1 <?xml version="1.0" standalone="no" ?>
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>(not final)</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>
55 <revnumber>current</revnumber>
56 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
57 <authorinitials></authorinitials>
58 <revremark></revremark>
61 <revnumber>0.16</revnumber>
62 <date>11 April 2011</date>
63 <authorinitials></authorinitials>
64 <revremark>add path_namespace, arg0namespace; argNpath matches object
68 <revnumber>0.15</revnumber>
69 <date>3 November 2010</date>
70 <authorinitials></authorinitials>
71 <revremark></revremark>
74 <revnumber>0.14</revnumber>
75 <date>12 May 2010</date>
76 <authorinitials></authorinitials>
77 <revremark></revremark>
80 <revnumber>0.13</revnumber>
81 <date>23 Dezember 2009</date>
82 <authorinitials></authorinitials>
83 <revremark></revremark>
86 <revnumber>0.12</revnumber>
87 <date>7 November, 2006</date>
88 <authorinitials></authorinitials>
89 <revremark></revremark>
92 <revnumber>0.11</revnumber>
93 <date>6 February 2005</date>
94 <authorinitials></authorinitials>
95 <revremark></revremark>
98 <revnumber>0.10</revnumber>
99 <date>28 January 2005</date>
100 <authorinitials></authorinitials>
101 <revremark></revremark>
104 <revnumber>0.9</revnumber>
105 <date>7 Januar 2005</date>
106 <authorinitials></authorinitials>
107 <revremark></revremark>
110 <revnumber>0.8</revnumber>
111 <date>06 September 2003</date>
112 <authorinitials></authorinitials>
113 <revremark>First released document.</revremark>
118 <sect1 id="introduction">
119 <title>Introduction</title>
121 D-Bus is a system for low-latency, low-overhead, easy to use
122 interprocess communication (IPC). In more detail:
126 D-Bus is <emphasis>low-latency</emphasis> because it is designed
127 to avoid round trips and allow asynchronous operation, much like
133 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
134 binary protocol, and does not have to convert to and from a text
135 format such as XML. Because D-Bus is intended for potentially
136 high-resolution same-machine IPC, not primarily for Internet IPC,
137 this is an interesting optimization.
142 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
143 of <firstterm>messages</firstterm> rather than byte streams, and
144 automatically handles a lot of the hard IPC issues. Also, the D-Bus
145 library is designed to be wrapped in a way that lets developers use
146 their framework's existing object/type system, rather than learning
147 a new one specifically for IPC.
154 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
155 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
156 a system for one application to talk to a single other
157 application. However, the primary intended application of the protocol is the
158 D-Bus <firstterm>message bus</firstterm>, specified in <xref
159 linkend="message-bus"/>. The message bus is a special application that
160 accepts connections from multiple other applications, and forwards
165 Uses of D-Bus include notification of system changes (notification of when
166 a camera is plugged in to a computer, or a new version of some software
167 has been installed), or desktop interoperability, for example a file
168 monitoring service or a configuration service.
172 D-Bus is designed for two specific use cases:
176 A "system bus" for notifications from the system to user sessions,
177 and to allow the system to request input from user sessions.
182 A "session bus" used to implement desktop environments such as
187 D-Bus is not intended to be a generic IPC system for any possible
188 application, and intentionally omits many features found in other
189 IPC systems for this reason.
193 At the same time, the bus daemons offer a number of features not found in
194 other IPC systems, such as single-owner "bus names" (similar to X
195 selections), on-demand startup of services, and security policies.
196 In many ways, these features are the primary motivation for developing
197 D-Bus; other systems would have sufficed if IPC were the only goal.
201 D-Bus may turn out to be useful in unanticipated applications, but future
202 versions of this spec and the reference implementation probably will not
203 incorporate features that interfere with the core use cases.
207 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
208 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
209 document are to be interpreted as described in RFC 2119. However, the
210 document could use a serious audit to be sure it makes sense to do
211 so. Also, they are not capitalized.
214 <sect2 id="stability">
215 <title>Protocol and Specification Stability</title>
217 The D-Bus protocol is frozen (only compatible extensions are allowed) as
218 of November 8, 2006. However, this specification could still use a fair
219 bit of work to make interoperable reimplementation possible without
220 reference to the D-Bus reference implementation. Thus, this
221 specification is not marked 1.0. To mark it 1.0, we'd like to see
222 someone invest significant effort in clarifying the specification
223 language, and growing the specification to cover more aspects of the
224 reference implementation's behavior.
227 Until this work is complete, any attempt to reimplement D-Bus will
228 probably require looking at the reference implementation and/or asking
229 questions on the D-Bus mailing list about intended behavior.
230 Questions on the list are very welcome.
233 Nonetheless, this document should be a useful starting point and is
234 to our knowledge accurate, though incomplete.
240 <sect1 id="message-protocol">
241 <title>Message Protocol</title>
244 A <firstterm>message</firstterm> consists of a
245 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
246 think of a message as a package, the header is the address, and the body
247 contains the package contents. The message delivery system uses the header
248 information to figure out where to send the message and how to interpret
249 it; the recipient interprets the body of the message.
253 The body of the message is made up of zero or more
254 <firstterm>arguments</firstterm>, which are typed values, such as an
255 integer or a byte array.
259 Both header and body use the same type system and format for
260 serializing data. Each type of value has a wire format.
261 Converting a value from some other representation into the wire
262 format is called <firstterm>marshaling</firstterm> and converting
263 it back from the wire format is <firstterm>unmarshaling</firstterm>.
266 <sect2 id="message-protocol-signatures">
267 <title>Type Signatures</title>
270 The D-Bus protocol does not include type tags in the marshaled data; a
271 block of marshaled values must have a known <firstterm>type
272 signature</firstterm>. The type signature is made up of <firstterm>type
273 codes</firstterm>. A type code is an ASCII character representing the
274 type of a value. Because ASCII characters are used, the type signature
275 will always form a valid ASCII string. A simple string compare
276 determines whether two type signatures are equivalent.
280 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
281 the ASCII character 'i'. So the signature for a block of values
282 containing a single <literal>INT32</literal> would be:
286 A block of values containing two <literal>INT32</literal> would have this signature:
293 All <firstterm>basic</firstterm> types work like
294 <literal>INT32</literal> in this example. To marshal and unmarshal
295 basic types, you simply read one value from the data
296 block corresponding to each type code in the signature.
297 In addition to basic types, there are four <firstterm>container</firstterm>
298 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
299 and <literal>DICT_ENTRY</literal>.
303 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
304 code does not appear in signatures. Instead, ASCII characters
305 '(' and ')' are used to mark the beginning and end of the struct.
306 So for example, a struct containing two integers would have this
311 Structs can be nested, so for example a struct containing
312 an integer and another struct:
316 The value block storing that struct would contain three integers; the
317 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
322 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
323 but is useful in code that implements the protocol. This type code
324 is specified to allow such code to interoperate in non-protocol contexts.
328 Empty structures are not allowed; there must be at least one
329 type code between the parentheses.
333 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
334 followed by a <firstterm>single complete type</firstterm>. The single
335 complete type following the array is the type of each array element. So
336 the simple example is:
340 which is an array of 32-bit integers. But an array can be of any type,
341 such as this array-of-struct-with-two-int32-fields:
345 Or this array of array of integer:
352 The phrase <firstterm>single complete type</firstterm> deserves some
353 definition. A single complete type is a basic type code, a variant type code,
354 an array with its element type, or a struct with its fields.
355 So the following signatures are not single complete types:
365 And the following signatures contain multiple complete types:
375 Note however that a single complete type may <emphasis>contain</emphasis>
376 multiple other single complete types.
380 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
381 type <literal>VARIANT</literal> will have the signature of a single complete type as part
382 of the <emphasis>value</emphasis>. This signature will be followed by a
383 marshaled value of that type.
387 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
388 than parentheses it uses curly braces, and it has more restrictions.
389 The restrictions are: it occurs only as an array element type; it has
390 exactly two single complete types inside the curly braces; the first
391 single complete type (the "key") must be a basic type rather than a
392 container type. Implementations must not accept dict entries outside of
393 arrays, must not accept dict entries with zero, one, or more than two
394 fields, and must not accept dict entries with non-basic-typed keys. A
395 dict entry is always a key-value pair.
399 The first field in the <literal>DICT_ENTRY</literal> is always the key.
400 A message is considered corrupt if the same key occurs twice in the same
401 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
402 implementations are not required to reject dicts with duplicate keys.
406 In most languages, an array of dict entry would be represented as a
407 map, hash table, or dict object.
411 The following table summarizes the D-Bus types.
416 <entry>Conventional Name</entry>
418 <entry>Description</entry>
423 <entry><literal>INVALID</literal></entry>
424 <entry>0 (ASCII NUL)</entry>
425 <entry>Not a valid type code, used to terminate signatures</entry>
427 <entry><literal>BYTE</literal></entry>
428 <entry>121 (ASCII 'y')</entry>
429 <entry>8-bit unsigned integer</entry>
431 <entry><literal>BOOLEAN</literal></entry>
432 <entry>98 (ASCII 'b')</entry>
433 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
435 <entry><literal>INT16</literal></entry>
436 <entry>110 (ASCII 'n')</entry>
437 <entry>16-bit signed integer</entry>
439 <entry><literal>UINT16</literal></entry>
440 <entry>113 (ASCII 'q')</entry>
441 <entry>16-bit unsigned integer</entry>
443 <entry><literal>INT32</literal></entry>
444 <entry>105 (ASCII 'i')</entry>
445 <entry>32-bit signed integer</entry>
447 <entry><literal>UINT32</literal></entry>
448 <entry>117 (ASCII 'u')</entry>
449 <entry>32-bit unsigned integer</entry>
451 <entry><literal>INT64</literal></entry>
452 <entry>120 (ASCII 'x')</entry>
453 <entry>64-bit signed integer</entry>
455 <entry><literal>UINT64</literal></entry>
456 <entry>116 (ASCII 't')</entry>
457 <entry>64-bit unsigned integer</entry>
459 <entry><literal>DOUBLE</literal></entry>
460 <entry>100 (ASCII 'd')</entry>
461 <entry>IEEE 754 double</entry>
463 <entry><literal>STRING</literal></entry>
464 <entry>115 (ASCII 's')</entry>
465 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
467 <entry><literal>OBJECT_PATH</literal></entry>
468 <entry>111 (ASCII 'o')</entry>
469 <entry>Name of an object instance</entry>
471 <entry><literal>SIGNATURE</literal></entry>
472 <entry>103 (ASCII 'g')</entry>
473 <entry>A type signature</entry>
475 <entry><literal>ARRAY</literal></entry>
476 <entry>97 (ASCII 'a')</entry>
479 <entry><literal>STRUCT</literal></entry>
480 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
481 <entry>Struct</entry>
483 <entry><literal>VARIANT</literal></entry>
484 <entry>118 (ASCII 'v') </entry>
485 <entry>Variant type (the type of the value is part of the value itself)</entry>
487 <entry><literal>DICT_ENTRY</literal></entry>
488 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
489 <entry>Entry in a dict or map (array of key-value pairs)</entry>
491 <entry><literal>UNIX_FD</literal></entry>
492 <entry>104 (ASCII 'h')</entry>
493 <entry>Unix file descriptor</entry>
502 <sect2 id="message-protocol-marshaling">
503 <title>Marshaling (Wire Format)</title>
506 Given a type signature, a block of bytes can be converted into typed
507 values. This section describes the format of the block of bytes. Byte
508 order and alignment issues are handled uniformly for all D-Bus types.
512 A block of bytes has an associated byte order. The byte order
513 has to be discovered in some way; for D-Bus messages, the
514 byte order is part of the message header as described in
515 <xref linkend="message-protocol-messages"/>. For now, assume
516 that the byte order is known to be either little endian or big
521 Each value in a block of bytes is aligned "naturally," for example
522 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
523 8-byte boundary. To properly align a value, <firstterm>alignment
524 padding</firstterm> may be necessary. The alignment padding must always
525 be the minimum required padding to properly align the following value;
526 and it must always be made up of nul bytes. The alignment padding must
527 not be left uninitialized (it can't contain garbage), and more padding
528 than required must not be used.
532 Given all this, the types are marshaled on the wire as follows:
537 <entry>Conventional Name</entry>
538 <entry>Encoding</entry>
539 <entry>Alignment</entry>
544 <entry><literal>INVALID</literal></entry>
545 <entry>Not applicable; cannot be marshaled.</entry>
548 <entry><literal>BYTE</literal></entry>
549 <entry>A single 8-bit byte.</entry>
552 <entry><literal>BOOLEAN</literal></entry>
553 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
556 <entry><literal>INT16</literal></entry>
557 <entry>16-bit signed integer in the message's byte order.</entry>
560 <entry><literal>UINT16</literal></entry>
561 <entry>16-bit unsigned integer in the message's byte order.</entry>
564 <entry><literal>INT32</literal></entry>
565 <entry>32-bit signed integer in the message's byte order.</entry>
568 <entry><literal>UINT32</literal></entry>
569 <entry>32-bit unsigned integer in the message's byte order.</entry>
572 <entry><literal>INT64</literal></entry>
573 <entry>64-bit signed integer in the message's byte order.</entry>
576 <entry><literal>UINT64</literal></entry>
577 <entry>64-bit unsigned integer in the message's byte order.</entry>
580 <entry><literal>DOUBLE</literal></entry>
581 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
584 <entry><literal>STRING</literal></entry>
585 <entry>A <literal>UINT32</literal> indicating the string's
586 length in bytes excluding its terminating nul, followed by
587 non-nul string data of the given length, followed by a terminating nul
594 <entry><literal>OBJECT_PATH</literal></entry>
595 <entry>Exactly the same as <literal>STRING</literal> except the
596 content must be a valid object path (see below).
