 "*install-path" - limit session bus to dbus installation path.
3532 The dbus installation path is determined from the location of
3533 the shared dbus library. If the library is located in a 'bin'
3534 subdirectory the installation root is the directory above,
3535 otherwise the directory where the library lives is taken as
3538 <install-root>/bin/[lib]dbus-1.dll
3539 <install-root>/[lib]dbus-1.dll
3545 "*user" - limit session bus to the recent user.
3550 other values - specify dedicated session bus like "release",
3562 <sect3 id="meta-transports-autolaunch-windows-implementation">
3563 <title>Windows implementation</title>
3565 On start, the server opens a platform specific transport, creates a mutex
3566 and a shared memory section containing the related session bus address.
3567 This mutex will be inspected by the dbus client library to detect a
3568 running dbus session bus. The access to the mutex and the shared memory
3569 section are protected by global locks.
3572 In the recent implementation the autolaunch transport uses a tcp transport
3573 on localhost with a port choosen from the operating system. This detail may
3574 change in the future.
3577 Disclaimer: The recent implementation is in an early state and may not
3578 work in all cirumstances and/or may have security issues. Because of this
3579 the implementation is not documentated yet.
3586 <title>UUIDs</title>
3588 A working D-Bus implementation uses universally-unique IDs in two places.
3589 First, each server address has a UUID identifying the address,
3590 as described in <xref linkend="addresses"/>. Second, each operating
3591 system kernel instance running a D-Bus client or server has a UUID
3592 identifying that kernel, retrieved by invoking the method
3593 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3594 linkend="standard-interfaces-peer"/>).
3597 The term "UUID" in this document is intended literally, i.e. an
3598 identifier that is universally unique. It is not intended to refer to
3599 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3602 The UUID must contain 128 bits of data and be hex-encoded. The
3603 hex-encoded string may not contain hyphens or other non-hex-digit
3604 characters, and it must be exactly 32 characters long. To generate a
3605 UUID, the current reference implementation concatenates 96 bits of random
3606 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3610 It would also be acceptable and probably better to simply generate 128
3611 bits of random data, as long as the random number generator is of high
3612 quality. The timestamp could conceivably help if the random bits are not
3613 very random. With a quality random number generator, collisions are
3614 extremely unlikely even with only 96 bits, so it's somewhat academic.
3617 Implementations should, however, stick to random data for the first 96 bits
3622 <sect1 id="standard-interfaces">
3623 <title>Standard Interfaces</title>
3625 See <xref linkend="message-protocol-types-notation"/> for details on
3626 the notation used in this section. There are some standard interfaces
3627 that may be useful across various D-Bus applications.
3629 <sect2 id="standard-interfaces-peer">
3630 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3632 The <literal>org.freedesktop.DBus.Peer</literal> interface
3635 org.freedesktop.DBus.Peer.Ping ()
3636 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3640 On receipt of the <literal>METHOD_CALL</literal> message
3641 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3642 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3643 usual. It does not matter which object path a ping is sent to. The
3644 reference implementation handles this method automatically.
3647 On receipt of the <literal>METHOD_CALL</literal> message
3648 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3649 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3650 UUID representing the identity of the machine the process is running on.
3651 This UUID must be the same for all processes on a single system at least
3652 until that system next reboots. It should be the same across reboots
3653 if possible, but this is not always possible to implement and is not
3655 It does not matter which object path a GetMachineId is sent to. The
3656 reference implementation handles this method automatically.
3659 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3660 a virtual machine running on a hypervisor, rather than a physical machine.
3661 Basically if two processes see the same UUID, they should also see the same
3662 shared memory, UNIX domain sockets, process IDs, and other features that require
3663 a running OS kernel in common between the processes.
3666 The UUID is often used where other programs might use a hostname. Hostnames
3667 can change without rebooting, however, or just be "localhost" - so the UUID
3671 <xref linkend="uuids"/> explains the format of the UUID.
3675 <sect2 id="standard-interfaces-introspectable">
3676 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3678 This interface has one method:
3680 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3684 Objects instances may implement
3685 <literal>Introspect</literal> which returns an XML description of
3686 the object, including its interfaces (with signals and methods), objects
3687 below it in the object path tree, and its properties.
3690 <xref linkend="introspection-format"/> describes the format of this XML string.
3693 <sect2 id="standard-interfaces-properties">
3694 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3696 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3697 or <firstterm>attributes</firstterm>. These can be exposed via the
3698 <literal>org.freedesktop.DBus.Properties</literal> interface.
3702 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3703 in STRING property_name,
3705 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3706 in STRING property_name,
3708 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3709 out DICT<STRING,VARIANT> props);
3713 It is conventional to give D-Bus properties names consisting of
3714 capitalized words without punctuation ("CamelCase"), like
3715 <link linkend="message-protocol-names-member">member names</link>.
3716 For instance, the GObject property
3717 <literal>connection-status</literal> or the Qt property
3718 <literal>connectionStatus</literal> could be represented on D-Bus
3719 as <literal>ConnectionStatus</literal>.
3722 Strictly speaking, D-Bus property names are not required to follow
3723 the same naming restrictions as member names, but D-Bus property
3724 names that would not be valid member names (in particular,
3725 GObject-style dash-separated property names) can cause interoperability
3726 problems and should be avoided.
3729 The available properties and whether they are writable can be determined
3730 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3731 see <xref linkend="standard-interfaces-introspectable"/>.
3734 An empty string may be provided for the interface name; in this case,
3735 if there are multiple properties on an object with the same name,
3736 the results are undefined (picking one by according to an arbitrary
3737 deterministic rule, or returning an error, are the reasonable
3741 If one or more properties change on an object, the
3742 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3743 signal may be emitted (this signal was added in 0.14):
3747 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3748 DICT<STRING,VARIANT> changed_properties,
3749 ARRAY<STRING> invalidated_properties);
3753 where <literal>changed_properties</literal> is a dictionary
3754 containing the changed properties with the new values and
3755 <literal>invalidated_properties</literal> is an array of
3756 properties that changed but the value is not conveyed.
3759 Whether the <literal>PropertiesChanged</literal> signal is
3760 supported can be determined by calling
3761 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3762 that the signal may be supported for an object but it may
3763 differ how whether and how it is used on a per-property basis
3764 (for e.g. performance or security reasons). Each property (or
3765 the parent interface) must be annotated with the
3766 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3767 annotation to convey this (usually the default value
3768 <literal>true</literal> is sufficient meaning that the
3769 annotation does not need to be used). See <xref
3770 linkend="introspection-format"/> for details on this
3775 <sect2 id="standard-interfaces-objectmanager">
3776 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3778 An API can optionally make use of this interface for one or
3779 more sub-trees of objects. The root of each sub-tree implements
3780 this interface so other applications can get all objects,
3781 interfaces and properties in a single method call. It is
3782 appropriate to use this interface if users of the tree of
3783 objects are expected to be interested in all interfaces of all
3784 objects in the tree; a more granular API should be used if
3785 users of the objects are expected to be interested in a small
3786 subset of the objects, a small subset of their interfaces, or
3790 The method that applications can use to get all objects and
3791 properties is <literal>GetManagedObjects</literal>:
3795 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3799 The return value of this method is a dict whose keys are
3800 object paths. All returned object paths are children of the
3801 object path implementing this interface, i.e. their object
3802 paths start with the ObjectManager's object path plus '/'.
