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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.17</releaseinfo>
10 <date>2011-06-01</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
43 <firstname>Sven</firstname>
44 <surname>Herzberg</surname>
46 <orgname>Imendio AB</orgname>
48 <email>sven@imendio.com</email>
53 <firstname>Simon</firstname>
54 <surname>McVittie</surname>
56 <orgname>Collabora Ltd.</orgname>
58 <email>simon.mcvittie@collabora.co.uk</email>
63 <firstname>David</firstname>
64 <surname>Zeuthen</surname>
66 <orgname>Red Hat, Inc.</orgname>
68 <email>davidz@redhat.com</email>
75 <revnumber>current</revnumber>
76 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
77 <authorinitials></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.17</revnumber>
82 <date>1 June 2011</date>
83 <authorinitials>smcv/davidz</authorinitials>
84 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
85 by GVariant</revremark>
88 <revnumber>0.16</revnumber>
89 <date>11 April 2011</date>
90 <authorinitials></authorinitials>
91 <revremark>add path_namespace, arg0namespace; argNpath matches object
95 <revnumber>0.15</revnumber>
96 <date>3 November 2010</date>
97 <authorinitials></authorinitials>
98 <revremark></revremark>
101 <revnumber>0.14</revnumber>
102 <date>12 May 2010</date>
103 <authorinitials></authorinitials>
104 <revremark></revremark>
107 <revnumber>0.13</revnumber>
108 <date>23 Dezember 2009</date>
109 <authorinitials></authorinitials>
110 <revremark></revremark>
113 <revnumber>0.12</revnumber>
114 <date>7 November, 2006</date>
115 <authorinitials></authorinitials>
116 <revremark></revremark>
119 <revnumber>0.11</revnumber>
120 <date>6 February 2005</date>
121 <authorinitials></authorinitials>
122 <revremark></revremark>
125 <revnumber>0.10</revnumber>
126 <date>28 January 2005</date>
127 <authorinitials></authorinitials>
128 <revremark></revremark>
131 <revnumber>0.9</revnumber>
132 <date>7 Januar 2005</date>
133 <authorinitials></authorinitials>
134 <revremark></revremark>
137 <revnumber>0.8</revnumber>
138 <date>06 September 2003</date>
139 <authorinitials></authorinitials>
140 <revremark>First released document.</revremark>
145 <sect1 id="introduction">
146 <title>Introduction</title>
148 D-Bus is a system for low-latency, low-overhead, easy to use
149 interprocess communication (IPC). In more detail:
153 D-Bus is <emphasis>low-latency</emphasis> because it is designed
154 to avoid round trips and allow asynchronous operation, much like
160 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
161 binary protocol, and does not have to convert to and from a text
162 format such as XML. Because D-Bus is intended for potentially
163 high-resolution same-machine IPC, not primarily for Internet IPC,
164 this is an interesting optimization.
169 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
170 of <firstterm>messages</firstterm> rather than byte streams, and
171 automatically handles a lot of the hard IPC issues. Also, the D-Bus
172 library is designed to be wrapped in a way that lets developers use
173 their framework's existing object/type system, rather than learning
174 a new one specifically for IPC.
181 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
182 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
183 a system for one application to talk to a single other
184 application. However, the primary intended application of the protocol is the
185 D-Bus <firstterm>message bus</firstterm>, specified in <xref
186 linkend="message-bus"/>. The message bus is a special application that
187 accepts connections from multiple other applications, and forwards
192 Uses of D-Bus include notification of system changes (notification of when
193 a camera is plugged in to a computer, or a new version of some software
194 has been installed), or desktop interoperability, for example a file
195 monitoring service or a configuration service.
199 D-Bus is designed for two specific use cases:
203 A "system bus" for notifications from the system to user sessions,
204 and to allow the system to request input from user sessions.
209 A "session bus" used to implement desktop environments such as
214 D-Bus is not intended to be a generic IPC system for any possible
215 application, and intentionally omits many features found in other
216 IPC systems for this reason.
220 At the same time, the bus daemons offer a number of features not found in
221 other IPC systems, such as single-owner "bus names" (similar to X
222 selections), on-demand startup of services, and security policies.
223 In many ways, these features are the primary motivation for developing
224 D-Bus; other systems would have sufficed if IPC were the only goal.
228 D-Bus may turn out to be useful in unanticipated applications, but future
229 versions of this spec and the reference implementation probably will not
230 incorporate features that interfere with the core use cases.
234 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
235 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
236 document are to be interpreted as described in RFC 2119. However, the
237 document could use a serious audit to be sure it makes sense to do
238 so. Also, they are not capitalized.
241 <sect2 id="stability">
242 <title>Protocol and Specification Stability</title>
244 The D-Bus protocol is frozen (only compatible extensions are allowed) as
245 of November 8, 2006. However, this specification could still use a fair
246 bit of work to make interoperable reimplementation possible without
247 reference to the D-Bus reference implementation. Thus, this
248 specification is not marked 1.0. To mark it 1.0, we'd like to see
249 someone invest significant effort in clarifying the specification
250 language, and growing the specification to cover more aspects of the
251 reference implementation's behavior.
254 Until this work is complete, any attempt to reimplement D-Bus will
255 probably require looking at the reference implementation and/or asking
256 questions on the D-Bus mailing list about intended behavior.
257 Questions on the list are very welcome.
260 Nonetheless, this document should be a useful starting point and is
261 to our knowledge accurate, though incomplete.
267 <sect1 id="message-protocol">
268 <title>Message Protocol</title>
271 A <firstterm>message</firstterm> consists of a
272 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
273 think of a message as a package, the header is the address, and the body
274 contains the package contents. The message delivery system uses the header
275 information to figure out where to send the message and how to interpret
276 it; the recipient interprets the body of the message.
280 The body of the message is made up of zero or more
281 <firstterm>arguments</firstterm>, which are typed values, such as an
282 integer or a byte array.
286 Both header and body use the same type system and format for
287 serializing data. Each type of value has a wire format.
288 Converting a value from some other representation into the wire
289 format is called <firstterm>marshaling</firstterm> and converting
290 it back from the wire format is <firstterm>unmarshaling</firstterm>.
293 <sect2 id="message-protocol-signatures">
294 <title>Type Signatures</title>
297 The D-Bus protocol does not include type tags in the marshaled data; a
298 block of marshaled values must have a known <firstterm>type
299 signature</firstterm>. The type signature is made up of <firstterm>type
300 codes</firstterm>. A type code is an ASCII character representing the
301 type of a value. Because ASCII characters are used, the type signature
302 will always form a valid ASCII string. A simple string compare
303 determines whether two type signatures are equivalent.
307 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
308 the ASCII character 'i'. So the signature for a block of values
309 containing a single <literal>INT32</literal> would be:
313 A block of values containing two <literal>INT32</literal> would have this signature:
320 All <firstterm>basic</firstterm> types work like
321 <literal>INT32</literal> in this example. To marshal and unmarshal
322 basic types, you simply read one value from the data
323 block corresponding to each type code in the signature.
324 In addition to basic types, there are four <firstterm>container</firstterm>
325 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
326 and <literal>DICT_ENTRY</literal>.
330 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
331 code does not appear in signatures. Instead, ASCII characters
332 '(' and ')' are used to mark the beginning and end of the struct.
333 So for example, a struct containing two integers would have this
338 Structs can be nested, so for example a struct containing
339 an integer and another struct:
343 The value block storing that struct would contain three integers; the
344 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
349 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
350 but is useful in code that implements the protocol. This type code
351 is specified to allow such code to interoperate in non-protocol contexts.
355 Empty structures are not allowed; there must be at least one
356 type code between the parentheses.
360 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
361 followed by a <firstterm>single complete type</firstterm>. The single
362 complete type following the array is the type of each array element. So
363 the simple example is:
367 which is an array of 32-bit integers. But an array can be of any type,
368 such as this array-of-struct-with-two-int32-fields:
372 Or this array of array of integer:
379 The phrase <firstterm>single complete type</firstterm> deserves some
380 definition. A single complete type is a basic type code, a variant type code,
381 an array with its element type, or a struct with its fields.
382 So the following signatures are not single complete types:
392 And the following signatures contain multiple complete types:
402 Note however that a single complete type may <emphasis>contain</emphasis>
403 multiple other single complete types.
407 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
408 type <literal>VARIANT</literal> will have the signature of a single complete type as part
409 of the <emphasis>value</emphasis>. This signature will be followed by a
410 marshaled value of that type.
414 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
415 than parentheses it uses curly braces, and it has more restrictions.
416 The restrictions are: it occurs only as an array element type; it has
417 exactly two single complete types inside the curly braces; the first
418 single complete type (the "key") must be a basic type rather than a
419 container type. Implementations must not accept dict entries outside of
420 arrays, must not accept dict entries with zero, one, or more than two
421 fields, and must not accept dict entries with non-basic-typed keys. A
422 dict entry is always a key-value pair.
426 The first field in the <literal>DICT_ENTRY</literal> is always the key.
427 A message is considered corrupt if the same key occurs twice in the same
428 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
429 implementations are not required to reject dicts with duplicate keys.
433 In most languages, an array of dict entry would be represented as a
434 map, hash table, or dict object.
438 The following table summarizes the D-Bus types.
443 <entry>Conventional Name</entry>
445 <entry>Description</entry>
450 <entry><literal>INVALID</literal></entry>
451 <entry>0 (ASCII NUL)</entry>
452 <entry>Not a valid type code, used to terminate signatures</entry>
454 <entry><literal>BYTE</literal></entry>
455 <entry>121 (ASCII 'y')</entry>
456 <entry>8-bit unsigned integer</entry>
458 <entry><literal>BOOLEAN</literal></entry>
459 <entry>98 (ASCII 'b')</entry>
460 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
462 <entry><literal>INT16</literal></entry>
463 <entry>110 (ASCII 'n')</entry>
464 <entry>16-bit signed integer</entry>
466 <entry><literal>UINT16</literal></entry>
467 <entry>113 (ASCII 'q')</entry>
468 <entry>16-bit unsigned integer</entry>
470 <entry><literal>INT32</literal></entry>
471 <entry>105 (ASCII 'i')</entry>
472 <entry>32-bit signed integer</entry>
474 <entry><literal>UINT32</literal></entry>
475 <entry>117 (ASCII 'u')</entry>
476 <entry>32-bit unsigned integer</entry>
478 <entry><literal>INT64</literal></entry>
479 <entry>120 (ASCII 'x')</entry>
480 <entry>64-bit signed integer</entry>
482 <entry><literal>UINT64</literal></entry>
483 <entry>116 (ASCII 't')</entry>
484 <entry>64-bit unsigned integer</entry>
486 <entry><literal>DOUBLE</literal></entry>
487 <entry>100 (ASCII 'd')</entry>
488 <entry>IEEE 754 double</entry>
490 <entry><literal>STRING</literal></entry>
491 <entry>115 (ASCII 's')</entry>
492 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
494 <entry><literal>OBJECT_PATH</literal></entry>
495 <entry>111 (ASCII 'o')</entry>
496 <entry>Name of an object instance</entry>
498 <entry><literal>SIGNATURE</literal></entry>
499 <entry>103 (ASCII 'g')</entry>
500 <entry>A type signature</entry>
502 <entry><literal>ARRAY</literal></entry>
503 <entry>97 (ASCII 'a')</entry>
506 <entry><literal>STRUCT</literal></entry>
507 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
508 <entry>Struct; type code 114 'r' is reserved for use in
509 bindings and implementations to represent the general
510 concept of a struct, and must not appear in signatures
511 used on D-Bus.</entry>
513 <entry><literal>VARIANT</literal></entry>
514 <entry>118 (ASCII 'v') </entry>
515 <entry>Variant type (the type of the value is part of the value itself)</entry>
517 <entry><literal>DICT_ENTRY</literal></entry>
518 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
519 <entry>Entry in a dict or map (array of key-value pairs).
520 Type code 101 'e' is reserved for use in bindings and
521 implementations to represent the general concept of a
522 dict or dict-entry, and must not appear in signatures
523 used on D-Bus.</entry>
525 <entry><literal>UNIX_FD</literal></entry>
526 <entry>104 (ASCII 'h')</entry>
527 <entry>Unix file descriptor</entry>
530 <entry>(reserved)</entry>
531 <entry>109 (ASCII 'm')</entry>
532 <entry>Reserved for <ulink
533 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
534 'maybe' type compatible with the one in GVariant</ulink>,
535 and must not appear in signatures used on D-Bus until
536 specified here</entry>
539 <entry>(reserved)</entry>
540 <entry>42 (ASCII '*')</entry>
541 <entry>Reserved for use in bindings/implementations to
542 represent any <firstterm>single complete type</firstterm>,
543 and must not appear in signatures used on D-Bus.</entry>
546 <entry>(reserved)</entry>
547 <entry>63 (ASCII '?')</entry>
548 <entry>Reserved for use in bindings/implementations to
549 represent any <firstterm>basic type</firstterm>, and must
550 not appear in signatures used on D-Bus.</entry>
553 <entry>(reserved)</entry>
554 <entry>64 (ASCII '@'), 38 (ASCII '&'),
555 94 (ASCII '^')</entry>
556 <entry>Reserved for internal use by bindings/implementations,
557 and must not appear in signatures used on D-Bus.
558 GVariant uses these type-codes to encode calling
568 <sect2 id="message-protocol-marshaling">
569 <title>Marshaling (Wire Format)</title>
572 Given a type signature, a block of bytes can be converted into typed
573 values. This section describes the format of the block of bytes. Byte
574 order and alignment issues are handled uniformly for all D-Bus types.
578 A block of bytes has an associated byte order. The byte order
579 has to be discovered in some way; for D-Bus messages, the
580 byte order is part of the message header as described in
581 <xref linkend="message-protocol-messages"/>. For now, assume
582 that the byte order is known to be either little endian or big
587 Each value in a block of bytes is aligned "naturally," for example
588 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
589 8-byte boundary. To properly align a value, <firstterm>alignment
590 padding</firstterm> may be necessary. The alignment padding must always
591 be the minimum required padding to properly align the following value;
592 and it must always be made up of nul bytes. The alignment padding must
593 not be left uninitialized (it can't contain garbage), and more padding
594 than required must not be used.
