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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.24</releaseinfo>
10 <date>2014-10-01</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.24</revnumber>
75 <date>2014-10-01</date>
76 <authorinitials>SMcV</authorinitials>
78 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
79 document how to quote match rules
83 <revnumber>0.23</revnumber>
84 <date>2014-01-06</date>
85 <authorinitials>SMcV, CY</authorinitials>
87 method call messages with no INTERFACE may be considered an error;
88 document tcp:bind=... and nonce-tcp:bind=...; define listenable
89 and connectable addresses
93 <revnumber>0.22</revnumber>
94 <date>2013-10-09</date>
95 <authorinitials></authorinitials>
96 <revremark>add GetConnectionCredentials, document
97 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
98 document and correct .service file syntax and naming
102 <revnumber>0.21</revnumber>
103 <date>2013-04-25</date>
104 <authorinitials>smcv</authorinitials>
105 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
106 Corrigendum #9)</revremark>
109 <revnumber>0.20</revnumber>
110 <date>22 February 2013</date>
111 <authorinitials>smcv, walters</authorinitials>
112 <revremark>reorganise for clarity, remove false claims about
113 basic types, mention /o/fd/DBus</revremark>
116 <revnumber>0.19</revnumber>
117 <date>20 February 2012</date>
118 <authorinitials>smcv/lp</authorinitials>
119 <revremark>formally define unique connection names and well-known
120 bus names; document best practices for interface, bus, member and
121 error names, and object paths; document the search path for session
122 and system services on Unix; document the systemd transport</revremark>
125 <revnumber>0.18</revnumber>
126 <date>29 July 2011</date>
127 <authorinitials>smcv</authorinitials>
128 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
129 match keyword; promote type system to a top-level section</revremark>
132 <revnumber>0.17</revnumber>
133 <date>1 June 2011</date>
134 <authorinitials>smcv/davidz</authorinitials>
135 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
136 by GVariant</revremark>
139 <revnumber>0.16</revnumber>
140 <date>11 April 2011</date>
141 <authorinitials></authorinitials>
142 <revremark>add path_namespace, arg0namespace; argNpath matches object
146 <revnumber>0.15</revnumber>
147 <date>3 November 2010</date>
148 <authorinitials></authorinitials>
149 <revremark></revremark>
152 <revnumber>0.14</revnumber>
153 <date>12 May 2010</date>
154 <authorinitials></authorinitials>
155 <revremark></revremark>
158 <revnumber>0.13</revnumber>
159 <date>23 Dezember 2009</date>
160 <authorinitials></authorinitials>
161 <revremark></revremark>
164 <revnumber>0.12</revnumber>
165 <date>7 November, 2006</date>
166 <authorinitials></authorinitials>
167 <revremark></revremark>
170 <revnumber>0.11</revnumber>
171 <date>6 February 2005</date>
172 <authorinitials></authorinitials>
173 <revremark></revremark>
176 <revnumber>0.10</revnumber>
177 <date>28 January 2005</date>
178 <authorinitials></authorinitials>
179 <revremark></revremark>
182 <revnumber>0.9</revnumber>
183 <date>7 Januar 2005</date>
184 <authorinitials></authorinitials>
185 <revremark></revremark>
188 <revnumber>0.8</revnumber>
189 <date>06 September 2003</date>
190 <authorinitials></authorinitials>
191 <revremark>First released document.</revremark>
196 <sect1 id="introduction">
197 <title>Introduction</title>
199 D-Bus is a system for low-overhead, easy to use
200 interprocess communication (IPC). In more detail:
204 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
205 binary protocol, and does not have to convert to and from a text
206 format such as XML. Because D-Bus is intended for potentially
207 high-resolution same-machine IPC, not primarily for Internet IPC,
208 this is an interesting optimization. D-Bus is also designed to
209 avoid round trips and allow asynchronous operation, much like
215 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
216 of <firstterm>messages</firstterm> rather than byte streams, and
217 automatically handles a lot of the hard IPC issues. Also, the D-Bus
218 library is designed to be wrapped in a way that lets developers use
219 their framework's existing object/type system, rather than learning
220 a new one specifically for IPC.
227 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
228 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
229 a system for one application to talk to a single other
230 application. However, the primary intended application of the protocol is the
231 D-Bus <firstterm>message bus</firstterm>, specified in <xref
232 linkend="message-bus"/>. The message bus is a special application that
233 accepts connections from multiple other applications, and forwards
238 Uses of D-Bus include notification of system changes (notification of when
239 a camera is plugged in to a computer, or a new version of some software
240 has been installed), or desktop interoperability, for example a file
241 monitoring service or a configuration service.
245 D-Bus is designed for two specific use cases:
249 A "system bus" for notifications from the system to user sessions,
250 and to allow the system to request input from user sessions.
255 A "session bus" used to implement desktop environments such as
260 D-Bus is not intended to be a generic IPC system for any possible
261 application, and intentionally omits many features found in other
262 IPC systems for this reason.
266 At the same time, the bus daemons offer a number of features not found in
267 other IPC systems, such as single-owner "bus names" (similar to X
268 selections), on-demand startup of services, and security policies.
269 In many ways, these features are the primary motivation for developing
270 D-Bus; other systems would have sufficed if IPC were the only goal.
274 D-Bus may turn out to be useful in unanticipated applications, but future
275 versions of this spec and the reference implementation probably will not
276 incorporate features that interfere with the core use cases.
280 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
281 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
282 document are to be interpreted as described in RFC 2119. However, the
283 document could use a serious audit to be sure it makes sense to do
284 so. Also, they are not capitalized.
287 <sect2 id="stability">
288 <title>Protocol and Specification Stability</title>
290 The D-Bus protocol is frozen (only compatible extensions are allowed) as
291 of November 8, 2006. However, this specification could still use a fair
292 bit of work to make interoperable reimplementation possible without
293 reference to the D-Bus reference implementation. Thus, this
294 specification is not marked 1.0. To mark it 1.0, we'd like to see
295 someone invest significant effort in clarifying the specification
296 language, and growing the specification to cover more aspects of the
297 reference implementation's behavior.
300 Until this work is complete, any attempt to reimplement D-Bus will
301 probably require looking at the reference implementation and/or asking
302 questions on the D-Bus mailing list about intended behavior.
303 Questions on the list are very welcome.
306 Nonetheless, this document should be a useful starting point and is
307 to our knowledge accurate, though incomplete.
313 <sect1 id="type-system">
314 <title>Type System</title>
317 D-Bus has a type system, in which values of various types can be
318 serialized into a sequence of bytes referred to as the
319 <firstterm>wire format</firstterm> in a standard way.
320 Converting a value from some other representation into the wire
321 format is called <firstterm>marshaling</firstterm> and converting
322 it back from the wire format is <firstterm>unmarshaling</firstterm>.
326 The D-Bus protocol does not include type tags in the marshaled data; a
327 block of marshaled values must have a known <firstterm>type
328 signature</firstterm>. The type signature is made up of zero or more
329 <firstterm id="term-single-complete-type">single complete
330 types</firstterm>, each made up of one or more
331 <firstterm>type codes</firstterm>.
335 A type code is an ASCII character representing the
336 type of a value. Because ASCII characters are used, the type signature
337 will always form a valid ASCII string. A simple string compare
338 determines whether two type signatures are equivalent.
342 A single complete type is a sequence of type codes that fully describes
343 one type: either a basic type, or a single fully-described container type.
344 A single complete type is a basic type code, a variant type code,
345 an array with its element type, or a struct with its fields (all of which
346 are defined below). So the following signatures are not single complete
357 And the following signatures contain multiple complete types:
367 Note however that a single complete type may <emphasis>contain</emphasis>
368 multiple other single complete types, by containing a struct or dict
372 <sect2 id="basic-types">
373 <title>Basic types</title>
376 The simplest type codes are the <firstterm id="term-basic-type">basic
377 types</firstterm>, which are the types whose structure is entirely
378 defined by their 1-character type code. Basic types consist of
379 fixed types and string-like types.
383 The <firstterm id="term-fixed-type">fixed types</firstterm>
384 are basic types whose values have a fixed length, namely BYTE,
385 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
390 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
391 the ASCII character 'i'. So the signature for a block of values
392 containing a single <literal>INT32</literal> would be:
396 A block of values containing two <literal>INT32</literal> would have this signature:
403 The characteristics of the fixed types are listed in this table.
409 <entry>Conventional name</entry>
410 <entry>ASCII type-code</entry>
411 <entry>Encoding</entry>
416 <entry><literal>BYTE</literal></entry>
417 <entry><literal>y</literal> (121)</entry>
418 <entry>Unsigned 8-bit integer</entry>
421 <entry><literal>BOOLEAN</literal></entry>
422 <entry><literal>b</literal> (98)</entry>
423 <entry>Boolean value: 0 is false, 1 is true, any other value
424 allowed by the marshalling format is invalid</entry>
427 <entry><literal>INT16</literal></entry>
428 <entry><literal>n</literal> (110)</entry>
429 <entry>Signed (two's complement) 16-bit integer</entry>
432 <entry><literal>UINT16</literal></entry>
433 <entry><literal>q</literal> (113)</entry>
434 <entry>Unsigned 16-bit integer</entry>
437 <entry><literal>INT32</literal></entry>
438 <entry><literal>i</literal> (105)</entry>
439 <entry>Signed (two's complement) 32-bit integer</entry>
442 <entry><literal>UINT32</literal></entry>
443 <entry><literal>u</literal> (117)</entry>
444 <entry>Unsigned 32-bit integer</entry>
447 <entry><literal>INT64</literal></entry>
448 <entry><literal>x</literal> (120)</entry>
449 <entry>Signed (two's complement) 64-bit integer
450 (mnemonic: x and t are the first characters in "sixty" not
451 already used for something more common)</entry>
454 <entry><literal>UINT64</literal></entry>
455 <entry><literal>t</literal> (116)</entry>
456 <entry>Unsigned 64-bit integer</entry>
459 <entry><literal>DOUBLE</literal></entry>
460 <entry><literal>d</literal> (100)</entry>
461 <entry>IEEE 754 double-precision floating point</entry>
464 <entry><literal>UNIX_FD</literal></entry>
465 <entry><literal>h</literal> (104)</entry>
466 <entry>Unsigned 32-bit integer representing an index into an
467 out-of-band array of file descriptors, transferred via some
468 platform-specific mechanism (mnemonic: h for handle)</entry>
476 The <firstterm id="term-string-like-type">string-like types</firstterm>
477 are basic types with a variable length. The value of any string-like
478 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
479 none of which may be U+0000. The UTF-8 text must be validated
480 strictly: in particular, it must not contain overlong sequences
481 or codepoints above U+10FFFF.
485 Since D-Bus Specification version 0.21, in accordance with Unicode
486 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
487 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
488 D-Bus rejected these noncharacters).
492 The marshalling formats for the string-like types all end with a
493 single zero (NUL) byte, but that byte is not considered to be part of
498 The characteristics of the string-like types are listed in this table.
504 <entry>Conventional name</entry>
505 <entry>ASCII type-code</entry>
506 <entry>Validity constraints</entry>
511 <entry><literal>STRING</literal></entry>
512 <entry><literal>s</literal> (115)</entry>
513 <entry>No extra constraints</entry>
516 <entry><literal>OBJECT_PATH</literal></entry>
517 <entry><literal>o</literal> (111)</entry>
519 <link linkend="message-protocol-marshaling-object-path">a
520 syntactically valid object path</link></entry>
523 <entry><literal>SIGNATURE</literal></entry>
524 <entry><literal>g</literal> (103)</entry>
526 <firstterm linkend="term-single-complete-type">single
527 complete types</firstterm></entry>
534 <sect3 id="message-protocol-marshaling-object-path">
535 <title>Valid Object Paths</title>
538 An object path is a name used to refer to an object instance.
539 Conceptually, each participant in a D-Bus message exchange may have
540 any number of object instances (think of C++ or Java objects) and each
541 such instance will have a path. Like a filesystem, the object
542 instances in an application form a hierarchical tree.
546 Object paths are often namespaced by starting with a reversed
547 domain name and containing an interface version number, in the
549 <link linkend="message-protocol-names-interface">interface
551 <link linkend="message-protocol-names-bus">well-known
553 This makes it possible to implement more than one service, or
554 more than one version of a service, in the same process,
555 even if the services share a connection but cannot otherwise
556 co-operate (for instance, if they are implemented by different
561 For instance, if the owner of <literal>example.com</literal> is
562 developing a D-Bus API for a music player, they might use the
563 hierarchy of object paths that start with
564 <literal>/com/example/MusicPlayer1</literal> for its objects.
568 The following rules define a valid object path. Implementations must
569 not send or accept messages with invalid object paths.
573 The path may be of any length.
578 The path must begin with an ASCII '/' (integer 47) character,
579 and must consist of elements separated by slash characters.
584 Each element must only contain the ASCII characters
590 No element may be the empty string.
595 Multiple '/' characters cannot occur in sequence.
600 A trailing '/' character is not allowed unless the
601 path is the root path (a single '/' character).
609 <sect3 id="message-protocol-marshaling-signature">
610 <title>Valid Signatures</title>
612 An implementation must not send or accept invalid signatures.
613 Valid signatures will conform to the following rules:
617 The signature is a list of single complete types.
618 Arrays must have element types, and structs must
619 have both open and close parentheses.
624 Only type codes, open and close parentheses, and open and
625 close curly brackets are allowed in the signature. The
626 <literal>STRUCT</literal> type code
627 is not allowed in signatures, because parentheses
628 are used instead. Similarly, the
629 <literal>DICT_ENTRY</literal> type code is not allowed in
630 signatures, because curly brackets are used instead.
635 The maximum depth of container type nesting is 32 array type
636 codes and 32 open parentheses. This implies that the maximum
637 total depth of recursion is 64, for an "array of array of array
638 of ... struct of struct of struct of ..." where there are 32
644 The maximum length of a signature is 255.
651 When signatures appear in messages, the marshalling format
652 guarantees that they will be followed by a nul byte (which can
653 be interpreted as either C-style string termination or the INVALID
654 type-code), but this is not conceptually part of the signature.
660 <sect2 id="container-types">
661 <title>Container types</title>
664 In addition to basic types, there are four <firstterm>container</firstterm>
665 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
666 and <literal>DICT_ENTRY</literal>.
670 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
671 code does not appear in signatures. Instead, ASCII characters
672 '(' and ')' are used to mark the beginning and end of the struct.
673 So for example, a struct containing two integers would have this
678 Structs can be nested, so for example a struct containing
679 an integer and another struct:
683 The value block storing that struct would contain three integers; the
684 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
689 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
690 but is useful in code that implements the protocol. This type code
691 is specified to allow such code to interoperate in non-protocol contexts.
695 Empty structures are not allowed; there must be at least one
696 type code between the parentheses.
700 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
701 followed by a <firstterm>single complete type</firstterm>. The single
702 complete type following the array is the type of each array element. So
703 the simple example is:
707 which is an array of 32-bit integers. But an array can be of any type,
708 such as this array-of-struct-with-two-int32-fields:
712 Or this array of array of integer:
719 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
720 type <literal>VARIANT</literal> will have the signature of a single complete type as part
721 of the <emphasis>value</emphasis>. This signature will be followed by a
722 marshaled value of that type.
726 Unlike a message signature, the variant signature can
727 contain only a single complete type. So "i", "ai"
728 or "(ii)" is OK, but "ii" is not. Use of variants may not
729 cause a total message depth to be larger than 64, including
730 other container types such as structures.
734 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
735 than parentheses it uses curly braces, and it has more restrictions.
736 The restrictions are: it occurs only as an array element type; it has
737 exactly two single complete types inside the curly braces; the first
738 single complete type (the "key") must be a basic type rather than a
739 container type. Implementations must not accept dict entries outside of
740 arrays, must not accept dict entries with zero, one, or more than two
741 fields, and must not accept dict entries with non-basic-typed keys. A
742 dict entry is always a key-value pair.
746 The first field in the <literal>DICT_ENTRY</literal> is always the key.
747 A message is considered corrupt if the same key occurs twice in the same
748 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
749 implementations are not required to reject dicts with duplicate keys.
753 In most languages, an array of dict entry would be represented as a
754 map, hash table, or dict object.