3805 Each value is a dict whose keys are interfaces names. Each
3806 value in this inner dict is the same dict that would be
3807 returned by the <link
3808 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3809 method for that combination of object path and interface. If
3810 an interface has no properties, the empty dict is returned.
3813 Changes are emitted using the following two signals:
3817 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3818 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3819 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3820 ARRAY<STRING> interfaces);
3824 The <literal>InterfacesAdded</literal> signal is emitted when
3825 either a new object is added or when an existing object gains
3826 one or more interfaces. The
3827 <literal>InterfacesRemoved</literal> signal is emitted
3828 whenever an object is removed or it loses one or more
3829 interfaces. The second parameter of the
3830 <literal>InterfacesAdded</literal> signal contains a dict with
3831 the interfaces and properties (if any) that have been added to
3832 the given object path. Similarly, the second parameter of the
3833 <literal>InterfacesRemoved</literal> signal contains an array
3834 of the interfaces that were removed. Note that changes on
3835 properties on existing interfaces are not reported using this
3836 interface - an application should also monitor the existing <link
3837 linkend="standard-interfaces-properties">PropertiesChanged</link>
3838 signal on each object.
3841 Applications SHOULD NOT export objects that are children of an
3842 object (directly or otherwise) implementing this interface but
3843 which are not returned in the reply from the
3844 <literal>GetManagedObjects()</literal> method of this
3845 interface on the given object.
3848 The intent of the <literal>ObjectManager</literal> interface
3849 is to make it easy to write a robust client
3850 implementation. The trivial client implementation only needs
3851 to make two method calls:
3855 org.freedesktop.DBus.AddMatch (bus_proxy,
3856 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3857 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3861 on the message bus and the remote application's
3862 <literal>ObjectManager</literal>, respectively. Whenever a new
3863 remote object is created (or an existing object gains a new
3864 interface), the <literal>InterfacesAdded</literal> signal is
3865 emitted, and since this signal contains all properties for the
3866 interfaces, no calls to the
3867 <literal>org.freedesktop.Properties</literal> interface on the
3868 remote object are needed. Additionally, since the initial
3869 <literal>AddMatch()</literal> rule already includes signal
3870 messages from the newly created child object, no new
3871 <literal>AddMatch()</literal> call is needed.
3876 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3877 interface was added in version 0.17 of the D-Bus
3884 <sect1 id="introspection-format">
3885 <title>Introspection Data Format</title>
3887 As described in <xref linkend="standard-interfaces-introspectable"/>,
3888 objects may be introspected at runtime, returning an XML string
3889 that describes the object. The same XML format may be used in
3890 other contexts as well, for example as an "IDL" for generating
3891 static language bindings.
3894 Here is an example of introspection data:
3896 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3897 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3898 <node name="/com/example/sample_object">
3899 <interface name="com.example.SampleInterface">
3900 <method name="Frobate">
3901 <arg name="foo" type="i" direction="in"/>
3902 <arg name="bar" type="s" direction="out"/>
3903 <arg name="baz" type="a{us}" direction="out"/>
3904 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3906 <method name="Bazify">
3907 <arg name="bar" type="(iiu)" direction="in"/>
3908 <arg name="bar" type="v" direction="out"/>
3910 <method name="Mogrify">
3911 <arg name="bar" type="(iiav)" direction="in"/>
3913 <signal name="Changed">
3914 <arg name="new_value" type="b"/>
3916 <property name="Bar" type="y" access="readwrite"/>
3918 <node name="child_of_sample_object"/>
3919 <node name="another_child_of_sample_object"/>
3924 A more formal DTD and spec needs writing, but here are some quick notes.
3928 Only the root <node> element can omit the node name, as it's
3929 known to be the object that was introspected. If the root
3930 <node> does have a name attribute, it must be an absolute
3931 object path. If child <node> have object paths, they must be
3937 If a child <node> has any sub-elements, then they
3938 must represent a complete introspection of the child.
3939 If a child <node> is empty, then it may or may
3940 not have sub-elements; the child must be introspected
3941 in order to find out. The intent is that if an object
3942 knows that its children are "fast" to introspect
3943 it can go ahead and return their information, but
3944 otherwise it can omit it.
3949 The direction element on <arg> may be omitted,
3950 in which case it defaults to "in" for method calls
3951 and "out" for signals. Signals only allow "out"
3952 so while direction may be specified, it's pointless.
3957 The possible directions are "in" and "out",
3958 unlike CORBA there is no "inout"
3963 The possible property access flags are
3964 "readwrite", "read", and "write"
3969 Multiple interfaces can of course be listed for
3975 The "name" attribute on arguments is optional.
3981 Method, interface, property, and signal elements may have
3982 "annotations", which are generic key/value pairs of metadata.
3983 They are similar conceptually to Java's annotations and C# attributes.
3984 Well-known annotations:
3991 <entry>Values (separated by ,)</entry>
3992 <entry>Description</entry>
3997 <entry>org.freedesktop.DBus.Deprecated</entry>
3998 <entry>true,false</entry>
3999 <entry>Whether or not the entity is deprecated; defaults to false</entry>
4002 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
4003 <entry>(string)</entry>
4004 <entry>The C symbol; may be used for methods and interfaces</entry>
4007 <entry>org.freedesktop.DBus.Method.NoReply</entry>
4008 <entry>true,false</entry>
4009 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
4012 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
4013 <entry>true,invalidates,const,false</entry>
4016 If set to <literal>false</literal>, the
4017 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
4019 linkend="standard-interfaces-properties"/> is not
4020 guaranteed to be emitted if the property changes.
4023 If set to <literal>const</literal> the property never
4024 changes value during the lifetime of the object it
4025 belongs to, and hence the signal is never emitted for
4029 If set to <literal>invalidates</literal> the signal
4030 is emitted but the value is not included in the
4034 If set to <literal>true</literal> the signal is
4035 emitted with the value included.
4038 The value for the annotation defaults to
4039 <literal>true</literal> if the enclosing interface
4040 element does not specify the annotation. Otherwise it
4041 defaults to the value specified in the enclosing
4045 This annotation is intended to be used by code
4046 generators to implement client-side caching of
4047 property values. For all properties for which the
4048 annotation is set to <literal>const</literal>,
4049 <literal>invalidates</literal> or
4050 <literal>true</literal> the client may
4051 unconditionally cache the values as the properties
4052 don't change or notifications are generated for them
4061 <sect1 id="message-bus">
4062 <title>Message Bus Specification</title>
4063 <sect2 id="message-bus-overview">
4064 <title>Message Bus Overview</title>
4066 The message bus accepts connections from one or more applications.
4067 Once connected, applications can exchange messages with other
4068 applications that are also connected to the bus.
4071 In order to route messages among connections, the message bus keeps a
4072 mapping from names to connections. Each connection has one
4073 unique-for-the-lifetime-of-the-bus name automatically assigned.
4074 Applications may request additional names for a connection. Additional
4075 names are usually "well-known names" such as
4076 "com.example.TextEditor". When a name is bound to a connection,
4077 that connection is said to <firstterm>own</firstterm> the name.
4080 The bus itself owns a special name,
4081 <literal>org.freedesktop.DBus</literal>, with an object
4082 located at <literal>/org/freedesktop/DBus</literal> that
4083 implements the <literal>org.freedesktop.DBus</literal>
4084 interface. This service allows applications to make
4085 administrative requests of the bus itself. For example,
4086 applications can ask the bus to assign a name to a connection.