598 Given all this, the types are marshaled on the wire as follows:
603 <entry>Conventional Name</entry>
604 <entry>Encoding</entry>
605 <entry>Alignment</entry>
610 <entry><literal>INVALID</literal></entry>
611 <entry>Not applicable; cannot be marshaled.</entry>
614 <entry><literal>BYTE</literal></entry>
615 <entry>A single 8-bit byte.</entry>
618 <entry><literal>BOOLEAN</literal></entry>
619 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
622 <entry><literal>INT16</literal></entry>
623 <entry>16-bit signed integer in the message's byte order.</entry>
626 <entry><literal>UINT16</literal></entry>
627 <entry>16-bit unsigned integer in the message's byte order.</entry>
630 <entry><literal>INT32</literal></entry>
631 <entry>32-bit signed integer in the message's byte order.</entry>
634 <entry><literal>UINT32</literal></entry>
635 <entry>32-bit unsigned integer in the message's byte order.</entry>
638 <entry><literal>INT64</literal></entry>
639 <entry>64-bit signed integer in the message's byte order.</entry>
642 <entry><literal>UINT64</literal></entry>
643 <entry>64-bit unsigned integer in the message's byte order.</entry>
646 <entry><literal>DOUBLE</literal></entry>
647 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
650 <entry><literal>STRING</literal></entry>
651 <entry>A <literal>UINT32</literal> indicating the string's
652 length in bytes excluding its terminating nul, followed by
653 non-nul string data of the given length, followed by a terminating nul
660 <entry><literal>OBJECT_PATH</literal></entry>
661 <entry>Exactly the same as <literal>STRING</literal> except the
662 content must be a valid object path (see below).
668 <entry><literal>SIGNATURE</literal></entry>
669 <entry>The same as <literal>STRING</literal> except the length is a single
670 byte (thus signatures have a maximum length of 255)
671 and the content must be a valid signature (see below).
677 <entry><literal>ARRAY</literal></entry>
679 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
680 alignment padding to the alignment boundary of the array element type,
681 followed by each array element. The array length is from the
682 end of the alignment padding to the end of the last element,
683 i.e. it does not include the padding after the length,
684 or any padding after the last element.
685 Arrays have a maximum length defined to be 2 to the 26th power or
686 67108864. Implementations must not send or accept arrays exceeding this
693 <entry><literal>STRUCT</literal></entry>
695 A struct must start on an 8-byte boundary regardless of the
696 type of the struct fields. The struct value consists of each
697 field marshaled in sequence starting from that 8-byte
704 <entry><literal>VARIANT</literal></entry>
706 A variant type has a marshaled
707 <literal>SIGNATURE</literal> followed by a marshaled
708 value with the type given in the signature. Unlike
709 a message signature, the variant signature can
710 contain only a single complete type. So "i", "ai"
711 or "(ii)" is OK, but "ii" is not. Use of variants may not
712 cause a total message depth to be larger than 64, including
713 other container types such as structures.
716 1 (alignment of the signature)
719 <entry><literal>DICT_ENTRY</literal></entry>
727 <entry><literal>UNIX_FD</literal></entry>
728 <entry>32-bit unsigned integer in the message's byte
729 order. The actual file descriptors need to be
730 transferred out-of-band via some platform specific
731 mechanism. On the wire, values of this type store the index to the
732 file descriptor in the array of file descriptors that
733 accompany the message.</entry>
741 <sect3 id="message-protocol-marshaling-object-path">
742 <title>Valid Object Paths</title>
745 An object path is a name used to refer to an object instance.
746 Conceptually, each participant in a D-Bus message exchange may have
747 any number of object instances (think of C++ or Java objects) and each
748 such instance will have a path. Like a filesystem, the object
749 instances in an application form a hierarchical tree.
753 The following rules define a valid object path. Implementations must
754 not send or accept messages with invalid object paths.
758 The path may be of any length.
763 The path must begin with an ASCII '/' (integer 47) character,
764 and must consist of elements separated by slash characters.
769 Each element must only contain the ASCII characters
775 No element may be the empty string.
780 Multiple '/' characters cannot occur in sequence.
785 A trailing '/' character is not allowed unless the
786 path is the root path (a single '/' character).
795 <sect3 id="message-protocol-marshaling-signature">
796 <title>Valid Signatures</title>
798 An implementation must not send or accept invalid signatures.
799 Valid signatures will conform to the following rules:
803 The signature ends with a nul byte.
808 The signature is a list of single complete types.
809 Arrays must have element types, and structs must
810 have both open and close parentheses.
815 Only type codes and open and close parentheses are
816 allowed in the signature. The <literal>STRUCT</literal> type code
817 is not allowed in signatures, because parentheses
823 The maximum depth of container type nesting is 32 array type
824 codes and 32 open parentheses. This implies that the maximum
825 total depth of recursion is 64, for an "array of array of array
826 of ... struct of struct of struct of ..." where there are 32
832 The maximum length of a signature is 255.
837 Signatures must be nul-terminated.
846 <sect2 id="message-protocol-messages">
847 <title>Message Format</title>
850 A message consists of a header and a body. The header is a block of
851 values with a fixed signature and meaning. The body is a separate block
852 of values, with a signature specified in the header.
856 The length of the header must be a multiple of 8, allowing the body to
857 begin on an 8-byte boundary when storing the entire message in a single
858 buffer. If the header does not naturally end on an 8-byte boundary
859 up to 7 bytes of nul-initialized alignment padding must be added.
863 The message body need not end on an 8-byte boundary.
867 The maximum length of a message, including header, header alignment padding,
868 and body is 2 to the 27th power or 134217728. Implementations must not
869 send or accept messages exceeding this size.
873 The signature of the header is:
877 Written out more readably, this is:
879 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
884 These values have the following meanings:
890 <entry>Description</entry>
895 <entry>1st <literal>BYTE</literal></entry>
896 <entry>Endianness flag; ASCII 'l' for little-endian
897 or ASCII 'B' for big-endian. Both header and body are
898 in this endianness.</entry>
901 <entry>2nd <literal>BYTE</literal></entry>
902 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
903 Currently-defined types are described below.
907 <entry>3rd <literal>BYTE</literal></entry>
908 <entry>Bitwise OR of flags. Unknown flags
909 must be ignored. Currently-defined flags are described below.
913 <entry>4th <literal>BYTE</literal></entry>
914 <entry>Major protocol version of the sending application. If
915 the major protocol version of the receiving application does not
916 match, the applications will not be able to communicate and the
917 D-Bus connection must be disconnected. The major protocol
918 version for this version of the specification is 1.
922 <entry>1st <literal>UINT32</literal></entry>
923 <entry>Length in bytes of the message body, starting
924 from the end of the header. The header ends after
925 its alignment padding to an 8-boundary.
929 <entry>2nd <literal>UINT32</literal></entry>
930 <entry>The serial of this message, used as a cookie
931 by the sender to identify the reply corresponding
932 to this request. This must not be zero.
936 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
937 <entry>An array of zero or more <firstterm>header
938 fields</firstterm> where the byte is the field code, and the
939 variant is the field value. The message type determines
940 which fields are required.
948 <firstterm>Message types</firstterm> that can appear in the second byte
954 <entry>Conventional name</entry>
955 <entry>Decimal value</entry>
956 <entry>Description</entry>
961 <entry><literal>INVALID</literal></entry>
963 <entry>This is an invalid type.</entry>
966 <entry><literal>METHOD_CALL</literal></entry>
968 <entry>Method call.</entry>
971 <entry><literal>METHOD_RETURN</literal></entry>
973 <entry>Method reply with returned data.</entry>
976 <entry><literal>ERROR</literal></entry>
978 <entry>Error reply. If the first argument exists and is a
979 string, it is an error message.</entry>
982 <entry><literal>SIGNAL</literal></entry>
984 <entry>Signal emission.</entry>
991 Flags that can appear in the third byte of the header:
996 <entry>Conventional name</entry>
997 <entry>Hex value</entry>
998 <entry>Description</entry>
1003 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1005 <entry>This message does not expect method return replies or
1006 error replies; the reply can be omitted as an
1007 optimization. However, it is compliant with this specification
1008 to return the reply despite this flag and the only harm
1009 from doing so is extra network traffic.
1013 <entry><literal>NO_AUTO_START</literal></entry>
1015 <entry>The bus must not launch an owner
1016 for the destination name in response to this message.
1024 <sect3 id="message-protocol-header-fields">
1025 <title>Header Fields</title>
1028 The array at the end of the header contains <firstterm>header
1029 fields</firstterm>, where each field is a 1-byte field code followed
1030 by a field value. A header must contain the required header fields for
1031 its message type, and zero or more of any optional header
1032 fields. Future versions of this protocol specification may add new
1033 fields. Implementations must ignore fields they do not
1034 understand. Implementations must not invent their own header fields;
1035 only changes to this specification may introduce new header fields.
1039 Again, if an implementation sees a header field code that it does not
1040 expect, it must ignore that field, as it will be part of a new
1041 (but compatible) version of this specification. This also applies
1042 to known header fields appearing in unexpected messages, for
1043 example: if a signal has a reply serial it must be ignored
1044 even though it has no meaning as of this version of the spec.
1048 However, implementations must not send or accept known header fields
1049 with the wrong type stored in the field value. So for example a
1050 message with an <literal>INTERFACE</literal> field of type
1051 <literal>UINT32</literal> would be considered corrupt.
1055 Here are the currently-defined header fields:
1060 <entry>Conventional Name</entry>
1061 <entry>Decimal Code</entry>
1063 <entry>Required In</entry>
1064 <entry>Description</entry>
1069 <entry><literal>INVALID</literal></entry>
1072 <entry>not allowed</entry>
1073 <entry>Not a valid field name (error if it appears in a message)</entry>
1076 <entry><literal>PATH</literal></entry>
1078 <entry><literal>OBJECT_PATH</literal></entry>
1079 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1080 <entry>The object to send a call to,
1081 or the object a signal is emitted from.
1083 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1084 implementations should not send messages with this path,
1085 and the reference implementation of the bus daemon will
1086 disconnect any application that attempts to do so.
1090 <entry><literal>INTERFACE</literal></entry>
1092 <entry><literal>STRING</literal></entry>
1093 <entry><literal>SIGNAL</literal></entry>
1095 The interface to invoke a method call on, or
1096 that a signal is emitted from. Optional for
1097 method calls, required for signals.
1098 The special interface
1099 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1100 implementations should not send messages with this
1101 interface, and the reference implementation of the bus
1102 daemon will disconnect any application that attempts to
1107 <entry><literal>MEMBER</literal></entry>
1109 <entry><literal>STRING</literal></entry>
1110 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1111 <entry>The member, either the method name or signal name.</entry>
1114 <entry><literal>ERROR_NAME</literal></entry>
1116 <entry><literal>STRING</literal></entry>
1117 <entry><literal>ERROR</literal></entry>
1118 <entry>The name of the error that occurred, for errors</entry>
1121 <entry><literal>REPLY_SERIAL</literal></entry>
1123 <entry><literal>UINT32</literal></entry>
1124 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1125 <entry>The serial number of the message this message is a reply
1126 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1129 <entry><literal>DESTINATION</literal></entry>
1131 <entry><literal>STRING</literal></entry>
1132 <entry>optional</entry>
1133 <entry>The name of the connection this message is intended for.
1134 Only used in combination with the message bus, see
1135 <xref linkend="message-bus"/>.</entry>
1138 <entry><literal>SENDER</literal></entry>
1140 <entry><literal>STRING</literal></entry>
1141 <entry>optional</entry>
1142 <entry>Unique name of the sending connection.
1143 The message bus fills in this field so it is reliable; the field is
1144 only meaningful in combination with the message bus.</entry>
1147 <entry><literal>SIGNATURE</literal></entry>
1149 <entry><literal>SIGNATURE</literal></entry>
1150 <entry>optional</entry>
1151 <entry>The signature of the message body.
1152 If omitted, it is assumed to be the
1153 empty signature "" (i.e. the body must be 0-length).</entry>
1156 <entry><literal>UNIX_FDS</literal></entry>
1158 <entry><literal>UINT32</literal></entry>
1159 <entry>optional</entry>
1160 <entry>The number of Unix file descriptors that
1161 accompany the message. If omitted, it is assumed
1162 that no Unix file descriptors accompany the
1163 message. The actual file descriptors need to be
1164 transferred via platform specific mechanism
1165 out-of-band. They must be sent at the same time as
1166 part of the message itself. They may not be sent
1167 before the first byte of the message itself is
1168 transferred or after the last byte of the message
1178 <sect2 id="message-protocol-names">
1179 <title>Valid Names</title>
1181 The various names in D-Bus messages have some restrictions.
1184 There is a <firstterm>maximum name length</firstterm>
1185 of 255 which applies to bus names, interfaces, and members.
1187 <sect3 id="message-protocol-names-interface">
1188 <title>Interface names</title>
1190 Interfaces have names with type <literal>STRING</literal>, meaning that
1191 they must be valid UTF-8. However, there are also some
1192 additional restrictions that apply to interface names
1195 <listitem><para>Interface names are composed of 1 or more elements separated by
1196 a period ('.') character. All elements must contain at least
1200 <listitem><para>Each element must only contain the ASCII characters
1201 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1205 <listitem><para>Interface names must contain at least one '.' (period)
1206 character (and thus at least two elements).
1209 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1210 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1214 <sect3 id="message-protocol-names-bus">
1215 <title>Bus names</title>
1217 Connections have one or more bus names associated with them.
1218 A connection has exactly one bus name that is a unique connection
1219 name. The unique connection name remains with the connection for
1220 its entire lifetime.
1221 A bus name is of type <literal>STRING</literal>,
1222 meaning that it must be valid UTF-8. However, there are also
1223 some additional restrictions that apply to bus names
1226 <listitem><para>Bus names that start with a colon (':')
1227 character are unique connection names.
