1 <?xml version="1.0" standalone="no" ?>
2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.19</releaseinfo>
10 <date>2012-02-21</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>0.19</revnumber>
82 <date>20 February 2012</date>
83 <authorinitials>smcv/lp</authorinitials>
84 <revremark>formally define unique connection names and well-known
85 bus names; document best practices for interface, bus, member and
86 error names, and object paths; document the search path for session
87 and system services on Unix; document the systemd transport</revremark>
90 <revnumber>0.18</revnumber>
91 <date>29 July 2011</date>
92 <authorinitials>smcv</authorinitials>
93 <revremark>define eavesdropping, unicast, broadcast; add eavesdrop
94 match keyword; promote type system to a top-level section</revremark>
97 <revnumber>0.17</revnumber>
98 <date>1 June 2011</date>
99 <authorinitials>smcv/davidz</authorinitials>
100 <revremark>define ObjectManager; reserve extra pseudo-type-codes used
101 by GVariant</revremark>
104 <revnumber>0.16</revnumber>
105 <date>11 April 2011</date>
106 <authorinitials></authorinitials>
107 <revremark>add path_namespace, arg0namespace; argNpath matches object
111 <revnumber>0.15</revnumber>
112 <date>3 November 2010</date>
113 <authorinitials></authorinitials>
114 <revremark></revremark>
117 <revnumber>0.14</revnumber>
118 <date>12 May 2010</date>
119 <authorinitials></authorinitials>
120 <revremark></revremark>
123 <revnumber>0.13</revnumber>
124 <date>23 Dezember 2009</date>
125 <authorinitials></authorinitials>
126 <revremark></revremark>
129 <revnumber>0.12</revnumber>
130 <date>7 November, 2006</date>
131 <authorinitials></authorinitials>
132 <revremark></revremark>
135 <revnumber>0.11</revnumber>
136 <date>6 February 2005</date>
137 <authorinitials></authorinitials>
138 <revremark></revremark>
141 <revnumber>0.10</revnumber>
142 <date>28 January 2005</date>
143 <authorinitials></authorinitials>
144 <revremark></revremark>
147 <revnumber>0.9</revnumber>
148 <date>7 Januar 2005</date>
149 <authorinitials></authorinitials>
150 <revremark></revremark>
153 <revnumber>0.8</revnumber>
154 <date>06 September 2003</date>
155 <authorinitials></authorinitials>
156 <revremark>First released document.</revremark>
161 <sect1 id="introduction">
162 <title>Introduction</title>
164 D-Bus is a system for low-latency, low-overhead, easy to use
165 interprocess communication (IPC). In more detail:
169 D-Bus is <emphasis>low-latency</emphasis> because it is designed
170 to avoid round trips and allow asynchronous operation, much like
176 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
177 binary protocol, and does not have to convert to and from a text
178 format such as XML. Because D-Bus is intended for potentially
179 high-resolution same-machine IPC, not primarily for Internet IPC,
180 this is an interesting optimization.
185 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
186 of <firstterm>messages</firstterm> rather than byte streams, and
187 automatically handles a lot of the hard IPC issues. Also, the D-Bus
188 library is designed to be wrapped in a way that lets developers use
189 their framework's existing object/type system, rather than learning
190 a new one specifically for IPC.
197 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
198 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
199 a system for one application to talk to a single other
200 application. However, the primary intended application of the protocol is the
201 D-Bus <firstterm>message bus</firstterm>, specified in <xref
202 linkend="message-bus"/>. The message bus is a special application that
203 accepts connections from multiple other applications, and forwards
208 Uses of D-Bus include notification of system changes (notification of when
209 a camera is plugged in to a computer, or a new version of some software
210 has been installed), or desktop interoperability, for example a file
211 monitoring service or a configuration service.
215 D-Bus is designed for two specific use cases:
219 A "system bus" for notifications from the system to user sessions,
220 and to allow the system to request input from user sessions.
225 A "session bus" used to implement desktop environments such as
230 D-Bus is not intended to be a generic IPC system for any possible
231 application, and intentionally omits many features found in other
232 IPC systems for this reason.
236 At the same time, the bus daemons offer a number of features not found in
237 other IPC systems, such as single-owner "bus names" (similar to X
238 selections), on-demand startup of services, and security policies.
239 In many ways, these features are the primary motivation for developing
240 D-Bus; other systems would have sufficed if IPC were the only goal.
244 D-Bus may turn out to be useful in unanticipated applications, but future
245 versions of this spec and the reference implementation probably will not
246 incorporate features that interfere with the core use cases.
250 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
251 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
252 document are to be interpreted as described in RFC 2119. However, the
253 document could use a serious audit to be sure it makes sense to do
254 so. Also, they are not capitalized.
257 <sect2 id="stability">
258 <title>Protocol and Specification Stability</title>
260 The D-Bus protocol is frozen (only compatible extensions are allowed) as
261 of November 8, 2006. However, this specification could still use a fair
262 bit of work to make interoperable reimplementation possible without
263 reference to the D-Bus reference implementation. Thus, this
264 specification is not marked 1.0. To mark it 1.0, we'd like to see
265 someone invest significant effort in clarifying the specification
266 language, and growing the specification to cover more aspects of the
267 reference implementation's behavior.
270 Until this work is complete, any attempt to reimplement D-Bus will
271 probably require looking at the reference implementation and/or asking
272 questions on the D-Bus mailing list about intended behavior.
273 Questions on the list are very welcome.
276 Nonetheless, this document should be a useful starting point and is
277 to our knowledge accurate, though incomplete.
283 <sect1 id="type-system">
284 <title>Type System</title>
287 D-Bus has a type system, in which values of various types can be
288 serialized into a sequence of bytes referred to as the
289 <firstterm>wire format</firstterm> in a standard way.
290 Converting a value from some other representation into the wire
291 format is called <firstterm>marshaling</firstterm> and converting
292 it back from the wire format is <firstterm>unmarshaling</firstterm>.
295 <sect2 id="message-protocol-signatures">
296 <title>Type Signatures</title>
299 The D-Bus protocol does not include type tags in the marshaled data; a
300 block of marshaled values must have a known <firstterm>type
301 signature</firstterm>. The type signature is made up of <firstterm>type
302 codes</firstterm>. A type code is an ASCII character representing the
303 type of a value. Because ASCII characters are used, the type signature
304 will always form a valid ASCII string. A simple string compare
305 determines whether two type signatures are equivalent.
309 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
310 the ASCII character 'i'. So the signature for a block of values
311 containing a single <literal>INT32</literal> would be:
315 A block of values containing two <literal>INT32</literal> would have this signature:
322 All <firstterm>basic</firstterm> types work like
323 <literal>INT32</literal> in this example. To marshal and unmarshal
324 basic types, you simply read one value from the data
325 block corresponding to each type code in the signature.
326 In addition to basic types, there are four <firstterm>container</firstterm>
327 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
328 and <literal>DICT_ENTRY</literal>.
332 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
333 code does not appear in signatures. Instead, ASCII characters
334 '(' and ')' are used to mark the beginning and end of the struct.
335 So for example, a struct containing two integers would have this
340 Structs can be nested, so for example a struct containing
341 an integer and another struct:
345 The value block storing that struct would contain three integers; the
346 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
351 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
352 but is useful in code that implements the protocol. This type code
353 is specified to allow such code to interoperate in non-protocol contexts.
357 Empty structures are not allowed; there must be at least one
358 type code between the parentheses.
362 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
363 followed by a <firstterm>single complete type</firstterm>. The single
364 complete type following the array is the type of each array element. So
365 the simple example is:
369 which is an array of 32-bit integers. But an array can be of any type,
370 such as this array-of-struct-with-two-int32-fields:
374 Or this array of array of integer:
381 The phrase <firstterm>single complete type</firstterm> deserves some
382 definition. A single complete type is a basic type code, a variant type code,
383 an array with its element type, or a struct with its fields.
384 So the following signatures are not single complete types:
394 And the following signatures contain multiple complete types:
404 Note however that a single complete type may <emphasis>contain</emphasis>
405 multiple other single complete types.
409 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
410 type <literal>VARIANT</literal> will have the signature of a single complete type as part
411 of the <emphasis>value</emphasis>. This signature will be followed by a
412 marshaled value of that type.
416 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
417 than parentheses it uses curly braces, and it has more restrictions.
