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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.20</releaseinfo>
10 <date>unreleased</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>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>current</revnumber>
76 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
77 <authorinitials>smcv, walters</authorinitials>
78 <revremark>reorganise for clarity, remove false claims about
79 basic types, mention /o/fd/DBus</revremark>
82 <revnumber>0.19</revnumber>
83 <date>20 February 2012</date>
84 <authorinitials>smcv/lp</authorinitials>
85 <revremark>formally define unique connection names and well-known
86 bus names; document best practices for interface, bus, member and
87 error names, and object paths; document the search path for session
88 and system services on Unix; document the systemd transport</revremark>
91 <revnumber>0.18</revnumber>
92 <date>29 July 2011</date>
93 <authorinitials>smcv</authorinitials>
94 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
95 match keyword; promote type system to a top-level section</revremark>
98 <revnumber>0.17</revnumber>
99 <date>1 June 2011</date>
100 <authorinitials>smcv/davidz</authorinitials>
101 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
102 by GVariant</revremark>
105 <revnumber>0.16</revnumber>
106 <date>11 April 2011</date>
107 <authorinitials></authorinitials>
108 <revremark>add path_namespace, arg0namespace; argNpath matches object
112 <revnumber>0.15</revnumber>
113 <date>3 November 2010</date>
114 <authorinitials></authorinitials>
115 <revremark></revremark>
118 <revnumber>0.14</revnumber>
119 <date>12 May 2010</date>
120 <authorinitials></authorinitials>
121 <revremark></revremark>
124 <revnumber>0.13</revnumber>
125 <date>23 Dezember 2009</date>
126 <authorinitials></authorinitials>
127 <revremark></revremark>
130 <revnumber>0.12</revnumber>
131 <date>7 November, 2006</date>
132 <authorinitials></authorinitials>
133 <revremark></revremark>
136 <revnumber>0.11</revnumber>
137 <date>6 February 2005</date>
138 <authorinitials></authorinitials>
139 <revremark></revremark>
142 <revnumber>0.10</revnumber>
143 <date>28 January 2005</date>
144 <authorinitials></authorinitials>
145 <revremark></revremark>
148 <revnumber>0.9</revnumber>
149 <date>7 Januar 2005</date>
150 <authorinitials></authorinitials>
151 <revremark></revremark>
154 <revnumber>0.8</revnumber>
155 <date>06 September 2003</date>
156 <authorinitials></authorinitials>
157 <revremark>First released document.</revremark>
162 <sect1 id="introduction">
163 <title>Introduction</title>
165 D-Bus is a system for low-latency, low-overhead, easy to use
166 interprocess communication (IPC). In more detail:
170 D-Bus is <emphasis>low-latency</emphasis> because it is designed
171 to avoid round trips and allow asynchronous operation, much like
177 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
178 binary protocol, and does not have to convert to and from a text
179 format such as XML. Because D-Bus is intended for potentially
180 high-resolution same-machine IPC, not primarily for Internet IPC,
181 this is an interesting optimization.
186 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
187 of <firstterm>messages</firstterm> rather than byte streams, and
188 automatically handles a lot of the hard IPC issues. Also, the D-Bus
189 library is designed to be wrapped in a way that lets developers use
190 their framework's existing object/type system, rather than learning
191 a new one specifically for IPC.
198 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
199 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
200 a system for one application to talk to a single other
201 application. However, the primary intended application of the protocol is the
202 D-Bus <firstterm>message bus</firstterm>, specified in <xref
203 linkend="message-bus"/>. The message bus is a special application that
204 accepts connections from multiple other applications, and forwards
209 Uses of D-Bus include notification of system changes (notification of when
210 a camera is plugged in to a computer, or a new version of some software
211 has been installed), or desktop interoperability, for example a file
212 monitoring service or a configuration service.
216 D-Bus is designed for two specific use cases:
220 A "system bus" for notifications from the system to user sessions,
221 and to allow the system to request input from user sessions.
226 A "session bus" used to implement desktop environments such as
231 D-Bus is not intended to be a generic IPC system for any possible
232 application, and intentionally omits many features found in other
233 IPC systems for this reason.
237 At the same time, the bus daemons offer a number of features not found in
238 other IPC systems, such as single-owner "bus names" (similar to X
239 selections), on-demand startup of services, and security policies.
240 In many ways, these features are the primary motivation for developing
241 D-Bus; other systems would have sufficed if IPC were the only goal.
245 D-Bus may turn out to be useful in unanticipated applications, but future
246 versions of this spec and the reference implementation probably will not
247 incorporate features that interfere with the core use cases.
251 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
252 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
253 document are to be interpreted as described in RFC 2119. However, the
254 document could use a serious audit to be sure it makes sense to do
255 so. Also, they are not capitalized.
258 <sect2 id="stability">
259 <title>Protocol and Specification Stability</title>
261 The D-Bus protocol is frozen (only compatible extensions are allowed) as
262 of November 8, 2006. However, this specification could still use a fair
263 bit of work to make interoperable reimplementation possible without
264 reference to the D-Bus reference implementation. Thus, this
265 specification is not marked 1.0. To mark it 1.0, we'd like to see
266 someone invest significant effort in clarifying the specification
267 language, and growing the specification to cover more aspects of the
268 reference implementation's behavior.
271 Until this work is complete, any attempt to reimplement D-Bus will
272 probably require looking at the reference implementation and/or asking
273 questions on the D-Bus mailing list about intended behavior.
274 Questions on the list are very welcome.
277 Nonetheless, this document should be a useful starting point and is
278 to our knowledge accurate, though incomplete.
284 <sect1 id="type-system">
285 <title>Type System</title>
288 D-Bus has a type system, in which values of various types can be
289 serialized into a sequence of bytes referred to as the
290 <firstterm>wire format</firstterm> in a standard way.
291 Converting a value from some other representation into the wire
292 format is called <firstterm>marshaling</firstterm> and converting
293 it back from the wire format is <firstterm>unmarshaling</firstterm>.
297 The D-Bus protocol does not include type tags in the marshaled data; a
298 block of marshaled values must have a known <firstterm>type
299 signature</firstterm>. The type signature is made up of zero or more
300 <firstterm id="term-single-complete-type">single complete
301 types</firstterm>, each made up of one or more
302 <firstterm>type codes</firstterm>.
306 A type code is an ASCII character representing the
307 type of a value. Because ASCII characters are used, the type signature
308 will always form a valid ASCII string. A simple string compare
309 determines whether two type signatures are equivalent.
313 A single complete type is a sequence of type codes that fully describes
314 one type: either a basic type, or a single fully-described container type.
315 A single complete type is a basic type code, a variant type code,
316 an array with its element type, or a struct with its fields (all of which
317 are defined below). So the following signatures are not single complete
328 And the following signatures contain multiple complete types:
338 Note however that a single complete type may <emphasis>contain</emphasis>
339 multiple other single complete types, by containing a struct or dict
343 <sect2 id="basic-types">
344 <title>Basic types</title>
347 The simplest type codes are the <firstterm id="term-basic-type">basic
348 types</firstterm>, which are the types whose structure is entirely
349 defined by their 1-character type code. Basic types consist of
350 fixed types and string-like types.
354 The <firstterm id="term-fixed-type">fixed types</firstterm>
355 are basic types whose values have a fixed length, namely BYTE,
356 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
361 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
362 the ASCII character 'i'. So the signature for a block of values
363 containing a single <literal>INT32</literal> would be:
367 A block of values containing two <literal>INT32</literal> would have this signature:
374 The characteristics of the fixed types are listed in this table.
380 <entry>Conventional name</entry>
381 <entry>ASCII type-code</entry>
382 <entry>Encoding</entry>
387 <entry><literal>BYTE</literal></entry>
388 <entry><literal>y</literal> (121)</entry>
389 <entry>Unsigned 8-bit integer</entry>
392 <entry><literal>BOOLEAN</literal></entry>
393 <entry><literal>b</literal> (98)</entry>
394 <entry>Boolean value: 0 is false, 1 is true, any other value
395 allowed by the marshalling format is invalid</entry>
398 <entry><literal>INT16</literal></entry>
399 <entry><literal>n</literal> (110)</entry>
400 <entry>Signed (two's complement) 16-bit integer</entry>
403 <entry><literal>UINT16</literal></entry>
404 <entry><literal>q</literal> (113)</entry>
405 <entry>Unsigned 16-bit integer</entry>
408 <entry><literal>INT32</literal></entry>
409 <entry><literal>i</literal> (105)</entry>
410 <entry>Signed (two's complement) 32-bit integer</entry>
413 <entry><literal>UINT32</literal></entry>
414 <entry><literal>u</literal> (117)</entry>
415 <entry>Unsigned 32-bit integer</entry>
418 <entry><literal>INT64</literal></entry>
419 <entry><literal>x</literal> (120)</entry>
420 <entry>Signed (two's complement) 64-bit integer
421 (mnemonic: x and t are the first characters in "sixty" not
422 already used for something more common)</entry>
425 <entry><literal>UINT64</literal></entry>
426 <entry><literal>t</literal> (116)</entry>
427 <entry>Unsigned 64-bit integer</entry>
430 <entry><literal>DOUBLE</literal></entry>
431 <entry><literal>d</literal> (100)</entry>
432 <entry>IEEE 754 double-precision floating point</entry>
435 <entry><literal>UNIX_FD</literal></entry>
436 <entry><literal>h</literal> (104)</entry>
437 <entry>Unsigned 32-bit integer representing an index into an
438 out-of-band array of file descriptors, transferred via some
439 platform-specific mechanism (mnemonic: h for handle)</entry>
447 The <firstterm id="term-string-like-type">string-like types</firstterm>
448 are basic types with a variable length. The value of any string-like
449 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
450 none of which may be U+0000. The UTF-8 text must be validated
451 strictly: in particular, it must not contain overlong sequences,
452 noncharacters such as U+FFFE, or codepoints above U+10FFFF.
456 The marshalling formats for the string-like types all end with a
457 single zero (NUL) byte, but that byte is not considered to be part of
462 The characteristics of the string-like types are listed in this table.
468 <entry>Conventional name</entry>
469 <entry>ASCII type-code</entry>
470 <entry>Validity constraints</entry>
475 <entry><literal>STRING</literal></entry>
476 <entry><literal>s</literal> (115)</entry>
477 <entry>No extra constraints</entry>
480 <entry><literal>OBJECT_PATH</literal></entry>
481 <entry><literal>o</literal> (111)</entry>
483 <link linkend="message-protocol-marshaling-object-path">a
484 syntactically valid object path</link></entry>
487 <entry><literal>SIGNATURE</literal></entry>
488 <entry><literal>g</literal> (103)</entry>
490 <firstterm linkend="term-single-complete-type">single
491 complete types</firstterm></entry>
498 <sect3 id="message-protocol-marshaling-object-path">
499 <title>Valid Object Paths</title>
502 An object path is a name used to refer to an object instance.
503 Conceptually, each participant in a D-Bus message exchange may have
504 any number of object instances (think of C++ or Java objects) and each
505 such instance will have a path. Like a filesystem, the object
506 instances in an application form a hierarchical tree.
510 Object paths are often namespaced by starting with a reversed
511 domain name and containing an interface version number, in the
513 <link linkend="message-protocol-names-interface">interface
515 <link linkend="message-protocol-names-bus">well-known
517 This makes it possible to implement more than one service, or
518 more than one version of a service, in the same process,
519 even if the services share a connection but cannot otherwise
520 co-operate (for instance, if they are implemented by different
525 For instance, if the owner of <literal>example.com</literal> is
526 developing a D-Bus API for a music player, they might use the
527 hierarchy of object paths that start with
528 <literal>/com/example/MusicPlayer1</literal> for its objects.
532 The following rules define a valid object path. Implementations must
533 not send or accept messages with invalid object paths.
537 The path may be of any length.
542 The path must begin with an ASCII '/' (integer 47) character,
543 and must consist of elements separated by slash characters.
548 Each element must only contain the ASCII characters
554 No element may be the empty string.
559 Multiple '/' characters cannot occur in sequence.
564 A trailing '/' character is not allowed unless the
565 path is the root path (a single '/' character).
573 <sect3 id="message-protocol-marshaling-signature">
574 <title>Valid Signatures</title>
576 An implementation must not send or accept invalid signatures.
577 Valid signatures will conform to the following rules:
581 The signature is a list of single complete types.
582 Arrays must have element types, and structs must
583 have both open and close parentheses.
588 Only type codes, open and close parentheses, and open and
589 close curly brackets are allowed in the signature. The
590 <literal>STRUCT</literal> type code
591 is not allowed in signatures, because parentheses
592 are used instead. Similarly, the
593 <literal>DICT_ENTRY</literal> type code is not allowed in
594 signatures, because curly brackets are used instead.
599 The maximum depth of container type nesting is 32 array type
600 codes and 32 open parentheses. This implies that the maximum
601 total depth of recursion is 64, for an "array of array of array
602 of ... struct of struct of struct of ..." where there are 32
608 The maximum length of a signature is 255.
615 When signatures appear in messages, the marshalling format
616 guarantees that they will be followed by a nul byte (which can
617 be interpreted as either C-style string termination or the INVALID
618 type-code), but this is not conceptually part of the signature.
624 <sect2 id="container-types">
625 <title>Container types</title>
628 In addition to basic types, there are four <firstterm>container</firstterm>
629 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
630 and <literal>DICT_ENTRY</literal>.
634 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
635 code does not appear in signatures. Instead, ASCII characters
636 '(' and ')' are used to mark the beginning and end of the struct.
637 So for example, a struct containing two integers would have this
642 Structs can be nested, so for example a struct containing
643 an integer and another struct:
647 The value block storing that struct would contain three integers; the
648 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
653 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
654 but is useful in code that implements the protocol. This type code
655 is specified to allow such code to interoperate in non-protocol contexts.
659 Empty structures are not allowed; there must be at least one
660 type code between the parentheses.
664 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
665 followed by a <firstterm>single complete type</firstterm>. The single
666 complete type following the array is the type of each array element. So
667 the simple example is:
671 which is an array of 32-bit integers. But an array can be of any type,
672 such as this array-of-struct-with-two-int32-fields:
676 Or this array of array of integer:
683 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
684 type <literal>VARIANT</literal> will have the signature of a single complete type as part
685 of the <emphasis>value</emphasis>. This signature will be followed by a
686 marshaled value of that type.
690 Unlike a message signature, the variant signature can
691 contain only a single complete type. So "i", "ai"
692 or "(ii)" is OK, but "ii" is not. Use of variants may not
693 cause a total message depth to be larger than 64, including
694 other container types such as structures.
698 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
699 than parentheses it uses curly braces, and it has more restrictions.
700 The restrictions are: it occurs only as an array element type; it has
701 exactly two single complete types inside the curly braces; the first
702 single complete type (the "key") must be a basic type rather than a
703 container type. Implementations must not accept dict entries outside of
704 arrays, must not accept dict entries with zero, one, or more than two
705 fields, and must not accept dict entries with non-basic-typed keys. A
706 dict entry is always a key-value pair.
710 The first field in the <literal>DICT_ENTRY</literal> is always the key.
711 A message is considered corrupt if the same key occurs twice in the same
712 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
713 implementations are not required to reject dicts with duplicate keys.
717 In most languages, an array of dict entry would be represented as a
718 map, hash table, or dict object.
