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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.23</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>0.23</revnumber>
76 <date>not yet released</date>
77 <authorinitials></authorinitials>
79 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
83 <revnumber>0.22</revnumber>
84 <date>2013-10-09</date>
85 <authorinitials></authorinitials>
86 <revremark>add GetConnectionCredentials, document
87 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
88 document and correct .service file syntax and naming
92 <revnumber>0.21</revnumber>
93 <date>2013-04-25</date>
94 <authorinitials>smcv</authorinitials>
95 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
96 Corrigendum #9)</revremark>
99 <revnumber>0.20</revnumber>
100 <date>22 February 2013</date>
101 <authorinitials>smcv, walters</authorinitials>
102 <revremark>reorganise for clarity, remove false claims about
103 basic types, mention /o/fd/DBus</revremark>
106 <revnumber>0.19</revnumber>
107 <date>20 February 2012</date>
108 <authorinitials>smcv/lp</authorinitials>
109 <revremark>formally define unique connection names and well-known
110 bus names; document best practices for interface, bus, member and
111 error names, and object paths; document the search path for session
112 and system services on Unix; document the systemd transport</revremark>
115 <revnumber>0.18</revnumber>
116 <date>29 July 2011</date>
117 <authorinitials>smcv</authorinitials>
118 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
119 match keyword; promote type system to a top-level section</revremark>
122 <revnumber>0.17</revnumber>
123 <date>1 June 2011</date>
124 <authorinitials>smcv/davidz</authorinitials>
125 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
126 by GVariant</revremark>
129 <revnumber>0.16</revnumber>
130 <date>11 April 2011</date>
131 <authorinitials></authorinitials>
132 <revremark>add path_namespace, arg0namespace; argNpath matches object
136 <revnumber>0.15</revnumber>
137 <date>3 November 2010</date>
138 <authorinitials></authorinitials>
139 <revremark></revremark>
142 <revnumber>0.14</revnumber>
143 <date>12 May 2010</date>
144 <authorinitials></authorinitials>
145 <revremark></revremark>
148 <revnumber>0.13</revnumber>
149 <date>23 Dezember 2009</date>
150 <authorinitials></authorinitials>
151 <revremark></revremark>
154 <revnumber>0.12</revnumber>
155 <date>7 November, 2006</date>
156 <authorinitials></authorinitials>
157 <revremark></revremark>
160 <revnumber>0.11</revnumber>
161 <date>6 February 2005</date>
162 <authorinitials></authorinitials>
163 <revremark></revremark>
166 <revnumber>0.10</revnumber>
167 <date>28 January 2005</date>
168 <authorinitials></authorinitials>
169 <revremark></revremark>
172 <revnumber>0.9</revnumber>
173 <date>7 Januar 2005</date>
174 <authorinitials></authorinitials>
175 <revremark></revremark>
178 <revnumber>0.8</revnumber>
179 <date>06 September 2003</date>
180 <authorinitials></authorinitials>
181 <revremark>First released document.</revremark>
186 <sect1 id="introduction">
187 <title>Introduction</title>
189 D-Bus is a system for low-overhead, easy to use
190 interprocess communication (IPC). In more detail:
194 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
195 binary protocol, and does not have to convert to and from a text
196 format such as XML. Because D-Bus is intended for potentially
197 high-resolution same-machine IPC, not primarily for Internet IPC,
198 this is an interesting optimization. D-Bus is also designed to
199 avoid round trips and allow asynchronous operation, much like
205 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
206 of <firstterm>messages</firstterm> rather than byte streams, and
207 automatically handles a lot of the hard IPC issues. Also, the D-Bus
208 library is designed to be wrapped in a way that lets developers use
209 their framework's existing object/type system, rather than learning
210 a new one specifically for IPC.
217 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
218 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
219 a system for one application to talk to a single other
220 application. However, the primary intended application of the protocol is the
221 D-Bus <firstterm>message bus</firstterm>, specified in <xref
222 linkend="message-bus"/>. The message bus is a special application that
223 accepts connections from multiple other applications, and forwards
228 Uses of D-Bus include notification of system changes (notification of when
229 a camera is plugged in to a computer, or a new version of some software
230 has been installed), or desktop interoperability, for example a file
231 monitoring service or a configuration service.
235 D-Bus is designed for two specific use cases:
239 A "system bus" for notifications from the system to user sessions,
240 and to allow the system to request input from user sessions.
245 A "session bus" used to implement desktop environments such as
250 D-Bus is not intended to be a generic IPC system for any possible
251 application, and intentionally omits many features found in other
252 IPC systems for this reason.
256 At the same time, the bus daemons offer a number of features not found in
257 other IPC systems, such as single-owner "bus names" (similar to X
258 selections), on-demand startup of services, and security policies.
259 In many ways, these features are the primary motivation for developing
260 D-Bus; other systems would have sufficed if IPC were the only goal.
264 D-Bus may turn out to be useful in unanticipated applications, but future
265 versions of this spec and the reference implementation probably will not
266 incorporate features that interfere with the core use cases.
270 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
271 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
272 document are to be interpreted as described in RFC 2119. However, the
273 document could use a serious audit to be sure it makes sense to do
274 so. Also, they are not capitalized.
277 <sect2 id="stability">
278 <title>Protocol and Specification Stability</title>
280 The D-Bus protocol is frozen (only compatible extensions are allowed) as
281 of November 8, 2006. However, this specification could still use a fair
282 bit of work to make interoperable reimplementation possible without
283 reference to the D-Bus reference implementation. Thus, this
284 specification is not marked 1.0. To mark it 1.0, we'd like to see
285 someone invest significant effort in clarifying the specification
286 language, and growing the specification to cover more aspects of the
287 reference implementation's behavior.
290 Until this work is complete, any attempt to reimplement D-Bus will
291 probably require looking at the reference implementation and/or asking
292 questions on the D-Bus mailing list about intended behavior.
293 Questions on the list are very welcome.
296 Nonetheless, this document should be a useful starting point and is
297 to our knowledge accurate, though incomplete.
303 <sect1 id="type-system">
304 <title>Type System</title>
307 D-Bus has a type system, in which values of various types can be
308 serialized into a sequence of bytes referred to as the
309 <firstterm>wire format</firstterm> in a standard way.
310 Converting a value from some other representation into the wire
311 format is called <firstterm>marshaling</firstterm> and converting
312 it back from the wire format is <firstterm>unmarshaling</firstterm>.
316 The D-Bus protocol does not include type tags in the marshaled data; a
317 block of marshaled values must have a known <firstterm>type
318 signature</firstterm>. The type signature is made up of zero or more
319 <firstterm id="term-single-complete-type">single complete
320 types</firstterm>, each made up of one or more
321 <firstterm>type codes</firstterm>.
325 A type code is an ASCII character representing the
326 type of a value. Because ASCII characters are used, the type signature
327 will always form a valid ASCII string. A simple string compare
328 determines whether two type signatures are equivalent.
332 A single complete type is a sequence of type codes that fully describes
333 one type: either a basic type, or a single fully-described container type.
334 A single complete type is a basic type code, a variant type code,
335 an array with its element type, or a struct with its fields (all of which
336 are defined below). So the following signatures are not single complete
347 And the following signatures contain multiple complete types:
357 Note however that a single complete type may <emphasis>contain</emphasis>
358 multiple other single complete types, by containing a struct or dict
362 <sect2 id="basic-types">
363 <title>Basic types</title>
366 The simplest type codes are the <firstterm id="term-basic-type">basic
367 types</firstterm>, which are the types whose structure is entirely
368 defined by their 1-character type code. Basic types consist of
369 fixed types and string-like types.
373 The <firstterm id="term-fixed-type">fixed types</firstterm>
374 are basic types whose values have a fixed length, namely BYTE,
375 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
380 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
381 the ASCII character 'i'. So the signature for a block of values
382 containing a single <literal>INT32</literal> would be:
386 A block of values containing two <literal>INT32</literal> would have this signature:
393 The characteristics of the fixed types are listed in this table.
399 <entry>Conventional name</entry>
400 <entry>ASCII type-code</entry>
401 <entry>Encoding</entry>
406 <entry><literal>BYTE</literal></entry>
407 <entry><literal>y</literal> (121)</entry>
408 <entry>Unsigned 8-bit integer</entry>
411 <entry><literal>BOOLEAN</literal></entry>
412 <entry><literal>b</literal> (98)</entry>
413 <entry>Boolean value: 0 is false, 1 is true, any other value
414 allowed by the marshalling format is invalid</entry>
417 <entry><literal>INT16</literal></entry>
418 <entry><literal>n</literal> (110)</entry>
419 <entry>Signed (two's complement) 16-bit integer</entry>
422 <entry><literal>UINT16</literal></entry>
423 <entry><literal>q</literal> (113)</entry>
424 <entry>Unsigned 16-bit integer</entry>
427 <entry><literal>INT32</literal></entry>
428 <entry><literal>i</literal> (105)</entry>
429 <entry>Signed (two's complement) 32-bit integer</entry>
432 <entry><literal>UINT32</literal></entry>
433 <entry><literal>u</literal> (117)</entry>
434 <entry>Unsigned 32-bit integer</entry>
437 <entry><literal>INT64</literal></entry>
438 <entry><literal>x</literal> (120)</entry>
439 <entry>Signed (two's complement) 64-bit integer
440 (mnemonic: x and t are the first characters in "sixty" not
441 already used for something more common)</entry>
444 <entry><literal>UINT64</literal></entry>
445 <entry><literal>t</literal> (116)</entry>
446 <entry>Unsigned 64-bit integer</entry>
449 <entry><literal>DOUBLE</literal></entry>
450 <entry><literal>d</literal> (100)</entry>
451 <entry>IEEE 754 double-precision floating point</entry>
454 <entry><literal>UNIX_FD</literal></entry>
455 <entry><literal>h</literal> (104)</entry>
456 <entry>Unsigned 32-bit integer representing an index into an
457 out-of-band array of file descriptors, transferred via some
458 platform-specific mechanism (mnemonic: h for handle)</entry>
466 The <firstterm id="term-string-like-type">string-like types</firstterm>
467 are basic types with a variable length. The value of any string-like
468 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
469 none of which may be U+0000. The UTF-8 text must be validated
470 strictly: in particular, it must not contain overlong sequences
471 or codepoints above U+10FFFF.
475 Since D-Bus Specification version 0.21, in accordance with Unicode
476 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
477 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
478 D-Bus rejected these noncharacters).
482 The marshalling formats for the string-like types all end with a
483 single zero (NUL) byte, but that byte is not considered to be part of
488 The characteristics of the string-like types are listed in this table.
494 <entry>Conventional name</entry>
495 <entry>ASCII type-code</entry>
496 <entry>Validity constraints</entry>
501 <entry><literal>STRING</literal></entry>
502 <entry><literal>s</literal> (115)</entry>
503 <entry>No extra constraints</entry>
506 <entry><literal>OBJECT_PATH</literal></entry>
507 <entry><literal>o</literal> (111)</entry>
509 <link linkend="message-protocol-marshaling-object-path">a
510 syntactically valid object path</link></entry>
513 <entry><literal>SIGNATURE</literal></entry>
514 <entry><literal>g</literal> (103)</entry>
516 <firstterm linkend="term-single-complete-type">single
517 complete types</firstterm></entry>
524 <sect3 id="message-protocol-marshaling-object-path">
525 <title>Valid Object Paths</title>
528 An object path is a name used to refer to an object instance.
529 Conceptually, each participant in a D-Bus message exchange may have
530 any number of object instances (think of C++ or Java objects) and each
531 such instance will have a path. Like a filesystem, the object
532 instances in an application form a hierarchical tree.
536 Object paths are often namespaced by starting with a reversed
537 domain name and containing an interface version number, in the
539 <link linkend="message-protocol-names-interface">interface
541 <link linkend="message-protocol-names-bus">well-known
543 This makes it possible to implement more than one service, or
544 more than one version of a service, in the same process,
545 even if the services share a connection but cannot otherwise
546 co-operate (for instance, if they are implemented by different
551 For instance, if the owner of <literal>example.com</literal> is
552 developing a D-Bus API for a music player, they might use the
553 hierarchy of object paths that start with
554 <literal>/com/example/MusicPlayer1</literal> for its objects.
558 The following rules define a valid object path. Implementations must
559 not send or accept messages with invalid object paths.
563 The path may be of any length.
568 The path must begin with an ASCII '/' (integer 47) character,
569 and must consist of elements separated by slash characters.
574 Each element must only contain the ASCII characters
580 No element may be the empty string.
585 Multiple '/' characters cannot occur in sequence.
590 A trailing '/' character is not allowed unless the
591 path is the root path (a single '/' character).
599 <sect3 id="message-protocol-marshaling-signature">
600 <title>Valid Signatures</title>
602 An implementation must not send or accept invalid signatures.
603 Valid signatures will conform to the following rules:
607 The signature is a list of single complete types.
608 Arrays must have element types, and structs must
609 have both open and close parentheses.
614 Only type codes, open and close parentheses, and open and
615 close curly brackets are allowed in the signature. The
616 <literal>STRUCT</literal> type code
617 is not allowed in signatures, because parentheses
618 are used instead. Similarly, the
619 <literal>DICT_ENTRY</literal> type code is not allowed in
620 signatures, because curly brackets are used instead.
625 The maximum depth of container type nesting is 32 array type
626 codes and 32 open parentheses. This implies that the maximum
627 total depth of recursion is 64, for an "array of array of array
628 of ... struct of struct of struct of ..." where there are 32
634 The maximum length of a signature is 255.
641 When signatures appear in messages, the marshalling format
642 guarantees that they will be followed by a nul byte (which can
643 be interpreted as either C-style string termination or the INVALID
644 type-code), but this is not conceptually part of the signature.
650 <sect2 id="container-types">
651 <title>Container types</title>
654 In addition to basic types, there are four <firstterm>container</firstterm>
655 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
656 and <literal>DICT_ENTRY</literal>.
660 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
661 code does not appear in signatures. Instead, ASCII characters
662 '(' and ')' are used to mark the beginning and end of the struct.
663 So for example, a struct containing two integers would have this
668 Structs can be nested, so for example a struct containing
669 an integer and another struct:
673 The value block storing that struct would contain three integers; the
674 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
679 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
680 but is useful in code that implements the protocol. This type code
681 is specified to allow such code to interoperate in non-protocol contexts.
685 Empty structures are not allowed; there must be at least one
686 type code between the parentheses.
690 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
691 followed by a <firstterm>single complete type</firstterm>. The single
692 complete type following the array is the type of each array element. So
693 the simple example is:
697 which is an array of 32-bit integers. But an array can be of any type,
698 such as this array-of-struct-with-two-int32-fields:
702 Or this array of array of integer:
709 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
710 type <literal>VARIANT</literal> will have the signature of a single complete type as part
711 of the <emphasis>value</emphasis>. This signature will be followed by a
712 marshaled value of that type.
716 Unlike a message signature, the variant signature can
717 contain only a single complete type. So "i", "ai"
718 or "(ii)" is OK, but "ii" is not. Use of variants may not
719 cause a total message depth to be larger than 64, including
720 other container types such as structures.
724 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
725 than parentheses it uses curly braces, and it has more restrictions.
726 The restrictions are: it occurs only as an array element type; it has
727 exactly two single complete types inside the curly braces; the first
728 single complete type (the "key") must be a basic type rather than a
729 container type. Implementations must not accept dict entries outside of
730 arrays, must not accept dict entries with zero, one, or more than two
731 fields, and must not accept dict entries with non-basic-typed keys. A
732 dict entry is always a key-value pair.
736 The first field in the <literal>DICT_ENTRY</literal> is always the key.
737 A message is considered corrupt if the same key occurs twice in the same
738 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
739 implementations are not required to reject dicts with duplicate keys.
743 In most languages, an array of dict entry would be represented as a
744 map, hash table, or dict object.
749 <title>Summary of types</title>
752 The following table summarizes the D-Bus types.
757 <entry>Conventional Name</entry>
759 <entry>Description</entry>
764 <entry><literal>INVALID</literal></entry>
765 <entry>0 (ASCII NUL)</entry>
766 <entry>Not a valid type code, used to terminate signatures</entry>
768 <entry><literal>BYTE</literal></entry>
769 <entry>121 (ASCII 'y')</entry>
770 <entry>8-bit unsigned integer</entry>
772 <entry><literal>BOOLEAN</literal></entry>
773 <entry>98 (ASCII 'b')</entry>
774 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
776 <entry><literal>INT16</literal></entry>
777 <entry>110 (ASCII 'n')</entry>
778 <entry>16-bit signed integer</entry>
780 <entry><literal>UINT16</literal></entry>
781 <entry>113 (ASCII 'q')</entry>
782 <entry>16-bit unsigned integer</entry>
784 <entry><literal>INT32</literal></entry>
785 <entry>105 (ASCII 'i')</entry>
786 <entry>32-bit signed integer</entry>
788 <entry><literal>UINT32</literal></entry>
789 <entry>117 (ASCII 'u')</entry>
790 <entry>32-bit unsigned integer</entry>
792 <entry><literal>INT64</literal></entry>
793 <entry>120 (ASCII 'x')</entry>
794 <entry>64-bit signed integer</entry>
796 <entry><literal>UINT64</literal></entry>
797 <entry>116 (ASCII 't')</entry>
798 <entry>64-bit unsigned integer</entry>
800 <entry><literal>DOUBLE</literal></entry>
801 <entry>100 (ASCII 'd')</entry>
802 <entry>IEEE 754 double</entry>
804 <entry><literal>STRING</literal></entry>
805 <entry>115 (ASCII 's')</entry>
806 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
808 <entry><literal>OBJECT_PATH</literal></entry>
809 <entry>111 (ASCII 'o')</entry>
810 <entry>Name of an object instance</entry>
812 <entry><literal>SIGNATURE</literal></entry>
813 <entry>103 (ASCII 'g')</entry>
814 <entry>A type signature</entry>
816 <entry><literal>ARRAY</literal></entry>
817 <entry>97 (ASCII 'a')</entry>
820 <entry><literal>STRUCT</literal></entry>
821 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
822 <entry>Struct; type code 114 'r' is reserved for use in
823 bindings and implementations to represent the general
824 concept of a struct, and must not appear in signatures
825 used on D-Bus.</entry>
827 <entry><literal>VARIANT</literal></entry>
828 <entry>118 (ASCII 'v') </entry>
829 <entry>Variant type (the type of the value is part of the value itself)</entry>
831 <entry><literal>DICT_ENTRY</literal></entry>
832 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
833 <entry>Entry in a dict or map (array of key-value pairs).