418 The restrictions are: it occurs only as an array element type; it has
419 exactly two single complete types inside the curly braces; the first
420 single complete type (the "key") must be a basic type rather than a
421 container type. Implementations must not accept dict entries outside of
422 arrays, must not accept dict entries with zero, one, or more than two
423 fields, and must not accept dict entries with non-basic-typed keys. A
424 dict entry is always a key-value pair.
428 The first field in the <literal>DICT_ENTRY</literal> is always the key.
429 A message is considered corrupt if the same key occurs twice in the same
430 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
431 implementations are not required to reject dicts with duplicate keys.
435 In most languages, an array of dict entry would be represented as a
436 map, hash table, or dict object.
440 The following table summarizes the D-Bus types.
445 <entry>Conventional Name</entry>
447 <entry>Description</entry>
452 <entry><literal>INVALID</literal></entry>
453 <entry>0 (ASCII NUL)</entry>
454 <entry>Not a valid type code, used to terminate signatures</entry>
456 <entry><literal>BYTE</literal></entry>
457 <entry>121 (ASCII 'y')</entry>
458 <entry>8-bit unsigned integer</entry>
460 <entry><literal>BOOLEAN</literal></entry>
461 <entry>98 (ASCII 'b')</entry>
462 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
464 <entry><literal>INT16</literal></entry>
465 <entry>110 (ASCII 'n')</entry>
466 <entry>16-bit signed integer</entry>
468 <entry><literal>UINT16</literal></entry>
469 <entry>113 (ASCII 'q')</entry>
470 <entry>16-bit unsigned integer</entry>
472 <entry><literal>INT32</literal></entry>
473 <entry>105 (ASCII 'i')</entry>
474 <entry>32-bit signed integer</entry>
476 <entry><literal>UINT32</literal></entry>
477 <entry>117 (ASCII 'u')</entry>
478 <entry>32-bit unsigned integer</entry>
480 <entry><literal>INT64</literal></entry>
481 <entry>120 (ASCII 'x')</entry>
482 <entry>64-bit signed integer</entry>
484 <entry><literal>UINT64</literal></entry>
485 <entry>116 (ASCII 't')</entry>
486 <entry>64-bit unsigned integer</entry>
488 <entry><literal>DOUBLE</literal></entry>
489 <entry>100 (ASCII 'd')</entry>
490 <entry>IEEE 754 double</entry>
492 <entry><literal>STRING</literal></entry>
493 <entry>115 (ASCII 's')</entry>
494 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
496 <entry><literal>OBJECT_PATH</literal></entry>
497 <entry>111 (ASCII 'o')</entry>
498 <entry>Name of an object instance</entry>
500 <entry><literal>SIGNATURE</literal></entry>
501 <entry>103 (ASCII 'g')</entry>
502 <entry>A type signature</entry>
504 <entry><literal>ARRAY</literal></entry>
505 <entry>97 (ASCII 'a')</entry>
508 <entry><literal>STRUCT</literal></entry>
509 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
510 <entry>Struct; type code 114 'r' is reserved for use in
511 bindings and implementations to represent the general
512 concept of a struct, and must not appear in signatures
513 used on D-Bus.</entry>
515 <entry><literal>VARIANT</literal></entry>
516 <entry>118 (ASCII 'v') </entry>
517 <entry>Variant type (the type of the value is part of the value itself)</entry>
519 <entry><literal>DICT_ENTRY</literal></entry>
520 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
521 <entry>Entry in a dict or map (array of key-value pairs).
522 Type code 101 'e' is reserved for use in bindings and
523 implementations to represent the general concept of a
524 dict or dict-entry, and must not appear in signatures
525 used on D-Bus.</entry>
527 <entry><literal>UNIX_FD</literal></entry>
528 <entry>104 (ASCII 'h')</entry>
529 <entry>Unix file descriptor</entry>
532 <entry>(reserved)</entry>
533 <entry>109 (ASCII 'm')</entry>
534 <entry>Reserved for <ulink
535 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
536 'maybe' type compatible with the one in GVariant</ulink>,
537 and must not appear in signatures used on D-Bus until
538 specified here</entry>
541 <entry>(reserved)</entry>
542 <entry>42 (ASCII '*')</entry>
543 <entry>Reserved for use in bindings/implementations to
544 represent any <firstterm>single complete type</firstterm>,
545 and must not appear in signatures used on D-Bus.</entry>
548 <entry>(reserved)</entry>
549 <entry>63 (ASCII '?')</entry>
550 <entry>Reserved for use in bindings/implementations to
551 represent any <firstterm>basic type</firstterm>, and must
552 not appear in signatures used on D-Bus.</entry>
555 <entry>(reserved)</entry>
556 <entry>64 (ASCII '@'), 38 (ASCII '&'),
557 94 (ASCII '^')</entry>
558 <entry>Reserved for internal use by bindings/implementations,
559 and must not appear in signatures used on D-Bus.
560 GVariant uses these type-codes to encode calling
570 <sect2 id="message-protocol-marshaling">
571 <title>Marshaling (Wire Format)</title>
574 Given a type signature, a block of bytes can be converted into typed
575 values. This section describes the format of the block of bytes. Byte
576 order and alignment issues are handled uniformly for all D-Bus types.
580 A block of bytes has an associated byte order. The byte order
581 has to be discovered in some way; for D-Bus messages, the
582 byte order is part of the message header as described in
583 <xref linkend="message-protocol-messages"/>. For now, assume
584 that the byte order is known to be either little endian or big
589 Each value in a block of bytes is aligned "naturally," for example
590 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
591 8-byte boundary. To properly align a value, <firstterm>alignment
592 padding</firstterm> may be necessary. The alignment padding must always
593 be the minimum required padding to properly align the following value;
594 and it must always be made up of nul bytes. The alignment padding must
595 not be left uninitialized (it can't contain garbage), and more padding
596 than required must not be used.
600 Given all this, the types are marshaled on the wire as follows:
605 <entry>Conventional Name</entry>
606 <entry>Encoding</entry>
607 <entry>Alignment</entry>
612 <entry><literal>INVALID</literal></entry>
613 <entry>Not applicable; cannot be marshaled.</entry>
616 <entry><literal>BYTE</literal></entry>
617 <entry>A single 8-bit byte.</entry>
620 <entry><literal>BOOLEAN</literal></entry>
621 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
624 <entry><literal>INT16</literal></entry>
625 <entry>16-bit signed integer in the message's byte order.</entry>
628 <entry><literal>UINT16</literal></entry>
629 <entry>16-bit unsigned integer in the message's byte order.</entry>
632 <entry><literal>INT32</literal></entry>
633 <entry>32-bit signed integer in the message's byte order.</entry>
636 <entry><literal>UINT32</literal></entry>
637 <entry>32-bit unsigned integer in the message's byte order.</entry>
640 <entry><literal>INT64</literal></entry>
641 <entry>64-bit signed integer in the message's byte order.</entry>
644 <entry><literal>UINT64</literal></entry>
645 <entry>64-bit unsigned integer in the message's byte order.</entry>
648 <entry><literal>DOUBLE</literal></entry>
649 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
652 <entry><literal>STRING</literal></entry>
653 <entry>A <literal>UINT32</literal> indicating the string's
654 length in bytes excluding its terminating nul, followed by
655 non-nul string data of the given length, followed by a terminating nul
662 <entry><literal>OBJECT_PATH</literal></entry>
663 <entry>Exactly the same as <literal>STRING</literal> except the
664 content must be a valid object path (see below).
670 <entry><literal>SIGNATURE</literal></entry>
671 <entry>The same as <literal>STRING</literal> except the length is a single
672 byte (thus signatures have a maximum length of 255)
673 and the content must be a valid signature (see below).
679 <entry><literal>ARRAY</literal></entry>
681 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
682 alignment padding to the alignment boundary of the array element type,
683 followed by each array element. The array length is from the
684 end of the alignment padding to the end of the last element,
685 i.e. it does not include the padding after the length,
686 or any padding after the last element.
687 Arrays have a maximum length defined to be 2 to the 26th power or
688 67108864. Implementations must not send or accept arrays exceeding this
695 <entry><literal>STRUCT</literal></entry>
697 A struct must start on an 8-byte boundary regardless of the
698 type of the struct fields. The struct value consists of each
699 field marshaled in sequence starting from that 8-byte
706 <entry><literal>VARIANT</literal></entry>
708 A variant type has a marshaled
709 <literal>SIGNATURE</literal> followed by a marshaled
710 value with the type given in the signature. Unlike
711 a message signature, the variant signature can
712 contain only a single complete type. So "i", "ai"
713 or "(ii)" is OK, but "ii" is not. Use of variants may not
714 cause a total message depth to be larger than 64, including
715 other container types such as structures.
