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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.19</releaseinfo>
10 <date>2012-02-21</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></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.19</revnumber>
82 <date>20 February 2012</date>
83 <authorinitials>smcv/lp</authorinitials>
84 <revremark>formally define unique connection names and well-known
85 bus names; document best practices for interface, bus, member and
86 error names, and object paths; document the search path for session
87 and system services on Unix; document the systemd transport</revremark>
90 <revnumber>0.18</revnumber>
91 <date>29 July 2011</date>
92 <authorinitials>smcv</authorinitials>
93 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
94 match keyword; promote type system to a top-level section</revremark>
97 <revnumber>0.17</revnumber>
98 <date>1 June 2011</date>
99 <authorinitials>smcv/davidz</authorinitials>
100 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
101 by GVariant</revremark>
104 <revnumber>0.16</revnumber>
105 <date>11 April 2011</date>
106 <authorinitials></authorinitials>
107 <revremark>add path_namespace, arg0namespace; argNpath matches object
111 <revnumber>0.15</revnumber>
112 <date>3 November 2010</date>
113 <authorinitials></authorinitials>
114 <revremark></revremark>
117 <revnumber>0.14</revnumber>
118 <date>12 May 2010</date>
119 <authorinitials></authorinitials>
120 <revremark></revremark>
123 <revnumber>0.13</revnumber>
124 <date>23 Dezember 2009</date>
125 <authorinitials></authorinitials>
126 <revremark></revremark>
129 <revnumber>0.12</revnumber>
130 <date>7 November, 2006</date>
131 <authorinitials></authorinitials>
132 <revremark></revremark>
135 <revnumber>0.11</revnumber>
136 <date>6 February 2005</date>
137 <authorinitials></authorinitials>
138 <revremark></revremark>
141 <revnumber>0.10</revnumber>
142 <date>28 January 2005</date>
143 <authorinitials></authorinitials>
144 <revremark></revremark>
147 <revnumber>0.9</revnumber>
148 <date>7 Januar 2005</date>
149 <authorinitials></authorinitials>
150 <revremark></revremark>
153 <revnumber>0.8</revnumber>
154 <date>06 September 2003</date>
155 <authorinitials></authorinitials>
156 <revremark>First released document.</revremark>
161 <sect1 id="introduction">
162 <title>Introduction</title>
164 D-Bus is a system for low-latency, low-overhead, easy to use
165 interprocess communication (IPC). In more detail:
169 D-Bus is <emphasis>low-latency</emphasis> because it is designed
170 to avoid round trips and allow asynchronous operation, much like
176 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
177 binary protocol, and does not have to convert to and from a text
178 format such as XML. Because D-Bus is intended for potentially
179 high-resolution same-machine IPC, not primarily for Internet IPC,
180 this is an interesting optimization.
185 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
186 of <firstterm>messages</firstterm> rather than byte streams, and
187 automatically handles a lot of the hard IPC issues. Also, the D-Bus
188 library is designed to be wrapped in a way that lets developers use
189 their framework's existing object/type system, rather than learning
190 a new one specifically for IPC.
197 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
198 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
199 a system for one application to talk to a single other
200 application. However, the primary intended application of the protocol is the
201 D-Bus <firstterm>message bus</firstterm>, specified in <xref
202 linkend="message-bus"/>. The message bus is a special application that
203 accepts connections from multiple other applications, and forwards
208 Uses of D-Bus include notification of system changes (notification of when
209 a camera is plugged in to a computer, or a new version of some software
210 has been installed), or desktop interoperability, for example a file
211 monitoring service or a configuration service.
215 D-Bus is designed for two specific use cases:
219 A "system bus" for notifications from the system to user sessions,
220 and to allow the system to request input from user sessions.
225 A "session bus" used to implement desktop environments such as
230 D-Bus is not intended to be a generic IPC system for any possible
231 application, and intentionally omits many features found in other
232 IPC systems for this reason.
236 At the same time, the bus daemons offer a number of features not found in
237 other IPC systems, such as single-owner "bus names" (similar to X
238 selections), on-demand startup of services, and security policies.
239 In many ways, these features are the primary motivation for developing
240 D-Bus; other systems would have sufficed if IPC were the only goal.
244 D-Bus may turn out to be useful in unanticipated applications, but future
245 versions of this spec and the reference implementation probably will not
246 incorporate features that interfere with the core use cases.
250 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
251 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
252 document are to be interpreted as described in RFC 2119. However, the
253 document could use a serious audit to be sure it makes sense to do
254 so. Also, they are not capitalized.
257 <sect2 id="stability">
258 <title>Protocol and Specification Stability</title>
260 The D-Bus protocol is frozen (only compatible extensions are allowed) as
261 of November 8, 2006. However, this specification could still use a fair
262 bit of work to make interoperable reimplementation possible without
263 reference to the D-Bus reference implementation. Thus, this
264 specification is not marked 1.0. To mark it 1.0, we'd like to see
265 someone invest significant effort in clarifying the specification
266 language, and growing the specification to cover more aspects of the
267 reference implementation's behavior.
270 Until this work is complete, any attempt to reimplement D-Bus will
271 probably require looking at the reference implementation and/or asking
272 questions on the D-Bus mailing list about intended behavior.
273 Questions on the list are very welcome.
276 Nonetheless, this document should be a useful starting point and is
277 to our knowledge accurate, though incomplete.
283 <sect1 id="type-system">
284 <title>Type System</title>
287 D-Bus has a type system, in which values of various types can be
288 serialized into a sequence of bytes referred to as the
289 <firstterm>wire format</firstterm> in a standard way.
290 Converting a value from some other representation into the wire
291 format is called <firstterm>marshaling</firstterm> and converting
292 it back from the wire format is <firstterm>unmarshaling</firstterm>.
296 The D-Bus protocol does not include type tags in the marshaled data; a
297 block of marshaled values must have a known <firstterm>type
298 signature</firstterm>. The type signature is made up of zero or more
299 <firstterm id="term-single-complete-type">single complete
300 types</firstterm>, each made up of one or more
301 <firstterm>type codes</firstterm>.
305 A type code is an ASCII character representing the
306 type of a value. Because ASCII characters are used, the type signature
307 will always form a valid ASCII string. A simple string compare
308 determines whether two type signatures are equivalent.
312 A single complete type is a sequence of type codes that fully describes
313 one type: either a basic type, or a single fully-described container type.
314 A single complete type is a basic type code, a variant type code,
315 an array with its element type, or a struct with its fields (all of which
316 are defined below). So the following signatures are not single complete
327 And the following signatures contain multiple complete types:
337 Note however that a single complete type may <emphasis>contain</emphasis>
338 multiple other single complete types, by containing a struct or dict
342 <sect2 id="basic-types">
343 <title>Basic types</title>
346 The simplest type codes are the <firstterm id="term-basic-type">basic
347 types</firstterm>, which are the types whose structure is entirely
348 defined by their 1-character type code. Basic types consist of
349 fixed types and string-like types.
353 The <firstterm id="term-fixed-type">fixed types</firstterm>
354 are basic types whose values have a fixed length, namely BYTE,
355 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
360 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
361 the ASCII character 'i'. So the signature for a block of values
362 containing a single <literal>INT32</literal> would be:
366 A block of values containing two <literal>INT32</literal> would have this signature:
373 The characteristics of the fixed types are listed in this table.
379 <entry>Conventional name</entry>
380 <entry>ASCII type-code</entry>
381 <entry>Encoding</entry>
386 <entry><literal>BYTE</literal></entry>
387 <entry><literal>y</literal> (121)</entry>
388 <entry>Unsigned 8-bit integer</entry>
391 <entry><literal>BOOLEAN</literal></entry>
392 <entry><literal>b</literal> (98)</entry>
393 <entry>Boolean value: 0 is false, 1 is true, any other value
394 allowed by the marshalling format is invalid</entry>
397 <entry><literal>INT16</literal></entry>
398 <entry><literal>n</literal> (110)</entry>
399 <entry>Signed (two's complement) 16-bit integer</entry>
402 <entry><literal>UINT16</literal></entry>
403 <entry><literal>q</literal> (113)</entry>
404 <entry>Unsigned 16-bit integer</entry>
407 <entry><literal>INT32</literal></entry>
408 <entry><literal>i</literal> (105)</entry>
409 <entry>Signed (two's complement) 32-bit integer</entry>
412 <entry><literal>UINT32</literal></entry>
413 <entry><literal>u</literal> (117)</entry>
414 <entry>Unsigned 32-bit integer</entry>
417 <entry><literal>INT64</literal></entry>
418 <entry><literal>x</literal> (120)</entry>
419 <entry>Signed (two's complement) 64-bit integer
420 (mnemonic: x and t are the first characters in "sixty" not
421 already used for something more common)</entry>
424 <entry><literal>UINT64</literal></entry>
425 <entry><literal>t</literal> (116)</entry>
426 <entry>Unsigned 64-bit integer</entry>
429 <entry><literal>DOUBLE</literal></entry>
430 <entry><literal>d</literal> (100)</entry>
431 <entry>IEEE 754 double-precision floating point</entry>
434 <entry><literal>UNIX_FD</literal></entry>
435 <entry><literal>h</literal> (104)</entry>
436 <entry>Unsigned 32-bit integer representing an index into an
437 out-of-band array of file descriptors, transferred via some
438 platform-specific mechanism (mnemonic: h for handle)</entry>
446 The <firstterm id="term-string-like-type">string-like types</firstterm>
447 are basic types with a variable length. The value of any string-like
448 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
449 none of which may be U+0000. The UTF-8 text must be validated
450 strictly: in particular, it must not contain overlong sequences,
451 noncharacters such as U+FFFE, or codepoints above U+10FFFF.
455 The marshalling formats for the string-like types all end with a
456 single zero (NUL) byte, but that byte is not considered to be part of
461 The characteristics of the string-like types are listed in this table.
467 <entry>Conventional name</entry>
468 <entry>ASCII type-code</entry>
469 <entry>Validity constraints</entry>
474 <entry><literal>STRING</literal></entry>
475 <entry><literal>s</literal> (115)</entry>
476 <entry>No extra constraints</entry>
479 <entry><literal>OBJECT_PATH</literal></entry>
480 <entry><literal>o</literal> (111)</entry>
482 <link linkend="message-protocol-marshaling-object-path">a
483 syntactically valid object path</link></entry>
486 <entry><literal>SIGNATURE</literal></entry>
487 <entry><literal>g</literal> (103)</entry>
489 <firstterm linkend="term-single-complete-type">single
490 complete types</firstterm></entry>
497 <sect3 id="message-protocol-marshaling-object-path">
498 <title>Valid Object Paths</title>
501 An object path is a name used to refer to an object instance.
502 Conceptually, each participant in a D-Bus message exchange may have
503 any number of object instances (think of C++ or Java objects) and each
504 such instance will have a path. Like a filesystem, the object
505 instances in an application form a hierarchical tree.
509 Object paths are often namespaced by starting with a reversed
510 domain name and containing an interface version number, in the
512 <link linkend="message-protocol-names-interface">interface
514 <link linkend="message-protocol-names-bus">well-known
516 This makes it possible to implement more than one service, or
517 more than one version of a service, in the same process,
518 even if the services share a connection but cannot otherwise
519 co-operate (for instance, if they are implemented by different
524 For instance, if the owner of <literal>example.com</literal> is
525 developing a D-Bus API for a music player, they might use the
526 hierarchy of object paths that start with
527 <literal>/com/example/MusicPlayer1</literal> for its objects.
531 The following rules define a valid object path. Implementations must
532 not send or accept messages with invalid object paths.
536 The path may be of any length.
541 The path must begin with an ASCII '/' (integer 47) character,
542 and must consist of elements separated by slash characters.
547 Each element must only contain the ASCII characters
553 No element may be the empty string.
558 Multiple '/' characters cannot occur in sequence.
563 A trailing '/' character is not allowed unless the
564 path is the root path (a single '/' character).
572 <sect3 id="message-protocol-marshaling-signature">
573 <title>Valid Signatures</title>
575 An implementation must not send or accept invalid signatures.
576 Valid signatures will conform to the following rules:
580 The signature is a list of single complete types.
581 Arrays must have element types, and structs must
582 have both open and close parentheses.
587 Only type codes, open and close parentheses, and open and
588 close curly brackets are allowed in the signature. The
589 <literal>STRUCT</literal> type code
590 is not allowed in signatures, because parentheses
591 are used instead. Similarly, the
592 <literal>DICT_ENTRY</literal> type code is not allowed in
593 signatures, because curly brackets are used instead.
598 The maximum depth of container type nesting is 32 array type
599 codes and 32 open parentheses. This implies that the maximum
600 total depth of recursion is 64, for an "array of array of array
601 of ... struct of struct of struct of ..." where there are 32
607 The maximum length of a signature is 255.
614 When signatures appear in messages, the marshalling format
615 guarantees that they will be followed by a nul byte (which can
616 be interpreted as either C-style string termination or the INVALID
617 type-code), but this is not conceptually part of the signature.
623 <sect2 id="container-types">
624 <title>Container types</title>
627 In addition to basic types, there are four <firstterm>container</firstterm>
628 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
629 and <literal>DICT_ENTRY</literal>.
633 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
634 code does not appear in signatures. Instead, ASCII characters
635 '(' and ')' are used to mark the beginning and end of the struct.
636 So for example, a struct containing two integers would have this
641 Structs can be nested, so for example a struct containing
642 an integer and another struct:
646 The value block storing that struct would contain three integers; the
647 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
652 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
653 but is useful in code that implements the protocol. This type code
654 is specified to allow such code to interoperate in non-protocol contexts.
658 Empty structures are not allowed; there must be at least one
659 type code between the parentheses.
663 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
664 followed by a <firstterm>single complete type</firstterm>. The single
665 complete type following the array is the type of each array element. So
666 the simple example is:
670 which is an array of 32-bit integers. But an array can be of any type,
671 such as this array-of-struct-with-two-int32-fields:
675 Or this array of array of integer:
682 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
683 type <literal>VARIANT</literal> will have the signature of a single complete type as part
684 of the <emphasis>value</emphasis>. This signature will be followed by a
685 marshaled value of that type.
689 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
690 than parentheses it uses curly braces, and it has more restrictions.
691 The restrictions are: it occurs only as an array element type; it has
692 exactly two single complete types inside the curly braces; the first
693 single complete type (the "key") must be a basic type rather than a
694 container type. Implementations must not accept dict entries outside of
695 arrays, must not accept dict entries with zero, one, or more than two
696 fields, and must not accept dict entries with non-basic-typed keys. A
697 dict entry is always a key-value pair.
701 The first field in the <literal>DICT_ENTRY</literal> is always the key.
702 A message is considered corrupt if the same key occurs twice in the same
703 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
704 implementations are not required to reject dicts with duplicate keys.
708 In most languages, an array of dict entry would be represented as a
709 map, hash table, or dict object.
