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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.25</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>0.26</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>0.25</revnumber>
83 <date>2014-11-10</date>
84 <authorinitials>smcv, lennart</authorinitials>
86 ALLOW_INTERACTIVE_AUTHORIZATION flag, EmitsChangedSignal=const
90 <revnumber>0.24</revnumber>
91 <date>2014-10-01</date>
92 <authorinitials>SMcV</authorinitials>
94 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
95 document how to quote match rules
99 <revnumber>0.23</revnumber>
100 <date>2014-01-06</date>
101 <authorinitials>SMcV, CY</authorinitials>
103 method call messages with no INTERFACE may be considered an error;
104 document tcp:bind=... and nonce-tcp:bind=...; define listenable
105 and connectable addresses
109 <revnumber>0.22</revnumber>
110 <date>2013-10-09</date>
111 <authorinitials></authorinitials>
112 <revremark>add GetConnectionCredentials, document
113 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
114 document and correct .service file syntax and naming
118 <revnumber>0.21</revnumber>
119 <date>2013-04-25</date>
120 <authorinitials>smcv</authorinitials>
121 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
122 Corrigendum #9)</revremark>
125 <revnumber>0.20</revnumber>
126 <date>22 February 2013</date>
127 <authorinitials>smcv, walters</authorinitials>
128 <revremark>reorganise for clarity, remove false claims about
129 basic types, mention /o/fd/DBus</revremark>
132 <revnumber>0.19</revnumber>
133 <date>20 February 2012</date>
134 <authorinitials>smcv/lp</authorinitials>
135 <revremark>formally define unique connection names and well-known
136 bus names; document best practices for interface, bus, member and
137 error names, and object paths; document the search path for session
138 and system services on Unix; document the systemd transport</revremark>
141 <revnumber>0.18</revnumber>
142 <date>29 July 2011</date>
143 <authorinitials>smcv</authorinitials>
144 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
145 match keyword; promote type system to a top-level section</revremark>
148 <revnumber>0.17</revnumber>
149 <date>1 June 2011</date>
150 <authorinitials>smcv/davidz</authorinitials>
151 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
152 by GVariant</revremark>
155 <revnumber>0.16</revnumber>
156 <date>11 April 2011</date>
157 <authorinitials></authorinitials>
158 <revremark>add path_namespace, arg0namespace; argNpath matches object
162 <revnumber>0.15</revnumber>
163 <date>3 November 2010</date>
164 <authorinitials></authorinitials>
165 <revremark></revremark>
168 <revnumber>0.14</revnumber>
169 <date>12 May 2010</date>
170 <authorinitials></authorinitials>
171 <revremark></revremark>
174 <revnumber>0.13</revnumber>
175 <date>23 Dezember 2009</date>
176 <authorinitials></authorinitials>
177 <revremark></revremark>
180 <revnumber>0.12</revnumber>
181 <date>7 November, 2006</date>
182 <authorinitials></authorinitials>
183 <revremark></revremark>
186 <revnumber>0.11</revnumber>
187 <date>6 February 2005</date>
188 <authorinitials></authorinitials>
189 <revremark></revremark>
192 <revnumber>0.10</revnumber>
193 <date>28 January 2005</date>
194 <authorinitials></authorinitials>
195 <revremark></revremark>
198 <revnumber>0.9</revnumber>
199 <date>7 Januar 2005</date>
200 <authorinitials></authorinitials>
201 <revremark></revremark>
204 <revnumber>0.8</revnumber>
205 <date>06 September 2003</date>
206 <authorinitials></authorinitials>
207 <revremark>First released document.</revremark>
212 <sect1 id="introduction">
213 <title>Introduction</title>
215 D-Bus is a system for low-overhead, easy to use
216 interprocess communication (IPC). In more detail:
220 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
221 binary protocol, and does not have to convert to and from a text
222 format such as XML. Because D-Bus is intended for potentially
223 high-resolution same-machine IPC, not primarily for Internet IPC,
224 this is an interesting optimization. D-Bus is also designed to
225 avoid round trips and allow asynchronous operation, much like
231 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
232 of <firstterm>messages</firstterm> rather than byte streams, and
233 automatically handles a lot of the hard IPC issues. Also, the D-Bus
234 library is designed to be wrapped in a way that lets developers use
235 their framework's existing object/type system, rather than learning
236 a new one specifically for IPC.
243 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
244 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
245 a system for one application to talk to a single other
246 application. However, the primary intended application of the protocol is the
247 D-Bus <firstterm>message bus</firstterm>, specified in <xref
248 linkend="message-bus"/>. The message bus is a special application that
249 accepts connections from multiple other applications, and forwards
254 Uses of D-Bus include notification of system changes (notification of when
255 a camera is plugged in to a computer, or a new version of some software
256 has been installed), or desktop interoperability, for example a file
257 monitoring service or a configuration service.
261 D-Bus is designed for two specific use cases:
265 A "system bus" for notifications from the system to user sessions,
266 and to allow the system to request input from user sessions.
271 A "session bus" used to implement desktop environments such as
276 D-Bus is not intended to be a generic IPC system for any possible
277 application, and intentionally omits many features found in other
278 IPC systems for this reason.
282 At the same time, the bus daemons offer a number of features not found in
283 other IPC systems, such as single-owner "bus names" (similar to X
284 selections), on-demand startup of services, and security policies.
285 In many ways, these features are the primary motivation for developing
286 D-Bus; other systems would have sufficed if IPC were the only goal.
290 D-Bus may turn out to be useful in unanticipated applications, but future
291 versions of this spec and the reference implementation probably will not
292 incorporate features that interfere with the core use cases.
296 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
297 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
298 document are to be interpreted as described in RFC 2119. However, the
299 document could use a serious audit to be sure it makes sense to do
300 so. Also, they are not capitalized.
303 <sect2 id="stability">
304 <title>Protocol and Specification Stability</title>
306 The D-Bus protocol is frozen (only compatible extensions are allowed) as
307 of November 8, 2006. However, this specification could still use a fair
308 bit of work to make interoperable reimplementation possible without
309 reference to the D-Bus reference implementation. Thus, this
310 specification is not marked 1.0. To mark it 1.0, we'd like to see
311 someone invest significant effort in clarifying the specification
312 language, and growing the specification to cover more aspects of the
313 reference implementation's behavior.
316 Until this work is complete, any attempt to reimplement D-Bus will
317 probably require looking at the reference implementation and/or asking
318 questions on the D-Bus mailing list about intended behavior.
319 Questions on the list are very welcome.
322 Nonetheless, this document should be a useful starting point and is
323 to our knowledge accurate, though incomplete.
329 <sect1 id="type-system">
330 <title>Type System</title>
333 D-Bus has a type system, in which values of various types can be
334 serialized into a sequence of bytes referred to as the
335 <firstterm>wire format</firstterm> in a standard way.
336 Converting a value from some other representation into the wire
337 format is called <firstterm>marshaling</firstterm> and converting
338 it back from the wire format is <firstterm>unmarshaling</firstterm>.
342 The D-Bus protocol does not include type tags in the marshaled data; a
343 block of marshaled values must have a known <firstterm>type
344 signature</firstterm>. The type signature is made up of zero or more
345 <firstterm id="term-single-complete-type">single complete
346 types</firstterm>, each made up of one or more
347 <firstterm>type codes</firstterm>.
351 A type code is an ASCII character representing the
352 type of a value. Because ASCII characters are used, the type signature
353 will always form a valid ASCII string. A simple string compare
354 determines whether two type signatures are equivalent.
358 A single complete type is a sequence of type codes that fully describes
359 one type: either a basic type, or a single fully-described container type.
360 A single complete type is a basic type code, a variant type code,
361 an array with its element type, or a struct with its fields (all of which
362 are defined below). So the following signatures are not single complete
373 And the following signatures contain multiple complete types:
383 Note however that a single complete type may <emphasis>contain</emphasis>
384 multiple other single complete types, by containing a struct or dict
388 <sect2 id="basic-types">
389 <title>Basic types</title>
392 The simplest type codes are the <firstterm id="term-basic-type">basic
393 types</firstterm>, which are the types whose structure is entirely
394 defined by their 1-character type code. Basic types consist of
395 fixed types and string-like types.
399 The <firstterm id="term-fixed-type">fixed types</firstterm>
400 are basic types whose values have a fixed length, namely BYTE,
401 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
406 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
407 the ASCII character 'i'. So the signature for a block of values
408 containing a single <literal>INT32</literal> would be:
412 A block of values containing two <literal>INT32</literal> would have this signature:
419 The characteristics of the fixed types are listed in this table.
425 <entry>Conventional name</entry>
426 <entry>ASCII type-code</entry>
427 <entry>Encoding</entry>
432 <entry><literal>BYTE</literal></entry>
433 <entry><literal>y</literal> (121)</entry>
434 <entry>Unsigned 8-bit integer</entry>
437 <entry><literal>BOOLEAN</literal></entry>
438 <entry><literal>b</literal> (98)</entry>
439 <entry>Boolean value: 0 is false, 1 is true, any other value
440 allowed by the marshalling format is invalid</entry>
443 <entry><literal>INT16</literal></entry>
444 <entry><literal>n</literal> (110)</entry>
445 <entry>Signed (two's complement) 16-bit integer</entry>
448 <entry><literal>UINT16</literal></entry>
449 <entry><literal>q</literal> (113)</entry>
450 <entry>Unsigned 16-bit integer</entry>
453 <entry><literal>INT32</literal></entry>
454 <entry><literal>i</literal> (105)</entry>
455 <entry>Signed (two's complement) 32-bit integer</entry>
458 <entry><literal>UINT32</literal></entry>
459 <entry><literal>u</literal> (117)</entry>
460 <entry>Unsigned 32-bit integer</entry>
463 <entry><literal>INT64</literal></entry>
464 <entry><literal>x</literal> (120)</entry>
465 <entry>Signed (two's complement) 64-bit integer
466 (mnemonic: x and t are the first characters in "sixty" not
467 already used for something more common)</entry>
470 <entry><literal>UINT64</literal></entry>
471 <entry><literal>t</literal> (116)</entry>
472 <entry>Unsigned 64-bit integer</entry>
475 <entry><literal>DOUBLE</literal></entry>
476 <entry><literal>d</literal> (100)</entry>
477 <entry>IEEE 754 double-precision floating point</entry>
480 <entry><literal>UNIX_FD</literal></entry>
481 <entry><literal>h</literal> (104)</entry>
482 <entry>Unsigned 32-bit integer representing an index into an
483 out-of-band array of file descriptors, transferred via some
484 platform-specific mechanism (mnemonic: h for handle)</entry>
492 The <firstterm id="term-string-like-type">string-like types</firstterm>
493 are basic types with a variable length. The value of any string-like
494 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
495 none of which may be U+0000. The UTF-8 text must be validated
496 strictly: in particular, it must not contain overlong sequences
497 or codepoints above U+10FFFF.
501 Since D-Bus Specification version 0.21, in accordance with Unicode
502 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
503 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
504 D-Bus rejected these noncharacters).
508 The marshalling formats for the string-like types all end with a
509 single zero (NUL) byte, but that byte is not considered to be part of
514 The characteristics of the string-like types are listed in this table.
520 <entry>Conventional name</entry>
521 <entry>ASCII type-code</entry>
522 <entry>Validity constraints</entry>
527 <entry><literal>STRING</literal></entry>
528 <entry><literal>s</literal> (115)</entry>
529 <entry>No extra constraints</entry>
532 <entry><literal>OBJECT_PATH</literal></entry>
533 <entry><literal>o</literal> (111)</entry>
535 <link linkend="message-protocol-marshaling-object-path">a
536 syntactically valid object path</link></entry>
539 <entry><literal>SIGNATURE</literal></entry>
540 <entry><literal>g</literal> (103)</entry>
542 <firstterm linkend="term-single-complete-type">single
543 complete types</firstterm></entry>
550 <sect3 id="message-protocol-marshaling-object-path">
551 <title>Valid Object Paths</title>
554 An object path is a name used to refer to an object instance.
555 Conceptually, each participant in a D-Bus message exchange may have
556 any number of object instances (think of C++ or Java objects) and each
557 such instance will have a path. Like a filesystem, the object
558 instances in an application form a hierarchical tree.
562 Object paths are often namespaced by starting with a reversed
563 domain name and containing an interface version number, in the
565 <link linkend="message-protocol-names-interface">interface
567 <link linkend="message-protocol-names-bus">well-known
569 This makes it possible to implement more than one service, or
570 more than one version of a service, in the same process,
571 even if the services share a connection but cannot otherwise
572 co-operate (for instance, if they are implemented by different
577 For instance, if the owner of <literal>example.com</literal> is
578 developing a D-Bus API for a music player, they might use the
579 hierarchy of object paths that start with
580 <literal>/com/example/MusicPlayer1</literal> for its objects.
584 The following rules define a valid object path. Implementations must
585 not send or accept messages with invalid object paths.
589 The path may be of any length.
594 The path must begin with an ASCII '/' (integer 47) character,
595 and must consist of elements separated by slash characters.
600 Each element must only contain the ASCII characters
606 No element may be the empty string.
611 Multiple '/' characters cannot occur in sequence.
616 A trailing '/' character is not allowed unless the
617 path is the root path (a single '/' character).
625 <sect3 id="message-protocol-marshaling-signature">
626 <title>Valid Signatures</title>
628 An implementation must not send or accept invalid signatures.
629 Valid signatures will conform to the following rules:
633 The signature is a list of single complete types.
634 Arrays must have element types, and structs must
635 have both open and close parentheses.
640 Only type codes, open and close parentheses, and open and
641 close curly brackets are allowed in the signature. The
642 <literal>STRUCT</literal> type code
643 is not allowed in signatures, because parentheses
644 are used instead. Similarly, the
645 <literal>DICT_ENTRY</literal> type code is not allowed in
646 signatures, because curly brackets are used instead.
651 The maximum depth of container type nesting is 32 array type
652 codes and 32 open parentheses. This implies that the maximum
653 total depth of recursion is 64, for an "array of array of array
654 of ... struct of struct of struct of ..." where there are 32
660 The maximum length of a signature is 255.
667 When signatures appear in messages, the marshalling format
668 guarantees that they will be followed by a nul byte (which can
669 be interpreted as either C-style string termination or the INVALID
670 type-code), but this is not conceptually part of the signature.
676 <sect2 id="container-types">
677 <title>Container types</title>
680 In addition to basic types, there are four <firstterm>container</firstterm>
681 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
682 and <literal>DICT_ENTRY</literal>.
686 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
687 code does not appear in signatures. Instead, ASCII characters
688 '(' and ')' are used to mark the beginning and end of the struct.
689 So for example, a struct containing two integers would have this
694 Structs can be nested, so for example a struct containing
695 an integer and another struct:
699 The value block storing that struct would contain three integers; the
700 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
705 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
706 but is useful in code that implements the protocol. This type code
707 is specified to allow such code to interoperate in non-protocol contexts.
711 Empty structures are not allowed; there must be at least one
712 type code between the parentheses.
716 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
717 followed by a <firstterm>single complete type</firstterm>. The single
718 complete type following the array is the type of each array element. So
719 the simple example is:
723 which is an array of 32-bit integers. But an array can be of any type,
724 such as this array-of-struct-with-two-int32-fields:
728 Or this array of array of integer:
735 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
736 type <literal>VARIANT</literal> will have the signature of a single complete type as part
737 of the <emphasis>value</emphasis>. This signature will be followed by a
738 marshaled value of that type.
742 Unlike a message signature, the variant signature can
743 contain only a single complete type. So "i", "ai"
744 or "(ii)" is OK, but "ii" is not. Use of variants may not
745 cause a total message depth to be larger than 64, including
746 other container types such as structures.
750 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
751 than parentheses it uses curly braces, and it has more restrictions.
752 The restrictions are: it occurs only as an array element type; it has
753 exactly two single complete types inside the curly braces; the first
754 single complete type (the "key") must be a basic type rather than a
755 container type. Implementations must not accept dict entries outside of
756 arrays, must not accept dict entries with zero, one, or more than two
757 fields, and must not accept dict entries with non-basic-typed keys. A
758 dict entry is always a key-value pair.
762 The first field in the <literal>DICT_ENTRY</literal> is always the key.
763 A message is considered corrupt if the same key occurs twice in the same
764 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
765 implementations are not required to reject dicts with duplicate keys.
769 In most languages, an array of dict entry would be represented as a
770 map, hash table, or dict object.
