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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.24</releaseinfo>
10 <date>2014-10-01</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
67 <email>zeuthen@gmail.com</email>
74 <revnumber>0.24</revnumber>
75 <date>2014-10-01</date>
76 <authorinitials>SMcV</authorinitials>
78 non-method-calls never expect a reply even without NO_REPLY_EXPECTED;
79 document how to quote match rules
83 <revnumber>0.23</revnumber>
84 <date>2014-01-06</date>
85 <authorinitials>SMcV, CY</authorinitials>
87 method call messages with no INTERFACE may be considered an error;
88 document tcp:bind=... and nonce-tcp:bind=...; define listenable
89 and connectable addresses
93 <revnumber>0.22</revnumber>
94 <date>2013-10-09</date>
95 <authorinitials></authorinitials>
96 <revremark>add GetConnectionCredentials, document
97 GetAtdAuditSessionData, document GetConnectionSELinuxSecurityContext,
98 document and correct .service file syntax and naming
102 <revnumber>0.21</revnumber>
103 <date>2013-04-25</date>
104 <authorinitials>smcv</authorinitials>
105 <revremark>allow Unicode noncharacters in UTF-8 (Unicode
106 Corrigendum #9)</revremark>
109 <revnumber>0.20</revnumber>
110 <date>22 February 2013</date>
111 <authorinitials>smcv, walters</authorinitials>
112 <revremark>reorganise for clarity, remove false claims about
113 basic types, mention /o/fd/DBus</revremark>
116 <revnumber>0.19</revnumber>
117 <date>20 February 2012</date>
118 <authorinitials>smcv/lp</authorinitials>
119 <revremark>formally define unique connection names and well-known
120 bus names; document best practices for interface, bus, member and
121 error names, and object paths; document the search path for session
122 and system services on Unix; document the systemd transport</revremark>
125 <revnumber>0.18</revnumber>
126 <date>29 July 2011</date>
127 <authorinitials>smcv</authorinitials>
128 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
129 match keyword; promote type system to a top-level section</revremark>
132 <revnumber>0.17</revnumber>
133 <date>1 June 2011</date>
134 <authorinitials>smcv/davidz</authorinitials>
135 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
136 by GVariant</revremark>
139 <revnumber>0.16</revnumber>
140 <date>11 April 2011</date>
141 <authorinitials></authorinitials>
142 <revremark>add path_namespace, arg0namespace; argNpath matches object
146 <revnumber>0.15</revnumber>
147 <date>3 November 2010</date>
148 <authorinitials></authorinitials>
149 <revremark></revremark>
152 <revnumber>0.14</revnumber>
153 <date>12 May 2010</date>
154 <authorinitials></authorinitials>
155 <revremark></revremark>
158 <revnumber>0.13</revnumber>
159 <date>23 Dezember 2009</date>
160 <authorinitials></authorinitials>
161 <revremark></revremark>
164 <revnumber>0.12</revnumber>
165 <date>7 November, 2006</date>
166 <authorinitials></authorinitials>
167 <revremark></revremark>
170 <revnumber>0.11</revnumber>
171 <date>6 February 2005</date>
172 <authorinitials></authorinitials>
173 <revremark></revremark>
176 <revnumber>0.10</revnumber>
177 <date>28 January 2005</date>
178 <authorinitials></authorinitials>
179 <revremark></revremark>
182 <revnumber>0.9</revnumber>
183 <date>7 Januar 2005</date>
184 <authorinitials></authorinitials>
185 <revremark></revremark>
188 <revnumber>0.8</revnumber>
189 <date>06 September 2003</date>
190 <authorinitials></authorinitials>
191 <revremark>First released document.</revremark>
196 <sect1 id="introduction">
197 <title>Introduction</title>
199 D-Bus is a system for low-overhead, easy to use
200 interprocess communication (IPC). In more detail:
204 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
205 binary protocol, and does not have to convert to and from a text
206 format such as XML. Because D-Bus is intended for potentially
207 high-resolution same-machine IPC, not primarily for Internet IPC,
208 this is an interesting optimization. D-Bus is also designed to
209 avoid round trips and allow asynchronous operation, much like
215 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
216 of <firstterm>messages</firstterm> rather than byte streams, and
217 automatically handles a lot of the hard IPC issues. Also, the D-Bus
218 library is designed to be wrapped in a way that lets developers use
219 their framework's existing object/type system, rather than learning
220 a new one specifically for IPC.
227 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
228 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
229 a system for one application to talk to a single other
230 application. However, the primary intended application of the protocol is the
231 D-Bus <firstterm>message bus</firstterm>, specified in <xref
232 linkend="message-bus"/>. The message bus is a special application that
233 accepts connections from multiple other applications, and forwards
238 Uses of D-Bus include notification of system changes (notification of when
239 a camera is plugged in to a computer, or a new version of some software
240 has been installed), or desktop interoperability, for example a file
241 monitoring service or a configuration service.
245 D-Bus is designed for two specific use cases:
249 A "system bus" for notifications from the system to user sessions,
250 and to allow the system to request input from user sessions.
255 A "session bus" used to implement desktop environments such as
260 D-Bus is not intended to be a generic IPC system for any possible
261 application, and intentionally omits many features found in other
262 IPC systems for this reason.
266 At the same time, the bus daemons offer a number of features not found in
267 other IPC systems, such as single-owner "bus names" (similar to X
268 selections), on-demand startup of services, and security policies.
269 In many ways, these features are the primary motivation for developing
270 D-Bus; other systems would have sufficed if IPC were the only goal.
274 D-Bus may turn out to be useful in unanticipated applications, but future
275 versions of this spec and the reference implementation probably will not
276 incorporate features that interfere with the core use cases.
280 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
281 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
282 document are to be interpreted as described in RFC 2119. However, the
283 document could use a serious audit to be sure it makes sense to do
284 so. Also, they are not capitalized.
287 <sect2 id="stability">
288 <title>Protocol and Specification Stability</title>
290 The D-Bus protocol is frozen (only compatible extensions are allowed) as
291 of November 8, 2006. However, this specification could still use a fair
292 bit of work to make interoperable reimplementation possible without
293 reference to the D-Bus reference implementation. Thus, this
294 specification is not marked 1.0. To mark it 1.0, we'd like to see
295 someone invest significant effort in clarifying the specification
296 language, and growing the specification to cover more aspects of the
297 reference implementation's behavior.
300 Until this work is complete, any attempt to reimplement D-Bus will
301 probably require looking at the reference implementation and/or asking
302 questions on the D-Bus mailing list about intended behavior.
303 Questions on the list are very welcome.
306 Nonetheless, this document should be a useful starting point and is
307 to our knowledge accurate, though incomplete.
313 <sect1 id="type-system">
314 <title>Type System</title>
317 D-Bus has a type system, in which values of various types can be
318 serialized into a sequence of bytes referred to as the
319 <firstterm>wire format</firstterm> in a standard way.
320 Converting a value from some other representation into the wire
321 format is called <firstterm>marshaling</firstterm> and converting
322 it back from the wire format is <firstterm>unmarshaling</firstterm>.
326 The D-Bus protocol does not include type tags in the marshaled data; a
327 block of marshaled values must have a known <firstterm>type
328 signature</firstterm>. The type signature is made up of zero or more
329 <firstterm id="term-single-complete-type">single complete
330 types</firstterm>, each made up of one or more
331 <firstterm>type codes</firstterm>.
335 A type code is an ASCII character representing the
336 type of a value. Because ASCII characters are used, the type signature
337 will always form a valid ASCII string. A simple string compare
338 determines whether two type signatures are equivalent.
342 A single complete type is a sequence of type codes that fully describes
343 one type: either a basic type, or a single fully-described container type.
344 A single complete type is a basic type code, a variant type code,
345 an array with its element type, or a struct with its fields (all of which
346 are defined below). So the following signatures are not single complete
357 And the following signatures contain multiple complete types:
367 Note however that a single complete type may <emphasis>contain</emphasis>
368 multiple other single complete types, by containing a struct or dict
372 <sect2 id="basic-types">
373 <title>Basic types</title>
376 The simplest type codes are the <firstterm id="term-basic-type">basic
377 types</firstterm>, which are the types whose structure is entirely
378 defined by their 1-character type code. Basic types consist of
379 fixed types and string-like types.
383 The <firstterm id="term-fixed-type">fixed types</firstterm>
384 are basic types whose values have a fixed length, namely BYTE,
385 BOOLEAN, DOUBLE, UNIX_FD, and signed or unsigned integers of length
390 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
391 the ASCII character 'i'. So the signature for a block of values
392 containing a single <literal>INT32</literal> would be:
396 A block of values containing two <literal>INT32</literal> would have this signature:
403 The characteristics of the fixed types are listed in this table.
409 <entry>Conventional name</entry>
410 <entry>ASCII type-code</entry>
411 <entry>Encoding</entry>
416 <entry><literal>BYTE</literal></entry>
417 <entry><literal>y</literal> (121)</entry>
418 <entry>Unsigned 8-bit integer</entry>
421 <entry><literal>BOOLEAN</literal></entry>
422 <entry><literal>b</literal> (98)</entry>
423 <entry>Boolean value: 0 is false, 1 is true, any other value
424 allowed by the marshalling format is invalid</entry>
427 <entry><literal>INT16</literal></entry>
428 <entry><literal>n</literal> (110)</entry>
429 <entry>Signed (two's complement) 16-bit integer</entry>
432 <entry><literal>UINT16</literal></entry>
433 <entry><literal>q</literal> (113)</entry>
434 <entry>Unsigned 16-bit integer</entry>
437 <entry><literal>INT32</literal></entry>
438 <entry><literal>i</literal> (105)</entry>
439 <entry>Signed (two's complement) 32-bit integer</entry>
442 <entry><literal>UINT32</literal></entry>
443 <entry><literal>u</literal> (117)</entry>
444 <entry>Unsigned 32-bit integer</entry>
447 <entry><literal>INT64</literal></entry>
448 <entry><literal>x</literal> (120)</entry>
449 <entry>Signed (two's complement) 64-bit integer
450 (mnemonic: x and t are the first characters in "sixty" not
451 already used for something more common)</entry>
454 <entry><literal>UINT64</literal></entry>
455 <entry><literal>t</literal> (116)</entry>
456 <entry>Unsigned 64-bit integer</entry>
459 <entry><literal>DOUBLE</literal></entry>
460 <entry><literal>d</literal> (100)</entry>
461 <entry>IEEE 754 double-precision floating point</entry>
464 <entry><literal>UNIX_FD</literal></entry>
465 <entry><literal>h</literal> (104)</entry>
466 <entry>Unsigned 32-bit integer representing an index into an
467 out-of-band array of file descriptors, transferred via some
468 platform-specific mechanism (mnemonic: h for handle)</entry>
476 The <firstterm id="term-string-like-type">string-like types</firstterm>
477 are basic types with a variable length. The value of any string-like
478 type is conceptually 0 or more Unicode codepoints encoded in UTF-8,
479 none of which may be U+0000. The UTF-8 text must be validated
480 strictly: in particular, it must not contain overlong sequences
481 or codepoints above U+10FFFF.
485 Since D-Bus Specification version 0.21, in accordance with Unicode
486 Corrigendum #9, the "noncharacters" U+FDD0..U+FDEF, U+nFFFE and
487 U+nFFFF are allowed in UTF-8 strings (but note that older versions of
488 D-Bus rejected these noncharacters).
492 The marshalling formats for the string-like types all end with a
493 single zero (NUL) byte, but that byte is not considered to be part of
498 The characteristics of the string-like types are listed in this table.
504 <entry>Conventional name</entry>
505 <entry>ASCII type-code</entry>
506 <entry>Validity constraints</entry>
511 <entry><literal>STRING</literal></entry>
512 <entry><literal>s</literal> (115)</entry>
513 <entry>No extra constraints</entry>
516 <entry><literal>OBJECT_PATH</literal></entry>
517 <entry><literal>o</literal> (111)</entry>
519 <link linkend="message-protocol-marshaling-object-path">a
520 syntactically valid object path</link></entry>
523 <entry><literal>SIGNATURE</literal></entry>
524 <entry><literal>g</literal> (103)</entry>
526 <firstterm linkend="term-single-complete-type">single
527 complete types</firstterm></entry>
534 <sect3 id="message-protocol-marshaling-object-path">
535 <title>Valid Object Paths</title>
538 An object path is a name used to refer to an object instance.
539 Conceptually, each participant in a D-Bus message exchange may have
540 any number of object instances (think of C++ or Java objects) and each
541 such instance will have a path. Like a filesystem, the object
542 instances in an application form a hierarchical tree.
546 Object paths are often namespaced by starting with a reversed
547 domain name and containing an interface version number, in the
549 <link linkend="message-protocol-names-interface">interface
551 <link linkend="message-protocol-names-bus">well-known
553 This makes it possible to implement more than one service, or
554 more than one version of a service, in the same process,
555 even if the services share a connection but cannot otherwise
556 co-operate (for instance, if they are implemented by different
561 For instance, if the owner of <literal>example.com</literal> is
562 developing a D-Bus API for a music player, they might use the
563 hierarchy of object paths that start with
564 <literal>/com/example/MusicPlayer1</literal> for its objects.
568 The following rules define a valid object path. Implementations must
569 not send or accept messages with invalid object paths.
573 The path may be of any length.
578 The path must begin with an ASCII '/' (integer 47) character,
579 and must consist of elements separated by slash characters.
584 Each element must only contain the ASCII characters
590 No element may be the empty string.
595 Multiple '/' characters cannot occur in sequence.
600 A trailing '/' character is not allowed unless the
601 path is the root path (a single '/' character).
609 <sect3 id="message-protocol-marshaling-signature">
610 <title>Valid Signatures</title>
612 An implementation must not send or accept invalid signatures.
613 Valid signatures will conform to the following rules:
617 The signature is a list of single complete types.
618 Arrays must have element types, and structs must
619 have both open and close parentheses.
624 Only type codes, open and close parentheses, and open and
625 close curly brackets are allowed in the signature. The
626 <literal>STRUCT</literal> type code
627 is not allowed in signatures, because parentheses
628 are used instead. Similarly, the
629 <literal>DICT_ENTRY</literal> type code is not allowed in
630 signatures, because curly brackets are used instead.
635 The maximum depth of container type nesting is 32 array type
636 codes and 32 open parentheses. This implies that the maximum
637 total depth of recursion is 64, for an "array of array of array
638 of ... struct of struct of struct of ..." where there are 32
644 The maximum length of a signature is 255.
651 When signatures appear in messages, the marshalling format
652 guarantees that they will be followed by a nul byte (which can
653 be interpreted as either C-style string termination or the INVALID
654 type-code), but this is not conceptually part of the signature.
660 <sect2 id="container-types">
661 <title>Container types</title>
664 In addition to basic types, there are four <firstterm>container</firstterm>
665 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
666 and <literal>DICT_ENTRY</literal>.
670 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
671 code does not appear in signatures. Instead, ASCII characters
672 '(' and ')' are used to mark the beginning and end of the struct.
673 So for example, a struct containing two integers would have this
678 Structs can be nested, so for example a struct containing
679 an integer and another struct:
683 The value block storing that struct would contain three integers; the
684 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
689 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
690 but is useful in code that implements the protocol. This type code
691 is specified to allow such code to interoperate in non-protocol contexts.
695 Empty structures are not allowed; there must be at least one
696 type code between the parentheses.
700 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
701 followed by a <firstterm>single complete type</firstterm>. The single
702 complete type following the array is the type of each array element. So
703 the simple example is:
707 which is an array of 32-bit integers. But an array can be of any type,
708 such as this array-of-struct-with-two-int32-fields:
712 Or this array of array of integer:
719 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
720 type <literal>VARIANT</literal> will have the signature of a single complete type as part
721 of the <emphasis>value</emphasis>. This signature will be followed by a
722 marshaled value of that type.
726 Unlike a message signature, the variant signature can
727 contain only a single complete type. So "i", "ai"
728 or "(ii)" is OK, but "ii" is not. Use of variants may not
729 cause a total message depth to be larger than 64, including
730 other container types such as structures.
734 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
735 than parentheses it uses curly braces, and it has more restrictions.
736 The restrictions are: it occurs only as an array element type; it has
737 exactly two single complete types inside the curly braces; the first
738 single complete type (the "key") must be a basic type rather than a
739 container type. Implementations must not accept dict entries outside of
740 arrays, must not accept dict entries with zero, one, or more than two
741 fields, and must not accept dict entries with non-basic-typed keys. A
742 dict entry is always a key-value pair.
746 The first field in the <literal>DICT_ENTRY</literal> is always the key.
747 A message is considered corrupt if the same key occurs twice in the same
748 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
749 implementations are not required to reject dicts with duplicate keys.
753 In most languages, an array of dict entry would be represented as a
754 map, hash table, or dict object.