723 <title>Summary of types</title>
726 The following table summarizes the D-Bus types.
731 <entry>Conventional Name</entry>
733 <entry>Description</entry>
738 <entry><literal>INVALID</literal></entry>
739 <entry>0 (ASCII NUL)</entry>
740 <entry>Not a valid type code, used to terminate signatures</entry>
742 <entry><literal>BYTE</literal></entry>
743 <entry>121 (ASCII 'y')</entry>
744 <entry>8-bit unsigned integer</entry>
746 <entry><literal>BOOLEAN</literal></entry>
747 <entry>98 (ASCII 'b')</entry>
748 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
750 <entry><literal>INT16</literal></entry>
751 <entry>110 (ASCII 'n')</entry>
752 <entry>16-bit signed integer</entry>
754 <entry><literal>UINT16</literal></entry>
755 <entry>113 (ASCII 'q')</entry>
756 <entry>16-bit unsigned integer</entry>
758 <entry><literal>INT32</literal></entry>
759 <entry>105 (ASCII 'i')</entry>
760 <entry>32-bit signed integer</entry>
762 <entry><literal>UINT32</literal></entry>
763 <entry>117 (ASCII 'u')</entry>
764 <entry>32-bit unsigned integer</entry>
766 <entry><literal>INT64</literal></entry>
767 <entry>120 (ASCII 'x')</entry>
768 <entry>64-bit signed integer</entry>
770 <entry><literal>UINT64</literal></entry>
771 <entry>116 (ASCII 't')</entry>
772 <entry>64-bit unsigned integer</entry>
774 <entry><literal>DOUBLE</literal></entry>
775 <entry>100 (ASCII 'd')</entry>
776 <entry>IEEE 754 double</entry>
778 <entry><literal>STRING</literal></entry>
779 <entry>115 (ASCII 's')</entry>
780 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
782 <entry><literal>OBJECT_PATH</literal></entry>
783 <entry>111 (ASCII 'o')</entry>
784 <entry>Name of an object instance</entry>
786 <entry><literal>SIGNATURE</literal></entry>
787 <entry>103 (ASCII 'g')</entry>
788 <entry>A type signature</entry>
790 <entry><literal>ARRAY</literal></entry>
791 <entry>97 (ASCII 'a')</entry>
794 <entry><literal>STRUCT</literal></entry>
795 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
796 <entry>Struct; type code 114 'r' is reserved for use in
797 bindings and implementations to represent the general
798 concept of a struct, and must not appear in signatures
799 used on D-Bus.</entry>
801 <entry><literal>VARIANT</literal></entry>
802 <entry>118 (ASCII 'v') </entry>
803 <entry>Variant type (the type of the value is part of the value itself)</entry>
805 <entry><literal>DICT_ENTRY</literal></entry>
806 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
807 <entry>Entry in a dict or map (array of key-value pairs).
808 Type code 101 'e' is reserved for use in bindings and
809 implementations to represent the general concept of a
810 dict or dict-entry, and must not appear in signatures
811 used on D-Bus.</entry>
813 <entry><literal>UNIX_FD</literal></entry>
814 <entry>104 (ASCII 'h')</entry>
815 <entry>Unix file descriptor</entry>
818 <entry>(reserved)</entry>
819 <entry>109 (ASCII 'm')</entry>
820 <entry>Reserved for <ulink
821 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
822 'maybe' type compatible with the one in GVariant</ulink>,
823 and must not appear in signatures used on D-Bus until
824 specified here</entry>
827 <entry>(reserved)</entry>
828 <entry>42 (ASCII '*')</entry>
829 <entry>Reserved for use in bindings/implementations to
830 represent any <firstterm>single complete type</firstterm>,
831 and must not appear in signatures used on D-Bus.</entry>
834 <entry>(reserved)</entry>
835 <entry>63 (ASCII '?')</entry>
836 <entry>Reserved for use in bindings/implementations to
837 represent any <firstterm>basic type</firstterm>, and must
838 not appear in signatures used on D-Bus.</entry>
841 <entry>(reserved)</entry>
842 <entry>64 (ASCII '@'), 38 (ASCII '&'),
843 94 (ASCII '^')</entry>
844 <entry>Reserved for internal use by bindings/implementations,
845 and must not appear in signatures used on D-Bus.
846 GVariant uses these type-codes to encode calling
857 <sect1 id="message-protocol-marshaling">
858 <title>Marshaling (Wire Format)</title>
861 D-Bus defines a marshalling format for its type system, which is
862 used in D-Bus messages. This is not the only possible marshalling
863 format for the type system: for instance, GVariant (part of GLib)
864 re-uses the D-Bus type system but implements an alternative marshalling
869 <title>Byte order and alignment</title>
872 Given a type signature, a block of bytes can be converted into typed
873 values. This section describes the format of the block of bytes. Byte
874 order and alignment issues are handled uniformly for all D-Bus types.
878 A block of bytes has an associated byte order. The byte order
879 has to be discovered in some way; for D-Bus messages, the
880 byte order is part of the message header as described in
881 <xref linkend="message-protocol-messages"/>. For now, assume
882 that the byte order is known to be either little endian or big
887 Each value in a block of bytes is aligned "naturally," for example
888 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
889 8-byte boundary. To properly align a value, <firstterm>alignment
890 padding</firstterm> may be necessary. The alignment padding must always
891 be the minimum required padding to properly align the following value;
892 and it must always be made up of nul bytes. The alignment padding must
893 not be left uninitialized (it can't contain garbage), and more padding
894 than required must not be used.
898 As an exception to natural alignment, <literal>STRUCT</literal> and
899 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
900 boundary, regardless of the alignments of their contents.
905 <title>Marshalling basic types</title>
908 To marshal and unmarshal fixed types, you simply read one value
909 from the data block corresponding to each type code in the signature.
910 All signed integer values are encoded in two's complement, DOUBLE
911 values are IEEE 754 double-precision floating-point, and BOOLEAN
912 values are encoded in 32 bits (of which only the least significant
917 The string-like types are all marshalled as a
918 fixed-length unsigned integer <varname>n</varname> giving the
919 length of the variable part, followed by <varname>n</varname>
920 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
921 which is not considered to be part of the text. The alignment
922 of the string-like type is the same as the alignment of
923 <varname>n</varname>.
927 For the STRING and OBJECT_PATH types, <varname>n</varname> is
928 encoded in 4 bytes, leading to 4-byte alignment.
929 For the SIGNATURE type, <varname>n</varname> is encoded as a single
930 byte. As a result, alignment padding is never required before a
936 <title>Marshalling containers</title>
939 Arrays are marshalled as a <literal>UINT32</literal>
940 <varname>n</varname> giving the length of the array data in bytes,
941 followed by alignment padding to the alignment boundary of the array
942 element type, followed by the <varname>n</varname> bytes of the
943 array elements marshalled in sequence. <varname>n</varname> does not
944 include the padding after the length, or any padding after the
949 For instance, if the current position in the message is a multiple
950 of 8 bytes and the byte-order is big-endian, an array containing only
951 the 64-bit integer 5 would be marshalled as:
954 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
955 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
956 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
961 Arrays have a maximum length defined to be 2 to the 26th power or
962 67108864. Implementations must not send or accept arrays exceeding this
967 Structs and dict entries are marshalled in the same way as their
968 contents, but their alignment is always to an 8-byte boundary,
969 even if their contents would normally be less strictly aligned.
973 Variants are marshalled as the <literal>SIGNATURE</literal> of
974 the contents (which must be a single complete type), followed by a
975 marshalled value with the type given by that signature. The
976 variant has the same 1-byte alignment as the signature, which means
977 that alignment padding before a variant is never needed.
978 Use of variants may not cause a total message depth to be larger
979 than 64, including other container types such as structures.
984 <title>Summary of D-Bus marshalling</title>
987 Given all this, the types are marshaled on the wire as follows:
992 <entry>Conventional Name</entry>
993 <entry>Encoding</entry>
994 <entry>Alignment</entry>
999 <entry><literal>INVALID</literal></entry>
1000 <entry>Not applicable; cannot be marshaled.</entry>
1003 <entry><literal>BYTE</literal></entry>
1004 <entry>A single 8-bit byte.</entry>
1007 <entry><literal>BOOLEAN</literal></entry>
1008 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1011 <entry><literal>INT16</literal></entry>
1012 <entry>16-bit signed integer in the message's byte order.</entry>
1015 <entry><literal>UINT16</literal></entry>
1016 <entry>16-bit unsigned integer in the message's byte order.</entry>
1019 <entry><literal>INT32</literal></entry>
1020 <entry>32-bit signed integer in the message's byte order.</entry>
1023 <entry><literal>UINT32</literal></entry>
1024 <entry>32-bit unsigned integer in the message's byte order.</entry>
1027 <entry><literal>INT64</literal></entry>
1028 <entry>64-bit signed integer in the message's byte order.</entry>
1031 <entry><literal>UINT64</literal></entry>
1032 <entry>64-bit unsigned integer in the message's byte order.</entry>
1035 <entry><literal>DOUBLE</literal></entry>
1036 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1039 <entry><literal>STRING</literal></entry>
1040 <entry>A <literal>UINT32</literal> indicating the string's
1041 length in bytes excluding its terminating nul, followed by
1042 non-nul string data of the given length, followed by a terminating nul
1049 <entry><literal>OBJECT_PATH</literal></entry>
1050 <entry>Exactly the same as <literal>STRING</literal> except the
1051 content must be a valid object path (see above).
1057 <entry><literal>SIGNATURE</literal></entry>
1058 <entry>The same as <literal>STRING</literal> except the length is a single
1059 byte (thus signatures have a maximum length of 255)
1060 and the content must be a valid signature (see above).
1066 <entry><literal>ARRAY</literal></entry>
1068 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1069 alignment padding to the alignment boundary of the array element type,
1070 followed by each array element.
1076 <entry><literal>STRUCT</literal></entry>
1078 A struct must start on an 8-byte boundary regardless of the
1079 type of the struct fields. The struct value consists of each
1080 field marshaled in sequence starting from that 8-byte
1087 <entry><literal>VARIANT</literal></entry>
1089 The marshaled <literal>SIGNATURE</literal> of a single
1090 complete type, followed by a marshaled value with the type
1091 given in the signature.
1094 1 (alignment of the signature)
1097 <entry><literal>DICT_ENTRY</literal></entry>
1099 Identical to STRUCT.
1105 <entry><literal>UNIX_FD</literal></entry>
1106 <entry>32-bit unsigned integer in the message's byte
1107 order. The actual file descriptors need to be
1108 transferred out-of-band via some platform specific
1109 mechanism. On the wire, values of this type store the index to the
1110 file descriptor in the array of file descriptors that
1111 accompany the message.</entry>
1123 <sect1 id="message-protocol">
1124 <title>Message Protocol</title>
1127 A <firstterm>message</firstterm> consists of a
1128 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1129 think of a message as a package, the header is the address, and the body
1130 contains the package contents. The message delivery system uses the header
1131 information to figure out where to send the message and how to interpret
1132 it; the recipient interprets the body of the message.
1136 The body of the message is made up of zero or more
1137 <firstterm>arguments</firstterm>, which are typed values, such as an
1138 integer or a byte array.
1142 Both header and body use the D-Bus <link linkend="type-system">type
1143 system</link> and format for serializing data.
1146 <sect2 id="message-protocol-messages">
1147 <title>Message Format</title>
1150 A message consists of a header and a body. The header is a block of
1151 values with a fixed signature and meaning. The body is a separate block
1152 of values, with a signature specified in the header.
1156 The length of the header must be a multiple of 8, allowing the body to
1157 begin on an 8-byte boundary when storing the entire message in a single
1158 buffer. If the header does not naturally end on an 8-byte boundary
1159 up to 7 bytes of nul-initialized alignment padding must be added.
1163 The message body need not end on an 8-byte boundary.
1167 The maximum length of a message, including header, header alignment padding,
1168 and body is 2 to the 27th power or 134217728. Implementations must not
1169 send or accept messages exceeding this size.
1173 The signature of the header is:
1177 Written out more readably, this is:
1179 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1184 These values have the following meanings:
1189 <entry>Value</entry>
1190 <entry>Description</entry>
1195 <entry>1st <literal>BYTE</literal></entry>
1196 <entry>Endianness flag; ASCII 'l' for little-endian
1197 or ASCII 'B' for big-endian. Both header and body are
1198 in this endianness.</entry>
1201 <entry>2nd <literal>BYTE</literal></entry>
1202 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1203 Currently-defined types are described below.
1207 <entry>3rd <literal>BYTE</literal></entry>
1208 <entry>Bitwise OR of flags. Unknown flags
1209 must be ignored. Currently-defined flags are described below.
1213 <entry>4th <literal>BYTE</literal></entry>
1214 <entry>Major protocol version of the sending application. If
1215 the major protocol version of the receiving application does not
1216 match, the applications will not be able to communicate and the
1217 D-Bus connection must be disconnected. The major protocol
1218 version for this version of the specification is 1.
1222 <entry>1st <literal>UINT32</literal></entry>
1223 <entry>Length in bytes of the message body, starting
1224 from the end of the header. The header ends after
1225 its alignment padding to an 8-boundary.
1229 <entry>2nd <literal>UINT32</literal></entry>
1230 <entry>The serial of this message, used as a cookie
1231 by the sender to identify the reply corresponding
1232 to this request. This must not be zero.
1236 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1237 <entry>An array of zero or more <firstterm>header
1238 fields</firstterm> where the byte is the field code, and the
1239 variant is the field value. The message type determines
1240 which fields are required.
1248 <firstterm>Message types</firstterm> that can appear in the second byte
1254 <entry>Conventional name</entry>
1255 <entry>Decimal value</entry>
1256 <entry>Description</entry>
1261 <entry><literal>INVALID</literal></entry>
1263 <entry>This is an invalid type.</entry>
1266 <entry><literal>METHOD_CALL</literal></entry>
1268 <entry>Method call.</entry>
1271 <entry><literal>METHOD_RETURN</literal></entry>
1273 <entry>Method reply with returned data.</entry>
1276 <entry><literal>ERROR</literal></entry>
1278 <entry>Error reply. If the first argument exists and is a
1279 string, it is an error message.</entry>
1282 <entry><literal>SIGNAL</literal></entry>
1284 <entry>Signal emission.</entry>
1291 Flags that can appear in the third byte of the header:
1296 <entry>Conventional name</entry>
1297 <entry>Hex value</entry>
1298 <entry>Description</entry>
1303 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1305 <entry>This message does not expect method return replies or
1306 error replies; the reply can be omitted as an
1307 optimization. However, it is compliant with this specification
1308 to return the reply despite this flag and the only harm
1309 from doing so is extra network traffic.
1313 <entry><literal>NO_AUTO_START</literal></entry>
1315 <entry>The bus must not launch an owner
1316 for the destination name in response to this message.
1324 <sect3 id="message-protocol-header-fields">
1325 <title>Header Fields</title>
1328 The array at the end of the header contains <firstterm>header
1329 fields</firstterm>, where each field is a 1-byte field code followed
1330 by a field value. A header must contain the required header fields for
1331 its message type, and zero or more of any optional header
1332 fields. Future versions of this protocol specification may add new
1333 fields. Implementations must ignore fields they do not
1334 understand. Implementations must not invent their own header fields;
1335 only changes to this specification may introduce new header fields.
1339 Again, if an implementation sees a header field code that it does not
1340 expect, it must ignore that field, as it will be part of a new
1341 (but compatible) version of this specification. This also applies
1342 to known header fields appearing in unexpected messages, for
1343 example: if a signal has a reply serial it must be ignored
1344 even though it has no meaning as of this version of the spec.
1348 However, implementations must not send or accept known header fields
1349 with the wrong type stored in the field value. So for example a
1350 message with an <literal>INTERFACE</literal> field of type
1351 <literal>UINT32</literal> would be considered corrupt.
1355 Here are the currently-defined header fields:
1360 <entry>Conventional Name</entry>
1361 <entry>Decimal Code</entry>
1363 <entry>Required In</entry>
1364 <entry>Description</entry>
1369 <entry><literal>INVALID</literal></entry>
1372 <entry>not allowed</entry>
1373 <entry>Not a valid field name (error if it appears in a message)</entry>
1376 <entry><literal>PATH</literal></entry>
1378 <entry><literal>OBJECT_PATH</literal></entry>
1379 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1380 <entry>The object to send a call to,
1381 or the object a signal is emitted from.
1383 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1384 implementations should not send messages with this path,
1385 and the reference implementation of the bus daemon will
1386 disconnect any application that attempts to do so.
1390 <entry><literal>INTERFACE</literal></entry>
1392 <entry><literal>STRING</literal></entry>
1393 <entry><literal>SIGNAL</literal></entry>
1395 The interface to invoke a method call on, or
1396 that a signal is emitted from. Optional for
1397 method calls, required for signals.
