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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.19</releaseinfo>
10 <date>UNRELEASED</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>current</revnumber>
76 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
77 <authorinitials></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.18</revnumber>
82 <date>29 July 2011</date>
83 <authorinitials>smcv</authorinitials>
84 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
85 match keyword; promote type system to a top-level section</revremark>
88 <revnumber>0.17</revnumber>
89 <date>1 June 2011</date>
90 <authorinitials>smcv/davidz</authorinitials>
91 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
92 by GVariant</revremark>
95 <revnumber>0.16</revnumber>
96 <date>11 April 2011</date>
97 <authorinitials></authorinitials>
98 <revremark>add path_namespace, arg0namespace; argNpath matches object
102 <revnumber>0.15</revnumber>
103 <date>3 November 2010</date>
104 <authorinitials></authorinitials>
105 <revremark></revremark>
108 <revnumber>0.14</revnumber>
109 <date>12 May 2010</date>
110 <authorinitials></authorinitials>
111 <revremark></revremark>
114 <revnumber>0.13</revnumber>
115 <date>23 Dezember 2009</date>
116 <authorinitials></authorinitials>
117 <revremark></revremark>
120 <revnumber>0.12</revnumber>
121 <date>7 November, 2006</date>
122 <authorinitials></authorinitials>
123 <revremark></revremark>
126 <revnumber>0.11</revnumber>
127 <date>6 February 2005</date>
128 <authorinitials></authorinitials>
129 <revremark></revremark>
132 <revnumber>0.10</revnumber>
133 <date>28 January 2005</date>
134 <authorinitials></authorinitials>
135 <revremark></revremark>
138 <revnumber>0.9</revnumber>
139 <date>7 Januar 2005</date>
140 <authorinitials></authorinitials>
141 <revremark></revremark>
144 <revnumber>0.8</revnumber>
145 <date>06 September 2003</date>
146 <authorinitials></authorinitials>
147 <revremark>First released document.</revremark>
152 <sect1 id="introduction">
153 <title>Introduction</title>
155 D-Bus is a system for low-latency, low-overhead, easy to use
156 interprocess communication (IPC). In more detail:
160 D-Bus is <emphasis>low-latency</emphasis> because it is designed
161 to avoid round trips and allow asynchronous operation, much like
167 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
168 binary protocol, and does not have to convert to and from a text
169 format such as XML. Because D-Bus is intended for potentially
170 high-resolution same-machine IPC, not primarily for Internet IPC,
171 this is an interesting optimization.
176 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
177 of <firstterm>messages</firstterm> rather than byte streams, and
178 automatically handles a lot of the hard IPC issues. Also, the D-Bus
179 library is designed to be wrapped in a way that lets developers use
180 their framework's existing object/type system, rather than learning
181 a new one specifically for IPC.
188 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
189 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
190 a system for one application to talk to a single other
191 application. However, the primary intended application of the protocol is the
192 D-Bus <firstterm>message bus</firstterm>, specified in <xref
193 linkend="message-bus"/>. The message bus is a special application that
194 accepts connections from multiple other applications, and forwards
199 Uses of D-Bus include notification of system changes (notification of when
200 a camera is plugged in to a computer, or a new version of some software
201 has been installed), or desktop interoperability, for example a file
202 monitoring service or a configuration service.
206 D-Bus is designed for two specific use cases:
210 A "system bus" for notifications from the system to user sessions,
211 and to allow the system to request input from user sessions.
216 A "session bus" used to implement desktop environments such as
221 D-Bus is not intended to be a generic IPC system for any possible
222 application, and intentionally omits many features found in other
223 IPC systems for this reason.
227 At the same time, the bus daemons offer a number of features not found in
228 other IPC systems, such as single-owner "bus names" (similar to X
229 selections), on-demand startup of services, and security policies.
230 In many ways, these features are the primary motivation for developing
231 D-Bus; other systems would have sufficed if IPC were the only goal.
235 D-Bus may turn out to be useful in unanticipated applications, but future
236 versions of this spec and the reference implementation probably will not
237 incorporate features that interfere with the core use cases.
241 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
242 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
243 document are to be interpreted as described in RFC 2119. However, the
244 document could use a serious audit to be sure it makes sense to do
245 so. Also, they are not capitalized.
248 <sect2 id="stability">
249 <title>Protocol and Specification Stability</title>
251 The D-Bus protocol is frozen (only compatible extensions are allowed) as
252 of November 8, 2006. However, this specification could still use a fair
253 bit of work to make interoperable reimplementation possible without
254 reference to the D-Bus reference implementation. Thus, this
255 specification is not marked 1.0. To mark it 1.0, we'd like to see
256 someone invest significant effort in clarifying the specification
257 language, and growing the specification to cover more aspects of the
258 reference implementation's behavior.
261 Until this work is complete, any attempt to reimplement D-Bus will
262 probably require looking at the reference implementation and/or asking
263 questions on the D-Bus mailing list about intended behavior.
264 Questions on the list are very welcome.
267 Nonetheless, this document should be a useful starting point and is
268 to our knowledge accurate, though incomplete.
274 <sect1 id="type-system">
275 <title>Type System</title>
278 D-Bus has a type system, in which values of various types can be
279 serialized into a sequence of bytes referred to as the
280 <firstterm>wire format</firstterm> in a standard way.
281 Converting a value from some other representation into the wire
282 format is called <firstterm>marshaling</firstterm> and converting
283 it back from the wire format is <firstterm>unmarshaling</firstterm>.
286 <sect2 id="message-protocol-signatures">
287 <title>Type Signatures</title>
290 The D-Bus protocol does not include type tags in the marshaled data; a
291 block of marshaled values must have a known <firstterm>type
292 signature</firstterm>. The type signature is made up of <firstterm>type
293 codes</firstterm>. A type code is an ASCII character representing the
294 type of a value. Because ASCII characters are used, the type signature
295 will always form a valid ASCII string. A simple string compare
296 determines whether two type signatures are equivalent.
300 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
301 the ASCII character 'i'. So the signature for a block of values
302 containing a single <literal>INT32</literal> would be:
306 A block of values containing two <literal>INT32</literal> would have this signature:
313 All <firstterm>basic</firstterm> types work like
314 <literal>INT32</literal> in this example. To marshal and unmarshal
315 basic types, you simply read one value from the data
316 block corresponding to each type code in the signature.
317 In addition to basic types, there are four <firstterm>container</firstterm>
318 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
319 and <literal>DICT_ENTRY</literal>.
323 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
324 code does not appear in signatures. Instead, ASCII characters
325 '(' and ')' are used to mark the beginning and end of the struct.
326 So for example, a struct containing two integers would have this
331 Structs can be nested, so for example a struct containing
332 an integer and another struct:
336 The value block storing that struct would contain three integers; the
337 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
342 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
343 but is useful in code that implements the protocol. This type code
344 is specified to allow such code to interoperate in non-protocol contexts.
348 Empty structures are not allowed; there must be at least one
349 type code between the parentheses.
353 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
354 followed by a <firstterm>single complete type</firstterm>. The single
355 complete type following the array is the type of each array element. So
356 the simple example is:
360 which is an array of 32-bit integers. But an array can be of any type,
361 such as this array-of-struct-with-two-int32-fields:
365 Or this array of array of integer:
372 The phrase <firstterm>single complete type</firstterm> deserves some
373 definition. A single complete type is a basic type code, a variant type code,
374 an array with its element type, or a struct with its fields.
375 So the following signatures are not single complete types:
385 And the following signatures contain multiple complete types:
395 Note however that a single complete type may <emphasis>contain</emphasis>
396 multiple other single complete types.
400 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
401 type <literal>VARIANT</literal> will have the signature of a single complete type as part
402 of the <emphasis>value</emphasis>. This signature will be followed by a
403 marshaled value of that type.
407 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
408 than parentheses it uses curly braces, and it has more restrictions.
409 The restrictions are: it occurs only as an array element type; it has
410 exactly two single complete types inside the curly braces; the first
411 single complete type (the "key") must be a basic type rather than a
412 container type. Implementations must not accept dict entries outside of
413 arrays, must not accept dict entries with zero, one, or more than two
414 fields, and must not accept dict entries with non-basic-typed keys. A
415 dict entry is always a key-value pair.
419 The first field in the <literal>DICT_ENTRY</literal> is always the key.
420 A message is considered corrupt if the same key occurs twice in the same
421 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
422 implementations are not required to reject dicts with duplicate keys.
426 In most languages, an array of dict entry would be represented as a
427 map, hash table, or dict object.
431 The following table summarizes the D-Bus types.
436 <entry>Conventional Name</entry>
438 <entry>Description</entry>
443 <entry><literal>INVALID</literal></entry>
444 <entry>0 (ASCII NUL)</entry>
445 <entry>Not a valid type code, used to terminate signatures</entry>
447 <entry><literal>BYTE</literal></entry>
448 <entry>121 (ASCII 'y')</entry>
449 <entry>8-bit unsigned integer</entry>
451 <entry><literal>BOOLEAN</literal></entry>
452 <entry>98 (ASCII 'b')</entry>
453 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
455 <entry><literal>INT16</literal></entry>
456 <entry>110 (ASCII 'n')</entry>
457 <entry>16-bit signed integer</entry>
459 <entry><literal>UINT16</literal></entry>
460 <entry>113 (ASCII 'q')</entry>
461 <entry>16-bit unsigned integer</entry>
463 <entry><literal>INT32</literal></entry>
464 <entry>105 (ASCII 'i')</entry>
465 <entry>32-bit signed integer</entry>
467 <entry><literal>UINT32</literal></entry>
468 <entry>117 (ASCII 'u')</entry>
469 <entry>32-bit unsigned integer</entry>
471 <entry><literal>INT64</literal></entry>
472 <entry>120 (ASCII 'x')</entry>
473 <entry>64-bit signed integer</entry>
475 <entry><literal>UINT64</literal></entry>
476 <entry>116 (ASCII 't')</entry>
477 <entry>64-bit unsigned integer</entry>
479 <entry><literal>DOUBLE</literal></entry>
480 <entry>100 (ASCII 'd')</entry>
481 <entry>IEEE 754 double</entry>
483 <entry><literal>STRING</literal></entry>
484 <entry>115 (ASCII 's')</entry>
485 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
487 <entry><literal>OBJECT_PATH</literal></entry>
488 <entry>111 (ASCII 'o')</entry>
489 <entry>Name of an object instance</entry>
491 <entry><literal>SIGNATURE</literal></entry>
492 <entry>103 (ASCII 'g')</entry>
493 <entry>A type signature</entry>
495 <entry><literal>ARRAY</literal></entry>
496 <entry>97 (ASCII 'a')</entry>
499 <entry><literal>STRUCT</literal></entry>
500 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
501 <entry>Struct; type code 114 'r' is reserved for use in
502 bindings and implementations to represent the general
503 concept of a struct, and must not appear in signatures
504 used on D-Bus.</entry>
506 <entry><literal>VARIANT</literal></entry>
507 <entry>118 (ASCII 'v') </entry>
508 <entry>Variant type (the type of the value is part of the value itself)</entry>
510 <entry><literal>DICT_ENTRY</literal></entry>
511 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
512 <entry>Entry in a dict or map (array of key-value pairs).
513 Type code 101 'e' is reserved for use in bindings and
514 implementations to represent the general concept of a
515 dict or dict-entry, and must not appear in signatures
516 used on D-Bus.</entry>
518 <entry><literal>UNIX_FD</literal></entry>
519 <entry>104 (ASCII 'h')</entry>
520 <entry>Unix file descriptor</entry>
523 <entry>(reserved)</entry>
524 <entry>109 (ASCII 'm')</entry>
525 <entry>Reserved for <ulink
526 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
527 'maybe' type compatible with the one in GVariant</ulink>,
528 and must not appear in signatures used on D-Bus until
529 specified here</entry>
532 <entry>(reserved)</entry>
533 <entry>42 (ASCII '*')</entry>
534 <entry>Reserved for use in bindings/implementations to
535 represent any <firstterm>single complete type</firstterm>,
536 and must not appear in signatures used on D-Bus.</entry>
539 <entry>(reserved)</entry>
540 <entry>63 (ASCII '?')</entry>
541 <entry>Reserved for use in bindings/implementations to
542 represent any <firstterm>basic type</firstterm>, and must
543 not appear in signatures used on D-Bus.</entry>
546 <entry>(reserved)</entry>
547 <entry>64 (ASCII '@'), 38 (ASCII '&'),
548 94 (ASCII '^')</entry>
549 <entry>Reserved for internal use by bindings/implementations,
550 and must not appear in signatures used on D-Bus.
551 GVariant uses these type-codes to encode calling
561 <sect2 id="message-protocol-marshaling">
562 <title>Marshaling (Wire Format)</title>
565 Given a type signature, a block of bytes can be converted into typed
566 values. This section describes the format of the block of bytes. Byte
567 order and alignment issues are handled uniformly for all D-Bus types.
571 A block of bytes has an associated byte order. The byte order
572 has to be discovered in some way; for D-Bus messages, the
573 byte order is part of the message header as described in
574 <xref linkend="message-protocol-messages"/>. For now, assume
575 that the byte order is known to be either little endian or big
580 Each value in a block of bytes is aligned "naturally," for example
581 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
582 8-byte boundary. To properly align a value, <firstterm>alignment
583 padding</firstterm> may be necessary. The alignment padding must always
584 be the minimum required padding to properly align the following value;
585 and it must always be made up of nul bytes. The alignment padding must
586 not be left uninitialized (it can't contain garbage), and more padding
587 than required must not be used.
591 Given all this, the types are marshaled on the wire as follows:
596 <entry>Conventional Name</entry>
597 <entry>Encoding</entry>
598 <entry>Alignment</entry>
603 <entry><literal>INVALID</literal></entry>
604 <entry>Not applicable; cannot be marshaled.</entry>
607 <entry><literal>BYTE</literal></entry>
608 <entry>A single 8-bit byte.</entry>
611 <entry><literal>BOOLEAN</literal></entry>
612 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
615 <entry><literal>INT16</literal></entry>
616 <entry>16-bit signed integer in the message's byte order.</entry>
619 <entry><literal>UINT16</literal></entry>
620 <entry>16-bit unsigned integer in the message's byte order.</entry>
623 <entry><literal>INT32</literal></entry>
624 <entry>32-bit signed integer in the message's byte order.</entry>
627 <entry><literal>UINT32</literal></entry>
628 <entry>32-bit unsigned integer in the message's byte order.</entry>
631 <entry><literal>INT64</literal></entry>
632 <entry>64-bit signed integer in the message's byte order.</entry>
635 <entry><literal>UINT64</literal></entry>
636 <entry>64-bit unsigned integer in the message's byte order.</entry>
639 <entry><literal>DOUBLE</literal></entry>
640 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
643 <entry><literal>STRING</literal></entry>
644 <entry>A <literal>UINT32</literal> indicating the string's
645 length in bytes excluding its terminating nul, followed by
646 non-nul string data of the given length, followed by a terminating nul
653 <entry><literal>OBJECT_PATH</literal></entry>
654 <entry>Exactly the same as <literal>STRING</literal> except the
655 content must be a valid object path (see below).
