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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
3 "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.24</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.24</revnumber>
75 <date>(not yet released)</date>
76 <authorinitials>n/a</authorinitials>
78 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
82 <revnumber>0.23</revnumber>
83 <date>2014-01-06</date>
84 <authorinitials>SMcV, CY</authorinitials>
86 method call messages with no INTERFACE may be considered an error;
87 document tcp:bind=... and nonce-tcp:bind=...; define listenable
88 and connectable addresses
92 <revnumber>0.22</revnumber>
93 <date>2013-10-09</date>
94 <authorinitials></authorinitials>
95 <revremark>add GetConnectionCredentials, document
96 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
97 document and correct .service file syntax and naming
101 <revnumber>0.21</revnumber>
102 <date>2013-04-25</date>
103 <authorinitials>smcv</authorinitials>
104 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
105 Corrigendum #9)</revremark>
108 <revnumber>0.20</revnumber>
109 <date>22 February 2013</date>
110 <authorinitials>smcv, walters</authorinitials>
111 <revremark>reorganise for clarity, remove false claims about
112 basic types, mention /o/fd/DBus</revremark>
115 <revnumber>0.19</revnumber>
116 <date>20 February 2012</date>
117 <authorinitials>smcv/lp</authorinitials>
118 <revremark>formally define unique connection names and well-known
119 bus names; document best practices for interface, bus, member and
120 error names, and object paths; document the search path for session
121 and system services on Unix; document the systemd transport</revremark>
124 <revnumber>0.18</revnumber>
125 <date>29 July 2011</date>
126 <authorinitials>smcv</authorinitials>
127 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
128 match keyword; promote type system to a top-level section</revremark>
131 <revnumber>0.17</revnumber>
132 <date>1 June 2011</date>
133 <authorinitials>smcv/davidz</authorinitials>
134 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
135 by GVariant</revremark>
138 <revnumber>0.16</revnumber>
139 <date>11 April 2011</date>
140 <authorinitials></authorinitials>
141 <revremark>add path_namespace, arg0namespace; argNpath matches object
145 <revnumber>0.15</revnumber>
146 <date>3 November 2010</date>
147 <authorinitials></authorinitials>
148 <revremark></revremark>
151 <revnumber>0.14</revnumber>
152 <date>12 May 2010</date>
153 <authorinitials></authorinitials>
154 <revremark></revremark>
157 <revnumber>0.13</revnumber>
158 <date>23 Dezember 2009</date>
159 <authorinitials></authorinitials>
160 <revremark></revremark>
163 <revnumber>0.12</revnumber>
164 <date>7 November, 2006</date>
165 <authorinitials></authorinitials>
166 <revremark></revremark>
169 <revnumber>0.11</revnumber>
170 <date>6 February 2005</date>
171 <authorinitials></authorinitials>
172 <revremark></revremark>
175 <revnumber>0.10</revnumber>
176 <date>28 January 2005</date>
177 <authorinitials></authorinitials>
178 <revremark></revremark>
181 <revnumber>0.9</revnumber>
182 <date>7 Januar 2005</date>
183 <authorinitials></authorinitials>
184 <revremark></revremark>
187 <revnumber>0.8</revnumber>
188 <date>06 September 2003</date>
189 <authorinitials></authorinitials>
190 <revremark>First released document.</revremark>
195 <sect1 id="introduction">
196 <title>Introduction</title>
198 D-Bus is a system for low-overhead, easy to use
199 interprocess communication (IPC). In more detail:
203 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
204 binary protocol, and does not have to convert to and from a text
205 format such as XML. Because D-Bus is intended for potentially
206 high-resolution same-machine IPC, not primarily for Internet IPC,
207 this is an interesting optimization. D-Bus is also designed to
208 avoid round trips and allow asynchronous operation, much like
214 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
215 of <firstterm>messages</firstterm> rather than byte streams, and
216 automatically handles a lot of the hard IPC issues. Also, the D-Bus
217 library is designed to be wrapped in a way that lets developers use
218 their framework's existing object/type system, rather than learning
219 a new one specifically for IPC.
226 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
227 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
228 a system for one application to talk to a single other
229 application. However, the primary intended application of the protocol is the
230 D-Bus <firstterm>message bus</firstterm>, specified in <xref
231 linkend="message-bus"/>. The message bus is a special application that
232 accepts connections from multiple other applications, and forwards
237 Uses of D-Bus include notification of system changes (notification of when
238 a camera is plugged in to a computer, or a new version of some software
239 has been installed), or desktop interoperability, for example a file
240 monitoring service or a configuration service.
244 D-Bus is designed for two specific use cases:
248 A "system bus" for notifications from the system to user sessions,
249 and to allow the system to request input from user sessions.
254 A "session bus" used to implement desktop environments such as
259 D-Bus is not intended to be a generic IPC system for any possible
260 application, and intentionally omits many features found in other
261 IPC systems for this reason.
265 At the same time, the bus daemons offer a number of features not found in
266 other IPC systems, such as single-owner "bus names" (similar to X
267 selections), on-demand startup of services, and security policies.
268 In many ways, these features are the primary motivation for developing
269 D-Bus; other systems would have sufficed if IPC were the only goal.
273 D-Bus may turn out to be useful in unanticipated applications, but future
274 versions of this spec and the reference implementation probably will not
275 incorporate features that interfere with the core use cases.
279 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
280 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
281 document are to be interpreted as described in RFC 2119. However, the
282 document could use a serious audit to be sure it makes sense to do
283 so. Also, they are not capitalized.
286 <sect2 id="stability">
287 <title>Protocol and Specification Stability</title>
289 The D-Bus protocol is frozen (only compatible extensions are allowed) as
290 of November 8, 2006. However, this specification could still use a fair
291 bit of work to make interoperable reimplementation possible without
292 reference to the D-Bus reference implementation. Thus, this
293 specification is not marked 1.0. To mark it 1.0, we'd like to see
294 someone invest significant effort in clarifying the specification
295 language, and growing the specification to cover more aspects of the
296 reference implementation's behavior.
299 Until this work is complete, any attempt to reimplement D-Bus will
300 probably require looking at the reference implementation and/or asking
301 questions on the D-Bus mailing list about intended behavior.
302 Questions on the list are very welcome.
305 Nonetheless, this document should be a useful starting point and is
306 to our knowledge accurate, though incomplete.
312 <sect1 id="type-system">
313 <title>Type System</title>
316 D-Bus has a type system, in which values of various types can be
317 serialized into a sequence of bytes referred to as the
318 <firstterm>wire format</firstterm> in a standard way.
319 Converting a value from some other representation into the wire
320 format is called <firstterm>marshaling</firstterm> and converting
321 it back from the wire format is <firstterm>unmarshaling</firstterm>.
325 The D-Bus protocol does not include type tags in the marshaled data; a
326 block of marshaled values must have a known <firstterm>type
327 signature</firstterm>. The type signature is made up of zero or more
328 <firstterm id="term-single-complete-type">single complete
329 types</firstterm>, each made up of one or more
330 <firstterm>type codes</firstterm>.
334 A type code is an ASCII character representing the
335 type of a value. Because ASCII characters are used, the type signature
336 will always form a valid ASCII string. A simple string compare
337 determines whether two type signatures are equivalent.
341 A single complete type is a sequence of type codes that fully describes
342 one type: either a basic type, or a single fully-described container type.
343 A single complete type is a basic type code, a variant type code,
344 an array with its element type, or a struct with its fields (all of which
345 are defined below). So the following signatures are not single complete
356 And the following signatures contain multiple complete types:
366 Note however that a single complete type may <emphasis>contain</emphasis>
367 multiple other single complete types, by containing a struct or dict
371 <sect2 id="basic-types">
372 <title>Basic types</title>
375 The simplest type codes are the <firstterm id="term-basic-type">basic
376 types</firstterm>, which are the types whose structure is entirely
377 defined by their 1-character type code. Basic types consist of
378 fixed types and string-like types.
382 The <firstterm id="term-fixed-type">fixed types</firstterm>
383 are basic types whose values have a fixed length, namely BYTE,
384 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
389 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
390 the ASCII character 'i'. So the signature for a block of values
391 containing a single <literal>INT32</literal> would be:
395 A block of values containing two <literal>INT32</literal> would have this signature:
402 The characteristics of the fixed types are listed in this table.
408 <entry>Conventional name</entry>
409 <entry>ASCII type-code</entry>
410 <entry>Encoding</entry>
415 <entry><literal>BYTE</literal></entry>
416 <entry><literal>y</literal> (121)</entry>
417 <entry>Unsigned 8-bit integer</entry>
420 <entry><literal>BOOLEAN</literal></entry>
421 <entry><literal>b</literal> (98)</entry>
422 <entry>Boolean value: 0 is false, 1 is true, any other value
423 allowed by the marshalling format is invalid</entry>
426 <entry><literal>INT16</literal></entry>
427 <entry><literal>n</literal> (110)</entry>
428 <entry>Signed (two's complement) 16-bit integer</entry>
431 <entry><literal>UINT16</literal></entry>
432 <entry><literal>q</literal> (113)</entry>
433 <entry>Unsigned 16-bit integer</entry>
436 <entry><literal>INT32</literal></entry>
437 <entry><literal>i</literal> (105)</entry>
438 <entry>Signed (two's complement) 32-bit integer</entry>
441 <entry><literal>UINT32</literal></entry>
442 <entry><literal>u</literal> (117)</entry>
443 <entry>Unsigned 32-bit integer</entry>
446 <entry><literal>INT64</literal></entry>
447 <entry><literal>x</literal> (120)</entry>
448 <entry>Signed (two's complement) 64-bit integer
449 (mnemonic: x and t are the first characters in "sixty" not
450 already used for something more common)</entry>
453 <entry><literal>UINT64</literal></entry>
454 <entry><literal>t</literal> (116)</entry>
455 <entry>Unsigned 64-bit integer</entry>
458 <entry><literal>DOUBLE</literal></entry>
459 <entry><literal>d</literal> (100)</entry>
460 <entry>IEEE 754 double-precision floating point</entry>
463 <entry><literal>UNIX_FD</literal></entry>
464 <entry><literal>h</literal> (104)</entry>
465 <entry>Unsigned 32-bit integer representing an index into an
466 out-of-band array of file descriptors, transferred via some
467 platform-specific mechanism (mnemonic: h for handle)</entry>
475 The <firstterm id="term-string-like-type">string-like types</firstterm>
476 are basic types with a variable length. The value of any string-like
477 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
478 none of which may be U+0000. The UTF-8 text must be validated
479 strictly: in particular, it must not contain overlong sequences
480 or codepoints above U+10FFFF.
484 Since D-Bus Specification version 0.21, in accordance with Unicode
485 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
486 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
487 D-Bus rejected these noncharacters).
491 The marshalling formats for the string-like types all end with a
492 single zero (NUL) byte, but that byte is not considered to be part of
497 The characteristics of the string-like types are listed in this table.
503 <entry>Conventional name</entry>
504 <entry>ASCII type-code</entry>
505 <entry>Validity constraints</entry>
510 <entry><literal>STRING</literal></entry>
511 <entry><literal>s</literal> (115)</entry>
512 <entry>No extra constraints</entry>
515 <entry><literal>OBJECT_PATH</literal></entry>
516 <entry><literal>o</literal> (111)</entry>
518 <link linkend="message-protocol-marshaling-object-path">a
519 syntactically valid object path</link></entry>
522 <entry><literal>SIGNATURE</literal></entry>
523 <entry><literal>g</literal> (103)</entry>
525 <firstterm linkend="term-single-complete-type">single
526 complete types</firstterm></entry>
533 <sect3 id="message-protocol-marshaling-object-path">
534 <title>Valid Object Paths</title>
537 An object path is a name used to refer to an object instance.
538 Conceptually, each participant in a D-Bus message exchange may have
539 any number of object instances (think of C++ or Java objects) and each
540 such instance will have a path. Like a filesystem, the object
541 instances in an application form a hierarchical tree.
545 Object paths are often namespaced by starting with a reversed
546 domain name and containing an interface version number, in the
548 <link linkend="message-protocol-names-interface">interface
550 <link linkend="message-protocol-names-bus">well-known
552 This makes it possible to implement more than one service, or
553 more than one version of a service, in the same process,
554 even if the services share a connection but cannot otherwise
555 co-operate (for instance, if they are implemented by different
560 For instance, if the owner of <literal>example.com</literal> is
561 developing a D-Bus API for a music player, they might use the
562 hierarchy of object paths that start with
563 <literal>/com/example/MusicPlayer1</literal> for its objects.
567 The following rules define a valid object path. Implementations must
568 not send or accept messages with invalid object paths.
572 The path may be of any length.
577 The path must begin with an ASCII '/' (integer 47) character,
578 and must consist of elements separated by slash characters.
583 Each element must only contain the ASCII characters
589 No element may be the empty string.
594 Multiple '/' characters cannot occur in sequence.
599 A trailing '/' character is not allowed unless the
600 path is the root path (a single '/' character).
608 <sect3 id="message-protocol-marshaling-signature">
609 <title>Valid Signatures</title>
611 An implementation must not send or accept invalid signatures.
612 Valid signatures will conform to the following rules:
616 The signature is a list of single complete types.
617 Arrays must have element types, and structs must
618 have both open and close parentheses.
623 Only type codes, open and close parentheses, and open and
624 close curly brackets are allowed in the signature. The
625 <literal>STRUCT</literal> type code
626 is not allowed in signatures, because parentheses
627 are used instead. Similarly, the
628 <literal>DICT_ENTRY</literal> type code is not allowed in
629 signatures, because curly brackets are used instead.
634 The maximum depth of container type nesting is 32 array type
635 codes and 32 open parentheses. This implies that the maximum
636 total depth of recursion is 64, for an "array of array of array
637 of ... struct of struct of struct of ..." where there are 32
643 The maximum length of a signature is 255.
650 When signatures appear in messages, the marshalling format
651 guarantees that they will be followed by a nul byte (which can
652 be interpreted as either C-style string termination or the INVALID
653 type-code), but this is not conceptually part of the signature.
659 <sect2 id="container-types">
660 <title>Container types</title>
663 In addition to basic types, there are four <firstterm>container</firstterm>
664 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
665 and <literal>DICT_ENTRY</literal>.
669 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
670 code does not appear in signatures. Instead, ASCII characters
671 '(' and ')' are used to mark the beginning and end of the struct.
672 So for example, a struct containing two integers would have this
677 Structs can be nested, so for example a struct containing
678 an integer and another struct:
682 The value block storing that struct would contain three integers; the
683 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
688 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
689 but is useful in code that implements the protocol. This type code
690 is specified to allow such code to interoperate in non-protocol contexts.
694 Empty structures are not allowed; there must be at least one
695 type code between the parentheses.
699 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
700 followed by a <firstterm>single complete type</firstterm>. The single
701 complete type following the array is the type of each array element. So
702 the simple example is:
706 which is an array of 32-bit integers. But an array can be of any type,
707 such as this array-of-struct-with-two-int32-fields:
711 Or this array of array of integer:
718 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
719 type <literal>VARIANT</literal> will have the signature of a single complete type as part
720 of the <emphasis>value</emphasis>. This signature will be followed by a
721 marshaled value of that type.
725 Unlike a message signature, the variant signature can
726 contain only a single complete type. So "i", "ai"
727 or "(ii)" is OK, but "ii" is not. Use of variants may not
728 cause a total message depth to be larger than 64, including
729 other container types such as structures.
733 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
734 than parentheses it uses curly braces, and it has more restrictions.
735 The restrictions are: it occurs only as an array element type; it has
736 exactly two single complete types inside the curly braces; the first
737 single complete type (the "key") must be a basic type rather than a
738 container type. Implementations must not accept dict entries outside of
739 arrays, must not accept dict entries with zero, one, or more than two
740 fields, and must not accept dict entries with non-basic-typed keys. A
741 dict entry is always a key-value pair.
745 The first field in the <literal>DICT_ENTRY</literal> is always the key.
746 A message is considered corrupt if the same key occurs twice in the same
747 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
748 implementations are not required to reject dicts with duplicate keys.
752 In most languages, an array of dict entry would be represented as a
753 map, hash table, or dict object.