4089 Each name may have <firstterm>queued owners</firstterm>. When an
4090 application requests a name for a connection and the name is already in
4091 use, the bus will optionally add the connection to a queue waiting for
4092 the name. If the current owner of the name disconnects or releases
4093 the name, the next connection in the queue will become the new owner.
4097 This feature causes the right thing to happen if you start two text
4098 editors for example; the first one may request "com.example.TextEditor",
4099 and the second will be queued as a possible owner of that name. When
4100 the first exits, the second will take over.
4104 Applications may send <firstterm>unicast messages</firstterm> to
4105 a specific recipient or to the message bus itself, or
4106 <firstterm>broadcast messages</firstterm> to all interested recipients.
4107 See <xref linkend="message-bus-routing"/> for details.
4111 <sect2 id="message-bus-names">
4112 <title>Message Bus Names</title>
4114 Each connection has at least one name, assigned at connection time and
4115 returned in response to the
4116 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4117 automatically-assigned name is called the connection's <firstterm>unique
4118 name</firstterm>. Unique names are never reused for two different
4119 connections to the same bus.
4122 Ownership of a unique name is a prerequisite for interaction with
4123 the message bus. It logically follows that the unique name is always
4124 the first name that an application comes to own, and the last
4125 one that it loses ownership of.
4128 Unique connection names must begin with the character ':' (ASCII colon
4129 character); bus names that are not unique names must not begin
4130 with this character. (The bus must reject any attempt by an application
4131 to manually request a name beginning with ':'.) This restriction
4132 categorically prevents "spoofing"; messages sent to a unique name
4133 will always go to the expected connection.
4136 When a connection is closed, all the names that it owns are deleted (or
4137 transferred to the next connection in the queue if any).
4140 A connection can request additional names to be associated with it using
4141 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4142 linkend="message-protocol-names-bus"/> describes the format of a valid
4143 name. These names can be released again using the
4144 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4147 <sect3 id="bus-messages-request-name">
4148 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4152 UINT32 RequestName (in STRING name, in UINT32 flags)
4159 <entry>Argument</entry>
4161 <entry>Description</entry>
4167 <entry>STRING</entry>
4168 <entry>Name to request</entry>
4172 <entry>UINT32</entry>
4173 <entry>Flags</entry>
4183 <entry>Argument</entry>
4185 <entry>Description</entry>
4191 <entry>UINT32</entry>
4192 <entry>Return value</entry>
4199 This method call should be sent to
4200 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4201 assign the given name to the method caller. Each name maintains a
4202 queue of possible owners, where the head of the queue is the primary
4203 or current owner of the name. Each potential owner in the queue
4204 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4205 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4206 call. When RequestName is invoked the following occurs:
4210 If the method caller is currently the primary owner of the name,
4211 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4212 values are updated with the values from the new RequestName call,
4213 and nothing further happens.
4219 If the current primary owner (head of the queue) has
4220 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4221 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4222 the caller of RequestName replaces the current primary owner at
4223 the head of the queue and the current primary owner moves to the
4224 second position in the queue. If the caller of RequestName was
4225 in the queue previously its flags are updated with the values from
4226 the new RequestName in addition to moving it to the head of the queue.
4232 If replacement is not possible, and the method caller is
4233 currently in the queue but not the primary owner, its flags are
4234 updated with the values from the new RequestName call.
4240 If replacement is not possible, and the method caller is
4241 currently not in the queue, the method caller is appended to the
4248 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4249 set and is not the primary owner, it is removed from the
4250 queue. This can apply to the previous primary owner (if it
4251 was replaced) or the method caller (if it updated the
4252 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4253 queue, or if it was just added to the queue with that flag set).
4259 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4260 queue," even if another application already in the queue had specified
4261 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4262 that does not allow replacement goes away, and the next primary owner
4263 does allow replacement. In this case, queued items that specified
4264 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4265 automatically replace the new primary owner. In other words,
4266 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4267 time RequestName is called. This is deliberate to avoid an infinite loop
4268 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4269 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4272 The flags argument contains any of the following values logically ORed
4279 <entry>Conventional Name</entry>
4280 <entry>Value</entry>
4281 <entry>Description</entry>
4286 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4290 If an application A specifies this flag and succeeds in
4291 becoming the owner of the name, and another application B
4292 later calls RequestName with the
4293 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4294 will lose ownership and receive a
4295 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4296 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4297 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4298 is not specified by application B, then application B will not replace
4299 application A as the owner.
4304 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4308 Try to replace the current owner if there is one. If this
4309 flag is not set the application will only become the owner of
4310 the name if there is no current owner. If this flag is set,
4311 the application will replace the current owner if
4312 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4317 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4321 Without this flag, if an application requests a name that is
4322 already owned, the application will be placed in a queue to
4323 own the name when the current owner gives it up. If this
4324 flag is given, the application will not be placed in the
4325 queue, the request for the name will simply fail. This flag
4326 also affects behavior when an application is replaced as
4327 name owner; by default the application moves back into the
4328 waiting queue, unless this flag was provided when the application
4329 became the name owner.
4337 The return code can be one of the following values:
4343 <entry>Conventional Name</entry>
4344 <entry>Value</entry>
4345 <entry>Description</entry>
4350 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4351 <entry>1</entry> <entry>The caller is now the primary owner of
4352 the name, replacing any previous owner. Either the name had no
4353 owner before, or the caller specified
4354 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4355 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4358 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4361 <entry>The name already had an owner,
4362 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4363 the current owner did not specify
4364 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4365 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4369 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4370 <entry>The name already has an owner,
4371 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4372 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4373 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4374 specified by the requesting application.</entry>
4377 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4379 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4387 <sect3 id="bus-messages-release-name">
4388 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4392 UINT32 ReleaseName (in STRING name)
4399 <entry>Argument</entry>
4401 <entry>Description</entry>
4407 <entry>STRING</entry>
4408 <entry>Name to release</entry>
4418 <entry>Argument</entry>
4420 <entry>Description</entry>
4426 <entry>UINT32</entry>
4427 <entry>Return value</entry>
4434 This method call should be sent to
4435 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4436 release the method caller's claim to the given name. If the caller is
4437 the primary owner, a new primary owner will be selected from the
4438 queue if any other owners are waiting. If the caller is waiting in
4439 the queue for the name, the caller will removed from the queue and
4440 will not be made an owner of the name if it later becomes available.
4441 If there are no other owners in the queue for the name, it will be
4442 removed from the bus entirely.
4444 The return code can be one of the following values:
4450 <entry>Conventional Name</entry>
4451 <entry>Value</entry>
4452 <entry>Description</entry>
4457 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4458 <entry>1</entry> <entry>The caller has released his claim on
4459 the given name. Either the caller was the primary owner of
4460 the name, and the name is now unused or taken by somebody
4461 waiting in the queue for the name, or the caller was waiting
4462 in the queue for the name and has now been removed from the
4466 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4468 <entry>The given name does not exist on this bus.</entry>
4471 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4473 <entry>The caller was not the primary owner of this name,
4474 and was also not waiting in the queue to own this name.</entry>
4482 <sect3 id="bus-messages-list-queued-owners">
4483 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4487 ARRAY of STRING ListQueuedOwners (in STRING name)
4494 <entry>Argument</entry>
4496 <entry>Description</entry>
4502 <entry>STRING</entry>
4503 <entry>The well-known bus name to query, such as
4504 <literal>com.example.cappuccino</literal></entry>
4514 <entry>Argument</entry>
4516 <entry>Description</entry>
4522 <entry>ARRAY of STRING</entry>
4523 <entry>The unique bus names of connections currently queued
4524 for the name</entry>
4531 This method call should be sent to
4532 <literal>org.freedesktop.DBus</literal> and lists the connections
4533 currently queued for a bus name (see
4534 <xref linkend="term-queued-owner"/>).
