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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 Unlike a message signature, the variant signature can
690 contain only a single complete type. So "i", "ai"
691 or "(ii)" is OK, but "ii" is not. Use of variants may not
692 cause a total message depth to be larger than 64, including
693 other container types such as structures.
697 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
698 than parentheses it uses curly braces, and it has more restrictions.
699 The restrictions are: it occurs only as an array element type; it has
700 exactly two single complete types inside the curly braces; the first
701 single complete type (the "key") must be a basic type rather than a
702 container type. Implementations must not accept dict entries outside of
703 arrays, must not accept dict entries with zero, one, or more than two
704 fields, and must not accept dict entries with non-basic-typed keys. A
705 dict entry is always a key-value pair.
709 The first field in the <literal>DICT_ENTRY</literal> is always the key.
710 A message is considered corrupt if the same key occurs twice in the same
711 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
712 implementations are not required to reject dicts with duplicate keys.
716 In most languages, an array of dict entry would be represented as a
717 map, hash table, or dict object.
722 <title>Summary of types</title>
725 The following table summarizes the D-Bus types.
730 <entry>Conventional Name</entry>
732 <entry>Description</entry>
737 <entry><literal>INVALID</literal></entry>
738 <entry>0 (ASCII NUL)</entry>
739 <entry>Not a valid type code, used to terminate signatures</entry>
741 <entry><literal>BYTE</literal></entry>
742 <entry>121 (ASCII 'y')</entry>
743 <entry>8-bit unsigned integer</entry>
745 <entry><literal>BOOLEAN</literal></entry>
746 <entry>98 (ASCII 'b')</entry>
747 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
749 <entry><literal>INT16</literal></entry>
750 <entry>110 (ASCII 'n')</entry>
751 <entry>16-bit signed integer</entry>
753 <entry><literal>UINT16</literal></entry>
754 <entry>113 (ASCII 'q')</entry>
755 <entry>16-bit unsigned integer</entry>
757 <entry><literal>INT32</literal></entry>
758 <entry>105 (ASCII 'i')</entry>
759 <entry>32-bit signed integer</entry>
761 <entry><literal>UINT32</literal></entry>
762 <entry>117 (ASCII 'u')</entry>
763 <entry>32-bit unsigned integer</entry>
765 <entry><literal>INT64</literal></entry>
766 <entry>120 (ASCII 'x')</entry>
767 <entry>64-bit signed integer</entry>
769 <entry><literal>UINT64</literal></entry>
770 <entry>116 (ASCII 't')</entry>
771 <entry>64-bit unsigned integer</entry>
773 <entry><literal>DOUBLE</literal></entry>
774 <entry>100 (ASCII 'd')</entry>
775 <entry>IEEE 754 double</entry>
777 <entry><literal>STRING</literal></entry>
778 <entry>115 (ASCII 's')</entry>
779 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
781 <entry><literal>OBJECT_PATH</literal></entry>
782 <entry>111 (ASCII 'o')</entry>
783 <entry>Name of an object instance</entry>
785 <entry><literal>SIGNATURE</literal></entry>
786 <entry>103 (ASCII 'g')</entry>
787 <entry>A type signature</entry>
789 <entry><literal>ARRAY</literal></entry>
790 <entry>97 (ASCII 'a')</entry>
793 <entry><literal>STRUCT</literal></entry>
794 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
795 <entry>Struct; type code 114 'r' is reserved for use in
796 bindings and implementations to represent the general
797 concept of a struct, and must not appear in signatures
798 used on D-Bus.</entry>
800 <entry><literal>VARIANT</literal></entry>
801 <entry>118 (ASCII 'v') </entry>
802 <entry>Variant type (the type of the value is part of the value itself)</entry>
804 <entry><literal>DICT_ENTRY</literal></entry>
805 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
806 <entry>Entry in a dict or map (array of key-value pairs).
807 Type code 101 'e' is reserved for use in bindings and
808 implementations to represent the general concept of a
809 dict or dict-entry, and must not appear in signatures
810 used on D-Bus.</entry>
812 <entry><literal>UNIX_FD</literal></entry>
813 <entry>104 (ASCII 'h')</entry>
814 <entry>Unix file descriptor</entry>
817 <entry>(reserved)</entry>
818 <entry>109 (ASCII 'm')</entry>
819 <entry>Reserved for <ulink
820 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
821 'maybe' type compatible with the one in GVariant</ulink>,
822 and must not appear in signatures used on D-Bus until
823 specified here</entry>
826 <entry>(reserved)</entry>
827 <entry>42 (ASCII '*')</entry>
828 <entry>Reserved for use in bindings/implementations to
829 represent any <firstterm>single complete type</firstterm>,
830 and must not appear in signatures used on D-Bus.</entry>
833 <entry>(reserved)</entry>
834 <entry>63 (ASCII '?')</entry>
835 <entry>Reserved for use in bindings/implementations to
836 represent any <firstterm>basic type</firstterm>, and must
837 not appear in signatures used on D-Bus.</entry>
840 <entry>(reserved)</entry>
841 <entry>64 (ASCII '@'), 38 (ASCII '&'),
842 94 (ASCII '^')</entry>
843 <entry>Reserved for internal use by bindings/implementations,
844 and must not appear in signatures used on D-Bus.
845 GVariant uses these type-codes to encode calling
856 <sect1 id="message-protocol-marshaling">
857 <title>Marshaling (Wire Format)</title>
860 D-Bus defines a marshalling format for its type system, which is
861 used in D-Bus messages. This is not the only possible marshalling
862 format for the type system: for instance, GVariant (part of GLib)
863 re-uses the D-Bus type system but implements an alternative marshalling
868 <title>Byte order and alignment</title>
871 Given a type signature, a block of bytes can be converted into typed
872 values. This section describes the format of the block of bytes. Byte
873 order and alignment issues are handled uniformly for all D-Bus types.
877 A block of bytes has an associated byte order. The byte order
878 has to be discovered in some way; for D-Bus messages, the
879 byte order is part of the message header as described in
880 <xref linkend="message-protocol-messages"/>. For now, assume
881 that the byte order is known to be either little endian or big
886 Each value in a block of bytes is aligned "naturally," for example
887 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
888 8-byte boundary. To properly align a value, <firstterm>alignment
889 padding</firstterm> may be necessary. The alignment padding must always
890 be the minimum required padding to properly align the following value;
891 and it must always be made up of nul bytes. The alignment padding must
892 not be left uninitialized (it can't contain garbage), and more padding
893 than required must not be used.
897 As an exception to natural alignment, <literal>STRUCT</literal> and
898 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
899 boundary, regardless of the alignments of their contents.
904 <title>Marshalling basic types</title>
907 To marshal and unmarshal fixed types, you simply read one value
908 from the data block corresponding to each type code in the signature.
909 All signed integer values are encoded in two's complement, DOUBLE
910 values are IEEE 754 double-precision floating-point, and BOOLEAN
911 values are encoded in 32 bits (of which only the least significant
916 The string-like types are all marshalled as a
917 fixed-length unsigned integer <varname>n</varname> giving the
918 length of the variable part, followed by <varname>n</varname>
919 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
920 which is not considered to be part of the text. The alignment
921 of the string-like type is the same as the alignment of
922 <varname>n</varname>.
926 For the STRING and OBJECT_PATH types, <varname>n</varname> is
927 encoded in 4 bytes, leading to 4-byte alignment.
928 For the SIGNATURE type, <varname>n</varname> is encoded as a single
929 byte. As a result, alignment padding is never required before a
935 <title>Marshalling containers</title>
938 Arrays are marshalled as a <literal>UINT32</literal>
939 <varname>n</varname> giving the length of the array data in bytes,
940 followed by alignment padding to the alignment boundary of the array
941 element type, followed by the <varname>n</varname> bytes of the
942 array elements marshalled in sequence. <varname>n</varname> does not
943 include the padding after the length, or any padding after the
948 For instance, if the current position in the message is a multiple
949 of 8 bytes and the byte-order is big-endian, an array containing only
950 the 64-bit integer 5 would be marshalled as:
953 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
954 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
955 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
960 Arrays have a maximum length defined to be 2 to the 26th power or
961 67108864. Implementations must not send or accept arrays exceeding this
966 Structs and dict entries are marshalled in the same way as their
967 contents, but their alignment is always to an 8-byte boundary,
968 even if their contents would normally be less strictly aligned.
972 Variants are marshalled as the <literal>SIGNATURE</literal> of
973 the contents (which must be a single complete type), followed by a
974 marshalled value with the type given by that signature. The
975 variant has the same 1-byte alignment as the signature, which means
976 that alignment padding before a variant is never needed.
977 Use of variants may not cause a total message depth to be larger
978 than 64, including other container types such as structures.
983 <title>Summary of D-Bus marshalling</title>
986 Given all this, the types are marshaled on the wire as follows:
991 <entry>Conventional Name</entry>
992 <entry>Encoding</entry>
993 <entry>Alignment</entry>
998 <entry><literal>INVALID</literal></entry>
999 <entry>Not applicable; cannot be marshaled.</entry>
1002 <entry><literal>BYTE</literal></entry>
1003 <entry>A single 8-bit byte.</entry>
1006 <entry><literal>BOOLEAN</literal></entry>
1007 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1010 <entry><literal>INT16</literal></entry>
1011 <entry>16-bit signed integer in the message's byte order.</entry>
1014 <entry><literal>UINT16</literal></entry>
1015 <entry>16-bit unsigned integer in the message's byte order.</entry>
1018 <entry><literal>INT32</literal></entry>
1019 <entry>32-bit signed integer in the message's byte order.</entry>
1022 <entry><literal>UINT32</literal></entry>
1023 <entry>32-bit unsigned integer in the message's byte order.</entry>
1026 <entry><literal>INT64</literal></entry>
1027 <entry>64-bit signed integer in the message's byte order.</entry>
1030 <entry><literal>UINT64</literal></entry>
1031 <entry>64-bit unsigned integer in the message's byte order.</entry>
1034 <entry><literal>DOUBLE</literal></entry>
1035 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1038 <entry><literal>STRING</literal></entry>
1039 <entry>A <literal>UINT32</literal> indicating the string's
1040 length in bytes excluding its terminating nul, followed by
1041 non-nul string data of the given length, followed by a terminating nul
1048 <entry><literal>OBJECT_PATH</literal></entry>
1049 <entry>Exactly the same as <literal>STRING</literal> except the
1050 content must be a valid object path (see above).
1056 <entry><literal>SIGNATURE</literal></entry>
1057 <entry>The same as <literal>STRING</literal> except the length is a single
1058 byte (thus signatures have a maximum length of 255)
1059 and the content must be a valid signature (see above).
1065 <entry><literal>ARRAY</literal></entry>
1067 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1068 alignment padding to the alignment boundary of the array element type,
1069 followed by each array element.
1075 <entry><literal>STRUCT</literal></entry>
1077 A struct must start on an 8-byte boundary regardless of the
1078 type of the struct fields. The struct value consists of each
1079 field marshaled in sequence starting from that 8-byte
1086 <entry><literal>VARIANT</literal></entry>
1088 The marshaled <literal>SIGNATURE</literal> of a single
1089 complete type, followed by a marshaled value with the type
1090 given in the signature.
1093 1 (alignment of the signature)
1096 <entry><literal>DICT_ENTRY</literal></entry>
1098 Identical to STRUCT.
1104 <entry><literal>UNIX_FD</literal></entry>
1105 <entry>32-bit unsigned integer in the message's byte
1106 order. The actual file descriptors need to be
1107 transferred out-of-band via some platform specific
1108 mechanism. On the wire, values of this type store the index to the
1109 file descriptor in the array of file descriptors that
1110 accompany the message.</entry>
1122 <sect1 id="message-protocol">
1123 <title>Message Protocol</title>
1126 A <firstterm>message</firstterm> consists of a
1127 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1128 think of a message as a package, the header is the address, and the body
1129 contains the package contents. The message delivery system uses the header
1130 information to figure out where to send the message and how to interpret
1131 it; the recipient interprets the body of the message.
1135 The body of the message is made up of zero or more
1136 <firstterm>arguments</firstterm>, which are typed values, such as an
1137 integer or a byte array.
1141 Both header and body use the D-Bus <link linkend="type-system">type
1142 system</link> and format for serializing data.
1145 <sect2 id="message-protocol-messages">
1146 <title>Message Format</title>
1149 A message consists of a header and a body. The header is a block of
1150 values with a fixed signature and meaning. The body is a separate block
1151 of values, with a signature specified in the header.
1155 The length of the header must be a multiple of 8, allowing the body to
1156 begin on an 8-byte boundary when storing the entire message in a single
1157 buffer. If the header does not naturally end on an 8-byte boundary
1158 up to 7 bytes of nul-initialized alignment padding must be added.
1162 The message body need not end on an 8-byte boundary.
1166 The maximum length of a message, including header, header alignment padding,
1167 and body is 2 to the 27th power or 134217728. Implementations must not
1168 send or accept messages exceeding this size.
1172 The signature of the header is:
1176 Written out more readably, this is:
1178 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1183 These values have the following meanings:
1188 <entry>Value</entry>
1189 <entry>Description</entry>
1194 <entry>1st <literal>BYTE</literal></entry>
1195 <entry>Endianness flag; ASCII 'l' for little-endian
1196 or ASCII 'B' for big-endian. Both header and body are
1197 in this endianness.</entry>
1200 <entry>2nd <literal>BYTE</literal></entry>
1201 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1202 Currently-defined types are described below.
1206 <entry>3rd <literal>BYTE</literal></entry>
1207 <entry>Bitwise OR of flags. Unknown flags
1208 must be ignored. Currently-defined flags are described below.
1212 <entry>4th <literal>BYTE</literal></entry>
1213 <entry>Major protocol version of the sending application. If
1214 the major protocol version of the receiving application does not
1215 match, the applications will not be able to communicate and the
1216 D-Bus connection must be disconnected. The major protocol
1217 version for this version of the specification is 1.
1221 <entry>1st <literal>UINT32</literal></entry>
1222 <entry>Length in bytes of the message body, starting
1223 from the end of the header. The header ends after
1224 its alignment padding to an 8-boundary.
1228 <entry>2nd <literal>UINT32</literal></entry>
1229 <entry>The serial of this message, used as a cookie
1230 by the sender to identify the reply corresponding
1231 to this request. This must not be zero.
1235 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1236 <entry>An array of zero or more <firstterm>header
1237 fields</firstterm> where the byte is the field code, and the
1238 variant is the field value. The message type determines
1239 which fields are required.