759 <title>Summary of types</title>
762 The following table summarizes the D-Bus types.
767 <entry>Conventional Name</entry>
769 <entry>Description</entry>
774 <entry><literal>INVALID</literal></entry>
775 <entry>0 (ASCII NUL)</entry>
776 <entry>Not a valid type code, used to terminate signatures</entry>
778 <entry><literal>BYTE</literal></entry>
779 <entry>121 (ASCII 'y')</entry>
780 <entry>8-bit unsigned integer</entry>
782 <entry><literal>BOOLEAN</literal></entry>
783 <entry>98 (ASCII 'b')</entry>
784 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
786 <entry><literal>INT16</literal></entry>
787 <entry>110 (ASCII 'n')</entry>
788 <entry>16-bit signed integer</entry>
790 <entry><literal>UINT16</literal></entry>
791 <entry>113 (ASCII 'q')</entry>
792 <entry>16-bit unsigned integer</entry>
794 <entry><literal>INT32</literal></entry>
795 <entry>105 (ASCII 'i')</entry>
796 <entry>32-bit signed integer</entry>
798 <entry><literal>UINT32</literal></entry>
799 <entry>117 (ASCII 'u')</entry>
800 <entry>32-bit unsigned integer</entry>
802 <entry><literal>INT64</literal></entry>
803 <entry>120 (ASCII 'x')</entry>
804 <entry>64-bit signed integer</entry>
806 <entry><literal>UINT64</literal></entry>
807 <entry>116 (ASCII 't')</entry>
808 <entry>64-bit unsigned integer</entry>
810 <entry><literal>DOUBLE</literal></entry>
811 <entry>100 (ASCII 'd')</entry>
812 <entry>IEEE 754 double</entry>
814 <entry><literal>STRING</literal></entry>
815 <entry>115 (ASCII 's')</entry>
816 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
818 <entry><literal>OBJECT_PATH</literal></entry>
819 <entry>111 (ASCII 'o')</entry>
820 <entry>Name of an object instance</entry>
822 <entry><literal>SIGNATURE</literal></entry>
823 <entry>103 (ASCII 'g')</entry>
824 <entry>A type signature</entry>
826 <entry><literal>ARRAY</literal></entry>
827 <entry>97 (ASCII 'a')</entry>
830 <entry><literal>STRUCT</literal></entry>
831 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
832 <entry>Struct; type code 114 'r' is reserved for use in
833 bindings and implementations to represent the general
834 concept of a struct, and must not appear in signatures
835 used on D-Bus.</entry>
837 <entry><literal>VARIANT</literal></entry>
838 <entry>118 (ASCII 'v') </entry>
839 <entry>Variant type (the type of the value is part of the value itself)</entry>
841 <entry><literal>DICT_ENTRY</literal></entry>
842 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
843 <entry>Entry in a dict or map (array of key-value pairs).
844 Type code 101 'e' is reserved for use in bindings and
845 implementations to represent the general concept of a
846 dict or dict-entry, and must not appear in signatures
847 used on D-Bus.</entry>
849 <entry><literal>UNIX_FD</literal></entry>
850 <entry>104 (ASCII 'h')</entry>
851 <entry>Unix file descriptor</entry>
854 <entry>(reserved)</entry>
855 <entry>109 (ASCII 'm')</entry>
856 <entry>Reserved for <ulink
857 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
858 'maybe' type compatible with the one in GVariant</ulink>,
859 and must not appear in signatures used on D-Bus until
860 specified here</entry>
863 <entry>(reserved)</entry>
864 <entry>42 (ASCII '*')</entry>
865 <entry>Reserved for use in bindings/implementations to
866 represent any <firstterm>single complete type</firstterm>,
867 and must not appear in signatures used on D-Bus.</entry>
870 <entry>(reserved)</entry>
871 <entry>63 (ASCII '?')</entry>
872 <entry>Reserved for use in bindings/implementations to
873 represent any <firstterm>basic type</firstterm>, and must
874 not appear in signatures used on D-Bus.</entry>
877 <entry>(reserved)</entry>
878 <entry>64 (ASCII '@'), 38 (ASCII '&'),
879 94 (ASCII '^')</entry>
880 <entry>Reserved for internal use by bindings/implementations,
881 and must not appear in signatures used on D-Bus.
882 GVariant uses these type-codes to encode calling
893 <sect1 id="message-protocol-marshaling">
894 <title>Marshaling (Wire Format)</title>
897 D-Bus defines a marshalling format for its type system, which is
898 used in D-Bus messages. This is not the only possible marshalling
899 format for the type system: for instance, GVariant (part of GLib)
900 re-uses the D-Bus type system but implements an alternative marshalling
905 <title>Byte order and alignment</title>
908 Given a type signature, a block of bytes can be converted into typed
909 values. This section describes the format of the block of bytes. Byte
910 order and alignment issues are handled uniformly for all D-Bus types.
914 A block of bytes has an associated byte order. The byte order
915 has to be discovered in some way; for D-Bus messages, the
916 byte order is part of the message header as described in
917 <xref linkend="message-protocol-messages"/>. For now, assume
918 that the byte order is known to be either little endian or big
923 Each value in a block of bytes is aligned "naturally," for example
924 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
925 8-byte boundary. To properly align a value, <firstterm>alignment
926 padding</firstterm> may be necessary. The alignment padding must always
927 be the minimum required padding to properly align the following value;
928 and it must always be made up of nul bytes. The alignment padding must
929 not be left uninitialized (it can't contain garbage), and more padding
930 than required must not be used.
934 As an exception to natural alignment, <literal>STRUCT</literal> and
935 <literal>DICT_ENTRY</literal> values are always aligned to an 8-byte
936 boundary, regardless of the alignments of their contents.
941 <title>Marshalling basic types</title>
944 To marshal and unmarshal fixed types, you simply read one value
945 from the data block corresponding to each type code in the signature.
946 All signed integer values are encoded in two's complement, DOUBLE
947 values are IEEE 754 double-precision floating-point, and BOOLEAN
948 values are encoded in 32 bits (of which only the least significant
953 The string-like types are all marshalled as a
954 fixed-length unsigned integer <varname>n</varname> giving the
955 length of the variable part, followed by <varname>n</varname>
956 nonzero bytes of UTF-8 text, followed by a single zero (nul) byte
957 which is not considered to be part of the text. The alignment
958 of the string-like type is the same as the alignment of
959 <varname>n</varname>.
963 For the STRING and OBJECT_PATH types, <varname>n</varname> is
964 encoded in 4 bytes, leading to 4-byte alignment.
965 For the SIGNATURE type, <varname>n</varname> is encoded as a single
966 byte. As a result, alignment padding is never required before a
972 <title>Marshalling containers</title>
975 Arrays are marshalled as a <literal>UINT32</literal>
976 <varname>n</varname> giving the length of the array data in bytes,
977 followed by alignment padding to the alignment boundary of the array
978 element type, followed by the <varname>n</varname> bytes of the
979 array elements marshalled in sequence. <varname>n</varname> does not
980 include the padding after the length, or any padding after the
985 For instance, if the current position in the message is a multiple
986 of 8 bytes and the byte-order is big-endian, an array containing only
987 the 64-bit integer 5 would be marshalled as:
990 00 00 00 08 <lineannotation>8 bytes of data</lineannotation>
991 00 00 00 00 <lineannotation>padding to 8-byte boundary</lineannotation>
992 00 00 00 00 00 00 00 05 <lineannotation>first element = 5</lineannotation>
997 Arrays have a maximum length defined to be 2 to the 26th power or
998 67108864. Implementations must not send or accept arrays exceeding this
1003 Structs and dict entries are marshalled in the same way as their
1004 contents, but their alignment is always to an 8-byte boundary,
1005 even if their contents would normally be less strictly aligned.
1009 Variants are marshalled as the <literal>SIGNATURE</literal> of
1010 the contents (which must be a single complete type), followed by a
1011 marshalled value with the type given by that signature. The
1012 variant has the same 1-byte alignment as the signature, which means
1013 that alignment padding before a variant is never needed.
1014 Use of variants may not cause a total message depth to be larger
1015 than 64, including other container types such as structures.
1020 <title>Summary of D-Bus marshalling</title>
1023 Given all this, the types are marshaled on the wire as follows:
1028 <entry>Conventional Name</entry>
1029 <entry>Encoding</entry>
1030 <entry>Alignment</entry>
1035 <entry><literal>INVALID</literal></entry>
1036 <entry>Not applicable; cannot be marshaled.</entry>
1039 <entry><literal>BYTE</literal></entry>
1040 <entry>A single 8-bit byte.</entry>
1043 <entry><literal>BOOLEAN</literal></entry>
1044 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
1047 <entry><literal>INT16</literal></entry>
1048 <entry>16-bit signed integer in the message's byte order.</entry>
1051 <entry><literal>UINT16</literal></entry>
1052 <entry>16-bit unsigned integer in the message's byte order.</entry>
1055 <entry><literal>INT32</literal></entry>
1056 <entry>32-bit signed integer in the message's byte order.</entry>
1059 <entry><literal>UINT32</literal></entry>
1060 <entry>32-bit unsigned integer in the message's byte order.</entry>
1063 <entry><literal>INT64</literal></entry>
1064 <entry>64-bit signed integer in the message's byte order.</entry>
1067 <entry><literal>UINT64</literal></entry>
1068 <entry>64-bit unsigned integer in the message's byte order.</entry>
1071 <entry><literal>DOUBLE</literal></entry>
1072 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
1075 <entry><literal>STRING</literal></entry>
1076 <entry>A <literal>UINT32</literal> indicating the string's
1077 length in bytes excluding its terminating nul, followed by
1078 non-nul string data of the given length, followed by a terminating nul
1085 <entry><literal>OBJECT_PATH</literal></entry>
1086 <entry>Exactly the same as <literal>STRING</literal> except the
1087 content must be a valid object path (see above).
1093 <entry><literal>SIGNATURE</literal></entry>
1094 <entry>The same as <literal>STRING</literal> except the length is a single
1095 byte (thus signatures have a maximum length of 255)
1096 and the content must be a valid signature (see above).
1102 <entry><literal>ARRAY</literal></entry>
1104 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
1105 alignment padding to the alignment boundary of the array element type,
1106 followed by each array element.
1112 <entry><literal>STRUCT</literal></entry>
1114 A struct must start on an 8-byte boundary regardless of the
1115 type of the struct fields. The struct value consists of each
1116 field marshaled in sequence starting from that 8-byte
1123 <entry><literal>VARIANT</literal></entry>
1125 The marshaled <literal>SIGNATURE</literal> of a single
1126 complete type, followed by a marshaled value with the type
1127 given in the signature.
1130 1 (alignment of the signature)
1133 <entry><literal>DICT_ENTRY</literal></entry>
1135 Identical to STRUCT.
1141 <entry><literal>UNIX_FD</literal></entry>
1142 <entry>32-bit unsigned integer in the message's byte
1143 order. The actual file descriptors need to be
1144 transferred out-of-band via some platform specific
1145 mechanism. On the wire, values of this type store the index to the
1146 file descriptor in the array of file descriptors that
1147 accompany the message.</entry>
1159 <sect1 id="message-protocol">
1160 <title>Message Protocol</title>
1163 A <firstterm>message</firstterm> consists of a
1164 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
1165 think of a message as a package, the header is the address, and the body
1166 contains the package contents. The message delivery system uses the header
1167 information to figure out where to send the message and how to interpret
1168 it; the recipient interprets the body of the message.
1172 The body of the message is made up of zero or more
1173 <firstterm>arguments</firstterm>, which are typed values, such as an
1174 integer or a byte array.
1178 Both header and body use the D-Bus <link linkend="type-system">type
1179 system</link> and format for serializing data.
1182 <sect2 id="message-protocol-messages">
1183 <title>Message Format</title>
1186 A message consists of a header and a body. The header is a block of
1187 values with a fixed signature and meaning. The body is a separate block
1188 of values, with a signature specified in the header.
1192 The length of the header must be a multiple of 8, allowing the body to
1193 begin on an 8-byte boundary when storing the entire message in a single
1194 buffer. If the header does not naturally end on an 8-byte boundary
1195 up to 7 bytes of nul-initialized alignment padding must be added.
1199 The message body need not end on an 8-byte boundary.
1203 The maximum length of a message, including header, header alignment padding,
1204 and body is 2 to the 27th power or 134217728. Implementations must not
1205 send or accept messages exceeding this size.
1209 The signature of the header is:
1213 Written out more readably, this is:
1215 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
1220 These values have the following meanings:
1225 <entry>Value</entry>
1226 <entry>Description</entry>
1231 <entry>1st <literal>BYTE</literal></entry>
1232 <entry>Endianness flag; ASCII 'l' for little-endian
1233 or ASCII 'B' for big-endian. Both header and body are
1234 in this endianness.</entry>
1237 <entry>2nd <literal>BYTE</literal></entry>
1238 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
1239 Currently-defined types are described below.
1243 <entry>3rd <literal>BYTE</literal></entry>
1244 <entry>Bitwise OR of flags. Unknown flags
1245 must be ignored. Currently-defined flags are described below.
1249 <entry>4th <literal>BYTE</literal></entry>
1250 <entry>Major protocol version of the sending application. If
1251 the major protocol version of the receiving application does not
1252 match, the applications will not be able to communicate and the
1253 D-Bus connection must be disconnected. The major protocol
1254 version for this version of the specification is 1.
1258 <entry>1st <literal>UINT32</literal></entry>
1259 <entry>Length in bytes of the message body, starting
1260 from the end of the header. The header ends after
1261 its alignment padding to an 8-boundary.
1265 <entry>2nd <literal>UINT32</literal></entry>
1266 <entry>The serial of this message, used as a cookie
1267 by the sender to identify the reply corresponding
1268 to this request. This must not be zero.
1272 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
1273 <entry>An array of zero or more <firstterm>header
1274 fields</firstterm> where the byte is the field code, and the
1275 variant is the field value. The message type determines
1276 which fields are required.
1284 <firstterm>Message types</firstterm> that can appear in the second byte
1290 <entry>Conventional name</entry>
1291 <entry>Decimal value</entry>
1292 <entry>Description</entry>
1297 <entry><literal>INVALID</literal></entry>
1299 <entry>This is an invalid type.</entry>
1302 <entry><literal>METHOD_CALL</literal></entry>
1304 <entry>Method call. This message type may prompt a
1308 <entry><literal>METHOD_RETURN</literal></entry>
1310 <entry>Method reply with returned data.</entry>
1313 <entry><literal>ERROR</literal></entry>
1315 <entry>Error reply. If the first argument exists and is a
1316 string, it is an error message.</entry>
1319 <entry><literal>SIGNAL</literal></entry>
1321 <entry>Signal emission.</entry>
1328 Flags that can appear in the third byte of the header:
1333 <entry>Conventional name</entry>
1334 <entry>Hex value</entry>
1335 <entry>Description</entry>
1340 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1344 This message does not expect method return replies or
1345 error replies, even if it is of a type that can
1346 have a reply; the reply can be omitted as an
1347 optimization. It is compliant with this specification
1348 to return the reply despite this flag, although doing
1349 so on a bus with a non-trivial security policy
1350 (such as the well-known system bus) may result in
1351 access denial messages being logged for the reply.
1354 Note that METHOD_CALL is the only message type currently
1355 defined in this specification that can expect a reply,
1356 so the presence or absence of this flag in the other
1357 three message types that are currently
1358 documented is meaningless: replies to those message
1359 types should not be sent, whether this flag is present
1365 <entry><literal>NO_AUTO_START</literal></entry>
1367 <entry>The bus must not launch an owner
1368 for the destination name in response to this message.
1376 <sect3 id="message-protocol-header-fields">
1377 <title>Header Fields</title>
1380 The array at the end of the header contains <firstterm>header
1381 fields</firstterm>, where each field is a 1-byte field code followed
1382 by a field value. A header must contain the required header fields for
1383 its message type, and zero or more of any optional header
1384 fields. Future versions of this protocol specification may add new
1385 fields. Implementations must ignore fields they do not
1386 understand. Implementations must not invent their own header fields;
1387 only changes to this specification may introduce new header fields.
1391 Again, if an implementation sees a header field code that it does not
1392 expect, it must ignore that field, as it will be part of a new
1393 (but compatible) version of this specification. This also applies
1394 to known header fields appearing in unexpected messages, for
1395 example: if a signal has a reply serial it must be ignored
1396 even though it has no meaning as of this version of the spec.
1400 However, implementations must not send or accept known header fields
1401 with the wrong type stored in the field value. So for example a
1402 message with an <literal>INTERFACE</literal> field of type
1403 <literal>UINT32</literal> would be considered corrupt.
1407 Here are the currently-defined header fields:
1412 <entry>Conventional Name</entry>
1413 <entry>Decimal Code</entry>
1415 <entry>Required In</entry>
1416 <entry>Description</entry>
1421 <entry><literal>INVALID</literal></entry>
1424 <entry>not allowed</entry>
1425 <entry>Not a valid field name (error if it appears in a message)</entry>
1428 <entry><literal>PATH</literal></entry>
1430 <entry><literal>OBJECT_PATH</literal></entry>
1431 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1432 <entry>The object to send a call to,
1433 or the object a signal is emitted from.
1435 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1436 implementations should not send messages with this path,
1437 and the reference implementation of the bus daemon will
1438 disconnect any application that attempts to do so.
1442 <entry><literal>INTERFACE</literal></entry>
1444 <entry><literal>STRING</literal></entry>
1445 <entry><literal>SIGNAL</literal></entry>
1447 The interface to invoke a method call on, or
1448 that a signal is emitted from. Optional for
1449 method calls, required for signals.
1450 The special interface
1451 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1452 implementations should not send messages with this
1453 interface, and the reference implementation of the bus
1454 daemon will disconnect any application that attempts to
1459 <entry><literal>MEMBER</literal></entry>
1461 <entry><literal>STRING</literal></entry>
1462 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1463 <entry>The member, either the method name or signal name.</entry>
1466 <entry><literal>ERROR_NAME</literal></entry>
1468 <entry><literal>STRING</literal></entry>
1469 <entry><literal>ERROR</literal></entry>
1470 <entry>The name of the error that occurred, for errors</entry>
1473 <entry><literal>REPLY_SERIAL</literal></entry>
1475 <entry><literal>UINT32</literal></entry>
1476 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1477 <entry>The serial number of the message this message is a reply
1478 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1481 <entry><literal>DESTINATION</literal></entry>
1483 <entry><literal>STRING</literal></entry>
1484 <entry>optional</entry>
1485 <entry>The name of the connection this message is intended for.
