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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.
1407 <sect3 id="message-protocol-header-fields">
1408 <title>Header Fields</title>
1411 The array at the end of the header contains <firstterm>header
1412 fields</firstterm>, where each field is a 1-byte field code followed
1413 by a field value. A header must contain the required header fields for
1414 its message type, and zero or more of any optional header
1415 fields. Future versions of this protocol specification may add new
1416 fields. Implementations must ignore fields they do not
1417 understand. Implementations must not invent their own header fields;
1418 only changes to this specification may introduce new header fields.
1422 Again, if an implementation sees a header field code that it does not
1423 expect, it must ignore that field, as it will be part of a new
1424 (but compatible) version of this specification. This also applies
1425 to known header fields appearing in unexpected messages, for
1426 example: if a signal has a reply serial it must be ignored
1427 even though it has no meaning as of this version of the spec.
1431 However, implementations must not send or accept known header fields
1432 with the wrong type stored in the field value. So for example a
1433 message with an <literal>INTERFACE</literal> field of type
1434 <literal>UINT32</literal> would be considered corrupt.
1438 Here are the currently-defined header fields:
1443 <entry>Conventional Name</entry>
1444 <entry>Decimal Code</entry>
1446 <entry>Required In</entry>
1447 <entry>Description</entry>
1452 <entry><literal>INVALID</literal></entry>
1455 <entry>not allowed</entry>
1456 <entry>Not a valid field name (error if it appears in a message)</entry>
1459 <entry><literal>PATH</literal></entry>
1461 <entry><literal>OBJECT_PATH</literal></entry>
1462 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1463 <entry>The object to send a call to,
1464 or the object a signal is emitted from.
1466 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1467 implementations should not send messages with this path,
1468 and the reference implementation of the bus daemon will
1469 disconnect any application that attempts to do so.
1473 <entry><literal>INTERFACE</literal></entry>
1475 <entry><literal>STRING</literal></entry>
1476 <entry><literal>SIGNAL</literal></entry>
1478 The interface to invoke a method call on, or
1479 that a signal is emitted from. Optional for
1480 method calls, required for signals.
1481 The special interface
1482 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1483 implementations should not send messages with this
1484 interface, and the reference implementation of the bus
1485 daemon will disconnect any application that attempts to
1490 <entry><literal>MEMBER</literal></entry>
1492 <entry><literal>STRING</literal></entry>
1493 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1494 <entry>The member, either the method name or signal name.</entry>
1497 <entry><literal>ERROR_NAME</literal></entry>
1499 <entry><literal>STRING</literal></entry>
1500 <entry><literal>ERROR</literal></entry>
1501 <entry>The name of the error that occurred, for errors</entry>
1504 <entry><literal>REPLY_SERIAL</literal></entry>
1506 <entry><literal>UINT32</literal></entry>
1507 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1508 <entry>The serial number of the message this message is a reply
1509 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1512 <entry><literal>DESTINATION</literal></entry>
1514 <entry><literal>STRING</literal></entry>
1515 <entry>optional</entry>
1516 <entry>The name of the connection this message is intended for.
1517 Only used in combination with the message bus, see
1518 <xref linkend="message-bus"/>.</entry>
1521 <entry><literal>SENDER</literal></entry>
1523 <entry><literal>STRING</literal></entry>
1524 <entry>optional</entry>
1525 <entry>Unique name of the sending connection.
1526 The message bus fills in this field so it is reliable; the field is
1527 only meaningful in combination with the message bus.</entry>
1530 <entry><literal>SIGNATURE</literal></entry>
1532 <entry><literal>SIGNATURE</literal></entry>
1533 <entry>optional</entry>
1534 <entry>The signature of the message body.
1535 If omitted, it is assumed to be the
1536 empty signature "" (i.e. the body must be 0-length).</entry>
1539 <entry><literal>UNIX_FDS</literal></entry>
1541 <entry><literal>UINT32</literal></entry>
1542 <entry>optional</entry>
1543 <entry>The number of Unix file descriptors that
1544 accompany the message. If omitted, it is assumed
1545 that no Unix file descriptors accompany the
1546 message. The actual file descriptors need to be
1547 transferred via platform specific mechanism
1548 out-of-band. They must be sent at the same time as
1549 part of the message itself. They may not be sent
1550 before the first byte of the message itself is
1551 transferred or after the last byte of the message
1561 <sect2 id="message-protocol-names">
1562 <title>Valid Names</title>
1564 The various names in D-Bus messages have some restrictions.
1567 There is a <firstterm>maximum name length</firstterm>
1568 of 255 which applies to bus names, interfaces, and members.
1570 <sect3 id="message-protocol-names-interface">
1571 <title>Interface names</title>
1573 Interfaces have names with type <literal>STRING</literal>, meaning that
1574 they must be valid UTF-8. However, there are also some
1575 additional restrictions that apply to interface names
1578 <listitem><para>Interface names are composed of 1 or more elements separated by
1579 a period ('.') character. All elements must contain at least
1583 <listitem><para>Each element must only contain the ASCII characters
1584 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1588 <listitem><para>Interface names must contain at least one '.' (period)
1589 character (and thus at least two elements).
1592 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1593 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1598 Interface names should start with the reversed DNS domain name of
1599 the author of the interface (in lower-case), like interface names
1600 in Java. It is conventional for the rest of the interface name
1601 to consist of words run together, with initial capital letters
1602 on all words ("CamelCase"). Several levels of hierarchy can be used.
1603 It is also a good idea to include the major version of the interface
1604 in the name, and increment it if incompatible changes are made;
1605 this way, a single object can implement several versions of an
1606 interface in parallel, if necessary.
1610 For instance, if the owner of <literal>example.com</literal> is
1611 developing a D-Bus API for a music player, they might define
1612 interfaces called <literal>com.example.MusicPlayer1</literal>,
1613 <literal>com.example.MusicPlayer1.Track</literal> and
1614 <literal>com.example.MusicPlayer1.Seekable</literal>.
1618 D-Bus does not distinguish between the concepts that would be
1619 called classes and interfaces in Java: either can be identified on
1620 D-Bus by an interface name.
1623 <sect3 id="message-protocol-names-bus">
1624 <title>Bus names</title>
1626 Connections have one or more bus names associated with them.
1627 A connection has exactly one bus name that is a <firstterm>unique
1628 connection name</firstterm>. The unique connection name remains
1629 with the connection for its entire lifetime.
1630 A bus name is of type <literal>STRING</literal>,
1631 meaning that it must be valid UTF-8. However, there are also
1632 some additional restrictions that apply to bus names
1635 <listitem><para>Bus names that start with a colon (':')
1636 character are unique connection names. Other bus names
1637 are called <firstterm>well-known bus names</firstterm>.
1640 <listitem><para>Bus names are composed of 1 or more elements separated by
1641 a period ('.') character. All elements must contain at least
1645 <listitem><para>Each element must only contain the ASCII characters
1646 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1647 connection name may begin with a digit, elements in
1648 other bus names must not begin with a digit.
1652 <listitem><para>Bus names must contain at least one '.' (period)
1653 character (and thus at least two elements).
1656 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1657 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1661 Note that the hyphen ('-') character is allowed in bus names but
1662 not in interface names.
1666 Like <link linkend="message-protocol-names-interface">interface
1667 names</link>, well-known bus names should start with the
1668 reversed DNS domain name of the author of the interface (in
1669 lower-case), and it is conventional for the rest of the well-known
1670 bus name to consist of words run together, with initial
1671 capital letters. As with interface names, including a version
1672 number in well-known bus names is a good idea; it's possible to
1673 have the well-known bus name for more than one version
1674 simultaneously if backwards compatibility is required.
1678 If a well-known bus name implies the presence of a "main" interface,
1679 that "main" interface is often given the same name as
1680 the well-known bus name, and situated at the corresponding object
1681 path. For instance, if the owner of <literal>example.com</literal>
1682 is developing a D-Bus API for a music player, they might define
1683 that any application that takes the well-known name
1684 <literal>com.example.MusicPlayer1</literal> should have an object
1685 at the object path <literal>/com/example/MusicPlayer1</literal>
1686 which implements the interface
1687 <literal>com.example.MusicPlayer1</literal>.
1690 <sect3 id="message-protocol-names-member">
1691 <title>Member names</title>
1693 Member (i.e. method or signal) names:
1695 <listitem><para>Must only contain the ASCII characters
1696 "[A-Z][a-z][0-9]_" and may not begin with a
1697 digit.</para></listitem>
1698 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1699 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1700 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1705 It is conventional for member names on D-Bus to consist of
1706 capitalized words with no punctuation ("camel-case").
1707 Method names should usually be verbs, such as
1708 <literal>GetItems</literal>, and signal names should usually be
1709 a description of an event, such as <literal>ItemsChanged</literal>.
1712 <sect3 id="message-protocol-names-error">
1713 <title>Error names</title>
1715 Error names have the same restrictions as interface names.
1719 Error names have the same naming conventions as interface
1720 names, and often contain <literal>.Error.</literal>; for instance,
1721 the owner of <literal>example.com</literal> might define the
1722 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1723 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1724 The errors defined by D-Bus itself, such as
1725 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1731 <sect2 id="message-protocol-types">
1732 <title>Message Types</title>
1734 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1735 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1736 This section describes these conventions.
1738 <sect3 id="message-protocol-types-method">
1739 <title>Method Calls</title>
1741 Some messages invoke an operation on a remote object. These are
1742 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1743 messages map naturally to methods on objects in a typical program.
1746 A method call message is required to have a <literal>MEMBER</literal> header field
1747 indicating the name of the method. Optionally, the message has an
1748 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1749 Including the <literal>INTERFACE</literal> in all method call
1750 messages is strongly recommended.
1753 In the absence of an <literal>INTERFACE</literal> field, if two
1754 or more interfaces on the same object have a method with the same
1755 name, it is undefined which of those methods will be invoked.
1756 Implementations may choose to either return an error, or deliver the
1757 message as though it had an arbitrary one of those interfaces.
1760 In some situations (such as the well-known system bus), messages
1761 are filtered through an access-control list external to the
1762 remote object implementation. If that filter rejects certain
1763 messages by matching their interface, or accepts only messages
1764 to specific interfaces, it must also reject messages that have no
1765 <literal>INTERFACE</literal>: otherwise, malicious
1766 applications could use this to bypass the filter.
1769 Method call messages also include a <literal>PATH</literal> field
1770 indicating the object to invoke the method on. If the call is passing
1771 through a message bus, the message will also have a
1772 <literal>DESTINATION</literal> field giving the name of the connection
1773 to receive the message.
1776 When an application handles a method call message, it is required to
1777 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1778 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1779 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1782 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1783 are the return value(s) or "out parameters" of the method call.
1784 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1785 and the call fails; no return value will be provided. It makes
1786 no sense to send multiple replies to the same method call.
1789 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1790 reply is required, so the caller will know the method
1791 was successfully processed.
1794 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1798 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1799 then as an optimization the application receiving the method
1800 call may choose to omit the reply message (regardless of
1801 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1802 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1803 flag and reply anyway.
1806 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1807 destination name does not exist then a program to own the destination
1808 name will be started before the message is delivered. The message
1809 will be held until the new program is successfully started or has
1810 failed to start; in case of failure, an error will be returned. This
1811 flag is only relevant in the context of a message bus, it is ignored
1812 during one-to-one communication with no intermediate bus.
1814 <sect4 id="message-protocol-types-method-apis">
1815 <title>Mapping method calls to native APIs</title>
1817 APIs for D-Bus may map method calls to a method call in a specific
1818 programming language, such as C++, or may map a method call written
1819 in an IDL to a D-Bus message.
1822 In APIs of this nature, arguments to a method are often termed "in"
1823 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1824 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1825 "inout" arguments, which are both sent and received, i.e. the caller
1826 passes in a value which is modified. Mapped to D-Bus, an "inout"
1827 argument is equivalent to an "in" argument, followed by an "out"
1828 argument. You can't pass things "by reference" over the wire, so
1829 "inout" is purely an illusion of the in-process API.
1832 Given a method with zero or one return values, followed by zero or more
1833 arguments, where each argument may be "in", "out", or "inout", the
1834 caller constructs a message by appending each "in" or "inout" argument,
1835 in order. "out" arguments are not represented in the caller's message.
1838 The recipient constructs a reply by appending first the return value
1839 if any, then each "out" or "inout" argument, in order.
1840 "in" arguments are not represented in the reply message.
1843 Error replies are normally mapped to exceptions in languages that have
1847 In converting from native APIs to D-Bus, it is perhaps nice to
1848 map D-Bus naming conventions ("FooBar") to native conventions
1849 such as "fooBar" or "foo_bar" automatically. This is OK
1850 as long as you can say that the native API is one that
1851 was specifically written for D-Bus. It makes the most sense
1852 when writing object implementations that will be exported
1853 over the bus. Object proxies used to invoke remote D-Bus
1854 objects probably need the ability to call any D-Bus method,
1855 and thus a magic name mapping like this could be a problem.
1858 This specification doesn't require anything of native API bindings;
1859 the preceding is only a suggested convention for consistency
1865 <sect3 id="message-protocol-types-signal">
1866 <title>Signal Emission</title>
1868 Unlike method calls, signal emissions have no replies.
1869 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1870 It must have three header fields: <literal>PATH</literal> giving the object
1871 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1872 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1873 for signals, though it is optional for method calls.
1877 <sect3 id="message-protocol-types-errors">
1878 <title>Errors</title>
1880 Messages of type <literal>ERROR</literal> are most commonly replies
1881 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1882 to any kind of message. The message bus for example
1883 will return an <literal>ERROR</literal> in reply to a signal emission if
1884 the bus does not have enough memory to send the signal.
