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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
3 "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.24</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.24</revnumber>
75 <date>(not yet released)</date>
76 <authorinitials>n/a</authorinitials>
78 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
82 <revnumber>0.23</revnumber>
83 <date>2014-01-06</date>
84 <authorinitials>SMcV, CY</authorinitials>
86 method call messages with no INTERFACE may be considered an error;
87 document tcp:bind=... and nonce-tcp:bind=...; define listenable
88 and connectable addresses
92 <revnumber>0.22</revnumber>
93 <date>2013-10-09</date>
94 <authorinitials></authorinitials>
95 <revremark>add GetConnectionCredentials, document
96 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
97 document and correct .service file syntax and naming
101 <revnumber>0.21</revnumber>
102 <date>2013-04-25</date>
103 <authorinitials>smcv</authorinitials>
104 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
105 Corrigendum #9)</revremark>
108 <revnumber>0.20</revnumber>
109 <date>22 February 2013</date>
110 <authorinitials>smcv, walters</authorinitials>
111 <revremark>reorganise for clarity, remove false claims about
112 basic types, mention /o/fd/DBus</revremark>
115 <revnumber>0.19</revnumber>
116 <date>20 February 2012</date>
117 <authorinitials>smcv/lp</authorinitials>
118 <revremark>formally define unique connection names and well-known
119 bus names; document best practices for interface, bus, member and
120 error names, and object paths; document the search path for session
121 and system services on Unix; document the systemd transport</revremark>
124 <revnumber>0.18</revnumber>
125 <date>29 July 2011</date>
126 <authorinitials>smcv</authorinitials>
127 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
128 match keyword; promote type system to a top-level section</revremark>
131 <revnumber>0.17</revnumber>
132 <date>1 June 2011</date>
133 <authorinitials>smcv/davidz</authorinitials>
134 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
135 by GVariant</revremark>
138 <revnumber>0.16</revnumber>
139 <date>11 April 2011</date>
140 <authorinitials></authorinitials>
141 <revremark>add path_namespace, arg0namespace; argNpath matches object
145 <revnumber>0.15</revnumber>
146 <date>3 November 2010</date>
147 <authorinitials></authorinitials>
148 <revremark></revremark>
151 <revnumber>0.14</revnumber>
152 <date>12 May 2010</date>
153 <authorinitials></authorinitials>
154 <revremark></revremark>
157 <revnumber>0.13</revnumber>
158 <date>23 Dezember 2009</date>
159 <authorinitials></authorinitials>
160 <revremark></revremark>
163 <revnumber>0.12</revnumber>
164 <date>7 November, 2006</date>
165 <authorinitials></authorinitials>
166 <revremark></revremark>
169 <revnumber>0.11</revnumber>
170 <date>6 February 2005</date>
171 <authorinitials></authorinitials>
172 <revremark></revremark>
175 <revnumber>0.10</revnumber>
176 <date>28 January 2005</date>
177 <authorinitials></authorinitials>
178 <revremark></revremark>
181 <revnumber>0.9</revnumber>
182 <date>7 Januar 2005</date>
183 <authorinitials></authorinitials>
184 <revremark></revremark>
187 <revnumber>0.8</revnumber>
188 <date>06 September 2003</date>
189 <authorinitials></authorinitials>
190 <revremark>First released document.</revremark>
195 <sect1 id="introduction">
196 <title>Introduction</title>
198 D-Bus is a system for low-overhead, easy to use
199 interprocess communication (IPC). In more detail:
203 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
204 binary protocol, and does not have to convert to and from a text
205 format such as XML. Because D-Bus is intended for potentially
206 high-resolution same-machine IPC, not primarily for Internet IPC,
207 this is an interesting optimization. D-Bus is also designed to
208 avoid round trips and allow asynchronous operation, much like
214 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
215 of <firstterm>messages</firstterm> rather than byte streams, and
216 automatically handles a lot of the hard IPC issues. Also, the D-Bus
217 library is designed to be wrapped in a way that lets developers use
218 their framework's existing object/type system, rather than learning
219 a new one specifically for IPC.
226 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
227 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
228 a system for one application to talk to a single other
229 application. However, the primary intended application of the protocol is the
230 D-Bus <firstterm>message bus</firstterm>, specified in <xref
231 linkend="message-bus"/>. The message bus is a special application that
232 accepts connections from multiple other applications, and forwards
237 Uses of D-Bus include notification of system changes (notification of when
238 a camera is plugged in to a computer, or a new version of some software
239 has been installed), or desktop interoperability, for example a file
240 monitoring service or a configuration service.
244 D-Bus is designed for two specific use cases:
248 A "system bus" for notifications from the system to user sessions,
249 and to allow the system to request input from user sessions.
254 A "session bus" used to implement desktop environments such as
259 D-Bus is not intended to be a generic IPC system for any possible
260 application, and intentionally omits many features found in other
261 IPC systems for this reason.
265 At the same time, the bus daemons offer a number of features not found in
266 other IPC systems, such as single-owner "bus names" (similar to X
267 selections), on-demand startup of services, and security policies.
268 In many ways, these features are the primary motivation for developing
269 D-Bus; other systems would have sufficed if IPC were the only goal.
273 D-Bus may turn out to be useful in unanticipated applications, but future
274 versions of this spec and the reference implementation probably will not
275 incorporate features that interfere with the core use cases.
279 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
280 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
281 document are to be interpreted as described in RFC 2119. However, the
282 document could use a serious audit to be sure it makes sense to do
283 so. Also, they are not capitalized.
286 <sect2 id="stability">
287 <title>Protocol and Specification Stability</title>
289 The D-Bus protocol is frozen (only compatible extensions are allowed) as
290 of November 8, 2006. However, this specification could still use a fair
291 bit of work to make interoperable reimplementation possible without
292 reference to the D-Bus reference implementation. Thus, this
293 specification is not marked 1.0. To mark it 1.0, we'd like to see
294 someone invest significant effort in clarifying the specification
295 language, and growing the specification to cover more aspects of the
296 reference implementation's behavior.
299 Until this work is complete, any attempt to reimplement D-Bus will
300 probably require looking at the reference implementation and/or asking
301 questions on the D-Bus mailing list about intended behavior.
302 Questions on the list are very welcome.
305 Nonetheless, this document should be a useful starting point and is
306 to our knowledge accurate, though incomplete.
312 <sect1 id="type-system">
313 <title>Type System</title>
316 D-Bus has a type system, in which values of various types can be
317 serialized into a sequence of bytes referred to as the
318 <firstterm>wire format</firstterm> in a standard way.
319 Converting a value from some other representation into the wire
320 format is called <firstterm>marshaling</firstterm> and converting
321 it back from the wire format is <firstterm>unmarshaling</firstterm>.
325 The D-Bus protocol does not include type tags in the marshaled data; a
326 block of marshaled values must have a known <firstterm>type
327 signature</firstterm>. The type signature is made up of zero or more
328 <firstterm id="term-single-complete-type">single complete
329 types</firstterm>, each made up of one or more
330 <firstterm>type codes</firstterm>.
334 A type code is an ASCII character representing the
335 type of a value. Because ASCII characters are used, the type signature
336 will always form a valid ASCII string. A simple string compare
337 determines whether two type signatures are equivalent.
341 A single complete type is a sequence of type codes that fully describes
342 one type: either a basic type, or a single fully-described container type.
343 A single complete type is a basic type code, a variant type code,
344 an array with its element type, or a struct with its fields (all of which
345 are defined below). So the following signatures are not single complete
356 And the following signatures contain multiple complete types:
366 Note however that a single complete type may <emphasis>contain</emphasis>
367 multiple other single complete types, by containing a struct or dict
371 <sect2 id="basic-types">
372 <title>Basic types</title>
375 The simplest type codes are the <firstterm id="term-basic-type">basic
376 types</firstterm>, which are the types whose structure is entirely
377 defined by their 1-character type code. Basic types consist of
378 fixed types and string-like types.
382 The <firstterm id="term-fixed-type">fixed types</firstterm>
383 are basic types whose values have a fixed length, namely BYTE,
384 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
389 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
390 the ASCII character 'i'. So the signature for a block of values
391 containing a single <literal>INT32</literal> would be:
395 A block of values containing two <literal>INT32</literal> would have this signature:
402 The characteristics of the fixed types are listed in this table.
408 <entry>Conventional name</entry>
409 <entry>ASCII type-code</entry>
410 <entry>Encoding</entry>
415 <entry><literal>BYTE</literal></entry>
416 <entry><literal>y</literal> (121)</entry>
417 <entry>Unsigned 8-bit integer</entry>
420 <entry><literal>BOOLEAN</literal></entry>
421 <entry><literal>b</literal> (98)</entry>
422 <entry>Boolean value: 0 is false, 1 is true, any other value
423 allowed by the marshalling format is invalid</entry>
426 <entry><literal>INT16</literal></entry>
427 <entry><literal>n</literal> (110)</entry>
428 <entry>Signed (two's complement) 16-bit integer</entry>
431 <entry><literal>UINT16</literal></entry>
432 <entry><literal>q</literal> (113)</entry>
433 <entry>Unsigned 16-bit integer</entry>
436 <entry><literal>INT32</literal></entry>
437 <entry><literal>i</literal> (105)</entry>
438 <entry>Signed (two's complement) 32-bit integer</entry>
441 <entry><literal>UINT32</literal></entry>
442 <entry><literal>u</literal> (117)</entry>
443 <entry>Unsigned 32-bit integer</entry>
446 <entry><literal>INT64</literal></entry>
447 <entry><literal>x</literal> (120)</entry>
448 <entry>Signed (two's complement) 64-bit integer
449 (mnemonic: x and t are the first characters in "sixty" not
450 already used for something more common)</entry>
453 <entry><literal>UINT64</literal></entry>
454 <entry><literal>t</literal> (116)</entry>
455 <entry>Unsigned 64-bit integer</entry>
458 <entry><literal>DOUBLE</literal></entry>
459 <entry><literal>d</literal> (100)</entry>
460 <entry>IEEE 754 double-precision floating point</entry>
463 <entry><literal>UNIX_FD</literal></entry>
464 <entry><literal>h</literal> (104)</entry>
465 <entry>Unsigned 32-bit integer representing an index into an
466 out-of-band array of file descriptors, transferred via some
467 platform-specific mechanism (mnemonic: h for handle)</entry>
475 The <firstterm id="term-string-like-type">string-like types</firstterm>
476 are basic types with a variable length. The value of any string-like
477 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
478 none of which may be U+0000. The UTF-8 text must be validated
479 strictly: in particular, it must not contain overlong sequences
480 or codepoints above U+10FFFF.
484 Since D-Bus Specification version 0.21, in accordance with Unicode
485 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
486 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
487 D-Bus rejected these noncharacters).
491 The marshalling formats for the string-like types all end with a
492 single zero (NUL) byte, but that byte is not considered to be part of
497 The characteristics of the string-like types are listed in this table.
503 <entry>Conventional name</entry>
504 <entry>ASCII type-code</entry>
505 <entry>Validity constraints</entry>
510 <entry><literal>STRING</literal></entry>
511 <entry><literal>s</literal> (115)</entry>
512 <entry>No extra constraints</entry>
515 <entry><literal>OBJECT_PATH</literal></entry>
516 <entry><literal>o</literal> (111)</entry>
518 <link linkend="message-protocol-marshaling-object-path">a
519 syntactically valid object path</link></entry>
522 <entry><literal>SIGNATURE</literal></entry>
523 <entry><literal>g</literal> (103)</entry>
525 <firstterm linkend="term-single-complete-type">single
526 complete types</firstterm></entry>
533 <sect3 id="message-protocol-marshaling-object-path">
534 <title>Valid Object Paths</title>
537 An object path is a name used to refer to an object instance.
538 Conceptually, each participant in a D-Bus message exchange may have
539 any number of object instances (think of C++ or Java objects) and each
540 such instance will have a path. Like a filesystem, the object
541 instances in an application form a hierarchical tree.
545 Object paths are often namespaced by starting with a reversed
546 domain name and containing an interface version number, in the
548 <link linkend="message-protocol-names-interface">interface
550 <link linkend="message-protocol-names-bus">well-known
552 This makes it possible to implement more than one service, or
553 more than one version of a service, in the same process,
554 even if the services share a connection but cannot otherwise
555 co-operate (for instance, if they are implemented by different
560 For instance, if the owner of <literal>example.com</literal> is
561 developing a D-Bus API for a music player, they might use the
562 hierarchy of object paths that start with
563 <literal>/com/example/MusicPlayer1</literal> for its objects.
567 The following rules define a valid object path. Implementations must
568 not send or accept messages with invalid object paths.
572 The path may be of any length.
577 The path must begin with an ASCII '/' (integer 47) character,
578 and must consist of elements separated by slash characters.
583 Each element must only contain the ASCII characters
589 No element may be the empty string.
594 Multiple '/' characters cannot occur in sequence.
599 A trailing '/' character is not allowed unless the
600 path is the root path (a single '/' character).
608 <sect3 id="message-protocol-marshaling-signature">
609 <title>Valid Signatures</title>
611 An implementation must not send or accept invalid signatures.
612 Valid signatures will conform to the following rules:
616 The signature is a list of single complete types.
617 Arrays must have element types, and structs must
618 have both open and close parentheses.
623 Only type codes, open and close parentheses, and open and
624 close curly brackets are allowed in the signature. The
625 <literal>STRUCT</literal> type code
626 is not allowed in signatures, because parentheses
627 are used instead. Similarly, the
628 <literal>DICT_ENTRY</literal> type code is not allowed in
629 signatures, because curly brackets are used instead.
634 The maximum depth of container type nesting is 32 array type
635 codes and 32 open parentheses. This implies that the maximum
636 total depth of recursion is 64, for an "array of array of array
637 of ... struct of struct of struct of ..." where there are 32
643 The maximum length of a signature is 255.
650 When signatures appear in messages, the marshalling format
651 guarantees that they will be followed by a nul byte (which can
652 be interpreted as either C-style string termination or the INVALID
653 type-code), but this is not conceptually part of the signature.
659 <sect2 id="container-types">
660 <title>Container types</title>
663 In addition to basic types, there are four <firstterm>container</firstterm>
664 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
665 and <literal>DICT_ENTRY</literal>.
669 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
670 code does not appear in signatures. Instead, ASCII characters
671 '(' and ')' are used to mark the beginning and end of the struct.
672 So for example, a struct containing two integers would have this
677 Structs can be nested, so for example a struct containing
678 an integer and another struct:
682 The value block storing that struct would contain three integers; the
683 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
688 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
689 but is useful in code that implements the protocol. This type code
690 is specified to allow such code to interoperate in non-protocol contexts.
694 Empty structures are not allowed; there must be at least one
695 type code between the parentheses.
699 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
700 followed by a <firstterm>single complete type</firstterm>. The single
701 complete type following the array is the type of each array element. So
702 the simple example is:
706 which is an array of 32-bit integers. But an array can be of any type,
707 such as this array-of-struct-with-two-int32-fields:
711 Or this array of array of integer:
718 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
719 type <literal>VARIANT</literal> will have the signature of a single complete type as part
720 of the <emphasis>value</emphasis>. This signature will be followed by a
721 marshaled value of that type.
725 Unlike a message signature, the variant signature can
726 contain only a single complete type. So "i", "ai"
727 or "(ii)" is OK, but "ii" is not. Use of variants may not
728 cause a total message depth to be larger than 64, including
729 other container types such as structures.
733 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
734 than parentheses it uses curly braces, and it has more restrictions.
735 The restrictions are: it occurs only as an array element type; it has
736 exactly two single complete types inside the curly braces; the first
737 single complete type (the "key") must be a basic type rather than a
738 container type. Implementations must not accept dict entries outside of
739 arrays, must not accept dict entries with zero, one, or more than two
740 fields, and must not accept dict entries with non-basic-typed keys. A
741 dict entry is always a key-value pair.
745 The first field in the <literal>DICT_ENTRY</literal> is always the key.
746 A message is considered corrupt if the same key occurs twice in the same
747 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
748 implementations are not required to reject dicts with duplicate keys.
752 In most languages, an array of dict entry would be represented as a
753 map, hash table, or dict object.