775 <title>Summary of types</title>
778 The following table summarizes the D-Bus types.
783 <entry>Category</entry>
784 <entry>Conventional Name</entry>
786 <entry>Description</entry>
791 <entry>reserved</entry>
792 <entry><literal>INVALID</literal></entry>
793 <entry>0 (ASCII NUL)</entry>
794 <entry>Not a valid type code, used to terminate signatures</entry>
796 <entry>fixed, basic</entry>
797 <entry><literal>BYTE</literal></entry>
798 <entry>121 (ASCII 'y')</entry>
799 <entry>8-bit unsigned integer</entry>
801 <entry>fixed, basic</entry>
802 <entry><literal>BOOLEAN</literal></entry>
803 <entry>98 (ASCII 'b')</entry>
804 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
806 <entry>fixed, basic</entry>
807 <entry><literal>INT16</literal></entry>
808 <entry>110 (ASCII 'n')</entry>
809 <entry>16-bit signed integer</entry>
811 <entry>fixed, basic</entry>
812 <entry><literal>UINT16</literal></entry>
813 <entry>113 (ASCII 'q')</entry>
814 <entry>16-bit unsigned integer</entry>
816 <entry>fixed, basic</entry>
817 <entry><literal>INT32</literal></entry>
818 <entry>105 (ASCII 'i')</entry>
819 <entry>32-bit signed integer</entry>
821 <entry>fixed, basic</entry>
822 <entry><literal>UINT32</literal></entry>
823 <entry>117 (ASCII 'u')</entry>
824 <entry>32-bit unsigned integer</entry>
826 <entry>fixed, basic</entry>
827 <entry><literal>INT64</literal></entry>
828 <entry>120 (ASCII 'x')</entry>
829 <entry>64-bit signed integer</entry>
831 <entry>fixed, basic</entry>
832 <entry><literal>UINT64</literal></entry>
833 <entry>116 (ASCII 't')</entry>
834 <entry>64-bit unsigned integer</entry>
836 <entry>fixed, basic</entry>
837 <entry><literal>DOUBLE</literal></entry>
838 <entry>100 (ASCII 'd')</entry>
839 <entry>IEEE 754 double</entry>
841 <entry>string-like, basic</entry>
842 <entry><literal>STRING</literal></entry>
843 <entry>115 (ASCII 's')</entry>
844 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
846 <entry>string-like, basic</entry>
847 <entry><literal>OBJECT_PATH</literal></entry>
848 <entry>111 (ASCII 'o')</entry>
849 <entry>Name of an object instance</entry>
851 <entry>string-like, basic</entry>
852 <entry><literal>SIGNATURE</literal></entry>
853 <entry>103 (ASCII 'g')</entry>
854 <entry>A type signature</entry>
856 <entry>container</entry>
857 <entry><literal>ARRAY</literal></entry>
858 <entry>97 (ASCII 'a')</entry>
861 <entry>container</entry>
862 <entry><literal>STRUCT</literal></entry>
863 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
864 <entry>Struct; type code 114 'r' is reserved for use in
865 bindings and implementations to represent the general
866 concept of a struct, and must not appear in signatures
867 used on D-Bus.</entry>
869 <entry>container</entry>
870 <entry><literal>VARIANT</literal></entry>
871 <entry>118 (ASCII 'v') </entry>
872 <entry>Variant type (the type of the value is part of the value itself)</entry>
874 <entry>container</entry>
875 <entry><literal>DICT_ENTRY</literal></entry>
876 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
877 <entry>Entry in a dict or map (array of key-value pairs).
878 Type code 101 'e' is reserved for use in bindings and
879 implementations to represent the general concept of a
880 dict or dict-entry, and must not appear in signatures
881 used on D-Bus.</entry>
883 <entry>fixed, basic</entry>
884 <entry><literal>UNIX_FD</literal></entry>
885 <entry>104 (ASCII 'h')</entry>
886 <entry>Unix file descriptor</entry>
889 <entry>reserved</entry>
890 <entry>(reserved)</entry>
891 <entry>109 (ASCII 'm')</entry>
892 <entry>Reserved for <ulink
893 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
894 'maybe' type compatible with the one in GVariant</ulink>,
895 and must not appear in signatures used on D-Bus until
896 specified here</entry>
899 <entry>reserved</entry>
900 <entry>(reserved)</entry>
901 <entry>42 (ASCII '*')</entry>
902 <entry>Reserved for use in bindings/implementations to
903 represent any <firstterm>single complete type</firstterm>,
904 and must not appear in signatures used on D-Bus.</entry>
907 <entry>reserved</entry>
908 <entry>(reserved)</entry>
909 <entry>63 (ASCII '?')</entry>
910 <entry>Reserved for use in bindings/implementations to
911 represent any <firstterm>basic type</firstterm>, and must
912 not appear in signatures used on D-Bus.</entry>
915 <entry>reserved</entry>
916 <entry>(reserved)</entry>
917 <entry>64 (ASCII '@'), 38 (ASCII '&'),
918 94 (ASCII '^')</entry>
919 <entry>Reserved for internal use by bindings/implementations,
920 and must not appear in signatures used on D-Bus.
921 GVariant uses these type-codes to encode calling
932 <sect1 id="message-protocol-marshaling">
933 <title>Marshaling (Wire Format)</title>
936 D-Bus defines a marshalling format for its type system, which is
937 used in D-Bus messages. This is not the only possible marshalling
938 format for the type system: for instance, GVariant (part of GLib)
939 re-uses the D-Bus type system but implements an alternative marshalling
944 <title>Byte order and alignment</title>
947 Given a type signature, a block of bytes can be converted into typed
948 values. This section describes the format of the block of bytes. Byte
949 order and alignment issues are handled uniformly for all D-Bus types.
953 A block of bytes has an associated byte order. The byte order
954 has to be discovered in some way; for D-Bus messages, the
955 byte order is part of the message header as described in
956 <xref linkend="message-protocol-messages"/>. For now, assume
957 that the byte order is known to be either little endian or big
962 Each value in a block of bytes is aligned "naturally," for example
963 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
964 8-byte boundary. To properly align a value, <firstterm>alignment
965 padding</firstterm> may be necessary. The alignment padding must always
966 be the minimum required padding to properly align the following value;
967 and it must always be made up of nul bytes. The alignment padding must
968 not be left uninitialized (it can't contain garbage), and more padding
969 than required must not be used.
973 As an exception to natural alignment, <literal>STRUCT</literal> and
974 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
975 boundary, regardless of the alignments of their contents.
980 <title>Marshalling basic types</title>
983 To marshal and unmarshal fixed types, you simply read one value
984 from the data block corresponding to each type code in the signature.
985 All signed integer values are encoded in two's complement, DOUBLE
986 values are IEEE 754 double-precision floating-point, and BOOLEAN
987 values are encoded in 32 bits (of which only the least significant
992 The string-like types are all marshalled as a
993 fixed-length unsigned integer <varname>n</varname> giving the
994 length of the variable part, followed by <varname>n</varname>
995 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
996 which is not considered to be part of the text. The alignment
997 of the string-like type is the same as the alignment of
998 <varname>n</varname>.
1002 For the STRING and OBJECT_PATH types, <varname>n</varname> is
1003 encoded in 4 bytes, leading to 4-byte alignment.
1004 For the SIGNATURE type, <varname>n</varname> is encoded as a single
1005 byte. As a result, alignment padding is never required before a
1011 <title>Marshalling containers</title>
1014 Arrays are marshalled as a <literal>UINT32</literal>
1015 <varname>n</varname> giving the length of the array data in bytes,
1016 followed by alignment padding to the alignment boundary of the array
1017 element type, followed by the <varname>n</varname> bytes of the
1018 array elements marshalled in sequence. <varname>n</varname> does not
1019 include the padding after the length, or any padding after the
1024 For instance, if the current position in the message is a multiple
1025 of 8 bytes and the byte-order is big-endian, an array containing only
1026 the 64-bit integer 5 would be marshalled as:
1029 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
1030 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1031 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1036 Arrays have a maximum length defined to be 2 to the 26th power or
1037 67108864 (64 MiB). Implementations must not send or accept arrays
1038 exceeding this length.
1042 Structs and dict entries are marshalled in the same way as their
1043 contents, but their alignment is always to an 8-byte boundary,
1044 even if their contents would normally be less strictly aligned.
1048 Variants are marshalled as the <literal>SIGNATURE</literal> of
1049 the contents (which must be a single complete type), followed by a
1050 marshalled value with the type given by that signature. The
1051 variant has the same 1-byte alignment as the signature, which means
1052 that alignment padding before a variant is never needed.
1053 Use of variants may not cause a total message depth to be larger
1054 than 64, including other container types such as structures.
1059 <title>Summary of D-Bus marshalling</title>
1062 Given all this, the types are marshaled on the wire as follows:
1067 <entry>Conventional Name</entry>
1068 <entry>Encoding</entry>
1069 <entry>Alignment</entry>
1074 <entry><literal>INVALID</literal></entry>
1075 <entry>Not applicable; cannot be marshaled.</entry>
1078 <entry><literal>BYTE</literal></entry>
1079 <entry>A single 8-bit byte.</entry>
1082 <entry><literal>BOOLEAN</literal></entry>
1083 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1086 <entry><literal>INT16</literal></entry>
1087 <entry>16-bit signed integer in the message's byte order.</entry>
1090 <entry><literal>UINT16</literal></entry>
1091 <entry>16-bit unsigned integer in the message's byte order.</entry>
1094 <entry><literal>INT32</literal></entry>
1095 <entry>32-bit signed integer in the message's byte order.</entry>
1098 <entry><literal>UINT32</literal></entry>
1099 <entry>32-bit unsigned integer in the message's byte order.</entry>
1102 <entry><literal>INT64</literal></entry>
1103 <entry>64-bit signed integer in the message's byte order.</entry>
1106 <entry><literal>UINT64</literal></entry>
1107 <entry>64-bit unsigned integer in the message's byte order.</entry>
1110 <entry><literal>DOUBLE</literal></entry>
1111 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1114 <entry><literal>STRING</literal></entry>
1115 <entry>A <literal>UINT32</literal> indicating the string's
1116 length in bytes excluding its terminating nul, followed by
1117 non-nul string data of the given length, followed by a terminating nul
1124 <entry><literal>OBJECT_PATH</literal></entry>
1125 <entry>Exactly the same as <literal>STRING</literal> except the
1126 content must be a valid object path (see above).
1132 <entry><literal>SIGNATURE</literal></entry>
1133 <entry>The same as <literal>STRING</literal> except the length is a single
1134 byte (thus signatures have a maximum length of 255)
1135 and the content must be a valid signature (see above).
1141 <entry><literal>ARRAY</literal></entry>
1143 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1144 alignment padding to the alignment boundary of the array element type,
1145 followed by each array element.
1151 <entry><literal>STRUCT</literal></entry>
1153 A struct must start on an 8-byte boundary regardless of the
1154 type of the struct fields. The struct value consists of each
1155 field marshaled in sequence starting from that 8-byte
1162 <entry><literal>VARIANT</literal></entry>
1164 The marshaled <literal>SIGNATURE</literal> of a single
1165 complete type, followed by a marshaled value with the type
1166 given in the signature.
1169 1 (alignment of the signature)
1172 <entry><literal>DICT_ENTRY</literal></entry>
1174 Identical to STRUCT.
1180 <entry><literal>UNIX_FD</literal></entry>
1181 <entry>32-bit unsigned integer in the message's byte
1182 order. The actual file descriptors need to be
1183 transferred out-of-band via some platform specific
1184 mechanism. On the wire, values of this type store the index to the
1185 file descriptor in the array of file descriptors that
1186 accompany the message.</entry>
1198 <sect1 id="message-protocol">
1199 <title>Message Protocol</title>
1202 A <firstterm>message</firstterm> consists of a
1203 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1204 think of a message as a package, the header is the address, and the body
1205 contains the package contents. The message delivery system uses the header
1206 information to figure out where to send the message and how to interpret
1207 it; the recipient interprets the body of the message.
1211 The body of the message is made up of zero or more
1212 <firstterm>arguments</firstterm>, which are typed values, such as an
1213 integer or a byte array.
1217 Both header and body use the D-Bus <link linkend="type-system">type
1218 system</link> and format for serializing data.
1221 <sect2 id="message-protocol-messages">
1222 <title>Message Format</title>
1225 A message consists of a header and a body. The header is a block of
1226 values with a fixed signature and meaning. The body is a separate block
1227 of values, with a signature specified in the header.
1231 The length of the header must be a multiple of 8, allowing the body to
1232 begin on an 8-byte boundary when storing the entire message in a single
1233 buffer. If the header does not naturally end on an 8-byte boundary
1234 up to 7 bytes of nul-initialized alignment padding must be added.
1238 The message body need not end on an 8-byte boundary.
1242 The maximum length of a message, including header, header alignment padding,
1243 and body is 2 to the 27th power or 134217728 (128 MiB).
1244 Implementations must not send or accept messages exceeding this size.
1248 The signature of the header is:
1252 Written out more readably, this is:
1254 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1259 These values have the following meanings:
1264 <entry>Value</entry>
1265 <entry>Description</entry>
1270 <entry>1st <literal>BYTE</literal></entry>
1271 <entry>Endianness flag; ASCII 'l' for little-endian
1272 or ASCII 'B' for big-endian. Both header and body are
1273 in this endianness.</entry>
1276 <entry>2nd <literal>BYTE</literal></entry>
1277 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1278 Currently-defined types are described below.
1282 <entry>3rd <literal>BYTE</literal></entry>
1283 <entry>Bitwise OR of flags. Unknown flags
1284 must be ignored. Currently-defined flags are described below.
1288 <entry>4th <literal>BYTE</literal></entry>
1289 <entry>Major protocol version of the sending application. If
1290 the major protocol version of the receiving application does not
1291 match, the applications will not be able to communicate and the
1292 D-Bus connection must be disconnected. The major protocol
1293 version for this version of the specification is 1.
1297 <entry>1st <literal>UINT32</literal></entry>
1298 <entry>Length in bytes of the message body, starting
1299 from the end of the header. The header ends after
1300 its alignment padding to an 8-boundary.
1304 <entry>2nd <literal>UINT32</literal></entry>
1305 <entry>The serial of this message, used as a cookie
1306 by the sender to identify the reply corresponding
1307 to this request. This must not be zero.
1311 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1312 <entry>An array of zero or more <firstterm>header
1313 fields</firstterm> where the byte is the field code, and the
1314 variant is the field value. The message type determines
1315 which fields are required.
1323 <firstterm>Message types</firstterm> that can appear in the second byte
1329 <entry>Conventional name</entry>
1330 <entry>Decimal value</entry>
1331 <entry>Description</entry>
1336 <entry><literal>INVALID</literal></entry>
1338 <entry>This is an invalid type.</entry>
1341 <entry><literal>METHOD_CALL</literal></entry>
1343 <entry>Method call. This message type may prompt a
1347 <entry><literal>METHOD_RETURN</literal></entry>
1349 <entry>Method reply with returned data.</entry>
1352 <entry><literal>ERROR</literal></entry>
1354 <entry>Error reply. If the first argument exists and is a
1355 string, it is an error message.</entry>
1358 <entry><literal>SIGNAL</literal></entry>
1360 <entry>Signal emission.</entry>
1367 Flags that can appear in the third byte of the header:
1372 <entry>Conventional name</entry>
1373 <entry>Hex value</entry>
1374 <entry>Description</entry>
1379 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1383 This message does not expect method return replies or
1384 error replies, even if it is of a type that can
1385 have a reply; the reply can be omitted as an
1386 optimization. It is compliant with this specification
1387 to return the reply despite this flag, although doing
1388 so on a bus with a non-trivial security policy
1389 (such as the well-known system bus) may result in
1390 access denial messages being logged for the reply.
1393 Note that METHOD_CALL is the only message type currently
1394 defined in this specification that can expect a reply,
1395 so the presence or absence of this flag in the other
1396 three message types that are currently
1397 documented is meaningless: replies to those message
1398 types should not be sent, whether this flag is present
1404 <entry><literal>NO_AUTO_START</literal></entry>
1406 <entry>The bus must not launch an owner
1407 for the destination name in response to this message.
1411 <entry><literal>ALLOW_INTERACTIVE_AUTHORIZATION</literal></entry>
1415 This flag may be set on a method call message to
1416 inform the receiving side that the caller is prepared
1417 to wait for interactive authorization, which might
1418 take a considerable time to complete. For instance,
1419 if this flag is set, it would be appropriate to
1420 query the user for passwords or confirmation via
1421 Polkit or a similar framework.
1424 This flag is only useful when
1425 unprivileged code calls a more privileged method call,
1426 and an authorization framework is deployed that allows
1427 possibly interactive authorization. If no such framework
1428 is deployed it has no effect. This flag should not
1429 be set by default by client implementations. If it is
1430 set, the caller should also set a suitably long timeout
1431 on the method call to make sure the user interaction
1432 may complete. This flag is only valid for method call
1433 messages, and shall be ignored otherwise.
1436 Interaction that takes place as a part of the
1437 effect of the method being called is outside the scope
1438 of this flag, even if it could also be characterized
1439 as authentication or authorization. For instance, in
1440 a method call that directs a network management service
1441 to attempt to connect to a virtual private network,
1442 this flag should control how the network management
1443 service makes the decision "is this user allowed to
1444 change system network configuration?", but it should
1445 not affect how or whether the network management
1446 service interacts with the user to obtain the credentials
1447 that are required for access to the VPN.
1450 If a this flag is not set on a method call, and a
1451 service determines that the requested operation is
1452 not allowed without interactive authorization, but
1453 could be allowed after successful interactive
1454 authorization, it may return the
1455 <literal>org.freedesktop.DBus.Error.InteractiveAuthorizationRequired</literal>
1459 The absence of this flag does not guarantee that
1460 interactive authorization will not be applied, since
1461 existing services that pre-date this flag might
1462 already use interactive authorization. However,
1463 existing D-Bus APIs that will use interactive
1464 authorization should document that the call may take
1465 longer than usual, and new D-Bus APIs should avoid
1466 interactive authorization in the absence of this flag.
1475 <sect3 id="message-protocol-header-fields">
1476 <title>Header Fields</title>
1479 The array at the end of the header contains <firstterm>header
1480 fields</firstterm>, where each field is a 1-byte field code followed
1481 by a field value. A header must contain the required header fields for
1482 its message type, and zero or more of any optional header
1483 fields. Future versions of this protocol specification may add new
1484 fields. Implementations must ignore fields they do not
1485 understand. Implementations must not invent their own header fields;
1486 only changes to this specification may introduce new header fields.
1490 Again, if an implementation sees a header field code that it does not
1491 expect, it must ignore that field, as it will be part of a new
1492 (but compatible) version of this specification. This also applies
1493 to known header fields appearing in unexpected messages, for
1494 example: if a signal has a reply serial it must be ignored
1495 even though it has no meaning as of this version of the spec.
1499 However, implementations must not send or accept known header fields
1500 with the wrong type stored in the field value. So for example a
1501 message with an <literal>INTERFACE</literal> field of type
1502 <literal>UINT32</literal> would be considered corrupt.
1506 Here are the currently-defined header fields:
1511 <entry>Conventional Name</entry>
1512 <entry>Decimal Code</entry>
1514 <entry>Required In</entry>
1515 <entry>Description</entry>
1520 <entry><literal>INVALID</literal></entry>
1523 <entry>not allowed</entry>
1524 <entry>Not a valid field name (error if it appears in a message)</entry>
1527 <entry><literal>PATH</literal></entry>
1529 <entry><literal>OBJECT_PATH</literal></entry>
1530 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1531 <entry>The object to send a call to,
1532 or the object a signal is emitted from.