1486 Only used in combination with the message bus, see
1487 <xref linkend="message-bus"/>.</entry>
1490 <entry><literal>SENDER</literal></entry>
1492 <entry><literal>STRING</literal></entry>
1493 <entry>optional</entry>
1494 <entry>Unique name of the sending connection.
1495 The message bus fills in this field so it is reliable; the field is
1496 only meaningful in combination with the message bus.</entry>
1499 <entry><literal>SIGNATURE</literal></entry>
1501 <entry><literal>SIGNATURE</literal></entry>
1502 <entry>optional</entry>
1503 <entry>The signature of the message body.
1504 If omitted, it is assumed to be the
1505 empty signature "" (i.e. the body must be 0-length).</entry>
1508 <entry><literal>UNIX_FDS</literal></entry>
1510 <entry><literal>UINT32</literal></entry>
1511 <entry>optional</entry>
1512 <entry>The number of Unix file descriptors that
1513 accompany the message. If omitted, it is assumed
1514 that no Unix file descriptors accompany the
1515 message. The actual file descriptors need to be
1516 transferred via platform specific mechanism
1517 out-of-band. They must be sent at the same time as
1518 part of the message itself. They may not be sent
1519 before the first byte of the message itself is
1520 transferred or after the last byte of the message
1530 <sect2 id="message-protocol-names">
1531 <title>Valid Names</title>
1533 The various names in D-Bus messages have some restrictions.
1536 There is a <firstterm>maximum name length</firstterm>
1537 of 255 which applies to bus names, interfaces, and members.
1539 <sect3 id="message-protocol-names-interface">
1540 <title>Interface names</title>
1542 Interfaces have names with type <literal>STRING</literal>, meaning that
1543 they must be valid UTF-8. However, there are also some
1544 additional restrictions that apply to interface names
1547 <listitem><para>Interface names are composed of 1 or more elements separated by
1548 a period ('.') character. All elements must contain at least
1552 <listitem><para>Each element must only contain the ASCII characters
1553 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1557 <listitem><para>Interface names must contain at least one '.' (period)
1558 character (and thus at least two elements).
1561 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1562 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1567 Interface names should start with the reversed DNS domain name of
1568 the author of the interface (in lower-case), like interface names
1569 in Java. It is conventional for the rest of the interface name
1570 to consist of words run together, with initial capital letters
1571 on all words ("CamelCase"). Several levels of hierarchy can be used.
1572 It is also a good idea to include the major version of the interface
1573 in the name, and increment it if incompatible changes are made;
1574 this way, a single object can implement several versions of an
1575 interface in parallel, if necessary.
1579 For instance, if the owner of <literal>example.com</literal> is
1580 developing a D-Bus API for a music player, they might define
1581 interfaces called <literal>com.example.MusicPlayer1</literal>,
1582 <literal>com.example.MusicPlayer1.Track</literal> and
1583 <literal>com.example.MusicPlayer1.Seekable</literal>.
1587 D-Bus does not distinguish between the concepts that would be
1588 called classes and interfaces in Java: either can be identified on
1589 D-Bus by an interface name.
1592 <sect3 id="message-protocol-names-bus">
1593 <title>Bus names</title>
1595 Connections have one or more bus names associated with them.
1596 A connection has exactly one bus name that is a <firstterm>unique
1597 connection name</firstterm>. The unique connection name remains
1598 with the connection for its entire lifetime.
1599 A bus name is of type <literal>STRING</literal>,
1600 meaning that it must be valid UTF-8. However, there are also
1601 some additional restrictions that apply to bus names
1604 <listitem><para>Bus names that start with a colon (':')
1605 character are unique connection names. Other bus names
1606 are called <firstterm>well-known bus names</firstterm>.
1609 <listitem><para>Bus names are composed of 1 or more elements separated by
1610 a period ('.') character. All elements must contain at least
1614 <listitem><para>Each element must only contain the ASCII characters
1615 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1616 connection name may begin with a digit, elements in
1617 other bus names must not begin with a digit.
1621 <listitem><para>Bus names must contain at least one '.' (period)
1622 character (and thus at least two elements).
1625 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1626 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1630 Note that the hyphen ('-') character is allowed in bus names but
1631 not in interface names.
1635 Like <link linkend="message-protocol-names-interface">interface
1636 names</link>, well-known bus names should start with the
1637 reversed DNS domain name of the author of the interface (in
1638 lower-case), and it is conventional for the rest of the well-known
1639 bus name to consist of words run together, with initial
1640 capital letters. As with interface names, including a version
1641 number in well-known bus names is a good idea; it's possible to
1642 have the well-known bus name for more than one version
1643 simultaneously if backwards compatibility is required.
1647 If a well-known bus name implies the presence of a "main" interface,
1648 that "main" interface is often given the same name as
1649 the well-known bus name, and situated at the corresponding object
1650 path. For instance, if the owner of <literal>example.com</literal>
1651 is developing a D-Bus API for a music player, they might define
1652 that any application that takes the well-known name
1653 <literal>com.example.MusicPlayer1</literal> should have an object
1654 at the object path <literal>/com/example/MusicPlayer1</literal>
1655 which implements the interface
1656 <literal>com.example.MusicPlayer1</literal>.
1659 <sect3 id="message-protocol-names-member">
1660 <title>Member names</title>
1662 Member (i.e. method or signal) names:
1664 <listitem><para>Must only contain the ASCII characters
1665 "[A-Z][a-z][0-9]_" and may not begin with a
1666 digit.</para></listitem>
1667 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1668 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1669 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1674 It is conventional for member names on D-Bus to consist of
1675 capitalized words with no punctuation ("camel-case").
1676 Method names should usually be verbs, such as
1677 <literal>GetItems</literal>, and signal names should usually be
1678 a description of an event, such as <literal>ItemsChanged</literal>.
1681 <sect3 id="message-protocol-names-error">
1682 <title>Error names</title>
1684 Error names have the same restrictions as interface names.
1688 Error names have the same naming conventions as interface
1689 names, and often contain <literal>.Error.</literal>; for instance,
1690 the owner of <literal>example.com</literal> might define the
1691 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1692 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1693 The errors defined by D-Bus itself, such as
1694 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1700 <sect2 id="message-protocol-types">
1701 <title>Message Types</title>
1703 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1704 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1705 This section describes these conventions.
1707 <sect3 id="message-protocol-types-method">
1708 <title>Method Calls</title>
1710 Some messages invoke an operation on a remote object. These are
1711 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1712 messages map naturally to methods on objects in a typical program.
1715 A method call message is required to have a <literal>MEMBER</literal> header field
1716 indicating the name of the method. Optionally, the message has an
1717 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1718 Including the <literal>INTERFACE</literal> in all method call
1719 messages is strongly recommended.
1722 In the absence of an <literal>INTERFACE</literal> field, if two
1723 or more interfaces on the same object have a method with the same
1724 name, it is undefined which of those methods will be invoked.
1725 Implementations may choose to either return an error, or deliver the
1726 message as though it had an arbitrary one of those interfaces.
1729 In some situations (such as the well-known system bus), messages
1730 are filtered through an access-control list external to the
1731 remote object implementation. If that filter rejects certain
1732 messages by matching their interface, or accepts only messages
1733 to specific interfaces, it must also reject messages that have no
1734 <literal>INTERFACE</literal>: otherwise, malicious
1735 applications could use this to bypass the filter.
1738 Method call messages also include a <literal>PATH</literal> field
1739 indicating the object to invoke the method on. If the call is passing
1740 through a message bus, the message will also have a
1741 <literal>DESTINATION</literal> field giving the name of the connection
1742 to receive the message.
1745 When an application handles a method call message, it is required to
1746 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1747 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1748 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1751 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1752 are the return value(s) or "out parameters" of the method call.
1753 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1754 and the call fails; no return value will be provided. It makes
1755 no sense to send multiple replies to the same method call.
1758 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1759 reply is required, so the caller will know the method
1760 was successfully processed.
1763 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1767 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1768 then as an optimization the application receiving the method
1769 call may choose to omit the reply message (regardless of
1770 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1771 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1772 flag and reply anyway.
1775 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1776 destination name does not exist then a program to own the destination
1777 name will be started before the message is delivered. The message
1778 will be held until the new program is successfully started or has
1779 failed to start; in case of failure, an error will be returned. This
1780 flag is only relevant in the context of a message bus, it is ignored
1781 during one-to-one communication with no intermediate bus.
1783 <sect4 id="message-protocol-types-method-apis">
1784 <title>Mapping method calls to native APIs</title>
1786 APIs for D-Bus may map method calls to a method call in a specific
1787 programming language, such as C++, or may map a method call written
1788 in an IDL to a D-Bus message.
1791 In APIs of this nature, arguments to a method are often termed "in"
1792 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1793 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1794 "inout" arguments, which are both sent and received, i.e. the caller
1795 passes in a value which is modified. Mapped to D-Bus, an "inout"
1796 argument is equivalent to an "in" argument, followed by an "out"
1797 argument. You can't pass things "by reference" over the wire, so
1798 "inout" is purely an illusion of the in-process API.
1801 Given a method with zero or one return values, followed by zero or more
1802 arguments, where each argument may be "in", "out", or "inout", the
1803 caller constructs a message by appending each "in" or "inout" argument,
1804 in order. "out" arguments are not represented in the caller's message.
1807 The recipient constructs a reply by appending first the return value
1808 if any, then each "out" or "inout" argument, in order.
1809 "in" arguments are not represented in the reply message.
1812 Error replies are normally mapped to exceptions in languages that have
1816 In converting from native APIs to D-Bus, it is perhaps nice to
1817 map D-Bus naming conventions ("FooBar") to native conventions
1818 such as "fooBar" or "foo_bar" automatically. This is OK
1819 as long as you can say that the native API is one that
1820 was specifically written for D-Bus. It makes the most sense
1821 when writing object implementations that will be exported
1822 over the bus. Object proxies used to invoke remote D-Bus
1823 objects probably need the ability to call any D-Bus method,
1824 and thus a magic name mapping like this could be a problem.
1827 This specification doesn't require anything of native API bindings;
1828 the preceding is only a suggested convention for consistency
1834 <sect3 id="message-protocol-types-signal">
1835 <title>Signal Emission</title>
1837 Unlike method calls, signal emissions have no replies.
1838 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1839 It must have three header fields: <literal>PATH</literal> giving the object
1840 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1841 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1842 for signals, though it is optional for method calls.
1846 <sect3 id="message-protocol-types-errors">
1847 <title>Errors</title>
1849 Messages of type <literal>ERROR</literal> are most commonly replies
1850 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1851 to any kind of message. The message bus for example
1852 will return an <literal>ERROR</literal> in reply to a signal emission if
1853 the bus does not have enough memory to send the signal.
1856 An <literal>ERROR</literal> may have any arguments, but if the first
1857 argument is a <literal>STRING</literal>, it must be an error message.
1858 The error message may be logged or shown to the user
1863 <sect3 id="message-protocol-types-notation">
1864 <title>Notation in this document</title>
1866 This document uses a simple pseudo-IDL to describe particular method
1867 calls and signals. Here is an example of a method call:
1869 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1870 out UINT32 resultcode)
1872 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1873 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1874 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1875 characters so it's known that the last part of the name in
1876 the "IDL" is the member name.
1879 In C++ that might end up looking like this:
1881 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1882 unsigned int flags);
1884 or equally valid, the return value could be done as an argument:
1886 void org::freedesktop::DBus::StartServiceByName (const char *name,
1888 unsigned int *resultcode);
1890 It's really up to the API designer how they want to make
1891 this look. You could design an API where the namespace wasn't used
1892 in C++, using STL or Qt, using varargs, or whatever you wanted.
1895 Signals are written as follows:
1897 org.freedesktop.DBus.NameLost (STRING name)
1899 Signals don't specify "in" vs. "out" because only
1900 a single direction is possible.
1903 It isn't especially encouraged to use this lame pseudo-IDL in actual
1904 API implementations; you might use the native notation for the
1905 language you're using, or you might use COM or CORBA IDL, for example.
1910 <sect2 id="message-protocol-handling-invalid">
1911 <title>Invalid Protocol and Spec Extensions</title>
1914 For security reasons, the D-Bus protocol should be strictly parsed and
1915 validated, with the exception of defined extension points. Any invalid
1916 protocol or spec violations should result in immediately dropping the
1917 connection without notice to the other end. Exceptions should be
1918 carefully considered, e.g. an exception may be warranted for a
1919 well-understood idiosyncrasy of a widely-deployed implementation. In
1920 cases where the other end of a connection is 100% trusted and known to
1921 be friendly, skipping validation for performance reasons could also make
1922 sense in certain cases.
1926 Generally speaking violations of the "must" requirements in this spec
1927 should be considered possible attempts to exploit security, and violations
1928 of the "should" suggestions should be considered legitimate (though perhaps
1929 they should generate an error in some cases).
1933 The following extension points are built in to D-Bus on purpose and must
1934 not be treated as invalid protocol. The extension points are intended
1935 for use by future versions of this spec, they are not intended for third
1936 parties. At the moment, the only way a third party could extend D-Bus
1937 without breaking interoperability would be to introduce a way to negotiate new
1938 feature support as part of the auth protocol, using EXTENSION_-prefixed
1939 commands. There is not yet a standard way to negotiate features.
1943 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1944 commands result in an ERROR rather than a disconnect. This enables
1945 future extensions to the protocol. Commands starting with EXTENSION_ are
1946 reserved for third parties.
1951 The authentication protocol supports pluggable auth mechanisms.
1956 The address format (see <xref linkend="addresses"/>) supports new
1962 Messages with an unknown type (something other than
1963 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1964 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1965 Unknown-type messages must still be well-formed in the same way
1966 as the known messages, however. They still have the normal
1972 Header fields with an unknown or unexpected field code must be ignored,
1973 though again they must still be well-formed.
1978 New standard interfaces (with new methods and signals) can of course be added.
1988 <sect1 id="auth-protocol">
1989 <title>Authentication Protocol</title>
1991 Before the flow of messages begins, two applications must
1992 authenticate. A simple plain-text protocol is used for
1993 authentication; this protocol is a SASL profile, and maps fairly
1994 directly from the SASL specification. The message encoding is
1995 NOT used here, only plain text messages.
1998 In examples, "C:" and "S:" indicate lines sent by the client and
1999 server respectively.
2001 <sect2 id="auth-protocol-overview">
2002 <title>Protocol Overview</title>
2004 The protocol is a line-based protocol, where each line ends with
2005 \r\n. Each line begins with an all-caps ASCII command name containing
2006 only the character range [A-Z_], a space, then any arguments for the
2007 command, then the \r\n ending the line. The protocol is
2008 case-sensitive. All bytes must be in the ASCII character set.
2010 Commands from the client to the server are as follows:
2013 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
2014 <listitem><para>CANCEL</para></listitem>
2015 <listitem><para>BEGIN</para></listitem>
2016 <listitem><para>DATA <data in hex encoding></para></listitem>
2017 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2018 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2021 From server to client are as follows:
2024 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2025 <listitem><para>OK <GUID in hex></para></listitem>
2026 <listitem><para>DATA <data in hex encoding></para></listitem>
2027 <listitem><para>ERROR</para></listitem>
2028 <listitem><para>AGREE_UNIX_FD</para></listitem>
2032 Unofficial extensions to the command set must begin with the letters
2033 "EXTENSION_", to avoid conflicts with future official commands.
2034 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2037 <sect2 id="auth-nul-byte">
2038 <title>Special credentials-passing nul byte</title>
2040 Immediately after connecting to the server, the client must send a
2041 single nul byte. This byte may be accompanied by credentials
2042 information on some operating systems that use sendmsg() with
2043 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2044 sockets. However, the nul byte must be sent even on other kinds of
2045 socket, and even on operating systems that do not require a byte to be
2046 sent in order to transmit credentials. The text protocol described in
2047 this document begins after the single nul byte. If the first byte
2048 received from the client is not a nul byte, the server may disconnect
2052 A nul byte in any context other than the initial byte is an error;
2053 the protocol is ASCII-only.
2056 The credentials sent along with the nul byte may be used with the
2057 SASL mechanism EXTERNAL.
2060 <sect2 id="auth-command-auth">
2061 <title>AUTH command</title>
2063 If an AUTH command has no arguments, it is a request to list
2064 available mechanisms. The server must respond with a REJECTED
2065 command listing the mechanisms it understands, or with an error.
2068 If an AUTH command specifies a mechanism, and the server supports
2069 said mechanism, the server should begin exchanging SASL
2070 challenge-response data with the client using DATA commands.
2073 If the server does not support the mechanism given in the AUTH
2074 command, it must send either a REJECTED command listing the mechanisms
2075 it does support, or an error.
2078 If the [initial-response] argument is provided, it is intended for use
2079 with mechanisms that have no initial challenge (or an empty initial
2080 challenge), as if it were the argument to an initial DATA command. If
2081 the selected mechanism has an initial challenge and [initial-response]
2082 was provided, the server should reject authentication by sending
2086 If authentication succeeds after exchanging DATA commands,
2087 an OK command must be sent to the client.
2090 The first octet received by the server after the \r\n of the BEGIN
2091 command from the client must be the first octet of the
2092 authenticated/encrypted stream of D-Bus messages.
2095 If BEGIN is received by the server, the first octet received
2096 by the client after the \r\n of the OK command must be the
2097 first octet of the authenticated/encrypted stream of D-Bus
2101 <sect2 id="auth-command-cancel">
2102 <title>CANCEL Command</title>
2104 At any time up to sending the BEGIN command, the client may send a
2105 CANCEL command. On receiving the CANCEL command, the server must
2106 send a REJECTED command and abort the current authentication
2110 <sect2 id="auth-command-data">
2111 <title>DATA Command</title>
2113 The DATA command may come from either client or server, and simply
2114 contains a hex-encoded block of data to be interpreted
2115 according to the SASL mechanism in use.