758 <title>Summary of types</title>
761 The following table summarizes the D-Bus types.
766 <entry>Conventional Name</entry>
768 <entry>Description</entry>
773 <entry><literal>INVALID</literal></entry>
774 <entry>0 (ASCII NUL)</entry>
775 <entry>Not a valid type code, used to terminate signatures</entry>
777 <entry><literal>BYTE</literal></entry>
778 <entry>121 (ASCII 'y')</entry>
779 <entry>8-bit unsigned integer</entry>
781 <entry><literal>BOOLEAN</literal></entry>
782 <entry>98 (ASCII 'b')</entry>
783 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
785 <entry><literal>INT16</literal></entry>
786 <entry>110 (ASCII 'n')</entry>
787 <entry>16-bit signed integer</entry>
789 <entry><literal>UINT16</literal></entry>
790 <entry>113 (ASCII 'q')</entry>
791 <entry>16-bit unsigned integer</entry>
793 <entry><literal>INT32</literal></entry>
794 <entry>105 (ASCII 'i')</entry>
795 <entry>32-bit signed integer</entry>
797 <entry><literal>UINT32</literal></entry>
798 <entry>117 (ASCII 'u')</entry>
799 <entry>32-bit unsigned integer</entry>
801 <entry><literal>INT64</literal></entry>
802 <entry>120 (ASCII 'x')</entry>
803 <entry>64-bit signed integer</entry>
805 <entry><literal>UINT64</literal></entry>
806 <entry>116 (ASCII 't')</entry>
807 <entry>64-bit unsigned integer</entry>
809 <entry><literal>DOUBLE</literal></entry>
810 <entry>100 (ASCII 'd')</entry>
811 <entry>IEEE 754 double</entry>
813 <entry><literal>STRING</literal></entry>
814 <entry>115 (ASCII 's')</entry>
815 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
817 <entry><literal>OBJECT_PATH</literal></entry>
818 <entry>111 (ASCII 'o')</entry>
819 <entry>Name of an object instance</entry>
821 <entry><literal>SIGNATURE</literal></entry>
822 <entry>103 (ASCII 'g')</entry>
823 <entry>A type signature</entry>
825 <entry><literal>ARRAY</literal></entry>
826 <entry>97 (ASCII 'a')</entry>
829 <entry><literal>STRUCT</literal></entry>
830 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
831 <entry>Struct; type code 114 'r' is reserved for use in
832 bindings and implementations to represent the general
833 concept of a struct, and must not appear in signatures
834 used on D-Bus.</entry>
836 <entry><literal>VARIANT</literal></entry>
837 <entry>118 (ASCII 'v') </entry>
838 <entry>Variant type (the type of the value is part of the value itself)</entry>
840 <entry><literal>DICT_ENTRY</literal></entry>
841 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
842 <entry>Entry in a dict or map (array of key-value pairs).
843 Type code 101 'e' is reserved for use in bindings and
844 implementations to represent the general concept of a
845 dict or dict-entry, and must not appear in signatures
846 used on D-Bus.</entry>
848 <entry><literal>UNIX_FD</literal></entry>
849 <entry>104 (ASCII 'h')</entry>
850 <entry>Unix file descriptor</entry>
853 <entry>(reserved)</entry>
854 <entry>109 (ASCII 'm')</entry>
855 <entry>Reserved for <ulink
856 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
857 'maybe' type compatible with the one in GVariant</ulink>,
858 and must not appear in signatures used on D-Bus until
859 specified here</entry>
862 <entry>(reserved)</entry>
863 <entry>42 (ASCII '*')</entry>
864 <entry>Reserved for use in bindings/implementations to
865 represent any <firstterm>single complete type</firstterm>,
866 and must not appear in signatures used on D-Bus.</entry>
869 <entry>(reserved)</entry>
870 <entry>63 (ASCII '?')</entry>
871 <entry>Reserved for use in bindings/implementations to
872 represent any <firstterm>basic type</firstterm>, and must
873 not appear in signatures used on D-Bus.</entry>
876 <entry>(reserved)</entry>
877 <entry>64 (ASCII '@'), 38 (ASCII '&'),
878 94 (ASCII '^')</entry>
879 <entry>Reserved for internal use by bindings/implementations,
880 and must not appear in signatures used on D-Bus.
881 GVariant uses these type-codes to encode calling
892 <sect1 id="message-protocol-marshaling">
893 <title>Marshaling (Wire Format)</title>
896 D-Bus defines a marshalling format for its type system, which is
897 used in D-Bus messages. This is not the only possible marshalling
898 format for the type system: for instance, GVariant (part of GLib)
899 re-uses the D-Bus type system but implements an alternative marshalling
904 <title>Byte order and alignment</title>
907 Given a type signature, a block of bytes can be converted into typed
908 values. This section describes the format of the block of bytes. Byte
909 order and alignment issues are handled uniformly for all D-Bus types.
913 A block of bytes has an associated byte order. The byte order
914 has to be discovered in some way; for D-Bus messages, the
915 byte order is part of the message header as described in
916 <xref linkend="message-protocol-messages"/>. For now, assume
917 that the byte order is known to be either little endian or big
922 Each value in a block of bytes is aligned "naturally," for example
923 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
924 8-byte boundary. To properly align a value, <firstterm>alignment
925 padding</firstterm> may be necessary. The alignment padding must always
926 be the minimum required padding to properly align the following value;
927 and it must always be made up of nul bytes. The alignment padding must
928 not be left uninitialized (it can't contain garbage), and more padding
929 than required must not be used.
933 As an exception to natural alignment, <literal>STRUCT</literal> and
934 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
935 boundary, regardless of the alignments of their contents.
940 <title>Marshalling basic types</title>
943 To marshal and unmarshal fixed types, you simply read one value
944 from the data block corresponding to each type code in the signature.
945 All signed integer values are encoded in two's complement, DOUBLE
946 values are IEEE 754 double-precision floating-point, and BOOLEAN
947 values are encoded in 32 bits (of which only the least significant
952 The string-like types are all marshalled as a
953 fixed-length unsigned integer <varname>n</varname> giving the
954 length of the variable part, followed by <varname>n</varname>
955 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
956 which is not considered to be part of the text. The alignment
957 of the string-like type is the same as the alignment of
958 <varname>n</varname>.
962 For the STRING and OBJECT_PATH types, <varname>n</varname> is
963 encoded in 4 bytes, leading to 4-byte alignment.
964 For the SIGNATURE type, <varname>n</varname> is encoded as a single
965 byte. As a result, alignment padding is never required before a
971 <title>Marshalling containers</title>
974 Arrays are marshalled as a <literal>UINT32</literal>
975 <varname>n</varname> giving the length of the array data in bytes,
976 followed by alignment padding to the alignment boundary of the array
977 element type, followed by the <varname>n</varname> bytes of the
978 array elements marshalled in sequence. <varname>n</varname> does not
979 include the padding after the length, or any padding after the
984 For instance, if the current position in the message is a multiple
985 of 8 bytes and the byte-order is big-endian, an array containing only
986 the 64-bit integer 5 would be marshalled as:
989 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
990 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
991 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
996 Arrays have a maximum length defined to be 2 to the 26th power or
997 67108864. Implementations must not send or accept arrays exceeding this
1002 Structs and dict entries are marshalled in the same way as their
1003 contents, but their alignment is always to an 8-byte boundary,
1004 even if their contents would normally be less strictly aligned.
1008 Variants are marshalled as the <literal>SIGNATURE</literal> of
1009 the contents (which must be a single complete type), followed by a
1010 marshalled value with the type given by that signature. The
1011 variant has the same 1-byte alignment as the signature, which means
1012 that alignment padding before a variant is never needed.
1013 Use of variants may not cause a total message depth to be larger
1014 than 64, including other container types such as structures.
1019 <title>Summary of D-Bus marshalling</title>
1022 Given all this, the types are marshaled on the wire as follows:
1027 <entry>Conventional Name</entry>
1028 <entry>Encoding</entry>
1029 <entry>Alignment</entry>
1034 <entry><literal>INVALID</literal></entry>
1035 <entry>Not applicable; cannot be marshaled.</entry>
1038 <entry><literal>BYTE</literal></entry>
1039 <entry>A single 8-bit byte.</entry>
1042 <entry><literal>BOOLEAN</literal></entry>
1043 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1046 <entry><literal>INT16</literal></entry>
1047 <entry>16-bit signed integer in the message's byte order.</entry>
1050 <entry><literal>UINT16</literal></entry>
1051 <entry>16-bit unsigned integer in the message's byte order.</entry>
1054 <entry><literal>INT32</literal></entry>
1055 <entry>32-bit signed integer in the message's byte order.</entry>
1058 <entry><literal>UINT32</literal></entry>
1059 <entry>32-bit unsigned integer in the message's byte order.</entry>
1062 <entry><literal>INT64</literal></entry>
1063 <entry>64-bit signed integer in the message's byte order.</entry>
1066 <entry><literal>UINT64</literal></entry>
1067 <entry>64-bit unsigned integer in the message's byte order.</entry>
1070 <entry><literal>DOUBLE</literal></entry>
1071 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1074 <entry><literal>STRING</literal></entry>
1075 <entry>A <literal>UINT32</literal> indicating the string's
1076 length in bytes excluding its terminating nul, followed by
1077 non-nul string data of the given length, followed by a terminating nul
1084 <entry><literal>OBJECT_PATH</literal></entry>
1085 <entry>Exactly the same as <literal>STRING</literal> except the
1086 content must be a valid object path (see above).
1092 <entry><literal>SIGNATURE</literal></entry>
1093 <entry>The same as <literal>STRING</literal> except the length is a single
1094 byte (thus signatures have a maximum length of 255)
1095 and the content must be a valid signature (see above).
1101 <entry><literal>ARRAY</literal></entry>
1103 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1104 alignment padding to the alignment boundary of the array element type,
1105 followed by each array element.
1111 <entry><literal>STRUCT</literal></entry>
1113 A struct must start on an 8-byte boundary regardless of the
1114 type of the struct fields. The struct value consists of each
1115 field marshaled in sequence starting from that 8-byte
1122 <entry><literal>VARIANT</literal></entry>
1124 The marshaled <literal>SIGNATURE</literal> of a single
1125 complete type, followed by a marshaled value with the type
1126 given in the signature.
1129 1 (alignment of the signature)
1132 <entry><literal>DICT_ENTRY</literal></entry>
1134 Identical to STRUCT.
1140 <entry><literal>UNIX_FD</literal></entry>
1141 <entry>32-bit unsigned integer in the message's byte
1142 order. The actual file descriptors need to be
1143 transferred out-of-band via some platform specific
1144 mechanism. On the wire, values of this type store the index to the
1145 file descriptor in the array of file descriptors that
1146 accompany the message.</entry>
1158 <sect1 id="message-protocol">
1159 <title>Message Protocol</title>
1162 A <firstterm>message</firstterm> consists of a
1163 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1164 think of a message as a package, the header is the address, and the body
1165 contains the package contents. The message delivery system uses the header
1166 information to figure out where to send the message and how to interpret
1167 it; the recipient interprets the body of the message.
1171 The body of the message is made up of zero or more
1172 <firstterm>arguments</firstterm>, which are typed values, such as an
1173 integer or a byte array.
1177 Both header and body use the D-Bus <link linkend="type-system">type
1178 system</link> and format for serializing data.
1181 <sect2 id="message-protocol-messages">
1182 <title>Message Format</title>
1185 A message consists of a header and a body. The header is a block of
1186 values with a fixed signature and meaning. The body is a separate block
1187 of values, with a signature specified in the header.
1191 The length of the header must be a multiple of 8, allowing the body to
1192 begin on an 8-byte boundary when storing the entire message in a single
1193 buffer. If the header does not naturally end on an 8-byte boundary
1194 up to 7 bytes of nul-initialized alignment padding must be added.
1198 The message body need not end on an 8-byte boundary.
1202 The maximum length of a message, including header, header alignment padding,
1203 and body is 2 to the 27th power or 134217728. Implementations must not
1204 send or accept messages exceeding this size.
1208 The signature of the header is:
1212 Written out more readably, this is:
1214 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1219 These values have the following meanings:
1224 <entry>Value</entry>
1225 <entry>Description</entry>
1230 <entry>1st <literal>BYTE</literal></entry>
1231 <entry>Endianness flag; ASCII 'l' for little-endian
1232 or ASCII 'B' for big-endian. Both header and body are
1233 in this endianness.</entry>
1236 <entry>2nd <literal>BYTE</literal></entry>
1237 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1238 Currently-defined types are described below.
1242 <entry>3rd <literal>BYTE</literal></entry>
1243 <entry>Bitwise OR of flags. Unknown flags
1244 must be ignored. Currently-defined flags are described below.
1248 <entry>4th <literal>BYTE</literal></entry>
1249 <entry>Major protocol version of the sending application. If
1250 the major protocol version of the receiving application does not
1251 match, the applications will not be able to communicate and the
1252 D-Bus connection must be disconnected. The major protocol
1253 version for this version of the specification is 1.
1257 <entry>1st <literal>UINT32</literal></entry>
1258 <entry>Length in bytes of the message body, starting
1259 from the end of the header. The header ends after
1260 its alignment padding to an 8-boundary.
1264 <entry>2nd <literal>UINT32</literal></entry>
1265 <entry>The serial of this message, used as a cookie
1266 by the sender to identify the reply corresponding
1267 to this request. This must not be zero.
1271 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1272 <entry>An array of zero or more <firstterm>header
1273 fields</firstterm> where the byte is the field code, and the
1274 variant is the field value. The message type determines
1275 which fields are required.
1283 <firstterm>Message types</firstterm> that can appear in the second byte
1289 <entry>Conventional name</entry>
1290 <entry>Decimal value</entry>
1291 <entry>Description</entry>
1296 <entry><literal>INVALID</literal></entry>
1298 <entry>This is an invalid type.</entry>
1301 <entry><literal>METHOD_CALL</literal></entry>
1303 <entry>Method call.</entry>
1306 <entry><literal>METHOD_RETURN</literal></entry>
1308 <entry>Method reply with returned data.</entry>
1311 <entry><literal>ERROR</literal></entry>
1313 <entry>Error reply. If the first argument exists and is a
1314 string, it is an error message.</entry>
1317 <entry><literal>SIGNAL</literal></entry>
1319 <entry>Signal emission.</entry>
1326 Flags that can appear in the third byte of the header:
1331 <entry>Conventional name</entry>
1332 <entry>Hex value</entry>
1333 <entry>Description</entry>
1338 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1340 <entry>This message does not expect method return replies or
1341 error replies; the reply can be omitted as an
1342 optimization. However, it is compliant with this specification
1343 to return the reply despite this flag and the only harm
1344 from doing so is extra network traffic.
1348 <entry><literal>NO_AUTO_START</literal></entry>
1350 <entry>The bus must not launch an owner
1351 for the destination name in response to this message.
1359 <sect3 id="message-protocol-header-fields">
1360 <title>Header Fields</title>
1363 The array at the end of the header contains <firstterm>header
1364 fields</firstterm>, where each field is a 1-byte field code followed
1365 by a field value. A header must contain the required header fields for
1366 its message type, and zero or more of any optional header
1367 fields. Future versions of this protocol specification may add new
1368 fields. Implementations must ignore fields they do not
1369 understand. Implementations must not invent their own header fields;
1370 only changes to this specification may introduce new header fields.
1374 Again, if an implementation sees a header field code that it does not
1375 expect, it must ignore that field, as it will be part of a new
1376 (but compatible) version of this specification. This also applies
1377 to known header fields appearing in unexpected messages, for
1378 example: if a signal has a reply serial it must be ignored
1379 even though it has no meaning as of this version of the spec.
1383 However, implementations must not send or accept known header fields
1384 with the wrong type stored in the field value. So for example a
1385 message with an <literal>INTERFACE</literal> field of type
1386 <literal>UINT32</literal> would be considered corrupt.
1390 Here are the currently-defined header fields:
1395 <entry>Conventional Name</entry>
1396 <entry>Decimal Code</entry>
1398 <entry>Required In</entry>
1399 <entry>Description</entry>
1404 <entry><literal>INVALID</literal></entry>
1407 <entry>not allowed</entry>
1408 <entry>Not a valid field name (error if it appears in a message)</entry>
1411 <entry><literal>PATH</literal></entry>
1413 <entry><literal>OBJECT_PATH</literal></entry>
1414 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1415 <entry>The object to send a call to,
1416 or the object a signal is emitted from.
1418 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1419 implementations should not send messages with this path,
1420 and the reference implementation of the bus daemon will
1421 disconnect any application that attempts to do so.
1425 <entry><literal>INTERFACE</literal></entry>
1427 <entry><literal>STRING</literal></entry>
1428 <entry><literal>SIGNAL</literal></entry>
1430 The interface to invoke a method call on, or
1431 that a signal is emitted from. Optional for
1432 method calls, required for signals.
1433 The special interface
1434 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1435 implementations should not send messages with this
1436 interface, and the reference implementation of the bus
1437 daemon will disconnect any application that attempts to
1442 <entry><literal>MEMBER</literal></entry>
1444 <entry><literal>STRING</literal></entry>
1445 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1446 <entry>The member, either the method name or signal name.</entry>
1449 <entry><literal>ERROR_NAME</literal></entry>
1451 <entry><literal>STRING</literal></entry>
1452 <entry><literal>ERROR</literal></entry>
1453 <entry>The name of the error that occurred, for errors</entry>
1456 <entry><literal>REPLY_SERIAL</literal></entry>
1458 <entry><literal>UINT32</literal></entry>
1459 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1460 <entry>The serial number of the message this message is a reply
1461 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1464 <entry><literal>DESTINATION</literal></entry>
1466 <entry><literal>STRING</literal></entry>
1467 <entry>optional</entry>
1468 <entry>The name of the connection this message is intended for.
1469 Only used in combination with the message bus, see
1470 <xref linkend="message-bus"/>.</entry>
1473 <entry><literal>SENDER</literal></entry>
1475 <entry><literal>STRING</literal></entry>
1476 <entry>optional</entry>
1477 <entry>Unique name of the sending connection.
1478 The message bus fills in this field so it is reliable; the field is
1479 only meaningful in combination with the message bus.</entry>
1482 <entry><literal>SIGNATURE</literal></entry>
1484 <entry><literal>SIGNATURE</literal></entry>
1485 <entry>optional</entry>
1486 <entry>The signature of the message body.
