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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.23</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>0.23</revnumber>
76 <date>not yet released</date>
77 <authorinitials></authorinitials>
79 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
83 <revnumber>0.22</revnumber>
84 <date>2013-10-09</date>
85 <authorinitials></authorinitials>
86 <revremark>add GetConnectionCredentials, document
87 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
88 document and correct .service file syntax and naming
92 <revnumber>0.21</revnumber>
93 <date>2013-04-25</date>
94 <authorinitials>smcv</authorinitials>
95 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
96 Corrigendum #9)</revremark>
99 <revnumber>0.20</revnumber>
100 <date>22 February 2013</date>
101 <authorinitials>smcv, walters</authorinitials>
102 <revremark>reorganise for clarity, remove false claims about
103 basic types, mention /o/fd/DBus</revremark>
106 <revnumber>0.19</revnumber>
107 <date>20 February 2012</date>
108 <authorinitials>smcv/lp</authorinitials>
109 <revremark>formally define unique connection names and well-known
110 bus names; document best practices for interface, bus, member and
111 error names, and object paths; document the search path for session
112 and system services on Unix; document the systemd transport</revremark>
115 <revnumber>0.18</revnumber>
116 <date>29 July 2011</date>
117 <authorinitials>smcv</authorinitials>
118 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
119 match keyword; promote type system to a top-level section</revremark>
122 <revnumber>0.17</revnumber>
123 <date>1 June 2011</date>
124 <authorinitials>smcv/davidz</authorinitials>
125 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
126 by GVariant</revremark>
129 <revnumber>0.16</revnumber>
130 <date>11 April 2011</date>
131 <authorinitials></authorinitials>
132 <revremark>add path_namespace, arg0namespace; argNpath matches object
136 <revnumber>0.15</revnumber>
137 <date>3 November 2010</date>
138 <authorinitials></authorinitials>
139 <revremark></revremark>
142 <revnumber>0.14</revnumber>
143 <date>12 May 2010</date>
144 <authorinitials></authorinitials>
145 <revremark></revremark>
148 <revnumber>0.13</revnumber>
149 <date>23 Dezember 2009</date>
150 <authorinitials></authorinitials>
151 <revremark></revremark>
154 <revnumber>0.12</revnumber>
155 <date>7 November, 2006</date>
156 <authorinitials></authorinitials>
157 <revremark></revremark>
160 <revnumber>0.11</revnumber>
161 <date>6 February 2005</date>
162 <authorinitials></authorinitials>
163 <revremark></revremark>
166 <revnumber>0.10</revnumber>
167 <date>28 January 2005</date>
168 <authorinitials></authorinitials>
169 <revremark></revremark>
172 <revnumber>0.9</revnumber>
173 <date>7 Januar 2005</date>
174 <authorinitials></authorinitials>
175 <revremark></revremark>
178 <revnumber>0.8</revnumber>
179 <date>06 September 2003</date>
180 <authorinitials></authorinitials>
181 <revremark>First released document.</revremark>
186 <sect1 id="introduction">
187 <title>Introduction</title>
189 D-Bus is a system for low-overhead, easy to use
190 interprocess communication (IPC). In more detail:
194 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
195 binary protocol, and does not have to convert to and from a text
196 format such as XML. Because D-Bus is intended for potentially
197 high-resolution same-machine IPC, not primarily for Internet IPC,
198 this is an interesting optimization. D-Bus is also designed to
199 avoid round trips and allow asynchronous operation, much like
205 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
206 of <firstterm>messages</firstterm> rather than byte streams, and
207 automatically handles a lot of the hard IPC issues. Also, the D-Bus
208 library is designed to be wrapped in a way that lets developers use
209 their framework's existing object/type system, rather than learning
210 a new one specifically for IPC.
217 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
218 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
219 a system for one application to talk to a single other
220 application. However, the primary intended application of the protocol is the
221 D-Bus <firstterm>message bus</firstterm>, specified in <xref
222 linkend="message-bus"/>. The message bus is a special application that
223 accepts connections from multiple other applications, and forwards
228 Uses of D-Bus include notification of system changes (notification of when
229 a camera is plugged in to a computer, or a new version of some software
230 has been installed), or desktop interoperability, for example a file
231 monitoring service or a configuration service.
235 D-Bus is designed for two specific use cases:
239 A "system bus" for notifications from the system to user sessions,
240 and to allow the system to request input from user sessions.
245 A "session bus" used to implement desktop environments such as
250 D-Bus is not intended to be a generic IPC system for any possible
251 application, and intentionally omits many features found in other
252 IPC systems for this reason.
256 At the same time, the bus daemons offer a number of features not found in
257 other IPC systems, such as single-owner "bus names" (similar to X
258 selections), on-demand startup of services, and security policies.
259 In many ways, these features are the primary motivation for developing
260 D-Bus; other systems would have sufficed if IPC were the only goal.
264 D-Bus may turn out to be useful in unanticipated applications, but future
265 versions of this spec and the reference implementation probably will not
266 incorporate features that interfere with the core use cases.
270 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
271 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
272 document are to be interpreted as described in RFC 2119. However, the
273 document could use a serious audit to be sure it makes sense to do
274 so. Also, they are not capitalized.
277 <sect2 id="stability">
278 <title>Protocol and Specification Stability</title>
280 The D-Bus protocol is frozen (only compatible extensions are allowed) as
281 of November 8, 2006. However, this specification could still use a fair
282 bit of work to make interoperable reimplementation possible without
283 reference to the D-Bus reference implementation. Thus, this
284 specification is not marked 1.0. To mark it 1.0, we'd like to see
285 someone invest significant effort in clarifying the specification
286 language, and growing the specification to cover more aspects of the
287 reference implementation's behavior.
290 Until this work is complete, any attempt to reimplement D-Bus will
291 probably require looking at the reference implementation and/or asking
292 questions on the D-Bus mailing list about intended behavior.
293 Questions on the list are very welcome.
296 Nonetheless, this document should be a useful starting point and is
297 to our knowledge accurate, though incomplete.
303 <sect1 id="type-system">
304 <title>Type System</title>
307 D-Bus has a type system, in which values of various types can be
308 serialized into a sequence of bytes referred to as the
309 <firstterm>wire format</firstterm> in a standard way.
310 Converting a value from some other representation into the wire
311 format is called <firstterm>marshaling</firstterm> and converting
312 it back from the wire format is <firstterm>unmarshaling</firstterm>.
316 The D-Bus protocol does not include type tags in the marshaled data; a
317 block of marshaled values must have a known <firstterm>type
318 signature</firstterm>. The type signature is made up of zero or more
319 <firstterm id="term-single-complete-type">single complete
320 types</firstterm>, each made up of one or more
321 <firstterm>type codes</firstterm>.
325 A type code is an ASCII character representing the
326 type of a value. Because ASCII characters are used, the type signature
327 will always form a valid ASCII string. A simple string compare
328 determines whether two type signatures are equivalent.
332 A single complete type is a sequence of type codes that fully describes
333 one type: either a basic type, or a single fully-described container type.
334 A single complete type is a basic type code, a variant type code,
335 an array with its element type, or a struct with its fields (all of which
336 are defined below). So the following signatures are not single complete
347 And the following signatures contain multiple complete types:
357 Note however that a single complete type may <emphasis>contain</emphasis>
358 multiple other single complete types, by containing a struct or dict
362 <sect2 id="basic-types">
363 <title>Basic types</title>
366 The simplest type codes are the <firstterm id="term-basic-type">basic
367 types</firstterm>, which are the types whose structure is entirely
368 defined by their 1-character type code. Basic types consist of
369 fixed types and string-like types.
373 The <firstterm id="term-fixed-type">fixed types</firstterm>
374 are basic types whose values have a fixed length, namely BYTE,
375 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
380 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
381 the ASCII character 'i'. So the signature for a block of values
382 containing a single <literal>INT32</literal> would be:
386 A block of values containing two <literal>INT32</literal> would have this signature:
393 The characteristics of the fixed types are listed in this table.
399 <entry>Conventional name</entry>
400 <entry>ASCII type-code</entry>
401 <entry>Encoding</entry>
406 <entry><literal>BYTE</literal></entry>
407 <entry><literal>y</literal> (121)</entry>
408 <entry>Unsigned 8-bit integer</entry>
411 <entry><literal>BOOLEAN</literal></entry>
412 <entry><literal>b</literal> (98)</entry>
413 <entry>Boolean value: 0 is false, 1 is true, any other value
414 allowed by the marshalling format is invalid</entry>
417 <entry><literal>INT16</literal></entry>
418 <entry><literal>n</literal> (110)</entry>
419 <entry>Signed (two's complement) 16-bit integer</entry>
422 <entry><literal>UINT16</literal></entry>
423 <entry><literal>q</literal> (113)</entry>
424 <entry>Unsigned 16-bit integer</entry>
427 <entry><literal>INT32</literal></entry>
428 <entry><literal>i</literal> (105)</entry>
429 <entry>Signed (two's complement) 32-bit integer</entry>
432 <entry><literal>UINT32</literal></entry>
433 <entry><literal>u</literal> (117)</entry>
434 <entry>Unsigned 32-bit integer</entry>
437 <entry><literal>INT64</literal></entry>
438 <entry><literal>x</literal> (120)</entry>
439 <entry>Signed (two's complement) 64-bit integer
440 (mnemonic: x and t are the first characters in "sixty" not
441 already used for something more common)</entry>
444 <entry><literal>UINT64</literal></entry>
445 <entry><literal>t</literal> (116)</entry>
446 <entry>Unsigned 64-bit integer</entry>
449 <entry><literal>DOUBLE</literal></entry>
450 <entry><literal>d</literal> (100)</entry>
451 <entry>IEEE 754 double-precision floating point</entry>
454 <entry><literal>UNIX_FD</literal></entry>
455 <entry><literal>h</literal> (104)</entry>
456 <entry>Unsigned 32-bit integer representing an index into an
457 out-of-band array of file descriptors, transferred via some
458 platform-specific mechanism (mnemonic: h for handle)</entry>
466 The <firstterm id="term-string-like-type">string-like types</firstterm>
467 are basic types with a variable length. The value of any string-like
468 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
469 none of which may be U+0000. The UTF-8 text must be validated
470 strictly: in particular, it must not contain overlong sequences
471 or codepoints above U+10FFFF.
475 Since D-Bus Specification version 0.21, in accordance with Unicode
476 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
477 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
478 D-Bus rejected these noncharacters).
482 The marshalling formats for the string-like types all end with a
483 single zero (NUL) byte, but that byte is not considered to be part of
488 The characteristics of the string-like types are listed in this table.
494 <entry>Conventional name</entry>
495 <entry>ASCII type-code</entry>
496 <entry>Validity constraints</entry>
501 <entry><literal>STRING</literal></entry>
502 <entry><literal>s</literal> (115)</entry>
503 <entry>No extra constraints</entry>
506 <entry><literal>OBJECT_PATH</literal></entry>
507 <entry><literal>o</literal> (111)</entry>
509 <link linkend="message-protocol-marshaling-object-path">a
510 syntactically valid object path</link></entry>
513 <entry><literal>SIGNATURE</literal></entry>
514 <entry><literal>g</literal> (103)</entry>
516 <firstterm linkend="term-single-complete-type">single
517 complete types</firstterm></entry>
524 <sect3 id="message-protocol-marshaling-object-path">
525 <title>Valid Object Paths</title>
528 An object path is a name used to refer to an object instance.
529 Conceptually, each participant in a D-Bus message exchange may have
530 any number of object instances (think of C++ or Java objects) and each
531 such instance will have a path. Like a filesystem, the object
532 instances in an application form a hierarchical tree.
536 Object paths are often namespaced by starting with a reversed
537 domain name and containing an interface version number, in the
539 <link linkend="message-protocol-names-interface">interface
541 <link linkend="message-protocol-names-bus">well-known
543 This makes it possible to implement more than one service, or
544 more than one version of a service, in the same process,
545 even if the services share a connection but cannot otherwise
546 co-operate (for instance, if they are implemented by different
551 For instance, if the owner of <literal>example.com</literal> is
552 developing a D-Bus API for a music player, they might use the
553 hierarchy of object paths that start with
554 <literal>/com/example/MusicPlayer1</literal> for its objects.
558 The following rules define a valid object path. Implementations must
559 not send or accept messages with invalid object paths.
563 The path may be of any length.
568 The path must begin with an ASCII '/' (integer 47) character,
569 and must consist of elements separated by slash characters.
574 Each element must only contain the ASCII characters
580 No element may be the empty string.
585 Multiple '/' characters cannot occur in sequence.
590 A trailing '/' character is not allowed unless the
591 path is the root path (a single '/' character).
599 <sect3 id="message-protocol-marshaling-signature">
600 <title>Valid Signatures</title>
602 An implementation must not send or accept invalid signatures.
603 Valid signatures will conform to the following rules:
607 The signature is a list of single complete types.
608 Arrays must have element types, and structs must
609 have both open and close parentheses.
614 Only type codes, open and close parentheses, and open and
615 close curly brackets are allowed in the signature. The
616 <literal>STRUCT</literal> type code
617 is not allowed in signatures, because parentheses
618 are used instead. Similarly, the
619 <literal>DICT_ENTRY</literal> type code is not allowed in
620 signatures, because curly brackets are used instead.
625 The maximum depth of container type nesting is 32 array type
626 codes and 32 open parentheses. This implies that the maximum
627 total depth of recursion is 64, for an "array of array of array
628 of ... struct of struct of struct of ..." where there are 32
634 The maximum length of a signature is 255.
641 When signatures appear in messages, the marshalling format
642 guarantees that they will be followed by a nul byte (which can
643 be interpreted as either C-style string termination or the INVALID
644 type-code), but this is not conceptually part of the signature.
650 <sect2 id="container-types">
651 <title>Container types</title>
654 In addition to basic types, there are four <firstterm>container</firstterm>
655 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
656 and <literal>DICT_ENTRY</literal>.
660 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
661 code does not appear in signatures. Instead, ASCII characters
662 '(' and ')' are used to mark the beginning and end of the struct.
663 So for example, a struct containing two integers would have this
668 Structs can be nested, so for example a struct containing
669 an integer and another struct:
673 The value block storing that struct would contain three integers; the
674 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
679 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
680 but is useful in code that implements the protocol. This type code
681 is specified to allow such code to interoperate in non-protocol contexts.
685 Empty structures are not allowed; there must be at least one
686 type code between the parentheses.
690 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
691 followed by a <firstterm>single complete type</firstterm>. The single
692 complete type following the array is the type of each array element. So
693 the simple example is:
697 which is an array of 32-bit integers. But an array can be of any type,
698 such as this array-of-struct-with-two-int32-fields:
702 Or this array of array of integer:
709 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
710 type <literal>VARIANT</literal> will have the signature of a single complete type as part
711 of the <emphasis>value</emphasis>. This signature will be followed by a
712 marshaled value of that type.
716 Unlike a message signature, the variant signature can
717 contain only a single complete type. So "i", "ai"
718 or "(ii)" is OK, but "ii" is not. Use of variants may not
719 cause a total message depth to be larger than 64, including
720 other container types such as structures.
724 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
725 than parentheses it uses curly braces, and it has more restrictions.
726 The restrictions are: it occurs only as an array element type; it has
727 exactly two single complete types inside the curly braces; the first
728 single complete type (the "key") must be a basic type rather than a
729 container type. Implementations must not accept dict entries outside of
730 arrays, must not accept dict entries with zero, one, or more than two
731 fields, and must not accept dict entries with non-basic-typed keys. A
732 dict entry is always a key-value pair.
736 The first field in the <literal>DICT_ENTRY</literal> is always the key.
737 A message is considered corrupt if the same key occurs twice in the same
738 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
739 implementations are not required to reject dicts with duplicate keys.
743 In most languages, an array of dict entry would be represented as a
744 map, hash table, or dict object.