661 <entry><literal>SIGNATURE</literal></entry>
662 <entry>The same as <literal>STRING</literal> except the length is a single
663 byte (thus signatures have a maximum length of 255)
664 and the content must be a valid signature (see below).
670 <entry><literal>ARRAY</literal></entry>
672 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
673 alignment padding to the alignment boundary of the array element type,
674 followed by each array element. The array length is from the
675 end of the alignment padding to the end of the last element,
676 i.e. it does not include the padding after the length,
677 or any padding after the last element.
678 Arrays have a maximum length defined to be 2 to the 26th power or
679 67108864. Implementations must not send or accept arrays exceeding this
686 <entry><literal>STRUCT</literal></entry>
688 A struct must start on an 8-byte boundary regardless of the
689 type of the struct fields. The struct value consists of each
690 field marshaled in sequence starting from that 8-byte
697 <entry><literal>VARIANT</literal></entry>
699 A variant type has a marshaled
700 <literal>SIGNATURE</literal> followed by a marshaled
701 value with the type given in the signature. Unlike
702 a message signature, the variant signature can
703 contain only a single complete type. So "i", "ai"
704 or "(ii)" is OK, but "ii" is not. Use of variants may not
705 cause a total message depth to be larger than 64, including
706 other container types such as structures.
709 1 (alignment of the signature)
712 <entry><literal>DICT_ENTRY</literal></entry>
720 <entry><literal>UNIX_FD</literal></entry>
721 <entry>32-bit unsigned integer in the message's byte
722 order. The actual file descriptors need to be
723 transferred out-of-band via some platform specific
724 mechanism. On the wire, values of this type store the index to the
725 file descriptor in the array of file descriptors that
726 accompany the message.</entry>
734 <sect3 id="message-protocol-marshaling-object-path">
735 <title>Valid Object Paths</title>
738 An object path is a name used to refer to an object instance.
739 Conceptually, each participant in a D-Bus message exchange may have
740 any number of object instances (think of C++ or Java objects) and each
741 such instance will have a path. Like a filesystem, the object
742 instances in an application form a hierarchical tree.
746 The following rules define a valid object path. Implementations must
747 not send or accept messages with invalid object paths.
751 The path may be of any length.
756 The path must begin with an ASCII '/' (integer 47) character,
757 and must consist of elements separated by slash characters.
762 Each element must only contain the ASCII characters
768 No element may be the empty string.
773 Multiple '/' characters cannot occur in sequence.
778 A trailing '/' character is not allowed unless the
779 path is the root path (a single '/' character).
788 <sect3 id="message-protocol-marshaling-signature">
789 <title>Valid Signatures</title>
791 An implementation must not send or accept invalid signatures.
792 Valid signatures will conform to the following rules:
796 The signature ends with a nul byte.
801 The signature is a list of single complete types.
802 Arrays must have element types, and structs must
803 have both open and close parentheses.
808 Only type codes and open and close parentheses are
809 allowed in the signature. The <literal>STRUCT</literal> type code
810 is not allowed in signatures, because parentheses
816 The maximum depth of container type nesting is 32 array type
817 codes and 32 open parentheses. This implies that the maximum
818 total depth of recursion is 64, for an "array of array of array
819 of ... struct of struct of struct of ..." where there are 32
825 The maximum length of a signature is 255.
830 Signatures must be nul-terminated.
841 <sect1 id="message-protocol">
842 <title>Message Protocol</title>
845 A <firstterm>message</firstterm> consists of a
846 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
847 think of a message as a package, the header is the address, and the body
848 contains the package contents. The message delivery system uses the header
849 information to figure out where to send the message and how to interpret
850 it; the recipient interprets the body of the message.
854 The body of the message is made up of zero or more
855 <firstterm>arguments</firstterm>, which are typed values, such as an
856 integer or a byte array.
860 Both header and body use the D-Bus <link linkend="type-system">type
861 system</link> and format for serializing data.
864 <sect2 id="message-protocol-messages">
865 <title>Message Format</title>
868 A message consists of a header and a body. The header is a block of
869 values with a fixed signature and meaning. The body is a separate block
870 of values, with a signature specified in the header.
874 The length of the header must be a multiple of 8, allowing the body to
875 begin on an 8-byte boundary when storing the entire message in a single
876 buffer. If the header does not naturally end on an 8-byte boundary
877 up to 7 bytes of nul-initialized alignment padding must be added.
881 The message body need not end on an 8-byte boundary.
885 The maximum length of a message, including header, header alignment padding,
886 and body is 2 to the 27th power or 134217728. Implementations must not
887 send or accept messages exceeding this size.
891 The signature of the header is:
895 Written out more readably, this is:
897 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
902 These values have the following meanings:
908 <entry>Description</entry>
913 <entry>1st <literal>BYTE</literal></entry>
914 <entry>Endianness flag; ASCII 'l' for little-endian
915 or ASCII 'B' for big-endian. Both header and body are
916 in this endianness.</entry>
919 <entry>2nd <literal>BYTE</literal></entry>
920 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
921 Currently-defined types are described below.
925 <entry>3rd <literal>BYTE</literal></entry>
926 <entry>Bitwise OR of flags. Unknown flags
927 must be ignored. Currently-defined flags are described below.
931 <entry>4th <literal>BYTE</literal></entry>
932 <entry>Major protocol version of the sending application. If
933 the major protocol version of the receiving application does not
934 match, the applications will not be able to communicate and the
935 D-Bus connection must be disconnected. The major protocol
936 version for this version of the specification is 1.
940 <entry>1st <literal>UINT32</literal></entry>
941 <entry>Length in bytes of the message body, starting
942 from the end of the header. The header ends after
943 its alignment padding to an 8-boundary.
947 <entry>2nd <literal>UINT32</literal></entry>
948 <entry>The serial of this message, used as a cookie
949 by the sender to identify the reply corresponding
950 to this request. This must not be zero.
954 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
955 <entry>An array of zero or more <firstterm>header
956 fields</firstterm> where the byte is the field code, and the
957 variant is the field value. The message type determines
958 which fields are required.
966 <firstterm>Message types</firstterm> that can appear in the second byte
972 <entry>Conventional name</entry>
973 <entry>Decimal value</entry>
974 <entry>Description</entry>
979 <entry><literal>INVALID</literal></entry>
981 <entry>This is an invalid type.</entry>
984 <entry><literal>METHOD_CALL</literal></entry>
986 <entry>Method call.</entry>
989 <entry><literal>METHOD_RETURN</literal></entry>
991 <entry>Method reply with returned data.</entry>
994 <entry><literal>ERROR</literal></entry>
996 <entry>Error reply. If the first argument exists and is a
997 string, it is an error message.</entry>
1000 <entry><literal>SIGNAL</literal></entry>
1002 <entry>Signal emission.</entry>
1009 Flags that can appear in the third byte of the header:
1014 <entry>Conventional name</entry>
1015 <entry>Hex value</entry>
1016 <entry>Description</entry>
1021 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1023 <entry>This message does not expect method return replies or
1024 error replies; the reply can be omitted as an
1025 optimization. However, it is compliant with this specification
1026 to return the reply despite this flag and the only harm
1027 from doing so is extra network traffic.
1031 <entry><literal>NO_AUTO_START</literal></entry>
1033 <entry>The bus must not launch an owner
1034 for the destination name in response to this message.
1042 <sect3 id="message-protocol-header-fields">
1043 <title>Header Fields</title>
1046 The array at the end of the header contains <firstterm>header
1047 fields</firstterm>, where each field is a 1-byte field code followed
1048 by a field value. A header must contain the required header fields for
1049 its message type, and zero or more of any optional header
1050 fields. Future versions of this protocol specification may add new
1051 fields. Implementations must ignore fields they do not
1052 understand. Implementations must not invent their own header fields;
1053 only changes to this specification may introduce new header fields.
1057 Again, if an implementation sees a header field code that it does not
1058 expect, it must ignore that field, as it will be part of a new
1059 (but compatible) version of this specification. This also applies
1060 to known header fields appearing in unexpected messages, for
1061 example: if a signal has a reply serial it must be ignored
1062 even though it has no meaning as of this version of the spec.
1066 However, implementations must not send or accept known header fields
1067 with the wrong type stored in the field value. So for example a
1068 message with an <literal>INTERFACE</literal> field of type
1069 <literal>UINT32</literal> would be considered corrupt.
1073 Here are the currently-defined header fields:
1078 <entry>Conventional Name</entry>
1079 <entry>Decimal Code</entry>
1081 <entry>Required In</entry>
1082 <entry>Description</entry>
1087 <entry><literal>INVALID</literal></entry>
1090 <entry>not allowed</entry>
1091 <entry>Not a valid field name (error if it appears in a message)</entry>
1094 <entry><literal>PATH</literal></entry>
1096 <entry><literal>OBJECT_PATH</literal></entry>
1097 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1098 <entry>The object to send a call to,
1099 or the object a signal is emitted from.
1101 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1102 implementations should not send messages with this path,
1103 and the reference implementation of the bus daemon will
1104 disconnect any application that attempts to do so.
1108 <entry><literal>INTERFACE</literal></entry>
1110 <entry><literal>STRING</literal></entry>
1111 <entry><literal>SIGNAL</literal></entry>
1113 The interface to invoke a method call on, or
1114 that a signal is emitted from. Optional for
1115 method calls, required for signals.
1116 The special interface
1117 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1118 implementations should not send messages with this
1119 interface, and the reference implementation of the bus
1120 daemon will disconnect any application that attempts to
1125 <entry><literal>MEMBER</literal></entry>
1127 <entry><literal>STRING</literal></entry>
1128 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1129 <entry>The member, either the method name or signal name.</entry>
1132 <entry><literal>ERROR_NAME</literal></entry>
1134 <entry><literal>STRING</literal></entry>
1135 <entry><literal>ERROR</literal></entry>
1136 <entry>The name of the error that occurred, for errors</entry>
1139 <entry><literal>REPLY_SERIAL</literal></entry>
1141 <entry><literal>UINT32</literal></entry>
1142 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1143 <entry>The serial number of the message this message is a reply
1144 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1147 <entry><literal>DESTINATION</literal></entry>
1149 <entry><literal>STRING</literal></entry>
1150 <entry>optional</entry>
1151 <entry>The name of the connection this message is intended for.
1152 Only used in combination with the message bus, see
1153 <xref linkend="message-bus"/>.</entry>
1156 <entry><literal>SENDER</literal></entry>
1158 <entry><literal>STRING</literal></entry>
1159 <entry>optional</entry>
1160 <entry>Unique name of the sending connection.
1161 The message bus fills in this field so it is reliable; the field is
1162 only meaningful in combination with the message bus.</entry>
1165 <entry><literal>SIGNATURE</literal></entry>
1167 <entry><literal>SIGNATURE</literal></entry>
1168 <entry>optional</entry>
1169 <entry>The signature of the message body.
1170 If omitted, it is assumed to be the
1171 empty signature "" (i.e. the body must be 0-length).</entry>
1174 <entry><literal>UNIX_FDS</literal></entry>
1176 <entry><literal>UINT32</literal></entry>
1177 <entry>optional</entry>
1178 <entry>The number of Unix file descriptors that
1179 accompany the message. If omitted, it is assumed
1180 that no Unix file descriptors accompany the
1181 message. The actual file descriptors need to be
1182 transferred via platform specific mechanism
1183 out-of-band. They must be sent at the same time as
1184 part of the message itself. They may not be sent
1185 before the first byte of the message itself is
1186 transferred or after the last byte of the message
1196 <sect2 id="message-protocol-names">
1197 <title>Valid Names</title>
1199 The various names in D-Bus messages have some restrictions.
1202 There is a <firstterm>maximum name length</firstterm>
1203 of 255 which applies to bus names, interfaces, and members.
1205 <sect3 id="message-protocol-names-interface">
1206 <title>Interface names</title>
1208 Interfaces have names with type <literal>STRING</literal>, meaning that
1209 they must be valid UTF-8. However, there are also some
1210 additional restrictions that apply to interface names
1213 <listitem><para>Interface names are composed of 1 or more elements separated by
1214 a period ('.') character. All elements must contain at least
1218 <listitem><para>Each element must only contain the ASCII characters
1219 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1223 <listitem><para>Interface names must contain at least one '.' (period)
1224 character (and thus at least two elements).
1227 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1228 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1232 <sect3 id="message-protocol-names-bus">
1233 <title>Bus names</title>
1235 Connections have one or more bus names associated with them.
1236 A connection has exactly one bus name that is a unique connection
1237 name. The unique connection name remains with the connection for
1238 its entire lifetime.
1239 A bus name is of type <literal>STRING</literal>,
1240 meaning that it must be valid UTF-8. However, there are also
1241 some additional restrictions that apply to bus names
1244 <listitem><para>Bus names that start with a colon (':')
1245 character are unique connection names.
1248 <listitem><para>Bus names are composed of 1 or more elements separated by
1249 a period ('.') character. All elements must contain at least
1253 <listitem><para>Each element must only contain the ASCII characters
1254 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1255 connection name may begin with a digit, elements in
1256 other bus names must not begin with a digit.
1260 <listitem><para>Bus names must contain at least one '.' (period)
1261 character (and thus at least two elements).
1264 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1265 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1269 Note that the hyphen ('-') character is allowed in bus names but
1270 not in interface names.
1273 <sect3 id="message-protocol-names-member">
1274 <title>Member names</title>
1276 Member (i.e. method or signal) names:
1278 <listitem><para>Must only contain the ASCII characters
1279 "[A-Z][a-z][0-9]_" and may not begin with a
1280 digit.</para></listitem>
1281 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1282 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1283 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1287 <sect3 id="message-protocol-names-error">
1288 <title>Error names</title>
1290 Error names have the same restrictions as interface names.
1295 <sect2 id="message-protocol-types">
1296 <title>Message Types</title>
1298 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1299 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1300 This section describes these conventions.