1887 An <literal>ERROR</literal> may have any arguments, but if the first
1888 argument is a <literal>STRING</literal>, it must be an error message.
1889 The error message may be logged or shown to the user
1894 <sect3 id="message-protocol-types-notation">
1895 <title>Notation in this document</title>
1897 This document uses a simple pseudo-IDL to describe particular method
1898 calls and signals. Here is an example of a method call:
1900 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1901 out UINT32 resultcode)
1903 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1904 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1905 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1906 characters so it's known that the last part of the name in
1907 the "IDL" is the member name.
1910 In C++ that might end up looking like this:
1912 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1913 unsigned int flags);
1915 or equally valid, the return value could be done as an argument:
1917 void org::freedesktop::DBus::StartServiceByName (const char *name,
1919 unsigned int *resultcode);
1921 It's really up to the API designer how they want to make
1922 this look. You could design an API where the namespace wasn't used
1923 in C++, using STL or Qt, using varargs, or whatever you wanted.
1926 Signals are written as follows:
1928 org.freedesktop.DBus.NameLost (STRING name)
1930 Signals don't specify "in" vs. "out" because only
1931 a single direction is possible.
1934 It isn't especially encouraged to use this lame pseudo-IDL in actual
1935 API implementations; you might use the native notation for the
1936 language you're using, or you might use COM or CORBA IDL, for example.
1941 <sect2 id="message-protocol-handling-invalid">
1942 <title>Invalid Protocol and Spec Extensions</title>
1945 For security reasons, the D-Bus protocol should be strictly parsed and
1946 validated, with the exception of defined extension points. Any invalid
1947 protocol or spec violations should result in immediately dropping the
1948 connection without notice to the other end. Exceptions should be
1949 carefully considered, e.g. an exception may be warranted for a
1950 well-understood idiosyncrasy of a widely-deployed implementation. In
1951 cases where the other end of a connection is 100% trusted and known to
1952 be friendly, skipping validation for performance reasons could also make
1953 sense in certain cases.
1957 Generally speaking violations of the "must" requirements in this spec
1958 should be considered possible attempts to exploit security, and violations
1959 of the "should" suggestions should be considered legitimate (though perhaps
1960 they should generate an error in some cases).
1964 The following extension points are built in to D-Bus on purpose and must
1965 not be treated as invalid protocol. The extension points are intended
1966 for use by future versions of this spec, they are not intended for third
1967 parties. At the moment, the only way a third party could extend D-Bus
1968 without breaking interoperability would be to introduce a way to negotiate new
1969 feature support as part of the auth protocol, using EXTENSION_-prefixed
1970 commands. There is not yet a standard way to negotiate features.
1974 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1975 commands result in an ERROR rather than a disconnect. This enables
1976 future extensions to the protocol. Commands starting with EXTENSION_ are
1977 reserved for third parties.
1982 The authentication protocol supports pluggable auth mechanisms.
1987 The address format (see <xref linkend="addresses"/>) supports new
1993 Messages with an unknown type (something other than
1994 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1995 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1996 Unknown-type messages must still be well-formed in the same way
1997 as the known messages, however. They still have the normal
2003 Header fields with an unknown or unexpected field code must be ignored,
2004 though again they must still be well-formed.
2009 New standard interfaces (with new methods and signals) can of course be added.
2019 <sect1 id="auth-protocol">
2020 <title>Authentication Protocol</title>
2022 Before the flow of messages begins, two applications must
2023 authenticate. A simple plain-text protocol is used for
2024 authentication; this protocol is a SASL profile, and maps fairly
2025 directly from the SASL specification. The message encoding is
2026 NOT used here, only plain text messages.
2029 In examples, "C:" and "S:" indicate lines sent by the client and
2030 server respectively.
2032 <sect2 id="auth-protocol-overview">
2033 <title>Protocol Overview</title>
2035 The protocol is a line-based protocol, where each line ends with
2036 \r\n. Each line begins with an all-caps ASCII command name containing
2037 only the character range [A-Z_], a space, then any arguments for the
2038 command, then the \r\n ending the line. The protocol is
2039 case-sensitive. All bytes must be in the ASCII character set.
2041 Commands from the client to the server are as follows:
2044 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2045 <listitem><para>CANCEL</para></listitem>
2046 <listitem><para>BEGIN</para></listitem>
2047 <listitem><para>DATA <data in hex encoding></para></listitem>
2048 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2049 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2052 From server to client are as follows:
2055 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2056 <listitem><para>OK <GUID in hex></para></listitem>
2057 <listitem><para>DATA <data in hex encoding></para></listitem>
2058 <listitem><para>ERROR</para></listitem>
2059 <listitem><para>AGREE_UNIX_FD</para></listitem>
2063 Unofficial extensions to the command set must begin with the letters
2064 "EXTENSION_", to avoid conflicts with future official commands.
2065 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2068 <sect2 id="auth-nul-byte">
2069 <title>Special credentials-passing nul byte</title>
2071 Immediately after connecting to the server, the client must send a
2072 single nul byte. This byte may be accompanied by credentials
2073 information on some operating systems that use sendmsg() with
2074 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2075 sockets. However, the nul byte must be sent even on other kinds of
2076 socket, and even on operating systems that do not require a byte to be
2077 sent in order to transmit credentials. The text protocol described in
2078 this document begins after the single nul byte. If the first byte
2079 received from the client is not a nul byte, the server may disconnect
2083 A nul byte in any context other than the initial byte is an error;
2084 the protocol is ASCII-only.
2087 The credentials sent along with the nul byte may be used with the
2088 SASL mechanism EXTERNAL.
2091 <sect2 id="auth-command-auth">
2092 <title>AUTH command</title>
2094 If an AUTH command has no arguments, it is a request to list
2095 available mechanisms. The server must respond with a REJECTED
2096 command listing the mechanisms it understands, or with an error.
2099 If an AUTH command specifies a mechanism, and the server supports
2100 said mechanism, the server should begin exchanging SASL
2101 challenge-response data with the client using DATA commands.
2104 If the server does not support the mechanism given in the AUTH
2105 command, it must send either a REJECTED command listing the mechanisms
2106 it does support, or an error.
2109 If the [initial-response] argument is provided, it is intended for use
2110 with mechanisms that have no initial challenge (or an empty initial
2111 challenge), as if it were the argument to an initial DATA command. If
2112 the selected mechanism has an initial challenge and [initial-response]
2113 was provided, the server should reject authentication by sending
2117 If authentication succeeds after exchanging DATA commands,
2118 an OK command must be sent to the client.
2121 The first octet received by the server after the \r\n of the BEGIN
2122 command from the client must be the first octet of the
2123 authenticated/encrypted stream of D-Bus messages.
2126 If BEGIN is received by the server, the first octet received
2127 by the client after the \r\n of the OK command must be the
2128 first octet of the authenticated/encrypted stream of D-Bus
2132 <sect2 id="auth-command-cancel">
2133 <title>CANCEL Command</title>
2135 At any time up to sending the BEGIN command, the client may send a
2136 CANCEL command. On receiving the CANCEL command, the server must
2137 send a REJECTED command and abort the current authentication
2141 <sect2 id="auth-command-data">
2142 <title>DATA Command</title>
2144 The DATA command may come from either client or server, and simply
2145 contains a hex-encoded block of data to be interpreted
2146 according to the SASL mechanism in use.
2149 Some SASL mechanisms support sending an "empty string";
2150 FIXME we need some way to do this.
2153 <sect2 id="auth-command-begin">
2154 <title>BEGIN Command</title>
2156 The BEGIN command acknowledges that the client has received an
2157 OK command from the server, and that the stream of messages
2161 The first octet received by the server after the \r\n of the BEGIN
2162 command from the client must be the first octet of the
2163 authenticated/encrypted stream of D-Bus messages.
2166 <sect2 id="auth-command-rejected">
2167 <title>REJECTED Command</title>
2169 The REJECTED command indicates that the current authentication
2170 exchange has failed, and further exchange of DATA is inappropriate.
2171 The client would normally try another mechanism, or try providing
2172 different responses to challenges.
2174 Optionally, the REJECTED command has a space-separated list of
2175 available auth mechanisms as arguments. If a server ever provides
2176 a list of supported mechanisms, it must provide the same list
2177 each time it sends a REJECTED message. Clients are free to
2178 ignore all lists received after the first.
2181 <sect2 id="auth-command-ok">
2182 <title>OK Command</title>
2184 The OK command indicates that the client has been
2185 authenticated. The client may now proceed with negotiating
2186 Unix file descriptor passing. To do that it shall send
2187 NEGOTIATE_UNIX_FD to the server.
2190 Otherwise, the client must respond to the OK command by
2191 sending a BEGIN command, followed by its stream of messages,
2192 or by disconnecting. The server must not accept additional
2193 commands using this protocol after the BEGIN command has been
2194 received. Further communication will be a stream of D-Bus
2195 messages (optionally encrypted, as negotiated) rather than
2199 If a client sends BEGIN the first octet received by the client
2200 after the \r\n of the OK command must be the first octet of
2201 the authenticated/encrypted stream of D-Bus messages.
2204 The OK command has one argument, which is the GUID of the server.
2205 See <xref linkend="addresses"/> for more on server GUIDs.
2208 <sect2 id="auth-command-error">
2209 <title>ERROR Command</title>
2211 The ERROR command indicates that either server or client did not
2212 know a command, does not accept the given command in the current
2213 context, or did not understand the arguments to the command. This
2214 allows the protocol to be extended; a client or server can send a
2215 command present or permitted only in new protocol versions, and if
2216 an ERROR is received instead of an appropriate response, fall back
2217 to using some other technique.
2220 If an ERROR is sent, the server or client that sent the
2221 error must continue as if the command causing the ERROR had never been
2222 received. However, the the server or client receiving the error
2223 should try something other than whatever caused the error;
2224 if only canceling/rejecting the authentication.
2227 If the D-Bus protocol changes incompatibly at some future time,
2228 applications implementing the new protocol would probably be able to
2229 check for support of the new protocol by sending a new command and
2230 receiving an ERROR from applications that don't understand it. Thus the
2231 ERROR feature of the auth protocol is an escape hatch that lets us
2232 negotiate extensions or changes to the D-Bus protocol in the future.
2235 <sect2 id="auth-command-negotiate-unix-fd">
2236 <title>NEGOTIATE_UNIX_FD Command</title>
2238 The NEGOTIATE_UNIX_FD command indicates that the client
2239 supports Unix file descriptor passing. This command may only
2240 be sent after the connection is authenticated, i.e. after OK
2241 was received by the client. This command may only be sent on
2242 transports that support Unix file descriptor passing.
2245 On receiving NEGOTIATE_UNIX_FD the server must respond with
2246 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2247 the transport chosen supports Unix file descriptor passing and
2248 the server supports this feature. It shall respond the latter
2249 if the transport does not support Unix file descriptor
2250 passing, the server does not support this feature, or the
2251 server decides not to enable file descriptor passing due to
2252 security or other reasons.
2255 <sect2 id="auth-command-agree-unix-fd">
2256 <title>AGREE_UNIX_FD Command</title>
2258 The AGREE_UNIX_FD command indicates that the server supports
2259 Unix file descriptor passing. This command may only be sent
2260 after the connection is authenticated, and the client sent
2261 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2262 command may only be sent on transports that support Unix file
2266 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2267 followed by its stream of messages, or by disconnecting. The
2268 server must not accept additional commands using this protocol
2269 after the BEGIN command has been received. Further
2270 communication will be a stream of D-Bus messages (optionally
2271 encrypted, as negotiated) rather than this protocol.
2274 <sect2 id="auth-command-future">
2275 <title>Future Extensions</title>
2277 Future extensions to the authentication and negotiation
2278 protocol are possible. For that new commands may be
2279 introduced. If a client or server receives an unknown command
2280 it shall respond with ERROR and not consider this fatal. New
2281 commands may be introduced both before, and after
2282 authentication, i.e. both before and after the OK command.
2285 <sect2 id="auth-examples">
2286 <title>Authentication examples</title>
2290 <title>Example of successful magic cookie authentication</title>
2292 (MAGIC_COOKIE is a made up mechanism)
2294 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2300 <title>Example of finding out mechanisms then picking one</title>
2303 S: REJECTED KERBEROS_V4 SKEY
2304 C: AUTH SKEY 7ab83f32ee
2305 S: DATA 8799cabb2ea93e
2306 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2312 <title>Example of client sends unknown command then falls back to regular auth</title>
2316 C: AUTH MAGIC_COOKIE 3736343435313230333039
2322 <title>Example of server doesn't support initial auth mechanism</title>
2324 C: AUTH MAGIC_COOKIE 3736343435313230333039
2325 S: REJECTED KERBEROS_V4 SKEY
2326 C: AUTH SKEY 7ab83f32ee
2327 S: DATA 8799cabb2ea93e
2328 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2334 <title>Example of wrong password or the like followed by successful retry</title>
2336 C: AUTH MAGIC_COOKIE 3736343435313230333039
2337 S: REJECTED KERBEROS_V4 SKEY
2338 C: AUTH SKEY 7ab83f32ee
2339 S: DATA 8799cabb2ea93e
2340 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2342 C: AUTH SKEY 7ab83f32ee
2343 S: DATA 8799cabb2ea93e
2344 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2350 <title>Example of skey cancelled and restarted</title>
2352 C: AUTH MAGIC_COOKIE 3736343435313230333039
2353 S: REJECTED KERBEROS_V4 SKEY
2354 C: AUTH SKEY 7ab83f32ee
2355 S: DATA 8799cabb2ea93e
2358 C: AUTH SKEY 7ab83f32ee
2359 S: DATA 8799cabb2ea93e
2360 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2366 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2368 (MAGIC_COOKIE is a made up mechanism)
2370 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2372 C: NEGOTIATE_UNIX_FD
2378 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2380 (MAGIC_COOKIE is a made up mechanism)
2382 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2384 C: NEGOTIATE_UNIX_FD
2391 <sect2 id="auth-states">
2392 <title>Authentication state diagrams</title>
2395 This section documents the auth protocol in terms of
2396 a state machine for the client and the server. This is
2397 probably the most robust way to implement the protocol.