834 Type code 101 'e' is reserved for use in bindings and
835 implementations to represent the general concept of a
836 dict or dict-entry, and must not appear in signatures
837 used on D-Bus.</entry>
839 <entry><literal>UNIX_FD</literal></entry>
840 <entry>104 (ASCII 'h')</entry>
841 <entry>Unix file descriptor</entry>
844 <entry>(reserved)</entry>
845 <entry>109 (ASCII 'm')</entry>
846 <entry>Reserved for <ulink
847 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
848 'maybe' type compatible with the one in GVariant</ulink>,
849 and must not appear in signatures used on D-Bus until
850 specified here</entry>
853 <entry>(reserved)</entry>
854 <entry>42 (ASCII '*')</entry>
855 <entry>Reserved for use in bindings/implementations to
856 represent any <firstterm>single complete type</firstterm>,
857 and must not appear in signatures used on D-Bus.</entry>
860 <entry>(reserved)</entry>
861 <entry>63 (ASCII '?')</entry>
862 <entry>Reserved for use in bindings/implementations to
863 represent any <firstterm>basic type</firstterm>, and must
864 not appear in signatures used on D-Bus.</entry>
867 <entry>(reserved)</entry>
868 <entry>64 (ASCII '@'), 38 (ASCII '&'),
869 94 (ASCII '^')</entry>
870 <entry>Reserved for internal use by bindings/implementations,
871 and must not appear in signatures used on D-Bus.
872 GVariant uses these type-codes to encode calling
883 <sect1 id="message-protocol-marshaling">
884 <title>Marshaling (Wire Format)</title>
887 D-Bus defines a marshalling format for its type system, which is
888 used in D-Bus messages. This is not the only possible marshalling
889 format for the type system: for instance, GVariant (part of GLib)
890 re-uses the D-Bus type system but implements an alternative marshalling
895 <title>Byte order and alignment</title>
898 Given a type signature, a block of bytes can be converted into typed
899 values. This section describes the format of the block of bytes. Byte
900 order and alignment issues are handled uniformly for all D-Bus types.
904 A block of bytes has an associated byte order. The byte order
905 has to be discovered in some way; for D-Bus messages, the
906 byte order is part of the message header as described in
907 <xref linkend="message-protocol-messages"/>. For now, assume
908 that the byte order is known to be either little endian or big
913 Each value in a block of bytes is aligned "naturally," for example
914 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
915 8-byte boundary. To properly align a value, <firstterm>alignment
916 padding</firstterm> may be necessary. The alignment padding must always
917 be the minimum required padding to properly align the following value;
918 and it must always be made up of nul bytes. The alignment padding must
919 not be left uninitialized (it can't contain garbage), and more padding
920 than required must not be used.
924 As an exception to natural alignment, <literal>STRUCT</literal> and
925 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
926 boundary, regardless of the alignments of their contents.
931 <title>Marshalling basic types</title>
934 To marshal and unmarshal fixed types, you simply read one value
935 from the data block corresponding to each type code in the signature.
936 All signed integer values are encoded in two's complement, DOUBLE
937 values are IEEE 754 double-precision floating-point, and BOOLEAN
938 values are encoded in 32 bits (of which only the least significant
943 The string-like types are all marshalled as a
944 fixed-length unsigned integer <varname>n</varname> giving the
945 length of the variable part, followed by <varname>n</varname>
946 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
947 which is not considered to be part of the text. The alignment
948 of the string-like type is the same as the alignment of
949 <varname>n</varname>.
953 For the STRING and OBJECT_PATH types, <varname>n</varname> is
954 encoded in 4 bytes, leading to 4-byte alignment.
955 For the SIGNATURE type, <varname>n</varname> is encoded as a single
956 byte. As a result, alignment padding is never required before a
962 <title>Marshalling containers</title>
965 Arrays are marshalled as a <literal>UINT32</literal>
966 <varname>n</varname> giving the length of the array data in bytes,
967 followed by alignment padding to the alignment boundary of the array
968 element type, followed by the <varname>n</varname> bytes of the
969 array elements marshalled in sequence. <varname>n</varname> does not
970 include the padding after the length, or any padding after the
975 For instance, if the current position in the message is a multiple
976 of 8 bytes and the byte-order is big-endian, an array containing only
977 the 64-bit integer 5 would be marshalled as:
980 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
981 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
982 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
987 Arrays have a maximum length defined to be 2 to the 26th power or
988 67108864. Implementations must not send or accept arrays exceeding this
993 Structs and dict entries are marshalled in the same way as their
994 contents, but their alignment is always to an 8-byte boundary,
995 even if their contents would normally be less strictly aligned.
999 Variants are marshalled as the <literal>SIGNATURE</literal> of
1000 the contents (which must be a single complete type), followed by a
1001 marshalled value with the type given by that signature. The
1002 variant has the same 1-byte alignment as the signature, which means
1003 that alignment padding before a variant is never needed.
1004 Use of variants may not cause a total message depth to be larger
1005 than 64, including other container types such as structures.
1010 <title>Summary of D-Bus marshalling</title>
1013 Given all this, the types are marshaled on the wire as follows:
1018 <entry>Conventional Name</entry>
1019 <entry>Encoding</entry>
1020 <entry>Alignment</entry>
1025 <entry><literal>INVALID</literal></entry>
1026 <entry>Not applicable; cannot be marshaled.</entry>
1029 <entry><literal>BYTE</literal></entry>
1030 <entry>A single 8-bit byte.</entry>
1033 <entry><literal>BOOLEAN</literal></entry>
1034 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1037 <entry><literal>INT16</literal></entry>
1038 <entry>16-bit signed integer in the message's byte order.</entry>
1041 <entry><literal>UINT16</literal></entry>
1042 <entry>16-bit unsigned integer in the message's byte order.</entry>
1045 <entry><literal>INT32</literal></entry>
1046 <entry>32-bit signed integer in the message's byte order.</entry>
1049 <entry><literal>UINT32</literal></entry>
1050 <entry>32-bit unsigned integer in the message's byte order.</entry>
1053 <entry><literal>INT64</literal></entry>
1054 <entry>64-bit signed integer in the message's byte order.</entry>
1057 <entry><literal>UINT64</literal></entry>
1058 <entry>64-bit unsigned integer in the message's byte order.</entry>
1061 <entry><literal>DOUBLE</literal></entry>
1062 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1065 <entry><literal>STRING</literal></entry>
1066 <entry>A <literal>UINT32</literal> indicating the string's
1067 length in bytes excluding its terminating nul, followed by
1068 non-nul string data of the given length, followed by a terminating nul
1075 <entry><literal>OBJECT_PATH</literal></entry>
1076 <entry>Exactly the same as <literal>STRING</literal> except the
1077 content must be a valid object path (see above).
1083 <entry><literal>SIGNATURE</literal></entry>
1084 <entry>The same as <literal>STRING</literal> except the length is a single
1085 byte (thus signatures have a maximum length of 255)
1086 and the content must be a valid signature (see above).
1092 <entry><literal>ARRAY</literal></entry>
1094 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1095 alignment padding to the alignment boundary of the array element type,
1096 followed by each array element.
1102 <entry><literal>STRUCT</literal></entry>
1104 A struct must start on an 8-byte boundary regardless of the
1105 type of the struct fields. The struct value consists of each
1106 field marshaled in sequence starting from that 8-byte
1113 <entry><literal>VARIANT</literal></entry>
1115 The marshaled <literal>SIGNATURE</literal> of a single
1116 complete type, followed by a marshaled value with the type
1117 given in the signature.
1120 1 (alignment of the signature)
1123 <entry><literal>DICT_ENTRY</literal></entry>
1125 Identical to STRUCT.
1131 <entry><literal>UNIX_FD</literal></entry>
1132 <entry>32-bit unsigned integer in the message's byte
1133 order. The actual file descriptors need to be
1134 transferred out-of-band via some platform specific
1135 mechanism. On the wire, values of this type store the index to the
1136 file descriptor in the array of file descriptors that
1137 accompany the message.</entry>
1149 <sect1 id="message-protocol">
1150 <title>Message Protocol</title>
1153 A <firstterm>message</firstterm> consists of a
1154 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1155 think of a message as a package, the header is the address, and the body
1156 contains the package contents. The message delivery system uses the header
1157 information to figure out where to send the message and how to interpret
1158 it; the recipient interprets the body of the message.
1162 The body of the message is made up of zero or more
1163 <firstterm>arguments</firstterm>, which are typed values, such as an
1164 integer or a byte array.
1168 Both header and body use the D-Bus <link linkend="type-system">type
1169 system</link> and format for serializing data.
1172 <sect2 id="message-protocol-messages">
1173 <title>Message Format</title>
1176 A message consists of a header and a body. The header is a block of
1177 values with a fixed signature and meaning. The body is a separate block
1178 of values, with a signature specified in the header.
1182 The length of the header must be a multiple of 8, allowing the body to
1183 begin on an 8-byte boundary when storing the entire message in a single
1184 buffer. If the header does not naturally end on an 8-byte boundary
1185 up to 7 bytes of nul-initialized alignment padding must be added.
1189 The message body need not end on an 8-byte boundary.
1193 The maximum length of a message, including header, header alignment padding,
1194 and body is 2 to the 27th power or 134217728. Implementations must not
1195 send or accept messages exceeding this size.
1199 The signature of the header is:
1203 Written out more readably, this is:
1205 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1210 These values have the following meanings:
1215 <entry>Value</entry>
1216 <entry>Description</entry>
1221 <entry>1st <literal>BYTE</literal></entry>
1222 <entry>Endianness flag; ASCII 'l' for little-endian
1223 or ASCII 'B' for big-endian. Both header and body are
1224 in this endianness.</entry>
1227 <entry>2nd <literal>BYTE</literal></entry>
1228 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1229 Currently-defined types are described below.
1233 <entry>3rd <literal>BYTE</literal></entry>
1234 <entry>Bitwise OR of flags. Unknown flags
1235 must be ignored. Currently-defined flags are described below.
1239 <entry>4th <literal>BYTE</literal></entry>
1240 <entry>Major protocol version of the sending application. If
1241 the major protocol version of the receiving application does not
1242 match, the applications will not be able to communicate and the
1243 D-Bus connection must be disconnected. The major protocol
1244 version for this version of the specification is 1.
1248 <entry>1st <literal>UINT32</literal></entry>
1249 <entry>Length in bytes of the message body, starting
1250 from the end of the header. The header ends after
1251 its alignment padding to an 8-boundary.
1255 <entry>2nd <literal>UINT32</literal></entry>
1256 <entry>The serial of this message, used as a cookie
1257 by the sender to identify the reply corresponding
1258 to this request. This must not be zero.
1262 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1263 <entry>An array of zero or more <firstterm>header
1264 fields</firstterm> where the byte is the field code, and the
1265 variant is the field value. The message type determines
1266 which fields are required.
1274 <firstterm>Message types</firstterm> that can appear in the second byte
1280 <entry>Conventional name</entry>
1281 <entry>Decimal value</entry>
1282 <entry>Description</entry>
1287 <entry><literal>INVALID</literal></entry>
1289 <entry>This is an invalid type.</entry>
1292 <entry><literal>METHOD_CALL</literal></entry>
1294 <entry>Method call.</entry>
1297 <entry><literal>METHOD_RETURN</literal></entry>
1299 <entry>Method reply with returned data.</entry>
1302 <entry><literal>ERROR</literal></entry>
1304 <entry>Error reply. If the first argument exists and is a
1305 string, it is an error message.</entry>
1308 <entry><literal>SIGNAL</literal></entry>
1310 <entry>Signal emission.</entry>
1317 Flags that can appear in the third byte of the header:
1322 <entry>Conventional name</entry>
1323 <entry>Hex value</entry>
1324 <entry>Description</entry>
1329 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1331 <entry>This message does not expect method return replies or
1332 error replies; the reply can be omitted as an
1333 optimization. However, it is compliant with this specification
1334 to return the reply despite this flag and the only harm
1335 from doing so is extra network traffic.
1339 <entry><literal>NO_AUTO_START</literal></entry>
1341 <entry>The bus must not launch an owner
1342 for the destination name in response to this message.
1350 <sect3 id="message-protocol-header-fields">
1351 <title>Header Fields</title>
1354 The array at the end of the header contains <firstterm>header
1355 fields</firstterm>, where each field is a 1-byte field code followed
1356 by a field value. A header must contain the required header fields for
1357 its message type, and zero or more of any optional header
1358 fields. Future versions of this protocol specification may add new
1359 fields. Implementations must ignore fields they do not
1360 understand. Implementations must not invent their own header fields;
1361 only changes to this specification may introduce new header fields.
1365 Again, if an implementation sees a header field code that it does not
1366 expect, it must ignore that field, as it will be part of a new
1367 (but compatible) version of this specification. This also applies
1368 to known header fields appearing in unexpected messages, for
1369 example: if a signal has a reply serial it must be ignored
1370 even though it has no meaning as of this version of the spec.
1374 However, implementations must not send or accept known header fields
1375 with the wrong type stored in the field value. So for example a
1376 message with an <literal>INTERFACE</literal> field of type
1377 <literal>UINT32</literal> would be considered corrupt.
1381 Here are the currently-defined header fields:
1386 <entry>Conventional Name</entry>
1387 <entry>Decimal Code</entry>
1389 <entry>Required In</entry>
1390 <entry>Description</entry>
1395 <entry><literal>INVALID</literal></entry>
1398 <entry>not allowed</entry>
1399 <entry>Not a valid field name (error if it appears in a message)</entry>
1402 <entry><literal>PATH</literal></entry>
1404 <entry><literal>OBJECT_PATH</literal></entry>
1405 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1406 <entry>The object to send a call to,
1407 or the object a signal is emitted from.
1409 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1410 implementations should not send messages with this path,
1411 and the reference implementation of the bus daemon will
1412 disconnect any application that attempts to do so.
1416 <entry><literal>INTERFACE</literal></entry>
1418 <entry><literal>STRING</literal></entry>
1419 <entry><literal>SIGNAL</literal></entry>
1421 The interface to invoke a method call on, or
1422 that a signal is emitted from. Optional for
1423 method calls, required for signals.
1424 The special interface
1425 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1426 implementations should not send messages with this
1427 interface, and the reference implementation of the bus
1428 daemon will disconnect any application that attempts to
1433 <entry><literal>MEMBER</literal></entry>
1435 <entry><literal>STRING</literal></entry>
1436 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1437 <entry>The member, either the method name or signal name.</entry>
1440 <entry><literal>ERROR_NAME</literal></entry>
1442 <entry><literal>STRING</literal></entry>
1443 <entry><literal>ERROR</literal></entry>
1444 <entry>The name of the error that occurred, for errors</entry>
1447 <entry><literal>REPLY_SERIAL</literal></entry>
1449 <entry><literal>UINT32</literal></entry>
1450 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1451 <entry>The serial number of the message this message is a reply
1452 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1455 <entry><literal>DESTINATION</literal></entry>
1457 <entry><literal>STRING</literal></entry>
1458 <entry>optional</entry>
1459 <entry>The name of the connection this message is intended for.
1460 Only used in combination with the message bus, see
1461 <xref linkend="message-bus"/>.</entry>
1464 <entry><literal>SENDER</literal></entry>
1466 <entry><literal>STRING</literal></entry>
1467 <entry>optional</entry>
1468 <entry>Unique name of the sending connection.
1469 The message bus fills in this field so it is reliable; the field is
1470 only meaningful in combination with the message bus.</entry>
1473 <entry><literal>SIGNATURE</literal></entry>
1475 <entry><literal>SIGNATURE</literal></entry>
1476 <entry>optional</entry>
1477 <entry>The signature of the message body.