602 <entry><literal>SIGNATURE</literal></entry>
603 <entry>The same as <literal>STRING</literal> except the length is a single
604 byte (thus signatures have a maximum length of 255)
605 and the content must be a valid signature (see below).
611 <entry><literal>ARRAY</literal></entry>
613 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
614 alignment padding to the alignment boundary of the array element type,
615 followed by each array element. The array length is from the
616 end of the alignment padding to the end of the last element,
617 i.e. it does not include the padding after the length,
618 or any padding after the last element.
619 Arrays have a maximum length defined to be 2 to the 26th power or
620 67108864. Implementations must not send or accept arrays exceeding this
627 <entry><literal>STRUCT</literal></entry>
629 A struct must start on an 8-byte boundary regardless of the
630 type of the struct fields. The struct value consists of each
631 field marshaled in sequence starting from that 8-byte
638 <entry><literal>VARIANT</literal></entry>
640 A variant type has a marshaled
641 <literal>SIGNATURE</literal> followed by a marshaled
642 value with the type given in the signature. Unlike
643 a message signature, the variant signature can
644 contain only a single complete type. So "i", "ai"
645 or "(ii)" is OK, but "ii" is not. Use of variants may not
646 cause a total message depth to be larger than 64, including
647 other container types such as structures.
650 1 (alignment of the signature)
653 <entry><literal>DICT_ENTRY</literal></entry>
661 <entry><literal>UNIX_FD</literal></entry>
662 <entry>32-bit unsigned integer in the message's byte
663 order. The actual file descriptors need to be
664 transferred out-of-band via some platform specific
665 mechanism. On the wire, values of this type store the index to the
666 file descriptor in the array of file descriptors that
667 accompany the message.</entry>
675 <sect3 id="message-protocol-marshaling-object-path">
676 <title>Valid Object Paths</title>
679 An object path is a name used to refer to an object instance.
680 Conceptually, each participant in a D-Bus message exchange may have
681 any number of object instances (think of C++ or Java objects) and each
682 such instance will have a path. Like a filesystem, the object
683 instances in an application form a hierarchical tree.
687 The following rules define a valid object path. Implementations must
688 not send or accept messages with invalid object paths.
692 The path may be of any length.
697 The path must begin with an ASCII '/' (integer 47) character,
698 and must consist of elements separated by slash characters.
703 Each element must only contain the ASCII characters
709 No element may be the empty string.
714 Multiple '/' characters cannot occur in sequence.
719 A trailing '/' character is not allowed unless the
720 path is the root path (a single '/' character).
729 <sect3 id="message-protocol-marshaling-signature">
730 <title>Valid Signatures</title>
732 An implementation must not send or accept invalid signatures.
733 Valid signatures will conform to the following rules:
737 The signature ends with a nul byte.
742 The signature is a list of single complete types.
743 Arrays must have element types, and structs must
744 have both open and close parentheses.
749 Only type codes and open and close parentheses are
750 allowed in the signature. The <literal>STRUCT</literal> type code
751 is not allowed in signatures, because parentheses
757 The maximum depth of container type nesting is 32 array type
758 codes and 32 open parentheses. This implies that the maximum
759 total depth of recursion is 64, for an "array of array of array
760 of ... struct of struct of struct of ..." where there are 32
766 The maximum length of a signature is 255.
771 Signatures must be nul-terminated.
780 <sect2 id="message-protocol-messages">
781 <title>Message Format</title>
784 A message consists of a header and a body. The header is a block of
785 values with a fixed signature and meaning. The body is a separate block
786 of values, with a signature specified in the header.
790 The length of the header must be a multiple of 8, allowing the body to
791 begin on an 8-byte boundary when storing the entire message in a single
792 buffer. If the header does not naturally end on an 8-byte boundary
793 up to 7 bytes of nul-initialized alignment padding must be added.
797 The message body need not end on an 8-byte boundary.
801 The maximum length of a message, including header, header alignment padding,
802 and body is 2 to the 27th power or 134217728. Implementations must not
803 send or accept messages exceeding this size.
807 The signature of the header is:
811 Written out more readably, this is:
813 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
818 These values have the following meanings:
824 <entry>Description</entry>
829 <entry>1st <literal>BYTE</literal></entry>
830 <entry>Endianness flag; ASCII 'l' for little-endian
831 or ASCII 'B' for big-endian. Both header and body are
832 in this endianness.</entry>
835 <entry>2nd <literal>BYTE</literal></entry>
836 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
837 Currently-defined types are described below.
841 <entry>3rd <literal>BYTE</literal></entry>
842 <entry>Bitwise OR of flags. Unknown flags
843 must be ignored. Currently-defined flags are described below.
847 <entry>4th <literal>BYTE</literal></entry>
848 <entry>Major protocol version of the sending application. If
849 the major protocol version of the receiving application does not
850 match, the applications will not be able to communicate and the
851 D-Bus connection must be disconnected. The major protocol
852 version for this version of the specification is 1.
856 <entry>1st <literal>UINT32</literal></entry>
857 <entry>Length in bytes of the message body, starting
858 from the end of the header. The header ends after
859 its alignment padding to an 8-boundary.
863 <entry>2nd <literal>UINT32</literal></entry>
864 <entry>The serial of this message, used as a cookie
865 by the sender to identify the reply corresponding
866 to this request. This must not be zero.
870 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
871 <entry>An array of zero or more <firstterm>header
872 fields</firstterm> where the byte is the field code, and the
873 variant is the field value. The message type determines
874 which fields are required.
882 <firstterm>Message types</firstterm> that can appear in the second byte
888 <entry>Conventional name</entry>
889 <entry>Decimal value</entry>
890 <entry>Description</entry>
895 <entry><literal>INVALID</literal></entry>
897 <entry>This is an invalid type.</entry>
900 <entry><literal>METHOD_CALL</literal></entry>
902 <entry>Method call.</entry>
905 <entry><literal>METHOD_RETURN</literal></entry>
907 <entry>Method reply with returned data.</entry>
910 <entry><literal>ERROR</literal></entry>
912 <entry>Error reply. If the first argument exists and is a
913 string, it is an error message.</entry>
916 <entry><literal>SIGNAL</literal></entry>
918 <entry>Signal emission.</entry>
925 Flags that can appear in the third byte of the header:
930 <entry>Conventional name</entry>
931 <entry>Hex value</entry>
932 <entry>Description</entry>
937 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
939 <entry>This message does not expect method return replies or
940 error replies; the reply can be omitted as an
941 optimization. However, it is compliant with this specification
942 to return the reply despite this flag and the only harm
943 from doing so is extra network traffic.
947 <entry><literal>NO_AUTO_START</literal></entry>
949 <entry>The bus must not launch an owner
950 for the destination name in response to this message.
958 <sect3 id="message-protocol-header-fields">
959 <title>Header Fields</title>
962 The array at the end of the header contains <firstterm>header
963 fields</firstterm>, where each field is a 1-byte field code followed
964 by a field value. A header must contain the required header fields for
965 its message type, and zero or more of any optional header
966 fields. Future versions of this protocol specification may add new
967 fields. Implementations must ignore fields they do not
968 understand. Implementations must not invent their own header fields;
969 only changes to this specification may introduce new header fields.
973 Again, if an implementation sees a header field code that it does not
974 expect, it must ignore that field, as it will be part of a new
975 (but compatible) version of this specification. This also applies
976 to known header fields appearing in unexpected messages, for
977 example: if a signal has a reply serial it must be ignored
978 even though it has no meaning as of this version of the spec.
982 However, implementations must not send or accept known header fields
983 with the wrong type stored in the field value. So for example a
984 message with an <literal>INTERFACE</literal> field of type
985 <literal>UINT32</literal> would be considered corrupt.
989 Here are the currently-defined header fields:
994 <entry>Conventional Name</entry>
995 <entry>Decimal Code</entry>
997 <entry>Required In</entry>
998 <entry>Description</entry>
1003 <entry><literal>INVALID</literal></entry>
1006 <entry>not allowed</entry>
1007 <entry>Not a valid field name (error if it appears in a message)</entry>
1010 <entry><literal>PATH</literal></entry>
1012 <entry><literal>OBJECT_PATH</literal></entry>
1013 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1014 <entry>The object to send a call to,
1015 or the object a signal is emitted from.
1017 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1018 implementations should not send messages with this path,
1019 and the reference implementation of the bus daemon will
1020 disconnect any application that attempts to do so.
1024 <entry><literal>INTERFACE</literal></entry>
1026 <entry><literal>STRING</literal></entry>
1027 <entry><literal>SIGNAL</literal></entry>
1029 The interface to invoke a method call on, or
1030 that a signal is emitted from. Optional for
1031 method calls, required for signals.
1032 The special interface
1033 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1034 implementations should not send messages with this
1035 interface, and the reference implementation of the bus
1036 daemon will disconnect any application that attempts to
1041 <entry><literal>MEMBER</literal></entry>
1043 <entry><literal>STRING</literal></entry>
1044 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1045 <entry>The member, either the method name or signal name.</entry>
1048 <entry><literal>ERROR_NAME</literal></entry>
1050 <entry><literal>STRING</literal></entry>
1051 <entry><literal>ERROR</literal></entry>
1052 <entry>The name of the error that occurred, for errors</entry>
1055 <entry><literal>REPLY_SERIAL</literal></entry>
1057 <entry><literal>UINT32</literal></entry>
1058 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1059 <entry>The serial number of the message this message is a reply
1060 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1063 <entry><literal>DESTINATION</literal></entry>
1065 <entry><literal>STRING</literal></entry>
1066 <entry>optional</entry>
1067 <entry>The name of the connection this message is intended for.
1068 Only used in combination with the message bus, see
1069 <xref linkend="message-bus"/>.</entry>
1072 <entry><literal>SENDER</literal></entry>
1074 <entry><literal>STRING</literal></entry>
1075 <entry>optional</entry>
1076 <entry>Unique name of the sending connection.
1077 The message bus fills in this field so it is reliable; the field is
1078 only meaningful in combination with the message bus.</entry>
1081 <entry><literal>SIGNATURE</literal></entry>
1083 <entry><literal>SIGNATURE</literal></entry>
1084 <entry>optional</entry>
1085 <entry>The signature of the message body.
1086 If omitted, it is assumed to be the
1087 empty signature "" (i.e. the body must be 0-length).</entry>
1090 <entry><literal>UNIX_FDS</literal></entry>
1092 <entry><literal>UINT32</literal></entry>
1093 <entry>optional</entry>
1094 <entry>The number of Unix file descriptors that
1095 accompany the message. If omitted, it is assumed
1096 that no Unix file descriptors accompany the
1097 message. The actual file descriptors need to be
1098 transferred via platform specific mechanism
1099 out-of-band. They must be sent at the same time as
1100 part of the message itself. They may not be sent
1101 before the first byte of the message itself is
1102 transferred or after the last byte of the message
1112 <sect2 id="message-protocol-names">
1113 <title>Valid Names</title>
1115 The various names in D-Bus messages have some restrictions.
1118 There is a <firstterm>maximum name length</firstterm>
1119 of 255 which applies to bus names, interfaces, and members.
1121 <sect3 id="message-protocol-names-interface">
1122 <title>Interface names</title>
1124 Interfaces have names with type <literal>STRING</literal>, meaning that
1125 they must be valid UTF-8. However, there are also some
1126 additional restrictions that apply to interface names
1129 <listitem><para>Interface names are composed of 1 or more elements separated by
1130 a period ('.') character. All elements must contain at least
1134 <listitem><para>Each element must only contain the ASCII characters
1135 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1139 <listitem><para>Interface names must contain at least one '.' (period)
1140 character (and thus at least two elements).
1143 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1144 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1148 <sect3 id="message-protocol-names-bus">
1149 <title>Bus names</title>
1151 Connections have one or more bus names associated with them.
1152 A connection has exactly one bus name that is a unique connection
1153 name. The unique connection name remains with the connection for
1154 its entire lifetime.
1155 A bus name is of type <literal>STRING</literal>,
1156 meaning that it must be valid UTF-8. However, there are also
1157 some additional restrictions that apply to bus names
1160 <listitem><para>Bus names that start with a colon (':')
1161 character are unique connection names.
1164 <listitem><para>Bus names are composed of 1 or more elements separated by
1165 a period ('.') character. All elements must contain at least
1169 <listitem><para>Each element must only contain the ASCII characters
1170 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1171 connection name may begin with a digit, elements in
1172 other bus names must not begin with a digit.
1176 <listitem><para>Bus names must contain at least one '.' (period)
1177 character (and thus at least two elements).
1180 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1181 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1185 Note that the hyphen ('-') character is allowed in bus names but
1186 not in interface names.
1189 <sect3 id="message-protocol-names-member">
1190 <title>Member names</title>
1192 Member (i.e. method or signal) names:
1194 <listitem><para>Must only contain the ASCII characters
1195 "[A-Z][a-z][0-9]_" and may not begin with a
1196 digit.</para></listitem>
1197 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1198 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1199 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1203 <sect3 id="message-protocol-names-error">
1204 <title>Error names</title>
1206 Error names have the same restrictions as interface names.
1211 <sect2 id="message-protocol-types">
1212 <title>Message Types</title>
1214 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1215 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1216 This section describes these conventions.