1302 <sect3 id="message-protocol-types-method">
1303 <title>Method Calls</title>
1305 Some messages invoke an operation on a remote object. These are
1306 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1307 messages map naturally to methods on objects in a typical program.
1310 A method call message is required to have a <literal>MEMBER</literal> header field
1311 indicating the name of the method. Optionally, the message has an
1312 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1313 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1314 a method with the same name, it is undefined which of the two methods
1315 will be invoked. Implementations may also choose to return an error in
1316 this ambiguous case. However, if a method name is unique
1317 implementations must not require an interface field.
1320 Method call messages also include a <literal>PATH</literal> field
1321 indicating the object to invoke the method on. If the call is passing
1322 through a message bus, the message will also have a
1323 <literal>DESTINATION</literal> field giving the name of the connection
1324 to receive the message.
1327 When an application handles a method call message, it is required to
1328 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1329 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1330 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1333 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1334 are the return value(s) or "out parameters" of the method call.
1335 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1336 and the call fails; no return value will be provided. It makes
1337 no sense to send multiple replies to the same method call.
1340 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1341 reply is required, so the caller will know the method
1342 was successfully processed.
1345 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1349 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1350 then as an optimization the application receiving the method
1351 call may choose to omit the reply message (regardless of
1352 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1353 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1354 flag and reply anyway.
1357 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1358 destination name does not exist then a program to own the destination
1359 name will be started before the message is delivered. The message
1360 will be held until the new program is successfully started or has
1361 failed to start; in case of failure, an error will be returned. This
1362 flag is only relevant in the context of a message bus, it is ignored
1363 during one-to-one communication with no intermediate bus.
1365 <sect4 id="message-protocol-types-method-apis">
1366 <title>Mapping method calls to native APIs</title>
1368 APIs for D-Bus may map method calls to a method call in a specific
1369 programming language, such as C++, or may map a method call written
1370 in an IDL to a D-Bus message.
1373 In APIs of this nature, arguments to a method are often termed "in"
1374 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1375 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1376 "inout" arguments, which are both sent and received, i.e. the caller
1377 passes in a value which is modified. Mapped to D-Bus, an "inout"
1378 argument is equivalent to an "in" argument, followed by an "out"
1379 argument. You can't pass things "by reference" over the wire, so
1380 "inout" is purely an illusion of the in-process API.
1383 Given a method with zero or one return values, followed by zero or more
1384 arguments, where each argument may be "in", "out", or "inout", the
1385 caller constructs a message by appending each "in" or "inout" argument,
1386 in order. "out" arguments are not represented in the caller's message.
1389 The recipient constructs a reply by appending first the return value
1390 if any, then each "out" or "inout" argument, in order.
1391 "in" arguments are not represented in the reply message.
1394 Error replies are normally mapped to exceptions in languages that have
1398 In converting from native APIs to D-Bus, it is perhaps nice to
1399 map D-Bus naming conventions ("FooBar") to native conventions
1400 such as "fooBar" or "foo_bar" automatically. This is OK
1401 as long as you can say that the native API is one that
1402 was specifically written for D-Bus. It makes the most sense
1403 when writing object implementations that will be exported
1404 over the bus. Object proxies used to invoke remote D-Bus
1405 objects probably need the ability to call any D-Bus method,
1406 and thus a magic name mapping like this could be a problem.
1409 This specification doesn't require anything of native API bindings;
1410 the preceding is only a suggested convention for consistency
1416 <sect3 id="message-protocol-types-signal">
1417 <title>Signal Emission</title>
1419 Unlike method calls, signal emissions have no replies.
1420 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1421 It must have three header fields: <literal>PATH</literal> giving the object
1422 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1423 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1424 for signals, though it is optional for method calls.
1428 <sect3 id="message-protocol-types-errors">
1429 <title>Errors</title>
1431 Messages of type <literal>ERROR</literal> are most commonly replies
1432 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1433 to any kind of message. The message bus for example
1434 will return an <literal>ERROR</literal> in reply to a signal emission if
1435 the bus does not have enough memory to send the signal.
1438 An <literal>ERROR</literal> may have any arguments, but if the first
1439 argument is a <literal>STRING</literal>, it must be an error message.
1440 The error message may be logged or shown to the user
1445 <sect3 id="message-protocol-types-notation">
1446 <title>Notation in this document</title>
1448 This document uses a simple pseudo-IDL to describe particular method
1449 calls and signals. Here is an example of a method call:
1451 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1452 out UINT32 resultcode)
1454 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1455 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1456 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1457 characters so it's known that the last part of the name in
1458 the "IDL" is the member name.
1461 In C++ that might end up looking like this:
1463 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1464 unsigned int flags);
1466 or equally valid, the return value could be done as an argument:
1468 void org::freedesktop::DBus::StartServiceByName (const char *name,
1470 unsigned int *resultcode);
1472 It's really up to the API designer how they want to make
1473 this look. You could design an API where the namespace wasn't used
1474 in C++, using STL or Qt, using varargs, or whatever you wanted.
1477 Signals are written as follows:
1479 org.freedesktop.DBus.NameLost (STRING name)
1481 Signals don't specify "in" vs. "out" because only
1482 a single direction is possible.
1485 It isn't especially encouraged to use this lame pseudo-IDL in actual
1486 API implementations; you might use the native notation for the
1487 language you're using, or you might use COM or CORBA IDL, for example.
1492 <sect2 id="message-protocol-handling-invalid">
1493 <title>Invalid Protocol and Spec Extensions</title>
1496 For security reasons, the D-Bus protocol should be strictly parsed and
1497 validated, with the exception of defined extension points. Any invalid
1498 protocol or spec violations should result in immediately dropping the
1499 connection without notice to the other end. Exceptions should be
1500 carefully considered, e.g. an exception may be warranted for a
1501 well-understood idiosyncrasy of a widely-deployed implementation. In
1502 cases where the other end of a connection is 100% trusted and known to
1503 be friendly, skipping validation for performance reasons could also make
1504 sense in certain cases.
1508 Generally speaking violations of the "must" requirements in this spec
1509 should be considered possible attempts to exploit security, and violations
1510 of the "should" suggestions should be considered legitimate (though perhaps
1511 they should generate an error in some cases).
1515 The following extension points are built in to D-Bus on purpose and must
1516 not be treated as invalid protocol. The extension points are intended
1517 for use by future versions of this spec, they are not intended for third
1518 parties. At the moment, the only way a third party could extend D-Bus
1519 without breaking interoperability would be to introduce a way to negotiate new
1520 feature support as part of the auth protocol, using EXTENSION_-prefixed
1521 commands. There is not yet a standard way to negotiate features.
1525 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1526 commands result in an ERROR rather than a disconnect. This enables
1527 future extensions to the protocol. Commands starting with EXTENSION_ are
1528 reserved for third parties.
1533 The authentication protocol supports pluggable auth mechanisms.
1538 The address format (see <xref linkend="addresses"/>) supports new
1544 Messages with an unknown type (something other than
1545 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1546 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1547 Unknown-type messages must still be well-formed in the same way
1548 as the known messages, however. They still have the normal
1554 Header fields with an unknown or unexpected field code must be ignored,
1555 though again they must still be well-formed.
1560 New standard interfaces (with new methods and signals) can of course be added.
1570 <sect1 id="auth-protocol">
1571 <title>Authentication Protocol</title>
1573 Before the flow of messages begins, two applications must
1574 authenticate. A simple plain-text protocol is used for
1575 authentication; this protocol is a SASL profile, and maps fairly
1576 directly from the SASL specification. The message encoding is
1577 NOT used here, only plain text messages.
1580 In examples, "C:" and "S:" indicate lines sent by the client and
1581 server respectively.
1583 <sect2 id="auth-protocol-overview">
1584 <title>Protocol Overview</title>
1586 The protocol is a line-based protocol, where each line ends with
1587 \r\n. Each line begins with an all-caps ASCII command name containing
1588 only the character range [A-Z_], a space, then any arguments for the
1589 command, then the \r\n ending the line. The protocol is
1590 case-sensitive. All bytes must be in the ASCII character set.
1592 Commands from the client to the server are as follows:
1595 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1596 <listitem><para>CANCEL</para></listitem>
1597 <listitem><para>BEGIN</para></listitem>
1598 <listitem><para>DATA <data in hex encoding></para></listitem>
1599 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1600 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1603 From server to client are as follows:
1606 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1607 <listitem><para>OK <GUID in hex></para></listitem>
1608 <listitem><para>DATA <data in hex encoding></para></listitem>
1609 <listitem><para>ERROR</para></listitem>
1610 <listitem><para>AGREE_UNIX_FD</para></listitem>
1614 Unofficial extensions to the command set must begin with the letters
1615 "EXTENSION_", to avoid conflicts with future official commands.
1616 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1619 <sect2 id="auth-nul-byte">
1620 <title>Special credentials-passing nul byte</title>
1622 Immediately after connecting to the server, the client must send a
1623 single nul byte. This byte may be accompanied by credentials
1624 information on some operating systems that use sendmsg() with
1625 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1626 sockets. However, the nul byte must be sent even on other kinds of
1627 socket, and even on operating systems that do not require a byte to be
1628 sent in order to transmit credentials. The text protocol described in
1629 this document begins after the single nul byte. If the first byte
1630 received from the client is not a nul byte, the server may disconnect
1634 A nul byte in any context other than the initial byte is an error;
1635 the protocol is ASCII-only.
1638 The credentials sent along with the nul byte may be used with the
1639 SASL mechanism EXTERNAL.
1642 <sect2 id="auth-command-auth">
1643 <title>AUTH command</title>
1645 If an AUTH command has no arguments, it is a request to list
1646 available mechanisms. The server must respond with a REJECTED
1647 command listing the mechanisms it understands, or with an error.
1650 If an AUTH command specifies a mechanism, and the server supports
1651 said mechanism, the server should begin exchanging SASL
1652 challenge-response data with the client using DATA commands.
1655 If the server does not support the mechanism given in the AUTH
1656 command, it must send either a REJECTED command listing the mechanisms
1657 it does support, or an error.
1660 If the [initial-response] argument is provided, it is intended for use
1661 with mechanisms that have no initial challenge (or an empty initial
1662 challenge), as if it were the argument to an initial DATA command. If
1663 the selected mechanism has an initial challenge and [initial-response]
1664 was provided, the server should reject authentication by sending
1668 If authentication succeeds after exchanging DATA commands,
1669 an OK command must be sent to the client.
1672 The first octet received by the server after the \r\n of the BEGIN
1673 command from the client must be the first octet of the
1674 authenticated/encrypted stream of D-Bus messages.
1677 If BEGIN is received by the server, the first octet received
1678 by the client after the \r\n of the OK command must be the
1679 first octet of the authenticated/encrypted stream of D-Bus
1683 <sect2 id="auth-command-cancel">
1684 <title>CANCEL Command</title>
1686 At any time up to sending the BEGIN command, the client may send a
1687 CANCEL command. On receiving the CANCEL command, the server must
1688 send a REJECTED command and abort the current authentication
1692 <sect2 id="auth-command-data">
1693 <title>DATA Command</title>
1695 The DATA command may come from either client or server, and simply
1696 contains a hex-encoded block of data to be interpreted
1697 according to the SASL mechanism in use.
1700 Some SASL mechanisms support sending an "empty string";
1701 FIXME we need some way to do this.
1704 <sect2 id="auth-command-begin">
1705 <title>BEGIN Command</title>
1707 The BEGIN command acknowledges that the client has received an
1708 OK command from the server, and that the stream of messages
1712 The first octet received by the server after the \r\n of the BEGIN
1713 command from the client must be the first octet of the
1714 authenticated/encrypted stream of D-Bus messages.
1717 <sect2 id="auth-command-rejected">
1718 <title>REJECTED Command</title>
1720 The REJECTED command indicates that the current authentication
1721 exchange has failed, and further exchange of DATA is inappropriate.
1722 The client would normally try another mechanism, or try providing
1723 different responses to challenges.
1725 Optionally, the REJECTED command has a space-separated list of
1726 available auth mechanisms as arguments. If a server ever provides
1727 a list of supported mechanisms, it must provide the same list
1728 each time it sends a REJECTED message. Clients are free to
1729 ignore all lists received after the first.
1732 <sect2 id="auth-command-ok">
1733 <title>OK Command</title>
1735 The OK command indicates that the client has been
1736 authenticated. The client may now proceed with negotiating
1737 Unix file descriptor passing. To do that it shall send
1738 NEGOTIATE_UNIX_FD to the server.
1741 Otherwise, the client must respond to the OK command by
1742 sending a BEGIN command, followed by its stream of messages,
1743 or by disconnecting. The server must not accept additional
1744 commands using this protocol after the BEGIN command has been
1745 received. Further communication will be a stream of D-Bus
1746 messages (optionally encrypted, as negotiated) rather than
1750 If a client sends BEGIN the first octet received by the client
1751 after the \r\n of the OK command must be the first octet of
1752 the authenticated/encrypted stream of D-Bus messages.
1755 The OK command has one argument, which is the GUID of the server.
1756 See <xref linkend="addresses"/> for more on server GUIDs.
1759 <sect2 id="auth-command-error">
1760 <title>ERROR Command</title>
1762 The ERROR command indicates that either server or client did not
1763 know a command, does not accept the given command in the current
1764 context, or did not understand the arguments to the command. This
1765 allows the protocol to be extended; a client or server can send a
1766 command present or permitted only in new protocol versions, and if
1767 an ERROR is received instead of an appropriate response, fall back
1768 to using some other technique.
1771 If an ERROR is sent, the server or client that sent the
1772 error must continue as if the command causing the ERROR had never been
1773 received. However, the the server or client receiving the error
1774 should try something other than whatever caused the error;
1775 if only canceling/rejecting the authentication.
1778 If the D-Bus protocol changes incompatibly at some future time,
1779 applications implementing the new protocol would probably be able to
1780 check for support of the new protocol by sending a new command and
1781 receiving an ERROR from applications that don't understand it. Thus the
1782 ERROR feature of the auth protocol is an escape hatch that lets us
1783 negotiate extensions or changes to the D-Bus protocol in the future.
1786 <sect2 id="auth-command-negotiate-unix-fd">
1787 <title>NEGOTIATE_UNIX_FD Command</title>
1789 The NEGOTIATE_UNIX_FD command indicates that the client
1790 supports Unix file descriptor passing. This command may only
1791 be sent after the connection is authenticated, i.e. after OK
1792 was received by the client. This command may only be sent on
1793 transports that support Unix file descriptor passing.