4539 <sect2 id="message-bus-routing">
4540 <title>Message Bus Message Routing</title>
4543 Messages may have a <literal>DESTINATION</literal> field (see <xref
4544 linkend="message-protocol-header-fields"/>), resulting in a
4545 <firstterm>unicast message</firstterm>. If the
4546 <literal>DESTINATION</literal> field is present, it specifies a message
4547 recipient by name. Method calls and replies normally specify this field.
4548 The message bus must send messages (of any type) with the
4549 <literal>DESTINATION</literal> field set to the specified recipient,
4550 regardless of whether the recipient has set up a match rule matching
4555 When the message bus receives a signal, if the
4556 <literal>DESTINATION</literal> field is absent, it is considered to
4557 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4558 applications with <firstterm>message matching rules</firstterm> that
4559 match the message. Most signal messages are broadcasts, and
4560 no other message types currently defined in this specification
4565 Unicast signal messages (those with a <literal>DESTINATION</literal>
4566 field) are not commonly used, but they are treated like any unicast
4567 message: they are delivered to the specified receipient,
4568 regardless of its match rules. One use for unicast signals is to
4569 avoid a race condition in which a signal is emitted before the intended
4570 recipient can call <xref linkend="bus-messages-add-match"/> to
4571 receive that signal: if the signal is sent directly to that recipient
4572 using a unicast message, it does not need to add a match rule at all,
4573 and there is no race condition. Another use for unicast signals,
4574 on message buses whose security policy prevents eavesdropping, is to
4575 send sensitive information which should only be visible to one
4580 When the message bus receives a method call, if the
4581 <literal>DESTINATION</literal> field is absent, the call is taken to be
4582 a standard one-to-one message and interpreted by the message bus
4583 itself. For example, sending an
4584 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4585 <literal>DESTINATION</literal> will cause the message bus itself to
4586 reply to the ping immediately; the message bus will not make this
4587 message visible to other applications.
4591 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4592 the ping message were sent with a <literal>DESTINATION</literal> name of
4593 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4594 forwarded, and the Yoyodyne Corporation screensaver application would be
4595 expected to reply to the ping.
4599 Message bus implementations may impose a security policy which
4600 prevents certain messages from being sent or received.
4601 When a method call message cannot be sent or received due to a security
4602 policy, the message bus should send an error reply, unless the
4603 original message had the <literal>NO_REPLY</literal> flag.
4606 <sect3 id="message-bus-routing-eavesdropping">
4607 <title>Eavesdropping</title>
4609 Receiving a unicast message whose <literal>DESTINATION</literal>
4610 indicates a different recipient is called
4611 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4612 a security boundary (like the standard system bus), the security
4613 policy should usually prevent eavesdropping, since unicast messages
4614 are normally kept private and may contain security-sensitive
4619 Eavesdropping is mainly useful for debugging tools, such as
4620 the <literal>dbus-monitor</literal> tool in the reference
4621 implementation of D-Bus. Tools which eavesdrop on the message bus
4622 should be careful to avoid sending a reply or error in response to
4623 messages intended for a different client.
4627 Clients may attempt to eavesdrop by adding match rules
4628 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4629 the <literal>eavesdrop='true'</literal> match. If the message bus'
4630 security policy does not allow eavesdropping, the match rule can
4631 still be added, but will not have any practical effect. For
4632 compatibility with older message bus implementations, if adding such
4633 a match rule results in an error reply, the client may fall back to
4634 adding the same rule with the <literal>eavesdrop</literal> match
4639 <sect3 id="message-bus-routing-match-rules">
4640 <title>Match Rules</title>
4642 An important part of the message bus routing protocol is match
4643 rules. Match rules describe the messages that should be sent to a
4644 client, based on the contents of the message. Broadcast signals
4645 are only sent to clients which have a suitable match rule: this
4646 avoids waking up client processes to deal with signals that are
4647 not relevant to that client.
4650 Messages that list a client as their <literal>DESTINATION</literal>
4651 do not need to match the client's match rules, and are sent to that
4652 client regardless. As a result, match rules are mainly used to
4653 receive a subset of broadcast signals.
4656 Match rules can also be used for eavesdropping
4657 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4658 if the security policy of the message bus allows it.
4661 Match rules are added using the AddMatch bus method
4662 (see <xref linkend="bus-messages-add-match"/>). Rules are
4663 specified as a string of comma separated key/value pairs.
4664 Excluding a key from the rule indicates a wildcard match.
4665 For instance excluding the the member from a match rule but
4666 adding a sender would let all messages from that sender through.
4667 An example of a complete rule would be
4668 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4671 Within single quotes (ASCII apostrophe, U+0027), a backslash
4672 (U+005C) represents itself, and an apostrophe ends the quoted
4673 section. Outside single quotes, \' (backslash, apostrophe)
4674 represents an apostrophe, and any backslash not followed by
4675 an apostrophe represents itself. For instance, the match rules
4676 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4677 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4678 both match messages where the arguments are a 1-character string
4679 containing an apostrophe, a 1-character string containing a
4680 backslash, a 1-character string containing a comma, and a
4681 2-character string containing two backslashes<footnote>
4683 This idiosyncratic quoting style is based on the rules for
4684 escaping items to appear inside single-quoted strings
4685 in POSIX <literal>/bin/sh</literal>, but please
4686 note that backslashes that are not inside single quotes have
4687 different behaviour. This syntax does not offer any way to
4688 represent an apostrophe inside single quotes (it is necessary
4689 to leave the single-quoted section, backslash-escape the
4690 apostrophe and re-enter single quotes), or to represent a
4691 comma outside single quotes (it is necessary to wrap it in
4692 a single-quoted section).
4697 The following table describes the keys that can be used to create
4704 <entry>Possible Values</entry>
4705 <entry>Description</entry>
4710 <entry><literal>type</literal></entry>
4711 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4712 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4715 <entry><literal>sender</literal></entry>
4716 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4717 and <xref linkend="term-unique-name"/> respectively)
4719 <entry>Match messages sent by a particular sender. An example of a sender match
4720 is sender='org.freedesktop.Hal'</entry>
4723 <entry><literal>interface</literal></entry>
4724 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4725 <entry>Match messages sent over or to a particular interface. An example of an
4726 interface match is interface='org.freedesktop.Hal.Manager'.
4727 If a message omits the interface header, it must not match any rule
4728 that specifies this key.</entry>
4731 <entry><literal>member</literal></entry>
4732 <entry>Any valid method or signal name</entry>
4733 <entry>Matches messages which have the give method or signal name. An example of
4734 a member match is member='NameOwnerChanged'</entry>
4737 <entry><literal>path</literal></entry>
4738 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4739 <entry>Matches messages which are sent from or to the given object. An example of a
4740 path match is path='/org/freedesktop/Hal/Manager'</entry>
4743 <entry><literal>path_namespace</literal></entry>
4744 <entry>An object path</entry>
4747 Matches messages which are sent from or to an
4748 object for which the object path is either the
4749 given value, or that value followed by one or
4750 more path components.
4755 <literal>path_namespace='/com/example/foo'</literal>
4756 would match signals sent by
4757 <literal>/com/example/foo</literal>
4759 <literal>/com/example/foo/bar</literal>,
4761 <literal>/com/example/foobar</literal>.