1478 If omitted, it is assumed to be the
1479 empty signature "" (i.e. the body must be 0-length).</entry>
1482 <entry><literal>UNIX_FDS</literal></entry>
1484 <entry><literal>UINT32</literal></entry>
1485 <entry>optional</entry>
1486 <entry>The number of Unix file descriptors that
1487 accompany the message. If omitted, it is assumed
1488 that no Unix file descriptors accompany the
1489 message. The actual file descriptors need to be
1490 transferred via platform specific mechanism
1491 out-of-band. They must be sent at the same time as
1492 part of the message itself. They may not be sent
1493 before the first byte of the message itself is
1494 transferred or after the last byte of the message
1504 <sect2 id="message-protocol-names">
1505 <title>Valid Names</title>
1507 The various names in D-Bus messages have some restrictions.
1510 There is a <firstterm>maximum name length</firstterm>
1511 of 255 which applies to bus names, interfaces, and members.
1513 <sect3 id="message-protocol-names-interface">
1514 <title>Interface names</title>
1516 Interfaces have names with type <literal>STRING</literal>, meaning that
1517 they must be valid UTF-8. However, there are also some
1518 additional restrictions that apply to interface names
1521 <listitem><para>Interface names are composed of 1 or more elements separated by
1522 a period ('.') character. All elements must contain at least
1526 <listitem><para>Each element must only contain the ASCII characters
1527 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1531 <listitem><para>Interface names must contain at least one '.' (period)
1532 character (and thus at least two elements).
1535 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1536 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1541 Interface names should start with the reversed DNS domain name of
1542 the author of the interface (in lower-case), like interface names
1543 in Java. It is conventional for the rest of the interface name
1544 to consist of words run together, with initial capital letters
1545 on all words ("CamelCase"). Several levels of hierarchy can be used.
1546 It is also a good idea to include the major version of the interface
1547 in the name, and increment it if incompatible changes are made;
1548 this way, a single object can implement several versions of an
1549 interface in parallel, if necessary.
1553 For instance, if the owner of <literal>example.com</literal> is
1554 developing a D-Bus API for a music player, they might define
1555 interfaces called <literal>com.example.MusicPlayer1</literal>,
1556 <literal>com.example.MusicPlayer1.Track</literal> and
1557 <literal>com.example.MusicPlayer1.Seekable</literal>.
1561 D-Bus does not distinguish between the concepts that would be
1562 called classes and interfaces in Java: either can be identified on
1563 D-Bus by an interface name.
1566 <sect3 id="message-protocol-names-bus">
1567 <title>Bus names</title>
1569 Connections have one or more bus names associated with them.
1570 A connection has exactly one bus name that is a <firstterm>unique
1571 connection name</firstterm>. The unique connection name remains
1572 with the connection for its entire lifetime.
1573 A bus name is of type <literal>STRING</literal>,
1574 meaning that it must be valid UTF-8. However, there are also
1575 some additional restrictions that apply to bus names
1578 <listitem><para>Bus names that start with a colon (':')
1579 character are unique connection names. Other bus names
1580 are called <firstterm>well-known bus names</firstterm>.
1583 <listitem><para>Bus names are composed of 1 or more elements separated by
1584 a period ('.') character. All elements must contain at least
1588 <listitem><para>Each element must only contain the ASCII characters
1589 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1590 connection name may begin with a digit, elements in
1591 other bus names must not begin with a digit.
1595 <listitem><para>Bus names must contain at least one '.' (period)
1596 character (and thus at least two elements).
1599 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1600 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1604 Note that the hyphen ('-') character is allowed in bus names but
1605 not in interface names.
1609 Like <link linkend="message-protocol-names-interface">interface
1610 names</link>, well-known bus names should start with the
1611 reversed DNS domain name of the author of the interface (in
1612 lower-case), and it is conventional for the rest of the well-known
1613 bus name to consist of words run together, with initial
1614 capital letters. As with interface names, including a version
1615 number in well-known bus names is a good idea; it's possible to
1616 have the well-known bus name for more than one version
1617 simultaneously if backwards compatibility is required.
1621 If a well-known bus name implies the presence of a "main" interface,
1622 that "main" interface is often given the same name as
1623 the well-known bus name, and situated at the corresponding object
1624 path. For instance, if the owner of <literal>example.com</literal>
1625 is developing a D-Bus API for a music player, they might define
1626 that any application that takes the well-known name
1627 <literal>com.example.MusicPlayer1</literal> should have an object
1628 at the object path <literal>/com/example/MusicPlayer1</literal>
1629 which implements the interface
1630 <literal>com.example.MusicPlayer1</literal>.
1633 <sect3 id="message-protocol-names-member">
1634 <title>Member names</title>
1636 Member (i.e. method or signal) names:
1638 <listitem><para>Must only contain the ASCII characters
1639 "[A-Z][a-z][0-9]_" and may not begin with a
1640 digit.</para></listitem>
1641 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1642 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1643 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1648 It is conventional for member names on D-Bus to consist of
1649 capitalized words with no punctuation ("camel-case").
1650 Method names should usually be verbs, such as
1651 <literal>GetItems</literal>, and signal names should usually be
1652 a description of an event, such as <literal>ItemsChanged</literal>.
1655 <sect3 id="message-protocol-names-error">
1656 <title>Error names</title>
1658 Error names have the same restrictions as interface names.
1662 Error names have the same naming conventions as interface
1663 names, and often contain <literal>.Error.</literal>; for instance,
1664 the owner of <literal>example.com</literal> might define the
1665 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1666 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1667 The errors defined by D-Bus itself, such as
1668 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1674 <sect2 id="message-protocol-types">
1675 <title>Message Types</title>
1677 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1678 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1679 This section describes these conventions.
1681 <sect3 id="message-protocol-types-method">
1682 <title>Method Calls</title>
1684 Some messages invoke an operation on a remote object. These are
1685 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1686 messages map naturally to methods on objects in a typical program.
1689 A method call message is required to have a <literal>MEMBER</literal> header field
1690 indicating the name of the method. Optionally, the message has an
1691 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1692 Including the <literal>INTERFACE</literal> in all method call
1693 messages is strongly recommended.
1696 In the absence of an <literal>INTERFACE</literal> field, if two
1697 or more interfaces on the same object have a method with the same
1698 name, it is undefined which of those methods will be invoked.
1699 Implementations may choose to either return an error, or deliver the
1700 message as though it had an arbitrary one of those interfaces.
1703 In some situations (such as the well-known system bus), messages
1704 are filtered through an access-control list external to the
1705 remote object implementation. If that filter rejects certain
1706 messages by matching their interface, or accepts only messages
1707 to specific interfaces, it must also reject messages that have no
1708 <literal>INTERFACE</literal>: otherwise, malicious
1709 applications could use this to bypass the filter.
1712 Method call messages also include a <literal>PATH</literal> field
1713 indicating the object to invoke the method on. If the call is passing
1714 through a message bus, the message will also have a
1715 <literal>DESTINATION</literal> field giving the name of the connection
1716 to receive the message.
1719 When an application handles a method call message, it is required to
1720 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1721 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1722 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1725 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1726 are the return value(s) or "out parameters" of the method call.
1727 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1728 and the call fails; no return value will be provided. It makes
1729 no sense to send multiple replies to the same method call.
1732 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1733 reply is required, so the caller will know the method
1734 was successfully processed.
1737 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1741 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1742 then as an optimization the application receiving the method
1743 call may choose to omit the reply message (regardless of
1744 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1745 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1746 flag and reply anyway.
1749 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1750 destination name does not exist then a program to own the destination
1751 name will be started before the message is delivered. The message
1752 will be held until the new program is successfully started or has
1753 failed to start; in case of failure, an error will be returned. This
1754 flag is only relevant in the context of a message bus, it is ignored
1755 during one-to-one communication with no intermediate bus.
1757 <sect4 id="message-protocol-types-method-apis">
1758 <title>Mapping method calls to native APIs</title>
1760 APIs for D-Bus may map method calls to a method call in a specific
1761 programming language, such as C++, or may map a method call written
1762 in an IDL to a D-Bus message.
1765 In APIs of this nature, arguments to a method are often termed "in"
1766 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1767 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1768 "inout" arguments, which are both sent and received, i.e. the caller
1769 passes in a value which is modified. Mapped to D-Bus, an "inout"
1770 argument is equivalent to an "in" argument, followed by an "out"
1771 argument. You can't pass things "by reference" over the wire, so
1772 "inout" is purely an illusion of the in-process API.
1775 Given a method with zero or one return values, followed by zero or more
1776 arguments, where each argument may be "in", "out", or "inout", the
1777 caller constructs a message by appending each "in" or "inout" argument,
1778 in order. "out" arguments are not represented in the caller's message.
1781 The recipient constructs a reply by appending first the return value
1782 if any, then each "out" or "inout" argument, in order.
1783 "in" arguments are not represented in the reply message.
1786 Error replies are normally mapped to exceptions in languages that have
1790 In converting from native APIs to D-Bus, it is perhaps nice to
1791 map D-Bus naming conventions ("FooBar") to native conventions
1792 such as "fooBar" or "foo_bar" automatically. This is OK
1793 as long as you can say that the native API is one that
1794 was specifically written for D-Bus. It makes the most sense
1795 when writing object implementations that will be exported
1796 over the bus. Object proxies used to invoke remote D-Bus
1797 objects probably need the ability to call any D-Bus method,
1798 and thus a magic name mapping like this could be a problem.
1801 This specification doesn't require anything of native API bindings;
1802 the preceding is only a suggested convention for consistency
1808 <sect3 id="message-protocol-types-signal">
1809 <title>Signal Emission</title>
1811 Unlike method calls, signal emissions have no replies.
1812 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1813 It must have three header fields: <literal>PATH</literal> giving the object
1814 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1815 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1816 for signals, though it is optional for method calls.
1820 <sect3 id="message-protocol-types-errors">
1821 <title>Errors</title>
1823 Messages of type <literal>ERROR</literal> are most commonly replies
1824 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1825 to any kind of message. The message bus for example
1826 will return an <literal>ERROR</literal> in reply to a signal emission if
1827 the bus does not have enough memory to send the signal.
1830 An <literal>ERROR</literal> may have any arguments, but if the first
1831 argument is a <literal>STRING</literal>, it must be an error message.
1832 The error message may be logged or shown to the user
1837 <sect3 id="message-protocol-types-notation">
1838 <title>Notation in this document</title>
1840 This document uses a simple pseudo-IDL to describe particular method
1841 calls and signals. Here is an example of a method call:
1843 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1844 out UINT32 resultcode)
1846 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1847 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1848 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1849 characters so it's known that the last part of the name in
1850 the "IDL" is the member name.
1853 In C++ that might end up looking like this:
1855 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1856 unsigned int flags);
1858 or equally valid, the return value could be done as an argument:
1860 void org::freedesktop::DBus::StartServiceByName (const char *name,
1862 unsigned int *resultcode);
1864 It's really up to the API designer how they want to make
1865 this look. You could design an API where the namespace wasn't used
1866 in C++, using STL or Qt, using varargs, or whatever you wanted.
1869 Signals are written as follows:
1871 org.freedesktop.DBus.NameLost (STRING name)
1873 Signals don't specify "in" vs. "out" because only
1874 a single direction is possible.
1877 It isn't especially encouraged to use this lame pseudo-IDL in actual
1878 API implementations; you might use the native notation for the
1879 language you're using, or you might use COM or CORBA IDL, for example.
1884 <sect2 id="message-protocol-handling-invalid">
1885 <title>Invalid Protocol and Spec Extensions</title>
1888 For security reasons, the D-Bus protocol should be strictly parsed and
1889 validated, with the exception of defined extension points. Any invalid
1890 protocol or spec violations should result in immediately dropping the
1891 connection without notice to the other end. Exceptions should be
1892 carefully considered, e.g. an exception may be warranted for a
1893 well-understood idiosyncrasy of a widely-deployed implementation. In
1894 cases where the other end of a connection is 100% trusted and known to
1895 be friendly, skipping validation for performance reasons could also make
1896 sense in certain cases.
1900 Generally speaking violations of the "must" requirements in this spec
1901 should be considered possible attempts to exploit security, and violations
1902 of the "should" suggestions should be considered legitimate (though perhaps
1903 they should generate an error in some cases).
1907 The following extension points are built in to D-Bus on purpose and must
1908 not be treated as invalid protocol. The extension points are intended
1909 for use by future versions of this spec, they are not intended for third
1910 parties. At the moment, the only way a third party could extend D-Bus
1911 without breaking interoperability would be to introduce a way to negotiate new
1912 feature support as part of the auth protocol, using EXTENSION_-prefixed
1913 commands. There is not yet a standard way to negotiate features.
1917 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1918 commands result in an ERROR rather than a disconnect. This enables
1919 future extensions to the protocol. Commands starting with EXTENSION_ are
1920 reserved for third parties.
1925 The authentication protocol supports pluggable auth mechanisms.
1930 The address format (see <xref linkend="addresses"/>) supports new
1936 Messages with an unknown type (something other than
1937 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1938 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1939 Unknown-type messages must still be well-formed in the same way
1940 as the known messages, however. They still have the normal
1946 Header fields with an unknown or unexpected field code must be ignored,
1947 though again they must still be well-formed.
1952 New standard interfaces (with new methods and signals) can of course be added.
1962 <sect1 id="auth-protocol">
1963 <title>Authentication Protocol</title>
1965 Before the flow of messages begins, two applications must
1966 authenticate. A simple plain-text protocol is used for
1967 authentication; this protocol is a SASL profile, and maps fairly
1968 directly from the SASL specification. The message encoding is
1969 NOT used here, only plain text messages.
1972 In examples, "C:" and "S:" indicate lines sent by the client and
1973 server respectively.
1975 <sect2 id="auth-protocol-overview">
1976 <title>Protocol Overview</title>
1978 The protocol is a line-based protocol, where each line ends with
1979 \r\n. Each line begins with an all-caps ASCII command name containing
1980 only the character range [A-Z_], a space, then any arguments for the
1981 command, then the \r\n ending the line. The protocol is
1982 case-sensitive. All bytes must be in the ASCII character set.
1984 Commands from the client to the server are as follows:
1987 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1988 <listitem><para>CANCEL</para></listitem>
1989 <listitem><para>BEGIN</para></listitem>
1990 <listitem><para>DATA <data in hex encoding></para></listitem>
1991 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1992 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1995 From server to client are as follows:
1998 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1999 <listitem><para>OK <GUID in hex></para></listitem>
2000 <listitem><para>DATA <data in hex encoding></para></listitem>
2001 <listitem><para>ERROR</para></listitem>
2002 <listitem><para>AGREE_UNIX_FD</para></listitem>
2006 Unofficial extensions to the command set must begin with the letters
2007 "EXTENSION_", to avoid conflicts with future official commands.
2008 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2011 <sect2 id="auth-nul-byte">
2012 <title>Special credentials-passing nul byte</title>
2014 Immediately after connecting to the server, the client must send a
2015 single nul byte. This byte may be accompanied by credentials
2016 information on some operating systems that use sendmsg() with
2017 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2018 sockets. However, the nul byte must be sent even on other kinds of
2019 socket, and even on operating systems that do not require a byte to be
2020 sent in order to transmit credentials. The text protocol described in
2021 this document begins after the single nul byte. If the first byte
2022 received from the client is not a nul byte, the server may disconnect
2026 A nul byte in any context other than the initial byte is an error;
2027 the protocol is ASCII-only.
2030 The credentials sent along with the nul byte may be used with the
2031 SASL mechanism EXTERNAL.
2034 <sect2 id="auth-command-auth">
2035 <title>AUTH command</title>
2037 If an AUTH command has no arguments, it is a request to list
2038 available mechanisms. The server must respond with a REJECTED
2039 command listing the mechanisms it understands, or with an error.
2042 If an AUTH command specifies a mechanism, and the server supports
2043 said mechanism, the server should begin exchanging SASL
2044 challenge-response data with the client using DATA commands.
2047 If the server does not support the mechanism given in the AUTH
2048 command, it must send either a REJECTED command listing the mechanisms
2049 it does support, or an error.
2052 If the [initial-response] argument is provided, it is intended for use
2053 with mechanisms that have no initial challenge (or an empty initial
2054 challenge), as if it were the argument to an initial DATA command. If
2055 the selected mechanism has an initial challenge and [initial-response]
2056 was provided, the server should reject authentication by sending
2060 If authentication succeeds after exchanging DATA commands,
2061 an OK command must be sent to the client.
2064 The first octet received by the server after the \r\n of the BEGIN
2065 command from the client must be the first octet of the
2066 authenticated/encrypted stream of D-Bus messages.
2069 If BEGIN is received by the server, the first octet received
2070 by the client after the \r\n of the OK command must be the
2071 first octet of the authenticated/encrypted stream of D-Bus
2075 <sect2 id="auth-command-cancel">
2076 <title>CANCEL Command</title>
2078 At any time up to sending the BEGIN command, the client may send a
2079 CANCEL command. On receiving the CANCEL command, the server must
2080 send a REJECTED command and abort the current authentication
2084 <sect2 id="auth-command-data">
2085 <title>DATA Command</title>
2087 The DATA command may come from either client or server, and simply
2088 contains a hex-encoded block of data to be interpreted
2089 according to the SASL mechanism in use.
2092 Some SASL mechanisms support sending an "empty string";
2093 FIXME we need some way to do this.
2096 <sect2 id="auth-command-begin">
2097 <title>BEGIN Command</title>
2099 The BEGIN command acknowledges that the client has received an
2100 OK command from the server, and that the stream of messages
2104 The first octet received by the server after the \r\n of the BEGIN
2105 command from the client must be the first octet of the
2106 authenticated/encrypted stream of D-Bus messages.
