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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.17</releaseinfo>
10 <date>2011-06-01</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
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>0.17</revnumber>
82 <date>1 June 2011</date>
83 <authorinitials>smcv/davidz</authorinitials>
84 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
85 by GVariant</revremark>
88 <revnumber>0.16</revnumber>
89 <date>11 April 2011</date>
90 <authorinitials></authorinitials>
91 <revremark>add path_namespace, arg0namespace; argNpath matches object
95 <revnumber>0.15</revnumber>
96 <date>3 November 2010</date>
97 <authorinitials></authorinitials>
98 <revremark></revremark>
101 <revnumber>0.14</revnumber>
102 <date>12 May 2010</date>
103 <authorinitials></authorinitials>
104 <revremark></revremark>
107 <revnumber>0.13</revnumber>
108 <date>23 Dezember 2009</date>
109 <authorinitials></authorinitials>
110 <revremark></revremark>
113 <revnumber>0.12</revnumber>
114 <date>7 November, 2006</date>
115 <authorinitials></authorinitials>
116 <revremark></revremark>
119 <revnumber>0.11</revnumber>
120 <date>6 February 2005</date>
121 <authorinitials></authorinitials>
122 <revremark></revremark>
125 <revnumber>0.10</revnumber>
126 <date>28 January 2005</date>
127 <authorinitials></authorinitials>
128 <revremark></revremark>
131 <revnumber>0.9</revnumber>
132 <date>7 Januar 2005</date>
133 <authorinitials></authorinitials>
134 <revremark></revremark>
137 <revnumber>0.8</revnumber>
138 <date>06 September 2003</date>
139 <authorinitials></authorinitials>
140 <revremark>First released document.</revremark>
145 <sect1 id="introduction">
146 <title>Introduction</title>
148 D-Bus is a system for low-latency, low-overhead, easy to use
149 interprocess communication (IPC). In more detail:
153 D-Bus is <emphasis>low-latency</emphasis> because it is designed
154 to avoid round trips and allow asynchronous operation, much like
160 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
161 binary protocol, and does not have to convert to and from a text
162 format such as XML. Because D-Bus is intended for potentially
163 high-resolution same-machine IPC, not primarily for Internet IPC,
164 this is an interesting optimization.
169 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
170 of <firstterm>messages</firstterm> rather than byte streams, and
171 automatically handles a lot of the hard IPC issues. Also, the D-Bus
172 library is designed to be wrapped in a way that lets developers use
173 their framework's existing object/type system, rather than learning
174 a new one specifically for IPC.
181 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
182 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
183 a system for one application to talk to a single other
184 application. However, the primary intended application of the protocol is the
185 D-Bus <firstterm>message bus</firstterm>, specified in <xref
186 linkend="message-bus"/>. The message bus is a special application that
187 accepts connections from multiple other applications, and forwards
192 Uses of D-Bus include notification of system changes (notification of when
193 a camera is plugged in to a computer, or a new version of some software
194 has been installed), or desktop interoperability, for example a file
195 monitoring service or a configuration service.
199 D-Bus is designed for two specific use cases:
203 A "system bus" for notifications from the system to user sessions,
204 and to allow the system to request input from user sessions.
209 A "session bus" used to implement desktop environments such as
214 D-Bus is not intended to be a generic IPC system for any possible
215 application, and intentionally omits many features found in other
216 IPC systems for this reason.
220 At the same time, the bus daemons offer a number of features not found in
221 other IPC systems, such as single-owner "bus names" (similar to X
222 selections), on-demand startup of services, and security policies.
223 In many ways, these features are the primary motivation for developing
224 D-Bus; other systems would have sufficed if IPC were the only goal.
228 D-Bus may turn out to be useful in unanticipated applications, but future
229 versions of this spec and the reference implementation probably will not
230 incorporate features that interfere with the core use cases.
234 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
235 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
236 document are to be interpreted as described in RFC 2119. However, the
237 document could use a serious audit to be sure it makes sense to do
238 so. Also, they are not capitalized.
241 <sect2 id="stability">
242 <title>Protocol and Specification Stability</title>
244 The D-Bus protocol is frozen (only compatible extensions are allowed) as
245 of November 8, 2006. However, this specification could still use a fair
246 bit of work to make interoperable reimplementation possible without
247 reference to the D-Bus reference implementation. Thus, this
248 specification is not marked 1.0. To mark it 1.0, we'd like to see
249 someone invest significant effort in clarifying the specification
250 language, and growing the specification to cover more aspects of the
251 reference implementation's behavior.
254 Until this work is complete, any attempt to reimplement D-Bus will
255 probably require looking at the reference implementation and/or asking
256 questions on the D-Bus mailing list about intended behavior.
257 Questions on the list are very welcome.
260 Nonetheless, this document should be a useful starting point and is
261 to our knowledge accurate, though incomplete.
267 <sect1 id="type-system">
268 <title>Type System</title>
271 D-Bus has a type system, in which values of various types can be
272 serialized into a sequence of bytes referred to as the
273 <firstterm>wire format</firstterm> in a standard way.
274 Converting a value from some other representation into the wire
275 format is called <firstterm>marshaling</firstterm> and converting
276 it back from the wire format is <firstterm>unmarshaling</firstterm>.
279 <sect2 id="message-protocol-signatures">
280 <title>Type Signatures</title>
283 The D-Bus protocol does not include type tags in the marshaled data; a
284 block of marshaled values must have a known <firstterm>type
285 signature</firstterm>. The type signature is made up of <firstterm>type
286 codes</firstterm>. A type code is an ASCII character representing the
287 type of a value. Because ASCII characters are used, the type signature
288 will always form a valid ASCII string. A simple string compare
289 determines whether two type signatures are equivalent.
293 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
294 the ASCII character 'i'. So the signature for a block of values
295 containing a single <literal>INT32</literal> would be:
299 A block of values containing two <literal>INT32</literal> would have this signature:
306 All <firstterm>basic</firstterm> types work like
307 <literal>INT32</literal> in this example. To marshal and unmarshal
308 basic types, you simply read one value from the data
309 block corresponding to each type code in the signature.
310 In addition to basic types, there are four <firstterm>container</firstterm>
311 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
312 and <literal>DICT_ENTRY</literal>.
316 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
317 code does not appear in signatures. Instead, ASCII characters
318 '(' and ')' are used to mark the beginning and end of the struct.
319 So for example, a struct containing two integers would have this
324 Structs can be nested, so for example a struct containing
325 an integer and another struct:
329 The value block storing that struct would contain three integers; the
330 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
335 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
336 but is useful in code that implements the protocol. This type code
337 is specified to allow such code to interoperate in non-protocol contexts.
341 Empty structures are not allowed; there must be at least one
342 type code between the parentheses.
346 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
347 followed by a <firstterm>single complete type</firstterm>. The single
348 complete type following the array is the type of each array element. So
349 the simple example is:
353 which is an array of 32-bit integers. But an array can be of any type,
354 such as this array-of-struct-with-two-int32-fields:
358 Or this array of array of integer:
365 The phrase <firstterm>single complete type</firstterm> deserves some
366 definition. A single complete type is a basic type code, a variant type code,
367 an array with its element type, or a struct with its fields.
368 So the following signatures are not single complete types:
378 And the following signatures contain multiple complete types:
388 Note however that a single complete type may <emphasis>contain</emphasis>
389 multiple other single complete types.
393 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
394 type <literal>VARIANT</literal> will have the signature of a single complete type as part
395 of the <emphasis>value</emphasis>. This signature will be followed by a
396 marshaled value of that type.
400 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
401 than parentheses it uses curly braces, and it has more restrictions.
402 The restrictions are: it occurs only as an array element type; it has
403 exactly two single complete types inside the curly braces; the first
404 single complete type (the "key") must be a basic type rather than a
405 container type. Implementations must not accept dict entries outside of
406 arrays, must not accept dict entries with zero, one, or more than two
407 fields, and must not accept dict entries with non-basic-typed keys. A
408 dict entry is always a key-value pair.
412 The first field in the <literal>DICT_ENTRY</literal> is always the key.
413 A message is considered corrupt if the same key occurs twice in the same
414 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
415 implementations are not required to reject dicts with duplicate keys.
419 In most languages, an array of dict entry would be represented as a
420 map, hash table, or dict object.
424 The following table summarizes the D-Bus types.
429 <entry>Conventional Name</entry>
431 <entry>Description</entry>
436 <entry><literal>INVALID</literal></entry>
437 <entry>0 (ASCII NUL)</entry>
438 <entry>Not a valid type code, used to terminate signatures</entry>
440 <entry><literal>BYTE</literal></entry>
441 <entry>121 (ASCII 'y')</entry>
442 <entry>8-bit unsigned integer</entry>
444 <entry><literal>BOOLEAN</literal></entry>
445 <entry>98 (ASCII 'b')</entry>
446 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
448 <entry><literal>INT16</literal></entry>
449 <entry>110 (ASCII 'n')</entry>
450 <entry>16-bit signed integer</entry>
452 <entry><literal>UINT16</literal></entry>
453 <entry>113 (ASCII 'q')</entry>
454 <entry>16-bit unsigned integer</entry>
456 <entry><literal>INT32</literal></entry>
457 <entry>105 (ASCII 'i')</entry>
458 <entry>32-bit signed integer</entry>
460 <entry><literal>UINT32</literal></entry>
461 <entry>117 (ASCII 'u')</entry>
462 <entry>32-bit unsigned integer</entry>
464 <entry><literal>INT64</literal></entry>
465 <entry>120 (ASCII 'x')</entry>
466 <entry>64-bit signed integer</entry>
468 <entry><literal>UINT64</literal></entry>
469 <entry>116 (ASCII 't')</entry>
470 <entry>64-bit unsigned integer</entry>
472 <entry><literal>DOUBLE</literal></entry>
473 <entry>100 (ASCII 'd')</entry>
474 <entry>IEEE 754 double</entry>
476 <entry><literal>STRING</literal></entry>
477 <entry>115 (ASCII 's')</entry>
478 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
480 <entry><literal>OBJECT_PATH</literal></entry>
481 <entry>111 (ASCII 'o')</entry>
482 <entry>Name of an object instance</entry>
484 <entry><literal>SIGNATURE</literal></entry>
485 <entry>103 (ASCII 'g')</entry>
486 <entry>A type signature</entry>
488 <entry><literal>ARRAY</literal></entry>
489 <entry>97 (ASCII 'a')</entry>
492 <entry><literal>STRUCT</literal></entry>
493 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
494 <entry>Struct; type code 114 'r' is reserved for use in
495 bindings and implementations to represent the general
496 concept of a struct, and must not appear in signatures
497 used on D-Bus.</entry>
499 <entry><literal>VARIANT</literal></entry>
500 <entry>118 (ASCII 'v') </entry>
501 <entry>Variant type (the type of the value is part of the value itself)</entry>
503 <entry><literal>DICT_ENTRY</literal></entry>
504 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
505 <entry>Entry in a dict or map (array of key-value pairs).
506 Type code 101 'e' is reserved for use in bindings and
507 implementations to represent the general concept of a
508 dict or dict-entry, and must not appear in signatures
509 used on D-Bus.</entry>
511 <entry><literal>UNIX_FD</literal></entry>
512 <entry>104 (ASCII 'h')</entry>
513 <entry>Unix file descriptor</entry>
516 <entry>(reserved)</entry>
517 <entry>109 (ASCII 'm')</entry>
518 <entry>Reserved for <ulink
519 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
520 'maybe' type compatible with the one in GVariant</ulink>,
521 and must not appear in signatures used on D-Bus until
522 specified here</entry>
525 <entry>(reserved)</entry>
526 <entry>42 (ASCII '*')</entry>
527 <entry>Reserved for use in bindings/implementations to
528 represent any <firstterm>single complete type</firstterm>,
529 and must not appear in signatures used on D-Bus.</entry>
532 <entry>(reserved)</entry>
533 <entry>63 (ASCII '?')</entry>
534 <entry>Reserved for use in bindings/implementations to
535 represent any <firstterm>basic type</firstterm>, and must
536 not appear in signatures used on D-Bus.</entry>
539 <entry>(reserved)</entry>
540 <entry>64 (ASCII '@'), 38 (ASCII '&'),
541 94 (ASCII '^')</entry>
542 <entry>Reserved for internal use by bindings/implementations,
543 and must not appear in signatures used on D-Bus.
544 GVariant uses these type-codes to encode calling
554 <sect2 id="message-protocol-marshaling">
555 <title>Marshaling (Wire Format)</title>
558 Given a type signature, a block of bytes can be converted into typed
559 values. This section describes the format of the block of bytes. Byte
560 order and alignment issues are handled uniformly for all D-Bus types.
564 A block of bytes has an associated byte order. The byte order
565 has to be discovered in some way; for D-Bus messages, the
566 byte order is part of the message header as described in
567 <xref linkend="message-protocol-messages"/>. For now, assume
568 that the byte order is known to be either little endian or big
573 Each value in a block of bytes is aligned "naturally," for example
574 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
575 8-byte boundary. To properly align a value, <firstterm>alignment
576 padding</firstterm> may be necessary. The alignment padding must always
577 be the minimum required padding to properly align the following value;
578 and it must always be made up of nul bytes. The alignment padding must
579 not be left uninitialized (it can't contain garbage), and more padding
580 than required must not be used.
584 Given all this, the types are marshaled on the wire as follows:
589 <entry>Conventional Name</entry>
590 <entry>Encoding</entry>
591 <entry>Alignment</entry>
596 <entry><literal>INVALID</literal></entry>
597 <entry>Not applicable; cannot be marshaled.</entry>
600 <entry><literal>BYTE</literal></entry>
601 <entry>A single 8-bit byte.</entry>
604 <entry><literal>BOOLEAN</literal></entry>
605 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
608 <entry><literal>INT16</literal></entry>
609 <entry>16-bit signed integer in the message's byte order.</entry>
612 <entry><literal>UINT16</literal></entry>
613 <entry>16-bit unsigned integer in the message's byte order.</entry>
616 <entry><literal>INT32</literal></entry>
617 <entry>32-bit signed integer in the message's byte order.</entry>
620 <entry><literal>UINT32</literal></entry>
621 <entry>32-bit unsigned integer in the message's byte order.</entry>
624 <entry><literal>INT64</literal></entry>
625 <entry>64-bit signed integer in the message's byte order.</entry>
628 <entry><literal>UINT64</literal></entry>
629 <entry>64-bit unsigned integer in the message's byte order.</entry>
632 <entry><literal>DOUBLE</literal></entry>
633 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
636 <entry><literal>STRING</literal></entry>
637 <entry>A <literal>UINT32</literal> indicating the string's
638 length in bytes excluding its terminating nul, followed by
639 non-nul string data of the given length, followed by a terminating nul
646 <entry><literal>OBJECT_PATH</literal></entry>
647 <entry>Exactly the same as <literal>STRING</literal> except the
648 content must be a valid object path (see below).
