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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.24</releaseinfo>
10 <date>(not yet released)</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>0.24</revnumber>
76 <date>(not yet released)</date>
77 <authorinitials>n/a</authorinitials>
79 see <ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink>
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.</entry>
1307 <entry><literal>METHOD_RETURN</literal></entry>
1309 <entry>Method reply with returned data.</entry>
1312 <entry><literal>ERROR</literal></entry>
1314 <entry>Error reply. If the first argument exists and is a
1315 string, it is an error message.</entry>
1318 <entry><literal>SIGNAL</literal></entry>
1320 <entry>Signal emission.</entry>
1327 Flags that can appear in the third byte of the header:
1332 <entry>Conventional name</entry>
1333 <entry>Hex value</entry>
1334 <entry>Description</entry>
1339 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1341 <entry>This message does not expect method return replies or
1342 error replies; the reply can be omitted as an
1343 optimization. However, it is compliant with this specification
1344 to return the reply despite this flag and the only harm
1345 from doing so is extra network traffic.
1349 <entry><literal>NO_AUTO_START</literal></entry>
1351 <entry>The bus must not launch an owner
1352 for the destination name in response to this message.
1360 <sect3 id="message-protocol-header-fields">
1361 <title>Header Fields</title>
1364 The array at the end of the header contains <firstterm>header
1365 fields</firstterm>, where each field is a 1-byte field code followed
1366 by a field value. A header must contain the required header fields for
1367 its message type, and zero or more of any optional header
1368 fields. Future versions of this protocol specification may add new
1369 fields. Implementations must ignore fields they do not
1370 understand. Implementations must not invent their own header fields;
1371 only changes to this specification may introduce new header fields.
1375 Again, if an implementation sees a header field code that it does not
1376 expect, it must ignore that field, as it will be part of a new
1377 (but compatible) version of this specification. This also applies
1378 to known header fields appearing in unexpected messages, for
1379 example: if a signal has a reply serial it must be ignored
1380 even though it has no meaning as of this version of the spec.
1384 However, implementations must not send or accept known header fields
1385 with the wrong type stored in the field value. So for example a
1386 message with an <literal>INTERFACE</literal> field of type
1387 <literal>UINT32</literal> would be considered corrupt.
1391 Here are the currently-defined header fields:
1396 <entry>Conventional Name</entry>
1397 <entry>Decimal Code</entry>
1399 <entry>Required In</entry>
1400 <entry>Description</entry>
1405 <entry><literal>INVALID</literal></entry>
1408 <entry>not allowed</entry>
1409 <entry>Not a valid field name (error if it appears in a message)</entry>
1412 <entry><literal>PATH</literal></entry>
1414 <entry><literal>OBJECT_PATH</literal></entry>
1415 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1416 <entry>The object to send a call to,
1417 or the object a signal is emitted from.
1419 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1420 implementations should not send messages with this path,
1421 and the reference implementation of the bus daemon will
1422 disconnect any application that attempts to do so.
1426 <entry><literal>INTERFACE</literal></entry>
1428 <entry><literal>STRING</literal></entry>
1429 <entry><literal>SIGNAL</literal></entry>
1431 The interface to invoke a method call on, or
1432 that a signal is emitted from. Optional for
1433 method calls, required for signals.
1434 The special interface
1435 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1436 implementations should not send messages with this
1437 interface, and the reference implementation of the bus
1438 daemon will disconnect any application that attempts to
1443 <entry><literal>MEMBER</literal></entry>
1445 <entry><literal>STRING</literal></entry>
1446 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1447 <entry>The member, either the method name or signal name.</entry>
1450 <entry><literal>ERROR_NAME</literal></entry>
1452 <entry><literal>STRING</literal></entry>
1453 <entry><literal>ERROR</literal></entry>
1454 <entry>The name of the error that occurred, for errors</entry>
1457 <entry><literal>REPLY_SERIAL</literal></entry>
1459 <entry><literal>UINT32</literal></entry>
1460 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1461 <entry>The serial number of the message this message is a reply
1462 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1465 <entry><literal>DESTINATION</literal></entry>
1467 <entry><literal>STRING</literal></entry>
1468 <entry>optional</entry>
1469 <entry>The name of the connection this message is intended for.
1470 Only used in combination with the message bus, see
1471 <xref linkend="message-bus"/>.</entry>
1474 <entry><literal>SENDER</literal></entry>
1476 <entry><literal>STRING</literal></entry>
1477 <entry>optional</entry>
1478 <entry>Unique name of the sending connection.
1479 The message bus fills in this field so it is reliable; the field is
1480 only meaningful in combination with the message bus.</entry>
1483 <entry><literal>SIGNATURE</literal></entry>
1485 <entry><literal>SIGNATURE</literal></entry>
1486 <entry>optional</entry>
1487 <entry>The signature of the message body.
1488 If omitted, it is assumed to be the
1489 empty signature "" (i.e. the body must be 0-length).</entry>
1492 <entry><literal>UNIX_FDS</literal></entry>
1494 <entry><literal>UINT32</literal></entry>
1495 <entry>optional</entry>
1496 <entry>The number of Unix file descriptors that
1497 accompany the message. If omitted, it is assumed
1498 that no Unix file descriptors accompany the
1499 message. The actual file descriptors need to be
1500 transferred via platform specific mechanism
1501 out-of-band. They must be sent at the same time as
1502 part of the message itself. They may not be sent
1503 before the first byte of the message itself is
1504 transferred or after the last byte of the message
1514 <sect2 id="message-protocol-names">
1515 <title>Valid Names</title>
1517 The various names in D-Bus messages have some restrictions.
1520 There is a <firstterm>maximum name length</firstterm>
1521 of 255 which applies to bus names, interfaces, and members.
1523 <sect3 id="message-protocol-names-interface">
1524 <title>Interface names</title>
1526 Interfaces have names with type <literal>STRING</literal>, meaning that
1527 they must be valid UTF-8. However, there are also some
1528 additional restrictions that apply to interface names
1531 <listitem><para>Interface names are composed of 1 or more elements separated by
1532 a period ('.') character. All elements must contain at least
1536 <listitem><para>Each element must only contain the ASCII characters
1537 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1541 <listitem><para>Interface names must contain at least one '.' (period)
1542 character (and thus at least two elements).
1545 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1546 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1551 Interface names should start with the reversed DNS domain name of
1552 the author of the interface (in lower-case), like interface names
1553 in Java. It is conventional for the rest of the interface name
1554 to consist of words run together, with initial capital letters
1555 on all words ("CamelCase"). Several levels of hierarchy can be used.
1556 It is also a good idea to include the major version of the interface
1557 in the name, and increment it if incompatible changes are made;
1558 this way, a single object can implement several versions of an
1559 interface in parallel, if necessary.
1563 For instance, if the owner of <literal>example.com</literal> is
1564 developing a D-Bus API for a music player, they might define
1565 interfaces called <literal>com.example.MusicPlayer1</literal>,
1566 <literal>com.example.MusicPlayer1.Track</literal> and
1567 <literal>com.example.MusicPlayer1.Seekable</literal>.
1571 D-Bus does not distinguish between the concepts that would be
1572 called classes and interfaces in Java: either can be identified on
1573 D-Bus by an interface name.
1576 <sect3 id="message-protocol-names-bus">
1577 <title>Bus names</title>
1579 Connections have one or more bus names associated with them.
1580 A connection has exactly one bus name that is a <firstterm>unique
1581 connection name</firstterm>. The unique connection name remains
1582 with the connection for its entire lifetime.
1583 A bus name is of type <literal>STRING</literal>,
1584 meaning that it must be valid UTF-8. However, there are also
1585 some additional restrictions that apply to bus names
1588 <listitem><para>Bus names that start with a colon (':')
1589 character are unique connection names. Other bus names
1590 are called <firstterm>well-known bus names</firstterm>.
1593 <listitem><para>Bus names are composed of 1 or more elements separated by
1594 a period ('.') character. All elements must contain at least
1598 <listitem><para>Each element must only contain the ASCII characters
1599 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1600 connection name may begin with a digit, elements in
1601 other bus names must not begin with a digit.
1605 <listitem><para>Bus names must contain at least one '.' (period)
1606 character (and thus at least two elements).
1609 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1610 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1614 Note that the hyphen ('-') character is allowed in bus names but
1615 not in interface names.
1619 Like <link linkend="message-protocol-names-interface">interface
1620 names</link>, well-known bus names should start with the
1621 reversed DNS domain name of the author of the interface (in
1622 lower-case), and it is conventional for the rest of the well-known
1623 bus name to consist of words run together, with initial
1624 capital letters. As with interface names, including a version
1625 number in well-known bus names is a good idea; it's possible to
1626 have the well-known bus name for more than one version
1627 simultaneously if backwards compatibility is required.
1631 If a well-known bus name implies the presence of a "main" interface,
1632 that "main" interface is often given the same name as
1633 the well-known bus name, and situated at the corresponding object
1634 path. For instance, if the owner of <literal>example.com</literal>
1635 is developing a D-Bus API for a music player, they might define
1636 that any application that takes the well-known name
1637 <literal>com.example.MusicPlayer1</literal> should have an object
1638 at the object path <literal>/com/example/MusicPlayer1</literal>
1639 which implements the interface
1640 <literal>com.example.MusicPlayer1</literal>.
1643 <sect3 id="message-protocol-names-member">
1644 <title>Member names</title>
1646 Member (i.e. method or signal) names:
1648 <listitem><para>Must only contain the ASCII characters
1649 "[A-Z][a-z][0-9]_" and may not begin with a
1650 digit.</para></listitem>
1651 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1652 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1653 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1658 It is conventional for member names on D-Bus to consist of
1659 capitalized words with no punctuation ("camel-case").
1660 Method names should usually be verbs, such as
1661 <literal>GetItems</literal>, and signal names should usually be
1662 a description of an event, such as <literal>ItemsChanged</literal>.
1665 <sect3 id="message-protocol-names-error">
1666 <title>Error names</title>
1668 Error names have the same restrictions as interface names.
1672 Error names have the same naming conventions as interface
1673 names, and often contain <literal>.Error.</literal>; for instance,
1674 the owner of <literal>example.com</literal> might define the
1675 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1676 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1677 The errors defined by D-Bus itself, such as
1678 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1684 <sect2 id="message-protocol-types">
1685 <title>Message Types</title>
1687 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1688 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1689 This section describes these conventions.
1691 <sect3 id="message-protocol-types-method">
1692 <title>Method Calls</title>
1694 Some messages invoke an operation on a remote object. These are
1695 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1696 messages map naturally to methods on objects in a typical program.
1699 A method call message is required to have a <literal>MEMBER</literal> header field
1700 indicating the name of the method. Optionally, the message has an
1701 <literal>INTERFACE</literal> field giving the interface the method is a part of.
1702 Including the <literal>INTERFACE</literal> in all method call
1703 messages is strongly recommended.
1706 In the absence of an <literal>INTERFACE</literal> field, if two
1707 or more interfaces on the same object have a method with the same
1708 name, it is undefined which of those methods will be invoked.
1709 Implementations may choose to either return an error, or deliver the
1710 message as though it had an arbitrary one of those interfaces.
1713 In some situations (such as the well-known system bus), messages
1714 are filtered through an access-control list external to the
1715 remote object implementation. If that filter rejects certain
1716 messages by matching their interface, or accepts only messages
1717 to specific interfaces, it must also reject messages that have no
1718 <literal>INTERFACE</literal>: otherwise, malicious
1719 applications could use this to bypass the filter.
1722 Method call messages also include a <literal>PATH</literal> field
1723 indicating the object to invoke the method on. If the call is passing
1724 through a message bus, the message will also have a
1725 <literal>DESTINATION</literal> field giving the name of the connection
1726 to receive the message.
1729 When an application handles a method call message, it is required to
1730 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1731 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1732 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1735 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1736 are the return value(s) or "out parameters" of the method call.
1737 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1738 and the call fails; no return value will be provided. It makes
1739 no sense to send multiple replies to the same method call.
1742 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1743 reply is required, so the caller will know the method
1744 was successfully processed.
1747 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1751 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1752 then as an optimization the application receiving the method
1753 call may choose to omit the reply message (regardless of
1754 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1755 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1756 flag and reply anyway.
1759 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1760 destination name does not exist then a program to own the destination
1761 name will be started before the message is delivered. The message
1762 will be held until the new program is successfully started or has
1763 failed to start; in case of failure, an error will be returned. This
1764 flag is only relevant in the context of a message bus, it is ignored
1765 during one-to-one communication with no intermediate bus.
1767 <sect4 id="message-protocol-types-method-apis">
1768 <title>Mapping method calls to native APIs</title>
1770 APIs for D-Bus may map method calls to a method call in a specific
1771 programming language, such as C++, or may map a method call written
1772 in an IDL to a D-Bus message.
1775 In APIs of this nature, arguments to a method are often termed "in"
1776 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1777 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1778 "inout" arguments, which are both sent and received, i.e. the caller
1779 passes in a value which is modified. Mapped to D-Bus, an "inout"
1780 argument is equivalent to an "in" argument, followed by an "out"
1781 argument. You can't pass things "by reference" over the wire, so
1782 "inout" is purely an illusion of the in-process API.
1785 Given a method with zero or one return values, followed by zero or more
1786 arguments, where each argument may be "in", "out", or "inout", the
1787 caller constructs a message by appending each "in" or "inout" argument,
1788 in order. "out" arguments are not represented in the caller's message.
1791 The recipient constructs a reply by appending first the return value
1792 if any, then each "out" or "inout" argument, in order.
1793 "in" arguments are not represented in the reply message.