1230 <listitem><para>Bus names are composed of 1 or more elements separated by
1231 a period ('.') character. All elements must contain at least
1235 <listitem><para>Each element must only contain the ASCII characters
1236 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1237 connection name may begin with a digit, elements in
1238 other bus names must not begin with a digit.
1242 <listitem><para>Bus names must contain at least one '.' (period)
1243 character (and thus at least two elements).
1246 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1247 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1251 Note that the hyphen ('-') character is allowed in bus names but
1252 not in interface names.
1255 <sect3 id="message-protocol-names-member">
1256 <title>Member names</title>
1258 Member (i.e. method or signal) names:
1260 <listitem><para>Must only contain the ASCII characters
1261 "[A-Z][a-z][0-9]_" and may not begin with a
1262 digit.</para></listitem>
1263 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1264 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1265 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1269 <sect3 id="message-protocol-names-error">
1270 <title>Error names</title>
1272 Error names have the same restrictions as interface names.
1277 <sect2 id="message-protocol-types">
1278 <title>Message Types</title>
1280 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1281 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1282 This section describes these conventions.
1284 <sect3 id="message-protocol-types-method">
1285 <title>Method Calls</title>
1287 Some messages invoke an operation on a remote object. These are
1288 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1289 messages map naturally to methods on objects in a typical program.
1292 A method call message is required to have a <literal>MEMBER</literal> header field
1293 indicating the name of the method. Optionally, the message has an
1294 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1295 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1296 a method with the same name, it is undefined which of the two methods
1297 will be invoked. Implementations may also choose to return an error in
1298 this ambiguous case. However, if a method name is unique
1299 implementations must not require an interface field.
1302 Method call messages also include a <literal>PATH</literal> field
1303 indicating the object to invoke the method on. If the call is passing
1304 through a message bus, the message will also have a
1305 <literal>DESTINATION</literal> field giving the name of the connection
1306 to receive the message.
1309 When an application handles a method call message, it is required to
1310 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1311 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1312 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1315 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1316 are the return value(s) or "out parameters" of the method call.
1317 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1318 and the call fails; no return value will be provided. It makes
1319 no sense to send multiple replies to the same method call.
1322 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1323 reply is required, so the caller will know the method
1324 was successfully processed.
1327 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1331 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1332 then as an optimization the application receiving the method
1333 call may choose to omit the reply message (regardless of
1334 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1335 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1336 flag and reply anyway.
1339 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1340 destination name does not exist then a program to own the destination
1341 name will be started before the message is delivered. The message
1342 will be held until the new program is successfully started or has
1343 failed to start; in case of failure, an error will be returned. This
1344 flag is only relevant in the context of a message bus, it is ignored
1345 during one-to-one communication with no intermediate bus.
1347 <sect4 id="message-protocol-types-method-apis">
1348 <title>Mapping method calls to native APIs</title>
1350 APIs for D-Bus may map method calls to a method call in a specific
1351 programming language, such as C++, or may map a method call written
1352 in an IDL to a D-Bus message.
1355 In APIs of this nature, arguments to a method are often termed "in"
1356 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1357 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1358 "inout" arguments, which are both sent and received, i.e. the caller
1359 passes in a value which is modified. Mapped to D-Bus, an "inout"
1360 argument is equivalent to an "in" argument, followed by an "out"
1361 argument. You can't pass things "by reference" over the wire, so
1362 "inout" is purely an illusion of the in-process API.
1365 Given a method with zero or one return values, followed by zero or more
1366 arguments, where each argument may be "in", "out", or "inout", the
1367 caller constructs a message by appending each "in" or "inout" argument,
1368 in order. "out" arguments are not represented in the caller's message.
1371 The recipient constructs a reply by appending first the return value
1372 if any, then each "out" or "inout" argument, in order.
1373 "in" arguments are not represented in the reply message.
1376 Error replies are normally mapped to exceptions in languages that have
1380 In converting from native APIs to D-Bus, it is perhaps nice to
1381 map D-Bus naming conventions ("FooBar") to native conventions
1382 such as "fooBar" or "foo_bar" automatically. This is OK
1383 as long as you can say that the native API is one that
1384 was specifically written for D-Bus. It makes the most sense
1385 when writing object implementations that will be exported
1386 over the bus. Object proxies used to invoke remote D-Bus
1387 objects probably need the ability to call any D-Bus method,
1388 and thus a magic name mapping like this could be a problem.
1391 This specification doesn't require anything of native API bindings;
1392 the preceding is only a suggested convention for consistency
1398 <sect3 id="message-protocol-types-signal">
1399 <title>Signal Emission</title>
1401 Unlike method calls, signal emissions have no replies.
1402 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1403 It must have three header fields: <literal>PATH</literal> giving the object
1404 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1405 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1406 for signals, though it is optional for method calls.
1410 <sect3 id="message-protocol-types-errors">
1411 <title>Errors</title>
1413 Messages of type <literal>ERROR</literal> are most commonly replies
1414 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1415 to any kind of message. The message bus for example
1416 will return an <literal>ERROR</literal> in reply to a signal emission if
1417 the bus does not have enough memory to send the signal.
1420 An <literal>ERROR</literal> may have any arguments, but if the first
1421 argument is a <literal>STRING</literal>, it must be an error message.
1422 The error message may be logged or shown to the user
1427 <sect3 id="message-protocol-types-notation">
1428 <title>Notation in this document</title>
1430 This document uses a simple pseudo-IDL to describe particular method
1431 calls and signals. Here is an example of a method call:
1433 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1434 out UINT32 resultcode)
1436 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1437 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1438 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1439 characters so it's known that the last part of the name in
1440 the "IDL" is the member name.
1443 In C++ that might end up looking like this:
1445 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1446 unsigned int flags);
1448 or equally valid, the return value could be done as an argument:
1450 void org::freedesktop::DBus::StartServiceByName (const char *name,
1452 unsigned int *resultcode);
1454 It's really up to the API designer how they want to make
1455 this look. You could design an API where the namespace wasn't used
1456 in C++, using STL or Qt, using varargs, or whatever you wanted.
1459 Signals are written as follows:
1461 org.freedesktop.DBus.NameLost (STRING name)
1463 Signals don't specify "in" vs. "out" because only
1464 a single direction is possible.
1467 It isn't especially encouraged to use this lame pseudo-IDL in actual
1468 API implementations; you might use the native notation for the
1469 language you're using, or you might use COM or CORBA IDL, for example.
1474 <sect2 id="message-protocol-handling-invalid">
1475 <title>Invalid Protocol and Spec Extensions</title>
1478 For security reasons, the D-Bus protocol should be strictly parsed and
1479 validated, with the exception of defined extension points. Any invalid
1480 protocol or spec violations should result in immediately dropping the
1481 connection without notice to the other end. Exceptions should be
1482 carefully considered, e.g. an exception may be warranted for a
1483 well-understood idiosyncrasy of a widely-deployed implementation. In
1484 cases where the other end of a connection is 100% trusted and known to
1485 be friendly, skipping validation for performance reasons could also make
1486 sense in certain cases.
1490 Generally speaking violations of the "must" requirements in this spec
1491 should be considered possible attempts to exploit security, and violations
1492 of the "should" suggestions should be considered legitimate (though perhaps
1493 they should generate an error in some cases).
1497 The following extension points are built in to D-Bus on purpose and must
1498 not be treated as invalid protocol. The extension points are intended
1499 for use by future versions of this spec, they are not intended for third
1500 parties. At the moment, the only way a third party could extend D-Bus
1501 without breaking interoperability would be to introduce a way to negotiate new
1502 feature support as part of the auth protocol, using EXTENSION_-prefixed
1503 commands. There is not yet a standard way to negotiate features.
1507 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1508 commands result in an ERROR rather than a disconnect. This enables
1509 future extensions to the protocol. Commands starting with EXTENSION_ are
1510 reserved for third parties.
1515 The authentication protocol supports pluggable auth mechanisms.
1520 The address format (see <xref linkend="addresses"/>) supports new
1526 Messages with an unknown type (something other than
1527 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1528 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1529 Unknown-type messages must still be well-formed in the same way
1530 as the known messages, however. They still have the normal
1536 Header fields with an unknown or unexpected field code must be ignored,
1537 though again they must still be well-formed.
1542 New standard interfaces (with new methods and signals) can of course be added.
1552 <sect1 id="auth-protocol">
1553 <title>Authentication Protocol</title>
1555 Before the flow of messages begins, two applications must
1556 authenticate. A simple plain-text protocol is used for
1557 authentication; this protocol is a SASL profile, and maps fairly
1558 directly from the SASL specification. The message encoding is
1559 NOT used here, only plain text messages.
1562 In examples, "C:" and "S:" indicate lines sent by the client and
1563 server respectively.
1565 <sect2 id="auth-protocol-overview">
1566 <title>Protocol Overview</title>
1568 The protocol is a line-based protocol, where each line ends with
1569 \r\n. Each line begins with an all-caps ASCII command name containing
1570 only the character range [A-Z_], a space, then any arguments for the
1571 command, then the \r\n ending the line. The protocol is
1572 case-sensitive. All bytes must be in the ASCII character set.
1574 Commands from the client to the server are as follows:
1577 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1578 <listitem><para>CANCEL</para></listitem>
1579 <listitem><para>BEGIN</para></listitem>
1580 <listitem><para>DATA <data in hex encoding></para></listitem>
1581 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1582 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1585 From server to client are as follows:
1588 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1589 <listitem><para>OK <GUID in hex></para></listitem>
1590 <listitem><para>DATA <data in hex encoding></para></listitem>
1591 <listitem><para>ERROR</para></listitem>
1592 <listitem><para>AGREE_UNIX_FD</para></listitem>
1596 Unofficial extensions to the command set must begin with the letters
1597 "EXTENSION_", to avoid conflicts with future official commands.
1598 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1601 <sect2 id="auth-nul-byte">
1602 <title>Special credentials-passing nul byte</title>
1604 Immediately after connecting to the server, the client must send a
1605 single nul byte. This byte may be accompanied by credentials
1606 information on some operating systems that use sendmsg() with
1607 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1608 sockets. However, the nul byte must be sent even on other kinds of
1609 socket, and even on operating systems that do not require a byte to be
1610 sent in order to transmit credentials. The text protocol described in
1611 this document begins after the single nul byte. If the first byte
1612 received from the client is not a nul byte, the server may disconnect
1616 A nul byte in any context other than the initial byte is an error;
1617 the protocol is ASCII-only.
1620 The credentials sent along with the nul byte may be used with the
1621 SASL mechanism EXTERNAL.
1624 <sect2 id="auth-command-auth">
1625 <title>AUTH command</title>
1627 If an AUTH command has no arguments, it is a request to list
1628 available mechanisms. The server must respond with a REJECTED
1629 command listing the mechanisms it understands, or with an error.
1632 If an AUTH command specifies a mechanism, and the server supports
1633 said mechanism, the server should begin exchanging SASL
1634 challenge-response data with the client using DATA commands.
1637 If the server does not support the mechanism given in the AUTH
1638 command, it must send either a REJECTED command listing the mechanisms
1639 it does support, or an error.
1642 If the [initial-response] argument is provided, it is intended for use
1643 with mechanisms that have no initial challenge (or an empty initial
1644 challenge), as if it were the argument to an initial DATA command. If
1645 the selected mechanism has an initial challenge and [initial-response]
1646 was provided, the server should reject authentication by sending
1650 If authentication succeeds after exchanging DATA commands,
1651 an OK command must be sent to the client.
1654 The first octet received by the server after the \r\n of the BEGIN
1655 command from the client must be the first octet of the
1656 authenticated/encrypted stream of D-Bus messages.
1659 If BEGIN is received by the server, the first octet received
1660 by the client after the \r\n of the OK command must be the
1661 first octet of the authenticated/encrypted stream of D-Bus
1665 <sect2 id="auth-command-cancel">
1666 <title>CANCEL Command</title>
1668 At any time up to sending the BEGIN command, the client may send a
1669 CANCEL command. On receiving the CANCEL command, the server must
1670 send a REJECTED command and abort the current authentication
1674 <sect2 id="auth-command-data">
1675 <title>DATA Command</title>
1677 The DATA command may come from either client or server, and simply
1678 contains a hex-encoded block of data to be interpreted
1679 according to the SASL mechanism in use.
1682 Some SASL mechanisms support sending an "empty string";
1683 FIXME we need some way to do this.
1686 <sect2 id="auth-command-begin">
1687 <title>BEGIN Command</title>
1689 The BEGIN command acknowledges that the client has received an
1690 OK command from the server, and that the stream of messages
1694 The first octet received by the server after the \r\n of the BEGIN
1695 command from the client must be the first octet of the
1696 authenticated/encrypted stream of D-Bus messages.
1699 <sect2 id="auth-command-rejected">
1700 <title>REJECTED Command</title>
1702 The REJECTED command indicates that the current authentication
1703 exchange has failed, and further exchange of DATA is inappropriate.
1704 The client would normally try another mechanism, or try providing
1705 different responses to challenges.
1707 Optionally, the REJECTED command has a space-separated list of
1708 available auth mechanisms as arguments. If a server ever provides
1709 a list of supported mechanisms, it must provide the same list
1710 each time it sends a REJECTED message. Clients are free to
1711 ignore all lists received after the first.
1714 <sect2 id="auth-command-ok">
1715 <title>OK Command</title>
1717 The OK command indicates that the client has been
1718 authenticated. The client may now proceed with negotiating
1719 Unix file descriptor passing. To do that it shall send
1720 NEGOTIATE_UNIX_FD to the server.
1723 Otherwise, the client must respond to the OK command by
1724 sending a BEGIN command, followed by its stream of messages,
1725 or by disconnecting. The server must not accept additional
1726 commands using this protocol after the BEGIN command has been
1727 received. Further communication will be a stream of D-Bus
1728 messages (optionally encrypted, as negotiated) rather than
1732 If a client sends BEGIN the first octet received by the client
1733 after the \r\n of the OK command must be the first octet of
1734 the authenticated/encrypted stream of D-Bus messages.