654 <entry><literal>SIGNATURE</literal></entry>
655 <entry>The same as <literal>STRING</literal> except the length is a single
656 byte (thus signatures have a maximum length of 255)
657 and the content must be a valid signature (see below).
663 <entry><literal>ARRAY</literal></entry>
665 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
666 alignment padding to the alignment boundary of the array element type,
667 followed by each array element. The array length is from the
668 end of the alignment padding to the end of the last element,
669 i.e. it does not include the padding after the length,
670 or any padding after the last element.
671 Arrays have a maximum length defined to be 2 to the 26th power or
672 67108864. Implementations must not send or accept arrays exceeding this
679 <entry><literal>STRUCT</literal></entry>
681 A struct must start on an 8-byte boundary regardless of the
682 type of the struct fields. The struct value consists of each
683 field marshaled in sequence starting from that 8-byte
690 <entry><literal>VARIANT</literal></entry>
692 A variant type has a marshaled
693 <literal>SIGNATURE</literal> followed by a marshaled
694 value with the type given in the signature. Unlike
695 a message signature, the variant signature can
696 contain only a single complete type. So "i", "ai"
697 or "(ii)" is OK, but "ii" is not. Use of variants may not
698 cause a total message depth to be larger than 64, including
699 other container types such as structures.
702 1 (alignment of the signature)
705 <entry><literal>DICT_ENTRY</literal></entry>
713 <entry><literal>UNIX_FD</literal></entry>
714 <entry>32-bit unsigned integer in the message's byte
715 order. The actual file descriptors need to be
716 transferred out-of-band via some platform specific
717 mechanism. On the wire, values of this type store the index to the
718 file descriptor in the array of file descriptors that
719 accompany the message.</entry>
727 <sect3 id="message-protocol-marshaling-object-path">
728 <title>Valid Object Paths</title>
731 An object path is a name used to refer to an object instance.
732 Conceptually, each participant in a D-Bus message exchange may have
733 any number of object instances (think of C++ or Java objects) and each
734 such instance will have a path. Like a filesystem, the object
735 instances in an application form a hierarchical tree.
739 The following rules define a valid object path. Implementations must
740 not send or accept messages with invalid object paths.
744 The path may be of any length.
749 The path must begin with an ASCII '/' (integer 47) character,
750 and must consist of elements separated by slash characters.
755 Each element must only contain the ASCII characters
761 No element may be the empty string.
766 Multiple '/' characters cannot occur in sequence.
771 A trailing '/' character is not allowed unless the
772 path is the root path (a single '/' character).
781 <sect3 id="message-protocol-marshaling-signature">
782 <title>Valid Signatures</title>
784 An implementation must not send or accept invalid signatures.
785 Valid signatures will conform to the following rules:
789 The signature ends with a nul byte.
794 The signature is a list of single complete types.
795 Arrays must have element types, and structs must
796 have both open and close parentheses.
801 Only type codes and open and close parentheses are
802 allowed in the signature. The <literal>STRUCT</literal> type code
803 is not allowed in signatures, because parentheses
809 The maximum depth of container type nesting is 32 array type
810 codes and 32 open parentheses. This implies that the maximum
811 total depth of recursion is 64, for an "array of array of array
812 of ... struct of struct of struct of ..." where there are 32
818 The maximum length of a signature is 255.
823 Signatures must be nul-terminated.
834 <sect1 id="message-protocol">
835 <title>Message Protocol</title>
838 A <firstterm>message</firstterm> consists of a
839 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
840 think of a message as a package, the header is the address, and the body
841 contains the package contents. The message delivery system uses the header
842 information to figure out where to send the message and how to interpret
843 it; the recipient interprets the body of the message.
847 The body of the message is made up of zero or more
848 <firstterm>arguments</firstterm>, which are typed values, such as an
849 integer or a byte array.
853 Both header and body use the D-Bus <link linkend="type-system">type
854 system</link> and format for serializing data.
857 <sect2 id="message-protocol-messages">
858 <title>Message Format</title>
861 A message consists of a header and a body. The header is a block of
862 values with a fixed signature and meaning. The body is a separate block
863 of values, with a signature specified in the header.
867 The length of the header must be a multiple of 8, allowing the body to
868 begin on an 8-byte boundary when storing the entire message in a single
869 buffer. If the header does not naturally end on an 8-byte boundary
870 up to 7 bytes of nul-initialized alignment padding must be added.
874 The message body need not end on an 8-byte boundary.
878 The maximum length of a message, including header, header alignment padding,
879 and body is 2 to the 27th power or 134217728. Implementations must not
880 send or accept messages exceeding this size.
884 The signature of the header is:
888 Written out more readably, this is:
890 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
895 These values have the following meanings:
901 <entry>Description</entry>
906 <entry>1st <literal>BYTE</literal></entry>
907 <entry>Endianness flag; ASCII 'l' for little-endian
908 or ASCII 'B' for big-endian. Both header and body are
909 in this endianness.</entry>
912 <entry>2nd <literal>BYTE</literal></entry>
913 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
914 Currently-defined types are described below.
918 <entry>3rd <literal>BYTE</literal></entry>
919 <entry>Bitwise OR of flags. Unknown flags
920 must be ignored. Currently-defined flags are described below.
924 <entry>4th <literal>BYTE</literal></entry>
925 <entry>Major protocol version of the sending application. If
926 the major protocol version of the receiving application does not
927 match, the applications will not be able to communicate and the
928 D-Bus connection must be disconnected. The major protocol
929 version for this version of the specification is 1.
933 <entry>1st <literal>UINT32</literal></entry>
934 <entry>Length in bytes of the message body, starting
935 from the end of the header. The header ends after
936 its alignment padding to an 8-boundary.
940 <entry>2nd <literal>UINT32</literal></entry>
941 <entry>The serial of this message, used as a cookie
942 by the sender to identify the reply corresponding
943 to this request. This must not be zero.
947 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
948 <entry>An array of zero or more <firstterm>header
949 fields</firstterm> where the byte is the field code, and the
950 variant is the field value. The message type determines
951 which fields are required.
959 <firstterm>Message types</firstterm> that can appear in the second byte
965 <entry>Conventional name</entry>
966 <entry>Decimal value</entry>
967 <entry>Description</entry>
972 <entry><literal>INVALID</literal></entry>
974 <entry>This is an invalid type.</entry>
977 <entry><literal>METHOD_CALL</literal></entry>
979 <entry>Method call.</entry>
982 <entry><literal>METHOD_RETURN</literal></entry>
984 <entry>Method reply with returned data.</entry>
987 <entry><literal>ERROR</literal></entry>
989 <entry>Error reply. If the first argument exists and is a
990 string, it is an error message.</entry>
993 <entry><literal>SIGNAL</literal></entry>
995 <entry>Signal emission.</entry>
1002 Flags that can appear in the third byte of the header:
1007 <entry>Conventional name</entry>
1008 <entry>Hex value</entry>
1009 <entry>Description</entry>
1014 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1016 <entry>This message does not expect method return replies or
1017 error replies; the reply can be omitted as an
1018 optimization. However, it is compliant with this specification
1019 to return the reply despite this flag and the only harm
1020 from doing so is extra network traffic.
1024 <entry><literal>NO_AUTO_START</literal></entry>
1026 <entry>The bus must not launch an owner
1027 for the destination name in response to this message.
1035 <sect3 id="message-protocol-header-fields">
1036 <title>Header Fields</title>
1039 The array at the end of the header contains <firstterm>header
1040 fields</firstterm>, where each field is a 1-byte field code followed
1041 by a field value. A header must contain the required header fields for
1042 its message type, and zero or more of any optional header
1043 fields. Future versions of this protocol specification may add new
1044 fields. Implementations must ignore fields they do not
1045 understand. Implementations must not invent their own header fields;
1046 only changes to this specification may introduce new header fields.
1050 Again, if an implementation sees a header field code that it does not
1051 expect, it must ignore that field, as it will be part of a new
1052 (but compatible) version of this specification. This also applies
1053 to known header fields appearing in unexpected messages, for
1054 example: if a signal has a reply serial it must be ignored
1055 even though it has no meaning as of this version of the spec.
1059 However, implementations must not send or accept known header fields
1060 with the wrong type stored in the field value. So for example a
1061 message with an <literal>INTERFACE</literal> field of type
1062 <literal>UINT32</literal> would be considered corrupt.
1066 Here are the currently-defined header fields:
1071 <entry>Conventional Name</entry>
1072 <entry>Decimal Code</entry>
1074 <entry>Required In</entry>
1075 <entry>Description</entry>
1080 <entry><literal>INVALID</literal></entry>
1083 <entry>not allowed</entry>
1084 <entry>Not a valid field name (error if it appears in a message)</entry>
1087 <entry><literal>PATH</literal></entry>
1089 <entry><literal>OBJECT_PATH</literal></entry>
1090 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1091 <entry>The object to send a call to,
1092 or the object a signal is emitted from.
1094 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1095 implementations should not send messages with this path,
1096 and the reference implementation of the bus daemon will
1097 disconnect any application that attempts to do so.
1101 <entry><literal>INTERFACE</literal></entry>
1103 <entry><literal>STRING</literal></entry>
1104 <entry><literal>SIGNAL</literal></entry>
1106 The interface to invoke a method call on, or
1107 that a signal is emitted from. Optional for
1108 method calls, required for signals.
1109 The special interface
1110 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1111 implementations should not send messages with this
1112 interface, and the reference implementation of the bus
1113 daemon will disconnect any application that attempts to
1118 <entry><literal>MEMBER</literal></entry>
1120 <entry><literal>STRING</literal></entry>
1121 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1122 <entry>The member, either the method name or signal name.</entry>
1125 <entry><literal>ERROR_NAME</literal></entry>
1127 <entry><literal>STRING</literal></entry>
1128 <entry><literal>ERROR</literal></entry>
1129 <entry>The name of the error that occurred, for errors</entry>
1132 <entry><literal>REPLY_SERIAL</literal></entry>
1134 <entry><literal>UINT32</literal></entry>
1135 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1136 <entry>The serial number of the message this message is a reply
1137 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1140 <entry><literal>DESTINATION</literal></entry>
1142 <entry><literal>STRING</literal></entry>
1143 <entry>optional</entry>
1144 <entry>The name of the connection this message is intended for.
1145 Only used in combination with the message bus, see
1146 <xref linkend="message-bus"/>.</entry>
1149 <entry><literal>SENDER</literal></entry>
1151 <entry><literal>STRING</literal></entry>
1152 <entry>optional</entry>
1153 <entry>Unique name of the sending connection.
1154 The message bus fills in this field so it is reliable; the field is
1155 only meaningful in combination with the message bus.</entry>
1158 <entry><literal>SIGNATURE</literal></entry>
1160 <entry><literal>SIGNATURE</literal></entry>
1161 <entry>optional</entry>
1162 <entry>The signature of the message body.
1163 If omitted, it is assumed to be the
1164 empty signature "" (i.e. the body must be 0-length).</entry>
1167 <entry><literal>UNIX_FDS</literal></entry>
1169 <entry><literal>UINT32</literal></entry>
1170 <entry>optional</entry>
1171 <entry>The number of Unix file descriptors that
1172 accompany the message. If omitted, it is assumed
1173 that no Unix file descriptors accompany the
1174 message. The actual file descriptors need to be
1175 transferred via platform specific mechanism
1176 out-of-band. They must be sent at the same time as
1177 part of the message itself. They may not be sent
1178 before the first byte of the message itself is
1179 transferred or after the last byte of the message
1189 <sect2 id="message-protocol-names">
1190 <title>Valid Names</title>
1192 The various names in D-Bus messages have some restrictions.
1195 There is a <firstterm>maximum name length</firstterm>
1196 of 255 which applies to bus names, interfaces, and members.
1198 <sect3 id="message-protocol-names-interface">
1199 <title>Interface names</title>
1201 Interfaces have names with type <literal>STRING</literal>, meaning that
1202 they must be valid UTF-8. However, there are also some
1203 additional restrictions that apply to interface names
1206 <listitem><para>Interface names are composed of 1 or more elements separated by
1207 a period ('.') character. All elements must contain at least
1211 <listitem><para>Each element must only contain the ASCII characters
1212 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1216 <listitem><para>Interface names must contain at least one '.' (period)
1217 character (and thus at least two elements).
1220 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1221 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1225 <sect3 id="message-protocol-names-bus">
1226 <title>Bus names</title>
1228 Connections have one or more bus names associated with them.
1229 A connection has exactly one bus name that is a unique connection
1230 name. The unique connection name remains with the connection for
1231 its entire lifetime.
1232 A bus name is of type <literal>STRING</literal>,
1233 meaning that it must be valid UTF-8. However, there are also
1234 some additional restrictions that apply to bus names
1237 <listitem><para>Bus names that start with a colon (':')
1238 character are unique connection names.
1241 <listitem><para>Bus names are composed of 1 or more elements separated by
1242 a period ('.') character. All elements must contain at least
1246 <listitem><para>Each element must only contain the ASCII characters
1247 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1248 connection name may begin with a digit, elements in
1249 other bus names must not begin with a digit.
1253 <listitem><para>Bus names must contain at least one '.' (period)
1254 character (and thus at least two elements).
1257 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1258 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1262 Note that the hyphen ('-') character is allowed in bus names but
1263 not in interface names.
1266 <sect3 id="message-protocol-names-member">
1267 <title>Member names</title>
1269 Member (i.e. method or signal) names:
1271 <listitem><para>Must only contain the ASCII characters
1272 "[A-Z][a-z][0-9]_" and may not begin with a
1273 digit.</para></listitem>
1274 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1275 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1276 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1280 <sect3 id="message-protocol-names-error">
1281 <title>Error names</title>
1283 Error names have the same restrictions as interface names.
1288 <sect2 id="message-protocol-types">
1289 <title>Message Types</title>
1291 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1292 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1293 This section describes these conventions.
1295 <sect3 id="message-protocol-types-method">
1296 <title>Method Calls</title>
1298 Some messages invoke an operation on a remote object. These are
1299 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1300 messages map naturally to methods on objects in a typical program.