1796 On receiving NEGOTIATE_UNIX_FD the server must respond with
1797 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1798 the transport chosen supports Unix file descriptor passing and
1799 the server supports this feature. It shall respond the latter
1800 if the transport does not support Unix file descriptor
1801 passing, the server does not support this feature, or the
1802 server decides not to enable file descriptor passing due to
1803 security or other reasons.
1806 <sect2 id="auth-command-agree-unix-fd">
1807 <title>AGREE_UNIX_FD Command</title>
1809 The AGREE_UNIX_FD command indicates that the server supports
1810 Unix file descriptor passing. This command may only be sent
1811 after the connection is authenticated, and the client sent
1812 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1813 command may only be sent on transports that support Unix file
1817 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1818 followed by its stream of messages, or by disconnecting. The
1819 server must not accept additional commands using this protocol
1820 after the BEGIN command has been received. Further
1821 communication will be a stream of D-Bus messages (optionally
1822 encrypted, as negotiated) rather than this protocol.
1825 <sect2 id="auth-command-future">
1826 <title>Future Extensions</title>
1828 Future extensions to the authentication and negotiation
1829 protocol are possible. For that new commands may be
1830 introduced. If a client or server receives an unknown command
1831 it shall respond with ERROR and not consider this fatal. New
1832 commands may be introduced both before, and after
1833 authentication, i.e. both before and after the OK command.
1836 <sect2 id="auth-examples">
1837 <title>Authentication examples</title>
1841 <title>Example of successful magic cookie authentication</title>
1843 (MAGIC_COOKIE is a made up mechanism)
1845 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1851 <title>Example of finding out mechanisms then picking one</title>
1854 S: REJECTED KERBEROS_V4 SKEY
1855 C: AUTH SKEY 7ab83f32ee
1856 S: DATA 8799cabb2ea93e
1857 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1863 <title>Example of client sends unknown command then falls back to regular auth</title>
1867 C: AUTH MAGIC_COOKIE 3736343435313230333039
1873 <title>Example of server doesn't support initial auth mechanism</title>
1875 C: AUTH MAGIC_COOKIE 3736343435313230333039
1876 S: REJECTED KERBEROS_V4 SKEY
1877 C: AUTH SKEY 7ab83f32ee
1878 S: DATA 8799cabb2ea93e
1879 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1885 <title>Example of wrong password or the like followed by successful retry</title>
1887 C: AUTH MAGIC_COOKIE 3736343435313230333039
1888 S: REJECTED KERBEROS_V4 SKEY
1889 C: AUTH SKEY 7ab83f32ee
1890 S: DATA 8799cabb2ea93e
1891 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1893 C: AUTH SKEY 7ab83f32ee
1894 S: DATA 8799cabb2ea93e
1895 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1901 <title>Example of skey cancelled and restarted</title>
1903 C: AUTH MAGIC_COOKIE 3736343435313230333039
1904 S: REJECTED KERBEROS_V4 SKEY
1905 C: AUTH SKEY 7ab83f32ee
1906 S: DATA 8799cabb2ea93e
1909 C: AUTH SKEY 7ab83f32ee
1910 S: DATA 8799cabb2ea93e
1911 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1917 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1919 (MAGIC_COOKIE is a made up mechanism)
1921 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1923 C: NEGOTIATE_UNIX_FD
1929 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1931 (MAGIC_COOKIE is a made up mechanism)
1933 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1935 C: NEGOTIATE_UNIX_FD
1942 <sect2 id="auth-states">
1943 <title>Authentication state diagrams</title>
1946 This section documents the auth protocol in terms of
1947 a state machine for the client and the server. This is
1948 probably the most robust way to implement the protocol.
1951 <sect3 id="auth-states-client">
1952 <title>Client states</title>
1955 To more precisely describe the interaction between the
1956 protocol state machine and the authentication mechanisms the
1957 following notation is used: MECH(CHALL) means that the
1958 server challenge CHALL was fed to the mechanism MECH, which
1964 CONTINUE(RESP) means continue the auth conversation
1965 and send RESP as the response to the server;
1971 OK(RESP) means that after sending RESP to the server
1972 the client side of the auth conversation is finished
1973 and the server should return "OK";
1979 ERROR means that CHALL was invalid and could not be
1985 Both RESP and CHALL may be empty.
1989 The Client starts by getting an initial response from the
1990 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1991 the mechanism did not provide an initial response. If the
1992 mechanism returns CONTINUE, the client starts in state
1993 <emphasis>WaitingForData</emphasis>, if the mechanism
1994 returns OK the client starts in state
1995 <emphasis>WaitingForOK</emphasis>.
1999 The client should keep track of available mechanisms and
2000 which it mechanisms it has already attempted. This list is
2001 used to decide which AUTH command to send. When the list is
2002 exhausted, the client should give up and close the
2007 <title><emphasis>WaitingForData</emphasis></title>
2015 MECH(CHALL) returns CONTINUE(RESP) → send
2017 <emphasis>WaitingForData</emphasis>
2021 MECH(CHALL) returns OK(RESP) → send DATA
2022 RESP, goto <emphasis>WaitingForOK</emphasis>
2026 MECH(CHALL) returns ERROR → send ERROR
2027 [msg], goto <emphasis>WaitingForData</emphasis>
2035 Receive REJECTED [mechs] →
2036 send AUTH [next mech], goto
2037 WaitingForData or <emphasis>WaitingForOK</emphasis>
2042 Receive ERROR → send
2044 <emphasis>WaitingForReject</emphasis>
2049 Receive OK → send
2050 BEGIN, terminate auth
2051 conversation, authenticated
2056 Receive anything else → send
2058 <emphasis>WaitingForData</emphasis>
2066 <title><emphasis>WaitingForOK</emphasis></title>
2071 Receive OK → send BEGIN, terminate auth
2072 conversation, <emphasis>authenticated</emphasis>
2077 Receive REJECT [mechs] → send AUTH [next mech],
2078 goto <emphasis>WaitingForData</emphasis> or
2079 <emphasis>WaitingForOK</emphasis>
2085 Receive DATA → send CANCEL, goto
2086 <emphasis>WaitingForReject</emphasis>
2092 Receive ERROR → send CANCEL, goto
2093 <emphasis>WaitingForReject</emphasis>
2099 Receive anything else → send ERROR, goto
2100 <emphasis>WaitingForOK</emphasis>
2108 <title><emphasis>WaitingForReject</emphasis></title>
2113 Receive REJECT [mechs] → send AUTH [next mech],
2114 goto <emphasis>WaitingForData</emphasis> or
2115 <emphasis>WaitingForOK</emphasis>
2121 Receive anything else → terminate auth
2122 conversation, disconnect
2131 <sect3 id="auth-states-server">
2132 <title>Server states</title>
2135 For the server MECH(RESP) means that the client response
2136 RESP was fed to the the mechanism MECH, which returns one of
2141 CONTINUE(CHALL) means continue the auth conversation and
2142 send CHALL as the challenge to the client;
2148 OK means that the client has been successfully
2155 REJECT means that the client failed to authenticate or
2156 there was an error in RESP.
2161 The server starts out in state
2162 <emphasis>WaitingForAuth</emphasis>. If the client is
2163 rejected too many times the server must disconnect the
2168 <title><emphasis>WaitingForAuth</emphasis></title>
2174 Receive AUTH → send REJECTED [mechs], goto
2175 <emphasis>WaitingForAuth</emphasis>
2181 Receive AUTH MECH RESP
2185 MECH not valid mechanism → send REJECTED
2187 <emphasis>WaitingForAuth</emphasis>
2191 MECH(RESP) returns CONTINUE(CHALL) → send
2193 <emphasis>WaitingForData</emphasis>
2197 MECH(RESP) returns OK → send OK, goto
2198 <emphasis>WaitingForBegin</emphasis>
2202 MECH(RESP) returns REJECT → send REJECTED
2204 <emphasis>WaitingForAuth</emphasis>
2212 Receive BEGIN → terminate
2213 auth conversation, disconnect
2219 Receive ERROR → send REJECTED [mechs], goto
2220 <emphasis>WaitingForAuth</emphasis>
2226 Receive anything else → send
2228 <emphasis>WaitingForAuth</emphasis>
2237 <title><emphasis>WaitingForData</emphasis></title>
2245 MECH(RESP) returns CONTINUE(CHALL) → send
2247 <emphasis>WaitingForData</emphasis>
2251 MECH(RESP) returns OK → send OK, goto
2252 <emphasis>WaitingForBegin</emphasis>
2256 MECH(RESP) returns REJECT → send REJECTED
2258 <emphasis>WaitingForAuth</emphasis>
2266 Receive BEGIN → terminate auth conversation,
2273 Receive CANCEL → send REJECTED [mechs], goto
2274 <emphasis>WaitingForAuth</emphasis>
2280 Receive ERROR → send REJECTED [mechs], goto
2281 <emphasis>WaitingForAuth</emphasis>
2287 Receive anything else → send ERROR, goto
2288 <emphasis>WaitingForData</emphasis>
2296 <title><emphasis>WaitingForBegin</emphasis></title>
2301 Receive BEGIN → terminate auth conversation,
2302 client authenticated
2308 Receive CANCEL → send REJECTED [mechs], goto
2309 <emphasis>WaitingForAuth</emphasis>
2315 Receive ERROR → send REJECTED [mechs], goto
2316 <emphasis>WaitingForAuth</emphasis>
2322 Receive anything else → send ERROR, goto
2323 <emphasis>WaitingForBegin</emphasis>
2333 <sect2 id="auth-mechanisms">
2334 <title>Authentication mechanisms</title>
2336 This section describes some new authentication mechanisms.
2337 D-Bus also allows any standard SASL mechanism of course.
2339 <sect3 id="auth-mechanisms-sha">
2340 <title>DBUS_COOKIE_SHA1</title>
2342 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2343 has the ability to read a private file owned by the user being
2344 authenticated. If the client can prove that it has access to a secret
2345 cookie stored in this file, then the client is authenticated.
2346 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2350 Throughout this description, "hex encoding" must output the digits
2351 from a to f in lower-case; the digits A to F must not be used
2352 in the DBUS_COOKIE_SHA1 mechanism.
2355 Authentication proceeds as follows:
2359 The client sends the username it would like to authenticate
2365 The server sends the name of its "cookie context" (see below); a
2366 space character; the integer ID of the secret cookie the client
2367 must demonstrate knowledge of; a space character; then a
2368 randomly-generated challenge string, all of this hex-encoded into
2374 The client locates the cookie and generates its own
2375 randomly-generated challenge string. The client then concatenates
2376 the server's decoded challenge, a ":" character, its own challenge,
2377 another ":" character, and the cookie. It computes the SHA-1 hash
2378 of this composite string as a hex digest. It concatenates the
2379 client's challenge string, a space character, and the SHA-1 hex
2380 digest, hex-encodes the result and sends it back to the server.
2385 The server generates the same concatenated string used by the
2386 client and computes its SHA-1 hash. It compares the hash with
2387 the hash received from the client; if the two hashes match, the
2388 client is authenticated.
2394 Each server has a "cookie context," which is a name that identifies a
2395 set of cookies that apply to that server. A sample context might be
2396 "org_freedesktop_session_bus". Context names must be valid ASCII,
2397 nonzero length, and may not contain the characters slash ("/"),
2398 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2399 tab ("\t"), or period ("."). There is a default context,
2400 "org_freedesktop_general" that's used by servers that do not specify
2404 Cookies are stored in a user's home directory, in the directory
2405 <filename>~/.dbus-keyrings/</filename>. This directory must
2406 not be readable or writable by other users. If it is,
2407 clients and servers must ignore it. The directory
2408 contains cookie files named after the cookie context.
2411 A cookie file contains one cookie per line. Each line
2412 has three space-separated fields:
2416 The cookie ID number, which must be a non-negative integer and
2417 may not be used twice in the same file.
2422 The cookie's creation time, in UNIX seconds-since-the-epoch
2428 The cookie itself, a hex-encoded random block of bytes. The cookie
2429 may be of any length, though obviously security increases
2430 as the length increases.
2436 Only server processes modify the cookie file.
2437 They must do so with this procedure:
2441 Create a lockfile name by appending ".lock" to the name of the
2442 cookie file. The server should attempt to create this file
2443 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2444 fails, the lock fails. Servers should retry for a reasonable
2445 period of time, then they may choose to delete an existing lock
2446 to keep users from having to manually delete a stale
2447 lock. <footnote><para>Lockfiles are used instead of real file
2448 locking <literal>fcntl()</literal> because real locking
2449 implementations are still flaky on network
2450 filesystems.</para></footnote>
2455 Once the lockfile has been created, the server loads the cookie
2456 file. It should then delete any cookies that are old (the
2457 timeout can be fairly short), or more than a reasonable
2458 time in the future (so that cookies never accidentally
2459 become permanent, if the clock was set far into the future
2460 at some point). If no recent keys remain, the
2461 server may generate a new key.
2466 The pruned and possibly added-to cookie file
2467 must be resaved atomically (using a temporary
2468 file which is rename()'d).
2473 The lock must be dropped by deleting the lockfile.
2479 Clients need not lock the file in order to load it,
2480 because servers are required to save the file atomically.
2485 <sect1 id="addresses">
2486 <title>Server Addresses</title>
2488 Server addresses consist of a transport name followed by a colon, and
2489 then an optional, comma-separated list of keys and values in the form key=value.
2490 Each value is escaped.
2494 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2495 Which is the address to a unix socket with the path /tmp/dbus-test.
2498 Value escaping is similar to URI escaping but simpler.
2502 The set of optionally-escaped bytes is:
2503 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2504 <emphasis>byte</emphasis> (note, not character) which is not in the
2505 set of optionally-escaped bytes must be replaced with an ASCII
2506 percent (<literal>%</literal>) and the value of the byte in hex.
2507 The hex value must always be two digits, even if the first digit is
2508 zero. The optionally-escaped bytes may be escaped if desired.
2513 To unescape, append each byte in the value; if a byte is an ASCII
2514 percent (<literal>%</literal>) character then append the following
2515 hex value instead. It is an error if a <literal>%</literal> byte
2516 does not have two hex digits following. It is an error if a
2517 non-optionally-escaped byte is seen unescaped.
2521 The set of optionally-escaped bytes is intended to preserve address
2522 readability and convenience.