1218 <sect3 id="message-protocol-types-method">
1219 <title>Method Calls</title>
1221 Some messages invoke an operation on a remote object. These are
1222 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1223 messages map naturally to methods on objects in a typical program.
1226 A method call message is required to have a <literal>MEMBER</literal> header field
1227 indicating the name of the method. Optionally, the message has an
1228 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1229 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1230 a method with the same name, it is undefined which of the two methods
1231 will be invoked. Implementations may also choose to return an error in
1232 this ambiguous case. However, if a method name is unique
1233 implementations must not require an interface field.
1236 Method call messages also include a <literal>PATH</literal> field
1237 indicating the object to invoke the method on. If the call is passing
1238 through a message bus, the message will also have a
1239 <literal>DESTINATION</literal> field giving the name of the connection
1240 to receive the message.
1243 When an application handles a method call message, it is required to
1244 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1245 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1246 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1249 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1250 are the return value(s) or "out parameters" of the method call.
1251 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1252 and the call fails; no return value will be provided. It makes
1253 no sense to send multiple replies to the same method call.
1256 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1257 reply is required, so the caller will know the method
1258 was successfully processed.
1261 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1265 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1266 then as an optimization the application receiving the method
1267 call may choose to omit the reply message (regardless of
1268 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1269 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1270 flag and reply anyway.
1273 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1274 destination name does not exist then a program to own the destination
1275 name will be started before the message is delivered. The message
1276 will be held until the new program is successfully started or has
1277 failed to start; in case of failure, an error will be returned. This
1278 flag is only relevant in the context of a message bus, it is ignored
1279 during one-to-one communication with no intermediate bus.
1281 <sect4 id="message-protocol-types-method-apis">
1282 <title>Mapping method calls to native APIs</title>
1284 APIs for D-Bus may map method calls to a method call in a specific
1285 programming language, such as C++, or may map a method call written
1286 in an IDL to a D-Bus message.
1289 In APIs of this nature, arguments to a method are often termed "in"
1290 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1291 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1292 "inout" arguments, which are both sent and received, i.e. the caller
1293 passes in a value which is modified. Mapped to D-Bus, an "inout"
1294 argument is equivalent to an "in" argument, followed by an "out"
1295 argument. You can't pass things "by reference" over the wire, so
1296 "inout" is purely an illusion of the in-process API.
1299 Given a method with zero or one return values, followed by zero or more
1300 arguments, where each argument may be "in", "out", or "inout", the
1301 caller constructs a message by appending each "in" or "inout" argument,
1302 in order. "out" arguments are not represented in the caller's message.
1305 The recipient constructs a reply by appending first the return value
1306 if any, then each "out" or "inout" argument, in order.
1307 "in" arguments are not represented in the reply message.
1310 Error replies are normally mapped to exceptions in languages that have
1314 In converting from native APIs to D-Bus, it is perhaps nice to
1315 map D-Bus naming conventions ("FooBar") to native conventions
1316 such as "fooBar" or "foo_bar" automatically. This is OK
1317 as long as you can say that the native API is one that
1318 was specifically written for D-Bus. It makes the most sense
1319 when writing object implementations that will be exported
1320 over the bus. Object proxies used to invoke remote D-Bus
1321 objects probably need the ability to call any D-Bus method,
1322 and thus a magic name mapping like this could be a problem.
1325 This specification doesn't require anything of native API bindings;
1326 the preceding is only a suggested convention for consistency
1332 <sect3 id="message-protocol-types-signal">
1333 <title>Signal Emission</title>
1335 Unlike method calls, signal emissions have no replies.
1336 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1337 It must have three header fields: <literal>PATH</literal> giving the object
1338 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1339 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1340 for signals, though it is optional for method calls.
1344 <sect3 id="message-protocol-types-errors">
1345 <title>Errors</title>
1347 Messages of type <literal>ERROR</literal> are most commonly replies
1348 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1349 to any kind of message. The message bus for example
1350 will return an <literal>ERROR</literal> in reply to a signal emission if
1351 the bus does not have enough memory to send the signal.
1354 An <literal>ERROR</literal> may have any arguments, but if the first
1355 argument is a <literal>STRING</literal>, it must be an error message.
1356 The error message may be logged or shown to the user
1361 <sect3 id="message-protocol-types-notation">
1362 <title>Notation in this document</title>
1364 This document uses a simple pseudo-IDL to describe particular method
1365 calls and signals. Here is an example of a method call:
1367 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1368 out UINT32 resultcode)
1370 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1371 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1372 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1373 characters so it's known that the last part of the name in
1374 the "IDL" is the member name.
1377 In C++ that might end up looking like this:
1379 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1380 unsigned int flags);
1382 or equally valid, the return value could be done as an argument:
1384 void org::freedesktop::DBus::StartServiceByName (const char *name,
1386 unsigned int *resultcode);
1388 It's really up to the API designer how they want to make
1389 this look. You could design an API where the namespace wasn't used
1390 in C++, using STL or Qt, using varargs, or whatever you wanted.
1393 Signals are written as follows:
1395 org.freedesktop.DBus.NameLost (STRING name)
1397 Signals don't specify "in" vs. "out" because only
1398 a single direction is possible.
1401 It isn't especially encouraged to use this lame pseudo-IDL in actual
1402 API implementations; you might use the native notation for the
1403 language you're using, or you might use COM or CORBA IDL, for example.
1408 <sect2 id="message-protocol-handling-invalid">
1409 <title>Invalid Protocol and Spec Extensions</title>
1412 For security reasons, the D-Bus protocol should be strictly parsed and
1413 validated, with the exception of defined extension points. Any invalid
1414 protocol or spec violations should result in immediately dropping the
1415 connection without notice to the other end. Exceptions should be
1416 carefully considered, e.g. an exception may be warranted for a
1417 well-understood idiosyncrasy of a widely-deployed implementation. In
1418 cases where the other end of a connection is 100% trusted and known to
1419 be friendly, skipping validation for performance reasons could also make
1420 sense in certain cases.
1424 Generally speaking violations of the "must" requirements in this spec
1425 should be considered possible attempts to exploit security, and violations
1426 of the "should" suggestions should be considered legitimate (though perhaps
1427 they should generate an error in some cases).
1431 The following extension points are built in to D-Bus on purpose and must
1432 not be treated as invalid protocol. The extension points are intended
1433 for use by future versions of this spec, they are not intended for third
1434 parties. At the moment, the only way a third party could extend D-Bus
1435 without breaking interoperability would be to introduce a way to negotiate new
1436 feature support as part of the auth protocol, using EXTENSION_-prefixed
1437 commands. There is not yet a standard way to negotiate features.
1441 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1442 commands result in an ERROR rather than a disconnect. This enables
1443 future extensions to the protocol. Commands starting with EXTENSION_ are
1444 reserved for third parties.
1449 The authentication protocol supports pluggable auth mechanisms.
1454 The address format (see <xref linkend="addresses"/>) supports new
1460 Messages with an unknown type (something other than
1461 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1462 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1463 Unknown-type messages must still be well-formed in the same way
1464 as the known messages, however. They still have the normal
1470 Header fields with an unknown or unexpected field code must be ignored,
1471 though again they must still be well-formed.
1476 New standard interfaces (with new methods and signals) can of course be added.
1486 <sect1 id="auth-protocol">
1487 <title>Authentication Protocol</title>
1489 Before the flow of messages begins, two applications must
1490 authenticate. A simple plain-text protocol is used for
1491 authentication; this protocol is a SASL profile, and maps fairly
1492 directly from the SASL specification. The message encoding is
1493 NOT used here, only plain text messages.
1496 In examples, "C:" and "S:" indicate lines sent by the client and
1497 server respectively.
1499 <sect2 id="auth-protocol-overview">
1500 <title>Protocol Overview</title>
1502 The protocol is a line-based protocol, where each line ends with
1503 \r\n. Each line begins with an all-caps ASCII command name containing
1504 only the character range [A-Z_], a space, then any arguments for the
1505 command, then the \r\n ending the line. The protocol is
1506 case-sensitive. All bytes must be in the ASCII character set.
1508 Commands from the client to the server are as follows:
1511 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1512 <listitem><para>CANCEL</para></listitem>
1513 <listitem><para>BEGIN</para></listitem>
1514 <listitem><para>DATA <data in hex encoding></para></listitem>
1515 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1516 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1519 From server to client are as follows:
1522 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1523 <listitem><para>OK <GUID in hex></para></listitem>
1524 <listitem><para>DATA <data in hex encoding></para></listitem>
1525 <listitem><para>ERROR</para></listitem>
1526 <listitem><para>AGREE_UNIX_FD</para></listitem>
1530 Unofficial extensions to the command set must begin with the letters
1531 "EXTENSION_", to avoid conflicts with future official commands.
1532 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1535 <sect2 id="auth-nul-byte">
1536 <title>Special credentials-passing nul byte</title>
1538 Immediately after connecting to the server, the client must send a
1539 single nul byte. This byte may be accompanied by credentials
1540 information on some operating systems that use sendmsg() with
1541 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1542 sockets. However, the nul byte must be sent even on other kinds of
1543 socket, and even on operating systems that do not require a byte to be
1544 sent in order to transmit credentials. The text protocol described in
1545 this document begins after the single nul byte. If the first byte
1546 received from the client is not a nul byte, the server may disconnect
1550 A nul byte in any context other than the initial byte is an error;
1551 the protocol is ASCII-only.
1554 The credentials sent along with the nul byte may be used with the
1555 SASL mechanism EXTERNAL.
1558 <sect2 id="auth-command-auth">
1559 <title>AUTH command</title>
1561 If an AUTH command has no arguments, it is a request to list
1562 available mechanisms. The server must respond with a REJECTED
1563 command listing the mechanisms it understands, or with an error.
1566 If an AUTH command specifies a mechanism, and the server supports
1567 said mechanism, the server should begin exchanging SASL
1568 challenge-response data with the client using DATA commands.
1571 If the server does not support the mechanism given in the AUTH
1572 command, it must send either a REJECTED command listing the mechanisms
1573 it does support, or an error.
1576 If the [initial-response] argument is provided, it is intended for use
1577 with mechanisms that have no initial challenge (or an empty initial
1578 challenge), as if it were the argument to an initial DATA command. If
1579 the selected mechanism has an initial challenge and [initial-response]
1580 was provided, the server should reject authentication by sending
1584 If authentication succeeds after exchanging DATA commands,
1585 an OK command must be sent to the client.
1588 The first octet received by the server after the \r\n of the BEGIN
1589 command from the client must be the first octet of the
1590 authenticated/encrypted stream of D-Bus messages.
1593 If BEGIN is received by the server, the first octet received
1594 by the client after the \r\n of the OK command must be the
1595 first octet of the authenticated/encrypted stream of D-Bus
1599 <sect2 id="auth-command-cancel">
1600 <title>CANCEL Command</title>
1602 At any time up to sending the BEGIN command, the client may send a
1603 CANCEL command. On receiving the CANCEL command, the server must
1604 send a REJECTED command and abort the current authentication
1608 <sect2 id="auth-command-data">
1609 <title>DATA Command</title>
1611 The DATA command may come from either client or server, and simply
1612 contains a hex-encoded block of data to be interpreted
1613 according to the SASL mechanism in use.
1616 Some SASL mechanisms support sending an "empty string";
1617 FIXME we need some way to do this.
1620 <sect2 id="auth-command-begin">
1621 <title>BEGIN Command</title>
1623 The BEGIN command acknowledges that the client has received an
1624 OK command from the server, and that the stream of messages
1628 The first octet received by the server after the \r\n of the BEGIN
1629 command from the client must be the first octet of the
1630 authenticated/encrypted stream of D-Bus messages.
1633 <sect2 id="auth-command-rejected">
1634 <title>REJECTED Command</title>
1636 The REJECTED command indicates that the current authentication
1637 exchange has failed, and further exchange of DATA is inappropriate.
1638 The client would normally try another mechanism, or try providing
1639 different responses to challenges.
1641 Optionally, the REJECTED command has a space-separated list of
1642 available auth mechanisms as arguments. If a server ever provides
1643 a list of supported mechanisms, it must provide the same list
1644 each time it sends a REJECTED message. Clients are free to
1645 ignore all lists received after the first.
1648 <sect2 id="auth-command-ok">
1649 <title>OK Command</title>
1651 The OK command indicates that the client has been
1652 authenticated. The client may now proceed with negotiating
1653 Unix file descriptor passing. To do that it shall send
1654 NEGOTIATE_UNIX_FD to the server.
1657 Otherwise, the client must respond to the OK command by
1658 sending a BEGIN command, followed by its stream of messages,
1659 or by disconnecting. The server must not accept additional
1660 commands using this protocol after the BEGIN command has been
1661 received. Further communication will be a stream of D-Bus
1662 messages (optionally encrypted, as negotiated) rather than
1666 If a client sends BEGIN the first octet received by the client
1667 after the \r\n of the OK command must be the first octet of
1668 the authenticated/encrypted stream of D-Bus messages.
1671 The OK command has one argument, which is the GUID of the server.
1672 See <xref linkend="addresses"/> for more on server GUIDs.
1675 <sect2 id="auth-command-error">
1676 <title>ERROR Command</title>
1678 The ERROR command indicates that either server or client did not
1679 know a command, does not accept the given command in the current
1680 context, or did not understand the arguments to the command. This
1681 allows the protocol to be extended; a client or server can send a
1682 command present or permitted only in new protocol versions, and if
1683 an ERROR is received instead of an appropriate response, fall back
1684 to using some other technique.