718 1 (alignment of the signature)
721 <entry><literal>DICT_ENTRY</literal></entry>
729 <entry><literal>UNIX_FD</literal></entry>
730 <entry>32-bit unsigned integer in the message's byte
731 order. The actual file descriptors need to be
732 transferred out-of-band via some platform specific
733 mechanism. On the wire, values of this type store the index to the
734 file descriptor in the array of file descriptors that
735 accompany the message.</entry>
743 <sect3 id="message-protocol-marshaling-object-path">
744 <title>Valid Object Paths</title>
747 An object path is a name used to refer to an object instance.
748 Conceptually, each participant in a D-Bus message exchange may have
749 any number of object instances (think of C++ or Java objects) and each
750 such instance will have a path. Like a filesystem, the object
751 instances in an application form a hierarchical tree.
755 The following rules define a valid object path. Implementations must
756 not send or accept messages with invalid object paths.
760 The path may be of any length.
765 The path must begin with an ASCII '/' (integer 47) character,
766 and must consist of elements separated by slash characters.
771 Each element must only contain the ASCII characters
777 No element may be the empty string.
782 Multiple '/' characters cannot occur in sequence.
787 A trailing '/' character is not allowed unless the
788 path is the root path (a single '/' character).
795 Object paths are often namespaced by starting with a reversed
796 domain name and containing an interface version number, in the
798 <link linkend="message-protocol-names-interface">interface
800 <link linkend="message-protocol-names-bus">well-known
802 This makes it possible to implement more than one service, or
803 more than one version of a service, in the same process,
804 even if the services share a connection but cannot otherwise
805 co-operate (for instance, if they are implemented by different
810 For instance, if the owner of <literal>example.com</literal> is
811 developing a D-Bus API for a music player, they might use the
812 hierarchy of object paths that start with
813 <literal>/com/example/MusicPlayer1</literal> for its objects.
817 <sect3 id="message-protocol-marshaling-signature">
818 <title>Valid Signatures</title>
820 An implementation must not send or accept invalid signatures.
821 Valid signatures will conform to the following rules:
825 The signature ends with a nul byte.
830 The signature is a list of single complete types.
831 Arrays must have element types, and structs must
832 have both open and close parentheses.
837 Only type codes and open and close parentheses are
838 allowed in the signature. The <literal>STRUCT</literal> type code
839 is not allowed in signatures, because parentheses
845 The maximum depth of container type nesting is 32 array type
846 codes and 32 open parentheses. This implies that the maximum
847 total depth of recursion is 64, for an "array of array of array
848 of ... struct of struct of struct of ..." where there are 32
854 The maximum length of a signature is 255.
859 Signatures must be nul-terminated.
870 <sect1 id="message-protocol">
871 <title>Message Protocol</title>
874 A <firstterm>message</firstterm> consists of a
875 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
876 think of a message as a package, the header is the address, and the body
877 contains the package contents. The message delivery system uses the header
878 information to figure out where to send the message and how to interpret
879 it; the recipient interprets the body of the message.
883 The body of the message is made up of zero or more
884 <firstterm>arguments</firstterm>, which are typed values, such as an
885 integer or a byte array.
889 Both header and body use the D-Bus <link linkend="type-system">type
890 system</link> and format for serializing data.
893 <sect2 id="message-protocol-messages">
894 <title>Message Format</title>
897 A message consists of a header and a body. The header is a block of
898 values with a fixed signature and meaning. The body is a separate block
899 of values, with a signature specified in the header.
903 The length of the header must be a multiple of 8, allowing the body to
904 begin on an 8-byte boundary when storing the entire message in a single
905 buffer. If the header does not naturally end on an 8-byte boundary
906 up to 7 bytes of nul-initialized alignment padding must be added.
910 The message body need not end on an 8-byte boundary.
914 The maximum length of a message, including header, header alignment padding,
915 and body is 2 to the 27th power or 134217728. Implementations must not
916 send or accept messages exceeding this size.
920 The signature of the header is:
924 Written out more readably, this is:
926 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
931 These values have the following meanings:
937 <entry>Description</entry>
942 <entry>1st <literal>BYTE</literal></entry>
943 <entry>Endianness flag; ASCII 'l' for little-endian
944 or ASCII 'B' for big-endian. Both header and body are
945 in this endianness.</entry>
948 <entry>2nd <literal>BYTE</literal></entry>
949 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
950 Currently-defined types are described below.
954 <entry>3rd <literal>BYTE</literal></entry>
955 <entry>Bitwise OR of flags. Unknown flags
956 must be ignored. Currently-defined flags are described below.
960 <entry>4th <literal>BYTE</literal></entry>
961 <entry>Major protocol version of the sending application. If
962 the major protocol version of the receiving application does not
963 match, the applications will not be able to communicate and the
964 D-Bus connection must be disconnected. The major protocol
965 version for this version of the specification is 1.
969 <entry>1st <literal>UINT32</literal></entry>
970 <entry>Length in bytes of the message body, starting
971 from the end of the header. The header ends after
972 its alignment padding to an 8-boundary.
976 <entry>2nd <literal>UINT32</literal></entry>
977 <entry>The serial of this message, used as a cookie
978 by the sender to identify the reply corresponding
979 to this request. This must not be zero.
983 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
984 <entry>An array of zero or more <firstterm>header
985 fields</firstterm> where the byte is the field code, and the
986 variant is the field value. The message type determines
987 which fields are required.
995 <firstterm>Message types</firstterm> that can appear in the second byte
1001 <entry>Conventional name</entry>
1002 <entry>Decimal value</entry>
1003 <entry>Description</entry>
1008 <entry><literal>INVALID</literal></entry>
1010 <entry>This is an invalid type.</entry>
1013 <entry><literal>METHOD_CALL</literal></entry>
1015 <entry>Method call.</entry>
1018 <entry><literal>METHOD_RETURN</literal></entry>
1020 <entry>Method reply with returned data.</entry>
1023 <entry><literal>ERROR</literal></entry>
1025 <entry>Error reply. If the first argument exists and is a
1026 string, it is an error message.</entry>
1029 <entry><literal>SIGNAL</literal></entry>
1031 <entry>Signal emission.</entry>
1038 Flags that can appear in the third byte of the header:
1043 <entry>Conventional name</entry>
1044 <entry>Hex value</entry>
1045 <entry>Description</entry>
1050 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1052 <entry>This message does not expect method return replies or
1053 error replies; the reply can be omitted as an
1054 optimization. However, it is compliant with this specification
1055 to return the reply despite this flag and the only harm
1056 from doing so is extra network traffic.
1060 <entry><literal>NO_AUTO_START</literal></entry>
1062 <entry>The bus must not launch an owner
1063 for the destination name in response to this message.
1071 <sect3 id="message-protocol-header-fields">
1072 <title>Header Fields</title>
1075 The array at the end of the header contains <firstterm>header
1076 fields</firstterm>, where each field is a 1-byte field code followed
1077 by a field value. A header must contain the required header fields for
1078 its message type, and zero or more of any optional header
1079 fields. Future versions of this protocol specification may add new
1080 fields. Implementations must ignore fields they do not
1081 understand. Implementations must not invent their own header fields;
1082 only changes to this specification may introduce new header fields.
1086 Again, if an implementation sees a header field code that it does not
1087 expect, it must ignore that field, as it will be part of a new
1088 (but compatible) version of this specification. This also applies
1089 to known header fields appearing in unexpected messages, for
1090 example: if a signal has a reply serial it must be ignored
1091 even though it has no meaning as of this version of the spec.
1095 However, implementations must not send or accept known header fields
1096 with the wrong type stored in the field value. So for example a
1097 message with an <literal>INTERFACE</literal> field of type
1098 <literal>UINT32</literal> would be considered corrupt.
1102 Here are the currently-defined header fields:
1107 <entry>Conventional Name</entry>
1108 <entry>Decimal Code</entry>
1110 <entry>Required In</entry>
1111 <entry>Description</entry>
1116 <entry><literal>INVALID</literal></entry>
1119 <entry>not allowed</entry>
1120 <entry>Not a valid field name (error if it appears in a message)</entry>
1123 <entry><literal>PATH</literal></entry>
1125 <entry><literal>OBJECT_PATH</literal></entry>
1126 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1127 <entry>The object to send a call to,
1128 or the object a signal is emitted from.