1534 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1535 implementations should not send messages with this path,
1536 and the reference implementation of the bus daemon will
1537 disconnect any application that attempts to do so.
1541 <entry><literal>INTERFACE</literal></entry>
1543 <entry><literal>STRING</literal></entry>
1544 <entry><literal>SIGNAL</literal></entry>
1546 The interface to invoke a method call on, or
1547 that a signal is emitted from. Optional for
1548 method calls, required for signals.
1549 The special interface
1550 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1551 implementations should not send messages with this
1552 interface, and the reference implementation of the bus
1553 daemon will disconnect any application that attempts to
1558 <entry><literal>MEMBER</literal></entry>
1560 <entry><literal>STRING</literal></entry>
1561 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1562 <entry>The member, either the method name or signal name.</entry>
1565 <entry><literal>ERROR_NAME</literal></entry>
1567 <entry><literal>STRING</literal></entry>
1568 <entry><literal>ERROR</literal></entry>
1569 <entry>The name of the error that occurred, for errors</entry>
1572 <entry><literal>REPLY_SERIAL</literal></entry>
1574 <entry><literal>UINT32</literal></entry>
1575 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1576 <entry>The serial number of the message this message is a reply
1577 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1580 <entry><literal>DESTINATION</literal></entry>
1582 <entry><literal>STRING</literal></entry>
1583 <entry>optional</entry>
1584 <entry>The name of the connection this message is intended for.
1585 Only used in combination with the message bus, see
1586 <xref linkend="message-bus"/>.</entry>
1589 <entry><literal>SENDER</literal></entry>
1591 <entry><literal>STRING</literal></entry>
1592 <entry>optional</entry>
1593 <entry>Unique name of the sending connection.
1594 The message bus fills in this field so it is reliable; the field is
1595 only meaningful in combination with the message bus.</entry>
1598 <entry><literal>SIGNATURE</literal></entry>
1600 <entry><literal>SIGNATURE</literal></entry>
1601 <entry>optional</entry>
1602 <entry>The signature of the message body.
1603 If omitted, it is assumed to be the
1604 empty signature "" (i.e. the body must be 0-length).</entry>
1607 <entry><literal>UNIX_FDS</literal></entry>
1609 <entry><literal>UINT32</literal></entry>
1610 <entry>optional</entry>
1611 <entry>The number of Unix file descriptors that
1612 accompany the message. If omitted, it is assumed
1613 that no Unix file descriptors accompany the
1614 message. The actual file descriptors need to be
1615 transferred via platform specific mechanism
1616 out-of-band. They must be sent at the same time as
1617 part of the message itself. They may not be sent
1618 before the first byte of the message itself is
1619 transferred or after the last byte of the message
1629 <sect2 id="message-protocol-names">
1630 <title>Valid Names</title>
1632 The various names in D-Bus messages have some restrictions.
1635 There is a <firstterm>maximum name length</firstterm>
1636 of 255 which applies to bus names, interfaces, and members.
1638 <sect3 id="message-protocol-names-interface">
1639 <title>Interface names</title>
1641 Interfaces have names with type <literal>STRING</literal>, meaning that
1642 they must be valid UTF-8. However, there are also some
1643 additional restrictions that apply to interface names
1646 <listitem><para>Interface names are composed of 1 or more elements separated by
1647 a period ('.') character. All elements must contain at least
1651 <listitem><para>Each element must only contain the ASCII characters
1652 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1656 <listitem><para>Interface names must contain at least one '.' (period)
1657 character (and thus at least two elements).
1660 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1661 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1666 Interface names should start with the reversed DNS domain name of
1667 the author of the interface (in lower-case), like interface names
1668 in Java. It is conventional for the rest of the interface name
1669 to consist of words run together, with initial capital letters
1670 on all words ("CamelCase"). Several levels of hierarchy can be used.
1671 It is also a good idea to include the major version of the interface
1672 in the name, and increment it if incompatible changes are made;
1673 this way, a single object can implement several versions of an
1674 interface in parallel, if necessary.
1678 For instance, if the owner of <literal>example.com</literal> is
1679 developing a D-Bus API for a music player, they might define
1680 interfaces called <literal>com.example.MusicPlayer1</literal>,
1681 <literal>com.example.MusicPlayer1.Track</literal> and
1682 <literal>com.example.MusicPlayer1.Seekable</literal>.
1686 D-Bus does not distinguish between the concepts that would be
1687 called classes and interfaces in Java: either can be identified on
1688 D-Bus by an interface name.
1691 <sect3 id="message-protocol-names-bus">
1692 <title>Bus names</title>
1694 Connections have one or more bus names associated with them.
1695 A connection has exactly one bus name that is a <firstterm>unique
1696 connection name</firstterm>. The unique connection name remains
1697 with the connection for its entire lifetime.
1698 A bus name is of type <literal>STRING</literal>,
1699 meaning that it must be valid UTF-8. However, there are also
1700 some additional restrictions that apply to bus names
1703 <listitem><para>Bus names that start with a colon (':')
1704 character are unique connection names. Other bus names
1705 are called <firstterm>well-known bus names</firstterm>.
1708 <listitem><para>Bus names are composed of 1 or more elements separated by
1709 a period ('.') character. All elements must contain at least
1713 <listitem><para>Each element must only contain the ASCII characters
1714 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1715 connection name may begin with a digit, elements in
1716 other bus names must not begin with a digit.
1720 <listitem><para>Bus names must contain at least one '.' (period)
1721 character (and thus at least two elements).
1724 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1725 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1729 Note that the hyphen ('-') character is allowed in bus names but
1730 not in interface names.
1734 Like <link linkend="message-protocol-names-interface">interface
1735 names</link>, well-known bus names should start with the
1736 reversed DNS domain name of the author of the interface (in
1737 lower-case), and it is conventional for the rest of the well-known
1738 bus name to consist of words run together, with initial
1739 capital letters. As with interface names, including a version
1740 number in well-known bus names is a good idea; it's possible to
1741 have the well-known bus name for more than one version
1742 simultaneously if backwards compatibility is required.
1746 If a well-known bus name implies the presence of a "main" interface,
1747 that "main" interface is often given the same name as
1748 the well-known bus name, and situated at the corresponding object
1749 path. For instance, if the owner of <literal>example.com</literal>
1750 is developing a D-Bus API for a music player, they might define
1751 that any application that takes the well-known name
1752 <literal>com.example.MusicPlayer1</literal> should have an object
1753 at the object path <literal>/com/example/MusicPlayer1</literal>
1754 which implements the interface
1755 <literal>com.example.MusicPlayer1</literal>.
1758 <sect3 id="message-protocol-names-member">
1759 <title>Member names</title>
1761 Member (i.e. method or signal) names:
1763 <listitem><para>Must only contain the ASCII characters
1764 "[A-Z][a-z][0-9]_" and may not begin with a
1765 digit.</para></listitem>
1766 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1767 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1768 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1773 It is conventional for member names on D-Bus to consist of
1774 capitalized words with no punctuation ("camel-case").
1775 Method names should usually be verbs, such as
1776 <literal>GetItems</literal>, and signal names should usually be
1777 a description of an event, such as <literal>ItemsChanged</literal>.
1780 <sect3 id="message-protocol-names-error">
1781 <title>Error names</title>
1783 Error names have the same restrictions as interface names.
1787 Error names have the same naming conventions as interface
1788 names, and often contain <literal>.Error.</literal>; for instance,
1789 the owner of <literal>example.com</literal> might define the
1790 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1791 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1792 The errors defined by D-Bus itself, such as
1793 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1799 <sect2 id="message-protocol-types">
1800 <title>Message Types</title>
1802 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1803 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1804 This section describes these conventions.
1806 <sect3 id="message-protocol-types-method">
1807 <title>Method Calls</title>
1809 Some messages invoke an operation on a remote object. These are
1810 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1811 messages map naturally to methods on objects in a typical program.
1814 A method call message is required to have a <literal>MEMBER</literal> header field
1815 indicating the name of the method. Optionally, the message has an
1816 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1817 Including the <literal>INTERFACE</literal> in all method call
1818 messages is strongly recommended.
1821 In the absence of an <literal>INTERFACE</literal> field, if two
1822 or more interfaces on the same object have a method with the same
1823 name, it is undefined which of those methods will be invoked.
1824 Implementations may choose to either return an error, or deliver the
1825 message as though it had an arbitrary one of those interfaces.
1828 In some situations (such as the well-known system bus), messages
1829 are filtered through an access-control list external to the
1830 remote object implementation. If that filter rejects certain
1831 messages by matching their interface, or accepts only messages
1832 to specific interfaces, it must also reject messages that have no
1833 <literal>INTERFACE</literal>: otherwise, malicious
1834 applications could use this to bypass the filter.
1837 Method call messages also include a <literal>PATH</literal> field
1838 indicating the object to invoke the method on. If the call is passing
1839 through a message bus, the message will also have a
1840 <literal>DESTINATION</literal> field giving the name of the connection
1841 to receive the message.
1844 When an application handles a method call message, it is required to
1845 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1846 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1847 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1850 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1851 are the return value(s) or "out parameters" of the method call.
1852 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1853 and the call fails; no return value will be provided. It makes
1854 no sense to send multiple replies to the same method call.
1857 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1858 reply is required, so the caller will know the method
1859 was successfully processed.
1862 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1866 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1867 then as an optimization the application receiving the method
1868 call may choose to omit the reply message (regardless of
1869 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1870 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1871 flag and reply anyway.
1874 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1875 destination name does not exist then a program to own the destination
1876 name will be started before the message is delivered. The message
1877 will be held until the new program is successfully started or has
1878 failed to start; in case of failure, an error will be returned. This
1879 flag is only relevant in the context of a message bus, it is ignored
1880 during one-to-one communication with no intermediate bus.
1882 <sect4 id="message-protocol-types-method-apis">
1883 <title>Mapping method calls to native APIs</title>
1885 APIs for D-Bus may map method calls to a method call in a specific
1886 programming language, such as C++, or may map a method call written
1887 in an IDL to a D-Bus message.
1890 In APIs of this nature, arguments to a method are often termed "in"
1891 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1892 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1893 "inout" arguments, which are both sent and received, i.e. the caller
1894 passes in a value which is modified. Mapped to D-Bus, an "inout"
1895 argument is equivalent to an "in" argument, followed by an "out"
1896 argument. You can't pass things "by reference" over the wire, so
1897 "inout" is purely an illusion of the in-process API.
1900 Given a method with zero or one return values, followed by zero or more
1901 arguments, where each argument may be "in", "out", or "inout", the
1902 caller constructs a message by appending each "in" or "inout" argument,
1903 in order. "out" arguments are not represented in the caller's message.
1906 The recipient constructs a reply by appending first the return value
1907 if any, then each "out" or "inout" argument, in order.
1908 "in" arguments are not represented in the reply message.
1911 Error replies are normally mapped to exceptions in languages that have
1915 In converting from native APIs to D-Bus, it is perhaps nice to
1916 map D-Bus naming conventions ("FooBar") to native conventions
1917 such as "fooBar" or "foo_bar" automatically. This is OK
1918 as long as you can say that the native API is one that
1919 was specifically written for D-Bus. It makes the most sense
1920 when writing object implementations that will be exported
1921 over the bus. Object proxies used to invoke remote D-Bus
1922 objects probably need the ability to call any D-Bus method,
1923 and thus a magic name mapping like this could be a problem.
1926 This specification doesn't require anything of native API bindings;
1927 the preceding is only a suggested convention for consistency
1933 <sect3 id="message-protocol-types-signal">
1934 <title>Signal Emission</title>
1936 Unlike method calls, signal emissions have no replies.
1937 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1938 It must have three header fields: <literal>PATH</literal> giving the object
1939 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1940 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1941 for signals, though it is optional for method calls.
1945 <sect3 id="message-protocol-types-errors">
1946 <title>Errors</title>
1948 Messages of type <literal>ERROR</literal> are most commonly replies
1949 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1950 to any kind of message. The message bus for example
1951 will return an <literal>ERROR</literal> in reply to a signal emission if
1952 the bus does not have enough memory to send the signal.
1955 An <literal>ERROR</literal> may have any arguments, but if the first
1956 argument is a <literal>STRING</literal>, it must be an error message.
1957 The error message may be logged or shown to the user
1962 <sect3 id="message-protocol-types-notation">
1963 <title>Notation in this document</title>
1965 This document uses a simple pseudo-IDL to describe particular method
1966 calls and signals. Here is an example of a method call:
1968 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1969 out UINT32 resultcode)
1971 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1972 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1973 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1974 characters so it's known that the last part of the name in
1975 the "IDL" is the member name.
1978 In C++ that might end up looking like this:
1980 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1981 unsigned int flags);
1983 or equally valid, the return value could be done as an argument:
1985 void org::freedesktop::DBus::StartServiceByName (const char *name,
1987 unsigned int *resultcode);
1989 It's really up to the API designer how they want to make
1990 this look. You could design an API where the namespace wasn't used
1991 in C++, using STL or Qt, using varargs, or whatever you wanted.
1994 Signals are written as follows:
1996 org.freedesktop.DBus.NameLost (STRING name)
1998 Signals don't specify "in" vs. "out" because only
1999 a single direction is possible.
2002 It isn't especially encouraged to use this lame pseudo-IDL in actual
2003 API implementations; you might use the native notation for the
2004 language you're using, or you might use COM or CORBA IDL, for example.
2009 <sect2 id="message-protocol-handling-invalid">
2010 <title>Invalid Protocol and Spec Extensions</title>
2013 For security reasons, the D-Bus protocol should be strictly parsed and
2014 validated, with the exception of defined extension points. Any invalid
2015 protocol or spec violations should result in immediately dropping the
2016 connection without notice to the other end. Exceptions should be
2017 carefully considered, e.g. an exception may be warranted for a
2018 well-understood idiosyncrasy of a widely-deployed implementation. In
2019 cases where the other end of a connection is 100% trusted and known to
2020 be friendly, skipping validation for performance reasons could also make
2021 sense in certain cases.
2025 Generally speaking violations of the "must" requirements in this spec
2026 should be considered possible attempts to exploit security, and violations
2027 of the "should" suggestions should be considered legitimate (though perhaps
2028 they should generate an error in some cases).
2032 The following extension points are built in to D-Bus on purpose and must
2033 not be treated as invalid protocol. The extension points are intended
2034 for use by future versions of this spec, they are not intended for third
2035 parties. At the moment, the only way a third party could extend D-Bus
2036 without breaking interoperability would be to introduce a way to negotiate new
2037 feature support as part of the auth protocol, using EXTENSION_-prefixed
2038 commands. There is not yet a standard way to negotiate features.
2042 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
2043 commands result in an ERROR rather than a disconnect. This enables
2044 future extensions to the protocol. Commands starting with EXTENSION_ are
2045 reserved for third parties.
2050 The authentication protocol supports pluggable auth mechanisms.
2055 The address format (see <xref linkend="addresses"/>) supports new
2061 Messages with an unknown type (something other than
2062 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
2063 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
2064 Unknown-type messages must still be well-formed in the same way
2065 as the known messages, however. They still have the normal
2071 Header fields with an unknown or unexpected field code must be ignored,
2072 though again they must still be well-formed.
2077 New standard interfaces (with new methods and signals) can of course be added.
2087 <sect1 id="auth-protocol">
2088 <title>Authentication Protocol</title>
2090 Before the flow of messages begins, two applications must
2091 authenticate. A simple plain-text protocol is used for
2092 authentication; this protocol is a SASL profile, and maps fairly
2093 directly from the SASL specification. The message encoding is
2094 NOT used here, only plain text messages.
2097 In examples, "C:" and "S:" indicate lines sent by the client and
2098 server respectively.
2100 <sect2 id="auth-protocol-overview">
2101 <title>Protocol Overview</title>
2103 The protocol is a line-based protocol, where each line ends with
2104 \r\n. Each line begins with an all-caps ASCII command name containing
2105 only the character range [A-Z_], a space, then any arguments for the
2106 command, then the \r\n ending the line. The protocol is
2107 case-sensitive. All bytes must be in the ASCII character set.
2109 Commands from the client to the server are as follows:
2112 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2113 <listitem><para>CANCEL</para></listitem>
2114 <listitem><para>BEGIN</para></listitem>
2115 <listitem><para>DATA <data in hex encoding></para></listitem>
2116 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2117 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2120 From server to client are as follows:
2123 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2124 <listitem><para>OK <GUID in hex></para></listitem>
2125 <listitem><para>DATA <data in hex encoding></para></listitem>
2126 <listitem><para>ERROR</para></listitem>
2127 <listitem><para>AGREE_UNIX_FD</para></listitem>
2131 Unofficial extensions to the command set must begin with the letters
2132 "EXTENSION_", to avoid conflicts with future official commands.
2133 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2136 <sect2 id="auth-nul-byte">
2137 <title>Special credentials-passing nul byte</title>
2139 Immediately after connecting to the server, the client must send a
2140 single nul byte. This byte may be accompanied by credentials
2141 information on some operating systems that use sendmsg() with
2142 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2143 sockets. However, the nul byte must be sent even on other kinds of
2144 socket, and even on operating systems that do not require a byte to be
2145 sent in order to transmit credentials. The text protocol described in
2146 this document begins after the single nul byte. If the first byte
2147 received from the client is not a nul byte, the server may disconnect
2151 A nul byte in any context other than the initial byte is an error;
2152 the protocol is ASCII-only.
2155 The credentials sent along with the nul byte may be used with the
2156 SASL mechanism EXTERNAL.
2159 <sect2 id="auth-command-auth">
2160 <title>AUTH command</title>
2162 If an AUTH command has no arguments, it is a request to list
2163 available mechanisms. The server must respond with a REJECTED
2164 command listing the mechanisms it understands, or with an error.
2167 If an AUTH command specifies a mechanism, and the server supports
2168 said mechanism, the server should begin exchanging SASL
2169 challenge-response data with the client using DATA commands.