4765 Using both <literal>path</literal> and
4766 <literal>path_namespace</literal> in the same match
4767 rule is not allowed.
4772 This match key was added in version 0.16 of the
4773 D-Bus specification and implemented by the bus
4774 daemon in dbus 1.5.0 and later.
4780 <entry><literal>destination</literal></entry>
4781 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4782 <entry>Matches messages which are being sent to the given unique name. An
4783 example of a destination match is destination=':1.0'</entry>
4786 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4787 <entry>Any string</entry>
4788 <entry>Arg matches are special and are used for further restricting the
4789 match based on the arguments in the body of a message. Only arguments of type
4790 STRING can be matched in this way. An example of an argument match
4791 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4795 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4796 <entry>Any string</entry>
4798 <para>Argument path matches provide a specialised form of wildcard matching for
4799 path-like namespaces. They can match arguments whose type is either STRING or
4800 OBJECT_PATH. As with normal argument matches,
4801 if the argument is exactly equal to the string given in the match
4802 rule then the rule is satisfied. Additionally, there is also a
4803 match when either the string given in the match rule or the
4804 appropriate message argument ends with '/' and is a prefix of the
4805 other. An example argument path match is arg0path='/aa/bb/'. This
4806 would match messages with first arguments of '/', '/aa/',
4807 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4808 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4810 <para>This is intended for monitoring “directories” in file system-like
4811 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4812 system. An application interested in all nodes in a particular hierarchy would
4813 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4814 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4815 represent a modification to the “bar” property, or a signal with zeroth
4816 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4817 many properties within that directory, and the interested application would be
4818 notified in both cases.</para>
4821 This match key was added in version 0.12 of the
4822 D-Bus specification, implemented for STRING
4823 arguments by the bus daemon in dbus 1.2.0 and later,
4824 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4831 <entry><literal>arg0namespace</literal></entry>
4832 <entry>Like a bus name, except that the string is not
4833 required to contain a '.' (period)</entry>
4835 <para>Match messages whose first argument is of type STRING, and is a bus name
4836 or interface name within the specified namespace. This is primarily intended
4837 for watching name owner changes for a group of related bus names, rather than
4838 for a single name or all name changes.</para>
4840 <para>Because every valid interface name is also a valid
4841 bus name, this can also be used for messages whose
4842 first argument is an interface name.</para>
4844 <para>For example, the match rule
4845 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4846 matches name owner changes for bus names such as
4847 <literal>com.example.backend.foo</literal>,
4848 <literal>com.example.backend.foo.bar</literal>, and
4849 <literal>com.example.backend</literal> itself.</para>
4851 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4854 This match key was added in version 0.16 of the
4855 D-Bus specification and implemented by the bus
4856 daemon in dbus 1.5.0 and later.
4862 <entry><literal>eavesdrop</literal></entry>
4863 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4864 <entry>Since D-Bus 1.5.6, match rules do not
4865 match messages which have a <literal>DESTINATION</literal>
4866 field unless the match rule specifically
4868 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4869 by specifying <literal>eavesdrop='true'</literal>
4870 in the match rule. <literal>eavesdrop='false'</literal>
4871 restores the default behaviour. Messages are
4872 delivered to their <literal>DESTINATION</literal>
4873 regardless of match rules, so this match does not
4874 affect normal delivery of unicast messages.
4875 If the message bus has a security policy which forbids
4876 eavesdropping, this match may still be used without error,
4877 but will not have any practical effect.
4878 In older versions of D-Bus, this match was not allowed
4879 in match rules, and all match rules behaved as if
4880 <literal>eavesdrop='true'</literal> had been used.
4889 <sect2 id="message-bus-starting-services">
4890 <title>Message Bus Starting Services</title>
4892 The message bus can start applications on behalf of other applications.
4893 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4894 An application that can be started in this way is called a
4895 <firstterm>service</firstterm>.
4898 With D-Bus, starting a service is normally done by name. That is,
4899 applications ask the message bus to start some program that will own a
4900 well-known name, such as <literal>com.example.TextEditor</literal>.
4901 This implies a contract documented along with the name
4902 <literal>com.example.TextEditor</literal> for which object
4903 the owner of that name will provide, and what interfaces those
4907 To find an executable corresponding to a particular name, the bus daemon
4908 looks for <firstterm>service description files</firstterm>. Service
4909 description files define a mapping from names to executables. Different
4910 kinds of message bus will look for these files in different places, see
4911 <xref linkend="message-bus-types"/>.
4914 Service description files have the ".service" file
4915 extension. The message bus will only load service description files
4916 ending with .service; all other files will be ignored. The file format
4917 is similar to that of <ulink
4918 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4919 entries</ulink>. All service description files must be in UTF-8
4920 encoding. To ensure that there will be no name collisions, service files
4921 must be namespaced using the same mechanism as messages and service
4926 On the well-known system bus, the name of a service description file
4927 must be its well-known name plus <literal>.service</literal>,
4929 <literal>com.example.ConfigurationDatabase.service</literal>.
4933 On the well-known session bus, services should follow the same
4934 service description file naming convention as on the system bus,
4935 but for backwards compatibility they are not required to do so.
4939 [FIXME the file format should be much better specified than "similar to
4940 .desktop entries" esp. since desktop entries are already
4941 badly-specified. ;-)]
4942 These sections from the specification apply to service files as well:
4945 <listitem><para>General syntax</para></listitem>
4946 <listitem><para>Comment format</para></listitem>
4949 Service description files must contain a
4950 <literal>D-BUS Service</literal> group with at least the keys
4951 <literal>Name</literal> (the well-known name of the service)
4952 and <literal>Exec</literal> (the command to be executed).
4955 <title>Example service description file</title>
4957 # Sample service description file
4959 Name=com.example.ConfigurationDatabase
4960 Exec=/usr/bin/sample-configd
4966 Additionally, service description files for the well-known system
4967 bus on Unix must contain a <literal>User</literal> key, whose value
4968 is the name of a user account (e.g. <literal>root</literal>).
4969 The system service will be run as that user.
4973 When an application asks to start a service by name, the bus daemon tries to
4974 find a service that will own that name. It then tries to spawn the
4975 executable associated with it. If this fails, it will report an
4980 On the well-known system bus, it is not possible for two .service files
4981 in the same directory to offer the same service, because they are
4982 constrained to have names that match the service name.
4986 On the well-known session bus, if two .service files in the same
4987 directory offer the same service name, the result is undefined.
4988 Distributors should avoid this situation, for instance by naming
4989 session services' .service files according to their service name.
4993 If two .service files in different directories offer the same
4994 service name, the one in the higher-priority directory is used:
4995 for instance, on the system bus, .service files in
4996 /usr/local/share/dbus-1/system-services take precedence over those
4997 in /usr/share/dbus-1/system-services.
5000 The executable launched will have the environment variable
5001 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
5002 message bus so it can connect and request the appropriate names.
5005 The executable being launched may want to know whether the message bus
5006 starting it is one of the well-known message buses (see <xref
5007 linkend="message-bus-types"/>). To facilitate this, the bus must also set
5008 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
5009 of the well-known buses. The currently-defined values for this variable
5010 are <literal>system</literal> for the systemwide message bus,
5011 and <literal>session</literal> for the per-login-session message
5012 bus. The new executable must still connect to the address given
5013 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
5014 resulting connection is to the well-known bus.
5017 [FIXME there should be a timeout somewhere, either specified
5018 in the .service file, by the client, or just a global value
5019 and if the client being activated fails to connect within that
5020 timeout, an error should be sent back.]