1737 The OK command has one argument, which is the GUID of the server.
1738 See <xref linkend="addresses"/> for more on server GUIDs.
1741 <sect2 id="auth-command-error">
1742 <title>ERROR Command</title>
1744 The ERROR command indicates that either server or client did not
1745 know a command, does not accept the given command in the current
1746 context, or did not understand the arguments to the command. This
1747 allows the protocol to be extended; a client or server can send a
1748 command present or permitted only in new protocol versions, and if
1749 an ERROR is received instead of an appropriate response, fall back
1750 to using some other technique.
1753 If an ERROR is sent, the server or client that sent the
1754 error must continue as if the command causing the ERROR had never been
1755 received. However, the the server or client receiving the error
1756 should try something other than whatever caused the error;
1757 if only canceling/rejecting the authentication.
1760 If the D-Bus protocol changes incompatibly at some future time,
1761 applications implementing the new protocol would probably be able to
1762 check for support of the new protocol by sending a new command and
1763 receiving an ERROR from applications that don't understand it. Thus the
1764 ERROR feature of the auth protocol is an escape hatch that lets us
1765 negotiate extensions or changes to the D-Bus protocol in the future.
1768 <sect2 id="auth-command-negotiate-unix-fd">
1769 <title>NEGOTIATE_UNIX_FD Command</title>
1771 The NEGOTIATE_UNIX_FD command indicates that the client
1772 supports Unix file descriptor passing. This command may only
1773 be sent after the connection is authenticated, i.e. after OK
1774 was received by the client. This command may only be sent on
1775 transports that support Unix file descriptor passing.
1778 On receiving NEGOTIATE_UNIX_FD the server must respond with
1779 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1780 the transport chosen supports Unix file descriptor passing and
1781 the server supports this feature. It shall respond the latter
1782 if the transport does not support Unix file descriptor
1783 passing, the server does not support this feature, or the
1784 server decides not to enable file descriptor passing due to
1785 security or other reasons.
1788 <sect2 id="auth-command-agree-unix-fd">
1789 <title>AGREE_UNIX_FD Command</title>
1791 The AGREE_UNIX_FD command indicates that the server supports
1792 Unix file descriptor passing. This command may only be sent
1793 after the connection is authenticated, and the client sent
1794 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1795 command may only be sent on transports that support Unix file
1799 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1800 followed by its stream of messages, or by disconnecting. The
1801 server must not accept additional commands using this protocol
1802 after the BEGIN command has been received. Further
1803 communication will be a stream of D-Bus messages (optionally
1804 encrypted, as negotiated) rather than this protocol.
1807 <sect2 id="auth-command-future">
1808 <title>Future Extensions</title>
1810 Future extensions to the authentication and negotiation
1811 protocol are possible. For that new commands may be
1812 introduced. If a client or server receives an unknown command
1813 it shall respond with ERROR and not consider this fatal. New
1814 commands may be introduced both before, and after
1815 authentication, i.e. both before and after the OK command.
1818 <sect2 id="auth-examples">
1819 <title>Authentication examples</title>
1823 <title>Example of successful magic cookie authentication</title>
1825 (MAGIC_COOKIE is a made up mechanism)
1827 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1833 <title>Example of finding out mechanisms then picking one</title>
1836 S: REJECTED KERBEROS_V4 SKEY
1837 C: AUTH SKEY 7ab83f32ee
1838 S: DATA 8799cabb2ea93e
1839 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1845 <title>Example of client sends unknown command then falls back to regular auth</title>
1849 C: AUTH MAGIC_COOKIE 3736343435313230333039
1855 <title>Example of server doesn't support initial auth mechanism</title>
1857 C: AUTH MAGIC_COOKIE 3736343435313230333039
1858 S: REJECTED KERBEROS_V4 SKEY
1859 C: AUTH SKEY 7ab83f32ee
1860 S: DATA 8799cabb2ea93e
1861 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1867 <title>Example of wrong password or the like followed by successful retry</title>
1869 C: AUTH MAGIC_COOKIE 3736343435313230333039
1870 S: REJECTED KERBEROS_V4 SKEY
1871 C: AUTH SKEY 7ab83f32ee
1872 S: DATA 8799cabb2ea93e
1873 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1875 C: AUTH SKEY 7ab83f32ee
1876 S: DATA 8799cabb2ea93e
1877 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1883 <title>Example of skey cancelled and restarted</title>
1885 C: AUTH MAGIC_COOKIE 3736343435313230333039
1886 S: REJECTED KERBEROS_V4 SKEY
1887 C: AUTH SKEY 7ab83f32ee
1888 S: DATA 8799cabb2ea93e
1891 C: AUTH SKEY 7ab83f32ee
1892 S: DATA 8799cabb2ea93e
1893 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1899 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1901 (MAGIC_COOKIE is a made up mechanism)
1903 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1905 C: NEGOTIATE_UNIX_FD
1911 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1913 (MAGIC_COOKIE is a made up mechanism)
1915 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1917 C: NEGOTIATE_UNIX_FD
1924 <sect2 id="auth-states">
1925 <title>Authentication state diagrams</title>
1928 This section documents the auth protocol in terms of
1929 a state machine for the client and the server. This is
1930 probably the most robust way to implement the protocol.
1933 <sect3 id="auth-states-client">
1934 <title>Client states</title>
1937 To more precisely describe the interaction between the
1938 protocol state machine and the authentication mechanisms the
1939 following notation is used: MECH(CHALL) means that the
1940 server challenge CHALL was fed to the mechanism MECH, which
1946 CONTINUE(RESP) means continue the auth conversation
1947 and send RESP as the response to the server;
1953 OK(RESP) means that after sending RESP to the server
1954 the client side of the auth conversation is finished
1955 and the server should return "OK";
1961 ERROR means that CHALL was invalid and could not be
1967 Both RESP and CHALL may be empty.
1971 The Client starts by getting an initial response from the
1972 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1973 the mechanism did not provide an initial response. If the
1974 mechanism returns CONTINUE, the client starts in state
1975 <emphasis>WaitingForData</emphasis>, if the mechanism
1976 returns OK the client starts in state
1977 <emphasis>WaitingForOK</emphasis>.
1981 The client should keep track of available mechanisms and
1982 which it mechanisms it has already attempted. This list is
1983 used to decide which AUTH command to send. When the list is
1984 exhausted, the client should give up and close the
1989 <title><emphasis>WaitingForData</emphasis></title>
1997 MECH(CHALL) returns CONTINUE(RESP) → send
1999 <emphasis>WaitingForData</emphasis>
2003 MECH(CHALL) returns OK(RESP) → send DATA
2004 RESP, goto <emphasis>WaitingForOK</emphasis>
2008 MECH(CHALL) returns ERROR → send ERROR
2009 [msg], goto <emphasis>WaitingForData</emphasis>
2017 Receive REJECTED [mechs] →
2018 send AUTH [next mech], goto
2019 WaitingForData or <emphasis>WaitingForOK</emphasis>
2024 Receive ERROR → send
2026 <emphasis>WaitingForReject</emphasis>
2031 Receive OK → send
2032 BEGIN, terminate auth
2033 conversation, authenticated
2038 Receive anything else → send
2040 <emphasis>WaitingForData</emphasis>
2048 <title><emphasis>WaitingForOK</emphasis></title>
2053 Receive OK → send BEGIN, terminate auth
2054 conversation, <emphasis>authenticated</emphasis>
2059 Receive REJECT [mechs] → send AUTH [next mech],
2060 goto <emphasis>WaitingForData</emphasis> or
2061 <emphasis>WaitingForOK</emphasis>
2067 Receive DATA → send CANCEL, goto
2068 <emphasis>WaitingForReject</emphasis>
2074 Receive ERROR → send CANCEL, goto
2075 <emphasis>WaitingForReject</emphasis>
2081 Receive anything else → send ERROR, goto
2082 <emphasis>WaitingForOK</emphasis>
2090 <title><emphasis>WaitingForReject</emphasis></title>
2095 Receive REJECT [mechs] → send AUTH [next mech],
2096 goto <emphasis>WaitingForData</emphasis> or
2097 <emphasis>WaitingForOK</emphasis>
2103 Receive anything else → terminate auth
2104 conversation, disconnect
2113 <sect3 id="auth-states-server">
2114 <title>Server states</title>
2117 For the server MECH(RESP) means that the client response
2118 RESP was fed to the the mechanism MECH, which returns one of
2123 CONTINUE(CHALL) means continue the auth conversation and
2124 send CHALL as the challenge to the client;
2130 OK means that the client has been successfully
2137 REJECT means that the client failed to authenticate or
2138 there was an error in RESP.
2143 The server starts out in state
2144 <emphasis>WaitingForAuth</emphasis>. If the client is
2145 rejected too many times the server must disconnect the
2150 <title><emphasis>WaitingForAuth</emphasis></title>
2156 Receive AUTH → send REJECTED [mechs], goto
2157 <emphasis>WaitingForAuth</emphasis>
2163 Receive AUTH MECH RESP
2167 MECH not valid mechanism → send REJECTED
2169 <emphasis>WaitingForAuth</emphasis>
2173 MECH(RESP) returns CONTINUE(CHALL) → send
2175 <emphasis>WaitingForData</emphasis>
2179 MECH(RESP) returns OK → send OK, goto
2180 <emphasis>WaitingForBegin</emphasis>
2184 MECH(RESP) returns REJECT → send REJECTED
2186 <emphasis>WaitingForAuth</emphasis>
2194 Receive BEGIN → terminate
2195 auth conversation, disconnect
2201 Receive ERROR → send REJECTED [mechs], goto
2202 <emphasis>WaitingForAuth</emphasis>
2208 Receive anything else → send
2210 <emphasis>WaitingForAuth</emphasis>
2219 <title><emphasis>WaitingForData</emphasis></title>
2227 MECH(RESP) returns CONTINUE(CHALL) → send
2229 <emphasis>WaitingForData</emphasis>
2233 MECH(RESP) returns OK → send OK, goto
2234 <emphasis>WaitingForBegin</emphasis>
2238 MECH(RESP) returns REJECT → send REJECTED
2240 <emphasis>WaitingForAuth</emphasis>
2248 Receive BEGIN → terminate auth conversation,
2255 Receive CANCEL → send REJECTED [mechs], goto
2256 <emphasis>WaitingForAuth</emphasis>
2262 Receive ERROR → send REJECTED [mechs], goto
2263 <emphasis>WaitingForAuth</emphasis>
2269 Receive anything else → send ERROR, goto
2270 <emphasis>WaitingForData</emphasis>
2278 <title><emphasis>WaitingForBegin</emphasis></title>
2283 Receive BEGIN → terminate auth conversation,
2284 client authenticated
2290 Receive CANCEL → send REJECTED [mechs], goto
2291 <emphasis>WaitingForAuth</emphasis>
2297 Receive ERROR → send REJECTED [mechs], goto
2298 <emphasis>WaitingForAuth</emphasis>
2304 Receive anything else → send ERROR, goto
2305 <emphasis>WaitingForBegin</emphasis>
2315 <sect2 id="auth-mechanisms">
2316 <title>Authentication mechanisms</title>
2318 This section describes some new authentication mechanisms.
2319 D-Bus also allows any standard SASL mechanism of course.
2321 <sect3 id="auth-mechanisms-sha">
2322 <title>DBUS_COOKIE_SHA1</title>
2324 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2325 has the ability to read a private file owned by the user being
2326 authenticated. If the client can prove that it has access to a secret
2327 cookie stored in this file, then the client is authenticated.
2328 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2332 Throughout this description, "hex encoding" must output the digits
2333 from a to f in lower-case; the digits A to F must not be used
2334 in the DBUS_COOKIE_SHA1 mechanism.
2337 Authentication proceeds as follows:
2341 The client sends the username it would like to authenticate
2347 The server sends the name of its "cookie context" (see below); a
2348 space character; the integer ID of the secret cookie the client
2349 must demonstrate knowledge of; a space character; then a
2350 randomly-generated challenge string, all of this hex-encoded into
2356 The client locates the cookie and generates its own
2357 randomly-generated challenge string. The client then concatenates
2358 the server's decoded challenge, a ":" character, its own challenge,
2359 another ":" character, and the cookie. It computes the SHA-1 hash
2360 of this composite string as a hex digest. It concatenates the
2361 client's challenge string, a space character, and the SHA-1 hex
2362 digest, hex-encodes the result and sends it back to the server.
2367 The server generates the same concatenated string used by the
2368 client and computes its SHA-1 hash. It compares the hash with
2369 the hash received from the client; if the two hashes match, the
2370 client is authenticated.
2376 Each server has a "cookie context," which is a name that identifies a
2377 set of cookies that apply to that server. A sample context might be
2378 "org_freedesktop_session_bus". Context names must be valid ASCII,
2379 nonzero length, and may not contain the characters slash ("/"),
2380 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2381 tab ("\t"), or period ("."). There is a default context,
2382 "org_freedesktop_general" that's used by servers that do not specify
2386 Cookies are stored in a user's home directory, in the directory
2387 <filename>~/.dbus-keyrings/</filename>. This directory must
2388 not be readable or writable by other users. If it is,
2389 clients and servers must ignore it. The directory
2390 contains cookie files named after the cookie context.
2393 A cookie file contains one cookie per line. Each line
2394 has three space-separated fields:
2398 The cookie ID number, which must be a non-negative integer and
2399 may not be used twice in the same file.
2404 The cookie's creation time, in UNIX seconds-since-the-epoch
2410 The cookie itself, a hex-encoded random block of bytes. The cookie
2411 may be of any length, though obviously security increases
2412 as the length increases.
2418 Only server processes modify the cookie file.
2419 They must do so with this procedure:
2423 Create a lockfile name by appending ".lock" to the name of the
2424 cookie file. The server should attempt to create this file
2425 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2426 fails, the lock fails. Servers should retry for a reasonable
2427 period of time, then they may choose to delete an existing lock
2428 to keep users from having to manually delete a stale
2429 lock. <footnote><para>Lockfiles are used instead of real file
2430 locking <literal>fcntl()</literal> because real locking
2431 implementations are still flaky on network
2432 filesystems.</para></footnote>
2437 Once the lockfile has been created, the server loads the cookie
2438 file. It should then delete any cookies that are old (the
2439 timeout can be fairly short), or more than a reasonable
2440 time in the future (so that cookies never accidentally
2441 become permanent, if the clock was set far into the future
2442 at some point). If no recent keys remain, the
2443 server may generate a new key.