758 <title>Summary of types</title>
761 The following table summarizes the D-Bus types.
766 <entry>Conventional Name</entry>
768 <entry>Description</entry>
773 <entry><literal>INVALID</literal></entry>
774 <entry>0 (ASCII NUL)</entry>
775 <entry>Not a valid type code, used to terminate signatures</entry>
777 <entry><literal>BYTE</literal></entry>
778 <entry>121 (ASCII 'y')</entry>
779 <entry>8-bit unsigned integer</entry>
781 <entry><literal>BOOLEAN</literal></entry>
782 <entry>98 (ASCII 'b')</entry>
783 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
785 <entry><literal>INT16</literal></entry>
786 <entry>110 (ASCII 'n')</entry>
787 <entry>16-bit signed integer</entry>
789 <entry><literal>UINT16</literal></entry>
790 <entry>113 (ASCII 'q')</entry>
791 <entry>16-bit unsigned integer</entry>
793 <entry><literal>INT32</literal></entry>
794 <entry>105 (ASCII 'i')</entry>
795 <entry>32-bit signed integer</entry>
797 <entry><literal>UINT32</literal></entry>
798 <entry>117 (ASCII 'u')</entry>
799 <entry>32-bit unsigned integer</entry>
801 <entry><literal>INT64</literal></entry>
802 <entry>120 (ASCII 'x')</entry>
803 <entry>64-bit signed integer</entry>
805 <entry><literal>UINT64</literal></entry>
806 <entry>116 (ASCII 't')</entry>
807 <entry>64-bit unsigned integer</entry>
809 <entry><literal>DOUBLE</literal></entry>
810 <entry>100 (ASCII 'd')</entry>
811 <entry>IEEE 754 double</entry>
813 <entry><literal>STRING</literal></entry>
814 <entry>115 (ASCII 's')</entry>
815 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
817 <entry><literal>OBJECT_PATH</literal></entry>
818 <entry>111 (ASCII 'o')</entry>
819 <entry>Name of an object instance</entry>
821 <entry><literal>SIGNATURE</literal></entry>
822 <entry>103 (ASCII 'g')</entry>
823 <entry>A type signature</entry>
825 <entry><literal>ARRAY</literal></entry>
826 <entry>97 (ASCII 'a')</entry>
829 <entry><literal>STRUCT</literal></entry>
830 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
831 <entry>Struct; type code 114 'r' is reserved for use in
832 bindings and implementations to represent the general
833 concept of a struct, and must not appear in signatures
834 used on D-Bus.</entry>
836 <entry><literal>VARIANT</literal></entry>
837 <entry>118 (ASCII 'v') </entry>
838 <entry>Variant type (the type of the value is part of the value itself)</entry>
840 <entry><literal>DICT_ENTRY</literal></entry>
841 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
842 <entry>Entry in a dict or map (array of key-value pairs).
843 Type code 101 'e' is reserved for use in bindings and
844 implementations to represent the general concept of a
845 dict or dict-entry, and must not appear in signatures
846 used on D-Bus.</entry>
848 <entry><literal>UNIX_FD</literal></entry>
849 <entry>104 (ASCII 'h')</entry>
850 <entry>Unix file descriptor</entry>
853 <entry>(reserved)</entry>
854 <entry>109 (ASCII 'm')</entry>
855 <entry>Reserved for <ulink
856 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
857 'maybe' type compatible with the one in GVariant</ulink>,
858 and must not appear in signatures used on D-Bus until
859 specified here</entry>
862 <entry>(reserved)</entry>
863 <entry>42 (ASCII '*')</entry>
864 <entry>Reserved for use in bindings/implementations to
865 represent any <firstterm>single complete type</firstterm>,
866 and must not appear in signatures used on D-Bus.</entry>
869 <entry>(reserved)</entry>
870 <entry>63 (ASCII '?')</entry>
871 <entry>Reserved for use in bindings/implementations to
872 represent any <firstterm>basic type</firstterm>, and must
873 not appear in signatures used on D-Bus.</entry>
876 <entry>(reserved)</entry>
877 <entry>64 (ASCII '@'), 38 (ASCII '&'),
878 94 (ASCII '^')</entry>
879 <entry>Reserved for internal use by bindings/implementations,
880 and must not appear in signatures used on D-Bus.
881 GVariant uses these type-codes to encode calling
892 <sect1 id="message-protocol-marshaling">
893 <title>Marshaling (Wire Format)</title>
896 D-Bus defines a marshalling format for its type system, which is
897 used in D-Bus messages. This is not the only possible marshalling
898 format for the type system: for instance, GVariant (part of GLib)
899 re-uses the D-Bus type system but implements an alternative marshalling
904 <title>Byte order and alignment</title>
907 Given a type signature, a block of bytes can be converted into typed
908 values. This section describes the format of the block of bytes. Byte
909 order and alignment issues are handled uniformly for all D-Bus types.
913 A block of bytes has an associated byte order. The byte order
914 has to be discovered in some way; for D-Bus messages, the
915 byte order is part of the message header as described in
916 <xref linkend="message-protocol-messages"/>. For now, assume
917 that the byte order is known to be either little endian or big
922 Each value in a block of bytes is aligned "naturally," for example
923 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
924 8-byte boundary. To properly align a value, <firstterm>alignment
925 padding</firstterm> may be necessary. The alignment padding must always
926 be the minimum required padding to properly align the following value;
927 and it must always be made up of nul bytes. The alignment padding must
928 not be left uninitialized (it can't contain garbage), and more padding
929 than required must not be used.
933 As an exception to natural alignment, <literal>STRUCT</literal> and
934 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
935 boundary, regardless of the alignments of their contents.
940 <title>Marshalling basic types</title>
943 To marshal and unmarshal fixed types, you simply read one value
944 from the data block corresponding to each type code in the signature.
945 All signed integer values are encoded in two's complement, DOUBLE
946 values are IEEE 754 double-precision floating-point, and BOOLEAN
947 values are encoded in 32 bits (of which only the least significant
952 The string-like types are all marshalled as a
953 fixed-length unsigned integer <varname>n</varname> giving the
954 length of the variable part, followed by <varname>n</varname>
955 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
956 which is not considered to be part of the text. The alignment
957 of the string-like type is the same as the alignment of
958 <varname>n</varname>.
962 For the STRING and OBJECT_PATH types, <varname>n</varname> is
963 encoded in 4 bytes, leading to 4-byte alignment.
964 For the SIGNATURE type, <varname>n</varname> is encoded as a single
965 byte. As a result, alignment padding is never required before a
971 <title>Marshalling containers</title>
974 Arrays are marshalled as a <literal>UINT32</literal>
975 <varname>n</varname> giving the length of the array data in bytes,
976 followed by alignment padding to the alignment boundary of the array
977 element type, followed by the <varname>n</varname> bytes of the
978 array elements marshalled in sequence. <varname>n</varname> does not
979 include the padding after the length, or any padding after the
984 For instance, if the current position in the message is a multiple
985 of 8 bytes and the byte-order is big-endian, an array containing only
986 the 64-bit integer 5 would be marshalled as:
989 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
990 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
991 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
996 Arrays have a maximum length defined to be 2 to the 26th power or
997 67108864. Implementations must not send or accept arrays exceeding this
1002 Structs and dict entries are marshalled in the same way as their
1003 contents, but their alignment is always to an 8-byte boundary,
1004 even if their contents would normally be less strictly aligned.
1008 Variants are marshalled as the <literal>SIGNATURE</literal> of
1009 the contents (which must be a single complete type), followed by a
1010 marshalled value with the type given by that signature. The
1011 variant has the same 1-byte alignment as the signature, which means
1012 that alignment padding before a variant is never needed.
1013 Use of variants may not cause a total message depth to be larger
1014 than 64, including other container types such as structures.
1019 <title>Summary of D-Bus marshalling</title>
1022 Given all this, the types are marshaled on the wire as follows:
1027 <entry>Conventional Name</entry>
1028 <entry>Encoding</entry>
1029 <entry>Alignment</entry>
1034 <entry><literal>INVALID</literal></entry>
1035 <entry>Not applicable; cannot be marshaled.</entry>
1038 <entry><literal>BYTE</literal></entry>
1039 <entry>A single 8-bit byte.</entry>
1042 <entry><literal>BOOLEAN</literal></entry>
1043 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1046 <entry><literal>INT16</literal></entry>
1047 <entry>16-bit signed integer in the message's byte order.</entry>
1050 <entry><literal>UINT16</literal></entry>
1051 <entry>16-bit unsigned integer in the message's byte order.</entry>
1054 <entry><literal>INT32</literal></entry>
1055 <entry>32-bit signed integer in the message's byte order.</entry>
1058 <entry><literal>UINT32</literal></entry>
1059 <entry>32-bit unsigned integer in the message's byte order.</entry>
1062 <entry><literal>INT64</literal></entry>
1063 <entry>64-bit signed integer in the message's byte order.</entry>
1066 <entry><literal>UINT64</literal></entry>
1067 <entry>64-bit unsigned integer in the message's byte order.</entry>
1070 <entry><literal>DOUBLE</literal></entry>
1071 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1074 <entry><literal>STRING</literal></entry>
1075 <entry>A <literal>UINT32</literal> indicating the string's
1076 length in bytes excluding its terminating nul, followed by
1077 non-nul string data of the given length, followed by a terminating nul
1084 <entry><literal>OBJECT_PATH</literal></entry>
1085 <entry>Exactly the same as <literal>STRING</literal> except the
1086 content must be a valid object path (see above).
1092 <entry><literal>SIGNATURE</literal></entry>
1093 <entry>The same as <literal>STRING</literal> except the length is a single
1094 byte (thus signatures have a maximum length of 255)
1095 and the content must be a valid signature (see above).
1101 <entry><literal>ARRAY</literal></entry>
1103 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1104 alignment padding to the alignment boundary of the array element type,
1105 followed by each array element.
1111 <entry><literal>STRUCT</literal></entry>
1113 A struct must start on an 8-byte boundary regardless of the
1114 type of the struct fields. The struct value consists of each
1115 field marshaled in sequence starting from that 8-byte
1122 <entry><literal>VARIANT</literal></entry>
1124 The marshaled <literal>SIGNATURE</literal> of a single
1125 complete type, followed by a marshaled value with the type
1126 given in the signature.
1129 1 (alignment of the signature)
1132 <entry><literal>DICT_ENTRY</literal></entry>
1134 Identical to STRUCT.
1140 <entry><literal>UNIX_FD</literal></entry>
1141 <entry>32-bit unsigned integer in the message's byte
1142 order. The actual file descriptors need to be
1143 transferred out-of-band via some platform specific
1144 mechanism. On the wire, values of this type store the index to the
1145 file descriptor in the array of file descriptors that
1146 accompany the message.</entry>
1158 <sect1 id="message-protocol">
1159 <title>Message Protocol</title>
1162 A <firstterm>message</firstterm> consists of a
1163 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1164 think of a message as a package, the header is the address, and the body
1165 contains the package contents. The message delivery system uses the header
1166 information to figure out where to send the message and how to interpret
1167 it; the recipient interprets the body of the message.
1171 The body of the message is made up of zero or more
1172 <firstterm>arguments</firstterm>, which are typed values, such as an
1173 integer or a byte array.
1177 Both header and body use the D-Bus <link linkend="type-system">type
1178 system</link> and format for serializing data.
1181 <sect2 id="message-protocol-messages">
1182 <title>Message Format</title>
1185 A message consists of a header and a body. The header is a block of
1186 values with a fixed signature and meaning. The body is a separate block
1187 of values, with a signature specified in the header.
1191 The length of the header must be a multiple of 8, allowing the body to
1192 begin on an 8-byte boundary when storing the entire message in a single
1193 buffer. If the header does not naturally end on an 8-byte boundary
1194 up to 7 bytes of nul-initialized alignment padding must be added.
1198 The message body need not end on an 8-byte boundary.
1202 The maximum length of a message, including header, header alignment padding,
1203 and body is 2 to the 27th power or 134217728. Implementations must not
1204 send or accept messages exceeding this size.
1208 The signature of the header is:
1212 Written out more readably, this is:
1214 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1219 These values have the following meanings:
1224 <entry>Value</entry>
1225 <entry>Description</entry>
1230 <entry>1st <literal>BYTE</literal></entry>
1231 <entry>Endianness flag; ASCII 'l' for little-endian
1232 or ASCII 'B' for big-endian. Both header and body are
1233 in this endianness.</entry>
1236 <entry>2nd <literal>BYTE</literal></entry>
1237 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1238 Currently-defined types are described below.
1242 <entry>3rd <literal>BYTE</literal></entry>
1243 <entry>Bitwise OR of flags. Unknown flags
1244 must be ignored. Currently-defined flags are described below.
1248 <entry>4th <literal>BYTE</literal></entry>
1249 <entry>Major protocol version of the sending application. If
1250 the major protocol version of the receiving application does not
1251 match, the applications will not be able to communicate and the
1252 D-Bus connection must be disconnected. The major protocol
1253 version for this version of the specification is 1.
1257 <entry>1st <literal>UINT32</literal></entry>
1258 <entry>Length in bytes of the message body, starting
1259 from the end of the header. The header ends after
1260 its alignment padding to an 8-boundary.
1264 <entry>2nd <literal>UINT32</literal></entry>
1265 <entry>The serial of this message, used as a cookie
1266 by the sender to identify the reply corresponding
1267 to this request. This must not be zero.
1271 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1272 <entry>An array of zero or more <firstterm>header
1273 fields</firstterm> where the byte is the field code, and the
1274 variant is the field value. The message type determines
1275 which fields are required.
1283 <firstterm>Message types</firstterm> that can appear in the second byte
1289 <entry>Conventional name</entry>
1290 <entry>Decimal value</entry>
1291 <entry>Description</entry>
1296 <entry><literal>INVALID</literal></entry>
1298 <entry>This is an invalid type.</entry>
1301 <entry><literal>METHOD_CALL</literal></entry>
1303 <entry>Method call. This message type may prompt a
1307 <entry><literal>METHOD_RETURN</literal></entry>
1309 <entry>Method reply with returned data.</entry>
1312 <entry><literal>ERROR</literal></entry>
1314 <entry>Error reply. If the first argument exists and is a
1315 string, it is an error message.</entry>
1318 <entry><literal>SIGNAL</literal></entry>
1320 <entry>Signal emission.</entry>
1327 Flags that can appear in the third byte of the header:
1332 <entry>Conventional name</entry>
1333 <entry>Hex value</entry>
1334 <entry>Description</entry>
1339 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1343 This message does not expect method return replies or
1344 error replies, even if it is of a type that can
1345 have a reply; the reply can be omitted as an
1346 optimization. It is compliant with this specification
1347 to return the reply despite this flag, although doing
1348 so on a bus with a non-trivial security policy
1349 (such as the well-known system bus) may result in
1350 access denial messages being logged for the reply.
1353 Note that METHOD_CALL is the only message type currently
1354 defined in this specification that can expect a reply,
1355 so the presence or absence of this flag in the other
1356 three message types that are currently
1357 documented is meaningless: replies to those message
1358 types should not be sent, whether this flag is present
1364 <entry><literal>NO_AUTO_START</literal></entry>
1366 <entry>The bus must not launch an owner
1367 for the destination name in response to this message.
1375 <sect3 id="message-protocol-header-fields">
1376 <title>Header Fields</title>
1379 The array at the end of the header contains <firstterm>header
1380 fields</firstterm>, where each field is a 1-byte field code followed
1381 by a field value. A header must contain the required header fields for
1382 its message type, and zero or more of any optional header
1383 fields. Future versions of this protocol specification may add new
1384 fields. Implementations must ignore fields they do not
1385 understand. Implementations must not invent their own header fields;
1386 only changes to this specification may introduce new header fields.
1390 Again, if an implementation sees a header field code that it does not
1391 expect, it must ignore that field, as it will be part of a new
1392 (but compatible) version of this specification. This also applies
1393 to known header fields appearing in unexpected messages, for
1394 example: if a signal has a reply serial it must be ignored
1395 even though it has no meaning as of this version of the spec.
1399 However, implementations must not send or accept known header fields
1400 with the wrong type stored in the field value. So for example a
1401 message with an <literal>INTERFACE</literal> field of type
1402 <literal>UINT32</literal> would be considered corrupt.
1406 Here are the currently-defined header fields:
1411 <entry>Conventional Name</entry>
1412 <entry>Decimal Code</entry>
1414 <entry>Required In</entry>
1415 <entry>Description</entry>
1420 <entry><literal>INVALID</literal></entry>
1423 <entry>not allowed</entry>
1424 <entry>Not a valid field name (error if it appears in a message)</entry>
1427 <entry><literal>PATH</literal></entry>
1429 <entry><literal>OBJECT_PATH</literal></entry>
1430 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1431 <entry>The object to send a call to,
1432 or the object a signal is emitted from.
1434 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1435 implementations should not send messages with this path,
1436 and the reference implementation of the bus daemon will
1437 disconnect any application that attempts to do so.
1441 <entry><literal>INTERFACE</literal></entry>
1443 <entry><literal>STRING</literal></entry>
1444 <entry><literal>SIGNAL</literal></entry>
1446 The interface to invoke a method call on, or
1447 that a signal is emitted from. Optional for
1448 method calls, required for signals.
1449 The special interface
1450 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1451 implementations should not send messages with this
1452 interface, and the reference implementation of the bus
1453 daemon will disconnect any application that attempts to
1458 <entry><literal>MEMBER</literal></entry>
1460 <entry><literal>STRING</literal></entry>
1461 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1462 <entry>The member, either the method name or signal name.</entry>
1465 <entry><literal>ERROR_NAME</literal></entry>
1467 <entry><literal>STRING</literal></entry>
1468 <entry><literal>ERROR</literal></entry>
1469 <entry>The name of the error that occurred, for errors</entry>
1472 <entry><literal>REPLY_SERIAL</literal></entry>
1474 <entry><literal>UINT32</literal></entry>
1475 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1476 <entry>The serial number of the message this message is a reply
1477 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1480 <entry><literal>DESTINATION</literal></entry>
1482 <entry><literal>STRING</literal></entry>
1483 <entry>optional</entry>
1484 <entry>The name of the connection this message is intended for.
1485 Only used in combination with the message bus, see
1486 <xref linkend="message-bus"/>.</entry>
1489 <entry><literal>SENDER</literal></entry>
1491 <entry><literal>STRING</literal></entry>
1492 <entry>optional</entry>
1493 <entry>Unique name of the sending connection.
1494 The message bus fills in this field so it is reliable; the field is
1495 only meaningful in combination with the message bus.</entry>
1498 <entry><literal>SIGNATURE</literal></entry>
1500 <entry><literal>SIGNATURE</literal></entry>
1501 <entry>optional</entry>
1502 <entry>The signature of the message body.