2400 <sect3 id="auth-states-client">
2401 <title>Client states</title>
2404 To more precisely describe the interaction between the
2405 protocol state machine and the authentication mechanisms the
2406 following notation is used: MECH(CHALL) means that the
2407 server challenge CHALL was fed to the mechanism MECH, which
2413 CONTINUE(RESP) means continue the auth conversation
2414 and send RESP as the response to the server;
2420 OK(RESP) means that after sending RESP to the server
2421 the client side of the auth conversation is finished
2422 and the server should return "OK";
2428 ERROR means that CHALL was invalid and could not be
2434 Both RESP and CHALL may be empty.
2438 The Client starts by getting an initial response from the
2439 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2440 the mechanism did not provide an initial response. If the
2441 mechanism returns CONTINUE, the client starts in state
2442 <emphasis>WaitingForData</emphasis>, if the mechanism
2443 returns OK the client starts in state
2444 <emphasis>WaitingForOK</emphasis>.
2448 The client should keep track of available mechanisms and
2449 which it mechanisms it has already attempted. This list is
2450 used to decide which AUTH command to send. When the list is
2451 exhausted, the client should give up and close the
2456 <title><emphasis>WaitingForData</emphasis></title>
2464 MECH(CHALL) returns CONTINUE(RESP) → send
2466 <emphasis>WaitingForData</emphasis>
2470 MECH(CHALL) returns OK(RESP) → send DATA
2471 RESP, goto <emphasis>WaitingForOK</emphasis>
2475 MECH(CHALL) returns ERROR → send ERROR
2476 [msg], goto <emphasis>WaitingForData</emphasis>
2484 Receive REJECTED [mechs] →
2485 send AUTH [next mech], goto
2486 WaitingForData or <emphasis>WaitingForOK</emphasis>
2491 Receive ERROR → send
2493 <emphasis>WaitingForReject</emphasis>
2498 Receive OK → send
2499 BEGIN, terminate auth
2500 conversation, authenticated
2505 Receive anything else → send
2507 <emphasis>WaitingForData</emphasis>
2515 <title><emphasis>WaitingForOK</emphasis></title>
2520 Receive OK → send BEGIN, terminate auth
2521 conversation, <emphasis>authenticated</emphasis>
2526 Receive REJECTED [mechs] → send AUTH [next mech],
2527 goto <emphasis>WaitingForData</emphasis> or
2528 <emphasis>WaitingForOK</emphasis>
2534 Receive DATA → send CANCEL, goto
2535 <emphasis>WaitingForReject</emphasis>
2541 Receive ERROR → send CANCEL, goto
2542 <emphasis>WaitingForReject</emphasis>
2548 Receive anything else → send ERROR, goto
2549 <emphasis>WaitingForOK</emphasis>
2557 <title><emphasis>WaitingForReject</emphasis></title>
2562 Receive REJECTED [mechs] → send AUTH [next mech],
2563 goto <emphasis>WaitingForData</emphasis> or
2564 <emphasis>WaitingForOK</emphasis>
2570 Receive anything else → terminate auth
2571 conversation, disconnect
2580 <sect3 id="auth-states-server">
2581 <title>Server states</title>
2584 For the server MECH(RESP) means that the client response
2585 RESP was fed to the the mechanism MECH, which returns one of
2590 CONTINUE(CHALL) means continue the auth conversation and
2591 send CHALL as the challenge to the client;
2597 OK means that the client has been successfully
2604 REJECTED means that the client failed to authenticate or
2605 there was an error in RESP.
2610 The server starts out in state
2611 <emphasis>WaitingForAuth</emphasis>. If the client is
2612 rejected too many times the server must disconnect the
2617 <title><emphasis>WaitingForAuth</emphasis></title>
2623 Receive AUTH → send REJECTED [mechs], goto
2624 <emphasis>WaitingForAuth</emphasis>
2630 Receive AUTH MECH RESP
2634 MECH not valid mechanism → send REJECTED
2636 <emphasis>WaitingForAuth</emphasis>
2640 MECH(RESP) returns CONTINUE(CHALL) → send
2642 <emphasis>WaitingForData</emphasis>
2646 MECH(RESP) returns OK → send OK, goto
2647 <emphasis>WaitingForBegin</emphasis>
2651 MECH(RESP) returns REJECTED → send REJECTED
2653 <emphasis>WaitingForAuth</emphasis>
2661 Receive BEGIN → terminate
2662 auth conversation, disconnect
2668 Receive ERROR → send REJECTED [mechs], goto
2669 <emphasis>WaitingForAuth</emphasis>
2675 Receive anything else → send
2677 <emphasis>WaitingForAuth</emphasis>
2686 <title><emphasis>WaitingForData</emphasis></title>
2694 MECH(RESP) returns CONTINUE(CHALL) → send
2696 <emphasis>WaitingForData</emphasis>
2700 MECH(RESP) returns OK → send OK, goto
2701 <emphasis>WaitingForBegin</emphasis>
2705 MECH(RESP) returns REJECTED → send REJECTED
2707 <emphasis>WaitingForAuth</emphasis>
2715 Receive BEGIN → terminate auth conversation,
2722 Receive CANCEL → send REJECTED [mechs], goto
2723 <emphasis>WaitingForAuth</emphasis>
2729 Receive ERROR → send REJECTED [mechs], goto
2730 <emphasis>WaitingForAuth</emphasis>
2736 Receive anything else → send ERROR, goto
2737 <emphasis>WaitingForData</emphasis>
2745 <title><emphasis>WaitingForBegin</emphasis></title>
2750 Receive BEGIN → terminate auth conversation,
2751 client authenticated
2757 Receive CANCEL → send REJECTED [mechs], goto
2758 <emphasis>WaitingForAuth</emphasis>
2764 Receive ERROR → send REJECTED [mechs], goto
2765 <emphasis>WaitingForAuth</emphasis>
2771 Receive anything else → send ERROR, goto
2772 <emphasis>WaitingForBegin</emphasis>
2782 <sect2 id="auth-mechanisms">
2783 <title>Authentication mechanisms</title>
2785 This section describes some new authentication mechanisms.
2786 D-Bus also allows any standard SASL mechanism of course.
2788 <sect3 id="auth-mechanisms-sha">
2789 <title>DBUS_COOKIE_SHA1</title>
2791 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2792 has the ability to read a private file owned by the user being
2793 authenticated. If the client can prove that it has access to a secret
2794 cookie stored in this file, then the client is authenticated.
2795 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2799 Throughout this description, "hex encoding" must output the digits
2800 from a to f in lower-case; the digits A to F must not be used
2801 in the DBUS_COOKIE_SHA1 mechanism.
2804 Authentication proceeds as follows:
2808 The client sends the username it would like to authenticate
2814 The server sends the name of its "cookie context" (see below); a
2815 space character; the integer ID of the secret cookie the client
2816 must demonstrate knowledge of; a space character; then a
2817 randomly-generated challenge string, all of this hex-encoded into
2823 The client locates the cookie and generates its own
2824 randomly-generated challenge string. The client then concatenates
2825 the server's decoded challenge, a ":" character, its own challenge,
2826 another ":" character, and the cookie. It computes the SHA-1 hash
2827 of this composite string as a hex digest. It concatenates the
2828 client's challenge string, a space character, and the SHA-1 hex
2829 digest, hex-encodes the result and sends it back to the server.
2834 The server generates the same concatenated string used by the
2835 client and computes its SHA-1 hash. It compares the hash with
2836 the hash received from the client; if the two hashes match, the
2837 client is authenticated.
2843 Each server has a "cookie context," which is a name that identifies a
2844 set of cookies that apply to that server. A sample context might be
2845 "org_freedesktop_session_bus". Context names must be valid ASCII,
2846 nonzero length, and may not contain the characters slash ("/"),
2847 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2848 tab ("\t"), or period ("."). There is a default context,
2849 "org_freedesktop_general" that's used by servers that do not specify
2853 Cookies are stored in a user's home directory, in the directory
2854 <filename>~/.dbus-keyrings/</filename>. This directory must
2855 not be readable or writable by other users. If it is,
2856 clients and servers must ignore it. The directory
2857 contains cookie files named after the cookie context.
2860 A cookie file contains one cookie per line. Each line
2861 has three space-separated fields:
2865 The cookie ID number, which must be a non-negative integer and
2866 may not be used twice in the same file.
2871 The cookie's creation time, in UNIX seconds-since-the-epoch
2877 The cookie itself, a hex-encoded random block of bytes. The cookie
2878 may be of any length, though obviously security increases
2879 as the length increases.
2885 Only server processes modify the cookie file.
2886 They must do so with this procedure:
2890 Create a lockfile name by appending ".lock" to the name of the
2891 cookie file. The server should attempt to create this file
2892 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2893 fails, the lock fails. Servers should retry for a reasonable
2894 period of time, then they may choose to delete an existing lock
2895 to keep users from having to manually delete a stale
2896 lock. <footnote><para>Lockfiles are used instead of real file
2897 locking <literal>fcntl()</literal> because real locking
2898 implementations are still flaky on network
2899 filesystems.</para></footnote>
2904 Once the lockfile has been created, the server loads the cookie
2905 file. It should then delete any cookies that are old (the
2906 timeout can be fairly short), or more than a reasonable
2907 time in the future (so that cookies never accidentally
2908 become permanent, if the clock was set far into the future
2909 at some point). If no recent keys remain, the
2910 server may generate a new key.
2915 The pruned and possibly added-to cookie file
2916 must be resaved atomically (using a temporary
2917 file which is rename()'d).
2922 The lock must be dropped by deleting the lockfile.
2928 Clients need not lock the file in order to load it,
2929 because servers are required to save the file atomically.
2934 <sect1 id="addresses">
2935 <title>Server Addresses</title>
2937 Server addresses consist of a transport name followed by a colon, and
2938 then an optional, comma-separated list of keys and values in the form key=value.
2939 Each value is escaped.
2943 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2944 Which is the address to a unix socket with the path /tmp/dbus-test.
2947 Value escaping is similar to URI escaping but simpler.
2951 The set of optionally-escaped bytes is:
2952 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2953 <emphasis>byte</emphasis> (note, not character) which is not in the
2954 set of optionally-escaped bytes must be replaced with an ASCII
2955 percent (<literal>%</literal>) and the value of the byte in hex.
2956 The hex value must always be two digits, even if the first digit is
2957 zero. The optionally-escaped bytes may be escaped if desired.
2962 To unescape, append each byte in the value; if a byte is an ASCII
2963 percent (<literal>%</literal>) character then append the following
2964 hex value instead. It is an error if a <literal>%</literal> byte
2965 does not have two hex digits following. It is an error if a
2966 non-optionally-escaped byte is seen unescaped.
2970 The set of optionally-escaped bytes is intended to preserve address
2971 readability and convenience.
2975 A server may specify a key-value pair with the key <literal>guid</literal>
2976 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2977 describes the format of the <literal>guid</literal> field. If present,
2978 this UUID may be used to distinguish one server address from another. A
2979 server should use a different UUID for each address it listens on. For
2980 example, if a message bus daemon offers both UNIX domain socket and TCP
2981 connections, but treats clients the same regardless of how they connect,
2982 those two connections are equivalent post-connection but should have
2983 distinct UUIDs to distinguish the kinds of connection.
2987 The intent of the address UUID feature is to allow a client to avoid
2988 opening multiple identical connections to the same server, by allowing the
2989 client to check whether an address corresponds to an already-existing
2990 connection. Comparing two addresses is insufficient, because addresses
2991 can be recycled by distinct servers, and equivalent addresses may look
2992 different if simply compared as strings (for example, the host in a TCP
2993 address can be given as an IP address or as a hostname).
2997 Note that the address key is <literal>guid</literal> even though the
2998 rest of the API and documentation says "UUID," for historical reasons.
3002 [FIXME clarify if attempting to connect to each is a requirement
3003 or just a suggestion]
3004 When connecting to a server, multiple server addresses can be
3005 separated by a semi-colon. The library will then try to connect
3006 to the first address and if that fails, it'll try to connect to
3007 the next one specified, and so forth. For example
3008 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3012 Some addresses are <firstterm>connectable</firstterm>. A connectable
3013 address is one containing enough information for a client to connect
3014 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3015 is a connectable address. It is not necessarily possible to listen
3016 on every connectable address: for instance, it is not possible to
3017 listen on a <literal>unixexec:</literal> address.
3021 Some addresses are <firstterm>listenable</firstterm>. A listenable
3022 address is one containing enough information for a server to listen on
3023 it, producing a connectable address (which may differ from the
3024 original address). Many listenable addresses are not connectable:
3025 for instance, <literal>tcp:host=127.0.0.1</literal>
3026 is listenable, but not connectable (because it does not specify
3031 Listening on an address that is not connectable will result in a
3032 connectable address that is not the same as the listenable address.
3033 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3034 might result in the connectable address
3035 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3036 or listening on <literal>unix:tmpdir=/tmp</literal>
3037 might result in the connectable address
3038 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3042 <sect1 id="transports">
3043 <title>Transports</title>
3045 [FIXME we need to specify in detail each transport and its possible arguments]
3047 Current transports include: unix domain sockets (including
3048 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3049 using in-process pipes. Future possible transports include one that
3050 tunnels over X11 protocol.