1796 Error replies are normally mapped to exceptions in languages that have
1800 In converting from native APIs to D-Bus, it is perhaps nice to
1801 map D-Bus naming conventions ("FooBar") to native conventions
1802 such as "fooBar" or "foo_bar" automatically. This is OK
1803 as long as you can say that the native API is one that
1804 was specifically written for D-Bus. It makes the most sense
1805 when writing object implementations that will be exported
1806 over the bus. Object proxies used to invoke remote D-Bus
1807 objects probably need the ability to call any D-Bus method,
1808 and thus a magic name mapping like this could be a problem.
1811 This specification doesn't require anything of native API bindings;
1812 the preceding is only a suggested convention for consistency
1818 <sect3 id="message-protocol-types-signal">
1819 <title>Signal Emission</title>
1821 Unlike method calls, signal emissions have no replies.
1822 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1823 It must have three header fields: <literal>PATH</literal> giving the object
1824 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1825 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1826 for signals, though it is optional for method calls.
1830 <sect3 id="message-protocol-types-errors">
1831 <title>Errors</title>
1833 Messages of type <literal>ERROR</literal> are most commonly replies
1834 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1835 to any kind of message. The message bus for example
1836 will return an <literal>ERROR</literal> in reply to a signal emission if
1837 the bus does not have enough memory to send the signal.
1840 An <literal>ERROR</literal> may have any arguments, but if the first
1841 argument is a <literal>STRING</literal>, it must be an error message.
1842 The error message may be logged or shown to the user
1847 <sect3 id="message-protocol-types-notation">
1848 <title>Notation in this document</title>
1850 This document uses a simple pseudo-IDL to describe particular method
1851 calls and signals. Here is an example of a method call:
1853 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1854 out UINT32 resultcode)
1856 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1857 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1858 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1859 characters so it's known that the last part of the name in
1860 the "IDL" is the member name.
1863 In C++ that might end up looking like this:
1865 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1866 unsigned int flags);
1868 or equally valid, the return value could be done as an argument:
1870 void org::freedesktop::DBus::StartServiceByName (const char *name,
1872 unsigned int *resultcode);
1874 It's really up to the API designer how they want to make
1875 this look. You could design an API where the namespace wasn't used
1876 in C++, using STL or Qt, using varargs, or whatever you wanted.
1879 Signals are written as follows:
1881 org.freedesktop.DBus.NameLost (STRING name)
1883 Signals don't specify "in" vs. "out" because only
1884 a single direction is possible.
1887 It isn't especially encouraged to use this lame pseudo-IDL in actual
1888 API implementations; you might use the native notation for the
1889 language you're using, or you might use COM or CORBA IDL, for example.
1894 <sect2 id="message-protocol-handling-invalid">
1895 <title>Invalid Protocol and Spec Extensions</title>
1898 For security reasons, the D-Bus protocol should be strictly parsed and
1899 validated, with the exception of defined extension points. Any invalid
1900 protocol or spec violations should result in immediately dropping the
1901 connection without notice to the other end. Exceptions should be
1902 carefully considered, e.g. an exception may be warranted for a
1903 well-understood idiosyncrasy of a widely-deployed implementation. In
1904 cases where the other end of a connection is 100% trusted and known to
1905 be friendly, skipping validation for performance reasons could also make
1906 sense in certain cases.
1910 Generally speaking violations of the "must" requirements in this spec
1911 should be considered possible attempts to exploit security, and violations
1912 of the "should" suggestions should be considered legitimate (though perhaps
1913 they should generate an error in some cases).
1917 The following extension points are built in to D-Bus on purpose and must
1918 not be treated as invalid protocol. The extension points are intended
1919 for use by future versions of this spec, they are not intended for third
1920 parties. At the moment, the only way a third party could extend D-Bus
1921 without breaking interoperability would be to introduce a way to negotiate new
1922 feature support as part of the auth protocol, using EXTENSION_-prefixed
1923 commands. There is not yet a standard way to negotiate features.
1927 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1928 commands result in an ERROR rather than a disconnect. This enables
1929 future extensions to the protocol. Commands starting with EXTENSION_ are
1930 reserved for third parties.
1935 The authentication protocol supports pluggable auth mechanisms.
1940 The address format (see <xref linkend="addresses"/>) supports new
1946 Messages with an unknown type (something other than
1947 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1948 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1949 Unknown-type messages must still be well-formed in the same way
1950 as the known messages, however. They still have the normal
1956 Header fields with an unknown or unexpected field code must be ignored,
1957 though again they must still be well-formed.
1962 New standard interfaces (with new methods and signals) can of course be added.
1972 <sect1 id="auth-protocol">
1973 <title>Authentication Protocol</title>
1975 Before the flow of messages begins, two applications must
1976 authenticate. A simple plain-text protocol is used for
1977 authentication; this protocol is a SASL profile, and maps fairly
1978 directly from the SASL specification. The message encoding is
1979 NOT used here, only plain text messages.
1982 In examples, "C:" and "S:" indicate lines sent by the client and
1983 server respectively.
1985 <sect2 id="auth-protocol-overview">
1986 <title>Protocol Overview</title>
1988 The protocol is a line-based protocol, where each line ends with
1989 \r\n. Each line begins with an all-caps ASCII command name containing
1990 only the character range [A-Z_], a space, then any arguments for the
1991 command, then the \r\n ending the line. The protocol is
1992 case-sensitive. All bytes must be in the ASCII character set.
1994 Commands from the client to the server are as follows:
1997 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1998 <listitem><para>CANCEL</para></listitem>
1999 <listitem><para>BEGIN</para></listitem>
2000 <listitem><para>DATA <data in hex encoding></para></listitem>
2001 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
2002 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
2005 From server to client are as follows:
2008 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
2009 <listitem><para>OK <GUID in hex></para></listitem>
2010 <listitem><para>DATA <data in hex encoding></para></listitem>
2011 <listitem><para>ERROR</para></listitem>
2012 <listitem><para>AGREE_UNIX_FD</para></listitem>
2016 Unofficial extensions to the command set must begin with the letters
2017 "EXTENSION_", to avoid conflicts with future official commands.
2018 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
2021 <sect2 id="auth-nul-byte">
2022 <title>Special credentials-passing nul byte</title>
2024 Immediately after connecting to the server, the client must send a
2025 single nul byte. This byte may be accompanied by credentials
2026 information on some operating systems that use sendmsg() with
2027 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
2028 sockets. However, the nul byte must be sent even on other kinds of
2029 socket, and even on operating systems that do not require a byte to be
2030 sent in order to transmit credentials. The text protocol described in
2031 this document begins after the single nul byte. If the first byte
2032 received from the client is not a nul byte, the server may disconnect
2036 A nul byte in any context other than the initial byte is an error;
2037 the protocol is ASCII-only.
2040 The credentials sent along with the nul byte may be used with the
2041 SASL mechanism EXTERNAL.
2044 <sect2 id="auth-command-auth">
2045 <title>AUTH command</title>
2047 If an AUTH command has no arguments, it is a request to list
2048 available mechanisms. The server must respond with a REJECTED
2049 command listing the mechanisms it understands, or with an error.
2052 If an AUTH command specifies a mechanism, and the server supports
2053 said mechanism, the server should begin exchanging SASL
2054 challenge-response data with the client using DATA commands.
2057 If the server does not support the mechanism given in the AUTH
2058 command, it must send either a REJECTED command listing the mechanisms
2059 it does support, or an error.
2062 If the [initial-response] argument is provided, it is intended for use
2063 with mechanisms that have no initial challenge (or an empty initial
2064 challenge), as if it were the argument to an initial DATA command. If
2065 the selected mechanism has an initial challenge and [initial-response]
2066 was provided, the server should reject authentication by sending
2070 If authentication succeeds after exchanging DATA commands,
2071 an OK command must be sent to the client.
2074 The first octet received by the server after the \r\n of the BEGIN
2075 command from the client must be the first octet of the
2076 authenticated/encrypted stream of D-Bus messages.
2079 If BEGIN is received by the server, the first octet received
2080 by the client after the \r\n of the OK command must be the
2081 first octet of the authenticated/encrypted stream of D-Bus
2085 <sect2 id="auth-command-cancel">
2086 <title>CANCEL Command</title>
2088 At any time up to sending the BEGIN command, the client may send a
2089 CANCEL command. On receiving the CANCEL command, the server must
2090 send a REJECTED command and abort the current authentication
2094 <sect2 id="auth-command-data">
2095 <title>DATA Command</title>
2097 The DATA command may come from either client or server, and simply
2098 contains a hex-encoded block of data to be interpreted
2099 according to the SASL mechanism in use.
2102 Some SASL mechanisms support sending an "empty string";
2103 FIXME we need some way to do this.
2106 <sect2 id="auth-command-begin">
2107 <title>BEGIN Command</title>
2109 The BEGIN command acknowledges that the client has received an
2110 OK command from the server, and that the stream of messages
2114 The first octet received by the server after the \r\n of the BEGIN
2115 command from the client must be the first octet of the
2116 authenticated/encrypted stream of D-Bus messages.
2119 <sect2 id="auth-command-rejected">
2120 <title>REJECTED Command</title>
2122 The REJECTED command indicates that the current authentication
2123 exchange has failed, and further exchange of DATA is inappropriate.
2124 The client would normally try another mechanism, or try providing
2125 different responses to challenges.
2127 Optionally, the REJECTED command has a space-separated list of
2128 available auth mechanisms as arguments. If a server ever provides
2129 a list of supported mechanisms, it must provide the same list
2130 each time it sends a REJECTED message. Clients are free to
2131 ignore all lists received after the first.
2134 <sect2 id="auth-command-ok">
2135 <title>OK Command</title>
2137 The OK command indicates that the client has been
2138 authenticated. The client may now proceed with negotiating
2139 Unix file descriptor passing. To do that it shall send
2140 NEGOTIATE_UNIX_FD to the server.
2143 Otherwise, the client must respond to the OK command by
2144 sending a BEGIN command, followed by its stream of messages,
2145 or by disconnecting. The server must not accept additional
2146 commands using this protocol after the BEGIN command has been
2147 received. Further communication will be a stream of D-Bus
2148 messages (optionally encrypted, as negotiated) rather than
2152 If a client sends BEGIN the first octet received by the client
2153 after the \r\n of the OK command must be the first octet of
2154 the authenticated/encrypted stream of D-Bus messages.
2157 The OK command has one argument, which is the GUID of the server.
2158 See <xref linkend="addresses"/> for more on server GUIDs.
2161 <sect2 id="auth-command-error">
2162 <title>ERROR Command</title>
2164 The ERROR command indicates that either server or client did not
2165 know a command, does not accept the given command in the current
2166 context, or did not understand the arguments to the command. This
2167 allows the protocol to be extended; a client or server can send a
2168 command present or permitted only in new protocol versions, and if
2169 an ERROR is received instead of an appropriate response, fall back
2170 to using some other technique.
2173 If an ERROR is sent, the server or client that sent the
2174 error must continue as if the command causing the ERROR had never been
2175 received. However, the the server or client receiving the error
2176 should try something other than whatever caused the error;
2177 if only canceling/rejecting the authentication.
2180 If the D-Bus protocol changes incompatibly at some future time,
2181 applications implementing the new protocol would probably be able to
2182 check for support of the new protocol by sending a new command and
2183 receiving an ERROR from applications that don't understand it. Thus the
2184 ERROR feature of the auth protocol is an escape hatch that lets us
2185 negotiate extensions or changes to the D-Bus protocol in the future.
2188 <sect2 id="auth-command-negotiate-unix-fd">
2189 <title>NEGOTIATE_UNIX_FD Command</title>
2191 The NEGOTIATE_UNIX_FD command indicates that the client
2192 supports Unix file descriptor passing. This command may only
2193 be sent after the connection is authenticated, i.e. after OK
2194 was received by the client. This command may only be sent on
2195 transports that support Unix file descriptor passing.
2198 On receiving NEGOTIATE_UNIX_FD the server must respond with
2199 either AGREE_UNIX_FD or ERROR. It shall respond the former if
2200 the transport chosen supports Unix file descriptor passing and
2201 the server supports this feature. It shall respond the latter
2202 if the transport does not support Unix file descriptor
2203 passing, the server does not support this feature, or the
2204 server decides not to enable file descriptor passing due to
2205 security or other reasons.
2208 <sect2 id="auth-command-agree-unix-fd">
2209 <title>AGREE_UNIX_FD Command</title>
2211 The AGREE_UNIX_FD command indicates that the server supports
2212 Unix file descriptor passing. This command may only be sent
2213 after the connection is authenticated, and the client sent
2214 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
2215 command may only be sent on transports that support Unix file
2219 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
2220 followed by its stream of messages, or by disconnecting. The
2221 server must not accept additional commands using this protocol
2222 after the BEGIN command has been received. Further
2223 communication will be a stream of D-Bus messages (optionally
2224 encrypted, as negotiated) rather than this protocol.
2227 <sect2 id="auth-command-future">
2228 <title>Future Extensions</title>
2230 Future extensions to the authentication and negotiation
2231 protocol are possible. For that new commands may be
2232 introduced. If a client or server receives an unknown command
2233 it shall respond with ERROR and not consider this fatal. New
2234 commands may be introduced both before, and after
2235 authentication, i.e. both before and after the OK command.