1503 If omitted, it is assumed to be the
1504 empty signature "" (i.e. the body must be 0-length).</entry>
1507 <entry><literal>UNIX_FDS</literal></entry>
1509 <entry><literal>UINT32</literal></entry>
1510 <entry>optional</entry>
1511 <entry>The number of Unix file descriptors that
1512 accompany the message. If omitted, it is assumed
1513 that no Unix file descriptors accompany the
1514 message. The actual file descriptors need to be
1515 transferred via platform specific mechanism
1516 out-of-band. They must be sent at the same time as
1517 part of the message itself. They may not be sent
1518 before the first byte of the message itself is
1519 transferred or after the last byte of the message
1529 <sect2 id="message-protocol-names">
1530 <title>Valid Names</title>
1532 The various names in D-Bus messages have some restrictions.
1535 There is a <firstterm>maximum name length</firstterm>
1536 of 255 which applies to bus names, interfaces, and members.
1538 <sect3 id="message-protocol-names-interface">
1539 <title>Interface names</title>
1541 Interfaces have names with type <literal>STRING</literal>, meaning that
1542 they must be valid UTF-8. However, there are also some
1543 additional restrictions that apply to interface names
1546 <listitem><para>Interface names are composed of 1 or more elements separated by
1547 a period ('.') character. All elements must contain at least
1551 <listitem><para>Each element must only contain the ASCII characters
1552 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1556 <listitem><para>Interface names must contain at least one '.' (period)
1557 character (and thus at least two elements).
1560 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1561 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1566 Interface names should start with the reversed DNS domain name of
1567 the author of the interface (in lower-case), like interface names
1568 in Java. It is conventional for the rest of the interface name
1569 to consist of words run together, with initial capital letters
1570 on all words ("CamelCase"). Several levels of hierarchy can be used.
1571 It is also a good idea to include the major version of the interface
1572 in the name, and increment it if incompatible changes are made;
1573 this way, a single object can implement several versions of an
1574 interface in parallel, if necessary.
1578 For instance, if the owner of <literal>example.com</literal> is
1579 developing a D-Bus API for a music player, they might define
1580 interfaces called <literal>com.example.MusicPlayer1</literal>,
1581 <literal>com.example.MusicPlayer1.Track</literal> and
1582 <literal>com.example.MusicPlayer1.Seekable</literal>.
1586 D-Bus does not distinguish between the concepts that would be
1587 called classes and interfaces in Java: either can be identified on
1588 D-Bus by an interface name.
1591 <sect3 id="message-protocol-names-bus">
1592 <title>Bus names</title>
1594 Connections have one or more bus names associated with them.
1595 A connection has exactly one bus name that is a <firstterm>unique
1596 connection name</firstterm>. The unique connection name remains
1597 with the connection for its entire lifetime.
1598 A bus name is of type <literal>STRING</literal>,
1599 meaning that it must be valid UTF-8. However, there are also
1600 some additional restrictions that apply to bus names
1603 <listitem><para>Bus names that start with a colon (':')
1604 character are unique connection names. Other bus names
1605 are called <firstterm>well-known bus names</firstterm>.
1608 <listitem><para>Bus names are composed of 1 or more elements separated by
1609 a period ('.') character. All elements must contain at least
1613 <listitem><para>Each element must only contain the ASCII characters
1614 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1615 connection name may begin with a digit, elements in
1616 other bus names must not begin with a digit.
1620 <listitem><para>Bus names must contain at least one '.' (period)
1621 character (and thus at least two elements).
1624 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1625 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1629 Note that the hyphen ('-') character is allowed in bus names but
1630 not in interface names.
1634 Like <link linkend="message-protocol-names-interface">interface
1635 names</link>, well-known bus names should start with the
1636 reversed DNS domain name of the author of the interface (in
1637 lower-case), and it is conventional for the rest of the well-known
1638 bus name to consist of words run together, with initial
1639 capital letters. As with interface names, including a version
1640 number in well-known bus names is a good idea; it's possible to
1641 have the well-known bus name for more than one version
1642 simultaneously if backwards compatibility is required.
1646 If a well-known bus name implies the presence of a "main" interface,
1647 that "main" interface is often given the same name as
1648 the well-known bus name, and situated at the corresponding object
1649 path. For instance, if the owner of <literal>example.com</literal>
1650 is developing a D-Bus API for a music player, they might define
1651 that any application that takes the well-known name
1652 <literal>com.example.MusicPlayer1</literal> should have an object
1653 at the object path <literal>/com/example/MusicPlayer1</literal>
1654 which implements the interface
1655 <literal>com.example.MusicPlayer1</literal>.
1658 <sect3 id="message-protocol-names-member">
1659 <title>Member names</title>
1661 Member (i.e. method or signal) names:
1663 <listitem><para>Must only contain the ASCII characters
1664 "[A-Z][a-z][0-9]_" and may not begin with a
1665 digit.</para></listitem>
1666 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1667 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1668 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1673 It is conventional for member names on D-Bus to consist of
1674 capitalized words with no punctuation ("camel-case").
1675 Method names should usually be verbs, such as
1676 <literal>GetItems</literal>, and signal names should usually be
1677 a description of an event, such as <literal>ItemsChanged</literal>.
1680 <sect3 id="message-protocol-names-error">
1681 <title>Error names</title>
1683 Error names have the same restrictions as interface names.
1687 Error names have the same naming conventions as interface
1688 names, and often contain <literal>.Error.</literal>; for instance,
1689 the owner of <literal>example.com</literal> might define the
1690 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1691 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1692 The errors defined by D-Bus itself, such as
1693 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1699 <sect2 id="message-protocol-types">
1700 <title>Message Types</title>
1702 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1703 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1704 This section describes these conventions.
1706 <sect3 id="message-protocol-types-method">
1707 <title>Method Calls</title>
1709 Some messages invoke an operation on a remote object. These are
1710 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1711 messages map naturally to methods on objects in a typical program.
1714 A method call message is required to have a <literal>MEMBER</literal> header field
1715 indicating the name of the method. Optionally, the message has an
1716 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1717 Including the <literal>INTERFACE</literal> in all method call
1718 messages is strongly recommended.
1721 In the absence of an <literal>INTERFACE</literal> field, if two
1722 or more interfaces on the same object have a method with the same
1723 name, it is undefined which of those methods will be invoked.
1724 Implementations may choose to either return an error, or deliver the
1725 message as though it had an arbitrary one of those interfaces.
1728 In some situations (such as the well-known system bus), messages
1729 are filtered through an access-control list external to the
1730 remote object implementation. If that filter rejects certain
1731 messages by matching their interface, or accepts only messages
1732 to specific interfaces, it must also reject messages that have no
1733 <literal>INTERFACE</literal>: otherwise, malicious
1734 applications could use this to bypass the filter.
1737 Method call messages also include a <literal>PATH</literal> field
1738 indicating the object to invoke the method on. If the call is passing
1739 through a message bus, the message will also have a
1740 <literal>DESTINATION</literal> field giving the name of the connection
1741 to receive the message.
1744 When an application handles a method call message, it is required to
1745 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1746 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1747 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1750 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1751 are the return value(s) or "out parameters" of the method call.
1752 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1753 and the call fails; no return value will be provided. It makes
1754 no sense to send multiple replies to the same method call.
1757 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1758 reply is required, so the caller will know the method
1759 was successfully processed.
1762 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1766 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1767 then as an optimization the application receiving the method
1768 call may choose to omit the reply message (regardless of
1769 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1770 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1771 flag and reply anyway.
1774 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1775 destination name does not exist then a program to own the destination
1776 name will be started before the message is delivered. The message
1777 will be held until the new program is successfully started or has
1778 failed to start; in case of failure, an error will be returned. This
1779 flag is only relevant in the context of a message bus, it is ignored
1780 during one-to-one communication with no intermediate bus.
1782 <sect4 id="message-protocol-types-method-apis">
1783 <title>Mapping method calls to native APIs</title>
1785 APIs for D-Bus may map method calls to a method call in a specific
1786 programming language, such as C++, or may map a method call written
1787 in an IDL to a D-Bus message.
1790 In APIs of this nature, arguments to a method are often termed "in"
1791 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1792 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1793 "inout" arguments, which are both sent and received, i.e. the caller
1794 passes in a value which is modified. Mapped to D-Bus, an "inout"
1795 argument is equivalent to an "in" argument, followed by an "out"
1796 argument. You can't pass things "by reference" over the wire, so
1797 "inout" is purely an illusion of the in-process API.
1800 Given a method with zero or one return values, followed by zero or more
1801 arguments, where each argument may be "in", "out", or "inout", the
1802 caller constructs a message by appending each "in" or "inout" argument,
1803 in order. "out" arguments are not represented in the caller's message.
1806 The recipient constructs a reply by appending first the return value
1807 if any, then each "out" or "inout" argument, in order.
1808 "in" arguments are not represented in the reply message.
1811 Error replies are normally mapped to exceptions in languages that have
1815 In converting from native APIs to D-Bus, it is perhaps nice to
1816 map D-Bus naming conventions ("FooBar") to native conventions
1817 such as "fooBar" or "foo_bar" automatically. This is OK
1818 as long as you can say that the native API is one that
1819 was specifically written for D-Bus. It makes the most sense
1820 when writing object implementations that will be exported
1821 over the bus. Object proxies used to invoke remote D-Bus
1822 objects probably need the ability to call any D-Bus method,
1823 and thus a magic name mapping like this could be a problem.
1826 This specification doesn't require anything of native API bindings;
1827 the preceding is only a suggested convention for consistency
1833 <sect3 id="message-protocol-types-signal">
1834 <title>Signal Emission</title>
1836 Unlike method calls, signal emissions have no replies.
1837 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1838 It must have three header fields: <literal>PATH</literal> giving the object
1839 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1840 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1841 for signals, though it is optional for method calls.
1845 <sect3 id="message-protocol-types-errors">
1846 <title>Errors</title>
1848 Messages of type <literal>ERROR</literal> are most commonly replies
1849 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1850 to any kind of message. The message bus for example
1851 will return an <literal>ERROR</literal> in reply to a signal emission if
1852 the bus does not have enough memory to send the signal.
1855 An <literal>ERROR</literal> may have any arguments, but if the first
1856 argument is a <literal>STRING</literal>, it must be an error message.
1857 The error message may be logged or shown to the user
1862 <sect3 id="message-protocol-types-notation">
1863 <title>Notation in this document</title>
1865 This document uses a simple pseudo-IDL to describe particular method
1866 calls and signals. Here is an example of a method call:
1868 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1869 out UINT32 resultcode)
1871 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1872 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1873 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1874 characters so it's known that the last part of the name in
1875 the "IDL" is the member name.
1878 In C++ that might end up looking like this:
1880 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1881 unsigned int flags);
1883 or equally valid, the return value could be done as an argument:
1885 void org::freedesktop::DBus::StartServiceByName (const char *name,
1887 unsigned int *resultcode);
1889 It's really up to the API designer how they want to make
1890 this look. You could design an API where the namespace wasn't used
1891 in C++, using STL or Qt, using varargs, or whatever you wanted.
1894 Signals are written as follows:
1896 org.freedesktop.DBus.NameLost (STRING name)
1898 Signals don't specify "in" vs. "out" because only
1899 a single direction is possible.
1902 It isn't especially encouraged to use this lame pseudo-IDL in actual
1903 API implementations; you might use the native notation for the
1904 language you're using, or you might use COM or CORBA IDL, for example.
1909 <sect2 id="message-protocol-handling-invalid">
1910 <title>Invalid Protocol and Spec Extensions</title>
1913 For security reasons, the D-Bus protocol should be strictly parsed and
1914 validated, with the exception of defined extension points. Any invalid
1915 protocol or spec violations should result in immediately dropping the
1916 connection without notice to the other end. Exceptions should be
1917 carefully considered, e.g. an exception may be warranted for a
1918 well-understood idiosyncrasy of a widely-deployed implementation. In
1919 cases where the other end of a connection is 100% trusted and known to
1920 be friendly, skipping validation for performance reasons could also make
1921 sense in certain cases.
1925 Generally speaking violations of the "must" requirements in this spec
1926 should be considered possible attempts to exploit security, and violations
1927 of the "should" suggestions should be considered legitimate (though perhaps
1928 they should generate an error in some cases).
1932 The following extension points are built in to D-Bus on purpose and must
1933 not be treated as invalid protocol. The extension points are intended
1934 for use by future versions of this spec, they are not intended for third
1935 parties. At the moment, the only way a third party could extend D-Bus
1936 without breaking interoperability would be to introduce a way to negotiate new
1937 feature support as part of the auth protocol, using EXTENSION_-prefixed
1938 commands. There is not yet a standard way to negotiate features.
1942 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1943 commands result in an ERROR rather than a disconnect. This enables
1944 future extensions to the protocol. Commands starting with EXTENSION_ are
1945 reserved for third parties.
1950 The authentication protocol supports pluggable auth mechanisms.
1955 The address format (see <xref linkend="addresses"/>) supports new
1961 Messages with an unknown type (something other than
1962 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1963 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1964 Unknown-type messages must still be well-formed in the same way
1965 as the known messages, however. They still have the normal
1971 Header fields with an unknown or unexpected field code must be ignored,
1972 though again they must still be well-formed.
1977 New standard interfaces (with new methods and signals) can of course be added.
1987 <sect1 id="auth-protocol">
1988 <title>Authentication Protocol</title>
1990 Before the flow of messages begins, two applications must
1991 authenticate. A simple plain-text protocol is used for
1992 authentication; this protocol is a SASL profile, and maps fairly
1993 directly from the SASL specification. The message encoding is
1994 NOT used here, only plain text messages.
1997 In examples, "C:" and "S:" indicate lines sent by the client and
1998 server respectively.
2000 <sect2 id="auth-protocol-overview">
2001 <title>Protocol Overview</title>
2003 The protocol is a line-based protocol, where each line ends with
2004 \r\n. Each line begins with an all-caps ASCII command name containing
2005 only the character range [A-Z_], a space, then any arguments for the
2006 command, then the \r\n ending the line. The protocol is
2007 case-sensitive. All bytes must be in the ASCII character set.
2009 Commands from the client to the server are as follows:
2012 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2013 <listitem><para>CANCEL</para></listitem>
2014 <listitem><para>BEGIN</para></listitem>
2015 <listitem><para>DATA <data in hex encoding></para></listitem>
2016 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2017 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2020 From server to client are as follows:
2023 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2024 <listitem><para>OK <GUID in hex></para></listitem>
2025 <listitem><para>DATA <data in hex encoding></para></listitem>
2026 <listitem><para>ERROR</para></listitem>
2027 <listitem><para>AGREE_UNIX_FD</para></listitem>
2031 Unofficial extensions to the command set must begin with the letters
2032 "EXTENSION_", to avoid conflicts with future official commands.
2033 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2036 <sect2 id="auth-nul-byte">
2037 <title>Special credentials-passing nul byte</title>
2039 Immediately after connecting to the server, the client must send a
2040 single nul byte. This byte may be accompanied by credentials
2041 information on some operating systems that use sendmsg() with
2042 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2043 sockets. However, the nul byte must be sent even on other kinds of
2044 socket, and even on operating systems that do not require a byte to be
2045 sent in order to transmit credentials. The text protocol described in
2046 this document begins after the single nul byte. If the first byte
2047 received from the client is not a nul byte, the server may disconnect
2051 A nul byte in any context other than the initial byte is an error;
2052 the protocol is ASCII-only.
2055 The credentials sent along with the nul byte may be used with the
2056 SASL mechanism EXTERNAL.
2059 <sect2 id="auth-command-auth">
2060 <title>AUTH command</title>
2062 If an AUTH command has no arguments, it is a request to list
2063 available mechanisms. The server must respond with a REJECTED
2064 command listing the mechanisms it understands, or with an error.
2067 If an AUTH command specifies a mechanism, and the server supports
2068 said mechanism, the server should begin exchanging SASL
2069 challenge-response data with the client using DATA commands.
2072 If the server does not support the mechanism given in the AUTH
2073 command, it must send either a REJECTED command listing the mechanisms
2074 it does support, or an error.
2077 If the [initial-response] argument is provided, it is intended for use
2078 with mechanisms that have no initial challenge (or an empty initial
2079 challenge), as if it were the argument to an initial DATA command. If
2080 the selected mechanism has an initial challenge and [initial-response]
2081 was provided, the server should reject authentication by sending
2085 If authentication succeeds after exchanging DATA commands,
2086 an OK command must be sent to the client.
2089 The first octet received by the server after the \r\n of the BEGIN
2090 command from the client must be the first octet of the
2091 authenticated/encrypted stream of D-Bus messages.
2094 If BEGIN is received by the server, the first octet received
2095 by the client after the \r\n of the OK command must be the
2096 first octet of the authenticated/encrypted stream of D-Bus
2100 <sect2 id="auth-command-cancel">
2101 <title>CANCEL Command</title>
2103 At any time up to sending the BEGIN command, the client may send a
2104 CANCEL command. On receiving the CANCEL command, the server must
2105 send a REJECTED command and abort the current authentication
2109 <sect2 id="auth-command-data">
2110 <title>DATA Command</title>
2112 The DATA command may come from either client or server, and simply
2113 contains a hex-encoded block of data to be interpreted
2114 according to the SASL mechanism in use.
2117 Some SASL mechanisms support sending an "empty string";
2118 FIXME we need some way to do this.
2121 <sect2 id="auth-command-begin">
2122 <title>BEGIN Command</title>
2124 The BEGIN command acknowledges that the client has received an
2125 OK command from the server, and that the stream of messages
2129 The first octet received by the server after the \r\n of the BEGIN
2130 command from the client must be the first octet of the
2131 authenticated/encrypted stream of D-Bus messages.