5023 <sect3 id="message-bus-starting-services-scope">
5024 <title>Message Bus Service Scope</title>
5026 The "scope" of a service is its "per-", such as per-session,
5027 per-machine, per-home-directory, or per-display. The reference
5028 implementation doesn't yet support starting services in a different
5029 scope from the message bus itself. So e.g. if you start a service
5030 on the session bus its scope is per-session.
5033 We could add an optional scope to a bus name. For example, for
5034 per-(display,session pair), we could have a unique ID for each display
5035 generated automatically at login and set on screen 0 by executing a
5036 special "set display ID" binary. The ID would be stored in a
5037 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
5038 random bytes. This ID would then be used to scope names.
5039 Starting/locating a service could be done by ID-name pair rather than
5043 Contrast this with a per-display scope. To achieve that, we would
5044 want a single bus spanning all sessions using a given display.
5045 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
5046 property on screen 0 of the display, pointing to this bus.
5051 <sect2 id="message-bus-types">
5052 <title>Well-known Message Bus Instances</title>
5054 Two standard message bus instances are defined here, along with how
5055 to locate them and where their service files live.
5057 <sect3 id="message-bus-types-login">
5058 <title>Login session message bus</title>
5060 Each time a user logs in, a <firstterm>login session message
5061 bus</firstterm> may be started. All applications in the user's login
5062 session may interact with one another using this message bus.
5065 The address of the login session message bus is given
5066 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5067 variable. If that variable is not set, applications may
5068 also try to read the address from the X Window System root
5069 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5070 The root window property must have type <literal>STRING</literal>.
5071 The environment variable should have precedence over the
5072 root window property.
5074 <para>The address of the login session message bus is given in the
5075 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5076 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5077 "autolaunch:", the system should use platform-specific methods of
5078 locating a running D-Bus session server, or starting one if a running
5079 instance cannot be found. Note that this mechanism is not recommended
5080 for attempting to determine if a daemon is running. It is inherently
5081 racy to attempt to make this determination, since the bus daemon may
5082 be started just before or just after the determination is made.
5083 Therefore, it is recommended that applications do not try to make this
5084 determination for their functionality purposes, and instead they
5085 should attempt to start the server.</para>
5087 <sect4 id="message-bus-types-login-x-windows">
5088 <title>X Windowing System</title>
5090 For the X Windowing System, the application must locate the
5091 window owner of the selection represented by the atom formed by
5095 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5099 <para>the current user's username</para>
5103 <para>the literal character '_' (underscore)</para>
5107 <para>the machine's ID</para>
5113 The following properties are defined for the window that owns
5115 <informaltable frame="all">
5124 <para>meaning</para>
5130 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5134 <para>the actual address of the server socket</para>
5140 <para>_DBUS_SESSION_BUS_PID</para>
5144 <para>the PID of the server process</para>
5153 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5154 present in this window.
5158 If the X selection cannot be located or if reading the
5159 properties from the window fails, the implementation MUST conclude
5160 that there is no D-Bus server running and proceed to start a new
5161 server. (See below on concurrency issues)
5165 Failure to connect to the D-Bus server address thus obtained
5166 MUST be treated as a fatal connection error and should be reported
5171 As an alternative, an implementation MAY find the information
5172 in the following file located in the current user's home directory,
5173 in subdirectory .dbus/session-bus/:
5176 <para>the machine's ID</para>
5180 <para>the literal character '-' (dash)</para>
5184 <para>the X display without the screen number, with the
5185 following prefixes removed, if present: ":", "localhost:"
5186 ."localhost.localdomain:". That is, a display of
5187 "localhost:10.0" produces just the number "10"</para>
5193 The contents of this file NAME=value assignment pairs and
5194 lines starting with # are comments (no comments are allowed
5195 otherwise). The following variable names are defined:
5202 <para>Variable</para>
5206 <para>meaning</para>
5212 <para>DBUS_SESSION_BUS_ADDRESS</para>
5216 <para>the actual address of the server socket</para>
5222 <para>DBUS_SESSION_BUS_PID</para>
5226 <para>the PID of the server process</para>
5232 <para>DBUS_SESSION_BUS_WINDOWID</para>
5236 <para>the window ID</para>
5245 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5250 Failure to open this file MUST be interpreted as absence of a
5251 running server. Therefore, the implementation MUST proceed to
5252 attempting to launch a new bus server if the file cannot be
5257 However, success in opening this file MUST NOT lead to the
5258 conclusion that the server is running. Thus, a failure to connect to
5259 the bus address obtained by the alternative method MUST NOT be
5260 considered a fatal error. If the connection cannot be established,
5261 the implementation MUST proceed to check the X selection settings or
5262 to start the server on its own.
5266 If the implementation concludes that the D-Bus server is not
5267 running it MUST attempt to start a new server and it MUST also
5268 ensure that the daemon started as an effect of the "autolaunch"
5269 mechanism provides the lookup mechanisms described above, so
5270 subsequent calls can locate the newly started server. The
5271 implementation MUST also ensure that if two or more concurrent
5272 initiations happen, only one server remains running and all other
5273 initiations are able to obtain the address of this server and
5274 connect to it. In other words, the implementation MUST ensure that
5275 the X selection is not present when it attempts to set it, without
5276 allowing another process to set the selection between the
5277 verification and the setting (e.g., by using XGrabServer /
5284 On Unix systems, the session bus should search for .service files
5285 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5287 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5288 Implementations may also search additional locations, which
5289 should be searched with lower priority than anything in
5290 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5291 for example, the reference implementation also
5292 looks in <literal>${datadir}/dbus-1/services</literal> as
5293 set at compile time.
5296 As described in the XDG Base Directory Specification, software
5297 packages should install their session .service files to their
5298 configured <literal>${datadir}/dbus-1/services</literal>,
5299 where <literal>${datadir}</literal> is as defined by the GNU
5300 coding standards. System administrators or users can arrange
5301 for these service files to be read by setting XDG_DATA_DIRS or by
5302 symlinking them into the default locations.
5306 <sect3 id="message-bus-types-system">
5307 <title>System message bus</title>
5309 A computer may have a <firstterm>system message bus</firstterm>,
5310 accessible to all applications on the system. This message bus may be
5311 used to broadcast system events, such as adding new hardware devices,
5312 changes in the printer queue, and so forth.
5315 The address of the system message bus is given
5316 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5317 variable. If that variable is not set, applications should try
5318 to connect to the well-known address
5319 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5322 The D-Bus reference implementation actually honors the
5323 <literal>$(localstatedir)</literal> configure option
5324 for this address, on both client and server side.
5329 On Unix systems, the system bus should default to searching
5330 for .service files in
5331 <literal>/usr/local/share/dbus-1/system-services</literal>,
5332 <literal>/usr/share/dbus-1/system-services</literal> and
5333 <literal>/lib/dbus-1/system-services</literal>, with that order
5334 of precedence. It may also search other implementation-specific
5335 locations, but should not vary these locations based on environment
5339 The system bus is security-sensitive and is typically executed
5340 by an init system with a clean environment. Its launch helper
5341 process is particularly security-sensitive, and specifically
5342 clears its own environment.
5347 Software packages should install their system .service
5348 files to their configured
5349 <literal>${datadir}/dbus-1/system-services</literal>,
5350 where <literal>${datadir}</literal> is as defined by the GNU
5351 coding standards. System administrators can arrange
5352 for these service files to be read by editing the system bus'
5353 configuration file or by symlinking them into the default
5359 <sect2 id="message-bus-messages">
5360 <title>Message Bus Messages</title>
5362 The special message bus name <literal>org.freedesktop.DBus</literal>
5363 responds to a number of additional messages.