2238 <sect2 id="auth-examples">
2239 <title>Authentication examples</title>
2243 <title>Example of successful magic cookie authentication</title>
2245 (MAGIC_COOKIE is a made up mechanism)
2247 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2253 <title>Example of finding out mechanisms then picking one</title>
2256 S: REJECTED KERBEROS_V4 SKEY
2257 C: AUTH SKEY 7ab83f32ee
2258 S: DATA 8799cabb2ea93e
2259 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2265 <title>Example of client sends unknown command then falls back to regular auth</title>
2269 C: AUTH MAGIC_COOKIE 3736343435313230333039
2275 <title>Example of server doesn't support initial auth mechanism</title>
2277 C: AUTH MAGIC_COOKIE 3736343435313230333039
2278 S: REJECTED KERBEROS_V4 SKEY
2279 C: AUTH SKEY 7ab83f32ee
2280 S: DATA 8799cabb2ea93e
2281 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2287 <title>Example of wrong password or the like followed by successful retry</title>
2289 C: AUTH MAGIC_COOKIE 3736343435313230333039
2290 S: REJECTED KERBEROS_V4 SKEY
2291 C: AUTH SKEY 7ab83f32ee
2292 S: DATA 8799cabb2ea93e
2293 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2295 C: AUTH SKEY 7ab83f32ee
2296 S: DATA 8799cabb2ea93e
2297 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2303 <title>Example of skey cancelled and restarted</title>
2305 C: AUTH MAGIC_COOKIE 3736343435313230333039
2306 S: REJECTED KERBEROS_V4 SKEY
2307 C: AUTH SKEY 7ab83f32ee
2308 S: DATA 8799cabb2ea93e
2311 C: AUTH SKEY 7ab83f32ee
2312 S: DATA 8799cabb2ea93e
2313 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2319 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2321 (MAGIC_COOKIE is a made up mechanism)
2323 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2325 C: NEGOTIATE_UNIX_FD
2331 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2333 (MAGIC_COOKIE is a made up mechanism)
2335 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2337 C: NEGOTIATE_UNIX_FD
2344 <sect2 id="auth-states">
2345 <title>Authentication state diagrams</title>
2348 This section documents the auth protocol in terms of
2349 a state machine for the client and the server. This is
2350 probably the most robust way to implement the protocol.
2353 <sect3 id="auth-states-client">
2354 <title>Client states</title>
2357 To more precisely describe the interaction between the
2358 protocol state machine and the authentication mechanisms the
2359 following notation is used: MECH(CHALL) means that the
2360 server challenge CHALL was fed to the mechanism MECH, which
2366 CONTINUE(RESP) means continue the auth conversation
2367 and send RESP as the response to the server;
2373 OK(RESP) means that after sending RESP to the server
2374 the client side of the auth conversation is finished
2375 and the server should return "OK";
2381 ERROR means that CHALL was invalid and could not be
2387 Both RESP and CHALL may be empty.
2391 The Client starts by getting an initial response from the
2392 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2393 the mechanism did not provide an initial response. If the
2394 mechanism returns CONTINUE, the client starts in state
2395 <emphasis>WaitingForData</emphasis>, if the mechanism
2396 returns OK the client starts in state
2397 <emphasis>WaitingForOK</emphasis>.
2401 The client should keep track of available mechanisms and
2402 which it mechanisms it has already attempted. This list is
2403 used to decide which AUTH command to send. When the list is
2404 exhausted, the client should give up and close the
2409 <title><emphasis>WaitingForData</emphasis></title>
2417 MECH(CHALL) returns CONTINUE(RESP) → send
2419 <emphasis>WaitingForData</emphasis>
2423 MECH(CHALL) returns OK(RESP) → send DATA
2424 RESP, goto <emphasis>WaitingForOK</emphasis>
2428 MECH(CHALL) returns ERROR → send ERROR
2429 [msg], goto <emphasis>WaitingForData</emphasis>
2437 Receive REJECTED [mechs] →
2438 send AUTH [next mech], goto
2439 WaitingForData or <emphasis>WaitingForOK</emphasis>
2444 Receive ERROR → send
2446 <emphasis>WaitingForReject</emphasis>
2451 Receive OK → send
2452 BEGIN, terminate auth
2453 conversation, authenticated
2458 Receive anything else → send
2460 <emphasis>WaitingForData</emphasis>
2468 <title><emphasis>WaitingForOK</emphasis></title>
2473 Receive OK → send BEGIN, terminate auth
2474 conversation, <emphasis>authenticated</emphasis>
2479 Receive REJECTED [mechs] → send AUTH [next mech],
2480 goto <emphasis>WaitingForData</emphasis> or
2481 <emphasis>WaitingForOK</emphasis>
2487 Receive DATA → send CANCEL, goto
2488 <emphasis>WaitingForReject</emphasis>
2494 Receive ERROR → send CANCEL, goto
2495 <emphasis>WaitingForReject</emphasis>
2501 Receive anything else → send ERROR, goto
2502 <emphasis>WaitingForOK</emphasis>
2510 <title><emphasis>WaitingForReject</emphasis></title>
2515 Receive REJECTED [mechs] → send AUTH [next mech],
2516 goto <emphasis>WaitingForData</emphasis> or
2517 <emphasis>WaitingForOK</emphasis>
2523 Receive anything else → terminate auth
2524 conversation, disconnect
2533 <sect3 id="auth-states-server">
2534 <title>Server states</title>
2537 For the server MECH(RESP) means that the client response
2538 RESP was fed to the the mechanism MECH, which returns one of
2543 CONTINUE(CHALL) means continue the auth conversation and
2544 send CHALL as the challenge to the client;
2550 OK means that the client has been successfully
2557 REJECTED means that the client failed to authenticate or
2558 there was an error in RESP.
2563 The server starts out in state
2564 <emphasis>WaitingForAuth</emphasis>. If the client is
2565 rejected too many times the server must disconnect the
2570 <title><emphasis>WaitingForAuth</emphasis></title>
2576 Receive AUTH → send REJECTED [mechs], goto
2577 <emphasis>WaitingForAuth</emphasis>
2583 Receive AUTH MECH RESP
2587 MECH not valid mechanism → send REJECTED
2589 <emphasis>WaitingForAuth</emphasis>
2593 MECH(RESP) returns CONTINUE(CHALL) → send
2595 <emphasis>WaitingForData</emphasis>
2599 MECH(RESP) returns OK → send OK, goto
2600 <emphasis>WaitingForBegin</emphasis>
2604 MECH(RESP) returns REJECTED → send REJECTED
2606 <emphasis>WaitingForAuth</emphasis>
2614 Receive BEGIN → terminate
2615 auth conversation, disconnect
2621 Receive ERROR → send REJECTED [mechs], goto
2622 <emphasis>WaitingForAuth</emphasis>
2628 Receive anything else → send
2630 <emphasis>WaitingForAuth</emphasis>
2639 <title><emphasis>WaitingForData</emphasis></title>
2647 MECH(RESP) returns CONTINUE(CHALL) → send
2649 <emphasis>WaitingForData</emphasis>
2653 MECH(RESP) returns OK → send OK, goto
2654 <emphasis>WaitingForBegin</emphasis>
2658 MECH(RESP) returns REJECTED → send REJECTED
2660 <emphasis>WaitingForAuth</emphasis>
2668 Receive BEGIN → terminate auth conversation,
2675 Receive CANCEL → send REJECTED [mechs], goto
2676 <emphasis>WaitingForAuth</emphasis>
2682 Receive ERROR → send REJECTED [mechs], goto
2683 <emphasis>WaitingForAuth</emphasis>
2689 Receive anything else → send ERROR, goto
2690 <emphasis>WaitingForData</emphasis>
2698 <title><emphasis>WaitingForBegin</emphasis></title>
2703 Receive BEGIN → terminate auth conversation,
2704 client authenticated
2710 Receive CANCEL → send REJECTED [mechs], goto
2711 <emphasis>WaitingForAuth</emphasis>
2717 Receive ERROR → send REJECTED [mechs], goto
2718 <emphasis>WaitingForAuth</emphasis>
2724 Receive anything else → send ERROR, goto
2725 <emphasis>WaitingForBegin</emphasis>
2735 <sect2 id="auth-mechanisms">
2736 <title>Authentication mechanisms</title>
2738 This section describes some new authentication mechanisms.
2739 D-Bus also allows any standard SASL mechanism of course.
2741 <sect3 id="auth-mechanisms-sha">
2742 <title>DBUS_COOKIE_SHA1</title>
2744 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2745 has the ability to read a private file owned by the user being
2746 authenticated. If the client can prove that it has access to a secret
2747 cookie stored in this file, then the client is authenticated.
2748 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2752 Throughout this description, "hex encoding" must output the digits
2753 from a to f in lower-case; the digits A to F must not be used
2754 in the DBUS_COOKIE_SHA1 mechanism.
2757 Authentication proceeds as follows:
2761 The client sends the username it would like to authenticate
2767 The server sends the name of its "cookie context" (see below); a
2768 space character; the integer ID of the secret cookie the client
2769 must demonstrate knowledge of; a space character; then a
2770 randomly-generated challenge string, all of this hex-encoded into
2776 The client locates the cookie and generates its own
2777 randomly-generated challenge string. The client then concatenates
2778 the server's decoded challenge, a ":" character, its own challenge,
2779 another ":" character, and the cookie. It computes the SHA-1 hash
2780 of this composite string as a hex digest. It concatenates the
2781 client's challenge string, a space character, and the SHA-1 hex
2782 digest, hex-encodes the result and sends it back to the server.
2787 The server generates the same concatenated string used by the
2788 client and computes its SHA-1 hash. It compares the hash with
2789 the hash received from the client; if the two hashes match, the
2790 client is authenticated.
2796 Each server has a "cookie context," which is a name that identifies a
2797 set of cookies that apply to that server. A sample context might be
2798 "org_freedesktop_session_bus". Context names must be valid ASCII,
2799 nonzero length, and may not contain the characters slash ("/"),
2800 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2801 tab ("\t"), or period ("."). There is a default context,
2802 "org_freedesktop_general" that's used by servers that do not specify
2806 Cookies are stored in a user's home directory, in the directory
2807 <filename>~/.dbus-keyrings/</filename>. This directory must
2808 not be readable or writable by other users. If it is,
2809 clients and servers must ignore it. The directory
2810 contains cookie files named after the cookie context.
2813 A cookie file contains one cookie per line. Each line
2814 has three space-separated fields:
2818 The cookie ID number, which must be a non-negative integer and
2819 may not be used twice in the same file.
2824 The cookie's creation time, in UNIX seconds-since-the-epoch
2830 The cookie itself, a hex-encoded random block of bytes. The cookie
2831 may be of any length, though obviously security increases
2832 as the length increases.
2838 Only server processes modify the cookie file.
2839 They must do so with this procedure:
2843 Create a lockfile name by appending ".lock" to the name of the
2844 cookie file. The server should attempt to create this file
2845 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2846 fails, the lock fails. Servers should retry for a reasonable
2847 period of time, then they may choose to delete an existing lock
2848 to keep users from having to manually delete a stale
2849 lock. <footnote><para>Lockfiles are used instead of real file
2850 locking <literal>fcntl()</literal> because real locking
2851 implementations are still flaky on network
2852 filesystems.</para></footnote>
2857 Once the lockfile has been created, the server loads the cookie
2858 file. It should then delete any cookies that are old (the
2859 timeout can be fairly short), or more than a reasonable
2860 time in the future (so that cookies never accidentally
2861 become permanent, if the clock was set far into the future
2862 at some point). If no recent keys remain, the
2863 server may generate a new key.
2868 The pruned and possibly added-to cookie file
2869 must be resaved atomically (using a temporary
2870 file which is rename()'d).
2875 The lock must be dropped by deleting the lockfile.
2881 Clients need not lock the file in order to load it,
2882 because servers are required to save the file atomically.
2887 <sect1 id="addresses">
2888 <title>Server Addresses</title>
2890 Server addresses consist of a transport name followed by a colon, and
2891 then an optional, comma-separated list of keys and values in the form key=value.
2892 Each value is escaped.
2896 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2897 Which is the address to a unix socket with the path /tmp/dbus-test.
2900 Value escaping is similar to URI escaping but simpler.
2904 The set of optionally-escaped bytes is:
2905 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2906 <emphasis>byte</emphasis> (note, not character) which is not in the
2907 set of optionally-escaped bytes must be replaced with an ASCII
2908 percent (<literal>%</literal>) and the value of the byte in hex.
2909 The hex value must always be two digits, even if the first digit is
2910 zero. The optionally-escaped bytes may be escaped if desired.
2915 To unescape, append each byte in the value; if a byte is an ASCII
2916 percent (<literal>%</literal>) character then append the following
2917 hex value instead. It is an error if a <literal>%</literal> byte
2918 does not have two hex digits following. It is an error if a
2919 non-optionally-escaped byte is seen unescaped.
2923 The set of optionally-escaped bytes is intended to preserve address
2924 readability and convenience.
2928 A server may specify a key-value pair with the key <literal>guid</literal>
2929 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2930 describes the format of the <literal>guid</literal> field. If present,
2931 this UUID may be used to distinguish one server address from another. A
2932 server should use a different UUID for each address it listens on. For
2933 example, if a message bus daemon offers both UNIX domain socket and TCP
2934 connections, but treats clients the same regardless of how they connect,
2935 those two connections are equivalent post-connection but should have
2936 distinct UUIDs to distinguish the kinds of connection.
2940 The intent of the address UUID feature is to allow a client to avoid
2941 opening multiple identical connections to the same server, by allowing the
2942 client to check whether an address corresponds to an already-existing
2943 connection. Comparing two addresses is insufficient, because addresses
2944 can be recycled by distinct servers, and equivalent addresses may look
2945 different if simply compared as strings (for example, the host in a TCP
2946 address can be given as an IP address or as a hostname).
2950 Note that the address key is <literal>guid</literal> even though the
2951 rest of the API and documentation says "UUID," for historical reasons.
2955 [FIXME clarify if attempting to connect to each is a requirement
2956 or just a suggestion]
2957 When connecting to a server, multiple server addresses can be
2958 separated by a semi-colon. The library will then try to connect
2959 to the first address and if that fails, it'll try to connect to
2960 the next one specified, and so forth. For example
2961 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2965 Some addresses are <firstterm>connectable</firstterm>. A connectable
2966 address is one containing enough information for a client to connect
2967 to it. For instance, <literal>tcp:host=127.0.0.1,port=4242</literal>
2968 is a connectable address. It is not necessarily possible to listen
2969 on every connectable address: for instance, it is not possible to
2970 listen on a <literal>unixexec:</literal> address.