2172 If the server does not support the mechanism given in the AUTH
2173 command, it must send either a REJECTED command listing the mechanisms
2174 it does support, or an error.
2177 If the [initial-response] argument is provided, it is intended for use
2178 with mechanisms that have no initial challenge (or an empty initial
2179 challenge), as if it were the argument to an initial DATA command. If
2180 the selected mechanism has an initial challenge and [initial-response]
2181 was provided, the server should reject authentication by sending
2185 If authentication succeeds after exchanging DATA commands,
2186 an OK command must be sent to the client.
2189 The first octet received by the server after the \r\n of the BEGIN
2190 command from the client must be the first octet of the
2191 authenticated/encrypted stream of D-Bus messages.
2194 If BEGIN is received by the server, the first octet received
2195 by the client after the \r\n of the OK command must be the
2196 first octet of the authenticated/encrypted stream of D-Bus
2200 <sect2 id="auth-command-cancel">
2201 <title>CANCEL Command</title>
2203 At any time up to sending the BEGIN command, the client may send a
2204 CANCEL command. On receiving the CANCEL command, the server must
2205 send a REJECTED command and abort the current authentication
2209 <sect2 id="auth-command-data">
2210 <title>DATA Command</title>
2212 The DATA command may come from either client or server, and simply
2213 contains a hex-encoded block of data to be interpreted
2214 according to the SASL mechanism in use.
2217 Some SASL mechanisms support sending an "empty string";
2218 FIXME we need some way to do this.
2221 <sect2 id="auth-command-begin">
2222 <title>BEGIN Command</title>
2224 The BEGIN command acknowledges that the client has received an
2225 OK command from the server, and that the stream of messages
2229 The first octet received by the server after the \r\n of the BEGIN
2230 command from the client must be the first octet of the
2231 authenticated/encrypted stream of D-Bus messages.
2234 <sect2 id="auth-command-rejected">
2235 <title>REJECTED Command</title>
2237 The REJECTED command indicates that the current authentication
2238 exchange has failed, and further exchange of DATA is inappropriate.
2239 The client would normally try another mechanism, or try providing
2240 different responses to challenges.
2242 Optionally, the REJECTED command has a space-separated list of
2243 available auth mechanisms as arguments. If a server ever provides
2244 a list of supported mechanisms, it must provide the same list
2245 each time it sends a REJECTED message. Clients are free to
2246 ignore all lists received after the first.
2249 <sect2 id="auth-command-ok">
2250 <title>OK Command</title>
2252 The OK command indicates that the client has been
2253 authenticated. The client may now proceed with negotiating
2254 Unix file descriptor passing. To do that it shall send
2255 NEGOTIATE_UNIX_FD to the server.
2258 Otherwise, the client must respond to the OK command by
2259 sending a BEGIN command, followed by its stream of messages,
2260 or by disconnecting. The server must not accept additional
2261 commands using this protocol after the BEGIN command has been
2262 received. Further communication will be a stream of D-Bus
2263 messages (optionally encrypted, as negotiated) rather than
2267 If a client sends BEGIN the first octet received by the client
2268 after the \r\n of the OK command must be the first octet of
2269 the authenticated/encrypted stream of D-Bus messages.
2272 The OK command has one argument, which is the GUID of the server.
2273 See <xref linkend="addresses"/> for more on server GUIDs.
2276 <sect2 id="auth-command-error">
2277 <title>ERROR Command</title>
2279 The ERROR command indicates that either server or client did not
2280 know a command, does not accept the given command in the current
2281 context, or did not understand the arguments to the command. This
2282 allows the protocol to be extended; a client or server can send a
2283 command present or permitted only in new protocol versions, and if
2284 an ERROR is received instead of an appropriate response, fall back
2285 to using some other technique.
2288 If an ERROR is sent, the server or client that sent the
2289 error must continue as if the command causing the ERROR had never been
2290 received. However, the the server or client receiving the error
2291 should try something other than whatever caused the error;
2292 if only canceling/rejecting the authentication.
2295 If the D-Bus protocol changes incompatibly at some future time,
2296 applications implementing the new protocol would probably be able to
2297 check for support of the new protocol by sending a new command and
2298 receiving an ERROR from applications that don't understand it. Thus the
2299 ERROR feature of the auth protocol is an escape hatch that lets us
2300 negotiate extensions or changes to the D-Bus protocol in the future.
2303 <sect2 id="auth-command-negotiate-unix-fd">
2304 <title>NEGOTIATE_UNIX_FD Command</title>
2306 The NEGOTIATE_UNIX_FD command indicates that the client
2307 supports Unix file descriptor passing. This command may only
2308 be sent after the connection is authenticated, i.e. after OK
2309 was received by the client. This command may only be sent on
2310 transports that support Unix file descriptor passing.
2313 On receiving NEGOTIATE_UNIX_FD the server must respond with
2314 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2315 the transport chosen supports Unix file descriptor passing and
2316 the server supports this feature. It shall respond the latter
2317 if the transport does not support Unix file descriptor
2318 passing, the server does not support this feature, or the
2319 server decides not to enable file descriptor passing due to
2320 security or other reasons.
2323 <sect2 id="auth-command-agree-unix-fd">
2324 <title>AGREE_UNIX_FD Command</title>
2326 The AGREE_UNIX_FD command indicates that the server supports
2327 Unix file descriptor passing. This command may only be sent
2328 after the connection is authenticated, and the client sent
2329 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2330 command may only be sent on transports that support Unix file
2334 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2335 followed by its stream of messages, or by disconnecting. The
2336 server must not accept additional commands using this protocol
2337 after the BEGIN command has been received. Further
2338 communication will be a stream of D-Bus messages (optionally
2339 encrypted, as negotiated) rather than this protocol.
2342 <sect2 id="auth-command-future">
2343 <title>Future Extensions</title>
2345 Future extensions to the authentication and negotiation
2346 protocol are possible. For that new commands may be
2347 introduced. If a client or server receives an unknown command
2348 it shall respond with ERROR and not consider this fatal. New
2349 commands may be introduced both before, and after
2350 authentication, i.e. both before and after the OK command.
2353 <sect2 id="auth-examples">
2354 <title>Authentication examples</title>
2358 <title>Example of successful magic cookie authentication</title>
2360 (MAGIC_COOKIE is a made up mechanism)
2362 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2368 <title>Example of finding out mechanisms then picking one</title>
2371 S: REJECTED KERBEROS_V4 SKEY
2372 C: AUTH SKEY 7ab83f32ee
2373 S: DATA 8799cabb2ea93e
2374 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2380 <title>Example of client sends unknown command then falls back to regular auth</title>
2384 C: AUTH MAGIC_COOKIE 3736343435313230333039
2390 <title>Example of server doesn't support initial auth mechanism</title>
2392 C: AUTH MAGIC_COOKIE 3736343435313230333039
2393 S: REJECTED KERBEROS_V4 SKEY
2394 C: AUTH SKEY 7ab83f32ee
2395 S: DATA 8799cabb2ea93e
2396 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2402 <title>Example of wrong password or the like followed by successful retry</title>
2404 C: AUTH MAGIC_COOKIE 3736343435313230333039
2405 S: REJECTED KERBEROS_V4 SKEY
2406 C: AUTH SKEY 7ab83f32ee
2407 S: DATA 8799cabb2ea93e
2408 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2410 C: AUTH SKEY 7ab83f32ee
2411 S: DATA 8799cabb2ea93e
2412 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2418 <title>Example of skey cancelled and restarted</title>
2420 C: AUTH MAGIC_COOKIE 3736343435313230333039
2421 S: REJECTED KERBEROS_V4 SKEY
2422 C: AUTH SKEY 7ab83f32ee
2423 S: DATA 8799cabb2ea93e
2426 C: AUTH SKEY 7ab83f32ee
2427 S: DATA 8799cabb2ea93e
2428 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2434 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2436 (MAGIC_COOKIE is a made up mechanism)
2438 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2440 C: NEGOTIATE_UNIX_FD
2446 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2448 (MAGIC_COOKIE is a made up mechanism)
2450 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2452 C: NEGOTIATE_UNIX_FD
2459 <sect2 id="auth-states">
2460 <title>Authentication state diagrams</title>
2463 This section documents the auth protocol in terms of
2464 a state machine for the client and the server. This is
2465 probably the most robust way to implement the protocol.
2468 <sect3 id="auth-states-client">
2469 <title>Client states</title>
2472 To more precisely describe the interaction between the
2473 protocol state machine and the authentication mechanisms the
2474 following notation is used: MECH(CHALL) means that the
2475 server challenge CHALL was fed to the mechanism MECH, which
2481 CONTINUE(RESP) means continue the auth conversation
2482 and send RESP as the response to the server;
2488 OK(RESP) means that after sending RESP to the server
2489 the client side of the auth conversation is finished
2490 and the server should return "OK";
2496 ERROR means that CHALL was invalid and could not be
2502 Both RESP and CHALL may be empty.
2506 The Client starts by getting an initial response from the
2507 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2508 the mechanism did not provide an initial response. If the
2509 mechanism returns CONTINUE, the client starts in state
2510 <emphasis>WaitingForData</emphasis>, if the mechanism
2511 returns OK the client starts in state
2512 <emphasis>WaitingForOK</emphasis>.
2516 The client should keep track of available mechanisms and
2517 which it mechanisms it has already attempted. This list is
2518 used to decide which AUTH command to send. When the list is
2519 exhausted, the client should give up and close the
2524 <title><emphasis>WaitingForData</emphasis></title>
2532 MECH(CHALL) returns CONTINUE(RESP) → send
2534 <emphasis>WaitingForData</emphasis>
2538 MECH(CHALL) returns OK(RESP) → send DATA
2539 RESP, goto <emphasis>WaitingForOK</emphasis>
2543 MECH(CHALL) returns ERROR → send ERROR
2544 [msg], goto <emphasis>WaitingForData</emphasis>
2552 Receive REJECTED [mechs] →
2553 send AUTH [next mech], goto
2554 WaitingForData or <emphasis>WaitingForOK</emphasis>
2559 Receive ERROR → send
2561 <emphasis>WaitingForReject</emphasis>
2566 Receive OK → send
2567 BEGIN, terminate auth
2568 conversation, authenticated
2573 Receive anything else → send
2575 <emphasis>WaitingForData</emphasis>
2583 <title><emphasis>WaitingForOK</emphasis></title>
2588 Receive OK → send BEGIN, terminate auth
2589 conversation, <emphasis>authenticated</emphasis>
2594 Receive REJECTED [mechs] → send AUTH [next mech],
2595 goto <emphasis>WaitingForData</emphasis> or
2596 <emphasis>WaitingForOK</emphasis>
2602 Receive DATA → send CANCEL, goto
2603 <emphasis>WaitingForReject</emphasis>
2609 Receive ERROR → send CANCEL, goto
2610 <emphasis>WaitingForReject</emphasis>
2616 Receive anything else → send ERROR, goto
2617 <emphasis>WaitingForOK</emphasis>
2625 <title><emphasis>WaitingForReject</emphasis></title>
2630 Receive REJECTED [mechs] → send AUTH [next mech],
2631 goto <emphasis>WaitingForData</emphasis> or
2632 <emphasis>WaitingForOK</emphasis>
2638 Receive anything else → terminate auth
2639 conversation, disconnect
2648 <sect3 id="auth-states-server">
2649 <title>Server states</title>
2652 For the server MECH(RESP) means that the client response
2653 RESP was fed to the the mechanism MECH, which returns one of
2658 CONTINUE(CHALL) means continue the auth conversation and
2659 send CHALL as the challenge to the client;
2665 OK means that the client has been successfully
2672 REJECTED means that the client failed to authenticate or
2673 there was an error in RESP.
2678 The server starts out in state
2679 <emphasis>WaitingForAuth</emphasis>. If the client is
2680 rejected too many times the server must disconnect the
2685 <title><emphasis>WaitingForAuth</emphasis></title>
2691 Receive AUTH → send REJECTED [mechs], goto
2692 <emphasis>WaitingForAuth</emphasis>
2698 Receive AUTH MECH RESP
2702 MECH not valid mechanism → send REJECTED
2704 <emphasis>WaitingForAuth</emphasis>
2708 MECH(RESP) returns CONTINUE(CHALL) → send
2710 <emphasis>WaitingForData</emphasis>
2714 MECH(RESP) returns OK → send OK, goto
2715 <emphasis>WaitingForBegin</emphasis>
2719 MECH(RESP) returns REJECTED → send REJECTED
2721 <emphasis>WaitingForAuth</emphasis>
2729 Receive BEGIN → terminate
2730 auth conversation, disconnect
2736 Receive ERROR → send REJECTED [mechs], goto
2737 <emphasis>WaitingForAuth</emphasis>
2743 Receive anything else → send
2745 <emphasis>WaitingForAuth</emphasis>
2754 <title><emphasis>WaitingForData</emphasis></title>
2762 MECH(RESP) returns CONTINUE(CHALL) → send
2764 <emphasis>WaitingForData</emphasis>
2768 MECH(RESP) returns OK → send OK, goto
2769 <emphasis>WaitingForBegin</emphasis>
2773 MECH(RESP) returns REJECTED → send REJECTED
2775 <emphasis>WaitingForAuth</emphasis>
2783 Receive BEGIN → terminate auth conversation,
2790 Receive CANCEL → send REJECTED [mechs], goto
2791 <emphasis>WaitingForAuth</emphasis>
2797 Receive ERROR → send REJECTED [mechs], goto
2798 <emphasis>WaitingForAuth</emphasis>
2804 Receive anything else → send ERROR, goto
2805 <emphasis>WaitingForData</emphasis>
2813 <title><emphasis>WaitingForBegin</emphasis></title>
2818 Receive BEGIN → terminate auth conversation,
2819 client authenticated
2825 Receive CANCEL → send REJECTED [mechs], goto
2826 <emphasis>WaitingForAuth</emphasis>
2832 Receive ERROR → send REJECTED [mechs], goto
2833 <emphasis>WaitingForAuth</emphasis>
2839 Receive anything else → send ERROR, goto
2840 <emphasis>WaitingForBegin</emphasis>
2850 <sect2 id="auth-mechanisms">
2851 <title>Authentication mechanisms</title>
2853 This section describes some new authentication mechanisms.
2854 D-Bus also allows any standard SASL mechanism of course.
2856 <sect3 id="auth-mechanisms-sha">
2857 <title>DBUS_COOKIE_SHA1</title>
2859 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2860 has the ability to read a private file owned by the user being
2861 authenticated. If the client can prove that it has access to a secret
2862 cookie stored in this file, then the client is authenticated.
2863 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2867 Throughout this description, "hex encoding" must output the digits
2868 from a to f in lower-case; the digits A to F must not be used
2869 in the DBUS_COOKIE_SHA1 mechanism.
2872 Authentication proceeds as follows:
2876 The client sends the username it would like to authenticate
2882 The server sends the name of its "cookie context" (see below); a
2883 space character; the integer ID of the secret cookie the client
2884 must demonstrate knowledge of; a space character; then a
2885 randomly-generated challenge string, all of this hex-encoded into
2891 The client locates the cookie and generates its own
2892 randomly-generated challenge string. The client then concatenates
2893 the server's decoded challenge, a ":" character, its own challenge,
2894 another ":" character, and the cookie. It computes the SHA-1 hash
2895 of this composite string as a hex digest. It concatenates the
2896 client's challenge string, a space character, and the SHA-1 hex
2897 digest, hex-encodes the result and sends it back to the server.
2902 The server generates the same concatenated string used by the
2903 client and computes its SHA-1 hash. It compares the hash with
2904 the hash received from the client; if the two hashes match, the
2905 client is authenticated.
2911 Each server has a "cookie context," which is a name that identifies a
2912 set of cookies that apply to that server. A sample context might be
2913 "org_freedesktop_session_bus". Context names must be valid ASCII,
2914 nonzero length, and may not contain the characters slash ("/"),
2915 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2916 tab ("\t"), or period ("."). There is a default context,
2917 "org_freedesktop_general" that's used by servers that do not specify
2921 Cookies are stored in a user's home directory, in the directory
2922 <filename>~/.dbus-keyrings/</filename>. This directory must
2923 not be readable or writable by other users. If it is,
2924 clients and servers must ignore it. The directory
2925 contains cookie files named after the cookie context.
2928 A cookie file contains one cookie per line. Each line
2929 has three space-separated fields:
2933 The cookie ID number, which must be a non-negative integer and
2934 may not be used twice in the same file.
2939 The cookie's creation time, in UNIX seconds-since-the-epoch
2945 The cookie itself, a hex-encoded random block of bytes. The cookie
2946 may be of any length, though obviously security increases
2947 as the length increases.
2953 Only server processes modify the cookie file.
2954 They must do so with this procedure:
2958 Create a lockfile name by appending ".lock" to the name of the
2959 cookie file. The server should attempt to create this file
2960 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2961 fails, the lock fails. Servers should retry for a reasonable
2962 period of time, then they may choose to delete an existing lock
2963 to keep users from having to manually delete a stale
2964 lock. <footnote><para>Lockfiles are used instead of real file
2965 locking <literal>fcntl()</literal> because real locking
2966 implementations are still flaky on network
2967 filesystems.</para></footnote>
2972 Once the lockfile has been created, the server loads the cookie
2973 file. It should then delete any cookies that are old (the
2974 timeout can be fairly short), or more than a reasonable
2975 time in the future (so that cookies never accidentally
2976 become permanent, if the clock was set far into the future
2977 at some point). If no recent keys remain, the
2978 server may generate a new key.
2983 The pruned and possibly added-to cookie file
2984 must be resaved atomically (using a temporary
2985 file which is rename()'d).
2990 The lock must be dropped by deleting the lockfile.
2996 Clients need not lock the file in order to load it,
2997 because servers are required to save the file atomically.
3002 <sect1 id="addresses">
3003 <title>Server Addresses</title>
3005 Server addresses consist of a transport name followed by a colon, and
3006 then an optional, comma-separated list of keys and values in the form key=value.