714 <title>Summary of types</title>
717 The following table summarizes the D-Bus types.
722 <entry>Conventional Name</entry>
724 <entry>Description</entry>
729 <entry><literal>INVALID</literal></entry>
730 <entry>0 (ASCII NUL)</entry>
731 <entry>Not a valid type code, used to terminate signatures</entry>
733 <entry><literal>BYTE</literal></entry>
734 <entry>121 (ASCII 'y')</entry>
735 <entry>8-bit unsigned integer</entry>
737 <entry><literal>BOOLEAN</literal></entry>
738 <entry>98 (ASCII 'b')</entry>
739 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
741 <entry><literal>INT16</literal></entry>
742 <entry>110 (ASCII 'n')</entry>
743 <entry>16-bit signed integer</entry>
745 <entry><literal>UINT16</literal></entry>
746 <entry>113 (ASCII 'q')</entry>
747 <entry>16-bit unsigned integer</entry>
749 <entry><literal>INT32</literal></entry>
750 <entry>105 (ASCII 'i')</entry>
751 <entry>32-bit signed integer</entry>
753 <entry><literal>UINT32</literal></entry>
754 <entry>117 (ASCII 'u')</entry>
755 <entry>32-bit unsigned integer</entry>
757 <entry><literal>INT64</literal></entry>
758 <entry>120 (ASCII 'x')</entry>
759 <entry>64-bit signed integer</entry>
761 <entry><literal>UINT64</literal></entry>
762 <entry>116 (ASCII 't')</entry>
763 <entry>64-bit unsigned integer</entry>
765 <entry><literal>DOUBLE</literal></entry>
766 <entry>100 (ASCII 'd')</entry>
767 <entry>IEEE 754 double</entry>
769 <entry><literal>STRING</literal></entry>
770 <entry>115 (ASCII 's')</entry>
771 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
773 <entry><literal>OBJECT_PATH</literal></entry>
774 <entry>111 (ASCII 'o')</entry>
775 <entry>Name of an object instance</entry>
777 <entry><literal>SIGNATURE</literal></entry>
778 <entry>103 (ASCII 'g')</entry>
779 <entry>A type signature</entry>
781 <entry><literal>ARRAY</literal></entry>
782 <entry>97 (ASCII 'a')</entry>
785 <entry><literal>STRUCT</literal></entry>
786 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
787 <entry>Struct; type code 114 'r' is reserved for use in
788 bindings and implementations to represent the general
789 concept of a struct, and must not appear in signatures
790 used on D-Bus.</entry>
792 <entry><literal>VARIANT</literal></entry>
793 <entry>118 (ASCII 'v') </entry>
794 <entry>Variant type (the type of the value is part of the value itself)</entry>
796 <entry><literal>DICT_ENTRY</literal></entry>
797 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
798 <entry>Entry in a dict or map (array of key-value pairs).
799 Type code 101 'e' is reserved for use in bindings and
800 implementations to represent the general concept of a
801 dict or dict-entry, and must not appear in signatures
802 used on D-Bus.</entry>
804 <entry><literal>UNIX_FD</literal></entry>
805 <entry>104 (ASCII 'h')</entry>
806 <entry>Unix file descriptor</entry>
809 <entry>(reserved)</entry>
810 <entry>109 (ASCII 'm')</entry>
811 <entry>Reserved for <ulink
812 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
813 'maybe' type compatible with the one in GVariant</ulink>,
814 and must not appear in signatures used on D-Bus until
815 specified here</entry>
818 <entry>(reserved)</entry>
819 <entry>42 (ASCII '*')</entry>
820 <entry>Reserved for use in bindings/implementations to
821 represent any <firstterm>single complete type</firstterm>,
822 and must not appear in signatures used on D-Bus.</entry>
825 <entry>(reserved)</entry>
826 <entry>63 (ASCII '?')</entry>
827 <entry>Reserved for use in bindings/implementations to
828 represent any <firstterm>basic type</firstterm>, and must
829 not appear in signatures used on D-Bus.</entry>
832 <entry>(reserved)</entry>
833 <entry>64 (ASCII '@'), 38 (ASCII '&'),
834 94 (ASCII '^')</entry>
835 <entry>Reserved for internal use by bindings/implementations,
836 and must not appear in signatures used on D-Bus.
837 GVariant uses these type-codes to encode calling
847 <sect2 id="message-protocol-marshaling">
848 <title>Marshaling (Wire Format)</title>
851 Given a type signature, a block of bytes can be converted into typed
852 values. This section describes the format of the block of bytes. Byte
853 order and alignment issues are handled uniformly for all D-Bus types.
857 A block of bytes has an associated byte order. The byte order
858 has to be discovered in some way; for D-Bus messages, the
859 byte order is part of the message header as described in
860 <xref linkend="message-protocol-messages"/>. For now, assume
861 that the byte order is known to be either little endian or big
866 Each value in a block of bytes is aligned "naturally," for example
867 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
868 8-byte boundary. To properly align a value, <firstterm>alignment
869 padding</firstterm> may be necessary. The alignment padding must always
870 be the minimum required padding to properly align the following value;
871 and it must always be made up of nul bytes. The alignment padding must
872 not be left uninitialized (it can't contain garbage), and more padding
873 than required must not be used.
877 To marshal and unmarshal fixed types, you simply read one value
878 from the data block corresponding to each type code in the signature.
879 All signed integer values are encoded in two's complement, DOUBLE
880 values are IEEE 754 double-precision floating-point, and BOOLEAN
881 values are encoded in 32 bits (of which only the least significant
886 The string-like types are all marshalled as a
887 fixed-length unsigned integer <varname>n</varname> giving the
888 length of the variable part, followed by <varname>n</varname>
889 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
890 which is not considered to be part of the text. The alignment
891 of the string-like type is the same as the alignment of
892 <varname>n</varname>.
896 For the STRING and OBJECT_PATH types, <varname>n</varname> is
897 encoded in 4 bytes, leading to 4-byte alignment.
898 For the SIGNATURE type, <varname>n</varname> is encoded as a single
899 byte. As a result, alignment padding is never required before a
904 Given all this, the types are marshaled on the wire as follows:
909 <entry>Conventional Name</entry>
910 <entry>Encoding</entry>
911 <entry>Alignment</entry>
916 <entry><literal>INVALID</literal></entry>
917 <entry>Not applicable; cannot be marshaled.</entry>
920 <entry><literal>BYTE</literal></entry>
921 <entry>A single 8-bit byte.</entry>
924 <entry><literal>BOOLEAN</literal></entry>
925 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
928 <entry><literal>INT16</literal></entry>
929 <entry>16-bit signed integer in the message's byte order.</entry>
932 <entry><literal>UINT16</literal></entry>
933 <entry>16-bit unsigned integer in the message's byte order.</entry>
936 <entry><literal>INT32</literal></entry>
937 <entry>32-bit signed integer in the message's byte order.</entry>
940 <entry><literal>UINT32</literal></entry>
941 <entry>32-bit unsigned integer in the message's byte order.</entry>
944 <entry><literal>INT64</literal></entry>
945 <entry>64-bit signed integer in the message's byte order.</entry>
948 <entry><literal>UINT64</literal></entry>
949 <entry>64-bit unsigned integer in the message's byte order.</entry>
952 <entry><literal>DOUBLE</literal></entry>
953 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
956 <entry><literal>STRING</literal></entry>
957 <entry>A <literal>UINT32</literal> indicating the string's
958 length in bytes excluding its terminating nul, followed by
959 non-nul string data of the given length, followed by a terminating nul
966 <entry><literal>OBJECT_PATH</literal></entry>
967 <entry>Exactly the same as <literal>STRING</literal> except the
968 content must be a valid object path (see above).
974 <entry><literal>SIGNATURE</literal></entry>
975 <entry>The same as <literal>STRING</literal> except the length is a single
976 byte (thus signatures have a maximum length of 255)
977 and the content must be a valid signature (see above).
983 <entry><literal>ARRAY</literal></entry>
985 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
986 alignment padding to the alignment boundary of the array element type,
987 followed by each array element. The array length is from the
988 end of the alignment padding to the end of the last element,
989 i.e. it does not include the padding after the length,
990 or any padding after the last element.
991 Arrays have a maximum length defined to be 2 to the 26th power or
992 67108864. Implementations must not send or accept arrays exceeding this
999 <entry><literal>STRUCT</literal></entry>
1001 A struct must start on an 8-byte boundary regardless of the
1002 type of the struct fields. The struct value consists of each
1003 field marshaled in sequence starting from that 8-byte
1010 <entry><literal>VARIANT</literal></entry>
1012 A variant type has a marshaled
1013 <literal>SIGNATURE</literal> followed by a marshaled
1014 value with the type given in the signature. Unlike
1015 a message signature, the variant signature can
1016 contain only a single complete type. So "i", "ai"
1017 or "(ii)" is OK, but "ii" is not. Use of variants may not
1018 cause a total message depth to be larger than 64, including
1019 other container types such as structures.
1022 1 (alignment of the signature)
1025 <entry><literal>DICT_ENTRY</literal></entry>
1027 Identical to STRUCT.
1033 <entry><literal>UNIX_FD</literal></entry>
1034 <entry>32-bit unsigned integer in the message's byte
1035 order. The actual file descriptors need to be
1036 transferred out-of-band via some platform specific
1037 mechanism. On the wire, values of this type store the index to the
1038 file descriptor in the array of file descriptors that
1039 accompany the message.</entry>
1051 <sect1 id="message-protocol">
1052 <title>Message Protocol</title>
1055 A <firstterm>message</firstterm> consists of a
1056 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1057 think of a message as a package, the header is the address, and the body
1058 contains the package contents. The message delivery system uses the header
1059 information to figure out where to send the message and how to interpret
1060 it; the recipient interprets the body of the message.
1064 The body of the message is made up of zero or more
1065 <firstterm>arguments</firstterm>, which are typed values, such as an
1066 integer or a byte array.
1070 Both header and body use the D-Bus <link linkend="type-system">type
1071 system</link> and format for serializing data.
1074 <sect2 id="message-protocol-messages">
1075 <title>Message Format</title>
1078 A message consists of a header and a body. The header is a block of
1079 values with a fixed signature and meaning. The body is a separate block
1080 of values, with a signature specified in the header.
1084 The length of the header must be a multiple of 8, allowing the body to
1085 begin on an 8-byte boundary when storing the entire message in a single
1086 buffer. If the header does not naturally end on an 8-byte boundary
1087 up to 7 bytes of nul-initialized alignment padding must be added.
1091 The message body need not end on an 8-byte boundary.
1095 The maximum length of a message, including header, header alignment padding,
1096 and body is 2 to the 27th power or 134217728. Implementations must not
1097 send or accept messages exceeding this size.
1101 The signature of the header is:
1105 Written out more readably, this is:
1107 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1112 These values have the following meanings:
1117 <entry>Value</entry>
1118 <entry>Description</entry>
1123 <entry>1st <literal>BYTE</literal></entry>
1124 <entry>Endianness flag; ASCII 'l' for little-endian
1125 or ASCII 'B' for big-endian. Both header and body are
1126 in this endianness.</entry>
1129 <entry>2nd <literal>BYTE</literal></entry>
1130 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1131 Currently-defined types are described below.
1135 <entry>3rd <literal>BYTE</literal></entry>
1136 <entry>Bitwise OR of flags. Unknown flags
1137 must be ignored. Currently-defined flags are described below.
1141 <entry>4th <literal>BYTE</literal></entry>
1142 <entry>Major protocol version of the sending application. If
1143 the major protocol version of the receiving application does not
1144 match, the applications will not be able to communicate and the
1145 D-Bus connection must be disconnected. The major protocol
1146 version for this version of the specification is 1.
1150 <entry>1st <literal>UINT32</literal></entry>
1151 <entry>Length in bytes of the message body, starting
1152 from the end of the header. The header ends after
1153 its alignment padding to an 8-boundary.
1157 <entry>2nd <literal>UINT32</literal></entry>
1158 <entry>The serial of this message, used as a cookie
1159 by the sender to identify the reply corresponding
1160 to this request. This must not be zero.
1164 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1165 <entry>An array of zero or more <firstterm>header
1166 fields</firstterm> where the byte is the field code, and the
1167 variant is the field value. The message type determines
1168 which fields are required.
1176 <firstterm>Message types</firstterm> that can appear in the second byte
1182 <entry>Conventional name</entry>
1183 <entry>Decimal value</entry>
1184 <entry>Description</entry>
1189 <entry><literal>INVALID</literal></entry>
1191 <entry>This is an invalid type.</entry>
1194 <entry><literal>METHOD_CALL</literal></entry>
1196 <entry>Method call.</entry>
1199 <entry><literal>METHOD_RETURN</literal></entry>
1201 <entry>Method reply with returned data.</entry>
1204 <entry><literal>ERROR</literal></entry>
1206 <entry>Error reply. If the first argument exists and is a
1207 string, it is an error message.</entry>
1210 <entry><literal>SIGNAL</literal></entry>
1212 <entry>Signal emission.</entry>
1219 Flags that can appear in the third byte of the header:
1224 <entry>Conventional name</entry>
1225 <entry>Hex value</entry>
1226 <entry>Description</entry>
1231 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1233 <entry>This message does not expect method return replies or
1234 error replies; the reply can be omitted as an
1235 optimization. However, it is compliant with this specification
1236 to return the reply despite this flag and the only harm
1237 from doing so is extra network traffic.
1241 <entry><literal>NO_AUTO_START</literal></entry>
1243 <entry>The bus must not launch an owner
1244 for the destination name in response to this message.
1252 <sect3 id="message-protocol-header-fields">
1253 <title>Header Fields</title>
1256 The array at the end of the header contains <firstterm>header
1257 fields</firstterm>, where each field is a 1-byte field code followed
1258 by a field value. A header must contain the required header fields for
1259 its message type, and zero or more of any optional header
1260 fields. Future versions of this protocol specification may add new
1261 fields. Implementations must ignore fields they do not
1262 understand. Implementations must not invent their own header fields;
1263 only changes to this specification may introduce new header fields.
1267 Again, if an implementation sees a header field code that it does not
1268 expect, it must ignore that field, as it will be part of a new
1269 (but compatible) version of this specification. This also applies
1270 to known header fields appearing in unexpected messages, for
1271 example: if a signal has a reply serial it must be ignored
1272 even though it has no meaning as of this version of the spec.
1276 However, implementations must not send or accept known header fields
1277 with the wrong type stored in the field value. So for example a
1278 message with an <literal>INTERFACE</literal> field of type
1279 <literal>UINT32</literal> would be considered corrupt.
1283 Here are the currently-defined header fields:
1288 <entry>Conventional Name</entry>
1289 <entry>Decimal Code</entry>
1291 <entry>Required In</entry>
1292 <entry>Description</entry>
1297 <entry><literal>INVALID</literal></entry>
1300 <entry>not allowed</entry>
1301 <entry>Not a valid field name (error if it appears in a message)</entry>
1304 <entry><literal>PATH</literal></entry>
1306 <entry><literal>OBJECT_PATH</literal></entry>
1307 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1308 <entry>The object to send a call to,
1309 or the object a signal is emitted from.
1311 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1312 implementations should not send messages with this path,
1313 and the reference implementation of the bus daemon will
1314 disconnect any application that attempts to do so.
1318 <entry><literal>INTERFACE</literal></entry>
1320 <entry><literal>STRING</literal></entry>
1321 <entry><literal>SIGNAL</literal></entry>
1323 The interface to invoke a method call on, or
1324 that a signal is emitted from. Optional for
1325 method calls, required for signals.
1326 The special interface
1327 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1328 implementations should not send messages with this
1329 interface, and the reference implementation of the bus
1330 daemon will disconnect any application that attempts to
1335 <entry><literal>MEMBER</literal></entry>
1337 <entry><literal>STRING</literal></entry>
1338 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1339 <entry>The member, either the method name or signal name.</entry>
1342 <entry><literal>ERROR_NAME</literal></entry>
1344 <entry><literal>STRING</literal></entry>
1345 <entry><literal>ERROR</literal></entry>
1346 <entry>The name of the error that occurred, for errors</entry>
1349 <entry><literal>REPLY_SERIAL</literal></entry>
1351 <entry><literal>UINT32</literal></entry>
1352 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1353 <entry>The serial number of the message this message is a reply
1354 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1357 <entry><literal>DESTINATION</literal></entry>
1359 <entry><literal>STRING</literal></entry>
1360 <entry>optional</entry>
1361 <entry>The name of the connection this message is intended for.
1362 Only used in combination with the message bus, see
1363 <xref linkend="message-bus"/>.</entry>
1366 <entry><literal>SENDER</literal></entry>
1368 <entry><literal>STRING</literal></entry>
1369 <entry>optional</entry>
1370 <entry>Unique name of the sending connection.
1371 The message bus fills in this field so it is reliable; the field is
1372 only meaningful in combination with the message bus.</entry>
1375 <entry><literal>SIGNATURE</literal></entry>
1377 <entry><literal>SIGNATURE</literal></entry>
1378 <entry>optional</entry>
1379 <entry>The signature of the message body.