1687 If an ERROR is sent, the server or client that sent the
1688 error must continue as if the command causing the ERROR had never been
1689 received. However, the the server or client receiving the error
1690 should try something other than whatever caused the error;
1691 if only canceling/rejecting the authentication.
1694 If the D-Bus protocol changes incompatibly at some future time,
1695 applications implementing the new protocol would probably be able to
1696 check for support of the new protocol by sending a new command and
1697 receiving an ERROR from applications that don't understand it. Thus the
1698 ERROR feature of the auth protocol is an escape hatch that lets us
1699 negotiate extensions or changes to the D-Bus protocol in the future.
1702 <sect2 id="auth-command-negotiate-unix-fd">
1703 <title>NEGOTIATE_UNIX_FD Command</title>
1705 The NEGOTIATE_UNIX_FD command indicates that the client
1706 supports Unix file descriptor passing. This command may only
1707 be sent after the connection is authenticated, i.e. after OK
1708 was received by the client. This command may only be sent on
1709 transports that support Unix file descriptor passing.
1712 On receiving NEGOTIATE_UNIX_FD the server must respond with
1713 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1714 the transport chosen supports Unix file descriptor passing and
1715 the server supports this feature. It shall respond the latter
1716 if the transport does not support Unix file descriptor
1717 passing, the server does not support this feature, or the
1718 server decides not to enable file descriptor passing due to
1719 security or other reasons.
1722 <sect2 id="auth-command-agree-unix-fd">
1723 <title>AGREE_UNIX_FD Command</title>
1725 The AGREE_UNIX_FD command indicates that the server supports
1726 Unix file descriptor passing. This command may only be sent
1727 after the connection is authenticated, and the client sent
1728 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1729 command may only be sent on transports that support Unix file
1733 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1734 followed by its stream of messages, or by disconnecting. The
1735 server must not accept additional commands using this protocol
1736 after the BEGIN command has been received. Further
1737 communication will be a stream of D-Bus messages (optionally
1738 encrypted, as negotiated) rather than this protocol.
1741 <sect2 id="auth-command-future">
1742 <title>Future Extensions</title>
1744 Future extensions to the authentication and negotiation
1745 protocol are possible. For that new commands may be
1746 introduced. If a client or server receives an unknown command
1747 it shall respond with ERROR and not consider this fatal. New
1748 commands may be introduced both before, and after
1749 authentication, i.e. both before and after the OK command.
1752 <sect2 id="auth-examples">
1753 <title>Authentication examples</title>
1757 <title>Example of successful magic cookie authentication</title>
1759 (MAGIC_COOKIE is a made up mechanism)
1761 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1767 <title>Example of finding out mechanisms then picking one</title>
1770 S: REJECTED KERBEROS_V4 SKEY
1771 C: AUTH SKEY 7ab83f32ee
1772 S: DATA 8799cabb2ea93e
1773 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1779 <title>Example of client sends unknown command then falls back to regular auth</title>
1783 C: AUTH MAGIC_COOKIE 3736343435313230333039
1789 <title>Example of server doesn't support initial auth mechanism</title>
1791 C: AUTH MAGIC_COOKIE 3736343435313230333039
1792 S: REJECTED KERBEROS_V4 SKEY
1793 C: AUTH SKEY 7ab83f32ee
1794 S: DATA 8799cabb2ea93e
1795 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1801 <title>Example of wrong password or the like followed by successful retry</title>
1803 C: AUTH MAGIC_COOKIE 3736343435313230333039
1804 S: REJECTED KERBEROS_V4 SKEY
1805 C: AUTH SKEY 7ab83f32ee
1806 S: DATA 8799cabb2ea93e
1807 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1809 C: AUTH SKEY 7ab83f32ee
1810 S: DATA 8799cabb2ea93e
1811 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1817 <title>Example of skey cancelled and restarted</title>
1819 C: AUTH MAGIC_COOKIE 3736343435313230333039
1820 S: REJECTED KERBEROS_V4 SKEY
1821 C: AUTH SKEY 7ab83f32ee
1822 S: DATA 8799cabb2ea93e
1825 C: AUTH SKEY 7ab83f32ee
1826 S: DATA 8799cabb2ea93e
1827 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1833 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1835 (MAGIC_COOKIE is a made up mechanism)
1837 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1839 C: NEGOTIATE_UNIX_FD
1845 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1847 (MAGIC_COOKIE is a made up mechanism)
1849 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1851 C: NEGOTIATE_UNIX_FD
1858 <sect2 id="auth-states">
1859 <title>Authentication state diagrams</title>
1862 This section documents the auth protocol in terms of
1863 a state machine for the client and the server. This is
1864 probably the most robust way to implement the protocol.
1867 <sect3 id="auth-states-client">
1868 <title>Client states</title>
1871 To more precisely describe the interaction between the
1872 protocol state machine and the authentication mechanisms the
1873 following notation is used: MECH(CHALL) means that the
1874 server challenge CHALL was fed to the mechanism MECH, which
1880 CONTINUE(RESP) means continue the auth conversation
1881 and send RESP as the response to the server;
1887 OK(RESP) means that after sending RESP to the server
1888 the client side of the auth conversation is finished
1889 and the server should return "OK";
1895 ERROR means that CHALL was invalid and could not be
1901 Both RESP and CHALL may be empty.
1905 The Client starts by getting an initial response from the
1906 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1907 the mechanism did not provide an initial response. If the
1908 mechanism returns CONTINUE, the client starts in state
1909 <emphasis>WaitingForData</emphasis>, if the mechanism
1910 returns OK the client starts in state
1911 <emphasis>WaitingForOK</emphasis>.
1915 The client should keep track of available mechanisms and
1916 which it mechanisms it has already attempted. This list is
1917 used to decide which AUTH command to send. When the list is
1918 exhausted, the client should give up and close the
1923 <title><emphasis>WaitingForData</emphasis></title>
1931 MECH(CHALL) returns CONTINUE(RESP) → send
1933 <emphasis>WaitingForData</emphasis>
1937 MECH(CHALL) returns OK(RESP) → send DATA
1938 RESP, goto <emphasis>WaitingForOK</emphasis>
1942 MECH(CHALL) returns ERROR → send ERROR
1943 [msg], goto <emphasis>WaitingForData</emphasis>
1951 Receive REJECTED [mechs] →
1952 send AUTH [next mech], goto
1953 WaitingForData or <emphasis>WaitingForOK</emphasis>
1958 Receive ERROR → send
1960 <emphasis>WaitingForReject</emphasis>
1965 Receive OK → send
1966 BEGIN, terminate auth
1967 conversation, authenticated
1972 Receive anything else → send
1974 <emphasis>WaitingForData</emphasis>
1982 <title><emphasis>WaitingForOK</emphasis></title>
1987 Receive OK → send BEGIN, terminate auth
1988 conversation, <emphasis>authenticated</emphasis>
1993 Receive REJECT [mechs] → send AUTH [next mech],
1994 goto <emphasis>WaitingForData</emphasis> or
1995 <emphasis>WaitingForOK</emphasis>
2001 Receive DATA → send CANCEL, goto
2002 <emphasis>WaitingForReject</emphasis>
2008 Receive ERROR → send CANCEL, goto
2009 <emphasis>WaitingForReject</emphasis>
2015 Receive anything else → send ERROR, goto
2016 <emphasis>WaitingForOK</emphasis>
2024 <title><emphasis>WaitingForReject</emphasis></title>
2029 Receive REJECT [mechs] → send AUTH [next mech],
2030 goto <emphasis>WaitingForData</emphasis> or
2031 <emphasis>WaitingForOK</emphasis>
2037 Receive anything else → terminate auth
2038 conversation, disconnect
2047 <sect3 id="auth-states-server">
2048 <title>Server states</title>
2051 For the server MECH(RESP) means that the client response
2052 RESP was fed to the the mechanism MECH, which returns one of
2057 CONTINUE(CHALL) means continue the auth conversation and
2058 send CHALL as the challenge to the client;
2064 OK means that the client has been successfully
2071 REJECT means that the client failed to authenticate or
2072 there was an error in RESP.
2077 The server starts out in state
2078 <emphasis>WaitingForAuth</emphasis>. If the client is
2079 rejected too many times the server must disconnect the
2084 <title><emphasis>WaitingForAuth</emphasis></title>
2090 Receive AUTH → send REJECTED [mechs], goto
2091 <emphasis>WaitingForAuth</emphasis>
2097 Receive AUTH MECH RESP
2101 MECH not valid mechanism → send REJECTED
2103 <emphasis>WaitingForAuth</emphasis>
2107 MECH(RESP) returns CONTINUE(CHALL) → send
2109 <emphasis>WaitingForData</emphasis>
2113 MECH(RESP) returns OK → send OK, goto
2114 <emphasis>WaitingForBegin</emphasis>
2118 MECH(RESP) returns REJECT → send REJECTED
2120 <emphasis>WaitingForAuth</emphasis>
2128 Receive BEGIN → terminate
2129 auth conversation, disconnect
2135 Receive ERROR → send REJECTED [mechs], goto
2136 <emphasis>WaitingForAuth</emphasis>
2142 Receive anything else → send
2144 <emphasis>WaitingForAuth</emphasis>
2153 <title><emphasis>WaitingForData</emphasis></title>
2161 MECH(RESP) returns CONTINUE(CHALL) → send
2163 <emphasis>WaitingForData</emphasis>
2167 MECH(RESP) returns OK → send OK, goto
2168 <emphasis>WaitingForBegin</emphasis>
2172 MECH(RESP) returns REJECT → send REJECTED
2174 <emphasis>WaitingForAuth</emphasis>
2182 Receive BEGIN → terminate auth conversation,
2189 Receive CANCEL → send REJECTED [mechs], goto
2190 <emphasis>WaitingForAuth</emphasis>
2196 Receive ERROR → send REJECTED [mechs], goto
2197 <emphasis>WaitingForAuth</emphasis>
2203 Receive anything else → send ERROR, goto
2204 <emphasis>WaitingForData</emphasis>
2212 <title><emphasis>WaitingForBegin</emphasis></title>
2217 Receive BEGIN → terminate auth conversation,
2218 client authenticated
2224 Receive CANCEL → send REJECTED [mechs], goto
2225 <emphasis>WaitingForAuth</emphasis>
2231 Receive ERROR → send REJECTED [mechs], goto
2232 <emphasis>WaitingForAuth</emphasis>
2238 Receive anything else → send ERROR, goto
2239 <emphasis>WaitingForBegin</emphasis>
2249 <sect2 id="auth-mechanisms">
2250 <title>Authentication mechanisms</title>
2252 This section describes some new authentication mechanisms.
2253 D-Bus also allows any standard SASL mechanism of course.
2255 <sect3 id="auth-mechanisms-sha">
2256 <title>DBUS_COOKIE_SHA1</title>
2258 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2259 has the ability to read a private file owned by the user being
2260 authenticated. If the client can prove that it has access to a secret
2261 cookie stored in this file, then the client is authenticated.
2262 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2266 Throughout this description, "hex encoding" must output the digits
2267 from a to f in lower-case; the digits A to F must not be used
2268 in the DBUS_COOKIE_SHA1 mechanism.
2271 Authentication proceeds as follows:
2275 The client sends the username it would like to authenticate
2281 The server sends the name of its "cookie context" (see below); a
2282 space character; the integer ID of the secret cookie the client
2283 must demonstrate knowledge of; a space character; then a
2284 randomly-generated challenge string, all of this hex-encoded into
2290 The client locates the cookie and generates its own
2291 randomly-generated challenge string. The client then concatenates
2292 the server's decoded challenge, a ":" character, its own challenge,
2293 another ":" character, and the cookie. It computes the SHA-1 hash
2294 of this composite string as a hex digest. It concatenates the
2295 client's challenge string, a space character, and the SHA-1 hex
2296 digest, hex-encodes the result and sends it back to the server.
2301 The server generates the same concatenated string used by the
2302 client and computes its SHA-1 hash. It compares the hash with
2303 the hash received from the client; if the two hashes match, the
2304 client is authenticated.
2310 Each server has a "cookie context," which is a name that identifies a
2311 set of cookies that apply to that server. A sample context might be
2312 "org_freedesktop_session_bus". Context names must be valid ASCII,
2313 nonzero length, and may not contain the characters slash ("/"),
2314 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2315 tab ("\t"), or period ("."). There is a default context,
2316 "org_freedesktop_general" that's used by servers that do not specify
2320 Cookies are stored in a user's home directory, in the directory
2321 <filename>~/.dbus-keyrings/</filename>. This directory must
2322 not be readable or writable by other users. If it is,
2323 clients and servers must ignore it. The directory
2324 contains cookie files named after the cookie context.
2327 A cookie file contains one cookie per line. Each line
2328 has three space-separated fields:
2332 The cookie ID number, which must be a non-negative integer and
2333 may not be used twice in the same file.
2338 The cookie's creation time, in UNIX seconds-since-the-epoch
2344 The cookie itself, a hex-encoded random block of bytes. The cookie
2345 may be of any length, though obviously security increases
2346 as the length increases.