759 <title>Summary of types</title>
762 The following table summarizes the D-Bus types.
767 <entry>Category</entry>
768 <entry>Conventional Name</entry>
770 <entry>Description</entry>
775 <entry>reserved</entry>
776 <entry><literal>INVALID</literal></entry>
777 <entry>0 (ASCII NUL)</entry>
778 <entry>Not a valid type code, used to terminate signatures</entry>
780 <entry>fixed, basic</entry>
781 <entry><literal>BYTE</literal></entry>
782 <entry>121 (ASCII 'y')</entry>
783 <entry>8-bit unsigned integer</entry>
785 <entry>fixed, basic</entry>
786 <entry><literal>BOOLEAN</literal></entry>
787 <entry>98 (ASCII 'b')</entry>
788 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
790 <entry>fixed, basic</entry>
791 <entry><literal>INT16</literal></entry>
792 <entry>110 (ASCII 'n')</entry>
793 <entry>16-bit signed integer</entry>
795 <entry>fixed, basic</entry>
796 <entry><literal>UINT16</literal></entry>
797 <entry>113 (ASCII 'q')</entry>
798 <entry>16-bit unsigned integer</entry>
800 <entry>fixed, basic</entry>
801 <entry><literal>INT32</literal></entry>
802 <entry>105 (ASCII 'i')</entry>
803 <entry>32-bit signed integer</entry>
805 <entry>fixed, basic</entry>
806 <entry><literal>UINT32</literal></entry>
807 <entry>117 (ASCII 'u')</entry>
808 <entry>32-bit unsigned integer</entry>
810 <entry>fixed, basic</entry>
811 <entry><literal>INT64</literal></entry>
812 <entry>120 (ASCII 'x')</entry>
813 <entry>64-bit signed integer</entry>
815 <entry>fixed, basic</entry>
816 <entry><literal>UINT64</literal></entry>
817 <entry>116 (ASCII 't')</entry>
818 <entry>64-bit unsigned integer</entry>
820 <entry>fixed, basic</entry>
821 <entry><literal>DOUBLE</literal></entry>
822 <entry>100 (ASCII 'd')</entry>
823 <entry>IEEE 754 double</entry>
825 <entry>string-like, basic</entry>
826 <entry><literal>STRING</literal></entry>
827 <entry>115 (ASCII 's')</entry>
828 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
830 <entry>string-like, basic</entry>
831 <entry><literal>OBJECT_PATH</literal></entry>
832 <entry>111 (ASCII 'o')</entry>
833 <entry>Name of an object instance</entry>
835 <entry>string-like, basic</entry>
836 <entry><literal>SIGNATURE</literal></entry>
837 <entry>103 (ASCII 'g')</entry>
838 <entry>A type signature</entry>
840 <entry>container</entry>
841 <entry><literal>ARRAY</literal></entry>
842 <entry>97 (ASCII 'a')</entry>
845 <entry>container</entry>
846 <entry><literal>STRUCT</literal></entry>
847 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
848 <entry>Struct; type code 114 'r' is reserved for use in
849 bindings and implementations to represent the general
850 concept of a struct, and must not appear in signatures
851 used on D-Bus.</entry>
853 <entry>container</entry>
854 <entry><literal>VARIANT</literal></entry>
855 <entry>118 (ASCII 'v') </entry>
856 <entry>Variant type (the type of the value is part of the value itself)</entry>
858 <entry>container</entry>
859 <entry><literal>DICT_ENTRY</literal></entry>
860 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
861 <entry>Entry in a dict or map (array of key-value pairs).
862 Type code 101 'e' is reserved for use in bindings and
863 implementations to represent the general concept of a
864 dict or dict-entry, and must not appear in signatures
865 used on D-Bus.</entry>
867 <entry>fixed, basic</entry>
868 <entry><literal>UNIX_FD</literal></entry>
869 <entry>104 (ASCII 'h')</entry>
870 <entry>Unix file descriptor</entry>
873 <entry>reserved</entry>
874 <entry>(reserved)</entry>
875 <entry>109 (ASCII 'm')</entry>
876 <entry>Reserved for <ulink
877 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
878 'maybe' type compatible with the one in GVariant</ulink>,
879 and must not appear in signatures used on D-Bus until
880 specified here</entry>
883 <entry>reserved</entry>
884 <entry>(reserved)</entry>
885 <entry>42 (ASCII '*')</entry>
886 <entry>Reserved for use in bindings/implementations to
887 represent any <firstterm>single complete type</firstterm>,
888 and must not appear in signatures used on D-Bus.</entry>
891 <entry>reserved</entry>
892 <entry>(reserved)</entry>
893 <entry>63 (ASCII '?')</entry>
894 <entry>Reserved for use in bindings/implementations to
895 represent any <firstterm>basic type</firstterm>, and must
896 not appear in signatures used on D-Bus.</entry>
899 <entry>reserved</entry>
900 <entry>(reserved)</entry>
901 <entry>64 (ASCII '@'), 38 (ASCII '&'),
902 94 (ASCII '^')</entry>
903 <entry>Reserved for internal use by bindings/implementations,
904 and must not appear in signatures used on D-Bus.
905 GVariant uses these type-codes to encode calling
916 <sect1 id="message-protocol-marshaling">
917 <title>Marshaling (Wire Format)</title>
920 D-Bus defines a marshalling format for its type system, which is
921 used in D-Bus messages. This is not the only possible marshalling
922 format for the type system: for instance, GVariant (part of GLib)
923 re-uses the D-Bus type system but implements an alternative marshalling
928 <title>Byte order and alignment</title>
931 Given a type signature, a block of bytes can be converted into typed
932 values. This section describes the format of the block of bytes. Byte
933 order and alignment issues are handled uniformly for all D-Bus types.
937 A block of bytes has an associated byte order. The byte order
938 has to be discovered in some way; for D-Bus messages, the
939 byte order is part of the message header as described in
940 <xref linkend="message-protocol-messages"/>. For now, assume
941 that the byte order is known to be either little endian or big
946 Each value in a block of bytes is aligned "naturally," for example
947 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
948 8-byte boundary. To properly align a value, <firstterm>alignment
949 padding</firstterm> may be necessary. The alignment padding must always
950 be the minimum required padding to properly align the following value;
951 and it must always be made up of nul bytes. The alignment padding must
952 not be left uninitialized (it can't contain garbage), and more padding
953 than required must not be used.
957 As an exception to natural alignment, <literal>STRUCT</literal> and
958 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
959 boundary, regardless of the alignments of their contents.
964 <title>Marshalling basic types</title>
967 To marshal and unmarshal fixed types, you simply read one value
968 from the data block corresponding to each type code in the signature.
969 All signed integer values are encoded in two's complement, DOUBLE
970 values are IEEE 754 double-precision floating-point, and BOOLEAN
971 values are encoded in 32 bits (of which only the least significant
976 The string-like types are all marshalled as a
977 fixed-length unsigned integer <varname>n</varname> giving the
978 length of the variable part, followed by <varname>n</varname>
979 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
980 which is not considered to be part of the text. The alignment
981 of the string-like type is the same as the alignment of
982 <varname>n</varname>.
986 For the STRING and OBJECT_PATH types, <varname>n</varname> is
987 encoded in 4 bytes, leading to 4-byte alignment.
988 For the SIGNATURE type, <varname>n</varname> is encoded as a single
989 byte. As a result, alignment padding is never required before a
995 <title>Marshalling containers</title>
998 Arrays are marshalled as a <literal>UINT32</literal>
999 <varname>n</varname> giving the length of the array data in bytes,
1000 followed by alignment padding to the alignment boundary of the array
1001 element type, followed by the <varname>n</varname> bytes of the
1002 array elements marshalled in sequence. <varname>n</varname> does not
1003 include the padding after the length, or any padding after the
1008 For instance, if the current position in the message is a multiple
1009 of 8 bytes and the byte-order is big-endian, an array containing only
1010 the 64-bit integer 5 would be marshalled as:
1013 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
1014 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
1015 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
1020 Arrays have a maximum length defined to be 2 to the 26th power or
1021 67108864 (64 MiB). Implementations must not send or accept arrays
1022 exceeding this length.
1026 Structs and dict entries are marshalled in the same way as their
1027 contents, but their alignment is always to an 8-byte boundary,
1028 even if their contents would normally be less strictly aligned.
1032 Variants are marshalled as the <literal>SIGNATURE</literal> of
1033 the contents (which must be a single complete type), followed by a
1034 marshalled value with the type given by that signature. The
1035 variant has the same 1-byte alignment as the signature, which means
1036 that alignment padding before a variant is never needed.
1037 Use of variants may not cause a total message depth to be larger
1038 than 64, including other container types such as structures.
1043 <title>Summary of D-Bus marshalling</title>
1046 Given all this, the types are marshaled on the wire as follows:
1051 <entry>Conventional Name</entry>
1052 <entry>Encoding</entry>
1053 <entry>Alignment</entry>
1058 <entry><literal>INVALID</literal></entry>
1059 <entry>Not applicable; cannot be marshaled.</entry>
1062 <entry><literal>BYTE</literal></entry>
1063 <entry>A single 8-bit byte.</entry>
1066 <entry><literal>BOOLEAN</literal></entry>
1067 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1070 <entry><literal>INT16</literal></entry>
1071 <entry>16-bit signed integer in the message's byte order.</entry>
1074 <entry><literal>UINT16</literal></entry>
1075 <entry>16-bit unsigned integer in the message's byte order.</entry>
1078 <entry><literal>INT32</literal></entry>
1079 <entry>32-bit signed integer in the message's byte order.</entry>
1082 <entry><literal>UINT32</literal></entry>
1083 <entry>32-bit unsigned integer in the message's byte order.</entry>
1086 <entry><literal>INT64</literal></entry>
1087 <entry>64-bit signed integer in the message's byte order.</entry>
1090 <entry><literal>UINT64</literal></entry>
1091 <entry>64-bit unsigned integer in the message's byte order.</entry>
1094 <entry><literal>DOUBLE</literal></entry>
1095 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1098 <entry><literal>STRING</literal></entry>
1099 <entry>A <literal>UINT32</literal> indicating the string's
1100 length in bytes excluding its terminating nul, followed by
1101 non-nul string data of the given length, followed by a terminating nul
1108 <entry><literal>OBJECT_PATH</literal></entry>
1109 <entry>Exactly the same as <literal>STRING</literal> except the
1110 content must be a valid object path (see above).
1116 <entry><literal>SIGNATURE</literal></entry>
1117 <entry>The same as <literal>STRING</literal> except the length is a single
1118 byte (thus signatures have a maximum length of 255)
1119 and the content must be a valid signature (see above).
1125 <entry><literal>ARRAY</literal></entry>
1127 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1128 alignment padding to the alignment boundary of the array element type,
1129 followed by each array element.
1135 <entry><literal>STRUCT</literal></entry>
1137 A struct must start on an 8-byte boundary regardless of the
1138 type of the struct fields. The struct value consists of each
1139 field marshaled in sequence starting from that 8-byte
1146 <entry><literal>VARIANT</literal></entry>
1148 The marshaled <literal>SIGNATURE</literal> of a single
1149 complete type, followed by a marshaled value with the type
1150 given in the signature.
1153 1 (alignment of the signature)
1156 <entry><literal>DICT_ENTRY</literal></entry>
1158 Identical to STRUCT.
1164 <entry><literal>UNIX_FD</literal></entry>
1165 <entry>32-bit unsigned integer in the message's byte
1166 order. The actual file descriptors need to be
1167 transferred out-of-band via some platform specific
1168 mechanism. On the wire, values of this type store the index to the
1169 file descriptor in the array of file descriptors that
1170 accompany the message.</entry>
1182 <sect1 id="message-protocol">
1183 <title>Message Protocol</title>
1186 A <firstterm>message</firstterm> consists of a
1187 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1188 think of a message as a package, the header is the address, and the body
1189 contains the package contents. The message delivery system uses the header
1190 information to figure out where to send the message and how to interpret
1191 it; the recipient interprets the body of the message.
1195 The body of the message is made up of zero or more
1196 <firstterm>arguments</firstterm>, which are typed values, such as an
1197 integer or a byte array.
1201 Both header and body use the D-Bus <link linkend="type-system">type
1202 system</link> and format for serializing data.
1205 <sect2 id="message-protocol-messages">
1206 <title>Message Format</title>
1209 A message consists of a header and a body. The header is a block of
1210 values with a fixed signature and meaning. The body is a separate block
1211 of values, with a signature specified in the header.
1215 The length of the header must be a multiple of 8, allowing the body to
1216 begin on an 8-byte boundary when storing the entire message in a single
1217 buffer. If the header does not naturally end on an 8-byte boundary
1218 up to 7 bytes of nul-initialized alignment padding must be added.
1222 The message body need not end on an 8-byte boundary.
1226 The maximum length of a message, including header, header alignment padding,
1227 and body is 2 to the 27th power or 134217728 (128 MiB).
1228 Implementations must not send or accept messages exceeding this size.
1232 The signature of the header is:
1236 Written out more readably, this is:
1238 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1243 These values have the following meanings:
1248 <entry>Value</entry>
1249 <entry>Description</entry>
1254 <entry>1st <literal>BYTE</literal></entry>
1255 <entry>Endianness flag; ASCII 'l' for little-endian
1256 or ASCII 'B' for big-endian. Both header and body are
1257 in this endianness.</entry>
1260 <entry>2nd <literal>BYTE</literal></entry>
1261 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1262 Currently-defined types are described below.
1266 <entry>3rd <literal>BYTE</literal></entry>
1267 <entry>Bitwise OR of flags. Unknown flags
1268 must be ignored. Currently-defined flags are described below.
1272 <entry>4th <literal>BYTE</literal></entry>
1273 <entry>Major protocol version of the sending application. If
1274 the major protocol version of the receiving application does not
1275 match, the applications will not be able to communicate and the
1276 D-Bus connection must be disconnected. The major protocol
1277 version for this version of the specification is 1.
1281 <entry>1st <literal>UINT32</literal></entry>
1282 <entry>Length in bytes of the message body, starting
1283 from the end of the header. The header ends after
1284 its alignment padding to an 8-boundary.
1288 <entry>2nd <literal>UINT32</literal></entry>
1289 <entry>The serial of this message, used as a cookie
1290 by the sender to identify the reply corresponding
1291 to this request. This must not be zero.
1295 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1296 <entry>An array of zero or more <firstterm>header
1297 fields</firstterm> where the byte is the field code, and the
1298 variant is the field value. The message type determines
1299 which fields are required.
1307 <firstterm>Message types</firstterm> that can appear in the second byte
1313 <entry>Conventional name</entry>
1314 <entry>Decimal value</entry>
1315 <entry>Description</entry>
1320 <entry><literal>INVALID</literal></entry>
1322 <entry>This is an invalid type.</entry>
1325 <entry><literal>METHOD_CALL</literal></entry>
1327 <entry>Method call. This message type may prompt a
1331 <entry><literal>METHOD_RETURN</literal></entry>
1333 <entry>Method reply with returned data.</entry>
1336 <entry><literal>ERROR</literal></entry>
1338 <entry>Error reply. If the first argument exists and is a
1339 string, it is an error message.</entry>
1342 <entry><literal>SIGNAL</literal></entry>
1344 <entry>Signal emission.</entry>
1351 Flags that can appear in the third byte of the header:
1356 <entry>Conventional name</entry>
1357 <entry>Hex value</entry>
1358 <entry>Description</entry>
1363 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1367 This message does not expect method return replies or
1368 error replies, even if it is of a type that can
1369 have a reply; the reply can be omitted as an
1370 optimization. It is compliant with this specification
1371 to return the reply despite this flag, although doing
1372 so on a bus with a non-trivial security policy
1373 (such as the well-known system bus) may result in
1374 access denial messages being logged for the reply.
1377 Note that METHOD_CALL is the only message type currently
1378 defined in this specification that can expect a reply,
1379 so the presence or absence of this flag in the other
1380 three message types that are currently
1381 documented is meaningless: replies to those message
1382 types should not be sent, whether this flag is present
1388 <entry><literal>NO_AUTO_START</literal></entry>
1390 <entry>The bus must not launch an owner
1391 for the destination name in response to this message.
1399 <sect3 id="message-protocol-header-fields">
1400 <title>Header Fields</title>
1403 The array at the end of the header contains <firstterm>header
1404 fields</firstterm>, where each field is a 1-byte field code followed
1405 by a field value. A header must contain the required header fields for
1406 its message type, and zero or more of any optional header
1407 fields. Future versions of this protocol specification may add new
1408 fields. Implementations must ignore fields they do not
1409 understand. Implementations must not invent their own header fields;
1410 only changes to this specification may introduce new header fields.
1414 Again, if an implementation sees a header field code that it does not
1415 expect, it must ignore that field, as it will be part of a new
1416 (but compatible) version of this specification. This also applies
1417 to known header fields appearing in unexpected messages, for
1418 example: if a signal has a reply serial it must be ignored
1419 even though it has no meaning as of this version of the spec.
1423 However, implementations must not send or accept known header fields
1424 with the wrong type stored in the field value. So for example a
1425 message with an <literal>INTERFACE</literal> field of type
1426 <literal>UINT32</literal> would be considered corrupt.
1430 Here are the currently-defined header fields:
1435 <entry>Conventional Name</entry>
1436 <entry>Decimal Code</entry>
1438 <entry>Required In</entry>
1439 <entry>Description</entry>
1444 <entry><literal>INVALID</literal></entry>
1447 <entry>not allowed</entry>
1448 <entry>Not a valid field name (error if it appears in a message)</entry>
1451 <entry><literal>PATH</literal></entry>
1453 <entry><literal>OBJECT_PATH</literal></entry>
1454 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1455 <entry>The object to send a call to,
1456 or the object a signal is emitted from.
1458 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1459 implementations should not send messages with this path,
1460 and the reference implementation of the bus daemon will
1461 disconnect any application that attempts to do so.
1465 <entry><literal>INTERFACE</literal></entry>
1467 <entry><literal>STRING</literal></entry>
1468 <entry><literal>SIGNAL</literal></entry>
1470 The interface to invoke a method call on, or
1471 that a signal is emitted from. Optional for
1472 method calls, required for signals.
1473 The special interface
1474 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1475 implementations should not send messages with this
1476 interface, and the reference implementation of the bus
1477 daemon will disconnect any application that attempts to
1482 <entry><literal>MEMBER</literal></entry>
1484 <entry><literal>STRING</literal></entry>
1485 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1486 <entry>The member, either the method name or signal name.</entry>
1489 <entry><literal>ERROR_NAME</literal></entry>
1491 <entry><literal>STRING</literal></entry>
1492 <entry><literal>ERROR</literal></entry>
1493 <entry>The name of the error that occurred, for errors</entry>
1496 <entry><literal>REPLY_SERIAL</literal></entry>
1498 <entry><literal>UINT32</literal></entry>
1499 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1500 <entry>The serial number of the message this message is a reply
1501 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1504 <entry><literal>DESTINATION</literal></entry>
1506 <entry><literal>STRING</literal></entry>
1507 <entry>optional</entry>
1508 <entry>The name of the connection this message is intended for.