1303 A method call message is required to have a <literal>MEMBER</literal> header field
1304 indicating the name of the method. Optionally, the message has an
1305 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1306 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1307 a method with the same name, it is undefined which of the two methods
1308 will be invoked. Implementations may also choose to return an error in
1309 this ambiguous case. However, if a method name is unique
1310 implementations must not require an interface field.
1313 Method call messages also include a <literal>PATH</literal> field
1314 indicating the object to invoke the method on. If the call is passing
1315 through a message bus, the message will also have a
1316 <literal>DESTINATION</literal> field giving the name of the connection
1317 to receive the message.
1320 When an application handles a method call message, it is required to
1321 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1322 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1323 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1326 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1327 are the return value(s) or "out parameters" of the method call.
1328 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1329 and the call fails; no return value will be provided. It makes
1330 no sense to send multiple replies to the same method call.
1333 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1334 reply is required, so the caller will know the method
1335 was successfully processed.
1338 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1342 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1343 then as an optimization the application receiving the method
1344 call may choose to omit the reply message (regardless of
1345 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1346 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1347 flag and reply anyway.
1350 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1351 destination name does not exist then a program to own the destination
1352 name will be started before the message is delivered. The message
1353 will be held until the new program is successfully started or has
1354 failed to start; in case of failure, an error will be returned. This
1355 flag is only relevant in the context of a message bus, it is ignored
1356 during one-to-one communication with no intermediate bus.
1358 <sect4 id="message-protocol-types-method-apis">
1359 <title>Mapping method calls to native APIs</title>
1361 APIs for D-Bus may map method calls to a method call in a specific
1362 programming language, such as C++, or may map a method call written
1363 in an IDL to a D-Bus message.
1366 In APIs of this nature, arguments to a method are often termed "in"
1367 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1368 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1369 "inout" arguments, which are both sent and received, i.e. the caller
1370 passes in a value which is modified. Mapped to D-Bus, an "inout"
1371 argument is equivalent to an "in" argument, followed by an "out"
1372 argument. You can't pass things "by reference" over the wire, so
1373 "inout" is purely an illusion of the in-process API.
1376 Given a method with zero or one return values, followed by zero or more
1377 arguments, where each argument may be "in", "out", or "inout", the
1378 caller constructs a message by appending each "in" or "inout" argument,
1379 in order. "out" arguments are not represented in the caller's message.
1382 The recipient constructs a reply by appending first the return value
1383 if any, then each "out" or "inout" argument, in order.
1384 "in" arguments are not represented in the reply message.
1387 Error replies are normally mapped to exceptions in languages that have
1391 In converting from native APIs to D-Bus, it is perhaps nice to
1392 map D-Bus naming conventions ("FooBar") to native conventions
1393 such as "fooBar" or "foo_bar" automatically. This is OK
1394 as long as you can say that the native API is one that
1395 was specifically written for D-Bus. It makes the most sense
1396 when writing object implementations that will be exported
1397 over the bus. Object proxies used to invoke remote D-Bus
1398 objects probably need the ability to call any D-Bus method,
1399 and thus a magic name mapping like this could be a problem.
1402 This specification doesn't require anything of native API bindings;
1403 the preceding is only a suggested convention for consistency
1409 <sect3 id="message-protocol-types-signal">
1410 <title>Signal Emission</title>
1412 Unlike method calls, signal emissions have no replies.
1413 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1414 It must have three header fields: <literal>PATH</literal> giving the object
1415 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1416 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1417 for signals, though it is optional for method calls.
1421 <sect3 id="message-protocol-types-errors">
1422 <title>Errors</title>
1424 Messages of type <literal>ERROR</literal> are most commonly replies
1425 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1426 to any kind of message. The message bus for example
1427 will return an <literal>ERROR</literal> in reply to a signal emission if
1428 the bus does not have enough memory to send the signal.
1431 An <literal>ERROR</literal> may have any arguments, but if the first
1432 argument is a <literal>STRING</literal>, it must be an error message.
1433 The error message may be logged or shown to the user
1438 <sect3 id="message-protocol-types-notation">
1439 <title>Notation in this document</title>
1441 This document uses a simple pseudo-IDL to describe particular method
1442 calls and signals. Here is an example of a method call:
1444 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1445 out UINT32 resultcode)
1447 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1448 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1449 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1450 characters so it's known that the last part of the name in
1451 the "IDL" is the member name.
1454 In C++ that might end up looking like this:
1456 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1457 unsigned int flags);
1459 or equally valid, the return value could be done as an argument:
1461 void org::freedesktop::DBus::StartServiceByName (const char *name,
1463 unsigned int *resultcode);
1465 It's really up to the API designer how they want to make
1466 this look. You could design an API where the namespace wasn't used
1467 in C++, using STL or Qt, using varargs, or whatever you wanted.
1470 Signals are written as follows:
1472 org.freedesktop.DBus.NameLost (STRING name)
1474 Signals don't specify "in" vs. "out" because only
1475 a single direction is possible.
1478 It isn't especially encouraged to use this lame pseudo-IDL in actual
1479 API implementations; you might use the native notation for the
1480 language you're using, or you might use COM or CORBA IDL, for example.
1485 <sect2 id="message-protocol-handling-invalid">
1486 <title>Invalid Protocol and Spec Extensions</title>
1489 For security reasons, the D-Bus protocol should be strictly parsed and
1490 validated, with the exception of defined extension points. Any invalid
1491 protocol or spec violations should result in immediately dropping the
1492 connection without notice to the other end. Exceptions should be
1493 carefully considered, e.g. an exception may be warranted for a
1494 well-understood idiosyncrasy of a widely-deployed implementation. In
1495 cases where the other end of a connection is 100% trusted and known to
1496 be friendly, skipping validation for performance reasons could also make
1497 sense in certain cases.
1501 Generally speaking violations of the "must" requirements in this spec
1502 should be considered possible attempts to exploit security, and violations
1503 of the "should" suggestions should be considered legitimate (though perhaps
1504 they should generate an error in some cases).
1508 The following extension points are built in to D-Bus on purpose and must
1509 not be treated as invalid protocol. The extension points are intended
1510 for use by future versions of this spec, they are not intended for third
1511 parties. At the moment, the only way a third party could extend D-Bus
1512 without breaking interoperability would be to introduce a way to negotiate new
1513 feature support as part of the auth protocol, using EXTENSION_-prefixed
1514 commands. There is not yet a standard way to negotiate features.
1518 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1519 commands result in an ERROR rather than a disconnect. This enables
1520 future extensions to the protocol. Commands starting with EXTENSION_ are
1521 reserved for third parties.
1526 The authentication protocol supports pluggable auth mechanisms.
1531 The address format (see <xref linkend="addresses"/>) supports new
1537 Messages with an unknown type (something other than
1538 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1539 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1540 Unknown-type messages must still be well-formed in the same way
1541 as the known messages, however. They still have the normal
1547 Header fields with an unknown or unexpected field code must be ignored,
1548 though again they must still be well-formed.
1553 New standard interfaces (with new methods and signals) can of course be added.
1563 <sect1 id="auth-protocol">
1564 <title>Authentication Protocol</title>
1566 Before the flow of messages begins, two applications must
1567 authenticate. A simple plain-text protocol is used for
1568 authentication; this protocol is a SASL profile, and maps fairly
1569 directly from the SASL specification. The message encoding is
1570 NOT used here, only plain text messages.
1573 In examples, "C:" and "S:" indicate lines sent by the client and
1574 server respectively.
1576 <sect2 id="auth-protocol-overview">
1577 <title>Protocol Overview</title>
1579 The protocol is a line-based protocol, where each line ends with
1580 \r\n. Each line begins with an all-caps ASCII command name containing
1581 only the character range [A-Z_], a space, then any arguments for the
1582 command, then the \r\n ending the line. The protocol is
1583 case-sensitive. All bytes must be in the ASCII character set.
1585 Commands from the client to the server are as follows:
1588 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1589 <listitem><para>CANCEL</para></listitem>
1590 <listitem><para>BEGIN</para></listitem>
1591 <listitem><para>DATA <data in hex encoding></para></listitem>
1592 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1593 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1596 From server to client are as follows:
1599 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1600 <listitem><para>OK <GUID in hex></para></listitem>
1601 <listitem><para>DATA <data in hex encoding></para></listitem>
1602 <listitem><para>ERROR</para></listitem>
1603 <listitem><para>AGREE_UNIX_FD</para></listitem>
1607 Unofficial extensions to the command set must begin with the letters
1608 "EXTENSION_", to avoid conflicts with future official commands.
1609 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1612 <sect2 id="auth-nul-byte">
1613 <title>Special credentials-passing nul byte</title>
1615 Immediately after connecting to the server, the client must send a
1616 single nul byte. This byte may be accompanied by credentials
1617 information on some operating systems that use sendmsg() with
1618 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1619 sockets. However, the nul byte must be sent even on other kinds of
1620 socket, and even on operating systems that do not require a byte to be
1621 sent in order to transmit credentials. The text protocol described in
1622 this document begins after the single nul byte. If the first byte
1623 received from the client is not a nul byte, the server may disconnect
1627 A nul byte in any context other than the initial byte is an error;
1628 the protocol is ASCII-only.
1631 The credentials sent along with the nul byte may be used with the
1632 SASL mechanism EXTERNAL.
1635 <sect2 id="auth-command-auth">
1636 <title>AUTH command</title>
1638 If an AUTH command has no arguments, it is a request to list
1639 available mechanisms. The server must respond with a REJECTED
1640 command listing the mechanisms it understands, or with an error.
1643 If an AUTH command specifies a mechanism, and the server supports
1644 said mechanism, the server should begin exchanging SASL
1645 challenge-response data with the client using DATA commands.
1648 If the server does not support the mechanism given in the AUTH
1649 command, it must send either a REJECTED command listing the mechanisms
1650 it does support, or an error.
1653 If the [initial-response] argument is provided, it is intended for use
1654 with mechanisms that have no initial challenge (or an empty initial
1655 challenge), as if it were the argument to an initial DATA command. If
1656 the selected mechanism has an initial challenge and [initial-response]
1657 was provided, the server should reject authentication by sending
1661 If authentication succeeds after exchanging DATA commands,
1662 an OK command must be sent to the client.
1665 The first octet received by the server after the \r\n of the BEGIN
1666 command from the client must be the first octet of the
1667 authenticated/encrypted stream of D-Bus messages.
1670 If BEGIN is received by the server, the first octet received
1671 by the client after the \r\n of the OK command must be the
1672 first octet of the authenticated/encrypted stream of D-Bus
1676 <sect2 id="auth-command-cancel">
1677 <title>CANCEL Command</title>
1679 At any time up to sending the BEGIN command, the client may send a
1680 CANCEL command. On receiving the CANCEL command, the server must
1681 send a REJECTED command and abort the current authentication
1685 <sect2 id="auth-command-data">
1686 <title>DATA Command</title>
1688 The DATA command may come from either client or server, and simply
1689 contains a hex-encoded block of data to be interpreted
1690 according to the SASL mechanism in use.
1693 Some SASL mechanisms support sending an "empty string";
1694 FIXME we need some way to do this.
1697 <sect2 id="auth-command-begin">
1698 <title>BEGIN Command</title>
1700 The BEGIN command acknowledges that the client has received an
1701 OK command from the server, and that the stream of messages
1705 The first octet received by the server after the \r\n of the BEGIN
1706 command from the client must be the first octet of the
1707 authenticated/encrypted stream of D-Bus messages.
1710 <sect2 id="auth-command-rejected">
1711 <title>REJECTED Command</title>
1713 The REJECTED command indicates that the current authentication
1714 exchange has failed, and further exchange of DATA is inappropriate.
1715 The client would normally try another mechanism, or try providing
1716 different responses to challenges.
1718 Optionally, the REJECTED command has a space-separated list of
1719 available auth mechanisms as arguments. If a server ever provides
1720 a list of supported mechanisms, it must provide the same list
1721 each time it sends a REJECTED message. Clients are free to
1722 ignore all lists received after the first.
1725 <sect2 id="auth-command-ok">
1726 <title>OK Command</title>
1728 The OK command indicates that the client has been
1729 authenticated. The client may now proceed with negotiating
1730 Unix file descriptor passing. To do that it shall send
1731 NEGOTIATE_UNIX_FD to the server.
1734 Otherwise, the client must respond to the OK command by
1735 sending a BEGIN command, followed by its stream of messages,
1736 or by disconnecting. The server must not accept additional
1737 commands using this protocol after the BEGIN command has been
1738 received. Further communication will be a stream of D-Bus
1739 messages (optionally encrypted, as negotiated) rather than
1743 If a client sends BEGIN the first octet received by the client
1744 after the \r\n of the OK command must be the first octet of
1745 the authenticated/encrypted stream of D-Bus messages.
1748 The OK command has one argument, which is the GUID of the server.
1749 See <xref linkend="addresses"/> for more on server GUIDs.
1752 <sect2 id="auth-command-error">
1753 <title>ERROR Command</title>
1755 The ERROR command indicates that either server or client did not
1756 know a command, does not accept the given command in the current
1757 context, or did not understand the arguments to the command. This
1758 allows the protocol to be extended; a client or server can send a
1759 command present or permitted only in new protocol versions, and if
1760 an ERROR is received instead of an appropriate response, fall back
1761 to using some other technique.
1764 If an ERROR is sent, the server or client that sent the
1765 error must continue as if the command causing the ERROR had never been
1766 received. However, the the server or client receiving the error
1767 should try something other than whatever caused the error;
1768 if only canceling/rejecting the authentication.
1771 If the D-Bus protocol changes incompatibly at some future time,
1772 applications implementing the new protocol would probably be able to
1773 check for support of the new protocol by sending a new command and
1774 receiving an ERROR from applications that don't understand it. Thus the
1775 ERROR feature of the auth protocol is an escape hatch that lets us
1776 negotiate extensions or changes to the D-Bus protocol in the future.
1779 <sect2 id="auth-command-negotiate-unix-fd">
1780 <title>NEGOTIATE_UNIX_FD Command</title>
1782 The NEGOTIATE_UNIX_FD command indicates that the client
1783 supports Unix file descriptor passing. This command may only
1784 be sent after the connection is authenticated, i.e. after OK
1785 was received by the client. This command may only be sent on
1786 transports that support Unix file descriptor passing.