2352 Only server processes modify the cookie file.
2353 They must do so with this procedure:
2357 Create a lockfile name by appending ".lock" to the name of the
2358 cookie file. The server should attempt to create this file
2359 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2360 fails, the lock fails. Servers should retry for a reasonable
2361 period of time, then they may choose to delete an existing lock
2362 to keep users from having to manually delete a stale
2363 lock. <footnote><para>Lockfiles are used instead of real file
2364 locking <literal>fcntl()</literal> because real locking
2365 implementations are still flaky on network
2366 filesystems.</para></footnote>
2371 Once the lockfile has been created, the server loads the cookie
2372 file. It should then delete any cookies that are old (the
2373 timeout can be fairly short), or more than a reasonable
2374 time in the future (so that cookies never accidentally
2375 become permanent, if the clock was set far into the future
2376 at some point). If no recent keys remain, the
2377 server may generate a new key.
2382 The pruned and possibly added-to cookie file
2383 must be resaved atomically (using a temporary
2384 file which is rename()'d).
2389 The lock must be dropped by deleting the lockfile.
2395 Clients need not lock the file in order to load it,
2396 because servers are required to save the file atomically.
2401 <sect1 id="addresses">
2402 <title>Server Addresses</title>
2404 Server addresses consist of a transport name followed by a colon, and
2405 then an optional, comma-separated list of keys and values in the form key=value.
2406 Each value is escaped.
2410 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2411 Which is the address to a unix socket with the path /tmp/dbus-test.
2414 Value escaping is similar to URI escaping but simpler.
2418 The set of optionally-escaped bytes is:
2419 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2420 <emphasis>byte</emphasis> (note, not character) which is not in the
2421 set of optionally-escaped bytes must be replaced with an ASCII
2422 percent (<literal>%</literal>) and the value of the byte in hex.
2423 The hex value must always be two digits, even if the first digit is
2424 zero. The optionally-escaped bytes may be escaped if desired.
2429 To unescape, append each byte in the value; if a byte is an ASCII
2430 percent (<literal>%</literal>) character then append the following
2431 hex value instead. It is an error if a <literal>%</literal> byte
2432 does not have two hex digits following. It is an error if a
2433 non-optionally-escaped byte is seen unescaped.
2437 The set of optionally-escaped bytes is intended to preserve address
2438 readability and convenience.
2442 A server may specify a key-value pair with the key <literal>guid</literal>
2443 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2444 describes the format of the <literal>guid</literal> field. If present,
2445 this UUID may be used to distinguish one server address from another. A
2446 server should use a different UUID for each address it listens on. For
2447 example, if a message bus daemon offers both UNIX domain socket and TCP
2448 connections, but treats clients the same regardless of how they connect,
2449 those two connections are equivalent post-connection but should have
2450 distinct UUIDs to distinguish the kinds of connection.
2454 The intent of the address UUID feature is to allow a client to avoid
2455 opening multiple identical connections to the same server, by allowing the
2456 client to check whether an address corresponds to an already-existing
2457 connection. Comparing two addresses is insufficient, because addresses
2458 can be recycled by distinct servers, and equivalent addresses may look
2459 different if simply compared as strings (for example, the host in a TCP
2460 address can be given as an IP address or as a hostname).
2464 Note that the address key is <literal>guid</literal> even though the
2465 rest of the API and documentation says "UUID," for historical reasons.
2469 [FIXME clarify if attempting to connect to each is a requirement
2470 or just a suggestion]
2471 When connecting to a server, multiple server addresses can be
2472 separated by a semi-colon. The library will then try to connect
2473 to the first address and if that fails, it'll try to connect to
2474 the next one specified, and so forth. For example
2475 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2480 <sect1 id="transports">
2481 <title>Transports</title>
2483 [FIXME we need to specify in detail each transport and its possible arguments]
2485 Current transports include: unix domain sockets (including
2486 abstract namespace on linux), launchd, TCP/IP, and a debug/testing transport
2487 using in-process pipes. Future possible transports include one that
2488 tunnels over X11 protocol.
2491 <sect2 id="transports-unix-domain-sockets">
2492 <title>Unix Domain Sockets</title>
2494 Unix domain sockets can be either paths in the file system or on Linux
2495 kernels, they can be abstract which are similar to paths but
2496 do not show up in the file system.
2500 When a socket is opened by the D-Bus library it truncates the path
2501 name right before the first trailing Nul byte. This is true for both
2502 normal paths and abstract paths. Note that this is a departure from
2503 previous versions of D-Bus that would create sockets with a fixed
2504 length path name. Names which were shorter than the fixed length
2505 would be padded by Nul bytes.
2508 Unix domain sockets are not available on windows.
2510 <sect3 id="transports-unix-domain-sockets-addresses">
2511 <title>Server Address Format</title>
2513 Unix domain socket addresses are identified by the "unix:" prefix
2514 and support the following key/value pairs:
2521 <entry>Values</entry>
2522 <entry>Description</entry>
2528 <entry>(path)</entry>
2529 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2532 <entry>tmpdir</entry>
2533 <entry>(path)</entry>
2534 <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>
2537 <entry>abstract</entry>
2538 <entry>(string)</entry>
2539 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2546 <sect2 id="transports-launchd">
2547 <title>launchd</title>
2549 launchd is a open-source server management system that replaces init, inetd
2550 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2551 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2555 launchd allocates a socket and provides it with the unix path through the
2556 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2557 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2558 it through its environment.
2559 Other processes can query for the launchd socket by executing:
2560 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2561 This is normally done by the D-Bus client library so doesn't have to be done
2565 launchd is not available on Microsoft Windows.
2567 <sect3 id="transports-launchd-addresses">
2568 <title>Server Address Format</title>
2570 launchd addresses are identified by the "launchd:" prefix
2571 and support the following key/value pairs:
2578 <entry>Values</entry>
2579 <entry>Description</entry>
2585 <entry>(environment variable)</entry>
2586 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2593 <sect2 id="transports-tcp-sockets">
2594 <title>TCP Sockets</title>
2596 The tcp transport provides TCP/IP based connections between clients
2597 located on the same or different hosts.
2600 Using tcp transport without any additional secure authentification mechanismus
2601 over a network is unsecure.
2604 Windows notes: Because of the tcp stack on windows does not provide sending
2605 credentials over a tcp connection, the EXTERNAL authentification
2606 mechanismus does not work.
2608 <sect3 id="transports-tcp-sockets-addresses">
2609 <title>Server Address Format</title>
2611 TCP/IP socket addresses are identified by the "tcp:" prefix
2612 and support the following key/value pairs:
2619 <entry>Values</entry>
2620 <entry>Description</entry>
2626 <entry>(string)</entry>
2627 <entry>dns name or ip address</entry>
2631 <entry>(number)</entry>
2632 <entry>The tcp port the server will open. A zero value let the server
2633 choose a free port provided from the underlaying operating system.
2634 libdbus is able to retrieve the real used port from the server.
2638 <entry>family</entry>
2639 <entry>(string)</entry>
2640 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2647 <sect2 id="transports-nonce-tcp-sockets">
2648 <title>Nonce-secured TCP Sockets</title>
2650 The nonce-tcp transport provides a secured TCP transport, using a
2651 simple authentication mechanism to ensure that only clients with read
2652 access to a certain location in the filesystem can connect to the server.
2653 The server writes a secret, the nonce, to a file and an incoming client
2654 connection is only accepted if the client sends the nonce right after
2655 the connect. The nonce mechanism requires no setup and is orthogonal to
2656 the higher-level authentication mechanisms described in the
2657 Authentication section.
2661 On start, the server generates a random 16 byte nonce and writes it
2662 to a file in the user's temporary directory. The nonce file location
2663 is published as part of the server's D-Bus address using the
2664 "noncefile" key-value pair.
2666 After an accept, the server reads 16 bytes from the socket. If the
2667 read bytes do not match the nonce stored in the nonce file, the
2668 server MUST immediately drop the connection.
2669 If the nonce match the received byte sequence, the client is accepted
2670 and the transport behaves like an unsecured tcp transport.
2673 After a successful connect to the server socket, the client MUST read
2674 the nonce from the file published by the server via the noncefile=
2675 key-value pair and send it over the socket. After that, the
2676 transport behaves like an unsecured tcp transport.
2678 <sect3 id="transports-nonce-tcp-sockets-addresses">
2679 <title>Server Address Format</title>
2681 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2682 and support the following key/value pairs:
2689 <entry>Values</entry>
2690 <entry>Description</entry>
2696 <entry>(string)</entry>
2697 <entry>dns name or ip address</entry>
2701 <entry>(number)</entry>
2702 <entry>The tcp port the server will open. A zero value let the server
2703 choose a free port provided from the underlaying operating system.
2704 libdbus is able to retrieve the real used port from the server.
2708 <entry>family</entry>
2709 <entry>(string)</entry>
2710 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2713 <entry>noncefile</entry>
2714 <entry>(path)</entry>
2715 <entry>file location containing the secret</entry>
2723 <sect1 id="meta-transports">
2724 <title>Meta Transports</title>
2726 Meta transports are a kind of transport with special enhancements or
2727 behavior. Currently available meta transports include: autolaunch
2730 <sect2 id="meta-transports-autolaunch">
2731 <title>Autolaunch</title>
2732 <para>The autolaunch transport provides a way for dbus clients to autodetect
2733 a running dbus session bus and to autolaunch a session bus if not present.
2735 <sect3 id="meta-transports-autolaunch-addresses">
2736 <title>Server Address Format</title>
2738 Autolaunch addresses uses the "autolaunch:" prefix and support the
2739 following key/value pairs:
2746 <entry>Values</entry>
2747 <entry>Description</entry>
2752 <entry>scope</entry>
2753 <entry>(string)</entry>
2754 <entry>scope of autolaunch (Windows only)
2758 "*install-path" - limit session bus to dbus installation path.
2759 The dbus installation path is determined from the location of
2760 the shared dbus library. If the library is located in a 'bin'
2761 subdirectory the installation root is the directory above,
2762 otherwise the directory where the library lives is taken as
2765 <install-root>/bin/[lib]dbus-1.dll
2766 <install-root>/[lib]dbus-1.dll
2772 "*user" - limit session bus to the recent user.
2777 other values - specify dedicated session bus like "release",
2789 <sect3 id="meta-transports-autolaunch-windows-implementation">
2790 <title>Windows implementation</title>
2792 On start, the server opens a platform specific transport, creates a mutex
2793 and a shared memory section containing the related session bus address.
2794 This mutex will be inspected by the dbus client library to detect a
2795 running dbus session bus. The access to the mutex and the shared memory
2796 section are protected by global locks.
2799 In the recent implementation the autolaunch transport uses a tcp transport
2800 on localhost with a port choosen from the operating system. This detail may
2801 change in the future.
2804 Disclaimer: The recent implementation is in an early state and may not
2805 work in all cirumstances and/or may have security issues. Because of this
2806 the implementation is not documentated yet.
2811 <sect1 id="naming-conventions">
2812 <title>Naming Conventions</title>
2815 D-Bus namespaces are all lowercase and correspond to reversed domain
2816 names, as with Java. e.g. "org.freedesktop"
2819 Interface, signal, method, and property names are "WindowsStyleCaps", note
2820 that the first letter is capitalized, unlike Java.
2823 Object paths are normally all lowercase with underscores used rather than
2829 <title>UUIDs</title>
2831 A working D-Bus implementation uses universally-unique IDs in two places.
2832 First, each server address has a UUID identifying the address,
2833 as described in <xref linkend="addresses"/>. Second, each operating
2834 system kernel instance running a D-Bus client or server has a UUID
2835 identifying that kernel, retrieved by invoking the method
2836 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2837 linkend="standard-interfaces-peer"/>).
2840 The term "UUID" in this document is intended literally, i.e. an
2841 identifier that is universally unique. It is not intended to refer to
2842 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2845 The UUID must contain 128 bits of data and be hex-encoded. The
2846 hex-encoded string may not contain hyphens or other non-hex-digit
2847 characters, and it must be exactly 32 characters long. To generate a
2848 UUID, the current reference implementation concatenates 96 bits of random
2849 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2853 It would also be acceptable and probably better to simply generate 128
2854 bits of random data, as long as the random number generator is of high
2855 quality. The timestamp could conceivably help if the random bits are not
2856 very random. With a quality random number generator, collisions are
2857 extremely unlikely even with only 96 bits, so it's somewhat academic.
2860 Implementations should, however, stick to random data for the first 96 bits
2865 <sect1 id="standard-interfaces">
2866 <title>Standard Interfaces</title>
2868 See <xref linkend="message-protocol-types-notation"/> for details on
2869 the notation used in this section. There are some standard interfaces
2870 that may be useful across various D-Bus applications.
2872 <sect2 id="standard-interfaces-peer">
2873 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2875 The <literal>org.freedesktop.DBus.Peer</literal> interface
2878 org.freedesktop.DBus.Peer.Ping ()
2879 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2883 On receipt of the <literal>METHOD_CALL</literal> message
2884 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2885 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2886 usual. It does not matter which object path a ping is sent to. The
2887 reference implementation handles this method automatically.
2890 On receipt of the <literal>METHOD_CALL</literal> message
2891 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2892 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2893 UUID representing the identity of the machine the process is running on.
2894 This UUID must be the same for all processes on a single system at least
2895 until that system next reboots. It should be the same across reboots
2896 if possible, but this is not always possible to implement and is not
2898 It does not matter which object path a GetMachineId is sent to. The
2899 reference implementation handles this method automatically.
2902 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2903 a virtual machine running on a hypervisor, rather than a physical machine.