2109 <sect2 id="auth-command-rejected">
2110 <title>REJECTED Command</title>
2112 The REJECTED command indicates that the current authentication
2113 exchange has failed, and further exchange of DATA is inappropriate.
2114 The client would normally try another mechanism, or try providing
2115 different responses to challenges.
2117 Optionally, the REJECTED command has a space-separated list of
2118 available auth mechanisms as arguments. If a server ever provides
2119 a list of supported mechanisms, it must provide the same list
2120 each time it sends a REJECTED message. Clients are free to
2121 ignore all lists received after the first.
2124 <sect2 id="auth-command-ok">
2125 <title>OK Command</title>
2127 The OK command indicates that the client has been
2128 authenticated. The client may now proceed with negotiating
2129 Unix file descriptor passing. To do that it shall send
2130 NEGOTIATE_UNIX_FD to the server.
2133 Otherwise, the client must respond to the OK command by
2134 sending a BEGIN command, followed by its stream of messages,
2135 or by disconnecting. The server must not accept additional
2136 commands using this protocol after the BEGIN command has been
2137 received. Further communication will be a stream of D-Bus
2138 messages (optionally encrypted, as negotiated) rather than
2142 If a client sends BEGIN the first octet received by the client
2143 after the \r\n of the OK command must be the first octet of
2144 the authenticated/encrypted stream of D-Bus messages.
2147 The OK command has one argument, which is the GUID of the server.
2148 See <xref linkend="addresses"/> for more on server GUIDs.
2151 <sect2 id="auth-command-error">
2152 <title>ERROR Command</title>
2154 The ERROR command indicates that either server or client did not
2155 know a command, does not accept the given command in the current
2156 context, or did not understand the arguments to the command. This
2157 allows the protocol to be extended; a client or server can send a
2158 command present or permitted only in new protocol versions, and if
2159 an ERROR is received instead of an appropriate response, fall back
2160 to using some other technique.
2163 If an ERROR is sent, the server or client that sent the
2164 error must continue as if the command causing the ERROR had never been
2165 received. However, the the server or client receiving the error
2166 should try something other than whatever caused the error;
2167 if only canceling/rejecting the authentication.
2170 If the D-Bus protocol changes incompatibly at some future time,
2171 applications implementing the new protocol would probably be able to
2172 check for support of the new protocol by sending a new command and
2173 receiving an ERROR from applications that don't understand it. Thus the
2174 ERROR feature of the auth protocol is an escape hatch that lets us
2175 negotiate extensions or changes to the D-Bus protocol in the future.
2178 <sect2 id="auth-command-negotiate-unix-fd">
2179 <title>NEGOTIATE_UNIX_FD Command</title>
2181 The NEGOTIATE_UNIX_FD command indicates that the client
2182 supports Unix file descriptor passing. This command may only
2183 be sent after the connection is authenticated, i.e. after OK
2184 was received by the client. This command may only be sent on
2185 transports that support Unix file descriptor passing.
2188 On receiving NEGOTIATE_UNIX_FD the server must respond with
2189 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2190 the transport chosen supports Unix file descriptor passing and
2191 the server supports this feature. It shall respond the latter
2192 if the transport does not support Unix file descriptor
2193 passing, the server does not support this feature, or the
2194 server decides not to enable file descriptor passing due to
2195 security or other reasons.
2198 <sect2 id="auth-command-agree-unix-fd">
2199 <title>AGREE_UNIX_FD Command</title>
2201 The AGREE_UNIX_FD command indicates that the server supports
2202 Unix file descriptor passing. This command may only be sent
2203 after the connection is authenticated, and the client sent
2204 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2205 command may only be sent on transports that support Unix file
2209 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2210 followed by its stream of messages, or by disconnecting. The
2211 server must not accept additional commands using this protocol
2212 after the BEGIN command has been received. Further
2213 communication will be a stream of D-Bus messages (optionally
2214 encrypted, as negotiated) rather than this protocol.
2217 <sect2 id="auth-command-future">
2218 <title>Future Extensions</title>
2220 Future extensions to the authentication and negotiation
2221 protocol are possible. For that new commands may be
2222 introduced. If a client or server receives an unknown command
2223 it shall respond with ERROR and not consider this fatal. New
2224 commands may be introduced both before, and after
2225 authentication, i.e. both before and after the OK command.
2228 <sect2 id="auth-examples">
2229 <title>Authentication examples</title>
2233 <title>Example of successful magic cookie authentication</title>
2235 (MAGIC_COOKIE is a made up mechanism)
2237 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2243 <title>Example of finding out mechanisms then picking one</title>
2246 S: REJECTED KERBEROS_V4 SKEY
2247 C: AUTH SKEY 7ab83f32ee
2248 S: DATA 8799cabb2ea93e
2249 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2255 <title>Example of client sends unknown command then falls back to regular auth</title>
2259 C: AUTH MAGIC_COOKIE 3736343435313230333039
2265 <title>Example of server doesn't support initial auth mechanism</title>
2267 C: AUTH MAGIC_COOKIE 3736343435313230333039
2268 S: REJECTED KERBEROS_V4 SKEY
2269 C: AUTH SKEY 7ab83f32ee
2270 S: DATA 8799cabb2ea93e
2271 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2277 <title>Example of wrong password or the like followed by successful retry</title>
2279 C: AUTH MAGIC_COOKIE 3736343435313230333039
2280 S: REJECTED KERBEROS_V4 SKEY
2281 C: AUTH SKEY 7ab83f32ee
2282 S: DATA 8799cabb2ea93e
2283 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2285 C: AUTH SKEY 7ab83f32ee
2286 S: DATA 8799cabb2ea93e
2287 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2293 <title>Example of skey cancelled and restarted</title>
2295 C: AUTH MAGIC_COOKIE 3736343435313230333039
2296 S: REJECTED KERBEROS_V4 SKEY
2297 C: AUTH SKEY 7ab83f32ee
2298 S: DATA 8799cabb2ea93e
2301 C: AUTH SKEY 7ab83f32ee
2302 S: DATA 8799cabb2ea93e
2303 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2309 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2311 (MAGIC_COOKIE is a made up mechanism)
2313 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2315 C: NEGOTIATE_UNIX_FD
2321 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2323 (MAGIC_COOKIE is a made up mechanism)
2325 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2327 C: NEGOTIATE_UNIX_FD
2334 <sect2 id="auth-states">
2335 <title>Authentication state diagrams</title>
2338 This section documents the auth protocol in terms of
2339 a state machine for the client and the server. This is
2340 probably the most robust way to implement the protocol.
2343 <sect3 id="auth-states-client">
2344 <title>Client states</title>
2347 To more precisely describe the interaction between the
2348 protocol state machine and the authentication mechanisms the
2349 following notation is used: MECH(CHALL) means that the
2350 server challenge CHALL was fed to the mechanism MECH, which
2356 CONTINUE(RESP) means continue the auth conversation
2357 and send RESP as the response to the server;
2363 OK(RESP) means that after sending RESP to the server
2364 the client side of the auth conversation is finished
2365 and the server should return "OK";
2371 ERROR means that CHALL was invalid and could not be
2377 Both RESP and CHALL may be empty.
2381 The Client starts by getting an initial response from the
2382 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2383 the mechanism did not provide an initial response. If the
2384 mechanism returns CONTINUE, the client starts in state
2385 <emphasis>WaitingForData</emphasis>, if the mechanism
2386 returns OK the client starts in state
2387 <emphasis>WaitingForOK</emphasis>.
2391 The client should keep track of available mechanisms and
2392 which it mechanisms it has already attempted. This list is
2393 used to decide which AUTH command to send. When the list is
2394 exhausted, the client should give up and close the
2399 <title><emphasis>WaitingForData</emphasis></title>
2407 MECH(CHALL) returns CONTINUE(RESP) → send
2409 <emphasis>WaitingForData</emphasis>
2413 MECH(CHALL) returns OK(RESP) → send DATA
2414 RESP, goto <emphasis>WaitingForOK</emphasis>
2418 MECH(CHALL) returns ERROR → send ERROR
2419 [msg], goto <emphasis>WaitingForData</emphasis>
2427 Receive REJECTED [mechs] →
2428 send AUTH [next mech], goto
2429 WaitingForData or <emphasis>WaitingForOK</emphasis>
2434 Receive ERROR → send
2436 <emphasis>WaitingForReject</emphasis>
2441 Receive OK → send
2442 BEGIN, terminate auth
2443 conversation, authenticated
2448 Receive anything else → send
2450 <emphasis>WaitingForData</emphasis>
2458 <title><emphasis>WaitingForOK</emphasis></title>
2463 Receive OK → send BEGIN, terminate auth
2464 conversation, <emphasis>authenticated</emphasis>
2469 Receive REJECTED [mechs] → send AUTH [next mech],
2470 goto <emphasis>WaitingForData</emphasis> or
2471 <emphasis>WaitingForOK</emphasis>
2477 Receive DATA → send CANCEL, goto
2478 <emphasis>WaitingForReject</emphasis>
2484 Receive ERROR → send CANCEL, goto
2485 <emphasis>WaitingForReject</emphasis>
2491 Receive anything else → send ERROR, goto
2492 <emphasis>WaitingForOK</emphasis>
2500 <title><emphasis>WaitingForReject</emphasis></title>
2505 Receive REJECTED [mechs] → send AUTH [next mech],
2506 goto <emphasis>WaitingForData</emphasis> or
2507 <emphasis>WaitingForOK</emphasis>
2513 Receive anything else → terminate auth
2514 conversation, disconnect
2523 <sect3 id="auth-states-server">
2524 <title>Server states</title>
2527 For the server MECH(RESP) means that the client response
2528 RESP was fed to the the mechanism MECH, which returns one of
2533 CONTINUE(CHALL) means continue the auth conversation and
2534 send CHALL as the challenge to the client;
2540 OK means that the client has been successfully
2547 REJECTED means that the client failed to authenticate or
2548 there was an error in RESP.
2553 The server starts out in state
2554 <emphasis>WaitingForAuth</emphasis>. If the client is
2555 rejected too many times the server must disconnect the
2560 <title><emphasis>WaitingForAuth</emphasis></title>
2566 Receive AUTH → send REJECTED [mechs], goto
2567 <emphasis>WaitingForAuth</emphasis>
2573 Receive AUTH MECH RESP
2577 MECH not valid mechanism → send REJECTED
2579 <emphasis>WaitingForAuth</emphasis>
2583 MECH(RESP) returns CONTINUE(CHALL) → send
2585 <emphasis>WaitingForData</emphasis>
2589 MECH(RESP) returns OK → send OK, goto
2590 <emphasis>WaitingForBegin</emphasis>
2594 MECH(RESP) returns REJECTED → send REJECTED
2596 <emphasis>WaitingForAuth</emphasis>
2604 Receive BEGIN → terminate
2605 auth conversation, disconnect
2611 Receive ERROR → send REJECTED [mechs], goto
2612 <emphasis>WaitingForAuth</emphasis>
2618 Receive anything else → send
2620 <emphasis>WaitingForAuth</emphasis>
2629 <title><emphasis>WaitingForData</emphasis></title>
2637 MECH(RESP) returns CONTINUE(CHALL) → send
2639 <emphasis>WaitingForData</emphasis>
2643 MECH(RESP) returns OK → send OK, goto
2644 <emphasis>WaitingForBegin</emphasis>
2648 MECH(RESP) returns REJECTED → send REJECTED
2650 <emphasis>WaitingForAuth</emphasis>
2658 Receive BEGIN → terminate auth conversation,
2665 Receive CANCEL → send REJECTED [mechs], goto
2666 <emphasis>WaitingForAuth</emphasis>
2672 Receive ERROR → send REJECTED [mechs], goto
2673 <emphasis>WaitingForAuth</emphasis>
2679 Receive anything else → send ERROR, goto
2680 <emphasis>WaitingForData</emphasis>
2688 <title><emphasis>WaitingForBegin</emphasis></title>
2693 Receive BEGIN → terminate auth conversation,
2694 client authenticated
2700 Receive CANCEL → send REJECTED [mechs], goto
2701 <emphasis>WaitingForAuth</emphasis>
2707 Receive ERROR → send REJECTED [mechs], goto
2708 <emphasis>WaitingForAuth</emphasis>
2714 Receive anything else → send ERROR, goto
2715 <emphasis>WaitingForBegin</emphasis>
2725 <sect2 id="auth-mechanisms">
2726 <title>Authentication mechanisms</title>
2728 This section describes some new authentication mechanisms.
2729 D-Bus also allows any standard SASL mechanism of course.
2731 <sect3 id="auth-mechanisms-sha">
2732 <title>DBUS_COOKIE_SHA1</title>
2734 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2735 has the ability to read a private file owned by the user being
2736 authenticated. If the client can prove that it has access to a secret
2737 cookie stored in this file, then the client is authenticated.
2738 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2742 Throughout this description, "hex encoding" must output the digits
2743 from a to f in lower-case; the digits A to F must not be used
2744 in the DBUS_COOKIE_SHA1 mechanism.
2747 Authentication proceeds as follows:
2751 The client sends the username it would like to authenticate
2757 The server sends the name of its "cookie context" (see below); a
2758 space character; the integer ID of the secret cookie the client
2759 must demonstrate knowledge of; a space character; then a
2760 randomly-generated challenge string, all of this hex-encoded into
2766 The client locates the cookie and generates its own
2767 randomly-generated challenge string. The client then concatenates
2768 the server's decoded challenge, a ":" character, its own challenge,
2769 another ":" character, and the cookie. It computes the SHA-1 hash
2770 of this composite string as a hex digest. It concatenates the
2771 client's challenge string, a space character, and the SHA-1 hex
2772 digest, hex-encodes the result and sends it back to the server.
2777 The server generates the same concatenated string used by the
2778 client and computes its SHA-1 hash. It compares the hash with
2779 the hash received from the client; if the two hashes match, the
2780 client is authenticated.
2786 Each server has a "cookie context," which is a name that identifies a
2787 set of cookies that apply to that server. A sample context might be
2788 "org_freedesktop_session_bus". Context names must be valid ASCII,
2789 nonzero length, and may not contain the characters slash ("/"),
2790 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2791 tab ("\t"), or period ("."). There is a default context,
2792 "org_freedesktop_general" that's used by servers that do not specify
2796 Cookies are stored in a user's home directory, in the directory
2797 <filename>~/.dbus-keyrings/</filename>. This directory must
2798 not be readable or writable by other users. If it is,
2799 clients and servers must ignore it. The directory
2800 contains cookie files named after the cookie context.
2803 A cookie file contains one cookie per line. Each line
2804 has three space-separated fields:
2808 The cookie ID number, which must be a non-negative integer and
2809 may not be used twice in the same file.
2814 The cookie's creation time, in UNIX seconds-since-the-epoch
2820 The cookie itself, a hex-encoded random block of bytes. The cookie
2821 may be of any length, though obviously security increases
2822 as the length increases.
2828 Only server processes modify the cookie file.
2829 They must do so with this procedure:
2833 Create a lockfile name by appending ".lock" to the name of the
2834 cookie file. The server should attempt to create this file
2835 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2836 fails, the lock fails. Servers should retry for a reasonable
2837 period of time, then they may choose to delete an existing lock
2838 to keep users from having to manually delete a stale
2839 lock. <footnote><para>Lockfiles are used instead of real file
2840 locking <literal>fcntl()</literal> because real locking
2841 implementations are still flaky on network
2842 filesystems.</para></footnote>
2847 Once the lockfile has been created, the server loads the cookie
2848 file. It should then delete any cookies that are old (the
2849 timeout can be fairly short), or more than a reasonable
2850 time in the future (so that cookies never accidentally
2851 become permanent, if the clock was set far into the future
2852 at some point). If no recent keys remain, the
2853 server may generate a new key.
2858 The pruned and possibly added-to cookie file
2859 must be resaved atomically (using a temporary
2860 file which is rename()'d).
2865 The lock must be dropped by deleting the lockfile.
2871 Clients need not lock the file in order to load it,
2872 because servers are required to save the file atomically.
2877 <sect1 id="addresses">
2878 <title>Server Addresses</title>
2880 Server addresses consist of a transport name followed by a colon, and
2881 then an optional, comma-separated list of keys and values in the form key=value.
2882 Each value is escaped.
2886 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2887 Which is the address to a unix socket with the path /tmp/dbus-test.
2890 Value escaping is similar to URI escaping but simpler.
2894 The set of optionally-escaped bytes is:
2895 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2896 <emphasis>byte</emphasis> (note, not character) which is not in the
2897 set of optionally-escaped bytes must be replaced with an ASCII
2898 percent (<literal>%</literal>) and the value of the byte in hex.
2899 The hex value must always be two digits, even if the first digit is
2900 zero. The optionally-escaped bytes may be escaped if desired.
2905 To unescape, append each byte in the value; if a byte is an ASCII
2906 percent (<literal>%</literal>) character then append the following
2907 hex value instead. It is an error if a <literal>%</literal> byte
2908 does not have two hex digits following. It is an error if a
2909 non-optionally-escaped byte is seen unescaped.
2913 The set of optionally-escaped bytes is intended to preserve address
2914 readability and convenience.