1509 Only used in combination with the message bus, see
1510 <xref linkend="message-bus"/>.</entry>
1513 <entry><literal>SENDER</literal></entry>
1515 <entry><literal>STRING</literal></entry>
1516 <entry>optional</entry>
1517 <entry>Unique name of the sending connection.
1518 The message bus fills in this field so it is reliable; the field is
1519 only meaningful in combination with the message bus.</entry>
1522 <entry><literal>SIGNATURE</literal></entry>
1524 <entry><literal>SIGNATURE</literal></entry>
1525 <entry>optional</entry>
1526 <entry>The signature of the message body.
1527 If omitted, it is assumed to be the
1528 empty signature "" (i.e. the body must be 0-length).</entry>
1531 <entry><literal>UNIX_FDS</literal></entry>
1533 <entry><literal>UINT32</literal></entry>
1534 <entry>optional</entry>
1535 <entry>The number of Unix file descriptors that
1536 accompany the message. If omitted, it is assumed
1537 that no Unix file descriptors accompany the
1538 message. The actual file descriptors need to be
1539 transferred via platform specific mechanism
1540 out-of-band. They must be sent at the same time as
1541 part of the message itself. They may not be sent
1542 before the first byte of the message itself is
1543 transferred or after the last byte of the message
1553 <sect2 id="message-protocol-names">
1554 <title>Valid Names</title>
1556 The various names in D-Bus messages have some restrictions.
1559 There is a <firstterm>maximum name length</firstterm>
1560 of 255 which applies to bus names, interfaces, and members.
1562 <sect3 id="message-protocol-names-interface">
1563 <title>Interface names</title>
1565 Interfaces have names with type <literal>STRING</literal>, meaning that
1566 they must be valid UTF-8. However, there are also some
1567 additional restrictions that apply to interface names
1570 <listitem><para>Interface names are composed of 1 or more elements separated by
1571 a period ('.') character. All elements must contain at least
1575 <listitem><para>Each element must only contain the ASCII characters
1576 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1580 <listitem><para>Interface names must contain at least one '.' (period)
1581 character (and thus at least two elements).
1584 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1585 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1590 Interface names should start with the reversed DNS domain name of
1591 the author of the interface (in lower-case), like interface names
1592 in Java. It is conventional for the rest of the interface name
1593 to consist of words run together, with initial capital letters
1594 on all words ("CamelCase"). Several levels of hierarchy can be used.
1595 It is also a good idea to include the major version of the interface
1596 in the name, and increment it if incompatible changes are made;
1597 this way, a single object can implement several versions of an
1598 interface in parallel, if necessary.
1602 For instance, if the owner of <literal>example.com</literal> is
1603 developing a D-Bus API for a music player, they might define
1604 interfaces called <literal>com.example.MusicPlayer1</literal>,
1605 <literal>com.example.MusicPlayer1.Track</literal> and
1606 <literal>com.example.MusicPlayer1.Seekable</literal>.
1610 D-Bus does not distinguish between the concepts that would be
1611 called classes and interfaces in Java: either can be identified on
1612 D-Bus by an interface name.
1615 <sect3 id="message-protocol-names-bus">
1616 <title>Bus names</title>
1618 Connections have one or more bus names associated with them.
1619 A connection has exactly one bus name that is a <firstterm>unique
1620 connection name</firstterm>. The unique connection name remains
1621 with the connection for its entire lifetime.
1622 A bus name is of type <literal>STRING</literal>,
1623 meaning that it must be valid UTF-8. However, there are also
1624 some additional restrictions that apply to bus names
1627 <listitem><para>Bus names that start with a colon (':')
1628 character are unique connection names. Other bus names
1629 are called <firstterm>well-known bus names</firstterm>.
1632 <listitem><para>Bus names are composed of 1 or more elements separated by
1633 a period ('.') character. All elements must contain at least
1637 <listitem><para>Each element must only contain the ASCII characters
1638 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1639 connection name may begin with a digit, elements in
1640 other bus names must not begin with a digit.
1644 <listitem><para>Bus names must contain at least one '.' (period)
1645 character (and thus at least two elements).
1648 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1649 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1653 Note that the hyphen ('-') character is allowed in bus names but
1654 not in interface names.
1658 Like <link linkend="message-protocol-names-interface">interface
1659 names</link>, well-known bus names should start with the
1660 reversed DNS domain name of the author of the interface (in
1661 lower-case), and it is conventional for the rest of the well-known
1662 bus name to consist of words run together, with initial
1663 capital letters. As with interface names, including a version
1664 number in well-known bus names is a good idea; it's possible to
1665 have the well-known bus name for more than one version
1666 simultaneously if backwards compatibility is required.
1670 If a well-known bus name implies the presence of a "main" interface,
1671 that "main" interface is often given the same name as
1672 the well-known bus name, and situated at the corresponding object
1673 path. For instance, if the owner of <literal>example.com</literal>
1674 is developing a D-Bus API for a music player, they might define
1675 that any application that takes the well-known name
1676 <literal>com.example.MusicPlayer1</literal> should have an object
1677 at the object path <literal>/com/example/MusicPlayer1</literal>
1678 which implements the interface
1679 <literal>com.example.MusicPlayer1</literal>.
1682 <sect3 id="message-protocol-names-member">
1683 <title>Member names</title>
1685 Member (i.e. method or signal) names:
1687 <listitem><para>Must only contain the ASCII characters
1688 "[A-Z][a-z][0-9]_" and may not begin with a
1689 digit.</para></listitem>
1690 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1691 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1692 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1697 It is conventional for member names on D-Bus to consist of
1698 capitalized words with no punctuation ("camel-case").
1699 Method names should usually be verbs, such as
1700 <literal>GetItems</literal>, and signal names should usually be
1701 a description of an event, such as <literal>ItemsChanged</literal>.
1704 <sect3 id="message-protocol-names-error">
1705 <title>Error names</title>
1707 Error names have the same restrictions as interface names.
1711 Error names have the same naming conventions as interface
1712 names, and often contain <literal>.Error.</literal>; for instance,
1713 the owner of <literal>example.com</literal> might define the
1714 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1715 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1716 The errors defined by D-Bus itself, such as
1717 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1723 <sect2 id="message-protocol-types">
1724 <title>Message Types</title>
1726 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1727 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1728 This section describes these conventions.
1730 <sect3 id="message-protocol-types-method">
1731 <title>Method Calls</title>
1733 Some messages invoke an operation on a remote object. These are
1734 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1735 messages map naturally to methods on objects in a typical program.
1738 A method call message is required to have a <literal>MEMBER</literal> header field
1739 indicating the name of the method. Optionally, the message has an
1740 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1741 Including the <literal>INTERFACE</literal> in all method call
1742 messages is strongly recommended.
1745 In the absence of an <literal>INTERFACE</literal> field, if two
1746 or more interfaces on the same object have a method with the same
1747 name, it is undefined which of those methods will be invoked.
1748 Implementations may choose to either return an error, or deliver the
1749 message as though it had an arbitrary one of those interfaces.
1752 In some situations (such as the well-known system bus), messages
1753 are filtered through an access-control list external to the
1754 remote object implementation. If that filter rejects certain
1755 messages by matching their interface, or accepts only messages
1756 to specific interfaces, it must also reject messages that have no
1757 <literal>INTERFACE</literal>: otherwise, malicious
1758 applications could use this to bypass the filter.
1761 Method call messages also include a <literal>PATH</literal> field
1762 indicating the object to invoke the method on. If the call is passing
1763 through a message bus, the message will also have a
1764 <literal>DESTINATION</literal> field giving the name of the connection
1765 to receive the message.
1768 When an application handles a method call message, it is required to
1769 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1770 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1771 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1774 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1775 are the return value(s) or "out parameters" of the method call.
1776 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1777 and the call fails; no return value will be provided. It makes
1778 no sense to send multiple replies to the same method call.
1781 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1782 reply is required, so the caller will know the method
1783 was successfully processed.
1786 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1790 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1791 then as an optimization the application receiving the method
1792 call may choose to omit the reply message (regardless of
1793 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1794 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1795 flag and reply anyway.
1798 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1799 destination name does not exist then a program to own the destination
1800 name will be started before the message is delivered. The message
1801 will be held until the new program is successfully started or has
1802 failed to start; in case of failure, an error will be returned. This
1803 flag is only relevant in the context of a message bus, it is ignored
1804 during one-to-one communication with no intermediate bus.
1806 <sect4 id="message-protocol-types-method-apis">
1807 <title>Mapping method calls to native APIs</title>
1809 APIs for D-Bus may map method calls to a method call in a specific
1810 programming language, such as C++, or may map a method call written
1811 in an IDL to a D-Bus message.
1814 In APIs of this nature, arguments to a method are often termed "in"
1815 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1816 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1817 "inout" arguments, which are both sent and received, i.e. the caller
1818 passes in a value which is modified. Mapped to D-Bus, an "inout"
1819 argument is equivalent to an "in" argument, followed by an "out"
1820 argument. You can't pass things "by reference" over the wire, so
1821 "inout" is purely an illusion of the in-process API.
1824 Given a method with zero or one return values, followed by zero or more
1825 arguments, where each argument may be "in", "out", or "inout", the
1826 caller constructs a message by appending each "in" or "inout" argument,
1827 in order. "out" arguments are not represented in the caller's message.
1830 The recipient constructs a reply by appending first the return value
1831 if any, then each "out" or "inout" argument, in order.
1832 "in" arguments are not represented in the reply message.
1835 Error replies are normally mapped to exceptions in languages that have
1839 In converting from native APIs to D-Bus, it is perhaps nice to
1840 map D-Bus naming conventions ("FooBar") to native conventions
1841 such as "fooBar" or "foo_bar" automatically. This is OK
1842 as long as you can say that the native API is one that
1843 was specifically written for D-Bus. It makes the most sense
1844 when writing object implementations that will be exported
1845 over the bus. Object proxies used to invoke remote D-Bus
1846 objects probably need the ability to call any D-Bus method,
1847 and thus a magic name mapping like this could be a problem.
1850 This specification doesn't require anything of native API bindings;
1851 the preceding is only a suggested convention for consistency
1857 <sect3 id="message-protocol-types-signal">
1858 <title>Signal Emission</title>
1860 Unlike method calls, signal emissions have no replies.
1861 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1862 It must have three header fields: <literal>PATH</literal> giving the object
1863 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1864 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1865 for signals, though it is optional for method calls.
1869 <sect3 id="message-protocol-types-errors">
1870 <title>Errors</title>
1872 Messages of type <literal>ERROR</literal> are most commonly replies
1873 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1874 to any kind of message. The message bus for example
1875 will return an <literal>ERROR</literal> in reply to a signal emission if
1876 the bus does not have enough memory to send the signal.
1879 An <literal>ERROR</literal> may have any arguments, but if the first
1880 argument is a <literal>STRING</literal>, it must be an error message.
1881 The error message may be logged or shown to the user
1886 <sect3 id="message-protocol-types-notation">
1887 <title>Notation in this document</title>
1889 This document uses a simple pseudo-IDL to describe particular method
1890 calls and signals. Here is an example of a method call:
1892 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1893 out UINT32 resultcode)
1895 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1896 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1897 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1898 characters so it's known that the last part of the name in
1899 the "IDL" is the member name.
1902 In C++ that might end up looking like this:
1904 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1905 unsigned int flags);
1907 or equally valid, the return value could be done as an argument:
1909 void org::freedesktop::DBus::StartServiceByName (const char *name,
1911 unsigned int *resultcode);
1913 It's really up to the API designer how they want to make
1914 this look. You could design an API where the namespace wasn't used
1915 in C++, using STL or Qt, using varargs, or whatever you wanted.
1918 Signals are written as follows:
1920 org.freedesktop.DBus.NameLost (STRING name)
1922 Signals don't specify "in" vs. "out" because only
1923 a single direction is possible.
1926 It isn't especially encouraged to use this lame pseudo-IDL in actual
1927 API implementations; you might use the native notation for the
1928 language you're using, or you might use COM or CORBA IDL, for example.
1933 <sect2 id="message-protocol-handling-invalid">
1934 <title>Invalid Protocol and Spec Extensions</title>
1937 For security reasons, the D-Bus protocol should be strictly parsed and
1938 validated, with the exception of defined extension points. Any invalid
1939 protocol or spec violations should result in immediately dropping the
1940 connection without notice to the other end. Exceptions should be
1941 carefully considered, e.g. an exception may be warranted for a
1942 well-understood idiosyncrasy of a widely-deployed implementation. In
1943 cases where the other end of a connection is 100% trusted and known to
1944 be friendly, skipping validation for performance reasons could also make
1945 sense in certain cases.
1949 Generally speaking violations of the "must" requirements in this spec
1950 should be considered possible attempts to exploit security, and violations
1951 of the "should" suggestions should be considered legitimate (though perhaps
1952 they should generate an error in some cases).
1956 The following extension points are built in to D-Bus on purpose and must
1957 not be treated as invalid protocol. The extension points are intended
1958 for use by future versions of this spec, they are not intended for third
1959 parties. At the moment, the only way a third party could extend D-Bus
1960 without breaking interoperability would be to introduce a way to negotiate new
1961 feature support as part of the auth protocol, using EXTENSION_-prefixed
1962 commands. There is not yet a standard way to negotiate features.
1966 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1967 commands result in an ERROR rather than a disconnect. This enables
1968 future extensions to the protocol. Commands starting with EXTENSION_ are
1969 reserved for third parties.
1974 The authentication protocol supports pluggable auth mechanisms.
1979 The address format (see <xref linkend="addresses"/>) supports new
1985 Messages with an unknown type (something other than
1986 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1987 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1988 Unknown-type messages must still be well-formed in the same way
1989 as the known messages, however. They still have the normal
1995 Header fields with an unknown or unexpected field code must be ignored,
1996 though again they must still be well-formed.
2001 New standard interfaces (with new methods and signals) can of course be added.
2011 <sect1 id="auth-protocol">
2012 <title>Authentication Protocol</title>
2014 Before the flow of messages begins, two applications must
2015 authenticate. A simple plain-text protocol is used for
2016 authentication; this protocol is a SASL profile, and maps fairly
2017 directly from the SASL specification. The message encoding is
2018 NOT used here, only plain text messages.
2021 In examples, "C:" and "S:" indicate lines sent by the client and
2022 server respectively.
2024 <sect2 id="auth-protocol-overview">
2025 <title>Protocol Overview</title>
2027 The protocol is a line-based protocol, where each line ends with
2028 \r\n. Each line begins with an all-caps ASCII command name containing
2029 only the character range [A-Z_], a space, then any arguments for the
2030 command, then the \r\n ending the line. The protocol is
2031 case-sensitive. All bytes must be in the ASCII character set.
2033 Commands from the client to the server are as follows:
2036 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2037 <listitem><para>CANCEL</para></listitem>
2038 <listitem><para>BEGIN</para></listitem>
2039 <listitem><para>DATA <data in hex encoding></para></listitem>
2040 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2041 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2044 From server to client are as follows:
2047 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2048 <listitem><para>OK <GUID in hex></para></listitem>
2049 <listitem><para>DATA <data in hex encoding></para></listitem>
2050 <listitem><para>ERROR</para></listitem>
2051 <listitem><para>AGREE_UNIX_FD</para></listitem>
2055 Unofficial extensions to the command set must begin with the letters
2056 "EXTENSION_", to avoid conflicts with future official commands.
2057 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2060 <sect2 id="auth-nul-byte">
2061 <title>Special credentials-passing nul byte</title>
2063 Immediately after connecting to the server, the client must send a
2064 single nul byte. This byte may be accompanied by credentials
2065 information on some operating systems that use sendmsg() with
2066 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2067 sockets. However, the nul byte must be sent even on other kinds of
2068 socket, and even on operating systems that do not require a byte to be
2069 sent in order to transmit credentials. The text protocol described in
2070 this document begins after the single nul byte. If the first byte
2071 received from the client is not a nul byte, the server may disconnect
2075 A nul byte in any context other than the initial byte is an error;
2076 the protocol is ASCII-only.
2079 The credentials sent along with the nul byte may be used with the
2080 SASL mechanism EXTERNAL.
2083 <sect2 id="auth-command-auth">
2084 <title>AUTH command</title>
2086 If an AUTH command has no arguments, it is a request to list
2087 available mechanisms. The server must respond with a REJECTED
2088 command listing the mechanisms it understands, or with an error.
2091 If an AUTH command specifies a mechanism, and the server supports
2092 said mechanism, the server should begin exchanging SASL
2093 challenge-response data with the client using DATA commands.
2096 If the server does not support the mechanism given in the AUTH
2097 command, it must send either a REJECTED command listing the mechanisms
2098 it does support, or an error.
2101 If the [initial-response] argument is provided, it is intended for use
2102 with mechanisms that have no initial challenge (or an empty initial
2103 challenge), as if it were the argument to an initial DATA command. If
2104 the selected mechanism has an initial challenge and [initial-response]
2105 was provided, the server should reject authentication by sending
2109 If authentication succeeds after exchanging DATA commands,
2110 an OK command must be sent to the client.
2113 The first octet received by the server after the \r\n of the BEGIN
2114 command from the client must be the first octet of the
2115 authenticated/encrypted stream of D-Bus messages.
2118 If BEGIN is received by the server, the first octet received
2119 by the client after the \r\n of the OK command must be the
2120 first octet of the authenticated/encrypted stream of D-Bus
2124 <sect2 id="auth-command-cancel">
2125 <title>CANCEL Command</title>
2127 At any time up to sending the BEGIN command, the client may send a
2128 CANCEL command. On receiving the CANCEL command, the server must
2129 send a REJECTED command and abort the current authentication
2133 <sect2 id="auth-command-data">
2134 <title>DATA Command</title>
2136 The DATA command may come from either client or server, and simply
2137 contains a hex-encoded block of data to be interpreted
2138 according to the SASL mechanism in use.