1789 On receiving NEGOTIATE_UNIX_FD the server must respond with
1790 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1791 the transport chosen supports Unix file descriptor passing and
1792 the server supports this feature. It shall respond the latter
1793 if the transport does not support Unix file descriptor
1794 passing, the server does not support this feature, or the
1795 server decides not to enable file descriptor passing due to
1796 security or other reasons.
1799 <sect2 id="auth-command-agree-unix-fd">
1800 <title>AGREE_UNIX_FD Command</title>
1802 The AGREE_UNIX_FD command indicates that the server supports
1803 Unix file descriptor passing. This command may only be sent
1804 after the connection is authenticated, and the client sent
1805 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1806 command may only be sent on transports that support Unix file
1810 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1811 followed by its stream of messages, or by disconnecting. The
1812 server must not accept additional commands using this protocol
1813 after the BEGIN command has been received. Further
1814 communication will be a stream of D-Bus messages (optionally
1815 encrypted, as negotiated) rather than this protocol.
1818 <sect2 id="auth-command-future">
1819 <title>Future Extensions</title>
1821 Future extensions to the authentication and negotiation
1822 protocol are possible. For that new commands may be
1823 introduced. If a client or server receives an unknown command
1824 it shall respond with ERROR and not consider this fatal. New
1825 commands may be introduced both before, and after
1826 authentication, i.e. both before and after the OK command.
1829 <sect2 id="auth-examples">
1830 <title>Authentication examples</title>
1834 <title>Example of successful magic cookie authentication</title>
1836 (MAGIC_COOKIE is a made up mechanism)
1838 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1844 <title>Example of finding out mechanisms then picking one</title>
1847 S: REJECTED KERBEROS_V4 SKEY
1848 C: AUTH SKEY 7ab83f32ee
1849 S: DATA 8799cabb2ea93e
1850 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1856 <title>Example of client sends unknown command then falls back to regular auth</title>
1860 C: AUTH MAGIC_COOKIE 3736343435313230333039
1866 <title>Example of server doesn't support initial auth mechanism</title>
1868 C: AUTH MAGIC_COOKIE 3736343435313230333039
1869 S: REJECTED KERBEROS_V4 SKEY
1870 C: AUTH SKEY 7ab83f32ee
1871 S: DATA 8799cabb2ea93e
1872 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1878 <title>Example of wrong password or the like followed by successful retry</title>
1880 C: AUTH MAGIC_COOKIE 3736343435313230333039
1881 S: REJECTED KERBEROS_V4 SKEY
1882 C: AUTH SKEY 7ab83f32ee
1883 S: DATA 8799cabb2ea93e
1884 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1886 C: AUTH SKEY 7ab83f32ee
1887 S: DATA 8799cabb2ea93e
1888 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1894 <title>Example of skey cancelled and restarted</title>
1896 C: AUTH MAGIC_COOKIE 3736343435313230333039
1897 S: REJECTED KERBEROS_V4 SKEY
1898 C: AUTH SKEY 7ab83f32ee
1899 S: DATA 8799cabb2ea93e
1902 C: AUTH SKEY 7ab83f32ee
1903 S: DATA 8799cabb2ea93e
1904 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1910 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1912 (MAGIC_COOKIE is a made up mechanism)
1914 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1916 C: NEGOTIATE_UNIX_FD
1922 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1924 (MAGIC_COOKIE is a made up mechanism)
1926 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1928 C: NEGOTIATE_UNIX_FD
1935 <sect2 id="auth-states">
1936 <title>Authentication state diagrams</title>
1939 This section documents the auth protocol in terms of
1940 a state machine for the client and the server. This is
1941 probably the most robust way to implement the protocol.
1944 <sect3 id="auth-states-client">
1945 <title>Client states</title>
1948 To more precisely describe the interaction between the
1949 protocol state machine and the authentication mechanisms the
1950 following notation is used: MECH(CHALL) means that the
1951 server challenge CHALL was fed to the mechanism MECH, which
1957 CONTINUE(RESP) means continue the auth conversation
1958 and send RESP as the response to the server;
1964 OK(RESP) means that after sending RESP to the server
1965 the client side of the auth conversation is finished
1966 and the server should return "OK";
1972 ERROR means that CHALL was invalid and could not be
1978 Both RESP and CHALL may be empty.
1982 The Client starts by getting an initial response from the
1983 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1984 the mechanism did not provide an initial response. If the
1985 mechanism returns CONTINUE, the client starts in state
1986 <emphasis>WaitingForData</emphasis>, if the mechanism
1987 returns OK the client starts in state
1988 <emphasis>WaitingForOK</emphasis>.
1992 The client should keep track of available mechanisms and
1993 which it mechanisms it has already attempted. This list is
1994 used to decide which AUTH command to send. When the list is
1995 exhausted, the client should give up and close the
2000 <title><emphasis>WaitingForData</emphasis></title>
2008 MECH(CHALL) returns CONTINUE(RESP) → send
2010 <emphasis>WaitingForData</emphasis>
2014 MECH(CHALL) returns OK(RESP) → send DATA
2015 RESP, goto <emphasis>WaitingForOK</emphasis>
2019 MECH(CHALL) returns ERROR → send ERROR
2020 [msg], goto <emphasis>WaitingForData</emphasis>
2028 Receive REJECTED [mechs] →
2029 send AUTH [next mech], goto
2030 WaitingForData or <emphasis>WaitingForOK</emphasis>
2035 Receive ERROR → send
2037 <emphasis>WaitingForReject</emphasis>
2042 Receive OK → send
2043 BEGIN, terminate auth
2044 conversation, authenticated
2049 Receive anything else → send
2051 <emphasis>WaitingForData</emphasis>
2059 <title><emphasis>WaitingForOK</emphasis></title>
2064 Receive OK → send BEGIN, terminate auth
2065 conversation, <emphasis>authenticated</emphasis>
2070 Receive REJECT [mechs] → send AUTH [next mech],
2071 goto <emphasis>WaitingForData</emphasis> or
2072 <emphasis>WaitingForOK</emphasis>
2078 Receive DATA → send CANCEL, goto
2079 <emphasis>WaitingForReject</emphasis>
2085 Receive ERROR → send CANCEL, goto
2086 <emphasis>WaitingForReject</emphasis>
2092 Receive anything else → send ERROR, goto
2093 <emphasis>WaitingForOK</emphasis>
2101 <title><emphasis>WaitingForReject</emphasis></title>
2106 Receive REJECT [mechs] → send AUTH [next mech],
2107 goto <emphasis>WaitingForData</emphasis> or
2108 <emphasis>WaitingForOK</emphasis>
2114 Receive anything else → terminate auth
2115 conversation, disconnect
2124 <sect3 id="auth-states-server">
2125 <title>Server states</title>
2128 For the server MECH(RESP) means that the client response
2129 RESP was fed to the the mechanism MECH, which returns one of
2134 CONTINUE(CHALL) means continue the auth conversation and
2135 send CHALL as the challenge to the client;
2141 OK means that the client has been successfully
2148 REJECT means that the client failed to authenticate or
2149 there was an error in RESP.
2154 The server starts out in state
2155 <emphasis>WaitingForAuth</emphasis>. If the client is
2156 rejected too many times the server must disconnect the
2161 <title><emphasis>WaitingForAuth</emphasis></title>
2167 Receive AUTH → send REJECTED [mechs], goto
2168 <emphasis>WaitingForAuth</emphasis>
2174 Receive AUTH MECH RESP
2178 MECH not valid mechanism → send REJECTED
2180 <emphasis>WaitingForAuth</emphasis>
2184 MECH(RESP) returns CONTINUE(CHALL) → send
2186 <emphasis>WaitingForData</emphasis>
2190 MECH(RESP) returns OK → send OK, goto
2191 <emphasis>WaitingForBegin</emphasis>
2195 MECH(RESP) returns REJECT → send REJECTED
2197 <emphasis>WaitingForAuth</emphasis>
2205 Receive BEGIN → terminate
2206 auth conversation, disconnect
2212 Receive ERROR → send REJECTED [mechs], goto
2213 <emphasis>WaitingForAuth</emphasis>
2219 Receive anything else → send
2221 <emphasis>WaitingForAuth</emphasis>
2230 <title><emphasis>WaitingForData</emphasis></title>
2238 MECH(RESP) returns CONTINUE(CHALL) → send
2240 <emphasis>WaitingForData</emphasis>
2244 MECH(RESP) returns OK → send OK, goto
2245 <emphasis>WaitingForBegin</emphasis>
2249 MECH(RESP) returns REJECT → send REJECTED
2251 <emphasis>WaitingForAuth</emphasis>
2259 Receive BEGIN → terminate auth conversation,
2266 Receive CANCEL → send REJECTED [mechs], goto
2267 <emphasis>WaitingForAuth</emphasis>
2273 Receive ERROR → send REJECTED [mechs], goto
2274 <emphasis>WaitingForAuth</emphasis>
2280 Receive anything else → send ERROR, goto
2281 <emphasis>WaitingForData</emphasis>
2289 <title><emphasis>WaitingForBegin</emphasis></title>
2294 Receive BEGIN → terminate auth conversation,
2295 client authenticated
2301 Receive CANCEL → send REJECTED [mechs], goto
2302 <emphasis>WaitingForAuth</emphasis>
2308 Receive ERROR → send REJECTED [mechs], goto
2309 <emphasis>WaitingForAuth</emphasis>
2315 Receive anything else → send ERROR, goto
2316 <emphasis>WaitingForBegin</emphasis>
2326 <sect2 id="auth-mechanisms">
2327 <title>Authentication mechanisms</title>
2329 This section describes some new authentication mechanisms.
2330 D-Bus also allows any standard SASL mechanism of course.
2332 <sect3 id="auth-mechanisms-sha">
2333 <title>DBUS_COOKIE_SHA1</title>
2335 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2336 has the ability to read a private file owned by the user being
2337 authenticated. If the client can prove that it has access to a secret
2338 cookie stored in this file, then the client is authenticated.
2339 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2343 Throughout this description, "hex encoding" must output the digits
2344 from a to f in lower-case; the digits A to F must not be used
2345 in the DBUS_COOKIE_SHA1 mechanism.
2348 Authentication proceeds as follows:
2352 The client sends the username it would like to authenticate
2358 The server sends the name of its "cookie context" (see below); a
2359 space character; the integer ID of the secret cookie the client
2360 must demonstrate knowledge of; a space character; then a
2361 randomly-generated challenge string, all of this hex-encoded into
2367 The client locates the cookie and generates its own
2368 randomly-generated challenge string. The client then concatenates
2369 the server's decoded challenge, a ":" character, its own challenge,
2370 another ":" character, and the cookie. It computes the SHA-1 hash
2371 of this composite string as a hex digest. It concatenates the
2372 client's challenge string, a space character, and the SHA-1 hex
2373 digest, hex-encodes the result and sends it back to the server.
2378 The server generates the same concatenated string used by the
2379 client and computes its SHA-1 hash. It compares the hash with
2380 the hash received from the client; if the two hashes match, the
2381 client is authenticated.
2387 Each server has a "cookie context," which is a name that identifies a
2388 set of cookies that apply to that server. A sample context might be
2389 "org_freedesktop_session_bus". Context names must be valid ASCII,
2390 nonzero length, and may not contain the characters slash ("/"),
2391 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2392 tab ("\t"), or period ("."). There is a default context,
2393 "org_freedesktop_general" that's used by servers that do not specify
2397 Cookies are stored in a user's home directory, in the directory
2398 <filename>~/.dbus-keyrings/</filename>. This directory must
2399 not be readable or writable by other users. If it is,
2400 clients and servers must ignore it. The directory
2401 contains cookie files named after the cookie context.
2404 A cookie file contains one cookie per line. Each line
2405 has three space-separated fields:
2409 The cookie ID number, which must be a non-negative integer and
2410 may not be used twice in the same file.
2415 The cookie's creation time, in UNIX seconds-since-the-epoch
2421 The cookie itself, a hex-encoded random block of bytes. The cookie
2422 may be of any length, though obviously security increases
2423 as the length increases.
2429 Only server processes modify the cookie file.
2430 They must do so with this procedure:
2434 Create a lockfile name by appending ".lock" to the name of the
2435 cookie file. The server should attempt to create this file
2436 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2437 fails, the lock fails. Servers should retry for a reasonable
2438 period of time, then they may choose to delete an existing lock
2439 to keep users from having to manually delete a stale
2440 lock. <footnote><para>Lockfiles are used instead of real file
2441 locking <literal>fcntl()</literal> because real locking
2442 implementations are still flaky on network
2443 filesystems.</para></footnote>
2448 Once the lockfile has been created, the server loads the cookie
2449 file. It should then delete any cookies that are old (the
2450 timeout can be fairly short), or more than a reasonable
2451 time in the future (so that cookies never accidentally
2452 become permanent, if the clock was set far into the future
2453 at some point). If no recent keys remain, the
2454 server may generate a new key.
2459 The pruned and possibly added-to cookie file
2460 must be resaved atomically (using a temporary
2461 file which is rename()'d).
2466 The lock must be dropped by deleting the lockfile.
2472 Clients need not lock the file in order to load it,
2473 because servers are required to save the file atomically.
2478 <sect1 id="addresses">
2479 <title>Server Addresses</title>
2481 Server addresses consist of a transport name followed by a colon, and
2482 then an optional, comma-separated list of keys and values in the form key=value.
2483 Each value is escaped.
2487 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2488 Which is the address to a unix socket with the path /tmp/dbus-test.
2491 Value escaping is similar to URI escaping but simpler.
2495 The set of optionally-escaped bytes is:
2496 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2497 <emphasis>byte</emphasis> (note, not character) which is not in the
2498 set of optionally-escaped bytes must be replaced with an ASCII
2499 percent (<literal>%</literal>) and the value of the byte in hex.
2500 The hex value must always be two digits, even if the first digit is
2501 zero. The optionally-escaped bytes may be escaped if desired.
2506 To unescape, append each byte in the value; if a byte is an ASCII
2507 percent (<literal>%</literal>) character then append the following
2508 hex value instead. It is an error if a <literal>%</literal> byte
2509 does not have two hex digits following. It is an error if a
2510 non-optionally-escaped byte is seen unescaped.
2514 The set of optionally-escaped bytes is intended to preserve address
2515 readability and convenience.
2519 A server may specify a key-value pair with the key <literal>guid</literal>
2520 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2521 describes the format of the <literal>guid</literal> field. If present,
2522 this UUID may be used to distinguish one server address from another. A
2523 server should use a different UUID for each address it listens on. For
2524 example, if a message bus daemon offers both UNIX domain socket and TCP
2525 connections, but treats clients the same regardless of how they connect,
2526 those two connections are equivalent post-connection but should have
2527 distinct UUIDs to distinguish the kinds of connection.