2918 A server may specify a key-value pair with the key <literal>guid</literal>
2919 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2920 describes the format of the <literal>guid</literal> field. If present,
2921 this UUID may be used to distinguish one server address from another. A
2922 server should use a different UUID for each address it listens on. For
2923 example, if a message bus daemon offers both UNIX domain socket and TCP
2924 connections, but treats clients the same regardless of how they connect,
2925 those two connections are equivalent post-connection but should have
2926 distinct UUIDs to distinguish the kinds of connection.
2930 The intent of the address UUID feature is to allow a client to avoid
2931 opening multiple identical connections to the same server, by allowing the
2932 client to check whether an address corresponds to an already-existing
2933 connection. Comparing two addresses is insufficient, because addresses
2934 can be recycled by distinct servers, and equivalent addresses may look
2935 different if simply compared as strings (for example, the host in a TCP
2936 address can be given as an IP address or as a hostname).
2940 Note that the address key is <literal>guid</literal> even though the
2941 rest of the API and documentation says "UUID," for historical reasons.
2945 [FIXME clarify if attempting to connect to each is a requirement
2946 or just a suggestion]
2947 When connecting to a server, multiple server addresses can be
2948 separated by a semi-colon. The library will then try to connect
2949 to the first address and if that fails, it'll try to connect to
2950 the next one specified, and so forth. For example
2951 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2955 Some addresses are <firstterm>connectable</firstterm>. A connectable
2956 address is one containing enough information for a client to connect
2957 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2958 is a connectable address. It is not necessarily possible to listen
2959 on every connectable address: for instance, it is not possible to
2960 listen on a <literal>unixexec:</literal> address.
2964 Some addresses are <firstterm>listenable</firstterm>. A listenable
2965 address is one containing enough information for a server to listen on
2966 it, producing a connectable address (which may differ from the
2967 original address). Many listenable addresses are not connectable:
2968 for instance, <literal>tcp:host=127.0.0.1</literal>
2969 is listenable, but not connectable (because it does not specify
2974 Listening on an address that is not connectable will result in a
2975 connectable address that is not the same as the listenable address.
2976 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
2977 might result in the connectable address
2978 <literal>tcp:host=127.0.0.1,port=30958</literal>,
2979 or listening on <literal>unix:tmpdir=/tmp</literal>
2980 might result in the connectable address
2981 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
2985 <sect1 id="transports">
2986 <title>Transports</title>
2988 [FIXME we need to specify in detail each transport and its possible arguments]
2990 Current transports include: unix domain sockets (including
2991 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2992 using in-process pipes. Future possible transports include one that
2993 tunnels over X11 protocol.
2996 <sect2 id="transports-unix-domain-sockets">
2997 <title>Unix Domain Sockets</title>
2999 Unix domain sockets can be either paths in the file system or on Linux
3000 kernels, they can be abstract which are similar to paths but
3001 do not show up in the file system.
3005 When a socket is opened by the D-Bus library it truncates the path
3006 name right before the first trailing Nul byte. This is true for both
3007 normal paths and abstract paths. Note that this is a departure from
3008 previous versions of D-Bus that would create sockets with a fixed
3009 length path name. Names which were shorter than the fixed length
3010 would be padded by Nul bytes.
3013 Unix domain sockets are not available on Windows.
3016 Unix addresses that specify <literal>path</literal> or
3017 <literal>abstract</literal> are both listenable and connectable.
3018 Unix addresses that specify <literal>tmpdir</literal> are only
3019 listenable: the corresponding connectable address will specify
3020 either <literal>path</literal> or <literal>abstract</literal>.
3022 <sect3 id="transports-unix-domain-sockets-addresses">
3023 <title>Server Address Format</title>
3025 Unix domain socket addresses are identified by the "unix:" prefix
3026 and support the following key/value pairs:
3033 <entry>Values</entry>
3034 <entry>Description</entry>
3040 <entry>(path)</entry>
3041 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3044 <entry>tmpdir</entry>
3045 <entry>(path)</entry>
3046 <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>
3049 <entry>abstract</entry>
3050 <entry>(string)</entry>
3051 <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>
3057 Exactly one of the keys <literal>path</literal>,
3058 <literal>abstract</literal> or
3059 <literal>tmpdir</literal> must be provided.
3063 <sect2 id="transports-launchd">
3064 <title>launchd</title>
3066 launchd is an open-source server management system that replaces init, inetd
3067 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3068 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3072 launchd allocates a socket and provides it with the unix path through the
3073 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3074 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3075 it through its environment.
3076 Other processes can query for the launchd socket by executing:
3077 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3078 This is normally done by the D-Bus client library so doesn't have to be done
3082 launchd is not available on Microsoft Windows.
3085 launchd addresses are listenable and connectable.
3087 <sect3 id="transports-launchd-addresses">
3088 <title>Server Address Format</title>
3090 launchd addresses are identified by the "launchd:" prefix
3091 and support the following key/value pairs:
3098 <entry>Values</entry>
3099 <entry>Description</entry>
3105 <entry>(environment variable)</entry>
3106 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3112 The <literal>env</literal> key is required.
3116 <sect2 id="transports-systemd">
3117 <title>systemd</title>
3119 systemd is an open-source server management system that
3120 replaces init and inetd on newer Linux systems. It supports
3121 socket activation. The D-Bus systemd transport is used to acquire
3122 socket activation file descriptors from systemd and use them
3123 as D-Bus transport when the current process is spawned by
3124 socket activation from it.
3127 The systemd transport accepts only one or more Unix domain or
3128 TCP streams sockets passed in via socket activation.
3131 The systemd transport is not available on non-Linux operating systems.
3134 The systemd transport defines no parameter keys.
3137 systemd addresses are listenable, but not connectable. The
3138 corresponding connectable address is the <literal>unix</literal>
3139 or <literal>tcp</literal> address of the socket.
3142 <sect2 id="transports-tcp-sockets">
3143 <title>TCP Sockets</title>
3145 The tcp transport provides TCP/IP based connections between clients
3146 located on the same or different hosts.
3149 Using tcp transport without any additional secure authentification mechanismus
3150 over a network is unsecure.
3153 On Windows and most Unix platforms, the TCP stack is unable to transfer
3154 credentials over a TCP connection, so the EXTERNAL authentication
3155 mechanism does not work for this transport.
3158 All <literal>tcp</literal> addresses are listenable.
3159 <literal>tcp</literal> addresses in which both
3160 <literal>host</literal> and <literal>port</literal> are
3161 specified, and <literal>port</literal> is non-zero,
3162 are also connectable.
3164 <sect3 id="transports-tcp-sockets-addresses">
3165 <title>Server Address Format</title>
3167 TCP/IP socket addresses are identified by the "tcp:" prefix
3168 and support the following key/value pairs:
3175 <entry>Values</entry>
3176 <entry>Description</entry>
3182 <entry>(string)</entry>
3183 <entry>DNS name or IP address</entry>
3187 <entry>(string)</entry>
3188 <entry>Used in a listenable address to configure the interface
3189 on which the server will listen: either the IP address of one of
3190 the local machine's interfaces (most commonly <literal>127.0.0.1
3191 </literal>), or a DNS name that resolves to one of those IP
3192 addresses, or '*' to listen on all interfaces simultaneously.
3193 If not specified, the default is the same value as "host".
3198 <entry>(number)</entry>
3199 <entry>The tcp port the server will open. A zero value let the server
3200 choose a free port provided from the underlaying operating system.
3201 libdbus is able to retrieve the real used port from the server.
3205 <entry>family</entry>
3206 <entry>(string)</entry>
3207 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3214 <sect2 id="transports-nonce-tcp-sockets">
3215 <title>Nonce-secured TCP Sockets</title>
3217 The nonce-tcp transport provides a secured TCP transport, using a
3218 simple authentication mechanism to ensure that only clients with read
3219 access to a certain location in the filesystem can connect to the server.
3220 The server writes a secret, the nonce, to a file and an incoming client
3221 connection is only accepted if the client sends the nonce right after
3222 the connect. The nonce mechanism requires no setup and is orthogonal to
3223 the higher-level authentication mechanisms described in the
3224 Authentication section.
3228 On start, the server generates a random 16 byte nonce and writes it
3229 to a file in the user's temporary directory. The nonce file location
3230 is published as part of the server's D-Bus address using the
3231 "noncefile" key-value pair.
3233 After an accept, the server reads 16 bytes from the socket. If the
3234 read bytes do not match the nonce stored in the nonce file, the
3235 server MUST immediately drop the connection.
3236 If the nonce match the received byte sequence, the client is accepted
3237 and the transport behaves like an unsecured tcp transport.
3240 After a successful connect to the server socket, the client MUST read
3241 the nonce from the file published by the server via the noncefile=
3242 key-value pair and send it over the socket. After that, the
3243 transport behaves like an unsecured tcp transport.
3246 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3247 <literal>host</literal>, <literal>port</literal> and
3248 <literal>noncefile</literal> are all specified,
3249 and <literal>port</literal> is nonzero, are also connectable.
3251 <sect3 id="transports-nonce-tcp-sockets-addresses">
3252 <title>Server Address Format</title>
3254 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3255 and support the following key/value pairs:
3262 <entry>Values</entry>
3263 <entry>Description</entry>
3269 <entry>(string)</entry>
3270 <entry>DNS name or IP address</entry>
3274 <entry>(string)</entry>
3275 <entry>The same as for tcp: addresses
3280 <entry>(number)</entry>
3281 <entry>The tcp port the server will open. A zero value let the server
3282 choose a free port provided from the underlaying operating system.
3283 libdbus is able to retrieve the real used port from the server.
3287 <entry>family</entry>
3288 <entry>(string)</entry>
3289 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3292 <entry>noncefile</entry>
3293 <entry>(path)</entry>
3294 <entry>File location containing the secret.
3295 This is only meaningful in connectable addresses:
3296 a listening D-Bus server that offers this transport
3297 will always create a new nonce file.</entry>
3304 <sect2 id="transports-exec">
3305 <title>Executed Subprocesses on Unix</title>
3307 This transport forks off a process and connects its standard
3308 input and standard output with an anonymous Unix domain
3309 socket. This socket is then used for communication by the
3310 transport. This transport may be used to use out-of-process
3311 forwarder programs as basis for the D-Bus protocol.
3314 The forked process will inherit the standard error output and
3315 process group from the parent process.
3318 Executed subprocesses are not available on Windows.
3321 <literal>unixexec</literal> addresses are connectable, but are not
3324 <sect3 id="transports-exec-addresses">
3325 <title>Server Address Format</title>
3327 Executed subprocess addresses are identified by the "unixexec:" prefix
3328 and support the following key/value pairs:
3335 <entry>Values</entry>
3336 <entry>Description</entry>
3342 <entry>(path)</entry>
3343 <entry>Path of the binary to execute, either an absolute
3344 path or a binary name that is searched for in the default
3345 search path of the OS. This corresponds to the first
3346 argument of execlp(). This key is mandatory.</entry>
3349 <entry>argv0</entry>
3350 <entry>(string)</entry>
3351 <entry>The program name to use when executing the
3352 binary. If omitted the same value as specified for path=
3353 will be used. This corresponds to the second argument of
3357 <entry>argv1, argv2, ...</entry>
3358 <entry>(string)</entry>
3359 <entry>Arguments to pass to the binary. This corresponds
3360 to the third and later arguments of execlp(). If a
3361 specific argvX is not specified no further argvY for Y > X
3362 are taken into account.</entry>
3370 <sect1 id="meta-transports">
3371 <title>Meta Transports</title>
3373 Meta transports are a kind of transport with special enhancements or
3374 behavior. Currently available meta transports include: autolaunch
3377 <sect2 id="meta-transports-autolaunch">
3378 <title>Autolaunch</title>
3379 <para>The autolaunch transport provides a way for dbus clients to autodetect
3380 a running dbus session bus and to autolaunch a session bus if not present.
3383 On Unix, <literal>autolaunch</literal> addresses are connectable,
3387 On Windows, <literal>autolaunch</literal> addresses are both
3388 connectable and listenable.
3391 <sect3 id="meta-transports-autolaunch-addresses">
3392 <title>Server Address Format</title>
3394 Autolaunch addresses uses the "autolaunch:" prefix and support the
3395 following key/value pairs:
3402 <entry>Values</entry>
3403 <entry>Description</entry>
3408 <entry>scope</entry>
3409 <entry>(string)</entry>
3410 <entry>scope of autolaunch (Windows only)
3414 "*install-path" - limit session bus to dbus installation path.
3415 The dbus installation path is determined from the location of
3416 the shared dbus library. If the library is located in a 'bin'
3417 subdirectory the installation root is the directory above,
3418 otherwise the directory where the library lives is taken as
3421 <install-root>/bin/[lib]dbus-1.dll
3422 <install-root>/[lib]dbus-1.dll
3428 "*user" - limit session bus to the recent user.
3433 other values - specify dedicated session bus like "release",
3445 <sect3 id="meta-transports-autolaunch-windows-implementation">
3446 <title>Windows implementation</title>
3448 On start, the server opens a platform specific transport, creates a mutex
3449 and a shared memory section containing the related session bus address.
3450 This mutex will be inspected by the dbus client library to detect a
3451 running dbus session bus. The access to the mutex and the shared memory
3452 section are protected by global locks.
3455 In the recent implementation the autolaunch transport uses a tcp transport
3456 on localhost with a port choosen from the operating system. This detail may
3457 change in the future.
3460 Disclaimer: The recent implementation is in an early state and may not
3461 work in all cirumstances and/or may have security issues. Because of this
3462 the implementation is not documentated yet.
3469 <title>UUIDs</title>
3471 A working D-Bus implementation uses universally-unique IDs in two places.
3472 First, each server address has a UUID identifying the address,
3473 as described in <xref linkend="addresses"/>. Second, each operating
3474 system kernel instance running a D-Bus client or server has a UUID
3475 identifying that kernel, retrieved by invoking the method
3476 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3477 linkend="standard-interfaces-peer"/>).
3480 The term "UUID" in this document is intended literally, i.e. an
3481 identifier that is universally unique. It is not intended to refer to
3482 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3485 The UUID must contain 128 bits of data and be hex-encoded. The
3486 hex-encoded string may not contain hyphens or other non-hex-digit
3487 characters, and it must be exactly 32 characters long. To generate a
3488 UUID, the current reference implementation concatenates 96 bits of random
3489 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3493 It would also be acceptable and probably better to simply generate 128
3494 bits of random data, as long as the random number generator is of high
3495 quality. The timestamp could conceivably help if the random bits are not
3496 very random. With a quality random number generator, collisions are
3497 extremely unlikely even with only 96 bits, so it's somewhat academic.
3500 Implementations should, however, stick to random data for the first 96 bits
3505 <sect1 id="standard-interfaces">
3506 <title>Standard Interfaces</title>
3508 See <xref linkend="message-protocol-types-notation"/> for details on
3509 the notation used in this section. There are some standard interfaces
3510 that may be useful across various D-Bus applications.
3512 <sect2 id="standard-interfaces-peer">
3513 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3515 The <literal>org.freedesktop.DBus.Peer</literal> interface
3518 org.freedesktop.DBus.Peer.Ping ()
3519 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3523 On receipt of the <literal>METHOD_CALL</literal> message
3524 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3525 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3526 usual. It does not matter which object path a ping is sent to. The
3527 reference implementation handles this method automatically.
3530 On receipt of the <literal>METHOD_CALL</literal> message
3531 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3532 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3533 UUID representing the identity of the machine the process is running on.
3534 This UUID must be the same for all processes on a single system at least
3535 until that system next reboots. It should be the same across reboots
3536 if possible, but this is not always possible to implement and is not
3538 It does not matter which object path a GetMachineId is sent to. The
3539 reference implementation handles this method automatically.
3542 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3543 a virtual machine running on a hypervisor, rather than a physical machine.
3544 Basically if two processes see the same UUID, they should also see the same
3545 shared memory, UNIX domain sockets, process IDs, and other features that require
3546 a running OS kernel in common between the processes.
3549 The UUID is often used where other programs might use a hostname. Hostnames
3550 can change without rebooting, however, or just be "localhost" - so the UUID
3554 <xref linkend="uuids"/> explains the format of the UUID.
3558 <sect2 id="standard-interfaces-introspectable">
3559 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3561 This interface has one method:
3563 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3567 Objects instances may implement
3568 <literal>Introspect</literal> which returns an XML description of
3569 the object, including its interfaces (with signals and methods), objects
3570 below it in the object path tree, and its properties.
3573 <xref linkend="introspection-format"/> describes the format of this XML string.
3576 <sect2 id="standard-interfaces-properties">
3577 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3579 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3580 or <firstterm>attributes</firstterm>. These can be exposed via the
3581 <literal>org.freedesktop.DBus.Properties</literal> interface.
3585 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3586 in STRING property_name,
3588 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3589 in STRING property_name,
3591 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3592 out DICT<STRING,VARIANT> props);
3596 It is conventional to give D-Bus properties names consisting of
3597 capitalized words without punctuation ("CamelCase"), like
3598 <link linkend="message-protocol-names-member">member names</link>.
3599 For instance, the GObject property
3600 <literal>connection-status</literal> or the Qt property
3601 <literal>connectionStatus</literal> could be represented on D-Bus
3602 as <literal>ConnectionStatus</literal>.
3605 Strictly speaking, D-Bus property names are not required to follow
3606 the same naming restrictions as member names, but D-Bus property
3607 names that would not be valid member names (in particular,
3608 GObject-style dash-separated property names) can cause interoperability
3609 problems and should be avoided.