2448 The pruned and possibly added-to cookie file
2449 must be resaved atomically (using a temporary
2450 file which is rename()'d).
2455 The lock must be dropped by deleting the lockfile.
2461 Clients need not lock the file in order to load it,
2462 because servers are required to save the file atomically.
2467 <sect1 id="addresses">
2468 <title>Server Addresses</title>
2470 Server addresses consist of a transport name followed by a colon, and
2471 then an optional, comma-separated list of keys and values in the form key=value.
2472 Each value is escaped.
2476 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2477 Which is the address to a unix socket with the path /tmp/dbus-test.
2480 Value escaping is similar to URI escaping but simpler.
2484 The set of optionally-escaped bytes is:
2485 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2486 <emphasis>byte</emphasis> (note, not character) which is not in the
2487 set of optionally-escaped bytes must be replaced with an ASCII
2488 percent (<literal>%</literal>) and the value of the byte in hex.
2489 The hex value must always be two digits, even if the first digit is
2490 zero. The optionally-escaped bytes may be escaped if desired.
2495 To unescape, append each byte in the value; if a byte is an ASCII
2496 percent (<literal>%</literal>) character then append the following
2497 hex value instead. It is an error if a <literal>%</literal> byte
2498 does not have two hex digits following. It is an error if a
2499 non-optionally-escaped byte is seen unescaped.
2503 The set of optionally-escaped bytes is intended to preserve address
2504 readability and convenience.
2508 A server may specify a key-value pair with the key <literal>guid</literal>
2509 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2510 describes the format of the <literal>guid</literal> field. If present,
2511 this UUID may be used to distinguish one server address from another. A
2512 server should use a different UUID for each address it listens on. For
2513 example, if a message bus daemon offers both UNIX domain socket and TCP
2514 connections, but treats clients the same regardless of how they connect,
2515 those two connections are equivalent post-connection but should have
2516 distinct UUIDs to distinguish the kinds of connection.
2520 The intent of the address UUID feature is to allow a client to avoid
2521 opening multiple identical connections to the same server, by allowing the
2522 client to check whether an address corresponds to an already-existing
2523 connection. Comparing two addresses is insufficient, because addresses
2524 can be recycled by distinct servers, and equivalent addresses may look
2525 different if simply compared as strings (for example, the host in a TCP
2526 address can be given as an IP address or as a hostname).
2530 Note that the address key is <literal>guid</literal> even though the
2531 rest of the API and documentation says "UUID," for historical reasons.
2535 [FIXME clarify if attempting to connect to each is a requirement
2536 or just a suggestion]
2537 When connecting to a server, multiple server addresses can be
2538 separated by a semi-colon. The library will then try to connect
2539 to the first address and if that fails, it'll try to connect to
2540 the next one specified, and so forth. For example
2541 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2546 <sect1 id="transports">
2547 <title>Transports</title>
2549 [FIXME we need to specify in detail each transport and its possible arguments]
2551 Current transports include: unix domain sockets (including
2552 abstract namespace on linux), launchd, TCP/IP, and a debug/testing transport
2553 using in-process pipes. Future possible transports include one that
2554 tunnels over X11 protocol.
2557 <sect2 id="transports-unix-domain-sockets">
2558 <title>Unix Domain Sockets</title>
2560 Unix domain sockets can be either paths in the file system or on Linux
2561 kernels, they can be abstract which are similar to paths but
2562 do not show up in the file system.
2566 When a socket is opened by the D-Bus library it truncates the path
2567 name right before the first trailing Nul byte. This is true for both
2568 normal paths and abstract paths. Note that this is a departure from
2569 previous versions of D-Bus that would create sockets with a fixed
2570 length path name. Names which were shorter than the fixed length
2571 would be padded by Nul bytes.
2574 Unix domain sockets are not available on windows.
2576 <sect3 id="transports-unix-domain-sockets-addresses">
2577 <title>Server Address Format</title>
2579 Unix domain socket addresses are identified by the "unix:" prefix
2580 and support the following key/value pairs:
2587 <entry>Values</entry>
2588 <entry>Description</entry>
2594 <entry>(path)</entry>
2595 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2598 <entry>tmpdir</entry>
2599 <entry>(path)</entry>
2600 <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>
2603 <entry>abstract</entry>
2604 <entry>(string)</entry>
2605 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2612 <sect2 id="transports-launchd">
2613 <title>launchd</title>
2615 launchd is a open-source server management system that replaces init, inetd
2616 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2617 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2621 launchd allocates a socket and provides it with the unix path through the
2622 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2623 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2624 it through its environment.
2625 Other processes can query for the launchd socket by executing:
2626 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2627 This is normally done by the D-Bus client library so doesn't have to be done
2631 launchd is not available on Microsoft Windows.
2633 <sect3 id="transports-launchd-addresses">
2634 <title>Server Address Format</title>
2636 launchd addresses are identified by the "launchd:" prefix
2637 and support the following key/value pairs:
2644 <entry>Values</entry>
2645 <entry>Description</entry>
2651 <entry>(environment variable)</entry>
2652 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2659 <sect2 id="transports-tcp-sockets">
2660 <title>TCP Sockets</title>
2662 The tcp transport provides TCP/IP based connections between clients
2663 located on the same or different hosts.
2666 Using tcp transport without any additional secure authentification mechanismus
2667 over a network is unsecure.
2670 Windows notes: Because of the tcp stack on windows does not provide sending
2671 credentials over a tcp connection, the EXTERNAL authentification
2672 mechanismus does not work.
2674 <sect3 id="transports-tcp-sockets-addresses">
2675 <title>Server Address Format</title>
2677 TCP/IP socket addresses are identified by the "tcp:" prefix
2678 and support the following key/value pairs:
2685 <entry>Values</entry>
2686 <entry>Description</entry>
2692 <entry>(string)</entry>
2693 <entry>dns name or ip address</entry>
2697 <entry>(number)</entry>
2698 <entry>The tcp port the server will open. A zero value let the server
2699 choose a free port provided from the underlaying operating system.
2700 libdbus is able to retrieve the real used port from the server.
2704 <entry>family</entry>
2705 <entry>(string)</entry>
2706 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2713 <sect2 id="transports-nonce-tcp-sockets">
2714 <title>Nonce-secured TCP Sockets</title>
2716 The nonce-tcp transport provides a secured TCP transport, using a
2717 simple authentication mechanism to ensure that only clients with read
2718 access to a certain location in the filesystem can connect to the server.
2719 The server writes a secret, the nonce, to a file and an incoming client
2720 connection is only accepted if the client sends the nonce right after
2721 the connect. The nonce mechanism requires no setup and is orthogonal to
2722 the higher-level authentication mechanisms described in the
2723 Authentication section.
2727 On start, the server generates a random 16 byte nonce and writes it
2728 to a file in the user's temporary directory. The nonce file location
2729 is published as part of the server's D-Bus address using the
2730 "noncefile" key-value pair.
2732 After an accept, the server reads 16 bytes from the socket. If the
2733 read bytes do not match the nonce stored in the nonce file, the
2734 server MUST immediately drop the connection.
2735 If the nonce match the received byte sequence, the client is accepted
2736 and the transport behaves like an unsecured tcp transport.
2739 After a successful connect to the server socket, the client MUST read
2740 the nonce from the file published by the server via the noncefile=
2741 key-value pair and send it over the socket. After that, the
2742 transport behaves like an unsecured tcp transport.
2744 <sect3 id="transports-nonce-tcp-sockets-addresses">
2745 <title>Server Address Format</title>
2747 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2748 and support the following key/value pairs:
2755 <entry>Values</entry>
2756 <entry>Description</entry>
2762 <entry>(string)</entry>
2763 <entry>dns name or ip address</entry>
2767 <entry>(number)</entry>
2768 <entry>The tcp port the server will open. A zero value let the server
2769 choose a free port provided from the underlaying operating system.
2770 libdbus is able to retrieve the real used port from the server.
2774 <entry>family</entry>
2775 <entry>(string)</entry>
2776 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2779 <entry>noncefile</entry>
2780 <entry>(path)</entry>
2781 <entry>file location containing the secret</entry>
2789 <sect1 id="meta-transports">
2790 <title>Meta Transports</title>
2792 Meta transports are a kind of transport with special enhancements or
2793 behavior. Currently available meta transports include: autolaunch
2796 <sect2 id="meta-transports-autolaunch">
2797 <title>Autolaunch</title>
2798 <para>The autolaunch transport provides a way for dbus clients to autodetect
2799 a running dbus session bus and to autolaunch a session bus if not present.
2801 <sect3 id="meta-transports-autolaunch-addresses">
2802 <title>Server Address Format</title>
2804 Autolaunch addresses uses the "autolaunch:" prefix and support the
2805 following key/value pairs:
2812 <entry>Values</entry>
2813 <entry>Description</entry>
2818 <entry>scope</entry>
2819 <entry>(string)</entry>
2820 <entry>scope of autolaunch (Windows only)
2824 "*install-path" - limit session bus to dbus installation path.
2825 The dbus installation path is determined from the location of
2826 the shared dbus library. If the library is located in a 'bin'
2827 subdirectory the installation root is the directory above,
2828 otherwise the directory where the library lives is taken as
2831 <install-root>/bin/[lib]dbus-1.dll
2832 <install-root>/[lib]dbus-1.dll
2838 "*user" - limit session bus to the recent user.
2843 other values - specify dedicated session bus like "release",
2855 <sect3 id="meta-transports-autolaunch-windows-implementation">
2856 <title>Windows implementation</title>
2858 On start, the server opens a platform specific transport, creates a mutex
2859 and a shared memory section containing the related session bus address.
2860 This mutex will be inspected by the dbus client library to detect a
2861 running dbus session bus. The access to the mutex and the shared memory
2862 section are protected by global locks.
2865 In the recent implementation the autolaunch transport uses a tcp transport
2866 on localhost with a port choosen from the operating system. This detail may
2867 change in the future.
2870 Disclaimer: The recent implementation is in an early state and may not
2871 work in all cirumstances and/or may have security issues. Because of this
2872 the implementation is not documentated yet.
2877 <sect1 id="naming-conventions">
2878 <title>Naming Conventions</title>
2881 D-Bus namespaces are all lowercase and correspond to reversed domain
2882 names, as with Java. e.g. "org.freedesktop"
2885 Interface, signal, method, and property names are "WindowsStyleCaps", note
2886 that the first letter is capitalized, unlike Java.
2889 Object paths are normally all lowercase with underscores used rather than
2895 <title>UUIDs</title>
2897 A working D-Bus implementation uses universally-unique IDs in two places.
2898 First, each server address has a UUID identifying the address,
2899 as described in <xref linkend="addresses"/>. Second, each operating
2900 system kernel instance running a D-Bus client or server has a UUID
2901 identifying that kernel, retrieved by invoking the method
2902 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2903 linkend="standard-interfaces-peer"/>).
2906 The term "UUID" in this document is intended literally, i.e. an
2907 identifier that is universally unique. It is not intended to refer to
2908 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2911 The UUID must contain 128 bits of data and be hex-encoded. The
2912 hex-encoded string may not contain hyphens or other non-hex-digit
2913 characters, and it must be exactly 32 characters long. To generate a
2914 UUID, the current reference implementation concatenates 96 bits of random
2915 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2919 It would also be acceptable and probably better to simply generate 128
2920 bits of random data, as long as the random number generator is of high
2921 quality. The timestamp could conceivably help if the random bits are not
2922 very random. With a quality random number generator, collisions are
2923 extremely unlikely even with only 96 bits, so it's somewhat academic.
2926 Implementations should, however, stick to random data for the first 96 bits
2931 <sect1 id="standard-interfaces">
2932 <title>Standard Interfaces</title>
2934 See <xref linkend="message-protocol-types-notation"/> for details on
2935 the notation used in this section. There are some standard interfaces
2936 that may be useful across various D-Bus applications.
2938 <sect2 id="standard-interfaces-peer">
2939 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2941 The <literal>org.freedesktop.DBus.Peer</literal> interface
2944 org.freedesktop.DBus.Peer.Ping ()
2945 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2949 On receipt of the <literal>METHOD_CALL</literal> message
2950 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2951 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2952 usual. It does not matter which object path a ping is sent to. The
2953 reference implementation handles this method automatically.
2956 On receipt of the <literal>METHOD_CALL</literal> message
2957 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2958 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2959 UUID representing the identity of the machine the process is running on.
2960 This UUID must be the same for all processes on a single system at least
2961 until that system next reboots. It should be the same across reboots
2962 if possible, but this is not always possible to implement and is not
2964 It does not matter which object path a GetMachineId is sent to. The
2965 reference implementation handles this method automatically.
2968 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2969 a virtual machine running on a hypervisor, rather than a physical machine.
2970 Basically if two processes see the same UUID, they should also see the same
2971 shared memory, UNIX domain sockets, process IDs, and other features that require
2972 a running OS kernel in common between the processes.
2975 The UUID is often used where other programs might use a hostname. Hostnames
2976 can change without rebooting, however, or just be "localhost" - so the UUID
2980 <xref linkend="uuids"/> explains the format of the UUID.
2984 <sect2 id="standard-interfaces-introspectable">
2985 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2987 This interface has one method:
2989 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2993 Objects instances may implement
2994 <literal>Introspect</literal> which returns an XML description of
2995 the object, including its interfaces (with signals and methods), objects
2996 below it in the object path tree, and its properties.
2999 <xref linkend="introspection-format"/> describes the format of this XML string.
3002 <sect2 id="standard-interfaces-properties">
3003 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3005 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3006 or <firstterm>attributes</firstterm>. These can be exposed via the
3007 <literal>org.freedesktop.DBus.Properties</literal> interface.