1247 <firstterm>Message types</firstterm> that can appear in the second byte
1253 <entry>Conventional name</entry>
1254 <entry>Decimal value</entry>
1255 <entry>Description</entry>
1260 <entry><literal>INVALID</literal></entry>
1262 <entry>This is an invalid type.</entry>
1265 <entry><literal>METHOD_CALL</literal></entry>
1267 <entry>Method call.</entry>
1270 <entry><literal>METHOD_RETURN</literal></entry>
1272 <entry>Method reply with returned data.</entry>
1275 <entry><literal>ERROR</literal></entry>
1277 <entry>Error reply. If the first argument exists and is a
1278 string, it is an error message.</entry>
1281 <entry><literal>SIGNAL</literal></entry>
1283 <entry>Signal emission.</entry>
1290 Flags that can appear in the third byte of the header:
1295 <entry>Conventional name</entry>
1296 <entry>Hex value</entry>
1297 <entry>Description</entry>
1302 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1304 <entry>This message does not expect method return replies or
1305 error replies; the reply can be omitted as an
1306 optimization. However, it is compliant with this specification
1307 to return the reply despite this flag and the only harm
1308 from doing so is extra network traffic.
1312 <entry><literal>NO_AUTO_START</literal></entry>
1314 <entry>The bus must not launch an owner
1315 for the destination name in response to this message.
1323 <sect3 id="message-protocol-header-fields">
1324 <title>Header Fields</title>
1327 The array at the end of the header contains <firstterm>header
1328 fields</firstterm>, where each field is a 1-byte field code followed
1329 by a field value. A header must contain the required header fields for
1330 its message type, and zero or more of any optional header
1331 fields. Future versions of this protocol specification may add new
1332 fields. Implementations must ignore fields they do not
1333 understand. Implementations must not invent their own header fields;
1334 only changes to this specification may introduce new header fields.
1338 Again, if an implementation sees a header field code that it does not
1339 expect, it must ignore that field, as it will be part of a new
1340 (but compatible) version of this specification. This also applies
1341 to known header fields appearing in unexpected messages, for
1342 example: if a signal has a reply serial it must be ignored
1343 even though it has no meaning as of this version of the spec.
1347 However, implementations must not send or accept known header fields
1348 with the wrong type stored in the field value. So for example a
1349 message with an <literal>INTERFACE</literal> field of type
1350 <literal>UINT32</literal> would be considered corrupt.
1354 Here are the currently-defined header fields:
1359 <entry>Conventional Name</entry>
1360 <entry>Decimal Code</entry>
1362 <entry>Required In</entry>
1363 <entry>Description</entry>
1368 <entry><literal>INVALID</literal></entry>
1371 <entry>not allowed</entry>
1372 <entry>Not a valid field name (error if it appears in a message)</entry>
1375 <entry><literal>PATH</literal></entry>
1377 <entry><literal>OBJECT_PATH</literal></entry>
1378 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1379 <entry>The object to send a call to,
1380 or the object a signal is emitted from.
1382 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1383 implementations should not send messages with this path,
1384 and the reference implementation of the bus daemon will
1385 disconnect any application that attempts to do so.
1389 <entry><literal>INTERFACE</literal></entry>
1391 <entry><literal>STRING</literal></entry>
1392 <entry><literal>SIGNAL</literal></entry>
1394 The interface to invoke a method call on, or
1395 that a signal is emitted from. Optional for
1396 method calls, required for signals.
1397 The special interface
1398 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1399 implementations should not send messages with this
1400 interface, and the reference implementation of the bus
1401 daemon will disconnect any application that attempts to
1406 <entry><literal>MEMBER</literal></entry>
1408 <entry><literal>STRING</literal></entry>
1409 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1410 <entry>The member, either the method name or signal name.</entry>
1413 <entry><literal>ERROR_NAME</literal></entry>
1415 <entry><literal>STRING</literal></entry>
1416 <entry><literal>ERROR</literal></entry>
1417 <entry>The name of the error that occurred, for errors</entry>
1420 <entry><literal>REPLY_SERIAL</literal></entry>
1422 <entry><literal>UINT32</literal></entry>
1423 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1424 <entry>The serial number of the message this message is a reply
1425 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1428 <entry><literal>DESTINATION</literal></entry>
1430 <entry><literal>STRING</literal></entry>
1431 <entry>optional</entry>
1432 <entry>The name of the connection this message is intended for.
1433 Only used in combination with the message bus, see
1434 <xref linkend="message-bus"/>.</entry>
1437 <entry><literal>SENDER</literal></entry>
1439 <entry><literal>STRING</literal></entry>
1440 <entry>optional</entry>
1441 <entry>Unique name of the sending connection.
1442 The message bus fills in this field so it is reliable; the field is
1443 only meaningful in combination with the message bus.</entry>
1446 <entry><literal>SIGNATURE</literal></entry>
1448 <entry><literal>SIGNATURE</literal></entry>
1449 <entry>optional</entry>
1450 <entry>The signature of the message body.
1451 If omitted, it is assumed to be the
1452 empty signature "" (i.e. the body must be 0-length).</entry>
1455 <entry><literal>UNIX_FDS</literal></entry>
1457 <entry><literal>UINT32</literal></entry>
1458 <entry>optional</entry>
1459 <entry>The number of Unix file descriptors that
1460 accompany the message. If omitted, it is assumed
1461 that no Unix file descriptors accompany the
1462 message. The actual file descriptors need to be
1463 transferred via platform specific mechanism
1464 out-of-band. They must be sent at the same time as
1465 part of the message itself. They may not be sent
1466 before the first byte of the message itself is
1467 transferred or after the last byte of the message
1477 <sect2 id="message-protocol-names">
1478 <title>Valid Names</title>
1480 The various names in D-Bus messages have some restrictions.
1483 There is a <firstterm>maximum name length</firstterm>
1484 of 255 which applies to bus names, interfaces, and members.
1486 <sect3 id="message-protocol-names-interface">
1487 <title>Interface names</title>
1489 Interfaces have names with type <literal>STRING</literal>, meaning that
1490 they must be valid UTF-8. However, there are also some
1491 additional restrictions that apply to interface names
1494 <listitem><para>Interface names are composed of 1 or more elements separated by
1495 a period ('.') character. All elements must contain at least
1499 <listitem><para>Each element must only contain the ASCII characters
1500 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1504 <listitem><para>Interface names must contain at least one '.' (period)
1505 character (and thus at least two elements).
1508 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1509 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1514 Interface names should start with the reversed DNS domain name of
1515 the author of the interface (in lower-case), like interface names
1516 in Java. It is conventional for the rest of the interface name
1517 to consist of words run together, with initial capital letters
1518 on all words ("CamelCase"). Several levels of hierarchy can be used.
1519 It is also a good idea to include the major version of the interface
1520 in the name, and increment it if incompatible changes are made;
1521 this way, a single object can implement several versions of an
1522 interface in parallel, if necessary.
1526 For instance, if the owner of <literal>example.com</literal> is
1527 developing a D-Bus API for a music player, they might define
1528 interfaces called <literal>com.example.MusicPlayer1</literal>,
1529 <literal>com.example.MusicPlayer1.Track</literal> and
1530 <literal>com.example.MusicPlayer1.Seekable</literal>.
1534 D-Bus does not distinguish between the concepts that would be
1535 called classes and interfaces in Java: either can be identified on
1536 D-Bus by an interface name.
1539 <sect3 id="message-protocol-names-bus">
1540 <title>Bus names</title>
1542 Connections have one or more bus names associated with them.
1543 A connection has exactly one bus name that is a <firstterm>unique
1544 connection name</firstterm>. The unique connection name remains
1545 with the connection for its entire lifetime.
1546 A bus name is of type <literal>STRING</literal>,
1547 meaning that it must be valid UTF-8. However, there are also
1548 some additional restrictions that apply to bus names
1551 <listitem><para>Bus names that start with a colon (':')
1552 character are unique connection names. Other bus names
1553 are called <firstterm>well-known bus names</firstterm>.
1556 <listitem><para>Bus names are composed of 1 or more elements separated by
1557 a period ('.') character. All elements must contain at least
1561 <listitem><para>Each element must only contain the ASCII characters
1562 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1563 connection name may begin with a digit, elements in
1564 other bus names must not begin with a digit.
1568 <listitem><para>Bus names must contain at least one '.' (period)
1569 character (and thus at least two elements).
1572 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1573 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1577 Note that the hyphen ('-') character is allowed in bus names but
1578 not in interface names.
1582 Like <link linkend="message-protocol-names-interface">interface
1583 names</link>, well-known bus names should start with the
1584 reversed DNS domain name of the author of the interface (in
1585 lower-case), and it is conventional for the rest of the well-known
1586 bus name to consist of words run together, with initial
1587 capital letters. As with interface names, including a version
1588 number in well-known bus names is a good idea; it's possible to
1589 have the well-known bus name for more than one version
1590 simultaneously if backwards compatibility is required.
1594 If a well-known bus name implies the presence of a "main" interface,
1595 that "main" interface is often given the same name as
1596 the well-known bus name, and situated at the corresponding object
1597 path. For instance, if the owner of <literal>example.com</literal>
1598 is developing a D-Bus API for a music player, they might define
1599 that any application that takes the well-known name
1600 <literal>com.example.MusicPlayer1</literal> should have an object
1601 at the object path <literal>/com/example/MusicPlayer1</literal>
1602 which implements the interface
1603 <literal>com.example.MusicPlayer1</literal>.
1606 <sect3 id="message-protocol-names-member">
1607 <title>Member names</title>
1609 Member (i.e. method or signal) names:
1611 <listitem><para>Must only contain the ASCII characters
1612 "[A-Z][a-z][0-9]_" and may not begin with a
1613 digit.</para></listitem>
1614 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1615 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1616 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1621 It is conventional for member names on D-Bus to consist of
1622 capitalized words with no punctuation ("camel-case").
1623 Method names should usually be verbs, such as
1624 <literal>GetItems</literal>, and signal names should usually be
1625 a description of an event, such as <literal>ItemsChanged</literal>.
1628 <sect3 id="message-protocol-names-error">
1629 <title>Error names</title>
1631 Error names have the same restrictions as interface names.
1635 Error names have the same naming conventions as interface
1636 names, and often contain <literal>.Error.</literal>; for instance,
1637 the owner of <literal>example.com</literal> might define the
1638 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1639 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1640 The errors defined by D-Bus itself, such as
1641 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1647 <sect2 id="message-protocol-types">
1648 <title>Message Types</title>
1650 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1651 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1652 This section describes these conventions.
1654 <sect3 id="message-protocol-types-method">
1655 <title>Method Calls</title>
1657 Some messages invoke an operation on a remote object. These are
1658 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1659 messages map naturally to methods on objects in a typical program.
1662 A method call message is required to have a <literal>MEMBER</literal> header field
1663 indicating the name of the method. Optionally, the message has an
1664 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1665 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1666 a method with the same name, it is undefined which of the two methods
1667 will be invoked. Implementations may also choose to return an error in
1668 this ambiguous case. However, if a method name is unique
1669 implementations must not require an interface field.
1672 Method call messages also include a <literal>PATH</literal> field
1673 indicating the object to invoke the method on. If the call is passing
1674 through a message bus, the message will also have a
1675 <literal>DESTINATION</literal> field giving the name of the connection
1676 to receive the message.
1679 When an application handles a method call message, it is required to
1680 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1681 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1682 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1685 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1686 are the return value(s) or "out parameters" of the method call.
1687 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1688 and the call fails; no return value will be provided. It makes
1689 no sense to send multiple replies to the same method call.
1692 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1693 reply is required, so the caller will know the method
1694 was successfully processed.
1697 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1701 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1702 then as an optimization the application receiving the method
1703 call may choose to omit the reply message (regardless of
1704 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1705 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1706 flag and reply anyway.
1709 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1710 destination name does not exist then a program to own the destination
1711 name will be started before the message is delivered. The message
1712 will be held until the new program is successfully started or has
1713 failed to start; in case of failure, an error will be returned. This
1714 flag is only relevant in the context of a message bus, it is ignored
1715 during one-to-one communication with no intermediate bus.
1717 <sect4 id="message-protocol-types-method-apis">
1718 <title>Mapping method calls to native APIs</title>
1720 APIs for D-Bus may map method calls to a method call in a specific
1721 programming language, such as C++, or may map a method call written
1722 in an IDL to a D-Bus message.
1725 In APIs of this nature, arguments to a method are often termed "in"
1726 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1727 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1728 "inout" arguments, which are both sent and received, i.e. the caller
1729 passes in a value which is modified. Mapped to D-Bus, an "inout"
1730 argument is equivalent to an "in" argument, followed by an "out"
1731 argument. You can't pass things "by reference" over the wire, so
1732 "inout" is purely an illusion of the in-process API.
1735 Given a method with zero or one return values, followed by zero or more
1736 arguments, where each argument may be "in", "out", or "inout", the
1737 caller constructs a message by appending each "in" or "inout" argument,
1738 in order. "out" arguments are not represented in the caller's message.
1741 The recipient constructs a reply by appending first the return value
1742 if any, then each "out" or "inout" argument, in order.
1743 "in" arguments are not represented in the reply message.
1746 Error replies are normally mapped to exceptions in languages that have
1750 In converting from native APIs to D-Bus, it is perhaps nice to
1751 map D-Bus naming conventions ("FooBar") to native conventions
1752 such as "fooBar" or "foo_bar" automatically. This is OK
1753 as long as you can say that the native API is one that
1754 was specifically written for D-Bus. It makes the most sense
1755 when writing object implementations that will be exported
1756 over the bus. Object proxies used to invoke remote D-Bus
1757 objects probably need the ability to call any D-Bus method,
1758 and thus a magic name mapping like this could be a problem.
1761 This specification doesn't require anything of native API bindings;
1762 the preceding is only a suggested convention for consistency
1768 <sect3 id="message-protocol-types-signal">
1769 <title>Signal Emission</title>
1771 Unlike method calls, signal emissions have no replies.
1772 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1773 It must have three header fields: <literal>PATH</literal> giving the object
1774 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1775 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1776 for signals, though it is optional for method calls.
1780 <sect3 id="message-protocol-types-errors">
1781 <title>Errors</title>
1783 Messages of type <literal>ERROR</literal> are most commonly replies
1784 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1785 to any kind of message. The message bus for example
1786 will return an <literal>ERROR</literal> in reply to a signal emission if
1787 the bus does not have enough memory to send the signal.