1398 The special interface
1399 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1400 implementations should not send messages with this
1401 interface, and the reference implementation of the bus
1402 daemon will disconnect any application that attempts to
1407 <entry><literal>MEMBER</literal></entry>
1409 <entry><literal>STRING</literal></entry>
1410 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1411 <entry>The member, either the method name or signal name.</entry>
1414 <entry><literal>ERROR_NAME</literal></entry>
1416 <entry><literal>STRING</literal></entry>
1417 <entry><literal>ERROR</literal></entry>
1418 <entry>The name of the error that occurred, for errors</entry>
1421 <entry><literal>REPLY_SERIAL</literal></entry>
1423 <entry><literal>UINT32</literal></entry>
1424 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1425 <entry>The serial number of the message this message is a reply
1426 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1429 <entry><literal>DESTINATION</literal></entry>
1431 <entry><literal>STRING</literal></entry>
1432 <entry>optional</entry>
1433 <entry>The name of the connection this message is intended for.
1434 Only used in combination with the message bus, see
1435 <xref linkend="message-bus"/>.</entry>
1438 <entry><literal>SENDER</literal></entry>
1440 <entry><literal>STRING</literal></entry>
1441 <entry>optional</entry>
1442 <entry>Unique name of the sending connection.
1443 The message bus fills in this field so it is reliable; the field is
1444 only meaningful in combination with the message bus.</entry>
1447 <entry><literal>SIGNATURE</literal></entry>
1449 <entry><literal>SIGNATURE</literal></entry>
1450 <entry>optional</entry>
1451 <entry>The signature of the message body.
1452 If omitted, it is assumed to be the
1453 empty signature "" (i.e. the body must be 0-length).</entry>
1456 <entry><literal>UNIX_FDS</literal></entry>
1458 <entry><literal>UINT32</literal></entry>
1459 <entry>optional</entry>
1460 <entry>The number of Unix file descriptors that
1461 accompany the message. If omitted, it is assumed
1462 that no Unix file descriptors accompany the
1463 message. The actual file descriptors need to be
1464 transferred via platform specific mechanism
1465 out-of-band. They must be sent at the same time as
1466 part of the message itself. They may not be sent
1467 before the first byte of the message itself is
1468 transferred or after the last byte of the message
1478 <sect2 id="message-protocol-names">
1479 <title>Valid Names</title>
1481 The various names in D-Bus messages have some restrictions.
1484 There is a <firstterm>maximum name length</firstterm>
1485 of 255 which applies to bus names, interfaces, and members.
1487 <sect3 id="message-protocol-names-interface">
1488 <title>Interface names</title>
1490 Interfaces have names with type <literal>STRING</literal>, meaning that
1491 they must be valid UTF-8. However, there are also some
1492 additional restrictions that apply to interface names
1495 <listitem><para>Interface names are composed of 1 or more elements separated by
1496 a period ('.') character. All elements must contain at least
1500 <listitem><para>Each element must only contain the ASCII characters
1501 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1505 <listitem><para>Interface names must contain at least one '.' (period)
1506 character (and thus at least two elements).
1509 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1510 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1515 Interface names should start with the reversed DNS domain name of
1516 the author of the interface (in lower-case), like interface names
1517 in Java. It is conventional for the rest of the interface name
1518 to consist of words run together, with initial capital letters
1519 on all words ("CamelCase"). Several levels of hierarchy can be used.
1520 It is also a good idea to include the major version of the interface
1521 in the name, and increment it if incompatible changes are made;
1522 this way, a single object can implement several versions of an
1523 interface in parallel, if necessary.
1527 For instance, if the owner of <literal>example.com</literal> is
1528 developing a D-Bus API for a music player, they might define
1529 interfaces called <literal>com.example.MusicPlayer1</literal>,
1530 <literal>com.example.MusicPlayer1.Track</literal> and
1531 <literal>com.example.MusicPlayer1.Seekable</literal>.
1535 D-Bus does not distinguish between the concepts that would be
1536 called classes and interfaces in Java: either can be identified on
1537 D-Bus by an interface name.
1540 <sect3 id="message-protocol-names-bus">
1541 <title>Bus names</title>
1543 Connections have one or more bus names associated with them.
1544 A connection has exactly one bus name that is a <firstterm>unique
1545 connection name</firstterm>. The unique connection name remains
1546 with the connection for its entire lifetime.
1547 A bus name is of type <literal>STRING</literal>,
1548 meaning that it must be valid UTF-8. However, there are also
1549 some additional restrictions that apply to bus names
1552 <listitem><para>Bus names that start with a colon (':')
1553 character are unique connection names. Other bus names
1554 are called <firstterm>well-known bus names</firstterm>.
1557 <listitem><para>Bus names are composed of 1 or more elements separated by
1558 a period ('.') character. All elements must contain at least
1562 <listitem><para>Each element must only contain the ASCII characters
1563 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1564 connection name may begin with a digit, elements in
1565 other bus names must not begin with a digit.
1569 <listitem><para>Bus names must contain at least one '.' (period)
1570 character (and thus at least two elements).
1573 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1574 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1578 Note that the hyphen ('-') character is allowed in bus names but
1579 not in interface names.
1583 Like <link linkend="message-protocol-names-interface">interface
1584 names</link>, well-known bus names should start with the
1585 reversed DNS domain name of the author of the interface (in
1586 lower-case), and it is conventional for the rest of the well-known
1587 bus name to consist of words run together, with initial
1588 capital letters. As with interface names, including a version
1589 number in well-known bus names is a good idea; it's possible to
1590 have the well-known bus name for more than one version
1591 simultaneously if backwards compatibility is required.
1595 If a well-known bus name implies the presence of a "main" interface,
1596 that "main" interface is often given the same name as
1597 the well-known bus name, and situated at the corresponding object
1598 path. For instance, if the owner of <literal>example.com</literal>
1599 is developing a D-Bus API for a music player, they might define
1600 that any application that takes the well-known name
1601 <literal>com.example.MusicPlayer1</literal> should have an object
1602 at the object path <literal>/com/example/MusicPlayer1</literal>
1603 which implements the interface
1604 <literal>com.example.MusicPlayer1</literal>.
1607 <sect3 id="message-protocol-names-member">
1608 <title>Member names</title>
1610 Member (i.e. method or signal) names:
1612 <listitem><para>Must only contain the ASCII characters
1613 "[A-Z][a-z][0-9]_" and may not begin with a
1614 digit.</para></listitem>
1615 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1616 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1617 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1622 It is conventional for member names on D-Bus to consist of
1623 capitalized words with no punctuation ("camel-case").
1624 Method names should usually be verbs, such as
1625 <literal>GetItems</literal>, and signal names should usually be
1626 a description of an event, such as <literal>ItemsChanged</literal>.
1629 <sect3 id="message-protocol-names-error">
1630 <title>Error names</title>
1632 Error names have the same restrictions as interface names.
1636 Error names have the same naming conventions as interface
1637 names, and often contain <literal>.Error.</literal>; for instance,
1638 the owner of <literal>example.com</literal> might define the
1639 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1640 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1641 The errors defined by D-Bus itself, such as
1642 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1648 <sect2 id="message-protocol-types">
1649 <title>Message Types</title>
1651 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1652 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1653 This section describes these conventions.
1655 <sect3 id="message-protocol-types-method">
1656 <title>Method Calls</title>
1658 Some messages invoke an operation on a remote object. These are
1659 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1660 messages map naturally to methods on objects in a typical program.
1663 A method call message is required to have a <literal>MEMBER</literal> header field
1664 indicating the name of the method. Optionally, the message has an
1665 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1666 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1667 a method with the same name, it is undefined which of the two methods
1668 will be invoked. Implementations may also choose to return an error in
1669 this ambiguous case. However, if a method name is unique
1670 implementations must not require an interface field.
1673 Method call messages also include a <literal>PATH</literal> field
1674 indicating the object to invoke the method on. If the call is passing
1675 through a message bus, the message will also have a
1676 <literal>DESTINATION</literal> field giving the name of the connection
1677 to receive the message.
1680 When an application handles a method call message, it is required to
1681 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1682 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1683 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1686 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1687 are the return value(s) or "out parameters" of the method call.
1688 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1689 and the call fails; no return value will be provided. It makes
1690 no sense to send multiple replies to the same method call.
1693 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1694 reply is required, so the caller will know the method
1695 was successfully processed.
1698 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1702 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1703 then as an optimization the application receiving the method
1704 call may choose to omit the reply message (regardless of
1705 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1706 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1707 flag and reply anyway.
1710 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1711 destination name does not exist then a program to own the destination
1712 name will be started before the message is delivered. The message
1713 will be held until the new program is successfully started or has
1714 failed to start; in case of failure, an error will be returned. This
1715 flag is only relevant in the context of a message bus, it is ignored
1716 during one-to-one communication with no intermediate bus.
1718 <sect4 id="message-protocol-types-method-apis">
1719 <title>Mapping method calls to native APIs</title>
1721 APIs for D-Bus may map method calls to a method call in a specific
1722 programming language, such as C++, or may map a method call written
1723 in an IDL to a D-Bus message.
1726 In APIs of this nature, arguments to a method are often termed "in"
1727 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1728 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1729 "inout" arguments, which are both sent and received, i.e. the caller
1730 passes in a value which is modified. Mapped to D-Bus, an "inout"
1731 argument is equivalent to an "in" argument, followed by an "out"
1732 argument. You can't pass things "by reference" over the wire, so
1733 "inout" is purely an illusion of the in-process API.
1736 Given a method with zero or one return values, followed by zero or more
1737 arguments, where each argument may be "in", "out", or "inout", the
1738 caller constructs a message by appending each "in" or "inout" argument,
1739 in order. "out" arguments are not represented in the caller's message.
1742 The recipient constructs a reply by appending first the return value
1743 if any, then each "out" or "inout" argument, in order.
1744 "in" arguments are not represented in the reply message.
1747 Error replies are normally mapped to exceptions in languages that have
1751 In converting from native APIs to D-Bus, it is perhaps nice to
1752 map D-Bus naming conventions ("FooBar") to native conventions
1753 such as "fooBar" or "foo_bar" automatically. This is OK
1754 as long as you can say that the native API is one that
1755 was specifically written for D-Bus. It makes the most sense
1756 when writing object implementations that will be exported
1757 over the bus. Object proxies used to invoke remote D-Bus
1758 objects probably need the ability to call any D-Bus method,
1759 and thus a magic name mapping like this could be a problem.
1762 This specification doesn't require anything of native API bindings;
1763 the preceding is only a suggested convention for consistency
1769 <sect3 id="message-protocol-types-signal">
1770 <title>Signal Emission</title>
1772 Unlike method calls, signal emissions have no replies.
1773 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1774 It must have three header fields: <literal>PATH</literal> giving the object
1775 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1776 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1777 for signals, though it is optional for method calls.
1781 <sect3 id="message-protocol-types-errors">
1782 <title>Errors</title>
1784 Messages of type <literal>ERROR</literal> are most commonly replies
1785 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1786 to any kind of message. The message bus for example
1787 will return an <literal>ERROR</literal> in reply to a signal emission if
1788 the bus does not have enough memory to send the signal.
1791 An <literal>ERROR</literal> may have any arguments, but if the first
1792 argument is a <literal>STRING</literal>, it must be an error message.
1793 The error message may be logged or shown to the user
1798 <sect3 id="message-protocol-types-notation">
1799 <title>Notation in this document</title>
1801 This document uses a simple pseudo-IDL to describe particular method
1802 calls and signals. Here is an example of a method call:
1804 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1805 out UINT32 resultcode)
1807 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1808 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1809 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1810 characters so it's known that the last part of the name in
1811 the "IDL" is the member name.
1814 In C++ that might end up looking like this:
1816 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1817 unsigned int flags);
1819 or equally valid, the return value could be done as an argument:
1821 void org::freedesktop::DBus::StartServiceByName (const char *name,
1823 unsigned int *resultcode);
1825 It's really up to the API designer how they want to make
1826 this look. You could design an API where the namespace wasn't used
1827 in C++, using STL or Qt, using varargs, or whatever you wanted.
1830 Signals are written as follows:
1832 org.freedesktop.DBus.NameLost (STRING name)
1834 Signals don't specify "in" vs. "out" because only
1835 a single direction is possible.
1838 It isn't especially encouraged to use this lame pseudo-IDL in actual
1839 API implementations; you might use the native notation for the
1840 language you're using, or you might use COM or CORBA IDL, for example.
1845 <sect2 id="message-protocol-handling-invalid">
1846 <title>Invalid Protocol and Spec Extensions</title>
1849 For security reasons, the D-Bus protocol should be strictly parsed and
1850 validated, with the exception of defined extension points. Any invalid
1851 protocol or spec violations should result in immediately dropping the
1852 connection without notice to the other end. Exceptions should be
1853 carefully considered, e.g. an exception may be warranted for a
1854 well-understood idiosyncrasy of a widely-deployed implementation. In
1855 cases where the other end of a connection is 100% trusted and known to
1856 be friendly, skipping validation for performance reasons could also make
1857 sense in certain cases.
1861 Generally speaking violations of the "must" requirements in this spec
1862 should be considered possible attempts to exploit security, and violations
1863 of the "should" suggestions should be considered legitimate (though perhaps
1864 they should generate an error in some cases).
1868 The following extension points are built in to D-Bus on purpose and must
1869 not be treated as invalid protocol. The extension points are intended
1870 for use by future versions of this spec, they are not intended for third
1871 parties. At the moment, the only way a third party could extend D-Bus
1872 without breaking interoperability would be to introduce a way to negotiate new
1873 feature support as part of the auth protocol, using EXTENSION_-prefixed
1874 commands. There is not yet a standard way to negotiate features.
1878 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1879 commands result in an ERROR rather than a disconnect. This enables
1880 future extensions to the protocol. Commands starting with EXTENSION_ are
1881 reserved for third parties.
1886 The authentication protocol supports pluggable auth mechanisms.
1891 The address format (see <xref linkend="addresses"/>) supports new
1897 Messages with an unknown type (something other than
1898 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1899 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1900 Unknown-type messages must still be well-formed in the same way
1901 as the known messages, however. They still have the normal
1907 Header fields with an unknown or unexpected field code must be ignored,
1908 though again they must still be well-formed.
1913 New standard interfaces (with new methods and signals) can of course be added.
1923 <sect1 id="auth-protocol">
1924 <title>Authentication Protocol</title>
1926 Before the flow of messages begins, two applications must
1927 authenticate. A simple plain-text protocol is used for
1928 authentication; this protocol is a SASL profile, and maps fairly
1929 directly from the SASL specification. The message encoding is
1930 NOT used here, only plain text messages.
1933 In examples, "C:" and "S:" indicate lines sent by the client and
1934 server respectively.
1936 <sect2 id="auth-protocol-overview">
1937 <title>Protocol Overview</title>
1939 The protocol is a line-based protocol, where each line ends with
1940 \r\n. Each line begins with an all-caps ASCII command name containing
1941 only the character range [A-Z_], a space, then any arguments for the
1942 command, then the \r\n ending the line. The protocol is
1943 case-sensitive. All bytes must be in the ASCII character set.
1945 Commands from the client to the server are as follows:
1948 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1949 <listitem><para>CANCEL</para></listitem>
1950 <listitem><para>BEGIN</para></listitem>
1951 <listitem><para>DATA <data in hex encoding></para></listitem>
1952 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1953 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1956 From server to client are as follows:
1959 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1960 <listitem><para>OK <GUID in hex></para></listitem>
1961 <listitem><para>DATA <data in hex encoding></para></listitem>
1962 <listitem><para>ERROR</para></listitem>
1963 <listitem><para>AGREE_UNIX_FD</para></listitem>
1967 Unofficial extensions to the command set must begin with the letters
1968 "EXTENSION_", to avoid conflicts with future official commands.
1969 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1972 <sect2 id="auth-nul-byte">
1973 <title>Special credentials-passing nul byte</title>
1975 Immediately after connecting to the server, the client must send a
1976 single nul byte. This byte may be accompanied by credentials
1977 information on some operating systems that use sendmsg() with
1978 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1979 sockets. However, the nul byte must be sent even on other kinds of
1980 socket, and even on operating systems that do not require a byte to be
1981 sent in order to transmit credentials. The text protocol described in
1982 this document begins after the single nul byte. If the first byte
1983 received from the client is not a nul byte, the server may disconnect
1987 A nul byte in any context other than the initial byte is an error;
1988 the protocol is ASCII-only.