1130 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1131 implementations should not send messages with this path,
1132 and the reference implementation of the bus daemon will
1133 disconnect any application that attempts to do so.
1137 <entry><literal>INTERFACE</literal></entry>
1139 <entry><literal>STRING</literal></entry>
1140 <entry><literal>SIGNAL</literal></entry>
1142 The interface to invoke a method call on, or
1143 that a signal is emitted from. Optional for
1144 method calls, required for signals.
1145 The special interface
1146 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1147 implementations should not send messages with this
1148 interface, and the reference implementation of the bus
1149 daemon will disconnect any application that attempts to
1154 <entry><literal>MEMBER</literal></entry>
1156 <entry><literal>STRING</literal></entry>
1157 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1158 <entry>The member, either the method name or signal name.</entry>
1161 <entry><literal>ERROR_NAME</literal></entry>
1163 <entry><literal>STRING</literal></entry>
1164 <entry><literal>ERROR</literal></entry>
1165 <entry>The name of the error that occurred, for errors</entry>
1168 <entry><literal>REPLY_SERIAL</literal></entry>
1170 <entry><literal>UINT32</literal></entry>
1171 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1172 <entry>The serial number of the message this message is a reply
1173 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1176 <entry><literal>DESTINATION</literal></entry>
1178 <entry><literal>STRING</literal></entry>
1179 <entry>optional</entry>
1180 <entry>The name of the connection this message is intended for.
1181 Only used in combination with the message bus, see
1182 <xref linkend="message-bus"/>.</entry>
1185 <entry><literal>SENDER</literal></entry>
1187 <entry><literal>STRING</literal></entry>
1188 <entry>optional</entry>
1189 <entry>Unique name of the sending connection.
1190 The message bus fills in this field so it is reliable; the field is
1191 only meaningful in combination with the message bus.</entry>
1194 <entry><literal>SIGNATURE</literal></entry>
1196 <entry><literal>SIGNATURE</literal></entry>
1197 <entry>optional</entry>
1198 <entry>The signature of the message body.
1199 If omitted, it is assumed to be the
1200 empty signature "" (i.e. the body must be 0-length).</entry>
1203 <entry><literal>UNIX_FDS</literal></entry>
1205 <entry><literal>UINT32</literal></entry>
1206 <entry>optional</entry>
1207 <entry>The number of Unix file descriptors that
1208 accompany the message. If omitted, it is assumed
1209 that no Unix file descriptors accompany the
1210 message. The actual file descriptors need to be
1211 transferred via platform specific mechanism
1212 out-of-band. They must be sent at the same time as
1213 part of the message itself. They may not be sent
1214 before the first byte of the message itself is
1215 transferred or after the last byte of the message
1225 <sect2 id="message-protocol-names">
1226 <title>Valid Names</title>
1228 The various names in D-Bus messages have some restrictions.
1231 There is a <firstterm>maximum name length</firstterm>
1232 of 255 which applies to bus names, interfaces, and members.
1234 <sect3 id="message-protocol-names-interface">
1235 <title>Interface names</title>
1237 Interfaces have names with type <literal>STRING</literal>, meaning that
1238 they must be valid UTF-8. However, there are also some
1239 additional restrictions that apply to interface names
1242 <listitem><para>Interface names are composed of 1 or more elements separated by
1243 a period ('.') character. All elements must contain at least
1247 <listitem><para>Each element must only contain the ASCII characters
1248 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1252 <listitem><para>Interface names must contain at least one '.' (period)
1253 character (and thus at least two elements).
1256 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1257 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1262 Interface names should start with the reversed DNS domain name of
1263 the author of the interface (in lower-case), like interface names
1264 in Java. It is conventional for the rest of the interface name
1265 to consist of words run together, with initial capital letters
1266 on all words ("CamelCase"). Several levels of hierarchy can be used.
1267 It is also a good idea to include the major version of the interface
1268 in the name, and increment it if incompatible changes are made;
1269 this way, a single object can implement several versions of an
1270 interface in parallel, if necessary.
1274 For instance, if the owner of <literal>example.com</literal> is
1275 developing a D-Bus API for a music player, they might define
1276 interfaces called <literal>com.example.MusicPlayer1</literal>,
1277 <literal>com.example.MusicPlayer1.Track</literal> and
1278 <literal>com.example.MusicPlayer1.Seekable</literal>.
1282 D-Bus does not distinguish between the concepts that would be
1283 called classes and interfaces in Java: either can be identified on
1284 D-Bus by an interface name.
1287 <sect3 id="message-protocol-names-bus">
1288 <title>Bus names</title>
1290 Connections have one or more bus names associated with them.
1291 A connection has exactly one bus name that is a <firstterm>unique
1292 connection name</firstterm>. The unique connection name remains
1293 with the connection for its entire lifetime.
1294 A bus name is of type <literal>STRING</literal>,
1295 meaning that it must be valid UTF-8. However, there are also
1296 some additional restrictions that apply to bus names
1299 <listitem><para>Bus names that start with a colon (':')
1300 character are unique connection names. Other bus names
1301 are called <firstterm>well-known bus names</firstterm>.
1304 <listitem><para>Bus names are composed of 1 or more elements separated by
1305 a period ('.') character. All elements must contain at least
1309 <listitem><para>Each element must only contain the ASCII characters
1310 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1311 connection name may begin with a digit, elements in
1312 other bus names must not begin with a digit.
1316 <listitem><para>Bus names must contain at least one '.' (period)
1317 character (and thus at least two elements).
1320 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1321 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1325 Note that the hyphen ('-') character is allowed in bus names but
1326 not in interface names.
1330 Like <link linkend="message-protocol-names-interface">interface
1331 names</link>, well-known bus names should start with the
1332 reversed DNS domain name of the author of the interface (in
1333 lower-case), and it is conventional for the rest of the well-known
1334 bus name to consist of words run together, with initial
1335 capital letters. As with interface names, including a version
1336 number in well-known bus names is a good idea; it's possible to
1337 have the well-known bus name for more than one version
1338 simultaneously if backwards compatibility is required.
1342 If a well-known bus name implies the presence of a "main" interface,
1343 that "main" interface is often given the same name as
1344 the well-known bus name, and situated at the corresponding object
1345 path. For instance, if the owner of <literal>example.com</literal>
1346 is developing a D-Bus API for a music player, they might define
1347 that any application that takes the well-known name
1348 <literal>com.example.MusicPlayer1</literal> should have an object
1349 at the object path <literal>/com/example/MusicPlayer1</literal>
1350 which implements the interface
1351 <literal>com.example.MusicPlayer1</literal>.
1354 <sect3 id="message-protocol-names-member">
1355 <title>Member names</title>
1357 Member (i.e. method or signal) names:
1359 <listitem><para>Must only contain the ASCII characters
1360 "[A-Z][a-z][0-9]_" and may not begin with a
1361 digit.</para></listitem>
1362 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1363 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1364 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1369 It is conventional for member names on D-Bus to consist of
1370 capitalized words with no punctuation ("camel-case").
1371 Method names should usually be verbs, such as
1372 <literal>GetItems</literal>, and signal names should usually be
1373 a description of an event, such as <literal>ItemsChanged</literal>.
1376 <sect3 id="message-protocol-names-error">
1377 <title>Error names</title>
1379 Error names have the same restrictions as interface names.
1383 Error names have the same naming conventions as interface
1384 names, and often contain <literal>.Error.</literal>; for instance,
1385 the owner of <literal>example.com</literal> might define the
1386 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1387 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1388 The errors defined by D-Bus itself, such as
1389 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1395 <sect2 id="message-protocol-types">
1396 <title>Message Types</title>
1398 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1399 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1400 This section describes these conventions.
1402 <sect3 id="message-protocol-types-method">
1403 <title>Method Calls</title>
1405 Some messages invoke an operation on a remote object. These are
1406 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1407 messages map naturally to methods on objects in a typical program.
1410 A method call message is required to have a <literal>MEMBER</literal> header field
1411 indicating the name of the method. Optionally, the message has an
1412 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1413 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1414 a method with the same name, it is undefined which of the two methods
1415 will be invoked. Implementations may also choose to return an error in
1416 this ambiguous case. However, if a method name is unique
1417 implementations must not require an interface field.