2531 The intent of the address UUID feature is to allow a client to avoid
2532 opening multiple identical connections to the same server, by allowing the
2533 client to check whether an address corresponds to an already-existing
2534 connection. Comparing two addresses is insufficient, because addresses
2535 can be recycled by distinct servers, and equivalent addresses may look
2536 different if simply compared as strings (for example, the host in a TCP
2537 address can be given as an IP address or as a hostname).
2541 Note that the address key is <literal>guid</literal> even though the
2542 rest of the API and documentation says "UUID," for historical reasons.
2546 [FIXME clarify if attempting to connect to each is a requirement
2547 or just a suggestion]
2548 When connecting to a server, multiple server addresses can be
2549 separated by a semi-colon. The library will then try to connect
2550 to the first address and if that fails, it'll try to connect to
2551 the next one specified, and so forth. For example
2552 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2557 <sect1 id="transports">
2558 <title>Transports</title>
2560 [FIXME we need to specify in detail each transport and its possible arguments]
2562 Current transports include: unix domain sockets (including
2563 abstract namespace on linux), launchd, TCP/IP, and a debug/testing transport
2564 using in-process pipes. Future possible transports include one that
2565 tunnels over X11 protocol.
2568 <sect2 id="transports-unix-domain-sockets">
2569 <title>Unix Domain Sockets</title>
2571 Unix domain sockets can be either paths in the file system or on Linux
2572 kernels, they can be abstract which are similar to paths but
2573 do not show up in the file system.
2577 When a socket is opened by the D-Bus library it truncates the path
2578 name right before the first trailing Nul byte. This is true for both
2579 normal paths and abstract paths. Note that this is a departure from
2580 previous versions of D-Bus that would create sockets with a fixed
2581 length path name. Names which were shorter than the fixed length
2582 would be padded by Nul bytes.
2585 Unix domain sockets are not available on windows.
2587 <sect3 id="transports-unix-domain-sockets-addresses">
2588 <title>Server Address Format</title>
2590 Unix domain socket addresses are identified by the "unix:" prefix
2591 and support the following key/value pairs:
2598 <entry>Values</entry>
2599 <entry>Description</entry>
2605 <entry>(path)</entry>
2606 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2609 <entry>tmpdir</entry>
2610 <entry>(path)</entry>
2611 <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>
2614 <entry>abstract</entry>
2615 <entry>(string)</entry>
2616 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2623 <sect2 id="transports-launchd">
2624 <title>launchd</title>
2626 launchd is a open-source server management system that replaces init, inetd
2627 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2628 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2632 launchd allocates a socket and provides it with the unix path through the
2633 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2634 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2635 it through its environment.
2636 Other processes can query for the launchd socket by executing:
2637 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2638 This is normally done by the D-Bus client library so doesn't have to be done
2642 launchd is not available on Microsoft Windows.
2644 <sect3 id="transports-launchd-addresses">
2645 <title>Server Address Format</title>
2647 launchd addresses are identified by the "launchd:" prefix
2648 and support the following key/value pairs:
2655 <entry>Values</entry>
2656 <entry>Description</entry>
2662 <entry>(environment variable)</entry>
2663 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2670 <sect2 id="transports-tcp-sockets">
2671 <title>TCP Sockets</title>
2673 The tcp transport provides TCP/IP based connections between clients
2674 located on the same or different hosts.
2677 Using tcp transport without any additional secure authentification mechanismus
2678 over a network is unsecure.
2681 Windows notes: Because of the tcp stack on windows does not provide sending
2682 credentials over a tcp connection, the EXTERNAL authentification
2683 mechanismus does not work.
2685 <sect3 id="transports-tcp-sockets-addresses">
2686 <title>Server Address Format</title>
2688 TCP/IP socket addresses are identified by the "tcp:" prefix
2689 and support the following key/value pairs:
2696 <entry>Values</entry>
2697 <entry>Description</entry>
2703 <entry>(string)</entry>
2704 <entry>dns name or ip address</entry>
2708 <entry>(number)</entry>
2709 <entry>The tcp port the server will open. A zero value let the server
2710 choose a free port provided from the underlaying operating system.
2711 libdbus is able to retrieve the real used port from the server.
2715 <entry>family</entry>
2716 <entry>(string)</entry>
2717 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2724 <sect2 id="transports-nonce-tcp-sockets">
2725 <title>Nonce-secured TCP Sockets</title>
2727 The nonce-tcp transport provides a secured TCP transport, using a
2728 simple authentication mechanism to ensure that only clients with read
2729 access to a certain location in the filesystem can connect to the server.
2730 The server writes a secret, the nonce, to a file and an incoming client
2731 connection is only accepted if the client sends the nonce right after
2732 the connect. The nonce mechanism requires no setup and is orthogonal to
2733 the higher-level authentication mechanisms described in the
2734 Authentication section.
2738 On start, the server generates a random 16 byte nonce and writes it
2739 to a file in the user's temporary directory. The nonce file location
2740 is published as part of the server's D-Bus address using the
2741 "noncefile" key-value pair.
2743 After an accept, the server reads 16 bytes from the socket. If the
2744 read bytes do not match the nonce stored in the nonce file, the
2745 server MUST immediately drop the connection.
2746 If the nonce match the received byte sequence, the client is accepted
2747 and the transport behaves like an unsecured tcp transport.
2750 After a successful connect to the server socket, the client MUST read
2751 the nonce from the file published by the server via the noncefile=
2752 key-value pair and send it over the socket. After that, the
2753 transport behaves like an unsecured tcp transport.
2755 <sect3 id="transports-nonce-tcp-sockets-addresses">
2756 <title>Server Address Format</title>
2758 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2759 and support the following key/value pairs:
2766 <entry>Values</entry>
2767 <entry>Description</entry>
2773 <entry>(string)</entry>
2774 <entry>dns name or ip address</entry>
2778 <entry>(number)</entry>
2779 <entry>The tcp port the server will open. A zero value let the server
2780 choose a free port provided from the underlaying operating system.
2781 libdbus is able to retrieve the real used port from the server.
2785 <entry>family</entry>
2786 <entry>(string)</entry>
2787 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2790 <entry>noncefile</entry>
2791 <entry>(path)</entry>
2792 <entry>file location containing the secret</entry>
2800 <sect1 id="meta-transports">
2801 <title>Meta Transports</title>
2803 Meta transports are a kind of transport with special enhancements or
2804 behavior. Currently available meta transports include: autolaunch
2807 <sect2 id="meta-transports-autolaunch">
2808 <title>Autolaunch</title>
2809 <para>The autolaunch transport provides a way for dbus clients to autodetect
2810 a running dbus session bus and to autolaunch a session bus if not present.
2812 <sect3 id="meta-transports-autolaunch-addresses">
2813 <title>Server Address Format</title>
2815 Autolaunch addresses uses the "autolaunch:" prefix and support the
2816 following key/value pairs:
2823 <entry>Values</entry>
2824 <entry>Description</entry>
2829 <entry>scope</entry>
2830 <entry>(string)</entry>
2831 <entry>scope of autolaunch (Windows only)
2835 "*install-path" - limit session bus to dbus installation path.
2836 The dbus installation path is determined from the location of
2837 the shared dbus library. If the library is located in a 'bin'
2838 subdirectory the installation root is the directory above,
2839 otherwise the directory where the library lives is taken as
2842 <install-root>/bin/[lib]dbus-1.dll
2843 <install-root>/[lib]dbus-1.dll
2849 "*user" - limit session bus to the recent user.
2854 other values - specify dedicated session bus like "release",
2866 <sect3 id="meta-transports-autolaunch-windows-implementation">
2867 <title>Windows implementation</title>
2869 On start, the server opens a platform specific transport, creates a mutex
2870 and a shared memory section containing the related session bus address.
2871 This mutex will be inspected by the dbus client library to detect a
2872 running dbus session bus. The access to the mutex and the shared memory
2873 section are protected by global locks.
2876 In the recent implementation the autolaunch transport uses a tcp transport
2877 on localhost with a port choosen from the operating system. This detail may
2878 change in the future.
2881 Disclaimer: The recent implementation is in an early state and may not
2882 work in all cirumstances and/or may have security issues. Because of this
2883 the implementation is not documentated yet.
2888 <sect1 id="naming-conventions">
2889 <title>Naming Conventions</title>
2892 D-Bus namespaces are all lowercase and correspond to reversed domain
2893 names, as with Java. e.g. "org.freedesktop"
2896 Interface, signal, method, and property names are "WindowsStyleCaps", note
2897 that the first letter is capitalized, unlike Java.
2900 Object paths are normally all lowercase with underscores used rather than
2906 <title>UUIDs</title>
2908 A working D-Bus implementation uses universally-unique IDs in two places.
2909 First, each server address has a UUID identifying the address,
2910 as described in <xref linkend="addresses"/>. Second, each operating
2911 system kernel instance running a D-Bus client or server has a UUID
2912 identifying that kernel, retrieved by invoking the method
2913 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2914 linkend="standard-interfaces-peer"/>).
2917 The term "UUID" in this document is intended literally, i.e. an
2918 identifier that is universally unique. It is not intended to refer to
2919 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2922 The UUID must contain 128 bits of data and be hex-encoded. The
2923 hex-encoded string may not contain hyphens or other non-hex-digit
2924 characters, and it must be exactly 32 characters long. To generate a
2925 UUID, the current reference implementation concatenates 96 bits of random
2926 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2930 It would also be acceptable and probably better to simply generate 128
2931 bits of random data, as long as the random number generator is of high
2932 quality. The timestamp could conceivably help if the random bits are not
2933 very random. With a quality random number generator, collisions are
2934 extremely unlikely even with only 96 bits, so it's somewhat academic.
2937 Implementations should, however, stick to random data for the first 96 bits
2942 <sect1 id="standard-interfaces">
2943 <title>Standard Interfaces</title>
2945 See <xref linkend="message-protocol-types-notation"/> for details on
2946 the notation used in this section. There are some standard interfaces
2947 that may be useful across various D-Bus applications.
2949 <sect2 id="standard-interfaces-peer">
2950 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2952 The <literal>org.freedesktop.DBus.Peer</literal> interface
2955 org.freedesktop.DBus.Peer.Ping ()
2956 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2960 On receipt of the <literal>METHOD_CALL</literal> message
2961 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2962 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2963 usual. It does not matter which object path a ping is sent to. The
2964 reference implementation handles this method automatically.
2967 On receipt of the <literal>METHOD_CALL</literal> message
2968 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2969 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2970 UUID representing the identity of the machine the process is running on.
2971 This UUID must be the same for all processes on a single system at least
2972 until that system next reboots. It should be the same across reboots
2973 if possible, but this is not always possible to implement and is not
2975 It does not matter which object path a GetMachineId is sent to. The
2976 reference implementation handles this method automatically.
2979 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2980 a virtual machine running on a hypervisor, rather than a physical machine.
2981 Basically if two processes see the same UUID, they should also see the same
2982 shared memory, UNIX domain sockets, process IDs, and other features that require
2983 a running OS kernel in common between the processes.
2986 The UUID is often used where other programs might use a hostname. Hostnames
2987 can change without rebooting, however, or just be "localhost" - so the UUID
2991 <xref linkend="uuids"/> explains the format of the UUID.
2995 <sect2 id="standard-interfaces-introspectable">
2996 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2998 This interface has one method:
3000 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3004 Objects instances may implement
3005 <literal>Introspect</literal> which returns an XML description of
3006 the object, including its interfaces (with signals and methods), objects
3007 below it in the object path tree, and its properties.
3010 <xref linkend="introspection-format"/> describes the format of this XML string.
3013 <sect2 id="standard-interfaces-properties">
3014 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3016 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3017 or <firstterm>attributes</firstterm>. These can be exposed via the
3018 <literal>org.freedesktop.DBus.Properties</literal> interface.
3022 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3023 in STRING property_name,
3025 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3026 in STRING property_name,
3028 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3029 out DICT<STRING,VARIANT> props);
3033 The available properties and whether they are writable can be determined
3034 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3035 see <xref linkend="standard-interfaces-introspectable"/>.
3038 An empty string may be provided for the interface name; in this case,
3039 if there are multiple properties on an object with the same name,
3040 the results are undefined (picking one by according to an arbitrary
3041 deterministic rule, or returning an error, are the reasonable
3045 If one or more properties change on an object, the
3046 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3047 signal may be emitted (this signal was added in 0.14):
3051 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3052 DICT<STRING,VARIANT> changed_properties,
3053 ARRAY<STRING> invalidated_properties);
3057 where <literal>changed_properties</literal> is a dictionary
3058 containing the changed properties with the new values and
3059 <literal>invalidated_properties</literal> is an array of
3060 properties that changed but the value is not conveyed.
3063 Whether the <literal>PropertiesChanged</literal> signal is
3064 supported can be determined by calling
3065 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3066 that the signal may be supported for an object but it may
3067 differ how whether and how it is used on a per-property basis
3068 (for e.g. performance or security reasons). Each property (or
3069 the parent interface) must be annotated with the
3070 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3071 annotation to convey this (usually the default value
3072 <literal>true</literal> is sufficient meaning that the
3073 annotation does not need to be used). See <xref
3074 linkend="introspection-format"/> for details on this
3079 <sect2 id="standard-interfaces-objectmanager">
3080 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3082 An API can optionally make use of this interface for one or
3083 more sub-trees of objects. The root of each sub-tree implements
3084 this interface so other applications can get all objects,
3085 interfaces and properties in a single method call. It is
3086 appropriate to use this interface if users of the tree of
3087 objects are expected to be interested in all interfaces of all
3088 objects in the tree; a more granular API should be used if
3089 users of the objects are expected to be interested in a small
3090 subset of the objects, a small subset of their interfaces, or
3094 The method that applications can use to get all objects and
3095 properties is <literal>GetManagedObjects</literal>:
3099 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3103 The return value of this method is a dict whose keys are
3104 object paths. All returned object paths are children of the
3105 object path implementing this interface, i.e. their object
3106 paths start with the ObjectManager's object path plus '/'.
3109 Each value is a dict whose keys are interfaces names. Each
3110 value in this inner dict is the same dict that would be
3111 returned by the <link
3112 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3113 method for that combination of object path and interface. If
3114 an interface has no properties, the empty dict is returned.