2974 Some addresses are <firstterm>listenable</firstterm>. A listenable
2975 address is one containing enough information for a server to listen on
2976 it, producing a connectable address (which may differ from the
2977 original address). Many listenable addresses are not connectable:
2978 for instance, <literal>tcp:host=127.0.0.1</literal>
2979 is listenable, but not connectable (because it does not specify
2984 Listening on an address that is not connectable will result in a
2985 connectable address that is not the same as the listenable address.
2986 For instance, listening on <literal>tcp:host=127.0.0.1</literal>
2987 might result in the connectable address
2988 <literal>tcp:host=127.0.0.1,port=30958</literal>,
2989 or listening on <literal>unix:tmpdir=/tmp</literal>
2990 might result in the connectable address
2991 <literal>unix:abstract=/tmp/dbus-U8OSdmf7</literal>.
2995 <sect1 id="transports">
2996 <title>Transports</title>
2998 [FIXME we need to specify in detail each transport and its possible arguments]
3000 Current transports include: unix domain sockets (including
3001 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
3002 using in-process pipes. Future possible transports include one that
3003 tunnels over X11 protocol.
3006 <sect2 id="transports-unix-domain-sockets">
3007 <title>Unix Domain Sockets</title>
3009 Unix domain sockets can be either paths in the file system or on Linux
3010 kernels, they can be abstract which are similar to paths but
3011 do not show up in the file system.
3015 When a socket is opened by the D-Bus library it truncates the path
3016 name right before the first trailing Nul byte. This is true for both
3017 normal paths and abstract paths. Note that this is a departure from
3018 previous versions of D-Bus that would create sockets with a fixed
3019 length path name. Names which were shorter than the fixed length
3020 would be padded by Nul bytes.
3023 Unix domain sockets are not available on Windows.
3026 Unix addresses that specify <literal>path</literal> or
3027 <literal>abstract</literal> are both listenable and connectable.
3028 Unix addresses that specify <literal>tmpdir</literal> are only
3029 listenable: the corresponding connectable address will specify
3030 either <literal>path</literal> or <literal>abstract</literal>.
3032 <sect3 id="transports-unix-domain-sockets-addresses">
3033 <title>Server Address Format</title>
3035 Unix domain socket addresses are identified by the "unix:" prefix
3036 and support the following key/value pairs:
3043 <entry>Values</entry>
3044 <entry>Description</entry>
3050 <entry>(path)</entry>
3051 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
3054 <entry>tmpdir</entry>
3055 <entry>(path)</entry>
3056 <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>
3059 <entry>abstract</entry>
3060 <entry>(string)</entry>
3061 <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>
3067 Exactly one of the keys <literal>path</literal>,
3068 <literal>abstract</literal> or
3069 <literal>tmpdir</literal> must be provided.
3073 <sect2 id="transports-launchd">
3074 <title>launchd</title>
3076 launchd is an open-source server management system that replaces init, inetd
3077 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
3078 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
3082 launchd allocates a socket and provides it with the unix path through the
3083 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
3084 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
3085 it through its environment.
3086 Other processes can query for the launchd socket by executing:
3087 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
3088 This is normally done by the D-Bus client library so doesn't have to be done
3092 launchd is not available on Microsoft Windows.
3095 launchd addresses are listenable and connectable.
3097 <sect3 id="transports-launchd-addresses">
3098 <title>Server Address Format</title>
3100 launchd addresses are identified by the "launchd:" prefix
3101 and support the following key/value pairs:
3108 <entry>Values</entry>
3109 <entry>Description</entry>
3115 <entry>(environment variable)</entry>
3116 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
3122 The <literal>env</literal> key is required.
3126 <sect2 id="transports-systemd">
3127 <title>systemd</title>
3129 systemd is an open-source server management system that
3130 replaces init and inetd on newer Linux systems. It supports
3131 socket activation. The D-Bus systemd transport is used to acquire
3132 socket activation file descriptors from systemd and use them
3133 as D-Bus transport when the current process is spawned by
3134 socket activation from it.
3137 The systemd transport accepts only one or more Unix domain or
3138 TCP streams sockets passed in via socket activation.
3141 The systemd transport is not available on non-Linux operating systems.
3144 The systemd transport defines no parameter keys.
3147 systemd addresses are listenable, but not connectable. The
3148 corresponding connectable address is the <literal>unix</literal>
3149 or <literal>tcp</literal> address of the socket.
3152 <sect2 id="transports-tcp-sockets">
3153 <title>TCP Sockets</title>
3155 The tcp transport provides TCP/IP based connections between clients
3156 located on the same or different hosts.
3159 Using tcp transport without any additional secure authentification mechanismus
3160 over a network is unsecure.
3163 On Windows and most Unix platforms, the TCP stack is unable to transfer
3164 credentials over a TCP connection, so the EXTERNAL authentication
3165 mechanism does not work for this transport.
3168 All <literal>tcp</literal> addresses are listenable.
3169 <literal>tcp</literal> addresses in which both
3170 <literal>host</literal> and <literal>port</literal> are
3171 specified, and <literal>port</literal> is non-zero,
3172 are also connectable.
3174 <sect3 id="transports-tcp-sockets-addresses">
3175 <title>Server Address Format</title>
3177 TCP/IP socket addresses are identified by the "tcp:" prefix
3178 and support the following key/value pairs:
3185 <entry>Values</entry>
3186 <entry>Description</entry>
3192 <entry>(string)</entry>
3193 <entry>DNS name or IP address</entry>
3197 <entry>(string)</entry>
3198 <entry>Used in a listenable address to configure the interface
3199 on which the server will listen: either the IP address of one of
3200 the local machine's interfaces (most commonly <literal>127.0.0.1
3201 </literal>), or a DNS name that resolves to one of those IP
3202 addresses, or '*' to listen on all interfaces simultaneously.
3203 If not specified, the default is the same value as "host".
3208 <entry>(number)</entry>
3209 <entry>The tcp port the server will open. A zero value let the server
3210 choose a free port provided from the underlaying operating system.
3211 libdbus is able to retrieve the real used port from the server.
3215 <entry>family</entry>
3216 <entry>(string)</entry>
3217 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3224 <sect2 id="transports-nonce-tcp-sockets">
3225 <title>Nonce-secured TCP Sockets</title>
3227 The nonce-tcp transport provides a secured TCP transport, using a
3228 simple authentication mechanism to ensure that only clients with read
3229 access to a certain location in the filesystem can connect to the server.
3230 The server writes a secret, the nonce, to a file and an incoming client
3231 connection is only accepted if the client sends the nonce right after
3232 the connect. The nonce mechanism requires no setup and is orthogonal to
3233 the higher-level authentication mechanisms described in the
3234 Authentication section.
3238 On start, the server generates a random 16 byte nonce and writes it
3239 to a file in the user's temporary directory. The nonce file location
3240 is published as part of the server's D-Bus address using the
3241 "noncefile" key-value pair.
3243 After an accept, the server reads 16 bytes from the socket. If the
3244 read bytes do not match the nonce stored in the nonce file, the
3245 server MUST immediately drop the connection.
3246 If the nonce match the received byte sequence, the client is accepted
3247 and the transport behaves like an unsecured tcp transport.
3250 After a successful connect to the server socket, the client MUST read
3251 the nonce from the file published by the server via the noncefile=
3252 key-value pair and send it over the socket. After that, the
3253 transport behaves like an unsecured tcp transport.
3256 All nonce-tcp addresses are listenable. nonce-tcp addresses in which
3257 <literal>host</literal>, <literal>port</literal> and
3258 <literal>noncefile</literal> are all specified,
3259 and <literal>port</literal> is nonzero, are also connectable.
3261 <sect3 id="transports-nonce-tcp-sockets-addresses">
3262 <title>Server Address Format</title>
3264 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
3265 and support the following key/value pairs:
3272 <entry>Values</entry>
3273 <entry>Description</entry>
3279 <entry>(string)</entry>
3280 <entry>DNS name or IP address</entry>
3284 <entry>(string)</entry>
3285 <entry>The same as for tcp: addresses
3290 <entry>(number)</entry>
3291 <entry>The tcp port the server will open. A zero value let the server
3292 choose a free port provided from the underlaying operating system.
3293 libdbus is able to retrieve the real used port from the server.
3297 <entry>family</entry>
3298 <entry>(string)</entry>
3299 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
3302 <entry>noncefile</entry>
3303 <entry>(path)</entry>
3304 <entry>File location containing the secret.
3305 This is only meaningful in connectable addresses:
3306 a listening D-Bus server that offers this transport
3307 will always create a new nonce file.</entry>
3314 <sect2 id="transports-exec">
3315 <title>Executed Subprocesses on Unix</title>
3317 This transport forks off a process and connects its standard
3318 input and standard output with an anonymous Unix domain
3319 socket. This socket is then used for communication by the
3320 transport. This transport may be used to use out-of-process
3321 forwarder programs as basis for the D-Bus protocol.
3324 The forked process will inherit the standard error output and
3325 process group from the parent process.
3328 Executed subprocesses are not available on Windows.
3331 <literal>unixexec</literal> addresses are connectable, but are not
3334 <sect3 id="transports-exec-addresses">
3335 <title>Server Address Format</title>
3337 Executed subprocess addresses are identified by the "unixexec:" prefix
3338 and support the following key/value pairs:
3345 <entry>Values</entry>
3346 <entry>Description</entry>
3352 <entry>(path)</entry>
3353 <entry>Path of the binary to execute, either an absolute
3354 path or a binary name that is searched for in the default
3355 search path of the OS. This corresponds to the first
3356 argument of execlp(). This key is mandatory.</entry>
3359 <entry>argv0</entry>
3360 <entry>(string)</entry>
3361 <entry>The program name to use when executing the
3362 binary. If omitted the same value as specified for path=
3363 will be used. This corresponds to the second argument of
3367 <entry>argv1, argv2, ...</entry>
3368 <entry>(string)</entry>
3369 <entry>Arguments to pass to the binary. This corresponds
3370 to the third and later arguments of execlp(). If a
3371 specific argvX is not specified no further argvY for Y > X
3372 are taken into account.</entry>
3380 <sect1 id="meta-transports">
3381 <title>Meta Transports</title>
3383 Meta transports are a kind of transport with special enhancements or
3384 behavior. Currently available meta transports include: autolaunch
3387 <sect2 id="meta-transports-autolaunch">
3388 <title>Autolaunch</title>
3389 <para>The autolaunch transport provides a way for dbus clients to autodetect
3390 a running dbus session bus and to autolaunch a session bus if not present.
3393 On Unix, <literal>autolaunch</literal> addresses are connectable,
3397 On Windows, <literal>autolaunch</literal> addresses are both
3398 connectable and listenable.
3401 <sect3 id="meta-transports-autolaunch-addresses">
3402 <title>Server Address Format</title>
3404 Autolaunch addresses uses the "autolaunch:" prefix and support the
3405 following key/value pairs:
3412 <entry>Values</entry>
3413 <entry>Description</entry>
3418 <entry>scope</entry>
3419 <entry>(string)</entry>
3420 <entry>scope of autolaunch (Windows only)
3424 "*install-path" - limit session bus to dbus installation path.
3425 The dbus installation path is determined from the location of
3426 the shared dbus library. If the library is located in a 'bin'
3427 subdirectory the installation root is the directory above,
3428 otherwise the directory where the library lives is taken as
3431 <install-root>/bin/[lib]dbus-1.dll
3432 <install-root>/[lib]dbus-1.dll
3438 "*user" - limit session bus to the recent user.
3443 other values - specify dedicated session bus like "release",
3455 <sect3 id="meta-transports-autolaunch-windows-implementation">
3456 <title>Windows implementation</title>
3458 On start, the server opens a platform specific transport, creates a mutex
3459 and a shared memory section containing the related session bus address.
3460 This mutex will be inspected by the dbus client library to detect a
3461 running dbus session bus. The access to the mutex and the shared memory
3462 section are protected by global locks.
3465 In the recent implementation the autolaunch transport uses a tcp transport
3466 on localhost with a port choosen from the operating system. This detail may
3467 change in the future.
3470 Disclaimer: The recent implementation is in an early state and may not
3471 work in all cirumstances and/or may have security issues. Because of this
3472 the implementation is not documentated yet.
3479 <title>UUIDs</title>
3481 A working D-Bus implementation uses universally-unique IDs in two places.
3482 First, each server address has a UUID identifying the address,
3483 as described in <xref linkend="addresses"/>. Second, each operating
3484 system kernel instance running a D-Bus client or server has a UUID
3485 identifying that kernel, retrieved by invoking the method
3486 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3487 linkend="standard-interfaces-peer"/>).
3490 The term "UUID" in this document is intended literally, i.e. an
3491 identifier that is universally unique. It is not intended to refer to
3492 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3495 The UUID must contain 128 bits of data and be hex-encoded. The
3496 hex-encoded string may not contain hyphens or other non-hex-digit
3497 characters, and it must be exactly 32 characters long. To generate a
3498 UUID, the current reference implementation concatenates 96 bits of random
3499 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3503 It would also be acceptable and probably better to simply generate 128
3504 bits of random data, as long as the random number generator is of high
3505 quality. The timestamp could conceivably help if the random bits are not
3506 very random. With a quality random number generator, collisions are
3507 extremely unlikely even with only 96 bits, so it's somewhat academic.
3510 Implementations should, however, stick to random data for the first 96 bits
3515 <sect1 id="standard-interfaces">
3516 <title>Standard Interfaces</title>
3518 See <xref linkend="message-protocol-types-notation"/> for details on
3519 the notation used in this section. There are some standard interfaces
3520 that may be useful across various D-Bus applications.
3522 <sect2 id="standard-interfaces-peer">
3523 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3525 The <literal>org.freedesktop.DBus.Peer</literal> interface
3528 org.freedesktop.DBus.Peer.Ping ()
3529 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3533 On receipt of the <literal>METHOD_CALL</literal> message
3534 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3535 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3536 usual. It does not matter which object path a ping is sent to. The
3537 reference implementation handles this method automatically.