2134 <sect2 id="auth-command-rejected">
2135 <title>REJECTED Command</title>
2137 The REJECTED command indicates that the current authentication
2138 exchange has failed, and further exchange of DATA is inappropriate.
2139 The client would normally try another mechanism, or try providing
2140 different responses to challenges.
2142 Optionally, the REJECTED command has a space-separated list of
2143 available auth mechanisms as arguments. If a server ever provides
2144 a list of supported mechanisms, it must provide the same list
2145 each time it sends a REJECTED message. Clients are free to
2146 ignore all lists received after the first.
2149 <sect2 id="auth-command-ok">
2150 <title>OK Command</title>
2152 The OK command indicates that the client has been
2153 authenticated. The client may now proceed with negotiating
2154 Unix file descriptor passing. To do that it shall send
2155 NEGOTIATE_UNIX_FD to the server.
2158 Otherwise, the client must respond to the OK command by
2159 sending a BEGIN command, followed by its stream of messages,
2160 or by disconnecting. The server must not accept additional
2161 commands using this protocol after the BEGIN command has been
2162 received. Further communication will be a stream of D-Bus
2163 messages (optionally encrypted, as negotiated) rather than
2167 If a client sends BEGIN the first octet received by the client
2168 after the \r\n of the OK command must be the first octet of
2169 the authenticated/encrypted stream of D-Bus messages.
2172 The OK command has one argument, which is the GUID of the server.
2173 See <xref linkend="addresses"/> for more on server GUIDs.
2176 <sect2 id="auth-command-error">
2177 <title>ERROR Command</title>
2179 The ERROR command indicates that either server or client did not
2180 know a command, does not accept the given command in the current
2181 context, or did not understand the arguments to the command. This
2182 allows the protocol to be extended; a client or server can send a
2183 command present or permitted only in new protocol versions, and if
2184 an ERROR is received instead of an appropriate response, fall back
2185 to using some other technique.
2188 If an ERROR is sent, the server or client that sent the
2189 error must continue as if the command causing the ERROR had never been
2190 received. However, the the server or client receiving the error
2191 should try something other than whatever caused the error;
2192 if only canceling/rejecting the authentication.
2195 If the D-Bus protocol changes incompatibly at some future time,
2196 applications implementing the new protocol would probably be able to
2197 check for support of the new protocol by sending a new command and
2198 receiving an ERROR from applications that don't understand it. Thus the
2199 ERROR feature of the auth protocol is an escape hatch that lets us
2200 negotiate extensions or changes to the D-Bus protocol in the future.
2203 <sect2 id="auth-command-negotiate-unix-fd">
2204 <title>NEGOTIATE_UNIX_FD Command</title>
2206 The NEGOTIATE_UNIX_FD command indicates that the client
2207 supports Unix file descriptor passing. This command may only
2208 be sent after the connection is authenticated, i.e. after OK
2209 was received by the client. This command may only be sent on
2210 transports that support Unix file descriptor passing.
2213 On receiving NEGOTIATE_UNIX_FD the server must respond with
2214 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2215 the transport chosen supports Unix file descriptor passing and
2216 the server supports this feature. It shall respond the latter
2217 if the transport does not support Unix file descriptor
2218 passing, the server does not support this feature, or the
2219 server decides not to enable file descriptor passing due to
2220 security or other reasons.
2223 <sect2 id="auth-command-agree-unix-fd">
2224 <title>AGREE_UNIX_FD Command</title>
2226 The AGREE_UNIX_FD command indicates that the server supports
2227 Unix file descriptor passing. This command may only be sent
2228 after the connection is authenticated, and the client sent
2229 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2230 command may only be sent on transports that support Unix file
2234 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2235 followed by its stream of messages, or by disconnecting. The
2236 server must not accept additional commands using this protocol
2237 after the BEGIN command has been received. Further
2238 communication will be a stream of D-Bus messages (optionally
2239 encrypted, as negotiated) rather than this protocol.
2242 <sect2 id="auth-command-future">
2243 <title>Future Extensions</title>
2245 Future extensions to the authentication and negotiation
2246 protocol are possible. For that new commands may be
2247 introduced. If a client or server receives an unknown command
2248 it shall respond with ERROR and not consider this fatal. New
2249 commands may be introduced both before, and after
2250 authentication, i.e. both before and after the OK command.
2253 <sect2 id="auth-examples">
2254 <title>Authentication examples</title>
2258 <title>Example of successful magic cookie authentication</title>
2260 (MAGIC_COOKIE is a made up mechanism)
2262 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2268 <title>Example of finding out mechanisms then picking one</title>
2271 S: REJECTED KERBEROS_V4 SKEY
2272 C: AUTH SKEY 7ab83f32ee
2273 S: DATA 8799cabb2ea93e
2274 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2280 <title>Example of client sends unknown command then falls back to regular auth</title>
2284 C: AUTH MAGIC_COOKIE 3736343435313230333039
2290 <title>Example of server doesn't support initial auth mechanism</title>
2292 C: AUTH MAGIC_COOKIE 3736343435313230333039
2293 S: REJECTED KERBEROS_V4 SKEY
2294 C: AUTH SKEY 7ab83f32ee
2295 S: DATA 8799cabb2ea93e
2296 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2302 <title>Example of wrong password or the like followed by successful retry</title>
2304 C: AUTH MAGIC_COOKIE 3736343435313230333039
2305 S: REJECTED KERBEROS_V4 SKEY
2306 C: AUTH SKEY 7ab83f32ee
2307 S: DATA 8799cabb2ea93e
2308 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2310 C: AUTH SKEY 7ab83f32ee
2311 S: DATA 8799cabb2ea93e
2312 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2318 <title>Example of skey cancelled and restarted</title>
2320 C: AUTH MAGIC_COOKIE 3736343435313230333039
2321 S: REJECTED KERBEROS_V4 SKEY
2322 C: AUTH SKEY 7ab83f32ee
2323 S: DATA 8799cabb2ea93e
2326 C: AUTH SKEY 7ab83f32ee
2327 S: DATA 8799cabb2ea93e
2328 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2334 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2336 (MAGIC_COOKIE is a made up mechanism)
2338 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2340 C: NEGOTIATE_UNIX_FD
2346 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2348 (MAGIC_COOKIE is a made up mechanism)
2350 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2352 C: NEGOTIATE_UNIX_FD
2359 <sect2 id="auth-states">
2360 <title>Authentication state diagrams</title>
2363 This section documents the auth protocol in terms of
2364 a state machine for the client and the server. This is
2365 probably the most robust way to implement the protocol.
2368 <sect3 id="auth-states-client">
2369 <title>Client states</title>
2372 To more precisely describe the interaction between the
2373 protocol state machine and the authentication mechanisms the
2374 following notation is used: MECH(CHALL) means that the
2375 server challenge CHALL was fed to the mechanism MECH, which
2381 CONTINUE(RESP) means continue the auth conversation
2382 and send RESP as the response to the server;
2388 OK(RESP) means that after sending RESP to the server
2389 the client side of the auth conversation is finished
2390 and the server should return "OK";
2396 ERROR means that CHALL was invalid and could not be
2402 Both RESP and CHALL may be empty.
2406 The Client starts by getting an initial response from the
2407 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2408 the mechanism did not provide an initial response. If the
2409 mechanism returns CONTINUE, the client starts in state
2410 <emphasis>WaitingForData</emphasis>, if the mechanism
2411 returns OK the client starts in state
2412 <emphasis>WaitingForOK</emphasis>.
2416 The client should keep track of available mechanisms and
2417 which it mechanisms it has already attempted. This list is
2418 used to decide which AUTH command to send. When the list is
2419 exhausted, the client should give up and close the
2424 <title><emphasis>WaitingForData</emphasis></title>
2432 MECH(CHALL) returns CONTINUE(RESP) → send
2434 <emphasis>WaitingForData</emphasis>
2438 MECH(CHALL) returns OK(RESP) → send DATA
2439 RESP, goto <emphasis>WaitingForOK</emphasis>
2443 MECH(CHALL) returns ERROR → send ERROR
2444 [msg], goto <emphasis>WaitingForData</emphasis>
2452 Receive REJECTED [mechs] →
2453 send AUTH [next mech], goto
2454 WaitingForData or <emphasis>WaitingForOK</emphasis>
2459 Receive ERROR → send
2461 <emphasis>WaitingForReject</emphasis>
2466 Receive OK → send
2467 BEGIN, terminate auth
2468 conversation, authenticated
2473 Receive anything else → send
2475 <emphasis>WaitingForData</emphasis>
2483 <title><emphasis>WaitingForOK</emphasis></title>
2488 Receive OK → send BEGIN, terminate auth
2489 conversation, <emphasis>authenticated</emphasis>
2494 Receive REJECTED [mechs] → send AUTH [next mech],
2495 goto <emphasis>WaitingForData</emphasis> or
2496 <emphasis>WaitingForOK</emphasis>
2502 Receive DATA → send CANCEL, goto
2503 <emphasis>WaitingForReject</emphasis>
2509 Receive ERROR → send CANCEL, goto
2510 <emphasis>WaitingForReject</emphasis>
2516 Receive anything else → send ERROR, goto
2517 <emphasis>WaitingForOK</emphasis>
2525 <title><emphasis>WaitingForReject</emphasis></title>
2530 Receive REJECTED [mechs] → send AUTH [next mech],
2531 goto <emphasis>WaitingForData</emphasis> or
2532 <emphasis>WaitingForOK</emphasis>
2538 Receive anything else → terminate auth
2539 conversation, disconnect
2548 <sect3 id="auth-states-server">
2549 <title>Server states</title>
2552 For the server MECH(RESP) means that the client response
2553 RESP was fed to the the mechanism MECH, which returns one of
2558 CONTINUE(CHALL) means continue the auth conversation and
2559 send CHALL as the challenge to the client;
2565 OK means that the client has been successfully
2572 REJECTED means that the client failed to authenticate or
2573 there was an error in RESP.
2578 The server starts out in state
2579 <emphasis>WaitingForAuth</emphasis>. If the client is
2580 rejected too many times the server must disconnect the
2585 <title><emphasis>WaitingForAuth</emphasis></title>
2591 Receive AUTH → send REJECTED [mechs], goto
2592 <emphasis>WaitingForAuth</emphasis>
2598 Receive AUTH MECH RESP
2602 MECH not valid mechanism → send REJECTED
2604 <emphasis>WaitingForAuth</emphasis>
2608 MECH(RESP) returns CONTINUE(CHALL) → send
2610 <emphasis>WaitingForData</emphasis>
2614 MECH(RESP) returns OK → send OK, goto
2615 <emphasis>WaitingForBegin</emphasis>
2619 MECH(RESP) returns REJECTED → send REJECTED
2621 <emphasis>WaitingForAuth</emphasis>
2629 Receive BEGIN → terminate
2630 auth conversation, disconnect
2636 Receive ERROR → send REJECTED [mechs], goto
2637 <emphasis>WaitingForAuth</emphasis>
2643 Receive anything else → send
2645 <emphasis>WaitingForAuth</emphasis>
2654 <title><emphasis>WaitingForData</emphasis></title>
2662 MECH(RESP) returns CONTINUE(CHALL) → send
2664 <emphasis>WaitingForData</emphasis>
2668 MECH(RESP) returns OK → send OK, goto
2669 <emphasis>WaitingForBegin</emphasis>
2673 MECH(RESP) returns REJECTED → send REJECTED
2675 <emphasis>WaitingForAuth</emphasis>
2683 Receive BEGIN → terminate auth conversation,
2690 Receive CANCEL → send REJECTED [mechs], goto
2691 <emphasis>WaitingForAuth</emphasis>
2697 Receive ERROR → send REJECTED [mechs], goto
2698 <emphasis>WaitingForAuth</emphasis>
2704 Receive anything else → send ERROR, goto
2705 <emphasis>WaitingForData</emphasis>
2713 <title><emphasis>WaitingForBegin</emphasis></title>
2718 Receive BEGIN → terminate auth conversation,
2719 client authenticated
2725 Receive CANCEL → send REJECTED [mechs], goto
2726 <emphasis>WaitingForAuth</emphasis>
2732 Receive ERROR → send REJECTED [mechs], goto
2733 <emphasis>WaitingForAuth</emphasis>
2739 Receive anything else → send ERROR, goto
2740 <emphasis>WaitingForBegin</emphasis>
2750 <sect2 id="auth-mechanisms">
2751 <title>Authentication mechanisms</title>
2753 This section describes some new authentication mechanisms.
2754 D-Bus also allows any standard SASL mechanism of course.
2756 <sect3 id="auth-mechanisms-sha">
2757 <title>DBUS_COOKIE_SHA1</title>
2759 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2760 has the ability to read a private file owned by the user being
2761 authenticated. If the client can prove that it has access to a secret
2762 cookie stored in this file, then the client is authenticated.
2763 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2767 Throughout this description, "hex encoding" must output the digits
2768 from a to f in lower-case; the digits A to F must not be used
2769 in the DBUS_COOKIE_SHA1 mechanism.
2772 Authentication proceeds as follows:
2776 The client sends the username it would like to authenticate
2782 The server sends the name of its "cookie context" (see below); a
2783 space character; the integer ID of the secret cookie the client
2784 must demonstrate knowledge of; a space character; then a
2785 randomly-generated challenge string, all of this hex-encoded into
2791 The client locates the cookie and generates its own
2792 randomly-generated challenge string. The client then concatenates
2793 the server's decoded challenge, a ":" character, its own challenge,
2794 another ":" character, and the cookie. It computes the SHA-1 hash
2795 of this composite string as a hex digest. It concatenates the
2796 client's challenge string, a space character, and the SHA-1 hex
2797 digest, hex-encodes the result and sends it back to the server.
2802 The server generates the same concatenated string used by the
2803 client and computes its SHA-1 hash. It compares the hash with
2804 the hash received from the client; if the two hashes match, the
2805 client is authenticated.
2811 Each server has a "cookie context," which is a name that identifies a
2812 set of cookies that apply to that server. A sample context might be
2813 "org_freedesktop_session_bus". Context names must be valid ASCII,
2814 nonzero length, and may not contain the characters slash ("/"),
2815 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2816 tab ("\t"), or period ("."). There is a default context,
2817 "org_freedesktop_general" that's used by servers that do not specify
2821 Cookies are stored in a user's home directory, in the directory
2822 <filename>~/.dbus-keyrings/</filename>. This directory must
2823 not be readable or writable by other users. If it is,
2824 clients and servers must ignore it. The directory
2825 contains cookie files named after the cookie context.
2828 A cookie file contains one cookie per line. Each line
2829 has three space-separated fields:
2833 The cookie ID number, which must be a non-negative integer and
2834 may not be used twice in the same file.
2839 The cookie's creation time, in UNIX seconds-since-the-epoch
2845 The cookie itself, a hex-encoded random block of bytes. The cookie
2846 may be of any length, though obviously security increases
2847 as the length increases.
2853 Only server processes modify the cookie file.
2854 They must do so with this procedure:
2858 Create a lockfile name by appending ".lock" to the name of the
2859 cookie file. The server should attempt to create this file
2860 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2861 fails, the lock fails. Servers should retry for a reasonable
2862 period of time, then they may choose to delete an existing lock
2863 to keep users from having to manually delete a stale
2864 lock. <footnote><para>Lockfiles are used instead of real file
2865 locking <literal>fcntl()</literal> because real locking
2866 implementations are still flaky on network
2867 filesystems.</para></footnote>
2872 Once the lockfile has been created, the server loads the cookie
2873 file. It should then delete any cookies that are old (the
2874 timeout can be fairly short), or more than a reasonable
2875 time in the future (so that cookies never accidentally
2876 become permanent, if the clock was set far into the future
2877 at some point). If no recent keys remain, the
2878 server may generate a new key.
2883 The pruned and possibly added-to cookie file
2884 must be resaved atomically (using a temporary
2885 file which is rename()'d).
2890 The lock must be dropped by deleting the lockfile.
2896 Clients need not lock the file in order to load it,
2897 because servers are required to save the file atomically.
2902 <sect1 id="addresses">
2903 <title>Server Addresses</title>
2905 Server addresses consist of a transport name followed by a colon, and
2906 then an optional, comma-separated list of keys and values in the form key=value.
2907 Each value is escaped.
2911 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2912 Which is the address to a unix socket with the path /tmp/dbus-test.
2915 Value escaping is similar to URI escaping but simpler.
2919 The set of optionally-escaped bytes is:
2920 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2921 <emphasis>byte</emphasis> (note, not character) which is not in the
2922 set of optionally-escaped bytes must be replaced with an ASCII
2923 percent (<literal>%</literal>) and the value of the byte in hex.
2924 The hex value must always be two digits, even if the first digit is
2925 zero. The optionally-escaped bytes may be escaped if desired.
2930 To unescape, append each byte in the value; if a byte is an ASCII
2931 percent (<literal>%</literal>) character then append the following
2932 hex value instead. It is an error if a <literal>%</literal> byte
2933 does not have two hex digits following. It is an error if a
2934 non-optionally-escaped byte is seen unescaped.
2938 The set of optionally-escaped bytes is intended to preserve address
2939 readability and convenience.
2943 A server may specify a key-value pair with the key <literal>guid</literal>
2944 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2945 describes the format of the <literal>guid</literal> field. If present,
2946 this UUID may be used to distinguish one server address from another. A
2947 server should use a different UUID for each address it listens on. For
2948 example, if a message bus daemon offers both UNIX domain socket and TCP
2949 connections, but treats clients the same regardless of how they connect,
2950 those two connections are equivalent post-connection but should have
2951 distinct UUIDs to distinguish the kinds of connection.