2118 Some SASL mechanisms support sending an "empty string";
2119 FIXME we need some way to do this.
2122 <sect2 id="auth-command-begin">
2123 <title>BEGIN Command</title>
2125 The BEGIN command acknowledges that the client has received an
2126 OK command from the server, and that the stream of messages
2130 The first octet received by the server after the \r\n of the BEGIN
2131 command from the client must be the first octet of the
2132 authenticated/encrypted stream of D-Bus messages.
2135 <sect2 id="auth-command-rejected">
2136 <title>REJECTED Command</title>
2138 The REJECTED command indicates that the current authentication
2139 exchange has failed, and further exchange of DATA is inappropriate.
2140 The client would normally try another mechanism, or try providing
2141 different responses to challenges.
2143 Optionally, the REJECTED command has a space-separated list of
2144 available auth mechanisms as arguments. If a server ever provides
2145 a list of supported mechanisms, it must provide the same list
2146 each time it sends a REJECTED message. Clients are free to
2147 ignore all lists received after the first.
2150 <sect2 id="auth-command-ok">
2151 <title>OK Command</title>
2153 The OK command indicates that the client has been
2154 authenticated. The client may now proceed with negotiating
2155 Unix file descriptor passing. To do that it shall send
2156 NEGOTIATE_UNIX_FD to the server.
2159 Otherwise, the client must respond to the OK command by
2160 sending a BEGIN command, followed by its stream of messages,
2161 or by disconnecting. The server must not accept additional
2162 commands using this protocol after the BEGIN command has been
2163 received. Further communication will be a stream of D-Bus
2164 messages (optionally encrypted, as negotiated) rather than
2168 If a client sends BEGIN the first octet received by the client
2169 after the \r\n of the OK command must be the first octet of
2170 the authenticated/encrypted stream of D-Bus messages.
2173 The OK command has one argument, which is the GUID of the server.
2174 See <xref linkend="addresses"/> for more on server GUIDs.
2177 <sect2 id="auth-command-error">
2178 <title>ERROR Command</title>
2180 The ERROR command indicates that either server or client did not
2181 know a command, does not accept the given command in the current
2182 context, or did not understand the arguments to the command. This
2183 allows the protocol to be extended; a client or server can send a
2184 command present or permitted only in new protocol versions, and if
2185 an ERROR is received instead of an appropriate response, fall back
2186 to using some other technique.
2189 If an ERROR is sent, the server or client that sent the
2190 error must continue as if the command causing the ERROR had never been
2191 received. However, the the server or client receiving the error
2192 should try something other than whatever caused the error;
2193 if only canceling/rejecting the authentication.
2196 If the D-Bus protocol changes incompatibly at some future time,
2197 applications implementing the new protocol would probably be able to
2198 check for support of the new protocol by sending a new command and
2199 receiving an ERROR from applications that don't understand it. Thus the
2200 ERROR feature of the auth protocol is an escape hatch that lets us
2201 negotiate extensions or changes to the D-Bus protocol in the future.
2204 <sect2 id="auth-command-negotiate-unix-fd">
2205 <title>NEGOTIATE_UNIX_FD Command</title>
2207 The NEGOTIATE_UNIX_FD command indicates that the client
2208 supports Unix file descriptor passing. This command may only
2209 be sent after the connection is authenticated, i.e. after OK
2210 was received by the client. This command may only be sent on
2211 transports that support Unix file descriptor passing.
2214 On receiving NEGOTIATE_UNIX_FD the server must respond with
2215 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2216 the transport chosen supports Unix file descriptor passing and
2217 the server supports this feature. It shall respond the latter
2218 if the transport does not support Unix file descriptor
2219 passing, the server does not support this feature, or the
2220 server decides not to enable file descriptor passing due to
2221 security or other reasons.
2224 <sect2 id="auth-command-agree-unix-fd">
2225 <title>AGREE_UNIX_FD Command</title>
2227 The AGREE_UNIX_FD command indicates that the server supports
2228 Unix file descriptor passing. This command may only be sent
2229 after the connection is authenticated, and the client sent
2230 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2231 command may only be sent on transports that support Unix file
2235 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2236 followed by its stream of messages, or by disconnecting. The
2237 server must not accept additional commands using this protocol
2238 after the BEGIN command has been received. Further
2239 communication will be a stream of D-Bus messages (optionally
2240 encrypted, as negotiated) rather than this protocol.
2243 <sect2 id="auth-command-future">
2244 <title>Future Extensions</title>
2246 Future extensions to the authentication and negotiation
2247 protocol are possible. For that new commands may be
2248 introduced. If a client or server receives an unknown command
2249 it shall respond with ERROR and not consider this fatal. New
2250 commands may be introduced both before, and after
2251 authentication, i.e. both before and after the OK command.
2254 <sect2 id="auth-examples">
2255 <title>Authentication examples</title>
2259 <title>Example of successful magic cookie authentication</title>
2261 (MAGIC_COOKIE is a made up mechanism)
2263 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2269 <title>Example of finding out mechanisms then picking one</title>
2272 S: REJECTED KERBEROS_V4 SKEY
2273 C: AUTH SKEY 7ab83f32ee
2274 S: DATA 8799cabb2ea93e
2275 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2281 <title>Example of client sends unknown command then falls back to regular auth</title>
2285 C: AUTH MAGIC_COOKIE 3736343435313230333039
2291 <title>Example of server doesn't support initial auth mechanism</title>
2293 C: AUTH MAGIC_COOKIE 3736343435313230333039
2294 S: REJECTED KERBEROS_V4 SKEY
2295 C: AUTH SKEY 7ab83f32ee
2296 S: DATA 8799cabb2ea93e
2297 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2303 <title>Example of wrong password or the like followed by successful retry</title>
2305 C: AUTH MAGIC_COOKIE 3736343435313230333039
2306 S: REJECTED KERBEROS_V4 SKEY
2307 C: AUTH SKEY 7ab83f32ee
2308 S: DATA 8799cabb2ea93e
2309 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2311 C: AUTH SKEY 7ab83f32ee
2312 S: DATA 8799cabb2ea93e
2313 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2319 <title>Example of skey cancelled and restarted</title>
2321 C: AUTH MAGIC_COOKIE 3736343435313230333039
2322 S: REJECTED KERBEROS_V4 SKEY
2323 C: AUTH SKEY 7ab83f32ee
2324 S: DATA 8799cabb2ea93e
2327 C: AUTH SKEY 7ab83f32ee
2328 S: DATA 8799cabb2ea93e
2329 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2335 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2337 (MAGIC_COOKIE is a made up mechanism)
2339 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2341 C: NEGOTIATE_UNIX_FD
2347 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2349 (MAGIC_COOKIE is a made up mechanism)
2351 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2353 C: NEGOTIATE_UNIX_FD
2360 <sect2 id="auth-states">
2361 <title>Authentication state diagrams</title>
2364 This section documents the auth protocol in terms of
2365 a state machine for the client and the server. This is
2366 probably the most robust way to implement the protocol.
2369 <sect3 id="auth-states-client">
2370 <title>Client states</title>
2373 To more precisely describe the interaction between the
2374 protocol state machine and the authentication mechanisms the
2375 following notation is used: MECH(CHALL) means that the
2376 server challenge CHALL was fed to the mechanism MECH, which
2382 CONTINUE(RESP) means continue the auth conversation
2383 and send RESP as the response to the server;
2389 OK(RESP) means that after sending RESP to the server
2390 the client side of the auth conversation is finished
2391 and the server should return "OK";
2397 ERROR means that CHALL was invalid and could not be
2403 Both RESP and CHALL may be empty.
2407 The Client starts by getting an initial response from the
2408 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2409 the mechanism did not provide an initial response. If the
2410 mechanism returns CONTINUE, the client starts in state
2411 <emphasis>WaitingForData</emphasis>, if the mechanism
2412 returns OK the client starts in state
2413 <emphasis>WaitingForOK</emphasis>.
2417 The client should keep track of available mechanisms and
2418 which it mechanisms it has already attempted. This list is
2419 used to decide which AUTH command to send. When the list is
2420 exhausted, the client should give up and close the
2425 <title><emphasis>WaitingForData</emphasis></title>
2433 MECH(CHALL) returns CONTINUE(RESP) → send
2435 <emphasis>WaitingForData</emphasis>
2439 MECH(CHALL) returns OK(RESP) → send DATA
2440 RESP, goto <emphasis>WaitingForOK</emphasis>
2444 MECH(CHALL) returns ERROR → send ERROR
2445 [msg], goto <emphasis>WaitingForData</emphasis>
2453 Receive REJECTED [mechs] →
2454 send AUTH [next mech], goto
2455 WaitingForData or <emphasis>WaitingForOK</emphasis>
2460 Receive ERROR → send
2462 <emphasis>WaitingForReject</emphasis>
2467 Receive OK → send
2468 BEGIN, terminate auth
2469 conversation, authenticated
2474 Receive anything else → send
2476 <emphasis>WaitingForData</emphasis>
2484 <title><emphasis>WaitingForOK</emphasis></title>
2489 Receive OK → send BEGIN, terminate auth
2490 conversation, <emphasis>authenticated</emphasis>
2495 Receive REJECTED [mechs] → send AUTH [next mech],
2496 goto <emphasis>WaitingForData</emphasis> or
2497 <emphasis>WaitingForOK</emphasis>
2503 Receive DATA → send CANCEL, goto
2504 <emphasis>WaitingForReject</emphasis>
2510 Receive ERROR → send CANCEL, goto
2511 <emphasis>WaitingForReject</emphasis>
2517 Receive anything else → send ERROR, goto
2518 <emphasis>WaitingForOK</emphasis>
2526 <title><emphasis>WaitingForReject</emphasis></title>
2531 Receive REJECTED [mechs] → send AUTH [next mech],
2532 goto <emphasis>WaitingForData</emphasis> or
2533 <emphasis>WaitingForOK</emphasis>
2539 Receive anything else → terminate auth
2540 conversation, disconnect
2549 <sect3 id="auth-states-server">
2550 <title>Server states</title>
2553 For the server MECH(RESP) means that the client response
2554 RESP was fed to the the mechanism MECH, which returns one of
2559 CONTINUE(CHALL) means continue the auth conversation and
2560 send CHALL as the challenge to the client;
2566 OK means that the client has been successfully
2573 REJECTED means that the client failed to authenticate or
2574 there was an error in RESP.
2579 The server starts out in state
2580 <emphasis>WaitingForAuth</emphasis>. If the client is
2581 rejected too many times the server must disconnect the
2586 <title><emphasis>WaitingForAuth</emphasis></title>
2592 Receive AUTH → send REJECTED [mechs], goto
2593 <emphasis>WaitingForAuth</emphasis>
2599 Receive AUTH MECH RESP
2603 MECH not valid mechanism → send REJECTED
2605 <emphasis>WaitingForAuth</emphasis>
2609 MECH(RESP) returns CONTINUE(CHALL) → send
2611 <emphasis>WaitingForData</emphasis>
2615 MECH(RESP) returns OK → send OK, goto
2616 <emphasis>WaitingForBegin</emphasis>
2620 MECH(RESP) returns REJECTED → send REJECTED
2622 <emphasis>WaitingForAuth</emphasis>
2630 Receive BEGIN → terminate
2631 auth conversation, disconnect
2637 Receive ERROR → send REJECTED [mechs], goto
2638 <emphasis>WaitingForAuth</emphasis>
2644 Receive anything else → send
2646 <emphasis>WaitingForAuth</emphasis>
2655 <title><emphasis>WaitingForData</emphasis></title>
2663 MECH(RESP) returns CONTINUE(CHALL) → send
2665 <emphasis>WaitingForData</emphasis>
2669 MECH(RESP) returns OK → send OK, goto
2670 <emphasis>WaitingForBegin</emphasis>
2674 MECH(RESP) returns REJECTED → send REJECTED
2676 <emphasis>WaitingForAuth</emphasis>
2684 Receive BEGIN → terminate auth conversation,
2691 Receive CANCEL → send REJECTED [mechs], goto
2692 <emphasis>WaitingForAuth</emphasis>
2698 Receive ERROR → send REJECTED [mechs], goto
2699 <emphasis>WaitingForAuth</emphasis>
2705 Receive anything else → send ERROR, goto
2706 <emphasis>WaitingForData</emphasis>
2714 <title><emphasis>WaitingForBegin</emphasis></title>
2719 Receive BEGIN → terminate auth conversation,
2720 client authenticated
2726 Receive CANCEL → send REJECTED [mechs], goto
2727 <emphasis>WaitingForAuth</emphasis>
2733 Receive ERROR → send REJECTED [mechs], goto
2734 <emphasis>WaitingForAuth</emphasis>
2740 Receive anything else → send ERROR, goto
2741 <emphasis>WaitingForBegin</emphasis>
2751 <sect2 id="auth-mechanisms">
2752 <title>Authentication mechanisms</title>
2754 This section describes some new authentication mechanisms.
2755 D-Bus also allows any standard SASL mechanism of course.
2757 <sect3 id="auth-mechanisms-sha">
2758 <title>DBUS_COOKIE_SHA1</title>
2760 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2761 has the ability to read a private file owned by the user being
2762 authenticated. If the client can prove that it has access to a secret
2763 cookie stored in this file, then the client is authenticated.
2764 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2768 Throughout this description, "hex encoding" must output the digits
2769 from a to f in lower-case; the digits A to F must not be used
2770 in the DBUS_COOKIE_SHA1 mechanism.
2773 Authentication proceeds as follows:
2777 The client sends the username it would like to authenticate
2783 The server sends the name of its "cookie context" (see below); a
2784 space character; the integer ID of the secret cookie the client
2785 must demonstrate knowledge of; a space character; then a
2786 randomly-generated challenge string, all of this hex-encoded into
2792 The client locates the cookie and generates its own
2793 randomly-generated challenge string. The client then concatenates
2794 the server's decoded challenge, a ":" character, its own challenge,
2795 another ":" character, and the cookie. It computes the SHA-1 hash
2796 of this composite string as a hex digest. It concatenates the
2797 client's challenge string, a space character, and the SHA-1 hex
2798 digest, hex-encodes the result and sends it back to the server.
2803 The server generates the same concatenated string used by the
2804 client and computes its SHA-1 hash. It compares the hash with
2805 the hash received from the client; if the two hashes match, the
2806 client is authenticated.
2812 Each server has a "cookie context," which is a name that identifies a
2813 set of cookies that apply to that server. A sample context might be
2814 "org_freedesktop_session_bus". Context names must be valid ASCII,
2815 nonzero length, and may not contain the characters slash ("/"),
2816 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2817 tab ("\t"), or period ("."). There is a default context,
2818 "org_freedesktop_general" that's used by servers that do not specify
2822 Cookies are stored in a user's home directory, in the directory
2823 <filename>~/.dbus-keyrings/</filename>. This directory must
2824 not be readable or writable by other users. If it is,
2825 clients and servers must ignore it. The directory
2826 contains cookie files named after the cookie context.
2829 A cookie file contains one cookie per line. Each line
2830 has three space-separated fields:
2834 The cookie ID number, which must be a non-negative integer and
2835 may not be used twice in the same file.
2840 The cookie's creation time, in UNIX seconds-since-the-epoch
2846 The cookie itself, a hex-encoded random block of bytes. The cookie
2847 may be of any length, though obviously security increases
2848 as the length increases.
2854 Only server processes modify the cookie file.
2855 They must do so with this procedure:
2859 Create a lockfile name by appending ".lock" to the name of the
2860 cookie file. The server should attempt to create this file
2861 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2862 fails, the lock fails. Servers should retry for a reasonable
2863 period of time, then they may choose to delete an existing lock
2864 to keep users from having to manually delete a stale
2865 lock. <footnote><para>Lockfiles are used instead of real file
2866 locking <literal>fcntl()</literal> because real locking
2867 implementations are still flaky on network
2868 filesystems.</para></footnote>
2873 Once the lockfile has been created, the server loads the cookie
2874 file. It should then delete any cookies that are old (the
2875 timeout can be fairly short), or more than a reasonable
2876 time in the future (so that cookies never accidentally
2877 become permanent, if the clock was set far into the future
2878 at some point). If no recent keys remain, the
2879 server may generate a new key.
2884 The pruned and possibly added-to cookie file
2885 must be resaved atomically (using a temporary
2886 file which is rename()'d).
2891 The lock must be dropped by deleting the lockfile.
2897 Clients need not lock the file in order to load it,
2898 because servers are required to save the file atomically.
2903 <sect1 id="addresses">
2904 <title>Server Addresses</title>
2906 Server addresses consist of a transport name followed by a colon, and
2907 then an optional, comma-separated list of keys and values in the form key=value.
2908 Each value is escaped.
2912 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2913 Which is the address to a unix socket with the path /tmp/dbus-test.
2916 Value escaping is similar to URI escaping but simpler.
2920 The set of optionally-escaped bytes is:
2921 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2922 <emphasis>byte</emphasis> (note, not character) which is not in the
2923 set of optionally-escaped bytes must be replaced with an ASCII
2924 percent (<literal>%</literal>) and the value of the byte in hex.
2925 The hex value must always be two digits, even if the first digit is
2926 zero. The optionally-escaped bytes may be escaped if desired.
2931 To unescape, append each byte in the value; if a byte is an ASCII
2932 percent (<literal>%</literal>) character then append the following
2933 hex value instead. It is an error if a <literal>%</literal> byte
2934 does not have two hex digits following. It is an error if a
2935 non-optionally-escaped byte is seen unescaped.
2939 The set of optionally-escaped bytes is intended to preserve address
2940 readability and convenience.