1420 Method call messages also include a <literal>PATH</literal> field
1421 indicating the object to invoke the method on. If the call is passing
1422 through a message bus, the message will also have a
1423 <literal>DESTINATION</literal> field giving the name of the connection
1424 to receive the message.
1427 When an application handles a method call message, it is required to
1428 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1429 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1430 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1433 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1434 are the return value(s) or "out parameters" of the method call.
1435 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1436 and the call fails; no return value will be provided. It makes
1437 no sense to send multiple replies to the same method call.
1440 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1441 reply is required, so the caller will know the method
1442 was successfully processed.
1445 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1449 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1450 then as an optimization the application receiving the method
1451 call may choose to omit the reply message (regardless of
1452 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1453 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1454 flag and reply anyway.
1457 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1458 destination name does not exist then a program to own the destination
1459 name will be started before the message is delivered. The message
1460 will be held until the new program is successfully started or has
1461 failed to start; in case of failure, an error will be returned. This
1462 flag is only relevant in the context of a message bus, it is ignored
1463 during one-to-one communication with no intermediate bus.
1465 <sect4 id="message-protocol-types-method-apis">
1466 <title>Mapping method calls to native APIs</title>
1468 APIs for D-Bus may map method calls to a method call in a specific
1469 programming language, such as C++, or may map a method call written
1470 in an IDL to a D-Bus message.
1473 In APIs of this nature, arguments to a method are often termed "in"
1474 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1475 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1476 "inout" arguments, which are both sent and received, i.e. the caller
1477 passes in a value which is modified. Mapped to D-Bus, an "inout"
1478 argument is equivalent to an "in" argument, followed by an "out"
1479 argument. You can't pass things "by reference" over the wire, so
1480 "inout" is purely an illusion of the in-process API.
1483 Given a method with zero or one return values, followed by zero or more
1484 arguments, where each argument may be "in", "out", or "inout", the
1485 caller constructs a message by appending each "in" or "inout" argument,
1486 in order. "out" arguments are not represented in the caller's message.
1489 The recipient constructs a reply by appending first the return value
1490 if any, then each "out" or "inout" argument, in order.
1491 "in" arguments are not represented in the reply message.
1494 Error replies are normally mapped to exceptions in languages that have
1498 In converting from native APIs to D-Bus, it is perhaps nice to
1499 map D-Bus naming conventions ("FooBar") to native conventions
1500 such as "fooBar" or "foo_bar" automatically. This is OK
1501 as long as you can say that the native API is one that
1502 was specifically written for D-Bus. It makes the most sense
1503 when writing object implementations that will be exported
1504 over the bus. Object proxies used to invoke remote D-Bus
1505 objects probably need the ability to call any D-Bus method,
1506 and thus a magic name mapping like this could be a problem.
1509 This specification doesn't require anything of native API bindings;
1510 the preceding is only a suggested convention for consistency
1516 <sect3 id="message-protocol-types-signal">
1517 <title>Signal Emission</title>
1519 Unlike method calls, signal emissions have no replies.
1520 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1521 It must have three header fields: <literal>PATH</literal> giving the object
1522 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1523 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1524 for signals, though it is optional for method calls.
1528 <sect3 id="message-protocol-types-errors">
1529 <title>Errors</title>
1531 Messages of type <literal>ERROR</literal> are most commonly replies
1532 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1533 to any kind of message. The message bus for example
1534 will return an <literal>ERROR</literal> in reply to a signal emission if
1535 the bus does not have enough memory to send the signal.
1538 An <literal>ERROR</literal> may have any arguments, but if the first
1539 argument is a <literal>STRING</literal>, it must be an error message.
1540 The error message may be logged or shown to the user
1545 <sect3 id="message-protocol-types-notation">
1546 <title>Notation in this document</title>
1548 This document uses a simple pseudo-IDL to describe particular method
1549 calls and signals. Here is an example of a method call:
1551 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1552 out UINT32 resultcode)
1554 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1555 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1556 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1557 characters so it's known that the last part of the name in
1558 the "IDL" is the member name.
1561 In C++ that might end up looking like this:
1563 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1564 unsigned int flags);
1566 or equally valid, the return value could be done as an argument:
1568 void org::freedesktop::DBus::StartServiceByName (const char *name,
1570 unsigned int *resultcode);
1572 It's really up to the API designer how they want to make
1573 this look. You could design an API where the namespace wasn't used
1574 in C++, using STL or Qt, using varargs, or whatever you wanted.
1577 Signals are written as follows:
1579 org.freedesktop.DBus.NameLost (STRING name)
1581 Signals don't specify "in" vs. "out" because only
1582 a single direction is possible.
1585 It isn't especially encouraged to use this lame pseudo-IDL in actual
1586 API implementations; you might use the native notation for the
1587 language you're using, or you might use COM or CORBA IDL, for example.
1592 <sect2 id="message-protocol-handling-invalid">
1593 <title>Invalid Protocol and Spec Extensions</title>
1596 For security reasons, the D-Bus protocol should be strictly parsed and
1597 validated, with the exception of defined extension points. Any invalid
1598 protocol or spec violations should result in immediately dropping the
1599 connection without notice to the other end. Exceptions should be
1600 carefully considered, e.g. an exception may be warranted for a
1601 well-understood idiosyncrasy of a widely-deployed implementation. In
1602 cases where the other end of a connection is 100% trusted and known to
1603 be friendly, skipping validation for performance reasons could also make
1604 sense in certain cases.
1608 Generally speaking violations of the "must" requirements in this spec
1609 should be considered possible attempts to exploit security, and violations
1610 of the "should" suggestions should be considered legitimate (though perhaps
1611 they should generate an error in some cases).
1615 The following extension points are built in to D-Bus on purpose and must
1616 not be treated as invalid protocol. The extension points are intended
1617 for use by future versions of this spec, they are not intended for third
1618 parties. At the moment, the only way a third party could extend D-Bus
1619 without breaking interoperability would be to introduce a way to negotiate new
1620 feature support as part of the auth protocol, using EXTENSION_-prefixed
1621 commands. There is not yet a standard way to negotiate features.
1625 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1626 commands result in an ERROR rather than a disconnect. This enables
1627 future extensions to the protocol. Commands starting with EXTENSION_ are
1628 reserved for third parties.
1633 The authentication protocol supports pluggable auth mechanisms.
1638 The address format (see <xref linkend="addresses"/>) supports new
1644 Messages with an unknown type (something other than
1645 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1646 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1647 Unknown-type messages must still be well-formed in the same way
1648 as the known messages, however. They still have the normal
1654 Header fields with an unknown or unexpected field code must be ignored,
1655 though again they must still be well-formed.
1660 New standard interfaces (with new methods and signals) can of course be added.
1670 <sect1 id="auth-protocol">
1671 <title>Authentication Protocol</title>
1673 Before the flow of messages begins, two applications must
1674 authenticate. A simple plain-text protocol is used for
1675 authentication; this protocol is a SASL profile, and maps fairly
1676 directly from the SASL specification. The message encoding is
1677 NOT used here, only plain text messages.
1680 In examples, "C:" and "S:" indicate lines sent by the client and
1681 server respectively.
1683 <sect2 id="auth-protocol-overview">
1684 <title>Protocol Overview</title>
1686 The protocol is a line-based protocol, where each line ends with
1687 \r\n. Each line begins with an all-caps ASCII command name containing
1688 only the character range [A-Z_], a space, then any arguments for the
1689 command, then the \r\n ending the line. The protocol is
1690 case-sensitive. All bytes must be in the ASCII character set.
1692 Commands from the client to the server are as follows:
1695 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1696 <listitem><para>CANCEL</para></listitem>
1697 <listitem><para>BEGIN</para></listitem>
1698 <listitem><para>DATA <data in hex encoding></para></listitem>
1699 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1700 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1703 From server to client are as follows:
1706 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1707 <listitem><para>OK <GUID in hex></para></listitem>
1708 <listitem><para>DATA <data in hex encoding></para></listitem>
1709 <listitem><para>ERROR</para></listitem>
1710 <listitem><para>AGREE_UNIX_FD</para></listitem>
1714 Unofficial extensions to the command set must begin with the letters
1715 "EXTENSION_", to avoid conflicts with future official commands.