3540 On receipt of the <literal>METHOD_CALL</literal> message
3541 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3542 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3543 UUID representing the identity of the machine the process is running on.
3544 This UUID must be the same for all processes on a single system at least
3545 until that system next reboots. It should be the same across reboots
3546 if possible, but this is not always possible to implement and is not
3548 It does not matter which object path a GetMachineId is sent to. The
3549 reference implementation handles this method automatically.
3552 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3553 a virtual machine running on a hypervisor, rather than a physical machine.
3554 Basically if two processes see the same UUID, they should also see the same
3555 shared memory, UNIX domain sockets, process IDs, and other features that require
3556 a running OS kernel in common between the processes.
3559 The UUID is often used where other programs might use a hostname. Hostnames
3560 can change without rebooting, however, or just be "localhost" - so the UUID
3564 <xref linkend="uuids"/> explains the format of the UUID.
3568 <sect2 id="standard-interfaces-introspectable">
3569 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3571 This interface has one method:
3573 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3577 Objects instances may implement
3578 <literal>Introspect</literal> which returns an XML description of
3579 the object, including its interfaces (with signals and methods), objects
3580 below it in the object path tree, and its properties.
3583 <xref linkend="introspection-format"/> describes the format of this XML string.
3586 <sect2 id="standard-interfaces-properties">
3587 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3589 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3590 or <firstterm>attributes</firstterm>. These can be exposed via the
3591 <literal>org.freedesktop.DBus.Properties</literal> interface.
3595 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3596 in STRING property_name,
3598 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3599 in STRING property_name,
3601 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3602 out DICT<STRING,VARIANT> props);
3606 It is conventional to give D-Bus properties names consisting of
3607 capitalized words without punctuation ("CamelCase"), like
3608 <link linkend="message-protocol-names-member">member names</link>.
3609 For instance, the GObject property
3610 <literal>connection-status</literal> or the Qt property
3611 <literal>connectionStatus</literal> could be represented on D-Bus
3612 as <literal>ConnectionStatus</literal>.
3615 Strictly speaking, D-Bus property names are not required to follow
3616 the same naming restrictions as member names, but D-Bus property
3617 names that would not be valid member names (in particular,
3618 GObject-style dash-separated property names) can cause interoperability
3619 problems and should be avoided.
3622 The available properties and whether they are writable can be determined
3623 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3624 see <xref linkend="standard-interfaces-introspectable"/>.
3627 An empty string may be provided for the interface name; in this case,
3628 if there are multiple properties on an object with the same name,
3629 the results are undefined (picking one by according to an arbitrary
3630 deterministic rule, or returning an error, are the reasonable
3634 If one or more properties change on an object, the
3635 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3636 signal may be emitted (this signal was added in 0.14):
3640 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3641 DICT<STRING,VARIANT> changed_properties,
3642 ARRAY<STRING> invalidated_properties);
3646 where <literal>changed_properties</literal> is a dictionary
3647 containing the changed properties with the new values and
3648 <literal>invalidated_properties</literal> is an array of
3649 properties that changed but the value is not conveyed.
3652 Whether the <literal>PropertiesChanged</literal> signal is
3653 supported can be determined by calling
3654 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3655 that the signal may be supported for an object but it may
3656 differ how whether and how it is used on a per-property basis
3657 (for e.g. performance or security reasons). Each property (or
3658 the parent interface) must be annotated with the
3659 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3660 annotation to convey this (usually the default value
3661 <literal>true</literal> is sufficient meaning that the
3662 annotation does not need to be used). See <xref
3663 linkend="introspection-format"/> for details on this
3668 <sect2 id="standard-interfaces-objectmanager">
3669 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3671 An API can optionally make use of this interface for one or
3672 more sub-trees of objects. The root of each sub-tree implements
3673 this interface so other applications can get all objects,
3674 interfaces and properties in a single method call. It is
3675 appropriate to use this interface if users of the tree of
3676 objects are expected to be interested in all interfaces of all
3677 objects in the tree; a more granular API should be used if
3678 users of the objects are expected to be interested in a small
3679 subset of the objects, a small subset of their interfaces, or
3683 The method that applications can use to get all objects and
3684 properties is <literal>GetManagedObjects</literal>:
3688 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3692 The return value of this method is a dict whose keys are
3693 object paths. All returned object paths are children of the
3694 object path implementing this interface, i.e. their object
3695 paths start with the ObjectManager's object path plus '/'.
3698 Each value is a dict whose keys are interfaces names. Each
3699 value in this inner dict is the same dict that would be
3700 returned by the <link
3701 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3702 method for that combination of object path and interface. If
3703 an interface has no properties, the empty dict is returned.
3706 Changes are emitted using the following two signals:
3710 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3711 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3712 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3713 ARRAY<STRING> interfaces);
3717 The <literal>InterfacesAdded</literal> signal is emitted when
3718 either a new object is added or when an existing object gains
3719 one or more interfaces. The
3720 <literal>InterfacesRemoved</literal> signal is emitted
3721 whenever an object is removed or it loses one or more
3722 interfaces. The second parameter of the
3723 <literal>InterfacesAdded</literal> signal contains a dict with
3724 the interfaces and properties (if any) that have been added to
3725 the given object path. Similarly, the second parameter of the
3726 <literal>InterfacesRemoved</literal> signal contains an array
3727 of the interfaces that were removed. Note that changes on
3728 properties on existing interfaces are not reported using this
3729 interface - an application should also monitor the existing <link
3730 linkend="standard-interfaces-properties">PropertiesChanged</link>
3731 signal on each object.
3734 Applications SHOULD NOT export objects that are children of an
3735 object (directly or otherwise) implementing this interface but
3736 which are not returned in the reply from the
3737 <literal>GetManagedObjects()</literal> method of this
3738 interface on the given object.
3741 The intent of the <literal>ObjectManager</literal> interface
3742 is to make it easy to write a robust client
3743 implementation. The trivial client implementation only needs
3744 to make two method calls:
3748 org.freedesktop.DBus.AddMatch (bus_proxy,
3749 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3750 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3754 on the message bus and the remote application's
3755 <literal>ObjectManager</literal>, respectively. Whenever a new
3756 remote object is created (or an existing object gains a new
3757 interface), the <literal>InterfacesAdded</literal> signal is
3758 emitted, and since this signal contains all properties for the
3759 interfaces, no calls to the
3760 <literal>org.freedesktop.Properties</literal> interface on the
3761 remote object are needed. Additionally, since the initial
3762 <literal>AddMatch()</literal> rule already includes signal
3763 messages from the newly created child object, no new
3764 <literal>AddMatch()</literal> call is needed.
3769 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3770 interface was added in version 0.17 of the D-Bus
3777 <sect1 id="introspection-format">
3778 <title>Introspection Data Format</title>
3780 As described in <xref linkend="standard-interfaces-introspectable"/>,
3781 objects may be introspected at runtime, returning an XML string
3782 that describes the object. The same XML format may be used in
3783 other contexts as well, for example as an "IDL" for generating
3784 static language bindings.
3787 Here is an example of introspection data:
3789 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3790 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3791 <node name="/com/example/sample_object">
3792 <interface name="com.example.SampleInterface">
3793 <method name="Frobate">
3794 <arg name="foo" type="i" direction="in"/>
3795 <arg name="bar" type="s" direction="out"/>
3796 <arg name="baz" type="a{us}" direction="out"/>
3797 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3799 <method name="Bazify">
3800 <arg name="bar" type="(iiu)" direction="in"/>
3801 <arg name="bar" type="v" direction="out"/>
3803 <method name="Mogrify">
3804 <arg name="bar" type="(iiav)" direction="in"/>
3806 <signal name="Changed">
3807 <arg name="new_value" type="b"/>
3809 <property name="Bar" type="y" access="readwrite"/>
3811 <node name="child_of_sample_object"/>
3812 <node name="another_child_of_sample_object"/>
3817 A more formal DTD and spec needs writing, but here are some quick notes.
3821 Only the root <node> element can omit the node name, as it's
3822 known to be the object that was introspected. If the root
3823 <node> does have a name attribute, it must be an absolute
3824 object path. If child <node> have object paths, they must be
3830 If a child <node> has any sub-elements, then they
3831 must represent a complete introspection of the child.
3832 If a child <node> is empty, then it may or may
3833 not have sub-elements; the child must be introspected
3834 in order to find out. The intent is that if an object
3835 knows that its children are "fast" to introspect
3836 it can go ahead and return their information, but
3837 otherwise it can omit it.
3842 The direction element on <arg> may be omitted,
3843 in which case it defaults to "in" for method calls
3844 and "out" for signals. Signals only allow "out"
3845 so while direction may be specified, it's pointless.
3850 The possible directions are "in" and "out",
3851 unlike CORBA there is no "inout"
3856 The possible property access flags are
3857 "readwrite", "read", and "write"
3862 Multiple interfaces can of course be listed for
3868 The "name" attribute on arguments is optional.
3874 Method, interface, property, and signal elements may have
3875 "annotations", which are generic key/value pairs of metadata.
3876 They are similar conceptually to Java's annotations and C# attributes.
3877 Well-known annotations:
3884 <entry>Values (separated by ,)</entry>
3885 <entry>Description</entry>
3890 <entry>org.freedesktop.DBus.Deprecated</entry>
3891 <entry>true,false</entry>
3892 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3895 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3896 <entry>(string)</entry>
3897 <entry>The C symbol; may be used for methods and interfaces</entry>
3900 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3901 <entry>true,false</entry>
3902 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3905 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3906 <entry>true,invalidates,false</entry>
3909 If set to <literal>false</literal>, the
3910 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3912 linkend="standard-interfaces-properties"/> is not
3913 guaranteed to be emitted if the property changes.
3916 If set to <literal>invalidates</literal> the signal
3917 is emitted but the value is not included in the
3921 If set to <literal>true</literal> the signal is
3922 emitted with the value included.
3925 The value for the annotation defaults to
3926 <literal>true</literal> if the enclosing interface
3927 element does not specify the annotation. Otherwise it
3928 defaults to the value specified in the enclosing
3937 <sect1 id="message-bus">
3938 <title>Message Bus Specification</title>
3939 <sect2 id="message-bus-overview">
3940 <title>Message Bus Overview</title>
3942 The message bus accepts connections from one or more applications.
3943 Once connected, applications can exchange messages with other
3944 applications that are also connected to the bus.
3947 In order to route messages among connections, the message bus keeps a
3948 mapping from names to connections. Each connection has one
3949 unique-for-the-lifetime-of-the-bus name automatically assigned.
3950 Applications may request additional names for a connection. Additional
3951 names are usually "well-known names" such as
3952 "com.example.TextEditor". When a name is bound to a connection,
3953 that connection is said to <firstterm>own</firstterm> the name.
3956 The bus itself owns a special name,
3957 <literal>org.freedesktop.DBus</literal>, with an object
3958 located at <literal>/org/freedesktop/DBus</literal> that
3959 implements the <literal>org.freedesktop.DBus</literal>
3960 interface. This service allows applications to make
3961 administrative requests of the bus itself. For example,
3962 applications can ask the bus to assign a name to a connection.
3965 Each name may have <firstterm>queued owners</firstterm>. When an
3966 application requests a name for a connection and the name is already in
3967 use, the bus will optionally add the connection to a queue waiting for
3968 the name. If the current owner of the name disconnects or releases
3969 the name, the next connection in the queue will become the new owner.
3973 This feature causes the right thing to happen if you start two text
3974 editors for example; the first one may request "com.example.TextEditor",
3975 and the second will be queued as a possible owner of that name. When
3976 the first exits, the second will take over.
3980 Applications may send <firstterm>unicast messages</firstterm> to
3981 a specific recipient or to the message bus itself, or
3982 <firstterm>broadcast messages</firstterm> to all interested recipients.
3983 See <xref linkend="message-bus-routing"/> for details.
3987 <sect2 id="message-bus-names">
3988 <title>Message Bus Names</title>
3990 Each connection has at least one name, assigned at connection time and
3991 returned in response to the
3992 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3993 automatically-assigned name is called the connection's <firstterm>unique
3994 name</firstterm>. Unique names are never reused for two different
3995 connections to the same bus.
3998 Ownership of a unique name is a prerequisite for interaction with
3999 the message bus. It logically follows that the unique name is always
4000 the first name that an application comes to own, and the last
4001 one that it loses ownership of.
4004 Unique connection names must begin with the character ':' (ASCII colon
4005 character); bus names that are not unique names must not begin
4006 with this character. (The bus must reject any attempt by an application
4007 to manually request a name beginning with ':'.) This restriction
4008 categorically prevents "spoofing"; messages sent to a unique name
4009 will always go to the expected connection.
4012 When a connection is closed, all the names that it owns are deleted (or
4013 transferred to the next connection in the queue if any).
4016 A connection can request additional names to be associated with it using
4017 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
4018 linkend="message-protocol-names-bus"/> describes the format of a valid
4019 name. These names can be released again using the
4020 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
4023 <sect3 id="bus-messages-request-name">
4024 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
4028 UINT32 RequestName (in STRING name, in UINT32 flags)
4035 <entry>Argument</entry>
4037 <entry>Description</entry>
4043 <entry>STRING</entry>
4044 <entry>Name to request</entry>
4048 <entry>UINT32</entry>
4049 <entry>Flags</entry>
4059 <entry>Argument</entry>
4061 <entry>Description</entry>
4067 <entry>UINT32</entry>
4068 <entry>Return value</entry>
4075 This method call should be sent to
4076 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4077 assign the given name to the method caller. Each name maintains a
4078 queue of possible owners, where the head of the queue is the primary
4079 or current owner of the name. Each potential owner in the queue
4080 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
4081 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
4082 call. When RequestName is invoked the following occurs:
4086 If the method caller is currently the primary owner of the name,
4087 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
4088 values are updated with the values from the new RequestName call,
4089 and nothing further happens.