3612 The available properties and whether they are writable can be determined
3613 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3614 see <xref linkend="standard-interfaces-introspectable"/>.
3617 An empty string may be provided for the interface name; in this case,
3618 if there are multiple properties on an object with the same name,
3619 the results are undefined (picking one by according to an arbitrary
3620 deterministic rule, or returning an error, are the reasonable
3624 If one or more properties change on an object, the
3625 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3626 signal may be emitted (this signal was added in 0.14):
3630 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3631 DICT<STRING,VARIANT> changed_properties,
3632 ARRAY<STRING> invalidated_properties);
3636 where <literal>changed_properties</literal> is a dictionary
3637 containing the changed properties with the new values and
3638 <literal>invalidated_properties</literal> is an array of
3639 properties that changed but the value is not conveyed.
3642 Whether the <literal>PropertiesChanged</literal> signal is
3643 supported can be determined by calling
3644 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3645 that the signal may be supported for an object but it may
3646 differ how whether and how it is used on a per-property basis
3647 (for e.g. performance or security reasons). Each property (or
3648 the parent interface) must be annotated with the
3649 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3650 annotation to convey this (usually the default value
3651 <literal>true</literal> is sufficient meaning that the
3652 annotation does not need to be used). See <xref
3653 linkend="introspection-format"/> for details on this
3658 <sect2 id="standard-interfaces-objectmanager">
3659 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3661 An API can optionally make use of this interface for one or
3662 more sub-trees of objects. The root of each sub-tree implements
3663 this interface so other applications can get all objects,
3664 interfaces and properties in a single method call. It is
3665 appropriate to use this interface if users of the tree of
3666 objects are expected to be interested in all interfaces of all
3667 objects in the tree; a more granular API should be used if
3668 users of the objects are expected to be interested in a small
3669 subset of the objects, a small subset of their interfaces, or
3673 The method that applications can use to get all objects and
3674 properties is <literal>GetManagedObjects</literal>:
3678 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3682 The return value of this method is a dict whose keys are
3683 object paths. All returned object paths are children of the
3684 object path implementing this interface, i.e. their object
3685 paths start with the ObjectManager's object path plus '/'.
3688 Each value is a dict whose keys are interfaces names. Each
3689 value in this inner dict is the same dict that would be
3690 returned by the <link
3691 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3692 method for that combination of object path and interface. If
3693 an interface has no properties, the empty dict is returned.
3696 Changes are emitted using the following two signals:
3700 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3701 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3702 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3703 ARRAY<STRING> interfaces);
3707 The <literal>InterfacesAdded</literal> signal is emitted when
3708 either a new object is added or when an existing object gains
3709 one or more interfaces. The
3710 <literal>InterfacesRemoved</literal> signal is emitted
3711 whenever an object is removed or it loses one or more
3712 interfaces. The second parameter of the
3713 <literal>InterfacesAdded</literal> signal contains a dict with
3714 the interfaces and properties (if any) that have been added to
3715 the given object path. Similarly, the second parameter of the
3716 <literal>InterfacesRemoved</literal> signal contains an array
3717 of the interfaces that were removed. Note that changes on
3718 properties on existing interfaces are not reported using this
3719 interface - an application should also monitor the existing <link
3720 linkend="standard-interfaces-properties">PropertiesChanged</link>
3721 signal on each object.
3724 Applications SHOULD NOT export objects that are children of an
3725 object (directly or otherwise) implementing this interface but
3726 which are not returned in the reply from the
3727 <literal>GetManagedObjects()</literal> method of this
3728 interface on the given object.
3731 The intent of the <literal>ObjectManager</literal> interface
3732 is to make it easy to write a robust client
3733 implementation. The trivial client implementation only needs
3734 to make two method calls:
3738 org.freedesktop.DBus.AddMatch (bus_proxy,
3739 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3740 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3744 on the message bus and the remote application's
3745 <literal>ObjectManager</literal>, respectively. Whenever a new
3746 remote object is created (or an existing object gains a new
3747 interface), the <literal>InterfacesAdded</literal> signal is
3748 emitted, and since this signal contains all properties for the
3749 interfaces, no calls to the
3750 <literal>org.freedesktop.Properties</literal> interface on the
3751 remote object are needed. Additionally, since the initial
3752 <literal>AddMatch()</literal> rule already includes signal
3753 messages from the newly created child object, no new
3754 <literal>AddMatch()</literal> call is needed.
3759 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3760 interface was added in version 0.17 of the D-Bus
3767 <sect1 id="introspection-format">
3768 <title>Introspection Data Format</title>
3770 As described in <xref linkend="standard-interfaces-introspectable"/>,
3771 objects may be introspected at runtime, returning an XML string
3772 that describes the object. The same XML format may be used in
3773 other contexts as well, for example as an "IDL" for generating
3774 static language bindings.
3777 Here is an example of introspection data:
3779 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3780 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3781 <node name="/com/example/sample_object">
3782 <interface name="com.example.SampleInterface">
3783 <method name="Frobate">
3784 <arg name="foo" type="i" direction="in"/>
3785 <arg name="bar" type="s" direction="out"/>
3786 <arg name="baz" type="a{us}" direction="out"/>
3787 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3789 <method name="Bazify">
3790 <arg name="bar" type="(iiu)" direction="in"/>
3791 <arg name="bar" type="v" direction="out"/>
3793 <method name="Mogrify">
3794 <arg name="bar" type="(iiav)" direction="in"/>
3796 <signal name="Changed">
3797 <arg name="new_value" type="b"/>
3799 <property name="Bar" type="y" access="readwrite"/>
3801 <node name="child_of_sample_object"/>
3802 <node name="another_child_of_sample_object"/>
3807 A more formal DTD and spec needs writing, but here are some quick notes.
3811 Only the root <node> element can omit the node name, as it's
3812 known to be the object that was introspected. If the root
3813 <node> does have a name attribute, it must be an absolute
3814 object path. If child <node> have object paths, they must be
3820 If a child <node> has any sub-elements, then they
3821 must represent a complete introspection of the child.
3822 If a child <node> is empty, then it may or may
3823 not have sub-elements; the child must be introspected
3824 in order to find out. The intent is that if an object
3825 knows that its children are "fast" to introspect
3826 it can go ahead and return their information, but
3827 otherwise it can omit it.
3832 The direction element on <arg> may be omitted,
3833 in which case it defaults to "in" for method calls
3834 and "out" for signals. Signals only allow "out"
3835 so while direction may be specified, it's pointless.
3840 The possible directions are "in" and "out",
3841 unlike CORBA there is no "inout"
3846 The possible property access flags are
3847 "readwrite", "read", and "write"
3852 Multiple interfaces can of course be listed for
3858 The "name" attribute on arguments is optional.
3864 Method, interface, property, and signal elements may have
3865 "annotations", which are generic key/value pairs of metadata.
3866 They are similar conceptually to Java's annotations and C# attributes.
3867 Well-known annotations:
3874 <entry>Values (separated by ,)</entry>
3875 <entry>Description</entry>
3880 <entry>org.freedesktop.DBus.Deprecated</entry>
3881 <entry>true,false</entry>
3882 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3885 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3886 <entry>(string)</entry>
3887 <entry>The C symbol; may be used for methods and interfaces</entry>
3890 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3891 <entry>true,false</entry>
3892 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3895 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3896 <entry>true,invalidates,false</entry>
3899 If set to <literal>false</literal>, the
3900 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3902 linkend="standard-interfaces-properties"/> is not
3903 guaranteed to be emitted if the property changes.
3906 If set to <literal>invalidates</literal> the signal
3907 is emitted but the value is not included in the
3911 If set to <literal>true</literal> the signal is
3912 emitted with the value included.
3915 The value for the annotation defaults to
3916 <literal>true</literal> if the enclosing interface
3917 element does not specify the annotation. Otherwise it
3918 defaults to the value specified in the enclosing
3927 <sect1 id="message-bus">
3928 <title>Message Bus Specification</title>
3929 <sect2 id="message-bus-overview">
3930 <title>Message Bus Overview</title>
3932 The message bus accepts connections from one or more applications.
3933 Once connected, applications can exchange messages with other
3934 applications that are also connected to the bus.
3937 In order to route messages among connections, the message bus keeps a
3938 mapping from names to connections. Each connection has one
3939 unique-for-the-lifetime-of-the-bus name automatically assigned.
3940 Applications may request additional names for a connection. Additional
3941 names are usually "well-known names" such as
3942 "com.example.TextEditor". When a name is bound to a connection,
3943 that connection is said to <firstterm>own</firstterm> the name.
3946 The bus itself owns a special name,
3947 <literal>org.freedesktop.DBus</literal>, with an object
3948 located at <literal>/org/freedesktop/DBus</literal> that
3949 implements the <literal>org.freedesktop.DBus</literal>
3950 interface. This service allows applications to make
3951 administrative requests of the bus itself. For example,
3952 applications can ask the bus to assign a name to a connection.
3955 Each name may have <firstterm>queued owners</firstterm>. When an
3956 application requests a name for a connection and the name is already in
3957 use, the bus will optionally add the connection to a queue waiting for
3958 the name. If the current owner of the name disconnects or releases
3959 the name, the next connection in the queue will become the new owner.
3963 This feature causes the right thing to happen if you start two text
3964 editors for example; the first one may request "com.example.TextEditor",
3965 and the second will be queued as a possible owner of that name. When
3966 the first exits, the second will take over.
3970 Applications may send <firstterm>unicast messages</firstterm> to
3971 a specific recipient or to the message bus itself, or
3972 <firstterm>broadcast messages</firstterm> to all interested recipients.
3973 See <xref linkend="message-bus-routing"/> for details.
3977 <sect2 id="message-bus-names">
3978 <title>Message Bus Names</title>
3980 Each connection has at least one name, assigned at connection time and
3981 returned in response to the
3982 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3983 automatically-assigned name is called the connection's <firstterm>unique
3984 name</firstterm>. Unique names are never reused for two different
3985 connections to the same bus.
3988 Ownership of a unique name is a prerequisite for interaction with
3989 the message bus. It logically follows that the unique name is always
3990 the first name that an application comes to own, and the last
3991 one that it loses ownership of.
3994 Unique connection names must begin with the character ':' (ASCII colon
3995 character); bus names that are not unique names must not begin
3996 with this character. (The bus must reject any attempt by an application
3997 to manually request a name beginning with ':'.) This restriction
3998 categorically prevents "spoofing"; messages sent to a unique name
3999 will always go to the expected connection.
4002 When a connection is closed, all the names that it owns are deleted (or
4003 transferred to the next connection in the queue if any).
4006 A connection can request additional names to be associated with it using
4007 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4008 linkend="message-protocol-names-bus"/> describes the format of a valid
4009 name. These names can be released again using the
4010 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4013 <sect3 id="bus-messages-request-name">
4014 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4018 UINT32 RequestName (in STRING name, in UINT32 flags)
4025 <entry>Argument</entry>
4027 <entry>Description</entry>
4033 <entry>STRING</entry>
4034 <entry>Name to request</entry>
4038 <entry>UINT32</entry>
4039 <entry>Flags</entry>
4049 <entry>Argument</entry>
4051 <entry>Description</entry>
4057 <entry>UINT32</entry>
4058 <entry>Return value</entry>
4065 This method call should be sent to
4066 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4067 assign the given name to the method caller. Each name maintains a
4068 queue of possible owners, where the head of the queue is the primary
4069 or current owner of the name. Each potential owner in the queue
4070 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4071 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4072 call. When RequestName is invoked the following occurs:
4076 If the method caller is currently the primary owner of the name,
4077 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4078 values are updated with the values from the new RequestName call,
4079 and nothing further happens.
4085 If the current primary owner (head of the queue) has
4086 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4087 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4088 the caller of RequestName replaces the current primary owner at
4089 the head of the queue and the current primary owner moves to the
4090 second position in the queue. If the caller of RequestName was
4091 in the queue previously its flags are updated with the values from
4092 the new RequestName in addition to moving it to the head of the queue.
4098 If replacement is not possible, and the method caller is
4099 currently in the queue but not the primary owner, its flags are
4100 updated with the values from the new RequestName call.
4106 If replacement is not possible, and the method caller is
4107 currently not in the queue, the method caller is appended to the
4114 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4115 set and is not the primary owner, it is removed from the
4116 queue. This can apply to the previous primary owner (if it
4117 was replaced) or the method caller (if it updated the
4118 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4119 queue, or if it was just added to the queue with that flag set).
4125 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4126 queue," even if another application already in the queue had specified
4127 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4128 that does not allow replacement goes away, and the next primary owner
4129 does allow replacement. In this case, queued items that specified
4130 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4131 automatically replace the new primary owner. In other words,
4132 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4133 time RequestName is called. This is deliberate to avoid an infinite loop
4134 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4135 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4138 The flags argument contains any of the following values logically ORed
4145 <entry>Conventional Name</entry>
4146 <entry>Value</entry>
4147 <entry>Description</entry>
4152 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4156 If an application A specifies this flag and succeeds in
4157 becoming the owner of the name, and another application B
4158 later calls RequestName with the
4159 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4160 will lose ownership and receive a
4161 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4162 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4163 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4164 is not specified by application B, then application B will not replace
4165 application A as the owner.
4170 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4174 Try to replace the current owner if there is one. If this
4175 flag is not set the application will only become the owner of
4176 the name if there is no current owner. If this flag is set,
4177 the application will replace the current owner if
4178 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4183 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4187 Without this flag, if an application requests a name that is
4188 already owned, the application will be placed in a queue to
4189 own the name when the current owner gives it up. If this
4190 flag is given, the application will not be placed in the
4191 queue, the request for the name will simply fail. This flag
4192 also affects behavior when an application is replaced as
4193 name owner; by default the application moves back into the
4194 waiting queue, unless this flag was provided when the application
4195 became the name owner.
4203 The return code can be one of the following values:
4209 <entry>Conventional Name</entry>
4210 <entry>Value</entry>
4211 <entry>Description</entry>
4216 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4217 <entry>1</entry> <entry>The caller is now the primary owner of
4218 the name, replacing any previous owner. Either the name had no
4219 owner before, or the caller specified
4220 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4221 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4224 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4227 <entry>The name already had an owner,
4228 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4229 the current owner did not specify
4230 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4231 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4235 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4236 <entry>The name already has an owner,
4237 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4238 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4239 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4240 specified by the requesting application.</entry>
4243 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4245 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4253 <sect3 id="bus-messages-release-name">
4254 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4258 UINT32 ReleaseName (in STRING name)
4265 <entry>Argument</entry>
4267 <entry>Description</entry>
4273 <entry>STRING</entry>
4274 <entry>Name to release</entry>
4284 <entry>Argument</entry>
4286 <entry>Description</entry>
4292 <entry>UINT32</entry>
4293 <entry>Return value</entry>
4300 This method call should be sent to
4301 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4302 release the method caller's claim to the given name. If the caller is
4303 the primary owner, a new primary owner will be selected from the
4304 queue if any other owners are waiting. If the caller is waiting in
4305 the queue for the name, the caller will removed from the queue and
4306 will not be made an owner of the name if it later becomes available.
4307 If there are no other owners in the queue for the name, it will be
4308 removed from the bus entirely.
4310 The return code can be one of the following values:
4316 <entry>Conventional Name</entry>
4317 <entry>Value</entry>
4318 <entry>Description</entry>
4323 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4324 <entry>1</entry> <entry>The caller has released his claim on
4325 the given name. Either the caller was the primary owner of
4326 the name, and the name is now unused or taken by somebody
4327 waiting in the queue for the name, or the caller was waiting
4328 in the queue for the name and has now been removed from the
4332 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4334 <entry>The given name does not exist on this bus.</entry>
4337 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4339 <entry>The caller was not the primary owner of this name,
4340 and was also not waiting in the queue to own this name.</entry>
4348 <sect3 id="bus-messages-list-queued-owners">
4349 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4353 ARRAY of STRING ListQueuedOwners (in STRING name)
4360 <entry>Argument</entry>
4362 <entry>Description</entry>
4368 <entry>STRING</entry>
4369 <entry>The well-known bus name to query, such as
4370 <literal>com.example.cappuccino</literal></entry>
4380 <entry>Argument</entry>
4382 <entry>Description</entry>
4388 <entry>ARRAY of STRING</entry>
4389 <entry>The unique bus names of connections currently queued
4390 for the name</entry>
4397 This method call should be sent to
4398 <literal>org.freedesktop.DBus</literal> and lists the connections
4399 currently queued for a bus name (see
4400 <xref linkend="term-queued-owner"/>).
4405 <sect2 id="message-bus-routing">
4406 <title>Message Bus Message Routing</title>
4409 Messages may have a <literal>DESTINATION</literal> field (see <xref
4410 linkend="message-protocol-header-fields"/>), resulting in a
4411 <firstterm>unicast message</firstterm>. If the
4412 <literal>DESTINATION</literal> field is present, it specifies a message
4413 recipient by name. Method calls and replies normally specify this field.
4414 The message bus must send messages (of any type) with the
4415 <literal>DESTINATION</literal> field set to the specified recipient,
4416 regardless of whether the recipient has set up a match rule matching
4421 When the message bus receives a signal, if the
4422 <literal>DESTINATION</literal> field is absent, it is considered to
4423 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4424 applications with <firstterm>message matching rules</firstterm> that
4425 match the message. Most signal messages are broadcasts.