1380 If omitted, it is assumed to be the
1381 empty signature "" (i.e. the body must be 0-length).</entry>
1384 <entry><literal>UNIX_FDS</literal></entry>
1386 <entry><literal>UINT32</literal></entry>
1387 <entry>optional</entry>
1388 <entry>The number of Unix file descriptors that
1389 accompany the message. If omitted, it is assumed
1390 that no Unix file descriptors accompany the
1391 message. The actual file descriptors need to be
1392 transferred via platform specific mechanism
1393 out-of-band. They must be sent at the same time as
1394 part of the message itself. They may not be sent
1395 before the first byte of the message itself is
1396 transferred or after the last byte of the message
1406 <sect2 id="message-protocol-names">
1407 <title>Valid Names</title>
1409 The various names in D-Bus messages have some restrictions.
1412 There is a <firstterm>maximum name length</firstterm>
1413 of 255 which applies to bus names, interfaces, and members.
1415 <sect3 id="message-protocol-names-interface">
1416 <title>Interface names</title>
1418 Interfaces have names with type <literal>STRING</literal>, meaning that
1419 they must be valid UTF-8. However, there are also some
1420 additional restrictions that apply to interface names
1423 <listitem><para>Interface names are composed of 1 or more elements separated by
1424 a period ('.') character. All elements must contain at least
1428 <listitem><para>Each element must only contain the ASCII characters
1429 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1433 <listitem><para>Interface names must contain at least one '.' (period)
1434 character (and thus at least two elements).
1437 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1438 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1443 Interface names should start with the reversed DNS domain name of
1444 the author of the interface (in lower-case), like interface names
1445 in Java. It is conventional for the rest of the interface name
1446 to consist of words run together, with initial capital letters
1447 on all words ("CamelCase"). Several levels of hierarchy can be used.
1448 It is also a good idea to include the major version of the interface
1449 in the name, and increment it if incompatible changes are made;
1450 this way, a single object can implement several versions of an
1451 interface in parallel, if necessary.
1455 For instance, if the owner of <literal>example.com</literal> is
1456 developing a D-Bus API for a music player, they might define
1457 interfaces called <literal>com.example.MusicPlayer1</literal>,
1458 <literal>com.example.MusicPlayer1.Track</literal> and
1459 <literal>com.example.MusicPlayer1.Seekable</literal>.
1463 D-Bus does not distinguish between the concepts that would be
1464 called classes and interfaces in Java: either can be identified on
1465 D-Bus by an interface name.
1468 <sect3 id="message-protocol-names-bus">
1469 <title>Bus names</title>
1471 Connections have one or more bus names associated with them.
1472 A connection has exactly one bus name that is a <firstterm>unique
1473 connection name</firstterm>. The unique connection name remains
1474 with the connection for its entire lifetime.
1475 A bus name is of type <literal>STRING</literal>,
1476 meaning that it must be valid UTF-8. However, there are also
1477 some additional restrictions that apply to bus names
1480 <listitem><para>Bus names that start with a colon (':')
1481 character are unique connection names. Other bus names
1482 are called <firstterm>well-known bus names</firstterm>.
1485 <listitem><para>Bus names are composed of 1 or more elements separated by
1486 a period ('.') character. All elements must contain at least
1490 <listitem><para>Each element must only contain the ASCII characters
1491 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1492 connection name may begin with a digit, elements in
1493 other bus names must not begin with a digit.
1497 <listitem><para>Bus names must contain at least one '.' (period)
1498 character (and thus at least two elements).
1501 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1502 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1506 Note that the hyphen ('-') character is allowed in bus names but
1507 not in interface names.
1511 Like <link linkend="message-protocol-names-interface">interface
1512 names</link>, well-known bus names should start with the
1513 reversed DNS domain name of the author of the interface (in
1514 lower-case), and it is conventional for the rest of the well-known
1515 bus name to consist of words run together, with initial
1516 capital letters. As with interface names, including a version
1517 number in well-known bus names is a good idea; it's possible to
1518 have the well-known bus name for more than one version
1519 simultaneously if backwards compatibility is required.
1523 If a well-known bus name implies the presence of a "main" interface,
1524 that "main" interface is often given the same name as
1525 the well-known bus name, and situated at the corresponding object
1526 path. For instance, if the owner of <literal>example.com</literal>
1527 is developing a D-Bus API for a music player, they might define
1528 that any application that takes the well-known name
1529 <literal>com.example.MusicPlayer1</literal> should have an object
1530 at the object path <literal>/com/example/MusicPlayer1</literal>
1531 which implements the interface
1532 <literal>com.example.MusicPlayer1</literal>.
1535 <sect3 id="message-protocol-names-member">
1536 <title>Member names</title>
1538 Member (i.e. method or signal) names:
1540 <listitem><para>Must only contain the ASCII characters
1541 "[A-Z][a-z][0-9]_" and may not begin with a
1542 digit.</para></listitem>
1543 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1544 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1545 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1550 It is conventional for member names on D-Bus to consist of
1551 capitalized words with no punctuation ("camel-case").
1552 Method names should usually be verbs, such as
1553 <literal>GetItems</literal>, and signal names should usually be
1554 a description of an event, such as <literal>ItemsChanged</literal>.
1557 <sect3 id="message-protocol-names-error">
1558 <title>Error names</title>
1560 Error names have the same restrictions as interface names.
1564 Error names have the same naming conventions as interface
1565 names, and often contain <literal>.Error.</literal>; for instance,
1566 the owner of <literal>example.com</literal> might define the
1567 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1568 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1569 The errors defined by D-Bus itself, such as
1570 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1576 <sect2 id="message-protocol-types">
1577 <title>Message Types</title>
1579 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1580 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1581 This section describes these conventions.
1583 <sect3 id="message-protocol-types-method">
1584 <title>Method Calls</title>
1586 Some messages invoke an operation on a remote object. These are
1587 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1588 messages map naturally to methods on objects in a typical program.
1591 A method call message is required to have a <literal>MEMBER</literal> header field
1592 indicating the name of the method. Optionally, the message has an
1593 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1594 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1595 a method with the same name, it is undefined which of the two methods
1596 will be invoked. Implementations may also choose to return an error in
1597 this ambiguous case. However, if a method name is unique
1598 implementations must not require an interface field.
1601 Method call messages also include a <literal>PATH</literal> field
1602 indicating the object to invoke the method on. If the call is passing
1603 through a message bus, the message will also have a
1604 <literal>DESTINATION</literal> field giving the name of the connection
1605 to receive the message.
1608 When an application handles a method call message, it is required to
1609 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1610 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1611 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1614 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1615 are the return value(s) or "out parameters" of the method call.
1616 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1617 and the call fails; no return value will be provided. It makes
1618 no sense to send multiple replies to the same method call.
1621 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1622 reply is required, so the caller will know the method
1623 was successfully processed.
1626 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1630 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1631 then as an optimization the application receiving the method
1632 call may choose to omit the reply message (regardless of
1633 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1634 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1635 flag and reply anyway.
1638 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1639 destination name does not exist then a program to own the destination
1640 name will be started before the message is delivered. The message
1641 will be held until the new program is successfully started or has
1642 failed to start; in case of failure, an error will be returned. This
1643 flag is only relevant in the context of a message bus, it is ignored
1644 during one-to-one communication with no intermediate bus.
1646 <sect4 id="message-protocol-types-method-apis">
1647 <title>Mapping method calls to native APIs</title>
1649 APIs for D-Bus may map method calls to a method call in a specific
1650 programming language, such as C++, or may map a method call written
1651 in an IDL to a D-Bus message.
1654 In APIs of this nature, arguments to a method are often termed "in"
1655 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1656 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1657 "inout" arguments, which are both sent and received, i.e. the caller
1658 passes in a value which is modified. Mapped to D-Bus, an "inout"
1659 argument is equivalent to an "in" argument, followed by an "out"
1660 argument. You can't pass things "by reference" over the wire, so
1661 "inout" is purely an illusion of the in-process API.
1664 Given a method with zero or one return values, followed by zero or more
1665 arguments, where each argument may be "in", "out", or "inout", the
1666 caller constructs a message by appending each "in" or "inout" argument,
1667 in order. "out" arguments are not represented in the caller's message.
1670 The recipient constructs a reply by appending first the return value
1671 if any, then each "out" or "inout" argument, in order.
1672 "in" arguments are not represented in the reply message.
1675 Error replies are normally mapped to exceptions in languages that have
1679 In converting from native APIs to D-Bus, it is perhaps nice to
1680 map D-Bus naming conventions ("FooBar") to native conventions
1681 such as "fooBar" or "foo_bar" automatically. This is OK
1682 as long as you can say that the native API is one that
1683 was specifically written for D-Bus. It makes the most sense
1684 when writing object implementations that will be exported
1685 over the bus. Object proxies used to invoke remote D-Bus
1686 objects probably need the ability to call any D-Bus method,
1687 and thus a magic name mapping like this could be a problem.
1690 This specification doesn't require anything of native API bindings;
1691 the preceding is only a suggested convention for consistency
1697 <sect3 id="message-protocol-types-signal">
1698 <title>Signal Emission</title>
1700 Unlike method calls, signal emissions have no replies.
1701 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1702 It must have three header fields: <literal>PATH</literal> giving the object
1703 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1704 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1705 for signals, though it is optional for method calls.
1709 <sect3 id="message-protocol-types-errors">
1710 <title>Errors</title>
1712 Messages of type <literal>ERROR</literal> are most commonly replies
1713 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1714 to any kind of message. The message bus for example
1715 will return an <literal>ERROR</literal> in reply to a signal emission if
1716 the bus does not have enough memory to send the signal.
1719 An <literal>ERROR</literal> may have any arguments, but if the first
1720 argument is a <literal>STRING</literal>, it must be an error message.
1721 The error message may be logged or shown to the user
1726 <sect3 id="message-protocol-types-notation">
1727 <title>Notation in this document</title>
1729 This document uses a simple pseudo-IDL to describe particular method
1730 calls and signals. Here is an example of a method call:
1732 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1733 out UINT32 resultcode)
1735 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1736 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1737 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1738 characters so it's known that the last part of the name in
1739 the "IDL" is the member name.
1742 In C++ that might end up looking like this:
1744 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1745 unsigned int flags);
1747 or equally valid, the return value could be done as an argument:
1749 void org::freedesktop::DBus::StartServiceByName (const char *name,
1751 unsigned int *resultcode);
1753 It's really up to the API designer how they want to make
1754 this look. You could design an API where the namespace wasn't used
1755 in C++, using STL or Qt, using varargs, or whatever you wanted.
1758 Signals are written as follows:
1760 org.freedesktop.DBus.NameLost (STRING name)
1762 Signals don't specify "in" vs. "out" because only
1763 a single direction is possible.
1766 It isn't especially encouraged to use this lame pseudo-IDL in actual
1767 API implementations; you might use the native notation for the
1768 language you're using, or you might use COM or CORBA IDL, for example.
1773 <sect2 id="message-protocol-handling-invalid">
1774 <title>Invalid Protocol and Spec Extensions</title>
1777 For security reasons, the D-Bus protocol should be strictly parsed and
1778 validated, with the exception of defined extension points. Any invalid
1779 protocol or spec violations should result in immediately dropping the
1780 connection without notice to the other end. Exceptions should be
1781 carefully considered, e.g. an exception may be warranted for a
1782 well-understood idiosyncrasy of a widely-deployed implementation. In
1783 cases where the other end of a connection is 100% trusted and known to
1784 be friendly, skipping validation for performance reasons could also make
1785 sense in certain cases.
1789 Generally speaking violations of the "must" requirements in this spec
1790 should be considered possible attempts to exploit security, and violations
1791 of the "should" suggestions should be considered legitimate (though perhaps
1792 they should generate an error in some cases).
1796 The following extension points are built in to D-Bus on purpose and must
1797 not be treated as invalid protocol. The extension points are intended
1798 for use by future versions of this spec, they are not intended for third
1799 parties. At the moment, the only way a third party could extend D-Bus
1800 without breaking interoperability would be to introduce a way to negotiate new
1801 feature support as part of the auth protocol, using EXTENSION_-prefixed
1802 commands. There is not yet a standard way to negotiate features.
1806 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1807 commands result in an ERROR rather than a disconnect. This enables
1808 future extensions to the protocol. Commands starting with EXTENSION_ are
1809 reserved for third parties.
1814 The authentication protocol supports pluggable auth mechanisms.
1819 The address format (see <xref linkend="addresses"/>) supports new
1825 Messages with an unknown type (something other than
1826 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1827 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1828 Unknown-type messages must still be well-formed in the same way
1829 as the known messages, however. They still have the normal
1835 Header fields with an unknown or unexpected field code must be ignored,
1836 though again they must still be well-formed.
1841 New standard interfaces (with new methods and signals) can of course be added.
1851 <sect1 id="auth-protocol">
1852 <title>Authentication Protocol</title>
1854 Before the flow of messages begins, two applications must
1855 authenticate. A simple plain-text protocol is used for
1856 authentication; this protocol is a SASL profile, and maps fairly
1857 directly from the SASL specification. The message encoding is
1858 NOT used here, only plain text messages.
1861 In examples, "C:" and "S:" indicate lines sent by the client and
1862 server respectively.
1864 <sect2 id="auth-protocol-overview">
1865 <title>Protocol Overview</title>
1867 The protocol is a line-based protocol, where each line ends with
1868 \r\n. Each line begins with an all-caps ASCII command name containing
1869 only the character range [A-Z_], a space, then any arguments for the
1870 command, then the \r\n ending the line. The protocol is
1871 case-sensitive. All bytes must be in the ASCII character set.
1873 Commands from the client to the server are as follows:
1876 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1877 <listitem><para>CANCEL</para></listitem>
1878 <listitem><para>BEGIN</para></listitem>
1879 <listitem><para>DATA <data in hex encoding></para></listitem>
1880 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1881 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1884 From server to client are as follows:
1887 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1888 <listitem><para>OK <GUID in hex></para></listitem>
1889 <listitem><para>DATA <data in hex encoding></para></listitem>
1890 <listitem><para>ERROR</para></listitem>
1891 <listitem><para>AGREE_UNIX_FD</para></listitem>
1895 Unofficial extensions to the command set must begin with the letters
1896 "EXTENSION_", to avoid conflicts with future official commands.
1897 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1900 <sect2 id="auth-nul-byte">
1901 <title>Special credentials-passing nul byte</title>
1903 Immediately after connecting to the server, the client must send a
1904 single nul byte. This byte may be accompanied by credentials
1905 information on some operating systems that use sendmsg() with
1906 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1907 sockets. However, the nul byte must be sent even on other kinds of
1908 socket, and even on operating systems that do not require a byte to be
1909 sent in order to transmit credentials. The text protocol described in
1910 this document begins after the single nul byte. If the first byte
1911 received from the client is not a nul byte, the server may disconnect
1915 A nul byte in any context other than the initial byte is an error;
1916 the protocol is ASCII-only.
1919 The credentials sent along with the nul byte may be used with the
1920 SASL mechanism EXTERNAL.
1923 <sect2 id="auth-command-auth">
1924 <title>AUTH command</title>
1926 If an AUTH command has no arguments, it is a request to list
1927 available mechanisms. The server must respond with a REJECTED
1928 command listing the mechanisms it understands, or with an error.
1931 If an AUTH command specifies a mechanism, and the server supports
1932 said mechanism, the server should begin exchanging SASL
1933 challenge-response data with the client using DATA commands.
1936 If the server does not support the mechanism given in the AUTH
1937 command, it must send either a REJECTED command listing the mechanisms
1938 it does support, or an error.
1941 If the [initial-response] argument is provided, it is intended for use
1942 with mechanisms that have no initial challenge (or an empty initial
1943 challenge), as if it were the argument to an initial DATA command. If
1944 the selected mechanism has an initial challenge and [initial-response]
1945 was provided, the server should reject authentication by sending
1949 If authentication succeeds after exchanging DATA commands,
1950 an OK command must be sent to the client.
1953 The first octet received by the server after the \r\n of the BEGIN
1954 command from the client must be the first octet of the
1955 authenticated/encrypted stream of D-Bus messages.