2904 Basically if two processes see the same UUID, they should also see the same
2905 shared memory, UNIX domain sockets, process IDs, and other features that require
2906 a running OS kernel in common between the processes.
2909 The UUID is often used where other programs might use a hostname. Hostnames
2910 can change without rebooting, however, or just be "localhost" - so the UUID
2914 <xref linkend="uuids"/> explains the format of the UUID.
2918 <sect2 id="standard-interfaces-introspectable">
2919 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2921 This interface has one method:
2923 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2927 Objects instances may implement
2928 <literal>Introspect</literal> which returns an XML description of
2929 the object, including its interfaces (with signals and methods), objects
2930 below it in the object path tree, and its properties.
2933 <xref linkend="introspection-format"/> describes the format of this XML string.
2936 <sect2 id="standard-interfaces-properties">
2937 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2939 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2940 or <firstterm>attributes</firstterm>. These can be exposed via the
2941 <literal>org.freedesktop.DBus.Properties</literal> interface.
2945 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2946 in STRING property_name,
2948 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2949 in STRING property_name,
2951 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
2952 out DICT<STRING,VARIANT> props);
2956 The available properties and whether they are writable can be determined
2957 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2958 see <xref linkend="standard-interfaces-introspectable"/>.
2961 An empty string may be provided for the interface name; in this case,
2962 if there are multiple properties on an object with the same name,
2963 the results are undefined (picking one by according to an arbitrary
2964 deterministic rule, or returning an error, are the reasonable
2968 If one or more properties change on an object, the
2969 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
2970 signal may be emitted (this signal was added in 0.14):
2974 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
2975 DICT<STRING,VARIANT> changed_properties,
2976 ARRAY<STRING> invalidated_properties);
2980 where <literal>changed_properties</literal> is a dictionary
2981 containing the changed properties with the new values and
2982 <literal>invalidated_properties</literal> is an array of
2983 properties that changed but the value is not conveyed.
2986 Whether the <literal>PropertiesChanged</literal> signal is
2987 supported can be determined by calling
2988 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
2989 that the signal may be supported for an object but it may
2990 differ how whether and how it is used on a per-property basis
2991 (for e.g. performance or security reasons). Each property (or
2992 the parent interface) must be annotated with the
2993 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
2994 annotation to convey this (usually the default value
2995 <literal>true</literal> is sufficient meaning that the
2996 annotation does not need to be used). See <xref
2997 linkend="introspection-format"/> for details on this
3003 <sect1 id="introspection-format">
3004 <title>Introspection Data Format</title>
3006 As described in <xref linkend="standard-interfaces-introspectable"/>,
3007 objects may be introspected at runtime, returning an XML string
3008 that describes the object. The same XML format may be used in
3009 other contexts as well, for example as an "IDL" for generating
3010 static language bindings.
3013 Here is an example of introspection data:
3015 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3016 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3017 <node name="/org/freedesktop/sample_object">
3018 <interface name="org.freedesktop.SampleInterface">
3019 <method name="Frobate">
3020 <arg name="foo" type="i" direction="in"/>
3021 <arg name="bar" type="s" direction="out"/>
3022 <arg name="baz" type="a{us}" direction="out"/>
3023 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3025 <method name="Bazify">
3026 <arg name="bar" type="(iiu)" direction="in"/>
3027 <arg name="bar" type="v" direction="out"/>
3029 <method name="Mogrify">
3030 <arg name="bar" type="(iiav)" direction="in"/>
3032 <signal name="Changed">
3033 <arg name="new_value" type="b"/>
3035 <property name="Bar" type="y" access="readwrite"/>
3037 <node name="child_of_sample_object"/>
3038 <node name="another_child_of_sample_object"/>
3043 A more formal DTD and spec needs writing, but here are some quick notes.
3047 Only the root <node> element can omit the node name, as it's
3048 known to be the object that was introspected. If the root
3049 <node> does have a name attribute, it must be an absolute
3050 object path. If child <node> have object paths, they must be
3056 If a child <node> has any sub-elements, then they
3057 must represent a complete introspection of the child.
3058 If a child <node> is empty, then it may or may
3059 not have sub-elements; the child must be introspected
3060 in order to find out. The intent is that if an object
3061 knows that its children are "fast" to introspect
3062 it can go ahead and return their information, but
3063 otherwise it can omit it.
3068 The direction element on <arg> may be omitted,
3069 in which case it defaults to "in" for method calls
3070 and "out" for signals. Signals only allow "out"
3071 so while direction may be specified, it's pointless.
3076 The possible directions are "in" and "out",
3077 unlike CORBA there is no "inout"
3082 The possible property access flags are
3083 "readwrite", "read", and "write"
3088 Multiple interfaces can of course be listed for
3094 The "name" attribute on arguments is optional.
3100 Method, interface, property, and signal elements may have
3101 "annotations", which are generic key/value pairs of metadata.
3102 They are similar conceptually to Java's annotations and C# attributes.
3103 Well-known annotations:
3110 <entry>Values (separated by ,)</entry>
3111 <entry>Description</entry>
3116 <entry>org.freedesktop.DBus.Deprecated</entry>
3117 <entry>true,false</entry>
3118 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3121 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3122 <entry>(string)</entry>
3123 <entry>The C symbol; may be used for methods and interfaces</entry>
3126 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3127 <entry>true,false</entry>
3128 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3131 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3132 <entry>true,invalidates,false</entry>
3135 If set to <literal>false</literal>, the
3136 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3138 linkend="standard-interfaces-properties"/> is not
3139 guaranteed to be emitted if the property changes.
3142 If set to <literal>invalidates</literal> the signal
3143 is emitted but the value is not included in the
3147 If set to <literal>true</literal> the signal is
3148 emitted with the value included.
3151 The value for the annotation defaults to
3152 <literal>true</literal> if the enclosing interface
3153 element does not specify the annotation. Otherwise it
3154 defaults to the value specified in the enclosing
3163 <sect1 id="message-bus">
3164 <title>Message Bus Specification</title>
3165 <sect2 id="message-bus-overview">
3166 <title>Message Bus Overview</title>
3168 The message bus accepts connections from one or more applications.
3169 Once connected, applications can exchange messages with other
3170 applications that are also connected to the bus.
3173 In order to route messages among connections, the message bus keeps a
3174 mapping from names to connections. Each connection has one
3175 unique-for-the-lifetime-of-the-bus name automatically assigned.
3176 Applications may request additional names for a connection. Additional
3177 names are usually "well-known names" such as
3178 "org.freedesktop.TextEditor". When a name is bound to a connection,
3179 that connection is said to <firstterm>own</firstterm> the name.
3182 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3183 This name routes messages to the bus, allowing applications to make
3184 administrative requests. For example, applications can ask the bus
3185 to assign a name to a connection.
3188 Each name may have <firstterm>queued owners</firstterm>. When an
3189 application requests a name for a connection and the name is already in
3190 use, the bus will optionally add the connection to a queue waiting for
3191 the name. If the current owner of the name disconnects or releases
3192 the name, the next connection in the queue will become the new owner.
3196 This feature causes the right thing to happen if you start two text
3197 editors for example; the first one may request "org.freedesktop.TextEditor",
3198 and the second will be queued as a possible owner of that name. When
3199 the first exits, the second will take over.
3203 Messages may have a <literal>DESTINATION</literal> field (see <xref
3204 linkend="message-protocol-header-fields"/>). If the
3205 <literal>DESTINATION</literal> field is present, it specifies a message
3206 recipient by name. Method calls and replies normally specify this field.
3207 The message bus must send messages (of any type) with the
3208 <literal>DESTINATION</literal> field set to the specified recipient,
3209 regardless of whether the recipient has set up a match rule matching
3214 Signals normally do not specify a destination; they are sent to all
3215 applications with <firstterm>message matching rules</firstterm> that
3220 When the message bus receives a method call, if the
3221 <literal>DESTINATION</literal> field is absent, the call is taken to be
3222 a standard one-to-one message and interpreted by the message bus
3223 itself. For example, sending an
3224 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3225 <literal>DESTINATION</literal> will cause the message bus itself to
3226 reply to the ping immediately; the message bus will not make this
3227 message visible to other applications.
3231 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3232 the ping message were sent with a <literal>DESTINATION</literal> name of
3233 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3234 forwarded, and the Yoyodyne Corporation screensaver application would be
3235 expected to reply to the ping.
3239 <sect2 id="message-bus-names">
3240 <title>Message Bus Names</title>
3242 Each connection has at least one name, assigned at connection time and
3243 returned in response to the
3244 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3245 automatically-assigned name is called the connection's <firstterm>unique
3246 name</firstterm>. Unique names are never reused for two different
3247 connections to the same bus.
3250 Ownership of a unique name is a prerequisite for interaction with
3251 the message bus. It logically follows that the unique name is always
3252 the first name that an application comes to own, and the last
3253 one that it loses ownership of.
3256 Unique connection names must begin with the character ':' (ASCII colon
3257 character); bus names that are not unique names must not begin
3258 with this character. (The bus must reject any attempt by an application
3259 to manually request a name beginning with ':'.) This restriction
3260 categorically prevents "spoofing"; messages sent to a unique name
3261 will always go to the expected connection.
3264 When a connection is closed, all the names that it owns are deleted (or
3265 transferred to the next connection in the queue if any).
3268 A connection can request additional names to be associated with it using
3269 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3270 linkend="message-protocol-names-bus"/> describes the format of a valid
3271 name. These names can be released again using the
3272 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3275 <sect3 id="bus-messages-request-name">
3276 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3280 UINT32 RequestName (in STRING name, in UINT32 flags)
3287 <entry>Argument</entry>
3289 <entry>Description</entry>
3295 <entry>STRING</entry>
3296 <entry>Name to request</entry>
3300 <entry>UINT32</entry>
3301 <entry>Flags</entry>
3311 <entry>Argument</entry>
3313 <entry>Description</entry>
3319 <entry>UINT32</entry>
3320 <entry>Return value</entry>
3327 This method call should be sent to
3328 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3329 assign the given name to the method caller. Each name maintains a
3330 queue of possible owners, where the head of the queue is the primary
3331 or current owner of the name. Each potential owner in the queue
3332 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3333 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3334 call. When RequestName is invoked the following occurs:
3338 If the method caller is currently the primary owner of the name,
3339 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3340 values are updated with the values from the new RequestName call,
3341 and nothing further happens.
3347 If the current primary owner (head of the queue) has
3348 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3349 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3350 the caller of RequestName replaces the current primary owner at
3351 the head of the queue and the current primary owner moves to the
3352 second position in the queue. If the caller of RequestName was
3353 in the queue previously its flags are updated with the values from
3354 the new RequestName in addition to moving it to the head of the queue.
3360 If replacement is not possible, and the method caller is
3361 currently in the queue but not the primary owner, its flags are
3362 updated with the values from the new RequestName call.
3368 If replacement is not possible, and the method caller is
3369 currently not in the queue, the method caller is appended to the
3376 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3377 set and is not the primary owner, it is removed from the
3378 queue. This can apply to the previous primary owner (if it
3379 was replaced) or the method caller (if it updated the
3380 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3381 queue, or if it was just added to the queue with that flag set).
3387 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3388 queue," even if another application already in the queue had specified
3389 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3390 that does not allow replacement goes away, and the next primary owner
3391 does allow replacement. In this case, queued items that specified
3392 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3393 automatically replace the new primary owner. In other words,
3394 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3395 time RequestName is called. This is deliberate to avoid an infinite loop
3396 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3397 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3400 The flags argument contains any of the following values logically ORed
3407 <entry>Conventional Name</entry>
3408 <entry>Value</entry>
3409 <entry>Description</entry>
3414 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3418 If an application A specifies this flag and succeeds in
3419 becoming the owner of the name, and another application B
3420 later calls RequestName with the
3421 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3422 will lose ownership and receive a
3423 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3424 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3425 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3426 is not specified by application B, then application B will not replace
3427 application A as the owner.
3432 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3436 Try to replace the current owner if there is one. If this
3437 flag is not set the application will only become the owner of
3438 the name if there is no current owner. If this flag is set,
3439 the application will replace the current owner if
3440 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3445 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3449 Without this flag, if an application requests a name that is
3450 already owned, the application will be placed in a queue to
3451 own the name when the current owner gives it up. If this
3452 flag is given, the application will not be placed in the
3453 queue, the request for the name will simply fail. This flag
3454 also affects behavior when an application is replaced as
3455 name owner; by default the application moves back into the
3456 waiting queue, unless this flag was provided when the application
3457 became the name owner.
3465 The return code can be one of the following values:
3471 <entry>Conventional Name</entry>
3472 <entry>Value</entry>
3473 <entry>Description</entry>
3478 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3479 <entry>1</entry> <entry>The caller is now the primary owner of
3480 the name, replacing any previous owner. Either the name had no
3481 owner before, or the caller specified
3482 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3483 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3486 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3489 <entry>The name already had an owner,
3490 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3491 the current owner did not specify
3492 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3493 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3497 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3498 <entry>The name already has an owner,
3499 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3500 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3501 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3502 specified by the requesting application.</entry>
3505 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3507 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3515 <sect3 id="bus-messages-release-name">
3516 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3520 UINT32 ReleaseName (in STRING name)
3527 <entry>Argument</entry>
3529 <entry>Description</entry>
3535 <entry>STRING</entry>
3536 <entry>Name to release</entry>
3546 <entry>Argument</entry>
3548 <entry>Description</entry>
3554 <entry>UINT32</entry>
3555 <entry>Return value</entry>
3562 This method call should be sent to
3563 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3564 release the method caller's claim to the given name. If the caller is
3565 the primary owner, a new primary owner will be selected from the
3566 queue if any other owners are waiting. If the caller is waiting in
3567 the queue for the name, the caller will removed from the queue and
3568 will not be made an owner of the name if it later becomes available.