2955 The intent of the address UUID feature is to allow a client to avoid
2956 opening multiple identical connections to the same server, by allowing the
2957 client to check whether an address corresponds to an already-existing
2958 connection. Comparing two addresses is insufficient, because addresses
2959 can be recycled by distinct servers, and equivalent addresses may look
2960 different if simply compared as strings (for example, the host in a TCP
2961 address can be given as an IP address or as a hostname).
2965 Note that the address key is <literal>guid</literal> even though the
2966 rest of the API and documentation says "UUID," for historical reasons.
2970 [FIXME clarify if attempting to connect to each is a requirement
2971 or just a suggestion]
2972 When connecting to a server, multiple server addresses can be
2973 separated by a semi-colon. The library will then try to connect
2974 to the first address and if that fails, it'll try to connect to
2975 the next one specified, and so forth. For example
2976 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2980 Some addresses are <firstterm>connectable</firstterm>. A connectable
2981 address is one containing enough information for a client to connect
2982 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2983 is a connectable address. It is not necessarily possible to listen
2984 on every connectable address: for instance, it is not possible to
2985 listen on a <literal>unixexec:</literal> address.
2989 Some addresses are <firstterm>listenable</firstterm>. A listenable
2990 address is one containing enough information for a server to listen on
2991 it, producing a connectable address (which may differ from the
2992 original address). Many listenable addresses are not connectable:
2993 for instance, <literal>tcp:host=127.0.0.1</literal>
2994 is listenable, but not connectable (because it does not specify
2999 Listening on an address that is not connectable will result in a
3000 connectable address that is not the same as the listenable address.
3001 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3002 might result in the connectable address
3003 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3004 or listening on <literal>unix:tmpdir=/tmp</literal>
3005 might result in the connectable address
3006 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3010 <sect1 id="transports">
3011 <title>Transports</title>
3013 [FIXME we need to specify in detail each transport and its possible arguments]
3015 Current transports include: unix domain sockets (including
3016 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3017 using in-process pipes. Future possible transports include one that
3018 tunnels over X11 protocol.
3021 <sect2 id="transports-unix-domain-sockets">
3022 <title>Unix Domain Sockets</title>
3024 Unix domain sockets can be either paths in the file system or on Linux
3025 kernels, they can be abstract which are similar to paths but
3026 do not show up in the file system.
3030 When a socket is opened by the D-Bus library it truncates the path
3031 name right before the first trailing Nul byte. This is true for both
3032 normal paths and abstract paths. Note that this is a departure from
3033 previous versions of D-Bus that would create sockets with a fixed
3034 length path name. Names which were shorter than the fixed length
3035 would be padded by Nul bytes.
3038 Unix domain sockets are not available on Windows.
3041 Unix addresses that specify <literal>path</literal> or
3042 <literal>abstract</literal> are both listenable and connectable.
3043 Unix addresses that specify <literal>tmpdir</literal> are only
3044 listenable: the corresponding connectable address will specify
3045 either <literal>path</literal> or <literal>abstract</literal>.
3047 <sect3 id="transports-unix-domain-sockets-addresses">
3048 <title>Server Address Format</title>
3050 Unix domain socket addresses are identified by the "unix:" prefix
3051 and support the following key/value pairs:
3058 <entry>Values</entry>
3059 <entry>Description</entry>
3065 <entry>(path)</entry>
3066 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3069 <entry>tmpdir</entry>
3070 <entry>(path)</entry>
3071 <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>
3074 <entry>abstract</entry>
3075 <entry>(string)</entry>
3076 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tmpdir" key must not be set. This key is only supported on platforms with "abstract Unix sockets", of which Linux is the only known example.</entry>
3082 Exactly one of the keys <literal>path</literal>,
3083 <literal>abstract</literal> or
3084 <literal>tmpdir</literal> must be provided.
3088 <sect2 id="transports-launchd">
3089 <title>launchd</title>
3091 launchd is an open-source server management system that replaces init, inetd
3092 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3093 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3097 launchd allocates a socket and provides it with the unix path through the
3098 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3099 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3100 it through its environment.
3101 Other processes can query for the launchd socket by executing:
3102 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3103 This is normally done by the D-Bus client library so doesn't have to be done
3107 launchd is not available on Microsoft Windows.
3110 launchd addresses are listenable and connectable.
3112 <sect3 id="transports-launchd-addresses">
3113 <title>Server Address Format</title>
3115 launchd addresses are identified by the "launchd:" prefix
3116 and support the following key/value pairs:
3123 <entry>Values</entry>
3124 <entry>Description</entry>
3130 <entry>(environment variable)</entry>
3131 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3137 The <literal>env</literal> key is required.
3141 <sect2 id="transports-systemd">
3142 <title>systemd</title>
3144 systemd is an open-source server management system that
3145 replaces init and inetd on newer Linux systems. It supports
3146 socket activation. The D-Bus systemd transport is used to acquire
3147 socket activation file descriptors from systemd and use them
3148 as D-Bus transport when the current process is spawned by
3149 socket activation from it.
3152 The systemd transport accepts only one or more Unix domain or
3153 TCP streams sockets passed in via socket activation.
3156 The systemd transport is not available on non-Linux operating systems.
3159 The systemd transport defines no parameter keys.
3162 systemd addresses are listenable, but not connectable. The
3163 corresponding connectable address is the <literal>unix</literal>
3164 or <literal>tcp</literal> address of the socket.
3167 <sect2 id="transports-tcp-sockets">
3168 <title>TCP Sockets</title>
3170 The tcp transport provides TCP/IP based connections between clients
3171 located on the same or different hosts.
3174 Using tcp transport without any additional secure authentification mechanismus
3175 over a network is unsecure.
3178 On Windows and most Unix platforms, the TCP stack is unable to transfer
3179 credentials over a TCP connection, so the EXTERNAL authentication
3180 mechanism does not work for this transport.
3183 All <literal>tcp</literal> addresses are listenable.
3184 <literal>tcp</literal> addresses in which both
3185 <literal>host</literal> and <literal>port</literal> are
3186 specified, and <literal>port</literal> is non-zero,
3187 are also connectable.
3189 <sect3 id="transports-tcp-sockets-addresses">
3190 <title>Server Address Format</title>
3192 TCP/IP socket addresses are identified by the "tcp:" prefix
3193 and support the following key/value pairs:
3200 <entry>Values</entry>
3201 <entry>Description</entry>
3207 <entry>(string)</entry>
3208 <entry>DNS name or IP address</entry>
3212 <entry>(string)</entry>
3213 <entry>Used in a listenable address to configure the interface
3214 on which the server will listen: either the IP address of one of
3215 the local machine's interfaces (most commonly <literal>127.0.0.1
3216 </literal>), or a DNS name that resolves to one of those IP
3217 addresses, or '*' to listen on all interfaces simultaneously.
3218 If not specified, the default is the same value as "host".
3223 <entry>(number)</entry>
3224 <entry>The tcp port the server will open. A zero value let the server
3225 choose a free port provided from the underlaying operating system.
3226 libdbus is able to retrieve the real used port from the server.
3230 <entry>family</entry>
3231 <entry>(string)</entry>
3232 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3239 <sect2 id="transports-nonce-tcp-sockets">
3240 <title>Nonce-secured TCP Sockets</title>
3242 The nonce-tcp transport provides a secured TCP transport, using a
3243 simple authentication mechanism to ensure that only clients with read
3244 access to a certain location in the filesystem can connect to the server.
3245 The server writes a secret, the nonce, to a file and an incoming client
3246 connection is only accepted if the client sends the nonce right after
3247 the connect. The nonce mechanism requires no setup and is orthogonal to
3248 the higher-level authentication mechanisms described in the
3249 Authentication section.
3253 On start, the server generates a random 16 byte nonce and writes it
3254 to a file in the user's temporary directory. The nonce file location
3255 is published as part of the server's D-Bus address using the
3256 "noncefile" key-value pair.
3258 After an accept, the server reads 16 bytes from the socket. If the
3259 read bytes do not match the nonce stored in the nonce file, the
3260 server MUST immediately drop the connection.
3261 If the nonce match the received byte sequence, the client is accepted
3262 and the transport behaves like an unsecured tcp transport.
3265 After a successful connect to the server socket, the client MUST read
3266 the nonce from the file published by the server via the noncefile=
3267 key-value pair and send it over the socket. After that, the
3268 transport behaves like an unsecured tcp transport.
3271 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3272 <literal>host</literal>, <literal>port</literal> and
3273 <literal>noncefile</literal> are all specified,
3274 and <literal>port</literal> is nonzero, are also connectable.
3276 <sect3 id="transports-nonce-tcp-sockets-addresses">
3277 <title>Server Address Format</title>
3279 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3280 and support the following key/value pairs:
3287 <entry>Values</entry>
3288 <entry>Description</entry>
3294 <entry>(string)</entry>
3295 <entry>DNS name or IP address</entry>
3299 <entry>(string)</entry>
3300 <entry>The same as for tcp: addresses
3305 <entry>(number)</entry>
3306 <entry>The tcp port the server will open. A zero value let the server
3307 choose a free port provided from the underlaying operating system.
3308 libdbus is able to retrieve the real used port from the server.
3312 <entry>family</entry>
3313 <entry>(string)</entry>
3314 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3317 <entry>noncefile</entry>
3318 <entry>(path)</entry>
3319 <entry>File location containing the secret.
3320 This is only meaningful in connectable addresses:
3321 a listening D-Bus server that offers this transport
3322 will always create a new nonce file.</entry>
3329 <sect2 id="transports-exec">
3330 <title>Executed Subprocesses on Unix</title>
3332 This transport forks off a process and connects its standard
3333 input and standard output with an anonymous Unix domain
3334 socket. This socket is then used for communication by the
3335 transport. This transport may be used to use out-of-process
3336 forwarder programs as basis for the D-Bus protocol.
3339 The forked process will inherit the standard error output and
3340 process group from the parent process.
3343 Executed subprocesses are not available on Windows.
3346 <literal>unixexec</literal> addresses are connectable, but are not
3349 <sect3 id="transports-exec-addresses">
3350 <title>Server Address Format</title>
3352 Executed subprocess addresses are identified by the "unixexec:" prefix
3353 and support the following key/value pairs:
3360 <entry>Values</entry>
3361 <entry>Description</entry>
3367 <entry>(path)</entry>
3368 <entry>Path of the binary to execute, either an absolute
3369 path or a binary name that is searched for in the default
3370 search path of the OS. This corresponds to the first
3371 argument of execlp(). This key is mandatory.</entry>
3374 <entry>argv0</entry>
3375 <entry>(string)</entry>
3376 <entry>The program name to use when executing the
3377 binary. If omitted the same value as specified for path=
3378 will be used. This corresponds to the second argument of
3382 <entry>argv1, argv2, ...</entry>
3383 <entry>(string)</entry>
3384 <entry>Arguments to pass to the binary. This corresponds
3385 to the third and later arguments of execlp(). If a
3386 specific argvX is not specified no further argvY for Y > X
3387 are taken into account.</entry>
3395 <sect1 id="meta-transports">
3396 <title>Meta Transports</title>
3398 Meta transports are a kind of transport with special enhancements or
3399 behavior. Currently available meta transports include: autolaunch
3402 <sect2 id="meta-transports-autolaunch">
3403 <title>Autolaunch</title>
3404 <para>The autolaunch transport provides a way for dbus clients to autodetect
3405 a running dbus session bus and to autolaunch a session bus if not present.
3408 On Unix, <literal>autolaunch</literal> addresses are connectable,
3412 On Windows, <literal>autolaunch</literal> addresses are both
3413 connectable and listenable.
3416 <sect3 id="meta-transports-autolaunch-addresses">
3417 <title>Server Address Format</title>
3419 Autolaunch addresses uses the "autolaunch:" prefix and support the
3420 following key/value pairs:
3427 <entry>Values</entry>
3428 <entry>Description</entry>
3433 <entry>scope</entry>
3434 <entry>(string)</entry>
3435 <entry>scope of autolaunch (Windows only)
3439 "*install-path" - limit session bus to dbus installation path.
3440 The dbus installation path is determined from the location of
3441 the shared dbus library. If the library is located in a 'bin'
3442 subdirectory the installation root is the directory above,
3443 otherwise the directory where the library lives is taken as
3446 <install-root>/bin/[lib]dbus-1.dll
3447 <install-root>/[lib]dbus-1.dll
3453 "*user" - limit session bus to the recent user.
3458 other values - specify dedicated session bus like "release",
3470 <sect3 id="meta-transports-autolaunch-windows-implementation">
3471 <title>Windows implementation</title>
3473 On start, the server opens a platform specific transport, creates a mutex
3474 and a shared memory section containing the related session bus address.
3475 This mutex will be inspected by the dbus client library to detect a
3476 running dbus session bus. The access to the mutex and the shared memory
3477 section are protected by global locks.
3480 In the recent implementation the autolaunch transport uses a tcp transport
3481 on localhost with a port choosen from the operating system. This detail may
3482 change in the future.
3485 Disclaimer: The recent implementation is in an early state and may not
3486 work in all cirumstances and/or may have security issues. Because of this
3487 the implementation is not documentated yet.
3494 <title>UUIDs</title>
3496 A working D-Bus implementation uses universally-unique IDs in two places.
3497 First, each server address has a UUID identifying the address,
3498 as described in <xref linkend="addresses"/>. Second, each operating
3499 system kernel instance running a D-Bus client or server has a UUID
3500 identifying that kernel, retrieved by invoking the method
3501 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3502 linkend="standard-interfaces-peer"/>).
3505 The term "UUID" in this document is intended literally, i.e. an
3506 identifier that is universally unique. It is not intended to refer to
3507 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3510 The UUID must contain 128 bits of data and be hex-encoded. The
3511 hex-encoded string may not contain hyphens or other non-hex-digit
3512 characters, and it must be exactly 32 characters long. To generate a
3513 UUID, the current reference implementation concatenates 96 bits of random
3514 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3518 It would also be acceptable and probably better to simply generate 128
3519 bits of random data, as long as the random number generator is of high
3520 quality. The timestamp could conceivably help if the random bits are not
3521 very random. With a quality random number generator, collisions are
3522 extremely unlikely even with only 96 bits, so it's somewhat academic.
3525 Implementations should, however, stick to random data for the first 96 bits
3530 <sect1 id="standard-interfaces">
3531 <title>Standard Interfaces</title>
3533 See <xref linkend="message-protocol-types-notation"/> for details on
3534 the notation used in this section. There are some standard interfaces
3535 that may be useful across various D-Bus applications.
3537 <sect2 id="standard-interfaces-peer">
3538 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3540 The <literal>org.freedesktop.DBus.Peer</literal> interface
3543 org.freedesktop.DBus.Peer.Ping ()
3544 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3548 On receipt of the <literal>METHOD_CALL</literal> message
3549 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3550 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3551 usual. It does not matter which object path a ping is sent to. The
3552 reference implementation handles this method automatically.
3555 On receipt of the <literal>METHOD_CALL</literal> message
3556 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3557 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3558 UUID representing the identity of the machine the process is running on.
3559 This UUID must be the same for all processes on a single system at least
3560 until that system next reboots. It should be the same across reboots
3561 if possible, but this is not always possible to implement and is not
3563 It does not matter which object path a GetMachineId is sent to. The
3564 reference implementation handles this method automatically.
3567 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3568 a virtual machine running on a hypervisor, rather than a physical machine.
3569 Basically if two processes see the same UUID, they should also see the same
3570 shared memory, UNIX domain sockets, process IDs, and other features that require
3571 a running OS kernel in common between the processes.
3574 The UUID is often used where other programs might use a hostname. Hostnames
3575 can change without rebooting, however, or just be "localhost" - so the UUID
3579 <xref linkend="uuids"/> explains the format of the UUID.
3583 <sect2 id="standard-interfaces-introspectable">
3584 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3586 This interface has one method:
3588 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3592 Objects instances may implement
3593 <literal>Introspect</literal> which returns an XML description of
3594 the object, including its interfaces (with signals and methods), objects
3595 below it in the object path tree, and its properties.
3598 <xref linkend="introspection-format"/> describes the format of this XML string.
3601 <sect2 id="standard-interfaces-properties">
3602 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3604 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3605 or <firstterm>attributes</firstterm>. These can be exposed via the
3606 <literal>org.freedesktop.DBus.Properties</literal> interface.
3610 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3611 in STRING property_name,
3613 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3614 in STRING property_name,
3616 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3617 out DICT<STRING,VARIANT> props);
3621 It is conventional to give D-Bus properties names consisting of
3622 capitalized words without punctuation ("CamelCase"), like
3623 <link linkend="message-protocol-names-member">member names</link>.
3624 For instance, the GObject property
3625 <literal>connection-status</literal> or the Qt property
3626 <literal>connectionStatus</literal> could be represented on D-Bus
3627 as <literal>ConnectionStatus</literal>.
3630 Strictly speaking, D-Bus property names are not required to follow
3631 the same naming restrictions as member names, but D-Bus property
3632 names that would not be valid member names (in particular,
3633 GObject-style dash-separated property names) can cause interoperability
3634 problems and should be avoided.
3637 The available properties and whether they are writable can be determined
3638 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3639 see <xref linkend="standard-interfaces-introspectable"/>.