1958 If BEGIN is received by the server, the first octet received
1959 by the client after the \r\n of the OK command must be the
1960 first octet of the authenticated/encrypted stream of D-Bus
1964 <sect2 id="auth-command-cancel">
1965 <title>CANCEL Command</title>
1967 At any time up to sending the BEGIN command, the client may send a
1968 CANCEL command. On receiving the CANCEL command, the server must
1969 send a REJECTED command and abort the current authentication
1973 <sect2 id="auth-command-data">
1974 <title>DATA Command</title>
1976 The DATA command may come from either client or server, and simply
1977 contains a hex-encoded block of data to be interpreted
1978 according to the SASL mechanism in use.
1981 Some SASL mechanisms support sending an "empty string";
1982 FIXME we need some way to do this.
1985 <sect2 id="auth-command-begin">
1986 <title>BEGIN Command</title>
1988 The BEGIN command acknowledges that the client has received an
1989 OK command from the server, and that the stream of messages
1993 The first octet received by the server after the \r\n of the BEGIN
1994 command from the client must be the first octet of the
1995 authenticated/encrypted stream of D-Bus messages.
1998 <sect2 id="auth-command-rejected">
1999 <title>REJECTED Command</title>
2001 The REJECTED command indicates that the current authentication
2002 exchange has failed, and further exchange of DATA is inappropriate.
2003 The client would normally try another mechanism, or try providing
2004 different responses to challenges.
2006 Optionally, the REJECTED command has a space-separated list of
2007 available auth mechanisms as arguments. If a server ever provides
2008 a list of supported mechanisms, it must provide the same list
2009 each time it sends a REJECTED message. Clients are free to
2010 ignore all lists received after the first.
2013 <sect2 id="auth-command-ok">
2014 <title>OK Command</title>
2016 The OK command indicates that the client has been
2017 authenticated. The client may now proceed with negotiating
2018 Unix file descriptor passing. To do that it shall send
2019 NEGOTIATE_UNIX_FD to the server.
2022 Otherwise, the client must respond to the OK command by
2023 sending a BEGIN command, followed by its stream of messages,
2024 or by disconnecting. The server must not accept additional
2025 commands using this protocol after the BEGIN command has been
2026 received. Further communication will be a stream of D-Bus
2027 messages (optionally encrypted, as negotiated) rather than
2031 If a client sends BEGIN the first octet received by the client
2032 after the \r\n of the OK command must be the first octet of
2033 the authenticated/encrypted stream of D-Bus messages.
2036 The OK command has one argument, which is the GUID of the server.
2037 See <xref linkend="addresses"/> for more on server GUIDs.
2040 <sect2 id="auth-command-error">
2041 <title>ERROR Command</title>
2043 The ERROR command indicates that either server or client did not
2044 know a command, does not accept the given command in the current
2045 context, or did not understand the arguments to the command. This
2046 allows the protocol to be extended; a client or server can send a
2047 command present or permitted only in new protocol versions, and if
2048 an ERROR is received instead of an appropriate response, fall back
2049 to using some other technique.
2052 If an ERROR is sent, the server or client that sent the
2053 error must continue as if the command causing the ERROR had never been
2054 received. However, the the server or client receiving the error
2055 should try something other than whatever caused the error;
2056 if only canceling/rejecting the authentication.
2059 If the D-Bus protocol changes incompatibly at some future time,
2060 applications implementing the new protocol would probably be able to
2061 check for support of the new protocol by sending a new command and
2062 receiving an ERROR from applications that don't understand it. Thus the
2063 ERROR feature of the auth protocol is an escape hatch that lets us
2064 negotiate extensions or changes to the D-Bus protocol in the future.
2067 <sect2 id="auth-command-negotiate-unix-fd">
2068 <title>NEGOTIATE_UNIX_FD Command</title>
2070 The NEGOTIATE_UNIX_FD command indicates that the client
2071 supports Unix file descriptor passing. This command may only
2072 be sent after the connection is authenticated, i.e. after OK
2073 was received by the client. This command may only be sent on
2074 transports that support Unix file descriptor passing.
2077 On receiving NEGOTIATE_UNIX_FD the server must respond with
2078 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2079 the transport chosen supports Unix file descriptor passing and
2080 the server supports this feature. It shall respond the latter
2081 if the transport does not support Unix file descriptor
2082 passing, the server does not support this feature, or the
2083 server decides not to enable file descriptor passing due to
2084 security or other reasons.
2087 <sect2 id="auth-command-agree-unix-fd">
2088 <title>AGREE_UNIX_FD Command</title>
2090 The AGREE_UNIX_FD command indicates that the server supports
2091 Unix file descriptor passing. This command may only be sent
2092 after the connection is authenticated, and the client sent
2093 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2094 command may only be sent on transports that support Unix file
2098 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2099 followed by its stream of messages, or by disconnecting. The
2100 server must not accept additional commands using this protocol
2101 after the BEGIN command has been received. Further
2102 communication will be a stream of D-Bus messages (optionally
2103 encrypted, as negotiated) rather than this protocol.
2106 <sect2 id="auth-command-future">
2107 <title>Future Extensions</title>
2109 Future extensions to the authentication and negotiation
2110 protocol are possible. For that new commands may be
2111 introduced. If a client or server receives an unknown command
2112 it shall respond with ERROR and not consider this fatal. New
2113 commands may be introduced both before, and after
2114 authentication, i.e. both before and after the OK command.
2117 <sect2 id="auth-examples">
2118 <title>Authentication examples</title>
2122 <title>Example of successful magic cookie authentication</title>
2124 (MAGIC_COOKIE is a made up mechanism)
2126 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2132 <title>Example of finding out mechanisms then picking one</title>
2135 S: REJECTED KERBEROS_V4 SKEY
2136 C: AUTH SKEY 7ab83f32ee
2137 S: DATA 8799cabb2ea93e
2138 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2144 <title>Example of client sends unknown command then falls back to regular auth</title>
2148 C: AUTH MAGIC_COOKIE 3736343435313230333039
2154 <title>Example of server doesn't support initial auth mechanism</title>
2156 C: AUTH MAGIC_COOKIE 3736343435313230333039
2157 S: REJECTED KERBEROS_V4 SKEY
2158 C: AUTH SKEY 7ab83f32ee
2159 S: DATA 8799cabb2ea93e
2160 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2166 <title>Example of wrong password or the like followed by successful retry</title>
2168 C: AUTH MAGIC_COOKIE 3736343435313230333039
2169 S: REJECTED KERBEROS_V4 SKEY
2170 C: AUTH SKEY 7ab83f32ee
2171 S: DATA 8799cabb2ea93e
2172 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2174 C: AUTH SKEY 7ab83f32ee
2175 S: DATA 8799cabb2ea93e
2176 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2182 <title>Example of skey cancelled and restarted</title>
2184 C: AUTH MAGIC_COOKIE 3736343435313230333039
2185 S: REJECTED KERBEROS_V4 SKEY
2186 C: AUTH SKEY 7ab83f32ee
2187 S: DATA 8799cabb2ea93e
2190 C: AUTH SKEY 7ab83f32ee
2191 S: DATA 8799cabb2ea93e
2192 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2198 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2200 (MAGIC_COOKIE is a made up mechanism)
2202 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2204 C: NEGOTIATE_UNIX_FD
2210 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2212 (MAGIC_COOKIE is a made up mechanism)
2214 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2216 C: NEGOTIATE_UNIX_FD
2223 <sect2 id="auth-states">
2224 <title>Authentication state diagrams</title>
2227 This section documents the auth protocol in terms of
2228 a state machine for the client and the server. This is
2229 probably the most robust way to implement the protocol.
2232 <sect3 id="auth-states-client">
2233 <title>Client states</title>
2236 To more precisely describe the interaction between the
2237 protocol state machine and the authentication mechanisms the
2238 following notation is used: MECH(CHALL) means that the
2239 server challenge CHALL was fed to the mechanism MECH, which
2245 CONTINUE(RESP) means continue the auth conversation
2246 and send RESP as the response to the server;
2252 OK(RESP) means that after sending RESP to the server
2253 the client side of the auth conversation is finished
2254 and the server should return "OK";
2260 ERROR means that CHALL was invalid and could not be
2266 Both RESP and CHALL may be empty.
2270 The Client starts by getting an initial response from the
2271 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2272 the mechanism did not provide an initial response. If the
2273 mechanism returns CONTINUE, the client starts in state
2274 <emphasis>WaitingForData</emphasis>, if the mechanism
2275 returns OK the client starts in state
2276 <emphasis>WaitingForOK</emphasis>.
2280 The client should keep track of available mechanisms and
2281 which it mechanisms it has already attempted. This list is
2282 used to decide which AUTH command to send. When the list is
2283 exhausted, the client should give up and close the
2288 <title><emphasis>WaitingForData</emphasis></title>
2296 MECH(CHALL) returns CONTINUE(RESP) → send
2298 <emphasis>WaitingForData</emphasis>
2302 MECH(CHALL) returns OK(RESP) → send DATA
2303 RESP, goto <emphasis>WaitingForOK</emphasis>
2307 MECH(CHALL) returns ERROR → send ERROR
2308 [msg], goto <emphasis>WaitingForData</emphasis>
2316 Receive REJECTED [mechs] →
2317 send AUTH [next mech], goto
2318 WaitingForData or <emphasis>WaitingForOK</emphasis>
2323 Receive ERROR → send
2325 <emphasis>WaitingForReject</emphasis>
2330 Receive OK → send
2331 BEGIN, terminate auth
2332 conversation, authenticated
2337 Receive anything else → send
2339 <emphasis>WaitingForData</emphasis>
2347 <title><emphasis>WaitingForOK</emphasis></title>
2352 Receive OK → send BEGIN, terminate auth
2353 conversation, <emphasis>authenticated</emphasis>
2358 Receive REJECT [mechs] → send AUTH [next mech],
2359 goto <emphasis>WaitingForData</emphasis> or
2360 <emphasis>WaitingForOK</emphasis>
2366 Receive DATA → send CANCEL, goto
2367 <emphasis>WaitingForReject</emphasis>
2373 Receive ERROR → send CANCEL, goto
2374 <emphasis>WaitingForReject</emphasis>
2380 Receive anything else → send ERROR, goto
2381 <emphasis>WaitingForOK</emphasis>
2389 <title><emphasis>WaitingForReject</emphasis></title>
2394 Receive REJECT [mechs] → send AUTH [next mech],
2395 goto <emphasis>WaitingForData</emphasis> or
2396 <emphasis>WaitingForOK</emphasis>
2402 Receive anything else → terminate auth
2403 conversation, disconnect
2412 <sect3 id="auth-states-server">
2413 <title>Server states</title>
2416 For the server MECH(RESP) means that the client response
2417 RESP was fed to the the mechanism MECH, which returns one of
2422 CONTINUE(CHALL) means continue the auth conversation and
2423 send CHALL as the challenge to the client;
2429 OK means that the client has been successfully
2436 REJECT means that the client failed to authenticate or
2437 there was an error in RESP.
2442 The server starts out in state
2443 <emphasis>WaitingForAuth</emphasis>. If the client is
2444 rejected too many times the server must disconnect the
2449 <title><emphasis>WaitingForAuth</emphasis></title>
2455 Receive AUTH → send REJECTED [mechs], goto
2456 <emphasis>WaitingForAuth</emphasis>
2462 Receive AUTH MECH RESP
2466 MECH not valid mechanism → send REJECTED
2468 <emphasis>WaitingForAuth</emphasis>
2472 MECH(RESP) returns CONTINUE(CHALL) → send
2474 <emphasis>WaitingForData</emphasis>
2478 MECH(RESP) returns OK → send OK, goto
2479 <emphasis>WaitingForBegin</emphasis>
2483 MECH(RESP) returns REJECT → send REJECTED
2485 <emphasis>WaitingForAuth</emphasis>
2493 Receive BEGIN → terminate
2494 auth conversation, disconnect
2500 Receive ERROR → send REJECTED [mechs], goto
2501 <emphasis>WaitingForAuth</emphasis>
2507 Receive anything else → send
2509 <emphasis>WaitingForAuth</emphasis>
2518 <title><emphasis>WaitingForData</emphasis></title>
2526 MECH(RESP) returns CONTINUE(CHALL) → send
2528 <emphasis>WaitingForData</emphasis>
2532 MECH(RESP) returns OK → send OK, goto
2533 <emphasis>WaitingForBegin</emphasis>
2537 MECH(RESP) returns REJECT → send REJECTED
2539 <emphasis>WaitingForAuth</emphasis>
2547 Receive BEGIN → terminate auth conversation,
2554 Receive CANCEL → send REJECTED [mechs], goto
2555 <emphasis>WaitingForAuth</emphasis>
2561 Receive ERROR → send REJECTED [mechs], goto
2562 <emphasis>WaitingForAuth</emphasis>
2568 Receive anything else → send ERROR, goto
2569 <emphasis>WaitingForData</emphasis>
2577 <title><emphasis>WaitingForBegin</emphasis></title>
2582 Receive BEGIN → terminate auth conversation,
2583 client authenticated
2589 Receive CANCEL → send REJECTED [mechs], goto
2590 <emphasis>WaitingForAuth</emphasis>
2596 Receive ERROR → send REJECTED [mechs], goto
2597 <emphasis>WaitingForAuth</emphasis>
2603 Receive anything else → send ERROR, goto
2604 <emphasis>WaitingForBegin</emphasis>
2614 <sect2 id="auth-mechanisms">
2615 <title>Authentication mechanisms</title>
2617 This section describes some new authentication mechanisms.
2618 D-Bus also allows any standard SASL mechanism of course.
2620 <sect3 id="auth-mechanisms-sha">
2621 <title>DBUS_COOKIE_SHA1</title>
2623 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2624 has the ability to read a private file owned by the user being
2625 authenticated. If the client can prove that it has access to a secret
2626 cookie stored in this file, then the client is authenticated.
2627 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2631 Throughout this description, "hex encoding" must output the digits
2632 from a to f in lower-case; the digits A to F must not be used
2633 in the DBUS_COOKIE_SHA1 mechanism.
2636 Authentication proceeds as follows:
2640 The client sends the username it would like to authenticate
2646 The server sends the name of its "cookie context" (see below); a
2647 space character; the integer ID of the secret cookie the client
2648 must demonstrate knowledge of; a space character; then a
2649 randomly-generated challenge string, all of this hex-encoded into
2655 The client locates the cookie and generates its own
2656 randomly-generated challenge string. The client then concatenates
2657 the server's decoded challenge, a ":" character, its own challenge,
2658 another ":" character, and the cookie. It computes the SHA-1 hash
2659 of this composite string as a hex digest. It concatenates the
2660 client's challenge string, a space character, and the SHA-1 hex
2661 digest, hex-encodes the result and sends it back to the server.
2666 The server generates the same concatenated string used by the
2667 client and computes its SHA-1 hash. It compares the hash with
2668 the hash received from the client; if the two hashes match, the
2669 client is authenticated.
2675 Each server has a "cookie context," which is a name that identifies a
2676 set of cookies that apply to that server. A sample context might be
2677 "org_freedesktop_session_bus". Context names must be valid ASCII,
2678 nonzero length, and may not contain the characters slash ("/"),
2679 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2680 tab ("\t"), or period ("."). There is a default context,
2681 "org_freedesktop_general" that's used by servers that do not specify
2685 Cookies are stored in a user's home directory, in the directory
2686 <filename>~/.dbus-keyrings/</filename>. This directory must
2687 not be readable or writable by other users. If it is,
2688 clients and servers must ignore it. The directory
2689 contains cookie files named after the cookie context.
2692 A cookie file contains one cookie per line. Each line
2693 has three space-separated fields:
2697 The cookie ID number, which must be a non-negative integer and
2698 may not be used twice in the same file.
2703 The cookie's creation time, in UNIX seconds-since-the-epoch
2709 The cookie itself, a hex-encoded random block of bytes. The cookie
2710 may be of any length, though obviously security increases
2711 as the length increases.