3007 Each value is escaped.
3011 <programlisting>unix:path=/tmp/dbus-test</programlisting>
3012 Which is the address to a unix socket with the path /tmp/dbus-test.
3015 Value escaping is similar to URI escaping but simpler.
3019 The set of optionally-escaped bytes is:
3020 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
3021 <emphasis>byte</emphasis> (note, not character) which is not in the
3022 set of optionally-escaped bytes must be replaced with an ASCII
3023 percent (<literal>%</literal>) and the value of the byte in hex.
3024 The hex value must always be two digits, even if the first digit is
3025 zero. The optionally-escaped bytes may be escaped if desired.
3030 To unescape, append each byte in the value; if a byte is an ASCII
3031 percent (<literal>%</literal>) character then append the following
3032 hex value instead. It is an error if a <literal>%</literal> byte
3033 does not have two hex digits following. It is an error if a
3034 non-optionally-escaped byte is seen unescaped.
3038 The set of optionally-escaped bytes is intended to preserve address
3039 readability and convenience.
3043 A server may specify a key-value pair with the key <literal>guid</literal>
3044 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
3045 describes the format of the <literal>guid</literal> field. If present,
3046 this UUID may be used to distinguish one server address from another. A
3047 server should use a different UUID for each address it listens on. For
3048 example, if a message bus daemon offers both UNIX domain socket and TCP
3049 connections, but treats clients the same regardless of how they connect,
3050 those two connections are equivalent post-connection but should have
3051 distinct UUIDs to distinguish the kinds of connection.
3055 The intent of the address UUID feature is to allow a client to avoid
3056 opening multiple identical connections to the same server, by allowing the
3057 client to check whether an address corresponds to an already-existing
3058 connection. Comparing two addresses is insufficient, because addresses
3059 can be recycled by distinct servers, and equivalent addresses may look
3060 different if simply compared as strings (for example, the host in a TCP
3061 address can be given as an IP address or as a hostname).
3065 Note that the address key is <literal>guid</literal> even though the
3066 rest of the API and documentation says "UUID," for historical reasons.
3070 [FIXME clarify if attempting to connect to each is a requirement
3071 or just a suggestion]
3072 When connecting to a server, multiple server addresses can be
3073 separated by a semi-colon. The library will then try to connect
3074 to the first address and if that fails, it'll try to connect to
3075 the next one specified, and so forth. For example
3076 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3080 Some addresses are <firstterm>connectable</firstterm>. A connectable
3081 address is one containing enough information for a client to connect
3082 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3083 is a connectable address. It is not necessarily possible to listen
3084 on every connectable address: for instance, it is not possible to
3085 listen on a <literal>unixexec:</literal> address.
3089 Some addresses are <firstterm>listenable</firstterm>. A listenable
3090 address is one containing enough information for a server to listen on
3091 it, producing a connectable address (which may differ from the
3092 original address). Many listenable addresses are not connectable:
3093 for instance, <literal>tcp:host=127.0.0.1</literal>
3094 is listenable, but not connectable (because it does not specify
3099 Listening on an address that is not connectable will result in a
3100 connectable address that is not the same as the listenable address.
3101 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3102 might result in the connectable address
3103 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3104 or listening on <literal>unix:tmpdir=/tmp</literal>
3105 might result in the connectable address
3106 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3110 <sect1 id="transports">
3111 <title>Transports</title>
3113 [FIXME we need to specify in detail each transport and its possible arguments]
3115 Current transports include: unix domain sockets (including
3116 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3117 using in-process pipes. Future possible transports include one that
3118 tunnels over X11 protocol.
3121 <sect2 id="transports-unix-domain-sockets">
3122 <title>Unix Domain Sockets</title>
3124 Unix domain sockets can be either paths in the file system or on Linux
3125 kernels, they can be abstract which are similar to paths but
3126 do not show up in the file system.
3130 When a socket is opened by the D-Bus library it truncates the path
3131 name right before the first trailing Nul byte. This is true for both
3132 normal paths and abstract paths. Note that this is a departure from
3133 previous versions of D-Bus that would create sockets with a fixed
3134 length path name. Names which were shorter than the fixed length
3135 would be padded by Nul bytes.
3138 Unix domain sockets are not available on Windows.
3141 Unix addresses that specify <literal>path</literal> or
3142 <literal>abstract</literal> are both listenable and connectable.
3143 Unix addresses that specify <literal>tmpdir</literal> are only
3144 listenable: the corresponding connectable address will specify
3145 either <literal>path</literal> or <literal>abstract</literal>.
3147 <sect3 id="transports-unix-domain-sockets-addresses">
3148 <title>Server Address Format</title>
3150 Unix domain socket addresses are identified by the "unix:" prefix
3151 and support the following key/value pairs:
3158 <entry>Values</entry>
3159 <entry>Description</entry>
3165 <entry>(path)</entry>
3166 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3169 <entry>tmpdir</entry>
3170 <entry>(path)</entry>
3171 <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>
3174 <entry>abstract</entry>
3175 <entry>(string)</entry>
3176 <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>
3182 Exactly one of the keys <literal>path</literal>,
3183 <literal>abstract</literal> or
3184 <literal>tmpdir</literal> must be provided.
3188 <sect2 id="transports-launchd">
3189 <title>launchd</title>
3191 launchd is an open-source server management system that replaces init, inetd
3192 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3193 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3197 launchd allocates a socket and provides it with the unix path through the
3198 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3199 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3200 it through its environment.
3201 Other processes can query for the launchd socket by executing:
3202 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3203 This is normally done by the D-Bus client library so doesn't have to be done
3207 launchd is not available on Microsoft Windows.
3210 launchd addresses are listenable and connectable.
3212 <sect3 id="transports-launchd-addresses">
3213 <title>Server Address Format</title>
3215 launchd addresses are identified by the "launchd:" prefix
3216 and support the following key/value pairs:
3223 <entry>Values</entry>
3224 <entry>Description</entry>
3230 <entry>(environment variable)</entry>
3231 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3237 The <literal>env</literal> key is required.
3241 <sect2 id="transports-systemd">
3242 <title>systemd</title>
3244 systemd is an open-source server management system that
3245 replaces init and inetd on newer Linux systems. It supports
3246 socket activation. The D-Bus systemd transport is used to acquire
3247 socket activation file descriptors from systemd and use them
3248 as D-Bus transport when the current process is spawned by
3249 socket activation from it.
3252 The systemd transport accepts only one or more Unix domain or
3253 TCP streams sockets passed in via socket activation.
3256 The systemd transport is not available on non-Linux operating systems.
3259 The systemd transport defines no parameter keys.
3262 systemd addresses are listenable, but not connectable. The
3263 corresponding connectable address is the <literal>unix</literal>
3264 or <literal>tcp</literal> address of the socket.
3267 <sect2 id="transports-tcp-sockets">
3268 <title>TCP Sockets</title>
3270 The tcp transport provides TCP/IP based connections between clients
3271 located on the same or different hosts.
3274 Using tcp transport without any additional secure authentification mechanismus
3275 over a network is unsecure.
3278 On Windows and most Unix platforms, the TCP stack is unable to transfer
3279 credentials over a TCP connection, so the EXTERNAL authentication
3280 mechanism does not work for this transport.
3283 All <literal>tcp</literal> addresses are listenable.
3284 <literal>tcp</literal> addresses in which both
3285 <literal>host</literal> and <literal>port</literal> are
3286 specified, and <literal>port</literal> is non-zero,
3287 are also connectable.
3289 <sect3 id="transports-tcp-sockets-addresses">
3290 <title>Server Address Format</title>
3292 TCP/IP socket addresses are identified by the "tcp:" prefix
3293 and support the following key/value pairs:
3300 <entry>Values</entry>
3301 <entry>Description</entry>
3307 <entry>(string)</entry>
3308 <entry>DNS name or IP address</entry>
3312 <entry>(string)</entry>
3313 <entry>Used in a listenable address to configure the interface
3314 on which the server will listen: either the IP address of one of
3315 the local machine's interfaces (most commonly <literal>127.0.0.1
3316 </literal>), or a DNS name that resolves to one of those IP
3317 addresses, or '*' to listen on all interfaces simultaneously.
3318 If not specified, the default is the same value as "host".
3323 <entry>(number)</entry>
3324 <entry>The tcp port the server will open. A zero value let the server
3325 choose a free port provided from the underlaying operating system.
3326 libdbus is able to retrieve the real used port from the server.
3330 <entry>family</entry>
3331 <entry>(string)</entry>
3332 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3339 <sect2 id="transports-nonce-tcp-sockets">
3340 <title>Nonce-secured TCP Sockets</title>
3342 The nonce-tcp transport provides a secured TCP transport, using a
3343 simple authentication mechanism to ensure that only clients with read
3344 access to a certain location in the filesystem can connect to the server.
3345 The server writes a secret, the nonce, to a file and an incoming client
3346 connection is only accepted if the client sends the nonce right after
3347 the connect. The nonce mechanism requires no setup and is orthogonal to
3348 the higher-level authentication mechanisms described in the
3349 Authentication section.
3353 On start, the server generates a random 16 byte nonce and writes it
3354 to a file in the user's temporary directory. The nonce file location
3355 is published as part of the server's D-Bus address using the
3356 "noncefile" key-value pair.
3358 After an accept, the server reads 16 bytes from the socket. If the
3359 read bytes do not match the nonce stored in the nonce file, the
3360 server MUST immediately drop the connection.
3361 If the nonce match the received byte sequence, the client is accepted
3362 and the transport behaves like an unsecured tcp transport.
3365 After a successful connect to the server socket, the client MUST read
3366 the nonce from the file published by the server via the noncefile=
3367 key-value pair and send it over the socket. After that, the
3368 transport behaves like an unsecured tcp transport.
3371 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3372 <literal>host</literal>, <literal>port</literal> and
3373 <literal>noncefile</literal> are all specified,
3374 and <literal>port</literal> is nonzero, are also connectable.
3376 <sect3 id="transports-nonce-tcp-sockets-addresses">
3377 <title>Server Address Format</title>
3379 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3380 and support the following key/value pairs:
3387 <entry>Values</entry>
3388 <entry>Description</entry>
3394 <entry>(string)</entry>
3395 <entry>DNS name or IP address</entry>
3399 <entry>(string)</entry>
3400 <entry>The same as for tcp: addresses
3405 <entry>(number)</entry>
3406 <entry>The tcp port the server will open. A zero value let the server
3407 choose a free port provided from the underlaying operating system.
3408 libdbus is able to retrieve the real used port from the server.
3412 <entry>family</entry>
3413 <entry>(string)</entry>
3414 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3417 <entry>noncefile</entry>
3418 <entry>(path)</entry>
3419 <entry>File location containing the secret.
3420 This is only meaningful in connectable addresses:
3421 a listening D-Bus server that offers this transport
3422 will always create a new nonce file.</entry>
3429 <sect2 id="transports-exec">
3430 <title>Executed Subprocesses on Unix</title>
3432 This transport forks off a process and connects its standard
3433 input and standard output with an anonymous Unix domain
3434 socket. This socket is then used for communication by the
3435 transport. This transport may be used to use out-of-process
3436 forwarder programs as basis for the D-Bus protocol.
3439 The forked process will inherit the standard error output and
3440 process group from the parent process.
3443 Executed subprocesses are not available on Windows.
3446 <literal>unixexec</literal> addresses are connectable, but are not
3449 <sect3 id="transports-exec-addresses">
3450 <title>Server Address Format</title>
3452 Executed subprocess addresses are identified by the "unixexec:" prefix
3453 and support the following key/value pairs:
3460 <entry>Values</entry>
3461 <entry>Description</entry>
3467 <entry>(path)</entry>
3468 <entry>Path of the binary to execute, either an absolute
3469 path or a binary name that is searched for in the default
3470 search path of the OS. This corresponds to the first
3471 argument of execlp(). This key is mandatory.</entry>
3474 <entry>argv0</entry>
3475 <entry>(string)</entry>
3476 <entry>The program name to use when executing the
3477 binary. If omitted the same value as specified for path=
3478 will be used. This corresponds to the second argument of
3482 <entry>argv1, argv2, ...</entry>
3483 <entry>(string)</entry>
3484 <entry>Arguments to pass to the binary. This corresponds
3485 to the third and later arguments of execlp(). If a
3486 specific argvX is not specified no further argvY for Y > X
3487 are taken into account.</entry>
3495 <sect1 id="meta-transports">
3496 <title>Meta Transports</title>
3498 Meta transports are a kind of transport with special enhancements or
3499 behavior. Currently available meta transports include: autolaunch
3502 <sect2 id="meta-transports-autolaunch">
3503 <title>Autolaunch</title>
3504 <para>The autolaunch transport provides a way for dbus clients to autodetect
3505 a running dbus session bus and to autolaunch a session bus if not present.
3508 On Unix, <literal>autolaunch</literal> addresses are connectable,
3512 On Windows, <literal>autolaunch</literal> addresses are both
3513 connectable and listenable.
3516 <sect3 id="meta-transports-autolaunch-addresses">
3517 <title>Server Address Format</title>
3519 Autolaunch addresses uses the "autolaunch:" prefix and support the
3520 following key/value pairs:
3527 <entry>Values</entry>
3528 <entry>Description</entry>
3533 <entry>scope</entry>
3534 <entry>(string)</entry>
3535 <entry>scope of autolaunch (Windows only)
3539 "*install-path" - limit session bus to dbus installation path.
3540 The dbus installation path is determined from the location of
3541 the shared dbus library. If the library is located in a 'bin'
3542 subdirectory the installation root is the directory above,
3543 otherwise the directory where the library lives is taken as
3546 <install-root>/bin/[lib]dbus-1.dll
3547 <install-root>/[lib]dbus-1.dll
3553 "*user" - limit session bus to the recent user.
3558 other values - specify dedicated session bus like "release",
3570 <sect3 id="meta-transports-autolaunch-windows-implementation">
3571 <title>Windows implementation</title>
3573 On start, the server opens a platform specific transport, creates a mutex
3574 and a shared memory section containing the related session bus address.
3575 This mutex will be inspected by the dbus client library to detect a
3576 running dbus session bus. The access to the mutex and the shared memory
3577 section are protected by global locks.
3580 In the recent implementation the autolaunch transport uses a tcp transport
3581 on localhost with a port choosen from the operating system. This detail may
3582 change in the future.
3585 Disclaimer: The recent implementation is in an early state and may not
3586 work in all cirumstances and/or may have security issues. Because of this
3587 the implementation is not documentated yet.
3594 <title>UUIDs</title>
3596 A working D-Bus implementation uses universally-unique IDs in two places.
3597 First, each server address has a UUID identifying the address,
3598 as described in <xref linkend="addresses"/>. Second, each operating
3599 system kernel instance running a D-Bus client or server has a UUID
3600 identifying that kernel, retrieved by invoking the method
3601 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3602 linkend="standard-interfaces-peer"/>).
3605 The term "UUID" in this document is intended literally, i.e. an
3606 identifier that is universally unique. It is not intended to refer to
3607 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3610 The UUID must contain 128 bits of data and be hex-encoded. The
3611 hex-encoded string may not contain hyphens or other non-hex-digit
3612 characters, and it must be exactly 32 characters long. To generate a
3613 UUID, the current reference implementation concatenates 96 bits of random
3614 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3618 It would also be acceptable and probably better to simply generate 128
3619 bits of random data, as long as the random number generator is of high
3620 quality. The timestamp could conceivably help if the random bits are not
3621 very random. With a quality random number generator, collisions are
3622 extremely unlikely even with only 96 bits, so it's somewhat academic.
3625 Implementations should, however, stick to random data for the first 96 bits
3630 <sect1 id="standard-interfaces">
3631 <title>Standard Interfaces</title>
3633 See <xref linkend="message-protocol-types-notation"/> for details on
3634 the notation used in this section. There are some standard interfaces
3635 that may be useful across various D-Bus applications.
3637 <sect2 id="standard-interfaces-peer">
3638 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3640 The <literal>org.freedesktop.DBus.Peer</literal> interface
3643 org.freedesktop.DBus.Peer.Ping ()
3644 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3648 On receipt of the <literal>METHOD_CALL</literal> message
3649 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3650 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3651 usual. It does not matter which object path a ping is sent to. The
3652 reference implementation handles this method automatically.
3655 On receipt of the <literal>METHOD_CALL</literal> message
3656 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3657 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3658 UUID representing the identity of the machine the process is running on.
3659 This UUID must be the same for all processes on a single system at least
3660 until that system next reboots. It should be the same across reboots
3661 if possible, but this is not always possible to implement and is not
3663 It does not matter which object path a GetMachineId is sent to. The
3664 reference implementation handles this method automatically.
3667 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3668 a virtual machine running on a hypervisor, rather than a physical machine.
3669 Basically if two processes see the same UUID, they should also see the same
3670 shared memory, UNIX domain sockets, process IDs, and other features that require
3671 a running OS kernel in common between the processes.
3674 The UUID is often used where other programs might use a hostname. Hostnames
3675 can change without rebooting, however, or just be "localhost" - so the UUID
3679 <xref linkend="uuids"/> explains the format of the UUID.
3683 <sect2 id="standard-interfaces-introspectable">
3684 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3686 This interface has one method:
3688 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3692 Objects instances may implement
3693 <literal>Introspect</literal> which returns an XML description of
3694 the object, including its interfaces (with signals and methods), objects
3695 below it in the object path tree, and its properties.
3698 <xref linkend="introspection-format"/> describes the format of this XML string.
3701 <sect2 id="standard-interfaces-properties">
3702 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3704 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3705 or <firstterm>attributes</firstterm>. These can be exposed via the
3706 <literal>org.freedesktop.DBus.Properties</literal> interface.
3710 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3711 in STRING property_name,
3713 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3714 in STRING property_name,
3716 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3717 out DICT<STRING,VARIANT> props);
3721 It is conventional to give D-Bus properties names consisting of
3722 capitalized words without punctuation ("CamelCase"), like
3723 <link linkend="message-protocol-names-member">member names</link>.