2944 A server may specify a key-value pair with the key <literal>guid</literal>
2945 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2946 describes the format of the <literal>guid</literal> field. If present,
2947 this UUID may be used to distinguish one server address from another. A
2948 server should use a different UUID for each address it listens on. For
2949 example, if a message bus daemon offers both UNIX domain socket and TCP
2950 connections, but treats clients the same regardless of how they connect,
2951 those two connections are equivalent post-connection but should have
2952 distinct UUIDs to distinguish the kinds of connection.
2956 The intent of the address UUID feature is to allow a client to avoid
2957 opening multiple identical connections to the same server, by allowing the
2958 client to check whether an address corresponds to an already-existing
2959 connection. Comparing two addresses is insufficient, because addresses
2960 can be recycled by distinct servers, and equivalent addresses may look
2961 different if simply compared as strings (for example, the host in a TCP
2962 address can be given as an IP address or as a hostname).
2966 Note that the address key is <literal>guid</literal> even though the
2967 rest of the API and documentation says "UUID," for historical reasons.
2971 [FIXME clarify if attempting to connect to each is a requirement
2972 or just a suggestion]
2973 When connecting to a server, multiple server addresses can be
2974 separated by a semi-colon. The library will then try to connect
2975 to the first address and if that fails, it'll try to connect to
2976 the next one specified, and so forth. For example
2977 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2981 Some addresses are <firstterm>connectable</firstterm>. A connectable
2982 address is one containing enough information for a client to connect
2983 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2984 is a connectable address. It is not necessarily possible to listen
2985 on every connectable address: for instance, it is not possible to
2986 listen on a <literal>unixexec:</literal> address.
2990 Some addresses are <firstterm>listenable</firstterm>. A listenable
2991 address is one containing enough information for a server to listen on
2992 it, producing a connectable address (which may differ from the
2993 original address). Many listenable addresses are not connectable:
2994 for instance, <literal>tcp:host=127.0.0.1</literal>
2995 is listenable, but not connectable (because it does not specify
3000 Listening on an address that is not connectable will result in a
3001 connectable address that is not the same as the listenable address.
3002 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
3003 might result in the connectable address
3004 <literal>tcp:host=127.0.0.1,port=30958</literal>,
3005 or listening on <literal>unix:tmpdir=/tmp</literal>
3006 might result in the connectable address
3007 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
3011 <sect1 id="transports">
3012 <title>Transports</title>
3014 [FIXME we need to specify in detail each transport and its possible arguments]
3016 Current transports include: unix domain sockets (including
3017 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3018 using in-process pipes. Future possible transports include one that
3019 tunnels over X11 protocol.
3022 <sect2 id="transports-unix-domain-sockets">
3023 <title>Unix Domain Sockets</title>
3025 Unix domain sockets can be either paths in the file system or on Linux
3026 kernels, they can be abstract which are similar to paths but
3027 do not show up in the file system.
3031 When a socket is opened by the D-Bus library it truncates the path
3032 name right before the first trailing Nul byte. This is true for both
3033 normal paths and abstract paths. Note that this is a departure from
3034 previous versions of D-Bus that would create sockets with a fixed
3035 length path name. Names which were shorter than the fixed length
3036 would be padded by Nul bytes.
3039 Unix domain sockets are not available on Windows.
3042 Unix addresses that specify <literal>path</literal> or
3043 <literal>abstract</literal> are both listenable and connectable.
3044 Unix addresses that specify <literal>tmpdir</literal> are only
3045 listenable: the corresponding connectable address will specify
3046 either <literal>path</literal> or <literal>abstract</literal>.
3048 <sect3 id="transports-unix-domain-sockets-addresses">
3049 <title>Server Address Format</title>
3051 Unix domain socket addresses are identified by the "unix:" prefix
3052 and support the following key/value pairs:
3059 <entry>Values</entry>
3060 <entry>Description</entry>
3066 <entry>(path)</entry>
3067 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3070 <entry>tmpdir</entry>
3071 <entry>(path)</entry>
3072 <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>
3075 <entry>abstract</entry>
3076 <entry>(string)</entry>
3077 <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>
3083 Exactly one of the keys <literal>path</literal>,
3084 <literal>abstract</literal> or
3085 <literal>tmpdir</literal> must be provided.
3089 <sect2 id="transports-launchd">
3090 <title>launchd</title>
3092 launchd is an open-source server management system that replaces init, inetd
3093 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3094 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3098 launchd allocates a socket and provides it with the unix path through the
3099 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3100 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3101 it through its environment.
3102 Other processes can query for the launchd socket by executing:
3103 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3104 This is normally done by the D-Bus client library so doesn't have to be done
3108 launchd is not available on Microsoft Windows.
3111 launchd addresses are listenable and connectable.
3113 <sect3 id="transports-launchd-addresses">
3114 <title>Server Address Format</title>
3116 launchd addresses are identified by the "launchd:" prefix
3117 and support the following key/value pairs:
3124 <entry>Values</entry>
3125 <entry>Description</entry>
3131 <entry>(environment variable)</entry>
3132 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3138 The <literal>env</literal> key is required.
3142 <sect2 id="transports-systemd">
3143 <title>systemd</title>
3145 systemd is an open-source server management system that
3146 replaces init and inetd on newer Linux systems. It supports
3147 socket activation. The D-Bus systemd transport is used to acquire
3148 socket activation file descriptors from systemd and use them
3149 as D-Bus transport when the current process is spawned by
3150 socket activation from it.
3153 The systemd transport accepts only one or more Unix domain or
3154 TCP streams sockets passed in via socket activation.
3157 The systemd transport is not available on non-Linux operating systems.
3160 The systemd transport defines no parameter keys.
3163 systemd addresses are listenable, but not connectable. The
3164 corresponding connectable address is the <literal>unix</literal>
3165 or <literal>tcp</literal> address of the socket.
3168 <sect2 id="transports-tcp-sockets">
3169 <title>TCP Sockets</title>
3171 The tcp transport provides TCP/IP based connections between clients
3172 located on the same or different hosts.
3175 Using tcp transport without any additional secure authentification mechanismus
3176 over a network is unsecure.
3179 On Windows and most Unix platforms, the TCP stack is unable to transfer
3180 credentials over a TCP connection, so the EXTERNAL authentication
3181 mechanism does not work for this transport.
3184 All <literal>tcp</literal> addresses are listenable.
3185 <literal>tcp</literal> addresses in which both
3186 <literal>host</literal> and <literal>port</literal> are
3187 specified, and <literal>port</literal> is non-zero,
3188 are also connectable.
3190 <sect3 id="transports-tcp-sockets-addresses">
3191 <title>Server Address Format</title>
3193 TCP/IP socket addresses are identified by the "tcp:" prefix
3194 and support the following key/value pairs:
3201 <entry>Values</entry>
3202 <entry>Description</entry>
3208 <entry>(string)</entry>
3209 <entry>DNS name or IP address</entry>
3213 <entry>(string)</entry>
3214 <entry>Used in a listenable address to configure the interface
3215 on which the server will listen: either the IP address of one of
3216 the local machine's interfaces (most commonly <literal>127.0.0.1
3217 </literal>), or a DNS name that resolves to one of those IP
3218 addresses, or '*' to listen on all interfaces simultaneously.
3219 If not specified, the default is the same value as "host".
3224 <entry>(number)</entry>
3225 <entry>The tcp port the server will open. A zero value let the server
3226 choose a free port provided from the underlaying operating system.
3227 libdbus is able to retrieve the real used port from the server.
3231 <entry>family</entry>
3232 <entry>(string)</entry>
3233 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3240 <sect2 id="transports-nonce-tcp-sockets">
3241 <title>Nonce-secured TCP Sockets</title>
3243 The nonce-tcp transport provides a secured TCP transport, using a
3244 simple authentication mechanism to ensure that only clients with read
3245 access to a certain location in the filesystem can connect to the server.
3246 The server writes a secret, the nonce, to a file and an incoming client
3247 connection is only accepted if the client sends the nonce right after
3248 the connect. The nonce mechanism requires no setup and is orthogonal to
3249 the higher-level authentication mechanisms described in the
3250 Authentication section.
3254 On start, the server generates a random 16 byte nonce and writes it
3255 to a file in the user's temporary directory. The nonce file location
3256 is published as part of the server's D-Bus address using the
3257 "noncefile" key-value pair.
3259 After an accept, the server reads 16 bytes from the socket. If the
3260 read bytes do not match the nonce stored in the nonce file, the
3261 server MUST immediately drop the connection.
3262 If the nonce match the received byte sequence, the client is accepted
3263 and the transport behaves like an unsecured tcp transport.
3266 After a successful connect to the server socket, the client MUST read
3267 the nonce from the file published by the server via the noncefile=
3268 key-value pair and send it over the socket. After that, the
3269 transport behaves like an unsecured tcp transport.
3272 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3273 <literal>host</literal>, <literal>port</literal> and
3274 <literal>noncefile</literal> are all specified,
3275 and <literal>port</literal> is nonzero, are also connectable.
3277 <sect3 id="transports-nonce-tcp-sockets-addresses">
3278 <title>Server Address Format</title>
3280 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3281 and support the following key/value pairs:
3288 <entry>Values</entry>
3289 <entry>Description</entry>
3295 <entry>(string)</entry>
3296 <entry>DNS name or IP address</entry>
3300 <entry>(string)</entry>
3301 <entry>The same as for tcp: addresses
3306 <entry>(number)</entry>
3307 <entry>The tcp port the server will open. A zero value let the server
3308 choose a free port provided from the underlaying operating system.
3309 libdbus is able to retrieve the real used port from the server.
3313 <entry>family</entry>
3314 <entry>(string)</entry>
3315 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3318 <entry>noncefile</entry>
3319 <entry>(path)</entry>
3320 <entry>File location containing the secret.
3321 This is only meaningful in connectable addresses:
3322 a listening D-Bus server that offers this transport
3323 will always create a new nonce file.</entry>
3330 <sect2 id="transports-exec">
3331 <title>Executed Subprocesses on Unix</title>
3333 This transport forks off a process and connects its standard
3334 input and standard output with an anonymous Unix domain
3335 socket. This socket is then used for communication by the
3336 transport. This transport may be used to use out-of-process
3337 forwarder programs as basis for the D-Bus protocol.
3340 The forked process will inherit the standard error output and
3341 process group from the parent process.
3344 Executed subprocesses are not available on Windows.
3347 <literal>unixexec</literal> addresses are connectable, but are not
3350 <sect3 id="transports-exec-addresses">
3351 <title>Server Address Format</title>
3353 Executed subprocess addresses are identified by the "unixexec:" prefix
3354 and support the following key/value pairs:
3361 <entry>Values</entry>
3362 <entry>Description</entry>
3368 <entry>(path)</entry>
3369 <entry>Path of the binary to execute, either an absolute
3370 path or a binary name that is searched for in the default
3371 search path of the OS. This corresponds to the first
3372 argument of execlp(). This key is mandatory.</entry>
3375 <entry>argv0</entry>
3376 <entry>(string)</entry>
3377 <entry>The program name to use when executing the
3378 binary. If omitted the same value as specified for path=
3379 will be used. This corresponds to the second argument of
3383 <entry>argv1, argv2, ...</entry>
3384 <entry>(string)</entry>
3385 <entry>Arguments to pass to the binary. This corresponds
3386 to the third and later arguments of execlp(). If a
3387 specific argvX is not specified no further argvY for Y > X
3388 are taken into account.</entry>
3396 <sect1 id="meta-transports">
3397 <title>Meta Transports</title>
3399 Meta transports are a kind of transport with special enhancements or
3400 behavior. Currently available meta transports include: autolaunch
3403 <sect2 id="meta-transports-autolaunch">
3404 <title>Autolaunch</title>
3405 <para>The autolaunch transport provides a way for dbus clients to autodetect
3406 a running dbus session bus and to autolaunch a session bus if not present.
3409 On Unix, <literal>autolaunch</literal> addresses are connectable,
3413 On Windows, <literal>autolaunch</literal> addresses are both
3414 connectable and listenable.
3417 <sect3 id="meta-transports-autolaunch-addresses">
3418 <title>Server Address Format</title>
3420 Autolaunch addresses uses the "autolaunch:" prefix and support the
3421 following key/value pairs:
3428 <entry>Values</entry>
3429 <entry>Description</entry>
3434 <entry>scope</entry>
3435 <entry>(string)</entry>
3436 <entry>scope of autolaunch (Windows only)
3440 "*install-path" - limit session bus to dbus installation path.
3441 The dbus installation path is determined from the location of
3442 the shared dbus library. If the library is located in a 'bin'
3443 subdirectory the installation root is the directory above,
3444 otherwise the directory where the library lives is taken as
3447 <install-root>/bin/[lib]dbus-1.dll
3448 <install-root>/[lib]dbus-1.dll
3454 "*user" - limit session bus to the recent user.
3459 other values - specify dedicated session bus like "release",
3471 <sect3 id="meta-transports-autolaunch-windows-implementation">
3472 <title>Windows implementation</title>
3474 On start, the server opens a platform specific transport, creates a mutex
3475 and a shared memory section containing the related session bus address.
3476 This mutex will be inspected by the dbus client library to detect a
3477 running dbus session bus. The access to the mutex and the shared memory
3478 section are protected by global locks.
3481 In the recent implementation the autolaunch transport uses a tcp transport
3482 on localhost with a port choosen from the operating system. This detail may
3483 change in the future.
3486 Disclaimer: The recent implementation is in an early state and may not
3487 work in all cirumstances and/or may have security issues. Because of this
3488 the implementation is not documentated yet.
3495 <title>UUIDs</title>
3497 A working D-Bus implementation uses universally-unique IDs in two places.
3498 First, each server address has a UUID identifying the address,
3499 as described in <xref linkend="addresses"/>. Second, each operating
3500 system kernel instance running a D-Bus client or server has a UUID
3501 identifying that kernel, retrieved by invoking the method
3502 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3503 linkend="standard-interfaces-peer"/>).
3506 The term "UUID" in this document is intended literally, i.e. an
3507 identifier that is universally unique. It is not intended to refer to
3508 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3511 The UUID must contain 128 bits of data and be hex-encoded. The
3512 hex-encoded string may not contain hyphens or other non-hex-digit
3513 characters, and it must be exactly 32 characters long. To generate a
3514 UUID, the current reference implementation concatenates 96 bits of random
3515 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3519 It would also be acceptable and probably better to simply generate 128
3520 bits of random data, as long as the random number generator is of high
3521 quality. The timestamp could conceivably help if the random bits are not
3522 very random. With a quality random number generator, collisions are
3523 extremely unlikely even with only 96 bits, so it's somewhat academic.
3526 Implementations should, however, stick to random data for the first 96 bits
3531 <sect1 id="standard-interfaces">
3532 <title>Standard Interfaces</title>
3534 See <xref linkend="message-protocol-types-notation"/> for details on
3535 the notation used in this section. There are some standard interfaces
3536 that may be useful across various D-Bus applications.
3538 <sect2 id="standard-interfaces-peer">
3539 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3541 The <literal>org.freedesktop.DBus.Peer</literal> interface
3544 org.freedesktop.DBus.Peer.Ping ()
3545 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3549 On receipt of the <literal>METHOD_CALL</literal> message
3550 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3551 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3552 usual. It does not matter which object path a ping is sent to. The
3553 reference implementation handles this method automatically.
3556 On receipt of the <literal>METHOD_CALL</literal> message
3557 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3558 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3559 UUID representing the identity of the machine the process is running on.
3560 This UUID must be the same for all processes on a single system at least
3561 until that system next reboots. It should be the same across reboots
3562 if possible, but this is not always possible to implement and is not
3564 It does not matter which object path a GetMachineId is sent to. The
3565 reference implementation handles this method automatically.
3568 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3569 a virtual machine running on a hypervisor, rather than a physical machine.
3570 Basically if two processes see the same UUID, they should also see the same
3571 shared memory, UNIX domain sockets, process IDs, and other features that require
3572 a running OS kernel in common between the processes.
3575 The UUID is often used where other programs might use a hostname. Hostnames
3576 can change without rebooting, however, or just be "localhost" - so the UUID
3580 <xref linkend="uuids"/> explains the format of the UUID.
3584 <sect2 id="standard-interfaces-introspectable">
3585 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3587 This interface has one method:
3589 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3593 Objects instances may implement
3594 <literal>Introspect</literal> which returns an XML description of
3595 the object, including its interfaces (with signals and methods), objects
3596 below it in the object path tree, and its properties.
3599 <xref linkend="introspection-format"/> describes the format of this XML string.
3602 <sect2 id="standard-interfaces-properties">
3603 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3605 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3606 or <firstterm>attributes</firstterm>. These can be exposed via the
3607 <literal>org.freedesktop.DBus.Properties</literal> interface.
3611 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3612 in STRING property_name,
3614 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3615 in STRING property_name,
3617 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3618 out DICT<STRING,VARIANT> props);
3622 It is conventional to give D-Bus properties names consisting of
3623 capitalized words without punctuation ("CamelCase"), like
3624 <link linkend="message-protocol-names-member">member names</link>.
3625 For instance, the GObject property
3626 <literal>connection-status</literal> or the Qt property
3627 <literal>connectionStatus</literal> could be represented on D-Bus
3628 as <literal>ConnectionStatus</literal>.
3631 Strictly speaking, D-Bus property names are not required to follow
3632 the same naming restrictions as member names, but D-Bus property
3633 names that would not be valid member names (in particular,
3634 GObject-style dash-separated property names) can cause interoperability
3635 problems and should be avoided.
3638 The available properties and whether they are writable can be determined
3639 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3640 see <xref linkend="standard-interfaces-introspectable"/>.