3569 If there are no other owners in the queue for the name, it will be
3570 removed from the bus entirely.
3572 The return code can be one of the following values:
3578 <entry>Conventional Name</entry>
3579 <entry>Value</entry>
3580 <entry>Description</entry>
3585 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3586 <entry>1</entry> <entry>The caller has released his claim on
3587 the given name. Either the caller was the primary owner of
3588 the name, and the name is now unused or taken by somebody
3589 waiting in the queue for the name, or the caller was waiting
3590 in the queue for the name and has now been removed from the
3594 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3596 <entry>The given name does not exist on this bus.</entry>
3599 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3601 <entry>The caller was not the primary owner of this name,
3602 and was also not waiting in the queue to own this name.</entry>
3610 <sect3 id="bus-messages-list-queued-owners">
3611 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3615 ARRAY of STRING ListQueuedOwners (in STRING name)
3622 <entry>Argument</entry>
3624 <entry>Description</entry>
3630 <entry>STRING</entry>
3631 <entry>The well-known bus name to query, such as
3632 <literal>com.example.cappuccino</literal></entry>
3642 <entry>Argument</entry>
3644 <entry>Description</entry>
3650 <entry>ARRAY of STRING</entry>
3651 <entry>The unique bus names of connections currently queued
3652 for the name</entry>
3659 This method call should be sent to
3660 <literal>org.freedesktop.DBus</literal> and lists the connections
3661 currently queued for a bus name (see
3662 <xref linkend="term-queued-owner"/>).
3667 <sect2 id="message-bus-routing">
3668 <title>Message Bus Message Routing</title>
3672 <sect3 id="message-bus-routing-match-rules">
3673 <title>Match Rules</title>
3675 An important part of the message bus routing protocol is match
3676 rules. Match rules describe what messages can be sent to a client
3677 based on the contents of the message. When a message is routed
3678 through the bus it is compared to clients' match rules. If any
3679 of the rules match, the message is dispatched to the client.
3680 If none of the rules match the message never leaves the bus. This
3681 is an effective way to control traffic over the bus and to make sure
3682 only relevant message need to be processed by the client.
3685 Match rules are added using the AddMatch bus method
3686 (see <xref linkend="bus-messages-add-match"/>). Rules are
3687 specified as a string of comma separated key/value pairs.
3688 Excluding a key from the rule indicates a wildcard match.
3689 For instance excluding the the member from a match rule but
3690 adding a sender would let all messages from that sender through.
3691 An example of a complete rule would be
3692 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3695 The following table describes the keys that can be used to create
3697 The following table summarizes the D-Bus types.
3703 <entry>Possible Values</entry>
3704 <entry>Description</entry>
3709 <entry><literal>type</literal></entry>
3710 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3711 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3714 <entry><literal>sender</literal></entry>
3715 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3716 and <xref linkend="term-unique-name"/> respectively)
3718 <entry>Match messages sent by a particular sender. An example of a sender match
3719 is sender='org.freedesktop.Hal'</entry>
3722 <entry><literal>interface</literal></entry>
3723 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3724 <entry>Match messages sent over or to a particular interface. An example of an
3725 interface match is interface='org.freedesktop.Hal.Manager'.
3726 If a message omits the interface header, it must not match any rule
3727 that specifies this key.</entry>
3730 <entry><literal>member</literal></entry>
3731 <entry>Any valid method or signal name</entry>
3732 <entry>Matches messages which have the give method or signal name. An example of
3733 a member match is member='NameOwnerChanged'</entry>
3736 <entry><literal>path</literal></entry>
3737 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3738 <entry>Matches messages which are sent from or to the given object. An example of a
3739 path match is path='/org/freedesktop/Hal/Manager'</entry>
3742 <entry><literal>path_namespace</literal></entry>
3743 <entry>An object path</entry>
3746 Matches messages which are sent from or to an
3747 object for which the object path is either the
3748 given value, or that value followed by one or
3749 more path components.
3754 <literal>path_namespace='/com/example/foo'</literal>
3755 would match signals sent by
3756 <literal>/com/example/foo</literal>
3758 <literal>/com/example/foo/bar</literal>,
3760 <literal>/com/example/foobar</literal>.
3764 Using both <literal>path</literal> and
3765 <literal>path_namespace</literal> in the same match
3766 rule is not allowed.
3771 This match key was added in version 0.16 of the
3772 D-Bus specification and implemented by the bus
3773 daemon in dbus 1.5.0 and later.
3779 <entry><literal>destination</literal></entry>
3780 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3781 <entry>Matches messages which are being sent to the given unique name. An
3782 example of a destination match is destination=':1.0'</entry>
3785 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3786 <entry>Any string</entry>
3787 <entry>Arg matches are special and are used for further restricting the
3788 match based on the arguments in the body of a message. Only arguments of type
3789 STRING can be matched in this way. An example of an argument match
3790 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3794 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
3795 <entry>Any string</entry>
3797 <para>Argument path matches provide a specialised form of wildcard matching for
3798 path-like namespaces. They can match arguments whose type is either STRING or
3799 OBJECT_PATH. As with normal argument matches,
3800 if the argument is exactly equal to the string given in the match
3801 rule then the rule is satisfied. Additionally, there is also a
3802 match when either the string given in the match rule or the
3803 appropriate message argument ends with '/' and is a prefix of the
3804 other. An example argument path match is arg0path='/aa/bb/'. This
3805 would match messages with first arguments of '/', '/aa/',
3806 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
3807 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
3809 <para>This is intended for monitoring “directories” in file system-like
3810 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
3811 system. An application interested in all nodes in a particular hierarchy would
3812 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
3813 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
3814 represent a modification to the “bar” property, or a signal with zeroth
3815 argument <literal>"/ca/example/"</literal> to represent atomic modification of
3816 many properties within that directory, and the interested application would be
3817 notified in both cases.</para>
3820 This match key was added in version 0.12 of the
3821 D-Bus specification, implemented for STRING
3822 arguments by the bus daemon in dbus 1.2.0 and later,
3823 and implemented for OBJECT_PATH arguments in dbus 1.5.0
3830 <entry><literal>arg0namespace</literal></entry>
3831 <entry>Like a bus name, except that the string is not
3832 required to contain a '.' (period)</entry>
3834 <para>Match messages whose first argument is of type STRING, and is a bus name
3835 or interface name within the specified namespace. This is primarily intended
3836 for watching name owner changes for a group of related bus names, rather than
3837 for a single name or all name changes.</para>
3839 <para>Because every valid interface name is also a valid
3840 bus name, this can also be used for messages whose
3841 first argument is an interface name.</para>
3843 <para>For example, the match rule
3844 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
3845 matches name owner changes for bus names such as
3846 <literal>com.example.backend.foo</literal>,
3847 <literal>com.example.backend.foo.bar</literal>, and
3848 <literal>com.example.backend</literal> itself.</para>
3850 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
3853 This match key was added in version 0.16 of the
3854 D-Bus specification and implemented by the bus
3855 daemon in dbus 1.5.0 and later.
3866 <sect2 id="message-bus-starting-services">
3867 <title>Message Bus Starting Services</title>
3869 The message bus can start applications on behalf of other applications.
3870 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3871 An application that can be started in this way is called a
3872 <firstterm>service</firstterm>.
3875 With D-Bus, starting a service is normally done by name. That is,
3876 applications ask the message bus to start some program that will own a
3877 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3878 This implies a contract documented along with the name
3879 <literal>org.freedesktop.TextEditor</literal> for which objects
3880 the owner of that name will provide, and what interfaces those
3884 To find an executable corresponding to a particular name, the bus daemon
3885 looks for <firstterm>service description files</firstterm>. Service
3886 description files define a mapping from names to executables. Different
3887 kinds of message bus will look for these files in different places, see
3888 <xref linkend="message-bus-types"/>.
3891 Service description files have the ".service" file
3892 extension. The message bus will only load service description files
3893 ending with .service; all other files will be ignored. The file format
3894 is similar to that of <ulink
3895 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
3896 entries</ulink>. All service description files must be in UTF-8
3897 encoding. To ensure that there will be no name collisions, service files
3898 must be namespaced using the same mechanism as messages and service
3903 [FIXME the file format should be much better specified than "similar to
3904 .desktop entries" esp. since desktop entries are already
3905 badly-specified. ;-)]
3906 These sections from the specification apply to service files as well:
3909 <listitem><para>General syntax</para></listitem>
3910 <listitem><para>Comment format</para></listitem>
3914 <title>Example service description file</title>
3916 # Sample service description file
3918 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3919 Exec=/usr/libexec/gconfd-2
3924 When an application asks to start a service by name, the bus daemon tries to
3925 find a service that will own that name. It then tries to spawn the
3926 executable associated with it. If this fails, it will report an
3927 error. [FIXME what happens if two .service files offer the same service;
3928 what kind of error is reported, should we have a way for the client to
3932 The executable launched will have the environment variable
3933 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3934 message bus so it can connect and request the appropriate names.
3937 The executable being launched may want to know whether the message bus
3938 starting it is one of the well-known message buses (see <xref
3939 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3940 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3941 of the well-known buses. The currently-defined values for this variable
3942 are <literal>system</literal> for the systemwide message bus,
3943 and <literal>session</literal> for the per-login-session message
3944 bus. The new executable must still connect to the address given
3945 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3946 resulting connection is to the well-known bus.
3949 [FIXME there should be a timeout somewhere, either specified
3950 in the .service file, by the client, or just a global value
3951 and if the client being activated fails to connect within that
3952 timeout, an error should be sent back.]
3955 <sect3 id="message-bus-starting-services-scope">
3956 <title>Message Bus Service Scope</title>
3958 The "scope" of a service is its "per-", such as per-session,
3959 per-machine, per-home-directory, or per-display. The reference
3960 implementation doesn't yet support starting services in a different
3961 scope from the message bus itself. So e.g. if you start a service
3962 on the session bus its scope is per-session.
3965 We could add an optional scope to a bus name. For example, for
3966 per-(display,session pair), we could have a unique ID for each display
3967 generated automatically at login and set on screen 0 by executing a
3968 special "set display ID" binary. The ID would be stored in a
3969 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3970 random bytes. This ID would then be used to scope names.
3971 Starting/locating a service could be done by ID-name pair rather than
3975 Contrast this with a per-display scope. To achieve that, we would
3976 want a single bus spanning all sessions using a given display.
3977 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3978 property on screen 0 of the display, pointing to this bus.
3983 <sect2 id="message-bus-types">
3984 <title>Well-known Message Bus Instances</title>
3986 Two standard message bus instances are defined here, along with how
3987 to locate them and where their service files live.
3989 <sect3 id="message-bus-types-login">
3990 <title>Login session message bus</title>
3992 Each time a user logs in, a <firstterm>login session message
3993 bus</firstterm> may be started. All applications in the user's login
3994 session may interact with one another using this message bus.
3997 The address of the login session message bus is given
3998 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3999 variable. If that variable is not set, applications may
4000 also try to read the address from the X Window System root
4001 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4002 The root window property must have type <literal>STRING</literal>.
4003 The environment variable should have precedence over the
4004 root window property.
4006 <para>The address of the login session message bus is given in the
4007 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4008 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4009 "autolaunch:", the system should use platform-specific methods of
4010 locating a running D-Bus session server, or starting one if a running
4011 instance cannot be found. Note that this mechanism is not recommended
4012 for attempting to determine if a daemon is running. It is inherently
4013 racy to attempt to make this determination, since the bus daemon may
4014 be started just before or just after the determination is made.
4015 Therefore, it is recommended that applications do not try to make this
4016 determination for their functionality purposes, and instead they
4017 should attempt to start the server.</para>
4019 <sect4 id="message-bus-types-login-x-windows">
4020 <title>X Windowing System</title>
4022 For the X Windowing System, the application must locate the
4023 window owner of the selection represented by the atom formed by
4027 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4031 <para>the current user's username</para>
4035 <para>the literal character '_' (underscore)</para>
4039 <para>the machine's ID</para>
4045 The following properties are defined for the window that owns
4047 <informaltable frame="all">
4056 <para>meaning</para>
4062 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4066 <para>the actual address of the server socket</para>
4072 <para>_DBUS_SESSION_BUS_PID</para>
4076 <para>the PID of the server process</para>
4085 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4086 present in this window.