2526 A server may specify a key-value pair with the key <literal>guid</literal>
2527 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2528 describes the format of the <literal>guid</literal> field. If present,
2529 this UUID may be used to distinguish one server address from another. A
2530 server should use a different UUID for each address it listens on. For
2531 example, if a message bus daemon offers both UNIX domain socket and TCP
2532 connections, but treats clients the same regardless of how they connect,
2533 those two connections are equivalent post-connection but should have
2534 distinct UUIDs to distinguish the kinds of connection.
2538 The intent of the address UUID feature is to allow a client to avoid
2539 opening multiple identical connections to the same server, by allowing the
2540 client to check whether an address corresponds to an already-existing
2541 connection. Comparing two addresses is insufficient, because addresses
2542 can be recycled by distinct servers, and equivalent addresses may look
2543 different if simply compared as strings (for example, the host in a TCP
2544 address can be given as an IP address or as a hostname).
2548 Note that the address key is <literal>guid</literal> even though the
2549 rest of the API and documentation says "UUID," for historical reasons.
2553 [FIXME clarify if attempting to connect to each is a requirement
2554 or just a suggestion]
2555 When connecting to a server, multiple server addresses can be
2556 separated by a semi-colon. The library will then try to connect
2557 to the first address and if that fails, it'll try to connect to
2558 the next one specified, and so forth. For example
2559 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2564 <sect1 id="transports">
2565 <title>Transports</title>
2567 [FIXME we need to specify in detail each transport and its possible arguments]
2569 Current transports include: unix domain sockets (including
2570 abstract namespace on linux), launchd, TCP/IP, and a debug/testing transport
2571 using in-process pipes. Future possible transports include one that
2572 tunnels over X11 protocol.
2575 <sect2 id="transports-unix-domain-sockets">
2576 <title>Unix Domain Sockets</title>
2578 Unix domain sockets can be either paths in the file system or on Linux
2579 kernels, they can be abstract which are similar to paths but
2580 do not show up in the file system.
2584 When a socket is opened by the D-Bus library it truncates the path
2585 name right before the first trailing Nul byte. This is true for both
2586 normal paths and abstract paths. Note that this is a departure from
2587 previous versions of D-Bus that would create sockets with a fixed
2588 length path name. Names which were shorter than the fixed length
2589 would be padded by Nul bytes.
2592 Unix domain sockets are not available on windows.
2594 <sect3 id="transports-unix-domain-sockets-addresses">
2595 <title>Server Address Format</title>
2597 Unix domain socket addresses are identified by the "unix:" prefix
2598 and support the following key/value pairs:
2605 <entry>Values</entry>
2606 <entry>Description</entry>
2612 <entry>(path)</entry>
2613 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2616 <entry>tmpdir</entry>
2617 <entry>(path)</entry>
2618 <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>
2621 <entry>abstract</entry>
2622 <entry>(string)</entry>
2623 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2630 <sect2 id="transports-launchd">
2631 <title>launchd</title>
2633 launchd is a open-source server management system that replaces init, inetd
2634 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2635 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2639 launchd allocates a socket and provides it with the unix path through the
2640 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2641 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2642 it through its environment.
2643 Other processes can query for the launchd socket by executing:
2644 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2645 This is normally done by the D-Bus client library so doesn't have to be done
2649 launchd is not available on Microsoft Windows.
2651 <sect3 id="transports-launchd-addresses">
2652 <title>Server Address Format</title>
2654 launchd addresses are identified by the "launchd:" prefix
2655 and support the following key/value pairs:
2662 <entry>Values</entry>
2663 <entry>Description</entry>
2669 <entry>(environment variable)</entry>
2670 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2677 <sect2 id="transports-tcp-sockets">
2678 <title>TCP Sockets</title>
2680 The tcp transport provides TCP/IP based connections between clients
2681 located on the same or different hosts.
2684 Using tcp transport without any additional secure authentification mechanismus
2685 over a network is unsecure.
2688 Windows notes: Because of the tcp stack on windows does not provide sending
2689 credentials over a tcp connection, the EXTERNAL authentification
2690 mechanismus does not work.
2692 <sect3 id="transports-tcp-sockets-addresses">
2693 <title>Server Address Format</title>
2695 TCP/IP socket addresses are identified by the "tcp:" prefix
2696 and support the following key/value pairs:
2703 <entry>Values</entry>
2704 <entry>Description</entry>
2710 <entry>(string)</entry>
2711 <entry>dns name or ip address</entry>
2715 <entry>(number)</entry>
2716 <entry>The tcp port the server will open. A zero value let the server
2717 choose a free port provided from the underlaying operating system.
2718 libdbus is able to retrieve the real used port from the server.
2722 <entry>family</entry>
2723 <entry>(string)</entry>
2724 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2731 <sect2 id="transports-nonce-tcp-sockets">
2732 <title>Nonce-secured TCP Sockets</title>
2734 The nonce-tcp transport provides a secured TCP transport, using a
2735 simple authentication mechanism to ensure that only clients with read
2736 access to a certain location in the filesystem can connect to the server.
2737 The server writes a secret, the nonce, to a file and an incoming client
2738 connection is only accepted if the client sends the nonce right after
2739 the connect. The nonce mechanism requires no setup and is orthogonal to
2740 the higher-level authentication mechanisms described in the
2741 Authentication section.
2745 On start, the server generates a random 16 byte nonce and writes it
2746 to a file in the user's temporary directory. The nonce file location
2747 is published as part of the server's D-Bus address using the
2748 "noncefile" key-value pair.
2750 After an accept, the server reads 16 bytes from the socket. If the
2751 read bytes do not match the nonce stored in the nonce file, the
2752 server MUST immediately drop the connection.
2753 If the nonce match the received byte sequence, the client is accepted
2754 and the transport behaves like an unsecured tcp transport.
2757 After a successful connect to the server socket, the client MUST read
2758 the nonce from the file published by the server via the noncefile=
2759 key-value pair and send it over the socket. After that, the
2760 transport behaves like an unsecured tcp transport.
2762 <sect3 id="transports-nonce-tcp-sockets-addresses">
2763 <title>Server Address Format</title>
2765 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2766 and support the following key/value pairs:
2773 <entry>Values</entry>
2774 <entry>Description</entry>
2780 <entry>(string)</entry>
2781 <entry>dns name or ip address</entry>
2785 <entry>(number)</entry>
2786 <entry>The tcp port the server will open. A zero value let the server
2787 choose a free port provided from the underlaying operating system.
2788 libdbus is able to retrieve the real used port from the server.
2792 <entry>family</entry>
2793 <entry>(string)</entry>
2794 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2797 <entry>noncefile</entry>
2798 <entry>(path)</entry>
2799 <entry>file location containing the secret</entry>
2807 <sect1 id="meta-transports">
2808 <title>Meta Transports</title>
2810 Meta transports are a kind of transport with special enhancements or
2811 behavior. Currently available meta transports include: autolaunch
2814 <sect2 id="meta-transports-autolaunch">
2815 <title>Autolaunch</title>
2816 <para>The autolaunch transport provides a way for dbus clients to autodetect
2817 a running dbus session bus and to autolaunch a session bus if not present.
2819 <sect3 id="meta-transports-autolaunch-addresses">
2820 <title>Server Address Format</title>
2822 Autolaunch addresses uses the "autolaunch:" prefix and support the
2823 following key/value pairs:
2830 <entry>Values</entry>
2831 <entry>Description</entry>
2836 <entry>scope</entry>
2837 <entry>(string)</entry>
2838 <entry>scope of autolaunch (Windows only)
2842 "*install-path" - limit session bus to dbus installation path.
2843 The dbus installation path is determined from the location of
2844 the shared dbus library. If the library is located in a 'bin'
2845 subdirectory the installation root is the directory above,
2846 otherwise the directory where the library lives is taken as
2849 <install-root>/bin/[lib]dbus-1.dll
2850 <install-root>/[lib]dbus-1.dll
2856 "*user" - limit session bus to the recent user.
2861 other values - specify dedicated session bus like "release",
2873 <sect3 id="meta-transports-autolaunch-windows-implementation">
2874 <title>Windows implementation</title>
2876 On start, the server opens a platform specific transport, creates a mutex
2877 and a shared memory section containing the related session bus address.
2878 This mutex will be inspected by the dbus client library to detect a
2879 running dbus session bus. The access to the mutex and the shared memory
2880 section are protected by global locks.
2883 In the recent implementation the autolaunch transport uses a tcp transport
2884 on localhost with a port choosen from the operating system. This detail may
2885 change in the future.
2888 Disclaimer: The recent implementation is in an early state and may not
2889 work in all cirumstances and/or may have security issues. Because of this
2890 the implementation is not documentated yet.
2895 <sect1 id="naming-conventions">
2896 <title>Naming Conventions</title>
2899 D-Bus namespaces are all lowercase and correspond to reversed domain
2900 names, as with Java. e.g. "org.freedesktop"
2903 Interface, signal, method, and property names are "WindowsStyleCaps", note
2904 that the first letter is capitalized, unlike Java.
2907 Object paths are normally all lowercase with underscores used rather than
2913 <title>UUIDs</title>
2915 A working D-Bus implementation uses universally-unique IDs in two places.
2916 First, each server address has a UUID identifying the address,
2917 as described in <xref linkend="addresses"/>. Second, each operating
2918 system kernel instance running a D-Bus client or server has a UUID
2919 identifying that kernel, retrieved by invoking the method
2920 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2921 linkend="standard-interfaces-peer"/>).
2924 The term "UUID" in this document is intended literally, i.e. an
2925 identifier that is universally unique. It is not intended to refer to
2926 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2929 The UUID must contain 128 bits of data and be hex-encoded. The
2930 hex-encoded string may not contain hyphens or other non-hex-digit
2931 characters, and it must be exactly 32 characters long. To generate a
2932 UUID, the current reference implementation concatenates 96 bits of random
2933 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2937 It would also be acceptable and probably better to simply generate 128
2938 bits of random data, as long as the random number generator is of high
2939 quality. The timestamp could conceivably help if the random bits are not
2940 very random. With a quality random number generator, collisions are
2941 extremely unlikely even with only 96 bits, so it's somewhat academic.
2944 Implementations should, however, stick to random data for the first 96 bits
2949 <sect1 id="standard-interfaces">
2950 <title>Standard Interfaces</title>
2952 See <xref linkend="message-protocol-types-notation"/> for details on
2953 the notation used in this section. There are some standard interfaces
2954 that may be useful across various D-Bus applications.
2956 <sect2 id="standard-interfaces-peer">
2957 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2959 The <literal>org.freedesktop.DBus.Peer</literal> interface
2962 org.freedesktop.DBus.Peer.Ping ()
2963 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2967 On receipt of the <literal>METHOD_CALL</literal> message
2968 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2969 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2970 usual. It does not matter which object path a ping is sent to. The
2971 reference implementation handles this method automatically.
2974 On receipt of the <literal>METHOD_CALL</literal> message
2975 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2976 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2977 UUID representing the identity of the machine the process is running on.
2978 This UUID must be the same for all processes on a single system at least
2979 until that system next reboots. It should be the same across reboots
2980 if possible, but this is not always possible to implement and is not
2982 It does not matter which object path a GetMachineId is sent to. The
2983 reference implementation handles this method automatically.
2986 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2987 a virtual machine running on a hypervisor, rather than a physical machine.
2988 Basically if two processes see the same UUID, they should also see the same
2989 shared memory, UNIX domain sockets, process IDs, and other features that require
2990 a running OS kernel in common between the processes.
2993 The UUID is often used where other programs might use a hostname. Hostnames
2994 can change without rebooting, however, or just be "localhost" - so the UUID
2998 <xref linkend="uuids"/> explains the format of the UUID.
3002 <sect2 id="standard-interfaces-introspectable">
3003 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3005 This interface has one method:
3007 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3011 Objects instances may implement
3012 <literal>Introspect</literal> which returns an XML description of
3013 the object, including its interfaces (with signals and methods), objects
3014 below it in the object path tree, and its properties.
3017 <xref linkend="introspection-format"/> describes the format of this XML string.
3020 <sect2 id="standard-interfaces-properties">
3021 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3023 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3024 or <firstterm>attributes</firstterm>. These can be exposed via the
3025 <literal>org.freedesktop.DBus.Properties</literal> interface.
3029 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3030 in STRING property_name,
3032 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3033 in STRING property_name,
3035 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3036 out DICT<STRING,VARIANT> props);
3040 The available properties and whether they are writable can be determined
3041 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3042 see <xref linkend="standard-interfaces-introspectable"/>.
3045 An empty string may be provided for the interface name; in this case,
3046 if there are multiple properties on an object with the same name,
3047 the results are undefined (picking one by according to an arbitrary
3048 deterministic rule, or returning an error, are the reasonable
3052 If one or more properties change on an object, the
3053 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3054 signal may be emitted (this signal was added in 0.14):
3058 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3059 DICT<STRING,VARIANT> changed_properties,
3060 ARRAY<STRING> invalidated_properties);
3064 where <literal>changed_properties</literal> is a dictionary
3065 containing the changed properties with the new values and
3066 <literal>invalidated_properties</literal> is an array of
3067 properties that changed but the value is not conveyed.
3070 Whether the <literal>PropertiesChanged</literal> signal is
3071 supported can be determined by calling
3072 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3073 that the signal may be supported for an object but it may
3074 differ how whether and how it is used on a per-property basis
3075 (for e.g. performance or security reasons). Each property (or
3076 the parent interface) must be annotated with the
3077 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3078 annotation to convey this (usually the default value
3079 <literal>true</literal> is sufficient meaning that the
3080 annotation does not need to be used). See <xref
3081 linkend="introspection-format"/> for details on this
3086 <sect2 id="standard-interfaces-objectmanager">
3087 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3089 An API can optionally make use of this interface for one or
3090 more sub-trees of objects. The root of each sub-tree implements
3091 this interface so other applications can get all objects,
3092 interfaces and properties in a single method call. It is
3093 appropriate to use this interface if users of the tree of
3094 objects are expected to be interested in all interfaces of all
3095 objects in the tree; a more granular API should be used if
3096 users of the objects are expected to be interested in a small
3097 subset of the objects, a small subset of their interfaces, or
3101 The method that applications can use to get all objects and
3102 properties is <literal>GetManagedObjects</literal>:
3106 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3110 The return value of this method is a dict whose keys are
3111 object paths. All returned object paths are children of the
3112 object path implementing this interface, i.e. their object
3113 paths start with the ObjectManager's object path plus '/'.
3116 Each value is a dict whose keys are interfaces names. Each
3117 value in this inner dict is the same dict that would be
3118 returned by the <link
3119 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3120 method for that combination of object path and interface. If
3121 an interface has no properties, the empty dict is returned.