3053 <sect2 id="transports-unix-domain-sockets">
3054 <title>Unix Domain Sockets</title>
3056 Unix domain sockets can be either paths in the file system or on Linux
3057 kernels, they can be abstract which are similar to paths but
3058 do not show up in the file system.
3062 When a socket is opened by the D-Bus library it truncates the path
3063 name right before the first trailing Nul byte. This is true for both
3064 normal paths and abstract paths. Note that this is a departure from
3065 previous versions of D-Bus that would create sockets with a fixed
3066 length path name. Names which were shorter than the fixed length
3067 would be padded by Nul bytes.
3070 Unix domain sockets are not available on Windows.
3073 Unix addresses that specify <literal>path</literal> or
3074 <literal>abstract</literal> are both listenable and connectable.
3075 Unix addresses that specify <literal>tmpdir</literal> are only
3076 listenable: the corresponding connectable address will specify
3077 either <literal>path</literal> or <literal>abstract</literal>.
3079 <sect3 id="transports-unix-domain-sockets-addresses">
3080 <title>Server Address Format</title>
3082 Unix domain socket addresses are identified by the "unix:" prefix
3083 and support the following key/value pairs:
3090 <entry>Values</entry>
3091 <entry>Description</entry>
3097 <entry>(path)</entry>
3098 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3101 <entry>tmpdir</entry>
3102 <entry>(path)</entry>
3103 <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>
3106 <entry>abstract</entry>
3107 <entry>(string)</entry>
3108 <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>
3114 Exactly one of the keys <literal>path</literal>,
3115 <literal>abstract</literal> or
3116 <literal>tmpdir</literal> must be provided.
3120 <sect2 id="transports-launchd">
3121 <title>launchd</title>
3123 launchd is an open-source server management system that replaces init, inetd
3124 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3125 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3129 launchd allocates a socket and provides it with the unix path through the
3130 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3131 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3132 it through its environment.
3133 Other processes can query for the launchd socket by executing:
3134 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3135 This is normally done by the D-Bus client library so doesn't have to be done
3139 launchd is not available on Microsoft Windows.
3142 launchd addresses are listenable and connectable.
3144 <sect3 id="transports-launchd-addresses">
3145 <title>Server Address Format</title>
3147 launchd addresses are identified by the "launchd:" prefix
3148 and support the following key/value pairs:
3155 <entry>Values</entry>
3156 <entry>Description</entry>
3162 <entry>(environment variable)</entry>
3163 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3169 The <literal>env</literal> key is required.
3173 <sect2 id="transports-systemd">
3174 <title>systemd</title>
3176 systemd is an open-source server management system that
3177 replaces init and inetd on newer Linux systems. It supports
3178 socket activation. The D-Bus systemd transport is used to acquire
3179 socket activation file descriptors from systemd and use them
3180 as D-Bus transport when the current process is spawned by
3181 socket activation from it.
3184 The systemd transport accepts only one or more Unix domain or
3185 TCP streams sockets passed in via socket activation.
3188 The systemd transport is not available on non-Linux operating systems.
3191 The systemd transport defines no parameter keys.
3194 systemd addresses are listenable, but not connectable. The
3195 corresponding connectable address is the <literal>unix</literal>
3196 or <literal>tcp</literal> address of the socket.
3199 <sect2 id="transports-tcp-sockets">
3200 <title>TCP Sockets</title>
3202 The tcp transport provides TCP/IP based connections between clients
3203 located on the same or different hosts.
3206 Using tcp transport without any additional secure authentification mechanismus
3207 over a network is unsecure.
3210 On Windows and most Unix platforms, the TCP stack is unable to transfer
3211 credentials over a TCP connection, so the EXTERNAL authentication
3212 mechanism does not work for this transport.
3215 All <literal>tcp</literal> addresses are listenable.
3216 <literal>tcp</literal> addresses in which both
3217 <literal>host</literal> and <literal>port</literal> are
3218 specified, and <literal>port</literal> is non-zero,
3219 are also connectable.
3221 <sect3 id="transports-tcp-sockets-addresses">
3222 <title>Server Address Format</title>
3224 TCP/IP socket addresses are identified by the "tcp:" prefix
3225 and support the following key/value pairs:
3232 <entry>Values</entry>
3233 <entry>Description</entry>
3239 <entry>(string)</entry>
3240 <entry>DNS name or IP address</entry>
3244 <entry>(string)</entry>
3245 <entry>Used in a listenable address to configure the interface
3246 on which the server will listen: either the IP address of one of
3247 the local machine's interfaces (most commonly <literal>127.0.0.1
3248 </literal>), or a DNS name that resolves to one of those IP
3249 addresses, or '*' to listen on all interfaces simultaneously.
3250 If not specified, the default is the same value as "host".
3255 <entry>(number)</entry>
3256 <entry>The tcp port the server will open. A zero value let the server
3257 choose a free port provided from the underlaying operating system.
3258 libdbus is able to retrieve the real used port from the server.
3262 <entry>family</entry>
3263 <entry>(string)</entry>
3264 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3271 <sect2 id="transports-nonce-tcp-sockets">
3272 <title>Nonce-secured TCP Sockets</title>
3274 The nonce-tcp transport provides a secured TCP transport, using a
3275 simple authentication mechanism to ensure that only clients with read
3276 access to a certain location in the filesystem can connect to the server.
3277 The server writes a secret, the nonce, to a file and an incoming client
3278 connection is only accepted if the client sends the nonce right after
3279 the connect. The nonce mechanism requires no setup and is orthogonal to
3280 the higher-level authentication mechanisms described in the
3281 Authentication section.
3285 On start, the server generates a random 16 byte nonce and writes it
3286 to a file in the user's temporary directory. The nonce file location
3287 is published as part of the server's D-Bus address using the
3288 "noncefile" key-value pair.
3290 After an accept, the server reads 16 bytes from the socket. If the
3291 read bytes do not match the nonce stored in the nonce file, the
3292 server MUST immediately drop the connection.
3293 If the nonce match the received byte sequence, the client is accepted
3294 and the transport behaves like an unsecured tcp transport.
3297 After a successful connect to the server socket, the client MUST read
3298 the nonce from the file published by the server via the noncefile=
3299 key-value pair and send it over the socket. After that, the
3300 transport behaves like an unsecured tcp transport.
3303 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3304 <literal>host</literal>, <literal>port</literal> and
3305 <literal>noncefile</literal> are all specified,
3306 and <literal>port</literal> is nonzero, are also connectable.
3308 <sect3 id="transports-nonce-tcp-sockets-addresses">
3309 <title>Server Address Format</title>
3311 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3312 and support the following key/value pairs:
3319 <entry>Values</entry>
3320 <entry>Description</entry>
3326 <entry>(string)</entry>
3327 <entry>DNS name or IP address</entry>
3331 <entry>(string)</entry>
3332 <entry>The same as for tcp: addresses
3337 <entry>(number)</entry>
3338 <entry>The tcp port the server will open. A zero value let the server
3339 choose a free port provided from the underlaying operating system.
3340 libdbus is able to retrieve the real used port from the server.
3344 <entry>family</entry>
3345 <entry>(string)</entry>
3346 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3349 <entry>noncefile</entry>
3350 <entry>(path)</entry>
3351 <entry>File location containing the secret.
3352 This is only meaningful in connectable addresses:
3353 a listening D-Bus server that offers this transport
3354 will always create a new nonce file.</entry>
3361 <sect2 id="transports-exec">
3362 <title>Executed Subprocesses on Unix</title>
3364 This transport forks off a process and connects its standard
3365 input and standard output with an anonymous Unix domain
3366 socket. This socket is then used for communication by the
3367 transport. This transport may be used to use out-of-process
3368 forwarder programs as basis for the D-Bus protocol.
3371 The forked process will inherit the standard error output and
3372 process group from the parent process.
3375 Executed subprocesses are not available on Windows.
3378 <literal>unixexec</literal> addresses are connectable, but are not
3381 <sect3 id="transports-exec-addresses">
3382 <title>Server Address Format</title>
3384 Executed subprocess addresses are identified by the "unixexec:" prefix
3385 and support the following key/value pairs:
3392 <entry>Values</entry>
3393 <entry>Description</entry>
3399 <entry>(path)</entry>
3400 <entry>Path of the binary to execute, either an absolute
3401 path or a binary name that is searched for in the default
3402 search path of the OS. This corresponds to the first
3403 argument of execlp(). This key is mandatory.</entry>
3406 <entry>argv0</entry>
3407 <entry>(string)</entry>
3408 <entry>The program name to use when executing the
3409 binary. If omitted the same value as specified for path=
3410 will be used. This corresponds to the second argument of
3414 <entry>argv1, argv2, ...</entry>
3415 <entry>(string)</entry>
3416 <entry>Arguments to pass to the binary. This corresponds
3417 to the third and later arguments of execlp(). If a
3418 specific argvX is not specified no further argvY for Y > X
3419 are taken into account.</entry>
3427 <sect1 id="meta-transports">
3428 <title>Meta Transports</title>
3430 Meta transports are a kind of transport with special enhancements or
3431 behavior. Currently available meta transports include: autolaunch
3434 <sect2 id="meta-transports-autolaunch">
3435 <title>Autolaunch</title>
3436 <para>The autolaunch transport provides a way for dbus clients to autodetect
3437 a running dbus session bus and to autolaunch a session bus if not present.
3440 On Unix, <literal>autolaunch</literal> addresses are connectable,
3444 On Windows, <literal>autolaunch</literal> addresses are both
3445 connectable and listenable.
3448 <sect3 id="meta-transports-autolaunch-addresses">
3449 <title>Server Address Format</title>
3451 Autolaunch addresses uses the "autolaunch:" prefix and support the
3452 following key/value pairs:
3459 <entry>Values</entry>
3460 <entry>Description</entry>
3465 <entry>scope</entry>
3466 <entry>(string)</entry>
3467 <entry>scope of autolaunch (Windows only)
3471 "*install-path" - limit session bus to dbus installation path.
3472 The dbus installation path is determined from the location of
3473 the shared dbus library. If the library is located in a 'bin'
3474 subdirectory the installation root is the directory above,
3475 otherwise the directory where the library lives is taken as
3478 <install-root>/bin/[lib]dbus-1.dll
3479 <install-root>/[lib]dbus-1.dll
3485 "*user" - limit session bus to the recent user.
3490 other values - specify dedicated session bus like "release",
3502 <sect3 id="meta-transports-autolaunch-windows-implementation">
3503 <title>Windows implementation</title>
3505 On start, the server opens a platform specific transport, creates a mutex
3506 and a shared memory section containing the related session bus address.
3507 This mutex will be inspected by the dbus client library to detect a
3508 running dbus session bus. The access to the mutex and the shared memory
3509 section are protected by global locks.
3512 In the recent implementation the autolaunch transport uses a tcp transport
3513 on localhost with a port choosen from the operating system. This detail may
3514 change in the future.
3517 Disclaimer: The recent implementation is in an early state and may not
3518 work in all cirumstances and/or may have security issues. Because of this
3519 the implementation is not documentated yet.
3526 <title>UUIDs</title>
3528 A working D-Bus implementation uses universally-unique IDs in two places.
3529 First, each server address has a UUID identifying the address,
3530 as described in <xref linkend="addresses"/>. Second, each operating
3531 system kernel instance running a D-Bus client or server has a UUID
3532 identifying that kernel, retrieved by invoking the method
3533 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3534 linkend="standard-interfaces-peer"/>).
3537 The term "UUID" in this document is intended literally, i.e. an
3538 identifier that is universally unique. It is not intended to refer to
3539 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3542 The UUID must contain 128 bits of data and be hex-encoded. The
3543 hex-encoded string may not contain hyphens or other non-hex-digit
3544 characters, and it must be exactly 32 characters long. To generate a
3545 UUID, the current reference implementation concatenates 96 bits of random
3546 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3550 It would also be acceptable and probably better to simply generate 128
3551 bits of random data, as long as the random number generator is of high
3552 quality. The timestamp could conceivably help if the random bits are not
3553 very random. With a quality random number generator, collisions are
3554 extremely unlikely even with only 96 bits, so it's somewhat academic.
3557 Implementations should, however, stick to random data for the first 96 bits
3562 <sect1 id="standard-interfaces">
3563 <title>Standard Interfaces</title>
3565 See <xref linkend="message-protocol-types-notation"/> for details on
3566 the notation used in this section. There are some standard interfaces
3567 that may be useful across various D-Bus applications.
3569 <sect2 id="standard-interfaces-peer">
3570 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3572 The <literal>org.freedesktop.DBus.Peer</literal> interface
3575 org.freedesktop.DBus.Peer.Ping ()
3576 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3580 On receipt of the <literal>METHOD_CALL</literal> message
3581 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3582 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3583 usual. It does not matter which object path a ping is sent to. The
3584 reference implementation handles this method automatically.
3587 On receipt of the <literal>METHOD_CALL</literal> message
3588 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3589 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3590 UUID representing the identity of the machine the process is running on.
3591 This UUID must be the same for all processes on a single system at least
3592 until that system next reboots. It should be the same across reboots
3593 if possible, but this is not always possible to implement and is not
3595 It does not matter which object path a GetMachineId is sent to. The
3596 reference implementation handles this method automatically.
3599 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3600 a virtual machine running on a hypervisor, rather than a physical machine.
3601 Basically if two processes see the same UUID, they should also see the same
3602 shared memory, UNIX domain sockets, process IDs, and other features that require
3603 a running OS kernel in common between the processes.
3606 The UUID is often used where other programs might use a hostname. Hostnames
3607 can change without rebooting, however, or just be "localhost" - so the UUID
3611 <xref linkend="uuids"/> explains the format of the UUID.
3615 <sect2 id="standard-interfaces-introspectable">
3616 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3618 This interface has one method:
3620 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3624 Objects instances may implement
3625 <literal>Introspect</literal> which returns an XML description of
3626 the object, including its interfaces (with signals and methods), objects
3627 below it in the object path tree, and its properties.