1487 If omitted, it is assumed to be the
1488 empty signature "" (i.e. the body must be 0-length).</entry>
1491 <entry><literal>UNIX_FDS</literal></entry>
1493 <entry><literal>UINT32</literal></entry>
1494 <entry>optional</entry>
1495 <entry>The number of Unix file descriptors that
1496 accompany the message. If omitted, it is assumed
1497 that no Unix file descriptors accompany the
1498 message. The actual file descriptors need to be
1499 transferred via platform specific mechanism
1500 out-of-band. They must be sent at the same time as
1501 part of the message itself. They may not be sent
1502 before the first byte of the message itself is
1503 transferred or after the last byte of the message
1513 <sect2 id="message-protocol-names">
1514 <title>Valid Names</title>
1516 The various names in D-Bus messages have some restrictions.
1519 There is a <firstterm>maximum name length</firstterm>
1520 of 255 which applies to bus names, interfaces, and members.
1522 <sect3 id="message-protocol-names-interface">
1523 <title>Interface names</title>
1525 Interfaces have names with type <literal>STRING</literal>, meaning that
1526 they must be valid UTF-8. However, there are also some
1527 additional restrictions that apply to interface names
1530 <listitem><para>Interface names are composed of 1 or more elements separated by
1531 a period ('.') character. All elements must contain at least
1535 <listitem><para>Each element must only contain the ASCII characters
1536 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1540 <listitem><para>Interface names must contain at least one '.' (period)
1541 character (and thus at least two elements).
1544 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1545 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1550 Interface names should start with the reversed DNS domain name of
1551 the author of the interface (in lower-case), like interface names
1552 in Java. It is conventional for the rest of the interface name
1553 to consist of words run together, with initial capital letters
1554 on all words ("CamelCase"). Several levels of hierarchy can be used.
1555 It is also a good idea to include the major version of the interface
1556 in the name, and increment it if incompatible changes are made;
1557 this way, a single object can implement several versions of an
1558 interface in parallel, if necessary.
1562 For instance, if the owner of <literal>example.com</literal> is
1563 developing a D-Bus API for a music player, they might define
1564 interfaces called <literal>com.example.MusicPlayer1</literal>,
1565 <literal>com.example.MusicPlayer1.Track</literal> and
1566 <literal>com.example.MusicPlayer1.Seekable</literal>.
1570 D-Bus does not distinguish between the concepts that would be
1571 called classes and interfaces in Java: either can be identified on
1572 D-Bus by an interface name.
1575 <sect3 id="message-protocol-names-bus">
1576 <title>Bus names</title>
1578 Connections have one or more bus names associated with them.
1579 A connection has exactly one bus name that is a <firstterm>unique
1580 connection name</firstterm>. The unique connection name remains
1581 with the connection for its entire lifetime.
1582 A bus name is of type <literal>STRING</literal>,
1583 meaning that it must be valid UTF-8. However, there are also
1584 some additional restrictions that apply to bus names
1587 <listitem><para>Bus names that start with a colon (':')
1588 character are unique connection names. Other bus names
1589 are called <firstterm>well-known bus names</firstterm>.
1592 <listitem><para>Bus names are composed of 1 or more elements separated by
1593 a period ('.') character. All elements must contain at least
1597 <listitem><para>Each element must only contain the ASCII characters
1598 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1599 connection name may begin with a digit, elements in
1600 other bus names must not begin with a digit.
1604 <listitem><para>Bus names must contain at least one '.' (period)
1605 character (and thus at least two elements).
1608 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1609 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1613 Note that the hyphen ('-') character is allowed in bus names but
1614 not in interface names.
1618 Like <link linkend="message-protocol-names-interface">interface
1619 names</link>, well-known bus names should start with the
1620 reversed DNS domain name of the author of the interface (in
1621 lower-case), and it is conventional for the rest of the well-known
1622 bus name to consist of words run together, with initial
1623 capital letters. As with interface names, including a version
1624 number in well-known bus names is a good idea; it's possible to
1625 have the well-known bus name for more than one version
1626 simultaneously if backwards compatibility is required.
1630 If a well-known bus name implies the presence of a "main" interface,
1631 that "main" interface is often given the same name as
1632 the well-known bus name, and situated at the corresponding object
1633 path. For instance, if the owner of <literal>example.com</literal>
1634 is developing a D-Bus API for a music player, they might define
1635 that any application that takes the well-known name
1636 <literal>com.example.MusicPlayer1</literal> should have an object
1637 at the object path <literal>/com/example/MusicPlayer1</literal>
1638 which implements the interface
1639 <literal>com.example.MusicPlayer1</literal>.
1642 <sect3 id="message-protocol-names-member">
1643 <title>Member names</title>
1645 Member (i.e. method or signal) names:
1647 <listitem><para>Must only contain the ASCII characters
1648 "[A-Z][a-z][0-9]_" and may not begin with a
1649 digit.</para></listitem>
1650 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1651 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1652 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1657 It is conventional for member names on D-Bus to consist of
1658 capitalized words with no punctuation ("camel-case").
1659 Method names should usually be verbs, such as
1660 <literal>GetItems</literal>, and signal names should usually be
1661 a description of an event, such as <literal>ItemsChanged</literal>.
1664 <sect3 id="message-protocol-names-error">
1665 <title>Error names</title>
1667 Error names have the same restrictions as interface names.
1671 Error names have the same naming conventions as interface
1672 names, and often contain <literal>.Error.</literal>; for instance,
1673 the owner of <literal>example.com</literal> might define the
1674 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1675 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1676 The errors defined by D-Bus itself, such as
1677 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1683 <sect2 id="message-protocol-types">
1684 <title>Message Types</title>
1686 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1687 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1688 This section describes these conventions.
1690 <sect3 id="message-protocol-types-method">
1691 <title>Method Calls</title>
1693 Some messages invoke an operation on a remote object. These are
1694 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1695 messages map naturally to methods on objects in a typical program.
1698 A method call message is required to have a <literal>MEMBER</literal> header field
1699 indicating the name of the method. Optionally, the message has an
1700 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1701 Including the <literal>INTERFACE</literal> in all method call
1702 messages is strongly recommended.
1705 In the absence of an <literal>INTERFACE</literal> field, if two
1706 or more interfaces on the same object have a method with the same
1707 name, it is undefined which of those methods will be invoked.
1708 Implementations may choose to either return an error, or deliver the
1709 message as though it had an arbitrary one of those interfaces.
1712 In some situations (such as the well-known system bus), messages
1713 are filtered through an access-control list external to the
1714 remote object implementation. If that filter rejects certain
1715 messages by matching their interface, or accepts only messages
1716 to specific interfaces, it must also reject messages that have no
1717 <literal>INTERFACE</literal>: otherwise, malicious
1718 applications could use this to bypass the filter.
1721 Method call messages also include a <literal>PATH</literal> field
1722 indicating the object to invoke the method on. If the call is passing
1723 through a message bus, the message will also have a
1724 <literal>DESTINATION</literal> field giving the name of the connection
1725 to receive the message.
1728 When an application handles a method call message, it is required to
1729 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1730 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1731 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1734 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1735 are the return value(s) or "out parameters" of the method call.
1736 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1737 and the call fails; no return value will be provided. It makes
1738 no sense to send multiple replies to the same method call.
1741 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1742 reply is required, so the caller will know the method
1743 was successfully processed.
1746 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1750 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1751 then as an optimization the application receiving the method
1752 call may choose to omit the reply message (regardless of
1753 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1754 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1755 flag and reply anyway.
1758 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1759 destination name does not exist then a program to own the destination
1760 name will be started before the message is delivered. The message
1761 will be held until the new program is successfully started or has
1762 failed to start; in case of failure, an error will be returned. This
1763 flag is only relevant in the context of a message bus, it is ignored
1764 during one-to-one communication with no intermediate bus.
1766 <sect4 id="message-protocol-types-method-apis">
1767 <title>Mapping method calls to native APIs</title>
1769 APIs for D-Bus may map method calls to a method call in a specific
1770 programming language, such as C++, or may map a method call written
1771 in an IDL to a D-Bus message.
1774 In APIs of this nature, arguments to a method are often termed "in"
1775 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1776 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1777 "inout" arguments, which are both sent and received, i.e. the caller
1778 passes in a value which is modified. Mapped to D-Bus, an "inout"
1779 argument is equivalent to an "in" argument, followed by an "out"
1780 argument. You can't pass things "by reference" over the wire, so
1781 "inout" is purely an illusion of the in-process API.
1784 Given a method with zero or one return values, followed by zero or more
1785 arguments, where each argument may be "in", "out", or "inout", the
1786 caller constructs a message by appending each "in" or "inout" argument,
1787 in order. "out" arguments are not represented in the caller's message.
1790 The recipient constructs a reply by appending first the return value
1791 if any, then each "out" or "inout" argument, in order.
1792 "in" arguments are not represented in the reply message.
1795 Error replies are normally mapped to exceptions in languages that have
1799 In converting from native APIs to D-Bus, it is perhaps nice to
1800 map D-Bus naming conventions ("FooBar") to native conventions
1801 such as "fooBar" or "foo_bar" automatically. This is OK
1802 as long as you can say that the native API is one that
1803 was specifically written for D-Bus. It makes the most sense
1804 when writing object implementations that will be exported
1805 over the bus. Object proxies used to invoke remote D-Bus
1806 objects probably need the ability to call any D-Bus method,
1807 and thus a magic name mapping like this could be a problem.
1810 This specification doesn't require anything of native API bindings;
1811 the preceding is only a suggested convention for consistency
1817 <sect3 id="message-protocol-types-signal">
1818 <title>Signal Emission</title>
1820 Unlike method calls, signal emissions have no replies.
1821 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1822 It must have three header fields: <literal>PATH</literal> giving the object
1823 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1824 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1825 for signals, though it is optional for method calls.
1829 <sect3 id="message-protocol-types-errors">
1830 <title>Errors</title>
1832 Messages of type <literal>ERROR</literal> are most commonly replies
1833 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1834 to any kind of message. The message bus for example
1835 will return an <literal>ERROR</literal> in reply to a signal emission if
1836 the bus does not have enough memory to send the signal.
1839 An <literal>ERROR</literal> may have any arguments, but if the first
1840 argument is a <literal>STRING</literal>, it must be an error message.
1841 The error message may be logged or shown to the user
1846 <sect3 id="message-protocol-types-notation">
1847 <title>Notation in this document</title>
1849 This document uses a simple pseudo-IDL to describe particular method
1850 calls and signals. Here is an example of a method call:
1852 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1853 out UINT32 resultcode)
1855 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1856 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1857 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1858 characters so it's known that the last part of the name in
1859 the "IDL" is the member name.
1862 In C++ that might end up looking like this:
1864 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1865 unsigned int flags);
1867 or equally valid, the return value could be done as an argument:
1869 void org::freedesktop::DBus::StartServiceByName (const char *name,
1871 unsigned int *resultcode);
1873 It's really up to the API designer how they want to make
1874 this look. You could design an API where the namespace wasn't used
1875 in C++, using STL or Qt, using varargs, or whatever you wanted.
1878 Signals are written as follows:
1880 org.freedesktop.DBus.NameLost (STRING name)
1882 Signals don't specify "in" vs. "out" because only
1883 a single direction is possible.
1886 It isn't especially encouraged to use this lame pseudo-IDL in actual
1887 API implementations; you might use the native notation for the
1888 language you're using, or you might use COM or CORBA IDL, for example.
1893 <sect2 id="message-protocol-handling-invalid">
1894 <title>Invalid Protocol and Spec Extensions</title>
1897 For security reasons, the D-Bus protocol should be strictly parsed and
1898 validated, with the exception of defined extension points. Any invalid
1899 protocol or spec violations should result in immediately dropping the
1900 connection without notice to the other end. Exceptions should be
1901 carefully considered, e.g. an exception may be warranted for a
1902 well-understood idiosyncrasy of a widely-deployed implementation. In
1903 cases where the other end of a connection is 100% trusted and known to
1904 be friendly, skipping validation for performance reasons could also make
1905 sense in certain cases.
1909 Generally speaking violations of the "must" requirements in this spec
1910 should be considered possible attempts to exploit security, and violations
1911 of the "should" suggestions should be considered legitimate (though perhaps
1912 they should generate an error in some cases).
1916 The following extension points are built in to D-Bus on purpose and must
1917 not be treated as invalid protocol. The extension points are intended
1918 for use by future versions of this spec, they are not intended for third
1919 parties. At the moment, the only way a third party could extend D-Bus
1920 without breaking interoperability would be to introduce a way to negotiate new
1921 feature support as part of the auth protocol, using EXTENSION_-prefixed
1922 commands. There is not yet a standard way to negotiate features.
1926 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1927 commands result in an ERROR rather than a disconnect. This enables
1928 future extensions to the protocol. Commands starting with EXTENSION_ are
1929 reserved for third parties.
1934 The authentication protocol supports pluggable auth mechanisms.
1939 The address format (see <xref linkend="addresses"/>) supports new
1945 Messages with an unknown type (something other than
1946 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1947 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1948 Unknown-type messages must still be well-formed in the same way
1949 as the known messages, however. They still have the normal
1955 Header fields with an unknown or unexpected field code must be ignored,
1956 though again they must still be well-formed.
1961 New standard interfaces (with new methods and signals) can of course be added.
1971 <sect1 id="auth-protocol">
1972 <title>Authentication Protocol</title>
1974 Before the flow of messages begins, two applications must
1975 authenticate. A simple plain-text protocol is used for
1976 authentication; this protocol is a SASL profile, and maps fairly
1977 directly from the SASL specification. The message encoding is
1978 NOT used here, only plain text messages.
1981 In examples, "C:" and "S:" indicate lines sent by the client and
1982 server respectively.
1984 <sect2 id="auth-protocol-overview">
1985 <title>Protocol Overview</title>
1987 The protocol is a line-based protocol, where each line ends with
1988 \r\n. Each line begins with an all-caps ASCII command name containing
1989 only the character range [A-Z_], a space, then any arguments for the
1990 command, then the \r\n ending the line. The protocol is
1991 case-sensitive. All bytes must be in the ASCII character set.
1993 Commands from the client to the server are as follows:
1996 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1997 <listitem><para>CANCEL</para></listitem>
1998 <listitem><para>BEGIN</para></listitem>
1999 <listitem><para>DATA <data in hex encoding></para></listitem>
2000 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2001 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2004 From server to client are as follows:
2007 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2008 <listitem><para>OK <GUID in hex></para></listitem>
2009 <listitem><para>DATA <data in hex encoding></para></listitem>
2010 <listitem><para>ERROR</para></listitem>
2011 <listitem><para>AGREE_UNIX_FD</para></listitem>
2015 Unofficial extensions to the command set must begin with the letters
2016 "EXTENSION_", to avoid conflicts with future official commands.
2017 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2020 <sect2 id="auth-nul-byte">
2021 <title>Special credentials-passing nul byte</title>
2023 Immediately after connecting to the server, the client must send a
2024 single nul byte. This byte may be accompanied by credentials
2025 information on some operating systems that use sendmsg() with
2026 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2027 sockets. However, the nul byte must be sent even on other kinds of
2028 socket, and even on operating systems that do not require a byte to be
2029 sent in order to transmit credentials. The text protocol described in
2030 this document begins after the single nul byte. If the first byte
2031 received from the client is not a nul byte, the server may disconnect
2035 A nul byte in any context other than the initial byte is an error;
2036 the protocol is ASCII-only.
2039 The credentials sent along with the nul byte may be used with the
2040 SASL mechanism EXTERNAL.
2043 <sect2 id="auth-command-auth">
2044 <title>AUTH command</title>
2046 If an AUTH command has no arguments, it is a request to list
2047 available mechanisms. The server must respond with a REJECTED
2048 command listing the mechanisms it understands, or with an error.
2051 If an AUTH command specifies a mechanism, and the server supports
2052 said mechanism, the server should begin exchanging SASL
2053 challenge-response data with the client using DATA commands.
2056 If the server does not support the mechanism given in the AUTH
2057 command, it must send either a REJECTED command listing the mechanisms
2058 it does support, or an error.
2061 If the [initial-response] argument is provided, it is intended for use
2062 with mechanisms that have no initial challenge (or an empty initial
2063 challenge), as if it were the argument to an initial DATA command. If
2064 the selected mechanism has an initial challenge and [initial-response]
2065 was provided, the server should reject authentication by sending
2069 If authentication succeeds after exchanging DATA commands,
2070 an OK command must be sent to the client.
2073 The first octet received by the server after the \r\n of the BEGIN
2074 command from the client must be the first octet of the
2075 authenticated/encrypted stream of D-Bus messages.
2078 If BEGIN is received by the server, the first octet received
2079 by the client after the \r\n of the OK command must be the
2080 first octet of the authenticated/encrypted stream of D-Bus
2084 <sect2 id="auth-command-cancel">
2085 <title>CANCEL Command</title>
2087 At any time up to sending the BEGIN command, the client may send a
2088 CANCEL command. On receiving the CANCEL command, the server must
2089 send a REJECTED command and abort the current authentication
2093 <sect2 id="auth-command-data">
2094 <title>DATA Command</title>
2096 The DATA command may come from either client or server, and simply
2097 contains a hex-encoded block of data to be interpreted
2098 according to the SASL mechanism in use.