3124 Changes are emitted using the following two signals:
3128 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3129 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3130 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3131 ARRAY<STRING> interfaces);
3135 The <literal>InterfacesAdded</literal> signal is emitted when
3136 either a new object is added or when an existing object gains
3137 one or more interfaces. The
3138 <literal>InterfacesRemoved</literal> signal is emitted
3139 whenever an object is removed or it loses one or more
3140 interfaces. The second parameter of the
3141 <literal>InterfacesAdded</literal> signal contains a dict with
3142 the interfaces and properties (if any) that have been added to
3143 the given object path. Similarly, the second parameter of the
3144 <literal>InterfacesRemoved</literal> signal contains an array
3145 of the interfaces that were removed. Note that changes on
3146 properties on existing interfaces are not reported using this
3147 interface - an application should also monitor the existing <link
3148 linkend="standard-interfaces-properties">PropertiesChanged</link>
3149 signal on each object.
3152 Applications SHOULD NOT export objects that are children of an
3153 object (directly or otherwise) implementing this interface but
3154 which are not returned in the reply from the
3155 <literal>GetManagedObjects()</literal> method of this
3156 interface on the given object.
3159 The intent of the <literal>ObjectManager</literal> interface
3160 is to make it easy to write a robust client
3161 implementation. The trivial client implementation only needs
3162 to make two method calls:
3166 org.freedesktop.DBus.AddMatch (bus_proxy,
3167 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3168 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3172 on the message bus and the remote application's
3173 <literal>ObjectManager</literal>, respectively. Whenever a new
3174 remote object is created (or an existing object gains a new
3175 interface), the <literal>InterfacesAdded</literal> signal is
3176 emitted, and since this signal contains all properties for the
3177 interfaces, no calls to the
3178 <literal>org.freedesktop.Properties</literal> interface on the
3179 remote object are needed. Additionally, since the initial
3180 <literal>AddMatch()</literal> rule already includes signal
3181 messages from the newly created child object, no new
3182 <literal>AddMatch()</literal> call is needed.
3187 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3188 interface was added in version 0.17 of the D-Bus
3195 <sect1 id="introspection-format">
3196 <title>Introspection Data Format</title>
3198 As described in <xref linkend="standard-interfaces-introspectable"/>,
3199 objects may be introspected at runtime, returning an XML string
3200 that describes the object. The same XML format may be used in
3201 other contexts as well, for example as an "IDL" for generating
3202 static language bindings.
3205 Here is an example of introspection data:
3207 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3208 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3209 <node name="/org/freedesktop/sample_object">
3210 <interface name="org.freedesktop.SampleInterface">
3211 <method name="Frobate">
3212 <arg name="foo" type="i" direction="in"/>
3213 <arg name="bar" type="s" direction="out"/>
3214 <arg name="baz" type="a{us}" direction="out"/>
3215 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3217 <method name="Bazify">
3218 <arg name="bar" type="(iiu)" direction="in"/>
3219 <arg name="bar" type="v" direction="out"/>
3221 <method name="Mogrify">
3222 <arg name="bar" type="(iiav)" direction="in"/>
3224 <signal name="Changed">
3225 <arg name="new_value" type="b"/>
3227 <property name="Bar" type="y" access="readwrite"/>
3229 <node name="child_of_sample_object"/>
3230 <node name="another_child_of_sample_object"/>
3235 A more formal DTD and spec needs writing, but here are some quick notes.
3239 Only the root <node> element can omit the node name, as it's
3240 known to be the object that was introspected. If the root
3241 <node> does have a name attribute, it must be an absolute
3242 object path. If child <node> have object paths, they must be
3248 If a child <node> has any sub-elements, then they
3249 must represent a complete introspection of the child.
3250 If a child <node> is empty, then it may or may
3251 not have sub-elements; the child must be introspected
3252 in order to find out. The intent is that if an object
3253 knows that its children are "fast" to introspect
3254 it can go ahead and return their information, but
3255 otherwise it can omit it.
3260 The direction element on <arg> may be omitted,
3261 in which case it defaults to "in" for method calls
3262 and "out" for signals. Signals only allow "out"
3263 so while direction may be specified, it's pointless.
3268 The possible directions are "in" and "out",
3269 unlike CORBA there is no "inout"
3274 The possible property access flags are
3275 "readwrite", "read", and "write"
3280 Multiple interfaces can of course be listed for
3286 The "name" attribute on arguments is optional.
3292 Method, interface, property, and signal elements may have
3293 "annotations", which are generic key/value pairs of metadata.
3294 They are similar conceptually to Java's annotations and C# attributes.
3295 Well-known annotations:
3302 <entry>Values (separated by ,)</entry>
3303 <entry>Description</entry>
3308 <entry>org.freedesktop.DBus.Deprecated</entry>
3309 <entry>true,false</entry>
3310 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3313 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3314 <entry>(string)</entry>
3315 <entry>The C symbol; may be used for methods and interfaces</entry>
3318 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3319 <entry>true,false</entry>
3320 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3323 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3324 <entry>true,invalidates,false</entry>
3327 If set to <literal>false</literal>, the
3328 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3330 linkend="standard-interfaces-properties"/> is not
3331 guaranteed to be emitted if the property changes.
3334 If set to <literal>invalidates</literal> the signal
3335 is emitted but the value is not included in the
3339 If set to <literal>true</literal> the signal is
3340 emitted with the value included.
3343 The value for the annotation defaults to
3344 <literal>true</literal> if the enclosing interface
3345 element does not specify the annotation. Otherwise it
3346 defaults to the value specified in the enclosing
3355 <sect1 id="message-bus">
3356 <title>Message Bus Specification</title>
3357 <sect2 id="message-bus-overview">
3358 <title>Message Bus Overview</title>
3360 The message bus accepts connections from one or more applications.
3361 Once connected, applications can exchange messages with other
3362 applications that are also connected to the bus.
3365 In order to route messages among connections, the message bus keeps a
3366 mapping from names to connections. Each connection has one
3367 unique-for-the-lifetime-of-the-bus name automatically assigned.
3368 Applications may request additional names for a connection. Additional
3369 names are usually "well-known names" such as
3370 "org.freedesktop.TextEditor". When a name is bound to a connection,
3371 that connection is said to <firstterm>own</firstterm> the name.
3374 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3375 This name routes messages to the bus, allowing applications to make
3376 administrative requests. For example, applications can ask the bus
3377 to assign a name to a connection.
3380 Each name may have <firstterm>queued owners</firstterm>. When an
3381 application requests a name for a connection and the name is already in
3382 use, the bus will optionally add the connection to a queue waiting for
3383 the name. If the current owner of the name disconnects or releases
3384 the name, the next connection in the queue will become the new owner.
3388 This feature causes the right thing to happen if you start two text
3389 editors for example; the first one may request "org.freedesktop.TextEditor",
3390 and the second will be queued as a possible owner of that name. When
3391 the first exits, the second will take over.
3395 Applications may send <firstterm>unicast messages</firstterm> to
3396 a specific recipient or to the message bus itself, or
3397 <firstterm>broadcast messages</firstterm> to all interested recipients.
3398 See <xref linkend="message-bus-routing"/> for details.
3402 <sect2 id="message-bus-names">
3403 <title>Message Bus Names</title>
3405 Each connection has at least one name, assigned at connection time and
3406 returned in response to the
3407 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3408 automatically-assigned name is called the connection's <firstterm>unique
3409 name</firstterm>. Unique names are never reused for two different
3410 connections to the same bus.
3413 Ownership of a unique name is a prerequisite for interaction with
3414 the message bus. It logically follows that the unique name is always
3415 the first name that an application comes to own, and the last
3416 one that it loses ownership of.
3419 Unique connection names must begin with the character ':' (ASCII colon
3420 character); bus names that are not unique names must not begin
3421 with this character. (The bus must reject any attempt by an application
3422 to manually request a name beginning with ':'.) This restriction
3423 categorically prevents "spoofing"; messages sent to a unique name
3424 will always go to the expected connection.
3427 When a connection is closed, all the names that it owns are deleted (or
3428 transferred to the next connection in the queue if any).
3431 A connection can request additional names to be associated with it using
3432 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3433 linkend="message-protocol-names-bus"/> describes the format of a valid
3434 name. These names can be released again using the
3435 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3438 <sect3 id="bus-messages-request-name">
3439 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3443 UINT32 RequestName (in STRING name, in UINT32 flags)
3450 <entry>Argument</entry>
3452 <entry>Description</entry>
3458 <entry>STRING</entry>
3459 <entry>Name to request</entry>
3463 <entry>UINT32</entry>
3464 <entry>Flags</entry>
3474 <entry>Argument</entry>
3476 <entry>Description</entry>
3482 <entry>UINT32</entry>
3483 <entry>Return value</entry>
3490 This method call should be sent to
3491 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3492 assign the given name to the method caller. Each name maintains a
3493 queue of possible owners, where the head of the queue is the primary
3494 or current owner of the name. Each potential owner in the queue
3495 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3496 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3497 call. When RequestName is invoked the following occurs:
3501 If the method caller is currently the primary owner of the name,
3502 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3503 values are updated with the values from the new RequestName call,
3504 and nothing further happens.
3510 If the current primary owner (head of the queue) has
3511 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3512 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3513 the caller of RequestName replaces the current primary owner at
3514 the head of the queue and the current primary owner moves to the
3515 second position in the queue. If the caller of RequestName was
3516 in the queue previously its flags are updated with the values from
3517 the new RequestName in addition to moving it to the head of the queue.
3523 If replacement is not possible, and the method caller is
3524 currently in the queue but not the primary owner, its flags are
3525 updated with the values from the new RequestName call.
3531 If replacement is not possible, and the method caller is
3532 currently not in the queue, the method caller is appended to the
3539 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3540 set and is not the primary owner, it is removed from the
3541 queue. This can apply to the previous primary owner (if it
3542 was replaced) or the method caller (if it updated the
3543 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3544 queue, or if it was just added to the queue with that flag set).
3550 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3551 queue," even if another application already in the queue had specified
3552 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3553 that does not allow replacement goes away, and the next primary owner
3554 does allow replacement. In this case, queued items that specified
3555 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3556 automatically replace the new primary owner. In other words,
3557 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3558 time RequestName is called. This is deliberate to avoid an infinite loop
3559 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3560 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3563 The flags argument contains any of the following values logically ORed
3570 <entry>Conventional Name</entry>
3571 <entry>Value</entry>
3572 <entry>Description</entry>
3577 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3581 If an application A specifies this flag and succeeds in
3582 becoming the owner of the name, and another application B
3583 later calls RequestName with the
3584 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3585 will lose ownership and receive a
3586 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3587 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3588 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3589 is not specified by application B, then application B will not replace
3590 application A as the owner.
3595 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3599 Try to replace the current owner if there is one. If this
3600 flag is not set the application will only become the owner of
3601 the name if there is no current owner. If this flag is set,
3602 the application will replace the current owner if
3603 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3608 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3612 Without this flag, if an application requests a name that is
3613 already owned, the application will be placed in a queue to
3614 own the name when the current owner gives it up. If this
3615 flag is given, the application will not be placed in the
3616 queue, the request for the name will simply fail. This flag
3617 also affects behavior when an application is replaced as
3618 name owner; by default the application moves back into the
3619 waiting queue, unless this flag was provided when the application
3620 became the name owner.
3628 The return code can be one of the following values:
3634 <entry>Conventional Name</entry>
3635 <entry>Value</entry>
3636 <entry>Description</entry>
3641 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3642 <entry>1</entry> <entry>The caller is now the primary owner of
3643 the name, replacing any previous owner. Either the name had no
3644 owner before, or the caller specified
3645 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3646 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3649 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3652 <entry>The name already had an owner,
3653 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3654 the current owner did not specify
3655 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3656 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3660 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3661 <entry>The name already has an owner,
3662 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3663 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3664 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3665 specified by the requesting application.</entry>
3668 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3670 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3678 <sect3 id="bus-messages-release-name">
3679 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3683 UINT32 ReleaseName (in STRING name)
3690 <entry>Argument</entry>
3692 <entry>Description</entry>
3698 <entry>STRING</entry>
3699 <entry>Name to release</entry>
3709 <entry>Argument</entry>
3711 <entry>Description</entry>
3717 <entry>UINT32</entry>
3718 <entry>Return value</entry>
3725 This method call should be sent to
3726 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3727 release the method caller's claim to the given name. If the caller is
3728 the primary owner, a new primary owner will be selected from the
3729 queue if any other owners are waiting. If the caller is waiting in
3730 the queue for the name, the caller will removed from the queue and
3731 will not be made an owner of the name if it later becomes available.
3732 If there are no other owners in the queue for the name, it will be
3733 removed from the bus entirely.
3735 The return code can be one of the following values:
3741 <entry>Conventional Name</entry>
3742 <entry>Value</entry>
3743 <entry>Description</entry>
3748 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3749 <entry>1</entry> <entry>The caller has released his claim on
3750 the given name. Either the caller was the primary owner of
3751 the name, and the name is now unused or taken by somebody
3752 waiting in the queue for the name, or the caller was waiting
3753 in the queue for the name and has now been removed from the
3757 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3759 <entry>The given name does not exist on this bus.</entry>
3762 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3764 <entry>The caller was not the primary owner of this name,
3765 and was also not waiting in the queue to own this name.</entry>
3773 <sect3 id="bus-messages-list-queued-owners">
3774 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3778 ARRAY of STRING ListQueuedOwners (in STRING name)
3785 <entry>Argument</entry>
3787 <entry>Description</entry>
3793 <entry>STRING</entry>
3794 <entry>The well-known bus name to query, such as
3795 <literal>com.example.cappuccino</literal></entry>
3805 <entry>Argument</entry>
3807 <entry>Description</entry>
3813 <entry>ARRAY of STRING</entry>
3814 <entry>The unique bus names of connections currently queued
3815 for the name</entry>
3822 This method call should be sent to
3823 <literal>org.freedesktop.DBus</literal> and lists the connections
3824 currently queued for a bus name (see
3825 <xref linkend="term-queued-owner"/>).
3830 <sect2 id="message-bus-routing">
3831 <title>Message Bus Message Routing</title>
3834 Messages may have a <literal>DESTINATION</literal> field (see <xref
3835 linkend="message-protocol-header-fields"/>), resulting in a
3836 <firstterm>unicast message</firstterm>. If the
3837 <literal>DESTINATION</literal> field is present, it specifies a message
3838 recipient by name. Method calls and replies normally specify this field.