5366 <sect3 id="bus-messages-hello">
5367 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5378 <entry>Argument</entry>
5380 <entry>Description</entry>
5386 <entry>STRING</entry>
5387 <entry>Unique name assigned to the connection</entry>
5394 Before an application is able to send messages to other applications
5395 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5396 to the message bus to obtain a unique name. If an application without
5397 a unique name tries to send a message to another application, or a
5398 message to the message bus itself that isn't the
5399 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5400 disconnected from the bus.
5403 There is no corresponding "disconnect" request; if a client wishes to
5404 disconnect from the bus, it simply closes the socket (or other
5405 communication channel).
5408 <sect3 id="bus-messages-list-names">
5409 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5413 ARRAY of STRING ListNames ()
5420 <entry>Argument</entry>
5422 <entry>Description</entry>
5428 <entry>ARRAY of STRING</entry>
5429 <entry>Array of strings where each string is a bus name</entry>
5436 Returns a list of all currently-owned names on the bus.
5439 <sect3 id="bus-messages-list-activatable-names">
5440 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5444 ARRAY of STRING ListActivatableNames ()
5451 <entry>Argument</entry>
5453 <entry>Description</entry>
5459 <entry>ARRAY of STRING</entry>
5460 <entry>Array of strings where each string is a bus name</entry>
5467 Returns a list of all names that can be activated on the bus.
5470 <sect3 id="bus-messages-name-exists">
5471 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5475 BOOLEAN NameHasOwner (in STRING name)
5482 <entry>Argument</entry>
5484 <entry>Description</entry>
5490 <entry>STRING</entry>
5491 <entry>Name to check</entry>
5501 <entry>Argument</entry>
5503 <entry>Description</entry>
5509 <entry>BOOLEAN</entry>
5510 <entry>Return value, true if the name exists</entry>
5517 Checks if the specified name exists (currently has an owner).
5521 <sect3 id="bus-messages-name-owner-changed">
5522 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5526 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5533 <entry>Argument</entry>
5535 <entry>Description</entry>
5541 <entry>STRING</entry>
5542 <entry>Name with a new owner</entry>
5546 <entry>STRING</entry>
5547 <entry>Old owner or empty string if none</entry>
5551 <entry>STRING</entry>
5552 <entry>New owner or empty string if none</entry>
5559 This signal indicates that the owner of a name has changed.
5560 It's also the signal to use to detect the appearance of
5561 new names on the bus.
5564 <sect3 id="bus-messages-name-lost">
5565 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5569 NameLost (STRING name)
5576 <entry>Argument</entry>
5578 <entry>Description</entry>
5584 <entry>STRING</entry>
5585 <entry>Name which was lost</entry>
5592 This signal is sent to a specific application when it loses
5593 ownership of a name.
5597 <sect3 id="bus-messages-name-acquired">
5598 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5602 NameAcquired (STRING name)
5609 <entry>Argument</entry>
5611 <entry>Description</entry>
5617 <entry>STRING</entry>
5618 <entry>Name which was acquired</entry>
5625 This signal is sent to a specific application when it gains
5626 ownership of a name.
5630 <sect3 id="bus-messages-start-service-by-name">
5631 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5635 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5642 <entry>Argument</entry>
5644 <entry>Description</entry>
5650 <entry>STRING</entry>
5651 <entry>Name of the service to start</entry>
5655 <entry>UINT32</entry>
5656 <entry>Flags (currently not used)</entry>
5666 <entry>Argument</entry>
5668 <entry>Description</entry>
5674 <entry>UINT32</entry>
5675 <entry>Return value</entry>
5680 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5684 The return value can be one of the following values:
5689 <entry>Identifier</entry>
5690 <entry>Value</entry>
5691 <entry>Description</entry>
5696 <entry>DBUS_START_REPLY_SUCCESS</entry>
5698 <entry>The service was successfully started.</entry>
5701 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5703 <entry>A connection already owns the given name.</entry>
5712 <sect3 id="bus-messages-update-activation-environment">
5713 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5717 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5724 <entry>Argument</entry>
5726 <entry>Description</entry>
5732 <entry>ARRAY of DICT<STRING,STRING></entry>
5733 <entry>Environment to add or update</entry>
5738 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5741 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5744 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.
5749 <sect3 id="bus-messages-get-name-owner">
5750 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5754 STRING GetNameOwner (in STRING name)
5761 <entry>Argument</entry>
5763 <entry>Description</entry>
5769 <entry>STRING</entry>
5770 <entry>Name to get the owner of</entry>
5780 <entry>Argument</entry>
5782 <entry>Description</entry>
5788 <entry>STRING</entry>
5789 <entry>Return value, a unique connection name</entry>
5794 Returns the unique connection name of the primary owner of the name
5795 given. If the requested name doesn't have an owner, returns a
5796 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5800 <sect3 id="bus-messages-get-connection-unix-user">
5801 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5805 UINT32 GetConnectionUnixUser (in STRING bus_name)
5812 <entry>Argument</entry>
5814 <entry>Description</entry>
5820 <entry>STRING</entry>
5821 <entry>Unique or well-known bus name of the connection to
5822 query, such as <literal>:12.34</literal> or
5823 <literal>com.example.tea</literal></entry>
5833 <entry>Argument</entry>
5835 <entry>Description</entry>
5841 <entry>UINT32</entry>
5842 <entry>Unix user ID</entry>
5847 Returns the Unix user ID of the process connected to the server. If
5848 unable to determine it (for instance, because the process is not on the
5849 same machine as the bus daemon), an error is returned.
5853 <sect3 id="bus-messages-get-connection-unix-process-id">
5854 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5858 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5865 <entry>Argument</entry>
5867 <entry>Description</entry>
5873 <entry>STRING</entry>
5874 <entry>Unique or well-known bus name of the connection to
5875 query, such as <literal>:12.34</literal> or
5876 <literal>com.example.tea</literal></entry>
5886 <entry>Argument</entry>
5888 <entry>Description</entry>
5894 <entry>UINT32</entry>
5895 <entry>Unix process id</entry>
5900 Returns the Unix process ID of the process connected to the server. If
5901 unable to determine it (for instance, because the process is not on the
5902 same machine as the bus daemon), an error is returned.
5906 <sect3 id="bus-messages-get-connection-credentials">
5907 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5911 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5918 <entry>Argument</entry>
5920 <entry>Description</entry>
5926 <entry>STRING</entry>
5927 <entry>Unique or well-known bus name of the connection to
5928 query, such as <literal>:12.34</literal> or
5929 <literal>com.example.tea</literal></entry>
5939 <entry>Argument</entry>
5941 <entry>Description</entry>
5947 <entry>DICT<STRING,VARIANT></entry>
5948 <entry>Credentials</entry>
5956 Returns as many credentials as possible for the process connected to
5957 the server. If unable to determine certain credentials (for instance,
5958 because the process is not on the same machine as the bus daemon,
5959 or because this version of the bus daemon does not support a
5960 particular security framework), or if the values of those credentials
5961 cannot be represented as documented here, then those credentials
5966 Keys in the returned dictionary not containing "." are defined
5967 by this specification. Bus daemon implementors supporting
5968 credentials frameworks not mentioned in this document should either
5969 contribute patches to this specification, or use keys containing
5970 "." and starting with a reversed domain name.