3011 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3012 in STRING property_name,
3014 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3015 in STRING property_name,
3017 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3018 out DICT<STRING,VARIANT> props);
3022 The available properties and whether they are writable can be determined
3023 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3024 see <xref linkend="standard-interfaces-introspectable"/>.
3027 An empty string may be provided for the interface name; in this case,
3028 if there are multiple properties on an object with the same name,
3029 the results are undefined (picking one by according to an arbitrary
3030 deterministic rule, or returning an error, are the reasonable
3034 If one or more properties change on an object, the
3035 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3036 signal may be emitted (this signal was added in 0.14):
3040 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3041 DICT<STRING,VARIANT> changed_properties,
3042 ARRAY<STRING> invalidated_properties);
3046 where <literal>changed_properties</literal> is a dictionary
3047 containing the changed properties with the new values and
3048 <literal>invalidated_properties</literal> is an array of
3049 properties that changed but the value is not conveyed.
3052 Whether the <literal>PropertiesChanged</literal> signal is
3053 supported can be determined by calling
3054 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3055 that the signal may be supported for an object but it may
3056 differ how whether and how it is used on a per-property basis
3057 (for e.g. performance or security reasons). Each property (or
3058 the parent interface) must be annotated with the
3059 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3060 annotation to convey this (usually the default value
3061 <literal>true</literal> is sufficient meaning that the
3062 annotation does not need to be used). See <xref
3063 linkend="introspection-format"/> for details on this
3068 <sect2 id="standard-interfaces-objectmanager">
3069 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3071 An API can optionally make use of this interface for one or
3072 more sub-trees of objects. The root of each sub-tree implements
3073 this interface so other applications can get all objects,
3074 interfaces and properties in a single method call. It is
3075 appropriate to use this interface if users of the tree of
3076 objects are expected to be interested in all interfaces of all
3077 objects in the tree; a more granular API should be used if
3078 users of the objects are expected to be interested in a small
3079 subset of the objects, a small subset of their interfaces, or
3083 The method that applications can use to get all objects and
3084 properties is <literal>GetManagedObjects</literal>:
3088 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3092 The return value of this method is a dict whose keys are
3093 object paths. All returned object paths are children of the
3094 object path implementing this interface, i.e. their object
3095 paths start with the ObjectManager's object path plus '/'.
3098 Each value is a dict whose keys are interfaces names. Each
3099 value in this inner dict is the same dict that would be
3100 returned by the <link
3101 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3102 method for that combination of object path and interface. If
3103 an interface has no properties, the empty dict is returned.
3106 Changes are emitted using the following two signals:
3110 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3111 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3112 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3113 ARRAY<STRING> interfaces);
3117 The <literal>InterfacesAdded</literal> signal is emitted when
3118 either a new object is added or when an existing object gains
3119 one or more interfaces. The
3120 <literal>InterfacesRemoved</literal> signal is emitted
3121 whenever an object is removed or it loses one or more
3122 interfaces. The second parameter of the
3123 <literal>InterfacesAdded</literal> signal contains a dict with
3124 the interfaces and properties (if any) that have been added to
3125 the given object path. Similarly, the second parameter of the
3126 <literal>InterfacesRemoved</literal> signal contains an array
3127 of the interfaces that were removed. Note that changes on
3128 properties on existing interfaces are not reported using this
3129 interface - an application should also monitor the existing <link
3130 linkend="standard-interfaces-properties">PropertiesChanged</link>
3131 signal on each object.
3134 Applications SHOULD NOT export objects that are children of an
3135 object (directly or otherwise) implementing this interface but
3136 which are not returned in the reply from the
3137 <literal>GetManagedObjects()</literal> method of this
3138 interface on the given object.
3141 The intent of the <literal>ObjectManager</literal> interface
3142 is to make it easy to write a robust client
3143 implementation. The trivial client implementation only needs
3144 to make two method calls:
3148 org.freedesktop.DBus.AddMatch (bus_proxy,
3149 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3150 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3154 on the message bus and the remote application's
3155 <literal>ObjectManager</literal>, respectively. Whenever a new
3156 remote object is created (or an existing object gains a new
3157 interface), the <literal>InterfacesAdded</literal> signal is
3158 emitted, and since this signal contains all properties for the
3159 interfaces, no calls to the
3160 <literal>org.freedesktop.Properties</literal> interface on the
3161 remote object are needed. Additionally, since the initial
3162 <literal>AddMatch()</literal> rule already includes signal
3163 messages from the newly created child object, no new
3164 <literal>AddMatch()</literal> call is needed.
3169 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3170 interface was added in version 0.17 of the D-Bus
3177 <sect1 id="introspection-format">
3178 <title>Introspection Data Format</title>
3180 As described in <xref linkend="standard-interfaces-introspectable"/>,
3181 objects may be introspected at runtime, returning an XML string
3182 that describes the object. The same XML format may be used in
3183 other contexts as well, for example as an "IDL" for generating
3184 static language bindings.
3187 Here is an example of introspection data:
3189 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3190 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3191 <node name="/org/freedesktop/sample_object">
3192 <interface name="org.freedesktop.SampleInterface">
3193 <method name="Frobate">
3194 <arg name="foo" type="i" direction="in"/>
3195 <arg name="bar" type="s" direction="out"/>
3196 <arg name="baz" type="a{us}" direction="out"/>
3197 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3199 <method name="Bazify">
3200 <arg name="bar" type="(iiu)" direction="in"/>
3201 <arg name="bar" type="v" direction="out"/>
3203 <method name="Mogrify">
3204 <arg name="bar" type="(iiav)" direction="in"/>
3206 <signal name="Changed">
3207 <arg name="new_value" type="b"/>
3209 <property name="Bar" type="y" access="readwrite"/>
3211 <node name="child_of_sample_object"/>
3212 <node name="another_child_of_sample_object"/>
3217 A more formal DTD and spec needs writing, but here are some quick notes.
3221 Only the root <node> element can omit the node name, as it's
3222 known to be the object that was introspected. If the root
3223 <node> does have a name attribute, it must be an absolute
3224 object path. If child <node> have object paths, they must be
3230 If a child <node> has any sub-elements, then they
3231 must represent a complete introspection of the child.
3232 If a child <node> is empty, then it may or may
3233 not have sub-elements; the child must be introspected
3234 in order to find out. The intent is that if an object
3235 knows that its children are "fast" to introspect
3236 it can go ahead and return their information, but
3237 otherwise it can omit it.
3242 The direction element on <arg> may be omitted,
3243 in which case it defaults to "in" for method calls
3244 and "out" for signals. Signals only allow "out"
3245 so while direction may be specified, it's pointless.
3250 The possible directions are "in" and "out",
3251 unlike CORBA there is no "inout"
3256 The possible property access flags are
3257 "readwrite", "read", and "write"
3262 Multiple interfaces can of course be listed for
3268 The "name" attribute on arguments is optional.
3274 Method, interface, property, and signal elements may have
3275 "annotations", which are generic key/value pairs of metadata.
3276 They are similar conceptually to Java's annotations and C# attributes.
3277 Well-known annotations:
3284 <entry>Values (separated by ,)</entry>
3285 <entry>Description</entry>
3290 <entry>org.freedesktop.DBus.Deprecated</entry>
3291 <entry>true,false</entry>
3292 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3295 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3296 <entry>(string)</entry>
3297 <entry>The C symbol; may be used for methods and interfaces</entry>
3300 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3301 <entry>true,false</entry>
3302 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3305 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3306 <entry>true,invalidates,false</entry>
3309 If set to <literal>false</literal>, the
3310 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3312 linkend="standard-interfaces-properties"/> is not
3313 guaranteed to be emitted if the property changes.
3316 If set to <literal>invalidates</literal> the signal
3317 is emitted but the value is not included in the
3321 If set to <literal>true</literal> the signal is
3322 emitted with the value included.
3325 The value for the annotation defaults to
3326 <literal>true</literal> if the enclosing interface
3327 element does not specify the annotation. Otherwise it
3328 defaults to the value specified in the enclosing
3337 <sect1 id="message-bus">
3338 <title>Message Bus Specification</title>
3339 <sect2 id="message-bus-overview">
3340 <title>Message Bus Overview</title>
3342 The message bus accepts connections from one or more applications.
3343 Once connected, applications can exchange messages with other
3344 applications that are also connected to the bus.
3347 In order to route messages among connections, the message bus keeps a
3348 mapping from names to connections. Each connection has one
3349 unique-for-the-lifetime-of-the-bus name automatically assigned.
3350 Applications may request additional names for a connection. Additional
3351 names are usually "well-known names" such as
3352 "org.freedesktop.TextEditor". When a name is bound to a connection,
3353 that connection is said to <firstterm>own</firstterm> the name.
3356 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3357 This name routes messages to the bus, allowing applications to make
3358 administrative requests. For example, applications can ask the bus
3359 to assign a name to a connection.
3362 Each name may have <firstterm>queued owners</firstterm>. When an
3363 application requests a name for a connection and the name is already in
3364 use, the bus will optionally add the connection to a queue waiting for
3365 the name. If the current owner of the name disconnects or releases
3366 the name, the next connection in the queue will become the new owner.
3370 This feature causes the right thing to happen if you start two text
3371 editors for example; the first one may request "org.freedesktop.TextEditor",
3372 and the second will be queued as a possible owner of that name. When
3373 the first exits, the second will take over.
3377 Messages may have a <literal>DESTINATION</literal> field (see <xref
3378 linkend="message-protocol-header-fields"/>). If the
3379 <literal>DESTINATION</literal> field is present, it specifies a message
3380 recipient by name. Method calls and replies normally specify this field.
3381 The message bus must send messages (of any type) with the
3382 <literal>DESTINATION</literal> field set to the specified recipient,
3383 regardless of whether the recipient has set up a match rule matching
3388 Signals normally do not specify a destination; they are sent to all
3389 applications with <firstterm>message matching rules</firstterm> that
3394 When the message bus receives a method call, if the
3395 <literal>DESTINATION</literal> field is absent, the call is taken to be
3396 a standard one-to-one message and interpreted by the message bus
3397 itself. For example, sending an
3398 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3399 <literal>DESTINATION</literal> will cause the message bus itself to
3400 reply to the ping immediately; the message bus will not make this
3401 message visible to other applications.
3405 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3406 the ping message were sent with a <literal>DESTINATION</literal> name of
3407 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3408 forwarded, and the Yoyodyne Corporation screensaver application would be
3409 expected to reply to the ping.
3413 <sect2 id="message-bus-names">
3414 <title>Message Bus Names</title>
3416 Each connection has at least one name, assigned at connection time and
3417 returned in response to the
3418 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3419 automatically-assigned name is called the connection's <firstterm>unique
3420 name</firstterm>. Unique names are never reused for two different
3421 connections to the same bus.
3424 Ownership of a unique name is a prerequisite for interaction with
3425 the message bus. It logically follows that the unique name is always
3426 the first name that an application comes to own, and the last
3427 one that it loses ownership of.
3430 Unique connection names must begin with the character ':' (ASCII colon
3431 character); bus names that are not unique names must not begin
3432 with this character. (The bus must reject any attempt by an application
3433 to manually request a name beginning with ':'.) This restriction
3434 categorically prevents "spoofing"; messages sent to a unique name
3435 will always go to the expected connection.
3438 When a connection is closed, all the names that it owns are deleted (or
3439 transferred to the next connection in the queue if any).
3442 A connection can request additional names to be associated with it using
3443 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3444 linkend="message-protocol-names-bus"/> describes the format of a valid
3445 name. These names can be released again using the
3446 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3449 <sect3 id="bus-messages-request-name">
3450 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3454 UINT32 RequestName (in STRING name, in UINT32 flags)
3461 <entry>Argument</entry>
3463 <entry>Description</entry>
3469 <entry>STRING</entry>
3470 <entry>Name to request</entry>
3474 <entry>UINT32</entry>
3475 <entry>Flags</entry>
3485 <entry>Argument</entry>
3487 <entry>Description</entry>
3493 <entry>UINT32</entry>
3494 <entry>Return value</entry>
3501 This method call should be sent to
3502 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3503 assign the given name to the method caller. Each name maintains a
3504 queue of possible owners, where the head of the queue is the primary
3505 or current owner of the name. Each potential owner in the queue
3506 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3507 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3508 call. When RequestName is invoked the following occurs:
3512 If the method caller is currently the primary owner of the name,
3513 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3514 values are updated with the values from the new RequestName call,
3515 and nothing further happens.
3521 If the current primary owner (head of the queue) has
3522 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3523 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3524 the caller of RequestName replaces the current primary owner at
3525 the head of the queue and the current primary owner moves to the
3526 second position in the queue. If the caller of RequestName was
3527 in the queue previously its flags are updated with the values from
3528 the new RequestName in addition to moving it to the head of the queue.
3534 If replacement is not possible, and the method caller is
3535 currently in the queue but not the primary owner, its flags are
3536 updated with the values from the new RequestName call.
3542 If replacement is not possible, and the method caller is
3543 currently not in the queue, the method caller is appended to the
3550 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3551 set and is not the primary owner, it is removed from the
3552 queue. This can apply to the previous primary owner (if it
3553 was replaced) or the method caller (if it updated the
3554 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3555 queue, or if it was just added to the queue with that flag set).
3561 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3562 queue," even if another application already in the queue had specified
3563 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3564 that does not allow replacement goes away, and the next primary owner
3565 does allow replacement. In this case, queued items that specified
3566 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3567 automatically replace the new primary owner. In other words,
3568 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3569 time RequestName is called. This is deliberate to avoid an infinite loop
3570 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3571 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3574 The flags argument contains any of the following values logically ORed
3581 <entry>Conventional Name</entry>
3582 <entry>Value</entry>
3583 <entry>Description</entry>
3588 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3592 If an application A specifies this flag and succeeds in
3593 becoming the owner of the name, and another application B
3594 later calls RequestName with the
3595 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3596 will lose ownership and receive a
3597 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3598 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3599 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3600 is not specified by application B, then application B will not replace
3601 application A as the owner.