749 <title>Summary of types</title>
752 The following table summarizes the D-Bus types.
757 <entry>Conventional Name</entry>
759 <entry>Description</entry>
764 <entry><literal>INVALID</literal></entry>
765 <entry>0 (ASCII NUL)</entry>
766 <entry>Not a valid type code, used to terminate signatures</entry>
768 <entry><literal>BYTE</literal></entry>
769 <entry>121 (ASCII 'y')</entry>
770 <entry>8-bit unsigned integer</entry>
772 <entry><literal>BOOLEAN</literal></entry>
773 <entry>98 (ASCII 'b')</entry>
774 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
776 <entry><literal>INT16</literal></entry>
777 <entry>110 (ASCII 'n')</entry>
778 <entry>16-bit signed integer</entry>
780 <entry><literal>UINT16</literal></entry>
781 <entry>113 (ASCII 'q')</entry>
782 <entry>16-bit unsigned integer</entry>
784 <entry><literal>INT32</literal></entry>
785 <entry>105 (ASCII 'i')</entry>
786 <entry>32-bit signed integer</entry>
788 <entry><literal>UINT32</literal></entry>
789 <entry>117 (ASCII 'u')</entry>
790 <entry>32-bit unsigned integer</entry>
792 <entry><literal>INT64</literal></entry>
793 <entry>120 (ASCII 'x')</entry>
794 <entry>64-bit signed integer</entry>
796 <entry><literal>UINT64</literal></entry>
797 <entry>116 (ASCII 't')</entry>
798 <entry>64-bit unsigned integer</entry>
800 <entry><literal>DOUBLE</literal></entry>
801 <entry>100 (ASCII 'd')</entry>
802 <entry>IEEE 754 double</entry>
804 <entry><literal>STRING</literal></entry>
805 <entry>115 (ASCII 's')</entry>
806 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
808 <entry><literal>OBJECT_PATH</literal></entry>
809 <entry>111 (ASCII 'o')</entry>
810 <entry>Name of an object instance</entry>
812 <entry><literal>SIGNATURE</literal></entry>
813 <entry>103 (ASCII 'g')</entry>
814 <entry>A type signature</entry>
816 <entry><literal>ARRAY</literal></entry>
817 <entry>97 (ASCII 'a')</entry>
820 <entry><literal>STRUCT</literal></entry>
821 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
822 <entry>Struct; type code 114 'r' is reserved for use in
823 bindings and implementations to represent the general
824 concept of a struct, and must not appear in signatures
825 used on D-Bus.</entry>
827 <entry><literal>VARIANT</literal></entry>
828 <entry>118 (ASCII 'v') </entry>
829 <entry>Variant type (the type of the value is part of the value itself)</entry>
831 <entry><literal>DICT_ENTRY</literal></entry>
832 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
833 <entry>Entry in a dict or map (array of key-value pairs).
834 Type code 101 'e' is reserved for use in bindings and
835 implementations to represent the general concept of a
836 dict or dict-entry, and must not appear in signatures
837 used on D-Bus.</entry>
839 <entry><literal>UNIX_FD</literal></entry>
840 <entry>104 (ASCII 'h')</entry>
841 <entry>Unix file descriptor</entry>
844 <entry>(reserved)</entry>
845 <entry>109 (ASCII 'm')</entry>
846 <entry>Reserved for <ulink
847 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
848 'maybe' type compatible with the one in GVariant</ulink>,
849 and must not appear in signatures used on D-Bus until
850 specified here</entry>
853 <entry>(reserved)</entry>
854 <entry>42 (ASCII '*')</entry>
855 <entry>Reserved for use in bindings/implementations to
856 represent any <firstterm>single complete type</firstterm>,
857 and must not appear in signatures used on D-Bus.</entry>
860 <entry>(reserved)</entry>
861 <entry>63 (ASCII '?')</entry>
862 <entry>Reserved for use in bindings/implementations to
863 represent any <firstterm>basic type</firstterm>, and must
864 not appear in signatures used on D-Bus.</entry>
867 <entry>(reserved)</entry>
868 <entry>64 (ASCII '@'), 38 (ASCII '&'),
869 94 (ASCII '^')</entry>
870 <entry>Reserved for internal use by bindings/implementations,
871 and must not appear in signatures used on D-Bus.
872 GVariant uses these type-codes to encode calling
883 <sect1 id="message-protocol-marshaling">
884 <title>Marshaling (Wire Format)</title>
887 D-Bus defines a marshalling format for its type system, which is
888 used in D-Bus messages. This is not the only possible marshalling
889 format for the type system: for instance, GVariant (part of GLib)
890 re-uses the D-Bus type system but implements an alternative marshalling
895 <title>Byte order and alignment</title>
898 Given a type signature, a block of bytes can be converted into typed
899 values. This section describes the format of the block of bytes. Byte
900 order and alignment issues are handled uniformly for all D-Bus types.
904 A block of bytes has an associated byte order. The byte order
905 has to be discovered in some way; for D-Bus messages, the
906 byte order is part of the message header as described in
907 <xref linkend="message-protocol-messages"/>. For now, assume
908 that the byte order is known to be either little endian or big
913 Each value in a block of bytes is aligned "naturally," for example
914 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
915 8-byte boundary. To properly align a value, <firstterm>alignment
916 padding</firstterm> may be necessary. The alignment padding must always
917 be the minimum required padding to properly align the following value;
918 and it must always be made up of nul bytes. The alignment padding must
919 not be left uninitialized (it can't contain garbage), and more padding
920 than required must not be used.
924 As an exception to natural alignment, <literal>STRUCT</literal> and
925 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
926 boundary, regardless of the alignments of their contents.
931 <title>Marshalling basic types</title>
934 To marshal and unmarshal fixed types, you simply read one value
935 from the data block corresponding to each type code in the signature.
936 All signed integer values are encoded in two's complement, DOUBLE
937 values are IEEE 754 double-precision floating-point, and BOOLEAN
938 values are encoded in 32 bits (of which only the least significant
943 The string-like types are all marshalled as a
944 fixed-length unsigned integer <varname>n</varname> giving the
945 length of the variable part, followed by <varname>n</varname>
946 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
947 which is not considered to be part of the text. The alignment
948 of the string-like type is the same as the alignment of
949 <varname>n</varname>.
953 For the STRING and OBJECT_PATH types, <varname>n</varname> is
954 encoded in 4 bytes, leading to 4-byte alignment.
955 For the SIGNATURE type, <varname>n</varname> is encoded as a single
956 byte. As a result, alignment padding is never required before a
962 <title>Marshalling containers</title>
965 Arrays are marshalled as a <literal>UINT32</literal>
966 <varname>n</varname> giving the length of the array data in bytes,
967 followed by alignment padding to the alignment boundary of the array
968 element type, followed by the <varname>n</varname> bytes of the
969 array elements marshalled in sequence. <varname>n</varname> does not
970 include the padding after the length, or any padding after the
975 For instance, if the current position in the message is a multiple
976 of 8 bytes and the byte-order is big-endian, an array containing only
977 the 64-bit integer 5 would be marshalled as:
980 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
981 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
982 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
987 Arrays have a maximum length defined to be 2 to the 26th power or
988 67108864. Implementations must not send or accept arrays exceeding this
993 Structs and dict entries are marshalled in the same way as their
994 contents, but their alignment is always to an 8-byte boundary,
995 even if their contents would normally be less strictly aligned.
999 Variants are marshalled as the <literal>SIGNATURE</literal> of
1000 the contents (which must be a single complete type), followed by a
1001 marshalled value with the type given by that signature. The
1002 variant has the same 1-byte alignment as the signature, which means
1003 that alignment padding before a variant is never needed.
1004 Use of variants may not cause a total message depth to be larger
1005 than 64, including other container types such as structures.
1010 <title>Summary of D-Bus marshalling</title>
1013 Given all this, the types are marshaled on the wire as follows:
1018 <entry>Conventional Name</entry>
1019 <entry>Encoding</entry>
1020 <entry>Alignment</entry>
1025 <entry><literal>INVALID</literal></entry>
1026 <entry>Not applicable; cannot be marshaled.</entry>
1029 <entry><literal>BYTE</literal></entry>
1030 <entry>A single 8-bit byte.</entry>
1033 <entry><literal>BOOLEAN</literal></entry>
1034 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1037 <entry><literal>INT16</literal></entry>
1038 <entry>16-bit signed integer in the message's byte order.</entry>
1041 <entry><literal>UINT16</literal></entry>
1042 <entry>16-bit unsigned integer in the message's byte order.</entry>
1045 <entry><literal>INT32</literal></entry>
1046 <entry>32-bit signed integer in the message's byte order.</entry>
1049 <entry><literal>UINT32</literal></entry>
1050 <entry>32-bit unsigned integer in the message's byte order.</entry>
1053 <entry><literal>INT64</literal></entry>
1054 <entry>64-bit signed integer in the message's byte order.</entry>
1057 <entry><literal>UINT64</literal></entry>
1058 <entry>64-bit unsigned integer in the message's byte order.</entry>
1061 <entry><literal>DOUBLE</literal></entry>
1062 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1065 <entry><literal>STRING</literal></entry>
1066 <entry>A <literal>UINT32</literal> indicating the string's
1067 length in bytes excluding its terminating nul, followed by
1068 non-nul string data of the given length, followed by a terminating nul
1075 <entry><literal>OBJECT_PATH</literal></entry>
1076 <entry>Exactly the same as <literal>STRING</literal> except the
1077 content must be a valid object path (see above).
1083 <entry><literal>SIGNATURE</literal></entry>
1084 <entry>The same as <literal>STRING</literal> except the length is a single
1085 byte (thus signatures have a maximum length of 255)
1086 and the content must be a valid signature (see above).
1092 <entry><literal>ARRAY</literal></entry>
1094 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1095 alignment padding to the alignment boundary of the array element type,
1096 followed by each array element.
1102 <entry><literal>STRUCT</literal></entry>
1104 A struct must start on an 8-byte boundary regardless of the
1105 type of the struct fields. The struct value consists of each
1106 field marshaled in sequence starting from that 8-byte
1113 <entry><literal>VARIANT</literal></entry>
1115 The marshaled <literal>SIGNATURE</literal> of a single
1116 complete type, followed by a marshaled value with the type
1117 given in the signature.
1120 1 (alignment of the signature)
1123 <entry><literal>DICT_ENTRY</literal></entry>
1125 Identical to STRUCT.
1131 <entry><literal>UNIX_FD</literal></entry>
1132 <entry>32-bit unsigned integer in the message's byte
1133 order. The actual file descriptors need to be
1134 transferred out-of-band via some platform specific
1135 mechanism. On the wire, values of this type store the index to the
1136 file descriptor in the array of file descriptors that
1137 accompany the message.</entry>
1149 <sect1 id="message-protocol">
1150 <title>Message Protocol</title>
1153 A <firstterm>message</firstterm> consists of a
1154 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1155 think of a message as a package, the header is the address, and the body
1156 contains the package contents. The message delivery system uses the header
1157 information to figure out where to send the message and how to interpret
1158 it; the recipient interprets the body of the message.
1162 The body of the message is made up of zero or more
1163 <firstterm>arguments</firstterm>, which are typed values, such as an
1164 integer or a byte array.
1168 Both header and body use the D-Bus <link linkend="type-system">type
1169 system</link> and format for serializing data.
1172 <sect2 id="message-protocol-messages">
1173 <title>Message Format</title>
1176 A message consists of a header and a body. The header is a block of
1177 values with a fixed signature and meaning. The body is a separate block
1178 of values, with a signature specified in the header.
1182 The length of the header must be a multiple of 8, allowing the body to
1183 begin on an 8-byte boundary when storing the entire message in a single
1184 buffer. If the header does not naturally end on an 8-byte boundary
1185 up to 7 bytes of nul-initialized alignment padding must be added.
1189 The message body need not end on an 8-byte boundary.
1193 The maximum length of a message, including header, header alignment padding,
1194 and body is 2 to the 27th power or 134217728. Implementations must not
1195 send or accept messages exceeding this size.
1199 The signature of the header is:
1203 Written out more readably, this is:
1205 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1210 These values have the following meanings:
1215 <entry>Value</entry>
1216 <entry>Description</entry>
1221 <entry>1st <literal>BYTE</literal></entry>
1222 <entry>Endianness flag; ASCII 'l' for little-endian
1223 or ASCII 'B' for big-endian. Both header and body are
1224 in this endianness.</entry>
1227 <entry>2nd <literal>BYTE</literal></entry>
1228 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1229 Currently-defined types are described below.
1233 <entry>3rd <literal>BYTE</literal></entry>
1234 <entry>Bitwise OR of flags. Unknown flags
1235 must be ignored. Currently-defined flags are described below.
1239 <entry>4th <literal>BYTE</literal></entry>
1240 <entry>Major protocol version of the sending application. If
1241 the major protocol version of the receiving application does not
1242 match, the applications will not be able to communicate and the
1243 D-Bus connection must be disconnected. The major protocol
1244 version for this version of the specification is 1.
1248 <entry>1st <literal>UINT32</literal></entry>
1249 <entry>Length in bytes of the message body, starting
1250 from the end of the header. The header ends after
1251 its alignment padding to an 8-boundary.
1255 <entry>2nd <literal>UINT32</literal></entry>
1256 <entry>The serial of this message, used as a cookie
1257 by the sender to identify the reply corresponding
1258 to this request. This must not be zero.
1262 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1263 <entry>An array of zero or more <firstterm>header
1264 fields</firstterm> where the byte is the field code, and the
1265 variant is the field value. The message type determines
1266 which fields are required.
1274 <firstterm>Message types</firstterm> that can appear in the second byte
1280 <entry>Conventional name</entry>
1281 <entry>Decimal value</entry>
1282 <entry>Description</entry>
1287 <entry><literal>INVALID</literal></entry>
1289 <entry>This is an invalid type.</entry>
1292 <entry><literal>METHOD_CALL</literal></entry>
1294 <entry>Method call.</entry>
1297 <entry><literal>METHOD_RETURN</literal></entry>
1299 <entry>Method reply with returned data.</entry>
1302 <entry><literal>ERROR</literal></entry>
1304 <entry>Error reply. If the first argument exists and is a
1305 string, it is an error message.</entry>
1308 <entry><literal>SIGNAL</literal></entry>
1310 <entry>Signal emission.</entry>
1317 Flags that can appear in the third byte of the header:
1322 <entry>Conventional name</entry>
1323 <entry>Hex value</entry>
1324 <entry>Description</entry>
1329 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1331 <entry>This message does not expect method return replies or
1332 error replies; the reply can be omitted as an
1333 optimization. However, it is compliant with this specification
1334 to return the reply despite this flag and the only harm
1335 from doing so is extra network traffic.
1339 <entry><literal>NO_AUTO_START</literal></entry>
1341 <entry>The bus must not launch an owner
1342 for the destination name in response to this message.
1350 <sect3 id="message-protocol-header-fields">
1351 <title>Header Fields</title>
1354 The array at the end of the header contains <firstterm>header
1355 fields</firstterm>, where each field is a 1-byte field code followed
1356 by a field value. A header must contain the required header fields for
1357 its message type, and zero or more of any optional header
1358 fields. Future versions of this protocol specification may add new
1359 fields. Implementations must ignore fields they do not
1360 understand. Implementations must not invent their own header fields;
1361 only changes to this specification may introduce new header fields.
1365 Again, if an implementation sees a header field code that it does not
1366 expect, it must ignore that field, as it will be part of a new
1367 (but compatible) version of this specification. This also applies
1368 to known header fields appearing in unexpected messages, for
1369 example: if a signal has a reply serial it must be ignored
1370 even though it has no meaning as of this version of the spec.
1374 However, implementations must not send or accept known header fields
1375 with the wrong type stored in the field value. So for example a
1376 message with an <literal>INTERFACE</literal> field of type
1377 <literal>UINT32</literal> would be considered corrupt.
1381 Here are the currently-defined header fields:
1386 <entry>Conventional Name</entry>
1387 <entry>Decimal Code</entry>
1389 <entry>Required In</entry>
1390 <entry>Description</entry>
1395 <entry><literal>INVALID</literal></entry>
1398 <entry>not allowed</entry>
1399 <entry>Not a valid field name (error if it appears in a message)</entry>
1402 <entry><literal>PATH</literal></entry>
1404 <entry><literal>OBJECT_PATH</literal></entry>
1405 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1406 <entry>The object to send a call to,
1407 or the object a signal is emitted from.
1409 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1410 implementations should not send messages with this path,
1411 and the reference implementation of the bus daemon will
1412 disconnect any application that attempts to do so.
1416 <entry><literal>INTERFACE</literal></entry>
1418 <entry><literal>STRING</literal></entry>
1419 <entry><literal>SIGNAL</literal></entry>
1421 The interface to invoke a method call on, or
1422 that a signal is emitted from. Optional for
1423 method calls, required for signals.
1424 The special interface
1425 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1426 implementations should not send messages with this
1427 interface, and the reference implementation of the bus
1428 daemon will disconnect any application that attempts to
1433 <entry><literal>MEMBER</literal></entry>
1435 <entry><literal>STRING</literal></entry>
1436 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1437 <entry>The member, either the method name or signal name.</entry>
1440 <entry><literal>ERROR_NAME</literal></entry>
1442 <entry><literal>STRING</literal></entry>
1443 <entry><literal>ERROR</literal></entry>
1444 <entry>The name of the error that occurred, for errors</entry>
1447 <entry><literal>REPLY_SERIAL</literal></entry>
1449 <entry><literal>UINT32</literal></entry>
1450 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1451 <entry>The serial number of the message this message is a reply
1452 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1455 <entry><literal>DESTINATION</literal></entry>
1457 <entry><literal>STRING</literal></entry>
1458 <entry>optional</entry>
1459 <entry>The name of the connection this message is intended for.
1460 Only used in combination with the message bus, see
1461 <xref linkend="message-bus"/>.</entry>
1464 <entry><literal>SENDER</literal></entry>
1466 <entry><literal>STRING</literal></entry>
1467 <entry>optional</entry>
1468 <entry>Unique name of the sending connection.