3630 <xref linkend="introspection-format"/> describes the format of this XML string.
3633 <sect2 id="standard-interfaces-properties">
3634 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3636 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3637 or <firstterm>attributes</firstterm>. These can be exposed via the
3638 <literal>org.freedesktop.DBus.Properties</literal> interface.
3642 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3643 in STRING property_name,
3645 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3646 in STRING property_name,
3648 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3649 out DICT<STRING,VARIANT> props);
3653 It is conventional to give D-Bus properties names consisting of
3654 capitalized words without punctuation ("CamelCase"), like
3655 <link linkend="message-protocol-names-member">member names</link>.
3656 For instance, the GObject property
3657 <literal>connection-status</literal> or the Qt property
3658 <literal>connectionStatus</literal> could be represented on D-Bus
3659 as <literal>ConnectionStatus</literal>.
3662 Strictly speaking, D-Bus property names are not required to follow
3663 the same naming restrictions as member names, but D-Bus property
3664 names that would not be valid member names (in particular,
3665 GObject-style dash-separated property names) can cause interoperability
3666 problems and should be avoided.
3669 The available properties and whether they are writable can be determined
3670 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3671 see <xref linkend="standard-interfaces-introspectable"/>.
3674 An empty string may be provided for the interface name; in this case,
3675 if there are multiple properties on an object with the same name,
3676 the results are undefined (picking one by according to an arbitrary
3677 deterministic rule, or returning an error, are the reasonable
3681 If one or more properties change on an object, the
3682 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3683 signal may be emitted (this signal was added in 0.14):
3687 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3688 DICT<STRING,VARIANT> changed_properties,
3689 ARRAY<STRING> invalidated_properties);
3693 where <literal>changed_properties</literal> is a dictionary
3694 containing the changed properties with the new values and
3695 <literal>invalidated_properties</literal> is an array of
3696 properties that changed but the value is not conveyed.
3699 Whether the <literal>PropertiesChanged</literal> signal is
3700 supported can be determined by calling
3701 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3702 that the signal may be supported for an object but it may
3703 differ how whether and how it is used on a per-property basis
3704 (for e.g. performance or security reasons). Each property (or
3705 the parent interface) must be annotated with the
3706 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3707 annotation to convey this (usually the default value
3708 <literal>true</literal> is sufficient meaning that the
3709 annotation does not need to be used). See <xref
3710 linkend="introspection-format"/> for details on this
3715 <sect2 id="standard-interfaces-objectmanager">
3716 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3718 An API can optionally make use of this interface for one or
3719 more sub-trees of objects. The root of each sub-tree implements
3720 this interface so other applications can get all objects,
3721 interfaces and properties in a single method call. It is
3722 appropriate to use this interface if users of the tree of
3723 objects are expected to be interested in all interfaces of all
3724 objects in the tree; a more granular API should be used if
3725 users of the objects are expected to be interested in a small
3726 subset of the objects, a small subset of their interfaces, or
3730 The method that applications can use to get all objects and
3731 properties is <literal>GetManagedObjects</literal>:
3735 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3739 The return value of this method is a dict whose keys are
3740 object paths. All returned object paths are children of the
3741 object path implementing this interface, i.e. their object
3742 paths start with the ObjectManager's object path plus '/'.
3745 Each value is a dict whose keys are interfaces names. Each
3746 value in this inner dict is the same dict that would be
3747 returned by the <link
3748 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3749 method for that combination of object path and interface. If
3750 an interface has no properties, the empty dict is returned.
3753 Changes are emitted using the following two signals:
3757 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3758 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3759 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3760 ARRAY<STRING> interfaces);
3764 The <literal>InterfacesAdded</literal> signal is emitted when
3765 either a new object is added or when an existing object gains
3766 one or more interfaces. The
3767 <literal>InterfacesRemoved</literal> signal is emitted
3768 whenever an object is removed or it loses one or more
3769 interfaces. The second parameter of the
3770 <literal>InterfacesAdded</literal> signal contains a dict with
3771 the interfaces and properties (if any) that have been added to
3772 the given object path. Similarly, the second parameter of the
3773 <literal>InterfacesRemoved</literal> signal contains an array
3774 of the interfaces that were removed. Note that changes on
3775 properties on existing interfaces are not reported using this
3776 interface - an application should also monitor the existing <link
3777 linkend="standard-interfaces-properties">PropertiesChanged</link>
3778 signal on each object.
3781 Applications SHOULD NOT export objects that are children of an
3782 object (directly or otherwise) implementing this interface but
3783 which are not returned in the reply from the
3784 <literal>GetManagedObjects()</literal> method of this
3785 interface on the given object.
3788 The intent of the <literal>ObjectManager</literal> interface
3789 is to make it easy to write a robust client
3790 implementation. The trivial client implementation only needs
3791 to make two method calls:
3795 org.freedesktop.DBus.AddMatch (bus_proxy,
3796 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3797 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3801 on the message bus and the remote application's
3802 <literal>ObjectManager</literal>, respectively. Whenever a new
3803 remote object is created (or an existing object gains a new
3804 interface), the <literal>InterfacesAdded</literal> signal is
3805 emitted, and since this signal contains all properties for the
3806 interfaces, no calls to the
3807 <literal>org.freedesktop.Properties</literal> interface on the
3808 remote object are needed. Additionally, since the initial
3809 <literal>AddMatch()</literal> rule already includes signal
3810 messages from the newly created child object, no new
3811 <literal>AddMatch()</literal> call is needed.
3816 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3817 interface was added in version 0.17 of the D-Bus
3824 <sect1 id="introspection-format">
3825 <title>Introspection Data Format</title>
3827 As described in <xref linkend="standard-interfaces-introspectable"/>,
3828 objects may be introspected at runtime, returning an XML string
3829 that describes the object. The same XML format may be used in
3830 other contexts as well, for example as an "IDL" for generating
3831 static language bindings.
3834 Here is an example of introspection data:
3836 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3837 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3838 <node name="/com/example/sample_object">
3839 <interface name="com.example.SampleInterface">
3840 <method name="Frobate">
3841 <arg name="foo" type="i" direction="in"/>
3842 <arg name="bar" type="s" direction="out"/>
3843 <arg name="baz" type="a{us}" direction="out"/>
3844 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3846 <method name="Bazify">
3847 <arg name="bar" type="(iiu)" direction="in"/>
3848 <arg name="bar" type="v" direction="out"/>
3850 <method name="Mogrify">
3851 <arg name="bar" type="(iiav)" direction="in"/>
3853 <signal name="Changed">
3854 <arg name="new_value" type="b"/>
3856 <property name="Bar" type="y" access="readwrite"/>
3858 <node name="child_of_sample_object"/>
3859 <node name="another_child_of_sample_object"/>
3864 A more formal DTD and spec needs writing, but here are some quick notes.
3868 Only the root <node> element can omit the node name, as it's
3869 known to be the object that was introspected. If the root
3870 <node> does have a name attribute, it must be an absolute
3871 object path. If child <node> have object paths, they must be
3877 If a child <node> has any sub-elements, then they
3878 must represent a complete introspection of the child.
3879 If a child <node> is empty, then it may or may
3880 not have sub-elements; the child must be introspected
3881 in order to find out. The intent is that if an object
3882 knows that its children are "fast" to introspect
3883 it can go ahead and return their information, but
3884 otherwise it can omit it.
3889 The direction element on <arg> may be omitted,
3890 in which case it defaults to "in" for method calls
3891 and "out" for signals. Signals only allow "out"
3892 so while direction may be specified, it's pointless.
3897 The possible directions are "in" and "out",
3898 unlike CORBA there is no "inout"
3903 The possible property access flags are
3904 "readwrite", "read", and "write"
3909 Multiple interfaces can of course be listed for
3915 The "name" attribute on arguments is optional.
3921 Method, interface, property, and signal elements may have
3922 "annotations", which are generic key/value pairs of metadata.
3923 They are similar conceptually to Java's annotations and C# attributes.
3924 Well-known annotations:
3931 <entry>Values (separated by ,)</entry>
3932 <entry>Description</entry>
3937 <entry>org.freedesktop.DBus.Deprecated</entry>
3938 <entry>true,false</entry>
3939 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3942 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3943 <entry>(string)</entry>
3944 <entry>The C symbol; may be used for methods and interfaces</entry>
3947 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3948 <entry>true,false</entry>
3949 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3952 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3953 <entry>true,invalidates,const,false</entry>
3956 If set to <literal>false</literal>, the
3957 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3959 linkend="standard-interfaces-properties"/> is not
3960 guaranteed to be emitted if the property changes.
3963 If set to <literal>const</literal> the property never
3964 changes value during the lifetime of the object it
3965 belongs to, and hence the signal is never emitted for
3969 If set to <literal>invalidates</literal> the signal
3970 is emitted but the value is not included in the
3974 If set to <literal>true</literal> the signal is
3975 emitted with the value included.
3978 The value for the annotation defaults to
3979 <literal>true</literal> if the enclosing interface
3980 element does not specify the annotation. Otherwise it
3981 defaults to the value specified in the enclosing
3985 This annotation is intended to be used by code
3986 generators to implement client-side caching of
3987 property values. For all properties for which the
3988 annotation is set to <literal>const</literal>,
3989 <literal>invalidates</literal> or
3990 <literal>true</literal> the client may
3991 unconditionally cache the values as the properties
3992 don't change or notifications are generated for them
4001 <sect1 id="message-bus">
4002 <title>Message Bus Specification</title>
4003 <sect2 id="message-bus-overview">
4004 <title>Message Bus Overview</title>
4006 The message bus accepts connections from one or more applications.
4007 Once connected, applications can exchange messages with other
4008 applications that are also connected to the bus.
4011 In order to route messages among connections, the message bus keeps a
4012 mapping from names to connections. Each connection has one
4013 unique-for-the-lifetime-of-the-bus name automatically assigned.
4014 Applications may request additional names for a connection. Additional
4015 names are usually "well-known names" such as
4016 "com.example.TextEditor". When a name is bound to a connection,
4017 that connection is said to <firstterm>own</firstterm> the name.
4020 The bus itself owns a special name,
4021 <literal>org.freedesktop.DBus</literal>, with an object
4022 located at <literal>/org/freedesktop/DBus</literal> that
4023 implements the <literal>org.freedesktop.DBus</literal>
4024 interface. This service allows applications to make
4025 administrative requests of the bus itself. For example,
4026 applications can ask the bus to assign a name to a connection.
4029 Each name may have <firstterm>queued owners</firstterm>. When an
4030 application requests a name for a connection and the name is already in
4031 use, the bus will optionally add the connection to a queue waiting for
4032 the name. If the current owner of the name disconnects or releases
4033 the name, the next connection in the queue will become the new owner.
4037 This feature causes the right thing to happen if you start two text
4038 editors for example; the first one may request "com.example.TextEditor",
4039 and the second will be queued as a possible owner of that name. When
4040 the first exits, the second will take over.
4044 Applications may send <firstterm>unicast messages</firstterm> to
4045 a specific recipient or to the message bus itself, or
4046 <firstterm>broadcast messages</firstterm> to all interested recipients.
4047 See <xref linkend="message-bus-routing"/> for details.
4051 <sect2 id="message-bus-names">
4052 <title>Message Bus Names</title>
4054 Each connection has at least one name, assigned at connection time and
4055 returned in response to the
4056 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4057 automatically-assigned name is called the connection's <firstterm>unique
4058 name</firstterm>. Unique names are never reused for two different
4059 connections to the same bus.
4062 Ownership of a unique name is a prerequisite for interaction with
4063 the message bus. It logically follows that the unique name is always
4064 the first name that an application comes to own, and the last
4065 one that it loses ownership of.
4068 Unique connection names must begin with the character ':' (ASCII colon
4069 character); bus names that are not unique names must not begin
4070 with this character. (The bus must reject any attempt by an application
4071 to manually request a name beginning with ':'.) This restriction
4072 categorically prevents "spoofing"; messages sent to a unique name
4073 will always go to the expected connection.
4076 When a connection is closed, all the names that it owns are deleted (or
4077 transferred to the next connection in the queue if any).
4080 A connection can request additional names to be associated with it using
4081 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4082 linkend="message-protocol-names-bus"/> describes the format of a valid
4083 name. These names can be released again using the
4084 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4087 <sect3 id="bus-messages-request-name">
4088 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4092 UINT32 RequestName (in STRING name, in UINT32 flags)
4099 <entry>Argument</entry>
4101 <entry>Description</entry>
4107 <entry>STRING</entry>
4108 <entry>Name to request</entry>
4112 <entry>UINT32</entry>
4113 <entry>Flags</entry>
4123 <entry>Argument</entry>
4125 <entry>Description</entry>
4131 <entry>UINT32</entry>
4132 <entry>Return value</entry>
4139 This method call should be sent to
4140 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4141 assign the given name to the method caller. Each name maintains a
4142 queue of possible owners, where the head of the queue is the primary
4143 or current owner of the name. Each potential owner in the queue
4144 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4145 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4146 call. When RequestName is invoked the following occurs:
4150 If the method caller is currently the primary owner of the name,
4151 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4152 values are updated with the values from the new RequestName call,
4153 and nothing further happens.
4159 If the current primary owner (head of the queue) has
4160 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4161 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4162 the caller of RequestName replaces the current primary owner at
4163 the head of the queue and the current primary owner moves to the
4164 second position in the queue. If the caller of RequestName was
4165 in the queue previously its flags are updated with the values from
4166 the new RequestName in addition to moving it to the head of the queue.
4172 If replacement is not possible, and the method caller is
4173 currently in the queue but not the primary owner, its flags are
4174 updated with the values from the new RequestName call.