1790 An <literal>ERROR</literal> may have any arguments, but if the first
1791 argument is a <literal>STRING</literal>, it must be an error message.
1792 The error message may be logged or shown to the user
1797 <sect3 id="message-protocol-types-notation">
1798 <title>Notation in this document</title>
1800 This document uses a simple pseudo-IDL to describe particular method
1801 calls and signals. Here is an example of a method call:
1803 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1804 out UINT32 resultcode)
1806 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1807 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1808 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1809 characters so it's known that the last part of the name in
1810 the "IDL" is the member name.
1813 In C++ that might end up looking like this:
1815 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1816 unsigned int flags);
1818 or equally valid, the return value could be done as an argument:
1820 void org::freedesktop::DBus::StartServiceByName (const char *name,
1822 unsigned int *resultcode);
1824 It's really up to the API designer how they want to make
1825 this look. You could design an API where the namespace wasn't used
1826 in C++, using STL or Qt, using varargs, or whatever you wanted.
1829 Signals are written as follows:
1831 org.freedesktop.DBus.NameLost (STRING name)
1833 Signals don't specify "in" vs. "out" because only
1834 a single direction is possible.
1837 It isn't especially encouraged to use this lame pseudo-IDL in actual
1838 API implementations; you might use the native notation for the
1839 language you're using, or you might use COM or CORBA IDL, for example.
1844 <sect2 id="message-protocol-handling-invalid">
1845 <title>Invalid Protocol and Spec Extensions</title>
1848 For security reasons, the D-Bus protocol should be strictly parsed and
1849 validated, with the exception of defined extension points. Any invalid
1850 protocol or spec violations should result in immediately dropping the
1851 connection without notice to the other end. Exceptions should be
1852 carefully considered, e.g. an exception may be warranted for a
1853 well-understood idiosyncrasy of a widely-deployed implementation. In
1854 cases where the other end of a connection is 100% trusted and known to
1855 be friendly, skipping validation for performance reasons could also make
1856 sense in certain cases.
1860 Generally speaking violations of the "must" requirements in this spec
1861 should be considered possible attempts to exploit security, and violations
1862 of the "should" suggestions should be considered legitimate (though perhaps
1863 they should generate an error in some cases).
1867 The following extension points are built in to D-Bus on purpose and must
1868 not be treated as invalid protocol. The extension points are intended
1869 for use by future versions of this spec, they are not intended for third
1870 parties. At the moment, the only way a third party could extend D-Bus
1871 without breaking interoperability would be to introduce a way to negotiate new
1872 feature support as part of the auth protocol, using EXTENSION_-prefixed
1873 commands. There is not yet a standard way to negotiate features.
1877 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1878 commands result in an ERROR rather than a disconnect. This enables
1879 future extensions to the protocol. Commands starting with EXTENSION_ are
1880 reserved for third parties.
1885 The authentication protocol supports pluggable auth mechanisms.
1890 The address format (see <xref linkend="addresses"/>) supports new
1896 Messages with an unknown type (something other than
1897 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1898 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1899 Unknown-type messages must still be well-formed in the same way
1900 as the known messages, however. They still have the normal
1906 Header fields with an unknown or unexpected field code must be ignored,
1907 though again they must still be well-formed.
1912 New standard interfaces (with new methods and signals) can of course be added.
1922 <sect1 id="auth-protocol">
1923 <title>Authentication Protocol</title>
1925 Before the flow of messages begins, two applications must
1926 authenticate. A simple plain-text protocol is used for
1927 authentication; this protocol is a SASL profile, and maps fairly
1928 directly from the SASL specification. The message encoding is
1929 NOT used here, only plain text messages.
1932 In examples, "C:" and "S:" indicate lines sent by the client and
1933 server respectively.
1935 <sect2 id="auth-protocol-overview">
1936 <title>Protocol Overview</title>
1938 The protocol is a line-based protocol, where each line ends with
1939 \r\n. Each line begins with an all-caps ASCII command name containing
1940 only the character range [A-Z_], a space, then any arguments for the
1941 command, then the \r\n ending the line. The protocol is
1942 case-sensitive. All bytes must be in the ASCII character set.
1944 Commands from the client to the server are as follows:
1947 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1948 <listitem><para>CANCEL</para></listitem>
1949 <listitem><para>BEGIN</para></listitem>
1950 <listitem><para>DATA <data in hex encoding></para></listitem>
1951 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1952 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1955 From server to client are as follows:
1958 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1959 <listitem><para>OK <GUID in hex></para></listitem>
1960 <listitem><para>DATA <data in hex encoding></para></listitem>
1961 <listitem><para>ERROR</para></listitem>
1962 <listitem><para>AGREE_UNIX_FD</para></listitem>
1966 Unofficial extensions to the command set must begin with the letters
1967 "EXTENSION_", to avoid conflicts with future official commands.
1968 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1971 <sect2 id="auth-nul-byte">
1972 <title>Special credentials-passing nul byte</title>
1974 Immediately after connecting to the server, the client must send a
1975 single nul byte. This byte may be accompanied by credentials
1976 information on some operating systems that use sendmsg() with
1977 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1978 sockets. However, the nul byte must be sent even on other kinds of
1979 socket, and even on operating systems that do not require a byte to be
1980 sent in order to transmit credentials. The text protocol described in
1981 this document begins after the single nul byte. If the first byte
1982 received from the client is not a nul byte, the server may disconnect
1986 A nul byte in any context other than the initial byte is an error;
1987 the protocol is ASCII-only.
1990 The credentials sent along with the nul byte may be used with the
1991 SASL mechanism EXTERNAL.
1994 <sect2 id="auth-command-auth">
1995 <title>AUTH command</title>
1997 If an AUTH command has no arguments, it is a request to list
1998 available mechanisms. The server must respond with a REJECTED
1999 command listing the mechanisms it understands, or with an error.
2002 If an AUTH command specifies a mechanism, and the server supports
2003 said mechanism, the server should begin exchanging SASL
2004 challenge-response data with the client using DATA commands.
2007 If the server does not support the mechanism given in the AUTH
2008 command, it must send either a REJECTED command listing the mechanisms
2009 it does support, or an error.
2012 If the [initial-response] argument is provided, it is intended for use
2013 with mechanisms that have no initial challenge (or an empty initial
2014 challenge), as if it were the argument to an initial DATA command. If
2015 the selected mechanism has an initial challenge and [initial-response]
2016 was provided, the server should reject authentication by sending
2020 If authentication succeeds after exchanging DATA commands,
2021 an OK command must be sent to the client.
2024 The first octet received by the server after the \r\n of the BEGIN
2025 command from the client must be the first octet of the
2026 authenticated/encrypted stream of D-Bus messages.
2029 If BEGIN is received by the server, the first octet received
2030 by the client after the \r\n of the OK command must be the
2031 first octet of the authenticated/encrypted stream of D-Bus
2035 <sect2 id="auth-command-cancel">
2036 <title>CANCEL Command</title>
2038 At any time up to sending the BEGIN command, the client may send a
2039 CANCEL command. On receiving the CANCEL command, the server must
2040 send a REJECTED command and abort the current authentication
2044 <sect2 id="auth-command-data">
2045 <title>DATA Command</title>
2047 The DATA command may come from either client or server, and simply
2048 contains a hex-encoded block of data to be interpreted
2049 according to the SASL mechanism in use.
2052 Some SASL mechanisms support sending an "empty string";
2053 FIXME we need some way to do this.
2056 <sect2 id="auth-command-begin">
2057 <title>BEGIN Command</title>
2059 The BEGIN command acknowledges that the client has received an
2060 OK command from the server, and that the stream of messages
2064 The first octet received by the server after the \r\n of the BEGIN
2065 command from the client must be the first octet of the
2066 authenticated/encrypted stream of D-Bus messages.
2069 <sect2 id="auth-command-rejected">
2070 <title>REJECTED Command</title>
2072 The REJECTED command indicates that the current authentication
2073 exchange has failed, and further exchange of DATA is inappropriate.
2074 The client would normally try another mechanism, or try providing
2075 different responses to challenges.
2077 Optionally, the REJECTED command has a space-separated list of
2078 available auth mechanisms as arguments. If a server ever provides
2079 a list of supported mechanisms, it must provide the same list
2080 each time it sends a REJECTED message. Clients are free to
2081 ignore all lists received after the first.
2084 <sect2 id="auth-command-ok">
2085 <title>OK Command</title>
2087 The OK command indicates that the client has been
2088 authenticated. The client may now proceed with negotiating
2089 Unix file descriptor passing. To do that it shall send
2090 NEGOTIATE_UNIX_FD to the server.
2093 Otherwise, the client must respond to the OK command by
2094 sending a BEGIN command, followed by its stream of messages,
2095 or by disconnecting. The server must not accept additional
2096 commands using this protocol after the BEGIN command has been
2097 received. Further communication will be a stream of D-Bus
2098 messages (optionally encrypted, as negotiated) rather than
2102 If a client sends BEGIN the first octet received by the client
2103 after the \r\n of the OK command must be the first octet of
2104 the authenticated/encrypted stream of D-Bus messages.
2107 The OK command has one argument, which is the GUID of the server.
2108 See <xref linkend="addresses"/> for more on server GUIDs.
2111 <sect2 id="auth-command-error">
2112 <title>ERROR Command</title>
2114 The ERROR command indicates that either server or client did not
2115 know a command, does not accept the given command in the current
2116 context, or did not understand the arguments to the command. This
2117 allows the protocol to be extended; a client or server can send a
2118 command present or permitted only in new protocol versions, and if
2119 an ERROR is received instead of an appropriate response, fall back
2120 to using some other technique.
2123 If an ERROR is sent, the server or client that sent the
2124 error must continue as if the command causing the ERROR had never been
2125 received. However, the the server or client receiving the error
2126 should try something other than whatever caused the error;
2127 if only canceling/rejecting the authentication.
2130 If the D-Bus protocol changes incompatibly at some future time,
2131 applications implementing the new protocol would probably be able to
2132 check for support of the new protocol by sending a new command and
2133 receiving an ERROR from applications that don't understand it. Thus the
2134 ERROR feature of the auth protocol is an escape hatch that lets us
2135 negotiate extensions or changes to the D-Bus protocol in the future.
2138 <sect2 id="auth-command-negotiate-unix-fd">
2139 <title>NEGOTIATE_UNIX_FD Command</title>
2141 The NEGOTIATE_UNIX_FD command indicates that the client
2142 supports Unix file descriptor passing. This command may only
2143 be sent after the connection is authenticated, i.e. after OK
2144 was received by the client. This command may only be sent on
2145 transports that support Unix file descriptor passing.
2148 On receiving NEGOTIATE_UNIX_FD the server must respond with
2149 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2150 the transport chosen supports Unix file descriptor passing and
2151 the server supports this feature. It shall respond the latter
2152 if the transport does not support Unix file descriptor
2153 passing, the server does not support this feature, or the
2154 server decides not to enable file descriptor passing due to
2155 security or other reasons.
2158 <sect2 id="auth-command-agree-unix-fd">
2159 <title>AGREE_UNIX_FD Command</title>
2161 The AGREE_UNIX_FD command indicates that the server supports
2162 Unix file descriptor passing. This command may only be sent
2163 after the connection is authenticated, and the client sent
2164 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2165 command may only be sent on transports that support Unix file
2169 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2170 followed by its stream of messages, or by disconnecting. The
2171 server must not accept additional commands using this protocol
2172 after the BEGIN command has been received. Further
2173 communication will be a stream of D-Bus messages (optionally
2174 encrypted, as negotiated) rather than this protocol.
2177 <sect2 id="auth-command-future">
2178 <title>Future Extensions</title>
2180 Future extensions to the authentication and negotiation
2181 protocol are possible. For that new commands may be
2182 introduced. If a client or server receives an unknown command
2183 it shall respond with ERROR and not consider this fatal. New
2184 commands may be introduced both before, and after
2185 authentication, i.e. both before and after the OK command.
2188 <sect2 id="auth-examples">
2189 <title>Authentication examples</title>
2193 <title>Example of successful magic cookie authentication</title>
2195 (MAGIC_COOKIE is a made up mechanism)
2197 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2203 <title>Example of finding out mechanisms then picking one</title>
2206 S: REJECTED KERBEROS_V4 SKEY
2207 C: AUTH SKEY 7ab83f32ee
2208 S: DATA 8799cabb2ea93e
2209 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2215 <title>Example of client sends unknown command then falls back to regular auth</title>
2219 C: AUTH MAGIC_COOKIE 3736343435313230333039
2225 <title>Example of server doesn't support initial auth mechanism</title>
2227 C: AUTH MAGIC_COOKIE 3736343435313230333039
2228 S: REJECTED KERBEROS_V4 SKEY
2229 C: AUTH SKEY 7ab83f32ee
2230 S: DATA 8799cabb2ea93e
2231 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2237 <title>Example of wrong password or the like followed by successful retry</title>
2239 C: AUTH MAGIC_COOKIE 3736343435313230333039
2240 S: REJECTED KERBEROS_V4 SKEY
2241 C: AUTH SKEY 7ab83f32ee
2242 S: DATA 8799cabb2ea93e
2243 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2245 C: AUTH SKEY 7ab83f32ee
2246 S: DATA 8799cabb2ea93e
2247 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2253 <title>Example of skey cancelled and restarted</title>
2255 C: AUTH MAGIC_COOKIE 3736343435313230333039
2256 S: REJECTED KERBEROS_V4 SKEY
2257 C: AUTH SKEY 7ab83f32ee
2258 S: DATA 8799cabb2ea93e
2261 C: AUTH SKEY 7ab83f32ee
2262 S: DATA 8799cabb2ea93e
2263 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2269 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2271 (MAGIC_COOKIE is a made up mechanism)
2273 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2275 C: NEGOTIATE_UNIX_FD
2281 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2283 (MAGIC_COOKIE is a made up mechanism)
2285 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2287 C: NEGOTIATE_UNIX_FD
2294 <sect2 id="auth-states">
2295 <title>Authentication state diagrams</title>
2298 This section documents the auth protocol in terms of
2299 a state machine for the client and the server. This is
2300 probably the most robust way to implement the protocol.