1469 The message bus fills in this field so it is reliable; the field is
1470 only meaningful in combination with the message bus.</entry>
1473 <entry><literal>SIGNATURE</literal></entry>
1475 <entry><literal>SIGNATURE</literal></entry>
1476 <entry>optional</entry>
1477 <entry>The signature of the message body.
1478 If omitted, it is assumed to be the
1479 empty signature "" (i.e. the body must be 0-length).</entry>
1482 <entry><literal>UNIX_FDS</literal></entry>
1484 <entry><literal>UINT32</literal></entry>
1485 <entry>optional</entry>
1486 <entry>The number of Unix file descriptors that
1487 accompany the message. If omitted, it is assumed
1488 that no Unix file descriptors accompany the
1489 message. The actual file descriptors need to be
1490 transferred via platform specific mechanism
1491 out-of-band. They must be sent at the same time as
1492 part of the message itself. They may not be sent
1493 before the first byte of the message itself is
1494 transferred or after the last byte of the message
1504 <sect2 id="message-protocol-names">
1505 <title>Valid Names</title>
1507 The various names in D-Bus messages have some restrictions.
1510 There is a <firstterm>maximum name length</firstterm>
1511 of 255 which applies to bus names, interfaces, and members.
1513 <sect3 id="message-protocol-names-interface">
1514 <title>Interface names</title>
1516 Interfaces have names with type <literal>STRING</literal>, meaning that
1517 they must be valid UTF-8. However, there are also some
1518 additional restrictions that apply to interface names
1521 <listitem><para>Interface names are composed of 1 or more elements separated by
1522 a period ('.') character. All elements must contain at least
1526 <listitem><para>Each element must only contain the ASCII characters
1527 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1531 <listitem><para>Interface names must contain at least one '.' (period)
1532 character (and thus at least two elements).
1535 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1536 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1541 Interface names should start with the reversed DNS domain name of
1542 the author of the interface (in lower-case), like interface names
1543 in Java. It is conventional for the rest of the interface name
1544 to consist of words run together, with initial capital letters
1545 on all words ("CamelCase"). Several levels of hierarchy can be used.
1546 It is also a good idea to include the major version of the interface
1547 in the name, and increment it if incompatible changes are made;
1548 this way, a single object can implement several versions of an
1549 interface in parallel, if necessary.
1553 For instance, if the owner of <literal>example.com</literal> is
1554 developing a D-Bus API for a music player, they might define
1555 interfaces called <literal>com.example.MusicPlayer1</literal>,
1556 <literal>com.example.MusicPlayer1.Track</literal> and
1557 <literal>com.example.MusicPlayer1.Seekable</literal>.
1561 D-Bus does not distinguish between the concepts that would be
1562 called classes and interfaces in Java: either can be identified on
1563 D-Bus by an interface name.
1566 <sect3 id="message-protocol-names-bus">
1567 <title>Bus names</title>
1569 Connections have one or more bus names associated with them.
1570 A connection has exactly one bus name that is a <firstterm>unique
1571 connection name</firstterm>. The unique connection name remains
1572 with the connection for its entire lifetime.
1573 A bus name is of type <literal>STRING</literal>,
1574 meaning that it must be valid UTF-8. However, there are also
1575 some additional restrictions that apply to bus names
1578 <listitem><para>Bus names that start with a colon (':')
1579 character are unique connection names. Other bus names
1580 are called <firstterm>well-known bus names</firstterm>.
1583 <listitem><para>Bus names are composed of 1 or more elements separated by
1584 a period ('.') character. All elements must contain at least
1588 <listitem><para>Each element must only contain the ASCII characters
1589 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1590 connection name may begin with a digit, elements in
1591 other bus names must not begin with a digit.
1595 <listitem><para>Bus names must contain at least one '.' (period)
1596 character (and thus at least two elements).
1599 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1600 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1604 Note that the hyphen ('-') character is allowed in bus names but
1605 not in interface names.
1609 Like <link linkend="message-protocol-names-interface">interface
1610 names</link>, well-known bus names should start with the
1611 reversed DNS domain name of the author of the interface (in
1612 lower-case), and it is conventional for the rest of the well-known
1613 bus name to consist of words run together, with initial
1614 capital letters. As with interface names, including a version
1615 number in well-known bus names is a good idea; it's possible to
1616 have the well-known bus name for more than one version
1617 simultaneously if backwards compatibility is required.
1621 If a well-known bus name implies the presence of a "main" interface,
1622 that "main" interface is often given the same name as
1623 the well-known bus name, and situated at the corresponding object
1624 path. For instance, if the owner of <literal>example.com</literal>
1625 is developing a D-Bus API for a music player, they might define
1626 that any application that takes the well-known name
1627 <literal>com.example.MusicPlayer1</literal> should have an object
1628 at the object path <literal>/com/example/MusicPlayer1</literal>
1629 which implements the interface
1630 <literal>com.example.MusicPlayer1</literal>.
1633 <sect3 id="message-protocol-names-member">
1634 <title>Member names</title>
1636 Member (i.e. method or signal) names:
1638 <listitem><para>Must only contain the ASCII characters
1639 "[A-Z][a-z][0-9]_" and may not begin with a
1640 digit.</para></listitem>
1641 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1642 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1643 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1648 It is conventional for member names on D-Bus to consist of
1649 capitalized words with no punctuation ("camel-case").
1650 Method names should usually be verbs, such as
1651 <literal>GetItems</literal>, and signal names should usually be
1652 a description of an event, such as <literal>ItemsChanged</literal>.
1655 <sect3 id="message-protocol-names-error">
1656 <title>Error names</title>
1658 Error names have the same restrictions as interface names.
1662 Error names have the same naming conventions as interface
1663 names, and often contain <literal>.Error.</literal>; for instance,
1664 the owner of <literal>example.com</literal> might define the
1665 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1666 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1667 The errors defined by D-Bus itself, such as
1668 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1674 <sect2 id="message-protocol-types">
1675 <title>Message Types</title>
1677 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1678 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1679 This section describes these conventions.
1681 <sect3 id="message-protocol-types-method">
1682 <title>Method Calls</title>
1684 Some messages invoke an operation on a remote object. These are
1685 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1686 messages map naturally to methods on objects in a typical program.
1689 A method call message is required to have a <literal>MEMBER</literal> header field
1690 indicating the name of the method. Optionally, the message has an
1691 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1692 Including the <literal>INTERFACE</literal> in all method call
1693 messages is strongly recommended.
1696 In the absence of an <literal>INTERFACE</literal> field, if two
1697 or more interfaces on the same object have a method with the same
1698 name, it is undefined which of those methods will be invoked.
1699 Implementations may choose to either return an error, or deliver the
1700 message as though it had an arbitrary one of those interfaces.
1703 Method call messages also include a <literal>PATH</literal> field
1704 indicating the object to invoke the method on. If the call is passing
1705 through a message bus, the message will also have a
1706 <literal>DESTINATION</literal> field giving the name of the connection
1707 to receive the message.
1710 When an application handles a method call message, it is required to
1711 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1712 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1713 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1716 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1717 are the return value(s) or "out parameters" of the method call.
1718 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1719 and the call fails; no return value will be provided. It makes
1720 no sense to send multiple replies to the same method call.
1723 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1724 reply is required, so the caller will know the method
1725 was successfully processed.
1728 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1732 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1733 then as an optimization the application receiving the method
1734 call may choose to omit the reply message (regardless of
1735 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1736 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1737 flag and reply anyway.
1740 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1741 destination name does not exist then a program to own the destination
1742 name will be started before the message is delivered. The message
1743 will be held until the new program is successfully started or has
1744 failed to start; in case of failure, an error will be returned. This
1745 flag is only relevant in the context of a message bus, it is ignored
1746 during one-to-one communication with no intermediate bus.
1748 <sect4 id="message-protocol-types-method-apis">
1749 <title>Mapping method calls to native APIs</title>
1751 APIs for D-Bus may map method calls to a method call in a specific
1752 programming language, such as C++, or may map a method call written
1753 in an IDL to a D-Bus message.
1756 In APIs of this nature, arguments to a method are often termed "in"
1757 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1758 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1759 "inout" arguments, which are both sent and received, i.e. the caller
1760 passes in a value which is modified. Mapped to D-Bus, an "inout"
1761 argument is equivalent to an "in" argument, followed by an "out"
1762 argument. You can't pass things "by reference" over the wire, so
1763 "inout" is purely an illusion of the in-process API.
1766 Given a method with zero or one return values, followed by zero or more
1767 arguments, where each argument may be "in", "out", or "inout", the
1768 caller constructs a message by appending each "in" or "inout" argument,
1769 in order. "out" arguments are not represented in the caller's message.
1772 The recipient constructs a reply by appending first the return value
1773 if any, then each "out" or "inout" argument, in order.
1774 "in" arguments are not represented in the reply message.
1777 Error replies are normally mapped to exceptions in languages that have
1781 In converting from native APIs to D-Bus, it is perhaps nice to
1782 map D-Bus naming conventions ("FooBar") to native conventions
1783 such as "fooBar" or "foo_bar" automatically. This is OK
1784 as long as you can say that the native API is one that
1785 was specifically written for D-Bus. It makes the most sense
1786 when writing object implementations that will be exported
1787 over the bus. Object proxies used to invoke remote D-Bus
1788 objects probably need the ability to call any D-Bus method,
1789 and thus a magic name mapping like this could be a problem.
1792 This specification doesn't require anything of native API bindings;
1793 the preceding is only a suggested convention for consistency
1799 <sect3 id="message-protocol-types-signal">
1800 <title>Signal Emission</title>
1802 Unlike method calls, signal emissions have no replies.
1803 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1804 It must have three header fields: <literal>PATH</literal> giving the object
1805 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1806 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1807 for signals, though it is optional for method calls.
1811 <sect3 id="message-protocol-types-errors">
1812 <title>Errors</title>
1814 Messages of type <literal>ERROR</literal> are most commonly replies
1815 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1816 to any kind of message. The message bus for example
1817 will return an <literal>ERROR</literal> in reply to a signal emission if
1818 the bus does not have enough memory to send the signal.
1821 An <literal>ERROR</literal> may have any arguments, but if the first
1822 argument is a <literal>STRING</literal>, it must be an error message.
1823 The error message may be logged or shown to the user
1828 <sect3 id="message-protocol-types-notation">
1829 <title>Notation in this document</title>
1831 This document uses a simple pseudo-IDL to describe particular method
1832 calls and signals. Here is an example of a method call:
1834 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1835 out UINT32 resultcode)
1837 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1838 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1839 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1840 characters so it's known that the last part of the name in
1841 the "IDL" is the member name.
1844 In C++ that might end up looking like this:
1846 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1847 unsigned int flags);
1849 or equally valid, the return value could be done as an argument:
1851 void org::freedesktop::DBus::StartServiceByName (const char *name,
1853 unsigned int *resultcode);
1855 It's really up to the API designer how they want to make
1856 this look. You could design an API where the namespace wasn't used
1857 in C++, using STL or Qt, using varargs, or whatever you wanted.
1860 Signals are written as follows:
1862 org.freedesktop.DBus.NameLost (STRING name)
1864 Signals don't specify "in" vs. "out" because only
1865 a single direction is possible.
1868 It isn't especially encouraged to use this lame pseudo-IDL in actual
1869 API implementations; you might use the native notation for the
1870 language you're using, or you might use COM or CORBA IDL, for example.
1875 <sect2 id="message-protocol-handling-invalid">
1876 <title>Invalid Protocol and Spec Extensions</title>
1879 For security reasons, the D-Bus protocol should be strictly parsed and
1880 validated, with the exception of defined extension points. Any invalid
1881 protocol or spec violations should result in immediately dropping the
1882 connection without notice to the other end. Exceptions should be
1883 carefully considered, e.g. an exception may be warranted for a
1884 well-understood idiosyncrasy of a widely-deployed implementation. In
1885 cases where the other end of a connection is 100% trusted and known to
1886 be friendly, skipping validation for performance reasons could also make
1887 sense in certain cases.
1891 Generally speaking violations of the "must" requirements in this spec
1892 should be considered possible attempts to exploit security, and violations
1893 of the "should" suggestions should be considered legitimate (though perhaps
1894 they should generate an error in some cases).
1898 The following extension points are built in to D-Bus on purpose and must
1899 not be treated as invalid protocol. The extension points are intended
1900 for use by future versions of this spec, they are not intended for third
1901 parties. At the moment, the only way a third party could extend D-Bus
1902 without breaking interoperability would be to introduce a way to negotiate new
1903 feature support as part of the auth protocol, using EXTENSION_-prefixed
1904 commands. There is not yet a standard way to negotiate features.
1908 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1909 commands result in an ERROR rather than a disconnect. This enables
1910 future extensions to the protocol. Commands starting with EXTENSION_ are
1911 reserved for third parties.
1916 The authentication protocol supports pluggable auth mechanisms.
1921 The address format (see <xref linkend="addresses"/>) supports new
1927 Messages with an unknown type (something other than
1928 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1929 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1930 Unknown-type messages must still be well-formed in the same way
1931 as the known messages, however. They still have the normal
1937 Header fields with an unknown or unexpected field code must be ignored,
1938 though again they must still be well-formed.
1943 New standard interfaces (with new methods and signals) can of course be added.
1953 <sect1 id="auth-protocol">
1954 <title>Authentication Protocol</title>
1956 Before the flow of messages begins, two applications must
1957 authenticate. A simple plain-text protocol is used for
1958 authentication; this protocol is a SASL profile, and maps fairly
1959 directly from the SASL specification. The message encoding is
1960 NOT used here, only plain text messages.
1963 In examples, "C:" and "S:" indicate lines sent by the client and
1964 server respectively.
1966 <sect2 id="auth-protocol-overview">
1967 <title>Protocol Overview</title>
1969 The protocol is a line-based protocol, where each line ends with
1970 \r\n. Each line begins with an all-caps ASCII command name containing
1971 only the character range [A-Z_], a space, then any arguments for the
1972 command, then the \r\n ending the line. The protocol is
1973 case-sensitive. All bytes must be in the ASCII character set.
1975 Commands from the client to the server are as follows:
1978 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1979 <listitem><para>CANCEL</para></listitem>
1980 <listitem><para>BEGIN</para></listitem>
1981 <listitem><para>DATA <data in hex encoding></para></listitem>
1982 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1983 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1986 From server to client are as follows:
1989 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1990 <listitem><para>OK <GUID in hex></para></listitem>
1991 <listitem><para>DATA <data in hex encoding></para></listitem>
1992 <listitem><para>ERROR</para></listitem>
1993 <listitem><para>AGREE_UNIX_FD</para></listitem>
1997 Unofficial extensions to the command set must begin with the letters
1998 "EXTENSION_", to avoid conflicts with future official commands.
1999 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2002 <sect2 id="auth-nul-byte">
2003 <title>Special credentials-passing nul byte</title>
2005 Immediately after connecting to the server, the client must send a
2006 single nul byte. This byte may be accompanied by credentials
2007 information on some operating systems that use sendmsg() with
2008 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2009 sockets. However, the nul byte must be sent even on other kinds of
2010 socket, and even on operating systems that do not require a byte to be
2011 sent in order to transmit credentials. The text protocol described in
2012 this document begins after the single nul byte. If the first byte
2013 received from the client is not a nul byte, the server may disconnect
2017 A nul byte in any context other than the initial byte is an error;
2018 the protocol is ASCII-only.
2021 The credentials sent along with the nul byte may be used with the
2022 SASL mechanism EXTERNAL.
2025 <sect2 id="auth-command-auth">
2026 <title>AUTH command</title>
2028 If an AUTH command has no arguments, it is a request to list
2029 available mechanisms. The server must respond with a REJECTED
2030 command listing the mechanisms it understands, or with an error.
2033 If an AUTH command specifies a mechanism, and the server supports
2034 said mechanism, the server should begin exchanging SASL
2035 challenge-response data with the client using DATA commands.
2038 If the server does not support the mechanism given in the AUTH
2039 command, it must send either a REJECTED command listing the mechanisms
2040 it does support, or an error.
2043 If the [initial-response] argument is provided, it is intended for use
2044 with mechanisms that have no initial challenge (or an empty initial
2045 challenge), as if it were the argument to an initial DATA command. If
2046 the selected mechanism has an initial challenge and [initial-response]
2047 was provided, the server should reject authentication by sending
2051 If authentication succeeds after exchanging DATA commands,
2052 an OK command must be sent to the client.
2055 The first octet received by the server after the \r\n of the BEGIN
2056 command from the client must be the first octet of the
2057 authenticated/encrypted stream of D-Bus messages.