2101 Some SASL mechanisms support sending an "empty string";
2102 FIXME we need some way to do this.
2105 <sect2 id="auth-command-begin">
2106 <title>BEGIN Command</title>
2108 The BEGIN command acknowledges that the client has received an
2109 OK command from the server, and that the stream of messages
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 <sect2 id="auth-command-rejected">
2119 <title>REJECTED Command</title>
2121 The REJECTED command indicates that the current authentication
2122 exchange has failed, and further exchange of DATA is inappropriate.
2123 The client would normally try another mechanism, or try providing
2124 different responses to challenges.
2126 Optionally, the REJECTED command has a space-separated list of
2127 available auth mechanisms as arguments. If a server ever provides
2128 a list of supported mechanisms, it must provide the same list
2129 each time it sends a REJECTED message. Clients are free to
2130 ignore all lists received after the first.
2133 <sect2 id="auth-command-ok">
2134 <title>OK Command</title>
2136 The OK command indicates that the client has been
2137 authenticated. The client may now proceed with negotiating
2138 Unix file descriptor passing. To do that it shall send
2139 NEGOTIATE_UNIX_FD to the server.
2142 Otherwise, the client must respond to the OK command by
2143 sending a BEGIN command, followed by its stream of messages,
2144 or by disconnecting. The server must not accept additional
2145 commands using this protocol after the BEGIN command has been
2146 received. Further communication will be a stream of D-Bus
2147 messages (optionally encrypted, as negotiated) rather than
2151 If a client sends BEGIN the first octet received by the client
2152 after the \r\n of the OK command must be the first octet of
2153 the authenticated/encrypted stream of D-Bus messages.
2156 The OK command has one argument, which is the GUID of the server.
2157 See <xref linkend="addresses"/> for more on server GUIDs.
2160 <sect2 id="auth-command-error">
2161 <title>ERROR Command</title>
2163 The ERROR command indicates that either server or client did not
2164 know a command, does not accept the given command in the current
2165 context, or did not understand the arguments to the command. This
2166 allows the protocol to be extended; a client or server can send a
2167 command present or permitted only in new protocol versions, and if
2168 an ERROR is received instead of an appropriate response, fall back
2169 to using some other technique.
2172 If an ERROR is sent, the server or client that sent the
2173 error must continue as if the command causing the ERROR had never been
2174 received. However, the the server or client receiving the error
2175 should try something other than whatever caused the error;
2176 if only canceling/rejecting the authentication.
2179 If the D-Bus protocol changes incompatibly at some future time,
2180 applications implementing the new protocol would probably be able to
2181 check for support of the new protocol by sending a new command and
2182 receiving an ERROR from applications that don't understand it. Thus the
2183 ERROR feature of the auth protocol is an escape hatch that lets us
2184 negotiate extensions or changes to the D-Bus protocol in the future.
2187 <sect2 id="auth-command-negotiate-unix-fd">
2188 <title>NEGOTIATE_UNIX_FD Command</title>
2190 The NEGOTIATE_UNIX_FD command indicates that the client
2191 supports Unix file descriptor passing. This command may only
2192 be sent after the connection is authenticated, i.e. after OK
2193 was received by the client. This command may only be sent on
2194 transports that support Unix file descriptor passing.
2197 On receiving NEGOTIATE_UNIX_FD the server must respond with
2198 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2199 the transport chosen supports Unix file descriptor passing and
2200 the server supports this feature. It shall respond the latter
2201 if the transport does not support Unix file descriptor
2202 passing, the server does not support this feature, or the
2203 server decides not to enable file descriptor passing due to
2204 security or other reasons.
2207 <sect2 id="auth-command-agree-unix-fd">
2208 <title>AGREE_UNIX_FD Command</title>
2210 The AGREE_UNIX_FD command indicates that the server supports
2211 Unix file descriptor passing. This command may only be sent
2212 after the connection is authenticated, and the client sent
2213 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2214 command may only be sent on transports that support Unix file
2218 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2219 followed by its stream of messages, or by disconnecting. The
2220 server must not accept additional commands using this protocol
2221 after the BEGIN command has been received. Further
2222 communication will be a stream of D-Bus messages (optionally
2223 encrypted, as negotiated) rather than this protocol.
2226 <sect2 id="auth-command-future">
2227 <title>Future Extensions</title>
2229 Future extensions to the authentication and negotiation
2230 protocol are possible. For that new commands may be
2231 introduced. If a client or server receives an unknown command
2232 it shall respond with ERROR and not consider this fatal. New
2233 commands may be introduced both before, and after
2234 authentication, i.e. both before and after the OK command.
2237 <sect2 id="auth-examples">
2238 <title>Authentication examples</title>
2242 <title>Example of successful magic cookie authentication</title>
2244 (MAGIC_COOKIE is a made up mechanism)
2246 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2252 <title>Example of finding out mechanisms then picking one</title>
2255 S: REJECTED KERBEROS_V4 SKEY
2256 C: AUTH SKEY 7ab83f32ee
2257 S: DATA 8799cabb2ea93e
2258 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2264 <title>Example of client sends unknown command then falls back to regular auth</title>
2268 C: AUTH MAGIC_COOKIE 3736343435313230333039
2274 <title>Example of server doesn't support initial auth mechanism</title>
2276 C: AUTH MAGIC_COOKIE 3736343435313230333039
2277 S: REJECTED KERBEROS_V4 SKEY
2278 C: AUTH SKEY 7ab83f32ee
2279 S: DATA 8799cabb2ea93e
2280 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2286 <title>Example of wrong password or the like followed by successful retry</title>
2288 C: AUTH MAGIC_COOKIE 3736343435313230333039
2289 S: REJECTED KERBEROS_V4 SKEY
2290 C: AUTH SKEY 7ab83f32ee
2291 S: DATA 8799cabb2ea93e
2292 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2294 C: AUTH SKEY 7ab83f32ee
2295 S: DATA 8799cabb2ea93e
2296 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2302 <title>Example of skey cancelled and restarted</title>
2304 C: AUTH MAGIC_COOKIE 3736343435313230333039
2305 S: REJECTED KERBEROS_V4 SKEY
2306 C: AUTH SKEY 7ab83f32ee
2307 S: DATA 8799cabb2ea93e
2310 C: AUTH SKEY 7ab83f32ee
2311 S: DATA 8799cabb2ea93e
2312 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2318 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2320 (MAGIC_COOKIE is a made up mechanism)
2322 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2324 C: NEGOTIATE_UNIX_FD
2330 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2332 (MAGIC_COOKIE is a made up mechanism)
2334 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2336 C: NEGOTIATE_UNIX_FD
2343 <sect2 id="auth-states">
2344 <title>Authentication state diagrams</title>
2347 This section documents the auth protocol in terms of
2348 a state machine for the client and the server. This is
2349 probably the most robust way to implement the protocol.
2352 <sect3 id="auth-states-client">
2353 <title>Client states</title>
2356 To more precisely describe the interaction between the
2357 protocol state machine and the authentication mechanisms the
2358 following notation is used: MECH(CHALL) means that the
2359 server challenge CHALL was fed to the mechanism MECH, which
2365 CONTINUE(RESP) means continue the auth conversation
2366 and send RESP as the response to the server;
2372 OK(RESP) means that after sending RESP to the server
2373 the client side of the auth conversation is finished
2374 and the server should return "OK";
2380 ERROR means that CHALL was invalid and could not be
2386 Both RESP and CHALL may be empty.
2390 The Client starts by getting an initial response from the
2391 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2392 the mechanism did not provide an initial response. If the
2393 mechanism returns CONTINUE, the client starts in state
2394 <emphasis>WaitingForData</emphasis>, if the mechanism
2395 returns OK the client starts in state
2396 <emphasis>WaitingForOK</emphasis>.
2400 The client should keep track of available mechanisms and
2401 which it mechanisms it has already attempted. This list is
2402 used to decide which AUTH command to send. When the list is
2403 exhausted, the client should give up and close the
2408 <title><emphasis>WaitingForData</emphasis></title>
2416 MECH(CHALL) returns CONTINUE(RESP) → send
2418 <emphasis>WaitingForData</emphasis>
2422 MECH(CHALL) returns OK(RESP) → send DATA
2423 RESP, goto <emphasis>WaitingForOK</emphasis>
2427 MECH(CHALL) returns ERROR → send ERROR
2428 [msg], goto <emphasis>WaitingForData</emphasis>
2436 Receive REJECTED [mechs] →
2437 send AUTH [next mech], goto
2438 WaitingForData or <emphasis>WaitingForOK</emphasis>
2443 Receive ERROR → send
2445 <emphasis>WaitingForReject</emphasis>
2450 Receive OK → send
2451 BEGIN, terminate auth
2452 conversation, authenticated
2457 Receive anything else → send
2459 <emphasis>WaitingForData</emphasis>
2467 <title><emphasis>WaitingForOK</emphasis></title>
2472 Receive OK → send BEGIN, terminate auth
2473 conversation, <emphasis>authenticated</emphasis>
2478 Receive REJECTED [mechs] → send AUTH [next mech],
2479 goto <emphasis>WaitingForData</emphasis> or
2480 <emphasis>WaitingForOK</emphasis>
2486 Receive DATA → send CANCEL, goto
2487 <emphasis>WaitingForReject</emphasis>
2493 Receive ERROR → send CANCEL, goto
2494 <emphasis>WaitingForReject</emphasis>
2500 Receive anything else → send ERROR, goto
2501 <emphasis>WaitingForOK</emphasis>
2509 <title><emphasis>WaitingForReject</emphasis></title>
2514 Receive REJECTED [mechs] → send AUTH [next mech],
2515 goto <emphasis>WaitingForData</emphasis> or
2516 <emphasis>WaitingForOK</emphasis>
2522 Receive anything else → terminate auth
2523 conversation, disconnect
2532 <sect3 id="auth-states-server">
2533 <title>Server states</title>
2536 For the server MECH(RESP) means that the client response
2537 RESP was fed to the the mechanism MECH, which returns one of
2542 CONTINUE(CHALL) means continue the auth conversation and
2543 send CHALL as the challenge to the client;
2549 OK means that the client has been successfully
2556 REJECTED means that the client failed to authenticate or
2557 there was an error in RESP.
2562 The server starts out in state
2563 <emphasis>WaitingForAuth</emphasis>. If the client is
2564 rejected too many times the server must disconnect the
2569 <title><emphasis>WaitingForAuth</emphasis></title>
2575 Receive AUTH → send REJECTED [mechs], goto
2576 <emphasis>WaitingForAuth</emphasis>
2582 Receive AUTH MECH RESP
2586 MECH not valid mechanism → send REJECTED
2588 <emphasis>WaitingForAuth</emphasis>
2592 MECH(RESP) returns CONTINUE(CHALL) → send
2594 <emphasis>WaitingForData</emphasis>
2598 MECH(RESP) returns OK → send OK, goto
2599 <emphasis>WaitingForBegin</emphasis>
2603 MECH(RESP) returns REJECTED → send REJECTED
2605 <emphasis>WaitingForAuth</emphasis>
2613 Receive BEGIN → terminate
2614 auth conversation, disconnect
2620 Receive ERROR → send REJECTED [mechs], goto
2621 <emphasis>WaitingForAuth</emphasis>
2627 Receive anything else → send
2629 <emphasis>WaitingForAuth</emphasis>
2638 <title><emphasis>WaitingForData</emphasis></title>
2646 MECH(RESP) returns CONTINUE(CHALL) → send
2648 <emphasis>WaitingForData</emphasis>
2652 MECH(RESP) returns OK → send OK, goto
2653 <emphasis>WaitingForBegin</emphasis>
2657 MECH(RESP) returns REJECTED → send REJECTED
2659 <emphasis>WaitingForAuth</emphasis>
2667 Receive BEGIN → terminate auth conversation,
2674 Receive CANCEL → send REJECTED [mechs], goto
2675 <emphasis>WaitingForAuth</emphasis>
2681 Receive ERROR → send REJECTED [mechs], goto
2682 <emphasis>WaitingForAuth</emphasis>
2688 Receive anything else → send ERROR, goto
2689 <emphasis>WaitingForData</emphasis>
2697 <title><emphasis>WaitingForBegin</emphasis></title>
2702 Receive BEGIN → terminate auth conversation,
2703 client authenticated
2709 Receive CANCEL → send REJECTED [mechs], goto
2710 <emphasis>WaitingForAuth</emphasis>
2716 Receive ERROR → send REJECTED [mechs], goto
2717 <emphasis>WaitingForAuth</emphasis>
2723 Receive anything else → send ERROR, goto
2724 <emphasis>WaitingForBegin</emphasis>
2734 <sect2 id="auth-mechanisms">
2735 <title>Authentication mechanisms</title>
2737 This section describes some new authentication mechanisms.
2738 D-Bus also allows any standard SASL mechanism of course.
2740 <sect3 id="auth-mechanisms-sha">
2741 <title>DBUS_COOKIE_SHA1</title>
2743 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2744 has the ability to read a private file owned by the user being
2745 authenticated. If the client can prove that it has access to a secret
2746 cookie stored in this file, then the client is authenticated.
2747 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2751 Throughout this description, "hex encoding" must output the digits
2752 from a to f in lower-case; the digits A to F must not be used
2753 in the DBUS_COOKIE_SHA1 mechanism.
2756 Authentication proceeds as follows:
2760 The client sends the username it would like to authenticate
2766 The server sends the name of its "cookie context" (see below); a
2767 space character; the integer ID of the secret cookie the client
2768 must demonstrate knowledge of; a space character; then a
2769 randomly-generated challenge string, all of this hex-encoded into
2775 The client locates the cookie and generates its own
2776 randomly-generated challenge string. The client then concatenates
2777 the server's decoded challenge, a ":" character, its own challenge,
2778 another ":" character, and the cookie. It computes the SHA-1 hash
2779 of this composite string as a hex digest. It concatenates the
2780 client's challenge string, a space character, and the SHA-1 hex
2781 digest, hex-encodes the result and sends it back to the server.
2786 The server generates the same concatenated string used by the
2787 client and computes its SHA-1 hash. It compares the hash with
2788 the hash received from the client; if the two hashes match, the
2789 client is authenticated.
2795 Each server has a "cookie context," which is a name that identifies a
2796 set of cookies that apply to that server. A sample context might be
2797 "org_freedesktop_session_bus". Context names must be valid ASCII,
2798 nonzero length, and may not contain the characters slash ("/"),
2799 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2800 tab ("\t"), or period ("."). There is a default context,
2801 "org_freedesktop_general" that's used by servers that do not specify
2805 Cookies are stored in a user's home directory, in the directory
2806 <filename>~/.dbus-keyrings/</filename>. This directory must
2807 not be readable or writable by other users. If it is,
2808 clients and servers must ignore it. The directory
2809 contains cookie files named after the cookie context.
2812 A cookie file contains one cookie per line. Each line
2813 has three space-separated fields:
2817 The cookie ID number, which must be a non-negative integer and
2818 may not be used twice in the same file.
2823 The cookie's creation time, in UNIX seconds-since-the-epoch
2829 The cookie itself, a hex-encoded random block of bytes. The cookie
2830 may be of any length, though obviously security increases
2831 as the length increases.
2837 Only server processes modify the cookie file.
2838 They must do so with this procedure:
2842 Create a lockfile name by appending ".lock" to the name of the
2843 cookie file. The server should attempt to create this file
2844 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2845 fails, the lock fails. Servers should retry for a reasonable
2846 period of time, then they may choose to delete an existing lock
2847 to keep users from having to manually delete a stale
2848 lock. <footnote><para>Lockfiles are used instead of real file
2849 locking <literal>fcntl()</literal> because real locking
2850 implementations are still flaky on network
2851 filesystems.</para></footnote>
2856 Once the lockfile has been created, the server loads the cookie
2857 file. It should then delete any cookies that are old (the
2858 timeout can be fairly short), or more than a reasonable
2859 time in the future (so that cookies never accidentally
2860 become permanent, if the clock was set far into the future
2861 at some point). If no recent keys remain, the
2862 server may generate a new key.
2867 The pruned and possibly added-to cookie file
2868 must be resaved atomically (using a temporary
2869 file which is rename()'d).
2874 The lock must be dropped by deleting the lockfile.
2880 Clients need not lock the file in order to load it,
2881 because servers are required to save the file atomically.
2886 <sect1 id="addresses">
2887 <title>Server Addresses</title>
2889 Server addresses consist of a transport name followed by a colon, and
2890 then an optional, comma-separated list of keys and values in the form key=value.
2891 Each value is escaped.
2895 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2896 Which is the address to a unix socket with the path /tmp/dbus-test.
2899 Value escaping is similar to URI escaping but simpler.
2903 The set of optionally-escaped bytes is:
2904 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2905 <emphasis>byte</emphasis> (note, not character) which is not in the
2906 set of optionally-escaped bytes must be replaced with an ASCII
2907 percent (<literal>%</literal>) and the value of the byte in hex.
2908 The hex value must always be two digits, even if the first digit is
2909 zero. The optionally-escaped bytes may be escaped if desired.
2914 To unescape, append each byte in the value; if a byte is an ASCII
2915 percent (<literal>%</literal>) character then append the following
2916 hex value instead. It is an error if a <literal>%</literal> byte
2917 does not have two hex digits following. It is an error if a
2918 non-optionally-escaped byte is seen unescaped.
2922 The set of optionally-escaped bytes is intended to preserve address
2923 readability and convenience.