2303 <sect3 id="auth-states-client">
2304 <title>Client states</title>
2307 To more precisely describe the interaction between the
2308 protocol state machine and the authentication mechanisms the
2309 following notation is used: MECH(CHALL) means that the
2310 server challenge CHALL was fed to the mechanism MECH, which
2316 CONTINUE(RESP) means continue the auth conversation
2317 and send RESP as the response to the server;
2323 OK(RESP) means that after sending RESP to the server
2324 the client side of the auth conversation is finished
2325 and the server should return "OK";
2331 ERROR means that CHALL was invalid and could not be
2337 Both RESP and CHALL may be empty.
2341 The Client starts by getting an initial response from the
2342 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2343 the mechanism did not provide an initial response. If the
2344 mechanism returns CONTINUE, the client starts in state
2345 <emphasis>WaitingForData</emphasis>, if the mechanism
2346 returns OK the client starts in state
2347 <emphasis>WaitingForOK</emphasis>.
2351 The client should keep track of available mechanisms and
2352 which it mechanisms it has already attempted. This list is
2353 used to decide which AUTH command to send. When the list is
2354 exhausted, the client should give up and close the
2359 <title><emphasis>WaitingForData</emphasis></title>
2367 MECH(CHALL) returns CONTINUE(RESP) → send
2369 <emphasis>WaitingForData</emphasis>
2373 MECH(CHALL) returns OK(RESP) → send DATA
2374 RESP, goto <emphasis>WaitingForOK</emphasis>
2378 MECH(CHALL) returns ERROR → send ERROR
2379 [msg], goto <emphasis>WaitingForData</emphasis>
2387 Receive REJECTED [mechs] →
2388 send AUTH [next mech], goto
2389 WaitingForData or <emphasis>WaitingForOK</emphasis>
2394 Receive ERROR → send
2396 <emphasis>WaitingForReject</emphasis>
2401 Receive OK → send
2402 BEGIN, terminate auth
2403 conversation, authenticated
2408 Receive anything else → send
2410 <emphasis>WaitingForData</emphasis>
2418 <title><emphasis>WaitingForOK</emphasis></title>
2423 Receive OK → send BEGIN, terminate auth
2424 conversation, <emphasis>authenticated</emphasis>
2429 Receive REJECT [mechs] → send AUTH [next mech],
2430 goto <emphasis>WaitingForData</emphasis> or
2431 <emphasis>WaitingForOK</emphasis>
2437 Receive DATA → send CANCEL, goto
2438 <emphasis>WaitingForReject</emphasis>
2444 Receive ERROR → send CANCEL, goto
2445 <emphasis>WaitingForReject</emphasis>
2451 Receive anything else → send ERROR, goto
2452 <emphasis>WaitingForOK</emphasis>
2460 <title><emphasis>WaitingForReject</emphasis></title>
2465 Receive REJECT [mechs] → send AUTH [next mech],
2466 goto <emphasis>WaitingForData</emphasis> or
2467 <emphasis>WaitingForOK</emphasis>
2473 Receive anything else → terminate auth
2474 conversation, disconnect
2483 <sect3 id="auth-states-server">
2484 <title>Server states</title>
2487 For the server MECH(RESP) means that the client response
2488 RESP was fed to the the mechanism MECH, which returns one of
2493 CONTINUE(CHALL) means continue the auth conversation and
2494 send CHALL as the challenge to the client;
2500 OK means that the client has been successfully
2507 REJECT means that the client failed to authenticate or
2508 there was an error in RESP.
2513 The server starts out in state
2514 <emphasis>WaitingForAuth</emphasis>. If the client is
2515 rejected too many times the server must disconnect the
2520 <title><emphasis>WaitingForAuth</emphasis></title>
2526 Receive AUTH → send REJECTED [mechs], goto
2527 <emphasis>WaitingForAuth</emphasis>
2533 Receive AUTH MECH RESP
2537 MECH not valid mechanism → send REJECTED
2539 <emphasis>WaitingForAuth</emphasis>
2543 MECH(RESP) returns CONTINUE(CHALL) → send
2545 <emphasis>WaitingForData</emphasis>
2549 MECH(RESP) returns OK → send OK, goto
2550 <emphasis>WaitingForBegin</emphasis>
2554 MECH(RESP) returns REJECT → send REJECTED
2556 <emphasis>WaitingForAuth</emphasis>
2564 Receive BEGIN → terminate
2565 auth conversation, disconnect
2571 Receive ERROR → send REJECTED [mechs], goto
2572 <emphasis>WaitingForAuth</emphasis>
2578 Receive anything else → send
2580 <emphasis>WaitingForAuth</emphasis>
2589 <title><emphasis>WaitingForData</emphasis></title>
2597 MECH(RESP) returns CONTINUE(CHALL) → send
2599 <emphasis>WaitingForData</emphasis>
2603 MECH(RESP) returns OK → send OK, goto
2604 <emphasis>WaitingForBegin</emphasis>
2608 MECH(RESP) returns REJECT → send REJECTED
2610 <emphasis>WaitingForAuth</emphasis>
2618 Receive BEGIN → terminate auth conversation,
2625 Receive CANCEL → send REJECTED [mechs], goto
2626 <emphasis>WaitingForAuth</emphasis>
2632 Receive ERROR → send REJECTED [mechs], goto
2633 <emphasis>WaitingForAuth</emphasis>
2639 Receive anything else → send ERROR, goto
2640 <emphasis>WaitingForData</emphasis>
2648 <title><emphasis>WaitingForBegin</emphasis></title>
2653 Receive BEGIN → terminate auth conversation,
2654 client authenticated
2660 Receive CANCEL → send REJECTED [mechs], goto
2661 <emphasis>WaitingForAuth</emphasis>
2667 Receive ERROR → send REJECTED [mechs], goto
2668 <emphasis>WaitingForAuth</emphasis>
2674 Receive anything else → send ERROR, goto
2675 <emphasis>WaitingForBegin</emphasis>
2685 <sect2 id="auth-mechanisms">
2686 <title>Authentication mechanisms</title>
2688 This section describes some new authentication mechanisms.
2689 D-Bus also allows any standard SASL mechanism of course.
2691 <sect3 id="auth-mechanisms-sha">
2692 <title>DBUS_COOKIE_SHA1</title>
2694 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2695 has the ability to read a private file owned by the user being
2696 authenticated. If the client can prove that it has access to a secret
2697 cookie stored in this file, then the client is authenticated.
2698 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2702 Throughout this description, "hex encoding" must output the digits
2703 from a to f in lower-case; the digits A to F must not be used
2704 in the DBUS_COOKIE_SHA1 mechanism.
2707 Authentication proceeds as follows:
2711 The client sends the username it would like to authenticate
2717 The server sends the name of its "cookie context" (see below); a
2718 space character; the integer ID of the secret cookie the client
2719 must demonstrate knowledge of; a space character; then a
2720 randomly-generated challenge string, all of this hex-encoded into
2726 The client locates the cookie and generates its own
2727 randomly-generated challenge string. The client then concatenates
2728 the server's decoded challenge, a ":" character, its own challenge,
2729 another ":" character, and the cookie. It computes the SHA-1 hash
2730 of this composite string as a hex digest. It concatenates the
2731 client's challenge string, a space character, and the SHA-1 hex
2732 digest, hex-encodes the result and sends it back to the server.
2737 The server generates the same concatenated string used by the
2738 client and computes its SHA-1 hash. It compares the hash with
2739 the hash received from the client; if the two hashes match, the
2740 client is authenticated.
2746 Each server has a "cookie context," which is a name that identifies a
2747 set of cookies that apply to that server. A sample context might be
2748 "org_freedesktop_session_bus". Context names must be valid ASCII,
2749 nonzero length, and may not contain the characters slash ("/"),
2750 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2751 tab ("\t"), or period ("."). There is a default context,
2752 "org_freedesktop_general" that's used by servers that do not specify
2756 Cookies are stored in a user's home directory, in the directory
2757 <filename>~/.dbus-keyrings/</filename>. This directory must
2758 not be readable or writable by other users. If it is,
2759 clients and servers must ignore it. The directory
2760 contains cookie files named after the cookie context.
2763 A cookie file contains one cookie per line. Each line
2764 has three space-separated fields:
2768 The cookie ID number, which must be a non-negative integer and
2769 may not be used twice in the same file.
2774 The cookie's creation time, in UNIX seconds-since-the-epoch
2780 The cookie itself, a hex-encoded random block of bytes. The cookie
2781 may be of any length, though obviously security increases
2782 as the length increases.
2788 Only server processes modify the cookie file.
2789 They must do so with this procedure:
2793 Create a lockfile name by appending ".lock" to the name of the
2794 cookie file. The server should attempt to create this file
2795 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2796 fails, the lock fails. Servers should retry for a reasonable
2797 period of time, then they may choose to delete an existing lock
2798 to keep users from having to manually delete a stale
2799 lock. <footnote><para>Lockfiles are used instead of real file
2800 locking <literal>fcntl()</literal> because real locking
2801 implementations are still flaky on network
2802 filesystems.</para></footnote>
2807 Once the lockfile has been created, the server loads the cookie
2808 file. It should then delete any cookies that are old (the
2809 timeout can be fairly short), or more than a reasonable
2810 time in the future (so that cookies never accidentally
2811 become permanent, if the clock was set far into the future
2812 at some point). If no recent keys remain, the
2813 server may generate a new key.
2818 The pruned and possibly added-to cookie file
2819 must be resaved atomically (using a temporary
2820 file which is rename()'d).
2825 The lock must be dropped by deleting the lockfile.
2831 Clients need not lock the file in order to load it,
2832 because servers are required to save the file atomically.
2837 <sect1 id="addresses">
2838 <title>Server Addresses</title>
2840 Server addresses consist of a transport name followed by a colon, and
2841 then an optional, comma-separated list of keys and values in the form key=value.
2842 Each value is escaped.
2846 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2847 Which is the address to a unix socket with the path /tmp/dbus-test.
2850 Value escaping is similar to URI escaping but simpler.
2854 The set of optionally-escaped bytes is:
2855 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2856 <emphasis>byte</emphasis> (note, not character) which is not in the
2857 set of optionally-escaped bytes must be replaced with an ASCII
2858 percent (<literal>%</literal>) and the value of the byte in hex.
2859 The hex value must always be two digits, even if the first digit is
2860 zero. The optionally-escaped bytes may be escaped if desired.
2865 To unescape, append each byte in the value; if a byte is an ASCII
2866 percent (<literal>%</literal>) character then append the following
2867 hex value instead. It is an error if a <literal>%</literal> byte
2868 does not have two hex digits following. It is an error if a
2869 non-optionally-escaped byte is seen unescaped.
2873 The set of optionally-escaped bytes is intended to preserve address
2874 readability and convenience.
2878 A server may specify a key-value pair with the key <literal>guid</literal>
2879 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2880 describes the format of the <literal>guid</literal> field. If present,
2881 this UUID may be used to distinguish one server address from another. A
2882 server should use a different UUID for each address it listens on. For
2883 example, if a message bus daemon offers both UNIX domain socket and TCP
2884 connections, but treats clients the same regardless of how they connect,
2885 those two connections are equivalent post-connection but should have
2886 distinct UUIDs to distinguish the kinds of connection.
2890 The intent of the address UUID feature is to allow a client to avoid
2891 opening multiple identical connections to the same server, by allowing the
2892 client to check whether an address corresponds to an already-existing
2893 connection. Comparing two addresses is insufficient, because addresses
2894 can be recycled by distinct servers, and equivalent addresses may look
2895 different if simply compared as strings (for example, the host in a TCP
2896 address can be given as an IP address or as a hostname).
2900 Note that the address key is <literal>guid</literal> even though the
2901 rest of the API and documentation says "UUID," for historical reasons.
2905 [FIXME clarify if attempting to connect to each is a requirement
2906 or just a suggestion]
2907 When connecting to a server, multiple server addresses can be
2908 separated by a semi-colon. The library will then try to connect
2909 to the first address and if that fails, it'll try to connect to
2910 the next one specified, and so forth. For example
2911 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2916 <sect1 id="transports">
2917 <title>Transports</title>
2919 [FIXME we need to specify in detail each transport and its possible arguments]
2921 Current transports include: unix domain sockets (including
2922 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2923 using in-process pipes. Future possible transports include one that
2924 tunnels over X11 protocol.
2927 <sect2 id="transports-unix-domain-sockets">
2928 <title>Unix Domain Sockets</title>
2930 Unix domain sockets can be either paths in the file system or on Linux
2931 kernels, they can be abstract which are similar to paths but
2932 do not show up in the file system.
2936 When a socket is opened by the D-Bus library it truncates the path
2937 name right before the first trailing Nul byte. This is true for both
2938 normal paths and abstract paths. Note that this is a departure from
2939 previous versions of D-Bus that would create sockets with a fixed
2940 length path name. Names which were shorter than the fixed length
2941 would be padded by Nul bytes.
2944 Unix domain sockets are not available on Windows.
2946 <sect3 id="transports-unix-domain-sockets-addresses">
2947 <title>Server Address Format</title>
2949 Unix domain socket addresses are identified by the "unix:" prefix
2950 and support the following key/value pairs:
2957 <entry>Values</entry>
2958 <entry>Description</entry>
2964 <entry>(path)</entry>
2965 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2968 <entry>tmpdir</entry>
2969 <entry>(path)</entry>
2970 <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>
2973 <entry>abstract</entry>
2974 <entry>(string)</entry>
2975 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2982 <sect2 id="transports-launchd">
2983 <title>launchd</title>
2985 launchd is an open-source server management system that replaces init, inetd
2986 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2987 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2991 launchd allocates a socket and provides it with the unix path through the
2992 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2993 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2994 it through its environment.
2995 Other processes can query for the launchd socket by executing:
2996 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2997 This is normally done by the D-Bus client library so doesn't have to be done
3001 launchd is not available on Microsoft Windows.
3003 <sect3 id="transports-launchd-addresses">
3004 <title>Server Address Format</title>
3006 launchd addresses are identified by the "launchd:" prefix
3007 and support the following key/value pairs:
3014 <entry>Values</entry>
3015 <entry>Description</entry>
3021 <entry>(environment variable)</entry>
3022 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3029 <sect2 id="transports-systemd">
3030 <title>systemd</title>
3032 systemd is an open-source server management system that
3033 replaces init and inetd on newer Linux systems. It supports
3034 socket activation. The D-Bus systemd transport is used to acquire
3035 socket activation file descriptors from systemd and use them
3036 as D-Bus transport when the current process is spawned by
3037 socket activation from it.
3040 The systemd transport accepts only one or more Unix domain or
3041 TCP streams sockets passed in via socket activation.
3044 The systemd transport is not available on non-Linux operating systems.
3047 The systemd transport defines no parameter keys.
3050 <sect2 id="transports-tcp-sockets">
3051 <title>TCP Sockets</title>
3053 The tcp transport provides TCP/IP based connections between clients
3054 located on the same or different hosts.