2060 If BEGIN is received by the server, the first octet received
2061 by the client after the \r\n of the OK command must be the
2062 first octet of the authenticated/encrypted stream of D-Bus
2066 <sect2 id="auth-command-cancel">
2067 <title>CANCEL Command</title>
2069 At any time up to sending the BEGIN command, the client may send a
2070 CANCEL command. On receiving the CANCEL command, the server must
2071 send a REJECTED command and abort the current authentication
2075 <sect2 id="auth-command-data">
2076 <title>DATA Command</title>
2078 The DATA command may come from either client or server, and simply
2079 contains a hex-encoded block of data to be interpreted
2080 according to the SASL mechanism in use.
2083 Some SASL mechanisms support sending an "empty string";
2084 FIXME we need some way to do this.
2087 <sect2 id="auth-command-begin">
2088 <title>BEGIN Command</title>
2090 The BEGIN command acknowledges that the client has received an
2091 OK command from the server, and that the stream of messages
2095 The first octet received by the server after the \r\n of the BEGIN
2096 command from the client must be the first octet of the
2097 authenticated/encrypted stream of D-Bus messages.
2100 <sect2 id="auth-command-rejected">
2101 <title>REJECTED Command</title>
2103 The REJECTED command indicates that the current authentication
2104 exchange has failed, and further exchange of DATA is inappropriate.
2105 The client would normally try another mechanism, or try providing
2106 different responses to challenges.
2108 Optionally, the REJECTED command has a space-separated list of
2109 available auth mechanisms as arguments. If a server ever provides
2110 a list of supported mechanisms, it must provide the same list
2111 each time it sends a REJECTED message. Clients are free to
2112 ignore all lists received after the first.
2115 <sect2 id="auth-command-ok">
2116 <title>OK Command</title>
2118 The OK command indicates that the client has been
2119 authenticated. The client may now proceed with negotiating
2120 Unix file descriptor passing. To do that it shall send
2121 NEGOTIATE_UNIX_FD to the server.
2124 Otherwise, the client must respond to the OK command by
2125 sending a BEGIN command, followed by its stream of messages,
2126 or by disconnecting. The server must not accept additional
2127 commands using this protocol after the BEGIN command has been
2128 received. Further communication will be a stream of D-Bus
2129 messages (optionally encrypted, as negotiated) rather than
2133 If a client sends BEGIN the first octet received by the client
2134 after the \r\n of the OK command must be the first octet of
2135 the authenticated/encrypted stream of D-Bus messages.
2138 The OK command has one argument, which is the GUID of the server.
2139 See <xref linkend="addresses"/> for more on server GUIDs.
2142 <sect2 id="auth-command-error">
2143 <title>ERROR Command</title>
2145 The ERROR command indicates that either server or client did not
2146 know a command, does not accept the given command in the current
2147 context, or did not understand the arguments to the command. This
2148 allows the protocol to be extended; a client or server can send a
2149 command present or permitted only in new protocol versions, and if
2150 an ERROR is received instead of an appropriate response, fall back
2151 to using some other technique.
2154 If an ERROR is sent, the server or client that sent the
2155 error must continue as if the command causing the ERROR had never been
2156 received. However, the the server or client receiving the error
2157 should try something other than whatever caused the error;
2158 if only canceling/rejecting the authentication.
2161 If the D-Bus protocol changes incompatibly at some future time,
2162 applications implementing the new protocol would probably be able to
2163 check for support of the new protocol by sending a new command and
2164 receiving an ERROR from applications that don't understand it. Thus the
2165 ERROR feature of the auth protocol is an escape hatch that lets us
2166 negotiate extensions or changes to the D-Bus protocol in the future.
2169 <sect2 id="auth-command-negotiate-unix-fd">
2170 <title>NEGOTIATE_UNIX_FD Command</title>
2172 The NEGOTIATE_UNIX_FD command indicates that the client
2173 supports Unix file descriptor passing. This command may only
2174 be sent after the connection is authenticated, i.e. after OK
2175 was received by the client. This command may only be sent on
2176 transports that support Unix file descriptor passing.
2179 On receiving NEGOTIATE_UNIX_FD the server must respond with
2180 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2181 the transport chosen supports Unix file descriptor passing and
2182 the server supports this feature. It shall respond the latter
2183 if the transport does not support Unix file descriptor
2184 passing, the server does not support this feature, or the
2185 server decides not to enable file descriptor passing due to
2186 security or other reasons.
2189 <sect2 id="auth-command-agree-unix-fd">
2190 <title>AGREE_UNIX_FD Command</title>
2192 The AGREE_UNIX_FD command indicates that the server supports
2193 Unix file descriptor passing. This command may only be sent
2194 after the connection is authenticated, and the client sent
2195 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2196 command may only be sent on transports that support Unix file
2200 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2201 followed by its stream of messages, or by disconnecting. The
2202 server must not accept additional commands using this protocol
2203 after the BEGIN command has been received. Further
2204 communication will be a stream of D-Bus messages (optionally
2205 encrypted, as negotiated) rather than this protocol.
2208 <sect2 id="auth-command-future">
2209 <title>Future Extensions</title>
2211 Future extensions to the authentication and negotiation
2212 protocol are possible. For that new commands may be
2213 introduced. If a client or server receives an unknown command
2214 it shall respond with ERROR and not consider this fatal. New
2215 commands may be introduced both before, and after
2216 authentication, i.e. both before and after the OK command.
2219 <sect2 id="auth-examples">
2220 <title>Authentication examples</title>
2224 <title>Example of successful magic cookie authentication</title>
2226 (MAGIC_COOKIE is a made up mechanism)
2228 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2234 <title>Example of finding out mechanisms then picking one</title>
2237 S: REJECTED KERBEROS_V4 SKEY
2238 C: AUTH SKEY 7ab83f32ee
2239 S: DATA 8799cabb2ea93e
2240 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2246 <title>Example of client sends unknown command then falls back to regular auth</title>
2250 C: AUTH MAGIC_COOKIE 3736343435313230333039
2256 <title>Example of server doesn't support initial auth mechanism</title>
2258 C: AUTH MAGIC_COOKIE 3736343435313230333039
2259 S: REJECTED KERBEROS_V4 SKEY
2260 C: AUTH SKEY 7ab83f32ee
2261 S: DATA 8799cabb2ea93e
2262 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2268 <title>Example of wrong password or the like followed by successful retry</title>
2270 C: AUTH MAGIC_COOKIE 3736343435313230333039
2271 S: REJECTED KERBEROS_V4 SKEY
2272 C: AUTH SKEY 7ab83f32ee
2273 S: DATA 8799cabb2ea93e
2274 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2276 C: AUTH SKEY 7ab83f32ee
2277 S: DATA 8799cabb2ea93e
2278 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2284 <title>Example of skey cancelled and restarted</title>
2286 C: AUTH MAGIC_COOKIE 3736343435313230333039
2287 S: REJECTED KERBEROS_V4 SKEY
2288 C: AUTH SKEY 7ab83f32ee
2289 S: DATA 8799cabb2ea93e
2292 C: AUTH SKEY 7ab83f32ee
2293 S: DATA 8799cabb2ea93e
2294 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2300 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2302 (MAGIC_COOKIE is a made up mechanism)
2304 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2306 C: NEGOTIATE_UNIX_FD
2312 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2314 (MAGIC_COOKIE is a made up mechanism)
2316 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2318 C: NEGOTIATE_UNIX_FD
2325 <sect2 id="auth-states">
2326 <title>Authentication state diagrams</title>
2329 This section documents the auth protocol in terms of
2330 a state machine for the client and the server. This is
2331 probably the most robust way to implement the protocol.
2334 <sect3 id="auth-states-client">
2335 <title>Client states</title>
2338 To more precisely describe the interaction between the
2339 protocol state machine and the authentication mechanisms the
2340 following notation is used: MECH(CHALL) means that the
2341 server challenge CHALL was fed to the mechanism MECH, which
2347 CONTINUE(RESP) means continue the auth conversation
2348 and send RESP as the response to the server;
2354 OK(RESP) means that after sending RESP to the server
2355 the client side of the auth conversation is finished
2356 and the server should return "OK";
2362 ERROR means that CHALL was invalid and could not be
2368 Both RESP and CHALL may be empty.
2372 The Client starts by getting an initial response from the
2373 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2374 the mechanism did not provide an initial response. If the
2375 mechanism returns CONTINUE, the client starts in state
2376 <emphasis>WaitingForData</emphasis>, if the mechanism
2377 returns OK the client starts in state
2378 <emphasis>WaitingForOK</emphasis>.
2382 The client should keep track of available mechanisms and
2383 which it mechanisms it has already attempted. This list is
2384 used to decide which AUTH command to send. When the list is
2385 exhausted, the client should give up and close the
2390 <title><emphasis>WaitingForData</emphasis></title>
2398 MECH(CHALL) returns CONTINUE(RESP) → send
2400 <emphasis>WaitingForData</emphasis>
2404 MECH(CHALL) returns OK(RESP) → send DATA
2405 RESP, goto <emphasis>WaitingForOK</emphasis>
2409 MECH(CHALL) returns ERROR → send ERROR
2410 [msg], goto <emphasis>WaitingForData</emphasis>
2418 Receive REJECTED [mechs] →
2419 send AUTH [next mech], goto
2420 WaitingForData or <emphasis>WaitingForOK</emphasis>
2425 Receive ERROR → send
2427 <emphasis>WaitingForReject</emphasis>
2432 Receive OK → send
2433 BEGIN, terminate auth
2434 conversation, authenticated
2439 Receive anything else → send
2441 <emphasis>WaitingForData</emphasis>
2449 <title><emphasis>WaitingForOK</emphasis></title>
2454 Receive OK → send BEGIN, terminate auth
2455 conversation, <emphasis>authenticated</emphasis>
2460 Receive REJECTED [mechs] → send AUTH [next mech],
2461 goto <emphasis>WaitingForData</emphasis> or
2462 <emphasis>WaitingForOK</emphasis>
2468 Receive DATA → send CANCEL, goto
2469 <emphasis>WaitingForReject</emphasis>
2475 Receive ERROR → send CANCEL, goto
2476 <emphasis>WaitingForReject</emphasis>
2482 Receive anything else → send ERROR, goto
2483 <emphasis>WaitingForOK</emphasis>
2491 <title><emphasis>WaitingForReject</emphasis></title>
2496 Receive REJECTED [mechs] → send AUTH [next mech],
2497 goto <emphasis>WaitingForData</emphasis> or
2498 <emphasis>WaitingForOK</emphasis>
2504 Receive anything else → terminate auth
2505 conversation, disconnect
2514 <sect3 id="auth-states-server">
2515 <title>Server states</title>
2518 For the server MECH(RESP) means that the client response
2519 RESP was fed to the the mechanism MECH, which returns one of
2524 CONTINUE(CHALL) means continue the auth conversation and
2525 send CHALL as the challenge to the client;
2531 OK means that the client has been successfully
2538 REJECTED means that the client failed to authenticate or
2539 there was an error in RESP.
2544 The server starts out in state
2545 <emphasis>WaitingForAuth</emphasis>. If the client is
2546 rejected too many times the server must disconnect the
2551 <title><emphasis>WaitingForAuth</emphasis></title>
2557 Receive AUTH → send REJECTED [mechs], goto
2558 <emphasis>WaitingForAuth</emphasis>
2564 Receive AUTH MECH RESP
2568 MECH not valid mechanism → send REJECTED
2570 <emphasis>WaitingForAuth</emphasis>
2574 MECH(RESP) returns CONTINUE(CHALL) → send
2576 <emphasis>WaitingForData</emphasis>
2580 MECH(RESP) returns OK → send OK, goto
2581 <emphasis>WaitingForBegin</emphasis>
2585 MECH(RESP) returns REJECTED → send REJECTED
2587 <emphasis>WaitingForAuth</emphasis>
2595 Receive BEGIN → terminate
2596 auth conversation, disconnect
2602 Receive ERROR → send REJECTED [mechs], goto
2603 <emphasis>WaitingForAuth</emphasis>
2609 Receive anything else → send
2611 <emphasis>WaitingForAuth</emphasis>
2620 <title><emphasis>WaitingForData</emphasis></title>
2628 MECH(RESP) returns CONTINUE(CHALL) → send
2630 <emphasis>WaitingForData</emphasis>
2634 MECH(RESP) returns OK → send OK, goto
2635 <emphasis>WaitingForBegin</emphasis>
2639 MECH(RESP) returns REJECTED → send REJECTED
2641 <emphasis>WaitingForAuth</emphasis>
2649 Receive BEGIN → terminate auth conversation,
2656 Receive CANCEL → send REJECTED [mechs], goto
2657 <emphasis>WaitingForAuth</emphasis>
2663 Receive ERROR → send REJECTED [mechs], goto
2664 <emphasis>WaitingForAuth</emphasis>
2670 Receive anything else → send ERROR, goto
2671 <emphasis>WaitingForData</emphasis>
2679 <title><emphasis>WaitingForBegin</emphasis></title>
2684 Receive BEGIN → terminate auth conversation,
2685 client authenticated
2691 Receive CANCEL → send REJECTED [mechs], goto
2692 <emphasis>WaitingForAuth</emphasis>
2698 Receive ERROR → send REJECTED [mechs], goto
2699 <emphasis>WaitingForAuth</emphasis>
2705 Receive anything else → send ERROR, goto
2706 <emphasis>WaitingForBegin</emphasis>
2716 <sect2 id="auth-mechanisms">
2717 <title>Authentication mechanisms</title>
2719 This section describes some new authentication mechanisms.
2720 D-Bus also allows any standard SASL mechanism of course.
2722 <sect3 id="auth-mechanisms-sha">
2723 <title>DBUS_COOKIE_SHA1</title>
2725 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2726 has the ability to read a private file owned by the user being
2727 authenticated. If the client can prove that it has access to a secret
2728 cookie stored in this file, then the client is authenticated.
2729 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2733 Throughout this description, "hex encoding" must output the digits
2734 from a to f in lower-case; the digits A to F must not be used
2735 in the DBUS_COOKIE_SHA1 mechanism.
2738 Authentication proceeds as follows:
2742 The client sends the username it would like to authenticate
2748 The server sends the name of its "cookie context" (see below); a
2749 space character; the integer ID of the secret cookie the client
2750 must demonstrate knowledge of; a space character; then a
2751 randomly-generated challenge string, all of this hex-encoded into
2757 The client locates the cookie and generates its own
2758 randomly-generated challenge string. The client then concatenates
2759 the server's decoded challenge, a ":" character, its own challenge,
2760 another ":" character, and the cookie. It computes the SHA-1 hash
2761 of this composite string as a hex digest. It concatenates the
2762 client's challenge string, a space character, and the SHA-1 hex
2763 digest, hex-encodes the result and sends it back to the server.
2768 The server generates the same concatenated string used by the
2769 client and computes its SHA-1 hash. It compares the hash with
2770 the hash received from the client; if the two hashes match, the
2771 client is authenticated.
2777 Each server has a "cookie context," which is a name that identifies a
2778 set of cookies that apply to that server. A sample context might be
2779 "org_freedesktop_session_bus". Context names must be valid ASCII,
2780 nonzero length, and may not contain the characters slash ("/"),
2781 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2782 tab ("\t"), or period ("."). There is a default context,
2783 "org_freedesktop_general" that's used by servers that do not specify
2787 Cookies are stored in a user's home directory, in the directory
2788 <filename>~/.dbus-keyrings/</filename>. This directory must
2789 not be readable or writable by other users. If it is,
2790 clients and servers must ignore it. The directory
2791 contains cookie files named after the cookie context.
2794 A cookie file contains one cookie per line. Each line
2795 has three space-separated fields:
2799 The cookie ID number, which must be a non-negative integer and
2800 may not be used twice in the same file.
2805 The cookie's creation time, in UNIX seconds-since-the-epoch
2811 The cookie itself, a hex-encoded random block of bytes. The cookie
2812 may be of any length, though obviously security increases
2813 as the length increases.
2819 Only server processes modify the cookie file.
2820 They must do so with this procedure:
2824 Create a lockfile name by appending ".lock" to the name of the
2825 cookie file. The server should attempt to create this file
2826 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2827 fails, the lock fails. Servers should retry for a reasonable
2828 period of time, then they may choose to delete an existing lock
2829 to keep users from having to manually delete a stale
2830 lock. <footnote><para>Lockfiles are used instead of real file
2831 locking <literal>fcntl()</literal> because real locking
2832 implementations are still flaky on network
2833 filesystems.</para></footnote>
2838 Once the lockfile has been created, the server loads the cookie
2839 file. It should then delete any cookies that are old (the
2840 timeout can be fairly short), or more than a reasonable
2841 time in the future (so that cookies never accidentally
2842 become permanent, if the clock was set far into the future
2843 at some point). If no recent keys remain, the
2844 server may generate a new key.
2849 The pruned and possibly added-to cookie file
2850 must be resaved atomically (using a temporary
2851 file which is rename()'d).
2856 The lock must be dropped by deleting the lockfile.
2862 Clients need not lock the file in order to load it,
2863 because servers are required to save the file atomically.