1991 The credentials sent along with the nul byte may be used with the
1992 SASL mechanism EXTERNAL.
1995 <sect2 id="auth-command-auth">
1996 <title>AUTH command</title>
1998 If an AUTH command has no arguments, it is a request to list
1999 available mechanisms. The server must respond with a REJECTED
2000 command listing the mechanisms it understands, or with an error.
2003 If an AUTH command specifies a mechanism, and the server supports
2004 said mechanism, the server should begin exchanging SASL
2005 challenge-response data with the client using DATA commands.
2008 If the server does not support the mechanism given in the AUTH
2009 command, it must send either a REJECTED command listing the mechanisms
2010 it does support, or an error.
2013 If the [initial-response] argument is provided, it is intended for use
2014 with mechanisms that have no initial challenge (or an empty initial
2015 challenge), as if it were the argument to an initial DATA command. If
2016 the selected mechanism has an initial challenge and [initial-response]
2017 was provided, the server should reject authentication by sending
2021 If authentication succeeds after exchanging DATA commands,
2022 an OK command must be sent to the client.
2025 The first octet received by the server after the \r\n of the BEGIN
2026 command from the client must be the first octet of the
2027 authenticated/encrypted stream of D-Bus messages.
2030 If BEGIN is received by the server, the first octet received
2031 by the client after the \r\n of the OK command must be the
2032 first octet of the authenticated/encrypted stream of D-Bus
2036 <sect2 id="auth-command-cancel">
2037 <title>CANCEL Command</title>
2039 At any time up to sending the BEGIN command, the client may send a
2040 CANCEL command. On receiving the CANCEL command, the server must
2041 send a REJECTED command and abort the current authentication
2045 <sect2 id="auth-command-data">
2046 <title>DATA Command</title>
2048 The DATA command may come from either client or server, and simply
2049 contains a hex-encoded block of data to be interpreted
2050 according to the SASL mechanism in use.
2053 Some SASL mechanisms support sending an "empty string";
2054 FIXME we need some way to do this.
2057 <sect2 id="auth-command-begin">
2058 <title>BEGIN Command</title>
2060 The BEGIN command acknowledges that the client has received an
2061 OK command from the server, and that the stream of messages
2065 The first octet received by the server after the \r\n of the BEGIN
2066 command from the client must be the first octet of the
2067 authenticated/encrypted stream of D-Bus messages.
2070 <sect2 id="auth-command-rejected">
2071 <title>REJECTED Command</title>
2073 The REJECTED command indicates that the current authentication
2074 exchange has failed, and further exchange of DATA is inappropriate.
2075 The client would normally try another mechanism, or try providing
2076 different responses to challenges.
2078 Optionally, the REJECTED command has a space-separated list of
2079 available auth mechanisms as arguments. If a server ever provides
2080 a list of supported mechanisms, it must provide the same list
2081 each time it sends a REJECTED message. Clients are free to
2082 ignore all lists received after the first.
2085 <sect2 id="auth-command-ok">
2086 <title>OK Command</title>
2088 The OK command indicates that the client has been
2089 authenticated. The client may now proceed with negotiating
2090 Unix file descriptor passing. To do that it shall send
2091 NEGOTIATE_UNIX_FD to the server.
2094 Otherwise, the client must respond to the OK command by
2095 sending a BEGIN command, followed by its stream of messages,
2096 or by disconnecting. The server must not accept additional
2097 commands using this protocol after the BEGIN command has been
2098 received. Further communication will be a stream of D-Bus
2099 messages (optionally encrypted, as negotiated) rather than
2103 If a client sends BEGIN the first octet received by the client
2104 after the \r\n of the OK command must be the first octet of
2105 the authenticated/encrypted stream of D-Bus messages.
2108 The OK command has one argument, which is the GUID of the server.
2109 See <xref linkend="addresses"/> for more on server GUIDs.
2112 <sect2 id="auth-command-error">
2113 <title>ERROR Command</title>
2115 The ERROR command indicates that either server or client did not
2116 know a command, does not accept the given command in the current
2117 context, or did not understand the arguments to the command. This
2118 allows the protocol to be extended; a client or server can send a
2119 command present or permitted only in new protocol versions, and if
2120 an ERROR is received instead of an appropriate response, fall back
2121 to using some other technique.
2124 If an ERROR is sent, the server or client that sent the
2125 error must continue as if the command causing the ERROR had never been
2126 received. However, the the server or client receiving the error
2127 should try something other than whatever caused the error;
2128 if only canceling/rejecting the authentication.
2131 If the D-Bus protocol changes incompatibly at some future time,
2132 applications implementing the new protocol would probably be able to
2133 check for support of the new protocol by sending a new command and
2134 receiving an ERROR from applications that don't understand it. Thus the
2135 ERROR feature of the auth protocol is an escape hatch that lets us
2136 negotiate extensions or changes to the D-Bus protocol in the future.
2139 <sect2 id="auth-command-negotiate-unix-fd">
2140 <title>NEGOTIATE_UNIX_FD Command</title>
2142 The NEGOTIATE_UNIX_FD command indicates that the client
2143 supports Unix file descriptor passing. This command may only
2144 be sent after the connection is authenticated, i.e. after OK
2145 was received by the client. This command may only be sent on
2146 transports that support Unix file descriptor passing.
2149 On receiving NEGOTIATE_UNIX_FD the server must respond with
2150 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2151 the transport chosen supports Unix file descriptor passing and
2152 the server supports this feature. It shall respond the latter
2153 if the transport does not support Unix file descriptor
2154 passing, the server does not support this feature, or the
2155 server decides not to enable file descriptor passing due to
2156 security or other reasons.
2159 <sect2 id="auth-command-agree-unix-fd">
2160 <title>AGREE_UNIX_FD Command</title>
2162 The AGREE_UNIX_FD command indicates that the server supports
2163 Unix file descriptor passing. This command may only be sent
2164 after the connection is authenticated, and the client sent
2165 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2166 command may only be sent on transports that support Unix file
2170 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2171 followed by its stream of messages, or by disconnecting. The
2172 server must not accept additional commands using this protocol
2173 after the BEGIN command has been received. Further
2174 communication will be a stream of D-Bus messages (optionally
2175 encrypted, as negotiated) rather than this protocol.
2178 <sect2 id="auth-command-future">
2179 <title>Future Extensions</title>
2181 Future extensions to the authentication and negotiation
2182 protocol are possible. For that new commands may be
2183 introduced. If a client or server receives an unknown command
2184 it shall respond with ERROR and not consider this fatal. New
2185 commands may be introduced both before, and after
2186 authentication, i.e. both before and after the OK command.
2189 <sect2 id="auth-examples">
2190 <title>Authentication examples</title>
2194 <title>Example of successful magic cookie authentication</title>
2196 (MAGIC_COOKIE is a made up mechanism)
2198 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2204 <title>Example of finding out mechanisms then picking one</title>
2207 S: REJECTED KERBEROS_V4 SKEY
2208 C: AUTH SKEY 7ab83f32ee
2209 S: DATA 8799cabb2ea93e
2210 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2216 <title>Example of client sends unknown command then falls back to regular auth</title>
2220 C: AUTH MAGIC_COOKIE 3736343435313230333039
2226 <title>Example of server doesn't support initial auth mechanism</title>
2228 C: AUTH MAGIC_COOKIE 3736343435313230333039
2229 S: REJECTED KERBEROS_V4 SKEY
2230 C: AUTH SKEY 7ab83f32ee
2231 S: DATA 8799cabb2ea93e
2232 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2238 <title>Example of wrong password or the like followed by successful retry</title>
2240 C: AUTH MAGIC_COOKIE 3736343435313230333039
2241 S: REJECTED KERBEROS_V4 SKEY
2242 C: AUTH SKEY 7ab83f32ee
2243 S: DATA 8799cabb2ea93e
2244 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2246 C: AUTH SKEY 7ab83f32ee
2247 S: DATA 8799cabb2ea93e
2248 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2254 <title>Example of skey cancelled and restarted</title>
2256 C: AUTH MAGIC_COOKIE 3736343435313230333039
2257 S: REJECTED KERBEROS_V4 SKEY
2258 C: AUTH SKEY 7ab83f32ee
2259 S: DATA 8799cabb2ea93e
2262 C: AUTH SKEY 7ab83f32ee
2263 S: DATA 8799cabb2ea93e
2264 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2270 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2272 (MAGIC_COOKIE is a made up mechanism)
2274 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2276 C: NEGOTIATE_UNIX_FD
2282 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2284 (MAGIC_COOKIE is a made up mechanism)
2286 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2288 C: NEGOTIATE_UNIX_FD
2295 <sect2 id="auth-states">
2296 <title>Authentication state diagrams</title>
2299 This section documents the auth protocol in terms of
2300 a state machine for the client and the server. This is
2301 probably the most robust way to implement the protocol.
2304 <sect3 id="auth-states-client">
2305 <title>Client states</title>
2308 To more precisely describe the interaction between the
2309 protocol state machine and the authentication mechanisms the
2310 following notation is used: MECH(CHALL) means that the
2311 server challenge CHALL was fed to the mechanism MECH, which
2317 CONTINUE(RESP) means continue the auth conversation
2318 and send RESP as the response to the server;
2324 OK(RESP) means that after sending RESP to the server
2325 the client side of the auth conversation is finished
2326 and the server should return "OK";
2332 ERROR means that CHALL was invalid and could not be
2338 Both RESP and CHALL may be empty.
2342 The Client starts by getting an initial response from the
2343 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2344 the mechanism did not provide an initial response. If the
2345 mechanism returns CONTINUE, the client starts in state
2346 <emphasis>WaitingForData</emphasis>, if the mechanism
2347 returns OK the client starts in state
2348 <emphasis>WaitingForOK</emphasis>.
2352 The client should keep track of available mechanisms and
2353 which it mechanisms it has already attempted. This list is
2354 used to decide which AUTH command to send. When the list is
2355 exhausted, the client should give up and close the
2360 <title><emphasis>WaitingForData</emphasis></title>
2368 MECH(CHALL) returns CONTINUE(RESP) → send
2370 <emphasis>WaitingForData</emphasis>
2374 MECH(CHALL) returns OK(RESP) → send DATA
2375 RESP, goto <emphasis>WaitingForOK</emphasis>
2379 MECH(CHALL) returns ERROR → send ERROR
2380 [msg], goto <emphasis>WaitingForData</emphasis>
2388 Receive REJECTED [mechs] →
2389 send AUTH [next mech], goto
2390 WaitingForData or <emphasis>WaitingForOK</emphasis>
2395 Receive ERROR → send
2397 <emphasis>WaitingForReject</emphasis>
2402 Receive OK → send
2403 BEGIN, terminate auth
2404 conversation, authenticated
2409 Receive anything else → send
2411 <emphasis>WaitingForData</emphasis>
2419 <title><emphasis>WaitingForOK</emphasis></title>
2424 Receive OK → send BEGIN, terminate auth
2425 conversation, <emphasis>authenticated</emphasis>
2430 Receive REJECTED [mechs] → send AUTH [next mech],
2431 goto <emphasis>WaitingForData</emphasis> or
2432 <emphasis>WaitingForOK</emphasis>
2438 Receive DATA → send CANCEL, goto
2439 <emphasis>WaitingForReject</emphasis>
2445 Receive ERROR → send CANCEL, goto
2446 <emphasis>WaitingForReject</emphasis>
2452 Receive anything else → send ERROR, goto
2453 <emphasis>WaitingForOK</emphasis>
2461 <title><emphasis>WaitingForReject</emphasis></title>
2466 Receive REJECTED [mechs] → send AUTH [next mech],
2467 goto <emphasis>WaitingForData</emphasis> or
2468 <emphasis>WaitingForOK</emphasis>
2474 Receive anything else → terminate auth
2475 conversation, disconnect
2484 <sect3 id="auth-states-server">
2485 <title>Server states</title>
2488 For the server MECH(RESP) means that the client response
2489 RESP was fed to the the mechanism MECH, which returns one of
2494 CONTINUE(CHALL) means continue the auth conversation and
2495 send CHALL as the challenge to the client;
2501 OK means that the client has been successfully
2508 REJECTED means that the client failed to authenticate or
2509 there was an error in RESP.
2514 The server starts out in state
2515 <emphasis>WaitingForAuth</emphasis>. If the client is
2516 rejected too many times the server must disconnect the
2521 <title><emphasis>WaitingForAuth</emphasis></title>
2527 Receive AUTH → send REJECTED [mechs], goto
2528 <emphasis>WaitingForAuth</emphasis>
2534 Receive AUTH MECH RESP
2538 MECH not valid mechanism → send REJECTED
2540 <emphasis>WaitingForAuth</emphasis>
2544 MECH(RESP) returns CONTINUE(CHALL) → send
2546 <emphasis>WaitingForData</emphasis>
2550 MECH(RESP) returns OK → send OK, goto
2551 <emphasis>WaitingForBegin</emphasis>
2555 MECH(RESP) returns REJECTED → send REJECTED
2557 <emphasis>WaitingForAuth</emphasis>
2565 Receive BEGIN → terminate
2566 auth conversation, disconnect
2572 Receive ERROR → send REJECTED [mechs], goto
2573 <emphasis>WaitingForAuth</emphasis>
2579 Receive anything else → send
2581 <emphasis>WaitingForAuth</emphasis>
2590 <title><emphasis>WaitingForData</emphasis></title>
2598 MECH(RESP) returns CONTINUE(CHALL) → send
2600 <emphasis>WaitingForData</emphasis>
2604 MECH(RESP) returns OK → send OK, goto
2605 <emphasis>WaitingForBegin</emphasis>
2609 MECH(RESP) returns REJECTED → send REJECTED
2611 <emphasis>WaitingForAuth</emphasis>
2619 Receive BEGIN → terminate auth conversation,
2626 Receive CANCEL → send REJECTED [mechs], goto
2627 <emphasis>WaitingForAuth</emphasis>
2633 Receive ERROR → send REJECTED [mechs], goto
2634 <emphasis>WaitingForAuth</emphasis>
2640 Receive anything else → send ERROR, goto
2641 <emphasis>WaitingForData</emphasis>
2649 <title><emphasis>WaitingForBegin</emphasis></title>
2654 Receive BEGIN → terminate auth conversation,
2655 client authenticated
2661 Receive CANCEL → send REJECTED [mechs], goto
2662 <emphasis>WaitingForAuth</emphasis>
2668 Receive ERROR → send REJECTED [mechs], goto
2669 <emphasis>WaitingForAuth</emphasis>
2675 Receive anything else → send ERROR, goto
2676 <emphasis>WaitingForBegin</emphasis>
2686 <sect2 id="auth-mechanisms">
2687 <title>Authentication mechanisms</title>
2689 This section describes some new authentication mechanisms.
2690 D-Bus also allows any standard SASL mechanism of course.
2692 <sect3 id="auth-mechanisms-sha">
2693 <title>DBUS_COOKIE_SHA1</title>
2695 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2696 has the ability to read a private file owned by the user being
2697 authenticated. If the client can prove that it has access to a secret
2698 cookie stored in this file, then the client is authenticated.
2699 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2703 Throughout this description, "hex encoding" must output the digits
2704 from a to f in lower-case; the digits A to F must not be used
2705 in the DBUS_COOKIE_SHA1 mechanism.
2708 Authentication proceeds as follows:
2712 The client sends the username it would like to authenticate
2718 The server sends the name of its "cookie context" (see below); a
2719 space character; the integer ID of the secret cookie the client
2720 must demonstrate knowledge of; a space character; then a
2721 randomly-generated challenge string, all of this hex-encoded into
2727 The client locates the cookie and generates its own
2728 randomly-generated challenge string. The client then concatenates
2729 the server's decoded challenge, a ":" character, its own challenge,
2730 another ":" character, and the cookie. It computes the SHA-1 hash
2731 of this composite string as a hex digest. It concatenates the
2732 client's challenge string, a space character, and the SHA-1 hex
2733 digest, hex-encodes the result and sends it back to the server.
2738 The server generates the same concatenated string used by the
2739 client and computes its SHA-1 hash. It compares the hash with
2740 the hash received from the client; if the two hashes match, the
2741 client is authenticated.