3057 Using tcp transport without any additional secure authentification mechanismus
3058 over a network is unsecure.
3061 Windows notes: Because of the tcp stack on Windows does not provide sending
3062 credentials over a tcp connection, the EXTERNAL authentification
3063 mechanismus does not work.
3065 <sect3 id="transports-tcp-sockets-addresses">
3066 <title>Server Address Format</title>
3068 TCP/IP socket addresses are identified by the "tcp:" prefix
3069 and support the following key/value pairs:
3076 <entry>Values</entry>
3077 <entry>Description</entry>
3083 <entry>(string)</entry>
3084 <entry>dns name or ip address</entry>
3088 <entry>(number)</entry>
3089 <entry>The tcp port the server will open. A zero value let the server
3090 choose a free port provided from the underlaying operating system.
3091 libdbus is able to retrieve the real used port from the server.
3095 <entry>family</entry>
3096 <entry>(string)</entry>
3097 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3104 <sect2 id="transports-nonce-tcp-sockets">
3105 <title>Nonce-secured TCP Sockets</title>
3107 The nonce-tcp transport provides a secured TCP transport, using a
3108 simple authentication mechanism to ensure that only clients with read
3109 access to a certain location in the filesystem can connect to the server.
3110 The server writes a secret, the nonce, to a file and an incoming client
3111 connection is only accepted if the client sends the nonce right after
3112 the connect. The nonce mechanism requires no setup and is orthogonal to
3113 the higher-level authentication mechanisms described in the
3114 Authentication section.
3118 On start, the server generates a random 16 byte nonce and writes it
3119 to a file in the user's temporary directory. The nonce file location
3120 is published as part of the server's D-Bus address using the
3121 "noncefile" key-value pair.
3123 After an accept, the server reads 16 bytes from the socket. If the
3124 read bytes do not match the nonce stored in the nonce file, the
3125 server MUST immediately drop the connection.
3126 If the nonce match the received byte sequence, the client is accepted
3127 and the transport behaves like an unsecured tcp transport.
3130 After a successful connect to the server socket, the client MUST read
3131 the nonce from the file published by the server via the noncefile=
3132 key-value pair and send it over the socket. After that, the
3133 transport behaves like an unsecured tcp transport.
3135 <sect3 id="transports-nonce-tcp-sockets-addresses">
3136 <title>Server Address Format</title>
3138 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3139 and support the following key/value pairs:
3146 <entry>Values</entry>
3147 <entry>Description</entry>
3153 <entry>(string)</entry>
3154 <entry>dns name or ip address</entry>
3158 <entry>(number)</entry>
3159 <entry>The tcp port the server will open. A zero value let the server
3160 choose a free port provided from the underlaying operating system.
3161 libdbus is able to retrieve the real used port from the server.
3165 <entry>family</entry>
3166 <entry>(string)</entry>
3167 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3170 <entry>noncefile</entry>
3171 <entry>(path)</entry>
3172 <entry>file location containing the secret</entry>
3179 <sect2 id="transports-exec">
3180 <title>Executed Subprocesses on Unix</title>
3182 This transport forks off a process and connects its standard
3183 input and standard output with an anonymous Unix domain
3184 socket. This socket is then used for communication by the
3185 transport. This transport may be used to use out-of-process
3186 forwarder programs as basis for the D-Bus protocol.
3189 The forked process will inherit the standard error output and
3190 process group from the parent process.
3193 Executed subprocesses are not available on Windows.
3195 <sect3 id="transports-exec-addresses">
3196 <title>Server Address Format</title>
3198 Executed subprocess addresses are identified by the "unixexec:" prefix
3199 and support the following key/value pairs:
3206 <entry>Values</entry>
3207 <entry>Description</entry>
3213 <entry>(path)</entry>
3214 <entry>Path of the binary to execute, either an absolute
3215 path or a binary name that is searched for in the default
3216 search path of the OS. This corresponds to the first
3217 argument of execlp(). This key is mandatory.</entry>
3220 <entry>argv0</entry>
3221 <entry>(string)</entry>
3222 <entry>The program name to use when executing the
3223 binary. If omitted the same value as specified for path=
3224 will be used. This corresponds to the second argument of
3228 <entry>argv1, argv2, ...</entry>
3229 <entry>(string)</entry>
3230 <entry>Arguments to pass to the binary. This corresponds
3231 to the third and later arguments of execlp(). If a
3232 specific argvX is not specified no further argvY for Y > X
3233 are taken into account.</entry>
3241 <sect1 id="meta-transports">
3242 <title>Meta Transports</title>
3244 Meta transports are a kind of transport with special enhancements or
3245 behavior. Currently available meta transports include: autolaunch
3248 <sect2 id="meta-transports-autolaunch">
3249 <title>Autolaunch</title>
3250 <para>The autolaunch transport provides a way for dbus clients to autodetect
3251 a running dbus session bus and to autolaunch a session bus if not present.
3253 <sect3 id="meta-transports-autolaunch-addresses">
3254 <title>Server Address Format</title>
3256 Autolaunch addresses uses the "autolaunch:" prefix and support the
3257 following key/value pairs:
3264 <entry>Values</entry>
3265 <entry>Description</entry>
3270 <entry>scope</entry>
3271 <entry>(string)</entry>
3272 <entry>scope of autolaunch (Windows only)
3276 "*install-path" - limit session bus to dbus installation path.
3277 The dbus installation path is determined from the location of
3278 the shared dbus library. If the library is located in a 'bin'
3279 subdirectory the installation root is the directory above,
3280 otherwise the directory where the library lives is taken as
3283 <install-root>/bin/[lib]dbus-1.dll
3284 <install-root>/[lib]dbus-1.dll
3290 "*user" - limit session bus to the recent user.
3295 other values - specify dedicated session bus like "release",
3307 <sect3 id="meta-transports-autolaunch-windows-implementation">
3308 <title>Windows implementation</title>
3310 On start, the server opens a platform specific transport, creates a mutex
3311 and a shared memory section containing the related session bus address.
3312 This mutex will be inspected by the dbus client library to detect a
3313 running dbus session bus. The access to the mutex and the shared memory
3314 section are protected by global locks.
3317 In the recent implementation the autolaunch transport uses a tcp transport
3318 on localhost with a port choosen from the operating system. This detail may
3319 change in the future.
3322 Disclaimer: The recent implementation is in an early state and may not
3323 work in all cirumstances and/or may have security issues. Because of this
3324 the implementation is not documentated yet.
3331 <title>UUIDs</title>
3333 A working D-Bus implementation uses universally-unique IDs in two places.
3334 First, each server address has a UUID identifying the address,
3335 as described in <xref linkend="addresses"/>. Second, each operating
3336 system kernel instance running a D-Bus client or server has a UUID
3337 identifying that kernel, retrieved by invoking the method
3338 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3339 linkend="standard-interfaces-peer"/>).
3342 The term "UUID" in this document is intended literally, i.e. an
3343 identifier that is universally unique. It is not intended to refer to
3344 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3347 The UUID must contain 128 bits of data and be hex-encoded. The
3348 hex-encoded string may not contain hyphens or other non-hex-digit
3349 characters, and it must be exactly 32 characters long. To generate a
3350 UUID, the current reference implementation concatenates 96 bits of random
3351 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3355 It would also be acceptable and probably better to simply generate 128
3356 bits of random data, as long as the random number generator is of high
3357 quality. The timestamp could conceivably help if the random bits are not
3358 very random. With a quality random number generator, collisions are
3359 extremely unlikely even with only 96 bits, so it's somewhat academic.
3362 Implementations should, however, stick to random data for the first 96 bits
3367 <sect1 id="standard-interfaces">
3368 <title>Standard Interfaces</title>
3370 See <xref linkend="message-protocol-types-notation"/> for details on
3371 the notation used in this section. There are some standard interfaces
3372 that may be useful across various D-Bus applications.
3374 <sect2 id="standard-interfaces-peer">
3375 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3377 The <literal>org.freedesktop.DBus.Peer</literal> interface
3380 org.freedesktop.DBus.Peer.Ping ()
3381 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3385 On receipt of the <literal>METHOD_CALL</literal> message
3386 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3387 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3388 usual. It does not matter which object path a ping is sent to. The
3389 reference implementation handles this method automatically.
3392 On receipt of the <literal>METHOD_CALL</literal> message
3393 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3394 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3395 UUID representing the identity of the machine the process is running on.
3396 This UUID must be the same for all processes on a single system at least
3397 until that system next reboots. It should be the same across reboots
3398 if possible, but this is not always possible to implement and is not
3400 It does not matter which object path a GetMachineId is sent to. The
3401 reference implementation handles this method automatically.
3404 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3405 a virtual machine running on a hypervisor, rather than a physical machine.
3406 Basically if two processes see the same UUID, they should also see the same
3407 shared memory, UNIX domain sockets, process IDs, and other features that require
3408 a running OS kernel in common between the processes.
3411 The UUID is often used where other programs might use a hostname. Hostnames
3412 can change without rebooting, however, or just be "localhost" - so the UUID
3416 <xref linkend="uuids"/> explains the format of the UUID.
3420 <sect2 id="standard-interfaces-introspectable">
3421 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3423 This interface has one method:
3425 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3429 Objects instances may implement
3430 <literal>Introspect</literal> which returns an XML description of
3431 the object, including its interfaces (with signals and methods), objects
3432 below it in the object path tree, and its properties.
3435 <xref linkend="introspection-format"/> describes the format of this XML string.
3438 <sect2 id="standard-interfaces-properties">
3439 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3441 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3442 or <firstterm>attributes</firstterm>. These can be exposed via the
3443 <literal>org.freedesktop.DBus.Properties</literal> interface.
3447 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3448 in STRING property_name,
3450 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3451 in STRING property_name,
3453 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3454 out DICT<STRING,VARIANT> props);
3458 It is conventional to give D-Bus properties names consisting of
3459 capitalized words without punctuation ("CamelCase"), like
3460 <link linkend="message-protocol-names-member">member names</link>.
3461 For instance, the GObject property
3462 <literal>connection-status</literal> or the Qt property
3463 <literal>connectionStatus</literal> could be represented on D-Bus
3464 as <literal>ConnectionStatus</literal>.
3467 Strictly speaking, D-Bus property names are not required to follow
3468 the same naming restrictions as member names, but D-Bus property
3469 names that would not be valid member names (in particular,
3470 GObject-style dash-separated property names) can cause interoperability
3471 problems and should be avoided.
3474 The available properties and whether they are writable can be determined
3475 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3476 see <xref linkend="standard-interfaces-introspectable"/>.
3479 An empty string may be provided for the interface name; in this case,
3480 if there are multiple properties on an object with the same name,
3481 the results are undefined (picking one by according to an arbitrary
3482 deterministic rule, or returning an error, are the reasonable
3486 If one or more properties change on an object, the
3487 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3488 signal may be emitted (this signal was added in 0.14):
3492 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3493 DICT<STRING,VARIANT> changed_properties,
3494 ARRAY<STRING> invalidated_properties);
3498 where <literal>changed_properties</literal> is a dictionary
3499 containing the changed properties with the new values and
3500 <literal>invalidated_properties</literal> is an array of
3501 properties that changed but the value is not conveyed.
3504 Whether the <literal>PropertiesChanged</literal> signal is
3505 supported can be determined by calling
3506 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3507 that the signal may be supported for an object but it may
3508 differ how whether and how it is used on a per-property basis
3509 (for e.g. performance or security reasons). Each property (or
3510 the parent interface) must be annotated with the
3511 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3512 annotation to convey this (usually the default value
3513 <literal>true</literal> is sufficient meaning that the
3514 annotation does not need to be used). See <xref
3515 linkend="introspection-format"/> for details on this
3520 <sect2 id="standard-interfaces-objectmanager">
3521 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3523 An API can optionally make use of this interface for one or
3524 more sub-trees of objects. The root of each sub-tree implements
3525 this interface so other applications can get all objects,
3526 interfaces and properties in a single method call. It is
3527 appropriate to use this interface if users of the tree of
3528 objects are expected to be interested in all interfaces of all
3529 objects in the tree; a more granular API should be used if
3530 users of the objects are expected to be interested in a small
3531 subset of the objects, a small subset of their interfaces, or
3535 The method that applications can use to get all objects and
3536 properties is <literal>GetManagedObjects</literal>:
3540 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3544 The return value of this method is a dict whose keys are
3545 object paths. All returned object paths are children of the
3546 object path implementing this interface, i.e. their object
3547 paths start with the ObjectManager's object path plus '/'.
3550 Each value is a dict whose keys are interfaces names. Each
3551 value in this inner dict is the same dict that would be
3552 returned by the <link
3553 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3554 method for that combination of object path and interface. If
3555 an interface has no properties, the empty dict is returned.
3558 Changes are emitted using the following two signals:
3562 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3563 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3564 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3565 ARRAY<STRING> interfaces);
3569 The <literal>InterfacesAdded</literal> signal is emitted when
3570 either a new object is added or when an existing object gains
3571 one or more interfaces. The
3572 <literal>InterfacesRemoved</literal> signal is emitted
3573 whenever an object is removed or it loses one or more
3574 interfaces. The second parameter of the
3575 <literal>InterfacesAdded</literal> signal contains a dict with
3576 the interfaces and properties (if any) that have been added to
3577 the given object path. Similarly, the second parameter of the
3578 <literal>InterfacesRemoved</literal> signal contains an array
3579 of the interfaces that were removed. Note that changes on
3580 properties on existing interfaces are not reported using this
3581 interface - an application should also monitor the existing <link
3582 linkend="standard-interfaces-properties">PropertiesChanged</link>
3583 signal on each object.
3586 Applications SHOULD NOT export objects that are children of an
3587 object (directly or otherwise) implementing this interface but
3588 which are not returned in the reply from the
3589 <literal>GetManagedObjects()</literal> method of this
3590 interface on the given object.
3593 The intent of the <literal>ObjectManager</literal> interface
3594 is to make it easy to write a robust client
3595 implementation. The trivial client implementation only needs
3596 to make two method calls:
3600 org.freedesktop.DBus.AddMatch (bus_proxy,
3601 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3602 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3606 on the message bus and the remote application's
3607 <literal>ObjectManager</literal>, respectively. Whenever a new
3608 remote object is created (or an existing object gains a new
3609 interface), the <literal>InterfacesAdded</literal> signal is
3610 emitted, and since this signal contains all properties for the
3611 interfaces, no calls to the
3612 <literal>org.freedesktop.Properties</literal> interface on the
3613 remote object are needed. Additionally, since the initial
3614 <literal>AddMatch()</literal> rule already includes signal
3615 messages from the newly created child object, no new
3616 <literal>AddMatch()</literal> call is needed.