2717 Only server processes modify the cookie file.
2718 They must do so with this procedure:
2722 Create a lockfile name by appending ".lock" to the name of the
2723 cookie file. The server should attempt to create this file
2724 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2725 fails, the lock fails. Servers should retry for a reasonable
2726 period of time, then they may choose to delete an existing lock
2727 to keep users from having to manually delete a stale
2728 lock. <footnote><para>Lockfiles are used instead of real file
2729 locking <literal>fcntl()</literal> because real locking
2730 implementations are still flaky on network
2731 filesystems.</para></footnote>
2736 Once the lockfile has been created, the server loads the cookie
2737 file. It should then delete any cookies that are old (the
2738 timeout can be fairly short), or more than a reasonable
2739 time in the future (so that cookies never accidentally
2740 become permanent, if the clock was set far into the future
2741 at some point). If no recent keys remain, the
2742 server may generate a new key.
2747 The pruned and possibly added-to cookie file
2748 must be resaved atomically (using a temporary
2749 file which is rename()'d).
2754 The lock must be dropped by deleting the lockfile.
2760 Clients need not lock the file in order to load it,
2761 because servers are required to save the file atomically.
2766 <sect1 id="addresses">
2767 <title>Server Addresses</title>
2769 Server addresses consist of a transport name followed by a colon, and
2770 then an optional, comma-separated list of keys and values in the form key=value.
2771 Each value is escaped.
2775 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2776 Which is the address to a unix socket with the path /tmp/dbus-test.
2779 Value escaping is similar to URI escaping but simpler.
2783 The set of optionally-escaped bytes is:
2784 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2785 <emphasis>byte</emphasis> (note, not character) which is not in the
2786 set of optionally-escaped bytes must be replaced with an ASCII
2787 percent (<literal>%</literal>) and the value of the byte in hex.
2788 The hex value must always be two digits, even if the first digit is
2789 zero. The optionally-escaped bytes may be escaped if desired.
2794 To unescape, append each byte in the value; if a byte is an ASCII
2795 percent (<literal>%</literal>) character then append the following
2796 hex value instead. It is an error if a <literal>%</literal> byte
2797 does not have two hex digits following. It is an error if a
2798 non-optionally-escaped byte is seen unescaped.
2802 The set of optionally-escaped bytes is intended to preserve address
2803 readability and convenience.
2807 A server may specify a key-value pair with the key <literal>guid</literal>
2808 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2809 describes the format of the <literal>guid</literal> field. If present,
2810 this UUID may be used to distinguish one server address from another. A
2811 server should use a different UUID for each address it listens on. For
2812 example, if a message bus daemon offers both UNIX domain socket and TCP
2813 connections, but treats clients the same regardless of how they connect,
2814 those two connections are equivalent post-connection but should have
2815 distinct UUIDs to distinguish the kinds of connection.
2819 The intent of the address UUID feature is to allow a client to avoid
2820 opening multiple identical connections to the same server, by allowing the
2821 client to check whether an address corresponds to an already-existing
2822 connection. Comparing two addresses is insufficient, because addresses
2823 can be recycled by distinct servers, and equivalent addresses may look
2824 different if simply compared as strings (for example, the host in a TCP
2825 address can be given as an IP address or as a hostname).
2829 Note that the address key is <literal>guid</literal> even though the
2830 rest of the API and documentation says "UUID," for historical reasons.
2834 [FIXME clarify if attempting to connect to each is a requirement
2835 or just a suggestion]
2836 When connecting to a server, multiple server addresses can be
2837 separated by a semi-colon. The library will then try to connect
2838 to the first address and if that fails, it'll try to connect to
2839 the next one specified, and so forth. For example
2840 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2845 <sect1 id="transports">
2846 <title>Transports</title>
2848 [FIXME we need to specify in detail each transport and its possible arguments]
2850 Current transports include: unix domain sockets (including
2851 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2852 using in-process pipes. Future possible transports include one that
2853 tunnels over X11 protocol.
2856 <sect2 id="transports-unix-domain-sockets">
2857 <title>Unix Domain Sockets</title>
2859 Unix domain sockets can be either paths in the file system or on Linux
2860 kernels, they can be abstract which are similar to paths but
2861 do not show up in the file system.
2865 When a socket is opened by the D-Bus library it truncates the path
2866 name right before the first trailing Nul byte. This is true for both
2867 normal paths and abstract paths. Note that this is a departure from
2868 previous versions of D-Bus that would create sockets with a fixed
2869 length path name. Names which were shorter than the fixed length
2870 would be padded by Nul bytes.
2873 Unix domain sockets are not available on Windows.
2875 <sect3 id="transports-unix-domain-sockets-addresses">
2876 <title>Server Address Format</title>
2878 Unix domain socket addresses are identified by the "unix:" prefix
2879 and support the following key/value pairs:
2886 <entry>Values</entry>
2887 <entry>Description</entry>
2893 <entry>(path)</entry>
2894 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2897 <entry>tmpdir</entry>
2898 <entry>(path)</entry>
2899 <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>
2902 <entry>abstract</entry>
2903 <entry>(string)</entry>
2904 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2911 <sect2 id="transports-launchd">
2912 <title>launchd</title>
2914 launchd is an open-source server management system that replaces init, inetd
2915 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2916 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2920 launchd allocates a socket and provides it with the unix path through the
2921 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2922 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2923 it through its environment.
2924 Other processes can query for the launchd socket by executing:
2925 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2926 This is normally done by the D-Bus client library so doesn't have to be done
2930 launchd is not available on Microsoft Windows.
2932 <sect3 id="transports-launchd-addresses">
2933 <title>Server Address Format</title>
2935 launchd addresses are identified by the "launchd:" prefix
2936 and support the following key/value pairs:
2943 <entry>Values</entry>
2944 <entry>Description</entry>
2950 <entry>(environment variable)</entry>
2951 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2958 <sect2 id="transports-systemd">
2959 <title>systemd</title>
2961 systemd is an open-source server management system that
2962 replaces init and inetd on newer Linux systems. It supports
2963 socket activation. The D-Bus systemd transport is used to acquire
2964 socket activation file descriptors from systemd and use them
2965 as D-Bus transport when the current process is spawned by
2966 socket activation from it.
2969 The systemd transport accepts only one or more Unix domain or
2970 TCP streams sockets passed in via socket activation.
2973 The systemd transport is not available on non-Linux operating systems.
2976 The systemd transport defines no parameter keys.
2979 <sect2 id="transports-tcp-sockets">
2980 <title>TCP Sockets</title>
2982 The tcp transport provides TCP/IP based connections between clients
2983 located on the same or different hosts.
2986 Using tcp transport without any additional secure authentification mechanismus
2987 over a network is unsecure.
2990 Windows notes: Because of the tcp stack on Windows does not provide sending
2991 credentials over a tcp connection, the EXTERNAL authentification
2992 mechanismus does not work.
2994 <sect3 id="transports-tcp-sockets-addresses">
2995 <title>Server Address Format</title>
2997 TCP/IP socket addresses are identified by the "tcp:" prefix
2998 and support the following key/value pairs:
3005 <entry>Values</entry>
3006 <entry>Description</entry>
3012 <entry>(string)</entry>
3013 <entry>dns name or ip address</entry>
3017 <entry>(number)</entry>
3018 <entry>The tcp port the server will open. A zero value let the server
3019 choose a free port provided from the underlaying operating system.
3020 libdbus is able to retrieve the real used port from the server.
3024 <entry>family</entry>
3025 <entry>(string)</entry>
3026 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3033 <sect2 id="transports-nonce-tcp-sockets">
3034 <title>Nonce-secured TCP Sockets</title>
3036 The nonce-tcp transport provides a secured TCP transport, using a
3037 simple authentication mechanism to ensure that only clients with read
3038 access to a certain location in the filesystem can connect to the server.
3039 The server writes a secret, the nonce, to a file and an incoming client
3040 connection is only accepted if the client sends the nonce right after
3041 the connect. The nonce mechanism requires no setup and is orthogonal to
3042 the higher-level authentication mechanisms described in the
3043 Authentication section.
3047 On start, the server generates a random 16 byte nonce and writes it
3048 to a file in the user's temporary directory. The nonce file location
3049 is published as part of the server's D-Bus address using the
3050 "noncefile" key-value pair.
3052 After an accept, the server reads 16 bytes from the socket. If the
3053 read bytes do not match the nonce stored in the nonce file, the
3054 server MUST immediately drop the connection.
3055 If the nonce match the received byte sequence, the client is accepted
3056 and the transport behaves like an unsecured tcp transport.
3059 After a successful connect to the server socket, the client MUST read
3060 the nonce from the file published by the server via the noncefile=
3061 key-value pair and send it over the socket. After that, the
3062 transport behaves like an unsecured tcp transport.
3064 <sect3 id="transports-nonce-tcp-sockets-addresses">
3065 <title>Server Address Format</title>
3067 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3068 and support the following key/value pairs:
3075 <entry>Values</entry>
3076 <entry>Description</entry>
3082 <entry>(string)</entry>
3083 <entry>dns name or ip address</entry>
3087 <entry>(number)</entry>
3088 <entry>The tcp port the server will open. A zero value let the server
3089 choose a free port provided from the underlaying operating system.
3090 libdbus is able to retrieve the real used port from the server.
3094 <entry>family</entry>
3095 <entry>(string)</entry>
3096 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3099 <entry>noncefile</entry>
3100 <entry>(path)</entry>
3101 <entry>file location containing the secret</entry>
3108 <sect2 id="transports-exec">
3109 <title>Executed Subprocesses on Unix</title>
3111 This transport forks off a process and connects its standard
3112 input and standard output with an anonymous Unix domain
3113 socket. This socket is then used for communication by the
3114 transport. This transport may be used to use out-of-process
3115 forwarder programs as basis for the D-Bus protocol.
3118 The forked process will inherit the standard error output and
3119 process group from the parent process.
3122 Executed subprocesses are not available on Windows.
3124 <sect3 id="transports-exec-addresses">
3125 <title>Server Address Format</title>
3127 Executed subprocess addresses are identified by the "unixexec:" prefix
3128 and support the following key/value pairs:
3135 <entry>Values</entry>
3136 <entry>Description</entry>
3142 <entry>(path)</entry>
3143 <entry>Path of the binary to execute, either an absolute
3144 path or a binary name that is searched for in the default
3145 search path of the OS. This corresponds to the first
3146 argument of execlp(). This key is mandatory.</entry>
3149 <entry>argv0</entry>
3150 <entry>(string)</entry>
3151 <entry>The program name to use when executing the
3152 binary. If omitted the same value as specified for path=
3153 will be used. This corresponds to the second argument of
3157 <entry>argv1, argv2, ...</entry>
3158 <entry>(string)</entry>
3159 <entry>Arguments to pass to the binary. This corresponds
3160 to the third and later arguments of execlp(). If a
3161 specific argvX is not specified no further argvY for Y > X
3162 are taken into account.</entry>
3170 <sect1 id="meta-transports">
3171 <title>Meta Transports</title>
3173 Meta transports are a kind of transport with special enhancements or
3174 behavior. Currently available meta transports include: autolaunch
3177 <sect2 id="meta-transports-autolaunch">
3178 <title>Autolaunch</title>
3179 <para>The autolaunch transport provides a way for dbus clients to autodetect
3180 a running dbus session bus and to autolaunch a session bus if not present.
3182 <sect3 id="meta-transports-autolaunch-addresses">
3183 <title>Server Address Format</title>
3185 Autolaunch addresses uses the "autolaunch:" prefix and support the
3186 following key/value pairs:
3193 <entry>Values</entry>
3194 <entry>Description</entry>
3199 <entry>scope</entry>
3200 <entry>(string)</entry>
3201 <entry>scope of autolaunch (Windows only)
3205 "*install-path" - limit session bus to dbus installation path.
3206 The dbus installation path is determined from the location of
3207 the shared dbus library. If the library is located in a 'bin'
3208 subdirectory the installation root is the directory above,
3209 otherwise the directory where the library lives is taken as
3212 <install-root>/bin/[lib]dbus-1.dll
3213 <install-root>/[lib]dbus-1.dll
3219 "*user" - limit session bus to the recent user.
3224 other values - specify dedicated session bus like "release",
3236 <sect3 id="meta-transports-autolaunch-windows-implementation">
3237 <title>Windows implementation</title>
3239 On start, the server opens a platform specific transport, creates a mutex
3240 and a shared memory section containing the related session bus address.
3241 This mutex will be inspected by the dbus client library to detect a
3242 running dbus session bus. The access to the mutex and the shared memory
3243 section are protected by global locks.
3246 In the recent implementation the autolaunch transport uses a tcp transport
3247 on localhost with a port choosen from the operating system. This detail may
3248 change in the future.
3251 Disclaimer: The recent implementation is in an early state and may not
3252 work in all cirumstances and/or may have security issues. Because of this
3253 the implementation is not documentated yet.
3260 <title>UUIDs</title>
3262 A working D-Bus implementation uses universally-unique IDs in two places.
3263 First, each server address has a UUID identifying the address,
3264 as described in <xref linkend="addresses"/>. Second, each operating
3265 system kernel instance running a D-Bus client or server has a UUID
3266 identifying that kernel, retrieved by invoking the method
3267 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3268 linkend="standard-interfaces-peer"/>).
3271 The term "UUID" in this document is intended literally, i.e. an
3272 identifier that is universally unique. It is not intended to refer to
3273 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3276 The UUID must contain 128 bits of data and be hex-encoded. The
3277 hex-encoded string may not contain hyphens or other non-hex-digit
3278 characters, and it must be exactly 32 characters long. To generate a
3279 UUID, the current reference implementation concatenates 96 bits of random
3280 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3284 It would also be acceptable and probably better to simply generate 128
3285 bits of random data, as long as the random number generator is of high
3286 quality. The timestamp could conceivably help if the random bits are not
3287 very random. With a quality random number generator, collisions are
3288 extremely unlikely even with only 96 bits, so it's somewhat academic.
3291 Implementations should, however, stick to random data for the first 96 bits
3296 <sect1 id="standard-interfaces">
3297 <title>Standard Interfaces</title>
3299 See <xref linkend="message-protocol-types-notation"/> for details on
3300 the notation used in this section. There are some standard interfaces
3301 that may be useful across various D-Bus applications.
3303 <sect2 id="standard-interfaces-peer">
3304 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3306 The <literal>org.freedesktop.DBus.Peer</literal> interface
3309 org.freedesktop.DBus.Peer.Ping ()
3310 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3314 On receipt of the <literal>METHOD_CALL</literal> message
3315 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3316 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3317 usual. It does not matter which object path a ping is sent to. The
3318 reference implementation handles this method automatically.
3321 On receipt of the <literal>METHOD_CALL</literal> message
3322 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3323 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3324 UUID representing the identity of the machine the process is running on.
3325 This UUID must be the same for all processes on a single system at least
3326 until that system next reboots. It should be the same across reboots
3327 if possible, but this is not always possible to implement and is not
3329 It does not matter which object path a GetMachineId is sent to. The
3330 reference implementation handles this method automatically.
3333 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3334 a virtual machine running on a hypervisor, rather than a physical machine.
3335 Basically if two processes see the same UUID, they should also see the same
3336 shared memory, UNIX domain sockets, process IDs, and other features that require
3337 a running OS kernel in common between the processes.
3340 The UUID is often used where other programs might use a hostname. Hostnames
3341 can change without rebooting, however, or just be "localhost" - so the UUID
3345 <xref linkend="uuids"/> explains the format of the UUID.
3349 <sect2 id="standard-interfaces-introspectable">
3350 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3352 This interface has one method:
3354 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3358 Objects instances may implement
3359 <literal>Introspect</literal> which returns an XML description of
3360 the object, including its interfaces (with signals and methods), objects
3361 below it in the object path tree, and its properties.
3364 <xref linkend="introspection-format"/> describes the format of this XML string.