4180 If replacement is not possible, and the method caller is
4181 currently not in the queue, the method caller is appended to the
4188 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4189 set and is not the primary owner, it is removed from the
4190 queue. This can apply to the previous primary owner (if it
4191 was replaced) or the method caller (if it updated the
4192 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4193 queue, or if it was just added to the queue with that flag set).
4199 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4200 queue," even if another application already in the queue had specified
4201 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4202 that does not allow replacement goes away, and the next primary owner
4203 does allow replacement. In this case, queued items that specified
4204 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4205 automatically replace the new primary owner. In other words,
4206 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4207 time RequestName is called. This is deliberate to avoid an infinite loop
4208 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4209 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4212 The flags argument contains any of the following values logically ORed
4219 <entry>Conventional Name</entry>
4220 <entry>Value</entry>
4221 <entry>Description</entry>
4226 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4230 If an application A specifies this flag and succeeds in
4231 becoming the owner of the name, and another application B
4232 later calls RequestName with the
4233 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4234 will lose ownership and receive a
4235 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4236 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4237 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4238 is not specified by application B, then application B will not replace
4239 application A as the owner.
4244 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4248 Try to replace the current owner if there is one. If this
4249 flag is not set the application will only become the owner of
4250 the name if there is no current owner. If this flag is set,
4251 the application will replace the current owner if
4252 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4257 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4261 Without this flag, if an application requests a name that is
4262 already owned, the application will be placed in a queue to
4263 own the name when the current owner gives it up. If this
4264 flag is given, the application will not be placed in the
4265 queue, the request for the name will simply fail. This flag
4266 also affects behavior when an application is replaced as
4267 name owner; by default the application moves back into the
4268 waiting queue, unless this flag was provided when the application
4269 became the name owner.
4277 The return code can be one of the following values:
4283 <entry>Conventional Name</entry>
4284 <entry>Value</entry>
4285 <entry>Description</entry>
4290 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4291 <entry>1</entry> <entry>The caller is now the primary owner of
4292 the name, replacing any previous owner. Either the name had no
4293 owner before, or the caller specified
4294 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4295 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4298 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4301 <entry>The name already had an owner,
4302 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4303 the current owner did not specify
4304 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4305 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4309 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4310 <entry>The name already has an owner,
4311 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4312 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4313 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4314 specified by the requesting application.</entry>
4317 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4319 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4327 <sect3 id="bus-messages-release-name">
4328 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4332 UINT32 ReleaseName (in STRING name)
4339 <entry>Argument</entry>
4341 <entry>Description</entry>
4347 <entry>STRING</entry>
4348 <entry>Name to release</entry>
4358 <entry>Argument</entry>
4360 <entry>Description</entry>
4366 <entry>UINT32</entry>
4367 <entry>Return value</entry>
4374 This method call should be sent to
4375 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4376 release the method caller's claim to the given name. If the caller is
4377 the primary owner, a new primary owner will be selected from the
4378 queue if any other owners are waiting. If the caller is waiting in
4379 the queue for the name, the caller will removed from the queue and
4380 will not be made an owner of the name if it later becomes available.
4381 If there are no other owners in the queue for the name, it will be
4382 removed from the bus entirely.
4384 The return code can be one of the following values:
4390 <entry>Conventional Name</entry>
4391 <entry>Value</entry>
4392 <entry>Description</entry>
4397 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4398 <entry>1</entry> <entry>The caller has released his claim on
4399 the given name. Either the caller was the primary owner of
4400 the name, and the name is now unused or taken by somebody
4401 waiting in the queue for the name, or the caller was waiting
4402 in the queue for the name and has now been removed from the
4406 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4408 <entry>The given name does not exist on this bus.</entry>
4411 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4413 <entry>The caller was not the primary owner of this name,
4414 and was also not waiting in the queue to own this name.</entry>
4422 <sect3 id="bus-messages-list-queued-owners">
4423 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4427 ARRAY of STRING ListQueuedOwners (in STRING name)
4434 <entry>Argument</entry>
4436 <entry>Description</entry>
4442 <entry>STRING</entry>
4443 <entry>The well-known bus name to query, such as
4444 <literal>com.example.cappuccino</literal></entry>
4454 <entry>Argument</entry>
4456 <entry>Description</entry>
4462 <entry>ARRAY of STRING</entry>
4463 <entry>The unique bus names of connections currently queued
4464 for the name</entry>
4471 This method call should be sent to
4472 <literal>org.freedesktop.DBus</literal> and lists the connections
4473 currently queued for a bus name (see
4474 <xref linkend="term-queued-owner"/>).
4479 <sect2 id="message-bus-routing">
4480 <title>Message Bus Message Routing</title>
4483 Messages may have a <literal>DESTINATION</literal> field (see <xref
4484 linkend="message-protocol-header-fields"/>), resulting in a
4485 <firstterm>unicast message</firstterm>. If the
4486 <literal>DESTINATION</literal> field is present, it specifies a message
4487 recipient by name. Method calls and replies normally specify this field.
4488 The message bus must send messages (of any type) with the
4489 <literal>DESTINATION</literal> field set to the specified recipient,
4490 regardless of whether the recipient has set up a match rule matching
4495 When the message bus receives a signal, if the
4496 <literal>DESTINATION</literal> field is absent, it is considered to
4497 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4498 applications with <firstterm>message matching rules</firstterm> that
4499 match the message. Most signal messages are broadcasts, and
4500 no other message types currently defined in this specification
4505 Unicast signal messages (those with a <literal>DESTINATION</literal>
4506 field) are not commonly used, but they are treated like any unicast
4507 message: they are delivered to the specified receipient,
4508 regardless of its match rules. One use for unicast signals is to
4509 avoid a race condition in which a signal is emitted before the intended
4510 recipient can call <xref linkend="bus-messages-add-match"/> to
4511 receive that signal: if the signal is sent directly to that recipient
4512 using a unicast message, it does not need to add a match rule at all,
4513 and there is no race condition. Another use for unicast signals,
4514 on message buses whose security policy prevents eavesdropping, is to
4515 send sensitive information which should only be visible to one
4520 When the message bus receives a method call, if the
4521 <literal>DESTINATION</literal> field is absent, the call is taken to be
4522 a standard one-to-one message and interpreted by the message bus
4523 itself. For example, sending an
4524 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4525 <literal>DESTINATION</literal> will cause the message bus itself to
4526 reply to the ping immediately; the message bus will not make this
4527 message visible to other applications.
4531 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4532 the ping message were sent with a <literal>DESTINATION</literal> name of
4533 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4534 forwarded, and the Yoyodyne Corporation screensaver application would be
4535 expected to reply to the ping.
4539 Message bus implementations may impose a security policy which
4540 prevents certain messages from being sent or received.
4541 When a method call message cannot be sent or received due to a security
4542 policy, the message bus should send an error reply, unless the
4543 original message had the <literal>NO_REPLY</literal> flag.
4546 <sect3 id="message-bus-routing-eavesdropping">
4547 <title>Eavesdropping</title>
4549 Receiving a unicast message whose <literal>DESTINATION</literal>
4550 indicates a different recipient is called
4551 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4552 a security boundary (like the standard system bus), the security
4553 policy should usually prevent eavesdropping, since unicast messages
4554 are normally kept private and may contain security-sensitive
4559 Eavesdropping is mainly useful for debugging tools, such as
4560 the <literal>dbus-monitor</literal> tool in the reference
4561 implementation of D-Bus. Tools which eavesdrop on the message bus
4562 should be careful to avoid sending a reply or error in response to
4563 messages intended for a different client.
4567 Clients may attempt to eavesdrop by adding match rules
4568 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4569 the <literal>eavesdrop='true'</literal> match. If the message bus'
4570 security policy does not allow eavesdropping, the match rule can
4571 still be added, but will not have any practical effect. For
4572 compatibility with older message bus implementations, if adding such
4573 a match rule results in an error reply, the client may fall back to
4574 adding the same rule with the <literal>eavesdrop</literal> match
4579 <sect3 id="message-bus-routing-match-rules">
4580 <title>Match Rules</title>
4582 An important part of the message bus routing protocol is match
4583 rules. Match rules describe the messages that should be sent to a
4584 client, based on the contents of the message. Broadcast signals
4585 are only sent to clients which have a suitable match rule: this
4586 avoids waking up client processes to deal with signals that are
4587 not relevant to that client.
4590 Messages that list a client as their <literal>DESTINATION</literal>
4591 do not need to match the client's match rules, and are sent to that
4592 client regardless. As a result, match rules are mainly used to
4593 receive a subset of broadcast signals.
4596 Match rules can also be used for eavesdropping
4597 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4598 if the security policy of the message bus allows it.
4601 Match rules are added using the AddMatch bus method
4602 (see <xref linkend="bus-messages-add-match"/>). Rules are
4603 specified as a string of comma separated key/value pairs.
4604 Excluding a key from the rule indicates a wildcard match.
4605 For instance excluding the the member from a match rule but
4606 adding a sender would let all messages from that sender through.
4607 An example of a complete rule would be
4608 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4611 Within single quotes (ASCII apostrophe, U+0027), a backslash
4612 (U+005C) represents itself, and an apostrophe ends the quoted
4613 section. Outside single quotes, \' (backslash, apostrophe)
4614 represents an apostrophe, and any backslash not followed by
4615 an apostrophe represents itself. For instance, the match rules
4616 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4617 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4618 both match messages where the arguments are a 1-character string
4619 containing an apostrophe, a 1-character string containing a
4620 backslash, a 1-character string containing a comma, and a
4621 2-character string containing two backslashes<footnote>
4623 This idiosyncratic quoting style is based on the rules for
4624 escaping items to appear inside single-quoted strings
4625 in POSIX <literal>/bin/sh</literal>, but please
4626 note that backslashes that are not inside single quotes have
4627 different behaviour. This syntax does not offer any way to
4628 represent an apostrophe inside single quotes (it is necessary
4629 to leave the single-quoted section, backslash-escape the
4630 apostrophe and re-enter single quotes), or to represent a
4631 comma outside single quotes (it is necessary to wrap it in
4632 a single-quoted section).
4637 The following table describes the keys that can be used to create
4644 <entry>Possible Values</entry>
4645 <entry>Description</entry>
4650 <entry><literal>type</literal></entry>
4651 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4652 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4655 <entry><literal>sender</literal></entry>
4656 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4657 and <xref linkend="term-unique-name"/> respectively)
4659 <entry>Match messages sent by a particular sender. An example of a sender match
4660 is sender='org.freedesktop.Hal'</entry>
4663 <entry><literal>interface</literal></entry>
4664 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4665 <entry>Match messages sent over or to a particular interface. An example of an
4666 interface match is interface='org.freedesktop.Hal.Manager'.
4667 If a message omits the interface header, it must not match any rule
4668 that specifies this key.</entry>
4671 <entry><literal>member</literal></entry>
4672 <entry>Any valid method or signal name</entry>
4673 <entry>Matches messages which have the give method or signal name. An example of
4674 a member match is member='NameOwnerChanged'</entry>
4677 <entry><literal>path</literal></entry>
4678 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4679 <entry>Matches messages which are sent from or to the given object. An example of a
4680 path match is path='/org/freedesktop/Hal/Manager'</entry>
4683 <entry><literal>path_namespace</literal></entry>
4684 <entry>An object path</entry>
4687 Matches messages which are sent from or to an
4688 object for which the object path is either the
4689 given value, or that value followed by one or
4690 more path components.
4695 <literal>path_namespace='/com/example/foo'</literal>
4696 would match signals sent by
4697 <literal>/com/example/foo</literal>
4699 <literal>/com/example/foo/bar</literal>,
4701 <literal>/com/example/foobar</literal>.
4705 Using both <literal>path</literal> and
4706 <literal>path_namespace</literal> in the same match
4707 rule is not allowed.
4712 This match key was added in version 0.16 of the
4713 D-Bus specification and implemented by the bus
4714 daemon in dbus 1.5.0 and later.
4720 <entry><literal>destination</literal></entry>
4721 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4722 <entry>Matches messages which are being sent to the given unique name. An
4723 example of a destination match is destination=':1.0'</entry>
4726 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4727 <entry>Any string</entry>
4728 <entry>Arg matches are special and are used for further restricting the
4729 match based on the arguments in the body of a message. Only arguments of type
4730 STRING can be matched in this way. An example of an argument match
4731 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4735 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4736 <entry>Any string</entry>
4738 <para>Argument path matches provide a specialised form of wildcard matching for
4739 path-like namespaces. They can match arguments whose type is either STRING or
4740 OBJECT_PATH. As with normal argument matches,
4741 if the argument is exactly equal to the string given in the match
4742 rule then the rule is satisfied. Additionally, there is also a
4743 match when either the string given in the match rule or the
4744 appropriate message argument ends with '/' and is a prefix of the
4745 other. An example argument path match is arg0path='/aa/bb/'. This
4746 would match messages with first arguments of '/', '/aa/',
4747 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4748 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4750 <para>This is intended for monitoring “directories” in file system-like
4751 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4752 system. An application interested in all nodes in a particular hierarchy would
4753 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4754 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4755 represent a modification to the “bar” property, or a signal with zeroth
4756 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4757 many properties within that directory, and the interested application would be
4758 notified in both cases.</para>
4761 This match key was added in version 0.12 of the
4762 D-Bus specification, implemented for STRING
4763 arguments by the bus daemon in dbus 1.2.0 and later,
4764 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4771 <entry><literal>arg0namespace</literal></entry>
4772 <entry>Like a bus name, except that the string is not
4773 required to contain a '.' (period)</entry>
4775 <para>Match messages whose first argument is of type STRING, and is a bus name
4776 or interface name within the specified namespace. This is primarily intended
4777 for watching name owner changes for a group of related bus names, rather than
4778 for a single name or all name changes.</para>
4780 <para>Because every valid interface name is also a valid
4781 bus name, this can also be used for messages whose
4782 first argument is an interface name.</para>
4784 <para>For example, the match rule
4785 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4786 matches name owner changes for bus names such as
4787 <literal>com.example.backend.foo</literal>,
4788 <literal>com.example.backend.foo.bar</literal>, and
4789 <literal>com.example.backend</literal> itself.</para>
4791 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4794 This match key was added in version 0.16 of the
4795 D-Bus specification and implemented by the bus
4796 daemon in dbus 1.5.0 and later.