3621 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3622 interface was added in version 0.17 of the D-Bus
3629 <sect1 id="introspection-format">
3630 <title>Introspection Data Format</title>
3632 As described in <xref linkend="standard-interfaces-introspectable"/>,
3633 objects may be introspected at runtime, returning an XML string
3634 that describes the object. The same XML format may be used in
3635 other contexts as well, for example as an "IDL" for generating
3636 static language bindings.
3639 Here is an example of introspection data:
3641 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3642 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3643 <node name="/org/freedesktop/sample_object">
3644 <interface name="org.freedesktop.SampleInterface">
3645 <method name="Frobate">
3646 <arg name="foo" type="i" direction="in"/>
3647 <arg name="bar" type="s" direction="out"/>
3648 <arg name="baz" type="a{us}" direction="out"/>
3649 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3651 <method name="Bazify">
3652 <arg name="bar" type="(iiu)" direction="in"/>
3653 <arg name="bar" type="v" direction="out"/>
3655 <method name="Mogrify">
3656 <arg name="bar" type="(iiav)" direction="in"/>
3658 <signal name="Changed">
3659 <arg name="new_value" type="b"/>
3661 <property name="Bar" type="y" access="readwrite"/>
3663 <node name="child_of_sample_object"/>
3664 <node name="another_child_of_sample_object"/>
3669 A more formal DTD and spec needs writing, but here are some quick notes.
3673 Only the root <node> element can omit the node name, as it's
3674 known to be the object that was introspected. If the root
3675 <node> does have a name attribute, it must be an absolute
3676 object path. If child <node> have object paths, they must be
3682 If a child <node> has any sub-elements, then they
3683 must represent a complete introspection of the child.
3684 If a child <node> is empty, then it may or may
3685 not have sub-elements; the child must be introspected
3686 in order to find out. The intent is that if an object
3687 knows that its children are "fast" to introspect
3688 it can go ahead and return their information, but
3689 otherwise it can omit it.
3694 The direction element on <arg> may be omitted,
3695 in which case it defaults to "in" for method calls
3696 and "out" for signals. Signals only allow "out"
3697 so while direction may be specified, it's pointless.
3702 The possible directions are "in" and "out",
3703 unlike CORBA there is no "inout"
3708 The possible property access flags are
3709 "readwrite", "read", and "write"
3714 Multiple interfaces can of course be listed for
3720 The "name" attribute on arguments is optional.
3726 Method, interface, property, and signal elements may have
3727 "annotations", which are generic key/value pairs of metadata.
3728 They are similar conceptually to Java's annotations and C# attributes.
3729 Well-known annotations:
3736 <entry>Values (separated by ,)</entry>
3737 <entry>Description</entry>
3742 <entry>org.freedesktop.DBus.Deprecated</entry>
3743 <entry>true,false</entry>
3744 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3747 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3748 <entry>(string)</entry>
3749 <entry>The C symbol; may be used for methods and interfaces</entry>
3752 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3753 <entry>true,false</entry>
3754 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3757 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3758 <entry>true,invalidates,false</entry>
3761 If set to <literal>false</literal>, the
3762 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3764 linkend="standard-interfaces-properties"/> is not
3765 guaranteed to be emitted if the property changes.
3768 If set to <literal>invalidates</literal> the signal
3769 is emitted but the value is not included in the
3773 If set to <literal>true</literal> the signal is
3774 emitted with the value included.
3777 The value for the annotation defaults to
3778 <literal>true</literal> if the enclosing interface
3779 element does not specify the annotation. Otherwise it
3780 defaults to the value specified in the enclosing
3789 <sect1 id="message-bus">
3790 <title>Message Bus Specification</title>
3791 <sect2 id="message-bus-overview">
3792 <title>Message Bus Overview</title>
3794 The message bus accepts connections from one or more applications.
3795 Once connected, applications can exchange messages with other
3796 applications that are also connected to the bus.
3799 In order to route messages among connections, the message bus keeps a
3800 mapping from names to connections. Each connection has one
3801 unique-for-the-lifetime-of-the-bus name automatically assigned.
3802 Applications may request additional names for a connection. Additional
3803 names are usually "well-known names" such as
3804 "org.freedesktop.TextEditor". When a name is bound to a connection,
3805 that connection is said to <firstterm>own</firstterm> the name.
3808 The bus itself owns a special name,
3809 <literal>org.freedesktop.DBus</literal>, with an object
3810 located at <literal>/org/freedesktop/DBus</literal> that
3811 implements the <literal>org.freedesktop.DBus</literal>
3812 interface. This service allows applications to make
3813 administrative requests of the bus itself. For example,
3814 applications can ask the bus to assign a name to a connection.
3817 Each name may have <firstterm>queued owners</firstterm>. When an
3818 application requests a name for a connection and the name is already in
3819 use, the bus will optionally add the connection to a queue waiting for
3820 the name. If the current owner of the name disconnects or releases
3821 the name, the next connection in the queue will become the new owner.
3825 This feature causes the right thing to happen if you start two text
3826 editors for example; the first one may request "org.freedesktop.TextEditor",
3827 and the second will be queued as a possible owner of that name. When
3828 the first exits, the second will take over.
3832 Applications may send <firstterm>unicast messages</firstterm> to
3833 a specific recipient or to the message bus itself, or
3834 <firstterm>broadcast messages</firstterm> to all interested recipients.
3835 See <xref linkend="message-bus-routing"/> for details.
3839 <sect2 id="message-bus-names">
3840 <title>Message Bus Names</title>
3842 Each connection has at least one name, assigned at connection time and
3843 returned in response to the
3844 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3845 automatically-assigned name is called the connection's <firstterm>unique
3846 name</firstterm>. Unique names are never reused for two different
3847 connections to the same bus.
3850 Ownership of a unique name is a prerequisite for interaction with
3851 the message bus. It logically follows that the unique name is always
3852 the first name that an application comes to own, and the last
3853 one that it loses ownership of.
3856 Unique connection names must begin with the character ':' (ASCII colon
3857 character); bus names that are not unique names must not begin
3858 with this character. (The bus must reject any attempt by an application
3859 to manually request a name beginning with ':'.) This restriction
3860 categorically prevents "spoofing"; messages sent to a unique name
3861 will always go to the expected connection.
3864 When a connection is closed, all the names that it owns are deleted (or
3865 transferred to the next connection in the queue if any).
3868 A connection can request additional names to be associated with it using
3869 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3870 linkend="message-protocol-names-bus"/> describes the format of a valid
3871 name. These names can be released again using the
3872 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3875 <sect3 id="bus-messages-request-name">
3876 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3880 UINT32 RequestName (in STRING name, in UINT32 flags)
3887 <entry>Argument</entry>
3889 <entry>Description</entry>
3895 <entry>STRING</entry>
3896 <entry>Name to request</entry>
3900 <entry>UINT32</entry>
3901 <entry>Flags</entry>
3911 <entry>Argument</entry>
3913 <entry>Description</entry>
3919 <entry>UINT32</entry>
3920 <entry>Return value</entry>
3927 This method call should be sent to
3928 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3929 assign the given name to the method caller. Each name maintains a
3930 queue of possible owners, where the head of the queue is the primary
3931 or current owner of the name. Each potential owner in the queue
3932 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3933 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3934 call. When RequestName is invoked the following occurs:
3938 If the method caller is currently the primary owner of the name,
3939 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3940 values are updated with the values from the new RequestName call,
3941 and nothing further happens.
3947 If the current primary owner (head of the queue) has
3948 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3949 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3950 the caller of RequestName replaces the current primary owner at
3951 the head of the queue and the current primary owner moves to the
3952 second position in the queue. If the caller of RequestName was
3953 in the queue previously its flags are updated with the values from
3954 the new RequestName in addition to moving it to the head of the queue.
3960 If replacement is not possible, and the method caller is
3961 currently in the queue but not the primary owner, its flags are
3962 updated with the values from the new RequestName call.
3968 If replacement is not possible, and the method caller is
3969 currently not in the queue, the method caller is appended to the
3976 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3977 set and is not the primary owner, it is removed from the
3978 queue. This can apply to the previous primary owner (if it
3979 was replaced) or the method caller (if it updated the
3980 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3981 queue, or if it was just added to the queue with that flag set).
3987 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3988 queue," even if another application already in the queue had specified
3989 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3990 that does not allow replacement goes away, and the next primary owner
3991 does allow replacement. In this case, queued items that specified
3992 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3993 automatically replace the new primary owner. In other words,
3994 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3995 time RequestName is called. This is deliberate to avoid an infinite loop
3996 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3997 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4000 The flags argument contains any of the following values logically ORed
4007 <entry>Conventional Name</entry>
4008 <entry>Value</entry>
4009 <entry>Description</entry>
4014 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4018 If an application A specifies this flag and succeeds in
4019 becoming the owner of the name, and another application B
4020 later calls RequestName with the
4021 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4022 will lose ownership and receive a
4023 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4024 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4025 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4026 is not specified by application B, then application B will not replace
4027 application A as the owner.
4032 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4036 Try to replace the current owner if there is one. If this
4037 flag is not set the application will only become the owner of
4038 the name if there is no current owner. If this flag is set,
4039 the application will replace the current owner if
4040 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4045 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4049 Without this flag, if an application requests a name that is
4050 already owned, the application will be placed in a queue to
4051 own the name when the current owner gives it up. If this
4052 flag is given, the application will not be placed in the
4053 queue, the request for the name will simply fail. This flag
4054 also affects behavior when an application is replaced as
4055 name owner; by default the application moves back into the
4056 waiting queue, unless this flag was provided when the application
4057 became the name owner.
4065 The return code can be one of the following values:
4071 <entry>Conventional Name</entry>
4072 <entry>Value</entry>
4073 <entry>Description</entry>
4078 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4079 <entry>1</entry> <entry>The caller is now the primary owner of
4080 the name, replacing any previous owner. Either the name had no
4081 owner before, or the caller specified
4082 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4083 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4086 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4089 <entry>The name already had an owner,
4090 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4091 the current owner did not specify
4092 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4093 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4097 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4098 <entry>The name already has an owner,
4099 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4100 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4101 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4102 specified by the requesting application.</entry>
4105 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4107 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4115 <sect3 id="bus-messages-release-name">
4116 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4120 UINT32 ReleaseName (in STRING name)
4127 <entry>Argument</entry>
4129 <entry>Description</entry>
4135 <entry>STRING</entry>
4136 <entry>Name to release</entry>
4146 <entry>Argument</entry>
4148 <entry>Description</entry>
4154 <entry>UINT32</entry>
4155 <entry>Return value</entry>
4162 This method call should be sent to
4163 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4164 release the method caller's claim to the given name. If the caller is
4165 the primary owner, a new primary owner will be selected from the
4166 queue if any other owners are waiting. If the caller is waiting in
4167 the queue for the name, the caller will removed from the queue and
4168 will not be made an owner of the name if it later becomes available.
4169 If there are no other owners in the queue for the name, it will be
4170 removed from the bus entirely.
4172 The return code can be one of the following values:
4178 <entry>Conventional Name</entry>
4179 <entry>Value</entry>
4180 <entry>Description</entry>
4185 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4186 <entry>1</entry> <entry>The caller has released his claim on
4187 the given name. Either the caller was the primary owner of
4188 the name, and the name is now unused or taken by somebody
4189 waiting in the queue for the name, or the caller was waiting
4190 in the queue for the name and has now been removed from the
4194 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4196 <entry>The given name does not exist on this bus.</entry>
4199 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4201 <entry>The caller was not the primary owner of this name,
4202 and was also not waiting in the queue to own this name.</entry>
4210 <sect3 id="bus-messages-list-queued-owners">
4211 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4215 ARRAY of STRING ListQueuedOwners (in STRING name)
4222 <entry>Argument</entry>
4224 <entry>Description</entry>
4230 <entry>STRING</entry>
4231 <entry>The well-known bus name to query, such as
4232 <literal>com.example.cappuccino</literal></entry>
4242 <entry>Argument</entry>
4244 <entry>Description</entry>
4250 <entry>ARRAY of STRING</entry>
4251 <entry>The unique bus names of connections currently queued
4252 for the name</entry>
4259 This method call should be sent to
4260 <literal>org.freedesktop.DBus</literal> and lists the connections
4261 currently queued for a bus name (see
4262 <xref linkend="term-queued-owner"/>).
4267 <sect2 id="message-bus-routing">
4268 <title>Message Bus Message Routing</title>
4271 Messages may have a <literal>DESTINATION</literal> field (see <xref
4272 linkend="message-protocol-header-fields"/>), resulting in a
4273 <firstterm>unicast message</firstterm>. If the
4274 <literal>DESTINATION</literal> field is present, it specifies a message
4275 recipient by name. Method calls and replies normally specify this field.
4276 The message bus must send messages (of any type) with the
4277 <literal>DESTINATION</literal> field set to the specified recipient,
4278 regardless of whether the recipient has set up a match rule matching
4283 When the message bus receives a signal, if the
4284 <literal>DESTINATION</literal> field is absent, it is considered to
4285 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4286 applications with <firstterm>message matching rules</firstterm> that
4287 match the message. Most signal messages are broadcasts.
4291 Unicast signal messages (those with a <literal>DESTINATION</literal>
4292 field) are not commonly used, but they are treated like any unicast
4293 message: they are delivered to the specified receipient,
4294 regardless of its match rules. One use for unicast signals is to
4295 avoid a race condition in which a signal is emitted before the intended
4296 recipient can call <xref linkend="bus-messages-add-match"/> to
4297 receive that signal: if the signal is sent directly to that recipient
4298 using a unicast message, it does not need to add a match rule at all,
4299 and there is no race condition. Another use for unicast signals,
4300 on message buses whose security policy prevents eavesdropping, is to
4301 send sensitive information which should only be visible to one
4306 When the message bus receives a method call, if the
4307 <literal>DESTINATION</literal> field is absent, the call is taken to be
4308 a standard one-to-one message and interpreted by the message bus
4309 itself. For example, sending an
4310 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4311 <literal>DESTINATION</literal> will cause the message bus itself to
4312 reply to the ping immediately; the message bus will not make this
4313 message visible to other applications.