3724 For instance, the GObject property
3725 <literal>connection-status</literal> or the Qt property
3726 <literal>connectionStatus</literal> could be represented on D-Bus
3727 as <literal>ConnectionStatus</literal>.
3730 Strictly speaking, D-Bus property names are not required to follow
3731 the same naming restrictions as member names, but D-Bus property
3732 names that would not be valid member names (in particular,
3733 GObject-style dash-separated property names) can cause interoperability
3734 problems and should be avoided.
3737 The available properties and whether they are writable can be determined
3738 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3739 see <xref linkend="standard-interfaces-introspectable"/>.
3742 An empty string may be provided for the interface name; in this case,
3743 if there are multiple properties on an object with the same name,
3744 the results are undefined (picking one by according to an arbitrary
3745 deterministic rule, or returning an error, are the reasonable
3749 If one or more properties change on an object, the
3750 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3751 signal may be emitted (this signal was added in 0.14):
3755 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3756 DICT<STRING,VARIANT> changed_properties,
3757 ARRAY<STRING> invalidated_properties);
3761 where <literal>changed_properties</literal> is a dictionary
3762 containing the changed properties with the new values and
3763 <literal>invalidated_properties</literal> is an array of
3764 properties that changed but the value is not conveyed.
3767 Whether the <literal>PropertiesChanged</literal> signal is
3768 supported can be determined by calling
3769 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3770 that the signal may be supported for an object but it may
3771 differ how whether and how it is used on a per-property basis
3772 (for e.g. performance or security reasons). Each property (or
3773 the parent interface) must be annotated with the
3774 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3775 annotation to convey this (usually the default value
3776 <literal>true</literal> is sufficient meaning that the
3777 annotation does not need to be used). See <xref
3778 linkend="introspection-format"/> for details on this
3783 <sect2 id="standard-interfaces-objectmanager">
3784 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3786 An API can optionally make use of this interface for one or
3787 more sub-trees of objects. The root of each sub-tree implements
3788 this interface so other applications can get all objects,
3789 interfaces and properties in a single method call. It is
3790 appropriate to use this interface if users of the tree of
3791 objects are expected to be interested in all interfaces of all
3792 objects in the tree; a more granular API should be used if
3793 users of the objects are expected to be interested in a small
3794 subset of the objects, a small subset of their interfaces, or
3798 The method that applications can use to get all objects and
3799 properties is <literal>GetManagedObjects</literal>:
3803 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3807 The return value of this method is a dict whose keys are
3808 object paths. All returned object paths are children of the
3809 object path implementing this interface, i.e. their object
3810 paths start with the ObjectManager's object path plus '/'.
3813 Each value is a dict whose keys are interfaces names. Each
3814 value in this inner dict is the same dict that would be
3815 returned by the <link
3816 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3817 method for that combination of object path and interface. If
3818 an interface has no properties, the empty dict is returned.
3821 Changes are emitted using the following two signals:
3825 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3826 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3827 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3828 ARRAY<STRING> interfaces);
3832 The <literal>InterfacesAdded</literal> signal is emitted when
3833 either a new object is added or when an existing object gains
3834 one or more interfaces. The
3835 <literal>InterfacesRemoved</literal> signal is emitted
3836 whenever an object is removed or it loses one or more
3837 interfaces. The second parameter of the
3838 <literal>InterfacesAdded</literal> signal contains a dict with
3839 the interfaces and properties (if any) that have been added to
3840 the given object path. Similarly, the second parameter of the
3841 <literal>InterfacesRemoved</literal> signal contains an array
3842 of the interfaces that were removed. Note that changes on
3843 properties on existing interfaces are not reported using this
3844 interface - an application should also monitor the existing <link
3845 linkend="standard-interfaces-properties">PropertiesChanged</link>
3846 signal on each object.
3849 Applications SHOULD NOT export objects that are children of an
3850 object (directly or otherwise) implementing this interface but
3851 which are not returned in the reply from the
3852 <literal>GetManagedObjects()</literal> method of this
3853 interface on the given object.
3856 The intent of the <literal>ObjectManager</literal> interface
3857 is to make it easy to write a robust client
3858 implementation. The trivial client implementation only needs
3859 to make two method calls:
3863 org.freedesktop.DBus.AddMatch (bus_proxy,
3864 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3865 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3869 on the message bus and the remote application's
3870 <literal>ObjectManager</literal>, respectively. Whenever a new
3871 remote object is created (or an existing object gains a new
3872 interface), the <literal>InterfacesAdded</literal> signal is
3873 emitted, and since this signal contains all properties for the
3874 interfaces, no calls to the
3875 <literal>org.freedesktop.Properties</literal> interface on the
3876 remote object are needed. Additionally, since the initial
3877 <literal>AddMatch()</literal> rule already includes signal
3878 messages from the newly created child object, no new
3879 <literal>AddMatch()</literal> call is needed.
3884 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3885 interface was added in version 0.17 of the D-Bus
3892 <sect1 id="introspection-format">
3893 <title>Introspection Data Format</title>
3895 As described in <xref linkend="standard-interfaces-introspectable"/>,
3896 objects may be introspected at runtime, returning an XML string
3897 that describes the object. The same XML format may be used in
3898 other contexts as well, for example as an "IDL" for generating
3899 static language bindings.
3902 Here is an example of introspection data:
3904 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3905 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3906 <node name="/com/example/sample_object">
3907 <interface name="com.example.SampleInterface">
3908 <method name="Frobate">
3909 <arg name="foo" type="i" direction="in"/>
3910 <arg name="bar" type="s" direction="out"/>
3911 <arg name="baz" type="a{us}" direction="out"/>
3912 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3914 <method name="Bazify">
3915 <arg name="bar" type="(iiu)" direction="in"/>
3916 <arg name="bar" type="v" direction="out"/>
3918 <method name="Mogrify">
3919 <arg name="bar" type="(iiav)" direction="in"/>
3921 <signal name="Changed">
3922 <arg name="new_value" type="b"/>
3924 <property name="Bar" type="y" access="readwrite"/>
3926 <node name="child_of_sample_object"/>
3927 <node name="another_child_of_sample_object"/>
3932 A more formal DTD and spec needs writing, but here are some quick notes.
3936 Only the root <node> element can omit the node name, as it's
3937 known to be the object that was introspected. If the root
3938 <node> does have a name attribute, it must be an absolute
3939 object path. If child <node> have object paths, they must be
3945 If a child <node> has any sub-elements, then they
3946 must represent a complete introspection of the child.
3947 If a child <node> is empty, then it may or may
3948 not have sub-elements; the child must be introspected
3949 in order to find out. The intent is that if an object
3950 knows that its children are "fast" to introspect
3951 it can go ahead and return their information, but
3952 otherwise it can omit it.
3957 The direction element on <arg> may be omitted,
3958 in which case it defaults to "in" for method calls
3959 and "out" for signals. Signals only allow "out"
3960 so while direction may be specified, it's pointless.
3965 The possible directions are "in" and "out",
3966 unlike CORBA there is no "inout"
3971 The possible property access flags are
3972 "readwrite", "read", and "write"
3977 Multiple interfaces can of course be listed for
3983 The "name" attribute on arguments is optional.
3989 Method, interface, property, and signal elements may have
3990 "annotations", which are generic key/value pairs of metadata.
3991 They are similar conceptually to Java's annotations and C# attributes.
3992 Well-known annotations:
3999 <entry>Values (separated by ,)</entry>
4000 <entry>Description</entry>
4005 <entry>org.freedesktop.DBus.Deprecated</entry>
4006 <entry>true,false</entry>
4007 <entry>Whether or not the entity is deprecated; defaults to false</entry>
4010 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
4011 <entry>(string)</entry>
4012 <entry>The C symbol; may be used for methods and interfaces</entry>
4015 <entry>org.freedesktop.DBus.Method.NoReply</entry>
4016 <entry>true,false</entry>
4017 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
4020 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
4021 <entry>true,invalidates,const,false</entry>
4024 If set to <literal>false</literal>, the
4025 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
4027 linkend="standard-interfaces-properties"/> is not
4028 guaranteed to be emitted if the property changes.
4031 If set to <literal>const</literal> the property never
4032 changes value during the lifetime of the object it
4033 belongs to, and hence the signal is never emitted for
4037 If set to <literal>invalidates</literal> the signal
4038 is emitted but the value is not included in the
4042 If set to <literal>true</literal> the signal is
4043 emitted with the value included.
4046 The value for the annotation defaults to
4047 <literal>true</literal> if the enclosing interface
4048 element does not specify the annotation. Otherwise it
4049 defaults to the value specified in the enclosing
4053 This annotation is intended to be used by code
4054 generators to implement client-side caching of
4055 property values. For all properties for which the
4056 annotation is set to <literal>const</literal>,
4057 <literal>invalidates</literal> or
4058 <literal>true</literal> the client may
4059 unconditionally cache the values as the properties
4060 don't change or notifications are generated for them
4069 <sect1 id="message-bus">
4070 <title>Message Bus Specification</title>
4071 <sect2 id="message-bus-overview">
4072 <title>Message Bus Overview</title>
4074 The message bus accepts connections from one or more applications.
4075 Once connected, applications can exchange messages with other
4076 applications that are also connected to the bus.
4079 In order to route messages among connections, the message bus keeps a
4080 mapping from names to connections. Each connection has one
4081 unique-for-the-lifetime-of-the-bus name automatically assigned.
4082 Applications may request additional names for a connection. Additional
4083 names are usually "well-known names" such as
4084 "com.example.TextEditor". When a name is bound to a connection,
4085 that connection is said to <firstterm>own</firstterm> the name.
4088 The bus itself owns a special name,
4089 <literal>org.freedesktop.DBus</literal>, with an object
4090 located at <literal>/org/freedesktop/DBus</literal> that
4091 implements the <literal>org.freedesktop.DBus</literal>
4092 interface. This service allows applications to make
4093 administrative requests of the bus itself. For example,
4094 applications can ask the bus to assign a name to a connection.
4097 Each name may have <firstterm>queued owners</firstterm>. When an
4098 application requests a name for a connection and the name is already in
4099 use, the bus will optionally add the connection to a queue waiting for
4100 the name. If the current owner of the name disconnects or releases
4101 the name, the next connection in the queue will become the new owner.
4105 This feature causes the right thing to happen if you start two text
4106 editors for example; the first one may request "com.example.TextEditor",
4107 and the second will be queued as a possible owner of that name. When
4108 the first exits, the second will take over.
4112 Applications may send <firstterm>unicast messages</firstterm> to
4113 a specific recipient or to the message bus itself, or
4114 <firstterm>broadcast messages</firstterm> to all interested recipients.
4115 See <xref linkend="message-bus-routing"/> for details.
4119 <sect2 id="message-bus-names">
4120 <title>Message Bus Names</title>
4122 Each connection has at least one name, assigned at connection time and
4123 returned in response to the
4124 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4125 automatically-assigned name is called the connection's <firstterm>unique
4126 name</firstterm>. Unique names are never reused for two different
4127 connections to the same bus.
4130 Ownership of a unique name is a prerequisite for interaction with
4131 the message bus. It logically follows that the unique name is always
4132 the first name that an application comes to own, and the last
4133 one that it loses ownership of.
4136 Unique connection names must begin with the character ':' (ASCII colon
4137 character); bus names that are not unique names must not begin
4138 with this character. (The bus must reject any attempt by an application
4139 to manually request a name beginning with ':'.) This restriction
4140 categorically prevents "spoofing"; messages sent to a unique name
4141 will always go to the expected connection.
4144 When a connection is closed, all the names that it owns are deleted (or
4145 transferred to the next connection in the queue if any).
4148 A connection can request additional names to be associated with it using
4149 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4150 linkend="message-protocol-names-bus"/> describes the format of a valid
4151 name. These names can be released again using the
4152 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4155 <sect3 id="bus-messages-request-name">
4156 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4160 UINT32 RequestName (in STRING name, in UINT32 flags)
4167 <entry>Argument</entry>
4169 <entry>Description</entry>
4175 <entry>STRING</entry>
4176 <entry>Name to request</entry>
4180 <entry>UINT32</entry>
4181 <entry>Flags</entry>
4191 <entry>Argument</entry>
4193 <entry>Description</entry>
4199 <entry>UINT32</entry>
4200 <entry>Return value</entry>
4207 This method call should be sent to
4208 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4209 assign the given name to the method caller. Each name maintains a
4210 queue of possible owners, where the head of the queue is the primary
4211 or current owner of the name. Each potential owner in the queue
4212 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4213 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4214 call. When RequestName is invoked the following occurs:
4218 If the method caller is currently the primary owner of the name,
4219 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4220 values are updated with the values from the new RequestName call,
4221 and nothing further happens.
4227 If the current primary owner (head of the queue) has
4228 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4229 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4230 the caller of RequestName replaces the current primary owner at
4231 the head of the queue and the current primary owner moves to the
4232 second position in the queue. If the caller of RequestName was
4233 in the queue previously its flags are updated with the values from
4234 the new RequestName in addition to moving it to the head of the queue.
4240 If replacement is not possible, and the method caller is
4241 currently in the queue but not the primary owner, its flags are
4242 updated with the values from the new RequestName call.
4248 If replacement is not possible, and the method caller is
4249 currently not in the queue, the method caller is appended to the
4256 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4257 set and is not the primary owner, it is removed from the
4258 queue. This can apply to the previous primary owner (if it
4259 was replaced) or the method caller (if it updated the
4260 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4261 queue, or if it was just added to the queue with that flag set).
4267 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4268 queue," even if another application already in the queue had specified
4269 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4270 that does not allow replacement goes away, and the next primary owner
4271 does allow replacement. In this case, queued items that specified
4272 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4273 automatically replace the new primary owner. In other words,
4274 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4275 time RequestName is called. This is deliberate to avoid an infinite loop
4276 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4277 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4280 The flags argument contains any of the following values logically ORed
4287 <entry>Conventional Name</entry>
4288 <entry>Value</entry>
4289 <entry>Description</entry>
4294 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4298 If an application A specifies this flag and succeeds in
4299 becoming the owner of the name, and another application B
4300 later calls RequestName with the
4301 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4302 will lose ownership and receive a
4303 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4304 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4305 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4306 is not specified by application B, then application B will not replace
4307 application A as the owner.
4312 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4316 Try to replace the current owner if there is one. If this
4317 flag is not set the application will only become the owner of
4318 the name if there is no current owner. If this flag is set,
4319 the application will replace the current owner if
4320 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4325 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4329 Without this flag, if an application requests a name that is
4330 already owned, the application will be placed in a queue to
4331 own the name when the current owner gives it up. If this
4332 flag is given, the application will not be placed in the
4333 queue, the request for the name will simply fail. This flag
4334 also affects behavior when an application is replaced as
4335 name owner; by default the application moves back into the
4336 waiting queue, unless this flag was provided when the application
4337 became the name owner.
4345 The return code can be one of the following values:
4351 <entry>Conventional Name</entry>
4352 <entry>Value</entry>
4353 <entry>Description</entry>
4358 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4359 <entry>1</entry> <entry>The caller is now the primary owner of
4360 the name, replacing any previous owner. Either the name had no
4361 owner before, or the caller specified
4362 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4363 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4366 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4369 <entry>The name already had an owner,
4370 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4371 the current owner did not specify
4372 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4373 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4377 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4378 <entry>The name already has an owner,
4379 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4380 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4381 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4382 specified by the requesting application.</entry>
4385 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4387 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4395 <sect3 id="bus-messages-release-name">
4396 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4400 UINT32 ReleaseName (in STRING name)
4407 <entry>Argument</entry>
4409 <entry>Description</entry>
4415 <entry>STRING</entry>
4416 <entry>Name to release</entry>
4426 <entry>Argument</entry>
4428 <entry>Description</entry>
4434 <entry>UINT32</entry>
4435 <entry>Return value</entry>
4442 This method call should be sent to
4443 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4444 release the method caller's claim to the given name. If the caller is
4445 the primary owner, a new primary owner will be selected from the
4446 queue if any other owners are waiting. If the caller is waiting in
4447 the queue for the name, the caller will removed from the queue and
4448 will not be made an owner of the name if it later becomes available.
4449 If there are no other owners in the queue for the name, it will be
4450 removed from the bus entirely.
4452 The return code can be one of the following values:
4458 <entry>Conventional Name</entry>
4459 <entry>Value</entry>
4460 <entry>Description</entry>
4465 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4466 <entry>1</entry> <entry>The caller has released his claim on
4467 the given name. Either the caller was the primary owner of
4468 the name, and the name is now unused or taken by somebody
4469 waiting in the queue for the name, or the caller was waiting
4470 in the queue for the name and has now been removed from the
4474 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4476 <entry>The given name does not exist on this bus.</entry>
4479 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4481 <entry>The caller was not the primary owner of this name,
4482 and was also not waiting in the queue to own this name.</entry>
4490 <sect3 id="bus-messages-list-queued-owners">
4491 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4495 ARRAY of STRING ListQueuedOwners (in STRING name)
4502 <entry>Argument</entry>
4504 <entry>Description</entry>
4510 <entry>STRING</entry>
4511 <entry>The well-known bus name to query, such as
4512 <literal>com.example.cappuccino</literal></entry>
4522 <entry>Argument</entry>
4524 <entry>Description</entry>
4530 <entry>ARRAY of STRING</entry>
4531 <entry>The unique bus names of connections currently queued
4532 for the name</entry>
4539 This method call should be sent to
4540 <literal>org.freedesktop.DBus</literal> and lists the connections
4541 currently queued for a bus name (see
4542 <xref linkend="term-queued-owner"/>).
4547 <sect2 id="message-bus-routing">
4548 <title>Message Bus Message Routing</title>
4551 Messages may have a <literal>DESTINATION</literal> field (see <xref
4552 linkend="message-protocol-header-fields"/>), resulting in a
4553 <firstterm>unicast message</firstterm>. If the
4554 <literal>DESTINATION</literal> field is present, it specifies a message
4555 recipient by name. Method calls and replies normally specify this field.