3606 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3610 Try to replace the current owner if there is one. If this
3611 flag is not set the application will only become the owner of
3612 the name if there is no current owner. If this flag is set,
3613 the application will replace the current owner if
3614 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3619 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3623 Without this flag, if an application requests a name that is
3624 already owned, the application will be placed in a queue to
3625 own the name when the current owner gives it up. If this
3626 flag is given, the application will not be placed in the
3627 queue, the request for the name will simply fail. This flag
3628 also affects behavior when an application is replaced as
3629 name owner; by default the application moves back into the
3630 waiting queue, unless this flag was provided when the application
3631 became the name owner.
3639 The return code can be one of the following values:
3645 <entry>Conventional Name</entry>
3646 <entry>Value</entry>
3647 <entry>Description</entry>
3652 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3653 <entry>1</entry> <entry>The caller is now the primary owner of
3654 the name, replacing any previous owner. Either the name had no
3655 owner before, or the caller specified
3656 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3657 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3660 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3663 <entry>The name already had an owner,
3664 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3665 the current owner did not specify
3666 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3667 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3671 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3672 <entry>The name already has an owner,
3673 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3674 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3675 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3676 specified by the requesting application.</entry>
3679 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3681 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3689 <sect3 id="bus-messages-release-name">
3690 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3694 UINT32 ReleaseName (in STRING name)
3701 <entry>Argument</entry>
3703 <entry>Description</entry>
3709 <entry>STRING</entry>
3710 <entry>Name to release</entry>
3720 <entry>Argument</entry>
3722 <entry>Description</entry>
3728 <entry>UINT32</entry>
3729 <entry>Return value</entry>
3736 This method call should be sent to
3737 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3738 release the method caller's claim to the given name. If the caller is
3739 the primary owner, a new primary owner will be selected from the
3740 queue if any other owners are waiting. If the caller is waiting in
3741 the queue for the name, the caller will removed from the queue and
3742 will not be made an owner of the name if it later becomes available.
3743 If there are no other owners in the queue for the name, it will be
3744 removed from the bus entirely.
3746 The return code can be one of the following values:
3752 <entry>Conventional Name</entry>
3753 <entry>Value</entry>
3754 <entry>Description</entry>
3759 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3760 <entry>1</entry> <entry>The caller has released his claim on
3761 the given name. Either the caller was the primary owner of
3762 the name, and the name is now unused or taken by somebody
3763 waiting in the queue for the name, or the caller was waiting
3764 in the queue for the name and has now been removed from the
3768 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3770 <entry>The given name does not exist on this bus.</entry>
3773 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3775 <entry>The caller was not the primary owner of this name,
3776 and was also not waiting in the queue to own this name.</entry>
3784 <sect3 id="bus-messages-list-queued-owners">
3785 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3789 ARRAY of STRING ListQueuedOwners (in STRING name)
3796 <entry>Argument</entry>
3798 <entry>Description</entry>
3804 <entry>STRING</entry>
3805 <entry>The well-known bus name to query, such as
3806 <literal>com.example.cappuccino</literal></entry>
3816 <entry>Argument</entry>
3818 <entry>Description</entry>
3824 <entry>ARRAY of STRING</entry>
3825 <entry>The unique bus names of connections currently queued
3826 for the name</entry>
3833 This method call should be sent to
3834 <literal>org.freedesktop.DBus</literal> and lists the connections
3835 currently queued for a bus name (see
3836 <xref linkend="term-queued-owner"/>).
3841 <sect2 id="message-bus-routing">
3842 <title>Message Bus Message Routing</title>
3846 <sect3 id="message-bus-routing-match-rules">
3847 <title>Match Rules</title>
3849 An important part of the message bus routing protocol is match
3850 rules. Match rules describe what messages can be sent to a client
3851 based on the contents of the message. When a message is routed
3852 through the bus it is compared to clients' match rules. If any
3853 of the rules match, the message is dispatched to the client.
3854 If none of the rules match the message never leaves the bus. This
3855 is an effective way to control traffic over the bus and to make sure
3856 only relevant message need to be processed by the client.
3859 Match rules are added using the AddMatch bus method
3860 (see <xref linkend="bus-messages-add-match"/>). Rules are
3861 specified as a string of comma separated key/value pairs.
3862 Excluding a key from the rule indicates a wildcard match.
3863 For instance excluding the the member from a match rule but
3864 adding a sender would let all messages from that sender through.
3865 An example of a complete rule would be
3866 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3869 The following table describes the keys that can be used to create
3871 The following table summarizes the D-Bus types.
3877 <entry>Possible Values</entry>
3878 <entry>Description</entry>
3883 <entry><literal>type</literal></entry>
3884 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3885 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3888 <entry><literal>sender</literal></entry>
3889 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3890 and <xref linkend="term-unique-name"/> respectively)
3892 <entry>Match messages sent by a particular sender. An example of a sender match
3893 is sender='org.freedesktop.Hal'</entry>
3896 <entry><literal>interface</literal></entry>
3897 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3898 <entry>Match messages sent over or to a particular interface. An example of an
3899 interface match is interface='org.freedesktop.Hal.Manager'.
3900 If a message omits the interface header, it must not match any rule
3901 that specifies this key.</entry>
3904 <entry><literal>member</literal></entry>
3905 <entry>Any valid method or signal name</entry>
3906 <entry>Matches messages which have the give method or signal name. An example of
3907 a member match is member='NameOwnerChanged'</entry>
3910 <entry><literal>path</literal></entry>
3911 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3912 <entry>Matches messages which are sent from or to the given object. An example of a
3913 path match is path='/org/freedesktop/Hal/Manager'</entry>
3916 <entry><literal>path_namespace</literal></entry>
3917 <entry>An object path</entry>
3920 Matches messages which are sent from or to an
3921 object for which the object path is either the
3922 given value, or that value followed by one or
3923 more path components.
3928 <literal>path_namespace='/com/example/foo'</literal>
3929 would match signals sent by
3930 <literal>/com/example/foo</literal>
3932 <literal>/com/example/foo/bar</literal>,
3934 <literal>/com/example/foobar</literal>.
3938 Using both <literal>path</literal> and
3939 <literal>path_namespace</literal> in the same match
3940 rule is not allowed.
3945 This match key was added in version 0.16 of the
3946 D-Bus specification and implemented by the bus
3947 daemon in dbus 1.5.0 and later.
3953 <entry><literal>destination</literal></entry>
3954 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3955 <entry>Matches messages which are being sent to the given unique name. An
3956 example of a destination match is destination=':1.0'</entry>
3959 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3960 <entry>Any string</entry>
3961 <entry>Arg matches are special and are used for further restricting the
3962 match based on the arguments in the body of a message. Only arguments of type
3963 STRING can be matched in this way. An example of an argument match
3964 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3968 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
3969 <entry>Any string</entry>
3971 <para>Argument path matches provide a specialised form of wildcard matching for
3972 path-like namespaces. They can match arguments whose type is either STRING or
3973 OBJECT_PATH. As with normal argument matches,
3974 if the argument is exactly equal to the string given in the match
3975 rule then the rule is satisfied. Additionally, there is also a
3976 match when either the string given in the match rule or the
3977 appropriate message argument ends with '/' and is a prefix of the
3978 other. An example argument path match is arg0path='/aa/bb/'. This
3979 would match messages with first arguments of '/', '/aa/',
3980 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
3981 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
3983 <para>This is intended for monitoring “directories” in file system-like
3984 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
3985 system. An application interested in all nodes in a particular hierarchy would
3986 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
3987 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
3988 represent a modification to the “bar” property, or a signal with zeroth
3989 argument <literal>"/ca/example/"</literal> to represent atomic modification of
3990 many properties within that directory, and the interested application would be
3991 notified in both cases.</para>
3994 This match key was added in version 0.12 of the
3995 D-Bus specification, implemented for STRING
3996 arguments by the bus daemon in dbus 1.2.0 and later,
3997 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4004 <entry><literal>arg0namespace</literal></entry>
4005 <entry>Like a bus name, except that the string is not
4006 required to contain a '.' (period)</entry>
4008 <para>Match messages whose first argument is of type STRING, and is a bus name
4009 or interface name within the specified namespace. This is primarily intended
4010 for watching name owner changes for a group of related bus names, rather than
4011 for a single name or all name changes.</para>
4013 <para>Because every valid interface name is also a valid
4014 bus name, this can also be used for messages whose
4015 first argument is an interface name.</para>
4017 <para>For example, the match rule
4018 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4019 matches name owner changes for bus names such as
4020 <literal>com.example.backend.foo</literal>,
4021 <literal>com.example.backend.foo.bar</literal>, and
4022 <literal>com.example.backend</literal> itself.</para>
4024 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4027 This match key was added in version 0.16 of the
4028 D-Bus specification and implemented by the bus
4029 daemon in dbus 1.5.0 and later.
4040 <sect2 id="message-bus-starting-services">
4041 <title>Message Bus Starting Services</title>
4043 The message bus can start applications on behalf of other applications.
4044 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4045 An application that can be started in this way is called a
4046 <firstterm>service</firstterm>.
4049 With D-Bus, starting a service is normally done by name. That is,
4050 applications ask the message bus to start some program that will own a
4051 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4052 This implies a contract documented along with the name
4053 <literal>org.freedesktop.TextEditor</literal> for which objects
4054 the owner of that name will provide, and what interfaces those
4058 To find an executable corresponding to a particular name, the bus daemon
4059 looks for <firstterm>service description files</firstterm>. Service
4060 description files define a mapping from names to executables. Different
4061 kinds of message bus will look for these files in different places, see
4062 <xref linkend="message-bus-types"/>.
4065 Service description files have the ".service" file
4066 extension. The message bus will only load service description files
4067 ending with .service; all other files will be ignored. The file format
4068 is similar to that of <ulink
4069 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4070 entries</ulink>. All service description files must be in UTF-8
4071 encoding. To ensure that there will be no name collisions, service files
4072 must be namespaced using the same mechanism as messages and service
4077 [FIXME the file format should be much better specified than "similar to
4078 .desktop entries" esp. since desktop entries are already
4079 badly-specified. ;-)]
4080 These sections from the specification apply to service files as well:
4083 <listitem><para>General syntax</para></listitem>
4084 <listitem><para>Comment format</para></listitem>
4088 <title>Example service description file</title>
4090 # Sample service description file
4092 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4093 Exec=/usr/libexec/gconfd-2
4098 When an application asks to start a service by name, the bus daemon tries to
4099 find a service that will own that name. It then tries to spawn the
4100 executable associated with it. If this fails, it will report an
4101 error. [FIXME what happens if two .service files offer the same service;
4102 what kind of error is reported, should we have a way for the client to
4106 The executable launched will have the environment variable
4107 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4108 message bus so it can connect and request the appropriate names.
4111 The executable being launched may want to know whether the message bus
4112 starting it is one of the well-known message buses (see <xref
4113 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4114 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4115 of the well-known buses. The currently-defined values for this variable
4116 are <literal>system</literal> for the systemwide message bus,
4117 and <literal>session</literal> for the per-login-session message
4118 bus. The new executable must still connect to the address given
4119 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4120 resulting connection is to the well-known bus.
4123 [FIXME there should be a timeout somewhere, either specified
4124 in the .service file, by the client, or just a global value
4125 and if the client being activated fails to connect within that
4126 timeout, an error should be sent back.]
4129 <sect3 id="message-bus-starting-services-scope">
4130 <title>Message Bus Service Scope</title>
4132 The "scope" of a service is its "per-", such as per-session,
4133 per-machine, per-home-directory, or per-display. The reference
4134 implementation doesn't yet support starting services in a different
4135 scope from the message bus itself. So e.g. if you start a service
4136 on the session bus its scope is per-session.
4139 We could add an optional scope to a bus name. For example, for
4140 per-(display,session pair), we could have a unique ID for each display
4141 generated automatically at login and set on screen 0 by executing a
4142 special "set display ID" binary. The ID would be stored in a
4143 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4144 random bytes. This ID would then be used to scope names.
4145 Starting/locating a service could be done by ID-name pair rather than
4149 Contrast this with a per-display scope. To achieve that, we would
4150 want a single bus spanning all sessions using a given display.
4151 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4152 property on screen 0 of the display, pointing to this bus.
4157 <sect2 id="message-bus-types">
4158 <title>Well-known Message Bus Instances</title>
4160 Two standard message bus instances are defined here, along with how
4161 to locate them and where their service files live.
4163 <sect3 id="message-bus-types-login">
4164 <title>Login session message bus</title>
4166 Each time a user logs in, a <firstterm>login session message
4167 bus</firstterm> may be started. All applications in the user's login
4168 session may interact with one another using this message bus.
4171 The address of the login session message bus is given
4172 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4173 variable. If that variable is not set, applications may
4174 also try to read the address from the X Window System root
4175 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4176 The root window property must have type <literal>STRING</literal>.
4177 The environment variable should have precedence over the
4178 root window property.
4180 <para>The address of the login session message bus is given in the
4181 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4182 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4183 "autolaunch:", the system should use platform-specific methods of
4184 locating a running D-Bus session server, or starting one if a running
4185 instance cannot be found. Note that this mechanism is not recommended
4186 for attempting to determine if a daemon is running. It is inherently
4187 racy to attempt to make this determination, since the bus daemon may
4188 be started just before or just after the determination is made.
4189 Therefore, it is recommended that applications do not try to make this
4190 determination for their functionality purposes, and instead they
4191 should attempt to start the server.</para>
4193 <sect4 id="message-bus-types-login-x-windows">
4194 <title>X Windowing System</title>
4196 For the X Windowing System, the application must locate the
4197 window owner of the selection represented by the atom formed by
4201 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4205 <para>the current user's username</para>
4209 <para>the literal character '_' (underscore)</para>
4213 <para>the machine's ID</para>
4219 The following properties are defined for the window that owns
4221 <informaltable frame="all">
4230 <para>meaning</para>
4236 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4240 <para>the actual address of the server socket</para>
4246 <para>_DBUS_SESSION_BUS_PID</para>
4250 <para>the PID of the server process</para>
4259 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4260 present in this window.