3643 An empty string may be provided for the interface name; in this case,
3644 if there are multiple properties on an object with the same name,
3645 the results are undefined (picking one by according to an arbitrary
3646 deterministic rule, or returning an error, are the reasonable
3650 If one or more properties change on an object, the
3651 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3652 signal may be emitted (this signal was added in 0.14):
3656 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3657 DICT<STRING,VARIANT> changed_properties,
3658 ARRAY<STRING> invalidated_properties);
3662 where <literal>changed_properties</literal> is a dictionary
3663 containing the changed properties with the new values and
3664 <literal>invalidated_properties</literal> is an array of
3665 properties that changed but the value is not conveyed.
3668 Whether the <literal>PropertiesChanged</literal> signal is
3669 supported can be determined by calling
3670 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3671 that the signal may be supported for an object but it may
3672 differ how whether and how it is used on a per-property basis
3673 (for e.g. performance or security reasons). Each property (or
3674 the parent interface) must be annotated with the
3675 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3676 annotation to convey this (usually the default value
3677 <literal>true</literal> is sufficient meaning that the
3678 annotation does not need to be used). See <xref
3679 linkend="introspection-format"/> for details on this
3684 <sect2 id="standard-interfaces-objectmanager">
3685 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3687 An API can optionally make use of this interface for one or
3688 more sub-trees of objects. The root of each sub-tree implements
3689 this interface so other applications can get all objects,
3690 interfaces and properties in a single method call. It is
3691 appropriate to use this interface if users of the tree of
3692 objects are expected to be interested in all interfaces of all
3693 objects in the tree; a more granular API should be used if
3694 users of the objects are expected to be interested in a small
3695 subset of the objects, a small subset of their interfaces, or
3699 The method that applications can use to get all objects and
3700 properties is <literal>GetManagedObjects</literal>:
3704 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3708 The return value of this method is a dict whose keys are
3709 object paths. All returned object paths are children of the
3710 object path implementing this interface, i.e. their object
3711 paths start with the ObjectManager's object path plus '/'.
3714 Each value is a dict whose keys are interfaces names. Each
3715 value in this inner dict is the same dict that would be
3716 returned by the <link
3717 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3718 method for that combination of object path and interface. If
3719 an interface has no properties, the empty dict is returned.
3722 Changes are emitted using the following two signals:
3726 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3727 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3728 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3729 ARRAY<STRING> interfaces);
3733 The <literal>InterfacesAdded</literal> signal is emitted when
3734 either a new object is added or when an existing object gains
3735 one or more interfaces. The
3736 <literal>InterfacesRemoved</literal> signal is emitted
3737 whenever an object is removed or it loses one or more
3738 interfaces. The second parameter of the
3739 <literal>InterfacesAdded</literal> signal contains a dict with
3740 the interfaces and properties (if any) that have been added to
3741 the given object path. Similarly, the second parameter of the
3742 <literal>InterfacesRemoved</literal> signal contains an array
3743 of the interfaces that were removed. Note that changes on
3744 properties on existing interfaces are not reported using this
3745 interface - an application should also monitor the existing <link
3746 linkend="standard-interfaces-properties">PropertiesChanged</link>
3747 signal on each object.
3750 Applications SHOULD NOT export objects that are children of an
3751 object (directly or otherwise) implementing this interface but
3752 which are not returned in the reply from the
3753 <literal>GetManagedObjects()</literal> method of this
3754 interface on the given object.
3757 The intent of the <literal>ObjectManager</literal> interface
3758 is to make it easy to write a robust client
3759 implementation. The trivial client implementation only needs
3760 to make two method calls:
3764 org.freedesktop.DBus.AddMatch (bus_proxy,
3765 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3766 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3770 on the message bus and the remote application's
3771 <literal>ObjectManager</literal>, respectively. Whenever a new
3772 remote object is created (or an existing object gains a new
3773 interface), the <literal>InterfacesAdded</literal> signal is
3774 emitted, and since this signal contains all properties for the
3775 interfaces, no calls to the
3776 <literal>org.freedesktop.Properties</literal> interface on the
3777 remote object are needed. Additionally, since the initial
3778 <literal>AddMatch()</literal> rule already includes signal
3779 messages from the newly created child object, no new
3780 <literal>AddMatch()</literal> call is needed.
3785 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3786 interface was added in version 0.17 of the D-Bus
3793 <sect1 id="introspection-format">
3794 <title>Introspection Data Format</title>
3796 As described in <xref linkend="standard-interfaces-introspectable"/>,
3797 objects may be introspected at runtime, returning an XML string
3798 that describes the object. The same XML format may be used in
3799 other contexts as well, for example as an "IDL" for generating
3800 static language bindings.
3803 Here is an example of introspection data:
3805 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3806 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3807 <node name="/com/example/sample_object">
3808 <interface name="com.example.SampleInterface">
3809 <method name="Frobate">
3810 <arg name="foo" type="i" direction="in"/>
3811 <arg name="bar" type="s" direction="out"/>
3812 <arg name="baz" type="a{us}" direction="out"/>
3813 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3815 <method name="Bazify">
3816 <arg name="bar" type="(iiu)" direction="in"/>
3817 <arg name="bar" type="v" direction="out"/>
3819 <method name="Mogrify">
3820 <arg name="bar" type="(iiav)" direction="in"/>
3822 <signal name="Changed">
3823 <arg name="new_value" type="b"/>
3825 <property name="Bar" type="y" access="readwrite"/>
3827 <node name="child_of_sample_object"/>
3828 <node name="another_child_of_sample_object"/>
3833 A more formal DTD and spec needs writing, but here are some quick notes.
3837 Only the root <node> element can omit the node name, as it's
3838 known to be the object that was introspected. If the root
3839 <node> does have a name attribute, it must be an absolute
3840 object path. If child <node> have object paths, they must be
3846 If a child <node> has any sub-elements, then they
3847 must represent a complete introspection of the child.
3848 If a child <node> is empty, then it may or may
3849 not have sub-elements; the child must be introspected
3850 in order to find out. The intent is that if an object
3851 knows that its children are "fast" to introspect
3852 it can go ahead and return their information, but
3853 otherwise it can omit it.
3858 The direction element on <arg> may be omitted,
3859 in which case it defaults to "in" for method calls
3860 and "out" for signals. Signals only allow "out"
3861 so while direction may be specified, it's pointless.
3866 The possible directions are "in" and "out",
3867 unlike CORBA there is no "inout"
3872 The possible property access flags are
3873 "readwrite", "read", and "write"
3878 Multiple interfaces can of course be listed for
3884 The "name" attribute on arguments is optional.
3890 Method, interface, property, and signal elements may have
3891 "annotations", which are generic key/value pairs of metadata.
3892 They are similar conceptually to Java's annotations and C# attributes.
3893 Well-known annotations:
3900 <entry>Values (separated by ,)</entry>
3901 <entry>Description</entry>
3906 <entry>org.freedesktop.DBus.Deprecated</entry>
3907 <entry>true,false</entry>
3908 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3911 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3912 <entry>(string)</entry>
3913 <entry>The C symbol; may be used for methods and interfaces</entry>
3916 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3917 <entry>true,false</entry>
3918 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3921 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3922 <entry>true,invalidates,false</entry>
3925 If set to <literal>false</literal>, the
3926 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3928 linkend="standard-interfaces-properties"/> is not
3929 guaranteed to be emitted if the property changes.
3932 If set to <literal>invalidates</literal> the signal
3933 is emitted but the value is not included in the
3937 If set to <literal>true</literal> the signal is
3938 emitted with the value included.
3941 The value for the annotation defaults to
3942 <literal>true</literal> if the enclosing interface
3943 element does not specify the annotation. Otherwise it
3944 defaults to the value specified in the enclosing
3953 <sect1 id="message-bus">
3954 <title>Message Bus Specification</title>
3955 <sect2 id="message-bus-overview">
3956 <title>Message Bus Overview</title>
3958 The message bus accepts connections from one or more applications.
3959 Once connected, applications can exchange messages with other
3960 applications that are also connected to the bus.
3963 In order to route messages among connections, the message bus keeps a
3964 mapping from names to connections. Each connection has one
3965 unique-for-the-lifetime-of-the-bus name automatically assigned.
3966 Applications may request additional names for a connection. Additional
3967 names are usually "well-known names" such as
3968 "com.example.TextEditor". When a name is bound to a connection,
3969 that connection is said to <firstterm>own</firstterm> the name.
3972 The bus itself owns a special name,
3973 <literal>org.freedesktop.DBus</literal>, with an object
3974 located at <literal>/org/freedesktop/DBus</literal> that
3975 implements the <literal>org.freedesktop.DBus</literal>
3976 interface. This service allows applications to make
3977 administrative requests of the bus itself. For example,
3978 applications can ask the bus to assign a name to a connection.
3981 Each name may have <firstterm>queued owners</firstterm>. When an
3982 application requests a name for a connection and the name is already in
3983 use, the bus will optionally add the connection to a queue waiting for
3984 the name. If the current owner of the name disconnects or releases
3985 the name, the next connection in the queue will become the new owner.
3989 This feature causes the right thing to happen if you start two text
3990 editors for example; the first one may request "com.example.TextEditor",
3991 and the second will be queued as a possible owner of that name. When
3992 the first exits, the second will take over.
3996 Applications may send <firstterm>unicast messages</firstterm> to
3997 a specific recipient or to the message bus itself, or
3998 <firstterm>broadcast messages</firstterm> to all interested recipients.
3999 See <xref linkend="message-bus-routing"/> for details.
4003 <sect2 id="message-bus-names">
4004 <title>Message Bus Names</title>
4006 Each connection has at least one name, assigned at connection time and
4007 returned in response to the
4008 <literal>org.freedesktop.DBus.Hello</literal> method call. This
4009 automatically-assigned name is called the connection's <firstterm>unique
4010 name</firstterm>. Unique names are never reused for two different
4011 connections to the same bus.
4014 Ownership of a unique name is a prerequisite for interaction with
4015 the message bus. It logically follows that the unique name is always
4016 the first name that an application comes to own, and the last
4017 one that it loses ownership of.
4020 Unique connection names must begin with the character ':' (ASCII colon
4021 character); bus names that are not unique names must not begin
4022 with this character. (The bus must reject any attempt by an application
4023 to manually request a name beginning with ':'.) This restriction
4024 categorically prevents "spoofing"; messages sent to a unique name
4025 will always go to the expected connection.
4028 When a connection is closed, all the names that it owns are deleted (or
4029 transferred to the next connection in the queue if any).
4032 A connection can request additional names to be associated with it using
4033 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4034 linkend="message-protocol-names-bus"/> describes the format of a valid
4035 name. These names can be released again using the
4036 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4039 <sect3 id="bus-messages-request-name">
4040 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4044 UINT32 RequestName (in STRING name, in UINT32 flags)
4051 <entry>Argument</entry>
4053 <entry>Description</entry>
4059 <entry>STRING</entry>
4060 <entry>Name to request</entry>
4064 <entry>UINT32</entry>
4065 <entry>Flags</entry>
4075 <entry>Argument</entry>
4077 <entry>Description</entry>
4083 <entry>UINT32</entry>
4084 <entry>Return value</entry>
4091 This method call should be sent to
4092 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4093 assign the given name to the method caller. Each name maintains a
4094 queue of possible owners, where the head of the queue is the primary
4095 or current owner of the name. Each potential owner in the queue
4096 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4097 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4098 call. When RequestName is invoked the following occurs:
4102 If the method caller is currently the primary owner of the name,
4103 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4104 values are updated with the values from the new RequestName call,
4105 and nothing further happens.
4111 If the current primary owner (head of the queue) has
4112 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4113 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4114 the caller of RequestName replaces the current primary owner at
4115 the head of the queue and the current primary owner moves to the
4116 second position in the queue. If the caller of RequestName was
4117 in the queue previously its flags are updated with the values from
4118 the new RequestName in addition to moving it to the head of the queue.
4124 If replacement is not possible, and the method caller is
4125 currently in the queue but not the primary owner, its flags are
4126 updated with the values from the new RequestName call.
4132 If replacement is not possible, and the method caller is
4133 currently not in the queue, the method caller is appended to the
4140 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4141 set and is not the primary owner, it is removed from the
4142 queue. This can apply to the previous primary owner (if it
4143 was replaced) or the method caller (if it updated the
4144 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4145 queue, or if it was just added to the queue with that flag set).
4151 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4152 queue," even if another application already in the queue had specified
4153 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4154 that does not allow replacement goes away, and the next primary owner
4155 does allow replacement. In this case, queued items that specified
4156 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4157 automatically replace the new primary owner. In other words,
4158 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4159 time RequestName is called. This is deliberate to avoid an infinite loop
4160 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4161 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4164 The flags argument contains any of the following values logically ORed
4171 <entry>Conventional Name</entry>
4172 <entry>Value</entry>
4173 <entry>Description</entry>
4178 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4182 If an application A specifies this flag and succeeds in
4183 becoming the owner of the name, and another application B
4184 later calls RequestName with the
4185 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4186 will lose ownership and receive a
4187 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4188 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4189 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4190 is not specified by application B, then application B will not replace
4191 application A as the owner.
4196 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4200 Try to replace the current owner if there is one. If this
4201 flag is not set the application will only become the owner of
4202 the name if there is no current owner. If this flag is set,
4203 the application will replace the current owner if
4204 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4209 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4213 Without this flag, if an application requests a name that is
4214 already owned, the application will be placed in a queue to
4215 own the name when the current owner gives it up. If this
4216 flag is given, the application will not be placed in the
4217 queue, the request for the name will simply fail. This flag
4218 also affects behavior when an application is replaced as
4219 name owner; by default the application moves back into the
4220 waiting queue, unless this flag was provided when the application
4221 became the name owner.
4229 The return code can be one of the following values:
4235 <entry>Conventional Name</entry>
4236 <entry>Value</entry>
4237 <entry>Description</entry>
4242 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4243 <entry>1</entry> <entry>The caller is now the primary owner of
4244 the name, replacing any previous owner. Either the name had no
4245 owner before, or the caller specified
4246 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4247 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4250 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4253 <entry>The name already had an owner,
4254 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4255 the current owner did not specify
4256 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4257 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4261 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4262 <entry>The name already has an owner,
4263 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4264 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4265 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4266 specified by the requesting application.</entry>
4269 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4271 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4279 <sect3 id="bus-messages-release-name">
4280 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4284 UINT32 ReleaseName (in STRING name)
4291 <entry>Argument</entry>
4293 <entry>Description</entry>
4299 <entry>STRING</entry>
4300 <entry>Name to release</entry>
4310 <entry>Argument</entry>
4312 <entry>Description</entry>
4318 <entry>UINT32</entry>
4319 <entry>Return value</entry>
4326 This method call should be sent to
4327 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4328 release the method caller's claim to the given name. If the caller is
4329 the primary owner, a new primary owner will be selected from the
4330 queue if any other owners are waiting. If the caller is waiting in
4331 the queue for the name, the caller will removed from the queue and
4332 will not be made an owner of the name if it later becomes available.
4333 If there are no other owners in the queue for the name, it will be
4334 removed from the bus entirely.
4336 The return code can be one of the following values:
4342 <entry>Conventional Name</entry>
4343 <entry>Value</entry>
4344 <entry>Description</entry>
4349 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4350 <entry>1</entry> <entry>The caller has released his claim on
4351 the given name. Either the caller was the primary owner of
4352 the name, and the name is now unused or taken by somebody
4353 waiting in the queue for the name, or the caller was waiting
4354 in the queue for the name and has now been removed from the
4358 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4360 <entry>The given name does not exist on this bus.</entry>
4363 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4365 <entry>The caller was not the primary owner of this name,
4366 and was also not waiting in the queue to own this name.</entry>
4374 <sect3 id="bus-messages-list-queued-owners">
4375 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4379 ARRAY of STRING ListQueuedOwners (in STRING name)
4386 <entry>Argument</entry>
4388 <entry>Description</entry>
4394 <entry>STRING</entry>
4395 <entry>The well-known bus name to query, such as
4396 <literal>com.example.cappuccino</literal></entry>
4406 <entry>Argument</entry>
4408 <entry>Description</entry>
4414 <entry>ARRAY of STRING</entry>
4415 <entry>The unique bus names of connections currently queued
4416 for the name</entry>
4423 This method call should be sent to
4424 <literal>org.freedesktop.DBus</literal> and lists the connections
4425 currently queued for a bus name (see
4426 <xref linkend="term-queued-owner"/>).
4431 <sect2 id="message-bus-routing">
4432 <title>Message Bus Message Routing</title>
4435 Messages may have a <literal>DESTINATION</literal> field (see <xref
4436 linkend="message-protocol-header-fields"/>), resulting in a
4437 <firstterm>unicast message</firstterm>. If the
4438 <literal>DESTINATION</literal> field is present, it specifies a message
4439 recipient by name. Method calls and replies normally specify this field.
4440 The message bus must send messages (of any type) with the
4441 <literal>DESTINATION</literal> field set to the specified recipient,
4442 regardless of whether the recipient has set up a match rule matching
4447 When the message bus receives a signal, if the
4448 <literal>DESTINATION</literal> field is absent, it is considered to
4449 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4450 applications with <firstterm>message matching rules</firstterm> that
4451 match the message. Most signal messages are broadcasts, and
4452 no other message types currently defined in this specification
4457 Unicast signal messages (those with a <literal>DESTINATION</literal>
4458 field) are not commonly used, but they are treated like any unicast
4459 message: they are delivered to the specified receipient,
4460 regardless of its match rules. One use for unicast signals is to
4461 avoid a race condition in which a signal is emitted before the intended
4462 recipient can call <xref linkend="bus-messages-add-match"/> to
4463 receive that signal: if the signal is sent directly to that recipient
4464 using a unicast message, it does not need to add a match rule at all,
4465 and there is no race condition. Another use for unicast signals,
4466 on message buses whose security policy prevents eavesdropping, is to
4467 send sensitive information which should only be visible to one
4472 When the message bus receives a method call, if the
4473 <literal>DESTINATION</literal> field is absent, the call is taken to be
4474 a standard one-to-one message and interpreted by the message bus
4475 itself. For example, sending an
4476 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4477 <literal>DESTINATION</literal> will cause the message bus itself to
4478 reply to the ping immediately; the message bus will not make this
4479 message visible to other applications.