3367 <sect2 id="standard-interfaces-properties">
3368 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3370 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3371 or <firstterm>attributes</firstterm>. These can be exposed via the
3372 <literal>org.freedesktop.DBus.Properties</literal> interface.
3376 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3377 in STRING property_name,
3379 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3380 in STRING property_name,
3382 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3383 out DICT<STRING,VARIANT> props);
3387 It is conventional to give D-Bus properties names consisting of
3388 capitalized words without punctuation ("CamelCase"), like
3389 <link linkend="message-protocol-names-member">member names</link>.
3390 For instance, the GObject property
3391 <literal>connection-status</literal> or the Qt property
3392 <literal>connectionStatus</literal> could be represented on D-Bus
3393 as <literal>ConnectionStatus</literal>.
3396 Strictly speaking, D-Bus property names are not required to follow
3397 the same naming restrictions as member names, but D-Bus property
3398 names that would not be valid member names (in particular,
3399 GObject-style dash-separated property names) can cause interoperability
3400 problems and should be avoided.
3403 The available properties and whether they are writable can be determined
3404 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3405 see <xref linkend="standard-interfaces-introspectable"/>.
3408 An empty string may be provided for the interface name; in this case,
3409 if there are multiple properties on an object with the same name,
3410 the results are undefined (picking one by according to an arbitrary
3411 deterministic rule, or returning an error, are the reasonable
3415 If one or more properties change on an object, the
3416 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3417 signal may be emitted (this signal was added in 0.14):
3421 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3422 DICT<STRING,VARIANT> changed_properties,
3423 ARRAY<STRING> invalidated_properties);
3427 where <literal>changed_properties</literal> is a dictionary
3428 containing the changed properties with the new values and
3429 <literal>invalidated_properties</literal> is an array of
3430 properties that changed but the value is not conveyed.
3433 Whether the <literal>PropertiesChanged</literal> signal is
3434 supported can be determined by calling
3435 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3436 that the signal may be supported for an object but it may
3437 differ how whether and how it is used on a per-property basis
3438 (for e.g. performance or security reasons). Each property (or
3439 the parent interface) must be annotated with the
3440 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3441 annotation to convey this (usually the default value
3442 <literal>true</literal> is sufficient meaning that the
3443 annotation does not need to be used). See <xref
3444 linkend="introspection-format"/> for details on this
3449 <sect2 id="standard-interfaces-objectmanager">
3450 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3452 An API can optionally make use of this interface for one or
3453 more sub-trees of objects. The root of each sub-tree implements
3454 this interface so other applications can get all objects,
3455 interfaces and properties in a single method call. It is
3456 appropriate to use this interface if users of the tree of
3457 objects are expected to be interested in all interfaces of all
3458 objects in the tree; a more granular API should be used if
3459 users of the objects are expected to be interested in a small
3460 subset of the objects, a small subset of their interfaces, or
3464 The method that applications can use to get all objects and
3465 properties is <literal>GetManagedObjects</literal>:
3469 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3473 The return value of this method is a dict whose keys are
3474 object paths. All returned object paths are children of the
3475 object path implementing this interface, i.e. their object
3476 paths start with the ObjectManager's object path plus '/'.
3479 Each value is a dict whose keys are interfaces names. Each
3480 value in this inner dict is the same dict that would be
3481 returned by the <link
3482 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3483 method for that combination of object path and interface. If
3484 an interface has no properties, the empty dict is returned.
3487 Changes are emitted using the following two signals:
3491 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3492 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3493 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3494 ARRAY<STRING> interfaces);
3498 The <literal>InterfacesAdded</literal> signal is emitted when
3499 either a new object is added or when an existing object gains
3500 one or more interfaces. The
3501 <literal>InterfacesRemoved</literal> signal is emitted
3502 whenever an object is removed or it loses one or more
3503 interfaces. The second parameter of the
3504 <literal>InterfacesAdded</literal> signal contains a dict with
3505 the interfaces and properties (if any) that have been added to
3506 the given object path. Similarly, the second parameter of the
3507 <literal>InterfacesRemoved</literal> signal contains an array
3508 of the interfaces that were removed. Note that changes on
3509 properties on existing interfaces are not reported using this
3510 interface - an application should also monitor the existing <link
3511 linkend="standard-interfaces-properties">PropertiesChanged</link>
3512 signal on each object.
3515 Applications SHOULD NOT export objects that are children of an
3516 object (directly or otherwise) implementing this interface but
3517 which are not returned in the reply from the
3518 <literal>GetManagedObjects()</literal> method of this
3519 interface on the given object.
3522 The intent of the <literal>ObjectManager</literal> interface
3523 is to make it easy to write a robust client
3524 implementation. The trivial client implementation only needs
3525 to make two method calls:
3529 org.freedesktop.DBus.AddMatch (bus_proxy,
3530 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3531 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3535 on the message bus and the remote application's
3536 <literal>ObjectManager</literal>, respectively. Whenever a new
3537 remote object is created (or an existing object gains a new
3538 interface), the <literal>InterfacesAdded</literal> signal is
3539 emitted, and since this signal contains all properties for the
3540 interfaces, no calls to the
3541 <literal>org.freedesktop.Properties</literal> interface on the
3542 remote object are needed. Additionally, since the initial
3543 <literal>AddMatch()</literal> rule already includes signal
3544 messages from the newly created child object, no new
3545 <literal>AddMatch()</literal> call is needed.
3550 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3551 interface was added in version 0.17 of the D-Bus
3558 <sect1 id="introspection-format">
3559 <title>Introspection Data Format</title>
3561 As described in <xref linkend="standard-interfaces-introspectable"/>,
3562 objects may be introspected at runtime, returning an XML string
3563 that describes the object. The same XML format may be used in
3564 other contexts as well, for example as an "IDL" for generating
3565 static language bindings.
3568 Here is an example of introspection data:
3570 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3571 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3572 <node name="/org/freedesktop/sample_object">
3573 <interface name="org.freedesktop.SampleInterface">
3574 <method name="Frobate">
3575 <arg name="foo" type="i" direction="in"/>
3576 <arg name="bar" type="s" direction="out"/>
3577 <arg name="baz" type="a{us}" direction="out"/>
3578 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3580 <method name="Bazify">
3581 <arg name="bar" type="(iiu)" direction="in"/>
3582 <arg name="bar" type="v" direction="out"/>
3584 <method name="Mogrify">
3585 <arg name="bar" type="(iiav)" direction="in"/>
3587 <signal name="Changed">
3588 <arg name="new_value" type="b"/>
3590 <property name="Bar" type="y" access="readwrite"/>
3592 <node name="child_of_sample_object"/>
3593 <node name="another_child_of_sample_object"/>
3598 A more formal DTD and spec needs writing, but here are some quick notes.
3602 Only the root <node> element can omit the node name, as it's
3603 known to be the object that was introspected. If the root
3604 <node> does have a name attribute, it must be an absolute
3605 object path. If child <node> have object paths, they must be
3611 If a child <node> has any sub-elements, then they
3612 must represent a complete introspection of the child.
3613 If a child <node> is empty, then it may or may
3614 not have sub-elements; the child must be introspected
3615 in order to find out. The intent is that if an object
3616 knows that its children are "fast" to introspect
3617 it can go ahead and return their information, but
3618 otherwise it can omit it.
3623 The direction element on <arg> may be omitted,
3624 in which case it defaults to "in" for method calls
3625 and "out" for signals. Signals only allow "out"
3626 so while direction may be specified, it's pointless.
3631 The possible directions are "in" and "out",
3632 unlike CORBA there is no "inout"
3637 The possible property access flags are
3638 "readwrite", "read", and "write"
3643 Multiple interfaces can of course be listed for
3649 The "name" attribute on arguments is optional.
3655 Method, interface, property, and signal elements may have
3656 "annotations", which are generic key/value pairs of metadata.
3657 They are similar conceptually to Java's annotations and C# attributes.
3658 Well-known annotations:
3665 <entry>Values (separated by ,)</entry>
3666 <entry>Description</entry>
3671 <entry>org.freedesktop.DBus.Deprecated</entry>
3672 <entry>true,false</entry>
3673 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3676 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3677 <entry>(string)</entry>
3678 <entry>The C symbol; may be used for methods and interfaces</entry>
3681 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3682 <entry>true,false</entry>
3683 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3686 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3687 <entry>true,invalidates,false</entry>
3690 If set to <literal>false</literal>, the
3691 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3693 linkend="standard-interfaces-properties"/> is not
3694 guaranteed to be emitted if the property changes.
3697 If set to <literal>invalidates</literal> the signal
3698 is emitted but the value is not included in the
3702 If set to <literal>true</literal> the signal is
3703 emitted with the value included.
3706 The value for the annotation defaults to
3707 <literal>true</literal> if the enclosing interface
3708 element does not specify the annotation. Otherwise it
3709 defaults to the value specified in the enclosing
3718 <sect1 id="message-bus">
3719 <title>Message Bus Specification</title>
3720 <sect2 id="message-bus-overview">
3721 <title>Message Bus Overview</title>
3723 The message bus accepts connections from one or more applications.
3724 Once connected, applications can exchange messages with other
3725 applications that are also connected to the bus.
3728 In order to route messages among connections, the message bus keeps a
3729 mapping from names to connections. Each connection has one
3730 unique-for-the-lifetime-of-the-bus name automatically assigned.
3731 Applications may request additional names for a connection. Additional
3732 names are usually "well-known names" such as
3733 "org.freedesktop.TextEditor". When a name is bound to a connection,
3734 that connection is said to <firstterm>own</firstterm> the name.
3737 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3738 This name routes messages to the bus, allowing applications to make
3739 administrative requests. For example, applications can ask the bus
3740 to assign a name to a connection.
3743 Each name may have <firstterm>queued owners</firstterm>. When an
3744 application requests a name for a connection and the name is already in
3745 use, the bus will optionally add the connection to a queue waiting for
3746 the name. If the current owner of the name disconnects or releases
3747 the name, the next connection in the queue will become the new owner.
3751 This feature causes the right thing to happen if you start two text
3752 editors for example; the first one may request "org.freedesktop.TextEditor",
3753 and the second will be queued as a possible owner of that name. When
3754 the first exits, the second will take over.
3758 Applications may send <firstterm>unicast messages</firstterm> to
3759 a specific recipient or to the message bus itself, or
3760 <firstterm>broadcast messages</firstterm> to all interested recipients.
3761 See <xref linkend="message-bus-routing"/> for details.
3765 <sect2 id="message-bus-names">
3766 <title>Message Bus Names</title>
3768 Each connection has at least one name, assigned at connection time and
3769 returned in response to the
3770 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3771 automatically-assigned name is called the connection's <firstterm>unique
3772 name</firstterm>. Unique names are never reused for two different
3773 connections to the same bus.
3776 Ownership of a unique name is a prerequisite for interaction with
3777 the message bus. It logically follows that the unique name is always
3778 the first name that an application comes to own, and the last
3779 one that it loses ownership of.
3782 Unique connection names must begin with the character ':' (ASCII colon
3783 character); bus names that are not unique names must not begin
3784 with this character. (The bus must reject any attempt by an application
3785 to manually request a name beginning with ':'.) This restriction
3786 categorically prevents "spoofing"; messages sent to a unique name
3787 will always go to the expected connection.
3790 When a connection is closed, all the names that it owns are deleted (or
3791 transferred to the next connection in the queue if any).
3794 A connection can request additional names to be associated with it using
3795 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3796 linkend="message-protocol-names-bus"/> describes the format of a valid
3797 name. These names can be released again using the
3798 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3801 <sect3 id="bus-messages-request-name">
3802 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3806 UINT32 RequestName (in STRING name, in UINT32 flags)
3813 <entry>Argument</entry>
3815 <entry>Description</entry>
3821 <entry>STRING</entry>
3822 <entry>Name to request</entry>
3826 <entry>UINT32</entry>
3827 <entry>Flags</entry>
3837 <entry>Argument</entry>
3839 <entry>Description</entry>
3845 <entry>UINT32</entry>
3846 <entry>Return value</entry>
3853 This method call should be sent to
3854 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3855 assign the given name to the method caller. Each name maintains a
3856 queue of possible owners, where the head of the queue is the primary
3857 or current owner of the name. Each potential owner in the queue
3858 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3859 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3860 call. When RequestName is invoked the following occurs:
3864 If the method caller is currently the primary owner of the name,
3865 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3866 values are updated with the values from the new RequestName call,
3867 and nothing further happens.
3873 If the current primary owner (head of the queue) has
3874 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3875 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3876 the caller of RequestName replaces the current primary owner at
3877 the head of the queue and the current primary owner moves to the
3878 second position in the queue. If the caller of RequestName was
3879 in the queue previously its flags are updated with the values from
3880 the new RequestName in addition to moving it to the head of the queue.
3886 If replacement is not possible, and the method caller is
3887 currently in the queue but not the primary owner, its flags are
3888 updated with the values from the new RequestName call.
3894 If replacement is not possible, and the method caller is
3895 currently not in the queue, the method caller is appended to the
3902 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3903 set and is not the primary owner, it is removed from the
3904 queue. This can apply to the previous primary owner (if it
3905 was replaced) or the method caller (if it updated the
3906 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3907 queue, or if it was just added to the queue with that flag set).
3913 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3914 queue," even if another application already in the queue had specified
3915 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3916 that does not allow replacement goes away, and the next primary owner
3917 does allow replacement. In this case, queued items that specified
3918 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3919 automatically replace the new primary owner. In other words,
3920 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3921 time RequestName is called. This is deliberate to avoid an infinite loop
3922 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3923 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3926 The flags argument contains any of the following values logically ORed
3933 <entry>Conventional Name</entry>
3934 <entry>Value</entry>
3935 <entry>Description</entry>
3940 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3944 If an application A specifies this flag and succeeds in
3945 becoming the owner of the name, and another application B
3946 later calls RequestName with the
3947 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3948 will lose ownership and receive a
3949 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3950 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3951 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3952 is not specified by application B, then application B will not replace
3953 application A as the owner.
3958 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3962 Try to replace the current owner if there is one. If this
3963 flag is not set the application will only become the owner of
3964 the name if there is no current owner. If this flag is set,
3965 the application will replace the current owner if
3966 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3971 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3975 Without this flag, if an application requests a name that is
3976 already owned, the application will be placed in a queue to
3977 own the name when the current owner gives it up. If this
3978 flag is given, the application will not be placed in the
3979 queue, the request for the name will simply fail. This flag
3980 also affects behavior when an application is replaced as
3981 name owner; by default the application moves back into the
3982 waiting queue, unless this flag was provided when the application
3983 became the name owner.
3991 The return code can be one of the following values:
3997 <entry>Conventional Name</entry>
3998 <entry>Value</entry>
3999 <entry>Description</entry>
4004 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4005 <entry>1</entry> <entry>The caller is now the primary owner of
4006 the name, replacing any previous owner. Either the name had no
4007 owner before, or the caller specified
4008 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4009 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4012 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4015 <entry>The name already had an owner,
4016 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4017 the current owner did not specify
4018 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4019 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4023 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4024 <entry>The name already has an owner,
4025 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4026 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4027 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4028 specified by the requesting application.</entry>
4031 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4033 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4041 <sect3 id="bus-messages-release-name">
4042 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4046 UINT32 ReleaseName (in STRING name)
4053 <entry>Argument</entry>
4055 <entry>Description</entry>
4061 <entry>STRING</entry>
4062 <entry>Name to release</entry>
4072 <entry>Argument</entry>
4074 <entry>Description</entry>
4080 <entry>UINT32</entry>
4081 <entry>Return value</entry>
4088 This method call should be sent to
4089 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4090 release the method caller's claim to the given name. If the caller is
4091 the primary owner, a new primary owner will be selected from the
4092 queue if any other owners are waiting. If the caller is waiting in
4093 the queue for the name, the caller will removed from the queue and
4094 will not be made an owner of the name if it later becomes available.
4095 If there are no other owners in the queue for the name, it will be
4096 removed from the bus entirely.