4264 If the X selection cannot be located or if reading the
4265 properties from the window fails, the implementation MUST conclude
4266 that there is no D-Bus server running and proceed to start a new
4267 server. (See below on concurrency issues)
4271 Failure to connect to the D-Bus server address thus obtained
4272 MUST be treated as a fatal connection error and should be reported
4277 As an alternative, an implementation MAY find the information
4278 in the following file located in the current user's home directory,
4279 in subdirectory .dbus/session-bus/:
4282 <para>the machine's ID</para>
4286 <para>the literal character '-' (dash)</para>
4290 <para>the X display without the screen number, with the
4291 following prefixes removed, if present: ":", "localhost:"
4292 ."localhost.localdomain:". That is, a display of
4293 "localhost:10.0" produces just the number "10"</para>
4299 The contents of this file NAME=value assignment pairs and
4300 lines starting with # are comments (no comments are allowed
4301 otherwise). The following variable names are defined:
4308 <para>Variable</para>
4312 <para>meaning</para>
4318 <para>DBUS_SESSION_BUS_ADDRESS</para>
4322 <para>the actual address of the server socket</para>
4328 <para>DBUS_SESSION_BUS_PID</para>
4332 <para>the PID of the server process</para>
4338 <para>DBUS_SESSION_BUS_WINDOWID</para>
4342 <para>the window ID</para>
4351 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4356 Failure to open this file MUST be interpreted as absence of a
4357 running server. Therefore, the implementation MUST proceed to
4358 attempting to launch a new bus server if the file cannot be
4363 However, success in opening this file MUST NOT lead to the
4364 conclusion that the server is running. Thus, a failure to connect to
4365 the bus address obtained by the alternative method MUST NOT be
4366 considered a fatal error. If the connection cannot be established,
4367 the implementation MUST proceed to check the X selection settings or
4368 to start the server on its own.
4372 If the implementation concludes that the D-Bus server is not
4373 running it MUST attempt to start a new server and it MUST also
4374 ensure that the daemon started as an effect of the "autolaunch"
4375 mechanism provides the lookup mechanisms described above, so
4376 subsequent calls can locate the newly started server. The
4377 implementation MUST also ensure that if two or more concurrent
4378 initiations happen, only one server remains running and all other
4379 initiations are able to obtain the address of this server and
4380 connect to it. In other words, the implementation MUST ensure that
4381 the X selection is not present when it attempts to set it, without
4382 allowing another process to set the selection between the
4383 verification and the setting (e.g., by using XGrabServer /
4390 [FIXME specify location of .service files, probably using
4391 DESKTOP_DIRS etc. from basedir specification, though login session
4392 bus is not really desktop-specific]
4396 <sect3 id="message-bus-types-system">
4397 <title>System message bus</title>
4399 A computer may have a <firstterm>system message bus</firstterm>,
4400 accessible to all applications on the system. This message bus may be
4401 used to broadcast system events, such as adding new hardware devices,
4402 changes in the printer queue, and so forth.
4405 The address of the system message bus is given
4406 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4407 variable. If that variable is not set, applications should try
4408 to connect to the well-known address
4409 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4412 The D-Bus reference implementation actually honors the
4413 <literal>$(localstatedir)</literal> configure option
4414 for this address, on both client and server side.
4419 [FIXME specify location of system bus .service files]
4424 <sect2 id="message-bus-messages">
4425 <title>Message Bus Messages</title>
4427 The special message bus name <literal>org.freedesktop.DBus</literal>
4428 responds to a number of additional messages.
4431 <sect3 id="bus-messages-hello">
4432 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4443 <entry>Argument</entry>
4445 <entry>Description</entry>
4451 <entry>STRING</entry>
4452 <entry>Unique name assigned to the connection</entry>
4459 Before an application is able to send messages to other applications
4460 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4461 to the message bus to obtain a unique name. If an application without
4462 a unique name tries to send a message to another application, or a
4463 message to the message bus itself that isn't the
4464 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4465 disconnected from the bus.
4468 There is no corresponding "disconnect" request; if a client wishes to
4469 disconnect from the bus, it simply closes the socket (or other
4470 communication channel).
4473 <sect3 id="bus-messages-list-names">
4474 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4478 ARRAY of STRING ListNames ()
4485 <entry>Argument</entry>
4487 <entry>Description</entry>
4493 <entry>ARRAY of STRING</entry>
4494 <entry>Array of strings where each string is a bus name</entry>
4501 Returns a list of all currently-owned names on the bus.
4504 <sect3 id="bus-messages-list-activatable-names">
4505 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4509 ARRAY of STRING ListActivatableNames ()
4516 <entry>Argument</entry>
4518 <entry>Description</entry>
4524 <entry>ARRAY of STRING</entry>
4525 <entry>Array of strings where each string is a bus name</entry>
4532 Returns a list of all names that can be activated on the bus.
4535 <sect3 id="bus-messages-name-exists">
4536 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4540 BOOLEAN NameHasOwner (in STRING name)
4547 <entry>Argument</entry>
4549 <entry>Description</entry>
4555 <entry>STRING</entry>
4556 <entry>Name to check</entry>
4566 <entry>Argument</entry>
4568 <entry>Description</entry>
4574 <entry>BOOLEAN</entry>
4575 <entry>Return value, true if the name exists</entry>
4582 Checks if the specified name exists (currently has an owner).
4586 <sect3 id="bus-messages-name-owner-changed">
4587 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4591 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4598 <entry>Argument</entry>
4600 <entry>Description</entry>
4606 <entry>STRING</entry>
4607 <entry>Name with a new owner</entry>
4611 <entry>STRING</entry>
4612 <entry>Old owner or empty string if none</entry>
4616 <entry>STRING</entry>
4617 <entry>New owner or empty string if none</entry>
4624 This signal indicates that the owner of a name has changed.
4625 It's also the signal to use to detect the appearance of
4626 new names on the bus.
4629 <sect3 id="bus-messages-name-lost">
4630 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4634 NameLost (STRING name)
4641 <entry>Argument</entry>
4643 <entry>Description</entry>
4649 <entry>STRING</entry>
4650 <entry>Name which was lost</entry>
4657 This signal is sent to a specific application when it loses
4658 ownership of a name.
4662 <sect3 id="bus-messages-name-acquired">
4663 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4667 NameAcquired (STRING name)
4674 <entry>Argument</entry>
4676 <entry>Description</entry>
4682 <entry>STRING</entry>
4683 <entry>Name which was acquired</entry>
4690 This signal is sent to a specific application when it gains
4691 ownership of a name.
4695 <sect3 id="bus-messages-start-service-by-name">
4696 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4700 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4707 <entry>Argument</entry>
4709 <entry>Description</entry>
4715 <entry>STRING</entry>
4716 <entry>Name of the service to start</entry>
4720 <entry>UINT32</entry>
4721 <entry>Flags (currently not used)</entry>
4731 <entry>Argument</entry>
4733 <entry>Description</entry>
4739 <entry>UINT32</entry>
4740 <entry>Return value</entry>
4745 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4749 The return value can be one of the following values:
4754 <entry>Identifier</entry>
4755 <entry>Value</entry>
4756 <entry>Description</entry>
4761 <entry>DBUS_START_REPLY_SUCCESS</entry>
4763 <entry>The service was successfully started.</entry>
4766 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4768 <entry>A connection already owns the given name.</entry>
4777 <sect3 id="bus-messages-update-activation-environment">
4778 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4782 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4789 <entry>Argument</entry>
4791 <entry>Description</entry>
4797 <entry>ARRAY of DICT<STRING,STRING></entry>
4798 <entry>Environment to add or update</entry>
4803 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4806 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4809 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.
4814 <sect3 id="bus-messages-get-name-owner">
4815 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4819 STRING GetNameOwner (in STRING name)
4826 <entry>Argument</entry>
4828 <entry>Description</entry>
4834 <entry>STRING</entry>
4835 <entry>Name to get the owner of</entry>
4845 <entry>Argument</entry>
4847 <entry>Description</entry>
4853 <entry>STRING</entry>
4854 <entry>Return value, a unique connection name</entry>
4859 Returns the unique connection name of the primary owner of the name
4860 given. If the requested name doesn't have an owner, returns a
4861 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4865 <sect3 id="bus-messages-get-connection-unix-user">
4866 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4870 UINT32 GetConnectionUnixUser (in STRING bus_name)
4877 <entry>Argument</entry>
4879 <entry>Description</entry>
4885 <entry>STRING</entry>
4886 <entry>Unique or well-known bus name of the connection to
4887 query, such as <literal>:12.34</literal> or
4888 <literal>com.example.tea</literal></entry>
4898 <entry>Argument</entry>
4900 <entry>Description</entry>
4906 <entry>UINT32</entry>
4907 <entry>Unix user ID</entry>
4912 Returns the Unix user ID of the process connected to the server. If
4913 unable to determine it (for instance, because the process is not on the
4914 same machine as the bus daemon), an error is returned.
4918 <sect3 id="bus-messages-get-connection-unix-process-id">
4919 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
4923 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
4930 <entry>Argument</entry>
4932 <entry>Description</entry>
4938 <entry>STRING</entry>
4939 <entry>Unique or well-known bus name of the connection to
4940 query, such as <literal>:12.34</literal> or
4941 <literal>com.example.tea</literal></entry>
4951 <entry>Argument</entry>
4953 <entry>Description</entry>
4959 <entry>UINT32</entry>
4960 <entry>Unix process id</entry>
4965 Returns the Unix process ID of the process connected to the server. If
4966 unable to determine it (for instance, because the process is not on the
4967 same machine as the bus daemon), an error is returned.
4971 <sect3 id="bus-messages-add-match">
4972 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
4976 AddMatch (in STRING rule)
4983 <entry>Argument</entry>
4985 <entry>Description</entry>
4991 <entry>STRING</entry>
4992 <entry>Match rule to add to the connection</entry>
4997 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
4998 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5002 <sect3 id="bus-messages-remove-match">
5003 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5007 RemoveMatch (in STRING rule)
5014 <entry>Argument</entry>
5016 <entry>Description</entry>
5022 <entry>STRING</entry>
5023 <entry>Match rule to remove from the connection</entry>
5028 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5029 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5034 <sect3 id="bus-messages-get-id">
5035 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5039 GetId (out STRING id)
5046 <entry>Argument</entry>
5048 <entry>Description</entry>
5054 <entry>STRING</entry>
5055 <entry>Unique ID identifying the bus daemon</entry>
5060 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5061 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5062 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5063 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5064 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5065 by org.freedesktop.DBus.Peer.GetMachineId().
5066 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5074 <appendix id="implementation-notes">
5075 <title>Implementation notes</title>
5076 <sect1 id="implementation-notes-subsection">
5084 <glossary><title>Glossary</title>
5086 This glossary defines some of the terms used in this specification.
5089 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5092 The message bus maintains an association between names and
5093 connections. (Normally, there's one connection per application.) A
5094 bus name is simply an identifier used to locate connections. For
5095 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5096 name might be used to send a message to a screensaver from Yoyodyne
5097 Corporation. An application is said to <firstterm>own</firstterm> a
5098 name if the message bus has associated the application's connection
5099 with the name. Names may also have <firstterm>queued
5100 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5101 The bus assigns a unique name to each connection,
5102 see <xref linkend="term-unique-name"/>. Other names
5103 can be thought of as "well-known names" and are
5104 used to find applications that offer specific functionality.
5109 <glossentry id="term-message"><glossterm>Message</glossterm>
5112 A message is the atomic unit of communication via the D-Bus
5113 protocol. It consists of a <firstterm>header</firstterm> and a
5114 <firstterm>body</firstterm>; the body is made up of
5115 <firstterm>arguments</firstterm>.
5120 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5123 The message bus is a special application that forwards
5124 or routes messages between a group of applications
5125 connected to the message bus. It also manages
5126 <firstterm>names</firstterm> used for routing
5132 <glossentry id="term-name"><glossterm>Name</glossterm>
5135 See <xref linkend="term-bus-name"/>. "Name" may
5136 also be used to refer to some of the other names
5137 in D-Bus, such as interface names.
5142 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5145 Used to prevent collisions when defining new interfaces or bus
5146 names. The convention used is the same one Java uses for defining
5147 classes: a reversed domain name.
5152 <glossentry id="term-object"><glossterm>Object</glossterm>
5155 Each application contains <firstterm>objects</firstterm>, which have
5156 <firstterm>interfaces</firstterm> and
5157 <firstterm>methods</firstterm>. Objects are referred to by a name,
5158 called a <firstterm>path</firstterm>.
5163 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5166 An application talking directly to another application, without going
5167 through a message bus. One-to-one connections may be "peer to peer" or
5168 "client to server." The D-Bus protocol has no concept of client
5169 vs. server after a connection has authenticated; the flow of messages
5170 is symmetrical (full duplex).
5175 <glossentry id="term-path"><glossterm>Path</glossterm>
5178 Object references (object names) in D-Bus are organized into a
5179 filesystem-style hierarchy, so each object is named by a path. As in
5180 LDAP, there's no difference between "files" and "directories"; a path
5181 can refer to an object, while still having child objects below it.
5186 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5189 Each bus name has a primary owner; messages sent to the name go to the
5190 primary owner. However, certain names also maintain a queue of
5191 secondary owners "waiting in the wings." If the primary owner releases
5192 the name, then the first secondary owner in the queue automatically
5193 becomes the new owner of the name.
5198 <glossentry id="term-service"><glossterm>Service</glossterm>
5201 A service is an executable that can be launched by the bus daemon.
5202 Services normally guarantee some particular features, for example they
5203 may guarantee that they will request a specific name such as
5204 "org.freedesktop.Screensaver", have a singleton object
5205 "/org/freedesktop/Application", and that object will implement the
5206 interface "org.freedesktop.ScreensaverControl".
5211 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5214 ".service files" tell the bus about service applications that can be
5215 launched (see <xref linkend="term-service"/>). Most importantly they
5216 provide a mapping from bus names to services that will request those
5217 names when they start up.
5222 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5225 The special name automatically assigned to each connection by the
5226 message bus. This name will never change owner, and will be unique
5227 (never reused during the lifetime of the message bus).
5228 It will begin with a ':' character.