4095 If the current primary owner (head of the queue) has
4096 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
4097 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
4098 the caller of RequestName replaces the current primary owner at
4099 the head of the queue and the current primary owner moves to the
4100 second position in the queue. If the caller of RequestName was
4101 in the queue previously its flags are updated with the values from
4102 the new RequestName in addition to moving it to the head of the queue.
4108 If replacement is not possible, and the method caller is
4109 currently in the queue but not the primary owner, its flags are
4110 updated with the values from the new RequestName call.
4116 If replacement is not possible, and the method caller is
4117 currently not in the queue, the method caller is appended to the
4124 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
4125 set and is not the primary owner, it is removed from the
4126 queue. This can apply to the previous primary owner (if it
4127 was replaced) or the method caller (if it updated the
4128 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
4129 queue, or if it was just added to the queue with that flag set).
4135 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
4136 queue," even if another application already in the queue had specified
4137 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
4138 that does not allow replacement goes away, and the next primary owner
4139 does allow replacement. In this case, queued items that specified
4140 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
4141 automatically replace the new primary owner. In other words,
4142 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
4143 time RequestName is called. This is deliberate to avoid an infinite loop
4144 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4145 and DBUS_NAME_FLAG_REPLACE_EXISTING.
4148 The flags argument contains any of the following values logically ORed
4155 <entry>Conventional Name</entry>
4156 <entry>Value</entry>
4157 <entry>Description</entry>
4162 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
4166 If an application A specifies this flag and succeeds in
4167 becoming the owner of the name, and another application B
4168 later calls RequestName with the
4169 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
4170 will lose ownership and receive a
4171 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
4172 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
4173 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
4174 is not specified by application B, then application B will not replace
4175 application A as the owner.
4180 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
4184 Try to replace the current owner if there is one. If this
4185 flag is not set the application will only become the owner of
4186 the name if there is no current owner. If this flag is set,
4187 the application will replace the current owner if
4188 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
4193 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
4197 Without this flag, if an application requests a name that is
4198 already owned, the application will be placed in a queue to
4199 own the name when the current owner gives it up. If this
4200 flag is given, the application will not be placed in the
4201 queue, the request for the name will simply fail. This flag
4202 also affects behavior when an application is replaced as
4203 name owner; by default the application moves back into the
4204 waiting queue, unless this flag was provided when the application
4205 became the name owner.
4213 The return code can be one of the following values:
4219 <entry>Conventional Name</entry>
4220 <entry>Value</entry>
4221 <entry>Description</entry>
4226 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
4227 <entry>1</entry> <entry>The caller is now the primary owner of
4228 the name, replacing any previous owner. Either the name had no
4229 owner before, or the caller specified
4230 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
4231 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
4234 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
4237 <entry>The name already had an owner,
4238 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
4239 the current owner did not specify
4240 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
4241 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
4245 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
4246 <entry>The name already has an owner,
4247 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
4248 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
4249 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
4250 specified by the requesting application.</entry>
4253 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
4255 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
4263 <sect3 id="bus-messages-release-name">
4264 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
4268 UINT32 ReleaseName (in STRING name)
4275 <entry>Argument</entry>
4277 <entry>Description</entry>
4283 <entry>STRING</entry>
4284 <entry>Name to release</entry>
4294 <entry>Argument</entry>
4296 <entry>Description</entry>
4302 <entry>UINT32</entry>
4303 <entry>Return value</entry>
4310 This method call should be sent to
4311 <literal>org.freedesktop.DBus</literal> and asks the message bus to
4312 release the method caller's claim to the given name. If the caller is
4313 the primary owner, a new primary owner will be selected from the
4314 queue if any other owners are waiting. If the caller is waiting in
4315 the queue for the name, the caller will removed from the queue and
4316 will not be made an owner of the name if it later becomes available.
4317 If there are no other owners in the queue for the name, it will be
4318 removed from the bus entirely.
4320 The return code can be one of the following values:
4326 <entry>Conventional Name</entry>
4327 <entry>Value</entry>
4328 <entry>Description</entry>
4333 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
4334 <entry>1</entry> <entry>The caller has released his claim on
4335 the given name. Either the caller was the primary owner of
4336 the name, and the name is now unused or taken by somebody
4337 waiting in the queue for the name, or the caller was waiting
4338 in the queue for the name and has now been removed from the
4342 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
4344 <entry>The given name does not exist on this bus.</entry>
4347 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
4349 <entry>The caller was not the primary owner of this name,
4350 and was also not waiting in the queue to own this name.</entry>
4358 <sect3 id="bus-messages-list-queued-owners">
4359 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
4363 ARRAY of STRING ListQueuedOwners (in STRING name)
4370 <entry>Argument</entry>
4372 <entry>Description</entry>
4378 <entry>STRING</entry>
4379 <entry>The well-known bus name to query, such as
4380 <literal>com.example.cappuccino</literal></entry>
4390 <entry>Argument</entry>
4392 <entry>Description</entry>
4398 <entry>ARRAY of STRING</entry>
4399 <entry>The unique bus names of connections currently queued
4400 for the name</entry>
4407 This method call should be sent to
4408 <literal>org.freedesktop.DBus</literal> and lists the connections
4409 currently queued for a bus name (see
4410 <xref linkend="term-queued-owner"/>).
4415 <sect2 id="message-bus-routing">
4416 <title>Message Bus Message Routing</title>
4419 Messages may have a <literal>DESTINATION</literal> field (see <xref
4420 linkend="message-protocol-header-fields"/>), resulting in a
4421 <firstterm>unicast message</firstterm>. If the
4422 <literal>DESTINATION</literal> field is present, it specifies a message
4423 recipient by name. Method calls and replies normally specify this field.
4424 The message bus must send messages (of any type) with the
4425 <literal>DESTINATION</literal> field set to the specified recipient,
4426 regardless of whether the recipient has set up a match rule matching
4431 When the message bus receives a signal, if the
4432 <literal>DESTINATION</literal> field is absent, it is considered to
4433 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4434 applications with <firstterm>message matching rules</firstterm> that
4435 match the message. Most signal messages are broadcasts.
4439 Unicast signal messages (those with a <literal>DESTINATION</literal>
4440 field) are not commonly used, but they are treated like any unicast
4441 message: they are delivered to the specified receipient,
4442 regardless of its match rules. One use for unicast signals is to
4443 avoid a race condition in which a signal is emitted before the intended
4444 recipient can call <xref linkend="bus-messages-add-match"/> to
4445 receive that signal: if the signal is sent directly to that recipient
4446 using a unicast message, it does not need to add a match rule at all,
4447 and there is no race condition. Another use for unicast signals,
4448 on message buses whose security policy prevents eavesdropping, is to
4449 send sensitive information which should only be visible to one
4454 When the message bus receives a method call, if the
4455 <literal>DESTINATION</literal> field is absent, the call is taken to be
4456 a standard one-to-one message and interpreted by the message bus
4457 itself. For example, sending an
4458 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4459 <literal>DESTINATION</literal> will cause the message bus itself to
4460 reply to the ping immediately; the message bus will not make this
4461 message visible to other applications.
4465 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4466 the ping message were sent with a <literal>DESTINATION</literal> name of
4467 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4468 forwarded, and the Yoyodyne Corporation screensaver application would be
4469 expected to reply to the ping.
4473 Message bus implementations may impose a security policy which
4474 prevents certain messages from being sent or received.
4475 When a message cannot be sent or received due to a security
4476 policy, the message bus should send an error reply, unless the
4477 original message had the <literal>NO_REPLY</literal> flag.
4480 <sect3 id="message-bus-routing-eavesdropping">
4481 <title>Eavesdropping</title>
4483 Receiving a unicast message whose <literal>DESTINATION</literal>
4484 indicates a different recipient is called
4485 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4486 a security boundary (like the standard system bus), the security
4487 policy should usually prevent eavesdropping, since unicast messages
4488 are normally kept private and may contain security-sensitive
4493 Eavesdropping is mainly useful for debugging tools, such as
4494 the <literal>dbus-monitor</literal> tool in the reference
4495 implementation of D-Bus. Tools which eavesdrop on the message bus
4496 should be careful to avoid sending a reply or error in response to
4497 messages intended for a different client.
4501 Clients may attempt to eavesdrop by adding match rules
4502 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4503 the <literal>eavesdrop='true'</literal> match. If the message bus'
4504 security policy does not allow eavesdropping, the match rule can
4505 still be added, but will not have any practical effect. For
4506 compatibility with older message bus implementations, if adding such
4507 a match rule results in an error reply, the client may fall back to
4508 adding the same rule with the <literal>eavesdrop</literal> match
4513 <sect3 id="message-bus-routing-match-rules">
4514 <title>Match Rules</title>
4516 An important part of the message bus routing protocol is match
4517 rules. Match rules describe the messages that should be sent to a
4518 client, based on the contents of the message. Broadcast signals
4519 are only sent to clients which have a suitable match rule: this
4520 avoids waking up client processes to deal with signals that are
4521 not relevant to that client.
4524 Messages that list a client as their <literal>DESTINATION</literal>
4525 do not need to match the client's match rules, and are sent to that
4526 client regardless. As a result, match rules are mainly used to
4527 receive a subset of broadcast signals.
4530 Match rules can also be used for eavesdropping
4531 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4532 if the security policy of the message bus allows it.
4535 Match rules are added using the AddMatch bus method
4536 (see <xref linkend="bus-messages-add-match"/>). Rules are
4537 specified as a string of comma separated key/value pairs.
4538 Excluding a key from the rule indicates a wildcard match.
4539 For instance excluding the the member from a match rule but
4540 adding a sender would let all messages from that sender through.
4541 An example of a complete rule would be
4542 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4545 The following table describes the keys that can be used to create
4552 <entry>Possible Values</entry>
4553 <entry>Description</entry>
4558 <entry><literal>type</literal></entry>
4559 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4560 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4563 <entry><literal>sender</literal></entry>
4564 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4565 and <xref linkend="term-unique-name"/> respectively)
4567 <entry>Match messages sent by a particular sender. An example of a sender match
4568 is sender='org.freedesktop.Hal'</entry>
4571 <entry><literal>interface</literal></entry>
4572 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4573 <entry>Match messages sent over or to a particular interface. An example of an
4574 interface match is interface='org.freedesktop.Hal.Manager'.
4575 If a message omits the interface header, it must not match any rule
4576 that specifies this key.</entry>
4579 <entry><literal>member</literal></entry>
4580 <entry>Any valid method or signal name</entry>
4581 <entry>Matches messages which have the give method or signal name. An example of
4582 a member match is member='NameOwnerChanged'</entry>
4585 <entry><literal>path</literal></entry>
4586 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4587 <entry>Matches messages which are sent from or to the given object. An example of a
4588 path match is path='/org/freedesktop/Hal/Manager'</entry>
4591 <entry><literal>path_namespace</literal></entry>
4592 <entry>An object path</entry>
4595 Matches messages which are sent from or to an
4596 object for which the object path is either the
4597 given value, or that value followed by one or
4598 more path components.
4603 <literal>path_namespace='/com/example/foo'</literal>
4604 would match signals sent by
4605 <literal>/com/example/foo</literal>
4607 <literal>/com/example/foo/bar</literal>,
4609 <literal>/com/example/foobar</literal>.
4613 Using both <literal>path</literal> and
4614 <literal>path_namespace</literal> in the same match
4615 rule is not allowed.
4620 This match key was added in version 0.16 of the
4621 D-Bus specification and implemented by the bus
4622 daemon in dbus 1.5.0 and later.
4628 <entry><literal>destination</literal></entry>
4629 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4630 <entry>Matches messages which are being sent to the given unique name. An
4631 example of a destination match is destination=':1.0'</entry>
4634 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4635 <entry>Any string</entry>
4636 <entry>Arg matches are special and are used for further restricting the
4637 match based on the arguments in the body of a message. Only arguments of type
4638 STRING can be matched in this way. An example of an argument match
4639 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4643 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4644 <entry>Any string</entry>
4646 <para>Argument path matches provide a specialised form of wildcard matching for
4647 path-like namespaces. They can match arguments whose type is either STRING or
4648 OBJECT_PATH. As with normal argument matches,
4649 if the argument is exactly equal to the string given in the match
4650 rule then the rule is satisfied. Additionally, there is also a
4651 match when either the string given in the match rule or the
4652 appropriate message argument ends with '/' and is a prefix of the
4653 other. An example argument path match is arg0path='/aa/bb/'. This
4654 would match messages with first arguments of '/', '/aa/',
4655 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4656 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4658 <para>This is intended for monitoring “directories” in file system-like
4659 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4660 system. An application interested in all nodes in a particular hierarchy would
4661 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4662 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4663 represent a modification to the “bar” property, or a signal with zeroth
4664 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4665 many properties within that directory, and the interested application would be
4666 notified in both cases.</para>
4669 This match key was added in version 0.12 of the
4670 D-Bus specification, implemented for STRING
4671 arguments by the bus daemon in dbus 1.2.0 and later,
4672 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4679 <entry><literal>arg0namespace</literal></entry>
4680 <entry>Like a bus name, except that the string is not
4681 required to contain a '.' (period)</entry>
4683 <para>Match messages whose first argument is of type STRING, and is a bus name
4684 or interface name within the specified namespace. This is primarily intended
4685 for watching name owner changes for a group of related bus names, rather than
4686 for a single name or all name changes.</para>
4688 <para>Because every valid interface name is also a valid
4689 bus name, this can also be used for messages whose
4690 first argument is an interface name.</para>
4692 <para>For example, the match rule
4693 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4694 matches name owner changes for bus names such as
4695 <literal>com.example.backend.foo</literal>,
4696 <literal>com.example.backend.foo.bar</literal>, and
4697 <literal>com.example.backend</literal> itself.</para>
4699 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4702 This match key was added in version 0.16 of the
4703 D-Bus specification and implemented by the bus
4704 daemon in dbus 1.5.0 and later.