2927 A server may specify a key-value pair with the key <literal>guid</literal>
2928 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2929 describes the format of the <literal>guid</literal> field. If present,
2930 this UUID may be used to distinguish one server address from another. A
2931 server should use a different UUID for each address it listens on. For
2932 example, if a message bus daemon offers both UNIX domain socket and TCP
2933 connections, but treats clients the same regardless of how they connect,
2934 those two connections are equivalent post-connection but should have
2935 distinct UUIDs to distinguish the kinds of connection.
2939 The intent of the address UUID feature is to allow a client to avoid
2940 opening multiple identical connections to the same server, by allowing the
2941 client to check whether an address corresponds to an already-existing
2942 connection. Comparing two addresses is insufficient, because addresses
2943 can be recycled by distinct servers, and equivalent addresses may look
2944 different if simply compared as strings (for example, the host in a TCP
2945 address can be given as an IP address or as a hostname).
2949 Note that the address key is <literal>guid</literal> even though the
2950 rest of the API and documentation says "UUID," for historical reasons.
2954 [FIXME clarify if attempting to connect to each is a requirement
2955 or just a suggestion]
2956 When connecting to a server, multiple server addresses can be
2957 separated by a semi-colon. The library will then try to connect
2958 to the first address and if that fails, it'll try to connect to
2959 the next one specified, and so forth. For example
2960 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2964 Some addresses are <firstterm>connectable</firstterm>. A connectable
2965 address is one containing enough information for a client to connect
2966 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2967 is a connectable address. It is not necessarily possible to listen
2968 on every connectable address: for instance, it is not possible to
2969 listen on a <literal>unixexec:</literal> address.
2973 Some addresses are <firstterm>listenable</firstterm>. A listenable
2974 address is one containing enough information for a server to listen on
2975 it, producing a connectable address (which may differ from the
2976 original address). Many listenable addresses are not connectable:
2977 for instance, <literal>tcp:host=127.0.0.1</literal>
2978 is listenable, but not connectable (because it does not specify
2983 Listening on an address that is not connectable will result in a
2984 connectable address that is not the same as the listenable address.
2985 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
2986 might result in the connectable address
2987 <literal>tcp:host=127.0.0.1,port=30958</literal>,
2988 or listening on <literal>unix:tmpdir=/tmp</literal>
2989 might result in the connectable address
2990 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
2994 <sect1 id="transports">
2995 <title>Transports</title>
2997 [FIXME we need to specify in detail each transport and its possible arguments]
2999 Current transports include: unix domain sockets (including
3000 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3001 using in-process pipes. Future possible transports include one that
3002 tunnels over X11 protocol.
3005 <sect2 id="transports-unix-domain-sockets">
3006 <title>Unix Domain Sockets</title>
3008 Unix domain sockets can be either paths in the file system or on Linux
3009 kernels, they can be abstract which are similar to paths but
3010 do not show up in the file system.
3014 When a socket is opened by the D-Bus library it truncates the path
3015 name right before the first trailing Nul byte. This is true for both
3016 normal paths and abstract paths. Note that this is a departure from
3017 previous versions of D-Bus that would create sockets with a fixed
3018 length path name. Names which were shorter than the fixed length
3019 would be padded by Nul bytes.
3022 Unix domain sockets are not available on Windows.
3025 Unix addresses that specify <literal>path</literal> or
3026 <literal>abstract</literal> are both listenable and connectable.
3027 Unix addresses that specify <literal>tmpdir</literal> are only
3028 listenable: the corresponding connectable address will specify
3029 either <literal>path</literal> or <literal>abstract</literal>.
3031 <sect3 id="transports-unix-domain-sockets-addresses">
3032 <title>Server Address Format</title>
3034 Unix domain socket addresses are identified by the "unix:" prefix
3035 and support the following key/value pairs:
3042 <entry>Values</entry>
3043 <entry>Description</entry>
3049 <entry>(path)</entry>
3050 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3053 <entry>tmpdir</entry>
3054 <entry>(path)</entry>
3055 <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>
3058 <entry>abstract</entry>
3059 <entry>(string)</entry>
3060 <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>
3066 Exactly one of the keys <literal>path</literal>,
3067 <literal>abstract</literal> or
3068 <literal>tmpdir</literal> must be provided.
3072 <sect2 id="transports-launchd">
3073 <title>launchd</title>
3075 launchd is an open-source server management system that replaces init, inetd
3076 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3077 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3081 launchd allocates a socket and provides it with the unix path through the
3082 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3083 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3084 it through its environment.
3085 Other processes can query for the launchd socket by executing:
3086 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3087 This is normally done by the D-Bus client library so doesn't have to be done
3091 launchd is not available on Microsoft Windows.
3094 launchd addresses are listenable and connectable.
3096 <sect3 id="transports-launchd-addresses">
3097 <title>Server Address Format</title>
3099 launchd addresses are identified by the "launchd:" prefix
3100 and support the following key/value pairs:
3107 <entry>Values</entry>
3108 <entry>Description</entry>
3114 <entry>(environment variable)</entry>
3115 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3121 The <literal>env</literal> key is required.
3125 <sect2 id="transports-systemd">
3126 <title>systemd</title>
3128 systemd is an open-source server management system that
3129 replaces init and inetd on newer Linux systems. It supports
3130 socket activation. The D-Bus systemd transport is used to acquire
3131 socket activation file descriptors from systemd and use them
3132 as D-Bus transport when the current process is spawned by
3133 socket activation from it.
3136 The systemd transport accepts only one or more Unix domain or
3137 TCP streams sockets passed in via socket activation.
3140 The systemd transport is not available on non-Linux operating systems.
3143 The systemd transport defines no parameter keys.
3146 systemd addresses are listenable, but not connectable. The
3147 corresponding connectable address is the <literal>unix</literal>
3148 or <literal>tcp</literal> address of the socket.
3151 <sect2 id="transports-tcp-sockets">
3152 <title>TCP Sockets</title>
3154 The tcp transport provides TCP/IP based connections between clients
3155 located on the same or different hosts.
3158 Using tcp transport without any additional secure authentification mechanismus
3159 over a network is unsecure.
3162 On Windows and most Unix platforms, the TCP stack is unable to transfer
3163 credentials over a TCP connection, so the EXTERNAL authentication
3164 mechanism does not work for this transport.
3167 All <literal>tcp</literal> addresses are listenable.
3168 <literal>tcp</literal> addresses in which both
3169 <literal>host</literal> and <literal>port</literal> are
3170 specified, and <literal>port</literal> is non-zero,
3171 are also connectable.
3173 <sect3 id="transports-tcp-sockets-addresses">
3174 <title>Server Address Format</title>
3176 TCP/IP socket addresses are identified by the "tcp:" prefix
3177 and support the following key/value pairs:
3184 <entry>Values</entry>
3185 <entry>Description</entry>
3191 <entry>(string)</entry>
3192 <entry>DNS name or IP address</entry>
3196 <entry>(string)</entry>
3197 <entry>Used in a listenable address to configure the interface
3198 on which the server will listen: either the IP address of one of
3199 the local machine's interfaces (most commonly <literal>127.0.0.1
3200 </literal>), or a DNS name that resolves to one of those IP
3201 addresses, or '*' to listen on all interfaces simultaneously.
3202 If not specified, the default is the same value as "host".
3207 <entry>(number)</entry>
3208 <entry>The tcp port the server will open. A zero value let the server
3209 choose a free port provided from the underlaying operating system.
3210 libdbus is able to retrieve the real used port from the server.
3214 <entry>family</entry>
3215 <entry>(string)</entry>
3216 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3223 <sect2 id="transports-nonce-tcp-sockets">
3224 <title>Nonce-secured TCP Sockets</title>
3226 The nonce-tcp transport provides a secured TCP transport, using a
3227 simple authentication mechanism to ensure that only clients with read
3228 access to a certain location in the filesystem can connect to the server.
3229 The server writes a secret, the nonce, to a file and an incoming client
3230 connection is only accepted if the client sends the nonce right after
3231 the connect. The nonce mechanism requires no setup and is orthogonal to
3232 the higher-level authentication mechanisms described in the
3233 Authentication section.
3237 On start, the server generates a random 16 byte nonce and writes it
3238 to a file in the user's temporary directory. The nonce file location
3239 is published as part of the server's D-Bus address using the
3240 "noncefile" key-value pair.
3242 After an accept, the server reads 16 bytes from the socket. If the
3243 read bytes do not match the nonce stored in the nonce file, the
3244 server MUST immediately drop the connection.
3245 If the nonce match the received byte sequence, the client is accepted
3246 and the transport behaves like an unsecured tcp transport.
3249 After a successful connect to the server socket, the client MUST read
3250 the nonce from the file published by the server via the noncefile=
3251 key-value pair and send it over the socket. After that, the
3252 transport behaves like an unsecured tcp transport.
3255 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3256 <literal>host</literal>, <literal>port</literal> and
3257 <literal>noncefile</literal> are all specified,
3258 and <literal>port</literal> is nonzero, are also connectable.
3260 <sect3 id="transports-nonce-tcp-sockets-addresses">
3261 <title>Server Address Format</title>
3263 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3264 and support the following key/value pairs:
3271 <entry>Values</entry>
3272 <entry>Description</entry>
3278 <entry>(string)</entry>
3279 <entry>DNS name or IP address</entry>
3283 <entry>(string)</entry>
3284 <entry>The same as for tcp: addresses
3289 <entry>(number)</entry>
3290 <entry>The tcp port the server will open. A zero value let the server
3291 choose a free port provided from the underlaying operating system.
3292 libdbus is able to retrieve the real used port from the server.
3296 <entry>family</entry>
3297 <entry>(string)</entry>
3298 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3301 <entry>noncefile</entry>
3302 <entry>(path)</entry>
3303 <entry>File location containing the secret.
3304 This is only meaningful in connectable addresses:
3305 a listening D-Bus server that offers this transport
3306 will always create a new nonce file.</entry>
3313 <sect2 id="transports-exec">
3314 <title>Executed Subprocesses on Unix</title>
3316 This transport forks off a process and connects its standard
3317 input and standard output with an anonymous Unix domain
3318 socket. This socket is then used for communication by the
3319 transport. This transport may be used to use out-of-process
3320 forwarder programs as basis for the D-Bus protocol.
3323 The forked process will inherit the standard error output and
3324 process group from the parent process.
3327 Executed subprocesses are not available on Windows.
3330 <literal>unixexec</literal> addresses are connectable, but are not
3333 <sect3 id="transports-exec-addresses">
3334 <title>Server Address Format</title>
3336 Executed subprocess addresses are identified by the "unixexec:" prefix
3337 and support the following key/value pairs:
3344 <entry>Values</entry>
3345 <entry>Description</entry>
3351 <entry>(path)</entry>
3352 <entry>Path of the binary to execute, either an absolute
3353 path or a binary name that is searched for in the default
3354 search path of the OS. This corresponds to the first
3355 argument of execlp(). This key is mandatory.</entry>
3358 <entry>argv0</entry>
3359 <entry>(string)</entry>
3360 <entry>The program name to use when executing the
3361 binary. If omitted the same value as specified for path=
3362 will be used. This corresponds to the second argument of
3366 <entry>argv1, argv2, ...</entry>
3367 <entry>(string)</entry>
3368 <entry>Arguments to pass to the binary. This corresponds
3369 to the third and later arguments of execlp(). If a
3370 specific argvX is not specified no further argvY for Y > X
3371 are taken into account.</entry>
3379 <sect1 id="meta-transports">
3380 <title>Meta Transports</title>
3382 Meta transports are a kind of transport with special enhancements or
3383 behavior. Currently available meta transports include: autolaunch
3386 <sect2 id="meta-transports-autolaunch">
3387 <title>Autolaunch</title>
3388 <para>The autolaunch transport provides a way for dbus clients to autodetect
3389 a running dbus session bus and to autolaunch a session bus if not present.
3392 On Unix, <literal>autolaunch</literal> addresses are connectable,
3396 On Windows, <literal>autolaunch</literal> addresses are both
3397 connectable and listenable.
3400 <sect3 id="meta-transports-autolaunch-addresses">
3401 <title>Server Address Format</title>
3403 Autolaunch addresses uses the "autolaunch:" prefix and support the
3404 following key/value pairs:
3411 <entry>Values</entry>
3412 <entry>Description</entry>
3417 <entry>scope</entry>
3418 <entry>(string)</entry>
3419 <entry>scope of autolaunch (Windows only)
3423 "*install-path" - limit session bus to dbus installation path.
3424 The dbus installation path is determined from the location of
3425 the shared dbus library. If the library is located in a 'bin'
3426 subdirectory the installation root is the directory above,
3427 otherwise the directory where the library lives is taken as
3430 <install-root>/bin/[lib]dbus-1.dll
3431 <install-root>/[lib]dbus-1.dll
3437 "*user" - limit session bus to the recent user.
3442 other values - specify dedicated session bus like "release",
3454 <sect3 id="meta-transports-autolaunch-windows-implementation">
3455 <title>Windows implementation</title>
3457 On start, the server opens a platform specific transport, creates a mutex
3458 and a shared memory section containing the related session bus address.
3459 This mutex will be inspected by the dbus client library to detect a
3460 running dbus session bus. The access to the mutex and the shared memory
3461 section are protected by global locks.
3464 In the recent implementation the autolaunch transport uses a tcp transport
3465 on localhost with a port choosen from the operating system. This detail may
3466 change in the future.
3469 Disclaimer: The recent implementation is in an early state and may not
3470 work in all cirumstances and/or may have security issues. Because of this
3471 the implementation is not documentated yet.
3478 <title>UUIDs</title>
3480 A working D-Bus implementation uses universally-unique IDs in two places.
3481 First, each server address has a UUID identifying the address,
3482 as described in <xref linkend="addresses"/>. Second, each operating
3483 system kernel instance running a D-Bus client or server has a UUID
3484 identifying that kernel, retrieved by invoking the method
3485 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3486 linkend="standard-interfaces-peer"/>).
3489 The term "UUID" in this document is intended literally, i.e. an
3490 identifier that is universally unique. It is not intended to refer to
3491 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3494 The UUID must contain 128 bits of data and be hex-encoded. The
3495 hex-encoded string may not contain hyphens or other non-hex-digit
3496 characters, and it must be exactly 32 characters long. To generate a
3497 UUID, the current reference implementation concatenates 96 bits of random
3498 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3502 It would also be acceptable and probably better to simply generate 128
3503 bits of random data, as long as the random number generator is of high
3504 quality. The timestamp could conceivably help if the random bits are not
3505 very random. With a quality random number generator, collisions are
3506 extremely unlikely even with only 96 bits, so it's somewhat academic.
3509 Implementations should, however, stick to random data for the first 96 bits
3514 <sect1 id="standard-interfaces">
3515 <title>Standard Interfaces</title>
3517 See <xref linkend="message-protocol-types-notation"/> for details on
3518 the notation used in this section. There are some standard interfaces
3519 that may be useful across various D-Bus applications.
3521 <sect2 id="standard-interfaces-peer">
3522 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3524 The <literal>org.freedesktop.DBus.Peer</literal> interface
3527 org.freedesktop.DBus.Peer.Ping ()
3528 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3532 On receipt of the <literal>METHOD_CALL</literal> message
3533 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3534 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3535 usual. It does not matter which object path a ping is sent to. The
3536 reference implementation handles this method automatically.
3539 On receipt of the <literal>METHOD_CALL</literal> message
3540 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3541 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3542 UUID representing the identity of the machine the process is running on.
3543 This UUID must be the same for all processes on a single system at least
3544 until that system next reboots. It should be the same across reboots
3545 if possible, but this is not always possible to implement and is not
3547 It does not matter which object path a GetMachineId is sent to. The
3548 reference implementation handles this method automatically.
3551 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3552 a virtual machine running on a hypervisor, rather than a physical machine.
3553 Basically if two processes see the same UUID, they should also see the same
3554 shared memory, UNIX domain sockets, process IDs, and other features that require
3555 a running OS kernel in common between the processes.
3558 The UUID is often used where other programs might use a hostname. Hostnames
3559 can change without rebooting, however, or just be "localhost" - so the UUID
3563 <xref linkend="uuids"/> explains the format of the UUID.
3567 <sect2 id="standard-interfaces-introspectable">
3568 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3570 This interface has one method:
3572 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3576 Objects instances may implement
3577 <literal>Introspect</literal> which returns an XML description of
3578 the object, including its interfaces (with signals and methods), objects
3579 below it in the object path tree, and its properties.
3582 <xref linkend="introspection-format"/> describes the format of this XML string.
3585 <sect2 id="standard-interfaces-properties">
3586 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3588 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3589 or <firstterm>attributes</firstterm>. These can be exposed via the
3590 <literal>org.freedesktop.DBus.Properties</literal> interface.
3594 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3595 in STRING property_name,
3597 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3598 in STRING property_name,
3600 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3601 out DICT<STRING,VARIANT> props);
3605 It is conventional to give D-Bus properties names consisting of
3606 capitalized words without punctuation ("CamelCase"), like
3607 <link linkend="message-protocol-names-member">member names</link>.
3608 For instance, the GObject property
3609 <literal>connection-status</literal> or the Qt property
3610 <literal>connectionStatus</literal> could be represented on D-Bus
3611 as <literal>ConnectionStatus</literal>.
3614 Strictly speaking, D-Bus property names are not required to follow
3615 the same naming restrictions as member names, but D-Bus property
3616 names that would not be valid member names (in particular,
3617 GObject-style dash-separated property names) can cause interoperability
3618 problems and should be avoided.