1716 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1719 <sect2 id="auth-nul-byte">
1720 <title>Special credentials-passing nul byte</title>
1722 Immediately after connecting to the server, the client must send a
1723 single nul byte. This byte may be accompanied by credentials
1724 information on some operating systems that use sendmsg() with
1725 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1726 sockets. However, the nul byte must be sent even on other kinds of
1727 socket, and even on operating systems that do not require a byte to be
1728 sent in order to transmit credentials. The text protocol described in
1729 this document begins after the single nul byte. If the first byte
1730 received from the client is not a nul byte, the server may disconnect
1734 A nul byte in any context other than the initial byte is an error;
1735 the protocol is ASCII-only.
1738 The credentials sent along with the nul byte may be used with the
1739 SASL mechanism EXTERNAL.
1742 <sect2 id="auth-command-auth">
1743 <title>AUTH command</title>
1745 If an AUTH command has no arguments, it is a request to list
1746 available mechanisms. The server must respond with a REJECTED
1747 command listing the mechanisms it understands, or with an error.
1750 If an AUTH command specifies a mechanism, and the server supports
1751 said mechanism, the server should begin exchanging SASL
1752 challenge-response data with the client using DATA commands.
1755 If the server does not support the mechanism given in the AUTH
1756 command, it must send either a REJECTED command listing the mechanisms
1757 it does support, or an error.
1760 If the [initial-response] argument is provided, it is intended for use
1761 with mechanisms that have no initial challenge (or an empty initial
1762 challenge), as if it were the argument to an initial DATA command. If
1763 the selected mechanism has an initial challenge and [initial-response]
1764 was provided, the server should reject authentication by sending
1768 If authentication succeeds after exchanging DATA commands,
1769 an OK command must be sent to the client.
1772 The first octet received by the server after the \r\n of the BEGIN
1773 command from the client must be the first octet of the
1774 authenticated/encrypted stream of D-Bus messages.
1777 If BEGIN is received by the server, the first octet received
1778 by the client after the \r\n of the OK command must be the
1779 first octet of the authenticated/encrypted stream of D-Bus
1783 <sect2 id="auth-command-cancel">
1784 <title>CANCEL Command</title>
1786 At any time up to sending the BEGIN command, the client may send a
1787 CANCEL command. On receiving the CANCEL command, the server must
1788 send a REJECTED command and abort the current authentication
1792 <sect2 id="auth-command-data">
1793 <title>DATA Command</title>
1795 The DATA command may come from either client or server, and simply
1796 contains a hex-encoded block of data to be interpreted
1797 according to the SASL mechanism in use.
1800 Some SASL mechanisms support sending an "empty string";
1801 FIXME we need some way to do this.
1804 <sect2 id="auth-command-begin">
1805 <title>BEGIN Command</title>
1807 The BEGIN command acknowledges that the client has received an
1808 OK command from the server, and that the stream of messages
1812 The first octet received by the server after the \r\n of the BEGIN
1813 command from the client must be the first octet of the
1814 authenticated/encrypted stream of D-Bus messages.
1817 <sect2 id="auth-command-rejected">
1818 <title>REJECTED Command</title>
1820 The REJECTED command indicates that the current authentication
1821 exchange has failed, and further exchange of DATA is inappropriate.
1822 The client would normally try another mechanism, or try providing
1823 different responses to challenges.
1825 Optionally, the REJECTED command has a space-separated list of
1826 available auth mechanisms as arguments. If a server ever provides
1827 a list of supported mechanisms, it must provide the same list
1828 each time it sends a REJECTED message. Clients are free to
1829 ignore all lists received after the first.
1832 <sect2 id="auth-command-ok">
1833 <title>OK Command</title>
1835 The OK command indicates that the client has been
1836 authenticated. The client may now proceed with negotiating
1837 Unix file descriptor passing. To do that it shall send
1838 NEGOTIATE_UNIX_FD to the server.
1841 Otherwise, the client must respond to the OK command by
1842 sending a BEGIN command, followed by its stream of messages,
1843 or by disconnecting. The server must not accept additional
1844 commands using this protocol after the BEGIN command has been
1845 received. Further communication will be a stream of D-Bus
1846 messages (optionally encrypted, as negotiated) rather than
1850 If a client sends BEGIN the first octet received by the client
1851 after the \r\n of the OK command must be the first octet of
1852 the authenticated/encrypted stream of D-Bus messages.
1855 The OK command has one argument, which is the GUID of the server.
1856 See <xref linkend="addresses"/> for more on server GUIDs.
1859 <sect2 id="auth-command-error">
1860 <title>ERROR Command</title>
1862 The ERROR command indicates that either server or client did not
1863 know a command, does not accept the given command in the current
1864 context, or did not understand the arguments to the command. This
1865 allows the protocol to be extended; a client or server can send a
1866 command present or permitted only in new protocol versions, and if
1867 an ERROR is received instead of an appropriate response, fall back
1868 to using some other technique.
1871 If an ERROR is sent, the server or client that sent the
1872 error must continue as if the command causing the ERROR had never been
1873 received. However, the the server or client receiving the error
1874 should try something other than whatever caused the error;
1875 if only canceling/rejecting the authentication.
1878 If the D-Bus protocol changes incompatibly at some future time,
1879 applications implementing the new protocol would probably be able to
1880 check for support of the new protocol by sending a new command and
1881 receiving an ERROR from applications that don't understand it. Thus the
1882 ERROR feature of the auth protocol is an escape hatch that lets us
1883 negotiate extensions or changes to the D-Bus protocol in the future.
1886 <sect2 id="auth-command-negotiate-unix-fd">
1887 <title>NEGOTIATE_UNIX_FD Command</title>
1889 The NEGOTIATE_UNIX_FD command indicates that the client
1890 supports Unix file descriptor passing. This command may only
1891 be sent after the connection is authenticated, i.e. after OK
1892 was received by the client. This command may only be sent on
1893 transports that support Unix file descriptor passing.
1896 On receiving NEGOTIATE_UNIX_FD the server must respond with
1897 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1898 the transport chosen supports Unix file descriptor passing and
1899 the server supports this feature. It shall respond the latter
1900 if the transport does not support Unix file descriptor
1901 passing, the server does not support this feature, or the
1902 server decides not to enable file descriptor passing due to
1903 security or other reasons.
1906 <sect2 id="auth-command-agree-unix-fd">
1907 <title>AGREE_UNIX_FD Command</title>
1909 The AGREE_UNIX_FD command indicates that the server supports
1910 Unix file descriptor passing. This command may only be sent
1911 after the connection is authenticated, and the client sent
1912 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1913 command may only be sent on transports that support Unix file
1917 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1918 followed by its stream of messages, or by disconnecting. The
1919 server must not accept additional commands using this protocol
1920 after the BEGIN command has been received. Further
1921 communication will be a stream of D-Bus messages (optionally
1922 encrypted, as negotiated) rather than this protocol.
1925 <sect2 id="auth-command-future">
1926 <title>Future Extensions</title>
1928 Future extensions to the authentication and negotiation
1929 protocol are possible. For that new commands may be
1930 introduced. If a client or server receives an unknown command
1931 it shall respond with ERROR and not consider this fatal. New
1932 commands may be introduced both before, and after
1933 authentication, i.e. both before and after the OK command.
1936 <sect2 id="auth-examples">
1937 <title>Authentication examples</title>
1941 <title>Example of successful magic cookie authentication</title>
1943 (MAGIC_COOKIE is a made up mechanism)
1945 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1951 <title>Example of finding out mechanisms then picking one</title>
1954 S: REJECTED KERBEROS_V4 SKEY
1955 C: AUTH SKEY 7ab83f32ee
1956 S: DATA 8799cabb2ea93e
1957 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1963 <title>Example of client sends unknown command then falls back to regular auth</title>
1967 C: AUTH MAGIC_COOKIE 3736343435313230333039
1973 <title>Example of server doesn't support initial auth mechanism</title>
1975 C: AUTH MAGIC_COOKIE 3736343435313230333039
1976 S: REJECTED KERBEROS_V4 SKEY
1977 C: AUTH SKEY 7ab83f32ee
1978 S: DATA 8799cabb2ea93e
1979 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1985 <title>Example of wrong password or the like followed by successful retry</title>
1987 C: AUTH MAGIC_COOKIE 3736343435313230333039
1988 S: REJECTED KERBEROS_V4 SKEY
1989 C: AUTH SKEY 7ab83f32ee
1990 S: DATA 8799cabb2ea93e
1991 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1993 C: AUTH SKEY 7ab83f32ee
1994 S: DATA 8799cabb2ea93e
1995 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2001 <title>Example of skey cancelled and restarted</title>
2003 C: AUTH MAGIC_COOKIE 3736343435313230333039
2004 S: REJECTED KERBEROS_V4 SKEY
2005 C: AUTH SKEY 7ab83f32ee
2006 S: DATA 8799cabb2ea93e
2009 C: AUTH SKEY 7ab83f32ee
2010 S: DATA 8799cabb2ea93e
2011 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2017 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2019 (MAGIC_COOKIE is a made up mechanism)
2021 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2023 C: NEGOTIATE_UNIX_FD
2029 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2031 (MAGIC_COOKIE is a made up mechanism)
2033 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2035 C: NEGOTIATE_UNIX_FD
2042 <sect2 id="auth-states">
2043 <title>Authentication state diagrams</title>
2046 This section documents the auth protocol in terms of
2047 a state machine for the client and the server. This is
2048 probably the most robust way to implement the protocol.