4429 Unicast signal messages (those with a <literal>DESTINATION</literal>
4430 field) are not commonly used, but they are treated like any unicast
4431 message: they are delivered to the specified receipient,
4432 regardless of its match rules. One use for unicast signals is to
4433 avoid a race condition in which a signal is emitted before the intended
4434 recipient can call <xref linkend="bus-messages-add-match"/> to
4435 receive that signal: if the signal is sent directly to that recipient
4436 using a unicast message, it does not need to add a match rule at all,
4437 and there is no race condition. Another use for unicast signals,
4438 on message buses whose security policy prevents eavesdropping, is to
4439 send sensitive information which should only be visible to one
4444 When the message bus receives a method call, if the
4445 <literal>DESTINATION</literal> field is absent, the call is taken to be
4446 a standard one-to-one message and interpreted by the message bus
4447 itself. For example, sending an
4448 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4449 <literal>DESTINATION</literal> will cause the message bus itself to
4450 reply to the ping immediately; the message bus will not make this
4451 message visible to other applications.
4455 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4456 the ping message were sent with a <literal>DESTINATION</literal> name of
4457 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4458 forwarded, and the Yoyodyne Corporation screensaver application would be
4459 expected to reply to the ping.
4463 Message bus implementations may impose a security policy which
4464 prevents certain messages from being sent or received.
4465 When a message cannot be sent or received due to a security
4466 policy, the message bus should send an error reply, unless the
4467 original message had the <literal>NO_REPLY</literal> flag.
4470 <sect3 id="message-bus-routing-eavesdropping">
4471 <title>Eavesdropping</title>
4473 Receiving a unicast message whose <literal>DESTINATION</literal>
4474 indicates a different recipient is called
4475 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4476 a security boundary (like the standard system bus), the security
4477 policy should usually prevent eavesdropping, since unicast messages
4478 are normally kept private and may contain security-sensitive
4483 Eavesdropping is mainly useful for debugging tools, such as
4484 the <literal>dbus-monitor</literal> tool in the reference
4485 implementation of D-Bus. Tools which eavesdrop on the message bus
4486 should be careful to avoid sending a reply or error in response to
4487 messages intended for a different client.
4491 Clients may attempt to eavesdrop by adding match rules
4492 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4493 the <literal>eavesdrop='true'</literal> match. If the message bus'
4494 security policy does not allow eavesdropping, the match rule can
4495 still be added, but will not have any practical effect. For
4496 compatibility with older message bus implementations, if adding such
4497 a match rule results in an error reply, the client may fall back to
4498 adding the same rule with the <literal>eavesdrop</literal> match
4503 <sect3 id="message-bus-routing-match-rules">
4504 <title>Match Rules</title>
4506 An important part of the message bus routing protocol is match
4507 rules. Match rules describe the messages that should be sent to a
4508 client, based on the contents of the message. Broadcast signals
4509 are only sent to clients which have a suitable match rule: this
4510 avoids waking up client processes to deal with signals that are
4511 not relevant to that client.
4514 Messages that list a client as their <literal>DESTINATION</literal>
4515 do not need to match the client's match rules, and are sent to that
4516 client regardless. As a result, match rules are mainly used to
4517 receive a subset of broadcast signals.
4520 Match rules can also be used for eavesdropping
4521 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4522 if the security policy of the message bus allows it.
4525 Match rules are added using the AddMatch bus method
4526 (see <xref linkend="bus-messages-add-match"/>). Rules are
4527 specified as a string of comma separated key/value pairs.
4528 Excluding a key from the rule indicates a wildcard match.
4529 For instance excluding the the member from a match rule but
4530 adding a sender would let all messages from that sender through.
4531 An example of a complete rule would be
4532 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4535 The following table describes the keys that can be used to create
4542 <entry>Possible Values</entry>
4543 <entry>Description</entry>
4548 <entry><literal>type</literal></entry>
4549 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4550 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4553 <entry><literal>sender</literal></entry>
4554 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4555 and <xref linkend="term-unique-name"/> respectively)
4557 <entry>Match messages sent by a particular sender. An example of a sender match
4558 is sender='org.freedesktop.Hal'</entry>
4561 <entry><literal>interface</literal></entry>
4562 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4563 <entry>Match messages sent over or to a particular interface. An example of an
4564 interface match is interface='org.freedesktop.Hal.Manager'.
4565 If a message omits the interface header, it must not match any rule
4566 that specifies this key.</entry>
4569 <entry><literal>member</literal></entry>
4570 <entry>Any valid method or signal name</entry>
4571 <entry>Matches messages which have the give method or signal name. An example of
4572 a member match is member='NameOwnerChanged'</entry>
4575 <entry><literal>path</literal></entry>
4576 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4577 <entry>Matches messages which are sent from or to the given object. An example of a
4578 path match is path='/org/freedesktop/Hal/Manager'</entry>
4581 <entry><literal>path_namespace</literal></entry>
4582 <entry>An object path</entry>
4585 Matches messages which are sent from or to an
4586 object for which the object path is either the
4587 given value, or that value followed by one or
4588 more path components.
4593 <literal>path_namespace='/com/example/foo'</literal>
4594 would match signals sent by
4595 <literal>/com/example/foo</literal>
4597 <literal>/com/example/foo/bar</literal>,
4599 <literal>/com/example/foobar</literal>.
4603 Using both <literal>path</literal> and
4604 <literal>path_namespace</literal> in the same match
4605 rule is not allowed.
4610 This match key was added in version 0.16 of the
4611 D-Bus specification and implemented by the bus
4612 daemon in dbus 1.5.0 and later.
4618 <entry><literal>destination</literal></entry>
4619 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4620 <entry>Matches messages which are being sent to the given unique name. An
4621 example of a destination match is destination=':1.0'</entry>
4624 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4625 <entry>Any string</entry>
4626 <entry>Arg matches are special and are used for further restricting the
4627 match based on the arguments in the body of a message. Only arguments of type
4628 STRING can be matched in this way. An example of an argument match
4629 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4633 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4634 <entry>Any string</entry>
4636 <para>Argument path matches provide a specialised form of wildcard matching for
4637 path-like namespaces. They can match arguments whose type is either STRING or
4638 OBJECT_PATH. As with normal argument matches,
4639 if the argument is exactly equal to the string given in the match
4640 rule then the rule is satisfied. Additionally, there is also a
4641 match when either the string given in the match rule or the
4642 appropriate message argument ends with '/' and is a prefix of the
4643 other. An example argument path match is arg0path='/aa/bb/'. This
4644 would match messages with first arguments of '/', '/aa/',
4645 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4646 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4648 <para>This is intended for monitoring “directories” in file system-like
4649 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4650 system. An application interested in all nodes in a particular hierarchy would
4651 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4652 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4653 represent a modification to the “bar” property, or a signal with zeroth
4654 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4655 many properties within that directory, and the interested application would be
4656 notified in both cases.</para>
4659 This match key was added in version 0.12 of the
4660 D-Bus specification, implemented for STRING
4661 arguments by the bus daemon in dbus 1.2.0 and later,
4662 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4669 <entry><literal>arg0namespace</literal></entry>
4670 <entry>Like a bus name, except that the string is not
4671 required to contain a '.' (period)</entry>
4673 <para>Match messages whose first argument is of type STRING, and is a bus name
4674 or interface name within the specified namespace. This is primarily intended
4675 for watching name owner changes for a group of related bus names, rather than
4676 for a single name or all name changes.</para>
4678 <para>Because every valid interface name is also a valid
4679 bus name, this can also be used for messages whose
4680 first argument is an interface name.</para>
4682 <para>For example, the match rule
4683 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4684 matches name owner changes for bus names such as
4685 <literal>com.example.backend.foo</literal>,
4686 <literal>com.example.backend.foo.bar</literal>, and
4687 <literal>com.example.backend</literal> itself.</para>
4689 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4692 This match key was added in version 0.16 of the
4693 D-Bus specification and implemented by the bus
4694 daemon in dbus 1.5.0 and later.
4700 <entry><literal>eavesdrop</literal></entry>
4701 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4702 <entry>Since D-Bus 1.5.6, match rules do not
4703 match messages which have a <literal>DESTINATION</literal>
4704 field unless the match rule specifically
4706 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4707 by specifying <literal>eavesdrop='true'</literal>
4708 in the match rule. <literal>eavesdrop='false'</literal>
4709 restores the default behaviour. Messages are
4710 delivered to their <literal>DESTINATION</literal>
4711 regardless of match rules, so this match does not
4712 affect normal delivery of unicast messages.
4713 If the message bus has a security policy which forbids
4714 eavesdropping, this match may still be used without error,
4715 but will not have any practical effect.
4716 In older versions of D-Bus, this match was not allowed
4717 in match rules, and all match rules behaved as if
4718 <literal>eavesdrop='true'</literal> had been used.
4727 <sect2 id="message-bus-starting-services">
4728 <title>Message Bus Starting Services</title>
4730 The message bus can start applications on behalf of other applications.
4731 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4732 An application that can be started in this way is called a
4733 <firstterm>service</firstterm>.
4736 With D-Bus, starting a service is normally done by name. That is,
4737 applications ask the message bus to start some program that will own a
4738 well-known name, such as <literal>com.example.TextEditor</literal>.
4739 This implies a contract documented along with the name
4740 <literal>com.example.TextEditor</literal> for which object
4741 the owner of that name will provide, and what interfaces those
4745 To find an executable corresponding to a particular name, the bus daemon
4746 looks for <firstterm>service description files</firstterm>. Service
4747 description files define a mapping from names to executables. Different
4748 kinds of message bus will look for these files in different places, see
4749 <xref linkend="message-bus-types"/>.
4752 Service description files have the ".service" file
4753 extension. The message bus will only load service description files
4754 ending with .service; all other files will be ignored. The file format
4755 is similar to that of <ulink
4756 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4757 entries</ulink>. All service description files must be in UTF-8
4758 encoding. To ensure that there will be no name collisions, service files
4759 must be namespaced using the same mechanism as messages and service
4764 On the well-known system bus, the name of a service description file
4765 must be its well-known name plus <literal>.service</literal>,
4767 <literal>com.example.ConfigurationDatabase.service</literal>.
4771 On the well-known session bus, services should follow the same
4772 service description file naming convention as on the system bus,
4773 but for backwards compatibility they are not required to do so.
4777 [FIXME the file format should be much better specified than "similar to
4778 .desktop entries" esp. since desktop entries are already
4779 badly-specified. ;-)]
4780 These sections from the specification apply to service files as well:
4783 <listitem><para>General syntax</para></listitem>
4784 <listitem><para>Comment format</para></listitem>
4787 Service description files must contain a
4788 <literal>D-BUS Service</literal> group with at least the keys
4789 <literal>Name</literal> (the well-known name of the service)
4790 and <literal>Exec</literal> (the command to be executed).
4793 <title>Example service description file</title>
4795 # Sample service description file
4797 Name=com.example.ConfigurationDatabase
4798 Exec=/usr/bin/sample-configd
4804 Additionally, service description files for the well-known system
4805 bus on Unix must contain a <literal>User</literal> key, whose value
4806 is the name of a user account (e.g. <literal>root</literal>).
4807 The system service will be run as that user.
4811 When an application asks to start a service by name, the bus daemon tries to
4812 find a service that will own that name. It then tries to spawn the
4813 executable associated with it. If this fails, it will report an
4818 On the well-known system bus, it is not possible for two .service files
4819 in the same directory to offer the same service, because they are
4820 constrained to have names that match the service name.
4824 On the well-known session bus, if two .service files in the same
4825 directory offer the same service name, the result is undefined.
4826 Distributors should avoid this situation, for instance by naming
4827 session services' .service files according to their service name.
4831 If two .service files in different directories offer the same
4832 service name, the one in the higher-priority directory is used:
4833 for instance, on the system bus, .service files in
4834 /usr/local/share/dbus-1/system-services take precedence over those
4835 in /usr/share/dbus-1/system-services.
4838 The executable launched will have the environment variable
4839 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4840 message bus so it can connect and request the appropriate names.
4843 The executable being launched may want to know whether the message bus
4844 starting it is one of the well-known message buses (see <xref
4845 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4846 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4847 of the well-known buses. The currently-defined values for this variable
4848 are <literal>system</literal> for the systemwide message bus,
4849 and <literal>session</literal> for the per-login-session message
4850 bus. The new executable must still connect to the address given
4851 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4852 resulting connection is to the well-known bus.
4855 [FIXME there should be a timeout somewhere, either specified
4856 in the .service file, by the client, or just a global value
4857 and if the client being activated fails to connect within that
4858 timeout, an error should be sent back.]
4861 <sect3 id="message-bus-starting-services-scope">
4862 <title>Message Bus Service Scope</title>
4864 The "scope" of a service is its "per-", such as per-session,
4865 per-machine, per-home-directory, or per-display. The reference
4866 implementation doesn't yet support starting services in a different
4867 scope from the message bus itself. So e.g. if you start a service
4868 on the session bus its scope is per-session.
4871 We could add an optional scope to a bus name. For example, for
4872 per-(display,session pair), we could have a unique ID for each display
4873 generated automatically at login and set on screen 0 by executing a
4874 special "set display ID" binary. The ID would be stored in a
4875 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4876 random bytes. This ID would then be used to scope names.
4877 Starting/locating a service could be done by ID-name pair rather than
4881 Contrast this with a per-display scope. To achieve that, we would
4882 want a single bus spanning all sessions using a given display.
4883 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4884 property on screen 0 of the display, pointing to this bus.
4889 <sect2 id="message-bus-types">
4890 <title>Well-known Message Bus Instances</title>
4892 Two standard message bus instances are defined here, along with how
4893 to locate them and where their service files live.
4895 <sect3 id="message-bus-types-login">
4896 <title>Login session message bus</title>
4898 Each time a user logs in, a <firstterm>login session message
4899 bus</firstterm> may be started. All applications in the user's login
4900 session may interact with one another using this message bus.
4903 The address of the login session message bus is given
4904 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4905 variable. If that variable is not set, applications may
4906 also try to read the address from the X Window System root
4907 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4908 The root window property must have type <literal>STRING</literal>.
4909 The environment variable should have precedence over the
4910 root window property.
4912 <para>The address of the login session message bus is given in the
4913 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4914 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4915 "autolaunch:", the system should use platform-specific methods of
4916 locating a running D-Bus session server, or starting one if a running
4917 instance cannot be found. Note that this mechanism is not recommended
4918 for attempting to determine if a daemon is running. It is inherently
4919 racy to attempt to make this determination, since the bus daemon may
4920 be started just before or just after the determination is made.
4921 Therefore, it is recommended that applications do not try to make this
4922 determination for their functionality purposes, and instead they
4923 should attempt to start the server.</para>
4925 <sect4 id="message-bus-types-login-x-windows">
4926 <title>X Windowing System</title>
4928 For the X Windowing System, the application must locate the
4929 window owner of the selection represented by the atom formed by
4933 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4937 <para>the current user's username</para>
4941 <para>the literal character '_' (underscore)</para>
4945 <para>the machine's ID</para>
4951 The following properties are defined for the window that owns
4953 <informaltable frame="all">
4962 <para>meaning</para>
4968 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4972 <para>the actual address of the server socket</para>
4978 <para>_DBUS_SESSION_BUS_PID</para>
4982 <para>the PID of the server process</para>
4991 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4992 present in this window.
4996 If the X selection cannot be located or if reading the
4997 properties from the window fails, the implementation MUST conclude
4998 that there is no D-Bus server running and proceed to start a new
4999 server. (See below on concurrency issues)
5003 Failure to connect to the D-Bus server address thus obtained
5004 MUST be treated as a fatal connection error and should be reported
5009 As an alternative, an implementation MAY find the information
5010 in the following file located in the current user's home directory,
5011 in subdirectory .dbus/session-bus/:
5014 <para>the machine's ID</para>
5018 <para>the literal character '-' (dash)</para>
5022 <para>the X display without the screen number, with the
5023 following prefixes removed, if present: ":", "localhost:"
5024 ."localhost.localdomain:". That is, a display of
5025 "localhost:10.0" produces just the number "10"</para>
5031 The contents of this file NAME=value assignment pairs and
5032 lines starting with # are comments (no comments are allowed
5033 otherwise). The following variable names are defined:
5040 <para>Variable</para>
5044 <para>meaning</para>
5050 <para>DBUS_SESSION_BUS_ADDRESS</para>
5054 <para>the actual address of the server socket</para>
5060 <para>DBUS_SESSION_BUS_PID</para>
5064 <para>the PID of the server process</para>
5070 <para>DBUS_SESSION_BUS_WINDOWID</para>
5074 <para>the window ID</para>
5083 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5088 Failure to open this file MUST be interpreted as absence of a
5089 running server. Therefore, the implementation MUST proceed to
5090 attempting to launch a new bus server if the file cannot be
5095 However, success in opening this file MUST NOT lead to the
5096 conclusion that the server is running. Thus, a failure to connect to
5097 the bus address obtained by the alternative method MUST NOT be
5098 considered a fatal error. If the connection cannot be established,
5099 the implementation MUST proceed to check the X selection settings or
5100 to start the server on its own.