3839 The message bus must send messages (of any type) with the
3840 <literal>DESTINATION</literal> field set to the specified recipient,
3841 regardless of whether the recipient has set up a match rule matching
3846 When the message bus receives a signal, if the
3847 <literal>DESTINATION</literal> field is absent, it is considered to
3848 be a <firstterm>broadcast signal</firstterm>, and is sent to all
3849 applications with <firstterm>message matching rules</firstterm> that
3850 match the message. Most signal messages are broadcasts.
3854 Unicast signal messages (those with a <literal>DESTINATION</literal>
3855 field) are not commonly used, but they are treated like any unicast
3856 message: they are delivered to the specified receipient,
3857 regardless of its match rules. One use for unicast signals is to
3858 avoid a race condition in which a signal is emitted before the intended
3859 recipient can call <xref linkend="bus-messages-add-match"/> to
3860 receive that signal: if the signal is sent directly to that recipient
3861 using a unicast message, it does not need to add a match rule at all,
3862 and there is no race condition. Another use for unicast signals,
3863 on message buses whose security policy prevents eavesdropping, is to
3864 send sensitive information which should only be visible to one
3869 When the message bus receives a method call, if the
3870 <literal>DESTINATION</literal> field is absent, the call is taken to be
3871 a standard one-to-one message and interpreted by the message bus
3872 itself. For example, sending an
3873 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3874 <literal>DESTINATION</literal> will cause the message bus itself to
3875 reply to the ping immediately; the message bus will not make this
3876 message visible to other applications.
3880 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3881 the ping message were sent with a <literal>DESTINATION</literal> name of
3882 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3883 forwarded, and the Yoyodyne Corporation screensaver application would be
3884 expected to reply to the ping.
3888 Message bus implementations may impose a security policy which
3889 prevents certain messages from being sent or received.
3890 When a message cannot be sent or received due to a security
3891 policy, the message bus should send an error reply, unless the
3892 original message had the <literal>NO_REPLY</literal> flag.
3895 <sect3 id="message-bus-routing-eavesdropping">
3896 <title>Eavesdropping</title>
3898 Receiving a unicast message whose <literal>DESTINATION</literal>
3899 indicates a different recipient is called
3900 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
3901 a security boundary (like the standard system bus), the security
3902 policy should usually prevent eavesdropping, since unicast messages
3903 are normally kept private and may contain security-sensitive
3908 Eavesdropping is mainly useful for debugging tools, such as
3909 the <literal>dbus-monitor</literal> tool in the reference
3910 implementation of D-Bus. Tools which eavesdrop on the message bus
3911 should be careful to avoid sending a reply or error in response to
3912 messages intended for a different client.
3916 Clients may attempt to eavesdrop by adding match rules
3917 (see <xref linkend="message-bus-routing-match-rules"/>) containing
3918 the <literal>eavesdrop='true'</literal> match. If the message bus'
3919 security policy does not allow eavesdropping, the match rule can
3920 still be added, but will not have any practical effect. For
3921 compatibility with older message bus implementations, if adding such
3922 a match rule results in an error reply, the client may fall back to
3923 adding the same rule with the <literal>eavesdrop</literal> match
3928 <sect3 id="message-bus-routing-match-rules">
3929 <title>Match Rules</title>
3931 An important part of the message bus routing protocol is match
3932 rules. Match rules describe the messages that should be sent to a
3933 client, based on the contents of the message. Broadcast signals
3934 are only sent to clients which have a suitable match rule: this
3935 avoids waking up client processes to deal with signals that are
3936 not relevant to that client.
3939 Messages that list a client as their <literal>DESTINATION</literal>
3940 do not need to match the client's match rules, and are sent to that
3941 client regardless. As a result, match rules are mainly used to
3942 receive a subset of broadcast signals.
3945 Match rules can also be used for eavesdropping
3946 (see <xref linkend="message-bus-routing-eavesdropping"/>),
3947 if the security policy of the message bus allows it.
3950 Match rules are added using the AddMatch bus method
3951 (see <xref linkend="bus-messages-add-match"/>). Rules are
3952 specified as a string of comma separated key/value pairs.
3953 Excluding a key from the rule indicates a wildcard match.
3954 For instance excluding the the member from a match rule but
3955 adding a sender would let all messages from that sender through.
3956 An example of a complete rule would be
3957 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3960 The following table describes the keys that can be used to create
3962 The following table summarizes the D-Bus types.
3968 <entry>Possible Values</entry>
3969 <entry>Description</entry>
3974 <entry><literal>type</literal></entry>
3975 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3976 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3979 <entry><literal>sender</literal></entry>
3980 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3981 and <xref linkend="term-unique-name"/> respectively)
3983 <entry>Match messages sent by a particular sender. An example of a sender match
3984 is sender='org.freedesktop.Hal'</entry>
3987 <entry><literal>interface</literal></entry>
3988 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3989 <entry>Match messages sent over or to a particular interface. An example of an
3990 interface match is interface='org.freedesktop.Hal.Manager'.
3991 If a message omits the interface header, it must not match any rule
3992 that specifies this key.</entry>
3995 <entry><literal>member</literal></entry>
3996 <entry>Any valid method or signal name</entry>
3997 <entry>Matches messages which have the give method or signal name. An example of
3998 a member match is member='NameOwnerChanged'</entry>
4001 <entry><literal>path</literal></entry>
4002 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4003 <entry>Matches messages which are sent from or to the given object. An example of a
4004 path match is path='/org/freedesktop/Hal/Manager'</entry>
4007 <entry><literal>path_namespace</literal></entry>
4008 <entry>An object path</entry>
4011 Matches messages which are sent from or to an
4012 object for which the object path is either the
4013 given value, or that value followed by one or
4014 more path components.
4019 <literal>path_namespace='/com/example/foo'</literal>
4020 would match signals sent by
4021 <literal>/com/example/foo</literal>
4023 <literal>/com/example/foo/bar</literal>,
4025 <literal>/com/example/foobar</literal>.
4029 Using both <literal>path</literal> and
4030 <literal>path_namespace</literal> in the same match
4031 rule is not allowed.
4036 This match key was added in version 0.16 of the
4037 D-Bus specification and implemented by the bus
4038 daemon in dbus 1.5.0 and later.
4044 <entry><literal>destination</literal></entry>
4045 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4046 <entry>Matches messages which are being sent to the given unique name. An
4047 example of a destination match is destination=':1.0'</entry>
4050 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4051 <entry>Any string</entry>
4052 <entry>Arg matches are special and are used for further restricting the
4053 match based on the arguments in the body of a message. Only arguments of type
4054 STRING can be matched in this way. An example of an argument match
4055 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4059 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4060 <entry>Any string</entry>
4062 <para>Argument path matches provide a specialised form of wildcard matching for
4063 path-like namespaces. They can match arguments whose type is either STRING or
4064 OBJECT_PATH. As with normal argument matches,
4065 if the argument is exactly equal to the string given in the match
4066 rule then the rule is satisfied. Additionally, there is also a
4067 match when either the string given in the match rule or the
4068 appropriate message argument ends with '/' and is a prefix of the
4069 other. An example argument path match is arg0path='/aa/bb/'. This
4070 would match messages with first arguments of '/', '/aa/',
4071 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4072 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4074 <para>This is intended for monitoring “directories” in file system-like
4075 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4076 system. An application interested in all nodes in a particular hierarchy would
4077 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4078 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4079 represent a modification to the “bar” property, or a signal with zeroth
4080 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4081 many properties within that directory, and the interested application would be
4082 notified in both cases.</para>
4085 This match key was added in version 0.12 of the
4086 D-Bus specification, implemented for STRING
4087 arguments by the bus daemon in dbus 1.2.0 and later,
4088 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4095 <entry><literal>arg0namespace</literal></entry>
4096 <entry>Like a bus name, except that the string is not
4097 required to contain a '.' (period)</entry>
4099 <para>Match messages whose first argument is of type STRING, and is a bus name
4100 or interface name within the specified namespace. This is primarily intended
4101 for watching name owner changes for a group of related bus names, rather than
4102 for a single name or all name changes.</para>
4104 <para>Because every valid interface name is also a valid
4105 bus name, this can also be used for messages whose
4106 first argument is an interface name.</para>
4108 <para>For example, the match rule
4109 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4110 matches name owner changes for bus names such as
4111 <literal>com.example.backend.foo</literal>,
4112 <literal>com.example.backend.foo.bar</literal>, and
4113 <literal>com.example.backend</literal> itself.</para>
4115 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4118 This match key was added in version 0.16 of the
4119 D-Bus specification and implemented by the bus
4120 daemon in dbus 1.5.0 and later.
4126 <entry><literal>eavesdrop</literal></entry>
4127 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4128 <entry>Since D-Bus 1.5.6, match rules do not
4129 match messages which have a <literal>DESTINATION</literal>
4130 field unless the match rule specifically
4132 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4133 by specifying <literal>eavesdrop='true'</literal>
4134 in the match rule. <literal>eavesdrop='false'</literal>
4135 restores the default behaviour. Messages are
4136 delivered to their <literal>DESTINATION</literal>
4137 regardless of match rules, so this match does not
4138 affect normal delivery of unicast messages.
4139 If the message bus has a security policy which forbids
4140 eavesdropping, this match may still be used without error,
4141 but will not have any practical effect.
4142 In older versions of D-Bus, this match was not allowed
4143 in match rules, and all match rules behaved as if
4144 <literal>eavesdrop='true'</literal> had been used.
4153 <sect2 id="message-bus-starting-services">
4154 <title>Message Bus Starting Services</title>
4156 The message bus can start applications on behalf of other applications.
4157 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4158 An application that can be started in this way is called a
4159 <firstterm>service</firstterm>.
4162 With D-Bus, starting a service is normally done by name. That is,
4163 applications ask the message bus to start some program that will own a
4164 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4165 This implies a contract documented along with the name
4166 <literal>org.freedesktop.TextEditor</literal> for which objects
4167 the owner of that name will provide, and what interfaces those
4171 To find an executable corresponding to a particular name, the bus daemon
4172 looks for <firstterm>service description files</firstterm>. Service
4173 description files define a mapping from names to executables. Different
4174 kinds of message bus will look for these files in different places, see
4175 <xref linkend="message-bus-types"/>.
4178 Service description files have the ".service" file
4179 extension. The message bus will only load service description files
4180 ending with .service; all other files will be ignored. The file format
4181 is similar to that of <ulink
4182 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4183 entries</ulink>. All service description files must be in UTF-8
4184 encoding. To ensure that there will be no name collisions, service files
4185 must be namespaced using the same mechanism as messages and service
4190 [FIXME the file format should be much better specified than "similar to
4191 .desktop entries" esp. since desktop entries are already
4192 badly-specified. ;-)]
4193 These sections from the specification apply to service files as well:
4196 <listitem><para>General syntax</para></listitem>
4197 <listitem><para>Comment format</para></listitem>
4201 <title>Example service description file</title>
4203 # Sample service description file
4205 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4206 Exec=/usr/libexec/gconfd-2
4211 When an application asks to start a service by name, the bus daemon tries to
4212 find a service that will own that name. It then tries to spawn the
4213 executable associated with it. If this fails, it will report an
4214 error. [FIXME what happens if two .service files offer the same service;
4215 what kind of error is reported, should we have a way for the client to
4219 The executable launched will have the environment variable
4220 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4221 message bus so it can connect and request the appropriate names.
4224 The executable being launched may want to know whether the message bus
4225 starting it is one of the well-known message buses (see <xref
4226 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4227 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4228 of the well-known buses. The currently-defined values for this variable
4229 are <literal>system</literal> for the systemwide message bus,
4230 and <literal>session</literal> for the per-login-session message
4231 bus. The new executable must still connect to the address given
4232 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4233 resulting connection is to the well-known bus.
4236 [FIXME there should be a timeout somewhere, either specified
4237 in the .service file, by the client, or just a global value
4238 and if the client being activated fails to connect within that
4239 timeout, an error should be sent back.]
4242 <sect3 id="message-bus-starting-services-scope">
4243 <title>Message Bus Service Scope</title>
4245 The "scope" of a service is its "per-", such as per-session,
4246 per-machine, per-home-directory, or per-display. The reference
4247 implementation doesn't yet support starting services in a different
4248 scope from the message bus itself. So e.g. if you start a service
4249 on the session bus its scope is per-session.
4252 We could add an optional scope to a bus name. For example, for
4253 per-(display,session pair), we could have a unique ID for each display
4254 generated automatically at login and set on screen 0 by executing a
4255 special "set display ID" binary. The ID would be stored in a
4256 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4257 random bytes. This ID would then be used to scope names.
4258 Starting/locating a service could be done by ID-name pair rather than
4262 Contrast this with a per-display scope. To achieve that, we would
4263 want a single bus spanning all sessions using a given display.
4264 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4265 property on screen 0 of the display, pointing to this bus.
4270 <sect2 id="message-bus-types">
4271 <title>Well-known Message Bus Instances</title>
4273 Two standard message bus instances are defined here, along with how
4274 to locate them and where their service files live.
4276 <sect3 id="message-bus-types-login">
4277 <title>Login session message bus</title>
4279 Each time a user logs in, a <firstterm>login session message
4280 bus</firstterm> may be started. All applications in the user's login
4281 session may interact with one another using this message bus.
4284 The address of the login session message bus is given
4285 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4286 variable. If that variable is not set, applications may
4287 also try to read the address from the X Window System root
4288 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4289 The root window property must have type <literal>STRING</literal>.
4290 The environment variable should have precedence over the
4291 root window property.
4293 <para>The address of the login session message bus is given in the
4294 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4295 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4296 "autolaunch:", the system should use platform-specific methods of
4297 locating a running D-Bus session server, or starting one if a running
4298 instance cannot be found. Note that this mechanism is not recommended
4299 for attempting to determine if a daemon is running. It is inherently
4300 racy to attempt to make this determination, since the bus daemon may
4301 be started just before or just after the determination is made.
4302 Therefore, it is recommended that applications do not try to make this
4303 determination for their functionality purposes, and instead they
4304 should attempt to start the server.</para>
4306 <sect4 id="message-bus-types-login-x-windows">
4307 <title>X Windowing System</title>
4309 For the X Windowing System, the application must locate the
4310 window owner of the selection represented by the atom formed by
4314 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4318 <para>the current user's username</para>
4322 <para>the literal character '_' (underscore)</para>
4326 <para>the machine's ID</para>
4332 The following properties are defined for the window that owns
4334 <informaltable frame="all">
4343 <para>meaning</para>
4349 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4353 <para>the actual address of the server socket</para>
4359 <para>_DBUS_SESSION_BUS_PID</para>
4363 <para>the PID of the server process</para>
4372 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4373 present in this window.