3621 The available properties and whether they are writable can be determined
3622 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3623 see <xref linkend="standard-interfaces-introspectable"/>.
3626 An empty string may be provided for the interface name; in this case,
3627 if there are multiple properties on an object with the same name,
3628 the results are undefined (picking one by according to an arbitrary
3629 deterministic rule, or returning an error, are the reasonable
3633 If one or more properties change on an object, the
3634 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3635 signal may be emitted (this signal was added in 0.14):
3639 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3640 DICT<STRING,VARIANT> changed_properties,
3641 ARRAY<STRING> invalidated_properties);
3645 where <literal>changed_properties</literal> is a dictionary
3646 containing the changed properties with the new values and
3647 <literal>invalidated_properties</literal> is an array of
3648 properties that changed but the value is not conveyed.
3651 Whether the <literal>PropertiesChanged</literal> signal is
3652 supported can be determined by calling
3653 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3654 that the signal may be supported for an object but it may
3655 differ how whether and how it is used on a per-property basis
3656 (for e.g. performance or security reasons). Each property (or
3657 the parent interface) must be annotated with the
3658 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3659 annotation to convey this (usually the default value
3660 <literal>true</literal> is sufficient meaning that the
3661 annotation does not need to be used). See <xref
3662 linkend="introspection-format"/> for details on this
3667 <sect2 id="standard-interfaces-objectmanager">
3668 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3670 An API can optionally make use of this interface for one or
3671 more sub-trees of objects. The root of each sub-tree implements
3672 this interface so other applications can get all objects,
3673 interfaces and properties in a single method call. It is
3674 appropriate to use this interface if users of the tree of
3675 objects are expected to be interested in all interfaces of all
3676 objects in the tree; a more granular API should be used if
3677 users of the objects are expected to be interested in a small
3678 subset of the objects, a small subset of their interfaces, or
3682 The method that applications can use to get all objects and
3683 properties is <literal>GetManagedObjects</literal>:
3687 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3691 The return value of this method is a dict whose keys are
3692 object paths. All returned object paths are children of the
3693 object path implementing this interface, i.e. their object
3694 paths start with the ObjectManager's object path plus '/'.
3697 Each value is a dict whose keys are interfaces names. Each
3698 value in this inner dict is the same dict that would be
3699 returned by the <link
3700 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3701 method for that combination of object path and interface. If
3702 an interface has no properties, the empty dict is returned.
3705 Changes are emitted using the following two signals:
3709 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3710 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3711 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3712 ARRAY<STRING> interfaces);
3716 The <literal>InterfacesAdded</literal> signal is emitted when
3717 either a new object is added or when an existing object gains
3718 one or more interfaces. The
3719 <literal>InterfacesRemoved</literal> signal is emitted
3720 whenever an object is removed or it loses one or more
3721 interfaces. The second parameter of the
3722 <literal>InterfacesAdded</literal> signal contains a dict with
3723 the interfaces and properties (if any) that have been added to
3724 the given object path. Similarly, the second parameter of the
3725 <literal>InterfacesRemoved</literal> signal contains an array
3726 of the interfaces that were removed. Note that changes on
3727 properties on existing interfaces are not reported using this
3728 interface - an application should also monitor the existing <link
3729 linkend="standard-interfaces-properties">PropertiesChanged</link>
3730 signal on each object.
3733 Applications SHOULD NOT export objects that are children of an
3734 object (directly or otherwise) implementing this interface but
3735 which are not returned in the reply from the
3736 <literal>GetManagedObjects()</literal> method of this
3737 interface on the given object.
3740 The intent of the <literal>ObjectManager</literal> interface
3741 is to make it easy to write a robust client
3742 implementation. The trivial client implementation only needs
3743 to make two method calls:
3747 org.freedesktop.DBus.AddMatch (bus_proxy,
3748 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3749 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3753 on the message bus and the remote application's
3754 <literal>ObjectManager</literal>, respectively. Whenever a new
3755 remote object is created (or an existing object gains a new
3756 interface), the <literal>InterfacesAdded</literal> signal is
3757 emitted, and since this signal contains all properties for the
3758 interfaces, no calls to the
3759 <literal>org.freedesktop.Properties</literal> interface on the
3760 remote object are needed. Additionally, since the initial
3761 <literal>AddMatch()</literal> rule already includes signal
3762 messages from the newly created child object, no new
3763 <literal>AddMatch()</literal> call is needed.
3768 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3769 interface was added in version 0.17 of the D-Bus
3776 <sect1 id="introspection-format">
3777 <title>Introspection Data Format</title>
3779 As described in <xref linkend="standard-interfaces-introspectable"/>,
3780 objects may be introspected at runtime, returning an XML string
3781 that describes the object. The same XML format may be used in
3782 other contexts as well, for example as an "IDL" for generating
3783 static language bindings.
3786 Here is an example of introspection data:
3788 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3789 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3790 <node name="/com/example/sample_object">
3791 <interface name="com.example.SampleInterface">
3792 <method name="Frobate">
3793 <arg name="foo" type="i" direction="in"/>
3794 <arg name="bar" type="s" direction="out"/>
3795 <arg name="baz" type="a{us}" direction="out"/>
3796 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3798 <method name="Bazify">
3799 <arg name="bar" type="(iiu)" direction="in"/>
3800 <arg name="bar" type="v" direction="out"/>
3802 <method name="Mogrify">
3803 <arg name="bar" type="(iiav)" direction="in"/>
3805 <signal name="Changed">
3806 <arg name="new_value" type="b"/>
3808 <property name="Bar" type="y" access="readwrite"/>
3810 <node name="child_of_sample_object"/>
3811 <node name="another_child_of_sample_object"/>
3816 A more formal DTD and spec needs writing, but here are some quick notes.
3820 Only the root <node> element can omit the node name, as it's
3821 known to be the object that was introspected. If the root
3822 <node> does have a name attribute, it must be an absolute
3823 object path. If child <node> have object paths, they must be
3829 If a child <node> has any sub-elements, then they
3830 must represent a complete introspection of the child.
3831 If a child <node> is empty, then it may or may
3832 not have sub-elements; the child must be introspected
3833 in order to find out. The intent is that if an object
3834 knows that its children are "fast" to introspect
3835 it can go ahead and return their information, but
3836 otherwise it can omit it.
3841 The direction element on <arg> may be omitted,
3842 in which case it defaults to "in" for method calls
3843 and "out" for signals. Signals only allow "out"
3844 so while direction may be specified, it's pointless.
3849 The possible directions are "in" and "out",
3850 unlike CORBA there is no "inout"
3855 The possible property access flags are
3856 "readwrite", "read", and "write"
3861 Multiple interfaces can of course be listed for
3867 The "name" attribute on arguments is optional.
3873 Method, interface, property, and signal elements may have
3874 "annotations", which are generic key/value pairs of metadata.
3875 They are similar conceptually to Java's annotations and C# attributes.
3876 Well-known annotations:
3883 <entry>Values (separated by ,)</entry>
3884 <entry>Description</entry>
3889 <entry>org.freedesktop.DBus.Deprecated</entry>
3890 <entry>true,false</entry>
3891 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3894 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3895 <entry>(string)</entry>
3896 <entry>The C symbol; may be used for methods and interfaces</entry>
3899 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3900 <entry>true,false</entry>
3901 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3904 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3905 <entry>true,invalidates,false</entry>
3908 If set to <literal>false</literal>, the
3909 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3911 linkend="standard-interfaces-properties"/> is not
3912 guaranteed to be emitted if the property changes.
3915 If set to <literal>invalidates</literal> the signal
3916 is emitted but the value is not included in the
3920 If set to <literal>true</literal> the signal is
3921 emitted with the value included.
3924 The value for the annotation defaults to
3925 <literal>true</literal> if the enclosing interface
3926 element does not specify the annotation. Otherwise it
3927 defaults to the value specified in the enclosing
3936 <sect1 id="message-bus">
3937 <title>Message Bus Specification</title>
3938 <sect2 id="message-bus-overview">
3939 <title>Message Bus Overview</title>
3941 The message bus accepts connections from one or more applications.
3942 Once connected, applications can exchange messages with other
3943 applications that are also connected to the bus.
3946 In order to route messages among connections, the message bus keeps a
3947 mapping from names to connections. Each connection has one
3948 unique-for-the-lifetime-of-the-bus name automatically assigned.
3949 Applications may request additional names for a connection. Additional
3950 names are usually "well-known names" such as
3951 "com.example.TextEditor". When a name is bound to a connection,
3952 that connection is said to <firstterm>own</firstterm> the name.
3955 The bus itself owns a special name,
3956 <literal>org.freedesktop.DBus</literal>, with an object
3957 located at <literal>/org/freedesktop/DBus</literal> that
3958 implements the <literal>org.freedesktop.DBus</literal>
3959 interface. This service allows applications to make
3960 administrative requests of the bus itself. For example,
3961 applications can ask the bus to assign a name to a connection.
3964 Each name may have <firstterm>queued owners</firstterm>. When an
3965 application requests a name for a connection and the name is already in
3966 use, the bus will optionally add the connection to a queue waiting for
3967 the name. If the current owner of the name disconnects or releases
3968 the name, the next connection in the queue will become the new owner.
3972 This feature causes the right thing to happen if you start two text
3973 editors for example; the first one may request "com.example.TextEditor",
3974 and the second will be queued as a possible owner of that name. When
3975 the first exits, the second will take over.
3979 Applications may send <firstterm>unicast messages</firstterm> to
3980 a specific recipient or to the message bus itself, or
3981 <firstterm>broadcast messages</firstterm> to all interested recipients.
3982 See <xref linkend="message-bus-routing"/> for details.
3986 <sect2 id="message-bus-names">
3987 <title>Message Bus Names</title>
3989 Each connection has at least one name, assigned at connection time and
3990 returned in response to the
3991 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3992 automatically-assigned name is called the connection's <firstterm>unique
3993 name</firstterm>. Unique names are never reused for two different
3994 connections to the same bus.
3997 Ownership of a unique name is a prerequisite for interaction with
3998 the message bus. It logically follows that the unique name is always
3999 the first name that an application comes to own, and the last
4000 one that it loses ownership of.
4003 Unique connection names must begin with the character ':' (ASCII colon
4004 character); bus names that are not unique names must not begin
4005 with this character. (The bus must reject any attempt by an application
4006 to manually request a name beginning with ':'.) This restriction
4007 categorically prevents "spoofing"; messages sent to a unique name
4008 will always go to the expected connection.
4011 When a connection is closed, all the names that it owns are deleted (or
4012 transferred to the next connection in the queue if any).
4015 A connection can request additional names to be associated with it using
4016 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4017 linkend="message-protocol-names-bus"/> describes the format of a valid
4018 name. These names can be released again using the
4019 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4022 <sect3 id="bus-messages-request-name">
4023 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4027 UINT32 RequestName (in STRING name, in UINT32 flags)
4034 <entry>Argument</entry>
4036 <entry>Description</entry>
4042 <entry>STRING</entry>
4043 <entry>Name to request</entry>
4047 <entry>UINT32</entry>
4048 <entry>Flags</entry>
4058 <entry>Argument</entry>
4060 <entry>Description</entry>
4066 <entry>UINT32</entry>
4067 <entry>Return value</entry>
4074 This method call should be sent to
4075 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4076 assign the given name to the method caller. Each name maintains a
4077 queue of possible owners, where the head of the queue is the primary
4078 or current owner of the name. Each potential owner in the queue
4079 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4080 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4081 call. When RequestName is invoked the following occurs:
4085 If the method caller is currently the primary owner of the name,
4086 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4087 values are updated with the values from the new RequestName call,
4088 and nothing further happens.
4094 If the current primary owner (head of the queue) has
4095 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4096 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4097 the caller of RequestName replaces the current primary owner at
4098 the head of the queue and the current primary owner moves to the
4099 second position in the queue. If the caller of RequestName was
4100 in the queue previously its flags are updated with the values from
4101 the new RequestName in addition to moving it to the head of the queue.
4107 If replacement is not possible, and the method caller is
4108 currently in the queue but not the primary owner, its flags are
4109 updated with the values from the new RequestName call.
4115 If replacement is not possible, and the method caller is
4116 currently not in the queue, the method caller is appended to the
4123 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4124 set and is not the primary owner, it is removed from the
4125 queue. This can apply to the previous primary owner (if it
4126 was replaced) or the method caller (if it updated the
4127 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4128 queue, or if it was just added to the queue with that flag set).
4134 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4135 queue," even if another application already in the queue had specified
4136 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4137 that does not allow replacement goes away, and the next primary owner
4138 does allow replacement. In this case, queued items that specified
4139 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4140 automatically replace the new primary owner. In other words,
4141 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4142 time RequestName is called. This is deliberate to avoid an infinite loop
4143 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4144 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4147 The flags argument contains any of the following values logically ORed
4154 <entry>Conventional Name</entry>
4155 <entry>Value</entry>
4156 <entry>Description</entry>
4161 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4165 If an application A specifies this flag and succeeds in
4166 becoming the owner of the name, and another application B
4167 later calls RequestName with the
4168 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4169 will lose ownership and receive a
4170 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4171 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4172 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4173 is not specified by application B, then application B will not replace
4174 application A as the owner.
4179 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4183 Try to replace the current owner if there is one. If this
4184 flag is not set the application will only become the owner of
4185 the name if there is no current owner. If this flag is set,
4186 the application will replace the current owner if
4187 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4192 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4196 Without this flag, if an application requests a name that is
4197 already owned, the application will be placed in a queue to
4198 own the name when the current owner gives it up. If this
4199 flag is given, the application will not be placed in the
4200 queue, the request for the name will simply fail. This flag
4201 also affects behavior when an application is replaced as
4202 name owner; by default the application moves back into the
4203 waiting queue, unless this flag was provided when the application
4204 became the name owner.
4212 The return code can be one of the following values:
4218 <entry>Conventional Name</entry>
4219 <entry>Value</entry>
4220 <entry>Description</entry>
4225 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4226 <entry>1</entry> <entry>The caller is now the primary owner of
4227 the name, replacing any previous owner. Either the name had no
4228 owner before, or the caller specified
4229 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4230 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4233 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4236 <entry>The name already had an owner,
4237 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4238 the current owner did not specify
4239 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4240 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4244 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4245 <entry>The name already has an owner,
4246 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4247 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4248 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4249 specified by the requesting application.</entry>
4252 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4254 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4262 <sect3 id="bus-messages-release-name">
4263 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4267 UINT32 ReleaseName (in STRING name)
4274 <entry>Argument</entry>
4276 <entry>Description</entry>
4282 <entry>STRING</entry>
4283 <entry>Name to release</entry>
4293 <entry>Argument</entry>
4295 <entry>Description</entry>
4301 <entry>UINT32</entry>
4302 <entry>Return value</entry>
4309 This method call should be sent to
4310 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4311 release the method caller's claim to the given name. If the caller is
4312 the primary owner, a new primary owner will be selected from the
4313 queue if any other owners are waiting. If the caller is waiting in
4314 the queue for the name, the caller will removed from the queue and
4315 will not be made an owner of the name if it later becomes available.
4316 If there are no other owners in the queue for the name, it will be
4317 removed from the bus entirely.
4319 The return code can be one of the following values:
4325 <entry>Conventional Name</entry>
4326 <entry>Value</entry>
4327 <entry>Description</entry>
4332 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4333 <entry>1</entry> <entry>The caller has released his claim on
4334 the given name. Either the caller was the primary owner of
4335 the name, and the name is now unused or taken by somebody
4336 waiting in the queue for the name, or the caller was waiting
4337 in the queue for the name and has now been removed from the
4341 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4343 <entry>The given name does not exist on this bus.</entry>
4346 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4348 <entry>The caller was not the primary owner of this name,
4349 and was also not waiting in the queue to own this name.</entry>
4357 <sect3 id="bus-messages-list-queued-owners">
4358 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4362 ARRAY of STRING ListQueuedOwners (in STRING name)
4369 <entry>Argument</entry>
4371 <entry>Description</entry>
4377 <entry>STRING</entry>
4378 <entry>The well-known bus name to query, such as
4379 <literal>com.example.cappuccino</literal></entry>
4389 <entry>Argument</entry>
4391 <entry>Description</entry>
4397 <entry>ARRAY of STRING</entry>
4398 <entry>The unique bus names of connections currently queued
4399 for the name</entry>
4406 This method call should be sent to
4407 <literal>org.freedesktop.DBus</literal> and lists the connections
4408 currently queued for a bus name (see
4409 <xref linkend="term-queued-owner"/>).
4414 <sect2 id="message-bus-routing">
4415 <title>Message Bus Message Routing</title>
4418 Messages may have a <literal>DESTINATION</literal> field (see <xref
4419 linkend="message-protocol-header-fields"/>), resulting in a
4420 <firstterm>unicast message</firstterm>. If the
4421 <literal>DESTINATION</literal> field is present, it specifies a message
4422 recipient by name. Method calls and replies normally specify this field.
4423 The message bus must send messages (of any type) with the
4424 <literal>DESTINATION</literal> field set to the specified recipient,
4425 regardless of whether the recipient has set up a match rule matching
4430 When the message bus receives a signal, if the
4431 <literal>DESTINATION</literal> field is absent, it is considered to
4432 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4433 applications with <firstterm>message matching rules</firstterm> that
4434 match the message. Most signal messages are broadcasts.