2747 Each server has a "cookie context," which is a name that identifies a
2748 set of cookies that apply to that server. A sample context might be
2749 "org_freedesktop_session_bus". Context names must be valid ASCII,
2750 nonzero length, and may not contain the characters slash ("/"),
2751 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2752 tab ("\t"), or period ("."). There is a default context,
2753 "org_freedesktop_general" that's used by servers that do not specify
2757 Cookies are stored in a user's home directory, in the directory
2758 <filename>~/.dbus-keyrings/</filename>. This directory must
2759 not be readable or writable by other users. If it is,
2760 clients and servers must ignore it. The directory
2761 contains cookie files named after the cookie context.
2764 A cookie file contains one cookie per line. Each line
2765 has three space-separated fields:
2769 The cookie ID number, which must be a non-negative integer and
2770 may not be used twice in the same file.
2775 The cookie's creation time, in UNIX seconds-since-the-epoch
2781 The cookie itself, a hex-encoded random block of bytes. The cookie
2782 may be of any length, though obviously security increases
2783 as the length increases.
2789 Only server processes modify the cookie file.
2790 They must do so with this procedure:
2794 Create a lockfile name by appending ".lock" to the name of the
2795 cookie file. The server should attempt to create this file
2796 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2797 fails, the lock fails. Servers should retry for a reasonable
2798 period of time, then they may choose to delete an existing lock
2799 to keep users from having to manually delete a stale
2800 lock. <footnote><para>Lockfiles are used instead of real file
2801 locking <literal>fcntl()</literal> because real locking
2802 implementations are still flaky on network
2803 filesystems.</para></footnote>
2808 Once the lockfile has been created, the server loads the cookie
2809 file. It should then delete any cookies that are old (the
2810 timeout can be fairly short), or more than a reasonable
2811 time in the future (so that cookies never accidentally
2812 become permanent, if the clock was set far into the future
2813 at some point). If no recent keys remain, the
2814 server may generate a new key.
2819 The pruned and possibly added-to cookie file
2820 must be resaved atomically (using a temporary
2821 file which is rename()'d).
2826 The lock must be dropped by deleting the lockfile.
2832 Clients need not lock the file in order to load it,
2833 because servers are required to save the file atomically.
2838 <sect1 id="addresses">
2839 <title>Server Addresses</title>
2841 Server addresses consist of a transport name followed by a colon, and
2842 then an optional, comma-separated list of keys and values in the form key=value.
2843 Each value is escaped.
2847 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2848 Which is the address to a unix socket with the path /tmp/dbus-test.
2851 Value escaping is similar to URI escaping but simpler.
2855 The set of optionally-escaped bytes is:
2856 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2857 <emphasis>byte</emphasis> (note, not character) which is not in the
2858 set of optionally-escaped bytes must be replaced with an ASCII
2859 percent (<literal>%</literal>) and the value of the byte in hex.
2860 The hex value must always be two digits, even if the first digit is
2861 zero. The optionally-escaped bytes may be escaped if desired.
2866 To unescape, append each byte in the value; if a byte is an ASCII
2867 percent (<literal>%</literal>) character then append the following
2868 hex value instead. It is an error if a <literal>%</literal> byte
2869 does not have two hex digits following. It is an error if a
2870 non-optionally-escaped byte is seen unescaped.
2874 The set of optionally-escaped bytes is intended to preserve address
2875 readability and convenience.
2879 A server may specify a key-value pair with the key <literal>guid</literal>
2880 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2881 describes the format of the <literal>guid</literal> field. If present,
2882 this UUID may be used to distinguish one server address from another. A
2883 server should use a different UUID for each address it listens on. For
2884 example, if a message bus daemon offers both UNIX domain socket and TCP
2885 connections, but treats clients the same regardless of how they connect,
2886 those two connections are equivalent post-connection but should have
2887 distinct UUIDs to distinguish the kinds of connection.
2891 The intent of the address UUID feature is to allow a client to avoid
2892 opening multiple identical connections to the same server, by allowing the
2893 client to check whether an address corresponds to an already-existing
2894 connection. Comparing two addresses is insufficient, because addresses
2895 can be recycled by distinct servers, and equivalent addresses may look
2896 different if simply compared as strings (for example, the host in a TCP
2897 address can be given as an IP address or as a hostname).
2901 Note that the address key is <literal>guid</literal> even though the
2902 rest of the API and documentation says "UUID," for historical reasons.
2906 [FIXME clarify if attempting to connect to each is a requirement
2907 or just a suggestion]
2908 When connecting to a server, multiple server addresses can be
2909 separated by a semi-colon. The library will then try to connect
2910 to the first address and if that fails, it'll try to connect to
2911 the next one specified, and so forth. For example
2912 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2917 <sect1 id="transports">
2918 <title>Transports</title>
2920 [FIXME we need to specify in detail each transport and its possible arguments]
2922 Current transports include: unix domain sockets (including
2923 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2924 using in-process pipes. Future possible transports include one that
2925 tunnels over X11 protocol.
2928 <sect2 id="transports-unix-domain-sockets">
2929 <title>Unix Domain Sockets</title>
2931 Unix domain sockets can be either paths in the file system or on Linux
2932 kernels, they can be abstract which are similar to paths but
2933 do not show up in the file system.
2937 When a socket is opened by the D-Bus library it truncates the path
2938 name right before the first trailing Nul byte. This is true for both
2939 normal paths and abstract paths. Note that this is a departure from
2940 previous versions of D-Bus that would create sockets with a fixed
2941 length path name. Names which were shorter than the fixed length
2942 would be padded by Nul bytes.
2945 Unix domain sockets are not available on Windows.
2947 <sect3 id="transports-unix-domain-sockets-addresses">
2948 <title>Server Address Format</title>
2950 Unix domain socket addresses are identified by the "unix:" prefix
2951 and support the following key/value pairs:
2958 <entry>Values</entry>
2959 <entry>Description</entry>
2965 <entry>(path)</entry>
2966 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2969 <entry>tmpdir</entry>
2970 <entry>(path)</entry>
2971 <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>
2974 <entry>abstract</entry>
2975 <entry>(string)</entry>
2976 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2983 <sect2 id="transports-launchd">
2984 <title>launchd</title>
2986 launchd is an open-source server management system that replaces init, inetd
2987 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2988 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2992 launchd allocates a socket and provides it with the unix path through the
2993 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2994 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2995 it through its environment.
2996 Other processes can query for the launchd socket by executing:
2997 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2998 This is normally done by the D-Bus client library so doesn't have to be done
3002 launchd is not available on Microsoft Windows.
3004 <sect3 id="transports-launchd-addresses">
3005 <title>Server Address Format</title>
3007 launchd addresses are identified by the "launchd:" prefix
3008 and support the following key/value pairs:
3015 <entry>Values</entry>
3016 <entry>Description</entry>
3022 <entry>(environment variable)</entry>
3023 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3030 <sect2 id="transports-systemd">
3031 <title>systemd</title>
3033 systemd is an open-source server management system that
3034 replaces init and inetd on newer Linux systems. It supports
3035 socket activation. The D-Bus systemd transport is used to acquire
3036 socket activation file descriptors from systemd and use them
3037 as D-Bus transport when the current process is spawned by
3038 socket activation from it.
3041 The systemd transport accepts only one or more Unix domain or
3042 TCP streams sockets passed in via socket activation.
3045 The systemd transport is not available on non-Linux operating systems.
3048 The systemd transport defines no parameter keys.
3051 <sect2 id="transports-tcp-sockets">
3052 <title>TCP Sockets</title>
3054 The tcp transport provides TCP/IP based connections between clients
3055 located on the same or different hosts.
3058 Using tcp transport without any additional secure authentification mechanismus
3059 over a network is unsecure.
3062 Windows notes: Because of the tcp stack on Windows does not provide sending
3063 credentials over a tcp connection, the EXTERNAL authentification
3064 mechanismus does not work.
3066 <sect3 id="transports-tcp-sockets-addresses">
3067 <title>Server Address Format</title>
3069 TCP/IP socket addresses are identified by the "tcp:" prefix
3070 and support the following key/value pairs:
3077 <entry>Values</entry>
3078 <entry>Description</entry>
3084 <entry>(string)</entry>
3085 <entry>dns name or ip address</entry>
3089 <entry>(number)</entry>
3090 <entry>The tcp port the server will open. A zero value let the server
3091 choose a free port provided from the underlaying operating system.
3092 libdbus is able to retrieve the real used port from the server.
3096 <entry>family</entry>
3097 <entry>(string)</entry>
3098 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3105 <sect2 id="transports-nonce-tcp-sockets">
3106 <title>Nonce-secured TCP Sockets</title>
3108 The nonce-tcp transport provides a secured TCP transport, using a
3109 simple authentication mechanism to ensure that only clients with read
3110 access to a certain location in the filesystem can connect to the server.
3111 The server writes a secret, the nonce, to a file and an incoming client
3112 connection is only accepted if the client sends the nonce right after
3113 the connect. The nonce mechanism requires no setup and is orthogonal to
3114 the higher-level authentication mechanisms described in the
3115 Authentication section.
3119 On start, the server generates a random 16 byte nonce and writes it
3120 to a file in the user's temporary directory. The nonce file location
3121 is published as part of the server's D-Bus address using the
3122 "noncefile" key-value pair.
3124 After an accept, the server reads 16 bytes from the socket. If the
3125 read bytes do not match the nonce stored in the nonce file, the
3126 server MUST immediately drop the connection.
3127 If the nonce match the received byte sequence, the client is accepted
3128 and the transport behaves like an unsecured tcp transport.
3131 After a successful connect to the server socket, the client MUST read
3132 the nonce from the file published by the server via the noncefile=
3133 key-value pair and send it over the socket. After that, the
3134 transport behaves like an unsecured tcp transport.
3136 <sect3 id="transports-nonce-tcp-sockets-addresses">
3137 <title>Server Address Format</title>
3139 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3140 and support the following key/value pairs:
3147 <entry>Values</entry>
3148 <entry>Description</entry>
3154 <entry>(string)</entry>
3155 <entry>dns name or ip address</entry>
3159 <entry>(number)</entry>
3160 <entry>The tcp port the server will open. A zero value let the server
3161 choose a free port provided from the underlaying operating system.
3162 libdbus is able to retrieve the real used port from the server.
3166 <entry>family</entry>
3167 <entry>(string)</entry>
3168 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3171 <entry>noncefile</entry>
3172 <entry>(path)</entry>
3173 <entry>file location containing the secret</entry>
3180 <sect2 id="transports-exec">
3181 <title>Executed Subprocesses on Unix</title>
3183 This transport forks off a process and connects its standard
3184 input and standard output with an anonymous Unix domain
3185 socket. This socket is then used for communication by the
3186 transport. This transport may be used to use out-of-process
3187 forwarder programs as basis for the D-Bus protocol.
3190 The forked process will inherit the standard error output and
3191 process group from the parent process.
3194 Executed subprocesses are not available on Windows.
3196 <sect3 id="transports-exec-addresses">
3197 <title>Server Address Format</title>
3199 Executed subprocess addresses are identified by the "unixexec:" prefix
3200 and support the following key/value pairs:
3207 <entry>Values</entry>
3208 <entry>Description</entry>
3214 <entry>(path)</entry>
3215 <entry>Path of the binary to execute, either an absolute
3216 path or a binary name that is searched for in the default
3217 search path of the OS. This corresponds to the first
3218 argument of execlp(). This key is mandatory.</entry>
3221 <entry>argv0</entry>
3222 <entry>(string)</entry>
3223 <entry>The program name to use when executing the
3224 binary. If omitted the same value as specified for path=
3225 will be used. This corresponds to the second argument of
3229 <entry>argv1, argv2, ...</entry>
3230 <entry>(string)</entry>
3231 <entry>Arguments to pass to the binary. This corresponds
3232 to the third and later arguments of execlp(). If a
3233 specific argvX is not specified no further argvY for Y > X
3234 are taken into account.</entry>
3242 <sect1 id="meta-transports">
3243 <title>Meta Transports</title>
3245 Meta transports are a kind of transport with special enhancements or
3246 behavior. Currently available meta transports include: autolaunch
3249 <sect2 id="meta-transports-autolaunch">
3250 <title>Autolaunch</title>
3251 <para>The autolaunch transport provides a way for dbus clients to autodetect
3252 a running dbus session bus and to autolaunch a session bus if not present.
3254 <sect3 id="meta-transports-autolaunch-addresses">
3255 <title>Server Address Format</title>
3257 Autolaunch addresses uses the "autolaunch:" prefix and support the
3258 following key/value pairs:
3265 <entry>Values</entry>
3266 <entry>Description</entry>
3271 <entry>scope</entry>
3272 <entry>(string)</entry>
3273 <entry>scope of autolaunch (Windows only)
3277 "*install-path" - limit session bus to dbus installation path.
3278 The dbus installation path is determined from the location of
3279 the shared dbus library. If the library is located in a 'bin'
3280 subdirectory the installation root is the directory above,
3281 otherwise the directory where the library lives is taken as
3284 <install-root>/bin/[lib]dbus-1.dll
3285 <install-root>/[lib]dbus-1.dll
3291 "*user" - limit session bus to the recent user.
3296 other values - specify dedicated session bus like "release",
3308 <sect3 id="meta-transports-autolaunch-windows-implementation">
3309 <title>Windows implementation</title>
3311 On start, the server opens a platform specific transport, creates a mutex
3312 and a shared memory section containing the related session bus address.
3313 This mutex will be inspected by the dbus client library to detect a
3314 running dbus session bus. The access to the mutex and the shared memory
3315 section are protected by global locks.
3318 In the recent implementation the autolaunch transport uses a tcp transport
3319 on localhost with a port choosen from the operating system. This detail may
3320 change in the future.
3323 Disclaimer: The recent implementation is in an early state and may not
3324 work in all cirumstances and/or may have security issues. Because of this
3325 the implementation is not documentated yet.
3332 <title>UUIDs</title>
3334 A working D-Bus implementation uses universally-unique IDs in two places.
3335 First, each server address has a UUID identifying the address,
3336 as described in <xref linkend="addresses"/>. Second, each operating
3337 system kernel instance running a D-Bus client or server has a UUID
3338 identifying that kernel, retrieved by invoking the method
3339 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3340 linkend="standard-interfaces-peer"/>).
3343 The term "UUID" in this document is intended literally, i.e. an
3344 identifier that is universally unique. It is not intended to refer to
3345 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3348 The UUID must contain 128 bits of data and be hex-encoded. The
3349 hex-encoded string may not contain hyphens or other non-hex-digit
3350 characters, and it must be exactly 32 characters long. To generate a
3351 UUID, the current reference implementation concatenates 96 bits of random
3352 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3356 It would also be acceptable and probably better to simply generate 128
3357 bits of random data, as long as the random number generator is of high
3358 quality. The timestamp could conceivably help if the random bits are not
3359 very random. With a quality random number generator, collisions are
3360 extremely unlikely even with only 96 bits, so it's somewhat academic.
3363 Implementations should, however, stick to random data for the first 96 bits
3368 <sect1 id="standard-interfaces">
3369 <title>Standard Interfaces</title>
3371 See <xref linkend="message-protocol-types-notation"/> for details on
3372 the notation used in this section. There are some standard interfaces
3373 that may be useful across various D-Bus applications.
3375 <sect2 id="standard-interfaces-peer">
3376 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3378 The <literal>org.freedesktop.DBus.Peer</literal> interface
3381 org.freedesktop.DBus.Peer.Ping ()
3382 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3386 On receipt of the <literal>METHOD_CALL</literal> message
3387 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3388 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3389 usual. It does not matter which object path a ping is sent to. The
3390 reference implementation handles this method automatically.
3393 On receipt of the <literal>METHOD_CALL</literal> message
3394 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3395 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3396 UUID representing the identity of the machine the process is running on.
3397 This UUID must be the same for all processes on a single system at least
3398 until that system next reboots. It should be the same across reboots
3399 if possible, but this is not always possible to implement and is not
3401 It does not matter which object path a GetMachineId is sent to. The
3402 reference implementation handles this method automatically.
3405 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3406 a virtual machine running on a hypervisor, rather than a physical machine.
3407 Basically if two processes see the same UUID, they should also see the same
3408 shared memory, UNIX domain sockets, process IDs, and other features that require
3409 a running OS kernel in common between the processes.
3412 The UUID is often used where other programs might use a hostname. Hostnames
3413 can change without rebooting, however, or just be "localhost" - so the UUID
3417 <xref linkend="uuids"/> explains the format of the UUID.