2868 <sect1 id="addresses">
2869 <title>Server Addresses</title>
2871 Server addresses consist of a transport name followed by a colon, and
2872 then an optional, comma-separated list of keys and values in the form key=value.
2873 Each value is escaped.
2877 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2878 Which is the address to a unix socket with the path /tmp/dbus-test.
2881 Value escaping is similar to URI escaping but simpler.
2885 The set of optionally-escaped bytes is:
2886 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2887 <emphasis>byte</emphasis> (note, not character) which is not in the
2888 set of optionally-escaped bytes must be replaced with an ASCII
2889 percent (<literal>%</literal>) and the value of the byte in hex.
2890 The hex value must always be two digits, even if the first digit is
2891 zero. The optionally-escaped bytes may be escaped if desired.
2896 To unescape, append each byte in the value; if a byte is an ASCII
2897 percent (<literal>%</literal>) character then append the following
2898 hex value instead. It is an error if a <literal>%</literal> byte
2899 does not have two hex digits following. It is an error if a
2900 non-optionally-escaped byte is seen unescaped.
2904 The set of optionally-escaped bytes is intended to preserve address
2905 readability and convenience.
2909 A server may specify a key-value pair with the key <literal>guid</literal>
2910 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2911 describes the format of the <literal>guid</literal> field. If present,
2912 this UUID may be used to distinguish one server address from another. A
2913 server should use a different UUID for each address it listens on. For
2914 example, if a message bus daemon offers both UNIX domain socket and TCP
2915 connections, but treats clients the same regardless of how they connect,
2916 those two connections are equivalent post-connection but should have
2917 distinct UUIDs to distinguish the kinds of connection.
2921 The intent of the address UUID feature is to allow a client to avoid
2922 opening multiple identical connections to the same server, by allowing the
2923 client to check whether an address corresponds to an already-existing
2924 connection. Comparing two addresses is insufficient, because addresses
2925 can be recycled by distinct servers, and equivalent addresses may look
2926 different if simply compared as strings (for example, the host in a TCP
2927 address can be given as an IP address or as a hostname).
2931 Note that the address key is <literal>guid</literal> even though the
2932 rest of the API and documentation says "UUID," for historical reasons.
2936 [FIXME clarify if attempting to connect to each is a requirement
2937 or just a suggestion]
2938 When connecting to a server, multiple server addresses can be
2939 separated by a semi-colon. The library will then try to connect
2940 to the first address and if that fails, it'll try to connect to
2941 the next one specified, and so forth. For example
2942 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2947 <sect1 id="transports">
2948 <title>Transports</title>
2950 [FIXME we need to specify in detail each transport and its possible arguments]
2952 Current transports include: unix domain sockets (including
2953 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2954 using in-process pipes. Future possible transports include one that
2955 tunnels over X11 protocol.
2958 <sect2 id="transports-unix-domain-sockets">
2959 <title>Unix Domain Sockets</title>
2961 Unix domain sockets can be either paths in the file system or on Linux
2962 kernels, they can be abstract which are similar to paths but
2963 do not show up in the file system.
2967 When a socket is opened by the D-Bus library it truncates the path
2968 name right before the first trailing Nul byte. This is true for both
2969 normal paths and abstract paths. Note that this is a departure from
2970 previous versions of D-Bus that would create sockets with a fixed
2971 length path name. Names which were shorter than the fixed length
2972 would be padded by Nul bytes.
2975 Unix domain sockets are not available on Windows.
2977 <sect3 id="transports-unix-domain-sockets-addresses">
2978 <title>Server Address Format</title>
2980 Unix domain socket addresses are identified by the "unix:" prefix
2981 and support the following key/value pairs:
2988 <entry>Values</entry>
2989 <entry>Description</entry>
2995 <entry>(path)</entry>
2996 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2999 <entry>tmpdir</entry>
3000 <entry>(path)</entry>
3001 <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>
3004 <entry>abstract</entry>
3005 <entry>(string)</entry>
3006 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
3013 <sect2 id="transports-launchd">
3014 <title>launchd</title>
3016 launchd is an open-source server management system that replaces init, inetd
3017 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3018 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3022 launchd allocates a socket and provides it with the unix path through the
3023 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3024 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3025 it through its environment.
3026 Other processes can query for the launchd socket by executing:
3027 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3028 This is normally done by the D-Bus client library so doesn't have to be done
3032 launchd is not available on Microsoft Windows.
3034 <sect3 id="transports-launchd-addresses">
3035 <title>Server Address Format</title>
3037 launchd addresses are identified by the "launchd:" prefix
3038 and support the following key/value pairs:
3045 <entry>Values</entry>
3046 <entry>Description</entry>
3052 <entry>(environment variable)</entry>
3053 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3060 <sect2 id="transports-systemd">
3061 <title>systemd</title>
3063 systemd is an open-source server management system that
3064 replaces init and inetd on newer Linux systems. It supports
3065 socket activation. The D-Bus systemd transport is used to acquire
3066 socket activation file descriptors from systemd and use them
3067 as D-Bus transport when the current process is spawned by
3068 socket activation from it.
3071 The systemd transport accepts only one or more Unix domain or
3072 TCP streams sockets passed in via socket activation.
3075 The systemd transport is not available on non-Linux operating systems.
3078 The systemd transport defines no parameter keys.
3081 <sect2 id="transports-tcp-sockets">
3082 <title>TCP Sockets</title>
3084 The tcp transport provides TCP/IP based connections between clients
3085 located on the same or different hosts.
3088 Using tcp transport without any additional secure authentification mechanismus
3089 over a network is unsecure.
3092 Windows notes: Because of the tcp stack on Windows does not provide sending
3093 credentials over a tcp connection, the EXTERNAL authentification
3094 mechanismus does not work.
3096 <sect3 id="transports-tcp-sockets-addresses">
3097 <title>Server Address Format</title>
3099 TCP/IP socket addresses are identified by the "tcp:" prefix
3100 and support the following key/value pairs:
3107 <entry>Values</entry>
3108 <entry>Description</entry>
3114 <entry>(string)</entry>
3115 <entry>dns name or ip address</entry>
3119 <entry>(number)</entry>
3120 <entry>The tcp port the server will open. A zero value let the server
3121 choose a free port provided from the underlaying operating system.
3122 libdbus is able to retrieve the real used port from the server.
3126 <entry>family</entry>
3127 <entry>(string)</entry>
3128 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3135 <sect2 id="transports-nonce-tcp-sockets">
3136 <title>Nonce-secured TCP Sockets</title>
3138 The nonce-tcp transport provides a secured TCP transport, using a
3139 simple authentication mechanism to ensure that only clients with read
3140 access to a certain location in the filesystem can connect to the server.
3141 The server writes a secret, the nonce, to a file and an incoming client
3142 connection is only accepted if the client sends the nonce right after
3143 the connect. The nonce mechanism requires no setup and is orthogonal to
3144 the higher-level authentication mechanisms described in the
3145 Authentication section.
3149 On start, the server generates a random 16 byte nonce and writes it
3150 to a file in the user's temporary directory. The nonce file location
3151 is published as part of the server's D-Bus address using the
3152 "noncefile" key-value pair.
3154 After an accept, the server reads 16 bytes from the socket. If the
3155 read bytes do not match the nonce stored in the nonce file, the
3156 server MUST immediately drop the connection.
3157 If the nonce match the received byte sequence, the client is accepted
3158 and the transport behaves like an unsecured tcp transport.
3161 After a successful connect to the server socket, the client MUST read
3162 the nonce from the file published by the server via the noncefile=
3163 key-value pair and send it over the socket. After that, the
3164 transport behaves like an unsecured tcp transport.
3166 <sect3 id="transports-nonce-tcp-sockets-addresses">
3167 <title>Server Address Format</title>
3169 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3170 and support the following key/value pairs:
3177 <entry>Values</entry>
3178 <entry>Description</entry>
3184 <entry>(string)</entry>
3185 <entry>dns name or ip address</entry>
3189 <entry>(number)</entry>
3190 <entry>The tcp port the server will open. A zero value let the server
3191 choose a free port provided from the underlaying operating system.
3192 libdbus is able to retrieve the real used port from the server.
3196 <entry>family</entry>
3197 <entry>(string)</entry>
3198 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3201 <entry>noncefile</entry>
3202 <entry>(path)</entry>
3203 <entry>file location containing the secret</entry>
3210 <sect2 id="transports-exec">
3211 <title>Executed Subprocesses on Unix</title>
3213 This transport forks off a process and connects its standard
3214 input and standard output with an anonymous Unix domain
3215 socket. This socket is then used for communication by the
3216 transport. This transport may be used to use out-of-process
3217 forwarder programs as basis for the D-Bus protocol.
3220 The forked process will inherit the standard error output and
3221 process group from the parent process.
3224 Executed subprocesses are not available on Windows.
3226 <sect3 id="transports-exec-addresses">
3227 <title>Server Address Format</title>
3229 Executed subprocess addresses are identified by the "unixexec:" prefix
3230 and support the following key/value pairs:
3237 <entry>Values</entry>
3238 <entry>Description</entry>
3244 <entry>(path)</entry>
3245 <entry>Path of the binary to execute, either an absolute
3246 path or a binary name that is searched for in the default
3247 search path of the OS. This corresponds to the first
3248 argument of execlp(). This key is mandatory.</entry>
3251 <entry>argv0</entry>
3252 <entry>(string)</entry>
3253 <entry>The program name to use when executing the
3254 binary. If omitted the same value as specified for path=
3255 will be used. This corresponds to the second argument of
3259 <entry>argv1, argv2, ...</entry>
3260 <entry>(string)</entry>
3261 <entry>Arguments to pass to the binary. This corresponds
3262 to the third and later arguments of execlp(). If a
3263 specific argvX is not specified no further argvY for Y > X
3264 are taken into account.</entry>
3272 <sect1 id="meta-transports">
3273 <title>Meta Transports</title>
3275 Meta transports are a kind of transport with special enhancements or
3276 behavior. Currently available meta transports include: autolaunch
3279 <sect2 id="meta-transports-autolaunch">
3280 <title>Autolaunch</title>
3281 <para>The autolaunch transport provides a way for dbus clients to autodetect
3282 a running dbus session bus and to autolaunch a session bus if not present.
3284 <sect3 id="meta-transports-autolaunch-addresses">
3285 <title>Server Address Format</title>
3287 Autolaunch addresses uses the "autolaunch:" prefix and support the
3288 following key/value pairs:
3295 <entry>Values</entry>
3296 <entry>Description</entry>
3301 <entry>scope</entry>
3302 <entry>(string)</entry>
3303 <entry>scope of autolaunch (Windows only)
3307 "*install-path" - limit session bus to dbus installation path.
3308 The dbus installation path is determined from the location of
3309 the shared dbus library. If the library is located in a 'bin'
3310 subdirectory the installation root is the directory above,
3311 otherwise the directory where the library lives is taken as
3314 <install-root>/bin/[lib]dbus-1.dll
3315 <install-root>/[lib]dbus-1.dll
3321 "*user" - limit session bus to the recent user.
3326 other values - specify dedicated session bus like "release",
3338 <sect3 id="meta-transports-autolaunch-windows-implementation">
3339 <title>Windows implementation</title>
3341 On start, the server opens a platform specific transport, creates a mutex
3342 and a shared memory section containing the related session bus address.
3343 This mutex will be inspected by the dbus client library to detect a
3344 running dbus session bus. The access to the mutex and the shared memory
3345 section are protected by global locks.
3348 In the recent implementation the autolaunch transport uses a tcp transport
3349 on localhost with a port choosen from the operating system. This detail may
3350 change in the future.
3353 Disclaimer: The recent implementation is in an early state and may not
3354 work in all cirumstances and/or may have security issues. Because of this
3355 the implementation is not documentated yet.
3362 <title>UUIDs</title>
3364 A working D-Bus implementation uses universally-unique IDs in two places.
3365 First, each server address has a UUID identifying the address,
3366 as described in <xref linkend="addresses"/>. Second, each operating
3367 system kernel instance running a D-Bus client or server has a UUID
3368 identifying that kernel, retrieved by invoking the method
3369 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3370 linkend="standard-interfaces-peer"/>).
3373 The term "UUID" in this document is intended literally, i.e. an
3374 identifier that is universally unique. It is not intended to refer to
3375 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3378 The UUID must contain 128 bits of data and be hex-encoded. The
3379 hex-encoded string may not contain hyphens or other non-hex-digit
3380 characters, and it must be exactly 32 characters long. To generate a
3381 UUID, the current reference implementation concatenates 96 bits of random
3382 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3386 It would also be acceptable and probably better to simply generate 128
3387 bits of random data, as long as the random number generator is of high
3388 quality. The timestamp could conceivably help if the random bits are not
3389 very random. With a quality random number generator, collisions are
3390 extremely unlikely even with only 96 bits, so it's somewhat academic.
3393 Implementations should, however, stick to random data for the first 96 bits
3398 <sect1 id="standard-interfaces">
3399 <title>Standard Interfaces</title>
3401 See <xref linkend="message-protocol-types-notation"/> for details on
3402 the notation used in this section. There are some standard interfaces
3403 that may be useful across various D-Bus applications.
3405 <sect2 id="standard-interfaces-peer">
3406 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3408 The <literal>org.freedesktop.DBus.Peer</literal> interface
3411 org.freedesktop.DBus.Peer.Ping ()
3412 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3416 On receipt of the <literal>METHOD_CALL</literal> message
3417 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3418 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3419 usual. It does not matter which object path a ping is sent to. The
3420 reference implementation handles this method automatically.
3423 On receipt of the <literal>METHOD_CALL</literal> message
3424 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3425 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3426 UUID representing the identity of the machine the process is running on.
3427 This UUID must be the same for all processes on a single system at least
3428 until that system next reboots. It should be the same across reboots
3429 if possible, but this is not always possible to implement and is not
3431 It does not matter which object path a GetMachineId is sent to. The
3432 reference implementation handles this method automatically.
3435 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3436 a virtual machine running on a hypervisor, rather than a physical machine.
3437 Basically if two processes see the same UUID, they should also see the same
3438 shared memory, UNIX domain sockets, process IDs, and other features that require
3439 a running OS kernel in common between the processes.
3442 The UUID is often used where other programs might use a hostname. Hostnames
3443 can change without rebooting, however, or just be "localhost" - so the UUID
3447 <xref linkend="uuids"/> explains the format of the UUID.
3451 <sect2 id="standard-interfaces-introspectable">
3452 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3454 This interface has one method:
3456 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3460 Objects instances may implement
3461 <literal>Introspect</literal> which returns an XML description of
3462 the object, including its interfaces (with signals and methods), objects
3463 below it in the object path tree, and its properties.
3466 <xref linkend="introspection-format"/> describes the format of this XML string.
3469 <sect2 id="standard-interfaces-properties">
3470 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3472 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3473 or <firstterm>attributes</firstterm>. These can be exposed via the
3474 <literal>org.freedesktop.DBus.Properties</literal> interface.
3478 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3479 in STRING property_name,
3481 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3482 in STRING property_name,
3484 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3485 out DICT<STRING,VARIANT> props);
3489 It is conventional to give D-Bus properties names consisting of
3490 capitalized words without punctuation ("CamelCase"), like
3491 <link linkend="message-protocol-names-member">member names</link>.
3492 For instance, the GObject property
3493 <literal>connection-status</literal> or the Qt property
3494 <literal>connectionStatus</literal> could be represented on D-Bus
3495 as <literal>ConnectionStatus</literal>.
3498 Strictly speaking, D-Bus property names are not required to follow
3499 the same naming restrictions as member names, but D-Bus property
3500 names that would not be valid member names (in particular,
3501 GObject-style dash-separated property names) can cause interoperability
3502 problems and should be avoided.
3505 The available properties and whether they are writable can be determined
3506 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3507 see <xref linkend="standard-interfaces-introspectable"/>.
3510 An empty string may be provided for the interface name; in this case,
3511 if there are multiple properties on an object with the same name,
3512 the results are undefined (picking one by according to an arbitrary
3513 deterministic rule, or returning an error, are the reasonable
3517 If one or more properties change on an object, the
3518 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3519 signal may be emitted (this signal was added in 0.14):
3523 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3524 DICT<STRING,VARIANT> changed_properties,
3525 ARRAY<STRING> invalidated_properties);
3529 where <literal>changed_properties</literal> is a dictionary
3530 containing the changed properties with the new values and
3531 <literal>invalidated_properties</literal> is an array of
3532 properties that changed but the value is not conveyed.