2051 <sect3 id="auth-states-client">
2052 <title>Client states</title>
2055 To more precisely describe the interaction between the
2056 protocol state machine and the authentication mechanisms the
2057 following notation is used: MECH(CHALL) means that the
2058 server challenge CHALL was fed to the mechanism MECH, which
2064 CONTINUE(RESP) means continue the auth conversation
2065 and send RESP as the response to the server;
2071 OK(RESP) means that after sending RESP to the server
2072 the client side of the auth conversation is finished
2073 and the server should return "OK";
2079 ERROR means that CHALL was invalid and could not be
2085 Both RESP and CHALL may be empty.
2089 The Client starts by getting an initial response from the
2090 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2091 the mechanism did not provide an initial response. If the
2092 mechanism returns CONTINUE, the client starts in state
2093 <emphasis>WaitingForData</emphasis>, if the mechanism
2094 returns OK the client starts in state
2095 <emphasis>WaitingForOK</emphasis>.
2099 The client should keep track of available mechanisms and
2100 which it mechanisms it has already attempted. This list is
2101 used to decide which AUTH command to send. When the list is
2102 exhausted, the client should give up and close the
2107 <title><emphasis>WaitingForData</emphasis></title>
2115 MECH(CHALL) returns CONTINUE(RESP) → send
2117 <emphasis>WaitingForData</emphasis>
2121 MECH(CHALL) returns OK(RESP) → send DATA
2122 RESP, goto <emphasis>WaitingForOK</emphasis>
2126 MECH(CHALL) returns ERROR → send ERROR
2127 [msg], goto <emphasis>WaitingForData</emphasis>
2135 Receive REJECTED [mechs] →
2136 send AUTH [next mech], goto
2137 WaitingForData or <emphasis>WaitingForOK</emphasis>
2142 Receive ERROR → send
2144 <emphasis>WaitingForReject</emphasis>
2149 Receive OK → send
2150 BEGIN, terminate auth
2151 conversation, authenticated
2156 Receive anything else → send
2158 <emphasis>WaitingForData</emphasis>
2166 <title><emphasis>WaitingForOK</emphasis></title>
2171 Receive OK → send BEGIN, terminate auth
2172 conversation, <emphasis>authenticated</emphasis>
2177 Receive REJECT [mechs] → send AUTH [next mech],
2178 goto <emphasis>WaitingForData</emphasis> or
2179 <emphasis>WaitingForOK</emphasis>
2185 Receive DATA → send CANCEL, goto
2186 <emphasis>WaitingForReject</emphasis>
2192 Receive ERROR → send CANCEL, goto
2193 <emphasis>WaitingForReject</emphasis>
2199 Receive anything else → send ERROR, goto
2200 <emphasis>WaitingForOK</emphasis>
2208 <title><emphasis>WaitingForReject</emphasis></title>
2213 Receive REJECT [mechs] → send AUTH [next mech],
2214 goto <emphasis>WaitingForData</emphasis> or
2215 <emphasis>WaitingForOK</emphasis>
2221 Receive anything else → terminate auth
2222 conversation, disconnect
2231 <sect3 id="auth-states-server">
2232 <title>Server states</title>
2235 For the server MECH(RESP) means that the client response
2236 RESP was fed to the the mechanism MECH, which returns one of
2241 CONTINUE(CHALL) means continue the auth conversation and
2242 send CHALL as the challenge to the client;
2248 OK means that the client has been successfully
2255 REJECT means that the client failed to authenticate or
2256 there was an error in RESP.
2261 The server starts out in state
2262 <emphasis>WaitingForAuth</emphasis>. If the client is
2263 rejected too many times the server must disconnect the
2268 <title><emphasis>WaitingForAuth</emphasis></title>
2274 Receive AUTH → send REJECTED [mechs], goto
2275 <emphasis>WaitingForAuth</emphasis>
2281 Receive AUTH MECH RESP
2285 MECH not valid mechanism → send REJECTED
2287 <emphasis>WaitingForAuth</emphasis>
2291 MECH(RESP) returns CONTINUE(CHALL) → send
2293 <emphasis>WaitingForData</emphasis>
2297 MECH(RESP) returns OK → send OK, goto
2298 <emphasis>WaitingForBegin</emphasis>
2302 MECH(RESP) returns REJECT → send REJECTED
2304 <emphasis>WaitingForAuth</emphasis>
2312 Receive BEGIN → terminate
2313 auth conversation, disconnect
2319 Receive ERROR → send REJECTED [mechs], goto
2320 <emphasis>WaitingForAuth</emphasis>
2326 Receive anything else → send
2328 <emphasis>WaitingForAuth</emphasis>
2337 <title><emphasis>WaitingForData</emphasis></title>
2345 MECH(RESP) returns CONTINUE(CHALL) → send
2347 <emphasis>WaitingForData</emphasis>
2351 MECH(RESP) returns OK → send OK, goto
2352 <emphasis>WaitingForBegin</emphasis>
2356 MECH(RESP) returns REJECT → send REJECTED
2358 <emphasis>WaitingForAuth</emphasis>
2366 Receive BEGIN → terminate auth conversation,
2373 Receive CANCEL → send REJECTED [mechs], goto
2374 <emphasis>WaitingForAuth</emphasis>
2380 Receive ERROR → send REJECTED [mechs], goto
2381 <emphasis>WaitingForAuth</emphasis>
2387 Receive anything else → send ERROR, goto
2388 <emphasis>WaitingForData</emphasis>
2396 <title><emphasis>WaitingForBegin</emphasis></title>
2401 Receive BEGIN → terminate auth conversation,
2402 client authenticated
2408 Receive CANCEL → send REJECTED [mechs], goto
2409 <emphasis>WaitingForAuth</emphasis>
2415 Receive ERROR → send REJECTED [mechs], goto
2416 <emphasis>WaitingForAuth</emphasis>
2422 Receive anything else → send ERROR, goto
2423 <emphasis>WaitingForBegin</emphasis>
2433 <sect2 id="auth-mechanisms">
2434 <title>Authentication mechanisms</title>
2436 This section describes some new authentication mechanisms.
2437 D-Bus also allows any standard SASL mechanism of course.
2439 <sect3 id="auth-mechanisms-sha">
2440 <title>DBUS_COOKIE_SHA1</title>
2442 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2443 has the ability to read a private file owned by the user being
2444 authenticated. If the client can prove that it has access to a secret
2445 cookie stored in this file, then the client is authenticated.
2446 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2450 Throughout this description, "hex encoding" must output the digits
2451 from a to f in lower-case; the digits A to F must not be used
2452 in the DBUS_COOKIE_SHA1 mechanism.
2455 Authentication proceeds as follows:
2459 The client sends the username it would like to authenticate
2465 The server sends the name of its "cookie context" (see below); a
2466 space character; the integer ID of the secret cookie the client
2467 must demonstrate knowledge of; a space character; then a
2468 randomly-generated challenge string, all of this hex-encoded into
2474 The client locates the cookie and generates its own
2475 randomly-generated challenge string. The client then concatenates
2476 the server's decoded challenge, a ":" character, its own challenge,
2477 another ":" character, and the cookie. It computes the SHA-1 hash
2478 of this composite string as a hex digest. It concatenates the
2479 client's challenge string, a space character, and the SHA-1 hex
2480 digest, hex-encodes the result and sends it back to the server.