2141 Some SASL mechanisms support sending an "empty string";
2142 FIXME we need some way to do this.
2145 <sect2 id="auth-command-begin">
2146 <title>BEGIN Command</title>
2148 The BEGIN command acknowledges that the client has received an
2149 OK command from the server, and that the stream of messages
2153 The first octet received by the server after the \r\n of the BEGIN
2154 command from the client must be the first octet of the
2155 authenticated/encrypted stream of D-Bus messages.
2158 <sect2 id="auth-command-rejected">
2159 <title>REJECTED Command</title>
2161 The REJECTED command indicates that the current authentication
2162 exchange has failed, and further exchange of DATA is inappropriate.
2163 The client would normally try another mechanism, or try providing
2164 different responses to challenges.
2166 Optionally, the REJECTED command has a space-separated list of
2167 available auth mechanisms as arguments. If a server ever provides
2168 a list of supported mechanisms, it must provide the same list
2169 each time it sends a REJECTED message. Clients are free to
2170 ignore all lists received after the first.
2173 <sect2 id="auth-command-ok">
2174 <title>OK Command</title>
2176 The OK command indicates that the client has been
2177 authenticated. The client may now proceed with negotiating
2178 Unix file descriptor passing. To do that it shall send
2179 NEGOTIATE_UNIX_FD to the server.
2182 Otherwise, the client must respond to the OK command by
2183 sending a BEGIN command, followed by its stream of messages,
2184 or by disconnecting. The server must not accept additional
2185 commands using this protocol after the BEGIN command has been
2186 received. Further communication will be a stream of D-Bus
2187 messages (optionally encrypted, as negotiated) rather than
2191 If a client sends BEGIN the first octet received by the client
2192 after the \r\n of the OK command must be the first octet of
2193 the authenticated/encrypted stream of D-Bus messages.
2196 The OK command has one argument, which is the GUID of the server.
2197 See <xref linkend="addresses"/> for more on server GUIDs.
2200 <sect2 id="auth-command-error">
2201 <title>ERROR Command</title>
2203 The ERROR command indicates that either server or client did not
2204 know a command, does not accept the given command in the current
2205 context, or did not understand the arguments to the command. This
2206 allows the protocol to be extended; a client or server can send a
2207 command present or permitted only in new protocol versions, and if
2208 an ERROR is received instead of an appropriate response, fall back
2209 to using some other technique.
2212 If an ERROR is sent, the server or client that sent the
2213 error must continue as if the command causing the ERROR had never been
2214 received. However, the the server or client receiving the error
2215 should try something other than whatever caused the error;
2216 if only canceling/rejecting the authentication.
2219 If the D-Bus protocol changes incompatibly at some future time,
2220 applications implementing the new protocol would probably be able to
2221 check for support of the new protocol by sending a new command and
2222 receiving an ERROR from applications that don't understand it. Thus the
2223 ERROR feature of the auth protocol is an escape hatch that lets us
2224 negotiate extensions or changes to the D-Bus protocol in the future.
2227 <sect2 id="auth-command-negotiate-unix-fd">
2228 <title>NEGOTIATE_UNIX_FD Command</title>
2230 The NEGOTIATE_UNIX_FD command indicates that the client
2231 supports Unix file descriptor passing. This command may only
2232 be sent after the connection is authenticated, i.e. after OK
2233 was received by the client. This command may only be sent on
2234 transports that support Unix file descriptor passing.
2237 On receiving NEGOTIATE_UNIX_FD the server must respond with
2238 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2239 the transport chosen supports Unix file descriptor passing and
2240 the server supports this feature. It shall respond the latter
2241 if the transport does not support Unix file descriptor
2242 passing, the server does not support this feature, or the
2243 server decides not to enable file descriptor passing due to
2244 security or other reasons.
2247 <sect2 id="auth-command-agree-unix-fd">
2248 <title>AGREE_UNIX_FD Command</title>
2250 The AGREE_UNIX_FD command indicates that the server supports
2251 Unix file descriptor passing. This command may only be sent
2252 after the connection is authenticated, and the client sent
2253 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2254 command may only be sent on transports that support Unix file
2258 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2259 followed by its stream of messages, or by disconnecting. The
2260 server must not accept additional commands using this protocol
2261 after the BEGIN command has been received. Further
2262 communication will be a stream of D-Bus messages (optionally
2263 encrypted, as negotiated) rather than this protocol.
2266 <sect2 id="auth-command-future">
2267 <title>Future Extensions</title>
2269 Future extensions to the authentication and negotiation
2270 protocol are possible. For that new commands may be
2271 introduced. If a client or server receives an unknown command
2272 it shall respond with ERROR and not consider this fatal. New
2273 commands may be introduced both before, and after
2274 authentication, i.e. both before and after the OK command.
2277 <sect2 id="auth-examples">
2278 <title>Authentication examples</title>
2282 <title>Example of successful magic cookie authentication</title>
2284 (MAGIC_COOKIE is a made up mechanism)
2286 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2292 <title>Example of finding out mechanisms then picking one</title>
2295 S: REJECTED KERBEROS_V4 SKEY
2296 C: AUTH SKEY 7ab83f32ee
2297 S: DATA 8799cabb2ea93e
2298 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2304 <title>Example of client sends unknown command then falls back to regular auth</title>
2308 C: AUTH MAGIC_COOKIE 3736343435313230333039
2314 <title>Example of server doesn't support initial auth mechanism</title>
2316 C: AUTH MAGIC_COOKIE 3736343435313230333039
2317 S: REJECTED KERBEROS_V4 SKEY
2318 C: AUTH SKEY 7ab83f32ee
2319 S: DATA 8799cabb2ea93e
2320 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2326 <title>Example of wrong password or the like followed by successful retry</title>
2328 C: AUTH MAGIC_COOKIE 3736343435313230333039
2329 S: REJECTED KERBEROS_V4 SKEY
2330 C: AUTH SKEY 7ab83f32ee
2331 S: DATA 8799cabb2ea93e
2332 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2334 C: AUTH SKEY 7ab83f32ee
2335 S: DATA 8799cabb2ea93e
2336 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2342 <title>Example of skey cancelled and restarted</title>
2344 C: AUTH MAGIC_COOKIE 3736343435313230333039
2345 S: REJECTED KERBEROS_V4 SKEY
2346 C: AUTH SKEY 7ab83f32ee
2347 S: DATA 8799cabb2ea93e
2350 C: AUTH SKEY 7ab83f32ee
2351 S: DATA 8799cabb2ea93e
2352 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2358 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2360 (MAGIC_COOKIE is a made up mechanism)
2362 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2364 C: NEGOTIATE_UNIX_FD
2370 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2372 (MAGIC_COOKIE is a made up mechanism)
2374 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2376 C: NEGOTIATE_UNIX_FD
2383 <sect2 id="auth-states">
2384 <title>Authentication state diagrams</title>
2387 This section documents the auth protocol in terms of
2388 a state machine for the client and the server. This is
2389 probably the most robust way to implement the protocol.
2392 <sect3 id="auth-states-client">
2393 <title>Client states</title>
2396 To more precisely describe the interaction between the
2397 protocol state machine and the authentication mechanisms the
2398 following notation is used: MECH(CHALL) means that the
2399 server challenge CHALL was fed to the mechanism MECH, which
2405 CONTINUE(RESP) means continue the auth conversation
2406 and send RESP as the response to the server;
2412 OK(RESP) means that after sending RESP to the server
2413 the client side of the auth conversation is finished
2414 and the server should return "OK";
2420 ERROR means that CHALL was invalid and could not be
2426 Both RESP and CHALL may be empty.
2430 The Client starts by getting an initial response from the
2431 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2432 the mechanism did not provide an initial response. If the
2433 mechanism returns CONTINUE, the client starts in state
2434 <emphasis>WaitingForData</emphasis>, if the mechanism
2435 returns OK the client starts in state
2436 <emphasis>WaitingForOK</emphasis>.
2440 The client should keep track of available mechanisms and
2441 which it mechanisms it has already attempted. This list is
2442 used to decide which AUTH command to send. When the list is
2443 exhausted, the client should give up and close the
2448 <title><emphasis>WaitingForData</emphasis></title>
2456 MECH(CHALL) returns CONTINUE(RESP) → send
2458 <emphasis>WaitingForData</emphasis>
2462 MECH(CHALL) returns OK(RESP) → send DATA
2463 RESP, goto <emphasis>WaitingForOK</emphasis>
2467 MECH(CHALL) returns ERROR → send ERROR
2468 [msg], goto <emphasis>WaitingForData</emphasis>
2476 Receive REJECTED [mechs] →
2477 send AUTH [next mech], goto
2478 WaitingForData or <emphasis>WaitingForOK</emphasis>
2483 Receive ERROR → send
2485 <emphasis>WaitingForReject</emphasis>
2490 Receive OK → send
2491 BEGIN, terminate auth
2492 conversation, authenticated
2497 Receive anything else → send
2499 <emphasis>WaitingForData</emphasis>
2507 <title><emphasis>WaitingForOK</emphasis></title>
2512 Receive OK → send BEGIN, terminate auth
2513 conversation, <emphasis>authenticated</emphasis>
2518 Receive REJECTED [mechs] → send AUTH [next mech],
2519 goto <emphasis>WaitingForData</emphasis> or
2520 <emphasis>WaitingForOK</emphasis>
2526 Receive DATA → send CANCEL, goto
2527 <emphasis>WaitingForReject</emphasis>
2533 Receive ERROR → send CANCEL, goto
2534 <emphasis>WaitingForReject</emphasis>
2540 Receive anything else → send ERROR, goto
2541 <emphasis>WaitingForOK</emphasis>
2549 <title><emphasis>WaitingForReject</emphasis></title>
2554 Receive REJECTED [mechs] → send AUTH [next mech],
2555 goto <emphasis>WaitingForData</emphasis> or
2556 <emphasis>WaitingForOK</emphasis>
2562 Receive anything else → terminate auth
2563 conversation, disconnect
2572 <sect3 id="auth-states-server">
2573 <title>Server states</title>
2576 For the server MECH(RESP) means that the client response
2577 RESP was fed to the the mechanism MECH, which returns one of
2582 CONTINUE(CHALL) means continue the auth conversation and
2583 send CHALL as the challenge to the client;
2589 OK means that the client has been successfully
2596 REJECTED means that the client failed to authenticate or
2597 there was an error in RESP.
2602 The server starts out in state
2603 <emphasis>WaitingForAuth</emphasis>. If the client is
2604 rejected too many times the server must disconnect the
2609 <title><emphasis>WaitingForAuth</emphasis></title>
2615 Receive AUTH → send REJECTED [mechs], goto
2616 <emphasis>WaitingForAuth</emphasis>
2622 Receive AUTH MECH RESP
2626 MECH not valid mechanism → send REJECTED
2628 <emphasis>WaitingForAuth</emphasis>
2632 MECH(RESP) returns CONTINUE(CHALL) → send
2634 <emphasis>WaitingForData</emphasis>
2638 MECH(RESP) returns OK → send OK, goto
2639 <emphasis>WaitingForBegin</emphasis>
2643 MECH(RESP) returns REJECTED → send REJECTED
2645 <emphasis>WaitingForAuth</emphasis>
2653 Receive BEGIN → terminate
2654 auth conversation, disconnect
2660 Receive ERROR → send REJECTED [mechs], goto
2661 <emphasis>WaitingForAuth</emphasis>
2667 Receive anything else → send
2669 <emphasis>WaitingForAuth</emphasis>
2678 <title><emphasis>WaitingForData</emphasis></title>
2686 MECH(RESP) returns CONTINUE(CHALL) → send
2688 <emphasis>WaitingForData</emphasis>
2692 MECH(RESP) returns OK → send OK, goto
2693 <emphasis>WaitingForBegin</emphasis>
2697 MECH(RESP) returns REJECTED → send REJECTED
2699 <emphasis>WaitingForAuth</emphasis>
2707 Receive BEGIN → terminate auth conversation,
2714 Receive CANCEL → send REJECTED [mechs], goto
2715 <emphasis>WaitingForAuth</emphasis>
2721 Receive ERROR → send REJECTED [mechs], goto
2722 <emphasis>WaitingForAuth</emphasis>
2728 Receive anything else → send ERROR, goto
2729 <emphasis>WaitingForData</emphasis>
2737 <title><emphasis>WaitingForBegin</emphasis></title>
2742 Receive BEGIN → terminate auth conversation,
2743 client authenticated
2749 Receive CANCEL → send REJECTED [mechs], goto
2750 <emphasis>WaitingForAuth</emphasis>
2756 Receive ERROR → send REJECTED [mechs], goto
2757 <emphasis>WaitingForAuth</emphasis>
2763 Receive anything else → send ERROR, goto
2764 <emphasis>WaitingForBegin</emphasis>
2774 <sect2 id="auth-mechanisms">
2775 <title>Authentication mechanisms</title>
2777 This section describes some new authentication mechanisms.
2778 D-Bus also allows any standard SASL mechanism of course.
2780 <sect3 id="auth-mechanisms-sha">
2781 <title>DBUS_COOKIE_SHA1</title>
2783 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2784 has the ability to read a private file owned by the user being
2785 authenticated. If the client can prove that it has access to a secret
2786 cookie stored in this file, then the client is authenticated.
2787 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2791 Throughout this description, "hex encoding" must output the digits
2792 from a to f in lower-case; the digits A to F must not be used
2793 in the DBUS_COOKIE_SHA1 mechanism.
2796 Authentication proceeds as follows:
2800 The client sends the username it would like to authenticate
2806 The server sends the name of its "cookie context" (see below); a
2807 space character; the integer ID of the secret cookie the client
2808 must demonstrate knowledge of; a space character; then a
2809 randomly-generated challenge string, all of this hex-encoded into
2815 The client locates the cookie and generates its own
2816 randomly-generated challenge string. The client then concatenates
2817 the server's decoded challenge, a ":" character, its own challenge,
2818 another ":" character, and the cookie. It computes the SHA-1 hash
2819 of this composite string as a hex digest. It concatenates the
2820 client's challenge string, a space character, and the SHA-1 hex
2821 digest, hex-encodes the result and sends it back to the server.
2826 The server generates the same concatenated string used by the
2827 client and computes its SHA-1 hash. It compares the hash with
2828 the hash received from the client; if the two hashes match, the
2829 client is authenticated.
2835 Each server has a "cookie context," which is a name that identifies a
2836 set of cookies that apply to that server. A sample context might be
2837 "org_freedesktop_session_bus". Context names must be valid ASCII,
2838 nonzero length, and may not contain the characters slash ("/"),
2839 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2840 tab ("\t"), or period ("."). There is a default context,
2841 "org_freedesktop_general" that's used by servers that do not specify
2845 Cookies are stored in a user's home directory, in the directory
2846 <filename>~/.dbus-keyrings/</filename>. This directory must
2847 not be readable or writable by other users. If it is,
2848 clients and servers must ignore it. The directory
2849 contains cookie files named after the cookie context.
2852 A cookie file contains one cookie per line. Each line
2853 has three space-separated fields:
2857 The cookie ID number, which must be a non-negative integer and
2858 may not be used twice in the same file.
2863 The cookie's creation time, in UNIX seconds-since-the-epoch
2869 The cookie itself, a hex-encoded random block of bytes. The cookie
2870 may be of any length, though obviously security increases
2871 as the length increases.
2877 Only server processes modify the cookie file.
2878 They must do so with this procedure:
2882 Create a lockfile name by appending ".lock" to the name of the
2883 cookie file. The server should attempt to create this file
2884 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2885 fails, the lock fails. Servers should retry for a reasonable
2886 period of time, then they may choose to delete an existing lock
2887 to keep users from having to manually delete a stale
2888 lock. <footnote><para>Lockfiles are used instead of real file
2889 locking <literal>fcntl()</literal> because real locking
2890 implementations are still flaky on network
2891 filesystems.</para></footnote>
2896 Once the lockfile has been created, the server loads the cookie
2897 file. It should then delete any cookies that are old (the
2898 timeout can be fairly short), or more than a reasonable
2899 time in the future (so that cookies never accidentally
2900 become permanent, if the clock was set far into the future
2901 at some point). If no recent keys remain, the
2902 server may generate a new key.
2907 The pruned and possibly added-to cookie file
2908 must be resaved atomically (using a temporary
2909 file which is rename()'d).
2914 The lock must be dropped by deleting the lockfile.
2920 Clients need not lock the file in order to load it,
2921 because servers are required to save the file atomically.
2926 <sect1 id="addresses">
2927 <title>Server Addresses</title>
2929 Server addresses consist of a transport name followed by a colon, and
2930 then an optional, comma-separated list of keys and values in the form key=value.
2931 Each value is escaped.
2935 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2936 Which is the address to a unix socket with the path /tmp/dbus-test.
2939 Value escaping is similar to URI escaping but simpler.
2943 The set of optionally-escaped bytes is:
2944 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2945 <emphasis>byte</emphasis> (note, not character) which is not in the
2946 set of optionally-escaped bytes must be replaced with an ASCII
2947 percent (<literal>%</literal>) and the value of the byte in hex.
2948 The hex value must always be two digits, even if the first digit is
2949 zero. The optionally-escaped bytes may be escaped if desired.
2954 To unescape, append each byte in the value; if a byte is an ASCII
2955 percent (<literal>%</literal>) character then append the following
2956 hex value instead. It is an error if a <literal>%</literal> byte
2957 does not have two hex digits following. It is an error if a
2958 non-optionally-escaped byte is seen unescaped.