3535 Whether the <literal>PropertiesChanged</literal> signal is
3536 supported can be determined by calling
3537 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3538 that the signal may be supported for an object but it may
3539 differ how whether and how it is used on a per-property basis
3540 (for e.g. performance or security reasons). Each property (or
3541 the parent interface) must be annotated with the
3542 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3543 annotation to convey this (usually the default value
3544 <literal>true</literal> is sufficient meaning that the
3545 annotation does not need to be used). See <xref
3546 linkend="introspection-format"/> for details on this
3551 <sect2 id="standard-interfaces-objectmanager">
3552 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3554 An API can optionally make use of this interface for one or
3555 more sub-trees of objects. The root of each sub-tree implements
3556 this interface so other applications can get all objects,
3557 interfaces and properties in a single method call. It is
3558 appropriate to use this interface if users of the tree of
3559 objects are expected to be interested in all interfaces of all
3560 objects in the tree; a more granular API should be used if
3561 users of the objects are expected to be interested in a small
3562 subset of the objects, a small subset of their interfaces, or
3566 The method that applications can use to get all objects and
3567 properties is <literal>GetManagedObjects</literal>:
3571 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3575 The return value of this method is a dict whose keys are
3576 object paths. All returned object paths are children of the
3577 object path implementing this interface, i.e. their object
3578 paths start with the ObjectManager's object path plus '/'.
3581 Each value is a dict whose keys are interfaces names. Each
3582 value in this inner dict is the same dict that would be
3583 returned by the <link
3584 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3585 method for that combination of object path and interface. If
3586 an interface has no properties, the empty dict is returned.
3589 Changes are emitted using the following two signals:
3593 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3594 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3595 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3596 ARRAY<STRING> interfaces);
3600 The <literal>InterfacesAdded</literal> signal is emitted when
3601 either a new object is added or when an existing object gains
3602 one or more interfaces. The
3603 <literal>InterfacesRemoved</literal> signal is emitted
3604 whenever an object is removed or it loses one or more
3605 interfaces. The second parameter of the
3606 <literal>InterfacesAdded</literal> signal contains a dict with
3607 the interfaces and properties (if any) that have been added to
3608 the given object path. Similarly, the second parameter of the
3609 <literal>InterfacesRemoved</literal> signal contains an array
3610 of the interfaces that were removed. Note that changes on
3611 properties on existing interfaces are not reported using this
3612 interface - an application should also monitor the existing <link
3613 linkend="standard-interfaces-properties">PropertiesChanged</link>
3614 signal on each object.
3617 Applications SHOULD NOT export objects that are children of an
3618 object (directly or otherwise) implementing this interface but
3619 which are not returned in the reply from the
3620 <literal>GetManagedObjects()</literal> method of this
3621 interface on the given object.
3624 The intent of the <literal>ObjectManager</literal> interface
3625 is to make it easy to write a robust client
3626 implementation. The trivial client implementation only needs
3627 to make two method calls:
3631 org.freedesktop.DBus.AddMatch (bus_proxy,
3632 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3633 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3637 on the message bus and the remote application's
3638 <literal>ObjectManager</literal>, respectively. Whenever a new
3639 remote object is created (or an existing object gains a new
3640 interface), the <literal>InterfacesAdded</literal> signal is
3641 emitted, and since this signal contains all properties for the
3642 interfaces, no calls to the
3643 <literal>org.freedesktop.Properties</literal> interface on the
3644 remote object are needed. Additionally, since the initial
3645 <literal>AddMatch()</literal> rule already includes signal
3646 messages from the newly created child object, no new
3647 <literal>AddMatch()</literal> call is needed.
3652 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3653 interface was added in version 0.17 of the D-Bus
3660 <sect1 id="introspection-format">
3661 <title>Introspection Data Format</title>
3663 As described in <xref linkend="standard-interfaces-introspectable"/>,
3664 objects may be introspected at runtime, returning an XML string
3665 that describes the object. The same XML format may be used in
3666 other contexts as well, for example as an "IDL" for generating
3667 static language bindings.
3670 Here is an example of introspection data:
3672 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3673 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3674 <node name="/com/example/sample_object">
3675 <interface name="com.example.SampleInterface">
3676 <method name="Frobate">
3677 <arg name="foo" type="i" direction="in"/>
3678 <arg name="bar" type="s" direction="out"/>
3679 <arg name="baz" type="a{us}" direction="out"/>
3680 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3682 <method name="Bazify">
3683 <arg name="bar" type="(iiu)" direction="in"/>
3684 <arg name="bar" type="v" direction="out"/>
3686 <method name="Mogrify">
3687 <arg name="bar" type="(iiav)" direction="in"/>
3689 <signal name="Changed">
3690 <arg name="new_value" type="b"/>
3692 <property name="Bar" type="y" access="readwrite"/>
3694 <node name="child_of_sample_object"/>
3695 <node name="another_child_of_sample_object"/>
3700 A more formal DTD and spec needs writing, but here are some quick notes.
3704 Only the root <node> element can omit the node name, as it's
3705 known to be the object that was introspected. If the root
3706 <node> does have a name attribute, it must be an absolute
3707 object path. If child <node> have object paths, they must be
3713 If a child <node> has any sub-elements, then they
3714 must represent a complete introspection of the child.
3715 If a child <node> is empty, then it may or may
3716 not have sub-elements; the child must be introspected
3717 in order to find out. The intent is that if an object
3718 knows that its children are "fast" to introspect
3719 it can go ahead and return their information, but
3720 otherwise it can omit it.
3725 The direction element on <arg> may be omitted,
3726 in which case it defaults to "in" for method calls
3727 and "out" for signals. Signals only allow "out"
3728 so while direction may be specified, it's pointless.
3733 The possible directions are "in" and "out",
3734 unlike CORBA there is no "inout"
3739 The possible property access flags are
3740 "readwrite", "read", and "write"
3745 Multiple interfaces can of course be listed for
3751 The "name" attribute on arguments is optional.
3757 Method, interface, property, and signal elements may have
3758 "annotations", which are generic key/value pairs of metadata.
3759 They are similar conceptually to Java's annotations and C# attributes.
3760 Well-known annotations:
3767 <entry>Values (separated by ,)</entry>
3768 <entry>Description</entry>
3773 <entry>org.freedesktop.DBus.Deprecated</entry>
3774 <entry>true,false</entry>
3775 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3778 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3779 <entry>(string)</entry>
3780 <entry>The C symbol; may be used for methods and interfaces</entry>
3783 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3784 <entry>true,false</entry>
3785 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3788 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3789 <entry>true,invalidates,false</entry>
3792 If set to <literal>false</literal>, the
3793 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3795 linkend="standard-interfaces-properties"/> is not
3796 guaranteed to be emitted if the property changes.
3799 If set to <literal>invalidates</literal> the signal
3800 is emitted but the value is not included in the
3804 If set to <literal>true</literal> the signal is
3805 emitted with the value included.
3808 The value for the annotation defaults to
3809 <literal>true</literal> if the enclosing interface
3810 element does not specify the annotation. Otherwise it
3811 defaults to the value specified in the enclosing
3820 <sect1 id="message-bus">
3821 <title>Message Bus Specification</title>
3822 <sect2 id="message-bus-overview">
3823 <title>Message Bus Overview</title>
3825 The message bus accepts connections from one or more applications.
3826 Once connected, applications can exchange messages with other
3827 applications that are also connected to the bus.
3830 In order to route messages among connections, the message bus keeps a
3831 mapping from names to connections. Each connection has one
3832 unique-for-the-lifetime-of-the-bus name automatically assigned.
3833 Applications may request additional names for a connection. Additional
3834 names are usually "well-known names" such as
3835 "com.example.TextEditor". When a name is bound to a connection,
3836 that connection is said to <firstterm>own</firstterm> the name.
3839 The bus itself owns a special name,
3840 <literal>org.freedesktop.DBus</literal>, with an object
3841 located at <literal>/org/freedesktop/DBus</literal> that
3842 implements the <literal>org.freedesktop.DBus</literal>
3843 interface. This service allows applications to make
3844 administrative requests of the bus itself. For example,
3845 applications can ask the bus to assign a name to a connection.
3848 Each name may have <firstterm>queued owners</firstterm>. When an
3849 application requests a name for a connection and the name is already in
3850 use, the bus will optionally add the connection to a queue waiting for
3851 the name. If the current owner of the name disconnects or releases
3852 the name, the next connection in the queue will become the new owner.
3856 This feature causes the right thing to happen if you start two text
3857 editors for example; the first one may request "com.example.TextEditor",
3858 and the second will be queued as a possible owner of that name. When
3859 the first exits, the second will take over.
3863 Applications may send <firstterm>unicast messages</firstterm> to
3864 a specific recipient or to the message bus itself, or
3865 <firstterm>broadcast messages</firstterm> to all interested recipients.
3866 See <xref linkend="message-bus-routing"/> for details.
3870 <sect2 id="message-bus-names">
3871 <title>Message Bus Names</title>
3873 Each connection has at least one name, assigned at connection time and
3874 returned in response to the
3875 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3876 automatically-assigned name is called the connection's <firstterm>unique
3877 name</firstterm>. Unique names are never reused for two different
3878 connections to the same bus.
3881 Ownership of a unique name is a prerequisite for interaction with
3882 the message bus. It logically follows that the unique name is always
3883 the first name that an application comes to own, and the last
3884 one that it loses ownership of.
3887 Unique connection names must begin with the character ':' (ASCII colon
3888 character); bus names that are not unique names must not begin
3889 with this character. (The bus must reject any attempt by an application
3890 to manually request a name beginning with ':'.) This restriction
3891 categorically prevents "spoofing"; messages sent to a unique name
3892 will always go to the expected connection.
3895 When a connection is closed, all the names that it owns are deleted (or
3896 transferred to the next connection in the queue if any).
3899 A connection can request additional names to be associated with it using
3900 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3901 linkend="message-protocol-names-bus"/> describes the format of a valid
3902 name. These names can be released again using the
3903 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3906 <sect3 id="bus-messages-request-name">
3907 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3911 UINT32 RequestName (in STRING name, in UINT32 flags)
3918 <entry>Argument</entry>
3920 <entry>Description</entry>
3926 <entry>STRING</entry>
3927 <entry>Name to request</entry>
3931 <entry>UINT32</entry>
3932 <entry>Flags</entry>
3942 <entry>Argument</entry>
3944 <entry>Description</entry>
3950 <entry>UINT32</entry>
3951 <entry>Return value</entry>
3958 This method call should be sent to
3959 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3960 assign the given name to the method caller. Each name maintains a
3961 queue of possible owners, where the head of the queue is the primary
3962 or current owner of the name. Each potential owner in the queue
3963 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3964 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3965 call. When RequestName is invoked the following occurs:
3969 If the method caller is currently the primary owner of the name,
3970 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3971 values are updated with the values from the new RequestName call,
3972 and nothing further happens.
3978 If the current primary owner (head of the queue) has
3979 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3980 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3981 the caller of RequestName replaces the current primary owner at
3982 the head of the queue and the current primary owner moves to the
3983 second position in the queue. If the caller of RequestName was
3984 in the queue previously its flags are updated with the values from
3985 the new RequestName in addition to moving it to the head of the queue.
3991 If replacement is not possible, and the method caller is
3992 currently in the queue but not the primary owner, its flags are
3993 updated with the values from the new RequestName call.
3999 If replacement is not possible, and the method caller is
4000 currently not in the queue, the method caller is appended to the
4007 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4008 set and is not the primary owner, it is removed from the
4009 queue. This can apply to the previous primary owner (if it
4010 was replaced) or the method caller (if it updated the
4011 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4012 queue, or if it was just added to the queue with that flag set).
4018 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4019 queue," even if another application already in the queue had specified
4020 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4021 that does not allow replacement goes away, and the next primary owner
4022 does allow replacement. In this case, queued items that specified
4023 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4024 automatically replace the new primary owner. In other words,
4025 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4026 time RequestName is called. This is deliberate to avoid an infinite loop
4027 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4028 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4031 The flags argument contains any of the following values logically ORed
4038 <entry>Conventional Name</entry>
4039 <entry>Value</entry>
4040 <entry>Description</entry>
4045 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4049 If an application A specifies this flag and succeeds in
4050 becoming the owner of the name, and another application B
4051 later calls RequestName with the
4052 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4053 will lose ownership and receive a
4054 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4055 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4056 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4057 is not specified by application B, then application B will not replace
4058 application A as the owner.
4063 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4067 Try to replace the current owner if there is one. If this
4068 flag is not set the application will only become the owner of
4069 the name if there is no current owner. If this flag is set,
4070 the application will replace the current owner if
4071 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4076 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4080 Without this flag, if an application requests a name that is
4081 already owned, the application will be placed in a queue to
4082 own the name when the current owner gives it up. If this
4083 flag is given, the application will not be placed in the
4084 queue, the request for the name will simply fail. This flag
4085 also affects behavior when an application is replaced as
4086 name owner; by default the application moves back into the
4087 waiting queue, unless this flag was provided when the application
4088 became the name owner.
4096 The return code can be one of the following values:
4102 <entry>Conventional Name</entry>
4103 <entry>Value</entry>
4104 <entry>Description</entry>
4109 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4110 <entry>1</entry> <entry>The caller is now the primary owner of
4111 the name, replacing any previous owner. Either the name had no
4112 owner before, or the caller specified
4113 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4114 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4117 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4120 <entry>The name already had an owner,
4121 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4122 the current owner did not specify
4123 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4124 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4128 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4129 <entry>The name already has an owner,
4130 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4131 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4132 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4133 specified by the requesting application.</entry>
4136 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4138 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4146 <sect3 id="bus-messages-release-name">
4147 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4151 UINT32 ReleaseName (in STRING name)
4158 <entry>Argument</entry>
4160 <entry>Description</entry>
4166 <entry>STRING</entry>
4167 <entry>Name to release</entry>
4177 <entry>Argument</entry>
4179 <entry>Description</entry>
4185 <entry>UINT32</entry>
4186 <entry>Return value</entry>
4193 This method call should be sent to
4194 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4195 release the method caller's claim to the given name. If the caller is
4196 the primary owner, a new primary owner will be selected from the
4197 queue if any other owners are waiting. If the caller is waiting in
4198 the queue for the name, the caller will removed from the queue and
4199 will not be made an owner of the name if it later becomes available.
4200 If there are no other owners in the queue for the name, it will be
4201 removed from the bus entirely.
4203 The return code can be one of the following values:
4209 <entry>Conventional Name</entry>
4210 <entry>Value</entry>
4211 <entry>Description</entry>
4216 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4217 <entry>1</entry> <entry>The caller has released his claim on
4218 the given name. Either the caller was the primary owner of
4219 the name, and the name is now unused or taken by somebody
4220 waiting in the queue for the name, or the caller was waiting
4221 in the queue for the name and has now been removed from the
4225 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4227 <entry>The given name does not exist on this bus.</entry>
4230 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4232 <entry>The caller was not the primary owner of this name,
4233 and was also not waiting in the queue to own this name.</entry>
4241 <sect3 id="bus-messages-list-queued-owners">
4242 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4246 ARRAY of STRING ListQueuedOwners (in STRING name)
4253 <entry>Argument</entry>
4255 <entry>Description</entry>
4261 <entry>STRING</entry>
4262 <entry>The well-known bus name to query, such as
4263 <literal>com.example.cappuccino</literal></entry>
4273 <entry>Argument</entry>
4275 <entry>Description</entry>
4281 <entry>ARRAY of STRING</entry>
4282 <entry>The unique bus names of connections currently queued
4283 for the name</entry>
4290 This method call should be sent to
4291 <literal>org.freedesktop.DBus</literal> and lists the connections
4292 currently queued for a bus name (see
4293 <xref linkend="term-queued-owner"/>).
4298 <sect2 id="message-bus-routing">
4299 <title>Message Bus Message Routing</title>
4302 Messages may have a <literal>DESTINATION</literal> field (see <xref
4303 linkend="message-protocol-header-fields"/>), resulting in a
4304 <firstterm>unicast message</firstterm>. If the
4305 <literal>DESTINATION</literal> field is present, it specifies a message
4306 recipient by name. Method calls and replies normally specify this field.
4307 The message bus must send messages (of any type) with the
4308 <literal>DESTINATION</literal> field set to the specified recipient,
4309 regardless of whether the recipient has set up a match rule matching
4314 When the message bus receives a signal, if the
4315 <literal>DESTINATION</literal> field is absent, it is considered to
4316 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4317 applications with <firstterm>message matching rules</firstterm> that
4318 match the message. Most signal messages are broadcasts.
4322 Unicast signal messages (those with a <literal>DESTINATION</literal>
4323 field) are not commonly used, but they are treated like any unicast
4324 message: they are delivered to the specified receipient,
4325 regardless of its match rules. One use for unicast signals is to
4326 avoid a race condition in which a signal is emitted before the intended
4327 recipient can call <xref linkend="bus-messages-add-match"/> to
4328 receive that signal: if the signal is sent directly to that recipient
4329 using a unicast message, it does not need to add a match rule at all,
4330 and there is no race condition. Another use for unicast signals,
4331 on message buses whose security policy prevents eavesdropping, is to
4332 send sensitive information which should only be visible to one
4337 When the message bus receives a method call, if the
4338 <literal>DESTINATION</literal> field is absent, the call is taken to be
4339 a standard one-to-one message and interpreted by the message bus
4340 itself. For example, sending an
4341 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4342 <literal>DESTINATION</literal> will cause the message bus itself to
4343 reply to the ping immediately; the message bus will not make this
4344 message visible to other applications.