4098 The return code can be one of the following values:
4104 <entry>Conventional Name</entry>
4105 <entry>Value</entry>
4106 <entry>Description</entry>
4111 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4112 <entry>1</entry> <entry>The caller has released his claim on
4113 the given name. Either the caller was the primary owner of
4114 the name, and the name is now unused or taken by somebody
4115 waiting in the queue for the name, or the caller was waiting
4116 in the queue for the name and has now been removed from the
4120 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4122 <entry>The given name does not exist on this bus.</entry>
4125 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4127 <entry>The caller was not the primary owner of this name,
4128 and was also not waiting in the queue to own this name.</entry>
4136 <sect3 id="bus-messages-list-queued-owners">
4137 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4141 ARRAY of STRING ListQueuedOwners (in STRING name)
4148 <entry>Argument</entry>
4150 <entry>Description</entry>
4156 <entry>STRING</entry>
4157 <entry>The well-known bus name to query, such as
4158 <literal>com.example.cappuccino</literal></entry>
4168 <entry>Argument</entry>
4170 <entry>Description</entry>
4176 <entry>ARRAY of STRING</entry>
4177 <entry>The unique bus names of connections currently queued
4178 for the name</entry>
4185 This method call should be sent to
4186 <literal>org.freedesktop.DBus</literal> and lists the connections
4187 currently queued for a bus name (see
4188 <xref linkend="term-queued-owner"/>).
4193 <sect2 id="message-bus-routing">
4194 <title>Message Bus Message Routing</title>
4197 Messages may have a <literal>DESTINATION</literal> field (see <xref
4198 linkend="message-protocol-header-fields"/>), resulting in a
4199 <firstterm>unicast message</firstterm>. If the
4200 <literal>DESTINATION</literal> field is present, it specifies a message
4201 recipient by name. Method calls and replies normally specify this field.
4202 The message bus must send messages (of any type) with the
4203 <literal>DESTINATION</literal> field set to the specified recipient,
4204 regardless of whether the recipient has set up a match rule matching
4209 When the message bus receives a signal, if the
4210 <literal>DESTINATION</literal> field is absent, it is considered to
4211 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4212 applications with <firstterm>message matching rules</firstterm> that
4213 match the message. Most signal messages are broadcasts.
4217 Unicast signal messages (those with a <literal>DESTINATION</literal>
4218 field) are not commonly used, but they are treated like any unicast
4219 message: they are delivered to the specified receipient,
4220 regardless of its match rules. One use for unicast signals is to
4221 avoid a race condition in which a signal is emitted before the intended
4222 recipient can call <xref linkend="bus-messages-add-match"/> to
4223 receive that signal: if the signal is sent directly to that recipient
4224 using a unicast message, it does not need to add a match rule at all,
4225 and there is no race condition. Another use for unicast signals,
4226 on message buses whose security policy prevents eavesdropping, is to
4227 send sensitive information which should only be visible to one
4232 When the message bus receives a method call, if the
4233 <literal>DESTINATION</literal> field is absent, the call is taken to be
4234 a standard one-to-one message and interpreted by the message bus
4235 itself. For example, sending an
4236 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4237 <literal>DESTINATION</literal> will cause the message bus itself to
4238 reply to the ping immediately; the message bus will not make this
4239 message visible to other applications.
4243 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4244 the ping message were sent with a <literal>DESTINATION</literal> name of
4245 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4246 forwarded, and the Yoyodyne Corporation screensaver application would be
4247 expected to reply to the ping.
4251 Message bus implementations may impose a security policy which
4252 prevents certain messages from being sent or received.
4253 When a message cannot be sent or received due to a security
4254 policy, the message bus should send an error reply, unless the
4255 original message had the <literal>NO_REPLY</literal> flag.
4258 <sect3 id="message-bus-routing-eavesdropping">
4259 <title>Eavesdropping</title>
4261 Receiving a unicast message whose <literal>DESTINATION</literal>
4262 indicates a different recipient is called
4263 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4264 a security boundary (like the standard system bus), the security
4265 policy should usually prevent eavesdropping, since unicast messages
4266 are normally kept private and may contain security-sensitive
4271 Eavesdropping is mainly useful for debugging tools, such as
4272 the <literal>dbus-monitor</literal> tool in the reference
4273 implementation of D-Bus. Tools which eavesdrop on the message bus
4274 should be careful to avoid sending a reply or error in response to
4275 messages intended for a different client.
4279 Clients may attempt to eavesdrop by adding match rules
4280 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4281 the <literal>eavesdrop='true'</literal> match. If the message bus'
4282 security policy does not allow eavesdropping, the match rule can
4283 still be added, but will not have any practical effect. For
4284 compatibility with older message bus implementations, if adding such
4285 a match rule results in an error reply, the client may fall back to
4286 adding the same rule with the <literal>eavesdrop</literal> match
4291 <sect3 id="message-bus-routing-match-rules">
4292 <title>Match Rules</title>
4294 An important part of the message bus routing protocol is match
4295 rules. Match rules describe the messages that should be sent to a
4296 client, based on the contents of the message. Broadcast signals
4297 are only sent to clients which have a suitable match rule: this
4298 avoids waking up client processes to deal with signals that are
4299 not relevant to that client.
4302 Messages that list a client as their <literal>DESTINATION</literal>
4303 do not need to match the client's match rules, and are sent to that
4304 client regardless. As a result, match rules are mainly used to
4305 receive a subset of broadcast signals.
4308 Match rules can also be used for eavesdropping
4309 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4310 if the security policy of the message bus allows it.
4313 Match rules are added using the AddMatch bus method
4314 (see <xref linkend="bus-messages-add-match"/>). Rules are
4315 specified as a string of comma separated key/value pairs.
4316 Excluding a key from the rule indicates a wildcard match.
4317 For instance excluding the the member from a match rule but
4318 adding a sender would let all messages from that sender through.
4319 An example of a complete rule would be
4320 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4323 The following table describes the keys that can be used to create
4325 The following table summarizes the D-Bus types.
4331 <entry>Possible Values</entry>
4332 <entry>Description</entry>
4337 <entry><literal>type</literal></entry>
4338 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4339 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4342 <entry><literal>sender</literal></entry>
4343 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4344 and <xref linkend="term-unique-name"/> respectively)
4346 <entry>Match messages sent by a particular sender. An example of a sender match
4347 is sender='org.freedesktop.Hal'</entry>
4350 <entry><literal>interface</literal></entry>
4351 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4352 <entry>Match messages sent over or to a particular interface. An example of an
4353 interface match is interface='org.freedesktop.Hal.Manager'.
4354 If a message omits the interface header, it must not match any rule
4355 that specifies this key.</entry>
4358 <entry><literal>member</literal></entry>
4359 <entry>Any valid method or signal name</entry>
4360 <entry>Matches messages which have the give method or signal name. An example of
4361 a member match is member='NameOwnerChanged'</entry>
4364 <entry><literal>path</literal></entry>
4365 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4366 <entry>Matches messages which are sent from or to the given object. An example of a
4367 path match is path='/org/freedesktop/Hal/Manager'</entry>
4370 <entry><literal>path_namespace</literal></entry>
4371 <entry>An object path</entry>
4374 Matches messages which are sent from or to an
4375 object for which the object path is either the
4376 given value, or that value followed by one or
4377 more path components.
4382 <literal>path_namespace='/com/example/foo'</literal>
4383 would match signals sent by
4384 <literal>/com/example/foo</literal>
4386 <literal>/com/example/foo/bar</literal>,
4388 <literal>/com/example/foobar</literal>.
4392 Using both <literal>path</literal> and
4393 <literal>path_namespace</literal> in the same match
4394 rule is not allowed.
4399 This match key was added in version 0.16 of the
4400 D-Bus specification and implemented by the bus
4401 daemon in dbus 1.5.0 and later.
4407 <entry><literal>destination</literal></entry>
4408 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4409 <entry>Matches messages which are being sent to the given unique name. An
4410 example of a destination match is destination=':1.0'</entry>
4413 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4414 <entry>Any string</entry>
4415 <entry>Arg matches are special and are used for further restricting the
4416 match based on the arguments in the body of a message. Only arguments of type
4417 STRING can be matched in this way. An example of an argument match
4418 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4422 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4423 <entry>Any string</entry>
4425 <para>Argument path matches provide a specialised form of wildcard matching for
4426 path-like namespaces. They can match arguments whose type is either STRING or
4427 OBJECT_PATH. As with normal argument matches,
4428 if the argument is exactly equal to the string given in the match
4429 rule then the rule is satisfied. Additionally, there is also a
4430 match when either the string given in the match rule or the
4431 appropriate message argument ends with '/' and is a prefix of the
4432 other. An example argument path match is arg0path='/aa/bb/'. This
4433 would match messages with first arguments of '/', '/aa/',
4434 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4435 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4437 <para>This is intended for monitoring “directories” in file system-like
4438 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4439 system. An application interested in all nodes in a particular hierarchy would
4440 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4441 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4442 represent a modification to the “bar” property, or a signal with zeroth
4443 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4444 many properties within that directory, and the interested application would be
4445 notified in both cases.</para>
4448 This match key was added in version 0.12 of the
4449 D-Bus specification, implemented for STRING
4450 arguments by the bus daemon in dbus 1.2.0 and later,
4451 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4458 <entry><literal>arg0namespace</literal></entry>
4459 <entry>Like a bus name, except that the string is not
4460 required to contain a '.' (period)</entry>
4462 <para>Match messages whose first argument is of type STRING, and is a bus name
4463 or interface name within the specified namespace. This is primarily intended
4464 for watching name owner changes for a group of related bus names, rather than
4465 for a single name or all name changes.</para>
4467 <para>Because every valid interface name is also a valid
4468 bus name, this can also be used for messages whose
4469 first argument is an interface name.</para>
4471 <para>For example, the match rule
4472 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4473 matches name owner changes for bus names such as
4474 <literal>com.example.backend.foo</literal>,
4475 <literal>com.example.backend.foo.bar</literal>, and
4476 <literal>com.example.backend</literal> itself.</para>
4478 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4481 This match key was added in version 0.16 of the
4482 D-Bus specification and implemented by the bus
4483 daemon in dbus 1.5.0 and later.
4489 <entry><literal>eavesdrop</literal></entry>
4490 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4491 <entry>Since D-Bus 1.5.6, match rules do not
4492 match messages which have a <literal>DESTINATION</literal>
4493 field unless the match rule specifically
4495 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4496 by specifying <literal>eavesdrop='true'</literal>
4497 in the match rule. <literal>eavesdrop='false'</literal>
4498 restores the default behaviour. Messages are
4499 delivered to their <literal>DESTINATION</literal>
4500 regardless of match rules, so this match does not
4501 affect normal delivery of unicast messages.
4502 If the message bus has a security policy which forbids
4503 eavesdropping, this match may still be used without error,
4504 but will not have any practical effect.
4505 In older versions of D-Bus, this match was not allowed
4506 in match rules, and all match rules behaved as if
4507 <literal>eavesdrop='true'</literal> had been used.
4516 <sect2 id="message-bus-starting-services">
4517 <title>Message Bus Starting Services</title>
4519 The message bus can start applications on behalf of other applications.
4520 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4521 An application that can be started in this way is called a
4522 <firstterm>service</firstterm>.
4525 With D-Bus, starting a service is normally done by name. That is,
4526 applications ask the message bus to start some program that will own a
4527 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4528 This implies a contract documented along with the name
4529 <literal>org.freedesktop.TextEditor</literal> for which objects
4530 the owner of that name will provide, and what interfaces those
4534 To find an executable corresponding to a particular name, the bus daemon
4535 looks for <firstterm>service description files</firstterm>. Service
4536 description files define a mapping from names to executables. Different
4537 kinds of message bus will look for these files in different places, see
4538 <xref linkend="message-bus-types"/>.
4541 Service description files have the ".service" file
4542 extension. The message bus will only load service description files
4543 ending with .service; all other files will be ignored. The file format
4544 is similar to that of <ulink
4545 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4546 entries</ulink>. All service description files must be in UTF-8
4547 encoding. To ensure that there will be no name collisions, service files
4548 must be namespaced using the same mechanism as messages and service
4553 [FIXME the file format should be much better specified than "similar to
4554 .desktop entries" esp. since desktop entries are already
4555 badly-specified. ;-)]
4556 These sections from the specification apply to service files as well:
4559 <listitem><para>General syntax</para></listitem>
4560 <listitem><para>Comment format</para></listitem>
4564 <title>Example service description file</title>
4566 # Sample service description file
4568 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4569 Exec=/usr/libexec/gconfd-2
4574 When an application asks to start a service by name, the bus daemon tries to
4575 find a service that will own that name. It then tries to spawn the
4576 executable associated with it. If this fails, it will report an
4577 error. [FIXME what happens if two .service files offer the same service;
4578 what kind of error is reported, should we have a way for the client to
4582 The executable launched will have the environment variable
4583 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4584 message bus so it can connect and request the appropriate names.
4587 The executable being launched may want to know whether the message bus
4588 starting it is one of the well-known message buses (see <xref
4589 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4590 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4591 of the well-known buses. The currently-defined values for this variable
4592 are <literal>system</literal> for the systemwide message bus,
4593 and <literal>session</literal> for the per-login-session message
4594 bus. The new executable must still connect to the address given
4595 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4596 resulting connection is to the well-known bus.
4599 [FIXME there should be a timeout somewhere, either specified
4600 in the .service file, by the client, or just a global value
4601 and if the client being activated fails to connect within that
4602 timeout, an error should be sent back.]
4605 <sect3 id="message-bus-starting-services-scope">
4606 <title>Message Bus Service Scope</title>
4608 The "scope" of a service is its "per-", such as per-session,
4609 per-machine, per-home-directory, or per-display. The reference
4610 implementation doesn't yet support starting services in a different
4611 scope from the message bus itself. So e.g. if you start a service
4612 on the session bus its scope is per-session.
4615 We could add an optional scope to a bus name. For example, for
4616 per-(display,session pair), we could have a unique ID for each display
4617 generated automatically at login and set on screen 0 by executing a
4618 special "set display ID" binary. The ID would be stored in a
4619 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4620 random bytes. This ID would then be used to scope names.
4621 Starting/locating a service could be done by ID-name pair rather than
4625 Contrast this with a per-display scope. To achieve that, we would
4626 want a single bus spanning all sessions using a given display.
4627 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4628 property on screen 0 of the display, pointing to this bus.
4633 <sect2 id="message-bus-types">
4634 <title>Well-known Message Bus Instances</title>
4636 Two standard message bus instances are defined here, along with how
4637 to locate them and where their service files live.
4639 <sect3 id="message-bus-types-login">
4640 <title>Login session message bus</title>
4642 Each time a user logs in, a <firstterm>login session message
4643 bus</firstterm> may be started. All applications in the user's login
4644 session may interact with one another using this message bus.
4647 The address of the login session message bus is given
4648 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4649 variable. If that variable is not set, applications may
4650 also try to read the address from the X Window System root
4651 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4652 The root window property must have type <literal>STRING</literal>.
4653 The environment variable should have precedence over the
4654 root window property.
4656 <para>The address of the login session message bus is given in the
4657 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4658 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4659 "autolaunch:", the system should use platform-specific methods of
4660 locating a running D-Bus session server, or starting one if a running
4661 instance cannot be found. Note that this mechanism is not recommended
4662 for attempting to determine if a daemon is running. It is inherently
4663 racy to attempt to make this determination, since the bus daemon may
4664 be started just before or just after the determination is made.
4665 Therefore, it is recommended that applications do not try to make this
4666 determination for their functionality purposes, and instead they
4667 should attempt to start the server.</para>
4669 <sect4 id="message-bus-types-login-x-windows">
4670 <title>X Windowing System</title>
4672 For the X Windowing System, the application must locate the
4673 window owner of the selection represented by the atom formed by
4677 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4681 <para>the current user's username</para>
4685 <para>the literal character '_' (underscore)</para>
4689 <para>the machine's ID</para>
4695 The following properties are defined for the window that owns
4697 <informaltable frame="all">
4706 <para>meaning</para>
4712 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4716 <para>the actual address of the server socket</para>
4722 <para>_DBUS_SESSION_BUS_PID</para>
4726 <para>the PID of the server process</para>
4735 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4736 present in this window.