3642 An empty string may be provided for the interface name; in this case,
3643 if there are multiple properties on an object with the same name,
3644 the results are undefined (picking one by according to an arbitrary
3645 deterministic rule, or returning an error, are the reasonable
3649 If one or more properties change on an object, the
3650 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3651 signal may be emitted (this signal was added in 0.14):
3655 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3656 DICT<STRING,VARIANT> changed_properties,
3657 ARRAY<STRING> invalidated_properties);
3661 where <literal>changed_properties</literal> is a dictionary
3662 containing the changed properties with the new values and
3663 <literal>invalidated_properties</literal> is an array of
3664 properties that changed but the value is not conveyed.
3667 Whether the <literal>PropertiesChanged</literal> signal is
3668 supported can be determined by calling
3669 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3670 that the signal may be supported for an object but it may
3671 differ how whether and how it is used on a per-property basis
3672 (for e.g. performance or security reasons). Each property (or
3673 the parent interface) must be annotated with the
3674 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3675 annotation to convey this (usually the default value
3676 <literal>true</literal> is sufficient meaning that the
3677 annotation does not need to be used). See <xref
3678 linkend="introspection-format"/> for details on this
3683 <sect2 id="standard-interfaces-objectmanager">
3684 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3686 An API can optionally make use of this interface for one or
3687 more sub-trees of objects. The root of each sub-tree implements
3688 this interface so other applications can get all objects,
3689 interfaces and properties in a single method call. It is
3690 appropriate to use this interface if users of the tree of
3691 objects are expected to be interested in all interfaces of all
3692 objects in the tree; a more granular API should be used if
3693 users of the objects are expected to be interested in a small
3694 subset of the objects, a small subset of their interfaces, or
3698 The method that applications can use to get all objects and
3699 properties is <literal>GetManagedObjects</literal>:
3703 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3707 The return value of this method is a dict whose keys are
3708 object paths. All returned object paths are children of the
3709 object path implementing this interface, i.e. their object
3710 paths start with the ObjectManager's object path plus '/'.
3713 Each value is a dict whose keys are interfaces names. Each
3714 value in this inner dict is the same dict that would be
3715 returned by the <link
3716 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3717 method for that combination of object path and interface. If
3718 an interface has no properties, the empty dict is returned.
3721 Changes are emitted using the following two signals:
3725 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3726 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3727 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3728 ARRAY<STRING> interfaces);
3732 The <literal>InterfacesAdded</literal> signal is emitted when
3733 either a new object is added or when an existing object gains
3734 one or more interfaces. The
3735 <literal>InterfacesRemoved</literal> signal is emitted
3736 whenever an object is removed or it loses one or more
3737 interfaces. The second parameter of the
3738 <literal>InterfacesAdded</literal> signal contains a dict with
3739 the interfaces and properties (if any) that have been added to
3740 the given object path. Similarly, the second parameter of the
3741 <literal>InterfacesRemoved</literal> signal contains an array
3742 of the interfaces that were removed. Note that changes on
3743 properties on existing interfaces are not reported using this
3744 interface - an application should also monitor the existing <link
3745 linkend="standard-interfaces-properties">PropertiesChanged</link>
3746 signal on each object.
3749 Applications SHOULD NOT export objects that are children of an
3750 object (directly or otherwise) implementing this interface but
3751 which are not returned in the reply from the
3752 <literal>GetManagedObjects()</literal> method of this
3753 interface on the given object.
3756 The intent of the <literal>ObjectManager</literal> interface
3757 is to make it easy to write a robust client
3758 implementation. The trivial client implementation only needs
3759 to make two method calls:
3763 org.freedesktop.DBus.AddMatch (bus_proxy,
3764 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3765 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3769 on the message bus and the remote application's
3770 <literal>ObjectManager</literal>, respectively. Whenever a new
3771 remote object is created (or an existing object gains a new
3772 interface), the <literal>InterfacesAdded</literal> signal is
3773 emitted, and since this signal contains all properties for the
3774 interfaces, no calls to the
3775 <literal>org.freedesktop.Properties</literal> interface on the
3776 remote object are needed. Additionally, since the initial
3777 <literal>AddMatch()</literal> rule already includes signal
3778 messages from the newly created child object, no new
3779 <literal>AddMatch()</literal> call is needed.
3784 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3785 interface was added in version 0.17 of the D-Bus
3792 <sect1 id="introspection-format">
3793 <title>Introspection Data Format</title>
3795 As described in <xref linkend="standard-interfaces-introspectable"/>,
3796 objects may be introspected at runtime, returning an XML string
3797 that describes the object. The same XML format may be used in
3798 other contexts as well, for example as an "IDL" for generating
3799 static language bindings.
3802 Here is an example of introspection data:
3804 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3805 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3806 <node name="/com/example/sample_object">
3807 <interface name="com.example.SampleInterface">
3808 <method name="Frobate">
3809 <arg name="foo" type="i" direction="in"/>
3810 <arg name="bar" type="s" direction="out"/>
3811 <arg name="baz" type="a{us}" direction="out"/>
3812 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3814 <method name="Bazify">
3815 <arg name="bar" type="(iiu)" direction="in"/>
3816 <arg name="bar" type="v" direction="out"/>
3818 <method name="Mogrify">
3819 <arg name="bar" type="(iiav)" direction="in"/>
3821 <signal name="Changed">
3822 <arg name="new_value" type="b"/>
3824 <property name="Bar" type="y" access="readwrite"/>
3826 <node name="child_of_sample_object"/>
3827 <node name="another_child_of_sample_object"/>
3832 A more formal DTD and spec needs writing, but here are some quick notes.
3836 Only the root <node> element can omit the node name, as it's
3837 known to be the object that was introspected. If the root
3838 <node> does have a name attribute, it must be an absolute
3839 object path. If child <node> have object paths, they must be
3845 If a child <node> has any sub-elements, then they
3846 must represent a complete introspection of the child.
3847 If a child <node> is empty, then it may or may
3848 not have sub-elements; the child must be introspected
3849 in order to find out. The intent is that if an object
3850 knows that its children are "fast" to introspect
3851 it can go ahead and return their information, but
3852 otherwise it can omit it.
3857 The direction element on <arg> may be omitted,
3858 in which case it defaults to "in" for method calls
3859 and "out" for signals. Signals only allow "out"
3860 so while direction may be specified, it's pointless.
3865 The possible directions are "in" and "out",
3866 unlike CORBA there is no "inout"
3871 The possible property access flags are
3872 "readwrite", "read", and "write"
3877 Multiple interfaces can of course be listed for
3883 The "name" attribute on arguments is optional.
3889 Method, interface, property, and signal elements may have
3890 "annotations", which are generic key/value pairs of metadata.
3891 They are similar conceptually to Java's annotations and C# attributes.
3892 Well-known annotations:
3899 <entry>Values (separated by ,)</entry>
3900 <entry>Description</entry>
3905 <entry>org.freedesktop.DBus.Deprecated</entry>
3906 <entry>true,false</entry>
3907 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3910 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3911 <entry>(string)</entry>
3912 <entry>The C symbol; may be used for methods and interfaces</entry>
3915 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3916 <entry>true,false</entry>
3917 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3920 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3921 <entry>true,invalidates,false</entry>
3924 If set to <literal>false</literal>, the
3925 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3927 linkend="standard-interfaces-properties"/> is not
3928 guaranteed to be emitted if the property changes.
3931 If set to <literal>invalidates</literal> the signal
3932 is emitted but the value is not included in the
3936 If set to <literal>true</literal> the signal is
3937 emitted with the value included.
3940 The value for the annotation defaults to
3941 <literal>true</literal> if the enclosing interface
3942 element does not specify the annotation. Otherwise it
3943 defaults to the value specified in the enclosing
3952 <sect1 id="message-bus">
3953 <title>Message Bus Specification</title>
3954 <sect2 id="message-bus-overview">
3955 <title>Message Bus Overview</title>
3957 The message bus accepts connections from one or more applications.
3958 Once connected, applications can exchange messages with other
3959 applications that are also connected to the bus.
3962 In order to route messages among connections, the message bus keeps a
3963 mapping from names to connections. Each connection has one
3964 unique-for-the-lifetime-of-the-bus name automatically assigned.
3965 Applications may request additional names for a connection. Additional
3966 names are usually "well-known names" such as
3967 "com.example.TextEditor". When a name is bound to a connection,
3968 that connection is said to <firstterm>own</firstterm> the name.
3971 The bus itself owns a special name,
3972 <literal>org.freedesktop.DBus</literal>, with an object
3973 located at <literal>/org/freedesktop/DBus</literal> that
3974 implements the <literal>org.freedesktop.DBus</literal>
3975 interface. This service allows applications to make
3976 administrative requests of the bus itself. For example,
3977 applications can ask the bus to assign a name to a connection.
3980 Each name may have <firstterm>queued owners</firstterm>. When an
3981 application requests a name for a connection and the name is already in
3982 use, the bus will optionally add the connection to a queue waiting for
3983 the name. If the current owner of the name disconnects or releases
3984 the name, the next connection in the queue will become the new owner.
3988 This feature causes the right thing to happen if you start two text
3989 editors for example; the first one may request "com.example.TextEditor",
3990 and the second will be queued as a possible owner of that name. When
3991 the first exits, the second will take over.
3995 Applications may send <firstterm>unicast messages</firstterm> to
3996 a specific recipient or to the message bus itself, or
3997 <firstterm>broadcast messages</firstterm> to all interested recipients.
3998 See <xref linkend="message-bus-routing"/> for details.
4002 <sect2 id="message-bus-names">
4003 <title>Message Bus Names</title>
4005 Each connection has at least one name, assigned at connection time and
4006 returned in response to the
4007 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4008 automatically-assigned name is called the connection's <firstterm>unique
4009 name</firstterm>. Unique names are never reused for two different
4010 connections to the same bus.
4013 Ownership of a unique name is a prerequisite for interaction with
4014 the message bus. It logically follows that the unique name is always
4015 the first name that an application comes to own, and the last
4016 one that it loses ownership of.
4019 Unique connection names must begin with the character ':' (ASCII colon
4020 character); bus names that are not unique names must not begin
4021 with this character. (The bus must reject any attempt by an application
4022 to manually request a name beginning with ':'.) This restriction
4023 categorically prevents "spoofing"; messages sent to a unique name
4024 will always go to the expected connection.
4027 When a connection is closed, all the names that it owns are deleted (or
4028 transferred to the next connection in the queue if any).
4031 A connection can request additional names to be associated with it using
4032 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4033 linkend="message-protocol-names-bus"/> describes the format of a valid
4034 name. These names can be released again using the
4035 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4038 <sect3 id="bus-messages-request-name">
4039 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4043 UINT32 RequestName (in STRING name, in UINT32 flags)
4050 <entry>Argument</entry>
4052 <entry>Description</entry>
4058 <entry>STRING</entry>
4059 <entry>Name to request</entry>
4063 <entry>UINT32</entry>
4064 <entry>Flags</entry>
4074 <entry>Argument</entry>
4076 <entry>Description</entry>
4082 <entry>UINT32</entry>
4083 <entry>Return value</entry>
4090 This method call should be sent to
4091 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4092 assign the given name to the method caller. Each name maintains a
4093 queue of possible owners, where the head of the queue is the primary
4094 or current owner of the name. Each potential owner in the queue
4095 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4096 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4097 call. When RequestName is invoked the following occurs:
4101 If the method caller is currently the primary owner of the name,
4102 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4103 values are updated with the values from the new RequestName call,
4104 and nothing further happens.
4110 If the current primary owner (head of the queue) has
4111 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4112 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4113 the caller of RequestName replaces the current primary owner at
4114 the head of the queue and the current primary owner moves to the
4115 second position in the queue. If the caller of RequestName was
4116 in the queue previously its flags are updated with the values from
4117 the new RequestName in addition to moving it to the head of the queue.
4123 If replacement is not possible, and the method caller is
4124 currently in the queue but not the primary owner, its flags are
4125 updated with the values from the new RequestName call.
4131 If replacement is not possible, and the method caller is
4132 currently not in the queue, the method caller is appended to the
4139 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4140 set and is not the primary owner, it is removed from the
4141 queue. This can apply to the previous primary owner (if it
4142 was replaced) or the method caller (if it updated the
4143 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4144 queue, or if it was just added to the queue with that flag set).
4150 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4151 queue," even if another application already in the queue had specified
4152 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4153 that does not allow replacement goes away, and the next primary owner
4154 does allow replacement. In this case, queued items that specified
4155 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4156 automatically replace the new primary owner. In other words,
4157 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4158 time RequestName is called. This is deliberate to avoid an infinite loop
4159 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4160 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4163 The flags argument contains any of the following values logically ORed
4170 <entry>Conventional Name</entry>
4171 <entry>Value</entry>
4172 <entry>Description</entry>
4177 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4181 If an application A specifies this flag and succeeds in
4182 becoming the owner of the name, and another application B
4183 later calls RequestName with the
4184 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4185 will lose ownership and receive a
4186 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4187 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4188 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4189 is not specified by application B, then application B will not replace
4190 application A as the owner.
4195 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4199 Try to replace the current owner if there is one. If this
4200 flag is not set the application will only become the owner of
4201 the name if there is no current owner. If this flag is set,
4202 the application will replace the current owner if
4203 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4208 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4212 Without this flag, if an application requests a name that is
4213 already owned, the application will be placed in a queue to
4214 own the name when the current owner gives it up. If this
4215 flag is given, the application will not be placed in the
4216 queue, the request for the name will simply fail. This flag
4217 also affects behavior when an application is replaced as
4218 name owner; by default the application moves back into the
4219 waiting queue, unless this flag was provided when the application
4220 became the name owner.
4228 The return code can be one of the following values:
4234 <entry>Conventional Name</entry>
4235 <entry>Value</entry>
4236 <entry>Description</entry>
4241 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4242 <entry>1</entry> <entry>The caller is now the primary owner of
4243 the name, replacing any previous owner. Either the name had no
4244 owner before, or the caller specified
4245 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4246 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4249 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4252 <entry>The name already had an owner,
4253 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4254 the current owner did not specify
4255 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4256 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4260 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4261 <entry>The name already has an owner,
4262 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4263 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4264 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4265 specified by the requesting application.</entry>
4268 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4270 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4278 <sect3 id="bus-messages-release-name">
4279 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4283 UINT32 ReleaseName (in STRING name)
4290 <entry>Argument</entry>
4292 <entry>Description</entry>
4298 <entry>STRING</entry>
4299 <entry>Name to release</entry>
4309 <entry>Argument</entry>
4311 <entry>Description</entry>
4317 <entry>UINT32</entry>
4318 <entry>Return value</entry>
4325 This method call should be sent to
4326 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4327 release the method caller's claim to the given name. If the caller is
4328 the primary owner, a new primary owner will be selected from the
4329 queue if any other owners are waiting. If the caller is waiting in
4330 the queue for the name, the caller will removed from the queue and
4331 will not be made an owner of the name if it later becomes available.
4332 If there are no other owners in the queue for the name, it will be
4333 removed from the bus entirely.
4335 The return code can be one of the following values:
4341 <entry>Conventional Name</entry>
4342 <entry>Value</entry>
4343 <entry>Description</entry>
4348 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4349 <entry>1</entry> <entry>The caller has released his claim on
4350 the given name. Either the caller was the primary owner of
4351 the name, and the name is now unused or taken by somebody
4352 waiting in the queue for the name, or the caller was waiting
4353 in the queue for the name and has now been removed from the
4357 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4359 <entry>The given name does not exist on this bus.</entry>
4362 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4364 <entry>The caller was not the primary owner of this name,
4365 and was also not waiting in the queue to own this name.</entry>
4373 <sect3 id="bus-messages-list-queued-owners">
4374 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4378 ARRAY of STRING ListQueuedOwners (in STRING name)
4385 <entry>Argument</entry>
4387 <entry>Description</entry>
4393 <entry>STRING</entry>
4394 <entry>The well-known bus name to query, such as
4395 <literal>com.example.cappuccino</literal></entry>
4405 <entry>Argument</entry>
4407 <entry>Description</entry>
4413 <entry>ARRAY of STRING</entry>
4414 <entry>The unique bus names of connections currently queued
4415 for the name</entry>
4422 This method call should be sent to
4423 <literal>org.freedesktop.DBus</literal> and lists the connections
4424 currently queued for a bus name (see
4425 <xref linkend="term-queued-owner"/>).
4430 <sect2 id="message-bus-routing">
4431 <title>Message Bus Message Routing</title>
4434 Messages may have a <literal>DESTINATION</literal> field (see <xref
4435 linkend="message-protocol-header-fields"/>), resulting in a
4436 <firstterm>unicast message</firstterm>. If the
4437 <literal>DESTINATION</literal> field is present, it specifies a message
4438 recipient by name. Method calls and replies normally specify this field.
4439 The message bus must send messages (of any type) with the
4440 <literal>DESTINATION</literal> field set to the specified recipient,
4441 regardless of whether the recipient has set up a match rule matching
4446 When the message bus receives a signal, if the
4447 <literal>DESTINATION</literal> field is absent, it is considered to
4448 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4449 applications with <firstterm>message matching rules</firstterm> that
4450 match the message. Most signal messages are broadcasts, and
4451 no other message types currently defined in this specification
4456 Unicast signal messages (those with a <literal>DESTINATION</literal>
4457 field) are not commonly used, but they are treated like any unicast
4458 message: they are delivered to the specified receipient,
4459 regardless of its match rules. One use for unicast signals is to
4460 avoid a race condition in which a signal is emitted before the intended
4461 recipient can call <xref linkend="bus-messages-add-match"/> to
4462 receive that signal: if the signal is sent directly to that recipient
4463 using a unicast message, it does not need to add a match rule at all,
4464 and there is no race condition. Another use for unicast signals,
4465 on message buses whose security policy prevents eavesdropping, is to
4466 send sensitive information which should only be visible to one
4471 When the message bus receives a method call, if the
4472 <literal>DESTINATION</literal> field is absent, the call is taken to be
4473 a standard one-to-one message and interpreted by the message bus
4474 itself. For example, sending an
4475 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4476 <literal>DESTINATION</literal> will cause the message bus itself to
4477 reply to the ping immediately; the message bus will not make this
4478 message visible to other applications.