5104 If the implementation concludes that the D-Bus server is not
5105 running it MUST attempt to start a new server and it MUST also
5106 ensure that the daemon started as an effect of the "autolaunch"
5107 mechanism provides the lookup mechanisms described above, so
5108 subsequent calls can locate the newly started server. The
5109 implementation MUST also ensure that if two or more concurrent
5110 initiations happen, only one server remains running and all other
5111 initiations are able to obtain the address of this server and
5112 connect to it. In other words, the implementation MUST ensure that
5113 the X selection is not present when it attempts to set it, without
5114 allowing another process to set the selection between the
5115 verification and the setting (e.g., by using XGrabServer /
5122 On Unix systems, the session bus should search for .service files
5123 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5125 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5126 Implementations may also search additional locations, which
5127 should be searched with lower priority than anything in
5128 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5129 for example, the reference implementation also
5130 looks in <literal>${datadir}/dbus-1/services</literal> as
5131 set at compile time.
5134 As described in the XDG Base Directory Specification, software
5135 packages should install their session .service files to their
5136 configured <literal>${datadir}/dbus-1/services</literal>,
5137 where <literal>${datadir}</literal> is as defined by the GNU
5138 coding standards. System administrators or users can arrange
5139 for these service files to be read by setting XDG_DATA_DIRS or by
5140 symlinking them into the default locations.
5144 <sect3 id="message-bus-types-system">
5145 <title>System message bus</title>
5147 A computer may have a <firstterm>system message bus</firstterm>,
5148 accessible to all applications on the system. This message bus may be
5149 used to broadcast system events, such as adding new hardware devices,
5150 changes in the printer queue, and so forth.
5153 The address of the system message bus is given
5154 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5155 variable. If that variable is not set, applications should try
5156 to connect to the well-known address
5157 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5160 The D-Bus reference implementation actually honors the
5161 <literal>$(localstatedir)</literal> configure option
5162 for this address, on both client and server side.
5167 On Unix systems, the system bus should default to searching
5168 for .service files in
5169 <literal>/usr/local/share/dbus-1/system-services</literal>,
5170 <literal>/usr/share/dbus-1/system-services</literal> and
5171 <literal>/lib/dbus-1/system-services</literal>, with that order
5172 of precedence. It may also search other implementation-specific
5173 locations, but should not vary these locations based on environment
5177 The system bus is security-sensitive and is typically executed
5178 by an init system with a clean environment. Its launch helper
5179 process is particularly security-sensitive, and specifically
5180 clears its own environment.
5185 Software packages should install their system .service
5186 files to their configured
5187 <literal>${datadir}/dbus-1/system-services</literal>,
5188 where <literal>${datadir}</literal> is as defined by the GNU
5189 coding standards. System administrators can arrange
5190 for these service files to be read by editing the system bus'
5191 configuration file or by symlinking them into the default
5197 <sect2 id="message-bus-messages">
5198 <title>Message Bus Messages</title>
5200 The special message bus name <literal>org.freedesktop.DBus</literal>
5201 responds to a number of additional messages.
5204 <sect3 id="bus-messages-hello">
5205 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5216 <entry>Argument</entry>
5218 <entry>Description</entry>
5224 <entry>STRING</entry>
5225 <entry>Unique name assigned to the connection</entry>
5232 Before an application is able to send messages to other applications
5233 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5234 to the message bus to obtain a unique name. If an application without
5235 a unique name tries to send a message to another application, or a
5236 message to the message bus itself that isn't the
5237 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5238 disconnected from the bus.
5241 There is no corresponding "disconnect" request; if a client wishes to
5242 disconnect from the bus, it simply closes the socket (or other
5243 communication channel).
5246 <sect3 id="bus-messages-list-names">
5247 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5251 ARRAY of STRING ListNames ()
5258 <entry>Argument</entry>
5260 <entry>Description</entry>
5266 <entry>ARRAY of STRING</entry>
5267 <entry>Array of strings where each string is a bus name</entry>
5274 Returns a list of all currently-owned names on the bus.
5277 <sect3 id="bus-messages-list-activatable-names">
5278 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5282 ARRAY of STRING ListActivatableNames ()
5289 <entry>Argument</entry>
5291 <entry>Description</entry>
5297 <entry>ARRAY of STRING</entry>
5298 <entry>Array of strings where each string is a bus name</entry>
5305 Returns a list of all names that can be activated on the bus.
5308 <sect3 id="bus-messages-name-exists">
5309 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5313 BOOLEAN NameHasOwner (in STRING name)
5320 <entry>Argument</entry>
5322 <entry>Description</entry>
5328 <entry>STRING</entry>
5329 <entry>Name to check</entry>
5339 <entry>Argument</entry>
5341 <entry>Description</entry>
5347 <entry>BOOLEAN</entry>
5348 <entry>Return value, true if the name exists</entry>
5355 Checks if the specified name exists (currently has an owner).
5359 <sect3 id="bus-messages-name-owner-changed">
5360 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5364 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5371 <entry>Argument</entry>
5373 <entry>Description</entry>
5379 <entry>STRING</entry>
5380 <entry>Name with a new owner</entry>
5384 <entry>STRING</entry>
5385 <entry>Old owner or empty string if none</entry>
5389 <entry>STRING</entry>
5390 <entry>New owner or empty string if none</entry>
5397 This signal indicates that the owner of a name has changed.
5398 It's also the signal to use to detect the appearance of
5399 new names on the bus.
5402 <sect3 id="bus-messages-name-lost">
5403 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5407 NameLost (STRING name)
5414 <entry>Argument</entry>
5416 <entry>Description</entry>
5422 <entry>STRING</entry>
5423 <entry>Name which was lost</entry>
5430 This signal is sent to a specific application when it loses
5431 ownership of a name.
5435 <sect3 id="bus-messages-name-acquired">
5436 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5440 NameAcquired (STRING name)
5447 <entry>Argument</entry>
5449 <entry>Description</entry>
5455 <entry>STRING</entry>
5456 <entry>Name which was acquired</entry>
5463 This signal is sent to a specific application when it gains
5464 ownership of a name.
5468 <sect3 id="bus-messages-start-service-by-name">
5469 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5473 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5480 <entry>Argument</entry>
5482 <entry>Description</entry>
5488 <entry>STRING</entry>
5489 <entry>Name of the service to start</entry>
5493 <entry>UINT32</entry>
5494 <entry>Flags (currently not used)</entry>
5504 <entry>Argument</entry>
5506 <entry>Description</entry>
5512 <entry>UINT32</entry>
5513 <entry>Return value</entry>
5518 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5522 The return value can be one of the following values:
5527 <entry>Identifier</entry>
5528 <entry>Value</entry>
5529 <entry>Description</entry>
5534 <entry>DBUS_START_REPLY_SUCCESS</entry>
5536 <entry>The service was successfully started.</entry>
5539 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5541 <entry>A connection already owns the given name.</entry>
5550 <sect3 id="bus-messages-update-activation-environment">
5551 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5555 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5562 <entry>Argument</entry>
5564 <entry>Description</entry>
5570 <entry>ARRAY of DICT<STRING,STRING></entry>
5571 <entry>Environment to add or update</entry>
5576 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5579 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5582 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.
5587 <sect3 id="bus-messages-get-name-owner">
5588 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5592 STRING GetNameOwner (in STRING name)
5599 <entry>Argument</entry>
5601 <entry>Description</entry>
5607 <entry>STRING</entry>
5608 <entry>Name to get the owner of</entry>
5618 <entry>Argument</entry>
5620 <entry>Description</entry>
5626 <entry>STRING</entry>
5627 <entry>Return value, a unique connection name</entry>
5632 Returns the unique connection name of the primary owner of the name
5633 given. If the requested name doesn't have an owner, returns a
5634 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5638 <sect3 id="bus-messages-get-connection-unix-user">
5639 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5643 UINT32 GetConnectionUnixUser (in STRING bus_name)
5650 <entry>Argument</entry>
5652 <entry>Description</entry>
5658 <entry>STRING</entry>
5659 <entry>Unique or well-known bus name of the connection to
5660 query, such as <literal>:12.34</literal> or
5661 <literal>com.example.tea</literal></entry>
5671 <entry>Argument</entry>
5673 <entry>Description</entry>
5679 <entry>UINT32</entry>
5680 <entry>Unix user ID</entry>
5685 Returns the Unix user ID of the process connected to the server. If
5686 unable to determine it (for instance, because the process is not on the
5687 same machine as the bus daemon), an error is returned.
5691 <sect3 id="bus-messages-get-connection-unix-process-id">
5692 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5696 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5703 <entry>Argument</entry>
5705 <entry>Description</entry>
5711 <entry>STRING</entry>
5712 <entry>Unique or well-known bus name of the connection to
5713 query, such as <literal>:12.34</literal> or
5714 <literal>com.example.tea</literal></entry>
5724 <entry>Argument</entry>
5726 <entry>Description</entry>
5732 <entry>UINT32</entry>
5733 <entry>Unix process id</entry>
5738 Returns the Unix process ID of the process connected to the server. If
5739 unable to determine it (for instance, because the process is not on the
5740 same machine as the bus daemon), an error is returned.
5744 <sect3 id="bus-messages-get-connection-credentials">
5745 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5749 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5756 <entry>Argument</entry>
5758 <entry>Description</entry>
5764 <entry>STRING</entry>
5765 <entry>Unique or well-known bus name of the connection to
5766 query, such as <literal>:12.34</literal> or
5767 <literal>com.example.tea</literal></entry>
5777 <entry>Argument</entry>
5779 <entry>Description</entry>
5785 <entry>DICT<STRING,VARIANT></entry>
5786 <entry>Credentials</entry>
5794 Returns as many credentials as possible for the process connected to
5795 the server. If unable to determine certain credentials (for instance,
5796 because the process is not on the same machine as the bus daemon,
5797 or because this version of the bus daemon does not support a
5798 particular security framework), or if the values of those credentials
5799 cannot be represented as documented here, then those credentials
5804 Keys in the returned dictionary not containing "." are defined
5805 by this specification. Bus daemon implementors supporting
5806 credentials frameworks not mentioned in this document should either
5807 contribute patches to this specification, or use keys containing
5808 "." and starting with a reversed domain name.
5814 <entry>Value type</entry>
5815 <entry>Value</entry>
5820 <entry>UnixUserID</entry>
5821 <entry>UINT32</entry>
5822 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5825 <entry>ProcessID</entry>
5826 <entry>UINT32</entry>
5827 <entry>The numeric process ID, on platforms that have
5828 this concept. On Unix, this is the process ID defined by
5837 This method was added in D-Bus 1.7 to reduce the round-trips
5838 required to list a process's credentials. In older versions, calling
5839 this method will fail: applications should recover by using the
5840 separate methods such as
5841 <xref linkend="bus-messages-get-connection-unix-user"/>
5846 <sect3 id="bus-messages-get-adt-audit-session-data">
5847 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5851 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5858 <entry>Argument</entry>
5860 <entry>Description</entry>
5866 <entry>STRING</entry>
5867 <entry>Unique or well-known bus name of the connection to
5868 query, such as <literal>:12.34</literal> or
5869 <literal>com.example.tea</literal></entry>
5879 <entry>Argument</entry>
5881 <entry>Description</entry>
5887 <entry>ARRAY of BYTE</entry>
5888 <entry>auditing data as returned by
5889 adt_export_session_data()</entry>
5894 Returns auditing data used by Solaris ADT, in an unspecified
5895 binary format. If you know what this means, please contribute
5896 documentation via the D-Bus bug tracking system.
5897 This method is on the core DBus interface for historical reasons;
5898 the same information should be made available via
5899 <xref linkend="bus-messages-get-connection-credentials"/>
5904 <sect3 id="bus-messages-get-connection-selinux-security-context">
5905 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5909 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5916 <entry>Argument</entry>
5918 <entry>Description</entry>
5924 <entry>STRING</entry>
5925 <entry>Unique or well-known bus name of the connection to
5926 query, such as <literal>:12.34</literal> or
5927 <literal>com.example.tea</literal></entry>
5937 <entry>Argument</entry>
5939 <entry>Description</entry>
5945 <entry>ARRAY of BYTE</entry>
5946 <entry>some sort of string of bytes, not necessarily UTF-8,
5947 not including '\0'</entry>
5952 Returns the security context used by SELinux, in an unspecified
5953 format. If you know what this means, please contribute
5954 documentation via the D-Bus bug tracking system.
5955 This method is on the core DBus interface for historical reasons;
5956 the same information should be made available via
5957 <xref linkend="bus-messages-get-connection-credentials"/>
5963 <sect3 id="bus-messages-add-match">
5964 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5968 AddMatch (in STRING rule)
5975 <entry>Argument</entry>
5977 <entry>Description</entry>
5983 <entry>STRING</entry>
5984 <entry>Match rule to add to the connection</entry>
5989 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5990 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5994 <sect3 id="bus-messages-remove-match">
5995 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5999 RemoveMatch (in STRING rule)
6006 <entry>Argument</entry>
6008 <entry>Description</entry>
6014 <entry>STRING</entry>
6015 <entry>Match rule to remove from the connection</entry>
6020 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6021 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6026 <sect3 id="bus-messages-get-id">
6027 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6031 GetId (out STRING id)
6038 <entry>Argument</entry>
6040 <entry>Description</entry>
6046 <entry>STRING</entry>
6047 <entry>Unique ID identifying the bus daemon</entry>
6052 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6053 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6054 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6055 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6056 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6057 by org.freedesktop.DBus.Peer.GetMachineId().
6058 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6066 <appendix id="implementation-notes">
6067 <title>Implementation notes</title>
6068 <sect1 id="implementation-notes-subsection">
6076 <glossary><title>Glossary</title>
6078 This glossary defines some of the terms used in this specification.
6081 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6084 The message bus maintains an association between names and
6085 connections. (Normally, there's one connection per application.) A
6086 bus name is simply an identifier used to locate connections. For
6087 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6088 name might be used to send a message to a screensaver from Yoyodyne
6089 Corporation. An application is said to <firstterm>own</firstterm> a
6090 name if the message bus has associated the application's connection
6091 with the name. Names may also have <firstterm>queued
6092 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6093 The bus assigns a unique name to each connection,
6094 see <xref linkend="term-unique-name"/>. Other names
6095 can be thought of as "well-known names" and are
6096 used to find applications that offer specific functionality.
6100 See <xref linkend="message-protocol-names-bus"/> for details of
6101 the syntax and naming conventions for bus names.
6106 <glossentry id="term-message"><glossterm>Message</glossterm>
6109 A message is the atomic unit of communication via the D-Bus
6110 protocol. It consists of a <firstterm>header</firstterm> and a
6111 <firstterm>body</firstterm>; the body is made up of
6112 <firstterm>arguments</firstterm>.
6117 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6120 The message bus is a special application that forwards
6121 or routes messages between a group of applications
6122 connected to the message bus. It also manages
6123 <firstterm>names</firstterm> used for routing
6129 <glossentry id="term-name"><glossterm>Name</glossterm>
6132 See <xref linkend="term-bus-name"/>. "Name" may
6133 also be used to refer to some of the other names
6134 in D-Bus, such as interface names.
6139 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6142 Used to prevent collisions when defining new interfaces, bus names
6143 etc. The convention used is the same one Java uses for defining
6144 classes: a reversed domain name.
6145 See <xref linkend="message-protocol-names-bus"/>,
6146 <xref linkend="message-protocol-names-interface"/>,
6147 <xref linkend="message-protocol-names-error"/>,
6148 <xref linkend="message-protocol-marshaling-object-path"/>.
6153 <glossentry id="term-object"><glossterm>Object</glossterm>
6156 Each application contains <firstterm>objects</firstterm>, which have
6157 <firstterm>interfaces</firstterm> and
6158 <firstterm>methods</firstterm>. Objects are referred to by a name,
6159 called a <firstterm>path</firstterm>.
6164 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6167 An application talking directly to another application, without going
6168 through a message bus. One-to-one connections may be "peer to peer" or
6169 "client to server." The D-Bus protocol has no concept of client
6170 vs. server after a connection has authenticated; the flow of messages
6171 is symmetrical (full duplex).
6176 <glossentry id="term-path"><glossterm>Path</glossterm>
6179 Object references (object names) in D-Bus are organized into a
6180 filesystem-style hierarchy, so each object is named by a path. As in
6181 LDAP, there's no difference between "files" and "directories"; a path
6182 can refer to an object, while still having child objects below it.
6187 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6190 Each bus name has a primary owner; messages sent to the name go to the
6191 primary owner. However, certain names also maintain a queue of
6192 secondary owners "waiting in the wings." If the primary owner releases
6193 the name, then the first secondary owner in the queue automatically
6194 becomes the new owner of the name.
6199 <glossentry id="term-service"><glossterm>Service</glossterm>
6202 A service is an executable that can be launched by the bus daemon.
6203 Services normally guarantee some particular features, for example they
6204 may guarantee that they will request a specific name such as
6205 "com.example.Screensaver", have a singleton object
6206 "/com/example/Application", and that object will implement the
6207 interface "com.example.Screensaver.Control".
6212 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6215 ".service files" tell the bus about service applications that can be
6216 launched (see <xref linkend="term-service"/>). Most importantly they
6217 provide a mapping from bus names to services that will request those
6218 names when they start up.
6223 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6226 The special name automatically assigned to each connection by the
6227 message bus. This name will never change owner, and will be unique
6228 (never reused during the lifetime of the message bus).
6229 It will begin with a ':' character.
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