4710 <entry><literal>eavesdrop</literal></entry>
4711 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4712 <entry>Since D-Bus 1.5.6, match rules do not
4713 match messages which have a <literal>DESTINATION</literal>
4714 field unless the match rule specifically
4716 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4717 by specifying <literal>eavesdrop='true'</literal>
4718 in the match rule. <literal>eavesdrop='false'</literal>
4719 restores the default behaviour. Messages are
4720 delivered to their <literal>DESTINATION</literal>
4721 regardless of match rules, so this match does not
4722 affect normal delivery of unicast messages.
4723 If the message bus has a security policy which forbids
4724 eavesdropping, this match may still be used without error,
4725 but will not have any practical effect.
4726 In older versions of D-Bus, this match was not allowed
4727 in match rules, and all match rules behaved as if
4728 <literal>eavesdrop='true'</literal> had been used.
4737 <sect2 id="message-bus-starting-services">
4738 <title>Message Bus Starting Services</title>
4740 The message bus can start applications on behalf of other applications.
4741 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4742 An application that can be started in this way is called a
4743 <firstterm>service</firstterm>.
4746 With D-Bus, starting a service is normally done by name. That is,
4747 applications ask the message bus to start some program that will own a
4748 well-known name, such as <literal>com.example.TextEditor</literal>.
4749 This implies a contract documented along with the name
4750 <literal>com.example.TextEditor</literal> for which object
4751 the owner of that name will provide, and what interfaces those
4755 To find an executable corresponding to a particular name, the bus daemon
4756 looks for <firstterm>service description files</firstterm>. Service
4757 description files define a mapping from names to executables. Different
4758 kinds of message bus will look for these files in different places, see
4759 <xref linkend="message-bus-types"/>.
4762 Service description files have the ".service" file
4763 extension. The message bus will only load service description files
4764 ending with .service; all other files will be ignored. The file format
4765 is similar to that of <ulink
4766 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4767 entries</ulink>. All service description files must be in UTF-8
4768 encoding. To ensure that there will be no name collisions, service files
4769 must be namespaced using the same mechanism as messages and service
4774 On the well-known system bus, the name of a service description file
4775 must be its well-known name plus <literal>.service</literal>,
4777 <literal>com.example.ConfigurationDatabase.service</literal>.
4781 On the well-known session bus, services should follow the same
4782 service description file naming convention as on the system bus,
4783 but for backwards compatibility they are not required to do so.
4787 [FIXME the file format should be much better specified than "similar to
4788 .desktop entries" esp. since desktop entries are already
4789 badly-specified. ;-)]
4790 These sections from the specification apply to service files as well:
4793 <listitem><para>General syntax</para></listitem>
4794 <listitem><para>Comment format</para></listitem>
4797 Service description files must contain a
4798 <literal>D-BUS Service</literal> group with at least the keys
4799 <literal>Name</literal> (the well-known name of the service)
4800 and <literal>Exec</literal> (the command to be executed).
4803 <title>Example service description file</title>
4805 # Sample service description file
4807 Name=com.example.ConfigurationDatabase
4808 Exec=/usr/bin/sample-configd
4814 Additionally, service description files for the well-known system
4815 bus on Unix must contain a <literal>User</literal> key, whose value
4816 is the name of a user account (e.g. <literal>root</literal>).
4817 The system service will be run as that user.
4821 When an application asks to start a service by name, the bus daemon tries to
4822 find a service that will own that name. It then tries to spawn the
4823 executable associated with it. If this fails, it will report an
4828 On the well-known system bus, it is not possible for two .service files
4829 in the same directory to offer the same service, because they are
4830 constrained to have names that match the service name.
4834 On the well-known session bus, if two .service files in the same
4835 directory offer the same service name, the result is undefined.
4836 Distributors should avoid this situation, for instance by naming
4837 session services' .service files according to their service name.
4841 If two .service files in different directories offer the same
4842 service name, the one in the higher-priority directory is used:
4843 for instance, on the system bus, .service files in
4844 /usr/local/share/dbus-1/system-services take precedence over those
4845 in /usr/share/dbus-1/system-services.
4848 The executable launched will have the environment variable
4849 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4850 message bus so it can connect and request the appropriate names.
4853 The executable being launched may want to know whether the message bus
4854 starting it is one of the well-known message buses (see <xref
4855 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4856 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4857 of the well-known buses. The currently-defined values for this variable
4858 are <literal>system</literal> for the systemwide message bus,
4859 and <literal>session</literal> for the per-login-session message
4860 bus. The new executable must still connect to the address given
4861 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4862 resulting connection is to the well-known bus.
4865 [FIXME there should be a timeout somewhere, either specified
4866 in the .service file, by the client, or just a global value
4867 and if the client being activated fails to connect within that
4868 timeout, an error should be sent back.]
4871 <sect3 id="message-bus-starting-services-scope">
4872 <title>Message Bus Service Scope</title>
4874 The "scope" of a service is its "per-", such as per-session,
4875 per-machine, per-home-directory, or per-display. The reference
4876 implementation doesn't yet support starting services in a different
4877 scope from the message bus itself. So e.g. if you start a service
4878 on the session bus its scope is per-session.
4881 We could add an optional scope to a bus name. For example, for
4882 per-(display,session pair), we could have a unique ID for each display
4883 generated automatically at login and set on screen 0 by executing a
4884 special "set display ID" binary. The ID would be stored in a
4885 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4886 random bytes. This ID would then be used to scope names.
4887 Starting/locating a service could be done by ID-name pair rather than
4891 Contrast this with a per-display scope. To achieve that, we would
4892 want a single bus spanning all sessions using a given display.
4893 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4894 property on screen 0 of the display, pointing to this bus.
4899 <sect2 id="message-bus-types">
4900 <title>Well-known Message Bus Instances</title>
4902 Two standard message bus instances are defined here, along with how
4903 to locate them and where their service files live.
4905 <sect3 id="message-bus-types-login">
4906 <title>Login session message bus</title>
4908 Each time a user logs in, a <firstterm>login session message
4909 bus</firstterm> may be started. All applications in the user's login
4910 session may interact with one another using this message bus.
4913 The address of the login session message bus is given
4914 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4915 variable. If that variable is not set, applications may
4916 also try to read the address from the X Window System root
4917 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4918 The root window property must have type <literal>STRING</literal>.
4919 The environment variable should have precedence over the
4920 root window property.
4922 <para>The address of the login session message bus is given in the
4923 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4924 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4925 "autolaunch:", the system should use platform-specific methods of
4926 locating a running D-Bus session server, or starting one if a running
4927 instance cannot be found. Note that this mechanism is not recommended
4928 for attempting to determine if a daemon is running. It is inherently
4929 racy to attempt to make this determination, since the bus daemon may
4930 be started just before or just after the determination is made.
4931 Therefore, it is recommended that applications do not try to make this
4932 determination for their functionality purposes, and instead they
4933 should attempt to start the server.</para>
4935 <sect4 id="message-bus-types-login-x-windows">
4936 <title>X Windowing System</title>
4938 For the X Windowing System, the application must locate the
4939 window owner of the selection represented by the atom formed by
4943 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4947 <para>the current user's username</para>
4951 <para>the literal character '_' (underscore)</para>
4955 <para>the machine's ID</para>
4961 The following properties are defined for the window that owns
4963 <informaltable frame="all">
4972 <para>meaning</para>
4978 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4982 <para>the actual address of the server socket</para>
4988 <para>_DBUS_SESSION_BUS_PID</para>
4992 <para>the PID of the server process</para>
5001 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
5002 present in this window.
5006 If the X selection cannot be located or if reading the
5007 properties from the window fails, the implementation MUST conclude
5008 that there is no D-Bus server running and proceed to start a new
5009 server. (See below on concurrency issues)
5013 Failure to connect to the D-Bus server address thus obtained
5014 MUST be treated as a fatal connection error and should be reported
5019 As an alternative, an implementation MAY find the information
5020 in the following file located in the current user's home directory,
5021 in subdirectory .dbus/session-bus/:
5024 <para>the machine's ID</para>
5028 <para>the literal character '-' (dash)</para>
5032 <para>the X display without the screen number, with the
5033 following prefixes removed, if present: ":", "localhost:"
5034 ."localhost.localdomain:". That is, a display of
5035 "localhost:10.0" produces just the number "10"</para>
5041 The contents of this file NAME=value assignment pairs and
5042 lines starting with # are comments (no comments are allowed
5043 otherwise). The following variable names are defined:
5050 <para>Variable</para>
5054 <para>meaning</para>
5060 <para>DBUS_SESSION_BUS_ADDRESS</para>
5064 <para>the actual address of the server socket</para>
5070 <para>DBUS_SESSION_BUS_PID</para>
5074 <para>the PID of the server process</para>
5080 <para>DBUS_SESSION_BUS_WINDOWID</para>
5084 <para>the window ID</para>
5093 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
5098 Failure to open this file MUST be interpreted as absence of a
5099 running server. Therefore, the implementation MUST proceed to
5100 attempting to launch a new bus server if the file cannot be
5105 However, success in opening this file MUST NOT lead to the
5106 conclusion that the server is running. Thus, a failure to connect to
5107 the bus address obtained by the alternative method MUST NOT be
5108 considered a fatal error. If the connection cannot be established,
5109 the implementation MUST proceed to check the X selection settings or
5110 to start the server on its own.
5114 If the implementation concludes that the D-Bus server is not
5115 running it MUST attempt to start a new server and it MUST also
5116 ensure that the daemon started as an effect of the "autolaunch"
5117 mechanism provides the lookup mechanisms described above, so
5118 subsequent calls can locate the newly started server. The
5119 implementation MUST also ensure that if two or more concurrent
5120 initiations happen, only one server remains running and all other
5121 initiations are able to obtain the address of this server and
5122 connect to it. In other words, the implementation MUST ensure that
5123 the X selection is not present when it attempts to set it, without
5124 allowing another process to set the selection between the
5125 verification and the setting (e.g., by using XGrabServer /
5132 On Unix systems, the session bus should search for .service files
5133 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
5135 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
5136 Implementations may also search additional locations, which
5137 should be searched with lower priority than anything in
5138 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
5139 for example, the reference implementation also
5140 looks in <literal>${datadir}/dbus-1/services</literal> as
5141 set at compile time.
5144 As described in the XDG Base Directory Specification, software
5145 packages should install their session .service files to their
5146 configured <literal>${datadir}/dbus-1/services</literal>,
5147 where <literal>${datadir}</literal> is as defined by the GNU
5148 coding standards. System administrators or users can arrange
5149 for these service files to be read by setting XDG_DATA_DIRS or by
5150 symlinking them into the default locations.
5154 <sect3 id="message-bus-types-system">
5155 <title>System message bus</title>
5157 A computer may have a <firstterm>system message bus</firstterm>,
5158 accessible to all applications on the system. This message bus may be
5159 used to broadcast system events, such as adding new hardware devices,
5160 changes in the printer queue, and so forth.
5163 The address of the system message bus is given
5164 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
5165 variable. If that variable is not set, applications should try
5166 to connect to the well-known address
5167 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
5170 The D-Bus reference implementation actually honors the
5171 <literal>$(localstatedir)</literal> configure option
5172 for this address, on both client and server side.
5177 On Unix systems, the system bus should default to searching
5178 for .service files in
5179 <literal>/usr/local/share/dbus-1/system-services</literal>,
5180 <literal>/usr/share/dbus-1/system-services</literal> and
5181 <literal>/lib/dbus-1/system-services</literal>, with that order
5182 of precedence. It may also search other implementation-specific
5183 locations, but should not vary these locations based on environment
5187 The system bus is security-sensitive and is typically executed
5188 by an init system with a clean environment. Its launch helper
5189 process is particularly security-sensitive, and specifically
5190 clears its own environment.
5195 Software packages should install their system .service
5196 files to their configured
5197 <literal>${datadir}/dbus-1/system-services</literal>,
5198 where <literal>${datadir}</literal> is as defined by the GNU
5199 coding standards. System administrators can arrange
5200 for these service files to be read by editing the system bus'
5201 configuration file or by symlinking them into the default
5207 <sect2 id="message-bus-messages">
5208 <title>Message Bus Messages</title>
5210 The special message bus name <literal>org.freedesktop.DBus</literal>
5211 responds to a number of additional messages.
5214 <sect3 id="bus-messages-hello">
5215 <title><literal>org.freedesktop.DBus.Hello</literal></title>
5226 <entry>Argument</entry>
5228 <entry>Description</entry>
5234 <entry>STRING</entry>
5235 <entry>Unique name assigned to the connection</entry>
5242 Before an application is able to send messages to other applications
5243 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
5244 to the message bus to obtain a unique name. If an application without
5245 a unique name tries to send a message to another application, or a
5246 message to the message bus itself that isn't the
5247 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
5248 disconnected from the bus.
5251 There is no corresponding "disconnect" request; if a client wishes to
5252 disconnect from the bus, it simply closes the socket (or other
5253 communication channel).