4483 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4484 the ping message were sent with a <literal>DESTINATION</literal> name of
4485 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4486 forwarded, and the Yoyodyne Corporation screensaver application would be
4487 expected to reply to the ping.
4491 Message bus implementations may impose a security policy which
4492 prevents certain messages from being sent or received.
4493 When a method call message cannot be sent or received due to a security
4494 policy, the message bus should send an error reply, unless the
4495 original message had the <literal>NO_REPLY</literal> flag.
4498 <sect3 id="message-bus-routing-eavesdropping">
4499 <title>Eavesdropping</title>
4501 Receiving a unicast message whose <literal>DESTINATION</literal>
4502 indicates a different recipient is called
4503 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4504 a security boundary (like the standard system bus), the security
4505 policy should usually prevent eavesdropping, since unicast messages
4506 are normally kept private and may contain security-sensitive
4511 Eavesdropping is mainly useful for debugging tools, such as
4512 the <literal>dbus-monitor</literal> tool in the reference
4513 implementation of D-Bus. Tools which eavesdrop on the message bus
4514 should be careful to avoid sending a reply or error in response to
4515 messages intended for a different client.
4519 Clients may attempt to eavesdrop by adding match rules
4520 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4521 the <literal>eavesdrop='true'</literal> match. If the message bus'
4522 security policy does not allow eavesdropping, the match rule can
4523 still be added, but will not have any practical effect. For
4524 compatibility with older message bus implementations, if adding such
4525 a match rule results in an error reply, the client may fall back to
4526 adding the same rule with the <literal>eavesdrop</literal> match
4531 <sect3 id="message-bus-routing-match-rules">
4532 <title>Match Rules</title>
4534 An important part of the message bus routing protocol is match
4535 rules. Match rules describe the messages that should be sent to a
4536 client, based on the contents of the message. Broadcast signals
4537 are only sent to clients which have a suitable match rule: this
4538 avoids waking up client processes to deal with signals that are
4539 not relevant to that client.
4542 Messages that list a client as their <literal>DESTINATION</literal>
4543 do not need to match the client's match rules, and are sent to that
4544 client regardless. As a result, match rules are mainly used to
4545 receive a subset of broadcast signals.
4548 Match rules can also be used for eavesdropping
4549 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4550 if the security policy of the message bus allows it.
4553 Match rules are added using the AddMatch bus method
4554 (see <xref linkend="bus-messages-add-match"/>). Rules are
4555 specified as a string of comma separated key/value pairs.
4556 Excluding a key from the rule indicates a wildcard match.
4557 For instance excluding the the member from a match rule but
4558 adding a sender would let all messages from that sender through.
4559 An example of a complete rule would be
4560 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4563 Within single quotes (ASCII apostrophe, U+0027), a backslash
4564 (U+005C) represents itself, and an apostrophe ends the quoted
4565 section. Outside single quotes, \' (backslash, apostrophe)
4566 represents an apostrophe, and any backslash not followed by
4567 an apostrophe represents itself. For instance, the match rules
4568 <literal>arg0=''\''',arg1='\',arg2=',',arg3='\\'</literal> and
4569 <literal>arg0=\',arg1=\,arg2=',',arg3=\\</literal>
4570 both match messages where the arguments are a 1-character string
4571 containing an apostrophe, a 1-character string containing a
4572 backslash, a 1-character string containing a comma, and a
4573 2-character string containing two backslashes<footnote>
4575 This idiosyncratic quoting style is based on the rules for
4576 escaping items to appear inside single-quoted strings
4577 in POSIX <literal>/bin/sh</literal>, but please
4578 note that backslashes that are not inside single quotes have
4579 different behaviour. This syntax does not offer any way to
4580 represent an apostrophe inside single quotes (it is necessary
4581 to leave the single-quoted section, backslash-escape the
4582 apostrophe and re-enter single quotes), or to represent a
4583 comma outside single quotes (it is necessary to wrap it in
4584 a single-quoted section).
4589 The following table describes the keys that can be used to create
4596 <entry>Possible Values</entry>
4597 <entry>Description</entry>
4602 <entry><literal>type</literal></entry>
4603 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4604 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4607 <entry><literal>sender</literal></entry>
4608 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4609 and <xref linkend="term-unique-name"/> respectively)
4611 <entry>Match messages sent by a particular sender. An example of a sender match
4612 is sender='org.freedesktop.Hal'</entry>
4615 <entry><literal>interface</literal></entry>
4616 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4617 <entry>Match messages sent over or to a particular interface. An example of an
4618 interface match is interface='org.freedesktop.Hal.Manager'.
4619 If a message omits the interface header, it must not match any rule
4620 that specifies this key.</entry>
4623 <entry><literal>member</literal></entry>
4624 <entry>Any valid method or signal name</entry>
4625 <entry>Matches messages which have the give method or signal name. An example of
4626 a member match is member='NameOwnerChanged'</entry>
4629 <entry><literal>path</literal></entry>
4630 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4631 <entry>Matches messages which are sent from or to the given object. An example of a
4632 path match is path='/org/freedesktop/Hal/Manager'</entry>
4635 <entry><literal>path_namespace</literal></entry>
4636 <entry>An object path</entry>
4639 Matches messages which are sent from or to an
4640 object for which the object path is either the
4641 given value, or that value followed by one or
4642 more path components.
4647 <literal>path_namespace='/com/example/foo'</literal>
4648 would match signals sent by
4649 <literal>/com/example/foo</literal>
4651 <literal>/com/example/foo/bar</literal>,
4653 <literal>/com/example/foobar</literal>.
4657 Using both <literal>path</literal> and
4658 <literal>path_namespace</literal> in the same match
4659 rule is not allowed.
4664 This match key was added in version 0.16 of the
4665 D-Bus specification and implemented by the bus
4666 daemon in dbus 1.5.0 and later.
4672 <entry><literal>destination</literal></entry>
4673 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4674 <entry>Matches messages which are being sent to the given unique name. An
4675 example of a destination match is destination=':1.0'</entry>
4678 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4679 <entry>Any string</entry>
4680 <entry>Arg matches are special and are used for further restricting the
4681 match based on the arguments in the body of a message. Only arguments of type
4682 STRING can be matched in this way. An example of an argument match
4683 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4687 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4688 <entry>Any string</entry>
4690 <para>Argument path matches provide a specialised form of wildcard matching for
4691 path-like namespaces. They can match arguments whose type is either STRING or
4692 OBJECT_PATH. As with normal argument matches,
4693 if the argument is exactly equal to the string given in the match
4694 rule then the rule is satisfied. Additionally, there is also a
4695 match when either the string given in the match rule or the
4696 appropriate message argument ends with '/' and is a prefix of the
4697 other. An example argument path match is arg0path='/aa/bb/'. This
4698 would match messages with first arguments of '/', '/aa/',
4699 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4700 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4702 <para>This is intended for monitoring “directories” in file system-like
4703 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4704 system. An application interested in all nodes in a particular hierarchy would
4705 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4706 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4707 represent a modification to the “bar” property, or a signal with zeroth
4708 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4709 many properties within that directory, and the interested application would be
4710 notified in both cases.</para>
4713 This match key was added in version 0.12 of the
4714 D-Bus specification, implemented for STRING
4715 arguments by the bus daemon in dbus 1.2.0 and later,
4716 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4723 <entry><literal>arg0namespace</literal></entry>
4724 <entry>Like a bus name, except that the string is not
4725 required to contain a '.' (period)</entry>
4727 <para>Match messages whose first argument is of type STRING, and is a bus name
4728 or interface name within the specified namespace. This is primarily intended
4729 for watching name owner changes for a group of related bus names, rather than
4730 for a single name or all name changes.</para>
4732 <para>Because every valid interface name is also a valid
4733 bus name, this can also be used for messages whose
4734 first argument is an interface name.</para>
4736 <para>For example, the match rule
4737 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4738 matches name owner changes for bus names such as
4739 <literal>com.example.backend.foo</literal>,
4740 <literal>com.example.backend.foo.bar</literal>, and
4741 <literal>com.example.backend</literal> itself.</para>
4743 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4746 This match key was added in version 0.16 of the
4747 D-Bus specification and implemented by the bus
4748 daemon in dbus 1.5.0 and later.
4754 <entry><literal>eavesdrop</literal></entry>
4755 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4756 <entry>Since D-Bus 1.5.6, match rules do not
4757 match messages which have a <literal>DESTINATION</literal>
4758 field unless the match rule specifically
4760 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4761 by specifying <literal>eavesdrop='true'</literal>
4762 in the match rule. <literal>eavesdrop='false'</literal>
4763 restores the default behaviour. Messages are
4764 delivered to their <literal>DESTINATION</literal>
4765 regardless of match rules, so this match does not
4766 affect normal delivery of unicast messages.
4767 If the message bus has a security policy which forbids
4768 eavesdropping, this match may still be used without error,
4769 but will not have any practical effect.
4770 In older versions of D-Bus, this match was not allowed
4771 in match rules, and all match rules behaved as if
4772 <literal>eavesdrop='true'</literal> had been used.
4781 <sect2 id="message-bus-starting-services">
4782 <title>Message Bus Starting Services</title>
4784 The message bus can start applications on behalf of other applications.
4785 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4786 An application that can be started in this way is called a
4787 <firstterm>service</firstterm>.
4790 With D-Bus, starting a service is normally done by name. That is,
4791 applications ask the message bus to start some program that will own a
4792 well-known name, such as <literal>com.example.TextEditor</literal>.
4793 This implies a contract documented along with the name
4794 <literal>com.example.TextEditor</literal> for which object
4795 the owner of that name will provide, and what interfaces those
4799 To find an executable corresponding to a particular name, the bus daemon
4800 looks for <firstterm>service description files</firstterm>. Service
4801 description files define a mapping from names to executables. Different
4802 kinds of message bus will look for these files in different places, see
4803 <xref linkend="message-bus-types"/>.
4806 Service description files have the ".service" file
4807 extension. The message bus will only load service description files
4808 ending with .service; all other files will be ignored. The file format
4809 is similar to that of <ulink
4810 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4811 entries</ulink>. All service description files must be in UTF-8
4812 encoding. To ensure that there will be no name collisions, service files
4813 must be namespaced using the same mechanism as messages and service
4818 On the well-known system bus, the name of a service description file
4819 must be its well-known name plus <literal>.service</literal>,
4821 <literal>com.example.ConfigurationDatabase.service</literal>.
4825 On the well-known session bus, services should follow the same
4826 service description file naming convention as on the system bus,
4827 but for backwards compatibility they are not required to do so.
4831 [FIXME the file format should be much better specified than "similar to
4832 .desktop entries" esp. since desktop entries are already
4833 badly-specified. ;-)]
4834 These sections from the specification apply to service files as well:
4837 <listitem><para>General syntax</para></listitem>
4838 <listitem><para>Comment format</para></listitem>
4841 Service description files must contain a
4842 <literal>D-BUS Service</literal> group with at least the keys
4843 <literal>Name</literal> (the well-known name of the service)
4844 and <literal>Exec</literal> (the command to be executed).
4847 <title>Example service description file</title>
4849 # Sample service description file
4851 Name=com.example.ConfigurationDatabase
4852 Exec=/usr/bin/sample-configd
4858 Additionally, service description files for the well-known system
4859 bus on Unix must contain a <literal>User</literal> key, whose value
4860 is the name of a user account (e.g. <literal>root</literal>).
4861 The system service will be run as that user.
4865 When an application asks to start a service by name, the bus daemon tries to
4866 find a service that will own that name. It then tries to spawn the
4867 executable associated with it. If this fails, it will report an
4872 On the well-known system bus, it is not possible for two .service files
4873 in the same directory to offer the same service, because they are
4874 constrained to have names that match the service name.
4878 On the well-known session bus, if two .service files in the same
4879 directory offer the same service name, the result is undefined.
4880 Distributors should avoid this situation, for instance by naming
4881 session services' .service files according to their service name.
4885 If two .service files in different directories offer the same
4886 service name, the one in the higher-priority directory is used:
4887 for instance, on the system bus, .service files in
4888 /usr/local/share/dbus-1/system-services take precedence over those
4889 in /usr/share/dbus-1/system-services.
4892 The executable launched will have the environment variable
4893 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4894 message bus so it can connect and request the appropriate names.
4897 The executable being launched may want to know whether the message bus
4898 starting it is one of the well-known message buses (see <xref
4899 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4900 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4901 of the well-known buses. The currently-defined values for this variable
4902 are <literal>system</literal> for the systemwide message bus,
4903 and <literal>session</literal> for the per-login-session message
4904 bus. The new executable must still connect to the address given
4905 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4906 resulting connection is to the well-known bus.
4909 [FIXME there should be a timeout somewhere, either specified
4910 in the .service file, by the client, or just a global value
4911 and if the client being activated fails to connect within that
4912 timeout, an error should be sent back.]
4915 <sect3 id="message-bus-starting-services-scope">
4916 <title>Message Bus Service Scope</title>
4918 The "scope" of a service is its "per-", such as per-session,
4919 per-machine, per-home-directory, or per-display. The reference
4920 implementation doesn't yet support starting services in a different
4921 scope from the message bus itself. So e.g. if you start a service
4922 on the session bus its scope is per-session.
4925 We could add an optional scope to a bus name. For example, for
4926 per-(display,session pair), we could have a unique ID for each display
4927 generated automatically at login and set on screen 0 by executing a
4928 special "set display ID" binary. The ID would be stored in a
4929 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4930 random bytes. This ID would then be used to scope names.
4931 Starting/locating a service could be done by ID-name pair rather than
4935 Contrast this with a per-display scope. To achieve that, we would
4936 want a single bus spanning all sessions using a given display.
4937 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4938 property on screen 0 of the display, pointing to this bus.
4943 <sect2 id="message-bus-types">
4944 <title>Well-known Message Bus Instances</title>
4946 Two standard message bus instances are defined here, along with how
4947 to locate them and where their service files live.
4949 <sect3 id="message-bus-types-login">
4950 <title>Login session message bus</title>
4952 Each time a user logs in, a <firstterm>login session message
4953 bus</firstterm> may be started. All applications in the user's login
4954 session may interact with one another using this message bus.
4957 The address of the login session message bus is given
4958 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4959 variable. If that variable is not set, applications may
4960 also try to read the address from the X Window System root
4961 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4962 The root window property must have type <literal>STRING</literal>.
4963 The environment variable should have precedence over the
4964 root window property.
4966 <para>The address of the login session message bus is given in the
4967 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4968 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4969 "autolaunch:", the system should use platform-specific methods of
4970 locating a running D-Bus session server, or starting one if a running
4971 instance cannot be found. Note that this mechanism is not recommended
4972 for attempting to determine if a daemon is running. It is inherently
4973 racy to attempt to make this determination, since the bus daemon may
4974 be started just before or just after the determination is made.
4975 Therefore, it is recommended that applications do not try to make this
4976 determination for their functionality purposes, and instead they
4977 should attempt to start the server.</para>
4979 <sect4 id="message-bus-types-login-x-windows">
4980 <title>X Windowing System</title>
4982 For the X Windowing System, the application must locate the
4983 window owner of the selection represented by the atom formed by
4987 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4991 <para>the current user's username</para>
4995 <para>the literal character '_' (underscore)</para>
4999 <para>the machine's ID</para>
5005 The following properties are defined for the window that owns
5007 <informaltable frame="all">
5016 <para>meaning</para>
5022 <para>_DBUS_SESSION_BUS_ADDRESS</para>
5026 <para>the actual address of the server socket</para>
5032 <para>_DBUS_SESSION_BUS_PID</para>
5036 <para>the PID of the server process</para>
5045 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5046 present in this window.
5050 If the X selection cannot be located or if reading the
5051 properties from the window fails, the implementation MUST conclude
5052 that there is no D-Bus server running and proceed to start a new
5053 server. (See below on concurrency issues)
5057 Failure to connect to the D-Bus server address thus obtained
5058 MUST be treated as a fatal connection error and should be reported
5063 As an alternative, an implementation MAY find the information
5064 in the following file located in the current user's home directory,
5065 in subdirectory .dbus/session-bus/:
5068 <para>the machine's ID</para>
5072 <para>the literal character '-' (dash)</para>
5076 <para>the X display without the screen number, with the
5077 following prefixes removed, if present: ":", "localhost:"
5078 ."localhost.localdomain:". That is, a display of
5079 "localhost:10.0" produces just the number "10"</para>
5085 The contents of this file NAME=value assignment pairs and
5086 lines starting with # are comments (no comments are allowed
5087 otherwise). The following variable names are defined:
5094 <para>Variable</para>
5098 <para>meaning</para>
5104 <para>DBUS_SESSION_BUS_ADDRESS</para>
5108 <para>the actual address of the server socket</para>
5114 <para>DBUS_SESSION_BUS_PID</para>
5118 <para>the PID of the server process</para>
5124 <para>DBUS_SESSION_BUS_WINDOWID</para>
5128 <para>the window ID</para>
5137 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5142 Failure to open this file MUST be interpreted as absence of a
5143 running server. Therefore, the implementation MUST proceed to
5144 attempting to launch a new bus server if the file cannot be
5149 However, success in opening this file MUST NOT lead to the
5150 conclusion that the server is running. Thus, a failure to connect to
5151 the bus address obtained by the alternative method MUST NOT be
5152 considered a fatal error. If the connection cannot be established,
5153 the implementation MUST proceed to check the X selection settings or
5154 to start the server on its own.