3117 Changes are emitted using the following two signals:
3121 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3122 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3123 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3124 ARRAY<STRING> interfaces);
3128 The <literal>InterfacesAdded</literal> signal is emitted when
3129 either a new object is added or when an existing object gains
3130 one or more interfaces. The
3131 <literal>InterfacesRemoved</literal> signal is emitted
3132 whenever an object is removed or it loses one or more
3133 interfaces. The second parameter of the
3134 <literal>InterfacesAdded</literal> signal contains a dict with
3135 the interfaces and properties (if any) that have been added to
3136 the given object path. Similarly, the second parameter of the
3137 <literal>InterfacesRemoved</literal> signal contains an array
3138 of the interfaces that were removed. Note that changes on
3139 properties on existing interfaces are not reported using this
3140 interface - an application should also monitor the existing <link
3141 linkend="standard-interfaces-properties">PropertiesChanged</link>
3142 signal on each object.
3145 Applications SHOULD NOT export objects that are children of an
3146 object (directly or otherwise) implementing this interface but
3147 which are not returned in the reply from the
3148 <literal>GetManagedObjects()</literal> method of this
3149 interface on the given object.
3152 The intent of the <literal>ObjectManager</literal> interface
3153 is to make it easy to write a robust client
3154 implementation. The trivial client implementation only needs
3155 to make two method calls:
3159 org.freedesktop.DBus.AddMatch (bus_proxy,
3160 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3161 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3165 on the message bus and the remote application's
3166 <literal>ObjectManager</literal>, respectively. Whenever a new
3167 remote object is created (or an existing object gains a new
3168 interface), the <literal>InterfacesAdded</literal> signal is
3169 emitted, and since this signal contains all properties for the
3170 interfaces, no calls to the
3171 <literal>org.freedesktop.Properties</literal> interface on the
3172 remote object are needed. Additionally, since the initial
3173 <literal>AddMatch()</literal> rule already includes signal
3174 messages from the newly created child object, no new
3175 <literal>AddMatch()</literal> call is needed.
3180 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3181 interface was added in version 0.17 of the D-Bus
3188 <sect1 id="introspection-format">
3189 <title>Introspection Data Format</title>
3191 As described in <xref linkend="standard-interfaces-introspectable"/>,
3192 objects may be introspected at runtime, returning an XML string
3193 that describes the object. The same XML format may be used in
3194 other contexts as well, for example as an "IDL" for generating
3195 static language bindings.
3198 Here is an example of introspection data:
3200 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3201 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3202 <node name="/org/freedesktop/sample_object">
3203 <interface name="org.freedesktop.SampleInterface">
3204 <method name="Frobate">
3205 <arg name="foo" type="i" direction="in"/>
3206 <arg name="bar" type="s" direction="out"/>
3207 <arg name="baz" type="a{us}" direction="out"/>
3208 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3210 <method name="Bazify">
3211 <arg name="bar" type="(iiu)" direction="in"/>
3212 <arg name="bar" type="v" direction="out"/>
3214 <method name="Mogrify">
3215 <arg name="bar" type="(iiav)" direction="in"/>
3217 <signal name="Changed">
3218 <arg name="new_value" type="b"/>
3220 <property name="Bar" type="y" access="readwrite"/>
3222 <node name="child_of_sample_object"/>
3223 <node name="another_child_of_sample_object"/>
3228 A more formal DTD and spec needs writing, but here are some quick notes.
3232 Only the root <node> element can omit the node name, as it's
3233 known to be the object that was introspected. If the root
3234 <node> does have a name attribute, it must be an absolute
3235 object path. If child <node> have object paths, they must be
3241 If a child <node> has any sub-elements, then they
3242 must represent a complete introspection of the child.
3243 If a child <node> is empty, then it may or may
3244 not have sub-elements; the child must be introspected
3245 in order to find out. The intent is that if an object
3246 knows that its children are "fast" to introspect
3247 it can go ahead and return their information, but
3248 otherwise it can omit it.
3253 The direction element on <arg> may be omitted,
3254 in which case it defaults to "in" for method calls
3255 and "out" for signals. Signals only allow "out"
3256 so while direction may be specified, it's pointless.
3261 The possible directions are "in" and "out",
3262 unlike CORBA there is no "inout"
3267 The possible property access flags are
3268 "readwrite", "read", and "write"
3273 Multiple interfaces can of course be listed for
3279 The "name" attribute on arguments is optional.
3285 Method, interface, property, and signal elements may have
3286 "annotations", which are generic key/value pairs of metadata.
3287 They are similar conceptually to Java's annotations and C# attributes.
3288 Well-known annotations:
3295 <entry>Values (separated by ,)</entry>
3296 <entry>Description</entry>
3301 <entry>org.freedesktop.DBus.Deprecated</entry>
3302 <entry>true,false</entry>
3303 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3306 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3307 <entry>(string)</entry>
3308 <entry>The C symbol; may be used for methods and interfaces</entry>
3311 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3312 <entry>true,false</entry>
3313 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3316 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3317 <entry>true,invalidates,false</entry>
3320 If set to <literal>false</literal>, the
3321 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3323 linkend="standard-interfaces-properties"/> is not
3324 guaranteed to be emitted if the property changes.
3327 If set to <literal>invalidates</literal> the signal
3328 is emitted but the value is not included in the
3332 If set to <literal>true</literal> the signal is
3333 emitted with the value included.
3336 The value for the annotation defaults to
3337 <literal>true</literal> if the enclosing interface
3338 element does not specify the annotation. Otherwise it
3339 defaults to the value specified in the enclosing
3348 <sect1 id="message-bus">
3349 <title>Message Bus Specification</title>
3350 <sect2 id="message-bus-overview">
3351 <title>Message Bus Overview</title>
3353 The message bus accepts connections from one or more applications.
3354 Once connected, applications can exchange messages with other
3355 applications that are also connected to the bus.
3358 In order to route messages among connections, the message bus keeps a
3359 mapping from names to connections. Each connection has one
3360 unique-for-the-lifetime-of-the-bus name automatically assigned.
3361 Applications may request additional names for a connection. Additional
3362 names are usually "well-known names" such as
3363 "org.freedesktop.TextEditor". When a name is bound to a connection,
3364 that connection is said to <firstterm>own</firstterm> the name.
3367 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3368 This name routes messages to the bus, allowing applications to make
3369 administrative requests. For example, applications can ask the bus
3370 to assign a name to a connection.
3373 Each name may have <firstterm>queued owners</firstterm>. When an
3374 application requests a name for a connection and the name is already in
3375 use, the bus will optionally add the connection to a queue waiting for
3376 the name. If the current owner of the name disconnects or releases
3377 the name, the next connection in the queue will become the new owner.
3381 This feature causes the right thing to happen if you start two text
3382 editors for example; the first one may request "org.freedesktop.TextEditor",
3383 and the second will be queued as a possible owner of that name. When
3384 the first exits, the second will take over.
3388 Applications may send <firstterm>unicast messages</firstterm> to
3389 a specific recipient or to the message bus itself, or
3390 <firstterm>broadcast messages</firstterm> to all interested recipients.
3391 See <xref linkend="message-bus-routing"/> for details.
3395 <sect2 id="message-bus-names">
3396 <title>Message Bus Names</title>
3398 Each connection has at least one name, assigned at connection time and
3399 returned in response to the
3400 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3401 automatically-assigned name is called the connection's <firstterm>unique
3402 name</firstterm>. Unique names are never reused for two different
3403 connections to the same bus.
3406 Ownership of a unique name is a prerequisite for interaction with
3407 the message bus. It logically follows that the unique name is always
3408 the first name that an application comes to own, and the last
3409 one that it loses ownership of.
3412 Unique connection names must begin with the character ':' (ASCII colon
3413 character); bus names that are not unique names must not begin
3414 with this character. (The bus must reject any attempt by an application
3415 to manually request a name beginning with ':'.) This restriction
3416 categorically prevents "spoofing"; messages sent to a unique name
3417 will always go to the expected connection.
3420 When a connection is closed, all the names that it owns are deleted (or
3421 transferred to the next connection in the queue if any).
3424 A connection can request additional names to be associated with it using
3425 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3426 linkend="message-protocol-names-bus"/> describes the format of a valid
3427 name. These names can be released again using the
3428 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3431 <sect3 id="bus-messages-request-name">
3432 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3436 UINT32 RequestName (in STRING name, in UINT32 flags)
3443 <entry>Argument</entry>
3445 <entry>Description</entry>
3451 <entry>STRING</entry>
3452 <entry>Name to request</entry>
3456 <entry>UINT32</entry>
3457 <entry>Flags</entry>
3467 <entry>Argument</entry>
3469 <entry>Description</entry>
3475 <entry>UINT32</entry>
3476 <entry>Return value</entry>
3483 This method call should be sent to
3484 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3485 assign the given name to the method caller. Each name maintains a
3486 queue of possible owners, where the head of the queue is the primary
3487 or current owner of the name. Each potential owner in the queue
3488 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3489 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3490 call. When RequestName is invoked the following occurs:
3494 If the method caller is currently the primary owner of the name,
3495 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3496 values are updated with the values from the new RequestName call,
3497 and nothing further happens.
3503 If the current primary owner (head of the queue) has
3504 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3505 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3506 the caller of RequestName replaces the current primary owner at
3507 the head of the queue and the current primary owner moves to the
3508 second position in the queue. If the caller of RequestName was
3509 in the queue previously its flags are updated with the values from
3510 the new RequestName in addition to moving it to the head of the queue.
3516 If replacement is not possible, and the method caller is
3517 currently in the queue but not the primary owner, its flags are
3518 updated with the values from the new RequestName call.
3524 If replacement is not possible, and the method caller is
3525 currently not in the queue, the method caller is appended to the
3532 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3533 set and is not the primary owner, it is removed from the
3534 queue. This can apply to the previous primary owner (if it
3535 was replaced) or the method caller (if it updated the
3536 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3537 queue, or if it was just added to the queue with that flag set).
3543 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3544 queue," even if another application already in the queue had specified
3545 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3546 that does not allow replacement goes away, and the next primary owner
3547 does allow replacement. In this case, queued items that specified
3548 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3549 automatically replace the new primary owner. In other words,
3550 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3551 time RequestName is called. This is deliberate to avoid an infinite loop
3552 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3553 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3556 The flags argument contains any of the following values logically ORed
3563 <entry>Conventional Name</entry>
3564 <entry>Value</entry>
3565 <entry>Description</entry>
3570 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3574 If an application A specifies this flag and succeeds in
3575 becoming the owner of the name, and another application B
3576 later calls RequestName with the
3577 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3578 will lose ownership and receive a
3579 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3580 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3581 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3582 is not specified by application B, then application B will not replace
3583 application A as the owner.
3588 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3592 Try to replace the current owner if there is one. If this
3593 flag is not set the application will only become the owner of
3594 the name if there is no current owner. If this flag is set,
3595 the application will replace the current owner if
3596 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3601 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3605 Without this flag, if an application requests a name that is
3606 already owned, the application will be placed in a queue to
3607 own the name when the current owner gives it up. If this
3608 flag is given, the application will not be placed in the
3609 queue, the request for the name will simply fail. This flag
3610 also affects behavior when an application is replaced as
3611 name owner; by default the application moves back into the
3612 waiting queue, unless this flag was provided when the application
3613 became the name owner.
3621 The return code can be one of the following values:
3627 <entry>Conventional Name</entry>
3628 <entry>Value</entry>
3629 <entry>Description</entry>
3634 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3635 <entry>1</entry> <entry>The caller is now the primary owner of
3636 the name, replacing any previous owner. Either the name had no
3637 owner before, or the caller specified
3638 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3639 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3642 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3645 <entry>The name already had an owner,
3646 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3647 the current owner did not specify
3648 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3649 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3653 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3654 <entry>The name already has an owner,
3655 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3656 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3657 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3658 specified by the requesting application.</entry>
3661 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3663 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3671 <sect3 id="bus-messages-release-name">
3672 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3676 UINT32 ReleaseName (in STRING name)
3683 <entry>Argument</entry>
3685 <entry>Description</entry>
3691 <entry>STRING</entry>
3692 <entry>Name to release</entry>
3702 <entry>Argument</entry>
3704 <entry>Description</entry>
3710 <entry>UINT32</entry>
3711 <entry>Return value</entry>
3718 This method call should be sent to
3719 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3720 release the method caller's claim to the given name. If the caller is
3721 the primary owner, a new primary owner will be selected from the
3722 queue if any other owners are waiting. If the caller is waiting in
3723 the queue for the name, the caller will removed from the queue and
3724 will not be made an owner of the name if it later becomes available.
3725 If there are no other owners in the queue for the name, it will be
3726 removed from the bus entirely.
3728 The return code can be one of the following values:
3734 <entry>Conventional Name</entry>
3735 <entry>Value</entry>
3736 <entry>Description</entry>
3741 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3742 <entry>1</entry> <entry>The caller has released his claim on
3743 the given name. Either the caller was the primary owner of
3744 the name, and the name is now unused or taken by somebody
3745 waiting in the queue for the name, or the caller was waiting
3746 in the queue for the name and has now been removed from the
3750 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3752 <entry>The given name does not exist on this bus.</entry>
3755 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3757 <entry>The caller was not the primary owner of this name,
3758 and was also not waiting in the queue to own this name.</entry>
3766 <sect3 id="bus-messages-list-queued-owners">
3767 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3771 ARRAY of STRING ListQueuedOwners (in STRING name)
3778 <entry>Argument</entry>
3780 <entry>Description</entry>
3786 <entry>STRING</entry>
3787 <entry>The well-known bus name to query, such as
3788 <literal>com.example.cappuccino</literal></entry>
3798 <entry>Argument</entry>
3800 <entry>Description</entry>
3806 <entry>ARRAY of STRING</entry>
3807 <entry>The unique bus names of connections currently queued
3808 for the name</entry>
3815 This method call should be sent to
3816 <literal>org.freedesktop.DBus</literal> and lists the connections
3817 currently queued for a bus name (see
3818 <xref linkend="term-queued-owner"/>).
3823 <sect2 id="message-bus-routing">
3824 <title>Message Bus Message Routing</title>
3827 Messages may have a <literal>DESTINATION</literal> field (see <xref
3828 linkend="message-protocol-header-fields"/>), resulting in a
3829 <firstterm>unicast message</firstterm>. If the
3830 <literal>DESTINATION</literal> field is present, it specifies a message
3831 recipient by name. Method calls and replies normally specify this field.