5256 <sect3 id="bus-messages-list-names">
5257 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
5261 ARRAY of STRING ListNames ()
5268 <entry>Argument</entry>
5270 <entry>Description</entry>
5276 <entry>ARRAY of STRING</entry>
5277 <entry>Array of strings where each string is a bus name</entry>
5284 Returns a list of all currently-owned names on the bus.
5287 <sect3 id="bus-messages-list-activatable-names">
5288 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
5292 ARRAY of STRING ListActivatableNames ()
5299 <entry>Argument</entry>
5301 <entry>Description</entry>
5307 <entry>ARRAY of STRING</entry>
5308 <entry>Array of strings where each string is a bus name</entry>
5315 Returns a list of all names that can be activated on the bus.
5318 <sect3 id="bus-messages-name-exists">
5319 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
5323 BOOLEAN NameHasOwner (in STRING name)
5330 <entry>Argument</entry>
5332 <entry>Description</entry>
5338 <entry>STRING</entry>
5339 <entry>Name to check</entry>
5349 <entry>Argument</entry>
5351 <entry>Description</entry>
5357 <entry>BOOLEAN</entry>
5358 <entry>Return value, true if the name exists</entry>
5365 Checks if the specified name exists (currently has an owner).
5369 <sect3 id="bus-messages-name-owner-changed">
5370 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
5374 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
5381 <entry>Argument</entry>
5383 <entry>Description</entry>
5389 <entry>STRING</entry>
5390 <entry>Name with a new owner</entry>
5394 <entry>STRING</entry>
5395 <entry>Old owner or empty string if none</entry>
5399 <entry>STRING</entry>
5400 <entry>New owner or empty string if none</entry>
5407 This signal indicates that the owner of a name has changed.
5408 It's also the signal to use to detect the appearance of
5409 new names on the bus.
5412 <sect3 id="bus-messages-name-lost">
5413 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
5417 NameLost (STRING name)
5424 <entry>Argument</entry>
5426 <entry>Description</entry>
5432 <entry>STRING</entry>
5433 <entry>Name which was lost</entry>
5440 This signal is sent to a specific application when it loses
5441 ownership of a name.
5445 <sect3 id="bus-messages-name-acquired">
5446 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5450 NameAcquired (STRING name)
5457 <entry>Argument</entry>
5459 <entry>Description</entry>
5465 <entry>STRING</entry>
5466 <entry>Name which was acquired</entry>
5473 This signal is sent to a specific application when it gains
5474 ownership of a name.
5478 <sect3 id="bus-messages-start-service-by-name">
5479 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5483 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5490 <entry>Argument</entry>
5492 <entry>Description</entry>
5498 <entry>STRING</entry>
5499 <entry>Name of the service to start</entry>
5503 <entry>UINT32</entry>
5504 <entry>Flags (currently not used)</entry>
5514 <entry>Argument</entry>
5516 <entry>Description</entry>
5522 <entry>UINT32</entry>
5523 <entry>Return value</entry>
5528 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5532 The return value can be one of the following values:
5537 <entry>Identifier</entry>
5538 <entry>Value</entry>
5539 <entry>Description</entry>
5544 <entry>DBUS_START_REPLY_SUCCESS</entry>
5546 <entry>The service was successfully started.</entry>
5549 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5551 <entry>A connection already owns the given name.</entry>
5560 <sect3 id="bus-messages-update-activation-environment">
5561 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5565 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5572 <entry>Argument</entry>
5574 <entry>Description</entry>
5580 <entry>ARRAY of DICT<STRING,STRING></entry>
5581 <entry>Environment to add or update</entry>
5586 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5589 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5592 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.
5597 <sect3 id="bus-messages-get-name-owner">
5598 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5602 STRING GetNameOwner (in STRING name)
5609 <entry>Argument</entry>
5611 <entry>Description</entry>
5617 <entry>STRING</entry>
5618 <entry>Name to get the owner of</entry>
5628 <entry>Argument</entry>
5630 <entry>Description</entry>
5636 <entry>STRING</entry>
5637 <entry>Return value, a unique connection name</entry>
5642 Returns the unique connection name of the primary owner of the name
5643 given. If the requested name doesn't have an owner, returns a
5644 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5648 <sect3 id="bus-messages-get-connection-unix-user">
5649 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5653 UINT32 GetConnectionUnixUser (in STRING bus_name)
5660 <entry>Argument</entry>
5662 <entry>Description</entry>
5668 <entry>STRING</entry>
5669 <entry>Unique or well-known bus name of the connection to
5670 query, such as <literal>:12.34</literal> or
5671 <literal>com.example.tea</literal></entry>
5681 <entry>Argument</entry>
5683 <entry>Description</entry>
5689 <entry>UINT32</entry>
5690 <entry>Unix user ID</entry>
5695 Returns the Unix user ID of the process connected to the server. If
5696 unable to determine it (for instance, because the process is not on the
5697 same machine as the bus daemon), an error is returned.
5701 <sect3 id="bus-messages-get-connection-unix-process-id">
5702 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5706 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5713 <entry>Argument</entry>
5715 <entry>Description</entry>
5721 <entry>STRING</entry>
5722 <entry>Unique or well-known bus name of the connection to
5723 query, such as <literal>:12.34</literal> or
5724 <literal>com.example.tea</literal></entry>
5734 <entry>Argument</entry>
5736 <entry>Description</entry>
5742 <entry>UINT32</entry>
5743 <entry>Unix process id</entry>
5748 Returns the Unix process ID of the process connected to the server. If
5749 unable to determine it (for instance, because the process is not on the
5750 same machine as the bus daemon), an error is returned.
5754 <sect3 id="bus-messages-get-connection-credentials">
5755 <title><literal>org.freedesktop.DBus.GetConnectionCredentials</literal></title>
5759 DICT<STRING,VARIANT> GetConnectionCredentials (in STRING bus_name)
5766 <entry>Argument</entry>
5768 <entry>Description</entry>
5774 <entry>STRING</entry>
5775 <entry>Unique or well-known bus name of the connection to
5776 query, such as <literal>:12.34</literal> or
5777 <literal>com.example.tea</literal></entry>
5787 <entry>Argument</entry>
5789 <entry>Description</entry>
5795 <entry>DICT<STRING,VARIANT></entry>
5796 <entry>Credentials</entry>
5804 Returns as many credentials as possible for the process connected to
5805 the server. If unable to determine certain credentials (for instance,
5806 because the process is not on the same machine as the bus daemon,
5807 or because this version of the bus daemon does not support a
5808 particular security framework), or if the values of those credentials
5809 cannot be represented as documented here, then those credentials
5814 Keys in the returned dictionary not containing "." are defined
5815 by this specification. Bus daemon implementors supporting
5816 credentials frameworks not mentioned in this document should either
5817 contribute patches to this specification, or use keys containing
5818 "." and starting with a reversed domain name.
5824 <entry>Value type</entry>
5825 <entry>Value</entry>
5830 <entry>UnixUserID</entry>
5831 <entry>UINT32</entry>
5832 <entry>The numeric Unix user ID, as defined by POSIX</entry>
5835 <entry>ProcessID</entry>
5836 <entry>UINT32</entry>
5837 <entry>The numeric process ID, on platforms that have
5838 this concept. On Unix, this is the process ID defined by
5847 This method was added in D-Bus 1.7 to reduce the round-trips
5848 required to list a process's credentials. In older versions, calling
5849 this method will fail: applications should recover by using the
5850 separate methods such as
5851 <xref linkend="bus-messages-get-connection-unix-user"/>
5856 <sect3 id="bus-messages-get-adt-audit-session-data">
5857 <title><literal>org.freedesktop.DBus.GetAdtAuditSessionData</literal></title>
5861 ARRAY of BYTE GetAdtAuditSessionData (in STRING bus_name)
5868 <entry>Argument</entry>
5870 <entry>Description</entry>
5876 <entry>STRING</entry>
5877 <entry>Unique or well-known bus name of the connection to
5878 query, such as <literal>:12.34</literal> or
5879 <literal>com.example.tea</literal></entry>
5889 <entry>Argument</entry>
5891 <entry>Description</entry>
5897 <entry>ARRAY of BYTE</entry>
5898 <entry>auditing data as returned by
5899 adt_export_session_data()</entry>
5904 Returns auditing data used by Solaris ADT, in an unspecified
5905 binary format. If you know what this means, please contribute
5906 documentation via the D-Bus bug tracking system.
5907 This method is on the core DBus interface for historical reasons;
5908 the same information should be made available via
5909 <xref linkend="bus-messages-get-connection-credentials"/>
5914 <sect3 id="bus-messages-get-connection-selinux-security-context">
5915 <title><literal>org.freedesktop.DBus.GetConnectionSELinuxSecurityContext</literal></title>
5919 ARRAY of BYTE GetConnectionSELinuxSecurityContext (in STRING bus_name)
5926 <entry>Argument</entry>
5928 <entry>Description</entry>
5934 <entry>STRING</entry>
5935 <entry>Unique or well-known bus name of the connection to
5936 query, such as <literal>:12.34</literal> or
5937 <literal>com.example.tea</literal></entry>
5947 <entry>Argument</entry>
5949 <entry>Description</entry>
5955 <entry>ARRAY of BYTE</entry>
5956 <entry>some sort of string of bytes, not necessarily UTF-8,
5957 not including '\0'</entry>
5962 Returns the security context used by SELinux, in an unspecified
5963 format. If you know what this means, please contribute
5964 documentation via the D-Bus bug tracking system.
5965 This method is on the core DBus interface for historical reasons;
5966 the same information should be made available via
5967 <xref linkend="bus-messages-get-connection-credentials"/>
5973 <sect3 id="bus-messages-add-match">
5974 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5978 AddMatch (in STRING rule)
5985 <entry>Argument</entry>
5987 <entry>Description</entry>
5993 <entry>STRING</entry>
5994 <entry>Match rule to add to the connection</entry>
5999 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
6000 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
6004 <sect3 id="bus-messages-remove-match">
6005 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
6009 RemoveMatch (in STRING rule)
6016 <entry>Argument</entry>
6018 <entry>Description</entry>
6024 <entry>STRING</entry>
6025 <entry>Match rule to remove from the connection</entry>
6030 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
6031 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
6036 <sect3 id="bus-messages-get-id">
6037 <title><literal>org.freedesktop.DBus.GetId</literal></title>
6041 GetId (out STRING id)
6048 <entry>Argument</entry>
6050 <entry>Description</entry>
6056 <entry>STRING</entry>
6057 <entry>Unique ID identifying the bus daemon</entry>
6062 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
6063 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
6064 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
6065 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
6066 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
6067 by org.freedesktop.DBus.Peer.GetMachineId().
6068 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
6076 <appendix id="implementation-notes">
6077 <title>Implementation notes</title>
6078 <sect1 id="implementation-notes-subsection">
6086 <glossary><title>Glossary</title>
6088 This glossary defines some of the terms used in this specification.
6091 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
6094 The message bus maintains an association between names and
6095 connections. (Normally, there's one connection per application.) A
6096 bus name is simply an identifier used to locate connections. For
6097 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
6098 name might be used to send a message to a screensaver from Yoyodyne
6099 Corporation. An application is said to <firstterm>own</firstterm> a
6100 name if the message bus has associated the application's connection
6101 with the name. Names may also have <firstterm>queued
6102 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
6103 The bus assigns a unique name to each connection,
6104 see <xref linkend="term-unique-name"/>. Other names
6105 can be thought of as "well-known names" and are
6106 used to find applications that offer specific functionality.
6110 See <xref linkend="message-protocol-names-bus"/> for details of
6111 the syntax and naming conventions for bus names.
6116 <glossentry id="term-message"><glossterm>Message</glossterm>
6119 A message is the atomic unit of communication via the D-Bus
6120 protocol. It consists of a <firstterm>header</firstterm> and a
6121 <firstterm>body</firstterm>; the body is made up of
6122 <firstterm>arguments</firstterm>.
6127 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
6130 The message bus is a special application that forwards
6131 or routes messages between a group of applications
6132 connected to the message bus. It also manages
6133 <firstterm>names</firstterm> used for routing
6139 <glossentry id="term-name"><glossterm>Name</glossterm>
6142 See <xref linkend="term-bus-name"/>. "Name" may
6143 also be used to refer to some of the other names
6144 in D-Bus, such as interface names.
6149 <glossentry id="namespace"><glossterm>Namespace</glossterm>
6152 Used to prevent collisions when defining new interfaces, bus names
6153 etc. The convention used is the same one Java uses for defining
6154 classes: a reversed domain name.
6155 See <xref linkend="message-protocol-names-bus"/>,
6156 <xref linkend="message-protocol-names-interface"/>,
6157 <xref linkend="message-protocol-names-error"/>,
6158 <xref linkend="message-protocol-marshaling-object-path"/>.
6163 <glossentry id="term-object"><glossterm>Object</glossterm>
6166 Each application contains <firstterm>objects</firstterm>, which have
6167 <firstterm>interfaces</firstterm> and
6168 <firstterm>methods</firstterm>. Objects are referred to by a name,
6169 called a <firstterm>path</firstterm>.
6174 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
6177 An application talking directly to another application, without going
6178 through a message bus. One-to-one connections may be "peer to peer" or
6179 "client to server." The D-Bus protocol has no concept of client
6180 vs. server after a connection has authenticated; the flow of messages
6181 is symmetrical (full duplex).
6186 <glossentry id="term-path"><glossterm>Path</glossterm>
6189 Object references (object names) in D-Bus are organized into a
6190 filesystem-style hierarchy, so each object is named by a path. As in
6191 LDAP, there's no difference between "files" and "directories"; a path
6192 can refer to an object, while still having child objects below it.
6197 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
6200 Each bus name has a primary owner; messages sent to the name go to the
6201 primary owner. However, certain names also maintain a queue of
6202 secondary owners "waiting in the wings." If the primary owner releases
6203 the name, then the first secondary owner in the queue automatically
6204 becomes the new owner of the name.
6209 <glossentry id="term-service"><glossterm>Service</glossterm>
6212 A service is an executable that can be launched by the bus daemon.
6213 Services normally guarantee some particular features, for example they
6214 may guarantee that they will request a specific name such as
6215 "com.example.Screensaver", have a singleton object
6216 "/com/example/Application", and that object will implement the
6217 interface "com.example.Screensaver.Control".
6222 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
6225 ".service files" tell the bus about service applications that can be
6226 launched (see <xref linkend="term-service"/>). Most importantly they
6227 provide a mapping from bus names to services that will request those
6228 names when they start up.
6233 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
6236 The special name automatically assigned to each connection by the
6237 message bus. This name will never change owner, and will be unique
6238 (never reused during the lifetime of the message bus).
6239 It will begin with a ':' character.