5976 <entry>Value type</entry>
5977 <entry>Value</entry>
5982 <entry>UnixUserID</entry>
5983 <entry>UINT32</entry>
5984 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5987 <entry>ProcessID</entry>
5988 <entry>UINT32</entry>
5989 <entry>The numeric process ID, on platforms that have
5990 this concept. On Unix, this is the process ID defined by
5999 This method was added in D-Bus 1.7 to reduce the round-trips
6000 required to list a process's credentials. In older versions, calling
6001 this method will fail: applications should recover by using the
6002 separate methods such as
6003 <xref linkend="bus-messages-get-connection-unix-user"/>
6008 <sect3 id="bus-messages-get-adt-audit-session-data">
6009 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
6013 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
6020 <entry>Argument</entry>
6022 <entry>Description</entry>
6028 <entry>STRING</entry>
6029 <entry>Unique or well-known bus name of the connection to
6030 query, such as <literal>:12.34</literal> or
6031 <literal>com.example.tea</literal></entry>
6041 <entry>Argument</entry>
6043 <entry>Description</entry>
6049 <entry>ARRAY of BYTE</entry>
6050 <entry>auditing data as returned by
6051 adt_export_session_data()</entry>
6056 Returns auditing data used by Solaris ADT, in an unspecified
6057 binary format. If you know what this means, please contribute
6058 documentation via the D-Bus bug tracking system.
6059 This method is on the core DBus interface for historical reasons;
6060 the same information should be made available via
6061 <xref linkend="bus-messages-get-connection-credentials"/>
6066 <sect3 id="bus-messages-get-connection-selinux-security-context">
6067 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6071 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6078 <entry>Argument</entry>
6080 <entry>Description</entry>
6086 <entry>STRING</entry>
6087 <entry>Unique or well-known bus name of the connection to
6088 query, such as <literal>:12.34</literal> or
6089 <literal>com.example.tea</literal></entry>
6099 <entry>Argument</entry>
6101 <entry>Description</entry>
6107 <entry>ARRAY of BYTE</entry>
6108 <entry>some sort of string of bytes, not necessarily UTF-8,
6109 not including '\0'</entry>
6114 Returns the security context used by SELinux, in an unspecified
6115 format. If you know what this means, please contribute
6116 documentation via the D-Bus bug tracking system.
6117 This method is on the core DBus interface for historical reasons;
6118 the same information should be made available via
6119 <xref linkend="bus-messages-get-connection-credentials"/>
6125 <sect3 id="bus-messages-add-match">
6126 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6130 AddMatch (in STRING rule)
6137 <entry>Argument</entry>
6139 <entry>Description</entry>
6145 <entry>STRING</entry>
6146 <entry>Match rule to add to the connection</entry>
6151 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6152 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6156 <sect3 id="bus-messages-remove-match">
6157 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6161 RemoveMatch (in STRING rule)
6168 <entry>Argument</entry>
6170 <entry>Description</entry>
6176 <entry>STRING</entry>
6177 <entry>Match rule to remove from the connection</entry>
6182 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6183 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6188 <sect3 id="bus-messages-get-id">
6189 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6193 GetId (out STRING id)
6200 <entry>Argument</entry>
6202 <entry>Description</entry>
6208 <entry>STRING</entry>
6209 <entry>Unique ID identifying the bus daemon</entry>
6214 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6215 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6216 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6217 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6218 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6219 by org.freedesktop.DBus.Peer.GetMachineId().
6220 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6228 <appendix id="implementation-notes">
6229 <title>Implementation notes</title>
6230 <sect1 id="implementation-notes-subsection">
6238 <glossary><title>Glossary</title>
6240 This glossary defines some of the terms used in this specification.
6243 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6246 The message bus maintains an association between names and
6247 connections. (Normally, there's one connection per application.) A
6248 bus name is simply an identifier used to locate connections. For
6249 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6250 name might be used to send a message to a screensaver from Yoyodyne
6251 Corporation. An application is said to <firstterm>own</firstterm> a
6252 name if the message bus has associated the application's connection
6253 with the name. Names may also have <firstterm>queued
6254 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6255 The bus assigns a unique name to each connection,
6256 see <xref linkend="term-unique-name"/>. Other names
6257 can be thought of as "well-known names" and are
6258 used to find applications that offer specific functionality.
6262 See <xref linkend="message-protocol-names-bus"/> for details of
6263 the syntax and naming conventions for bus names.
6268 <glossentry id="term-message"><glossterm>Message</glossterm>
6271 A message is the atomic unit of communication via the D-Bus
6272 protocol. It consists of a <firstterm>header</firstterm> and a
6273 <firstterm>body</firstterm>; the body is made up of
6274 <firstterm>arguments</firstterm>.
6279 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6282 The message bus is a special application that forwards
6283 or routes messages between a group of applications
6284 connected to the message bus. It also manages
6285 <firstterm>names</firstterm> used for routing
6291 <glossentry id="term-name"><glossterm>Name</glossterm>
6294 See <xref linkend="term-bus-name"/>. "Name" may
6295 also be used to refer to some of the other names
6296 in D-Bus, such as interface names.
6301 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6304 Used to prevent collisions when defining new interfaces, bus names
6305 etc. The convention used is the same one Java uses for defining
6306 classes: a reversed domain name.
6307 See <xref linkend="message-protocol-names-bus"/>,
6308 <xref linkend="message-protocol-names-interface"/>,
6309 <xref linkend="message-protocol-names-error"/>,
6310 <xref linkend="message-protocol-marshaling-object-path"/>.
6315 <glossentry id="term-object"><glossterm>Object</glossterm>
6318 Each application contains <firstterm>objects</firstterm>, which have
6319 <firstterm>interfaces</firstterm> and
6320 <firstterm>methods</firstterm>. Objects are referred to by a name,
6321 called a <firstterm>path</firstterm>.
6326 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6329 An application talking directly to another application, without going
6330 through a message bus. One-to-one connections may be "peer to peer" or
6331 "client to server." The D-Bus protocol has no concept of client
6332 vs. server after a connection has authenticated; the flow of messages
6333 is symmetrical (full duplex).
6338 <glossentry id="term-path"><glossterm>Path</glossterm>
6341 Object references (object names) in D-Bus are organized into a
6342 filesystem-style hierarchy, so each object is named by a path. As in
6343 LDAP, there's no difference between "files" and "directories"; a path
6344 can refer to an object, while still having child objects below it.
6349 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6352 Each bus name has a primary owner; messages sent to the name go to the
6353 primary owner. However, certain names also maintain a queue of
6354 secondary owners "waiting in the wings." If the primary owner releases
6355 the name, then the first secondary owner in the queue automatically
6356 becomes the new owner of the name.
6361 <glossentry id="term-service"><glossterm>Service</glossterm>
6364 A service is an executable that can be launched by the bus daemon.
6365 Services normally guarantee some particular features, for example they
6366 may guarantee that they will request a specific name such as
6367 "com.example.Screensaver", have a singleton object
6368 "/com/example/Application", and that object will implement the
6369 interface "com.example.Screensaver.Control".
6374 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6377 ".service files" tell the bus about service applications that can be
6378 launched (see <xref linkend="term-service"/>). Most importantly they
6379 provide a mapping from bus names to services that will request those
6380 names when they start up.
6385 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6388 The special name automatically assigned to each connection by the
6389 message bus. This name will never change owner, and will be unique
6390 (never reused during the lifetime of the message bus).
6391 It will begin with a ':' character.