4317 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4318 the ping message were sent with a <literal>DESTINATION</literal> name of
4319 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4320 forwarded, and the Yoyodyne Corporation screensaver application would be
4321 expected to reply to the ping.
4325 Message bus implementations may impose a security policy which
4326 prevents certain messages from being sent or received.
4327 When a message cannot be sent or received due to a security
4328 policy, the message bus should send an error reply, unless the
4329 original message had the <literal>NO_REPLY</literal> flag.
4332 <sect3 id="message-bus-routing-eavesdropping">
4333 <title>Eavesdropping</title>
4335 Receiving a unicast message whose <literal>DESTINATION</literal>
4336 indicates a different recipient is called
4337 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4338 a security boundary (like the standard system bus), the security
4339 policy should usually prevent eavesdropping, since unicast messages
4340 are normally kept private and may contain security-sensitive
4345 Eavesdropping is mainly useful for debugging tools, such as
4346 the <literal>dbus-monitor</literal> tool in the reference
4347 implementation of D-Bus. Tools which eavesdrop on the message bus
4348 should be careful to avoid sending a reply or error in response to
4349 messages intended for a different client.
4353 Clients may attempt to eavesdrop by adding match rules
4354 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4355 the <literal>eavesdrop='true'</literal> match. If the message bus'
4356 security policy does not allow eavesdropping, the match rule can
4357 still be added, but will not have any practical effect. For
4358 compatibility with older message bus implementations, if adding such
4359 a match rule results in an error reply, the client may fall back to
4360 adding the same rule with the <literal>eavesdrop</literal> match
4365 <sect3 id="message-bus-routing-match-rules">
4366 <title>Match Rules</title>
4368 An important part of the message bus routing protocol is match
4369 rules. Match rules describe the messages that should be sent to a
4370 client, based on the contents of the message. Broadcast signals
4371 are only sent to clients which have a suitable match rule: this
4372 avoids waking up client processes to deal with signals that are
4373 not relevant to that client.
4376 Messages that list a client as their <literal>DESTINATION</literal>
4377 do not need to match the client's match rules, and are sent to that
4378 client regardless. As a result, match rules are mainly used to
4379 receive a subset of broadcast signals.
4382 Match rules can also be used for eavesdropping
4383 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4384 if the security policy of the message bus allows it.
4387 Match rules are added using the AddMatch bus method
4388 (see <xref linkend="bus-messages-add-match"/>). Rules are
4389 specified as a string of comma separated key/value pairs.
4390 Excluding a key from the rule indicates a wildcard match.
4391 For instance excluding the the member from a match rule but
4392 adding a sender would let all messages from that sender through.
4393 An example of a complete rule would be
4394 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4397 The following table describes the keys that can be used to create
4399 The following table summarizes the D-Bus types.
4405 <entry>Possible Values</entry>
4406 <entry>Description</entry>
4411 <entry><literal>type</literal></entry>
4412 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4413 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4416 <entry><literal>sender</literal></entry>
4417 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4418 and <xref linkend="term-unique-name"/> respectively)
4420 <entry>Match messages sent by a particular sender. An example of a sender match
4421 is sender='org.freedesktop.Hal'</entry>
4424 <entry><literal>interface</literal></entry>
4425 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4426 <entry>Match messages sent over or to a particular interface. An example of an
4427 interface match is interface='org.freedesktop.Hal.Manager'.
4428 If a message omits the interface header, it must not match any rule
4429 that specifies this key.</entry>
4432 <entry><literal>member</literal></entry>
4433 <entry>Any valid method or signal name</entry>
4434 <entry>Matches messages which have the give method or signal name. An example of
4435 a member match is member='NameOwnerChanged'</entry>
4438 <entry><literal>path</literal></entry>
4439 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4440 <entry>Matches messages which are sent from or to the given object. An example of a
4441 path match is path='/org/freedesktop/Hal/Manager'</entry>
4444 <entry><literal>path_namespace</literal></entry>
4445 <entry>An object path</entry>
4448 Matches messages which are sent from or to an
4449 object for which the object path is either the
4450 given value, or that value followed by one or
4451 more path components.
4456 <literal>path_namespace='/com/example/foo'</literal>
4457 would match signals sent by
4458 <literal>/com/example/foo</literal>
4460 <literal>/com/example/foo/bar</literal>,
4462 <literal>/com/example/foobar</literal>.
4466 Using both <literal>path</literal> and
4467 <literal>path_namespace</literal> in the same match
4468 rule is not allowed.
4473 This match key was added in version 0.16 of the
4474 D-Bus specification and implemented by the bus
4475 daemon in dbus 1.5.0 and later.
4481 <entry><literal>destination</literal></entry>
4482 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4483 <entry>Matches messages which are being sent to the given unique name. An
4484 example of a destination match is destination=':1.0'</entry>
4487 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4488 <entry>Any string</entry>
4489 <entry>Arg matches are special and are used for further restricting the
4490 match based on the arguments in the body of a message. Only arguments of type
4491 STRING can be matched in this way. An example of an argument match
4492 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4496 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4497 <entry>Any string</entry>
4499 <para>Argument path matches provide a specialised form of wildcard matching for
4500 path-like namespaces. They can match arguments whose type is either STRING or
4501 OBJECT_PATH. As with normal argument matches,
4502 if the argument is exactly equal to the string given in the match
4503 rule then the rule is satisfied. Additionally, there is also a
4504 match when either the string given in the match rule or the
4505 appropriate message argument ends with '/' and is a prefix of the
4506 other. An example argument path match is arg0path='/aa/bb/'. This
4507 would match messages with first arguments of '/', '/aa/',
4508 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4509 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4511 <para>This is intended for monitoring “directories” in file system-like
4512 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4513 system. An application interested in all nodes in a particular hierarchy would
4514 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4515 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4516 represent a modification to the “bar” property, or a signal with zeroth
4517 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4518 many properties within that directory, and the interested application would be
4519 notified in both cases.</para>
4522 This match key was added in version 0.12 of the
4523 D-Bus specification, implemented for STRING
4524 arguments by the bus daemon in dbus 1.2.0 and later,
4525 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4532 <entry><literal>arg0namespace</literal></entry>
4533 <entry>Like a bus name, except that the string is not
4534 required to contain a '.' (period)</entry>
4536 <para>Match messages whose first argument is of type STRING, and is a bus name
4537 or interface name within the specified namespace. This is primarily intended
4538 for watching name owner changes for a group of related bus names, rather than
4539 for a single name or all name changes.</para>
4541 <para>Because every valid interface name is also a valid
4542 bus name, this can also be used for messages whose
4543 first argument is an interface name.</para>
4545 <para>For example, the match rule
4546 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4547 matches name owner changes for bus names such as
4548 <literal>com.example.backend.foo</literal>,
4549 <literal>com.example.backend.foo.bar</literal>, and
4550 <literal>com.example.backend</literal> itself.</para>
4552 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4555 This match key was added in version 0.16 of the
4556 D-Bus specification and implemented by the bus
4557 daemon in dbus 1.5.0 and later.
4563 <entry><literal>eavesdrop</literal></entry>
4564 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4565 <entry>Since D-Bus 1.5.6, match rules do not
4566 match messages which have a <literal>DESTINATION</literal>
4567 field unless the match rule specifically
4569 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4570 by specifying <literal>eavesdrop='true'</literal>
4571 in the match rule. <literal>eavesdrop='false'</literal>
4572 restores the default behaviour. Messages are
4573 delivered to their <literal>DESTINATION</literal>
4574 regardless of match rules, so this match does not
4575 affect normal delivery of unicast messages.
4576 If the message bus has a security policy which forbids
4577 eavesdropping, this match may still be used without error,
4578 but will not have any practical effect.
4579 In older versions of D-Bus, this match was not allowed
4580 in match rules, and all match rules behaved as if
4581 <literal>eavesdrop='true'</literal> had been used.
4590 <sect2 id="message-bus-starting-services">
4591 <title>Message Bus Starting Services</title>
4593 The message bus can start applications on behalf of other applications.
4594 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4595 An application that can be started in this way is called a
4596 <firstterm>service</firstterm>.
4599 With D-Bus, starting a service is normally done by name. That is,
4600 applications ask the message bus to start some program that will own a
4601 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4602 This implies a contract documented along with the name
4603 <literal>org.freedesktop.TextEditor</literal> for which objects
4604 the owner of that name will provide, and what interfaces those
4608 To find an executable corresponding to a particular name, the bus daemon
4609 looks for <firstterm>service description files</firstterm>. Service
4610 description files define a mapping from names to executables. Different
4611 kinds of message bus will look for these files in different places, see
4612 <xref linkend="message-bus-types"/>.
4615 Service description files have the ".service" file
4616 extension. The message bus will only load service description files
4617 ending with .service; all other files will be ignored. The file format
4618 is similar to that of <ulink
4619 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4620 entries</ulink>. All service description files must be in UTF-8
4621 encoding. To ensure that there will be no name collisions, service files
4622 must be namespaced using the same mechanism as messages and service
4627 [FIXME the file format should be much better specified than "similar to
4628 .desktop entries" esp. since desktop entries are already
4629 badly-specified. ;-)]
4630 These sections from the specification apply to service files as well:
4633 <listitem><para>General syntax</para></listitem>
4634 <listitem><para>Comment format</para></listitem>
4638 <title>Example service description file</title>
4640 # Sample service description file
4642 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4643 Exec=/usr/libexec/gconfd-2
4648 When an application asks to start a service by name, the bus daemon tries to
4649 find a service that will own that name. It then tries to spawn the
4650 executable associated with it. If this fails, it will report an
4651 error. [FIXME what happens if two .service files offer the same service;
4652 what kind of error is reported, should we have a way for the client to
4656 The executable launched will have the environment variable
4657 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4658 message bus so it can connect and request the appropriate names.
4661 The executable being launched may want to know whether the message bus
4662 starting it is one of the well-known message buses (see <xref
4663 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4664 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4665 of the well-known buses. The currently-defined values for this variable
4666 are <literal>system</literal> for the systemwide message bus,
4667 and <literal>session</literal> for the per-login-session message
4668 bus. The new executable must still connect to the address given
4669 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4670 resulting connection is to the well-known bus.
4673 [FIXME there should be a timeout somewhere, either specified
4674 in the .service file, by the client, or just a global value
4675 and if the client being activated fails to connect within that
4676 timeout, an error should be sent back.]
4679 <sect3 id="message-bus-starting-services-scope">
4680 <title>Message Bus Service Scope</title>
4682 The "scope" of a service is its "per-", such as per-session,
4683 per-machine, per-home-directory, or per-display. The reference
4684 implementation doesn't yet support starting services in a different
4685 scope from the message bus itself. So e.g. if you start a service
4686 on the session bus its scope is per-session.
4689 We could add an optional scope to a bus name. For example, for
4690 per-(display,session pair), we could have a unique ID for each display
4691 generated automatically at login and set on screen 0 by executing a
4692 special "set display ID" binary. The ID would be stored in a
4693 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4694 random bytes. This ID would then be used to scope names.
4695 Starting/locating a service could be done by ID-name pair rather than
4699 Contrast this with a per-display scope. To achieve that, we would
4700 want a single bus spanning all sessions using a given display.
4701 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4702 property on screen 0 of the display, pointing to this bus.
4707 <sect2 id="message-bus-types">
4708 <title>Well-known Message Bus Instances</title>
4710 Two standard message bus instances are defined here, along with how
4711 to locate them and where their service files live.
4713 <sect3 id="message-bus-types-login">
4714 <title>Login session message bus</title>
4716 Each time a user logs in, a <firstterm>login session message
4717 bus</firstterm> may be started. All applications in the user's login
4718 session may interact with one another using this message bus.
4721 The address of the login session message bus is given
4722 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4723 variable. If that variable is not set, applications may
4724 also try to read the address from the X Window System root
4725 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4726 The root window property must have type <literal>STRING</literal>.
4727 The environment variable should have precedence over the
4728 root window property.
4730 <para>The address of the login session message bus is given in the
4731 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4732 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4733 "autolaunch:", the system should use platform-specific methods of
4734 locating a running D-Bus session server, or starting one if a running
4735 instance cannot be found. Note that this mechanism is not recommended
4736 for attempting to determine if a daemon is running. It is inherently
4737 racy to attempt to make this determination, since the bus daemon may
4738 be started just before or just after the determination is made.
4739 Therefore, it is recommended that applications do not try to make this
4740 determination for their functionality purposes, and instead they
4741 should attempt to start the server.</para>
4743 <sect4 id="message-bus-types-login-x-windows">
4744 <title>X Windowing System</title>
4746 For the X Windowing System, the application must locate the
4747 window owner of the selection represented by the atom formed by
4751 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4755 <para>the current user's username</para>
4759 <para>the literal character '_' (underscore)</para>
4763 <para>the machine's ID</para>
4769 The following properties are defined for the window that owns
4771 <informaltable frame="all">
4780 <para>meaning</para>
4786 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4790 <para>the actual address of the server socket</para>
4796 <para>_DBUS_SESSION_BUS_PID</para>
4800 <para>the PID of the server process</para>
4809 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4810 present in this window.
4814 If the X selection cannot be located or if reading the
4815 properties from the window fails, the implementation MUST conclude
4816 that there is no D-Bus server running and proceed to start a new
4817 server. (See below on concurrency issues)
4821 Failure to connect to the D-Bus server address thus obtained
4822 MUST be treated as a fatal connection error and should be reported
4827 As an alternative, an implementation MAY find the information
4828 in the following file located in the current user's home directory,
4829 in subdirectory .dbus/session-bus/:
4832 <para>the machine's ID</para>
4836 <para>the literal character '-' (dash)</para>
4840 <para>the X display without the screen number, with the
4841 following prefixes removed, if present: ":", "localhost:"
4842 ."localhost.localdomain:". That is, a display of
4843 "localhost:10.0" produces just the number "10"</para>
4849 The contents of this file NAME=value assignment pairs and
4850 lines starting with # are comments (no comments are allowed
4851 otherwise). The following variable names are defined:
4858 <para>Variable</para>
4862 <para>meaning</para>
4868 <para>DBUS_SESSION_BUS_ADDRESS</para>
4872 <para>the actual address of the server socket</para>
4878 <para>DBUS_SESSION_BUS_PID</para>
4882 <para>the PID of the server process</para>
4888 <para>DBUS_SESSION_BUS_WINDOWID</para>
4892 <para>the window ID</para>
4901 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4906 Failure to open this file MUST be interpreted as absence of a
4907 running server. Therefore, the implementation MUST proceed to
4908 attempting to launch a new bus server if the file cannot be
4913 However, success in opening this file MUST NOT lead to the
4914 conclusion that the server is running. Thus, a failure to connect to
4915 the bus address obtained by the alternative method MUST NOT be
4916 considered a fatal error. If the connection cannot be established,
4917 the implementation MUST proceed to check the X selection settings or
4918 to start the server on its own.