2962 The set of optionally-escaped bytes is intended to preserve address
2963 readability and convenience.
2967 A server may specify a key-value pair with the key <literal>guid</literal>
2968 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2969 describes the format of the <literal>guid</literal> field. If present,
2970 this UUID may be used to distinguish one server address from another. A
2971 server should use a different UUID for each address it listens on. For
2972 example, if a message bus daemon offers both UNIX domain socket and TCP
2973 connections, but treats clients the same regardless of how they connect,
2974 those two connections are equivalent post-connection but should have
2975 distinct UUIDs to distinguish the kinds of connection.
2979 The intent of the address UUID feature is to allow a client to avoid
2980 opening multiple identical connections to the same server, by allowing the
2981 client to check whether an address corresponds to an already-existing
2982 connection. Comparing two addresses is insufficient, because addresses
2983 can be recycled by distinct servers, and equivalent addresses may look
2984 different if simply compared as strings (for example, the host in a TCP
2985 address can be given as an IP address or as a hostname).
2989 Note that the address key is <literal>guid</literal> even though the
2990 rest of the API and documentation says "UUID," for historical reasons.
2994 [FIXME clarify if attempting to connect to each is a requirement
2995 or just a suggestion]
2996 When connecting to a server, multiple server addresses can be
2997 separated by a semi-colon. The library will then try to connect
2998 to the first address and if that fails, it'll try to connect to
2999 the next one specified, and so forth. For example
3000 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
3004 Some addresses are <firstterm>connectable</firstterm>. A connectable
3005 address is one containing enough information for a client to connect
3006 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
3007 is a connectable address. It is not necessarily possible to listen
3008 on every connectable address: for instance, it is not possible to
3009 listen on a <literal>unixexec:</literal> address.
3013 Some addresses are <firstterm>listenable</firstterm>. A listenable
3014 address is one containing enough information for a server to listen on
3015 it, producing a connectable address (which may differ from the
3016 original address). Many listenable addresses are not connectable:
3017 for instance, <literal>tcp:host=127.0.0.1</literal>
3018 is listenable, but not connectable (because it does not specify
3023 Listening on an address that is not connectable will result in a
3024 connectable address that is not the same as the listenable address.
3025 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3026 might result in the connectable address
3027 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3028 or listening on <literal>unix:tmpdir=/tmp</literal>
3029 might result in the connectable address
3030 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3034 <sect1 id="transports">
3035 <title>Transports</title>
3037 [FIXME we need to specify in detail each transport and its possible arguments]
3039 Current transports include: unix domain sockets (including
3040 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3041 using in-process pipes. Future possible transports include one that
3042 tunnels over X11 protocol.
3045 <sect2 id="transports-unix-domain-sockets">
3046 <title>Unix Domain Sockets</title>
3048 Unix domain sockets can be either paths in the file system or on Linux
3049 kernels, they can be abstract which are similar to paths but
3050 do not show up in the file system.
3054 When a socket is opened by the D-Bus library it truncates the path
3055 name right before the first trailing Nul byte. This is true for both
3056 normal paths and abstract paths. Note that this is a departure from
3057 previous versions of D-Bus that would create sockets with a fixed
3058 length path name. Names which were shorter than the fixed length
3059 would be padded by Nul bytes.
3062 Unix domain sockets are not available on Windows.
3065 Unix addresses that specify <literal>path</literal> or
3066 <literal>abstract</literal> are both listenable and connectable.
3067 Unix addresses that specify <literal>tmpdir</literal> are only
3068 listenable: the corresponding connectable address will specify
3069 either <literal>path</literal> or <literal>abstract</literal>.
3071 <sect3 id="transports-unix-domain-sockets-addresses">
3072 <title>Server Address Format</title>
3074 Unix domain socket addresses are identified by the "unix:" prefix
3075 and support the following key/value pairs:
3082 <entry>Values</entry>
3083 <entry>Description</entry>
3089 <entry>(path)</entry>
3090 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3093 <entry>tmpdir</entry>
3094 <entry>(path)</entry>
3095 <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>
3098 <entry>abstract</entry>
3099 <entry>(string)</entry>
3100 <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>
3106 Exactly one of the keys <literal>path</literal>,
3107 <literal>abstract</literal> or
3108 <literal>tmpdir</literal> must be provided.
3112 <sect2 id="transports-launchd">
3113 <title>launchd</title>
3115 launchd is an open-source server management system that replaces init, inetd
3116 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3117 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3121 launchd allocates a socket and provides it with the unix path through the
3122 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3123 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3124 it through its environment.
3125 Other processes can query for the launchd socket by executing:
3126 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3127 This is normally done by the D-Bus client library so doesn't have to be done
3131 launchd is not available on Microsoft Windows.
3134 launchd addresses are listenable and connectable.
3136 <sect3 id="transports-launchd-addresses">
3137 <title>Server Address Format</title>
3139 launchd addresses are identified by the "launchd:" prefix
3140 and support the following key/value pairs:
3147 <entry>Values</entry>
3148 <entry>Description</entry>
3154 <entry>(environment variable)</entry>
3155 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3161 The <literal>env</literal> key is required.
3165 <sect2 id="transports-systemd">
3166 <title>systemd</title>
3168 systemd is an open-source server management system that
3169 replaces init and inetd on newer Linux systems. It supports
3170 socket activation. The D-Bus systemd transport is used to acquire
3171 socket activation file descriptors from systemd and use them
3172 as D-Bus transport when the current process is spawned by
3173 socket activation from it.
3176 The systemd transport accepts only one or more Unix domain or
3177 TCP streams sockets passed in via socket activation.
3180 The systemd transport is not available on non-Linux operating systems.
3183 The systemd transport defines no parameter keys.
3186 systemd addresses are listenable, but not connectable. The
3187 corresponding connectable address is the <literal>unix</literal>
3188 or <literal>tcp</literal> address of the socket.
3191 <sect2 id="transports-tcp-sockets">
3192 <title>TCP Sockets</title>
3194 The tcp transport provides TCP/IP based connections between clients
3195 located on the same or different hosts.
3198 Using tcp transport without any additional secure authentification mechanismus
3199 over a network is unsecure.
3202 On Windows and most Unix platforms, the TCP stack is unable to transfer
3203 credentials over a TCP connection, so the EXTERNAL authentication
3204 mechanism does not work for this transport.
3207 All <literal>tcp</literal> addresses are listenable.
3208 <literal>tcp</literal> addresses in which both
3209 <literal>host</literal> and <literal>port</literal> are
3210 specified, and <literal>port</literal> is non-zero,
3211 are also connectable.
3213 <sect3 id="transports-tcp-sockets-addresses">
3214 <title>Server Address Format</title>
3216 TCP/IP socket addresses are identified by the "tcp:" prefix
3217 and support the following key/value pairs:
3224 <entry>Values</entry>
3225 <entry>Description</entry>
3231 <entry>(string)</entry>
3232 <entry>DNS name or IP address</entry>
3236 <entry>(string)</entry>
3237 <entry>Used in a listenable address to configure the interface
3238 on which the server will listen: either the IP address of one of
3239 the local machine's interfaces (most commonly <literal>127.0.0.1
3240 </literal>), or a DNS name that resolves to one of those IP
3241 addresses, or '*' to listen on all interfaces simultaneously.
3242 If not specified, the default is the same value as "host".
3247 <entry>(number)</entry>
3248 <entry>The tcp port the server will open. A zero value let the server
3249 choose a free port provided from the underlaying operating system.
3250 libdbus is able to retrieve the real used port from the server.
3254 <entry>family</entry>
3255 <entry>(string)</entry>
3256 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3263 <sect2 id="transports-nonce-tcp-sockets">
3264 <title>Nonce-secured TCP Sockets</title>
3266 The nonce-tcp transport provides a secured TCP transport, using a
3267 simple authentication mechanism to ensure that only clients with read
3268 access to a certain location in the filesystem can connect to the server.
3269 The server writes a secret, the nonce, to a file and an incoming client
3270 connection is only accepted if the client sends the nonce right after
3271 the connect. The nonce mechanism requires no setup and is orthogonal to
3272 the higher-level authentication mechanisms described in the
3273 Authentication section.
3277 On start, the server generates a random 16 byte nonce and writes it
3278 to a file in the user's temporary directory. The nonce file location
3279 is published as part of the server's D-Bus address using the
3280 "noncefile" key-value pair.
3282 After an accept, the server reads 16 bytes from the socket. If the
3283 read bytes do not match the nonce stored in the nonce file, the
3284 server MUST immediately drop the connection.
3285 If the nonce match the received byte sequence, the client is accepted
3286 and the transport behaves like an unsecured tcp transport.
3289 After a successful connect to the server socket, the client MUST read
3290 the nonce from the file published by the server via the noncefile=
3291 key-value pair and send it over the socket. After that, the
3292 transport behaves like an unsecured tcp transport.
3295 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3296 <literal>host</literal>, <literal>port</literal> and
3297 <literal>noncefile</literal> are all specified,
3298 and <literal>port</literal> is nonzero, are also connectable.
3300 <sect3 id="transports-nonce-tcp-sockets-addresses">
3301 <title>Server Address Format</title>
3303 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3304 and support the following key/value pairs:
3311 <entry>Values</entry>
3312 <entry>Description</entry>
3318 <entry>(string)</entry>
3319 <entry>DNS name or IP address</entry>
3323 <entry>(string)</entry>
3324 <entry>The same as for tcp: addresses
3329 <entry>(number)</entry>
3330 <entry>The tcp port the server will open. A zero value let the server
3331 choose a free port provided from the underlaying operating system.
3332 libdbus is able to retrieve the real used port from the server.
3336 <entry>family</entry>
3337 <entry>(string)</entry>
3338 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3341 <entry>noncefile</entry>
3342 <entry>(path)</entry>
3343 <entry>File location containing the secret.
3344 This is only meaningful in connectable addresses:
3345 a listening D-Bus server that offers this transport
3346 will always create a new nonce file.</entry>
3353 <sect2 id="transports-exec">
3354 <title>Executed Subprocesses on Unix</title>
3356 This transport forks off a process and connects its standard
3357 input and standard output with an anonymous Unix domain
3358 socket. This socket is then used for communication by the
3359 transport. This transport may be used to use out-of-process
3360 forwarder programs as basis for the D-Bus protocol.
3363 The forked process will inherit the standard error output and
3364 process group from the parent process.
3367 Executed subprocesses are not available on Windows.
3370 <literal>unixexec</literal> addresses are connectable, but are not
3373 <sect3 id="transports-exec-addresses">
3374 <title>Server Address Format</title>
3376 Executed subprocess addresses are identified by the "unixexec:" prefix
3377 and support the following key/value pairs:
3384 <entry>Values</entry>
3385 <entry>Description</entry>
3391 <entry>(path)</entry>
3392 <entry>Path of the binary to execute, either an absolute
3393 path or a binary name that is searched for in the default
3394 search path of the OS. This corresponds to the first
3395 argument of execlp(). This key is mandatory.</entry>
3398 <entry>argv0</entry>
3399 <entry>(string)</entry>
3400 <entry>The program name to use when executing the
3401 binary. If omitted the same value as specified for path=
3402 will be used. This corresponds to the second argument of
3406 <entry>argv1, argv2, ...</entry>
3407 <entry>(string)</entry>
3408 <entry>Arguments to pass to the binary. This corresponds
3409 to the third and later arguments of execlp(). If a
3410 specific argvX is not specified no further argvY for Y > X
3411 are taken into account.</entry>
3419 <sect1 id="meta-transports">
3420 <title>Meta Transports</title>
3422 Meta transports are a kind of transport with special enhancements or
3423 behavior. Currently available meta transports include: autolaunch
3426 <sect2 id="meta-transports-autolaunch">
3427 <title>Autolaunch</title>
3428 <para>The autolaunch transport provides a way for dbus clients to autodetect
3429 a running dbus session bus and to autolaunch a session bus if not present.
3432 On Unix, <literal>autolaunch</literal> addresses are connectable,
3436 On Windows, <literal>autolaunch</literal> addresses are both
3437 connectable and listenable.
3440 <sect3 id="meta-transports-autolaunch-addresses">
3441 <title>Server Address Format</title>
3443 Autolaunch addresses uses the "autolaunch:" prefix and support the
3444 following key/value pairs:
3451 <entry>Values</entry>
3452 <entry>Description</entry>
3457 <entry>scope</entry>
3458 <entry>(string)</entry>
3459 <entry>scope of autolaunch (Windows only)
3463 "*install-path" - limit session bus to dbus installation path.
3464 The dbus installation path is determined from the location of
3465 the shared dbus library. If the library is located in a 'bin'
3466 subdirectory the installation root is the directory above,
3467 otherwise the directory where the library lives is taken as
3470 <install-root>/bin/[lib]dbus-1.dll
3471 <install-root>/[lib]dbus-1.dll
3477 "*user" - limit session bus to the recent user.
3482 other values - specify dedicated session bus like "release",
3494 <sect3 id="meta-transports-autolaunch-windows-implementation">
3495 <title>Windows implementation</title>
3497 On start, the server opens a platform specific transport, creates a mutex
3498 and a shared memory section containing the related session bus address.
3499 This mutex will be inspected by the dbus client library to detect a
3500 running dbus session bus. The access to the mutex and the shared memory
3501 section are protected by global locks.
3504 In the recent implementation the autolaunch transport uses a tcp transport
3505 on localhost with a port choosen from the operating system. This detail may
3506 change in the future.
3509 Disclaimer: The recent implementation is in an early state and may not
3510 work in all cirumstances and/or may have security issues. Because of this
3511 the implementation is not documentated yet.
3518 <title>UUIDs</title>
3520 A working D-Bus implementation uses universally-unique IDs in two places.
3521 First, each server address has a UUID identifying the address,
3522 as described in <xref linkend="addresses"/>. Second, each operating
3523 system kernel instance running a D-Bus client or server has a UUID
3524 identifying that kernel, retrieved by invoking the method
3525 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3526 linkend="standard-interfaces-peer"/>).
3529 The term "UUID" in this document is intended literally, i.e. an
3530 identifier that is universally unique. It is not intended to refer to
3531 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3534 The UUID must contain 128 bits of data and be hex-encoded. The
3535 hex-encoded string may not contain hyphens or other non-hex-digit
3536 characters, and it must be exactly 32 characters long. To generate a
3537 UUID, the current reference implementation concatenates 96 bits of random
3538 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3542 It would also be acceptable and probably better to simply generate 128
3543 bits of random data, as long as the random number generator is of high
3544 quality. The timestamp could conceivably help if the random bits are not
3545 very random. With a quality random number generator, collisions are
3546 extremely unlikely even with only 96 bits, so it's somewhat academic.
3549 Implementations should, however, stick to random data for the first 96 bits
3554 <sect1 id="standard-interfaces">
3555 <title>Standard Interfaces</title>
3557 See <xref linkend="message-protocol-types-notation"/> for details on
3558 the notation used in this section. There are some standard interfaces
3559 that may be useful across various D-Bus applications.
3561 <sect2 id="standard-interfaces-peer">
3562 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3564 The <literal>org.freedesktop.DBus.Peer</literal> interface
3567 org.freedesktop.DBus.Peer.Ping ()
3568 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3572 On receipt of the <literal>METHOD_CALL</literal> message
3573 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3574 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3575 usual. It does not matter which object path a ping is sent to. The
3576 reference implementation handles this method automatically.
3579 On receipt of the <literal>METHOD_CALL</literal> message
3580 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3581 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3582 UUID representing the identity of the machine the process is running on.
3583 This UUID must be the same for all processes on a single system at least
3584 until that system next reboots. It should be the same across reboots
3585 if possible, but this is not always possible to implement and is not
3587 It does not matter which object path a GetMachineId is sent to. The
3588 reference implementation handles this method automatically.
3591 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3592 a virtual machine running on a hypervisor, rather than a physical machine.
3593 Basically if two processes see the same UUID, they should also see the same
3594 shared memory, UNIX domain sockets, process IDs, and other features that require
3595 a running OS kernel in common between the processes.
3598 The UUID is often used where other programs might use a hostname. Hostnames
3599 can change without rebooting, however, or just be "localhost" - so the UUID
3603 <xref linkend="uuids"/> explains the format of the UUID.
3607 <sect2 id="standard-interfaces-introspectable">
3608 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3610 This interface has one method:
3612 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3616 Objects instances may implement
3617 <literal>Introspect</literal> which returns an XML description of
3618 the object, including its interfaces (with signals and methods), objects
3619 below it in the object path tree, and its properties.
3622 <xref linkend="introspection-format"/> describes the format of this XML string.
3625 <sect2 id="standard-interfaces-properties">
3626 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3628 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3629 or <firstterm>attributes</firstterm>. These can be exposed via the
3630 <literal>org.freedesktop.DBus.Properties</literal> interface.
3634 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3635 in STRING property_name,
3637 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3638 in STRING property_name,
3640 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3641 out DICT<STRING,VARIANT> props);
3645 It is conventional to give D-Bus properties names consisting of
3646 capitalized words without punctuation ("CamelCase"), like
3647 <link linkend="message-protocol-names-member">member names</link>.
3648 For instance, the GObject property
3649 <literal>connection-status</literal> or the Qt property
3650 <literal>connectionStatus</literal> could be represented on D-Bus
3651 as <literal>ConnectionStatus</literal>.
3654 Strictly speaking, D-Bus property names are not required to follow
3655 the same naming restrictions as member names, but D-Bus property
3656 names that would not be valid member names (in particular,
3657 GObject-style dash-separated property names) can cause interoperability
3658 problems and should be avoided.