4377 If the X selection cannot be located or if reading the
4378 properties from the window fails, the implementation MUST conclude
4379 that there is no D-Bus server running and proceed to start a new
4380 server. (See below on concurrency issues)
4384 Failure to connect to the D-Bus server address thus obtained
4385 MUST be treated as a fatal connection error and should be reported
4390 As an alternative, an implementation MAY find the information
4391 in the following file located in the current user's home directory,
4392 in subdirectory .dbus/session-bus/:
4395 <para>the machine's ID</para>
4399 <para>the literal character '-' (dash)</para>
4403 <para>the X display without the screen number, with the
4404 following prefixes removed, if present: ":", "localhost:"
4405 ."localhost.localdomain:". That is, a display of
4406 "localhost:10.0" produces just the number "10"</para>
4412 The contents of this file NAME=value assignment pairs and
4413 lines starting with # are comments (no comments are allowed
4414 otherwise). The following variable names are defined:
4421 <para>Variable</para>
4425 <para>meaning</para>
4431 <para>DBUS_SESSION_BUS_ADDRESS</para>
4435 <para>the actual address of the server socket</para>
4441 <para>DBUS_SESSION_BUS_PID</para>
4445 <para>the PID of the server process</para>
4451 <para>DBUS_SESSION_BUS_WINDOWID</para>
4455 <para>the window ID</para>
4464 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4469 Failure to open this file MUST be interpreted as absence of a
4470 running server. Therefore, the implementation MUST proceed to
4471 attempting to launch a new bus server if the file cannot be
4476 However, success in opening this file MUST NOT lead to the
4477 conclusion that the server is running. Thus, a failure to connect to
4478 the bus address obtained by the alternative method MUST NOT be
4479 considered a fatal error. If the connection cannot be established,
4480 the implementation MUST proceed to check the X selection settings or
4481 to start the server on its own.
4485 If the implementation concludes that the D-Bus server is not
4486 running it MUST attempt to start a new server and it MUST also
4487 ensure that the daemon started as an effect of the "autolaunch"
4488 mechanism provides the lookup mechanisms described above, so
4489 subsequent calls can locate the newly started server. The
4490 implementation MUST also ensure that if two or more concurrent
4491 initiations happen, only one server remains running and all other
4492 initiations are able to obtain the address of this server and
4493 connect to it. In other words, the implementation MUST ensure that
4494 the X selection is not present when it attempts to set it, without
4495 allowing another process to set the selection between the
4496 verification and the setting (e.g., by using XGrabServer /
4503 On Unix systems, the session bus should search for .service files
4504 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4506 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4507 Implementations may also search additional locations, which
4508 should be searched with lower priority than anything in
4509 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4510 for example, the reference implementation also
4511 looks in <literal>${datadir}/dbus-1/services</literal> as
4512 set at compile time.
4516 <sect3 id="message-bus-types-system">
4517 <title>System message bus</title>
4519 A computer may have a <firstterm>system message bus</firstterm>,
4520 accessible to all applications on the system. This message bus may be
4521 used to broadcast system events, such as adding new hardware devices,
4522 changes in the printer queue, and so forth.
4525 The address of the system message bus is given
4526 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4527 variable. If that variable is not set, applications should try
4528 to connect to the well-known address
4529 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4532 The D-Bus reference implementation actually honors the
4533 <literal>$(localstatedir)</literal> configure option
4534 for this address, on both client and server side.
4539 [FIXME specify location of system bus .service files]
4544 <sect2 id="message-bus-messages">
4545 <title>Message Bus Messages</title>
4547 The special message bus name <literal>org.freedesktop.DBus</literal>
4548 responds to a number of additional messages.
4551 <sect3 id="bus-messages-hello">
4552 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4563 <entry>Argument</entry>
4565 <entry>Description</entry>
4571 <entry>STRING</entry>
4572 <entry>Unique name assigned to the connection</entry>
4579 Before an application is able to send messages to other applications
4580 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4581 to the message bus to obtain a unique name. If an application without
4582 a unique name tries to send a message to another application, or a
4583 message to the message bus itself that isn't the
4584 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4585 disconnected from the bus.
4588 There is no corresponding "disconnect" request; if a client wishes to
4589 disconnect from the bus, it simply closes the socket (or other
4590 communication channel).
4593 <sect3 id="bus-messages-list-names">
4594 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4598 ARRAY of STRING ListNames ()
4605 <entry>Argument</entry>
4607 <entry>Description</entry>
4613 <entry>ARRAY of STRING</entry>
4614 <entry>Array of strings where each string is a bus name</entry>
4621 Returns a list of all currently-owned names on the bus.
4624 <sect3 id="bus-messages-list-activatable-names">
4625 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4629 ARRAY of STRING ListActivatableNames ()
4636 <entry>Argument</entry>
4638 <entry>Description</entry>
4644 <entry>ARRAY of STRING</entry>
4645 <entry>Array of strings where each string is a bus name</entry>
4652 Returns a list of all names that can be activated on the bus.
4655 <sect3 id="bus-messages-name-exists">
4656 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4660 BOOLEAN NameHasOwner (in STRING name)
4667 <entry>Argument</entry>
4669 <entry>Description</entry>
4675 <entry>STRING</entry>
4676 <entry>Name to check</entry>
4686 <entry>Argument</entry>
4688 <entry>Description</entry>
4694 <entry>BOOLEAN</entry>
4695 <entry>Return value, true if the name exists</entry>
4702 Checks if the specified name exists (currently has an owner).
4706 <sect3 id="bus-messages-name-owner-changed">
4707 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4711 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4718 <entry>Argument</entry>
4720 <entry>Description</entry>
4726 <entry>STRING</entry>
4727 <entry>Name with a new owner</entry>
4731 <entry>STRING</entry>
4732 <entry>Old owner or empty string if none</entry>
4736 <entry>STRING</entry>
4737 <entry>New owner or empty string if none</entry>
4744 This signal indicates that the owner of a name has changed.
4745 It's also the signal to use to detect the appearance of
4746 new names on the bus.
4749 <sect3 id="bus-messages-name-lost">
4750 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4754 NameLost (STRING name)
4761 <entry>Argument</entry>
4763 <entry>Description</entry>
4769 <entry>STRING</entry>
4770 <entry>Name which was lost</entry>
4777 This signal is sent to a specific application when it loses
4778 ownership of a name.
4782 <sect3 id="bus-messages-name-acquired">
4783 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4787 NameAcquired (STRING name)
4794 <entry>Argument</entry>
4796 <entry>Description</entry>
4802 <entry>STRING</entry>
4803 <entry>Name which was acquired</entry>
4810 This signal is sent to a specific application when it gains
4811 ownership of a name.
4815 <sect3 id="bus-messages-start-service-by-name">
4816 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4820 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4827 <entry>Argument</entry>
4829 <entry>Description</entry>
4835 <entry>STRING</entry>
4836 <entry>Name of the service to start</entry>
4840 <entry>UINT32</entry>
4841 <entry>Flags (currently not used)</entry>
4851 <entry>Argument</entry>
4853 <entry>Description</entry>
4859 <entry>UINT32</entry>
4860 <entry>Return value</entry>
4865 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4869 The return value can be one of the following values:
4874 <entry>Identifier</entry>
4875 <entry>Value</entry>
4876 <entry>Description</entry>
4881 <entry>DBUS_START_REPLY_SUCCESS</entry>
4883 <entry>The service was successfully started.</entry>
4886 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4888 <entry>A connection already owns the given name.</entry>
4897 <sect3 id="bus-messages-update-activation-environment">
4898 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4902 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4909 <entry>Argument</entry>
4911 <entry>Description</entry>
4917 <entry>ARRAY of DICT<STRING,STRING></entry>
4918 <entry>Environment to add or update</entry>
4923 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4926 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4929 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.
4934 <sect3 id="bus-messages-get-name-owner">
4935 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4939 STRING GetNameOwner (in STRING name)
4946 <entry>Argument</entry>
4948 <entry>Description</entry>
4954 <entry>STRING</entry>
4955 <entry>Name to get the owner of</entry>
4965 <entry>Argument</entry>
4967 <entry>Description</entry>
4973 <entry>STRING</entry>
4974 <entry>Return value, a unique connection name</entry>
4979 Returns the unique connection name of the primary owner of the name
4980 given. If the requested name doesn't have an owner, returns a
4981 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4985 <sect3 id="bus-messages-get-connection-unix-user">
4986 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4990 UINT32 GetConnectionUnixUser (in STRING bus_name)
4997 <entry>Argument</entry>
4999 <entry>Description</entry>
5005 <entry>STRING</entry>
5006 <entry>Unique or well-known bus name of the connection to
5007 query, such as <literal>:12.34</literal> or
5008 <literal>com.example.tea</literal></entry>
5018 <entry>Argument</entry>
5020 <entry>Description</entry>
5026 <entry>UINT32</entry>
5027 <entry>Unix user ID</entry>
5032 Returns the Unix user ID of the process connected to the server. If
5033 unable to determine it (for instance, because the process is not on the
5034 same machine as the bus daemon), an error is returned.
5038 <sect3 id="bus-messages-get-connection-unix-process-id">
5039 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5043 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5050 <entry>Argument</entry>
5052 <entry>Description</entry>
5058 <entry>STRING</entry>
5059 <entry>Unique or well-known bus name of the connection to
5060 query, such as <literal>:12.34</literal> or
5061 <literal>com.example.tea</literal></entry>
5071 <entry>Argument</entry>
5073 <entry>Description</entry>
5079 <entry>UINT32</entry>
5080 <entry>Unix process id</entry>
5085 Returns the Unix process ID of the process connected to the server. If
5086 unable to determine it (for instance, because the process is not on the
5087 same machine as the bus daemon), an error is returned.
5091 <sect3 id="bus-messages-add-match">
5092 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5096 AddMatch (in STRING rule)
5103 <entry>Argument</entry>
5105 <entry>Description</entry>
5111 <entry>STRING</entry>
5112 <entry>Match rule to add to the connection</entry>
5117 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5118 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5122 <sect3 id="bus-messages-remove-match">
5123 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5127 RemoveMatch (in STRING rule)
5134 <entry>Argument</entry>
5136 <entry>Description</entry>
5142 <entry>STRING</entry>
5143 <entry>Match rule to remove from the connection</entry>
5148 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5149 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5154 <sect3 id="bus-messages-get-id">
5155 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5159 GetId (out STRING id)
5166 <entry>Argument</entry>
5168 <entry>Description</entry>
5174 <entry>STRING</entry>
5175 <entry>Unique ID identifying the bus daemon</entry>
5180 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5181 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5182 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5183 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5184 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5185 by org.freedesktop.DBus.Peer.GetMachineId().
5186 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5194 <appendix id="implementation-notes">
5195 <title>Implementation notes</title>
5196 <sect1 id="implementation-notes-subsection">
5204 <glossary><title>Glossary</title>
5206 This glossary defines some of the terms used in this specification.
5209 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5212 The message bus maintains an association between names and
5213 connections. (Normally, there's one connection per application.) A
5214 bus name is simply an identifier used to locate connections. For
5215 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5216 name might be used to send a message to a screensaver from Yoyodyne
5217 Corporation. An application is said to <firstterm>own</firstterm> a
5218 name if the message bus has associated the application's connection
5219 with the name. Names may also have <firstterm>queued
5220 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5221 The bus assigns a unique name to each connection,
5222 see <xref linkend="term-unique-name"/>. Other names
5223 can be thought of as "well-known names" and are
5224 used to find applications that offer specific functionality.
5229 <glossentry id="term-message"><glossterm>Message</glossterm>
5232 A message is the atomic unit of communication via the D-Bus
5233 protocol. It consists of a <firstterm>header</firstterm> and a
5234 <firstterm>body</firstterm>; the body is made up of
5235 <firstterm>arguments</firstterm>.
5240 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5243 The message bus is a special application that forwards
5244 or routes messages between a group of applications
5245 connected to the message bus. It also manages
5246 <firstterm>names</firstterm> used for routing
5252 <glossentry id="term-name"><glossterm>Name</glossterm>
5255 See <xref linkend="term-bus-name"/>. "Name" may
5256 also be used to refer to some of the other names
5257 in D-Bus, such as interface names.
5262 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5265 Used to prevent collisions when defining new interfaces or bus
5266 names. The convention used is the same one Java uses for defining
5267 classes: a reversed domain name.
5272 <glossentry id="term-object"><glossterm>Object</glossterm>
5275 Each application contains <firstterm>objects</firstterm>, which have
5276 <firstterm>interfaces</firstterm> and
5277 <firstterm>methods</firstterm>. Objects are referred to by a name,
5278 called a <firstterm>path</firstterm>.
5283 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5286 An application talking directly to another application, without going
5287 through a message bus. One-to-one connections may be "peer to peer" or
5288 "client to server." The D-Bus protocol has no concept of client
5289 vs. server after a connection has authenticated; the flow of messages
5290 is symmetrical (full duplex).
5295 <glossentry id="term-path"><glossterm>Path</glossterm>
5298 Object references (object names) in D-Bus are organized into a
5299 filesystem-style hierarchy, so each object is named by a path. As in
5300 LDAP, there's no difference between "files" and "directories"; a path
5301 can refer to an object, while still having child objects below it.
5306 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5309 Each bus name has a primary owner; messages sent to the name go to the
5310 primary owner. However, certain names also maintain a queue of
5311 secondary owners "waiting in the wings." If the primary owner releases
5312 the name, then the first secondary owner in the queue automatically
5313 becomes the new owner of the name.
5318 <glossentry id="term-service"><glossterm>Service</glossterm>
5321 A service is an executable that can be launched by the bus daemon.
5322 Services normally guarantee some particular features, for example they
5323 may guarantee that they will request a specific name such as
5324 "org.freedesktop.Screensaver", have a singleton object
5325 "/org/freedesktop/Application", and that object will implement the
5326 interface "org.freedesktop.ScreensaverControl".
5331 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5334 ".service files" tell the bus about service applications that can be
5335 launched (see <xref linkend="term-service"/>). Most importantly they
5336 provide a mapping from bus names to services that will request those
5337 names when they start up.
5342 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5345 The special name automatically assigned to each connection by the
5346 message bus. This name will never change owner, and will be unique
5347 (never reused during the lifetime of the message bus).
5348 It will begin with a ':' character.