4922 If the implementation concludes that the D-Bus server is not
4923 running it MUST attempt to start a new server and it MUST also
4924 ensure that the daemon started as an effect of the "autolaunch"
4925 mechanism provides the lookup mechanisms described above, so
4926 subsequent calls can locate the newly started server. The
4927 implementation MUST also ensure that if two or more concurrent
4928 initiations happen, only one server remains running and all other
4929 initiations are able to obtain the address of this server and
4930 connect to it. In other words, the implementation MUST ensure that
4931 the X selection is not present when it attempts to set it, without
4932 allowing another process to set the selection between the
4933 verification and the setting (e.g., by using XGrabServer /
4940 On Unix systems, the session bus should search for .service files
4941 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4943 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4944 Implementations may also search additional locations, which
4945 should be searched with lower priority than anything in
4946 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4947 for example, the reference implementation also
4948 looks in <literal>${datadir}/dbus-1/services</literal> as
4949 set at compile time.
4952 As described in the XDG Base Directory Specification, software
4953 packages should install their session .service files to their
4954 configured <literal>${datadir}/dbus-1/services</literal>,
4955 where <literal>${datadir}</literal> is as defined by the GNU
4956 coding standards. System administrators or users can arrange
4957 for these service files to be read by setting XDG_DATA_DIRS or by
4958 symlinking them into the default locations.
4962 <sect3 id="message-bus-types-system">
4963 <title>System message bus</title>
4965 A computer may have a <firstterm>system message bus</firstterm>,
4966 accessible to all applications on the system. This message bus may be
4967 used to broadcast system events, such as adding new hardware devices,
4968 changes in the printer queue, and so forth.
4971 The address of the system message bus is given
4972 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4973 variable. If that variable is not set, applications should try
4974 to connect to the well-known address
4975 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4978 The D-Bus reference implementation actually honors the
4979 <literal>$(localstatedir)</literal> configure option
4980 for this address, on both client and server side.
4985 On Unix systems, the system bus should default to searching
4986 for .service files in
4987 <literal>/usr/local/share/dbus-1/system-services</literal>,
4988 <literal>/usr/share/dbus-1/system-services</literal> and
4989 <literal>/lib/dbus-1/system-services</literal>, with that order
4990 of precedence. It may also search other implementation-specific
4991 locations, but should not vary these locations based on environment
4995 The system bus is security-sensitive and is typically executed
4996 by an init system with a clean environment. Its launch helper
4997 process is particularly security-sensitive, and specifically
4998 clears its own environment.
5003 Software packages should install their system .service
5004 files to their configured
5005 <literal>${datadir}/dbus-1/system-services</literal>,
5006 where <literal>${datadir}</literal> is as defined by the GNU
5007 coding standards. System administrators can arrange
5008 for these service files to be read by editing the system bus'
5009 configuration file or by symlinking them into the default
5015 <sect2 id="message-bus-messages">
5016 <title>Message Bus Messages</title>
5018 The special message bus name <literal>org.freedesktop.DBus</literal>
5019 responds to a number of additional messages.
5022 <sect3 id="bus-messages-hello">
5023 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5034 <entry>Argument</entry>
5036 <entry>Description</entry>
5042 <entry>STRING</entry>
5043 <entry>Unique name assigned to the connection</entry>
5050 Before an application is able to send messages to other applications
5051 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5052 to the message bus to obtain a unique name. If an application without
5053 a unique name tries to send a message to another application, or a
5054 message to the message bus itself that isn't the
5055 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5056 disconnected from the bus.
5059 There is no corresponding "disconnect" request; if a client wishes to
5060 disconnect from the bus, it simply closes the socket (or other
5061 communication channel).
5064 <sect3 id="bus-messages-list-names">
5065 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5069 ARRAY of STRING ListNames ()
5076 <entry>Argument</entry>
5078 <entry>Description</entry>
5084 <entry>ARRAY of STRING</entry>
5085 <entry>Array of strings where each string is a bus name</entry>
5092 Returns a list of all currently-owned names on the bus.
5095 <sect3 id="bus-messages-list-activatable-names">
5096 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5100 ARRAY of STRING ListActivatableNames ()
5107 <entry>Argument</entry>
5109 <entry>Description</entry>
5115 <entry>ARRAY of STRING</entry>
5116 <entry>Array of strings where each string is a bus name</entry>
5123 Returns a list of all names that can be activated on the bus.
5126 <sect3 id="bus-messages-name-exists">
5127 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5131 BOOLEAN NameHasOwner (in STRING name)
5138 <entry>Argument</entry>
5140 <entry>Description</entry>
5146 <entry>STRING</entry>
5147 <entry>Name to check</entry>
5157 <entry>Argument</entry>
5159 <entry>Description</entry>
5165 <entry>BOOLEAN</entry>
5166 <entry>Return value, true if the name exists</entry>
5173 Checks if the specified name exists (currently has an owner).
5177 <sect3 id="bus-messages-name-owner-changed">
5178 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5182 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5189 <entry>Argument</entry>
5191 <entry>Description</entry>
5197 <entry>STRING</entry>
5198 <entry>Name with a new owner</entry>
5202 <entry>STRING</entry>
5203 <entry>Old owner or empty string if none</entry>
5207 <entry>STRING</entry>
5208 <entry>New owner or empty string if none</entry>
5215 This signal indicates that the owner of a name has changed.
5216 It's also the signal to use to detect the appearance of
5217 new names on the bus.
5220 <sect3 id="bus-messages-name-lost">
5221 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5225 NameLost (STRING name)
5232 <entry>Argument</entry>
5234 <entry>Description</entry>
5240 <entry>STRING</entry>
5241 <entry>Name which was lost</entry>
5248 This signal is sent to a specific application when it loses
5249 ownership of a name.
5253 <sect3 id="bus-messages-name-acquired">
5254 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5258 NameAcquired (STRING name)
5265 <entry>Argument</entry>
5267 <entry>Description</entry>
5273 <entry>STRING</entry>
5274 <entry>Name which was acquired</entry>
5281 This signal is sent to a specific application when it gains
5282 ownership of a name.
5286 <sect3 id="bus-messages-start-service-by-name">
5287 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5291 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5298 <entry>Argument</entry>
5300 <entry>Description</entry>
5306 <entry>STRING</entry>
5307 <entry>Name of the service to start</entry>
5311 <entry>UINT32</entry>
5312 <entry>Flags (currently not used)</entry>
5322 <entry>Argument</entry>
5324 <entry>Description</entry>
5330 <entry>UINT32</entry>
5331 <entry>Return value</entry>
5336 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5340 The return value can be one of the following values:
5345 <entry>Identifier</entry>
5346 <entry>Value</entry>
5347 <entry>Description</entry>
5352 <entry>DBUS_START_REPLY_SUCCESS</entry>
5354 <entry>The service was successfully started.</entry>
5357 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5359 <entry>A connection already owns the given name.</entry>
5368 <sect3 id="bus-messages-update-activation-environment">
5369 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5373 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5380 <entry>Argument</entry>
5382 <entry>Description</entry>
5388 <entry>ARRAY of DICT<STRING,STRING></entry>
5389 <entry>Environment to add or update</entry>
5394 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5397 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5400 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.
5405 <sect3 id="bus-messages-get-name-owner">
5406 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5410 STRING GetNameOwner (in STRING name)
5417 <entry>Argument</entry>
5419 <entry>Description</entry>
5425 <entry>STRING</entry>
5426 <entry>Name to get the owner of</entry>
5436 <entry>Argument</entry>
5438 <entry>Description</entry>
5444 <entry>STRING</entry>
5445 <entry>Return value, a unique connection name</entry>
5450 Returns the unique connection name of the primary owner of the name
5451 given. If the requested name doesn't have an owner, returns a
5452 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5456 <sect3 id="bus-messages-get-connection-unix-user">
5457 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5461 UINT32 GetConnectionUnixUser (in STRING bus_name)
5468 <entry>Argument</entry>
5470 <entry>Description</entry>
5476 <entry>STRING</entry>
5477 <entry>Unique or well-known bus name of the connection to
5478 query, such as <literal>:12.34</literal> or
5479 <literal>com.example.tea</literal></entry>
5489 <entry>Argument</entry>
5491 <entry>Description</entry>
5497 <entry>UINT32</entry>
5498 <entry>Unix user ID</entry>
5503 Returns the Unix user ID of the process connected to the server. If
5504 unable to determine it (for instance, because the process is not on the
5505 same machine as the bus daemon), an error is returned.
5509 <sect3 id="bus-messages-get-connection-unix-process-id">
5510 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5514 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5521 <entry>Argument</entry>
5523 <entry>Description</entry>
5529 <entry>STRING</entry>
5530 <entry>Unique or well-known bus name of the connection to
5531 query, such as <literal>:12.34</literal> or
5532 <literal>com.example.tea</literal></entry>
5542 <entry>Argument</entry>
5544 <entry>Description</entry>
5550 <entry>UINT32</entry>
5551 <entry>Unix process id</entry>
5556 Returns the Unix process ID of the process connected to the server. If
5557 unable to determine it (for instance, because the process is not on the
5558 same machine as the bus daemon), an error is returned.
5562 <sect3 id="bus-messages-add-match">
5563 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5567 AddMatch (in STRING rule)
5574 <entry>Argument</entry>
5576 <entry>Description</entry>
5582 <entry>STRING</entry>
5583 <entry>Match rule to add to the connection</entry>
5588 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5589 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5593 <sect3 id="bus-messages-remove-match">
5594 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5598 RemoveMatch (in STRING rule)
5605 <entry>Argument</entry>
5607 <entry>Description</entry>
5613 <entry>STRING</entry>
5614 <entry>Match rule to remove from the connection</entry>
5619 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5620 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5625 <sect3 id="bus-messages-get-id">
5626 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5630 GetId (out STRING id)
5637 <entry>Argument</entry>
5639 <entry>Description</entry>
5645 <entry>STRING</entry>
5646 <entry>Unique ID identifying the bus daemon</entry>
5651 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5652 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5653 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5654 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5655 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5656 by org.freedesktop.DBus.Peer.GetMachineId().
5657 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5665 <appendix id="implementation-notes">
5666 <title>Implementation notes</title>
5667 <sect1 id="implementation-notes-subsection">
5675 <glossary><title>Glossary</title>
5677 This glossary defines some of the terms used in this specification.
5680 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5683 The message bus maintains an association between names and
5684 connections. (Normally, there's one connection per application.) A
5685 bus name is simply an identifier used to locate connections. For
5686 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5687 name might be used to send a message to a screensaver from Yoyodyne
5688 Corporation. An application is said to <firstterm>own</firstterm> a
5689 name if the message bus has associated the application's connection
5690 with the name. Names may also have <firstterm>queued
5691 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5692 The bus assigns a unique name to each connection,
5693 see <xref linkend="term-unique-name"/>. Other names
5694 can be thought of as "well-known names" and are
5695 used to find applications that offer specific functionality.
5699 See <xref linkend="message-protocol-names-bus"/> for details of
5700 the syntax and naming conventions for bus names.
5705 <glossentry id="term-message"><glossterm>Message</glossterm>
5708 A message is the atomic unit of communication via the D-Bus
5709 protocol. It consists of a <firstterm>header</firstterm> and a
5710 <firstterm>body</firstterm>; the body is made up of
5711 <firstterm>arguments</firstterm>.
5716 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5719 The message bus is a special application that forwards
5720 or routes messages between a group of applications
5721 connected to the message bus. It also manages
5722 <firstterm>names</firstterm> used for routing
5728 <glossentry id="term-name"><glossterm>Name</glossterm>
5731 See <xref linkend="term-bus-name"/>. "Name" may
5732 also be used to refer to some of the other names
5733 in D-Bus, such as interface names.
5738 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5741 Used to prevent collisions when defining new interfaces, bus names
5742 etc. The convention used is the same one Java uses for defining
5743 classes: a reversed domain name.
5744 See <xref linkend="message-protocol-names-bus"/>,
5745 <xref linkend="message-protocol-names-interface"/>,
5746 <xref linkend="message-protocol-names-error"/>,
5747 <xref linkend="message-protocol-marshaling-object-path"/>.
5752 <glossentry id="term-object"><glossterm>Object</glossterm>
5755 Each application contains <firstterm>objects</firstterm>, which have
5756 <firstterm>interfaces</firstterm> and
5757 <firstterm>methods</firstterm>. Objects are referred to by a name,
5758 called a <firstterm>path</firstterm>.
5763 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5766 An application talking directly to another application, without going
5767 through a message bus. One-to-one connections may be "peer to peer" or
5768 "client to server." The D-Bus protocol has no concept of client
5769 vs. server after a connection has authenticated; the flow of messages
5770 is symmetrical (full duplex).
5775 <glossentry id="term-path"><glossterm>Path</glossterm>
5778 Object references (object names) in D-Bus are organized into a
5779 filesystem-style hierarchy, so each object is named by a path. As in
5780 LDAP, there's no difference between "files" and "directories"; a path
5781 can refer to an object, while still having child objects below it.
5786 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5789 Each bus name has a primary owner; messages sent to the name go to the
5790 primary owner. However, certain names also maintain a queue of
5791 secondary owners "waiting in the wings." If the primary owner releases
5792 the name, then the first secondary owner in the queue automatically
5793 becomes the new owner of the name.
5798 <glossentry id="term-service"><glossterm>Service</glossterm>
5801 A service is an executable that can be launched by the bus daemon.
5802 Services normally guarantee some particular features, for example they
5803 may guarantee that they will request a specific name such as
5804 "org.freedesktop.Screensaver", have a singleton object
5805 "/org/freedesktop/Application", and that object will implement the
5806 interface "org.freedesktop.ScreensaverControl".
5811 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5814 ".service files" tell the bus about service applications that can be
5815 launched (see <xref linkend="term-service"/>). Most importantly they
5816 provide a mapping from bus names to services that will request those
5817 names when they start up.
5822 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5825 The special name automatically assigned to each connection by the
5826 message bus. This name will never change owner, and will be unique
5827 (never reused during the lifetime of the message bus).
5828 It will begin with a ':' character.
projects / platform / upstream / dbus.git / blob