4482 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4483 the ping message were sent with a <literal>DESTINATION</literal> name of
4484 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4485 forwarded, and the Yoyodyne Corporation screensaver application would be
4486 expected to reply to the ping.
4490 Message bus implementations may impose a security policy which
4491 prevents certain messages from being sent or received.
4492 When a method call message cannot be sent or received due to a security
4493 policy, the message bus should send an error reply, unless the
4494 original message had the <literal>NO_REPLY</literal> flag.
4497 <sect3 id="message-bus-routing-eavesdropping">
4498 <title>Eavesdropping</title>
4500 Receiving a unicast message whose <literal>DESTINATION</literal>
4501 indicates a different recipient is called
4502 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4503 a security boundary (like the standard system bus), the security
4504 policy should usually prevent eavesdropping, since unicast messages
4505 are normally kept private and may contain security-sensitive
4510 Eavesdropping is mainly useful for debugging tools, such as
4511 the <literal>dbus-monitor</literal> tool in the reference
4512 implementation of D-Bus. Tools which eavesdrop on the message bus
4513 should be careful to avoid sending a reply or error in response to
4514 messages intended for a different client.
4518 Clients may attempt to eavesdrop by adding match rules
4519 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4520 the <literal>eavesdrop='true'</literal> match. If the message bus'
4521 security policy does not allow eavesdropping, the match rule can
4522 still be added, but will not have any practical effect. For
4523 compatibility with older message bus implementations, if adding such
4524 a match rule results in an error reply, the client may fall back to
4525 adding the same rule with the <literal>eavesdrop</literal> match
4530 <sect3 id="message-bus-routing-match-rules">
4531 <title>Match Rules</title>
4533 An important part of the message bus routing protocol is match
4534 rules. Match rules describe the messages that should be sent to a
4535 client, based on the contents of the message. Broadcast signals
4536 are only sent to clients which have a suitable match rule: this
4537 avoids waking up client processes to deal with signals that are
4538 not relevant to that client.
4541 Messages that list a client as their <literal>DESTINATION</literal>
4542 do not need to match the client's match rules, and are sent to that
4543 client regardless. As a result, match rules are mainly used to
4544 receive a subset of broadcast signals.
4547 Match rules can also be used for eavesdropping
4548 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4549 if the security policy of the message bus allows it.
4552 Match rules are added using the AddMatch bus method
4553 (see <xref linkend="bus-messages-add-match"/>). Rules are
4554 specified as a string of comma separated key/value pairs.
4555 Excluding a key from the rule indicates a wildcard match.
4556 For instance excluding the the member from a match rule but
4557 adding a sender would let all messages from that sender through.
4558 An example of a complete rule would be
4559 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4562 Within single quotes (ASCII apostrophe, U+0027), a backslash
4563 (U+005C) represents itself, and an apostrophe ends the quoted
4564 section. Outside single quotes, \' (backslash, apostrophe)
4565 represents an apostrophe, and any backslash not followed by
4566 an apostrophe represents itself. For instance, the match rules
4567 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4568 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4569 both match messages where the arguments are a 1-character string
4570 containing an apostrophe, a 1-character string containing a
4571 backslash, a 1-character string containing a comma, and a
4572 2-character string containing two backslashes<footnote>
4574 This idiosyncratic quoting style is based on the rules for
4575 escaping items to appear inside single-quoted strings
4576 in POSIX <literal>/bin/sh</literal>, but please
4577 note that backslashes that are not inside single quotes have
4578 different behaviour. This syntax does not offer any way to
4579 represent an apostrophe inside single quotes (it is necessary
4580 to leave the single-quoted section, backslash-escape the
4581 apostrophe and re-enter single quotes), or to represent a
4582 comma outside single quotes (it is necessary to wrap it in
4583 a single-quoted section).
4588 The following table describes the keys that can be used to create
4595 <entry>Possible Values</entry>
4596 <entry>Description</entry>
4601 <entry><literal>type</literal></entry>
4602 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4603 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4606 <entry><literal>sender</literal></entry>
4607 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4608 and <xref linkend="term-unique-name"/> respectively)
4610 <entry>Match messages sent by a particular sender. An example of a sender match
4611 is sender='org.freedesktop.Hal'</entry>
4614 <entry><literal>interface</literal></entry>
4615 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4616 <entry>Match messages sent over or to a particular interface. An example of an
4617 interface match is interface='org.freedesktop.Hal.Manager'.
4618 If a message omits the interface header, it must not match any rule
4619 that specifies this key.</entry>
4622 <entry><literal>member</literal></entry>
4623 <entry>Any valid method or signal name</entry>
4624 <entry>Matches messages which have the give method or signal name. An example of
4625 a member match is member='NameOwnerChanged'</entry>
4628 <entry><literal>path</literal></entry>
4629 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4630 <entry>Matches messages which are sent from or to the given object. An example of a
4631 path match is path='/org/freedesktop/Hal/Manager'</entry>
4634 <entry><literal>path_namespace</literal></entry>
4635 <entry>An object path</entry>
4638 Matches messages which are sent from or to an
4639 object for which the object path is either the
4640 given value, or that value followed by one or
4641 more path components.
4646 <literal>path_namespace='/com/example/foo'</literal>
4647 would match signals sent by
4648 <literal>/com/example/foo</literal>
4650 <literal>/com/example/foo/bar</literal>,
4652 <literal>/com/example/foobar</literal>.
4656 Using both <literal>path</literal> and
4657 <literal>path_namespace</literal> in the same match
4658 rule is not allowed.
4663 This match key was added in version 0.16 of the
4664 D-Bus specification and implemented by the bus
4665 daemon in dbus 1.5.0 and later.
4671 <entry><literal>destination</literal></entry>
4672 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4673 <entry>Matches messages which are being sent to the given unique name. An
4674 example of a destination match is destination=':1.0'</entry>
4677 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4678 <entry>Any string</entry>
4679 <entry>Arg matches are special and are used for further restricting the
4680 match based on the arguments in the body of a message. Only arguments of type
4681 STRING can be matched in this way. An example of an argument match
4682 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4686 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4687 <entry>Any string</entry>
4689 <para>Argument path matches provide a specialised form of wildcard matching for
4690 path-like namespaces. They can match arguments whose type is either STRING or
4691 OBJECT_PATH. As with normal argument matches,
4692 if the argument is exactly equal to the string given in the match
4693 rule then the rule is satisfied. Additionally, there is also a
4694 match when either the string given in the match rule or the
4695 appropriate message argument ends with '/' and is a prefix of the
4696 other. An example argument path match is arg0path='/aa/bb/'. This
4697 would match messages with first arguments of '/', '/aa/',
4698 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4699 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4701 <para>This is intended for monitoring “directories” in file system-like
4702 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4703 system. An application interested in all nodes in a particular hierarchy would
4704 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4705 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4706 represent a modification to the “bar” property, or a signal with zeroth
4707 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4708 many properties within that directory, and the interested application would be
4709 notified in both cases.</para>
4712 This match key was added in version 0.12 of the
4713 D-Bus specification, implemented for STRING
4714 arguments by the bus daemon in dbus 1.2.0 and later,
4715 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4722 <entry><literal>arg0namespace</literal></entry>
4723 <entry>Like a bus name, except that the string is not
4724 required to contain a '.' (period)</entry>
4726 <para>Match messages whose first argument is of type STRING, and is a bus name
4727 or interface name within the specified namespace. This is primarily intended
4728 for watching name owner changes for a group of related bus names, rather than
4729 for a single name or all name changes.</para>
4731 <para>Because every valid interface name is also a valid
4732 bus name, this can also be used for messages whose
4733 first argument is an interface name.</para>
4735 <para>For example, the match rule
4736 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4737 matches name owner changes for bus names such as
4738 <literal>com.example.backend.foo</literal>,
4739 <literal>com.example.backend.foo.bar</literal>, and
4740 <literal>com.example.backend</literal> itself.</para>
4742 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4745 This match key was added in version 0.16 of the
4746 D-Bus specification and implemented by the bus
4747 daemon in dbus 1.5.0 and later.
4753 <entry><literal>eavesdrop</literal></entry>
4754 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4755 <entry>Since D-Bus 1.5.6, match rules do not
4756 match messages which have a <literal>DESTINATION</literal>
4757 field unless the match rule specifically
4759 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4760 by specifying <literal>eavesdrop='true'</literal>
4761 in the match rule. <literal>eavesdrop='false'</literal>
4762 restores the default behaviour. Messages are
4763 delivered to their <literal>DESTINATION</literal>
4764 regardless of match rules, so this match does not
4765 affect normal delivery of unicast messages.
4766 If the message bus has a security policy which forbids
4767 eavesdropping, this match may still be used without error,
4768 but will not have any practical effect.
4769 In older versions of D-Bus, this match was not allowed
4770 in match rules, and all match rules behaved as if
4771 <literal>eavesdrop='true'</literal> had been used.
4780 <sect2 id="message-bus-starting-services">
4781 <title>Message Bus Starting Services</title>
4783 The message bus can start applications on behalf of other applications.
4784 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4785 An application that can be started in this way is called a
4786 <firstterm>service</firstterm>.
4789 With D-Bus, starting a service is normally done by name. That is,
4790 applications ask the message bus to start some program that will own a
4791 well-known name, such as <literal>com.example.TextEditor</literal>.
4792 This implies a contract documented along with the name
4793 <literal>com.example.TextEditor</literal> for which object
4794 the owner of that name will provide, and what interfaces those
4798 To find an executable corresponding to a particular name, the bus daemon
4799 looks for <firstterm>service description files</firstterm>. Service
4800 description files define a mapping from names to executables. Different
4801 kinds of message bus will look for these files in different places, see
4802 <xref linkend="message-bus-types"/>.
4805 Service description files have the ".service" file
4806 extension. The message bus will only load service description files
4807 ending with .service; all other files will be ignored. The file format
4808 is similar to that of <ulink
4809 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4810 entries</ulink>. All service description files must be in UTF-8
4811 encoding. To ensure that there will be no name collisions, service files
4812 must be namespaced using the same mechanism as messages and service
4817 On the well-known system bus, the name of a service description file
4818 must be its well-known name plus <literal>.service</literal>,
4820 <literal>com.example.ConfigurationDatabase.service</literal>.
4824 On the well-known session bus, services should follow the same
4825 service description file naming convention as on the system bus,
4826 but for backwards compatibility they are not required to do so.
4830 [FIXME the file format should be much better specified than "similar to
4831 .desktop entries" esp. since desktop entries are already
4832 badly-specified. ;-)]
4833 These sections from the specification apply to service files as well:
4836 <listitem><para>General syntax</para></listitem>
4837 <listitem><para>Comment format</para></listitem>
4840 Service description files must contain a
4841 <literal>D-BUS Service</literal> group with at least the keys
4842 <literal>Name</literal> (the well-known name of the service)
4843 and <literal>Exec</literal> (the command to be executed).
4846 <title>Example service description file</title>
4848 # Sample service description file
4850 Name=com.example.ConfigurationDatabase
4851 Exec=/usr/bin/sample-configd
4857 Additionally, service description files for the well-known system
4858 bus on Unix must contain a <literal>User</literal> key, whose value
4859 is the name of a user account (e.g. <literal>root</literal>).
4860 The system service will be run as that user.
4864 When an application asks to start a service by name, the bus daemon tries to
4865 find a service that will own that name. It then tries to spawn the
4866 executable associated with it. If this fails, it will report an
4871 On the well-known system bus, it is not possible for two .service files
4872 in the same directory to offer the same service, because they are
4873 constrained to have names that match the service name.
4877 On the well-known session bus, if two .service files in the same
4878 directory offer the same service name, the result is undefined.
4879 Distributors should avoid this situation, for instance by naming
4880 session services' .service files according to their service name.
4884 If two .service files in different directories offer the same
4885 service name, the one in the higher-priority directory is used:
4886 for instance, on the system bus, .service files in
4887 /usr/local/share/dbus-1/system-services take precedence over those
4888 in /usr/share/dbus-1/system-services.
4891 The executable launched will have the environment variable
4892 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4893 message bus so it can connect and request the appropriate names.
4896 The executable being launched may want to know whether the message bus
4897 starting it is one of the well-known message buses (see <xref
4898 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4899 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4900 of the well-known buses. The currently-defined values for this variable
4901 are <literal>system</literal> for the systemwide message bus,
4902 and <literal>session</literal> for the per-login-session message
4903 bus. The new executable must still connect to the address given
4904 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4905 resulting connection is to the well-known bus.
4908 [FIXME there should be a timeout somewhere, either specified
4909 in the .service file, by the client, or just a global value
4910 and if the client being activated fails to connect within that
4911 timeout, an error should be sent back.]
4914 <sect3 id="message-bus-starting-services-scope">
4915 <title>Message Bus Service Scope</title>
4917 The "scope" of a service is its "per-", such as per-session,
4918 per-machine, per-home-directory, or per-display. The reference
4919 implementation doesn't yet support starting services in a different
4920 scope from the message bus itself. So e.g. if you start a service
4921 on the session bus its scope is per-session.
4924 We could add an optional scope to a bus name. For example, for
4925 per-(display,session pair), we could have a unique ID for each display
4926 generated automatically at login and set on screen 0 by executing a
4927 special "set display ID" binary. The ID would be stored in a
4928 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4929 random bytes. This ID would then be used to scope names.
4930 Starting/locating a service could be done by ID-name pair rather than
4934 Contrast this with a per-display scope. To achieve that, we would
4935 want a single bus spanning all sessions using a given display.
4936 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4937 property on screen 0 of the display, pointing to this bus.
4942 <sect2 id="message-bus-types">
4943 <title>Well-known Message Bus Instances</title>
4945 Two standard message bus instances are defined here, along with how
4946 to locate them and where their service files live.
4948 <sect3 id="message-bus-types-login">
4949 <title>Login session message bus</title>
4951 Each time a user logs in, a <firstterm>login session message
4952 bus</firstterm> may be started. All applications in the user's login
4953 session may interact with one another using this message bus.
4956 The address of the login session message bus is given
4957 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4958 variable. If that variable is not set, applications may
4959 also try to read the address from the X Window System root
4960 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4961 The root window property must have type <literal>STRING</literal>.
4962 The environment variable should have precedence over the
4963 root window property.
4965 <para>The address of the login session message bus is given in the
4966 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4967 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4968 "autolaunch:", the system should use platform-specific methods of
4969 locating a running D-Bus session server, or starting one if a running
4970 instance cannot be found. Note that this mechanism is not recommended
4971 for attempting to determine if a daemon is running. It is inherently
4972 racy to attempt to make this determination, since the bus daemon may
4973 be started just before or just after the determination is made.
4974 Therefore, it is recommended that applications do not try to make this
4975 determination for their functionality purposes, and instead they
4976 should attempt to start the server.</para>
4978 <sect4 id="message-bus-types-login-x-windows">
4979 <title>X Windowing System</title>
4981 For the X Windowing System, the application must locate the
4982 window owner of the selection represented by the atom formed by
4986 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4990 <para>the current user's username</para>
4994 <para>the literal character '_' (underscore)</para>
4998 <para>the machine's ID</para>
5004 The following properties are defined for the window that owns
5006 <informaltable frame="all">
5015 <para>meaning</para>
5021 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5025 <para>the actual address of the server socket</para>
5031 <para>_DBUS_SESSION_BUS_PID</para>
5035 <para>the PID of the server process</para>
5044 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5045 present in this window.
5049 If the X selection cannot be located or if reading the
5050 properties from the window fails, the implementation MUST conclude
5051 that there is no D-Bus server running and proceed to start a new
5052 server. (See below on concurrency issues)
5056 Failure to connect to the D-Bus server address thus obtained
5057 MUST be treated as a fatal connection error and should be reported
5062 As an alternative, an implementation MAY find the information
5063 in the following file located in the current user's home directory,
5064 in subdirectory .dbus/session-bus/:
5067 <para>the machine's ID</para>
5071 <para>the literal character '-' (dash)</para>
5075 <para>the X display without the screen number, with the
5076 following prefixes removed, if present: ":", "localhost:"
5077 ."localhost.localdomain:". That is, a display of
5078 "localhost:10.0" produces just the number "10"</para>
5084 The contents of this file NAME=value assignment pairs and
5085 lines starting with # are comments (no comments are allowed
5086 otherwise). The following variable names are defined:
5093 <para>Variable</para>
5097 <para>meaning</para>
5103 <para>DBUS_SESSION_BUS_ADDRESS</para>
5107 <para>the actual address of the server socket</para>
5113 <para>DBUS_SESSION_BUS_PID</para>
5117 <para>the PID of the server process</para>
5123 <para>DBUS_SESSION_BUS_WINDOWID</para>
5127 <para>the window ID</para>
5136 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5141 Failure to open this file MUST be interpreted as absence of a
5142 running server. Therefore, the implementation MUST proceed to
5143 attempting to launch a new bus server if the file cannot be
5148 However, success in opening this file MUST NOT lead to the
5149 conclusion that the server is running. Thus, a failure to connect to
5150 the bus address obtained by the alternative method MUST NOT be
5151 considered a fatal error. If the connection cannot be established,
5152 the implementation MUST proceed to check the X selection settings or
5153 to start the server on its own.