4802 <entry><literal>eavesdrop</literal></entry>
4803 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4804 <entry>Since D-Bus 1.5.6, match rules do not
4805 match messages which have a <literal>DESTINATION</literal>
4806 field unless the match rule specifically
4808 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4809 by specifying <literal>eavesdrop='true'</literal>
4810 in the match rule. <literal>eavesdrop='false'</literal>
4811 restores the default behaviour. Messages are
4812 delivered to their <literal>DESTINATION</literal>
4813 regardless of match rules, so this match does not
4814 affect normal delivery of unicast messages.
4815 If the message bus has a security policy which forbids
4816 eavesdropping, this match may still be used without error,
4817 but will not have any practical effect.
4818 In older versions of D-Bus, this match was not allowed
4819 in match rules, and all match rules behaved as if
4820 <literal>eavesdrop='true'</literal> had been used.
4829 <sect2 id="message-bus-starting-services">
4830 <title>Message Bus Starting Services</title>
4832 The message bus can start applications on behalf of other applications.
4833 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4834 An application that can be started in this way is called a
4835 <firstterm>service</firstterm>.
4838 With D-Bus, starting a service is normally done by name. That is,
4839 applications ask the message bus to start some program that will own a
4840 well-known name, such as <literal>com.example.TextEditor</literal>.
4841 This implies a contract documented along with the name
4842 <literal>com.example.TextEditor</literal> for which object
4843 the owner of that name will provide, and what interfaces those
4847 To find an executable corresponding to a particular name, the bus daemon
4848 looks for <firstterm>service description files</firstterm>. Service
4849 description files define a mapping from names to executables. Different
4850 kinds of message bus will look for these files in different places, see
4851 <xref linkend="message-bus-types"/>.
4854 Service description files have the ".service" file
4855 extension. The message bus will only load service description files
4856 ending with .service; all other files will be ignored. The file format
4857 is similar to that of <ulink
4858 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4859 entries</ulink>. All service description files must be in UTF-8
4860 encoding. To ensure that there will be no name collisions, service files
4861 must be namespaced using the same mechanism as messages and service
4866 On the well-known system bus, the name of a service description file
4867 must be its well-known name plus <literal>.service</literal>,
4869 <literal>com.example.ConfigurationDatabase.service</literal>.
4873 On the well-known session bus, services should follow the same
4874 service description file naming convention as on the system bus,
4875 but for backwards compatibility they are not required to do so.
4879 [FIXME the file format should be much better specified than "similar to
4880 .desktop entries" esp. since desktop entries are already
4881 badly-specified. ;-)]
4882 These sections from the specification apply to service files as well:
4885 <listitem><para>General syntax</para></listitem>
4886 <listitem><para>Comment format</para></listitem>
4889 Service description files must contain a
4890 <literal>D-BUS Service</literal> group with at least the keys
4891 <literal>Name</literal> (the well-known name of the service)
4892 and <literal>Exec</literal> (the command to be executed).
4895 <title>Example service description file</title>
4897 # Sample service description file
4899 Name=com.example.ConfigurationDatabase
4900 Exec=/usr/bin/sample-configd
4906 Additionally, service description files for the well-known system
4907 bus on Unix must contain a <literal>User</literal> key, whose value
4908 is the name of a user account (e.g. <literal>root</literal>).
4909 The system service will be run as that user.
4913 When an application asks to start a service by name, the bus daemon tries to
4914 find a service that will own that name. It then tries to spawn the
4915 executable associated with it. If this fails, it will report an
4920 On the well-known system bus, it is not possible for two .service files
4921 in the same directory to offer the same service, because they are
4922 constrained to have names that match the service name.
4926 On the well-known session bus, if two .service files in the same
4927 directory offer the same service name, the result is undefined.
4928 Distributors should avoid this situation, for instance by naming
4929 session services' .service files according to their service name.
4933 If two .service files in different directories offer the same
4934 service name, the one in the higher-priority directory is used:
4935 for instance, on the system bus, .service files in
4936 /usr/local/share/dbus-1/system-services take precedence over those
4937 in /usr/share/dbus-1/system-services.
4940 The executable launched will have the environment variable
4941 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4942 message bus so it can connect and request the appropriate names.
4945 The executable being launched may want to know whether the message bus
4946 starting it is one of the well-known message buses (see <xref
4947 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4948 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4949 of the well-known buses. The currently-defined values for this variable
4950 are <literal>system</literal> for the systemwide message bus,
4951 and <literal>session</literal> for the per-login-session message
4952 bus. The new executable must still connect to the address given
4953 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4954 resulting connection is to the well-known bus.
4957 [FIXME there should be a timeout somewhere, either specified
4958 in the .service file, by the client, or just a global value
4959 and if the client being activated fails to connect within that
4960 timeout, an error should be sent back.]
4963 <sect3 id="message-bus-starting-services-scope">
4964 <title>Message Bus Service Scope</title>
4966 The "scope" of a service is its "per-", such as per-session,
4967 per-machine, per-home-directory, or per-display. The reference
4968 implementation doesn't yet support starting services in a different
4969 scope from the message bus itself. So e.g. if you start a service
4970 on the session bus its scope is per-session.
4973 We could add an optional scope to a bus name. For example, for
4974 per-(display,session pair), we could have a unique ID for each display
4975 generated automatically at login and set on screen 0 by executing a
4976 special "set display ID" binary. The ID would be stored in a
4977 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4978 random bytes. This ID would then be used to scope names.
4979 Starting/locating a service could be done by ID-name pair rather than
4983 Contrast this with a per-display scope. To achieve that, we would
4984 want a single bus spanning all sessions using a given display.
4985 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4986 property on screen 0 of the display, pointing to this bus.
4991 <sect2 id="message-bus-types">
4992 <title>Well-known Message Bus Instances</title>
4994 Two standard message bus instances are defined here, along with how
4995 to locate them and where their service files live.
4997 <sect3 id="message-bus-types-login">
4998 <title>Login session message bus</title>
5000 Each time a user logs in, a <firstterm>login session message
5001 bus</firstterm> may be started. All applications in the user's login
5002 session may interact with one another using this message bus.
5005 The address of the login session message bus is given
5006 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5007 variable. If that variable is not set, applications may
5008 also try to read the address from the X Window System root
5009 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5010 The root window property must have type <literal>STRING</literal>.
5011 The environment variable should have precedence over the
5012 root window property.
5014 <para>The address of the login session message bus is given in the
5015 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5016 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5017 "autolaunch:", the system should use platform-specific methods of
5018 locating a running D-Bus session server, or starting one if a running
5019 instance cannot be found. Note that this mechanism is not recommended
5020 for attempting to determine if a daemon is running. It is inherently
5021 racy to attempt to make this determination, since the bus daemon may
5022 be started just before or just after the determination is made.
5023 Therefore, it is recommended that applications do not try to make this
5024 determination for their functionality purposes, and instead they
5025 should attempt to start the server.</para>
5027 <sect4 id="message-bus-types-login-x-windows">
5028 <title>X Windowing System</title>
5030 For the X Windowing System, the application must locate the
5031 window owner of the selection represented by the atom formed by
5035 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5039 <para>the current user's username</para>
5043 <para>the literal character '_' (underscore)</para>
5047 <para>the machine's ID</para>
5053 The following properties are defined for the window that owns
5055 <informaltable frame="all">
5064 <para>meaning</para>
5070 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5074 <para>the actual address of the server socket</para>
5080 <para>_DBUS_SESSION_BUS_PID</para>
5084 <para>the PID of the server process</para>
5093 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5094 present in this window.
5098 If the X selection cannot be located or if reading the
5099 properties from the window fails, the implementation MUST conclude
5100 that there is no D-Bus server running and proceed to start a new
5101 server. (See below on concurrency issues)
5105 Failure to connect to the D-Bus server address thus obtained
5106 MUST be treated as a fatal connection error and should be reported
5111 As an alternative, an implementation MAY find the information
5112 in the following file located in the current user's home directory,
5113 in subdirectory .dbus/session-bus/:
5116 <para>the machine's ID</para>
5120 <para>the literal character '-' (dash)</para>
5124 <para>the X display without the screen number, with the
5125 following prefixes removed, if present: ":", "localhost:"
5126 ."localhost.localdomain:". That is, a display of
5127 "localhost:10.0" produces just the number "10"</para>
5133 The contents of this file NAME=value assignment pairs and
5134 lines starting with # are comments (no comments are allowed
5135 otherwise). The following variable names are defined:
5142 <para>Variable</para>
5146 <para>meaning</para>
5152 <para>DBUS_SESSION_BUS_ADDRESS</para>
5156 <para>the actual address of the server socket</para>
5162 <para>DBUS_SESSION_BUS_PID</para>
5166 <para>the PID of the server process</para>
5172 <para>DBUS_SESSION_BUS_WINDOWID</para>
5176 <para>the window ID</para>
5185 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5190 Failure to open this file MUST be interpreted as absence of a
5191 running server. Therefore, the implementation MUST proceed to
5192 attempting to launch a new bus server if the file cannot be
5197 However, success in opening this file MUST NOT lead to the
5198 conclusion that the server is running. Thus, a failure to connect to
5199 the bus address obtained by the alternative method MUST NOT be
5200 considered a fatal error. If the connection cannot be established,
5201 the implementation MUST proceed to check the X selection settings or
5202 to start the server on its own.
5206 If the implementation concludes that the D-Bus server is not
5207 running it MUST attempt to start a new server and it MUST also
5208 ensure that the daemon started as an effect of the "autolaunch"
5209 mechanism provides the lookup mechanisms described above, so
5210 subsequent calls can locate the newly started server. The
5211 implementation MUST also ensure that if two or more concurrent
5212 initiations happen, only one server remains running and all other
5213 initiations are able to obtain the address of this server and
5214 connect to it. In other words, the implementation MUST ensure that
5215 the X selection is not present when it attempts to set it, without
5216 allowing another process to set the selection between the
5217 verification and the setting (e.g., by using XGrabServer /
5224 On Unix systems, the session bus should search for .service files
5225 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5227 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5228 Implementations may also search additional locations, which
5229 should be searched with lower priority than anything in
5230 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5231 for example, the reference implementation also
5232 looks in <literal>${datadir}/dbus-1/services</literal> as
5233 set at compile time.
5236 As described in the XDG Base Directory Specification, software
5237 packages should install their session .service files to their
5238 configured <literal>${datadir}/dbus-1/services</literal>,
5239 where <literal>${datadir}</literal> is as defined by the GNU
5240 coding standards. System administrators or users can arrange
5241 for these service files to be read by setting XDG_DATA_DIRS or by
5242 symlinking them into the default locations.
5246 <sect3 id="message-bus-types-system">
5247 <title>System message bus</title>
5249 A computer may have a <firstterm>system message bus</firstterm>,
5250 accessible to all applications on the system. This message bus may be
5251 used to broadcast system events, such as adding new hardware devices,
5252 changes in the printer queue, and so forth.
5255 The address of the system message bus is given
5256 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5257 variable. If that variable is not set, applications should try
5258 to connect to the well-known address
5259 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5262 The D-Bus reference implementation actually honors the
5263 <literal>$(localstatedir)</literal> configure option
5264 for this address, on both client and server side.
5269 On Unix systems, the system bus should default to searching
5270 for .service files in
5271 <literal>/usr/local/share/dbus-1/system-services</literal>,
5272 <literal>/usr/share/dbus-1/system-services</literal> and
5273 <literal>/lib/dbus-1/system-services</literal>, with that order
5274 of precedence. It may also search other implementation-specific
5275 locations, but should not vary these locations based on environment
5279 The system bus is security-sensitive and is typically executed
5280 by an init system with a clean environment. Its launch helper
5281 process is particularly security-sensitive, and specifically
5282 clears its own environment.
5287 Software packages should install their system .service
5288 files to their configured
5289 <literal>${datadir}/dbus-1/system-services</literal>,
5290 where <literal>${datadir}</literal> is as defined by the GNU
5291 coding standards. System administrators can arrange
5292 for these service files to be read by editing the system bus'
5293 configuration file or by symlinking them into the default
5299 <sect2 id="message-bus-messages">
5300 <title>Message Bus Messages</title>
5302 The special message bus name <literal>org.freedesktop.DBus</literal>
5303 responds to a number of additional messages.
5306 <sect3 id="bus-messages-hello">
5307 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5318 <entry>Argument</entry>
5320 <entry>Description</entry>
5326 <entry>STRING</entry>
5327 <entry>Unique name assigned to the connection</entry>
5334 Before an application is able to send messages to other applications
5335 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5336 to the message bus to obtain a unique name. If an application without
5337 a unique name tries to send a message to another application, or a
5338 message to the message bus itself that isn't the
5339 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5340 disconnected from the bus.
5343 There is no corresponding "disconnect" request; if a client wishes to
5344 disconnect from the bus, it simply closes the socket (or other
5345 communication channel).