3661 The available properties and whether they are writable can be determined
3662 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3663 see <xref linkend="standard-interfaces-introspectable"/>.
3666 An empty string may be provided for the interface name; in this case,
3667 if there are multiple properties on an object with the same name,
3668 the results are undefined (picking one by according to an arbitrary
3669 deterministic rule, or returning an error, are the reasonable
3673 If one or more properties change on an object, the
3674 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3675 signal may be emitted (this signal was added in 0.14):
3679 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3680 DICT<STRING,VARIANT> changed_properties,
3681 ARRAY<STRING> invalidated_properties);
3685 where <literal>changed_properties</literal> is a dictionary
3686 containing the changed properties with the new values and
3687 <literal>invalidated_properties</literal> is an array of
3688 properties that changed but the value is not conveyed.
3691 Whether the <literal>PropertiesChanged</literal> signal is
3692 supported can be determined by calling
3693 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3694 that the signal may be supported for an object but it may
3695 differ how whether and how it is used on a per-property basis
3696 (for e.g. performance or security reasons). Each property (or
3697 the parent interface) must be annotated with the
3698 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3699 annotation to convey this (usually the default value
3700 <literal>true</literal> is sufficient meaning that the
3701 annotation does not need to be used). See <xref
3702 linkend="introspection-format"/> for details on this
3707 <sect2 id="standard-interfaces-objectmanager">
3708 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3710 An API can optionally make use of this interface for one or
3711 more sub-trees of objects. The root of each sub-tree implements
3712 this interface so other applications can get all objects,
3713 interfaces and properties in a single method call. It is
3714 appropriate to use this interface if users of the tree of
3715 objects are expected to be interested in all interfaces of all
3716 objects in the tree; a more granular API should be used if
3717 users of the objects are expected to be interested in a small
3718 subset of the objects, a small subset of their interfaces, or
3722 The method that applications can use to get all objects and
3723 properties is <literal>GetManagedObjects</literal>:
3727 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3731 The return value of this method is a dict whose keys are
3732 object paths. All returned object paths are children of the
3733 object path implementing this interface, i.e. their object
3734 paths start with the ObjectManager's object path plus '/'.
3737 Each value is a dict whose keys are interfaces names. Each
3738 value in this inner dict is the same dict that would be
3739 returned by the <link
3740 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3741 method for that combination of object path and interface. If
3742 an interface has no properties, the empty dict is returned.
3745 Changes are emitted using the following two signals:
3749 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3750 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3751 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3752 ARRAY<STRING> interfaces);
3756 The <literal>InterfacesAdded</literal> signal is emitted when
3757 either a new object is added or when an existing object gains
3758 one or more interfaces. The
3759 <literal>InterfacesRemoved</literal> signal is emitted
3760 whenever an object is removed or it loses one or more
3761 interfaces. The second parameter of the
3762 <literal>InterfacesAdded</literal> signal contains a dict with
3763 the interfaces and properties (if any) that have been added to
3764 the given object path. Similarly, the second parameter of the
3765 <literal>InterfacesRemoved</literal> signal contains an array
3766 of the interfaces that were removed. Note that changes on
3767 properties on existing interfaces are not reported using this
3768 interface - an application should also monitor the existing <link
3769 linkend="standard-interfaces-properties">PropertiesChanged</link>
3770 signal on each object.
3773 Applications SHOULD NOT export objects that are children of an
3774 object (directly or otherwise) implementing this interface but
3775 which are not returned in the reply from the
3776 <literal>GetManagedObjects()</literal> method of this
3777 interface on the given object.
3780 The intent of the <literal>ObjectManager</literal> interface
3781 is to make it easy to write a robust client
3782 implementation. The trivial client implementation only needs
3783 to make two method calls:
3787 org.freedesktop.DBus.AddMatch (bus_proxy,
3788 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3789 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3793 on the message bus and the remote application's
3794 <literal>ObjectManager</literal>, respectively. Whenever a new
3795 remote object is created (or an existing object gains a new
3796 interface), the <literal>InterfacesAdded</literal> signal is
3797 emitted, and since this signal contains all properties for the
3798 interfaces, no calls to the
3799 <literal>org.freedesktop.Properties</literal> interface on the
3800 remote object are needed. Additionally, since the initial
3801 <literal>AddMatch()</literal> rule already includes signal
3802 messages from the newly created child object, no new
3803 <literal>AddMatch()</literal> call is needed.
3808 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3809 interface was added in version 0.17 of the D-Bus
3816 <sect1 id="introspection-format">
3817 <title>Introspection Data Format</title>
3819 As described in <xref linkend="standard-interfaces-introspectable"/>,
3820 objects may be introspected at runtime, returning an XML string
3821 that describes the object. The same XML format may be used in
3822 other contexts as well, for example as an "IDL" for generating
3823 static language bindings.
3826 Here is an example of introspection data:
3828 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3829 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3830 <node name="/com/example/sample_object">
3831 <interface name="com.example.SampleInterface">
3832 <method name="Frobate">
3833 <arg name="foo" type="i" direction="in"/>
3834 <arg name="bar" type="s" direction="out"/>
3835 <arg name="baz" type="a{us}" direction="out"/>
3836 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3838 <method name="Bazify">
3839 <arg name="bar" type="(iiu)" direction="in"/>
3840 <arg name="bar" type="v" direction="out"/>
3842 <method name="Mogrify">
3843 <arg name="bar" type="(iiav)" direction="in"/>
3845 <signal name="Changed">
3846 <arg name="new_value" type="b"/>
3848 <property name="Bar" type="y" access="readwrite"/>
3850 <node name="child_of_sample_object"/>
3851 <node name="another_child_of_sample_object"/>
3856 A more formal DTD and spec needs writing, but here are some quick notes.
3860 Only the root <node> element can omit the node name, as it's
3861 known to be the object that was introspected. If the root
3862 <node> does have a name attribute, it must be an absolute
3863 object path. If child <node> have object paths, they must be
3869 If a child <node> has any sub-elements, then they
3870 must represent a complete introspection of the child.
3871 If a child <node> is empty, then it may or may
3872 not have sub-elements; the child must be introspected
3873 in order to find out. The intent is that if an object
3874 knows that its children are "fast" to introspect
3875 it can go ahead and return their information, but
3876 otherwise it can omit it.
3881 The direction element on <arg> may be omitted,
3882 in which case it defaults to "in" for method calls
3883 and "out" for signals. Signals only allow "out"
3884 so while direction may be specified, it's pointless.
3889 The possible directions are "in" and "out",
3890 unlike CORBA there is no "inout"
3895 The possible property access flags are
3896 "readwrite", "read", and "write"
3901 Multiple interfaces can of course be listed for
3907 The "name" attribute on arguments is optional.
3913 Method, interface, property, and signal elements may have
3914 "annotations", which are generic key/value pairs of metadata.
3915 They are similar conceptually to Java's annotations and C# attributes.
3916 Well-known annotations:
3923 <entry>Values (separated by ,)</entry>
3924 <entry>Description</entry>
3929 <entry>org.freedesktop.DBus.Deprecated</entry>
3930 <entry>true,false</entry>
3931 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3934 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3935 <entry>(string)</entry>
3936 <entry>The C symbol; may be used for methods and interfaces</entry>
3939 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3940 <entry>true,false</entry>
3941 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3944 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3945 <entry>true,invalidates,false</entry>
3948 If set to <literal>false</literal>, the
3949 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3951 linkend="standard-interfaces-properties"/> is not
3952 guaranteed to be emitted if the property changes.
3955 If set to <literal>invalidates</literal> the signal
3956 is emitted but the value is not included in the
3960 If set to <literal>true</literal> the signal is
3961 emitted with the value included.
3964 The value for the annotation defaults to
3965 <literal>true</literal> if the enclosing interface
3966 element does not specify the annotation. Otherwise it
3967 defaults to the value specified in the enclosing
3976 <sect1 id="message-bus">
3977 <title>Message Bus Specification</title>
3978 <sect2 id="message-bus-overview">
3979 <title>Message Bus Overview</title>
3981 The message bus accepts connections from one or more applications.
3982 Once connected, applications can exchange messages with other
3983 applications that are also connected to the bus.
3986 In order to route messages among connections, the message bus keeps a
3987 mapping from names to connections. Each connection has one
3988 unique-for-the-lifetime-of-the-bus name automatically assigned.
3989 Applications may request additional names for a connection. Additional
3990 names are usually "well-known names" such as
3991 "com.example.TextEditor". When a name is bound to a connection,
3992 that connection is said to <firstterm>own</firstterm> the name.
3995 The bus itself owns a special name,
3996 <literal>org.freedesktop.DBus</literal>, with an object
3997 located at <literal>/org/freedesktop/DBus</literal> that
3998 implements the <literal>org.freedesktop.DBus</literal>
3999 interface. This service allows applications to make
4000 administrative requests of the bus itself. For example,
4001 applications can ask the bus to assign a name to a connection.
4004 Each name may have <firstterm>queued owners</firstterm>. When an
4005 application requests a name for a connection and the name is already in
4006 use, the bus will optionally add the connection to a queue waiting for
4007 the name. If the current owner of the name disconnects or releases
4008 the name, the next connection in the queue will become the new owner.
4012 This feature causes the right thing to happen if you start two text
4013 editors for example; the first one may request "com.example.TextEditor",
4014 and the second will be queued as a possible owner of that name. When
4015 the first exits, the second will take over.
4019 Applications may send <firstterm>unicast messages</firstterm> to
4020 a specific recipient or to the message bus itself, or
4021 <firstterm>broadcast messages</firstterm> to all interested recipients.
4022 See <xref linkend="message-bus-routing"/> for details.
4026 <sect2 id="message-bus-names">
4027 <title>Message Bus Names</title>
4029 Each connection has at least one name, assigned at connection time and
4030 returned in response to the
4031 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4032 automatically-assigned name is called the connection's <firstterm>unique
4033 name</firstterm>. Unique names are never reused for two different
4034 connections to the same bus.
4037 Ownership of a unique name is a prerequisite for interaction with
4038 the message bus. It logically follows that the unique name is always
4039 the first name that an application comes to own, and the last
4040 one that it loses ownership of.
4043 Unique connection names must begin with the character ':' (ASCII colon
4044 character); bus names that are not unique names must not begin
4045 with this character. (The bus must reject any attempt by an application
4046 to manually request a name beginning with ':'.) This restriction
4047 categorically prevents "spoofing"; messages sent to a unique name
4048 will always go to the expected connection.
4051 When a connection is closed, all the names that it owns are deleted (or
4052 transferred to the next connection in the queue if any).
4055 A connection can request additional names to be associated with it using
4056 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4057 linkend="message-protocol-names-bus"/> describes the format of a valid
4058 name. These names can be released again using the
4059 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4062 <sect3 id="bus-messages-request-name">
4063 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4067 UINT32 RequestName (in STRING name, in UINT32 flags)
4074 <entry>Argument</entry>
4076 <entry>Description</entry>
4082 <entry>STRING</entry>
4083 <entry>Name to request</entry>
4087 <entry>UINT32</entry>
4088 <entry>Flags</entry>
4098 <entry>Argument</entry>
4100 <entry>Description</entry>
4106 <entry>UINT32</entry>
4107 <entry>Return value</entry>
4114 This method call should be sent to
4115 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4116 assign the given name to the method caller. Each name maintains a
4117 queue of possible owners, where the head of the queue is the primary
4118 or current owner of the name. Each potential owner in the queue
4119 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4120 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4121 call. When RequestName is invoked the following occurs:
4125 If the method caller is currently the primary owner of the name,
4126 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4127 values are updated with the values from the new RequestName call,
4128 and nothing further happens.
4134 If the current primary owner (head of the queue) has
4135 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4136 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4137 the caller of RequestName replaces the current primary owner at
4138 the head of the queue and the current primary owner moves to the
4139 second position in the queue. If the caller of RequestName was
4140 in the queue previously its flags are updated with the values from
4141 the new RequestName in addition to moving it to the head of the queue.
4147 If replacement is not possible, and the method caller is
4148 currently in the queue but not the primary owner, its flags are
4149 updated with the values from the new RequestName call.
4155 If replacement is not possible, and the method caller is
4156 currently not in the queue, the method caller is appended to the
4163 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4164 set and is not the primary owner, it is removed from the
4165 queue. This can apply to the previous primary owner (if it
4166 was replaced) or the method caller (if it updated the
4167 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4168 queue, or if it was just added to the queue with that flag set).
4174 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4175 queue," even if another application already in the queue had specified
4176 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4177 that does not allow replacement goes away, and the next primary owner
4178 does allow replacement. In this case, queued items that specified
4179 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4180 automatically replace the new primary owner. In other words,
4181 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4182 time RequestName is called. This is deliberate to avoid an infinite loop
4183 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4184 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4187 The flags argument contains any of the following values logically ORed
4194 <entry>Conventional Name</entry>
4195 <entry>Value</entry>
4196 <entry>Description</entry>
4201 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4205 If an application A specifies this flag and succeeds in
4206 becoming the owner of the name, and another application B
4207 later calls RequestName with the
4208 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4209 will lose ownership and receive a
4210 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4211 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4212 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4213 is not specified by application B, then application B will not replace
4214 application A as the owner.
4219 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4223 Try to replace the current owner if there is one. If this
4224 flag is not set the application will only become the owner of
4225 the name if there is no current owner. If this flag is set,
4226 the application will replace the current owner if
4227 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4232 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4236 Without this flag, if an application requests a name that is
4237 already owned, the application will be placed in a queue to
4238 own the name when the current owner gives it up. If this
4239 flag is given, the application will not be placed in the
4240 queue, the request for the name will simply fail. This flag
4241 also affects behavior when an application is replaced as
4242 name owner; by default the application moves back into the
4243 waiting queue, unless this flag was provided when the application
4244 became the name owner.
4252 The return code can be one of the following values:
4258 <entry>Conventional Name</entry>
4259 <entry>Value</entry>
4260 <entry>Description</entry>
4265 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4266 <entry>1</entry> <entry>The caller is now the primary owner of
4267 the name, replacing any previous owner. Either the name had no
4268 owner before, or the caller specified
4269 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4270 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4273 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4276 <entry>The name already had an owner,
4277 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4278 the current owner did not specify
4279 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4280 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4284 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4285 <entry>The name already has an owner,
4286 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4287 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4288 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4289 specified by the requesting application.</entry>
4292 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4294 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4302 <sect3 id="bus-messages-release-name">
4303 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4307 UINT32 ReleaseName (in STRING name)
4314 <entry>Argument</entry>
4316 <entry>Description</entry>
4322 <entry>STRING</entry>
4323 <entry>Name to release</entry>
4333 <entry>Argument</entry>
4335 <entry>Description</entry>
4341 <entry>UINT32</entry>
4342 <entry>Return value</entry>
4349 This method call should be sent to
4350 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4351 release the method caller's claim to the given name. If the caller is
4352 the primary owner, a new primary owner will be selected from the
4353 queue if any other owners are waiting. If the caller is waiting in
4354 the queue for the name, the caller will removed from the queue and
4355 will not be made an owner of the name if it later becomes available.
4356 If there are no other owners in the queue for the name, it will be
4357 removed from the bus entirely.
4359 The return code can be one of the following values:
4365 <entry>Conventional Name</entry>
4366 <entry>Value</entry>
4367 <entry>Description</entry>
4372 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4373 <entry>1</entry> <entry>The caller has released his claim on
4374 the given name. Either the caller was the primary owner of
4375 the name, and the name is now unused or taken by somebody
4376 waiting in the queue for the name, or the caller was waiting
4377 in the queue for the name and has now been removed from the
4381 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4383 <entry>The given name does not exist on this bus.</entry>
4386 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4388 <entry>The caller was not the primary owner of this name,
4389 and was also not waiting in the queue to own this name.</entry>
4397 <sect3 id="bus-messages-list-queued-owners">
4398 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4402 ARRAY of STRING ListQueuedOwners (in STRING name)
4409 <entry>Argument</entry>
4411 <entry>Description</entry>
4417 <entry>STRING</entry>
4418 <entry>The well-known bus name to query, such as
4419 <literal>com.example.cappuccino</literal></entry>
4429 <entry>Argument</entry>
4431 <entry>Description</entry>
4437 <entry>ARRAY of STRING</entry>
4438 <entry>The unique bus names of connections currently queued
4439 for the name</entry>
4446 This method call should be sent to
4447 <literal>org.freedesktop.DBus</literal> and lists the connections
4448 currently queued for a bus name (see
4449 <xref linkend="term-queued-owner"/>).
4454 <sect2 id="message-bus-routing">
4455 <title>Message Bus Message Routing</title>
4458 Messages may have a <literal>DESTINATION</literal> field (see <xref
4459 linkend="message-protocol-header-fields"/>), resulting in a
4460 <firstterm>unicast message</firstterm>. If the
4461 <literal>DESTINATION</literal> field is present, it specifies a message
4462 recipient by name. Method calls and replies normally specify this field.
4463 The message bus must send messages (of any type) with the
4464 <literal>DESTINATION</literal> field set to the specified recipient,
4465 regardless of whether the recipient has set up a match rule matching
4470 When the message bus receives a signal, if the
4471 <literal>DESTINATION</literal> field is absent, it is considered to
4472 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4473 applications with <firstterm>message matching rules</firstterm> that
4474 match the message. Most signal messages are broadcasts, and
4475 no other message types currently defined in this specification
4480 Unicast signal messages (those with a <literal>DESTINATION</literal>
4481 field) are not commonly used, but they are treated like any unicast
4482 message: they are delivered to the specified receipient,
4483 regardless of its match rules. One use for unicast signals is to
4484 avoid a race condition in which a signal is emitted before the intended
4485 recipient can call <xref linkend="bus-messages-add-match"/> to
4486 receive that signal: if the signal is sent directly to that recipient
4487 using a unicast message, it does not need to add a match rule at all,
4488 and there is no race condition. Another use for unicast signals,
4489 on message buses whose security policy prevents eavesdropping, is to
4490 send sensitive information which should only be visible to one
4495 When the message bus receives a method call, if the
4496 <literal>DESTINATION</literal> field is absent, the call is taken to be
4497 a standard one-to-one message and interpreted by the message bus
4498 itself. For example, sending an
4499 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4500 <literal>DESTINATION</literal> will cause the message bus itself to
4501 reply to the ping immediately; the message bus will not make this
4502 message visible to other applications.