3421 <sect2 id="standard-interfaces-introspectable">
3422 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3424 This interface has one method:
3426 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3430 Objects instances may implement
3431 <literal>Introspect</literal> which returns an XML description of
3432 the object, including its interfaces (with signals and methods), objects
3433 below it in the object path tree, and its properties.
3436 <xref linkend="introspection-format"/> describes the format of this XML string.
3439 <sect2 id="standard-interfaces-properties">
3440 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3442 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3443 or <firstterm>attributes</firstterm>. These can be exposed via the
3444 <literal>org.freedesktop.DBus.Properties</literal> interface.
3448 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3449 in STRING property_name,
3451 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3452 in STRING property_name,
3454 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3455 out DICT<STRING,VARIANT> props);
3459 It is conventional to give D-Bus properties names consisting of
3460 capitalized words without punctuation ("CamelCase"), like
3461 <link linkend="message-protocol-names-member">member names</link>.
3462 For instance, the GObject property
3463 <literal>connection-status</literal> or the Qt property
3464 <literal>connectionStatus</literal> could be represented on D-Bus
3465 as <literal>ConnectionStatus</literal>.
3468 Strictly speaking, D-Bus property names are not required to follow
3469 the same naming restrictions as member names, but D-Bus property
3470 names that would not be valid member names (in particular,
3471 GObject-style dash-separated property names) can cause interoperability
3472 problems and should be avoided.
3475 The available properties and whether they are writable can be determined
3476 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3477 see <xref linkend="standard-interfaces-introspectable"/>.
3480 An empty string may be provided for the interface name; in this case,
3481 if there are multiple properties on an object with the same name,
3482 the results are undefined (picking one by according to an arbitrary
3483 deterministic rule, or returning an error, are the reasonable
3487 If one or more properties change on an object, the
3488 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3489 signal may be emitted (this signal was added in 0.14):
3493 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3494 DICT<STRING,VARIANT> changed_properties,
3495 ARRAY<STRING> invalidated_properties);
3499 where <literal>changed_properties</literal> is a dictionary
3500 containing the changed properties with the new values and
3501 <literal>invalidated_properties</literal> is an array of
3502 properties that changed but the value is not conveyed.
3505 Whether the <literal>PropertiesChanged</literal> signal is
3506 supported can be determined by calling
3507 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3508 that the signal may be supported for an object but it may
3509 differ how whether and how it is used on a per-property basis
3510 (for e.g. performance or security reasons). Each property (or
3511 the parent interface) must be annotated with the
3512 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3513 annotation to convey this (usually the default value
3514 <literal>true</literal> is sufficient meaning that the
3515 annotation does not need to be used). See <xref
3516 linkend="introspection-format"/> for details on this
3521 <sect2 id="standard-interfaces-objectmanager">
3522 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3524 An API can optionally make use of this interface for one or
3525 more sub-trees of objects. The root of each sub-tree implements
3526 this interface so other applications can get all objects,
3527 interfaces and properties in a single method call. It is
3528 appropriate to use this interface if users of the tree of
3529 objects are expected to be interested in all interfaces of all
3530 objects in the tree; a more granular API should be used if
3531 users of the objects are expected to be interested in a small
3532 subset of the objects, a small subset of their interfaces, or
3536 The method that applications can use to get all objects and
3537 properties is <literal>GetManagedObjects</literal>:
3541 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3545 The return value of this method is a dict whose keys are
3546 object paths. All returned object paths are children of the
3547 object path implementing this interface, i.e. their object
3548 paths start with the ObjectManager's object path plus '/'.
3551 Each value is a dict whose keys are interfaces names. Each
3552 value in this inner dict is the same dict that would be
3553 returned by the <link
3554 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3555 method for that combination of object path and interface. If
3556 an interface has no properties, the empty dict is returned.
3559 Changes are emitted using the following two signals:
3563 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3564 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3565 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3566 ARRAY<STRING> interfaces);
3570 The <literal>InterfacesAdded</literal> signal is emitted when
3571 either a new object is added or when an existing object gains
3572 one or more interfaces. The
3573 <literal>InterfacesRemoved</literal> signal is emitted
3574 whenever an object is removed or it loses one or more
3575 interfaces. The second parameter of the
3576 <literal>InterfacesAdded</literal> signal contains a dict with
3577 the interfaces and properties (if any) that have been added to
3578 the given object path. Similarly, the second parameter of the
3579 <literal>InterfacesRemoved</literal> signal contains an array
3580 of the interfaces that were removed. Note that changes on
3581 properties on existing interfaces are not reported using this
3582 interface - an application should also monitor the existing <link
3583 linkend="standard-interfaces-properties">PropertiesChanged</link>
3584 signal on each object.
3587 Applications SHOULD NOT export objects that are children of an
3588 object (directly or otherwise) implementing this interface but
3589 which are not returned in the reply from the
3590 <literal>GetManagedObjects()</literal> method of this
3591 interface on the given object.
3594 The intent of the <literal>ObjectManager</literal> interface
3595 is to make it easy to write a robust client
3596 implementation. The trivial client implementation only needs
3597 to make two method calls:
3601 org.freedesktop.DBus.AddMatch (bus_proxy,
3602 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3603 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3607 on the message bus and the remote application's
3608 <literal>ObjectManager</literal>, respectively. Whenever a new
3609 remote object is created (or an existing object gains a new
3610 interface), the <literal>InterfacesAdded</literal> signal is
3611 emitted, and since this signal contains all properties for the
3612 interfaces, no calls to the
3613 <literal>org.freedesktop.Properties</literal> interface on the
3614 remote object are needed. Additionally, since the initial
3615 <literal>AddMatch()</literal> rule already includes signal
3616 messages from the newly created child object, no new
3617 <literal>AddMatch()</literal> call is needed.
3622 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3623 interface was added in version 0.17 of the D-Bus
3630 <sect1 id="introspection-format">
3631 <title>Introspection Data Format</title>
3633 As described in <xref linkend="standard-interfaces-introspectable"/>,
3634 objects may be introspected at runtime, returning an XML string
3635 that describes the object. The same XML format may be used in
3636 other contexts as well, for example as an "IDL" for generating
3637 static language bindings.
3640 Here is an example of introspection data:
3642 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3643 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3644 <node name="/org/freedesktop/sample_object">
3645 <interface name="org.freedesktop.SampleInterface">
3646 <method name="Frobate">
3647 <arg name="foo" type="i" direction="in"/>
3648 <arg name="bar" type="s" direction="out"/>
3649 <arg name="baz" type="a{us}" direction="out"/>
3650 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3652 <method name="Bazify">
3653 <arg name="bar" type="(iiu)" direction="in"/>
3654 <arg name="bar" type="v" direction="out"/>
3656 <method name="Mogrify">
3657 <arg name="bar" type="(iiav)" direction="in"/>
3659 <signal name="Changed">
3660 <arg name="new_value" type="b"/>
3662 <property name="Bar" type="y" access="readwrite"/>
3664 <node name="child_of_sample_object"/>
3665 <node name="another_child_of_sample_object"/>
3670 A more formal DTD and spec needs writing, but here are some quick notes.
3674 Only the root <node> element can omit the node name, as it's
3675 known to be the object that was introspected. If the root
3676 <node> does have a name attribute, it must be an absolute
3677 object path. If child <node> have object paths, they must be
3683 If a child <node> has any sub-elements, then they
3684 must represent a complete introspection of the child.
3685 If a child <node> is empty, then it may or may
3686 not have sub-elements; the child must be introspected
3687 in order to find out. The intent is that if an object
3688 knows that its children are "fast" to introspect
3689 it can go ahead and return their information, but
3690 otherwise it can omit it.
3695 The direction element on <arg> may be omitted,
3696 in which case it defaults to "in" for method calls
3697 and "out" for signals. Signals only allow "out"
3698 so while direction may be specified, it's pointless.
3703 The possible directions are "in" and "out",
3704 unlike CORBA there is no "inout"
3709 The possible property access flags are
3710 "readwrite", "read", and "write"
3715 Multiple interfaces can of course be listed for
3721 The "name" attribute on arguments is optional.
3727 Method, interface, property, and signal elements may have
3728 "annotations", which are generic key/value pairs of metadata.
3729 They are similar conceptually to Java's annotations and C# attributes.
3730 Well-known annotations:
3737 <entry>Values (separated by ,)</entry>
3738 <entry>Description</entry>
3743 <entry>org.freedesktop.DBus.Deprecated</entry>
3744 <entry>true,false</entry>
3745 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3748 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3749 <entry>(string)</entry>
3750 <entry>The C symbol; may be used for methods and interfaces</entry>
3753 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3754 <entry>true,false</entry>
3755 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3758 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3759 <entry>true,invalidates,false</entry>
3762 If set to <literal>false</literal>, the
3763 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3765 linkend="standard-interfaces-properties"/> is not
3766 guaranteed to be emitted if the property changes.
3769 If set to <literal>invalidates</literal> the signal
3770 is emitted but the value is not included in the
3774 If set to <literal>true</literal> the signal is
3775 emitted with the value included.
3778 The value for the annotation defaults to
3779 <literal>true</literal> if the enclosing interface
3780 element does not specify the annotation. Otherwise it
3781 defaults to the value specified in the enclosing
3790 <sect1 id="message-bus">
3791 <title>Message Bus Specification</title>
3792 <sect2 id="message-bus-overview">
3793 <title>Message Bus Overview</title>
3795 The message bus accepts connections from one or more applications.
3796 Once connected, applications can exchange messages with other
3797 applications that are also connected to the bus.
3800 In order to route messages among connections, the message bus keeps a
3801 mapping from names to connections. Each connection has one
3802 unique-for-the-lifetime-of-the-bus name automatically assigned.
3803 Applications may request additional names for a connection. Additional
3804 names are usually "well-known names" such as
3805 "org.freedesktop.TextEditor". When a name is bound to a connection,
3806 that connection is said to <firstterm>own</firstterm> the name.
3809 The bus itself owns a special name,
3810 <literal>org.freedesktop.DBus</literal>, with an object
3811 located at <literal>/org/freedesktop/DBus</literal> that
3812 implements the <literal>org.freedesktop.DBus</literal>
3813 interface. This service allows applications to make
3814 administrative requests of the bus itself. For example,
3815 applications can ask the bus to assign a name to a connection.
3818 Each name may have <firstterm>queued owners</firstterm>. When an
3819 application requests a name for a connection and the name is already in
3820 use, the bus will optionally add the connection to a queue waiting for
3821 the name. If the current owner of the name disconnects or releases
3822 the name, the next connection in the queue will become the new owner.
3826 This feature causes the right thing to happen if you start two text
3827 editors for example; the first one may request "org.freedesktop.TextEditor",
3828 and the second will be queued as a possible owner of that name. When
3829 the first exits, the second will take over.
3833 Applications may send <firstterm>unicast messages</firstterm> to
3834 a specific recipient or to the message bus itself, or
3835 <firstterm>broadcast messages</firstterm> to all interested recipients.
3836 See <xref linkend="message-bus-routing"/> for details.
3840 <sect2 id="message-bus-names">
3841 <title>Message Bus Names</title>
3843 Each connection has at least one name, assigned at connection time and
3844 returned in response to the
3845 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3846 automatically-assigned name is called the connection's <firstterm>unique
3847 name</firstterm>. Unique names are never reused for two different
3848 connections to the same bus.
3851 Ownership of a unique name is a prerequisite for interaction with
3852 the message bus. It logically follows that the unique name is always
3853 the first name that an application comes to own, and the last
3854 one that it loses ownership of.
3857 Unique connection names must begin with the character ':' (ASCII colon
3858 character); bus names that are not unique names must not begin
3859 with this character. (The bus must reject any attempt by an application
3860 to manually request a name beginning with ':'.) This restriction
3861 categorically prevents "spoofing"; messages sent to a unique name
3862 will always go to the expected connection.
3865 When a connection is closed, all the names that it owns are deleted (or
3866 transferred to the next connection in the queue if any).
3869 A connection can request additional names to be associated with it using
3870 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3871 linkend="message-protocol-names-bus"/> describes the format of a valid
3872 name. These names can be released again using the
3873 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3876 <sect3 id="bus-messages-request-name">
3877 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3881 UINT32 RequestName (in STRING name, in UINT32 flags)
3888 <entry>Argument</entry>
3890 <entry>Description</entry>
3896 <entry>STRING</entry>
3897 <entry>Name to request</entry>
3901 <entry>UINT32</entry>
3902 <entry>Flags</entry>
3912 <entry>Argument</entry>
3914 <entry>Description</entry>
3920 <entry>UINT32</entry>
3921 <entry>Return value</entry>
3928 This method call should be sent to
3929 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3930 assign the given name to the method caller. Each name maintains a
3931 queue of possible owners, where the head of the queue is the primary
3932 or current owner of the name. Each potential owner in the queue
3933 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3934 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3935 call. When RequestName is invoked the following occurs:
3939 If the method caller is currently the primary owner of the name,
3940 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3941 values are updated with the values from the new RequestName call,
3942 and nothing further happens.
3948 If the current primary owner (head of the queue) has
3949 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3950 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3951 the caller of RequestName replaces the current primary owner at
3952 the head of the queue and the current primary owner moves to the
3953 second position in the queue. If the caller of RequestName was
3954 in the queue previously its flags are updated with the values from
3955 the new RequestName in addition to moving it to the head of the queue.
3961 If replacement is not possible, and the method caller is
3962 currently in the queue but not the primary owner, its flags are
3963 updated with the values from the new RequestName call.
3969 If replacement is not possible, and the method caller is
3970 currently not in the queue, the method caller is appended to the
3977 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3978 set and is not the primary owner, it is removed from the
3979 queue. This can apply to the previous primary owner (if it
3980 was replaced) or the method caller (if it updated the
3981 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3982 queue, or if it was just added to the queue with that flag set).
3988 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3989 queue," even if another application already in the queue had specified
3990 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3991 that does not allow replacement goes away, and the next primary owner
3992 does allow replacement. In this case, queued items that specified
3993 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3994 automatically replace the new primary owner. In other words,
3995 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3996 time RequestName is called. This is deliberate to avoid an infinite loop
3997 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3998 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4001 The flags argument contains any of the following values logically ORed
4008 <entry>Conventional Name</entry>
4009 <entry>Value</entry>
4010 <entry>Description</entry>
4015 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4019 If an application A specifies this flag and succeeds in
4020 becoming the owner of the name, and another application B
4021 later calls RequestName with the
4022 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4023 will lose ownership and receive a
4024 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4025 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4026 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4027 is not specified by application B, then application B will not replace
4028 application A as the owner.
4033 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4037 Try to replace the current owner if there is one. If this
4038 flag is not set the application will only become the owner of
4039 the name if there is no current owner. If this flag is set,
4040 the application will replace the current owner if
4041 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4046 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4050 Without this flag, if an application requests a name that is
4051 already owned, the application will be placed in a queue to
4052 own the name when the current owner gives it up. If this
4053 flag is given, the application will not be placed in the
4054 queue, the request for the name will simply fail. This flag
4055 also affects behavior when an application is replaced as
4056 name owner; by default the application moves back into the
4057 waiting queue, unless this flag was provided when the application
4058 became the name owner.
4066 The return code can be one of the following values:
4072 <entry>Conventional Name</entry>
4073 <entry>Value</entry>
4074 <entry>Description</entry>
4079 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4080 <entry>1</entry> <entry>The caller is now the primary owner of
4081 the name, replacing any previous owner. Either the name had no
4082 owner before, or the caller specified
4083 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4084 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4087 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4090 <entry>The name already had an owner,
4091 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4092 the current owner did not specify
4093 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4094 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4098 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4099 <entry>The name already has an owner,
4100 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4101 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4102 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4103 specified by the requesting application.</entry>
4106 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4108 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4116 <sect3 id="bus-messages-release-name">
4117 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4121 UINT32 ReleaseName (in STRING name)
4128 <entry>Argument</entry>
4130 <entry>Description</entry>
4136 <entry>STRING</entry>
4137 <entry>Name to release</entry>
4147 <entry>Argument</entry>
4149 <entry>Description</entry>
4155 <entry>UINT32</entry>
4156 <entry>Return value</entry>
4163 This method call should be sent to
4164 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4165 release the method caller's claim to the given name. If the caller is
4166 the primary owner, a new primary owner will be selected from the
4167 queue if any other owners are waiting. If the caller is waiting in
4168 the queue for the name, the caller will removed from the queue and
4169 will not be made an owner of the name if it later becomes available.