4090 If the X selection cannot be located or if reading the
4091 properties from the window fails, the implementation MUST conclude
4092 that there is no D-Bus server running and proceed to start a new
4093 server. (See below on concurrency issues)
4097 Failure to connect to the D-Bus server address thus obtained
4098 MUST be treated as a fatal connection error and should be reported
4103 As an alternative, an implementation MAY find the information
4104 in the following file located in the current user's home directory,
4105 in subdirectory .dbus/session-bus/:
4108 <para>the machine's ID</para>
4112 <para>the literal character '-' (dash)</para>
4116 <para>the X display without the screen number, with the
4117 following prefixes removed, if present: ":", "localhost:"
4118 ."localhost.localdomain:". That is, a display of
4119 "localhost:10.0" produces just the number "10"</para>
4125 The contents of this file NAME=value assignment pairs and
4126 lines starting with # are comments (no comments are allowed
4127 otherwise). The following variable names are defined:
4134 <para>Variable</para>
4138 <para>meaning</para>
4144 <para>DBUS_SESSION_BUS_ADDRESS</para>
4148 <para>the actual address of the server socket</para>
4154 <para>DBUS_SESSION_BUS_PID</para>
4158 <para>the PID of the server process</para>
4164 <para>DBUS_SESSION_BUS_WINDOWID</para>
4168 <para>the window ID</para>
4177 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4182 Failure to open this file MUST be interpreted as absence of a
4183 running server. Therefore, the implementation MUST proceed to
4184 attempting to launch a new bus server if the file cannot be
4189 However, success in opening this file MUST NOT lead to the
4190 conclusion that the server is running. Thus, a failure to connect to
4191 the bus address obtained by the alternative method MUST NOT be
4192 considered a fatal error. If the connection cannot be established,
4193 the implementation MUST proceed to check the X selection settings or
4194 to start the server on its own.
4198 If the implementation concludes that the D-Bus server is not
4199 running it MUST attempt to start a new server and it MUST also
4200 ensure that the daemon started as an effect of the "autolaunch"
4201 mechanism provides the lookup mechanisms described above, so
4202 subsequent calls can locate the newly started server. The
4203 implementation MUST also ensure that if two or more concurrent
4204 initiations happen, only one server remains running and all other
4205 initiations are able to obtain the address of this server and
4206 connect to it. In other words, the implementation MUST ensure that
4207 the X selection is not present when it attempts to set it, without
4208 allowing another process to set the selection between the
4209 verification and the setting (e.g., by using XGrabServer /
4216 [FIXME specify location of .service files, probably using
4217 DESKTOP_DIRS etc. from basedir specification, though login session
4218 bus is not really desktop-specific]
4222 <sect3 id="message-bus-types-system">
4223 <title>System message bus</title>
4225 A computer may have a <firstterm>system message bus</firstterm>,
4226 accessible to all applications on the system. This message bus may be
4227 used to broadcast system events, such as adding new hardware devices,
4228 changes in the printer queue, and so forth.
4231 The address of the system message bus is given
4232 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4233 variable. If that variable is not set, applications should try
4234 to connect to the well-known address
4235 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4238 The D-Bus reference implementation actually honors the
4239 <literal>$(localstatedir)</literal> configure option
4240 for this address, on both client and server side.
4245 [FIXME specify location of system bus .service files]
4250 <sect2 id="message-bus-messages">
4251 <title>Message Bus Messages</title>
4253 The special message bus name <literal>org.freedesktop.DBus</literal>
4254 responds to a number of additional messages.
4257 <sect3 id="bus-messages-hello">
4258 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4269 <entry>Argument</entry>
4271 <entry>Description</entry>
4277 <entry>STRING</entry>
4278 <entry>Unique name assigned to the connection</entry>
4285 Before an application is able to send messages to other applications
4286 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4287 to the message bus to obtain a unique name. If an application without
4288 a unique name tries to send a message to another application, or a
4289 message to the message bus itself that isn't the
4290 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4291 disconnected from the bus.
4294 There is no corresponding "disconnect" request; if a client wishes to
4295 disconnect from the bus, it simply closes the socket (or other
4296 communication channel).
4299 <sect3 id="bus-messages-list-names">
4300 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4304 ARRAY of STRING ListNames ()
4311 <entry>Argument</entry>
4313 <entry>Description</entry>
4319 <entry>ARRAY of STRING</entry>
4320 <entry>Array of strings where each string is a bus name</entry>
4327 Returns a list of all currently-owned names on the bus.
4330 <sect3 id="bus-messages-list-activatable-names">
4331 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4335 ARRAY of STRING ListActivatableNames ()
4342 <entry>Argument</entry>
4344 <entry>Description</entry>
4350 <entry>ARRAY of STRING</entry>
4351 <entry>Array of strings where each string is a bus name</entry>
4358 Returns a list of all names that can be activated on the bus.
4361 <sect3 id="bus-messages-name-exists">
4362 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4366 BOOLEAN NameHasOwner (in STRING name)
4373 <entry>Argument</entry>
4375 <entry>Description</entry>
4381 <entry>STRING</entry>
4382 <entry>Name to check</entry>
4392 <entry>Argument</entry>
4394 <entry>Description</entry>
4400 <entry>BOOLEAN</entry>
4401 <entry>Return value, true if the name exists</entry>
4408 Checks if the specified name exists (currently has an owner).
4412 <sect3 id="bus-messages-name-owner-changed">
4413 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4417 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4424 <entry>Argument</entry>
4426 <entry>Description</entry>
4432 <entry>STRING</entry>
4433 <entry>Name with a new owner</entry>
4437 <entry>STRING</entry>
4438 <entry>Old owner or empty string if none</entry>
4442 <entry>STRING</entry>
4443 <entry>New owner or empty string if none</entry>
4450 This signal indicates that the owner of a name has changed.
4451 It's also the signal to use to detect the appearance of
4452 new names on the bus.
4455 <sect3 id="bus-messages-name-lost">
4456 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4460 NameLost (STRING name)
4467 <entry>Argument</entry>
4469 <entry>Description</entry>
4475 <entry>STRING</entry>
4476 <entry>Name which was lost</entry>
4483 This signal is sent to a specific application when it loses
4484 ownership of a name.
4488 <sect3 id="bus-messages-name-acquired">
4489 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4493 NameAcquired (STRING name)
4500 <entry>Argument</entry>
4502 <entry>Description</entry>
4508 <entry>STRING</entry>
4509 <entry>Name which was acquired</entry>
4516 This signal is sent to a specific application when it gains
4517 ownership of a name.
4521 <sect3 id="bus-messages-start-service-by-name">
4522 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4526 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4533 <entry>Argument</entry>
4535 <entry>Description</entry>
4541 <entry>STRING</entry>
4542 <entry>Name of the service to start</entry>
4546 <entry>UINT32</entry>
4547 <entry>Flags (currently not used)</entry>
4557 <entry>Argument</entry>
4559 <entry>Description</entry>
4565 <entry>UINT32</entry>
4566 <entry>Return value</entry>
4571 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4575 The return value can be one of the following values:
4580 <entry>Identifier</entry>
4581 <entry>Value</entry>
4582 <entry>Description</entry>
4587 <entry>DBUS_START_REPLY_SUCCESS</entry>
4589 <entry>The service was successfully started.</entry>
4592 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4594 <entry>A connection already owns the given name.</entry>
4603 <sect3 id="bus-messages-update-activation-environment">
4604 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4608 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4615 <entry>Argument</entry>
4617 <entry>Description</entry>
4623 <entry>ARRAY of DICT<STRING,STRING></entry>
4624 <entry>Environment to add or update</entry>
4629 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4632 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4635 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.
4640 <sect3 id="bus-messages-get-name-owner">
4641 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4645 STRING GetNameOwner (in STRING name)
4652 <entry>Argument</entry>
4654 <entry>Description</entry>
4660 <entry>STRING</entry>
4661 <entry>Name to get the owner of</entry>
4671 <entry>Argument</entry>
4673 <entry>Description</entry>
4679 <entry>STRING</entry>
4680 <entry>Return value, a unique connection name</entry>
4685 Returns the unique connection name of the primary owner of the name
4686 given. If the requested name doesn't have an owner, returns a
4687 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4691 <sect3 id="bus-messages-get-connection-unix-user">
4692 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4696 UINT32 GetConnectionUnixUser (in STRING bus_name)
4703 <entry>Argument</entry>
4705 <entry>Description</entry>
4711 <entry>STRING</entry>
4712 <entry>Unique or well-known bus name of the connection to
4713 query, such as <literal>:12.34</literal> or
4714 <literal>com.example.tea</literal></entry>
4724 <entry>Argument</entry>
4726 <entry>Description</entry>
4732 <entry>UINT32</entry>
4733 <entry>Unix user ID</entry>
4738 Returns the Unix user ID of the process connected to the server. If
4739 unable to determine it (for instance, because the process is not on the
4740 same machine as the bus daemon), an error is returned.
4744 <sect3 id="bus-messages-get-connection-unix-process-id">
4745 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
4749 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
4756 <entry>Argument</entry>
4758 <entry>Description</entry>
4764 <entry>STRING</entry>
4765 <entry>Unique or well-known bus name of the connection to
4766 query, such as <literal>:12.34</literal> or
4767 <literal>com.example.tea</literal></entry>
4777 <entry>Argument</entry>
4779 <entry>Description</entry>
4785 <entry>UINT32</entry>
4786 <entry>Unix process id</entry>
4791 Returns the Unix process ID of the process connected to the server. If
4792 unable to determine it (for instance, because the process is not on the
4793 same machine as the bus daemon), an error is returned.
4797 <sect3 id="bus-messages-add-match">
4798 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
4802 AddMatch (in STRING rule)
4809 <entry>Argument</entry>
4811 <entry>Description</entry>
4817 <entry>STRING</entry>
4818 <entry>Match rule to add to the connection</entry>
4823 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
4824 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
4828 <sect3 id="bus-messages-remove-match">
4829 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
4833 RemoveMatch (in STRING rule)
4840 <entry>Argument</entry>
4842 <entry>Description</entry>
4848 <entry>STRING</entry>
4849 <entry>Match rule to remove from the connection</entry>
4854 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
4855 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
4860 <sect3 id="bus-messages-get-id">
4861 <title><literal>org.freedesktop.DBus.GetId</literal></title>
4865 GetId (out STRING id)
4872 <entry>Argument</entry>
4874 <entry>Description</entry>
4880 <entry>STRING</entry>
4881 <entry>Unique ID identifying the bus daemon</entry>
4886 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
4887 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
4888 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
4889 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
4890 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
4891 by org.freedesktop.DBus.Peer.GetMachineId().
4892 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
4900 <appendix id="implementation-notes">
4901 <title>Implementation notes</title>
4902 <sect1 id="implementation-notes-subsection">
4910 <glossary><title>Glossary</title>
4912 This glossary defines some of the terms used in this specification.
4915 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
4918 The message bus maintains an association between names and
4919 connections. (Normally, there's one connection per application.) A
4920 bus name is simply an identifier used to locate connections. For
4921 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
4922 name might be used to send a message to a screensaver from Yoyodyne
4923 Corporation. An application is said to <firstterm>own</firstterm> a
4924 name if the message bus has associated the application's connection
4925 with the name. Names may also have <firstterm>queued
4926 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
4927 The bus assigns a unique name to each connection,
4928 see <xref linkend="term-unique-name"/>. Other names
4929 can be thought of as "well-known names" and are
4930 used to find applications that offer specific functionality.
4935 <glossentry id="term-message"><glossterm>Message</glossterm>
4938 A message is the atomic unit of communication via the D-Bus
4939 protocol. It consists of a <firstterm>header</firstterm> and a
4940 <firstterm>body</firstterm>; the body is made up of
4941 <firstterm>arguments</firstterm>.
4946 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
4949 The message bus is a special application that forwards
4950 or routes messages between a group of applications
4951 connected to the message bus. It also manages
4952 <firstterm>names</firstterm> used for routing
4958 <glossentry id="term-name"><glossterm>Name</glossterm>
4961 See <xref linkend="term-bus-name"/>. "Name" may
4962 also be used to refer to some of the other names
4963 in D-Bus, such as interface names.
4968 <glossentry id="namespace"><glossterm>Namespace</glossterm>
4971 Used to prevent collisions when defining new interfaces or bus
4972 names. The convention used is the same one Java uses for defining
4973 classes: a reversed domain name.
4978 <glossentry id="term-object"><glossterm>Object</glossterm>
4981 Each application contains <firstterm>objects</firstterm>, which have
4982 <firstterm>interfaces</firstterm> and
4983 <firstterm>methods</firstterm>. Objects are referred to by a name,
4984 called a <firstterm>path</firstterm>.
4989 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
4992 An application talking directly to another application, without going
4993 through a message bus. One-to-one connections may be "peer to peer" or
4994 "client to server." The D-Bus protocol has no concept of client
4995 vs. server after a connection has authenticated; the flow of messages
4996 is symmetrical (full duplex).
5001 <glossentry id="term-path"><glossterm>Path</glossterm>
5004 Object references (object names) in D-Bus are organized into a
5005 filesystem-style hierarchy, so each object is named by a path. As in
5006 LDAP, there's no difference between "files" and "directories"; a path
5007 can refer to an object, while still having child objects below it.
5012 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5015 Each bus name has a primary owner; messages sent to the name go to the
5016 primary owner. However, certain names also maintain a queue of
5017 secondary owners "waiting in the wings." If the primary owner releases
5018 the name, then the first secondary owner in the queue automatically
5019 becomes the new owner of the name.
5024 <glossentry id="term-service"><glossterm>Service</glossterm>
5027 A service is an executable that can be launched by the bus daemon.
5028 Services normally guarantee some particular features, for example they
5029 may guarantee that they will request a specific name such as
5030 "org.freedesktop.Screensaver", have a singleton object
5031 "/org/freedesktop/Application", and that object will implement the
5032 interface "org.freedesktop.ScreensaverControl".
5037 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5040 ".service files" tell the bus about service applications that can be
5041 launched (see <xref linkend="term-service"/>). Most importantly they
5042 provide a mapping from bus names to services that will request those
5043 names when they start up.
5048 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5051 The special name automatically assigned to each connection by the
5052 message bus. This name will never change owner, and will be unique
5053 (never reused during the lifetime of the message bus).
5054 It will begin with a ':' character.