4506 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4507 the ping message were sent with a <literal>DESTINATION</literal> name of
4508 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4509 forwarded, and the Yoyodyne Corporation screensaver application would be
4510 expected to reply to the ping.
4514 Message bus implementations may impose a security policy which
4515 prevents certain messages from being sent or received.
4516 When a method call message cannot be sent or received due to a security
4517 policy, the message bus should send an error reply, unless the
4518 original message had the <literal>NO_REPLY</literal> flag.
4521 <sect3 id="message-bus-routing-eavesdropping">
4522 <title>Eavesdropping</title>
4524 Receiving a unicast message whose <literal>DESTINATION</literal>
4525 indicates a different recipient is called
4526 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4527 a security boundary (like the standard system bus), the security
4528 policy should usually prevent eavesdropping, since unicast messages
4529 are normally kept private and may contain security-sensitive
4534 Eavesdropping is mainly useful for debugging tools, such as
4535 the <literal>dbus-monitor</literal> tool in the reference
4536 implementation of D-Bus. Tools which eavesdrop on the message bus
4537 should be careful to avoid sending a reply or error in response to
4538 messages intended for a different client.
4542 Clients may attempt to eavesdrop by adding match rules
4543 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4544 the <literal>eavesdrop='true'</literal> match. If the message bus'
4545 security policy does not allow eavesdropping, the match rule can
4546 still be added, but will not have any practical effect. For
4547 compatibility with older message bus implementations, if adding such
4548 a match rule results in an error reply, the client may fall back to
4549 adding the same rule with the <literal>eavesdrop</literal> match
4554 <sect3 id="message-bus-routing-match-rules">
4555 <title>Match Rules</title>
4557 An important part of the message bus routing protocol is match
4558 rules. Match rules describe the messages that should be sent to a
4559 client, based on the contents of the message. Broadcast signals
4560 are only sent to clients which have a suitable match rule: this
4561 avoids waking up client processes to deal with signals that are
4562 not relevant to that client.
4565 Messages that list a client as their <literal>DESTINATION</literal>
4566 do not need to match the client's match rules, and are sent to that
4567 client regardless. As a result, match rules are mainly used to
4568 receive a subset of broadcast signals.
4571 Match rules can also be used for eavesdropping
4572 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4573 if the security policy of the message bus allows it.
4576 Match rules are added using the AddMatch bus method
4577 (see <xref linkend="bus-messages-add-match"/>). Rules are
4578 specified as a string of comma separated key/value pairs.
4579 Excluding a key from the rule indicates a wildcard match.
4580 For instance excluding the the member from a match rule but
4581 adding a sender would let all messages from that sender through.
4582 An example of a complete rule would be
4583 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4586 Within single quotes (ASCII apostrophe, U+0027), a backslash
4587 (U+005C) represents itself, and an apostrophe ends the quoted
4588 section. Outside single quotes, \' (backslash, apostrophe)
4589 represents an apostrophe, and any backslash not followed by
4590 an apostrophe represents itself. For instance, the match rules
4591 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4592 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4593 both match messages where the arguments are a 1-character string
4594 containing an apostrophe, a 1-character string containing a
4595 backslash, a 1-character string containing a comma, and a
4596 2-character string containing two backslashes<footnote>
4598 This idiosyncratic quoting style is based on the rules for
4599 escaping items to appear inside single-quoted strings
4600 in POSIX <literal>/bin/sh</literal>, but please
4601 note that backslashes that are not inside single quotes have
4602 different behaviour. This syntax does not offer any way to
4603 represent an apostrophe inside single quotes (it is necessary
4604 to leave the single-quoted section, backslash-escape the
4605 apostrophe and re-enter single quotes), or to represent a
4606 comma outside single quotes (it is necessary to wrap it in
4607 a single-quoted section).
4612 The following table describes the keys that can be used to create
4619 <entry>Possible Values</entry>
4620 <entry>Description</entry>
4625 <entry><literal>type</literal></entry>
4626 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4627 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4630 <entry><literal>sender</literal></entry>
4631 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4632 and <xref linkend="term-unique-name"/> respectively)
4634 <entry>Match messages sent by a particular sender. An example of a sender match
4635 is sender='org.freedesktop.Hal'</entry>
4638 <entry><literal>interface</literal></entry>
4639 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4640 <entry>Match messages sent over or to a particular interface. An example of an
4641 interface match is interface='org.freedesktop.Hal.Manager'.
4642 If a message omits the interface header, it must not match any rule
4643 that specifies this key.</entry>
4646 <entry><literal>member</literal></entry>
4647 <entry>Any valid method or signal name</entry>
4648 <entry>Matches messages which have the give method or signal name. An example of
4649 a member match is member='NameOwnerChanged'</entry>
4652 <entry><literal>path</literal></entry>
4653 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4654 <entry>Matches messages which are sent from or to the given object. An example of a
4655 path match is path='/org/freedesktop/Hal/Manager'</entry>
4658 <entry><literal>path_namespace</literal></entry>
4659 <entry>An object path</entry>
4662 Matches messages which are sent from or to an
4663 object for which the object path is either the
4664 given value, or that value followed by one or
4665 more path components.
4670 <literal>path_namespace='/com/example/foo'</literal>
4671 would match signals sent by
4672 <literal>/com/example/foo</literal>
4674 <literal>/com/example/foo/bar</literal>,
4676 <literal>/com/example/foobar</literal>.
4680 Using both <literal>path</literal> and
4681 <literal>path_namespace</literal> in the same match
4682 rule is not allowed.
4687 This match key was added in version 0.16 of the
4688 D-Bus specification and implemented by the bus
4689 daemon in dbus 1.5.0 and later.
4695 <entry><literal>destination</literal></entry>
4696 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4697 <entry>Matches messages which are being sent to the given unique name. An
4698 example of a destination match is destination=':1.0'</entry>
4701 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4702 <entry>Any string</entry>
4703 <entry>Arg matches are special and are used for further restricting the
4704 match based on the arguments in the body of a message. Only arguments of type
4705 STRING can be matched in this way. An example of an argument match
4706 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4710 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4711 <entry>Any string</entry>
4713 <para>Argument path matches provide a specialised form of wildcard matching for
4714 path-like namespaces. They can match arguments whose type is either STRING or
4715 OBJECT_PATH. As with normal argument matches,
4716 if the argument is exactly equal to the string given in the match
4717 rule then the rule is satisfied. Additionally, there is also a
4718 match when either the string given in the match rule or the
4719 appropriate message argument ends with '/' and is a prefix of the
4720 other. An example argument path match is arg0path='/aa/bb/'. This
4721 would match messages with first arguments of '/', '/aa/',
4722 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4723 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4725 <para>This is intended for monitoring “directories” in file system-like
4726 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4727 system. An application interested in all nodes in a particular hierarchy would
4728 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4729 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4730 represent a modification to the “bar” property, or a signal with zeroth
4731 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4732 many properties within that directory, and the interested application would be
4733 notified in both cases.</para>
4736 This match key was added in version 0.12 of the
4737 D-Bus specification, implemented for STRING
4738 arguments by the bus daemon in dbus 1.2.0 and later,
4739 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4746 <entry><literal>arg0namespace</literal></entry>
4747 <entry>Like a bus name, except that the string is not
4748 required to contain a '.' (period)</entry>
4750 <para>Match messages whose first argument is of type STRING, and is a bus name
4751 or interface name within the specified namespace. This is primarily intended
4752 for watching name owner changes for a group of related bus names, rather than
4753 for a single name or all name changes.</para>
4755 <para>Because every valid interface name is also a valid
4756 bus name, this can also be used for messages whose
4757 first argument is an interface name.</para>
4759 <para>For example, the match rule
4760 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4761 matches name owner changes for bus names such as
4762 <literal>com.example.backend.foo</literal>,
4763 <literal>com.example.backend.foo.bar</literal>, and
4764 <literal>com.example.backend</literal> itself.</para>
4766 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4769 This match key was added in version 0.16 of the
4770 D-Bus specification and implemented by the bus
4771 daemon in dbus 1.5.0 and later.
4777 <entry><literal>eavesdrop</literal></entry>
4778 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4779 <entry>Since D-Bus 1.5.6, match rules do not
4780 match messages which have a <literal>DESTINATION</literal>
4781 field unless the match rule specifically
4783 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4784 by specifying <literal>eavesdrop='true'</literal>
4785 in the match rule. <literal>eavesdrop='false'</literal>
4786 restores the default behaviour. Messages are
4787 delivered to their <literal>DESTINATION</literal>
4788 regardless of match rules, so this match does not
4789 affect normal delivery of unicast messages.
4790 If the message bus has a security policy which forbids
4791 eavesdropping, this match may still be used without error,
4792 but will not have any practical effect.
4793 In older versions of D-Bus, this match was not allowed
4794 in match rules, and all match rules behaved as if
4795 <literal>eavesdrop='true'</literal> had been used.
4804 <sect2 id="message-bus-starting-services">
4805 <title>Message Bus Starting Services</title>
4807 The message bus can start applications on behalf of other applications.
4808 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4809 An application that can be started in this way is called a
4810 <firstterm>service</firstterm>.
4813 With D-Bus, starting a service is normally done by name. That is,
4814 applications ask the message bus to start some program that will own a
4815 well-known name, such as <literal>com.example.TextEditor</literal>.
4816 This implies a contract documented along with the name
4817 <literal>com.example.TextEditor</literal> for which object
4818 the owner of that name will provide, and what interfaces those
4822 To find an executable corresponding to a particular name, the bus daemon
4823 looks for <firstterm>service description files</firstterm>. Service
4824 description files define a mapping from names to executables. Different
4825 kinds of message bus will look for these files in different places, see
4826 <xref linkend="message-bus-types"/>.
4829 Service description files have the ".service" file
4830 extension. The message bus will only load service description files
4831 ending with .service; all other files will be ignored. The file format
4832 is similar to that of <ulink
4833 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4834 entries</ulink>. All service description files must be in UTF-8
4835 encoding. To ensure that there will be no name collisions, service files
4836 must be namespaced using the same mechanism as messages and service
4841 On the well-known system bus, the name of a service description file
4842 must be its well-known name plus <literal>.service</literal>,
4844 <literal>com.example.ConfigurationDatabase.service</literal>.
4848 On the well-known session bus, services should follow the same
4849 service description file naming convention as on the system bus,
4850 but for backwards compatibility they are not required to do so.
4854 [FIXME the file format should be much better specified than "similar to
4855 .desktop entries" esp. since desktop entries are already
4856 badly-specified. ;-)]
4857 These sections from the specification apply to service files as well:
4860 <listitem><para>General syntax</para></listitem>
4861 <listitem><para>Comment format</para></listitem>
4864 Service description files must contain a
4865 <literal>D-BUS Service</literal> group with at least the keys
4866 <literal>Name</literal> (the well-known name of the service)
4867 and <literal>Exec</literal> (the command to be executed).
4870 <title>Example service description file</title>
4872 # Sample service description file
4874 Name=com.example.ConfigurationDatabase
4875 Exec=/usr/bin/sample-configd
4881 Additionally, service description files for the well-known system
4882 bus on Unix must contain a <literal>User</literal> key, whose value
4883 is the name of a user account (e.g. <literal>root</literal>).
4884 The system service will be run as that user.
4888 When an application asks to start a service by name, the bus daemon tries to
4889 find a service that will own that name. It then tries to spawn the
4890 executable associated with it. If this fails, it will report an
4895 On the well-known system bus, it is not possible for two .service files
4896 in the same directory to offer the same service, because they are
4897 constrained to have names that match the service name.
4901 On the well-known session bus, if two .service files in the same
4902 directory offer the same service name, the result is undefined.
4903 Distributors should avoid this situation, for instance by naming
4904 session services' .service files according to their service name.
4908 If two .service files in different directories offer the same
4909 service name, the one in the higher-priority directory is used:
4910 for instance, on the system bus, .service files in
4911 /usr/local/share/dbus-1/system-services take precedence over those
4912 in /usr/share/dbus-1/system-services.
4915 The executable launched will have the environment variable
4916 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4917 message bus so it can connect and request the appropriate names.
4920 The executable being launched may want to know whether the message bus
4921 starting it is one of the well-known message buses (see <xref
4922 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4923 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4924 of the well-known buses. The currently-defined values for this variable
4925 are <literal>system</literal> for the systemwide message bus,
4926 and <literal>session</literal> for the per-login-session message
4927 bus. The new executable must still connect to the address given
4928 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4929 resulting connection is to the well-known bus.
4932 [FIXME there should be a timeout somewhere, either specified
4933 in the .service file, by the client, or just a global value
4934 and if the client being activated fails to connect within that
4935 timeout, an error should be sent back.]
4938 <sect3 id="message-bus-starting-services-scope">
4939 <title>Message Bus Service Scope</title>
4941 The "scope" of a service is its "per-", such as per-session,
4942 per-machine, per-home-directory, or per-display. The reference
4943 implementation doesn't yet support starting services in a different
4944 scope from the message bus itself. So e.g. if you start a service
4945 on the session bus its scope is per-session.
4948 We could add an optional scope to a bus name. For example, for
4949 per-(display,session pair), we could have a unique ID for each display
4950 generated automatically at login and set on screen 0 by executing a
4951 special "set display ID" binary. The ID would be stored in a
4952 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4953 random bytes. This ID would then be used to scope names.
4954 Starting/locating a service could be done by ID-name pair rather than
4958 Contrast this with a per-display scope. To achieve that, we would
4959 want a single bus spanning all sessions using a given display.
4960 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4961 property on screen 0 of the display, pointing to this bus.
4966 <sect2 id="message-bus-types">
4967 <title>Well-known Message Bus Instances</title>
4969 Two standard message bus instances are defined here, along with how
4970 to locate them and where their service files live.
4972 <sect3 id="message-bus-types-login">
4973 <title>Login session message bus</title>
4975 Each time a user logs in, a <firstterm>login session message
4976 bus</firstterm> may be started. All applications in the user's login
4977 session may interact with one another using this message bus.
4980 The address of the login session message bus is given
4981 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4982 variable. If that variable is not set, applications may
4983 also try to read the address from the X Window System root
4984 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4985 The root window property must have type <literal>STRING</literal>.
4986 The environment variable should have precedence over the
4987 root window property.
4989 <para>The address of the login session message bus is given in the
4990 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4991 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4992 "autolaunch:", the system should use platform-specific methods of
4993 locating a running D-Bus session server, or starting one if a running
4994 instance cannot be found. Note that this mechanism is not recommended
4995 for attempting to determine if a daemon is running. It is inherently
4996 racy to attempt to make this determination, since the bus daemon may
4997 be started just before or just after the determination is made.
4998 Therefore, it is recommended that applications do not try to make this
4999 determination for their functionality purposes, and instead they
5000 should attempt to start the server.</para>
5002 <sect4 id="message-bus-types-login-x-windows">
5003 <title>X Windowing System</title>
5005 For the X Windowing System, the application must locate the
5006 window owner of the selection represented by the atom formed by
5010 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
5014 <para>the current user's username</para>
5018 <para>the literal character '_' (underscore)</para>
5022 <para>the machine's ID</para>
5028 The following properties are defined for the window that owns
5030 <informaltable frame="all">
5039 <para>meaning</para>
5045 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5049 <para>the actual address of the server socket</para>
5055 <para>_DBUS_SESSION_BUS_PID</para>
5059 <para>the PID of the server process</para>
5068 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5069 present in this window.
5073 If the X selection cannot be located or if reading the
5074 properties from the window fails, the implementation MUST conclude
5075 that there is no D-Bus server running and proceed to start a new
5076 server. (See below on concurrency issues)
5080 Failure to connect to the D-Bus server address thus obtained
5081 MUST be treated as a fatal connection error and should be reported
5086 As an alternative, an implementation MAY find the information
5087 in the following file located in the current user's home directory,
5088 in subdirectory .dbus/session-bus/:
5091 <para>the machine's ID</para>
5095 <para>the literal character '-' (dash)</para>
5099 <para>the X display without the screen number, with the
5100 following prefixes removed, if present: ":", "localhost:"
5101 ."localhost.localdomain:". That is, a display of
5102 "localhost:10.0" produces just the number "10"</para>
5108 The contents of this file NAME=value assignment pairs and
5109 lines starting with # are comments (no comments are allowed
5110 otherwise). The following variable names are defined:
5117 <para>Variable</para>
5121 <para>meaning</para>
5127 <para>DBUS_SESSION_BUS_ADDRESS</para>
5131 <para>the actual address of the server socket</para>
5137 <para>DBUS_SESSION_BUS_PID</para>
5141 <para>the PID of the server process</para>
5147 <para>DBUS_SESSION_BUS_WINDOWID</para>
5151 <para>the window ID</para>
5160 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5165 Failure to open this file MUST be interpreted as absence of a
5166 running server. Therefore, the implementation MUST proceed to
5167 attempting to launch a new bus server if the file cannot be
5172 However, success in opening this file MUST NOT lead to the
5173 conclusion that the server is running. Thus, a failure to connect to
5174 the bus address obtained by the alternative method MUST NOT be
5175 considered a fatal error. If the connection cannot be established,
5176 the implementation MUST proceed to check the X selection settings or
5177 to start the server on its own.