3832 The message bus must send messages (of any type) with the
3833 <literal>DESTINATION</literal> field set to the specified recipient,
3834 regardless of whether the recipient has set up a match rule matching
3839 When the message bus receives a signal, if the
3840 <literal>DESTINATION</literal> field is absent, it is considered to
3841 be a <firstterm>broadcast signal</firstterm>, and is sent to all
3842 applications with <firstterm>message matching rules</firstterm> that
3843 match the message. Most signal messages are broadcasts.
3847 Unicast signal messages (those with a <literal>DESTINATION</literal>
3848 field) are not commonly used, but they are treated like any unicast
3849 message: they are delivered to the specified receipient,
3850 regardless of its match rules. One use for unicast signals is to
3851 avoid a race condition in which a signal is emitted before the intended
3852 recipient can call <xref linkend="bus-messages-add-match"/> to
3853 receive that signal: if the signal is sent directly to that recipient
3854 using a unicast message, it does not need to add a match rule at all,
3855 and there is no race condition. Another use for unicast signals,
3856 on message buses whose security policy prevents eavesdropping, is to
3857 send sensitive information which should only be visible to one
3862 When the message bus receives a method call, if the
3863 <literal>DESTINATION</literal> field is absent, the call is taken to be
3864 a standard one-to-one message and interpreted by the message bus
3865 itself. For example, sending an
3866 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3867 <literal>DESTINATION</literal> will cause the message bus itself to
3868 reply to the ping immediately; the message bus will not make this
3869 message visible to other applications.
3873 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3874 the ping message were sent with a <literal>DESTINATION</literal> name of
3875 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3876 forwarded, and the Yoyodyne Corporation screensaver application would be
3877 expected to reply to the ping.
3881 Message bus implementations may impose a security policy which
3882 prevents certain messages from being sent or received.
3883 When a message cannot be sent or received due to a security
3884 policy, the message bus should send an error reply, unless the
3885 original message had the <literal>NO_REPLY</literal> flag.
3888 <sect3 id="message-bus-routing-eavesdropping">
3889 <title>Eavesdropping</title>
3891 Receiving a unicast message whose <literal>DESTINATION</literal>
3892 indicates a different recipient is called
3893 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
3894 a security boundary (like the standard system bus), the security
3895 policy should usually prevent eavesdropping, since unicast messages
3896 are normally kept private and may contain security-sensitive
3901 Eavesdropping is mainly useful for debugging tools, such as
3902 the <literal>dbus-monitor</literal> tool in the reference
3903 implementation of D-Bus. Tools which eavesdrop on the message bus
3904 should be careful to avoid sending a reply or error in response to
3905 messages intended for a different client.
3909 Clients may attempt to eavesdrop by adding match rules
3910 (see <xref linkend="message-bus-routing-match-rules"/>) containing
3911 the <literal>eavesdrop='true'</literal> match. If the message bus'
3912 security policy does not allow eavesdropping, the match rule can
3913 still be added, but will not have any practical effect. For
3914 compatibility with older message bus implementations, if adding such
3915 a match rule results in an error reply, the client may fall back to
3916 adding the same rule with the <literal>eavesdrop</literal> match
3921 <sect3 id="message-bus-routing-match-rules">
3922 <title>Match Rules</title>
3924 An important part of the message bus routing protocol is match
3925 rules. Match rules describe the messages that should be sent to a
3926 client, based on the contents of the message. Broadcast signals
3927 are only sent to clients which have a suitable match rule: this
3928 avoids waking up client processes to deal with signals that are
3929 not relevant to that client.
3932 Messages that list a client as their <literal>DESTINATION</literal>
3933 do not need to match the client's match rules, and are sent to that
3934 client regardless. As a result, match rules are mainly used to
3935 receive a subset of broadcast signals.
3938 Match rules can also be used for eavesdropping
3939 (see <xref linkend="message-bus-routing-eavesdropping"/>),
3940 if the security policy of the message bus allows it.
3943 Match rules are added using the AddMatch bus method
3944 (see <xref linkend="bus-messages-add-match"/>). Rules are
3945 specified as a string of comma separated key/value pairs.
3946 Excluding a key from the rule indicates a wildcard match.
3947 For instance excluding the the member from a match rule but
3948 adding a sender would let all messages from that sender through.
3949 An example of a complete rule would be
3950 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3953 The following table describes the keys that can be used to create
3955 The following table summarizes the D-Bus types.
3961 <entry>Possible Values</entry>
3962 <entry>Description</entry>
3967 <entry><literal>type</literal></entry>
3968 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3969 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3972 <entry><literal>sender</literal></entry>
3973 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3974 and <xref linkend="term-unique-name"/> respectively)
3976 <entry>Match messages sent by a particular sender. An example of a sender match
3977 is sender='org.freedesktop.Hal'</entry>
3980 <entry><literal>interface</literal></entry>
3981 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3982 <entry>Match messages sent over or to a particular interface. An example of an
3983 interface match is interface='org.freedesktop.Hal.Manager'.
3984 If a message omits the interface header, it must not match any rule
3985 that specifies this key.</entry>
3988 <entry><literal>member</literal></entry>
3989 <entry>Any valid method or signal name</entry>
3990 <entry>Matches messages which have the give method or signal name. An example of
3991 a member match is member='NameOwnerChanged'</entry>
3994 <entry><literal>path</literal></entry>
3995 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3996 <entry>Matches messages which are sent from or to the given object. An example of a
3997 path match is path='/org/freedesktop/Hal/Manager'</entry>
4000 <entry><literal>path_namespace</literal></entry>
4001 <entry>An object path</entry>
4004 Matches messages which are sent from or to an
4005 object for which the object path is either the
4006 given value, or that value followed by one or
4007 more path components.
4012 <literal>path_namespace='/com/example/foo'</literal>
4013 would match signals sent by
4014 <literal>/com/example/foo</literal>
4016 <literal>/com/example/foo/bar</literal>,
4018 <literal>/com/example/foobar</literal>.
4022 Using both <literal>path</literal> and
4023 <literal>path_namespace</literal> in the same match
4024 rule is not allowed.
4029 This match key was added in version 0.16 of the
4030 D-Bus specification and implemented by the bus
4031 daemon in dbus 1.5.0 and later.
4037 <entry><literal>destination</literal></entry>
4038 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4039 <entry>Matches messages which are being sent to the given unique name. An
4040 example of a destination match is destination=':1.0'</entry>
4043 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4044 <entry>Any string</entry>
4045 <entry>Arg matches are special and are used for further restricting the
4046 match based on the arguments in the body of a message. Only arguments of type
4047 STRING can be matched in this way. An example of an argument match
4048 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4052 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4053 <entry>Any string</entry>
4055 <para>Argument path matches provide a specialised form of wildcard matching for
4056 path-like namespaces. They can match arguments whose type is either STRING or
4057 OBJECT_PATH. As with normal argument matches,
4058 if the argument is exactly equal to the string given in the match
4059 rule then the rule is satisfied. Additionally, there is also a
4060 match when either the string given in the match rule or the
4061 appropriate message argument ends with '/' and is a prefix of the
4062 other. An example argument path match is arg0path='/aa/bb/'. This
4063 would match messages with first arguments of '/', '/aa/',
4064 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4065 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4067 <para>This is intended for monitoring “directories” in file system-like
4068 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4069 system. An application interested in all nodes in a particular hierarchy would
4070 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4071 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4072 represent a modification to the “bar” property, or a signal with zeroth
4073 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4074 many properties within that directory, and the interested application would be
4075 notified in both cases.</para>
4078 This match key was added in version 0.12 of the
4079 D-Bus specification, implemented for STRING
4080 arguments by the bus daemon in dbus 1.2.0 and later,
4081 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4088 <entry><literal>arg0namespace</literal></entry>
4089 <entry>Like a bus name, except that the string is not
4090 required to contain a '.' (period)</entry>
4092 <para>Match messages whose first argument is of type STRING, and is a bus name
4093 or interface name within the specified namespace. This is primarily intended
4094 for watching name owner changes for a group of related bus names, rather than
4095 for a single name or all name changes.</para>
4097 <para>Because every valid interface name is also a valid
4098 bus name, this can also be used for messages whose
4099 first argument is an interface name.</para>
4101 <para>For example, the match rule
4102 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4103 matches name owner changes for bus names such as
4104 <literal>com.example.backend.foo</literal>,
4105 <literal>com.example.backend.foo.bar</literal>, and
4106 <literal>com.example.backend</literal> itself.</para>
4108 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4111 This match key was added in version 0.16 of the
4112 D-Bus specification and implemented by the bus
4113 daemon in dbus 1.5.0 and later.
4119 <entry><literal>eavesdrop</literal></entry>
4120 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4121 <entry>Since D-Bus 1.5.UNRELEASED, match rules do not
4122 match messages which have a <literal>DESTINATION</literal>
4123 field unless the match rule specifically
4125 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4126 by specifying <literal>eavesdrop='true'</literal>
4127 in the match rule. <literal>eavesdrop='false'</literal>
4128 restores the default behaviour. Messages are
4129 delivered to their <literal>DESTINATION</literal>
4130 regardless of match rules, so this match does not
4131 affect normal delivery of unicast messages.
4132 If the message bus has a security policy which forbids
4133 eavesdropping, this match may still be used without error,
4134 but will not have any practical effect.
4135 In older versions of D-Bus, this match was not allowed
4136 in match rules, and all match rules behaved as if
4137 <literal>eavesdrop='true'</literal> had been used.
4146 <sect2 id="message-bus-starting-services">
4147 <title>Message Bus Starting Services</title>
4149 The message bus can start applications on behalf of other applications.
4150 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4151 An application that can be started in this way is called a
4152 <firstterm>service</firstterm>.
4155 With D-Bus, starting a service is normally done by name. That is,
4156 applications ask the message bus to start some program that will own a
4157 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4158 This implies a contract documented along with the name
4159 <literal>org.freedesktop.TextEditor</literal> for which objects
4160 the owner of that name will provide, and what interfaces those
4164 To find an executable corresponding to a particular name, the bus daemon
4165 looks for <firstterm>service description files</firstterm>. Service
4166 description files define a mapping from names to executables. Different
4167 kinds of message bus will look for these files in different places, see
4168 <xref linkend="message-bus-types"/>.
4171 Service description files have the ".service" file
4172 extension. The message bus will only load service description files
4173 ending with .service; all other files will be ignored. The file format
4174 is similar to that of <ulink
4175 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4176 entries</ulink>. All service description files must be in UTF-8
4177 encoding. To ensure that there will be no name collisions, service files
4178 must be namespaced using the same mechanism as messages and service
4183 [FIXME the file format should be much better specified than "similar to
4184 .desktop entries" esp. since desktop entries are already
4185 badly-specified. ;-)]
4186 These sections from the specification apply to service files as well:
4189 <listitem><para>General syntax</para></listitem>
4190 <listitem><para>Comment format</para></listitem>
4194 <title>Example service description file</title>
4196 # Sample service description file
4198 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4199 Exec=/usr/libexec/gconfd-2
4204 When an application asks to start a service by name, the bus daemon tries to
4205 find a service that will own that name. It then tries to spawn the
4206 executable associated with it. If this fails, it will report an
4207 error. [FIXME what happens if two .service files offer the same service;
4208 what kind of error is reported, should we have a way for the client to
4212 The executable launched will have the environment variable
4213 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4214 message bus so it can connect and request the appropriate names.
4217 The executable being launched may want to know whether the message bus
4218 starting it is one of the well-known message buses (see <xref
4219 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4220 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4221 of the well-known buses. The currently-defined values for this variable
4222 are <literal>system</literal> for the systemwide message bus,
4223 and <literal>session</literal> for the per-login-session message
4224 bus. The new executable must still connect to the address given
4225 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4226 resulting connection is to the well-known bus.
4229 [FIXME there should be a timeout somewhere, either specified
4230 in the .service file, by the client, or just a global value
4231 and if the client being activated fails to connect within that
4232 timeout, an error should be sent back.]
4235 <sect3 id="message-bus-starting-services-scope">
4236 <title>Message Bus Service Scope</title>
4238 The "scope" of a service is its "per-", such as per-session,
4239 per-machine, per-home-directory, or per-display. The reference
4240 implementation doesn't yet support starting services in a different
4241 scope from the message bus itself. So e.g. if you start a service
4242 on the session bus its scope is per-session.
4245 We could add an optional scope to a bus name. For example, for
4246 per-(display,session pair), we could have a unique ID for each display
4247 generated automatically at login and set on screen 0 by executing a
4248 special "set display ID" binary. The ID would be stored in a
4249 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4250 random bytes. This ID would then be used to scope names.
4251 Starting/locating a service could be done by ID-name pair rather than
4255 Contrast this with a per-display scope. To achieve that, we would
4256 want a single bus spanning all sessions using a given display.
4257 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4258 property on screen 0 of the display, pointing to this bus.
4263 <sect2 id="message-bus-types">
4264 <title>Well-known Message Bus Instances</title>
4266 Two standard message bus instances are defined here, along with how
4267 to locate them and where their service files live.
4269 <sect3 id="message-bus-types-login">
4270 <title>Login session message bus</title>
4272 Each time a user logs in, a <firstterm>login session message
4273 bus</firstterm> may be started. All applications in the user's login
4274 session may interact with one another using this message bus.
4277 The address of the login session message bus is given
4278 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4279 variable. If that variable is not set, applications may
4280 also try to read the address from the X Window System root
4281 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4282 The root window property must have type <literal>STRING</literal>.
4283 The environment variable should have precedence over the
4284 root window property.
4286 <para>The address of the login session message bus is given in the
4287 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4288 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4289 "autolaunch:", the system should use platform-specific methods of
4290 locating a running D-Bus session server, or starting one if a running
4291 instance cannot be found. Note that this mechanism is not recommended
4292 for attempting to determine if a daemon is running. It is inherently
4293 racy to attempt to make this determination, since the bus daemon may
4294 be started just before or just after the determination is made.
4295 Therefore, it is recommended that applications do not try to make this
4296 determination for their functionality purposes, and instead they
4297 should attempt to start the server.</para>
4299 <sect4 id="message-bus-types-login-x-windows">
4300 <title>X Windowing System</title>
4302 For the X Windowing System, the application must locate the
4303 window owner of the selection represented by the atom formed by
4307 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4311 <para>the current user's username</para>
4315 <para>the literal character '_' (underscore)</para>
4319 <para>the machine's ID</para>
4325 The following properties are defined for the window that owns
4327 <informaltable frame="all">
4336 <para>meaning</para>
4342 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4346 <para>the actual address of the server socket</para>
4352 <para>_DBUS_SESSION_BUS_PID</para>
4356 <para>the PID of the server process</para>
4365 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4366 present in this window.