4170 If there are no other owners in the queue for the name, it will be
4171 removed from the bus entirely.
4173 The return code can be one of the following values:
4179 <entry>Conventional Name</entry>
4180 <entry>Value</entry>
4181 <entry>Description</entry>
4186 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4187 <entry>1</entry> <entry>The caller has released his claim on
4188 the given name. Either the caller was the primary owner of
4189 the name, and the name is now unused or taken by somebody
4190 waiting in the queue for the name, or the caller was waiting
4191 in the queue for the name and has now been removed from the
4195 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4197 <entry>The given name does not exist on this bus.</entry>
4200 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4202 <entry>The caller was not the primary owner of this name,
4203 and was also not waiting in the queue to own this name.</entry>
4211 <sect3 id="bus-messages-list-queued-owners">
4212 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4216 ARRAY of STRING ListQueuedOwners (in STRING name)
4223 <entry>Argument</entry>
4225 <entry>Description</entry>
4231 <entry>STRING</entry>
4232 <entry>The well-known bus name to query, such as
4233 <literal>com.example.cappuccino</literal></entry>
4243 <entry>Argument</entry>
4245 <entry>Description</entry>
4251 <entry>ARRAY of STRING</entry>
4252 <entry>The unique bus names of connections currently queued
4253 for the name</entry>
4260 This method call should be sent to
4261 <literal>org.freedesktop.DBus</literal> and lists the connections
4262 currently queued for a bus name (see
4263 <xref linkend="term-queued-owner"/>).
4268 <sect2 id="message-bus-routing">
4269 <title>Message Bus Message Routing</title>
4272 Messages may have a <literal>DESTINATION</literal> field (see <xref
4273 linkend="message-protocol-header-fields"/>), resulting in a
4274 <firstterm>unicast message</firstterm>. If the
4275 <literal>DESTINATION</literal> field is present, it specifies a message
4276 recipient by name. Method calls and replies normally specify this field.
4277 The message bus must send messages (of any type) with the
4278 <literal>DESTINATION</literal> field set to the specified recipient,
4279 regardless of whether the recipient has set up a match rule matching
4284 When the message bus receives a signal, if the
4285 <literal>DESTINATION</literal> field is absent, it is considered to
4286 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4287 applications with <firstterm>message matching rules</firstterm> that
4288 match the message. Most signal messages are broadcasts.
4292 Unicast signal messages (those with a <literal>DESTINATION</literal>
4293 field) are not commonly used, but they are treated like any unicast
4294 message: they are delivered to the specified receipient,
4295 regardless of its match rules. One use for unicast signals is to
4296 avoid a race condition in which a signal is emitted before the intended
4297 recipient can call <xref linkend="bus-messages-add-match"/> to
4298 receive that signal: if the signal is sent directly to that recipient
4299 using a unicast message, it does not need to add a match rule at all,
4300 and there is no race condition. Another use for unicast signals,
4301 on message buses whose security policy prevents eavesdropping, is to
4302 send sensitive information which should only be visible to one
4307 When the message bus receives a method call, if the
4308 <literal>DESTINATION</literal> field is absent, the call is taken to be
4309 a standard one-to-one message and interpreted by the message bus
4310 itself. For example, sending an
4311 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4312 <literal>DESTINATION</literal> will cause the message bus itself to
4313 reply to the ping immediately; the message bus will not make this
4314 message visible to other applications.
4318 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4319 the ping message were sent with a <literal>DESTINATION</literal> name of
4320 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4321 forwarded, and the Yoyodyne Corporation screensaver application would be
4322 expected to reply to the ping.
4326 Message bus implementations may impose a security policy which
4327 prevents certain messages from being sent or received.
4328 When a message cannot be sent or received due to a security
4329 policy, the message bus should send an error reply, unless the
4330 original message had the <literal>NO_REPLY</literal> flag.
4333 <sect3 id="message-bus-routing-eavesdropping">
4334 <title>Eavesdropping</title>
4336 Receiving a unicast message whose <literal>DESTINATION</literal>
4337 indicates a different recipient is called
4338 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4339 a security boundary (like the standard system bus), the security
4340 policy should usually prevent eavesdropping, since unicast messages
4341 are normally kept private and may contain security-sensitive
4346 Eavesdropping is mainly useful for debugging tools, such as
4347 the <literal>dbus-monitor</literal> tool in the reference
4348 implementation of D-Bus. Tools which eavesdrop on the message bus
4349 should be careful to avoid sending a reply or error in response to
4350 messages intended for a different client.
4354 Clients may attempt to eavesdrop by adding match rules
4355 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4356 the <literal>eavesdrop='true'</literal> match. If the message bus'
4357 security policy does not allow eavesdropping, the match rule can
4358 still be added, but will not have any practical effect. For
4359 compatibility with older message bus implementations, if adding such
4360 a match rule results in an error reply, the client may fall back to
4361 adding the same rule with the <literal>eavesdrop</literal> match
4366 <sect3 id="message-bus-routing-match-rules">
4367 <title>Match Rules</title>
4369 An important part of the message bus routing protocol is match
4370 rules. Match rules describe the messages that should be sent to a
4371 client, based on the contents of the message. Broadcast signals
4372 are only sent to clients which have a suitable match rule: this
4373 avoids waking up client processes to deal with signals that are
4374 not relevant to that client.
4377 Messages that list a client as their <literal>DESTINATION</literal>
4378 do not need to match the client's match rules, and are sent to that
4379 client regardless. As a result, match rules are mainly used to
4380 receive a subset of broadcast signals.
4383 Match rules can also be used for eavesdropping
4384 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4385 if the security policy of the message bus allows it.
4388 Match rules are added using the AddMatch bus method
4389 (see <xref linkend="bus-messages-add-match"/>). Rules are
4390 specified as a string of comma separated key/value pairs.
4391 Excluding a key from the rule indicates a wildcard match.
4392 For instance excluding the the member from a match rule but
4393 adding a sender would let all messages from that sender through.
4394 An example of a complete rule would be
4395 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4398 The following table describes the keys that can be used to create
4400 The following table summarizes the D-Bus types.
4406 <entry>Possible Values</entry>
4407 <entry>Description</entry>
4412 <entry><literal>type</literal></entry>
4413 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4414 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4417 <entry><literal>sender</literal></entry>
4418 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4419 and <xref linkend="term-unique-name"/> respectively)
4421 <entry>Match messages sent by a particular sender. An example of a sender match
4422 is sender='org.freedesktop.Hal'</entry>
4425 <entry><literal>interface</literal></entry>
4426 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4427 <entry>Match messages sent over or to a particular interface. An example of an
4428 interface match is interface='org.freedesktop.Hal.Manager'.
4429 If a message omits the interface header, it must not match any rule
4430 that specifies this key.</entry>
4433 <entry><literal>member</literal></entry>
4434 <entry>Any valid method or signal name</entry>
4435 <entry>Matches messages which have the give method or signal name. An example of
4436 a member match is member='NameOwnerChanged'</entry>
4439 <entry><literal>path</literal></entry>
4440 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4441 <entry>Matches messages which are sent from or to the given object. An example of a
4442 path match is path='/org/freedesktop/Hal/Manager'</entry>
4445 <entry><literal>path_namespace</literal></entry>
4446 <entry>An object path</entry>
4449 Matches messages which are sent from or to an
4450 object for which the object path is either the
4451 given value, or that value followed by one or
4452 more path components.
4457 <literal>path_namespace='/com/example/foo'</literal>
4458 would match signals sent by
4459 <literal>/com/example/foo</literal>
4461 <literal>/com/example/foo/bar</literal>,
4463 <literal>/com/example/foobar</literal>.
4467 Using both <literal>path</literal> and
4468 <literal>path_namespace</literal> in the same match
4469 rule is not allowed.
4474 This match key was added in version 0.16 of the
4475 D-Bus specification and implemented by the bus
4476 daemon in dbus 1.5.0 and later.
4482 <entry><literal>destination</literal></entry>
4483 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4484 <entry>Matches messages which are being sent to the given unique name. An
4485 example of a destination match is destination=':1.0'</entry>
4488 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4489 <entry>Any string</entry>
4490 <entry>Arg matches are special and are used for further restricting the
4491 match based on the arguments in the body of a message. Only arguments of type
4492 STRING can be matched in this way. An example of an argument match
4493 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4497 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4498 <entry>Any string</entry>
4500 <para>Argument path matches provide a specialised form of wildcard matching for
4501 path-like namespaces. They can match arguments whose type is either STRING or
4502 OBJECT_PATH. As with normal argument matches,
4503 if the argument is exactly equal to the string given in the match
4504 rule then the rule is satisfied. Additionally, there is also a
4505 match when either the string given in the match rule or the
4506 appropriate message argument ends with '/' and is a prefix of the
4507 other. An example argument path match is arg0path='/aa/bb/'. This
4508 would match messages with first arguments of '/', '/aa/',
4509 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4510 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4512 <para>This is intended for monitoring “directories” in file system-like
4513 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4514 system. An application interested in all nodes in a particular hierarchy would
4515 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4516 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4517 represent a modification to the “bar” property, or a signal with zeroth
4518 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4519 many properties within that directory, and the interested application would be
4520 notified in both cases.</para>
4523 This match key was added in version 0.12 of the
4524 D-Bus specification, implemented for STRING
4525 arguments by the bus daemon in dbus 1.2.0 and later,
4526 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4533 <entry><literal>arg0namespace</literal></entry>
4534 <entry>Like a bus name, except that the string is not
4535 required to contain a '.' (period)</entry>
4537 <para>Match messages whose first argument is of type STRING, and is a bus name
4538 or interface name within the specified namespace. This is primarily intended
4539 for watching name owner changes for a group of related bus names, rather than
4540 for a single name or all name changes.</para>
4542 <para>Because every valid interface name is also a valid
4543 bus name, this can also be used for messages whose
4544 first argument is an interface name.</para>
4546 <para>For example, the match rule
4547 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4548 matches name owner changes for bus names such as
4549 <literal>com.example.backend.foo</literal>,
4550 <literal>com.example.backend.foo.bar</literal>, and
4551 <literal>com.example.backend</literal> itself.</para>
4553 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4556 This match key was added in version 0.16 of the
4557 D-Bus specification and implemented by the bus
4558 daemon in dbus 1.5.0 and later.
4564 <entry><literal>eavesdrop</literal></entry>
4565 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4566 <entry>Since D-Bus 1.5.6, match rules do not
4567 match messages which have a <literal>DESTINATION</literal>
4568 field unless the match rule specifically
4570 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4571 by specifying <literal>eavesdrop='true'</literal>
4572 in the match rule. <literal>eavesdrop='false'</literal>
4573 restores the default behaviour. Messages are
4574 delivered to their <literal>DESTINATION</literal>
4575 regardless of match rules, so this match does not
4576 affect normal delivery of unicast messages.
4577 If the message bus has a security policy which forbids
4578 eavesdropping, this match may still be used without error,
4579 but will not have any practical effect.
4580 In older versions of D-Bus, this match was not allowed
4581 in match rules, and all match rules behaved as if
4582 <literal>eavesdrop='true'</literal> had been used.
4591 <sect2 id="message-bus-starting-services">
4592 <title>Message Bus Starting Services</title>
4594 The message bus can start applications on behalf of other applications.
4595 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4596 An application that can be started in this way is called a
4597 <firstterm>service</firstterm>.
4600 With D-Bus, starting a service is normally done by name. That is,
4601 applications ask the message bus to start some program that will own a
4602 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4603 This implies a contract documented along with the name
4604 <literal>org.freedesktop.TextEditor</literal> for which objects
4605 the owner of that name will provide, and what interfaces those
4609 To find an executable corresponding to a particular name, the bus daemon
4610 looks for <firstterm>service description files</firstterm>. Service
4611 description files define a mapping from names to executables. Different
4612 kinds of message bus will look for these files in different places, see
4613 <xref linkend="message-bus-types"/>.
4616 Service description files have the ".service" file
4617 extension. The message bus will only load service description files
4618 ending with .service; all other files will be ignored. The file format
4619 is similar to that of <ulink
4620 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4621 entries</ulink>. All service description files must be in UTF-8
4622 encoding. To ensure that there will be no name collisions, service files
4623 must be namespaced using the same mechanism as messages and service
4628 [FIXME the file format should be much better specified than "similar to
4629 .desktop entries" esp. since desktop entries are already
4630 badly-specified. ;-)]
4631 These sections from the specification apply to service files as well:
4634 <listitem><para>General syntax</para></listitem>
4635 <listitem><para>Comment format</para></listitem>
4639 <title>Example service description file</title>
4641 # Sample service description file
4643 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4644 Exec=/usr/libexec/gconfd-2
4649 When an application asks to start a service by name, the bus daemon tries to
4650 find a service that will own that name. It then tries to spawn the
4651 executable associated with it. If this fails, it will report an
4652 error. [FIXME what happens if two .service files offer the same service;
4653 what kind of error is reported, should we have a way for the client to
4657 The executable launched will have the environment variable
4658 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4659 message bus so it can connect and request the appropriate names.
4662 The executable being launched may want to know whether the message bus
4663 starting it is one of the well-known message buses (see <xref
4664 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4665 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4666 of the well-known buses. The currently-defined values for this variable
4667 are <literal>system</literal> for the systemwide message bus,
4668 and <literal>session</literal> for the per-login-session message
4669 bus. The new executable must still connect to the address given
4670 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4671 resulting connection is to the well-known bus.
4674 [FIXME there should be a timeout somewhere, either specified
4675 in the .service file, by the client, or just a global value
4676 and if the client being activated fails to connect within that
4677 timeout, an error should be sent back.]
4680 <sect3 id="message-bus-starting-services-scope">
4681 <title>Message Bus Service Scope</title>
4683 The "scope" of a service is its "per-", such as per-session,
4684 per-machine, per-home-directory, or per-display. The reference
4685 implementation doesn't yet support starting services in a different
4686 scope from the message bus itself. So e.g. if you start a service
4687 on the session bus its scope is per-session.
4690 We could add an optional scope to a bus name. For example, for
4691 per-(display,session pair), we could have a unique ID for each display
4692 generated automatically at login and set on screen 0 by executing a
4693 special "set display ID" binary. The ID would be stored in a
4694 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4695 random bytes. This ID would then be used to scope names.
4696 Starting/locating a service could be done by ID-name pair rather than
4700 Contrast this with a per-display scope. To achieve that, we would
4701 want a single bus spanning all sessions using a given display.
4702 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4703 property on screen 0 of the display, pointing to this bus.
4708 <sect2 id="message-bus-types">
4709 <title>Well-known Message Bus Instances</title>
4711 Two standard message bus instances are defined here, along with how
4712 to locate them and where their service files live.
4714 <sect3 id="message-bus-types-login">
4715 <title>Login session message bus</title>
4717 Each time a user logs in, a <firstterm>login session message
4718 bus</firstterm> may be started. All applications in the user's login
4719 session may interact with one another using this message bus.
4722 The address of the login session message bus is given
4723 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4724 variable. If that variable is not set, applications may
4725 also try to read the address from the X Window System root
4726 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4727 The root window property must have type <literal>STRING</literal>.
4728 The environment variable should have precedence over the
4729 root window property.
4731 <para>The address of the login session message bus is given in the
4732 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4733 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4734 "autolaunch:", the system should use platform-specific methods of
4735 locating a running D-Bus session server, or starting one if a running
4736 instance cannot be found. Note that this mechanism is not recommended
4737 for attempting to determine if a daemon is running. It is inherently
4738 racy to attempt to make this determination, since the bus daemon may
4739 be started just before or just after the determination is made.
4740 Therefore, it is recommended that applications do not try to make this
4741 determination for their functionality purposes, and instead they
4742 should attempt to start the server.</para>
4744 <sect4 id="message-bus-types-login-x-windows">
4745 <title>X Windowing System</title>
4747 For the X Windowing System, the application must locate the
4748 window owner of the selection represented by the atom formed by
4752 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4756 <para>the current user's username</para>
4760 <para>the literal character '_' (underscore)</para>
4764 <para>the machine's ID</para>
4770 The following properties are defined for the window that owns
4772 <informaltable frame="all">
4781 <para>meaning</para>
4787 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4791 <para>the actual address of the server socket</para>
4797 <para>_DBUS_SESSION_BUS_PID</para>
4801 <para>the PID of the server process</para>
4810 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4811 present in this window.