4438 Unicast signal messages (those with a <literal>DESTINATION</literal>
4439 field) are not commonly used, but they are treated like any unicast
4440 message: they are delivered to the specified receipient,
4441 regardless of its match rules. One use for unicast signals is to
4442 avoid a race condition in which a signal is emitted before the intended
4443 recipient can call <xref linkend="bus-messages-add-match"/> to
4444 receive that signal: if the signal is sent directly to that recipient
4445 using a unicast message, it does not need to add a match rule at all,
4446 and there is no race condition. Another use for unicast signals,
4447 on message buses whose security policy prevents eavesdropping, is to
4448 send sensitive information which should only be visible to one
4453 When the message bus receives a method call, if the
4454 <literal>DESTINATION</literal> field is absent, the call is taken to be
4455 a standard one-to-one message and interpreted by the message bus
4456 itself. For example, sending an
4457 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4458 <literal>DESTINATION</literal> will cause the message bus itself to
4459 reply to the ping immediately; the message bus will not make this
4460 message visible to other applications.
4464 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4465 the ping message were sent with a <literal>DESTINATION</literal> name of
4466 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4467 forwarded, and the Yoyodyne Corporation screensaver application would be
4468 expected to reply to the ping.
4472 Message bus implementations may impose a security policy which
4473 prevents certain messages from being sent or received.
4474 When a message cannot be sent or received due to a security
4475 policy, the message bus should send an error reply, unless the
4476 original message had the <literal>NO_REPLY</literal> flag.
4479 <sect3 id="message-bus-routing-eavesdropping">
4480 <title>Eavesdropping</title>
4482 Receiving a unicast message whose <literal>DESTINATION</literal>
4483 indicates a different recipient is called
4484 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4485 a security boundary (like the standard system bus), the security
4486 policy should usually prevent eavesdropping, since unicast messages
4487 are normally kept private and may contain security-sensitive
4492 Eavesdropping is mainly useful for debugging tools, such as
4493 the <literal>dbus-monitor</literal> tool in the reference
4494 implementation of D-Bus. Tools which eavesdrop on the message bus
4495 should be careful to avoid sending a reply or error in response to
4496 messages intended for a different client.
4500 Clients may attempt to eavesdrop by adding match rules
4501 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4502 the <literal>eavesdrop='true'</literal> match. If the message bus'
4503 security policy does not allow eavesdropping, the match rule can
4504 still be added, but will not have any practical effect. For
4505 compatibility with older message bus implementations, if adding such
4506 a match rule results in an error reply, the client may fall back to
4507 adding the same rule with the <literal>eavesdrop</literal> match
4512 <sect3 id="message-bus-routing-match-rules">
4513 <title>Match Rules</title>
4515 An important part of the message bus routing protocol is match
4516 rules. Match rules describe the messages that should be sent to a
4517 client, based on the contents of the message. Broadcast signals
4518 are only sent to clients which have a suitable match rule: this
4519 avoids waking up client processes to deal with signals that are
4520 not relevant to that client.
4523 Messages that list a client as their <literal>DESTINATION</literal>
4524 do not need to match the client's match rules, and are sent to that
4525 client regardless. As a result, match rules are mainly used to
4526 receive a subset of broadcast signals.
4529 Match rules can also be used for eavesdropping
4530 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4531 if the security policy of the message bus allows it.
4534 Match rules are added using the AddMatch bus method
4535 (see <xref linkend="bus-messages-add-match"/>). Rules are
4536 specified as a string of comma separated key/value pairs.
4537 Excluding a key from the rule indicates a wildcard match.
4538 For instance excluding the the member from a match rule but
4539 adding a sender would let all messages from that sender through.
4540 An example of a complete rule would be
4541 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4544 The following table describes the keys that can be used to create
4551 <entry>Possible Values</entry>
4552 <entry>Description</entry>
4557 <entry><literal>type</literal></entry>
4558 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4559 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4562 <entry><literal>sender</literal></entry>
4563 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4564 and <xref linkend="term-unique-name"/> respectively)
4566 <entry>Match messages sent by a particular sender. An example of a sender match
4567 is sender='org.freedesktop.Hal'</entry>
4570 <entry><literal>interface</literal></entry>
4571 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4572 <entry>Match messages sent over or to a particular interface. An example of an
4573 interface match is interface='org.freedesktop.Hal.Manager'.
4574 If a message omits the interface header, it must not match any rule
4575 that specifies this key.</entry>
4578 <entry><literal>member</literal></entry>
4579 <entry>Any valid method or signal name</entry>
4580 <entry>Matches messages which have the give method or signal name. An example of
4581 a member match is member='NameOwnerChanged'</entry>
4584 <entry><literal>path</literal></entry>
4585 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4586 <entry>Matches messages which are sent from or to the given object. An example of a
4587 path match is path='/org/freedesktop/Hal/Manager'</entry>
4590 <entry><literal>path_namespace</literal></entry>
4591 <entry>An object path</entry>
4594 Matches messages which are sent from or to an
4595 object for which the object path is either the
4596 given value, or that value followed by one or
4597 more path components.
4602 <literal>path_namespace='/com/example/foo'</literal>
4603 would match signals sent by
4604 <literal>/com/example/foo</literal>
4606 <literal>/com/example/foo/bar</literal>,
4608 <literal>/com/example/foobar</literal>.
4612 Using both <literal>path</literal> and
4613 <literal>path_namespace</literal> in the same match
4614 rule is not allowed.
4619 This match key was added in version 0.16 of the
4620 D-Bus specification and implemented by the bus
4621 daemon in dbus 1.5.0 and later.
4627 <entry><literal>destination</literal></entry>
4628 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4629 <entry>Matches messages which are being sent to the given unique name. An
4630 example of a destination match is destination=':1.0'</entry>
4633 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4634 <entry>Any string</entry>
4635 <entry>Arg matches are special and are used for further restricting the
4636 match based on the arguments in the body of a message. Only arguments of type
4637 STRING can be matched in this way. An example of an argument match
4638 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4642 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4643 <entry>Any string</entry>
4645 <para>Argument path matches provide a specialised form of wildcard matching for
4646 path-like namespaces. They can match arguments whose type is either STRING or
4647 OBJECT_PATH. As with normal argument matches,
4648 if the argument is exactly equal to the string given in the match
4649 rule then the rule is satisfied. Additionally, there is also a
4650 match when either the string given in the match rule or the
4651 appropriate message argument ends with '/' and is a prefix of the
4652 other. An example argument path match is arg0path='/aa/bb/'. This
4653 would match messages with first arguments of '/', '/aa/',
4654 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4655 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4657 <para>This is intended for monitoring “directories” in file system-like
4658 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4659 system. An application interested in all nodes in a particular hierarchy would
4660 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4661 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4662 represent a modification to the “bar” property, or a signal with zeroth
4663 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4664 many properties within that directory, and the interested application would be
4665 notified in both cases.</para>
4668 This match key was added in version 0.12 of the
4669 D-Bus specification, implemented for STRING
4670 arguments by the bus daemon in dbus 1.2.0 and later,
4671 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4678 <entry><literal>arg0namespace</literal></entry>
4679 <entry>Like a bus name, except that the string is not
4680 required to contain a '.' (period)</entry>
4682 <para>Match messages whose first argument is of type STRING, and is a bus name
4683 or interface name within the specified namespace. This is primarily intended
4684 for watching name owner changes for a group of related bus names, rather than
4685 for a single name or all name changes.</para>
4687 <para>Because every valid interface name is also a valid
4688 bus name, this can also be used for messages whose
4689 first argument is an interface name.</para>
4691 <para>For example, the match rule
4692 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4693 matches name owner changes for bus names such as
4694 <literal>com.example.backend.foo</literal>,
4695 <literal>com.example.backend.foo.bar</literal>, and
4696 <literal>com.example.backend</literal> itself.</para>
4698 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4701 This match key was added in version 0.16 of the
4702 D-Bus specification and implemented by the bus
4703 daemon in dbus 1.5.0 and later.
4709 <entry><literal>eavesdrop</literal></entry>
4710 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4711 <entry>Since D-Bus 1.5.6, match rules do not
4712 match messages which have a <literal>DESTINATION</literal>
4713 field unless the match rule specifically
4715 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4716 by specifying <literal>eavesdrop='true'</literal>
4717 in the match rule. <literal>eavesdrop='false'</literal>
4718 restores the default behaviour. Messages are
4719 delivered to their <literal>DESTINATION</literal>
4720 regardless of match rules, so this match does not
4721 affect normal delivery of unicast messages.
4722 If the message bus has a security policy which forbids
4723 eavesdropping, this match may still be used without error,
4724 but will not have any practical effect.
4725 In older versions of D-Bus, this match was not allowed
4726 in match rules, and all match rules behaved as if
4727 <literal>eavesdrop='true'</literal> had been used.
4736 <sect2 id="message-bus-starting-services">
4737 <title>Message Bus Starting Services</title>
4739 The message bus can start applications on behalf of other applications.
4740 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4741 An application that can be started in this way is called a
4742 <firstterm>service</firstterm>.
4745 With D-Bus, starting a service is normally done by name. That is,
4746 applications ask the message bus to start some program that will own a
4747 well-known name, such as <literal>com.example.TextEditor</literal>.
4748 This implies a contract documented along with the name
4749 <literal>com.example.TextEditor</literal> for which object
4750 the owner of that name will provide, and what interfaces those
4754 To find an executable corresponding to a particular name, the bus daemon
4755 looks for <firstterm>service description files</firstterm>. Service
4756 description files define a mapping from names to executables. Different
4757 kinds of message bus will look for these files in different places, see
4758 <xref linkend="message-bus-types"/>.
4761 Service description files have the ".service" file
4762 extension. The message bus will only load service description files
4763 ending with .service; all other files will be ignored. The file format
4764 is similar to that of <ulink
4765 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4766 entries</ulink>. All service description files must be in UTF-8
4767 encoding. To ensure that there will be no name collisions, service files
4768 must be namespaced using the same mechanism as messages and service
4773 On the well-known system bus, the name of a service description file
4774 must be its well-known name plus <literal>.service</literal>,
4776 <literal>com.example.ConfigurationDatabase.service</literal>.
4780 On the well-known session bus, services should follow the same
4781 service description file naming convention as on the system bus,
4782 but for backwards compatibility they are not required to do so.
4786 [FIXME the file format should be much better specified than "similar to
4787 .desktop entries" esp. since desktop entries are already
4788 badly-specified. ;-)]
4789 These sections from the specification apply to service files as well:
4792 <listitem><para>General syntax</para></listitem>
4793 <listitem><para>Comment format</para></listitem>
4796 Service description files must contain a
4797 <literal>D-BUS Service</literal> group with at least the keys
4798 <literal>Name</literal> (the well-known name of the service)
4799 and <literal>Exec</literal> (the command to be executed).
4802 <title>Example service description file</title>
4804 # Sample service description file
4806 Name=com.example.ConfigurationDatabase
4807 Exec=/usr/bin/sample-configd
4813 Additionally, service description files for the well-known system
4814 bus on Unix must contain a <literal>User</literal> key, whose value
4815 is the name of a user account (e.g. <literal>root</literal>).
4816 The system service will be run as that user.
4820 When an application asks to start a service by name, the bus daemon tries to
4821 find a service that will own that name. It then tries to spawn the
4822 executable associated with it. If this fails, it will report an
4827 On the well-known system bus, it is not possible for two .service files
4828 in the same directory to offer the same service, because they are
4829 constrained to have names that match the service name.
4833 On the well-known session bus, if two .service files in the same
4834 directory offer the same service name, the result is undefined.
4835 Distributors should avoid this situation, for instance by naming
4836 session services' .service files according to their service name.
4840 If two .service files in different directories offer the same
4841 service name, the one in the higher-priority directory is used:
4842 for instance, on the system bus, .service files in
4843 /usr/local/share/dbus-1/system-services take precedence over those
4844 in /usr/share/dbus-1/system-services.
4847 The executable launched will have the environment variable
4848 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4849 message bus so it can connect and request the appropriate names.
4852 The executable being launched may want to know whether the message bus
4853 starting it is one of the well-known message buses (see <xref
4854 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4855 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4856 of the well-known buses. The currently-defined values for this variable
4857 are <literal>system</literal> for the systemwide message bus,
4858 and <literal>session</literal> for the per-login-session message
4859 bus. The new executable must still connect to the address given
4860 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4861 resulting connection is to the well-known bus.
4864 [FIXME there should be a timeout somewhere, either specified
4865 in the .service file, by the client, or just a global value
4866 and if the client being activated fails to connect within that
4867 timeout, an error should be sent back.]
4870 <sect3 id="message-bus-starting-services-scope">
4871 <title>Message Bus Service Scope</title>
4873 The "scope" of a service is its "per-", such as per-session,
4874 per-machine, per-home-directory, or per-display. The reference
4875 implementation doesn't yet support starting services in a different
4876 scope from the message bus itself. So e.g. if you start a service
4877 on the session bus its scope is per-session.
4880 We could add an optional scope to a bus name. For example, for
4881 per-(display,session pair), we could have a unique ID for each display
4882 generated automatically at login and set on screen 0 by executing a
4883 special "set display ID" binary. The ID would be stored in a
4884 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4885 random bytes. This ID would then be used to scope names.
4886 Starting/locating a service could be done by ID-name pair rather than
4890 Contrast this with a per-display scope. To achieve that, we would
4891 want a single bus spanning all sessions using a given display.
4892 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4893 property on screen 0 of the display, pointing to this bus.
4898 <sect2 id="message-bus-types">
4899 <title>Well-known Message Bus Instances</title>
4901 Two standard message bus instances are defined here, along with how
4902 to locate them and where their service files live.
4904 <sect3 id="message-bus-types-login">
4905 <title>Login session message bus</title>
4907 Each time a user logs in, a <firstterm>login session message
4908 bus</firstterm> may be started. All applications in the user's login
4909 session may interact with one another using this message bus.
4912 The address of the login session message bus is given
4913 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4914 variable. If that variable is not set, applications may
4915 also try to read the address from the X Window System root
4916 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4917 The root window property must have type <literal>STRING</literal>.
4918 The environment variable should have precedence over the
4919 root window property.
4921 <para>The address of the login session message bus is given in the
4922 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4923 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4924 "autolaunch:", the system should use platform-specific methods of
4925 locating a running D-Bus session server, or starting one if a running
4926 instance cannot be found. Note that this mechanism is not recommended
4927 for attempting to determine if a daemon is running. It is inherently
4928 racy to attempt to make this determination, since the bus daemon may
4929 be started just before or just after the determination is made.
4930 Therefore, it is recommended that applications do not try to make this
4931 determination for their functionality purposes, and instead they
4932 should attempt to start the server.</para>
4934 <sect4 id="message-bus-types-login-x-windows">
4935 <title>X Windowing System</title>
4937 For the X Windowing System, the application must locate the
4938 window owner of the selection represented by the atom formed by
4942 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4946 <para>the current user's username</para>
4950 <para>the literal character '_' (underscore)</para>
4954 <para>the machine's ID</para>
4960 The following properties are defined for the window that owns
4962 <informaltable frame="all">
4971 <para>meaning</para>
4977 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4981 <para>the actual address of the server socket</para>
4987 <para>_DBUS_SESSION_BUS_PID</para>
4991 <para>the PID of the server process</para>
5000 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5001 present in this window.
5005 If the X selection cannot be located or if reading the
5006 properties from the window fails, the implementation MUST conclude
5007 that there is no D-Bus server running and proceed to start a new
5008 server. (See below on concurrency issues)
5012 Failure to connect to the D-Bus server address thus obtained
5013 MUST be treated as a fatal connection error and should be reported
5018 As an alternative, an implementation MAY find the information
5019 in the following file located in the current user's home directory,
5020 in subdirectory .dbus/session-bus/:
5023 <para>the machine's ID</para>
5027 <para>the literal character '-' (dash)</para>
5031 <para>the X display without the screen number, with the
5032 following prefixes removed, if present: ":", "localhost:"
5033 ."localhost.localdomain:". That is, a display of
5034 "localhost:10.0" produces just the number "10"</para>
5040 The contents of this file NAME=value assignment pairs and
5041 lines starting with # are comments (no comments are allowed
5042 otherwise). The following variable names are defined:
5049 <para>Variable</para>
5053 <para>meaning</para>
5059 <para>DBUS_SESSION_BUS_ADDRESS</para>
5063 <para>the actual address of the server socket</para>
5069 <para>DBUS_SESSION_BUS_PID</para>
5073 <para>the PID of the server process</para>
5079 <para>DBUS_SESSION_BUS_WINDOWID</para>
5083 <para>the window ID</para>
5092 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5097 Failure to open this file MUST be interpreted as absence of a
5098 running server. Therefore, the implementation MUST proceed to
5099 attempting to launch a new bus server if the file cannot be
5104 However, success in opening this file MUST NOT lead to the
5105 conclusion that the server is running. Thus, a failure to connect to
5106 the bus address obtained by the alternative method MUST NOT be
5107 considered a fatal error. If the connection cannot be established,
5108 the implementation MUST proceed to check the X selection settings or
5109 to start the server on its own.