4370 If the X selection cannot be located or if reading the
4371 properties from the window fails, the implementation MUST conclude
4372 that there is no D-Bus server running and proceed to start a new
4373 server. (See below on concurrency issues)
4377 Failure to connect to the D-Bus server address thus obtained
4378 MUST be treated as a fatal connection error and should be reported
4383 As an alternative, an implementation MAY find the information
4384 in the following file located in the current user's home directory,
4385 in subdirectory .dbus/session-bus/:
4388 <para>the machine's ID</para>
4392 <para>the literal character '-' (dash)</para>
4396 <para>the X display without the screen number, with the
4397 following prefixes removed, if present: ":", "localhost:"
4398 ."localhost.localdomain:". That is, a display of
4399 "localhost:10.0" produces just the number "10"</para>
4405 The contents of this file NAME=value assignment pairs and
4406 lines starting with # are comments (no comments are allowed
4407 otherwise). The following variable names are defined:
4414 <para>Variable</para>
4418 <para>meaning</para>
4424 <para>DBUS_SESSION_BUS_ADDRESS</para>
4428 <para>the actual address of the server socket</para>
4434 <para>DBUS_SESSION_BUS_PID</para>
4438 <para>the PID of the server process</para>
4444 <para>DBUS_SESSION_BUS_WINDOWID</para>
4448 <para>the window ID</para>
4457 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4462 Failure to open this file MUST be interpreted as absence of a
4463 running server. Therefore, the implementation MUST proceed to
4464 attempting to launch a new bus server if the file cannot be
4469 However, success in opening this file MUST NOT lead to the
4470 conclusion that the server is running. Thus, a failure to connect to
4471 the bus address obtained by the alternative method MUST NOT be
4472 considered a fatal error. If the connection cannot be established,
4473 the implementation MUST proceed to check the X selection settings or
4474 to start the server on its own.
4478 If the implementation concludes that the D-Bus server is not
4479 running it MUST attempt to start a new server and it MUST also
4480 ensure that the daemon started as an effect of the "autolaunch"
4481 mechanism provides the lookup mechanisms described above, so
4482 subsequent calls can locate the newly started server. The
4483 implementation MUST also ensure that if two or more concurrent
4484 initiations happen, only one server remains running and all other
4485 initiations are able to obtain the address of this server and
4486 connect to it. In other words, the implementation MUST ensure that
4487 the X selection is not present when it attempts to set it, without
4488 allowing another process to set the selection between the
4489 verification and the setting (e.g., by using XGrabServer /
4496 [FIXME specify location of .service files, probably using
4497 DESKTOP_DIRS etc. from basedir specification, though login session
4498 bus is not really desktop-specific]
4502 <sect3 id="message-bus-types-system">
4503 <title>System message bus</title>
4505 A computer may have a <firstterm>system message bus</firstterm>,
4506 accessible to all applications on the system. This message bus may be
4507 used to broadcast system events, such as adding new hardware devices,
4508 changes in the printer queue, and so forth.
4511 The address of the system message bus is given
4512 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4513 variable. If that variable is not set, applications should try
4514 to connect to the well-known address
4515 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4518 The D-Bus reference implementation actually honors the
4519 <literal>$(localstatedir)</literal> configure option
4520 for this address, on both client and server side.
4525 [FIXME specify location of system bus .service files]
4530 <sect2 id="message-bus-messages">
4531 <title>Message Bus Messages</title>
4533 The special message bus name <literal>org.freedesktop.DBus</literal>
4534 responds to a number of additional messages.
4537 <sect3 id="bus-messages-hello">
4538 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4549 <entry>Argument</entry>
4551 <entry>Description</entry>
4557 <entry>STRING</entry>
4558 <entry>Unique name assigned to the connection</entry>
4565 Before an application is able to send messages to other applications
4566 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4567 to the message bus to obtain a unique name. If an application without
4568 a unique name tries to send a message to another application, or a
4569 message to the message bus itself that isn't the
4570 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4571 disconnected from the bus.
4574 There is no corresponding "disconnect" request; if a client wishes to
4575 disconnect from the bus, it simply closes the socket (or other
4576 communication channel).
4579 <sect3 id="bus-messages-list-names">
4580 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4584 ARRAY of STRING ListNames ()
4591 <entry>Argument</entry>
4593 <entry>Description</entry>
4599 <entry>ARRAY of STRING</entry>
4600 <entry>Array of strings where each string is a bus name</entry>
4607 Returns a list of all currently-owned names on the bus.
4610 <sect3 id="bus-messages-list-activatable-names">
4611 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4615 ARRAY of STRING ListActivatableNames ()
4622 <entry>Argument</entry>
4624 <entry>Description</entry>
4630 <entry>ARRAY of STRING</entry>
4631 <entry>Array of strings where each string is a bus name</entry>
4638 Returns a list of all names that can be activated on the bus.
4641 <sect3 id="bus-messages-name-exists">
4642 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4646 BOOLEAN NameHasOwner (in STRING name)
4653 <entry>Argument</entry>
4655 <entry>Description</entry>
4661 <entry>STRING</entry>
4662 <entry>Name to check</entry>
4672 <entry>Argument</entry>
4674 <entry>Description</entry>
4680 <entry>BOOLEAN</entry>
4681 <entry>Return value, true if the name exists</entry>
4688 Checks if the specified name exists (currently has an owner).
4692 <sect3 id="bus-messages-name-owner-changed">
4693 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4697 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4704 <entry>Argument</entry>
4706 <entry>Description</entry>
4712 <entry>STRING</entry>
4713 <entry>Name with a new owner</entry>
4717 <entry>STRING</entry>
4718 <entry>Old owner or empty string if none</entry>
4722 <entry>STRING</entry>
4723 <entry>New owner or empty string if none</entry>
4730 This signal indicates that the owner of a name has changed.
4731 It's also the signal to use to detect the appearance of
4732 new names on the bus.
4735 <sect3 id="bus-messages-name-lost">
4736 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4740 NameLost (STRING name)
4747 <entry>Argument</entry>
4749 <entry>Description</entry>
4755 <entry>STRING</entry>
4756 <entry>Name which was lost</entry>
4763 This signal is sent to a specific application when it loses
4764 ownership of a name.
4768 <sect3 id="bus-messages-name-acquired">
4769 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4773 NameAcquired (STRING name)
4780 <entry>Argument</entry>
4782 <entry>Description</entry>
4788 <entry>STRING</entry>
4789 <entry>Name which was acquired</entry>
4796 This signal is sent to a specific application when it gains
4797 ownership of a name.
4801 <sect3 id="bus-messages-start-service-by-name">
4802 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4806 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4813 <entry>Argument</entry>
4815 <entry>Description</entry>
4821 <entry>STRING</entry>
4822 <entry>Name of the service to start</entry>
4826 <entry>UINT32</entry>
4827 <entry>Flags (currently not used)</entry>
4837 <entry>Argument</entry>
4839 <entry>Description</entry>
4845 <entry>UINT32</entry>
4846 <entry>Return value</entry>
4851 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4855 The return value can be one of the following values:
4860 <entry>Identifier</entry>
4861 <entry>Value</entry>
4862 <entry>Description</entry>
4867 <entry>DBUS_START_REPLY_SUCCESS</entry>
4869 <entry>The service was successfully started.</entry>
4872 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4874 <entry>A connection already owns the given name.</entry>
4883 <sect3 id="bus-messages-update-activation-environment">
4884 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4888 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4895 <entry>Argument</entry>
4897 <entry>Description</entry>
4903 <entry>ARRAY of DICT<STRING,STRING></entry>
4904 <entry>Environment to add or update</entry>
4909 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4912 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4915 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.
4920 <sect3 id="bus-messages-get-name-owner">
4921 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4925 STRING GetNameOwner (in STRING name)
4932 <entry>Argument</entry>
4934 <entry>Description</entry>
4940 <entry>STRING</entry>
4941 <entry>Name to get the owner of</entry>
4951 <entry>Argument</entry>
4953 <entry>Description</entry>
4959 <entry>STRING</entry>
4960 <entry>Return value, a unique connection name</entry>
4965 Returns the unique connection name of the primary owner of the name
4966 given. If the requested name doesn't have an owner, returns a
4967 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4971 <sect3 id="bus-messages-get-connection-unix-user">
4972 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4976 UINT32 GetConnectionUnixUser (in STRING bus_name)
4983 <entry>Argument</entry>
4985 <entry>Description</entry>
4991 <entry>STRING</entry>
4992 <entry>Unique or well-known bus name of the connection to
4993 query, such as <literal>:12.34</literal> or
4994 <literal>com.example.tea</literal></entry>
5004 <entry>Argument</entry>
5006 <entry>Description</entry>
5012 <entry>UINT32</entry>
5013 <entry>Unix user ID</entry>
5018 Returns the Unix user ID of the process connected to the server. If
5019 unable to determine it (for instance, because the process is not on the
5020 same machine as the bus daemon), an error is returned.
5024 <sect3 id="bus-messages-get-connection-unix-process-id">
5025 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5029 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5036 <entry>Argument</entry>
5038 <entry>Description</entry>
5044 <entry>STRING</entry>
5045 <entry>Unique or well-known bus name of the connection to
5046 query, such as <literal>:12.34</literal> or
5047 <literal>com.example.tea</literal></entry>
5057 <entry>Argument</entry>
5059 <entry>Description</entry>
5065 <entry>UINT32</entry>
5066 <entry>Unix process id</entry>
5071 Returns the Unix process ID of the process connected to the server. If
5072 unable to determine it (for instance, because the process is not on the
5073 same machine as the bus daemon), an error is returned.
5077 <sect3 id="bus-messages-add-match">
5078 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5082 AddMatch (in STRING rule)
5089 <entry>Argument</entry>
5091 <entry>Description</entry>
5097 <entry>STRING</entry>
5098 <entry>Match rule to add to the connection</entry>
5103 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5104 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5108 <sect3 id="bus-messages-remove-match">
5109 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5113 RemoveMatch (in STRING rule)
5120 <entry>Argument</entry>
5122 <entry>Description</entry>
5128 <entry>STRING</entry>
5129 <entry>Match rule to remove from the connection</entry>
5134 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5135 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5140 <sect3 id="bus-messages-get-id">
5141 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5145 GetId (out STRING id)
5152 <entry>Argument</entry>
5154 <entry>Description</entry>
5160 <entry>STRING</entry>
5161 <entry>Unique ID identifying the bus daemon</entry>
5166 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5167 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5168 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5169 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5170 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5171 by org.freedesktop.DBus.Peer.GetMachineId().
5172 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5180 <appendix id="implementation-notes">
5181 <title>Implementation notes</title>
5182 <sect1 id="implementation-notes-subsection">
5190 <glossary><title>Glossary</title>
5192 This glossary defines some of the terms used in this specification.
5195 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5198 The message bus maintains an association between names and
5199 connections. (Normally, there's one connection per application.) A
5200 bus name is simply an identifier used to locate connections. For
5201 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5202 name might be used to send a message to a screensaver from Yoyodyne
5203 Corporation. An application is said to <firstterm>own</firstterm> a
5204 name if the message bus has associated the application's connection
5205 with the name. Names may also have <firstterm>queued
5206 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5207 The bus assigns a unique name to each connection,
5208 see <xref linkend="term-unique-name"/>. Other names
5209 can be thought of as "well-known names" and are
5210 used to find applications that offer specific functionality.
5215 <glossentry id="term-message"><glossterm>Message</glossterm>
5218 A message is the atomic unit of communication via the D-Bus
5219 protocol. It consists of a <firstterm>header</firstterm> and a
5220 <firstterm>body</firstterm>; the body is made up of
5221 <firstterm>arguments</firstterm>.
5226 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5229 The message bus is a special application that forwards
5230 or routes messages between a group of applications
5231 connected to the message bus. It also manages
5232 <firstterm>names</firstterm> used for routing
5238 <glossentry id="term-name"><glossterm>Name</glossterm>
5241 See <xref linkend="term-bus-name"/>. "Name" may
5242 also be used to refer to some of the other names
5243 in D-Bus, such as interface names.
5248 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5251 Used to prevent collisions when defining new interfaces or bus
5252 names. The convention used is the same one Java uses for defining
5253 classes: a reversed domain name.
5258 <glossentry id="term-object"><glossterm>Object</glossterm>
5261 Each application contains <firstterm>objects</firstterm>, which have
5262 <firstterm>interfaces</firstterm> and
5263 <firstterm>methods</firstterm>. Objects are referred to by a name,
5264 called a <firstterm>path</firstterm>.
5269 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5272 An application talking directly to another application, without going
5273 through a message bus. One-to-one connections may be "peer to peer" or
5274 "client to server." The D-Bus protocol has no concept of client
5275 vs. server after a connection has authenticated; the flow of messages
5276 is symmetrical (full duplex).
5281 <glossentry id="term-path"><glossterm>Path</glossterm>
5284 Object references (object names) in D-Bus are organized into a
5285 filesystem-style hierarchy, so each object is named by a path. As in
5286 LDAP, there's no difference between "files" and "directories"; a path
5287 can refer to an object, while still having child objects below it.
5292 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5295 Each bus name has a primary owner; messages sent to the name go to the
5296 primary owner. However, certain names also maintain a queue of
5297 secondary owners "waiting in the wings." If the primary owner releases
5298 the name, then the first secondary owner in the queue automatically
5299 becomes the new owner of the name.
5304 <glossentry id="term-service"><glossterm>Service</glossterm>
5307 A service is an executable that can be launched by the bus daemon.
5308 Services normally guarantee some particular features, for example they
5309 may guarantee that they will request a specific name such as
5310 "org.freedesktop.Screensaver", have a singleton object
5311 "/org/freedesktop/Application", and that object will implement the
5312 interface "org.freedesktop.ScreensaverControl".
5317 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5320 ".service files" tell the bus about service applications that can be
5321 launched (see <xref linkend="term-service"/>). Most importantly they
5322 provide a mapping from bus names to services that will request those
5323 names when they start up.
5328 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5331 The special name automatically assigned to each connection by the
5332 message bus. This name will never change owner, and will be unique
5333 (never reused during the lifetime of the message bus).
5334 It will begin with a ':' character.