5157 If the implementation concludes that the D-Bus server is not
5158 running it MUST attempt to start a new server and it MUST also
5159 ensure that the daemon started as an effect of the "autolaunch"
5160 mechanism provides the lookup mechanisms described above, so
5161 subsequent calls can locate the newly started server. The
5162 implementation MUST also ensure that if two or more concurrent
5163 initiations happen, only one server remains running and all other
5164 initiations are able to obtain the address of this server and
5165 connect to it. In other words, the implementation MUST ensure that
5166 the X selection is not present when it attempts to set it, without
5167 allowing another process to set the selection between the
5168 verification and the setting (e.g., by using XGrabServer /
5175 On Unix systems, the session bus should search for .service files
5176 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5178 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5179 Implementations may also search additional locations, which
5180 should be searched with lower priority than anything in
5181 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5182 for example, the reference implementation also
5183 looks in <literal>${datadir}/dbus-1/services</literal> as
5184 set at compile time.
5187 As described in the XDG Base Directory Specification, software
5188 packages should install their session .service files to their
5189 configured <literal>${datadir}/dbus-1/services</literal>,
5190 where <literal>${datadir}</literal> is as defined by the GNU
5191 coding standards. System administrators or users can arrange
5192 for these service files to be read by setting XDG_DATA_DIRS or by
5193 symlinking them into the default locations.
5197 <sect3 id="message-bus-types-system">
5198 <title>System message bus</title>
5200 A computer may have a <firstterm>system message bus</firstterm>,
5201 accessible to all applications on the system. This message bus may be
5202 used to broadcast system events, such as adding new hardware devices,
5203 changes in the printer queue, and so forth.
5206 The address of the system message bus is given
5207 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5208 variable. If that variable is not set, applications should try
5209 to connect to the well-known address
5210 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5213 The D-Bus reference implementation actually honors the
5214 <literal>$(localstatedir)</literal> configure option
5215 for this address, on both client and server side.
5220 On Unix systems, the system bus should default to searching
5221 for .service files in
5222 <literal>/usr/local/share/dbus-1/system-services</literal>,
5223 <literal>/usr/share/dbus-1/system-services</literal> and
5224 <literal>/lib/dbus-1/system-services</literal>, with that order
5225 of precedence. It may also search other implementation-specific
5226 locations, but should not vary these locations based on environment
5230 The system bus is security-sensitive and is typically executed
5231 by an init system with a clean environment. Its launch helper
5232 process is particularly security-sensitive, and specifically
5233 clears its own environment.
5238 Software packages should install their system .service
5239 files to their configured
5240 <literal>${datadir}/dbus-1/system-services</literal>,
5241 where <literal>${datadir}</literal> is as defined by the GNU
5242 coding standards. System administrators can arrange
5243 for these service files to be read by editing the system bus'
5244 configuration file or by symlinking them into the default
5250 <sect2 id="message-bus-messages">
5251 <title>Message Bus Messages</title>
5253 The special message bus name <literal>org.freedesktop.DBus</literal>
5254 responds to a number of additional messages.
5257 <sect3 id="bus-messages-hello">
5258 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5269 <entry>Argument</entry>
5271 <entry>Description</entry>
5277 <entry>STRING</entry>
5278 <entry>Unique name assigned to the connection</entry>
5285 Before an application is able to send messages to other applications
5286 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5287 to the message bus to obtain a unique name. If an application without
5288 a unique name tries to send a message to another application, or a
5289 message to the message bus itself that isn't the
5290 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5291 disconnected from the bus.
5294 There is no corresponding "disconnect" request; if a client wishes to
5295 disconnect from the bus, it simply closes the socket (or other
5296 communication channel).
5299 <sect3 id="bus-messages-list-names">
5300 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5304 ARRAY of STRING ListNames ()
5311 <entry>Argument</entry>
5313 <entry>Description</entry>
5319 <entry>ARRAY of STRING</entry>
5320 <entry>Array of strings where each string is a bus name</entry>
5327 Returns a list of all currently-owned names on the bus.
5330 <sect3 id="bus-messages-list-activatable-names">
5331 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5335 ARRAY of STRING ListActivatableNames ()
5342 <entry>Argument</entry>
5344 <entry>Description</entry>
5350 <entry>ARRAY of STRING</entry>
5351 <entry>Array of strings where each string is a bus name</entry>
5358 Returns a list of all names that can be activated on the bus.
5361 <sect3 id="bus-messages-name-exists">
5362 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5366 BOOLEAN NameHasOwner (in STRING name)
5373 <entry>Argument</entry>
5375 <entry>Description</entry>
5381 <entry>STRING</entry>
5382 <entry>Name to check</entry>
5392 <entry>Argument</entry>
5394 <entry>Description</entry>
5400 <entry>BOOLEAN</entry>
5401 <entry>Return value, true if the name exists</entry>
5408 Checks if the specified name exists (currently has an owner).
5412 <sect3 id="bus-messages-name-owner-changed">
5413 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5417 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5424 <entry>Argument</entry>
5426 <entry>Description</entry>
5432 <entry>STRING</entry>
5433 <entry>Name with a new owner</entry>
5437 <entry>STRING</entry>
5438 <entry>Old owner or empty string if none</entry>
5442 <entry>STRING</entry>
5443 <entry>New owner or empty string if none</entry>
5450 This signal indicates that the owner of a name has changed.
5451 It's also the signal to use to detect the appearance of
5452 new names on the bus.
5455 <sect3 id="bus-messages-name-lost">
5456 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5460 NameLost (STRING name)
5467 <entry>Argument</entry>
5469 <entry>Description</entry>
5475 <entry>STRING</entry>
5476 <entry>Name which was lost</entry>
5483 This signal is sent to a specific application when it loses
5484 ownership of a name.
5488 <sect3 id="bus-messages-name-acquired">
5489 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5493 NameAcquired (STRING name)
5500 <entry>Argument</entry>
5502 <entry>Description</entry>
5508 <entry>STRING</entry>
5509 <entry>Name which was acquired</entry>
5516 This signal is sent to a specific application when it gains
5517 ownership of a name.
5521 <sect3 id="bus-messages-start-service-by-name">
5522 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5526 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5533 <entry>Argument</entry>
5535 <entry>Description</entry>
5541 <entry>STRING</entry>
5542 <entry>Name of the service to start</entry>
5546 <entry>UINT32</entry>
5547 <entry>Flags (currently not used)</entry>
5557 <entry>Argument</entry>
5559 <entry>Description</entry>
5565 <entry>UINT32</entry>
5566 <entry>Return value</entry>
5571 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5575 The return value can be one of the following values:
5580 <entry>Identifier</entry>
5581 <entry>Value</entry>
5582 <entry>Description</entry>
5587 <entry>DBUS_START_REPLY_SUCCESS</entry>
5589 <entry>The service was successfully started.</entry>
5592 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5594 <entry>A connection already owns the given name.</entry>
5603 <sect3 id="bus-messages-update-activation-environment">
5604 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5608 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5615 <entry>Argument</entry>
5617 <entry>Description</entry>
5623 <entry>ARRAY of DICT<STRING,STRING></entry>
5624 <entry>Environment to add or update</entry>
5629 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5632 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5635 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.
5640 <sect3 id="bus-messages-get-name-owner">
5641 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5645 STRING GetNameOwner (in STRING name)
5652 <entry>Argument</entry>
5654 <entry>Description</entry>
5660 <entry>STRING</entry>
5661 <entry>Name to get the owner of</entry>
5671 <entry>Argument</entry>
5673 <entry>Description</entry>
5679 <entry>STRING</entry>
5680 <entry>Return value, a unique connection name</entry>
5685 Returns the unique connection name of the primary owner of the name
5686 given. If the requested name doesn't have an owner, returns a
5687 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5691 <sect3 id="bus-messages-get-connection-unix-user">
5692 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5696 UINT32 GetConnectionUnixUser (in STRING bus_name)
5703 <entry>Argument</entry>
5705 <entry>Description</entry>
5711 <entry>STRING</entry>
5712 <entry>Unique or well-known bus name of the connection to
5713 query, such as <literal>:12.34</literal> or
5714 <literal>com.example.tea</literal></entry>
5724 <entry>Argument</entry>
5726 <entry>Description</entry>
5732 <entry>UINT32</entry>
5733 <entry>Unix user ID</entry>
5738 Returns the Unix user ID of the process connected to the server. If
5739 unable to determine it (for instance, because the process is not on the
5740 same machine as the bus daemon), an error is returned.
5744 <sect3 id="bus-messages-get-connection-unix-process-id">
5745 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5749 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5756 <entry>Argument</entry>
5758 <entry>Description</entry>
5764 <entry>STRING</entry>
5765 <entry>Unique or well-known bus name of the connection to
5766 query, such as <literal>:12.34</literal> or
5767 <literal>com.example.tea</literal></entry>
5777 <entry>Argument</entry>
5779 <entry>Description</entry>
5785 <entry>UINT32</entry>
5786 <entry>Unix process id</entry>
5791 Returns the Unix process ID of the process connected to the server. If
5792 unable to determine it (for instance, because the process is not on the
5793 same machine as the bus daemon), an error is returned.
5797 <sect3 id="bus-messages-get-connection-credentials">
5798 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5802 DICT<STRING,VARIANT> GetConnectionCredentials (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>DICT<STRING,VARIANT></entry>
5839 <entry>Credentials</entry>
5847 Returns as many credentials as possible for the process connected to
5848 the server. If unable to determine certain credentials (for instance,
5849 because the process is not on the same machine as the bus daemon,
5850 or because this version of the bus daemon does not support a
5851 particular security framework), or if the values of those credentials
5852 cannot be represented as documented here, then those credentials
5857 Keys in the returned dictionary not containing "." are defined
5858 by this specification. Bus daemon implementors supporting
5859 credentials frameworks not mentioned in this document should either
5860 contribute patches to this specification, or use keys containing
5861 "." and starting with a reversed domain name.
5867 <entry>Value type</entry>
5868 <entry>Value</entry>
5873 <entry>UnixUserID</entry>
5874 <entry>UINT32</entry>
5875 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5878 <entry>ProcessID</entry>
5879 <entry>UINT32</entry>
5880 <entry>The numeric process ID, on platforms that have
5881 this concept. On Unix, this is the process ID defined by
5890 This method was added in D-Bus 1.7 to reduce the round-trips
5891 required to list a process's credentials. In older versions, calling
5892 this method will fail: applications should recover by using the
5893 separate methods such as
5894 <xref linkend="bus-messages-get-connection-unix-user"/>
5899 <sect3 id="bus-messages-get-adt-audit-session-data">
5900 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5904 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5911 <entry>Argument</entry>
5913 <entry>Description</entry>
5919 <entry>STRING</entry>
5920 <entry>Unique or well-known bus name of the connection to
5921 query, such as <literal>:12.34</literal> or
5922 <literal>com.example.tea</literal></entry>
5932 <entry>Argument</entry>
5934 <entry>Description</entry>
5940 <entry>ARRAY of BYTE</entry>
5941 <entry>auditing data as returned by
5942 adt_export_session_data()</entry>
5947 Returns auditing data used by Solaris ADT, in an unspecified
5948 binary format. If you know what this means, please contribute
5949 documentation via the D-Bus bug tracking system.
5950 This method is on the core DBus interface for historical reasons;
5951 the same information should be made available via
5952 <xref linkend="bus-messages-get-connection-credentials"/>
5957 <sect3 id="bus-messages-get-connection-selinux-security-context">
5958 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5962 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5969 <entry>Argument</entry>
5971 <entry>Description</entry>
5977 <entry>STRING</entry>
5978 <entry>Unique or well-known bus name of the connection to
5979 query, such as <literal>:12.34</literal> or
5980 <literal>com.example.tea</literal></entry>
5990 <entry>Argument</entry>
5992 <entry>Description</entry>
5998 <entry>ARRAY of BYTE</entry>
5999 <entry>some sort of string of bytes, not necessarily UTF-8,
6000 not including '\0'</entry>
6005 Returns the security context used by SELinux, in an unspecified
6006 format. If you know what this means, please contribute
6007 documentation via the D-Bus bug tracking system.
6008 This method is on the core DBus interface for historical reasons;
6009 the same information should be made available via
6010 <xref linkend="bus-messages-get-connection-credentials"/>
6016 <sect3 id="bus-messages-add-match">
6017 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6021 AddMatch (in STRING rule)
6028 <entry>Argument</entry>
6030 <entry>Description</entry>
6036 <entry>STRING</entry>
6037 <entry>Match rule to add to the connection</entry>
6042 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6043 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6047 <sect3 id="bus-messages-remove-match">
6048 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6052 RemoveMatch (in STRING rule)
6059 <entry>Argument</entry>
6061 <entry>Description</entry>
6067 <entry>STRING</entry>
6068 <entry>Match rule to remove from the connection</entry>
6073 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6074 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6079 <sect3 id="bus-messages-get-id">
6080 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6084 GetId (out STRING id)
6091 <entry>Argument</entry>
6093 <entry>Description</entry>
6099 <entry>STRING</entry>
6100 <entry>Unique ID identifying the bus daemon</entry>
6105 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6106 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6107 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6108 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6109 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6110 by org.freedesktop.DBus.Peer.GetMachineId().
6111 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6119 <appendix id="implementation-notes">
6120 <title>Implementation notes</title>
6121 <sect1 id="implementation-notes-subsection">
6129 <glossary><title>Glossary</title>
6131 This glossary defines some of the terms used in this specification.
6134 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6137 The message bus maintains an association between names and
6138 connections. (Normally, there's one connection per application.) A
6139 bus name is simply an identifier used to locate connections. For
6140 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6141 name might be used to send a message to a screensaver from Yoyodyne
6142 Corporation. An application is said to <firstterm>own</firstterm> a
6143 name if the message bus has associated the application's connection
6144 with the name. Names may also have <firstterm>queued
6145 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6146 The bus assigns a unique name to each connection,
6147 see <xref linkend="term-unique-name"/>. Other names
6148 can be thought of as "well-known names" and are
6149 used to find applications that offer specific functionality.
6153 See <xref linkend="message-protocol-names-bus"/> for details of
6154 the syntax and naming conventions for bus names.
6159 <glossentry id="term-message"><glossterm>Message</glossterm>
6162 A message is the atomic unit of communication via the D-Bus
6163 protocol. It consists of a <firstterm>header</firstterm> and a
6164 <firstterm>body</firstterm>; the body is made up of
6165 <firstterm>arguments</firstterm>.
6170 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6173 The message bus is a special application that forwards
6174 or routes messages between a group of applications
6175 connected to the message bus. It also manages
6176 <firstterm>names</firstterm> used for routing
6182 <glossentry id="term-name"><glossterm>Name</glossterm>
6185 See <xref linkend="term-bus-name"/>. "Name" may
6186 also be used to refer to some of the other names
6187 in D-Bus, such as interface names.
6192 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6195 Used to prevent collisions when defining new interfaces, bus names
6196 etc. The convention used is the same one Java uses for defining
6197 classes: a reversed domain name.
6198 See <xref linkend="message-protocol-names-bus"/>,
6199 <xref linkend="message-protocol-names-interface"/>,
6200 <xref linkend="message-protocol-names-error"/>,
6201 <xref linkend="message-protocol-marshaling-object-path"/>.
6206 <glossentry id="term-object"><glossterm>Object</glossterm>
6209 Each application contains <firstterm>objects</firstterm>, which have
6210 <firstterm>interfaces</firstterm> and
6211 <firstterm>methods</firstterm>. Objects are referred to by a name,
6212 called a <firstterm>path</firstterm>.
6217 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6220 An application talking directly to another application, without going
6221 through a message bus. One-to-one connections may be "peer to peer" or
6222 "client to server." The D-Bus protocol has no concept of client
6223 vs. server after a connection has authenticated; the flow of messages
6224 is symmetrical (full duplex).
6229 <glossentry id="term-path"><glossterm>Path</glossterm>
6232 Object references (object names) in D-Bus are organized into a
6233 filesystem-style hierarchy, so each object is named by a path. As in
6234 LDAP, there's no difference between "files" and "directories"; a path
6235 can refer to an object, while still having child objects below it.
6240 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6243 Each bus name has a primary owner; messages sent to the name go to the
6244 primary owner. However, certain names also maintain a queue of
6245 secondary owners "waiting in the wings." If the primary owner releases
6246 the name, then the first secondary owner in the queue automatically
6247 becomes the new owner of the name.
6252 <glossentry id="term-service"><glossterm>Service</glossterm>
6255 A service is an executable that can be launched by the bus daemon.
6256 Services normally guarantee some particular features, for example they
6257 may guarantee that they will request a specific name such as
6258 "com.example.Screensaver", have a singleton object
6259 "/com/example/Application", and that object will implement the
6260 interface "com.example.Screensaver.Control".
6265 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6268 ".service files" tell the bus about service applications that can be
6269 launched (see <xref linkend="term-service"/>). Most importantly they
6270 provide a mapping from bus names to services that will request those
6271 names when they start up.
6276 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6279 The special name automatically assigned to each connection by the
6280 message bus. This name will never change owner, and will be unique
6281 (never reused during the lifetime of the message bus).
6282 It will begin with a ':' character.
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