4556 The message bus must send messages (of any type) with the
4557 <literal>DESTINATION</literal> field set to the specified recipient,
4558 regardless of whether the recipient has set up a match rule matching
4563 When the message bus receives a signal, if the
4564 <literal>DESTINATION</literal> field is absent, it is considered to
4565 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4566 applications with <firstterm>message matching rules</firstterm> that
4567 match the message. Most signal messages are broadcasts, and
4568 no other message types currently defined in this specification
4573 Unicast signal messages (those with a <literal>DESTINATION</literal>
4574 field) are not commonly used, but they are treated like any unicast
4575 message: they are delivered to the specified receipient,
4576 regardless of its match rules. One use for unicast signals is to
4577 avoid a race condition in which a signal is emitted before the intended
4578 recipient can call <xref linkend="bus-messages-add-match"/> to
4579 receive that signal: if the signal is sent directly to that recipient
4580 using a unicast message, it does not need to add a match rule at all,
4581 and there is no race condition. Another use for unicast signals,
4582 on message buses whose security policy prevents eavesdropping, is to
4583 send sensitive information which should only be visible to one
4588 When the message bus receives a method call, if the
4589 <literal>DESTINATION</literal> field is absent, the call is taken to be
4590 a standard one-to-one message and interpreted by the message bus
4591 itself. For example, sending an
4592 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4593 <literal>DESTINATION</literal> will cause the message bus itself to
4594 reply to the ping immediately; the message bus will not make this
4595 message visible to other applications.
4599 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4600 the ping message were sent with a <literal>DESTINATION</literal> name of
4601 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4602 forwarded, and the Yoyodyne Corporation screensaver application would be
4603 expected to reply to the ping.
4607 Message bus implementations may impose a security policy which
4608 prevents certain messages from being sent or received.
4609 When a method call message cannot be sent or received due to a security
4610 policy, the message bus should send an error reply, unless the
4611 original message had the <literal>NO_REPLY</literal> flag.
4614 <sect3 id="message-bus-routing-eavesdropping">
4615 <title>Eavesdropping</title>
4617 Receiving a unicast message whose <literal>DESTINATION</literal>
4618 indicates a different recipient is called
4619 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4620 a security boundary (like the standard system bus), the security
4621 policy should usually prevent eavesdropping, since unicast messages
4622 are normally kept private and may contain security-sensitive
4627 Eavesdropping is mainly useful for debugging tools, such as
4628 the <literal>dbus-monitor</literal> tool in the reference
4629 implementation of D-Bus. Tools which eavesdrop on the message bus
4630 should be careful to avoid sending a reply or error in response to
4631 messages intended for a different client.
4635 Clients may attempt to eavesdrop by adding match rules
4636 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4637 the <literal>eavesdrop='true'</literal> match. If the message bus'
4638 security policy does not allow eavesdropping, the match rule can
4639 still be added, but will not have any practical effect. For
4640 compatibility with older message bus implementations, if adding such
4641 a match rule results in an error reply, the client may fall back to
4642 adding the same rule with the <literal>eavesdrop</literal> match
4647 <sect3 id="message-bus-routing-match-rules">
4648 <title>Match Rules</title>
4650 An important part of the message bus routing protocol is match
4651 rules. Match rules describe the messages that should be sent to a
4652 client, based on the contents of the message. Broadcast signals
4653 are only sent to clients which have a suitable match rule: this
4654 avoids waking up client processes to deal with signals that are
4655 not relevant to that client.
4658 Messages that list a client as their <literal>DESTINATION</literal>
4659 do not need to match the client's match rules, and are sent to that
4660 client regardless. As a result, match rules are mainly used to
4661 receive a subset of broadcast signals.
4664 Match rules can also be used for eavesdropping
4665 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4666 if the security policy of the message bus allows it.
4669 Match rules are added using the AddMatch bus method
4670 (see <xref linkend="bus-messages-add-match"/>). Rules are
4671 specified as a string of comma separated key/value pairs.
4672 Excluding a key from the rule indicates a wildcard match.
4673 For instance excluding the the member from a match rule but
4674 adding a sender would let all messages from that sender through.
4675 An example of a complete rule would be
4676 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4679 Within single quotes (ASCII apostrophe, U+0027), a backslash
4680 (U+005C) represents itself, and an apostrophe ends the quoted
4681 section. Outside single quotes, \' (backslash, apostrophe)
4682 represents an apostrophe, and any backslash not followed by
4683 an apostrophe represents itself. For instance, the match rules
4684 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4685 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4686 both match messages where the arguments are a 1-character string
4687 containing an apostrophe, a 1-character string containing a
4688 backslash, a 1-character string containing a comma, and a
4689 2-character string containing two backslashes<footnote>
4691 This idiosyncratic quoting style is based on the rules for
4692 escaping items to appear inside single-quoted strings
4693 in POSIX <literal>/bin/sh</literal>, but please
4694 note that backslashes that are not inside single quotes have
4695 different behaviour. This syntax does not offer any way to
4696 represent an apostrophe inside single quotes (it is necessary
4697 to leave the single-quoted section, backslash-escape the
4698 apostrophe and re-enter single quotes), or to represent a
4699 comma outside single quotes (it is necessary to wrap it in
4700 a single-quoted section).
4705 The following table describes the keys that can be used to create
4712 <entry>Possible Values</entry>
4713 <entry>Description</entry>
4718 <entry><literal>type</literal></entry>
4719 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4720 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4723 <entry><literal>sender</literal></entry>
4724 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4725 and <xref linkend="term-unique-name"/> respectively)
4727 <entry>Match messages sent by a particular sender. An example of a sender match
4728 is sender='org.freedesktop.Hal'</entry>
4731 <entry><literal>interface</literal></entry>
4732 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4733 <entry>Match messages sent over or to a particular interface. An example of an
4734 interface match is interface='org.freedesktop.Hal.Manager'.
4735 If a message omits the interface header, it must not match any rule
4736 that specifies this key.</entry>
4739 <entry><literal>member</literal></entry>
4740 <entry>Any valid method or signal name</entry>
4741 <entry>Matches messages which have the give method or signal name. An example of
4742 a member match is member='NameOwnerChanged'</entry>
4745 <entry><literal>path</literal></entry>
4746 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4747 <entry>Matches messages which are sent from or to the given object. An example of a
4748 path match is path='/org/freedesktop/Hal/Manager'</entry>
4751 <entry><literal>path_namespace</literal></entry>
4752 <entry>An object path</entry>
4755 Matches messages which are sent from or to an
4756 object for which the object path is either the
4757 given value, or that value followed by one or
4758 more path components.
4763 <literal>path_namespace='/com/example/foo'</literal>
4764 would match signals sent by
4765 <literal>/com/example/foo</literal>
4767 <literal>/com/example/foo/bar</literal>,
4769 <literal>/com/example/foobar</literal>.
4773 Using both <literal>path</literal> and
4774 <literal>path_namespace</literal> in the same match
4775 rule is not allowed.
4780 This match key was added in version 0.16 of the
4781 D-Bus specification and implemented by the bus
4782 daemon in dbus 1.5.0 and later.
4788 <entry><literal>destination</literal></entry>
4789 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4790 <entry>Matches messages which are being sent to the given unique name. An
4791 example of a destination match is destination=':1.0'</entry>
4794 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4795 <entry>Any string</entry>
4796 <entry>Arg matches are special and are used for further restricting the
4797 match based on the arguments in the body of a message. Only arguments of type
4798 STRING can be matched in this way. An example of an argument match
4799 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4803 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4804 <entry>Any string</entry>
4806 <para>Argument path matches provide a specialised form of wildcard matching for
4807 path-like namespaces. They can match arguments whose type is either STRING or
4808 OBJECT_PATH. As with normal argument matches,
4809 if the argument is exactly equal to the string given in the match
4810 rule then the rule is satisfied. Additionally, there is also a
4811 match when either the string given in the match rule or the
4812 appropriate message argument ends with '/' and is a prefix of the
4813 other. An example argument path match is arg0path='/aa/bb/'. This
4814 would match messages with first arguments of '/', '/aa/',
4815 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4816 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4818 <para>This is intended for monitoring “directories” in file system-like
4819 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4820 system. An application interested in all nodes in a particular hierarchy would
4821 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4822 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4823 represent a modification to the “bar” property, or a signal with zeroth
4824 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4825 many properties within that directory, and the interested application would be
4826 notified in both cases.</para>
4829 This match key was added in version 0.12 of the
4830 D-Bus specification, implemented for STRING
4831 arguments by the bus daemon in dbus 1.2.0 and later,
4832 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4839 <entry><literal>arg0namespace</literal></entry>
4840 <entry>Like a bus name, except that the string is not
4841 required to contain a '.' (period)</entry>
4843 <para>Match messages whose first argument is of type STRING, and is a bus name
4844 or interface name within the specified namespace. This is primarily intended
4845 for watching name owner changes for a group of related bus names, rather than
4846 for a single name or all name changes.</para>
4848 <para>Because every valid interface name is also a valid
4849 bus name, this can also be used for messages whose
4850 first argument is an interface name.</para>
4852 <para>For example, the match rule
4853 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4854 matches name owner changes for bus names such as
4855 <literal>com.example.backend.foo</literal>,
4856 <literal>com.example.backend.foo.bar</literal>, and
4857 <literal>com.example.backend</literal> itself.</para>
4859 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4862 This match key was added in version 0.16 of the
4863 D-Bus specification and implemented by the bus
4864 daemon in dbus 1.5.0 and later.
4870 <entry><literal>eavesdrop</literal></entry>
4871 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4872 <entry>Since D-Bus 1.5.6, match rules do not
4873 match messages which have a <literal>DESTINATION</literal>
4874 field unless the match rule specifically
4876 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4877 by specifying <literal>eavesdrop='true'</literal>
4878 in the match rule. <literal>eavesdrop='false'</literal>
4879 restores the default behaviour. Messages are
4880 delivered to their <literal>DESTINATION</literal>
4881 regardless of match rules, so this match does not
4882 affect normal delivery of unicast messages.
4883 If the message bus has a security policy which forbids
4884 eavesdropping, this match may still be used without error,
4885 but will not have any practical effect.
4886 In older versions of D-Bus, this match was not allowed
4887 in match rules, and all match rules behaved as if
4888 <literal>eavesdrop='true'</literal> had been used.
4897 <sect2 id="message-bus-starting-services">
4898 <title>Message Bus Starting Services</title>
4900 The message bus can start applications on behalf of other applications.
4901 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4902 An application that can be started in this way is called a
4903 <firstterm>service</firstterm>.
4906 With D-Bus, starting a service is normally done by name. That is,
4907 applications ask the message bus to start some program that will own a
4908 well-known name, such as <literal>com.example.TextEditor</literal>.
4909 This implies a contract documented along with the name
4910 <literal>com.example.TextEditor</literal> for which object
4911 the owner of that name will provide, and what interfaces those
4915 To find an executable corresponding to a particular name, the bus daemon
4916 looks for <firstterm>service description files</firstterm>. Service
4917 description files define a mapping from names to executables. Different
4918 kinds of message bus will look for these files in different places, see
4919 <xref linkend="message-bus-types"/>.
4922 Service description files have the ".service" file
4923 extension. The message bus will only load service description files
4924 ending with .service; all other files will be ignored. The file format
4925 is similar to that of <ulink
4926 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4927 entries</ulink>. All service description files must be in UTF-8
4928 encoding. To ensure that there will be no name collisions, service files
4929 must be namespaced using the same mechanism as messages and service
4934 On the well-known system bus, the name of a service description file
4935 must be its well-known name plus <literal>.service</literal>,
4937 <literal>com.example.ConfigurationDatabase.service</literal>.
4941 On the well-known session bus, services should follow the same
4942 service description file naming convention as on the system bus,
4943 but for backwards compatibility they are not required to do so.
4947 [FIXME the file format should be much better specified than "similar to
4948 .desktop entries" esp. since desktop entries are already
4949 badly-specified. ;-)]
4950 These sections from the specification apply to service files as well:
4953 <listitem><para>General syntax</para></listitem>
4954 <listitem><para>Comment format</para></listitem>
4957 Service description files must contain a
4958 <literal>D-BUS Service</literal> group with at least the keys
4959 <literal>Name</literal> (the well-known name of the service)
4960 and <literal>Exec</literal> (the command to be executed).
4963 <title>Example service description file</title>
4965 # Sample service description file
4967 Name=com.example.ConfigurationDatabase
4968 Exec=/usr/bin/sample-configd
4974 Additionally, service description files for the well-known system
4975 bus on Unix must contain a <literal>User</literal> key, whose value
4976 is the name of a user account (e.g. <literal>root</literal>).
4977 The system service will be run as that user.
4981 When an application asks to start a service by name, the bus daemon tries to
4982 find a service that will own that name. It then tries to spawn the
4983 executable associated with it. If this fails, it will report an
4988 On the well-known system bus, it is not possible for two .service files
4989 in the same directory to offer the same service, because they are
4990 constrained to have names that match the service name.
4994 On the well-known session bus, if two .service files in the same
4995 directory offer the same service name, the result is undefined.
4996 Distributors should avoid this situation, for instance by naming
4997 session services' .service files according to their service name.
5001 If two .service files in different directories offer the same
5002 service name, the one in the higher-priority directory is used:
5003 for instance, on the system bus, .service files in
5004 /usr/local/share/dbus-1/system-services take precedence over those
5005 in /usr/share/dbus-1/system-services.
5008 The executable launched will have the environment variable
5009 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
5010 message bus so it can connect and request the appropriate names.
5013 The executable being launched may want to know whether the message bus
5014 starting it is one of the well-known message buses (see <xref
5015 linkend="message-bus-types"/>). To facilitate this, the bus must also set
5016 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
5017 of the well-known buses. The currently-defined values for this variable
5018 are <literal>system</literal> for the systemwide message bus,
5019 and <literal>session</literal> for the per-login-session message
5020 bus. The new executable must still connect to the address given
5021 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
5022 resulting connection is to the well-known bus.
5025 [FIXME there should be a timeout somewhere, either specified
5026 in the .service file, by the client, or just a global value
5027 and if the client being activated fails to connect within that
5028 timeout, an error should be sent back.]
5031 <sect3 id="message-bus-starting-services-scope">
5032 <title>Message Bus Service Scope</title>
5034 The "scope" of a service is its "per-", such as per-session,
5035 per-machine, per-home-directory, or per-display. The reference
5036 implementation doesn't yet support starting services in a different
5037 scope from the message bus itself. So e.g. if you start a service
5038 on the session bus its scope is per-session.
5041 We could add an optional scope to a bus name. For example, for
5042 per-(display,session pair), we could have a unique ID for each display
5043 generated automatically at login and set on screen 0 by executing a
5044 special "set display ID" binary. The ID would be stored in a
5045 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
5046 random bytes. This ID would then be used to scope names.
5047 Starting/locating a service could be done by ID-name pair rather than
5051 Contrast this with a per-display scope. To achieve that, we would
5052 want a single bus spanning all sessions using a given display.
5053 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
5054 property on screen 0 of the display, pointing to this bus.
5059 <sect2 id="message-bus-types">
5060 <title>Well-known Message Bus Instances</title>
5062 Two standard message bus instances are defined here, along with how
5063 to locate them and where their service files live.
5065 <sect3 id="message-bus-types-login">
5066 <title>Login session message bus</title>
5068 Each time a user logs in, a <firstterm>login session message
5069 bus</firstterm> may be started. All applications in the user's login
5070 session may interact with one another using this message bus.
5073 The address of the login session message bus is given
5074 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
5075 variable. If that variable is not set, applications may
5076 also try to read the address from the X Window System root
5077 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
5078 The root window property must have type <literal>STRING</literal>.
5079 The environment variable should have precedence over the
5080 root window property.
5082 <para>The address of the login session message bus is given in the
5083 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
5084 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
5085 "autolaunch:", the system should use platform-specific methods of
5086 locating a running D-Bus session server, or starting one if a running
5087 instance cannot be found. Note that this mechanism is not recommended
5088 for attempting to determine if a daemon is running. It is inherently
5089 racy to attempt to make this determination, since the bus daemon may
5090 be started just before or just after the determination is made.
5091 Therefore, it is recommended that applications do not try to make this
5092 determination for their functionality purposes, and instead they
5093 should attempt to start the server.</para>
5095 <sect4 id="message-bus-types-login-x-windows">
5096 <title>X Windowing System</title>
5098 For the X Windowing System, the application must locate the
5099 window owner of the selection represented by the atom formed by
5103 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5107 <para>the current user's username</para>
5111 <para>the literal character '_' (underscore)</para>
5115 <para>the machine's ID</para>
5121 The following properties are defined for the window that owns
5123 <informaltable frame="all">
5132 <para>meaning</para>
5138 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5142 <para>the actual address of the server socket</para>
5148 <para>_DBUS_SESSION_BUS_PID</para>
5152 <para>the PID of the server process</para>
5161 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5162 present in this window.
5166 If the X selection cannot be located or if reading the
5167 properties from the window fails, the implementation MUST conclude
5168 that there is no D-Bus server running and proceed to start a new
5169 server. (See below on concurrency issues)
5173 Failure to connect to the D-Bus server address thus obtained
5174 MUST be treated as a fatal connection error and should be reported
5179 As an alternative, an implementation MAY find the information
5180 in the following file located in the current user's home directory,
5181 in subdirectory .dbus/session-bus/:
5184 <para>the machine's ID</para>
5188 <para>the literal character '-' (dash)</para>
5192 <para>the X display without the screen number, with the
5193 following prefixes removed, if present: ":", "localhost:"
5194 ."localhost.localdomain:". That is, a display of
5195 "localhost:10.0" produces just the number "10"</para>
5201 The contents of this file NAME=value assignment pairs and
5202 lines starting with # are comments (no comments are allowed
5203 otherwise). The following variable names are defined:
5210 <para>Variable</para>
5214 <para>meaning</para>
5220 <para>DBUS_SESSION_BUS_ADDRESS</para>
5224 <para>the actual address of the server socket</para>
5230 <para>DBUS_SESSION_BUS_PID</para>
5234 <para>the PID of the server process</para>
5240 <para>DBUS_SESSION_BUS_WINDOWID</para>
5244 <para>the window ID</para>
5253 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5258 Failure to open this file MUST be interpreted as absence of a
5259 running server. Therefore, the implementation MUST proceed to
5260 attempting to launch a new bus server if the file cannot be
5265 However, success in opening this file MUST NOT lead to the
5266 conclusion that the server is running. Thus, a failure to connect to
5267 the bus address obtained by the alternative method MUST NOT be
5268 considered a fatal error. If the connection cannot be established,
5269 the implementation MUST proceed to check the X selection settings or
5270 to start the server on its own.