5348 <sect3 id="bus-messages-list-names">
5349 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5353 ARRAY of STRING ListNames ()
5360 <entry>Argument</entry>
5362 <entry>Description</entry>
5368 <entry>ARRAY of STRING</entry>
5369 <entry>Array of strings where each string is a bus name</entry>
5376 Returns a list of all currently-owned names on the bus.
5379 <sect3 id="bus-messages-list-activatable-names">
5380 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5384 ARRAY of STRING ListActivatableNames ()
5391 <entry>Argument</entry>
5393 <entry>Description</entry>
5399 <entry>ARRAY of STRING</entry>
5400 <entry>Array of strings where each string is a bus name</entry>
5407 Returns a list of all names that can be activated on the bus.
5410 <sect3 id="bus-messages-name-exists">
5411 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5415 BOOLEAN NameHasOwner (in STRING name)
5422 <entry>Argument</entry>
5424 <entry>Description</entry>
5430 <entry>STRING</entry>
5431 <entry>Name to check</entry>
5441 <entry>Argument</entry>
5443 <entry>Description</entry>
5449 <entry>BOOLEAN</entry>
5450 <entry>Return value, true if the name exists</entry>
5457 Checks if the specified name exists (currently has an owner).
5461 <sect3 id="bus-messages-name-owner-changed">
5462 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5466 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5473 <entry>Argument</entry>
5475 <entry>Description</entry>
5481 <entry>STRING</entry>
5482 <entry>Name with a new owner</entry>
5486 <entry>STRING</entry>
5487 <entry>Old owner or empty string if none</entry>
5491 <entry>STRING</entry>
5492 <entry>New owner or empty string if none</entry>
5499 This signal indicates that the owner of a name has changed.
5500 It's also the signal to use to detect the appearance of
5501 new names on the bus.
5504 <sect3 id="bus-messages-name-lost">
5505 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5509 NameLost (STRING name)
5516 <entry>Argument</entry>
5518 <entry>Description</entry>
5524 <entry>STRING</entry>
5525 <entry>Name which was lost</entry>
5532 This signal is sent to a specific application when it loses
5533 ownership of a name.
5537 <sect3 id="bus-messages-name-acquired">
5538 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5542 NameAcquired (STRING name)
5549 <entry>Argument</entry>
5551 <entry>Description</entry>
5557 <entry>STRING</entry>
5558 <entry>Name which was acquired</entry>
5565 This signal is sent to a specific application when it gains
5566 ownership of a name.
5570 <sect3 id="bus-messages-start-service-by-name">
5571 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5575 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5582 <entry>Argument</entry>
5584 <entry>Description</entry>
5590 <entry>STRING</entry>
5591 <entry>Name of the service to start</entry>
5595 <entry>UINT32</entry>
5596 <entry>Flags (currently not used)</entry>
5606 <entry>Argument</entry>
5608 <entry>Description</entry>
5614 <entry>UINT32</entry>
5615 <entry>Return value</entry>
5620 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5624 The return value can be one of the following values:
5629 <entry>Identifier</entry>
5630 <entry>Value</entry>
5631 <entry>Description</entry>
5636 <entry>DBUS_START_REPLY_SUCCESS</entry>
5638 <entry>The service was successfully started.</entry>
5641 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5643 <entry>A connection already owns the given name.</entry>
5652 <sect3 id="bus-messages-update-activation-environment">
5653 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5657 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5664 <entry>Argument</entry>
5666 <entry>Description</entry>
5672 <entry>ARRAY of DICT<STRING,STRING></entry>
5673 <entry>Environment to add or update</entry>
5678 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5681 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5684 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.
5689 <sect3 id="bus-messages-get-name-owner">
5690 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5694 STRING GetNameOwner (in STRING name)
5701 <entry>Argument</entry>
5703 <entry>Description</entry>
5709 <entry>STRING</entry>
5710 <entry>Name to get the owner of</entry>
5720 <entry>Argument</entry>
5722 <entry>Description</entry>
5728 <entry>STRING</entry>
5729 <entry>Return value, a unique connection name</entry>
5734 Returns the unique connection name of the primary owner of the name
5735 given. If the requested name doesn't have an owner, returns a
5736 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5740 <sect3 id="bus-messages-get-connection-unix-user">
5741 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5745 UINT32 GetConnectionUnixUser (in STRING bus_name)
5752 <entry>Argument</entry>
5754 <entry>Description</entry>
5760 <entry>STRING</entry>
5761 <entry>Unique or well-known bus name of the connection to
5762 query, such as <literal>:12.34</literal> or
5763 <literal>com.example.tea</literal></entry>
5773 <entry>Argument</entry>
5775 <entry>Description</entry>
5781 <entry>UINT32</entry>
5782 <entry>Unix user ID</entry>
5787 Returns the Unix user ID of the process connected to the server. If
5788 unable to determine it (for instance, because the process is not on the
5789 same machine as the bus daemon), an error is returned.
5793 <sect3 id="bus-messages-get-connection-unix-process-id">
5794 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5798 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5805 <entry>Argument</entry>
5807 <entry>Description</entry>
5813 <entry>STRING</entry>
5814 <entry>Unique or well-known bus name of the connection to
5815 query, such as <literal>:12.34</literal> or
5816 <literal>com.example.tea</literal></entry>
5826 <entry>Argument</entry>
5828 <entry>Description</entry>
5834 <entry>UINT32</entry>
5835 <entry>Unix process id</entry>
5840 Returns the Unix process ID of the process connected to the server. If
5841 unable to determine it (for instance, because the process is not on the
5842 same machine as the bus daemon), an error is returned.
5846 <sect3 id="bus-messages-get-connection-credentials">
5847 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5851 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5858 <entry>Argument</entry>
5860 <entry>Description</entry>
5866 <entry>STRING</entry>
5867 <entry>Unique or well-known bus name of the connection to
5868 query, such as <literal>:12.34</literal> or
5869 <literal>com.example.tea</literal></entry>
5879 <entry>Argument</entry>
5881 <entry>Description</entry>
5887 <entry>DICT<STRING,VARIANT></entry>
5888 <entry>Credentials</entry>
5896 Returns as many credentials as possible for the process connected to
5897 the server. If unable to determine certain credentials (for instance,
5898 because the process is not on the same machine as the bus daemon,
5899 or because this version of the bus daemon does not support a
5900 particular security framework), or if the values of those credentials
5901 cannot be represented as documented here, then those credentials
5906 Keys in the returned dictionary not containing "." are defined
5907 by this specification. Bus daemon implementors supporting
5908 credentials frameworks not mentioned in this document should either
5909 contribute patches to this specification, or use keys containing
5910 "." and starting with a reversed domain name.
5916 <entry>Value type</entry>
5917 <entry>Value</entry>
5922 <entry>UnixUserID</entry>
5923 <entry>UINT32</entry>
5924 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5927 <entry>ProcessID</entry>
5928 <entry>UINT32</entry>
5929 <entry>The numeric process ID, on platforms that have
5930 this concept. On Unix, this is the process ID defined by
5939 This method was added in D-Bus 1.7 to reduce the round-trips
5940 required to list a process's credentials. In older versions, calling
5941 this method will fail: applications should recover by using the
5942 separate methods such as
5943 <xref linkend="bus-messages-get-connection-unix-user"/>
5948 <sect3 id="bus-messages-get-adt-audit-session-data">
5949 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5953 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5960 <entry>Argument</entry>
5962 <entry>Description</entry>
5968 <entry>STRING</entry>
5969 <entry>Unique or well-known bus name of the connection to
5970 query, such as <literal>:12.34</literal> or
5971 <literal>com.example.tea</literal></entry>
5981 <entry>Argument</entry>
5983 <entry>Description</entry>
5989 <entry>ARRAY of BYTE</entry>
5990 <entry>auditing data as returned by
5991 adt_export_session_data()</entry>
5996 Returns auditing data used by Solaris ADT, in an unspecified
5997 binary format. If you know what this means, please contribute
5998 documentation via the D-Bus bug tracking system.
5999 This method is on the core DBus interface for historical reasons;
6000 the same information should be made available via
6001 <xref linkend="bus-messages-get-connection-credentials"/>
6006 <sect3 id="bus-messages-get-connection-selinux-security-context">
6007 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6011 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6018 <entry>Argument</entry>
6020 <entry>Description</entry>
6026 <entry>STRING</entry>
6027 <entry>Unique or well-known bus name of the connection to
6028 query, such as <literal>:12.34</literal> or
6029 <literal>com.example.tea</literal></entry>
6039 <entry>Argument</entry>
6041 <entry>Description</entry>
6047 <entry>ARRAY of BYTE</entry>
6048 <entry>some sort of string of bytes, not necessarily UTF-8,
6049 not including '\0'</entry>
6054 Returns the security context used by SELinux, in an unspecified
6055 format. If you know what this means, please contribute
6056 documentation via the D-Bus bug tracking system.
6057 This method is on the core DBus interface for historical reasons;
6058 the same information should be made available via
6059 <xref linkend="bus-messages-get-connection-credentials"/>
6065 <sect3 id="bus-messages-add-match">
6066 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6070 AddMatch (in STRING rule)
6077 <entry>Argument</entry>
6079 <entry>Description</entry>
6085 <entry>STRING</entry>
6086 <entry>Match rule to add to the connection</entry>
6091 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6092 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6096 <sect3 id="bus-messages-remove-match">
6097 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6101 RemoveMatch (in STRING rule)
6108 <entry>Argument</entry>
6110 <entry>Description</entry>
6116 <entry>STRING</entry>
6117 <entry>Match rule to remove from the connection</entry>
6122 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6123 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6128 <sect3 id="bus-messages-get-id">
6129 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6133 GetId (out STRING id)
6140 <entry>Argument</entry>
6142 <entry>Description</entry>
6148 <entry>STRING</entry>
6149 <entry>Unique ID identifying the bus daemon</entry>
6154 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6155 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6156 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6157 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6158 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6159 by org.freedesktop.DBus.Peer.GetMachineId().
6160 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6168 <appendix id="implementation-notes">
6169 <title>Implementation notes</title>
6170 <sect1 id="implementation-notes-subsection">
6178 <glossary><title>Glossary</title>
6180 This glossary defines some of the terms used in this specification.
6183 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6186 The message bus maintains an association between names and
6187 connections. (Normally, there's one connection per application.) A
6188 bus name is simply an identifier used to locate connections. For
6189 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6190 name might be used to send a message to a screensaver from Yoyodyne
6191 Corporation. An application is said to <firstterm>own</firstterm> a
6192 name if the message bus has associated the application's connection
6193 with the name. Names may also have <firstterm>queued
6194 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6195 The bus assigns a unique name to each connection,
6196 see <xref linkend="term-unique-name"/>. Other names
6197 can be thought of as "well-known names" and are
6198 used to find applications that offer specific functionality.
6202 See <xref linkend="message-protocol-names-bus"/> for details of
6203 the syntax and naming conventions for bus names.
6208 <glossentry id="term-message"><glossterm>Message</glossterm>
6211 A message is the atomic unit of communication via the D-Bus
6212 protocol. It consists of a <firstterm>header</firstterm> and a
6213 <firstterm>body</firstterm>; the body is made up of
6214 <firstterm>arguments</firstterm>.
6219 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6222 The message bus is a special application that forwards
6223 or routes messages between a group of applications
6224 connected to the message bus. It also manages
6225 <firstterm>names</firstterm> used for routing
6231 <glossentry id="term-name"><glossterm>Name</glossterm>
6234 See <xref linkend="term-bus-name"/>. "Name" may
6235 also be used to refer to some of the other names
6236 in D-Bus, such as interface names.
6241 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6244 Used to prevent collisions when defining new interfaces, bus names
6245 etc. The convention used is the same one Java uses for defining
6246 classes: a reversed domain name.
6247 See <xref linkend="message-protocol-names-bus"/>,
6248 <xref linkend="message-protocol-names-interface"/>,
6249 <xref linkend="message-protocol-names-error"/>,
6250 <xref linkend="message-protocol-marshaling-object-path"/>.
6255 <glossentry id="term-object"><glossterm>Object</glossterm>
6258 Each application contains <firstterm>objects</firstterm>, which have
6259 <firstterm>interfaces</firstterm> and
6260 <firstterm>methods</firstterm>. Objects are referred to by a name,
6261 called a <firstterm>path</firstterm>.
6266 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6269 An application talking directly to another application, without going
6270 through a message bus. One-to-one connections may be "peer to peer" or
6271 "client to server." The D-Bus protocol has no concept of client
6272 vs. server after a connection has authenticated; the flow of messages
6273 is symmetrical (full duplex).
6278 <glossentry id="term-path"><glossterm>Path</glossterm>
6281 Object references (object names) in D-Bus are organized into a
6282 filesystem-style hierarchy, so each object is named by a path. As in
6283 LDAP, there's no difference between "files" and "directories"; a path
6284 can refer to an object, while still having child objects below it.
6289 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6292 Each bus name has a primary owner; messages sent to the name go to the
6293 primary owner. However, certain names also maintain a queue of
6294 secondary owners "waiting in the wings." If the primary owner releases
6295 the name, then the first secondary owner in the queue automatically
6296 becomes the new owner of the name.
6301 <glossentry id="term-service"><glossterm>Service</glossterm>
6304 A service is an executable that can be launched by the bus daemon.
6305 Services normally guarantee some particular features, for example they
6306 may guarantee that they will request a specific name such as
6307 "com.example.Screensaver", have a singleton object
6308 "/com/example/Application", and that object will implement the
6309 interface "com.example.Screensaver.Control".
6314 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6317 ".service files" tell the bus about service applications that can be
6318 launched (see <xref linkend="term-service"/>). Most importantly they
6319 provide a mapping from bus names to services that will request those
6320 names when they start up.
6325 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6328 The special name automatically assigned to each connection by the
6329 message bus. This name will never change owner, and will be unique
6330 (never reused during the lifetime of the message bus).
6331 It will begin with a ':' character.
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