5158 If the implementation concludes that the D-Bus server is not
5159 running it MUST attempt to start a new server and it MUST also
5160 ensure that the daemon started as an effect of the "autolaunch"
5161 mechanism provides the lookup mechanisms described above, so
5162 subsequent calls can locate the newly started server. The
5163 implementation MUST also ensure that if two or more concurrent
5164 initiations happen, only one server remains running and all other
5165 initiations are able to obtain the address of this server and
5166 connect to it. In other words, the implementation MUST ensure that
5167 the X selection is not present when it attempts to set it, without
5168 allowing another process to set the selection between the
5169 verification and the setting (e.g., by using XGrabServer /
5176 On Unix systems, the session bus should search for .service files
5177 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5179 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5180 Implementations may also search additional locations, which
5181 should be searched with lower priority than anything in
5182 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5183 for example, the reference implementation also
5184 looks in <literal>${datadir}/dbus-1/services</literal> as
5185 set at compile time.
5188 As described in the XDG Base Directory Specification, software
5189 packages should install their session .service files to their
5190 configured <literal>${datadir}/dbus-1/services</literal>,
5191 where <literal>${datadir}</literal> is as defined by the GNU
5192 coding standards. System administrators or users can arrange
5193 for these service files to be read by setting XDG_DATA_DIRS or by
5194 symlinking them into the default locations.
5198 <sect3 id="message-bus-types-system">
5199 <title>System message bus</title>
5201 A computer may have a <firstterm>system message bus</firstterm>,
5202 accessible to all applications on the system. This message bus may be
5203 used to broadcast system events, such as adding new hardware devices,
5204 changes in the printer queue, and so forth.
5207 The address of the system message bus is given
5208 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5209 variable. If that variable is not set, applications should try
5210 to connect to the well-known address
5211 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5214 The D-Bus reference implementation actually honors the
5215 <literal>$(localstatedir)</literal> configure option
5216 for this address, on both client and server side.
5221 On Unix systems, the system bus should default to searching
5222 for .service files in
5223 <literal>/usr/local/share/dbus-1/system-services</literal>,
5224 <literal>/usr/share/dbus-1/system-services</literal> and
5225 <literal>/lib/dbus-1/system-services</literal>, with that order
5226 of precedence. It may also search other implementation-specific
5227 locations, but should not vary these locations based on environment
5231 The system bus is security-sensitive and is typically executed
5232 by an init system with a clean environment. Its launch helper
5233 process is particularly security-sensitive, and specifically
5234 clears its own environment.
5239 Software packages should install their system .service
5240 files to their configured
5241 <literal>${datadir}/dbus-1/system-services</literal>,
5242 where <literal>${datadir}</literal> is as defined by the GNU
5243 coding standards. System administrators can arrange
5244 for these service files to be read by editing the system bus'
5245 configuration file or by symlinking them into the default
5251 <sect2 id="message-bus-messages">
5252 <title>Message Bus Messages</title>
5254 The special message bus name <literal>org.freedesktop.DBus</literal>
5255 responds to a number of additional messages.
5258 <sect3 id="bus-messages-hello">
5259 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5270 <entry>Argument</entry>
5272 <entry>Description</entry>
5278 <entry>STRING</entry>
5279 <entry>Unique name assigned to the connection</entry>
5286 Before an application is able to send messages to other applications
5287 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5288 to the message bus to obtain a unique name. If an application without
5289 a unique name tries to send a message to another application, or a
5290 message to the message bus itself that isn't the
5291 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5292 disconnected from the bus.
5295 There is no corresponding "disconnect" request; if a client wishes to
5296 disconnect from the bus, it simply closes the socket (or other
5297 communication channel).
5300 <sect3 id="bus-messages-list-names">
5301 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5305 ARRAY of STRING ListNames ()
5312 <entry>Argument</entry>
5314 <entry>Description</entry>
5320 <entry>ARRAY of STRING</entry>
5321 <entry>Array of strings where each string is a bus name</entry>
5328 Returns a list of all currently-owned names on the bus.
5331 <sect3 id="bus-messages-list-activatable-names">
5332 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5336 ARRAY of STRING ListActivatableNames ()
5343 <entry>Argument</entry>
5345 <entry>Description</entry>
5351 <entry>ARRAY of STRING</entry>
5352 <entry>Array of strings where each string is a bus name</entry>
5359 Returns a list of all names that can be activated on the bus.
5362 <sect3 id="bus-messages-name-exists">
5363 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5367 BOOLEAN NameHasOwner (in STRING name)
5374 <entry>Argument</entry>
5376 <entry>Description</entry>
5382 <entry>STRING</entry>
5383 <entry>Name to check</entry>
5393 <entry>Argument</entry>
5395 <entry>Description</entry>
5401 <entry>BOOLEAN</entry>
5402 <entry>Return value, true if the name exists</entry>
5409 Checks if the specified name exists (currently has an owner).
5413 <sect3 id="bus-messages-name-owner-changed">
5414 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5418 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5425 <entry>Argument</entry>
5427 <entry>Description</entry>
5433 <entry>STRING</entry>
5434 <entry>Name with a new owner</entry>
5438 <entry>STRING</entry>
5439 <entry>Old owner or empty string if none</entry>
5443 <entry>STRING</entry>
5444 <entry>New owner or empty string if none</entry>
5451 This signal indicates that the owner of a name has changed.
5452 It's also the signal to use to detect the appearance of
5453 new names on the bus.
5456 <sect3 id="bus-messages-name-lost">
5457 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5461 NameLost (STRING name)
5468 <entry>Argument</entry>
5470 <entry>Description</entry>
5476 <entry>STRING</entry>
5477 <entry>Name which was lost</entry>
5484 This signal is sent to a specific application when it loses
5485 ownership of a name.
5489 <sect3 id="bus-messages-name-acquired">
5490 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5494 NameAcquired (STRING name)
5501 <entry>Argument</entry>
5503 <entry>Description</entry>
5509 <entry>STRING</entry>
5510 <entry>Name which was acquired</entry>
5517 This signal is sent to a specific application when it gains
5518 ownership of a name.
5522 <sect3 id="bus-messages-start-service-by-name">
5523 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5527 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5534 <entry>Argument</entry>
5536 <entry>Description</entry>
5542 <entry>STRING</entry>
5543 <entry>Name of the service to start</entry>
5547 <entry>UINT32</entry>
5548 <entry>Flags (currently not used)</entry>
5558 <entry>Argument</entry>
5560 <entry>Description</entry>
5566 <entry>UINT32</entry>
5567 <entry>Return value</entry>
5572 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5576 The return value can be one of the following values:
5581 <entry>Identifier</entry>
5582 <entry>Value</entry>
5583 <entry>Description</entry>
5588 <entry>DBUS_START_REPLY_SUCCESS</entry>
5590 <entry>The service was successfully started.</entry>
5593 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5595 <entry>A connection already owns the given name.</entry>
5604 <sect3 id="bus-messages-update-activation-environment">
5605 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5609 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5616 <entry>Argument</entry>
5618 <entry>Description</entry>
5624 <entry>ARRAY of DICT<STRING,STRING></entry>
5625 <entry>Environment to add or update</entry>
5630 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5633 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5636 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.
5641 <sect3 id="bus-messages-get-name-owner">
5642 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5646 STRING GetNameOwner (in STRING name)
5653 <entry>Argument</entry>
5655 <entry>Description</entry>
5661 <entry>STRING</entry>
5662 <entry>Name to get the owner of</entry>
5672 <entry>Argument</entry>
5674 <entry>Description</entry>
5680 <entry>STRING</entry>
5681 <entry>Return value, a unique connection name</entry>
5686 Returns the unique connection name of the primary owner of the name
5687 given. If the requested name doesn't have an owner, returns a
5688 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5692 <sect3 id="bus-messages-get-connection-unix-user">
5693 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5697 UINT32 GetConnectionUnixUser (in STRING bus_name)
5704 <entry>Argument</entry>
5706 <entry>Description</entry>
5712 <entry>STRING</entry>
5713 <entry>Unique or well-known bus name of the connection to
5714 query, such as <literal>:12.34</literal> or
5715 <literal>com.example.tea</literal></entry>
5725 <entry>Argument</entry>
5727 <entry>Description</entry>
5733 <entry>UINT32</entry>
5734 <entry>Unix user ID</entry>
5739 Returns the Unix user ID of the process connected to the server. If
5740 unable to determine it (for instance, because the process is not on the
5741 same machine as the bus daemon), an error is returned.
5745 <sect3 id="bus-messages-get-connection-unix-process-id">
5746 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5750 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5757 <entry>Argument</entry>
5759 <entry>Description</entry>
5765 <entry>STRING</entry>
5766 <entry>Unique or well-known bus name of the connection to
5767 query, such as <literal>:12.34</literal> or
5768 <literal>com.example.tea</literal></entry>
5778 <entry>Argument</entry>
5780 <entry>Description</entry>
5786 <entry>UINT32</entry>
5787 <entry>Unix process id</entry>
5792 Returns the Unix process ID of the process connected to the server. If
5793 unable to determine it (for instance, because the process is not on the
5794 same machine as the bus daemon), an error is returned.
5798 <sect3 id="bus-messages-get-connection-credentials">
5799 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5803 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5810 <entry>Argument</entry>
5812 <entry>Description</entry>
5818 <entry>STRING</entry>
5819 <entry>Unique or well-known bus name of the connection to
5820 query, such as <literal>:12.34</literal> or
5821 <literal>com.example.tea</literal></entry>
5831 <entry>Argument</entry>
5833 <entry>Description</entry>
5839 <entry>DICT<STRING,VARIANT></entry>
5840 <entry>Credentials</entry>
5848 Returns as many credentials as possible for the process connected to
5849 the server. If unable to determine certain credentials (for instance,
5850 because the process is not on the same machine as the bus daemon,
5851 or because this version of the bus daemon does not support a
5852 particular security framework), or if the values of those credentials
5853 cannot be represented as documented here, then those credentials
5858 Keys in the returned dictionary not containing "." are defined
5859 by this specification. Bus daemon implementors supporting
5860 credentials frameworks not mentioned in this document should either
5861 contribute patches to this specification, or use keys containing
5862 "." and starting with a reversed domain name.
5868 <entry>Value type</entry>
5869 <entry>Value</entry>
5874 <entry>UnixUserID</entry>
5875 <entry>UINT32</entry>
5876 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5879 <entry>ProcessID</entry>
5880 <entry>UINT32</entry>
5881 <entry>The numeric process ID, on platforms that have
5882 this concept. On Unix, this is the process ID defined by
5891 This method was added in D-Bus 1.7 to reduce the round-trips
5892 required to list a process's credentials. In older versions, calling
5893 this method will fail: applications should recover by using the
5894 separate methods such as
5895 <xref linkend="bus-messages-get-connection-unix-user"/>
5900 <sect3 id="bus-messages-get-adt-audit-session-data">
5901 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5905 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5912 <entry>Argument</entry>
5914 <entry>Description</entry>
5920 <entry>STRING</entry>
5921 <entry>Unique or well-known bus name of the connection to
5922 query, such as <literal>:12.34</literal> or
5923 <literal>com.example.tea</literal></entry>
5933 <entry>Argument</entry>
5935 <entry>Description</entry>
5941 <entry>ARRAY of BYTE</entry>
5942 <entry>auditing data as returned by
5943 adt_export_session_data()</entry>
5948 Returns auditing data used by Solaris ADT, in an unspecified
5949 binary format. If you know what this means, please contribute
5950 documentation via the D-Bus bug tracking system.
5951 This method is on the core DBus interface for historical reasons;
5952 the same information should be made available via
5953 <xref linkend="bus-messages-get-connection-credentials"/>
5958 <sect3 id="bus-messages-get-connection-selinux-security-context">
5959 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5963 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5970 <entry>Argument</entry>
5972 <entry>Description</entry>
5978 <entry>STRING</entry>
5979 <entry>Unique or well-known bus name of the connection to
5980 query, such as <literal>:12.34</literal> or
5981 <literal>com.example.tea</literal></entry>
5991 <entry>Argument</entry>
5993 <entry>Description</entry>
5999 <entry>ARRAY of BYTE</entry>
6000 <entry>some sort of string of bytes, not necessarily UTF-8,
6001 not including '\0'</entry>
6006 Returns the security context used by SELinux, in an unspecified
6007 format. If you know what this means, please contribute
6008 documentation via the D-Bus bug tracking system.
6009 This method is on the core DBus interface for historical reasons;
6010 the same information should be made available via
6011 <xref linkend="bus-messages-get-connection-credentials"/>
6017 <sect3 id="bus-messages-add-match">
6018 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
6022 AddMatch (in STRING rule)
6029 <entry>Argument</entry>
6031 <entry>Description</entry>
6037 <entry>STRING</entry>
6038 <entry>Match rule to add to the connection</entry>
6043 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6044 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6048 <sect3 id="bus-messages-remove-match">
6049 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6053 RemoveMatch (in STRING rule)
6060 <entry>Argument</entry>
6062 <entry>Description</entry>
6068 <entry>STRING</entry>
6069 <entry>Match rule to remove from the connection</entry>
6074 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6075 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6080 <sect3 id="bus-messages-get-id">
6081 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6085 GetId (out STRING id)
6092 <entry>Argument</entry>
6094 <entry>Description</entry>
6100 <entry>STRING</entry>
6101 <entry>Unique ID identifying the bus daemon</entry>
6106 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6107 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6108 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6109 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6110 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6111 by org.freedesktop.DBus.Peer.GetMachineId().
6112 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6120 <appendix id="implementation-notes">
6121 <title>Implementation notes</title>
6122 <sect1 id="implementation-notes-subsection">
6130 <glossary><title>Glossary</title>
6132 This glossary defines some of the terms used in this specification.
6135 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6138 The message bus maintains an association between names and
6139 connections. (Normally, there's one connection per application.) A
6140 bus name is simply an identifier used to locate connections. For
6141 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6142 name might be used to send a message to a screensaver from Yoyodyne
6143 Corporation. An application is said to <firstterm>own</firstterm> a
6144 name if the message bus has associated the application's connection
6145 with the name. Names may also have <firstterm>queued
6146 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6147 The bus assigns a unique name to each connection,
6148 see <xref linkend="term-unique-name"/>. Other names
6149 can be thought of as "well-known names" and are
6150 used to find applications that offer specific functionality.
6154 See <xref linkend="message-protocol-names-bus"/> for details of
6155 the syntax and naming conventions for bus names.
6160 <glossentry id="term-message"><glossterm>Message</glossterm>
6163 A message is the atomic unit of communication via the D-Bus
6164 protocol. It consists of a <firstterm>header</firstterm> and a
6165 <firstterm>body</firstterm>; the body is made up of
6166 <firstterm>arguments</firstterm>.
6171 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6174 The message bus is a special application that forwards
6175 or routes messages between a group of applications
6176 connected to the message bus. It also manages
6177 <firstterm>names</firstterm> used for routing
6183 <glossentry id="term-name"><glossterm>Name</glossterm>
6186 See <xref linkend="term-bus-name"/>. "Name" may
6187 also be used to refer to some of the other names
6188 in D-Bus, such as interface names.
6193 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6196 Used to prevent collisions when defining new interfaces, bus names
6197 etc. The convention used is the same one Java uses for defining
6198 classes: a reversed domain name.
6199 See <xref linkend="message-protocol-names-bus"/>,
6200 <xref linkend="message-protocol-names-interface"/>,
6201 <xref linkend="message-protocol-names-error"/>,
6202 <xref linkend="message-protocol-marshaling-object-path"/>.
6207 <glossentry id="term-object"><glossterm>Object</glossterm>
6210 Each application contains <firstterm>objects</firstterm>, which have
6211 <firstterm>interfaces</firstterm> and
6212 <firstterm>methods</firstterm>. Objects are referred to by a name,
6213 called a <firstterm>path</firstterm>.
6218 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6221 An application talking directly to another application, without going
6222 through a message bus. One-to-one connections may be "peer to peer" or
6223 "client to server." The D-Bus protocol has no concept of client
6224 vs. server after a connection has authenticated; the flow of messages
6225 is symmetrical (full duplex).
6230 <glossentry id="term-path"><glossterm>Path</glossterm>
6233 Object references (object names) in D-Bus are organized into a
6234 filesystem-style hierarchy, so each object is named by a path. As in
6235 LDAP, there's no difference between "files" and "directories"; a path
6236 can refer to an object, while still having child objects below it.
6241 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6244 Each bus name has a primary owner; messages sent to the name go to the
6245 primary owner. However, certain names also maintain a queue of
6246 secondary owners "waiting in the wings." If the primary owner releases
6247 the name, then the first secondary owner in the queue automatically
6248 becomes the new owner of the name.
6253 <glossentry id="term-service"><glossterm>Service</glossterm>
6256 A service is an executable that can be launched by the bus daemon.
6257 Services normally guarantee some particular features, for example they
6258 may guarantee that they will request a specific name such as
6259 "com.example.Screensaver", have a singleton object
6260 "/com/example/Application", and that object will implement the
6261 interface "com.example.Screensaver.Control".
6266 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6269 ".service files" tell the bus about service applications that can be
6270 launched (see <xref linkend="term-service"/>). Most importantly they
6271 provide a mapping from bus names to services that will request those
6272 names when they start up.
6277 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6280 The special name automatically assigned to each connection by the
6281 message bus. This name will never change owner, and will be unique
6282 (never reused during the lifetime of the message bus).
6283 It will begin with a ':' character.