5113 If the implementation concludes that the D-Bus server is not
5114 running it MUST attempt to start a new server and it MUST also
5115 ensure that the daemon started as an effect of the "autolaunch"
5116 mechanism provides the lookup mechanisms described above, so
5117 subsequent calls can locate the newly started server. The
5118 implementation MUST also ensure that if two or more concurrent
5119 initiations happen, only one server remains running and all other
5120 initiations are able to obtain the address of this server and
5121 connect to it. In other words, the implementation MUST ensure that
5122 the X selection is not present when it attempts to set it, without
5123 allowing another process to set the selection between the
5124 verification and the setting (e.g., by using XGrabServer /
5131 On Unix systems, the session bus should search for .service files
5132 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5134 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5135 Implementations may also search additional locations, which
5136 should be searched with lower priority than anything in
5137 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5138 for example, the reference implementation also
5139 looks in <literal>${datadir}/dbus-1/services</literal> as
5140 set at compile time.
5143 As described in the XDG Base Directory Specification, software
5144 packages should install their session .service files to their
5145 configured <literal>${datadir}/dbus-1/services</literal>,
5146 where <literal>${datadir}</literal> is as defined by the GNU
5147 coding standards. System administrators or users can arrange
5148 for these service files to be read by setting XDG_DATA_DIRS or by
5149 symlinking them into the default locations.
5153 <sect3 id="message-bus-types-system">
5154 <title>System message bus</title>
5156 A computer may have a <firstterm>system message bus</firstterm>,
5157 accessible to all applications on the system. This message bus may be
5158 used to broadcast system events, such as adding new hardware devices,
5159 changes in the printer queue, and so forth.
5162 The address of the system message bus is given
5163 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5164 variable. If that variable is not set, applications should try
5165 to connect to the well-known address
5166 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5169 The D-Bus reference implementation actually honors the
5170 <literal>$(localstatedir)</literal> configure option
5171 for this address, on both client and server side.
5176 On Unix systems, the system bus should default to searching
5177 for .service files in
5178 <literal>/usr/local/share/dbus-1/system-services</literal>,
5179 <literal>/usr/share/dbus-1/system-services</literal> and
5180 <literal>/lib/dbus-1/system-services</literal>, with that order
5181 of precedence. It may also search other implementation-specific
5182 locations, but should not vary these locations based on environment
5186 The system bus is security-sensitive and is typically executed
5187 by an init system with a clean environment. Its launch helper
5188 process is particularly security-sensitive, and specifically
5189 clears its own environment.
5194 Software packages should install their system .service
5195 files to their configured
5196 <literal>${datadir}/dbus-1/system-services</literal>,
5197 where <literal>${datadir}</literal> is as defined by the GNU
5198 coding standards. System administrators can arrange
5199 for these service files to be read by editing the system bus'
5200 configuration file or by symlinking them into the default
5206 <sect2 id="message-bus-messages">
5207 <title>Message Bus Messages</title>
5209 The special message bus name <literal>org.freedesktop.DBus</literal>
5210 responds to a number of additional messages.
5213 <sect3 id="bus-messages-hello">
5214 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5225 <entry>Argument</entry>
5227 <entry>Description</entry>
5233 <entry>STRING</entry>
5234 <entry>Unique name assigned to the connection</entry>
5241 Before an application is able to send messages to other applications
5242 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5243 to the message bus to obtain a unique name. If an application without
5244 a unique name tries to send a message to another application, or a
5245 message to the message bus itself that isn't the
5246 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5247 disconnected from the bus.
5250 There is no corresponding "disconnect" request; if a client wishes to
5251 disconnect from the bus, it simply closes the socket (or other
5252 communication channel).
5255 <sect3 id="bus-messages-list-names">
5256 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5260 ARRAY of STRING ListNames ()
5267 <entry>Argument</entry>
5269 <entry>Description</entry>
5275 <entry>ARRAY of STRING</entry>
5276 <entry>Array of strings where each string is a bus name</entry>
5283 Returns a list of all currently-owned names on the bus.
5286 <sect3 id="bus-messages-list-activatable-names">
5287 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5291 ARRAY of STRING ListActivatableNames ()
5298 <entry>Argument</entry>
5300 <entry>Description</entry>
5306 <entry>ARRAY of STRING</entry>
5307 <entry>Array of strings where each string is a bus name</entry>
5314 Returns a list of all names that can be activated on the bus.
5317 <sect3 id="bus-messages-name-exists">
5318 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5322 BOOLEAN NameHasOwner (in STRING name)
5329 <entry>Argument</entry>
5331 <entry>Description</entry>
5337 <entry>STRING</entry>
5338 <entry>Name to check</entry>
5348 <entry>Argument</entry>
5350 <entry>Description</entry>
5356 <entry>BOOLEAN</entry>
5357 <entry>Return value, true if the name exists</entry>
5364 Checks if the specified name exists (currently has an owner).
5368 <sect3 id="bus-messages-name-owner-changed">
5369 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5373 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5380 <entry>Argument</entry>
5382 <entry>Description</entry>
5388 <entry>STRING</entry>
5389 <entry>Name with a new owner</entry>
5393 <entry>STRING</entry>
5394 <entry>Old owner or empty string if none</entry>
5398 <entry>STRING</entry>
5399 <entry>New owner or empty string if none</entry>
5406 This signal indicates that the owner of a name has changed.
5407 It's also the signal to use to detect the appearance of
5408 new names on the bus.
5411 <sect3 id="bus-messages-name-lost">
5412 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5416 NameLost (STRING name)
5423 <entry>Argument</entry>
5425 <entry>Description</entry>
5431 <entry>STRING</entry>
5432 <entry>Name which was lost</entry>
5439 This signal is sent to a specific application when it loses
5440 ownership of a name.
5444 <sect3 id="bus-messages-name-acquired">
5445 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5449 NameAcquired (STRING name)
5456 <entry>Argument</entry>
5458 <entry>Description</entry>
5464 <entry>STRING</entry>
5465 <entry>Name which was acquired</entry>
5472 This signal is sent to a specific application when it gains
5473 ownership of a name.
5477 <sect3 id="bus-messages-start-service-by-name">
5478 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5482 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5489 <entry>Argument</entry>
5491 <entry>Description</entry>
5497 <entry>STRING</entry>
5498 <entry>Name of the service to start</entry>
5502 <entry>UINT32</entry>
5503 <entry>Flags (currently not used)</entry>
5513 <entry>Argument</entry>
5515 <entry>Description</entry>
5521 <entry>UINT32</entry>
5522 <entry>Return value</entry>
5527 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5531 The return value can be one of the following values:
5536 <entry>Identifier</entry>
5537 <entry>Value</entry>
5538 <entry>Description</entry>
5543 <entry>DBUS_START_REPLY_SUCCESS</entry>
5545 <entry>The service was successfully started.</entry>
5548 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5550 <entry>A connection already owns the given name.</entry>
5559 <sect3 id="bus-messages-update-activation-environment">
5560 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5564 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5571 <entry>Argument</entry>
5573 <entry>Description</entry>
5579 <entry>ARRAY of DICT<STRING,STRING></entry>
5580 <entry>Environment to add or update</entry>
5585 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5588 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5591 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.
5596 <sect3 id="bus-messages-get-name-owner">
5597 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5601 STRING GetNameOwner (in STRING name)
5608 <entry>Argument</entry>
5610 <entry>Description</entry>
5616 <entry>STRING</entry>
5617 <entry>Name to get the owner of</entry>
5627 <entry>Argument</entry>
5629 <entry>Description</entry>
5635 <entry>STRING</entry>
5636 <entry>Return value, a unique connection name</entry>
5641 Returns the unique connection name of the primary owner of the name
5642 given. If the requested name doesn't have an owner, returns a
5643 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5647 <sect3 id="bus-messages-get-connection-unix-user">
5648 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5652 UINT32 GetConnectionUnixUser (in STRING bus_name)
5659 <entry>Argument</entry>
5661 <entry>Description</entry>
5667 <entry>STRING</entry>
5668 <entry>Unique or well-known bus name of the connection to
5669 query, such as <literal>:12.34</literal> or
5670 <literal>com.example.tea</literal></entry>
5680 <entry>Argument</entry>
5682 <entry>Description</entry>
5688 <entry>UINT32</entry>
5689 <entry>Unix user ID</entry>
5694 Returns the Unix user ID of the process connected to the server. If
5695 unable to determine it (for instance, because the process is not on the
5696 same machine as the bus daemon), an error is returned.
5700 <sect3 id="bus-messages-get-connection-unix-process-id">
5701 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5705 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5712 <entry>Argument</entry>
5714 <entry>Description</entry>
5720 <entry>STRING</entry>
5721 <entry>Unique or well-known bus name of the connection to
5722 query, such as <literal>:12.34</literal> or
5723 <literal>com.example.tea</literal></entry>
5733 <entry>Argument</entry>
5735 <entry>Description</entry>
5741 <entry>UINT32</entry>
5742 <entry>Unix process id</entry>
5747 Returns the Unix process ID of the process connected to the server. If
5748 unable to determine it (for instance, because the process is not on the
5749 same machine as the bus daemon), an error is returned.
5753 <sect3 id="bus-messages-get-connection-credentials">
5754 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5758 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5765 <entry>Argument</entry>
5767 <entry>Description</entry>
5773 <entry>STRING</entry>
5774 <entry>Unique or well-known bus name of the connection to
5775 query, such as <literal>:12.34</literal> or
5776 <literal>com.example.tea</literal></entry>
5786 <entry>Argument</entry>
5788 <entry>Description</entry>
5794 <entry>DICT<STRING,VARIANT></entry>
5795 <entry>Credentials</entry>
5803 Returns as many credentials as possible for the process connected to
5804 the server. If unable to determine certain credentials (for instance,
5805 because the process is not on the same machine as the bus daemon,
5806 or because this version of the bus daemon does not support a
5807 particular security framework), or if the values of those credentials
5808 cannot be represented as documented here, then those credentials
5813 Keys in the returned dictionary not containing "." are defined
5814 by this specification. Bus daemon implementors supporting
5815 credentials frameworks not mentioned in this document should either
5816 contribute patches to this specification, or use keys containing
5817 "." and starting with a reversed domain name.
5823 <entry>Value type</entry>
5824 <entry>Value</entry>
5829 <entry>UnixUserID</entry>
5830 <entry>UINT32</entry>
5831 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5834 <entry>ProcessID</entry>
5835 <entry>UINT32</entry>
5836 <entry>The numeric process ID, on platforms that have
5837 this concept. On Unix, this is the process ID defined by
5846 This method was added in D-Bus 1.7 to reduce the round-trips
5847 required to list a process's credentials. In older versions, calling
5848 this method will fail: applications should recover by using the
5849 separate methods such as
5850 <xref linkend="bus-messages-get-connection-unix-user"/>
5855 <sect3 id="bus-messages-get-adt-audit-session-data">
5856 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5860 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5867 <entry>Argument</entry>
5869 <entry>Description</entry>
5875 <entry>STRING</entry>
5876 <entry>Unique or well-known bus name of the connection to
5877 query, such as <literal>:12.34</literal> or
5878 <literal>com.example.tea</literal></entry>
5888 <entry>Argument</entry>
5890 <entry>Description</entry>
5896 <entry>ARRAY of BYTE</entry>
5897 <entry>auditing data as returned by
5898 adt_export_session_data()</entry>
5903 Returns auditing data used by Solaris ADT, in an unspecified
5904 binary format. If you know what this means, please contribute
5905 documentation via the D-Bus bug tracking system.
5906 This method is on the core DBus interface for historical reasons;
5907 the same information should be made available via
5908 <xref linkend="bus-messages-get-connection-credentials"/>
5913 <sect3 id="bus-messages-get-connection-selinux-security-context">
5914 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5918 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5925 <entry>Argument</entry>
5927 <entry>Description</entry>
5933 <entry>STRING</entry>
5934 <entry>Unique or well-known bus name of the connection to
5935 query, such as <literal>:12.34</literal> or
5936 <literal>com.example.tea</literal></entry>
5946 <entry>Argument</entry>
5948 <entry>Description</entry>
5954 <entry>ARRAY of BYTE</entry>
5955 <entry>some sort of string of bytes, not necessarily UTF-8,
5956 not including '\0'</entry>
5961 Returns the security context used by SELinux, in an unspecified
5962 format. If you know what this means, please contribute
5963 documentation via the D-Bus bug tracking system.
5964 This method is on the core DBus interface for historical reasons;
5965 the same information should be made available via
5966 <xref linkend="bus-messages-get-connection-credentials"/>
5972 <sect3 id="bus-messages-add-match">
5973 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5977 AddMatch (in STRING rule)
5984 <entry>Argument</entry>
5986 <entry>Description</entry>
5992 <entry>STRING</entry>
5993 <entry>Match rule to add to the connection</entry>
5998 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5999 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6003 <sect3 id="bus-messages-remove-match">
6004 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6008 RemoveMatch (in STRING rule)
6015 <entry>Argument</entry>
6017 <entry>Description</entry>
6023 <entry>STRING</entry>
6024 <entry>Match rule to remove from the connection</entry>
6029 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6030 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6035 <sect3 id="bus-messages-get-id">
6036 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6040 GetId (out STRING id)
6047 <entry>Argument</entry>
6049 <entry>Description</entry>
6055 <entry>STRING</entry>
6056 <entry>Unique ID identifying the bus daemon</entry>
6061 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6062 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6063 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6064 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6065 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6066 by org.freedesktop.DBus.Peer.GetMachineId().
6067 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6075 <appendix id="implementation-notes">
6076 <title>Implementation notes</title>
6077 <sect1 id="implementation-notes-subsection">
6085 <glossary><title>Glossary</title>
6087 This glossary defines some of the terms used in this specification.
6090 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6093 The message bus maintains an association between names and
6094 connections. (Normally, there's one connection per application.) A
6095 bus name is simply an identifier used to locate connections. For
6096 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6097 name might be used to send a message to a screensaver from Yoyodyne
6098 Corporation. An application is said to <firstterm>own</firstterm> a
6099 name if the message bus has associated the application's connection
6100 with the name. Names may also have <firstterm>queued
6101 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6102 The bus assigns a unique name to each connection,
6103 see <xref linkend="term-unique-name"/>. Other names
6104 can be thought of as "well-known names" and are
6105 used to find applications that offer specific functionality.
6109 See <xref linkend="message-protocol-names-bus"/> for details of
6110 the syntax and naming conventions for bus names.
6115 <glossentry id="term-message"><glossterm>Message</glossterm>
6118 A message is the atomic unit of communication via the D-Bus
6119 protocol. It consists of a <firstterm>header</firstterm> and a
6120 <firstterm>body</firstterm>; the body is made up of
6121 <firstterm>arguments</firstterm>.
6126 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6129 The message bus is a special application that forwards
6130 or routes messages between a group of applications
6131 connected to the message bus. It also manages
6132 <firstterm>names</firstterm> used for routing
6138 <glossentry id="term-name"><glossterm>Name</glossterm>
6141 See <xref linkend="term-bus-name"/>. "Name" may
6142 also be used to refer to some of the other names
6143 in D-Bus, such as interface names.
6148 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6151 Used to prevent collisions when defining new interfaces, bus names
6152 etc. The convention used is the same one Java uses for defining
6153 classes: a reversed domain name.
6154 See <xref linkend="message-protocol-names-bus"/>,
6155 <xref linkend="message-protocol-names-interface"/>,
6156 <xref linkend="message-protocol-names-error"/>,
6157 <xref linkend="message-protocol-marshaling-object-path"/>.
6162 <glossentry id="term-object"><glossterm>Object</glossterm>
6165 Each application contains <firstterm>objects</firstterm>, which have
6166 <firstterm>interfaces</firstterm> and
6167 <firstterm>methods</firstterm>. Objects are referred to by a name,
6168 called a <firstterm>path</firstterm>.
6173 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6176 An application talking directly to another application, without going
6177 through a message bus. One-to-one connections may be "peer to peer" or
6178 "client to server." The D-Bus protocol has no concept of client
6179 vs. server after a connection has authenticated; the flow of messages
6180 is symmetrical (full duplex).
6185 <glossentry id="term-path"><glossterm>Path</glossterm>
6188 Object references (object names) in D-Bus are organized into a
6189 filesystem-style hierarchy, so each object is named by a path. As in
6190 LDAP, there's no difference between "files" and "directories"; a path
6191 can refer to an object, while still having child objects below it.
6196 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6199 Each bus name has a primary owner; messages sent to the name go to the
6200 primary owner. However, certain names also maintain a queue of
6201 secondary owners "waiting in the wings." If the primary owner releases
6202 the name, then the first secondary owner in the queue automatically
6203 becomes the new owner of the name.
6208 <glossentry id="term-service"><glossterm>Service</glossterm>
6211 A service is an executable that can be launched by the bus daemon.
6212 Services normally guarantee some particular features, for example they
6213 may guarantee that they will request a specific name such as
6214 "com.example.Screensaver", have a singleton object
6215 "/com/example/Application", and that object will implement the
6216 interface "com.example.Screensaver.Control".
6221 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6224 ".service files" tell the bus about service applications that can be
6225 launched (see <xref linkend="term-service"/>). Most importantly they
6226 provide a mapping from bus names to services that will request those
6227 names when they start up.
6232 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6235 The special name automatically assigned to each connection by the
6236 message bus. This name will never change owner, and will be unique
6237 (never reused during the lifetime of the message bus).
6238 It will begin with a ':' character.
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