4348 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4349 the ping message were sent with a <literal>DESTINATION</literal> name of
4350 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4351 forwarded, and the Yoyodyne Corporation screensaver application would be
4352 expected to reply to the ping.
4356 Message bus implementations may impose a security policy which
4357 prevents certain messages from being sent or received.
4358 When a message cannot be sent or received due to a security
4359 policy, the message bus should send an error reply, unless the
4360 original message had the <literal>NO_REPLY</literal> flag.
4363 <sect3 id="message-bus-routing-eavesdropping">
4364 <title>Eavesdropping</title>
4366 Receiving a unicast message whose <literal>DESTINATION</literal>
4367 indicates a different recipient is called
4368 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4369 a security boundary (like the standard system bus), the security
4370 policy should usually prevent eavesdropping, since unicast messages
4371 are normally kept private and may contain security-sensitive
4376 Eavesdropping is mainly useful for debugging tools, such as
4377 the <literal>dbus-monitor</literal> tool in the reference
4378 implementation of D-Bus. Tools which eavesdrop on the message bus
4379 should be careful to avoid sending a reply or error in response to
4380 messages intended for a different client.
4384 Clients may attempt to eavesdrop by adding match rules
4385 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4386 the <literal>eavesdrop='true'</literal> match. If the message bus'
4387 security policy does not allow eavesdropping, the match rule can
4388 still be added, but will not have any practical effect. For
4389 compatibility with older message bus implementations, if adding such
4390 a match rule results in an error reply, the client may fall back to
4391 adding the same rule with the <literal>eavesdrop</literal> match
4396 <sect3 id="message-bus-routing-match-rules">
4397 <title>Match Rules</title>
4399 An important part of the message bus routing protocol is match
4400 rules. Match rules describe the messages that should be sent to a
4401 client, based on the contents of the message. Broadcast signals
4402 are only sent to clients which have a suitable match rule: this
4403 avoids waking up client processes to deal with signals that are
4404 not relevant to that client.
4407 Messages that list a client as their <literal>DESTINATION</literal>
4408 do not need to match the client's match rules, and are sent to that
4409 client regardless. As a result, match rules are mainly used to
4410 receive a subset of broadcast signals.
4413 Match rules can also be used for eavesdropping
4414 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4415 if the security policy of the message bus allows it.
4418 Match rules are added using the AddMatch bus method
4419 (see <xref linkend="bus-messages-add-match"/>). Rules are
4420 specified as a string of comma separated key/value pairs.
4421 Excluding a key from the rule indicates a wildcard match.
4422 For instance excluding the the member from a match rule but
4423 adding a sender would let all messages from that sender through.
4424 An example of a complete rule would be
4425 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4428 The following table describes the keys that can be used to create
4435 <entry>Possible Values</entry>
4436 <entry>Description</entry>
4441 <entry><literal>type</literal></entry>
4442 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4443 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4446 <entry><literal>sender</literal></entry>
4447 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4448 and <xref linkend="term-unique-name"/> respectively)
4450 <entry>Match messages sent by a particular sender. An example of a sender match
4451 is sender='org.freedesktop.Hal'</entry>
4454 <entry><literal>interface</literal></entry>
4455 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4456 <entry>Match messages sent over or to a particular interface. An example of an
4457 interface match is interface='org.freedesktop.Hal.Manager'.
4458 If a message omits the interface header, it must not match any rule
4459 that specifies this key.</entry>
4462 <entry><literal>member</literal></entry>
4463 <entry>Any valid method or signal name</entry>
4464 <entry>Matches messages which have the give method or signal name. An example of
4465 a member match is member='NameOwnerChanged'</entry>
4468 <entry><literal>path</literal></entry>
4469 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4470 <entry>Matches messages which are sent from or to the given object. An example of a
4471 path match is path='/org/freedesktop/Hal/Manager'</entry>
4474 <entry><literal>path_namespace</literal></entry>
4475 <entry>An object path</entry>
4478 Matches messages which are sent from or to an
4479 object for which the object path is either the
4480 given value, or that value followed by one or
4481 more path components.
4486 <literal>path_namespace='/com/example/foo'</literal>
4487 would match signals sent by
4488 <literal>/com/example/foo</literal>
4490 <literal>/com/example/foo/bar</literal>,
4492 <literal>/com/example/foobar</literal>.
4496 Using both <literal>path</literal> and
4497 <literal>path_namespace</literal> in the same match
4498 rule is not allowed.
4503 This match key was added in version 0.16 of the
4504 D-Bus specification and implemented by the bus
4505 daemon in dbus 1.5.0 and later.
4511 <entry><literal>destination</literal></entry>
4512 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4513 <entry>Matches messages which are being sent to the given unique name. An
4514 example of a destination match is destination=':1.0'</entry>
4517 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4518 <entry>Any string</entry>
4519 <entry>Arg matches are special and are used for further restricting the
4520 match based on the arguments in the body of a message. Only arguments of type
4521 STRING can be matched in this way. An example of an argument match
4522 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4526 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4527 <entry>Any string</entry>
4529 <para>Argument path matches provide a specialised form of wildcard matching for
4530 path-like namespaces. They can match arguments whose type is either STRING or
4531 OBJECT_PATH. As with normal argument matches,
4532 if the argument is exactly equal to the string given in the match
4533 rule then the rule is satisfied. Additionally, there is also a
4534 match when either the string given in the match rule or the
4535 appropriate message argument ends with '/' and is a prefix of the
4536 other. An example argument path match is arg0path='/aa/bb/'. This
4537 would match messages with first arguments of '/', '/aa/',
4538 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4539 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4541 <para>This is intended for monitoring “directories” in file system-like
4542 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4543 system. An application interested in all nodes in a particular hierarchy would
4544 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4545 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4546 represent a modification to the “bar” property, or a signal with zeroth
4547 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4548 many properties within that directory, and the interested application would be
4549 notified in both cases.</para>
4552 This match key was added in version 0.12 of the
4553 D-Bus specification, implemented for STRING
4554 arguments by the bus daemon in dbus 1.2.0 and later,
4555 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4562 <entry><literal>arg0namespace</literal></entry>
4563 <entry>Like a bus name, except that the string is not
4564 required to contain a '.' (period)</entry>
4566 <para>Match messages whose first argument is of type STRING, and is a bus name
4567 or interface name within the specified namespace. This is primarily intended
4568 for watching name owner changes for a group of related bus names, rather than
4569 for a single name or all name changes.</para>
4571 <para>Because every valid interface name is also a valid
4572 bus name, this can also be used for messages whose
4573 first argument is an interface name.</para>
4575 <para>For example, the match rule
4576 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4577 matches name owner changes for bus names such as
4578 <literal>com.example.backend.foo</literal>,
4579 <literal>com.example.backend.foo.bar</literal>, and
4580 <literal>com.example.backend</literal> itself.</para>
4582 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4585 This match key was added in version 0.16 of the
4586 D-Bus specification and implemented by the bus
4587 daemon in dbus 1.5.0 and later.
4593 <entry><literal>eavesdrop</literal></entry>
4594 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4595 <entry>Since D-Bus 1.5.6, match rules do not
4596 match messages which have a <literal>DESTINATION</literal>
4597 field unless the match rule specifically
4599 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4600 by specifying <literal>eavesdrop='true'</literal>
4601 in the match rule. <literal>eavesdrop='false'</literal>
4602 restores the default behaviour. Messages are
4603 delivered to their <literal>DESTINATION</literal>
4604 regardless of match rules, so this match does not
4605 affect normal delivery of unicast messages.
4606 If the message bus has a security policy which forbids
4607 eavesdropping, this match may still be used without error,
4608 but will not have any practical effect.
4609 In older versions of D-Bus, this match was not allowed
4610 in match rules, and all match rules behaved as if
4611 <literal>eavesdrop='true'</literal> had been used.
4620 <sect2 id="message-bus-starting-services">
4621 <title>Message Bus Starting Services</title>
4623 The message bus can start applications on behalf of other applications.
4624 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4625 An application that can be started in this way is called a
4626 <firstterm>service</firstterm>.
4629 With D-Bus, starting a service is normally done by name. That is,
4630 applications ask the message bus to start some program that will own a
4631 well-known name, such as <literal>com.example.TextEditor</literal>.
4632 This implies a contract documented along with the name
4633 <literal>com.example.TextEditor</literal> for which object
4634 the owner of that name will provide, and what interfaces those
4638 To find an executable corresponding to a particular name, the bus daemon
4639 looks for <firstterm>service description files</firstterm>. Service
4640 description files define a mapping from names to executables. Different
4641 kinds of message bus will look for these files in different places, see
4642 <xref linkend="message-bus-types"/>.
4645 Service description files have the ".service" file
4646 extension. The message bus will only load service description files
4647 ending with .service; all other files will be ignored. The file format
4648 is similar to that of <ulink
4649 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4650 entries</ulink>. All service description files must be in UTF-8
4651 encoding. To ensure that there will be no name collisions, service files
4652 must be namespaced using the same mechanism as messages and service
4657 On the well-known system bus, the name of a service description file
4658 must be its well-known name plus <literal>.service</literal>,
4660 <literal>com.example.ConfigurationDatabase.service</literal>.
4664 On the well-known session bus, services should follow the same
4665 service description file naming convention as on the system bus,
4666 but for backwards compatibility they are not required to do so.
4670 [FIXME the file format should be much better specified than "similar to
4671 .desktop entries" esp. since desktop entries are already
4672 badly-specified. ;-)]
4673 These sections from the specification apply to service files as well:
4676 <listitem><para>General syntax</para></listitem>
4677 <listitem><para>Comment format</para></listitem>
4680 Service description files must contain a
4681 <literal>D-BUS Service</literal> group with at least the keys
4682 <literal>Name</literal> (the well-known name of the service)
4683 and <literal>Exec</literal> (the command to be executed).
4686 <title>Example service description file</title>
4688 # Sample service description file
4690 Name=com.example.ConfigurationDatabase
4691 Exec=/usr/bin/sample-configd
4697 Additionally, service description files for the well-known system
4698 bus on Unix must contain a <literal>User</literal> key, whose value
4699 is the name of a user account (e.g. <literal>root</literal>).
4700 The system service will be run as that user.
4704 When an application asks to start a service by name, the bus daemon tries to
4705 find a service that will own that name. It then tries to spawn the
4706 executable associated with it. If this fails, it will report an
4711 On the well-known system bus, it is not possible for two .service files
4712 in the same directory to offer the same service, because they are
4713 constrained to have names that match the service name.
4717 On the well-known session bus, if two .service files in the same
4718 directory offer the same service name, the result is undefined.
4719 Distributors should avoid this situation, for instance by naming
4720 session services' .service files according to their service name.
4724 If two .service files in different directories offer the same
4725 service name, the one in the higher-priority directory is used:
4726 for instance, on the system bus, .service files in
4727 /usr/local/share/dbus-1/system-services take precedence over those
4728 in /usr/share/dbus-1/system-services.
4731 The executable launched will have the environment variable
4732 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4733 message bus so it can connect and request the appropriate names.
4736 The executable being launched may want to know whether the message bus
4737 starting it is one of the well-known message buses (see <xref
4738 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4739 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4740 of the well-known buses. The currently-defined values for this variable
4741 are <literal>system</literal> for the systemwide message bus,
4742 and <literal>session</literal> for the per-login-session message
4743 bus. The new executable must still connect to the address given
4744 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4745 resulting connection is to the well-known bus.
4748 [FIXME there should be a timeout somewhere, either specified
4749 in the .service file, by the client, or just a global value
4750 and if the client being activated fails to connect within that
4751 timeout, an error should be sent back.]
4754 <sect3 id="message-bus-starting-services-scope">
4755 <title>Message Bus Service Scope</title>
4757 The "scope" of a service is its "per-", such as per-session,
4758 per-machine, per-home-directory, or per-display. The reference
4759 implementation doesn't yet support starting services in a different
4760 scope from the message bus itself. So e.g. if you start a service
4761 on the session bus its scope is per-session.
4764 We could add an optional scope to a bus name. For example, for
4765 per-(display,session pair), we could have a unique ID for each display
4766 generated automatically at login and set on screen 0 by executing a
4767 special "set display ID" binary. The ID would be stored in a
4768 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4769 random bytes. This ID would then be used to scope names.
4770 Starting/locating a service could be done by ID-name pair rather than
4774 Contrast this with a per-display scope. To achieve that, we would
4775 want a single bus spanning all sessions using a given display.
4776 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4777 property on screen 0 of the display, pointing to this bus.
4782 <sect2 id="message-bus-types">
4783 <title>Well-known Message Bus Instances</title>
4785 Two standard message bus instances are defined here, along with how
4786 to locate them and where their service files live.
4788 <sect3 id="message-bus-types-login">
4789 <title>Login session message bus</title>
4791 Each time a user logs in, a <firstterm>login session message
4792 bus</firstterm> may be started. All applications in the user's login
4793 session may interact with one another using this message bus.
4796 The address of the login session message bus is given
4797 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4798 variable. If that variable is not set, applications may
4799 also try to read the address from the X Window System root
4800 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4801 The root window property must have type <literal>STRING</literal>.
4802 The environment variable should have precedence over the
4803 root window property.
4805 <para>The address of the login session message bus is given in the
4806 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4807 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4808 "autolaunch:", the system should use platform-specific methods of
4809 locating a running D-Bus session server, or starting one if a running
4810 instance cannot be found. Note that this mechanism is not recommended
4811 for attempting to determine if a daemon is running. It is inherently
4812 racy to attempt to make this determination, since the bus daemon may
4813 be started just before or just after the determination is made.
4814 Therefore, it is recommended that applications do not try to make this
4815 determination for their functionality purposes, and instead they
4816 should attempt to start the server.</para>
4818 <sect4 id="message-bus-types-login-x-windows">
4819 <title>X Windowing System</title>
4821 For the X Windowing System, the application must locate the
4822 window owner of the selection represented by the atom formed by
4826 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4830 <para>the current user's username</para>
4834 <para>the literal character '_' (underscore)</para>
4838 <para>the machine's ID</para>
4844 The following properties are defined for the window that owns
4846 <informaltable frame="all">
4855 <para>meaning</para>
4861 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4865 <para>the actual address of the server socket</para>
4871 <para>_DBUS_SESSION_BUS_PID</para>
4875 <para>the PID of the server process</para>
4884 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4885 present in this window.
4889 If the X selection cannot be located or if reading the
4890 properties from the window fails, the implementation MUST conclude
4891 that there is no D-Bus server running and proceed to start a new
4892 server. (See below on concurrency issues)
4896 Failure to connect to the D-Bus server address thus obtained
4897 MUST be treated as a fatal connection error and should be reported
4902 As an alternative, an implementation MAY find the information
4903 in the following file located in the current user's home directory,
4904 in subdirectory .dbus/session-bus/:
4907 <para>the machine's ID</para>
4911 <para>the literal character '-' (dash)</para>
4915 <para>the X display without the screen number, with the
4916 following prefixes removed, if present: ":", "localhost:"
4917 ."localhost.localdomain:". That is, a display of
4918 "localhost:10.0" produces just the number "10"</para>
4924 The contents of this file NAME=value assignment pairs and
4925 lines starting with # are comments (no comments are allowed
4926 otherwise). The following variable names are defined:
4933 <para>Variable</para>
4937 <para>meaning</para>
4943 <para>DBUS_SESSION_BUS_ADDRESS</para>
4947 <para>the actual address of the server socket</para>
4953 <para>DBUS_SESSION_BUS_PID</para>
4957 <para>the PID of the server process</para>
4963 <para>DBUS_SESSION_BUS_WINDOWID</para>
4967 <para>the window ID</para>
4976 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4981 Failure to open this file MUST be interpreted as absence of a
4982 running server. Therefore, the implementation MUST proceed to
4983 attempting to launch a new bus server if the file cannot be
4988 However, success in opening this file MUST NOT lead to the
4989 conclusion that the server is running. Thus, a failure to connect to
4990 the bus address obtained by the alternative method MUST NOT be
4991 considered a fatal error. If the connection cannot be established,
4992 the implementation MUST proceed to check the X selection settings or
4993 to start the server on its own.
4997 If the implementation concludes that the D-Bus server is not
4998 running it MUST attempt to start a new server and it MUST also
4999 ensure that the daemon started as an effect of the "autolaunch"
5000 mechanism provides the lookup mechanisms described above, so
5001 subsequent calls can locate the newly started server. The
5002 implementation MUST also ensure that if two or more concurrent
5003 initiations happen, only one server remains running and all other
5004 initiations are able to obtain the address of this server and
5005 connect to it. In other words, the implementation MUST ensure that
5006 the X selection is not present when it attempts to set it, without
5007 allowing another process to set the selection between the
5008 verification and the setting (e.g., by using XGrabServer /
5015 On Unix systems, the session bus should search for .service files
5016 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5018 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5019 Implementations may also search additional locations, which
5020 should be searched with lower priority than anything in
5021 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5022 for example, the reference implementation also
5023 looks in <literal>${datadir}/dbus-1/services</literal> as
5024 set at compile time.