5181 If the implementation concludes that the D-Bus server is not
5182 running it MUST attempt to start a new server and it MUST also
5183 ensure that the daemon started as an effect of the "autolaunch"
5184 mechanism provides the lookup mechanisms described above, so
5185 subsequent calls can locate the newly started server. The
5186 implementation MUST also ensure that if two or more concurrent
5187 initiations happen, only one server remains running and all other
5188 initiations are able to obtain the address of this server and
5189 connect to it. In other words, the implementation MUST ensure that
5190 the X selection is not present when it attempts to set it, without
5191 allowing another process to set the selection between the
5192 verification and the setting (e.g., by using XGrabServer /
5199 On Unix systems, the session bus should search for .service files
5200 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5202 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5203 Implementations may also search additional locations, which
5204 should be searched with lower priority than anything in
5205 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5206 for example, the reference implementation also
5207 looks in <literal>${datadir}/dbus-1/services</literal> as
5208 set at compile time.
5211 As described in the XDG Base Directory Specification, software
5212 packages should install their session .service files to their
5213 configured <literal>${datadir}/dbus-1/services</literal>,
5214 where <literal>${datadir}</literal> is as defined by the GNU
5215 coding standards. System administrators or users can arrange
5216 for these service files to be read by setting XDG_DATA_DIRS or by
5217 symlinking them into the default locations.
5221 <sect3 id="message-bus-types-system">
5222 <title>System message bus</title>
5224 A computer may have a <firstterm>system message bus</firstterm>,
5225 accessible to all applications on the system. This message bus may be
5226 used to broadcast system events, such as adding new hardware devices,
5227 changes in the printer queue, and so forth.
5230 The address of the system message bus is given
5231 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5232 variable. If that variable is not set, applications should try
5233 to connect to the well-known address
5234 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5237 The D-Bus reference implementation actually honors the
5238 <literal>$(localstatedir)</literal> configure option
5239 for this address, on both client and server side.
5244 On Unix systems, the system bus should default to searching
5245 for .service files in
5246 <literal>/usr/local/share/dbus-1/system-services</literal>,
5247 <literal>/usr/share/dbus-1/system-services</literal> and
5248 <literal>/lib/dbus-1/system-services</literal>, with that order
5249 of precedence. It may also search other implementation-specific
5250 locations, but should not vary these locations based on environment
5254 The system bus is security-sensitive and is typically executed
5255 by an init system with a clean environment. Its launch helper
5256 process is particularly security-sensitive, and specifically
5257 clears its own environment.
5262 Software packages should install their system .service
5263 files to their configured
5264 <literal>${datadir}/dbus-1/system-services</literal>,
5265 where <literal>${datadir}</literal> is as defined by the GNU
5266 coding standards. System administrators can arrange
5267 for these service files to be read by editing the system bus'
5268 configuration file or by symlinking them into the default
5274 <sect2 id="message-bus-messages">
5275 <title>Message Bus Messages</title>
5277 The special message bus name <literal>org.freedesktop.DBus</literal>
5278 responds to a number of additional messages.
5281 <sect3 id="bus-messages-hello">
5282 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5293 <entry>Argument</entry>
5295 <entry>Description</entry>
5301 <entry>STRING</entry>
5302 <entry>Unique name assigned to the connection</entry>
5309 Before an application is able to send messages to other applications
5310 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5311 to the message bus to obtain a unique name. If an application without
5312 a unique name tries to send a message to another application, or a
5313 message to the message bus itself that isn't the
5314 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5315 disconnected from the bus.
5318 There is no corresponding "disconnect" request; if a client wishes to
5319 disconnect from the bus, it simply closes the socket (or other
5320 communication channel).
5323 <sect3 id="bus-messages-list-names">
5324 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5328 ARRAY of STRING ListNames ()
5335 <entry>Argument</entry>
5337 <entry>Description</entry>
5343 <entry>ARRAY of STRING</entry>
5344 <entry>Array of strings where each string is a bus name</entry>
5351 Returns a list of all currently-owned names on the bus.
5354 <sect3 id="bus-messages-list-activatable-names">
5355 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5359 ARRAY of STRING ListActivatableNames ()
5366 <entry>Argument</entry>
5368 <entry>Description</entry>
5374 <entry>ARRAY of STRING</entry>
5375 <entry>Array of strings where each string is a bus name</entry>
5382 Returns a list of all names that can be activated on the bus.
5385 <sect3 id="bus-messages-name-exists">
5386 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5390 BOOLEAN NameHasOwner (in STRING name)
5397 <entry>Argument</entry>
5399 <entry>Description</entry>
5405 <entry>STRING</entry>
5406 <entry>Name to check</entry>
5416 <entry>Argument</entry>
5418 <entry>Description</entry>
5424 <entry>BOOLEAN</entry>
5425 <entry>Return value, true if the name exists</entry>
5432 Checks if the specified name exists (currently has an owner).
5436 <sect3 id="bus-messages-name-owner-changed">
5437 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5441 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5448 <entry>Argument</entry>
5450 <entry>Description</entry>
5456 <entry>STRING</entry>
5457 <entry>Name with a new owner</entry>
5461 <entry>STRING</entry>
5462 <entry>Old owner or empty string if none</entry>
5466 <entry>STRING</entry>
5467 <entry>New owner or empty string if none</entry>
5474 This signal indicates that the owner of a name has changed.
5475 It's also the signal to use to detect the appearance of
5476 new names on the bus.
5479 <sect3 id="bus-messages-name-lost">
5480 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5484 NameLost (STRING name)
5491 <entry>Argument</entry>
5493 <entry>Description</entry>
5499 <entry>STRING</entry>
5500 <entry>Name which was lost</entry>
5507 This signal is sent to a specific application when it loses
5508 ownership of a name.
5512 <sect3 id="bus-messages-name-acquired">
5513 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5517 NameAcquired (STRING name)
5524 <entry>Argument</entry>
5526 <entry>Description</entry>
5532 <entry>STRING</entry>
5533 <entry>Name which was acquired</entry>
5540 This signal is sent to a specific application when it gains
5541 ownership of a name.
5545 <sect3 id="bus-messages-start-service-by-name">
5546 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5550 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5557 <entry>Argument</entry>
5559 <entry>Description</entry>
5565 <entry>STRING</entry>
5566 <entry>Name of the service to start</entry>
5570 <entry>UINT32</entry>
5571 <entry>Flags (currently not used)</entry>
5581 <entry>Argument</entry>
5583 <entry>Description</entry>
5589 <entry>UINT32</entry>
5590 <entry>Return value</entry>
5595 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5599 The return value can be one of the following values:
5604 <entry>Identifier</entry>
5605 <entry>Value</entry>
5606 <entry>Description</entry>
5611 <entry>DBUS_START_REPLY_SUCCESS</entry>
5613 <entry>The service was successfully started.</entry>
5616 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5618 <entry>A connection already owns the given name.</entry>
5627 <sect3 id="bus-messages-update-activation-environment">
5628 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5632 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5639 <entry>Argument</entry>
5641 <entry>Description</entry>
5647 <entry>ARRAY of DICT<STRING,STRING></entry>
5648 <entry>Environment to add or update</entry>
5653 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5656 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5659 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.
5664 <sect3 id="bus-messages-get-name-owner">
5665 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5669 STRING GetNameOwner (in STRING name)
5676 <entry>Argument</entry>
5678 <entry>Description</entry>
5684 <entry>STRING</entry>
5685 <entry>Name to get the owner of</entry>
5695 <entry>Argument</entry>
5697 <entry>Description</entry>
5703 <entry>STRING</entry>
5704 <entry>Return value, a unique connection name</entry>
5709 Returns the unique connection name of the primary owner of the name
5710 given. If the requested name doesn't have an owner, returns a
5711 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5715 <sect3 id="bus-messages-get-connection-unix-user">
5716 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5720 UINT32 GetConnectionUnixUser (in STRING bus_name)
5727 <entry>Argument</entry>
5729 <entry>Description</entry>
5735 <entry>STRING</entry>
5736 <entry>Unique or well-known bus name of the connection to
5737 query, such as <literal>:12.34</literal> or
5738 <literal>com.example.tea</literal></entry>
5748 <entry>Argument</entry>
5750 <entry>Description</entry>
5756 <entry>UINT32</entry>
5757 <entry>Unix user ID</entry>
5762 Returns the Unix user ID of the process connected to the server. If
5763 unable to determine it (for instance, because the process is not on the
5764 same machine as the bus daemon), an error is returned.
5768 <sect3 id="bus-messages-get-connection-unix-process-id">
5769 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5773 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5780 <entry>Argument</entry>
5782 <entry>Description</entry>
5788 <entry>STRING</entry>
5789 <entry>Unique or well-known bus name of the connection to
5790 query, such as <literal>:12.34</literal> or
5791 <literal>com.example.tea</literal></entry>
5801 <entry>Argument</entry>
5803 <entry>Description</entry>
5809 <entry>UINT32</entry>
5810 <entry>Unix process id</entry>
5815 Returns the Unix process ID of the process connected to the server. If
5816 unable to determine it (for instance, because the process is not on the
5817 same machine as the bus daemon), an error is returned.
5821 <sect3 id="bus-messages-get-connection-credentials">
5822 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5826 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5833 <entry>Argument</entry>
5835 <entry>Description</entry>
5841 <entry>STRING</entry>
5842 <entry>Unique or well-known bus name of the connection to
5843 query, such as <literal>:12.34</literal> or
5844 <literal>com.example.tea</literal></entry>
5854 <entry>Argument</entry>
5856 <entry>Description</entry>
5862 <entry>DICT<STRING,VARIANT></entry>
5863 <entry>Credentials</entry>
5871 Returns as many credentials as possible for the process connected to
5872 the server. If unable to determine certain credentials (for instance,
5873 because the process is not on the same machine as the bus daemon,
5874 or because this version of the bus daemon does not support a
5875 particular security framework), or if the values of those credentials
5876 cannot be represented as documented here, then those credentials
5881 Keys in the returned dictionary not containing "." are defined
5882 by this specification. Bus daemon implementors supporting
5883 credentials frameworks not mentioned in this document should either
5884 contribute patches to this specification, or use keys containing
5885 "." and starting with a reversed domain name.
5891 <entry>Value type</entry>
5892 <entry>Value</entry>
5897 <entry>UnixUserID</entry>
5898 <entry>UINT32</entry>
5899 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5902 <entry>ProcessID</entry>
5903 <entry>UINT32</entry>
5904 <entry>The numeric process ID, on platforms that have
5905 this concept. On Unix, this is the process ID defined by
5914 This method was added in D-Bus 1.7 to reduce the round-trips
5915 required to list a process's credentials. In older versions, calling
5916 this method will fail: applications should recover by using the
5917 separate methods such as
5918 <xref linkend="bus-messages-get-connection-unix-user"/>
5923 <sect3 id="bus-messages-get-adt-audit-session-data">
5924 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5928 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5935 <entry>Argument</entry>
5937 <entry>Description</entry>
5943 <entry>STRING</entry>
5944 <entry>Unique or well-known bus name of the connection to
5945 query, such as <literal>:12.34</literal> or
5946 <literal>com.example.tea</literal></entry>
5956 <entry>Argument</entry>
5958 <entry>Description</entry>
5964 <entry>ARRAY of BYTE</entry>
5965 <entry>auditing data as returned by
5966 adt_export_session_data()</entry>
5971 Returns auditing data used by Solaris ADT, in an unspecified
5972 binary format. If you know what this means, please contribute
5973 documentation via the D-Bus bug tracking system.
5974 This method is on the core DBus interface for historical reasons;
5975 the same information should be made available via
5976 <xref linkend="bus-messages-get-connection-credentials"/>
5981 <sect3 id="bus-messages-get-connection-selinux-security-context">
5982 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5986 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5993 <entry>Argument</entry>
5995 <entry>Description</entry>
6001 <entry>STRING</entry>
6002 <entry>Unique or well-known bus name of the connection to
6003 query, such as <literal>:12.34</literal> or
6004 <literal>com.example.tea</literal></entry>
6014 <entry>Argument</entry>
6016 <entry>Description</entry>
6022 <entry>ARRAY of BYTE</entry>
6023 <entry>some sort of string of bytes, not necessarily UTF-8,
6024 not including '\0'</entry>
6029 Returns the security context used by SELinux, in an unspecified
6030 format. If you know what this means, please contribute
6031 documentation via the D-Bus bug tracking system.
6032 This method is on the core DBus interface for historical reasons;
6033 the same information should be made available via
6034 <xref linkend="bus-messages-get-connection-credentials"/>
6040 <sect3 id="bus-messages-add-match">
6041 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6045 AddMatch (in STRING rule)
6052 <entry>Argument</entry>
6054 <entry>Description</entry>
6060 <entry>STRING</entry>
6061 <entry>Match rule to add to the connection</entry>
6066 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6067 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6071 <sect3 id="bus-messages-remove-match">
6072 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6076 RemoveMatch (in STRING rule)
6083 <entry>Argument</entry>
6085 <entry>Description</entry>
6091 <entry>STRING</entry>
6092 <entry>Match rule to remove from the connection</entry>
6097 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6098 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6103 <sect3 id="bus-messages-get-id">
6104 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6108 GetId (out STRING id)
6115 <entry>Argument</entry>
6117 <entry>Description</entry>
6123 <entry>STRING</entry>
6124 <entry>Unique ID identifying the bus daemon</entry>
6129 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6130 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6131 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6132 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6133 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6134 by org.freedesktop.DBus.Peer.GetMachineId().
6135 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6143 <appendix id="implementation-notes">
6144 <title>Implementation notes</title>
6145 <sect1 id="implementation-notes-subsection">
6153 <glossary><title>Glossary</title>
6155 This glossary defines some of the terms used in this specification.
6158 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6161 The message bus maintains an association between names and
6162 connections. (Normally, there's one connection per application.) A
6163 bus name is simply an identifier used to locate connections. For
6164 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6165 name might be used to send a message to a screensaver from Yoyodyne
6166 Corporation. An application is said to <firstterm>own</firstterm> a
6167 name if the message bus has associated the application's connection
6168 with the name. Names may also have <firstterm>queued
6169 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6170 The bus assigns a unique name to each connection,
6171 see <xref linkend="term-unique-name"/>. Other names
6172 can be thought of as "well-known names" and are
6173 used to find applications that offer specific functionality.
6177 See <xref linkend="message-protocol-names-bus"/> for details of
6178 the syntax and naming conventions for bus names.
6183 <glossentry id="term-message"><glossterm>Message</glossterm>
6186 A message is the atomic unit of communication via the D-Bus
6187 protocol. It consists of a <firstterm>header</firstterm> and a
6188 <firstterm>body</firstterm>; the body is made up of
6189 <firstterm>arguments</firstterm>.
6194 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6197 The message bus is a special application that forwards
6198 or routes messages between a group of applications
6199 connected to the message bus. It also manages
6200 <firstterm>names</firstterm> used for routing
6206 <glossentry id="term-name"><glossterm>Name</glossterm>
6209 See <xref linkend="term-bus-name"/>. "Name" may
6210 also be used to refer to some of the other names
6211 in D-Bus, such as interface names.
6216 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6219 Used to prevent collisions when defining new interfaces, bus names
6220 etc. The convention used is the same one Java uses for defining
6221 classes: a reversed domain name.
6222 See <xref linkend="message-protocol-names-bus"/>,
6223 <xref linkend="message-protocol-names-interface"/>,
6224 <xref linkend="message-protocol-names-error"/>,
6225 <xref linkend="message-protocol-marshaling-object-path"/>.
6230 <glossentry id="term-object"><glossterm>Object</glossterm>
6233 Each application contains <firstterm>objects</firstterm>, which have
6234 <firstterm>interfaces</firstterm> and
6235 <firstterm>methods</firstterm>. Objects are referred to by a name,
6236 called a <firstterm>path</firstterm>.
6241 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6244 An application talking directly to another application, without going
6245 through a message bus. One-to-one connections may be "peer to peer" or
6246 "client to server." The D-Bus protocol has no concept of client
6247 vs. server after a connection has authenticated; the flow of messages
6248 is symmetrical (full duplex).
6253 <glossentry id="term-path"><glossterm>Path</glossterm>
6256 Object references (object names) in D-Bus are organized into a
6257 filesystem-style hierarchy, so each object is named by a path. As in
6258 LDAP, there's no difference between "files" and "directories"; a path
6259 can refer to an object, while still having child objects below it.
6264 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6267 Each bus name has a primary owner; messages sent to the name go to the
6268 primary owner. However, certain names also maintain a queue of
6269 secondary owners "waiting in the wings." If the primary owner releases
6270 the name, then the first secondary owner in the queue automatically
6271 becomes the new owner of the name.
6276 <glossentry id="term-service"><glossterm>Service</glossterm>
6279 A service is an executable that can be launched by the bus daemon.
6280 Services normally guarantee some particular features, for example they
6281 may guarantee that they will request a specific name such as
6282 "com.example.Screensaver", have a singleton object
6283 "/com/example/Application", and that object will implement the
6284 interface "com.example.Screensaver.Control".
6289 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6292 ".service files" tell the bus about service applications that can be
6293 launched (see <xref linkend="term-service"/>). Most importantly they
6294 provide a mapping from bus names to services that will request those
6295 names when they start up.
6300 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6303 The special name automatically assigned to each connection by the
6304 message bus. This name will never change owner, and will be unique
6305 (never reused during the lifetime of the message bus).
6306 It will begin with a ':' character.
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