5274 If the implementation concludes that the D-Bus server is not
5275 running it MUST attempt to start a new server and it MUST also
5276 ensure that the daemon started as an effect of the "autolaunch"
5277 mechanism provides the lookup mechanisms described above, so
5278 subsequent calls can locate the newly started server. The
5279 implementation MUST also ensure that if two or more concurrent
5280 initiations happen, only one server remains running and all other
5281 initiations are able to obtain the address of this server and
5282 connect to it. In other words, the implementation MUST ensure that
5283 the X selection is not present when it attempts to set it, without
5284 allowing another process to set the selection between the
5285 verification and the setting (e.g., by using XGrabServer /
5292 On Unix systems, the session bus should search for .service files
5293 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5295 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5296 Implementations may also search additional locations, which
5297 should be searched with lower priority than anything in
5298 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5299 for example, the reference implementation also
5300 looks in <literal>${datadir}/dbus-1/services</literal> as
5301 set at compile time.
5304 As described in the XDG Base Directory Specification, software
5305 packages should install their session .service files to their
5306 configured <literal>${datadir}/dbus-1/services</literal>,
5307 where <literal>${datadir}</literal> is as defined by the GNU
5308 coding standards. System administrators or users can arrange
5309 for these service files to be read by setting XDG_DATA_DIRS or by
5310 symlinking them into the default locations.
5314 <sect3 id="message-bus-types-system">
5315 <title>System message bus</title>
5317 A computer may have a <firstterm>system message bus</firstterm>,
5318 accessible to all applications on the system. This message bus may be
5319 used to broadcast system events, such as adding new hardware devices,
5320 changes in the printer queue, and so forth.
5323 The address of the system message bus is given
5324 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5325 variable. If that variable is not set, applications should try
5326 to connect to the well-known address
5327 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5330 The D-Bus reference implementation actually honors the
5331 <literal>$(localstatedir)</literal> configure option
5332 for this address, on both client and server side.
5337 On Unix systems, the system bus should default to searching
5338 for .service files in
5339 <literal>/usr/local/share/dbus-1/system-services</literal>,
5340 <literal>/usr/share/dbus-1/system-services</literal> and
5341 <literal>/lib/dbus-1/system-services</literal>, with that order
5342 of precedence. It may also search other implementation-specific
5343 locations, but should not vary these locations based on environment
5347 The system bus is security-sensitive and is typically executed
5348 by an init system with a clean environment. Its launch helper
5349 process is particularly security-sensitive, and specifically
5350 clears its own environment.
5355 Software packages should install their system .service
5356 files to their configured
5357 <literal>${datadir}/dbus-1/system-services</literal>,
5358 where <literal>${datadir}</literal> is as defined by the GNU
5359 coding standards. System administrators can arrange
5360 for these service files to be read by editing the system bus'
5361 configuration file or by symlinking them into the default
5367 <sect2 id="message-bus-messages">
5368 <title>Message Bus Messages</title>
5370 The special message bus name <literal>org.freedesktop.DBus</literal>
5371 responds to a number of additional messages.
5374 <sect3 id="bus-messages-hello">
5375 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5386 <entry>Argument</entry>
5388 <entry>Description</entry>
5394 <entry>STRING</entry>
5395 <entry>Unique name assigned to the connection</entry>
5402 Before an application is able to send messages to other applications
5403 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5404 to the message bus to obtain a unique name. If an application without
5405 a unique name tries to send a message to another application, or a
5406 message to the message bus itself that isn't the
5407 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5408 disconnected from the bus.
5411 There is no corresponding "disconnect" request; if a client wishes to
5412 disconnect from the bus, it simply closes the socket (or other
5413 communication channel).
5416 <sect3 id="bus-messages-list-names">
5417 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5421 ARRAY of STRING ListNames ()
5428 <entry>Argument</entry>
5430 <entry>Description</entry>
5436 <entry>ARRAY of STRING</entry>
5437 <entry>Array of strings where each string is a bus name</entry>
5444 Returns a list of all currently-owned names on the bus.
5447 <sect3 id="bus-messages-list-activatable-names">
5448 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5452 ARRAY of STRING ListActivatableNames ()
5459 <entry>Argument</entry>
5461 <entry>Description</entry>
5467 <entry>ARRAY of STRING</entry>
5468 <entry>Array of strings where each string is a bus name</entry>
5475 Returns a list of all names that can be activated on the bus.
5478 <sect3 id="bus-messages-name-exists">
5479 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5483 BOOLEAN NameHasOwner (in STRING name)
5490 <entry>Argument</entry>
5492 <entry>Description</entry>
5498 <entry>STRING</entry>
5499 <entry>Name to check</entry>
5509 <entry>Argument</entry>
5511 <entry>Description</entry>
5517 <entry>BOOLEAN</entry>
5518 <entry>Return value, true if the name exists</entry>
5525 Checks if the specified name exists (currently has an owner).
5529 <sect3 id="bus-messages-name-owner-changed">
5530 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5534 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5541 <entry>Argument</entry>
5543 <entry>Description</entry>
5549 <entry>STRING</entry>
5550 <entry>Name with a new owner</entry>
5554 <entry>STRING</entry>
5555 <entry>Old owner or empty string if none</entry>
5559 <entry>STRING</entry>
5560 <entry>New owner or empty string if none</entry>
5567 This signal indicates that the owner of a name has changed.
5568 It's also the signal to use to detect the appearance of
5569 new names on the bus.
5572 <sect3 id="bus-messages-name-lost">
5573 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5577 NameLost (STRING name)
5584 <entry>Argument</entry>
5586 <entry>Description</entry>
5592 <entry>STRING</entry>
5593 <entry>Name which was lost</entry>
5600 This signal is sent to a specific application when it loses
5601 ownership of a name.
5605 <sect3 id="bus-messages-name-acquired">
5606 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5610 NameAcquired (STRING name)
5617 <entry>Argument</entry>
5619 <entry>Description</entry>
5625 <entry>STRING</entry>
5626 <entry>Name which was acquired</entry>
5633 This signal is sent to a specific application when it gains
5634 ownership of a name.
5638 <sect3 id="bus-messages-start-service-by-name">
5639 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5643 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5650 <entry>Argument</entry>
5652 <entry>Description</entry>
5658 <entry>STRING</entry>
5659 <entry>Name of the service to start</entry>
5663 <entry>UINT32</entry>
5664 <entry>Flags (currently not used)</entry>
5674 <entry>Argument</entry>
5676 <entry>Description</entry>
5682 <entry>UINT32</entry>
5683 <entry>Return value</entry>
5688 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5692 The return value can be one of the following values:
5697 <entry>Identifier</entry>
5698 <entry>Value</entry>
5699 <entry>Description</entry>
5704 <entry>DBUS_START_REPLY_SUCCESS</entry>
5706 <entry>The service was successfully started.</entry>
5709 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5711 <entry>A connection already owns the given name.</entry>
5720 <sect3 id="bus-messages-update-activation-environment">
5721 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5725 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5732 <entry>Argument</entry>
5734 <entry>Description</entry>
5740 <entry>ARRAY of DICT<STRING,STRING></entry>
5741 <entry>Environment to add or update</entry>
5746 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5749 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5752 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.
5757 <sect3 id="bus-messages-get-name-owner">
5758 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5762 STRING GetNameOwner (in STRING name)
5769 <entry>Argument</entry>
5771 <entry>Description</entry>
5777 <entry>STRING</entry>
5778 <entry>Name to get the owner of</entry>
5788 <entry>Argument</entry>
5790 <entry>Description</entry>
5796 <entry>STRING</entry>
5797 <entry>Return value, a unique connection name</entry>
5802 Returns the unique connection name of the primary owner of the name
5803 given. If the requested name doesn't have an owner, returns a
5804 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5808 <sect3 id="bus-messages-get-connection-unix-user">
5809 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5813 UINT32 GetConnectionUnixUser (in STRING bus_name)
5820 <entry>Argument</entry>
5822 <entry>Description</entry>
5828 <entry>STRING</entry>
5829 <entry>Unique or well-known bus name of the connection to
5830 query, such as <literal>:12.34</literal> or
5831 <literal>com.example.tea</literal></entry>
5841 <entry>Argument</entry>
5843 <entry>Description</entry>
5849 <entry>UINT32</entry>
5850 <entry>Unix user ID</entry>
5855 Returns the Unix user ID of the process connected to the server. If
5856 unable to determine it (for instance, because the process is not on the
5857 same machine as the bus daemon), an error is returned.
5861 <sect3 id="bus-messages-get-connection-unix-process-id">
5862 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5866 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5873 <entry>Argument</entry>
5875 <entry>Description</entry>
5881 <entry>STRING</entry>
5882 <entry>Unique or well-known bus name of the connection to
5883 query, such as <literal>:12.34</literal> or
5884 <literal>com.example.tea</literal></entry>
5894 <entry>Argument</entry>
5896 <entry>Description</entry>
5902 <entry>UINT32</entry>
5903 <entry>Unix process id</entry>
5908 Returns the Unix process ID of the process connected to the server. If
5909 unable to determine it (for instance, because the process is not on the
5910 same machine as the bus daemon), an error is returned.
5914 <sect3 id="bus-messages-get-connection-credentials">
5915 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5919 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5926 <entry>Argument</entry>
5928 <entry>Description</entry>
5934 <entry>STRING</entry>
5935 <entry>Unique or well-known bus name of the connection to
5936 query, such as <literal>:12.34</literal> or
5937 <literal>com.example.tea</literal></entry>
5947 <entry>Argument</entry>
5949 <entry>Description</entry>
5955 <entry>DICT<STRING,VARIANT></entry>
5956 <entry>Credentials</entry>
5964 Returns as many credentials as possible for the process connected to
5965 the server. If unable to determine certain credentials (for instance,
5966 because the process is not on the same machine as the bus daemon,
5967 or because this version of the bus daemon does not support a
5968 particular security framework), or if the values of those credentials
5969 cannot be represented as documented here, then those credentials
5974 Keys in the returned dictionary not containing "." are defined
5975 by this specification. Bus daemon implementors supporting
5976 credentials frameworks not mentioned in this document should either
5977 contribute patches to this specification, or use keys containing
5978 "." and starting with a reversed domain name.
5984 <entry>Value type</entry>
5985 <entry>Value</entry>
5990 <entry>UnixUserID</entry>
5991 <entry>UINT32</entry>
5992 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5995 <entry>ProcessID</entry>
5996 <entry>UINT32</entry>
5997 <entry>The numeric process ID, on platforms that have
5998 this concept. On Unix, this is the process ID defined by
6007 This method was added in D-Bus 1.7 to reduce the round-trips
6008 required to list a process's credentials. In older versions, calling
6009 this method will fail: applications should recover by using the
6010 separate methods such as
6011 <xref linkend="bus-messages-get-connection-unix-user"/>
6016 <sect3 id="bus-messages-get-adt-audit-session-data">
6017 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
6021 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
6028 <entry>Argument</entry>
6030 <entry>Description</entry>
6036 <entry>STRING</entry>
6037 <entry>Unique or well-known bus name of the connection to
6038 query, such as <literal>:12.34</literal> or
6039 <literal>com.example.tea</literal></entry>
6049 <entry>Argument</entry>
6051 <entry>Description</entry>
6057 <entry>ARRAY of BYTE</entry>
6058 <entry>auditing data as returned by
6059 adt_export_session_data()</entry>
6064 Returns auditing data used by Solaris ADT, in an unspecified
6065 binary format. If you know what this means, please contribute
6066 documentation via the D-Bus bug tracking system.
6067 This method is on the core DBus interface for historical reasons;
6068 the same information should be made available via
6069 <xref linkend="bus-messages-get-connection-credentials"/>
6074 <sect3 id="bus-messages-get-connection-selinux-security-context">
6075 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
6079 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
6086 <entry>Argument</entry>
6088 <entry>Description</entry>
6094 <entry>STRING</entry>
6095 <entry>Unique or well-known bus name of the connection to
6096 query, such as <literal>:12.34</literal> or
6097 <literal>com.example.tea</literal></entry>
6107 <entry>Argument</entry>
6109 <entry>Description</entry>
6115 <entry>ARRAY of BYTE</entry>
6116 <entry>some sort of string of bytes, not necessarily UTF-8,
6117 not including '\0'</entry>
6122 Returns the security context used by SELinux, in an unspecified
6123 format. If you know what this means, please contribute
6124 documentation via the D-Bus bug tracking system.
6125 This method is on the core DBus interface for historical reasons;
6126 the same information should be made available via
6127 <xref linkend="bus-messages-get-connection-credentials"/>
6133 <sect3 id="bus-messages-add-match">
6134 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6138 AddMatch (in STRING rule)
6145 <entry>Argument</entry>
6147 <entry>Description</entry>
6153 <entry>STRING</entry>
6154 <entry>Match rule to add to the connection</entry>
6159 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6160 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6164 <sect3 id="bus-messages-remove-match">
6165 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6169 RemoveMatch (in STRING rule)
6176 <entry>Argument</entry>
6178 <entry>Description</entry>
6184 <entry>STRING</entry>
6185 <entry>Match rule to remove from the connection</entry>
6190 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6191 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6196 <sect3 id="bus-messages-get-id">
6197 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6201 GetId (out STRING id)
6208 <entry>Argument</entry>
6210 <entry>Description</entry>
6216 <entry>STRING</entry>
6217 <entry>Unique ID identifying the bus daemon</entry>
6222 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6223 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6224 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6225 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6226 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6227 by org.freedesktop.DBus.Peer.GetMachineId().
6228 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6236 <appendix id="implementation-notes">
6237 <title>Implementation notes</title>
6238 <sect1 id="implementation-notes-subsection">
6246 <glossary><title>Glossary</title>
6248 This glossary defines some of the terms used in this specification.
6251 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6254 The message bus maintains an association between names and
6255 connections. (Normally, there's one connection per application.) A
6256 bus name is simply an identifier used to locate connections. For
6257 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6258 name might be used to send a message to a screensaver from Yoyodyne
6259 Corporation. An application is said to <firstterm>own</firstterm> a
6260 name if the message bus has associated the application's connection
6261 with the name. Names may also have <firstterm>queued
6262 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6263 The bus assigns a unique name to each connection,
6264 see <xref linkend="term-unique-name"/>. Other names
6265 can be thought of as "well-known names" and are
6266 used to find applications that offer specific functionality.
6270 See <xref linkend="message-protocol-names-bus"/> for details of
6271 the syntax and naming conventions for bus names.
6276 <glossentry id="term-message"><glossterm>Message</glossterm>
6279 A message is the atomic unit of communication via the D-Bus
6280 protocol. It consists of a <firstterm>header</firstterm> and a
6281 <firstterm>body</firstterm>; the body is made up of
6282 <firstterm>arguments</firstterm>.
6287 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6290 The message bus is a special application that forwards
6291 or routes messages between a group of applications
6292 connected to the message bus. It also manages
6293 <firstterm>names</firstterm> used for routing
6299 <glossentry id="term-name"><glossterm>Name</glossterm>
6302 See <xref linkend="term-bus-name"/>. "Name" may
6303 also be used to refer to some of the other names
6304 in D-Bus, such as interface names.
6309 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6312 Used to prevent collisions when defining new interfaces, bus names
6313 etc. The convention used is the same one Java uses for defining
6314 classes: a reversed domain name.
6315 See <xref linkend="message-protocol-names-bus"/>,
6316 <xref linkend="message-protocol-names-interface"/>,
6317 <xref linkend="message-protocol-names-error"/>,
6318 <xref linkend="message-protocol-marshaling-object-path"/>.
6323 <glossentry id="term-object"><glossterm>Object</glossterm>
6326 Each application contains <firstterm>objects</firstterm>, which have
6327 <firstterm>interfaces</firstterm> and
6328 <firstterm>methods</firstterm>. Objects are referred to by a name,
6329 called a <firstterm>path</firstterm>.
6334 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6337 An application talking directly to another application, without going
6338 through a message bus. One-to-one connections may be "peer to peer" or
6339 "client to server." The D-Bus protocol has no concept of client
6340 vs. server after a connection has authenticated; the flow of messages
6341 is symmetrical (full duplex).
6346 <glossentry id="term-path"><glossterm>Path</glossterm>
6349 Object references (object names) in D-Bus are organized into a
6350 filesystem-style hierarchy, so each object is named by a path. As in
6351 LDAP, there's no difference between "files" and "directories"; a path
6352 can refer to an object, while still having child objects below it.
6357 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6360 Each bus name has a primary owner; messages sent to the name go to the
6361 primary owner. However, certain names also maintain a queue of
6362 secondary owners "waiting in the wings." If the primary owner releases
6363 the name, then the first secondary owner in the queue automatically
6364 becomes the new owner of the name.
6369 <glossentry id="term-service"><glossterm>Service</glossterm>
6372 A service is an executable that can be launched by the bus daemon.
6373 Services normally guarantee some particular features, for example they
6374 may guarantee that they will request a specific name such as
6375 "com.example.Screensaver", have a singleton object
6376 "/com/example/Application", and that object will implement the
6377 interface "com.example.Screensaver.Control".
6382 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6385 ".service files" tell the bus about service applications that can be
6386 launched (see <xref linkend="term-service"/>). Most importantly they
6387 provide a mapping from bus names to services that will request those
6388 names when they start up.
6393 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6396 The special name automatically assigned to each connection by the
6397 message bus. This name will never change owner, and will be unique
6398 (never reused during the lifetime of the message bus).
6399 It will begin with a ':' character.