4740 If the X selection cannot be located or if reading the
4741 properties from the window fails, the implementation MUST conclude
4742 that there is no D-Bus server running and proceed to start a new
4743 server. (See below on concurrency issues)
4747 Failure to connect to the D-Bus server address thus obtained
4748 MUST be treated as a fatal connection error and should be reported
4753 As an alternative, an implementation MAY find the information
4754 in the following file located in the current user's home directory,
4755 in subdirectory .dbus/session-bus/:
4758 <para>the machine's ID</para>
4762 <para>the literal character '-' (dash)</para>
4766 <para>the X display without the screen number, with the
4767 following prefixes removed, if present: ":", "localhost:"
4768 ."localhost.localdomain:". That is, a display of
4769 "localhost:10.0" produces just the number "10"</para>
4775 The contents of this file NAME=value assignment pairs and
4776 lines starting with # are comments (no comments are allowed
4777 otherwise). The following variable names are defined:
4784 <para>Variable</para>
4788 <para>meaning</para>
4794 <para>DBUS_SESSION_BUS_ADDRESS</para>
4798 <para>the actual address of the server socket</para>
4804 <para>DBUS_SESSION_BUS_PID</para>
4808 <para>the PID of the server process</para>
4814 <para>DBUS_SESSION_BUS_WINDOWID</para>
4818 <para>the window ID</para>
4827 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4832 Failure to open this file MUST be interpreted as absence of a
4833 running server. Therefore, the implementation MUST proceed to
4834 attempting to launch a new bus server if the file cannot be
4839 However, success in opening this file MUST NOT lead to the
4840 conclusion that the server is running. Thus, a failure to connect to
4841 the bus address obtained by the alternative method MUST NOT be
4842 considered a fatal error. If the connection cannot be established,
4843 the implementation MUST proceed to check the X selection settings or
4844 to start the server on its own.
4848 If the implementation concludes that the D-Bus server is not
4849 running it MUST attempt to start a new server and it MUST also
4850 ensure that the daemon started as an effect of the "autolaunch"
4851 mechanism provides the lookup mechanisms described above, so
4852 subsequent calls can locate the newly started server. The
4853 implementation MUST also ensure that if two or more concurrent
4854 initiations happen, only one server remains running and all other
4855 initiations are able to obtain the address of this server and
4856 connect to it. In other words, the implementation MUST ensure that
4857 the X selection is not present when it attempts to set it, without
4858 allowing another process to set the selection between the
4859 verification and the setting (e.g., by using XGrabServer /
4866 On Unix systems, the session bus should search for .service files
4867 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4869 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4870 Implementations may also search additional locations, which
4871 should be searched with lower priority than anything in
4872 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4873 for example, the reference implementation also
4874 looks in <literal>${datadir}/dbus-1/services</literal> as
4875 set at compile time.
4878 As described in the XDG Base Directory Specification, software
4879 packages should install their session .service files to their
4880 configured <literal>${datadir}/dbus-1/services</literal>,
4881 where <literal>${datadir}</literal> is as defined by the GNU
4882 coding standards. System administrators or users can arrange
4883 for these service files to be read by setting XDG_DATA_DIRS or by
4884 symlinking them into the default locations.
4888 <sect3 id="message-bus-types-system">
4889 <title>System message bus</title>
4891 A computer may have a <firstterm>system message bus</firstterm>,
4892 accessible to all applications on the system. This message bus may be
4893 used to broadcast system events, such as adding new hardware devices,
4894 changes in the printer queue, and so forth.
4897 The address of the system message bus is given
4898 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4899 variable. If that variable is not set, applications should try
4900 to connect to the well-known address
4901 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4904 The D-Bus reference implementation actually honors the
4905 <literal>$(localstatedir)</literal> configure option
4906 for this address, on both client and server side.
4911 On Unix systems, the system bus should default to searching
4912 for .service files in
4913 <literal>/usr/local/share/dbus-1/system-services</literal>,
4914 <literal>/usr/share/dbus-1/system-services</literal> and
4915 <literal>/lib/dbus-1/system-services</literal>, with that order
4916 of precedence. It may also search other implementation-specific
4917 locations, but should not vary these locations based on environment
4921 The system bus is security-sensitive and is typically executed
4922 by an init system with a clean environment. Its launch helper
4923 process is particularly security-sensitive, and specifically
4924 clears its own environment.
4929 Software packages should install their system .service
4930 files to their configured
4931 <literal>${datadir}/dbus-1/system-services</literal>,
4932 where <literal>${datadir}</literal> is as defined by the GNU
4933 coding standards. System administrators can arrange
4934 for these service files to be read by editing the system bus'
4935 configuration file or by symlinking them into the default
4941 <sect2 id="message-bus-messages">
4942 <title>Message Bus Messages</title>
4944 The special message bus name <literal>org.freedesktop.DBus</literal>
4945 responds to a number of additional messages.
4948 <sect3 id="bus-messages-hello">
4949 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4960 <entry>Argument</entry>
4962 <entry>Description</entry>
4968 <entry>STRING</entry>
4969 <entry>Unique name assigned to the connection</entry>
4976 Before an application is able to send messages to other applications
4977 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4978 to the message bus to obtain a unique name. If an application without
4979 a unique name tries to send a message to another application, or a
4980 message to the message bus itself that isn't the
4981 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4982 disconnected from the bus.
4985 There is no corresponding "disconnect" request; if a client wishes to
4986 disconnect from the bus, it simply closes the socket (or other
4987 communication channel).
4990 <sect3 id="bus-messages-list-names">
4991 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4995 ARRAY of STRING ListNames ()
5002 <entry>Argument</entry>
5004 <entry>Description</entry>
5010 <entry>ARRAY of STRING</entry>
5011 <entry>Array of strings where each string is a bus name</entry>
5018 Returns a list of all currently-owned names on the bus.
5021 <sect3 id="bus-messages-list-activatable-names">
5022 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5026 ARRAY of STRING ListActivatableNames ()
5033 <entry>Argument</entry>
5035 <entry>Description</entry>
5041 <entry>ARRAY of STRING</entry>
5042 <entry>Array of strings where each string is a bus name</entry>
5049 Returns a list of all names that can be activated on the bus.
5052 <sect3 id="bus-messages-name-exists">
5053 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5057 BOOLEAN NameHasOwner (in STRING name)
5064 <entry>Argument</entry>
5066 <entry>Description</entry>
5072 <entry>STRING</entry>
5073 <entry>Name to check</entry>
5083 <entry>Argument</entry>
5085 <entry>Description</entry>
5091 <entry>BOOLEAN</entry>
5092 <entry>Return value, true if the name exists</entry>
5099 Checks if the specified name exists (currently has an owner).
5103 <sect3 id="bus-messages-name-owner-changed">
5104 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5108 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5115 <entry>Argument</entry>
5117 <entry>Description</entry>
5123 <entry>STRING</entry>
5124 <entry>Name with a new owner</entry>
5128 <entry>STRING</entry>
5129 <entry>Old owner or empty string if none</entry>
5133 <entry>STRING</entry>
5134 <entry>New owner or empty string if none</entry>
5141 This signal indicates that the owner of a name has changed.
5142 It's also the signal to use to detect the appearance of
5143 new names on the bus.
5146 <sect3 id="bus-messages-name-lost">
5147 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5151 NameLost (STRING name)
5158 <entry>Argument</entry>
5160 <entry>Description</entry>
5166 <entry>STRING</entry>
5167 <entry>Name which was lost</entry>
5174 This signal is sent to a specific application when it loses
5175 ownership of a name.
5179 <sect3 id="bus-messages-name-acquired">
5180 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5184 NameAcquired (STRING name)
5191 <entry>Argument</entry>
5193 <entry>Description</entry>
5199 <entry>STRING</entry>
5200 <entry>Name which was acquired</entry>
5207 This signal is sent to a specific application when it gains
5208 ownership of a name.
5212 <sect3 id="bus-messages-start-service-by-name">
5213 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5217 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5224 <entry>Argument</entry>
5226 <entry>Description</entry>
5232 <entry>STRING</entry>
5233 <entry>Name of the service to start</entry>
5237 <entry>UINT32</entry>
5238 <entry>Flags (currently not used)</entry>
5248 <entry>Argument</entry>
5250 <entry>Description</entry>
5256 <entry>UINT32</entry>
5257 <entry>Return value</entry>
5262 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5266 The return value can be one of the following values:
5271 <entry>Identifier</entry>
5272 <entry>Value</entry>
5273 <entry>Description</entry>
5278 <entry>DBUS_START_REPLY_SUCCESS</entry>
5280 <entry>The service was successfully started.</entry>
5283 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5285 <entry>A connection already owns the given name.</entry>
5294 <sect3 id="bus-messages-update-activation-environment">
5295 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5299 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5306 <entry>Argument</entry>
5308 <entry>Description</entry>
5314 <entry>ARRAY of DICT<STRING,STRING></entry>
5315 <entry>Environment to add or update</entry>
5320 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5323 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5326 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.
5331 <sect3 id="bus-messages-get-name-owner">
5332 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5336 STRING GetNameOwner (in STRING name)
5343 <entry>Argument</entry>
5345 <entry>Description</entry>
5351 <entry>STRING</entry>
5352 <entry>Name to get the owner of</entry>
5362 <entry>Argument</entry>
5364 <entry>Description</entry>
5370 <entry>STRING</entry>
5371 <entry>Return value, a unique connection name</entry>
5376 Returns the unique connection name of the primary owner of the name
5377 given. If the requested name doesn't have an owner, returns a
5378 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5382 <sect3 id="bus-messages-get-connection-unix-user">
5383 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5387 UINT32 GetConnectionUnixUser (in STRING bus_name)
5394 <entry>Argument</entry>
5396 <entry>Description</entry>
5402 <entry>STRING</entry>
5403 <entry>Unique or well-known bus name of the connection to
5404 query, such as <literal>:12.34</literal> or
5405 <literal>com.example.tea</literal></entry>
5415 <entry>Argument</entry>
5417 <entry>Description</entry>
5423 <entry>UINT32</entry>
5424 <entry>Unix user ID</entry>
5429 Returns the Unix user ID of the process connected to the server. If
5430 unable to determine it (for instance, because the process is not on the
5431 same machine as the bus daemon), an error is returned.
5435 <sect3 id="bus-messages-get-connection-unix-process-id">
5436 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5440 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5447 <entry>Argument</entry>
5449 <entry>Description</entry>
5455 <entry>STRING</entry>
5456 <entry>Unique or well-known bus name of the connection to
5457 query, such as <literal>:12.34</literal> or
5458 <literal>com.example.tea</literal></entry>
5468 <entry>Argument</entry>
5470 <entry>Description</entry>
5476 <entry>UINT32</entry>
5477 <entry>Unix process id</entry>
5482 Returns the Unix process ID of the process connected to the server. If
5483 unable to determine it (for instance, because the process is not on the
5484 same machine as the bus daemon), an error is returned.
5488 <sect3 id="bus-messages-add-match">
5489 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5493 AddMatch (in STRING rule)
5500 <entry>Argument</entry>
5502 <entry>Description</entry>
5508 <entry>STRING</entry>
5509 <entry>Match rule to add to the connection</entry>
5514 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5515 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5519 <sect3 id="bus-messages-remove-match">
5520 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5524 RemoveMatch (in STRING rule)
5531 <entry>Argument</entry>
5533 <entry>Description</entry>
5539 <entry>STRING</entry>
5540 <entry>Match rule to remove from the connection</entry>
5545 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5546 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5551 <sect3 id="bus-messages-get-id">
5552 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5556 GetId (out STRING id)
5563 <entry>Argument</entry>
5565 <entry>Description</entry>
5571 <entry>STRING</entry>
5572 <entry>Unique ID identifying the bus daemon</entry>
5577 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5578 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5579 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5580 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5581 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5582 by org.freedesktop.DBus.Peer.GetMachineId().
5583 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5591 <appendix id="implementation-notes">
5592 <title>Implementation notes</title>
5593 <sect1 id="implementation-notes-subsection">
5601 <glossary><title>Glossary</title>
5603 This glossary defines some of the terms used in this specification.
5606 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5609 The message bus maintains an association between names and
5610 connections. (Normally, there's one connection per application.) A
5611 bus name is simply an identifier used to locate connections. For
5612 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5613 name might be used to send a message to a screensaver from Yoyodyne
5614 Corporation. An application is said to <firstterm>own</firstterm> a
5615 name if the message bus has associated the application's connection
5616 with the name. Names may also have <firstterm>queued
5617 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5618 The bus assigns a unique name to each connection,
5619 see <xref linkend="term-unique-name"/>. Other names
5620 can be thought of as "well-known names" and are
5621 used to find applications that offer specific functionality.
5625 See <xref linkend="message-protocol-names-bus"/> for details of
5626 the syntax and naming conventions for bus names.
5631 <glossentry id="term-message"><glossterm>Message</glossterm>
5634 A message is the atomic unit of communication via the D-Bus
5635 protocol. It consists of a <firstterm>header</firstterm> and a
5636 <firstterm>body</firstterm>; the body is made up of
5637 <firstterm>arguments</firstterm>.
5642 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5645 The message bus is a special application that forwards
5646 or routes messages between a group of applications
5647 connected to the message bus. It also manages
5648 <firstterm>names</firstterm> used for routing
5654 <glossentry id="term-name"><glossterm>Name</glossterm>
5657 See <xref linkend="term-bus-name"/>. "Name" may
5658 also be used to refer to some of the other names
5659 in D-Bus, such as interface names.
5664 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5667 Used to prevent collisions when defining new interfaces, bus names
5668 etc. The convention used is the same one Java uses for defining
5669 classes: a reversed domain name.
5670 See <xref linkend="message-protocol-names-bus"/>,
5671 <xref linkend="message-protocol-names-interface"/>,
5672 <xref linkend="message-protocol-names-error"/>,
5673 <xref linkend="message-protocol-marshaling-object-path"/>.
5678 <glossentry id="term-object"><glossterm>Object</glossterm>
5681 Each application contains <firstterm>objects</firstterm>, which have
5682 <firstterm>interfaces</firstterm> and
5683 <firstterm>methods</firstterm>. Objects are referred to by a name,
5684 called a <firstterm>path</firstterm>.
5689 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5692 An application talking directly to another application, without going
5693 through a message bus. One-to-one connections may be "peer to peer" or
5694 "client to server." The D-Bus protocol has no concept of client
5695 vs. server after a connection has authenticated; the flow of messages
5696 is symmetrical (full duplex).
5701 <glossentry id="term-path"><glossterm>Path</glossterm>
5704 Object references (object names) in D-Bus are organized into a
5705 filesystem-style hierarchy, so each object is named by a path. As in
5706 LDAP, there's no difference between "files" and "directories"; a path
5707 can refer to an object, while still having child objects below it.
5712 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5715 Each bus name has a primary owner; messages sent to the name go to the
5716 primary owner. However, certain names also maintain a queue of
5717 secondary owners "waiting in the wings." If the primary owner releases
5718 the name, then the first secondary owner in the queue automatically
5719 becomes the new owner of the name.
5724 <glossentry id="term-service"><glossterm>Service</glossterm>
5727 A service is an executable that can be launched by the bus daemon.
5728 Services normally guarantee some particular features, for example they
5729 may guarantee that they will request a specific name such as
5730 "org.freedesktop.Screensaver", have a singleton object
5731 "/org/freedesktop/Application", and that object will implement the
5732 interface "org.freedesktop.ScreensaverControl".
5737 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5740 ".service files" tell the bus about service applications that can be
5741 launched (see <xref linkend="term-service"/>). Most importantly they
5742 provide a mapping from bus names to services that will request those
5743 names when they start up.
5748 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5751 The special name automatically assigned to each connection by the
5752 message bus. This name will never change owner, and will be unique
5753 (never reused during the lifetime of the message bus).
5754 It will begin with a ':' character.