4815 If the X selection cannot be located or if reading the
4816 properties from the window fails, the implementation MUST conclude
4817 that there is no D-Bus server running and proceed to start a new
4818 server. (See below on concurrency issues)
4822 Failure to connect to the D-Bus server address thus obtained
4823 MUST be treated as a fatal connection error and should be reported
4828 As an alternative, an implementation MAY find the information
4829 in the following file located in the current user's home directory,
4830 in subdirectory .dbus/session-bus/:
4833 <para>the machine's ID</para>
4837 <para>the literal character '-' (dash)</para>
4841 <para>the X display without the screen number, with the
4842 following prefixes removed, if present: ":", "localhost:"
4843 ."localhost.localdomain:". That is, a display of
4844 "localhost:10.0" produces just the number "10"</para>
4850 The contents of this file NAME=value assignment pairs and
4851 lines starting with # are comments (no comments are allowed
4852 otherwise). The following variable names are defined:
4859 <para>Variable</para>
4863 <para>meaning</para>
4869 <para>DBUS_SESSION_BUS_ADDRESS</para>
4873 <para>the actual address of the server socket</para>
4879 <para>DBUS_SESSION_BUS_PID</para>
4883 <para>the PID of the server process</para>
4889 <para>DBUS_SESSION_BUS_WINDOWID</para>
4893 <para>the window ID</para>
4902 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4907 Failure to open this file MUST be interpreted as absence of a
4908 running server. Therefore, the implementation MUST proceed to
4909 attempting to launch a new bus server if the file cannot be
4914 However, success in opening this file MUST NOT lead to the
4915 conclusion that the server is running. Thus, a failure to connect to
4916 the bus address obtained by the alternative method MUST NOT be
4917 considered a fatal error. If the connection cannot be established,
4918 the implementation MUST proceed to check the X selection settings or
4919 to start the server on its own.
4923 If the implementation concludes that the D-Bus server is not
4924 running it MUST attempt to start a new server and it MUST also
4925 ensure that the daemon started as an effect of the "autolaunch"
4926 mechanism provides the lookup mechanisms described above, so
4927 subsequent calls can locate the newly started server. The
4928 implementation MUST also ensure that if two or more concurrent
4929 initiations happen, only one server remains running and all other
4930 initiations are able to obtain the address of this server and
4931 connect to it. In other words, the implementation MUST ensure that
4932 the X selection is not present when it attempts to set it, without
4933 allowing another process to set the selection between the
4934 verification and the setting (e.g., by using XGrabServer /
4941 On Unix systems, the session bus should search for .service files
4942 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4944 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4945 Implementations may also search additional locations, which
4946 should be searched with lower priority than anything in
4947 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4948 for example, the reference implementation also
4949 looks in <literal>${datadir}/dbus-1/services</literal> as
4950 set at compile time.
4953 As described in the XDG Base Directory Specification, software
4954 packages should install their session .service files to their
4955 configured <literal>${datadir}/dbus-1/services</literal>,
4956 where <literal>${datadir}</literal> is as defined by the GNU
4957 coding standards. System administrators or users can arrange
4958 for these service files to be read by setting XDG_DATA_DIRS or by
4959 symlinking them into the default locations.
4963 <sect3 id="message-bus-types-system">
4964 <title>System message bus</title>
4966 A computer may have a <firstterm>system message bus</firstterm>,
4967 accessible to all applications on the system. This message bus may be
4968 used to broadcast system events, such as adding new hardware devices,
4969 changes in the printer queue, and so forth.
4972 The address of the system message bus is given
4973 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4974 variable. If that variable is not set, applications should try
4975 to connect to the well-known address
4976 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4979 The D-Bus reference implementation actually honors the
4980 <literal>$(localstatedir)</literal> configure option
4981 for this address, on both client and server side.
4986 On Unix systems, the system bus should default to searching
4987 for .service files in
4988 <literal>/usr/local/share/dbus-1/system-services</literal>,
4989 <literal>/usr/share/dbus-1/system-services</literal> and
4990 <literal>/lib/dbus-1/system-services</literal>, with that order
4991 of precedence. It may also search other implementation-specific
4992 locations, but should not vary these locations based on environment
4996 The system bus is security-sensitive and is typically executed
4997 by an init system with a clean environment. Its launch helper
4998 process is particularly security-sensitive, and specifically
4999 clears its own environment.
5004 Software packages should install their system .service
5005 files to their configured
5006 <literal>${datadir}/dbus-1/system-services</literal>,
5007 where <literal>${datadir}</literal> is as defined by the GNU
5008 coding standards. System administrators can arrange
5009 for these service files to be read by editing the system bus'
5010 configuration file or by symlinking them into the default
5016 <sect2 id="message-bus-messages">
5017 <title>Message Bus Messages</title>
5019 The special message bus name <literal>org.freedesktop.DBus</literal>
5020 responds to a number of additional messages.
5023 <sect3 id="bus-messages-hello">
5024 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5035 <entry>Argument</entry>
5037 <entry>Description</entry>
5043 <entry>STRING</entry>
5044 <entry>Unique name assigned to the connection</entry>
5051 Before an application is able to send messages to other applications
5052 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5053 to the message bus to obtain a unique name. If an application without
5054 a unique name tries to send a message to another application, or a
5055 message to the message bus itself that isn't the
5056 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5057 disconnected from the bus.
5060 There is no corresponding "disconnect" request; if a client wishes to
5061 disconnect from the bus, it simply closes the socket (or other
5062 communication channel).
5065 <sect3 id="bus-messages-list-names">
5066 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5070 ARRAY of STRING ListNames ()
5077 <entry>Argument</entry>
5079 <entry>Description</entry>
5085 <entry>ARRAY of STRING</entry>
5086 <entry>Array of strings where each string is a bus name</entry>
5093 Returns a list of all currently-owned names on the bus.
5096 <sect3 id="bus-messages-list-activatable-names">
5097 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5101 ARRAY of STRING ListActivatableNames ()
5108 <entry>Argument</entry>
5110 <entry>Description</entry>
5116 <entry>ARRAY of STRING</entry>
5117 <entry>Array of strings where each string is a bus name</entry>
5124 Returns a list of all names that can be activated on the bus.
5127 <sect3 id="bus-messages-name-exists">
5128 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5132 BOOLEAN NameHasOwner (in STRING name)
5139 <entry>Argument</entry>
5141 <entry>Description</entry>
5147 <entry>STRING</entry>
5148 <entry>Name to check</entry>
5158 <entry>Argument</entry>
5160 <entry>Description</entry>
5166 <entry>BOOLEAN</entry>
5167 <entry>Return value, true if the name exists</entry>
5174 Checks if the specified name exists (currently has an owner).
5178 <sect3 id="bus-messages-name-owner-changed">
5179 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5183 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5190 <entry>Argument</entry>
5192 <entry>Description</entry>
5198 <entry>STRING</entry>
5199 <entry>Name with a new owner</entry>
5203 <entry>STRING</entry>
5204 <entry>Old owner or empty string if none</entry>
5208 <entry>STRING</entry>
5209 <entry>New owner or empty string if none</entry>
5216 This signal indicates that the owner of a name has changed.
5217 It's also the signal to use to detect the appearance of
5218 new names on the bus.
5221 <sect3 id="bus-messages-name-lost">
5222 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5226 NameLost (STRING name)
5233 <entry>Argument</entry>
5235 <entry>Description</entry>
5241 <entry>STRING</entry>
5242 <entry>Name which was lost</entry>
5249 This signal is sent to a specific application when it loses
5250 ownership of a name.
5254 <sect3 id="bus-messages-name-acquired">
5255 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5259 NameAcquired (STRING name)
5266 <entry>Argument</entry>
5268 <entry>Description</entry>
5274 <entry>STRING</entry>
5275 <entry>Name which was acquired</entry>
5282 This signal is sent to a specific application when it gains
5283 ownership of a name.
5287 <sect3 id="bus-messages-start-service-by-name">
5288 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5292 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5299 <entry>Argument</entry>
5301 <entry>Description</entry>
5307 <entry>STRING</entry>
5308 <entry>Name of the service to start</entry>
5312 <entry>UINT32</entry>
5313 <entry>Flags (currently not used)</entry>
5323 <entry>Argument</entry>
5325 <entry>Description</entry>
5331 <entry>UINT32</entry>
5332 <entry>Return value</entry>
5337 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5341 The return value can be one of the following values:
5346 <entry>Identifier</entry>
5347 <entry>Value</entry>
5348 <entry>Description</entry>
5353 <entry>DBUS_START_REPLY_SUCCESS</entry>
5355 <entry>The service was successfully started.</entry>
5358 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5360 <entry>A connection already owns the given name.</entry>
5369 <sect3 id="bus-messages-update-activation-environment">
5370 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5374 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5381 <entry>Argument</entry>
5383 <entry>Description</entry>
5389 <entry>ARRAY of DICT<STRING,STRING></entry>
5390 <entry>Environment to add or update</entry>
5395 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5398 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5401 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.
5406 <sect3 id="bus-messages-get-name-owner">
5407 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5411 STRING GetNameOwner (in STRING name)
5418 <entry>Argument</entry>
5420 <entry>Description</entry>
5426 <entry>STRING</entry>
5427 <entry>Name to get the owner of</entry>
5437 <entry>Argument</entry>
5439 <entry>Description</entry>
5445 <entry>STRING</entry>
5446 <entry>Return value, a unique connection name</entry>
5451 Returns the unique connection name of the primary owner of the name
5452 given. If the requested name doesn't have an owner, returns a
5453 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5457 <sect3 id="bus-messages-get-connection-unix-user">
5458 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5462 UINT32 GetConnectionUnixUser (in STRING bus_name)
5469 <entry>Argument</entry>
5471 <entry>Description</entry>
5477 <entry>STRING</entry>
5478 <entry>Unique or well-known bus name of the connection to
5479 query, such as <literal>:12.34</literal> or
5480 <literal>com.example.tea</literal></entry>
5490 <entry>Argument</entry>
5492 <entry>Description</entry>
5498 <entry>UINT32</entry>
5499 <entry>Unix user ID</entry>
5504 Returns the Unix user ID of the process connected to the server. If
5505 unable to determine it (for instance, because the process is not on the
5506 same machine as the bus daemon), an error is returned.
5510 <sect3 id="bus-messages-get-connection-unix-process-id">
5511 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5515 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5522 <entry>Argument</entry>
5524 <entry>Description</entry>
5530 <entry>STRING</entry>
5531 <entry>Unique or well-known bus name of the connection to
5532 query, such as <literal>:12.34</literal> or
5533 <literal>com.example.tea</literal></entry>
5543 <entry>Argument</entry>
5545 <entry>Description</entry>
5551 <entry>UINT32</entry>
5552 <entry>Unix process id</entry>
5557 Returns the Unix process ID of the process connected to the server. If
5558 unable to determine it (for instance, because the process is not on the
5559 same machine as the bus daemon), an error is returned.
5563 <sect3 id="bus-messages-add-match">
5564 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5568 AddMatch (in STRING rule)
5575 <entry>Argument</entry>
5577 <entry>Description</entry>
5583 <entry>STRING</entry>
5584 <entry>Match rule to add to the connection</entry>
5589 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5590 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5594 <sect3 id="bus-messages-remove-match">
5595 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5599 RemoveMatch (in STRING rule)
5606 <entry>Argument</entry>
5608 <entry>Description</entry>
5614 <entry>STRING</entry>
5615 <entry>Match rule to remove from the connection</entry>
5620 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5621 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5626 <sect3 id="bus-messages-get-id">
5627 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5631 GetId (out STRING id)
5638 <entry>Argument</entry>
5640 <entry>Description</entry>
5646 <entry>STRING</entry>
5647 <entry>Unique ID identifying the bus daemon</entry>
5652 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5653 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5654 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5655 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5656 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5657 by org.freedesktop.DBus.Peer.GetMachineId().
5658 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5666 <appendix id="implementation-notes">
5667 <title>Implementation notes</title>
5668 <sect1 id="implementation-notes-subsection">
5676 <glossary><title>Glossary</title>
5678 This glossary defines some of the terms used in this specification.
5681 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5684 The message bus maintains an association between names and
5685 connections. (Normally, there's one connection per application.) A
5686 bus name is simply an identifier used to locate connections. For
5687 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5688 name might be used to send a message to a screensaver from Yoyodyne
5689 Corporation. An application is said to <firstterm>own</firstterm> a
5690 name if the message bus has associated the application's connection
5691 with the name. Names may also have <firstterm>queued
5692 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5693 The bus assigns a unique name to each connection,
5694 see <xref linkend="term-unique-name"/>. Other names
5695 can be thought of as "well-known names" and are
5696 used to find applications that offer specific functionality.
5700 See <xref linkend="message-protocol-names-bus"/> for details of
5701 the syntax and naming conventions for bus names.
5706 <glossentry id="term-message"><glossterm>Message</glossterm>
5709 A message is the atomic unit of communication via the D-Bus
5710 protocol. It consists of a <firstterm>header</firstterm> and a
5711 <firstterm>body</firstterm>; the body is made up of
5712 <firstterm>arguments</firstterm>.
5717 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5720 The message bus is a special application that forwards
5721 or routes messages between a group of applications
5722 connected to the message bus. It also manages
5723 <firstterm>names</firstterm> used for routing
5729 <glossentry id="term-name"><glossterm>Name</glossterm>
5732 See <xref linkend="term-bus-name"/>. "Name" may
5733 also be used to refer to some of the other names
5734 in D-Bus, such as interface names.
5739 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5742 Used to prevent collisions when defining new interfaces, bus names
5743 etc. The convention used is the same one Java uses for defining
5744 classes: a reversed domain name.
5745 See <xref linkend="message-protocol-names-bus"/>,
5746 <xref linkend="message-protocol-names-interface"/>,
5747 <xref linkend="message-protocol-names-error"/>,
5748 <xref linkend="message-protocol-marshaling-object-path"/>.
5753 <glossentry id="term-object"><glossterm>Object</glossterm>
5756 Each application contains <firstterm>objects</firstterm>, which have
5757 <firstterm>interfaces</firstterm> and
5758 <firstterm>methods</firstterm>. Objects are referred to by a name,
5759 called a <firstterm>path</firstterm>.
5764 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5767 An application talking directly to another application, without going
5768 through a message bus. One-to-one connections may be "peer to peer" or
5769 "client to server." The D-Bus protocol has no concept of client
5770 vs. server after a connection has authenticated; the flow of messages
5771 is symmetrical (full duplex).
5776 <glossentry id="term-path"><glossterm>Path</glossterm>
5779 Object references (object names) in D-Bus are organized into a
5780 filesystem-style hierarchy, so each object is named by a path. As in
5781 LDAP, there's no difference between "files" and "directories"; a path
5782 can refer to an object, while still having child objects below it.
5787 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5790 Each bus name has a primary owner; messages sent to the name go to the
5791 primary owner. However, certain names also maintain a queue of
5792 secondary owners "waiting in the wings." If the primary owner releases
5793 the name, then the first secondary owner in the queue automatically
5794 becomes the new owner of the name.
5799 <glossentry id="term-service"><glossterm>Service</glossterm>
5802 A service is an executable that can be launched by the bus daemon.
5803 Services normally guarantee some particular features, for example they
5804 may guarantee that they will request a specific name such as
5805 "org.freedesktop.Screensaver", have a singleton object
5806 "/org/freedesktop/Application", and that object will implement the
5807 interface "org.freedesktop.ScreensaverControl".
5812 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5815 ".service files" tell the bus about service applications that can be
5816 launched (see <xref linkend="term-service"/>). Most importantly they
5817 provide a mapping from bus names to services that will request those
5818 names when they start up.
5823 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5826 The special name automatically assigned to each connection by the
5827 message bus. This name will never change owner, and will be unique
5828 (never reused during the lifetime of the message bus).
5829 It will begin with a ':' character.
projects / platform / upstream / dbus.git / blob