5027 As described in the XDG Base Directory Specification, software
5028 packages should install their session .service files to their
5029 configured <literal>${datadir}/dbus-1/services</literal>,
5030 where <literal>${datadir}</literal> is as defined by the GNU
5031 coding standards. System administrators or users can arrange
5032 for these service files to be read by setting XDG_DATA_DIRS or by
5033 symlinking them into the default locations.
5037 <sect3 id="message-bus-types-system">
5038 <title>System message bus</title>
5040 A computer may have a <firstterm>system message bus</firstterm>,
5041 accessible to all applications on the system. This message bus may be
5042 used to broadcast system events, such as adding new hardware devices,
5043 changes in the printer queue, and so forth.
5046 The address of the system message bus is given
5047 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5048 variable. If that variable is not set, applications should try
5049 to connect to the well-known address
5050 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5053 The D-Bus reference implementation actually honors the
5054 <literal>$(localstatedir)</literal> configure option
5055 for this address, on both client and server side.
5060 On Unix systems, the system bus should default to searching
5061 for .service files in
5062 <literal>/usr/local/share/dbus-1/system-services</literal>,
5063 <literal>/usr/share/dbus-1/system-services</literal> and
5064 <literal>/lib/dbus-1/system-services</literal>, with that order
5065 of precedence. It may also search other implementation-specific
5066 locations, but should not vary these locations based on environment
5070 The system bus is security-sensitive and is typically executed
5071 by an init system with a clean environment. Its launch helper
5072 process is particularly security-sensitive, and specifically
5073 clears its own environment.
5078 Software packages should install their system .service
5079 files to their configured
5080 <literal>${datadir}/dbus-1/system-services</literal>,
5081 where <literal>${datadir}</literal> is as defined by the GNU
5082 coding standards. System administrators can arrange
5083 for these service files to be read by editing the system bus'
5084 configuration file or by symlinking them into the default
5090 <sect2 id="message-bus-messages">
5091 <title>Message Bus Messages</title>
5093 The special message bus name <literal>org.freedesktop.DBus</literal>
5094 responds to a number of additional messages.
5097 <sect3 id="bus-messages-hello">
5098 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5109 <entry>Argument</entry>
5111 <entry>Description</entry>
5117 <entry>STRING</entry>
5118 <entry>Unique name assigned to the connection</entry>
5125 Before an application is able to send messages to other applications
5126 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5127 to the message bus to obtain a unique name. If an application without
5128 a unique name tries to send a message to another application, or a
5129 message to the message bus itself that isn't the
5130 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5131 disconnected from the bus.
5134 There is no corresponding "disconnect" request; if a client wishes to
5135 disconnect from the bus, it simply closes the socket (or other
5136 communication channel).
5139 <sect3 id="bus-messages-list-names">
5140 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5144 ARRAY of STRING ListNames ()
5151 <entry>Argument</entry>
5153 <entry>Description</entry>
5159 <entry>ARRAY of STRING</entry>
5160 <entry>Array of strings where each string is a bus name</entry>
5167 Returns a list of all currently-owned names on the bus.
5170 <sect3 id="bus-messages-list-activatable-names">
5171 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5175 ARRAY of STRING ListActivatableNames ()
5182 <entry>Argument</entry>
5184 <entry>Description</entry>
5190 <entry>ARRAY of STRING</entry>
5191 <entry>Array of strings where each string is a bus name</entry>
5198 Returns a list of all names that can be activated on the bus.
5201 <sect3 id="bus-messages-name-exists">
5202 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5206 BOOLEAN NameHasOwner (in STRING name)
5213 <entry>Argument</entry>
5215 <entry>Description</entry>
5221 <entry>STRING</entry>
5222 <entry>Name to check</entry>
5232 <entry>Argument</entry>
5234 <entry>Description</entry>
5240 <entry>BOOLEAN</entry>
5241 <entry>Return value, true if the name exists</entry>
5248 Checks if the specified name exists (currently has an owner).
5252 <sect3 id="bus-messages-name-owner-changed">
5253 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5257 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5264 <entry>Argument</entry>
5266 <entry>Description</entry>
5272 <entry>STRING</entry>
5273 <entry>Name with a new owner</entry>
5277 <entry>STRING</entry>
5278 <entry>Old owner or empty string if none</entry>
5282 <entry>STRING</entry>
5283 <entry>New owner or empty string if none</entry>
5290 This signal indicates that the owner of a name has changed.
5291 It's also the signal to use to detect the appearance of
5292 new names on the bus.
5295 <sect3 id="bus-messages-name-lost">
5296 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5300 NameLost (STRING name)
5307 <entry>Argument</entry>
5309 <entry>Description</entry>
5315 <entry>STRING</entry>
5316 <entry>Name which was lost</entry>
5323 This signal is sent to a specific application when it loses
5324 ownership of a name.
5328 <sect3 id="bus-messages-name-acquired">
5329 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5333 NameAcquired (STRING name)
5340 <entry>Argument</entry>
5342 <entry>Description</entry>
5348 <entry>STRING</entry>
5349 <entry>Name which was acquired</entry>
5356 This signal is sent to a specific application when it gains
5357 ownership of a name.
5361 <sect3 id="bus-messages-start-service-by-name">
5362 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5366 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5373 <entry>Argument</entry>
5375 <entry>Description</entry>
5381 <entry>STRING</entry>
5382 <entry>Name of the service to start</entry>
5386 <entry>UINT32</entry>
5387 <entry>Flags (currently not used)</entry>
5397 <entry>Argument</entry>
5399 <entry>Description</entry>
5405 <entry>UINT32</entry>
5406 <entry>Return value</entry>
5411 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5415 The return value can be one of the following values:
5420 <entry>Identifier</entry>
5421 <entry>Value</entry>
5422 <entry>Description</entry>
5427 <entry>DBUS_START_REPLY_SUCCESS</entry>
5429 <entry>The service was successfully started.</entry>
5432 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5434 <entry>A connection already owns the given name.</entry>
5443 <sect3 id="bus-messages-update-activation-environment">
5444 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5448 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5455 <entry>Argument</entry>
5457 <entry>Description</entry>
5463 <entry>ARRAY of DICT<STRING,STRING></entry>
5464 <entry>Environment to add or update</entry>
5469 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5472 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5475 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.
5480 <sect3 id="bus-messages-get-name-owner">
5481 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5485 STRING GetNameOwner (in STRING name)
5492 <entry>Argument</entry>
5494 <entry>Description</entry>
5500 <entry>STRING</entry>
5501 <entry>Name to get the owner of</entry>
5511 <entry>Argument</entry>
5513 <entry>Description</entry>
5519 <entry>STRING</entry>
5520 <entry>Return value, a unique connection name</entry>
5525 Returns the unique connection name of the primary owner of the name
5526 given. If the requested name doesn't have an owner, returns a
5527 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5531 <sect3 id="bus-messages-get-connection-unix-user">
5532 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5536 UINT32 GetConnectionUnixUser (in STRING bus_name)
5543 <entry>Argument</entry>
5545 <entry>Description</entry>
5551 <entry>STRING</entry>
5552 <entry>Unique or well-known bus name of the connection to
5553 query, such as <literal>:12.34</literal> or
5554 <literal>com.example.tea</literal></entry>
5564 <entry>Argument</entry>
5566 <entry>Description</entry>
5572 <entry>UINT32</entry>
5573 <entry>Unix user ID</entry>
5578 Returns the Unix user ID of the process connected to the server. If
5579 unable to determine it (for instance, because the process is not on the
5580 same machine as the bus daemon), an error is returned.
5584 <sect3 id="bus-messages-get-connection-unix-process-id">
5585 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5589 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5596 <entry>Argument</entry>
5598 <entry>Description</entry>
5604 <entry>STRING</entry>
5605 <entry>Unique or well-known bus name of the connection to
5606 query, such as <literal>:12.34</literal> or
5607 <literal>com.example.tea</literal></entry>
5617 <entry>Argument</entry>
5619 <entry>Description</entry>
5625 <entry>UINT32</entry>
5626 <entry>Unix process id</entry>
5631 Returns the Unix process ID of the process connected to the server. If
5632 unable to determine it (for instance, because the process is not on the
5633 same machine as the bus daemon), an error is returned.
5637 <sect3 id="bus-messages-get-connection-credentials">
5638 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5642 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5649 <entry>Argument</entry>
5651 <entry>Description</entry>
5657 <entry>STRING</entry>
5658 <entry>Unique or well-known bus name of the connection to
5659 query, such as <literal>:12.34</literal> or
5660 <literal>com.example.tea</literal></entry>
5670 <entry>Argument</entry>
5672 <entry>Description</entry>
5678 <entry>DICT<STRING,VARIANT></entry>
5679 <entry>Credentials</entry>
5687 Returns as many credentials as possible for the process connected to
5688 the server. If unable to determine certain credentials (for instance,
5689 because the process is not on the same machine as the bus daemon,
5690 or because this version of the bus daemon does not support a
5691 particular security framework), or if the values of those credentials
5692 cannot be represented as documented here, then those credentials
5697 Keys in the returned dictionary not containing "." are defined
5698 by this specification. Bus daemon implementors supporting
5699 credentials frameworks not mentioned in this document should either
5700 contribute patches to this specification, or use keys containing
5701 "." and starting with a reversed domain name.
5707 <entry>Value type</entry>
5708 <entry>Value</entry>
5713 <entry>UnixUserID</entry>
5714 <entry>UINT32</entry>
5715 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5718 <entry>ProcessID</entry>
5719 <entry>UINT32</entry>
5720 <entry>The numeric process ID, on platforms that have
5721 this concept. On Unix, this is the process ID defined by
5730 This method was added in D-Bus 1.7 to reduce the round-trips
5731 required to list a process's credentials. In older versions, calling
5732 this method will fail: applications should recover by using the
5733 separate methods such as
5734 <xref linkend="bus-messages-get-connection-unix-user"/>
5739 <sect3 id="bus-messages-get-adt-audit-session-data">
5740 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5744 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5751 <entry>Argument</entry>
5753 <entry>Description</entry>
5759 <entry>STRING</entry>
5760 <entry>Unique or well-known bus name of the connection to
5761 query, such as <literal>:12.34</literal> or
5762 <literal>com.example.tea</literal></entry>
5772 <entry>Argument</entry>
5774 <entry>Description</entry>
5780 <entry>ARRAY of BYTE</entry>
5781 <entry>auditing data as returned by
5782 adt_export_session_data()</entry>
5787 Returns auditing data used by Solaris ADT, in an unspecified
5788 binary format. If you know what this means, please contribute
5789 documentation via the D-Bus bug tracking system.
5790 This method is on the core DBus interface for historical reasons;
5791 the same information should be made available via
5792 <xref linkend="bus-messages-get-connection-credentials"/>
5797 <sect3 id="bus-messages-get-connection-selinux-security-context">
5798 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5802 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5809 <entry>Argument</entry>
5811 <entry>Description</entry>
5817 <entry>STRING</entry>
5818 <entry>Unique or well-known bus name of the connection to
5819 query, such as <literal>:12.34</literal> or
5820 <literal>com.example.tea</literal></entry>
5830 <entry>Argument</entry>
5832 <entry>Description</entry>
5838 <entry>ARRAY of BYTE</entry>
5839 <entry>some sort of string of bytes, not necessarily UTF-8,
5840 not including '\0'</entry>
5845 Returns the security context used by SELinux, in an unspecified
5846 format. If you know what this means, please contribute
5847 documentation via the D-Bus bug tracking system.
5848 This method is on the core DBus interface for historical reasons;
5849 the same information should be made available via
5850 <xref linkend="bus-messages-get-connection-credentials"/>
5856 <sect3 id="bus-messages-add-match">
5857 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5861 AddMatch (in STRING rule)
5868 <entry>Argument</entry>
5870 <entry>Description</entry>
5876 <entry>STRING</entry>
5877 <entry>Match rule to add to the connection</entry>
5882 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5883 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5887 <sect3 id="bus-messages-remove-match">
5888 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5892 RemoveMatch (in STRING rule)
5899 <entry>Argument</entry>
5901 <entry>Description</entry>
5907 <entry>STRING</entry>
5908 <entry>Match rule to remove from the connection</entry>
5913 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5914 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5919 <sect3 id="bus-messages-get-id">
5920 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5924 GetId (out STRING id)
5931 <entry>Argument</entry>
5933 <entry>Description</entry>
5939 <entry>STRING</entry>
5940 <entry>Unique ID identifying the bus daemon</entry>
5945 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5946 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5947 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5948 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5949 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5950 by org.freedesktop.DBus.Peer.GetMachineId().
5951 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5959 <appendix id="implementation-notes">
5960 <title>Implementation notes</title>
5961 <sect1 id="implementation-notes-subsection">
5969 <glossary><title>Glossary</title>
5971 This glossary defines some of the terms used in this specification.
5974 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5977 The message bus maintains an association between names and
5978 connections. (Normally, there's one connection per application.) A
5979 bus name is simply an identifier used to locate connections. For
5980 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5981 name might be used to send a message to a screensaver from Yoyodyne
5982 Corporation. An application is said to <firstterm>own</firstterm> a
5983 name if the message bus has associated the application's connection
5984 with the name. Names may also have <firstterm>queued
5985 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5986 The bus assigns a unique name to each connection,
5987 see <xref linkend="term-unique-name"/>. Other names
5988 can be thought of as "well-known names" and are
5989 used to find applications that offer specific functionality.
5993 See <xref linkend="message-protocol-names-bus"/> for details of
5994 the syntax and naming conventions for bus names.
5999 <glossentry id="term-message"><glossterm>Message</glossterm>
6002 A message is the atomic unit of communication via the D-Bus
6003 protocol. It consists of a <firstterm>header</firstterm> and a
6004 <firstterm>body</firstterm>; the body is made up of
6005 <firstterm>arguments</firstterm>.
6010 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6013 The message bus is a special application that forwards
6014 or routes messages between a group of applications
6015 connected to the message bus. It also manages
6016 <firstterm>names</firstterm> used for routing
6022 <glossentry id="term-name"><glossterm>Name</glossterm>
6025 See <xref linkend="term-bus-name"/>. "Name" may
6026 also be used to refer to some of the other names
6027 in D-Bus, such as interface names.
6032 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6035 Used to prevent collisions when defining new interfaces, bus names
6036 etc. The convention used is the same one Java uses for defining
6037 classes: a reversed domain name.
6038 See <xref linkend="message-protocol-names-bus"/>,
6039 <xref linkend="message-protocol-names-interface"/>,
6040 <xref linkend="message-protocol-names-error"/>,
6041 <xref linkend="message-protocol-marshaling-object-path"/>.
6046 <glossentry id="term-object"><glossterm>Object</glossterm>
6049 Each application contains <firstterm>objects</firstterm>, which have
6050 <firstterm>interfaces</firstterm> and
6051 <firstterm>methods</firstterm>. Objects are referred to by a name,
6052 called a <firstterm>path</firstterm>.
6057 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6060 An application talking directly to another application, without going
6061 through a message bus. One-to-one connections may be "peer to peer" or
6062 "client to server." The D-Bus protocol has no concept of client
6063 vs. server after a connection has authenticated; the flow of messages
6064 is symmetrical (full duplex).
6069 <glossentry id="term-path"><glossterm>Path</glossterm>
6072 Object references (object names) in D-Bus are organized into a
6073 filesystem-style hierarchy, so each object is named by a path. As in
6074 LDAP, there's no difference between "files" and "directories"; a path
6075 can refer to an object, while still having child objects below it.
6080 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6083 Each bus name has a primary owner; messages sent to the name go to the
6084 primary owner. However, certain names also maintain a queue of
6085 secondary owners "waiting in the wings." If the primary owner releases
6086 the name, then the first secondary owner in the queue automatically
6087 becomes the new owner of the name.
6092 <glossentry id="term-service"><glossterm>Service</glossterm>
6095 A service is an executable that can be launched by the bus daemon.
6096 Services normally guarantee some particular features, for example they
6097 may guarantee that they will request a specific name such as
6098 "com.example.Screensaver", have a singleton object
6099 "/com/example/Application", and that object will implement the
6100 interface "com.example.Screensaver.Control".
6105 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6108 ".service files" tell the bus about service applications that can be
6109 launched (see <xref linkend="term-service"/>). Most importantly they
6110 provide a mapping from bus names to services that will request those
6111 names when they start up.
6116 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6119 The special name automatically assigned to each connection by the
6120 message bus. This name will never change owner, and will be unique
6121 (never reused during the lifetime of the message bus).
6122 It will begin with a ':' character.
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