2485 The server generates the same concatenated string used by the
2486 client and computes its SHA-1 hash. It compares the hash with
2487 the hash received from the client; if the two hashes match, the
2488 client is authenticated.
2494 Each server has a "cookie context," which is a name that identifies a
2495 set of cookies that apply to that server. A sample context might be
2496 "org_freedesktop_session_bus". Context names must be valid ASCII,
2497 nonzero length, and may not contain the characters slash ("/"),
2498 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2499 tab ("\t"), or period ("."). There is a default context,
2500 "org_freedesktop_general" that's used by servers that do not specify
2504 Cookies are stored in a user's home directory, in the directory
2505 <filename>~/.dbus-keyrings/</filename>. This directory must
2506 not be readable or writable by other users. If it is,
2507 clients and servers must ignore it. The directory
2508 contains cookie files named after the cookie context.
2511 A cookie file contains one cookie per line. Each line
2512 has three space-separated fields:
2516 The cookie ID number, which must be a non-negative integer and
2517 may not be used twice in the same file.
2522 The cookie's creation time, in UNIX seconds-since-the-epoch
2528 The cookie itself, a hex-encoded random block of bytes. The cookie
2529 may be of any length, though obviously security increases
2530 as the length increases.
2536 Only server processes modify the cookie file.
2537 They must do so with this procedure:
2541 Create a lockfile name by appending ".lock" to the name of the
2542 cookie file. The server should attempt to create this file
2543 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2544 fails, the lock fails. Servers should retry for a reasonable
2545 period of time, then they may choose to delete an existing lock
2546 to keep users from having to manually delete a stale
2547 lock. <footnote><para>Lockfiles are used instead of real file
2548 locking <literal>fcntl()</literal> because real locking
2549 implementations are still flaky on network
2550 filesystems.</para></footnote>
2555 Once the lockfile has been created, the server loads the cookie
2556 file. It should then delete any cookies that are old (the
2557 timeout can be fairly short), or more than a reasonable
2558 time in the future (so that cookies never accidentally
2559 become permanent, if the clock was set far into the future
2560 at some point). If no recent keys remain, the
2561 server may generate a new key.
2566 The pruned and possibly added-to cookie file
2567 must be resaved atomically (using a temporary
2568 file which is rename()'d).
2573 The lock must be dropped by deleting the lockfile.
2579 Clients need not lock the file in order to load it,
2580 because servers are required to save the file atomically.
2585 <sect1 id="addresses">
2586 <title>Server Addresses</title>
2588 Server addresses consist of a transport name followed by a colon, and
2589 then an optional, comma-separated list of keys and values in the form key=value.
2590 Each value is escaped.
2594 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2595 Which is the address to a unix socket with the path /tmp/dbus-test.
2598 Value escaping is similar to URI escaping but simpler.
2602 The set of optionally-escaped bytes is:
2603 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2604 <emphasis>byte</emphasis> (note, not character) which is not in the
2605 set of optionally-escaped bytes must be replaced with an ASCII
2606 percent (<literal>%</literal>) and the value of the byte in hex.
2607 The hex value must always be two digits, even if the first digit is
2608 zero. The optionally-escaped bytes may be escaped if desired.
2613 To unescape, append each byte in the value; if a byte is an ASCII
2614 percent (<literal>%</literal>) character then append the following
2615 hex value instead. It is an error if a <literal>%</literal> byte
2616 does not have two hex digits following. It is an error if a
2617 non-optionally-escaped byte is seen unescaped.
2621 The set of optionally-escaped bytes is intended to preserve address
2622 readability and convenience.
2626 A server may specify a key-value pair with the key <literal>guid</literal>
2627 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2628 describes the format of the <literal>guid</literal> field. If present,
2629 this UUID may be used to distinguish one server address from another. A
2630 server should use a different UUID for each address it listens on. For
2631 example, if a message bus daemon offers both UNIX domain socket and TCP
2632 connections, but treats clients the same regardless of how they connect,
2633 those two connections are equivalent post-connection but should have
2634 distinct UUIDs to distinguish the kinds of connection.
2638 The intent of the address UUID feature is to allow a client to avoid
2639 opening multiple identical connections to the same server, by allowing the
2640 client to check whether an address corresponds to an already-existing
2641 connection. Comparing two addresses is insufficient, because addresses
2642 can be recycled by distinct servers, and equivalent addresses may look
2643 different if simply compared as strings (for example, the host in a TCP
2644 address can be given as an IP address or as a hostname).
2648 Note that the address key is <literal>guid</literal> even though the
2649 rest of the API and documentation says "UUID," for historical reasons.
2653 [FIXME clarify if attempting to connect to each is a requirement
2654 or just a suggestion]
2655 When connecting to a server, multiple server addresses can be
2656 separated by a semi-colon. The library will then try to connect
2657 to the first address and if that fails, it'll try to connect to
2658 the next one specified, and so forth. For example
2659 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2664 <sect1 id="transports">
2665 <title>Transports</title>
2667 [FIXME we need to specify in detail each transport and its possible arguments]
2669 Current transports include: unix domain sockets (including
2670 abstract namespace on linux), launchd, systemd, TCP/IP, and a debug/testing transport
2671 using in-process pipes. Future possible transports include one that
2672 tunnels over X11 protocol.
2675 <sect2 id="transports-unix-domain-sockets">
2676 <title>Unix Domain Sockets</title>
2678 Unix domain sockets can be either paths in the file system or on Linux
2679 kernels, they can be abstract which are similar to paths but
2680 do not show up in the file system.
2684 When a socket is opened by the D-Bus library it truncates the path
2685 name right before the first trailing Nul byte. This is true for both
2686 normal paths and abstract paths. Note that this is a departure from
2687 previous versions of D-Bus that would create sockets with a fixed
2688 length path name. Names which were shorter than the fixed length
2689 would be padded by Nul bytes.
2692 Unix domain sockets are not available on windows.
2694 <sect3 id="transports-unix-domain-sockets-addresses">
2695 <title>Server Address Format</title>
2697 Unix domain socket addresses are identified by the "unix:" prefix
2698 and support the following key/value pairs:
2705 <entry>Values</entry>
2706 <entry>Description</entry>
2712 <entry>(path)</entry>
2713 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2716 <entry>tmpdir</entry>
2717 <entry>(path)</entry>
2718 <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>
2721 <entry>abstract</entry>
2722 <entry>(string)</entry>
2723 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2730 <sect2 id="transports-launchd">
2731 <title>launchd</title>
2733 launchd is an open-source server management system that replaces init, inetd
2734 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2735 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2739 launchd allocates a socket and provides it with the unix path through the
2740 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2741 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2742 it through its environment.
2743 Other processes can query for the launchd socket by executing:
2744 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2745 This is normally done by the D-Bus client library so doesn't have to be done
2749 launchd is not available on Microsoft Windows.
2751 <sect3 id="transports-launchd-addresses">
2752 <title>Server Address Format</title>
2754 launchd addresses are identified by the "launchd:" prefix
2755 and support the following key/value pairs:
2762 <entry>Values</entry>
2763 <entry>Description</entry>
2769 <entry>(environment variable)</entry>
2770 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2777 <sect2 id="transports-systemd">
2778 <title>systemd</title>
2780 systemd is an open-source server management system that
2781 replaces init and inetd on newer Linux systems. It supports
2782 socket activation. The D-Bus systemd transport is used to acquire
2783 socket activation file descriptors from systemd and use them
2784 as D-Bus transport when the current process is spawned by
2785 socket activation from it.
2788 The systemd transport accepts only one or more Unix domain or
2789 TCP streams sockets passed in via socket activation.
2792 The systemd transport is not available on non-Linux operating systems.
2795 The systemd transport defines no parameter keys.
2798 <sect2 id="transports-tcp-sockets">
2799 <title>TCP Sockets</title>
2801 The tcp transport provides TCP/IP based connections between clients
2802 located on the same or different hosts.
2805 Using tcp transport without any additional secure authentification mechanismus
2806 over a network is unsecure.
2809 Windows notes: Because of the tcp stack on windows does not provide sending
2810 credentials over a tcp connection, the EXTERNAL authentification
2811 mechanismus does not work.
2813 <sect3 id="transports-tcp-sockets-addresses">
2814 <title>Server Address Format</title>
2816 TCP/IP socket addresses are identified by the "tcp:" prefix
2817 and support the following key/value pairs:
2824 <entry>Values</entry>
2825 <entry>Description</entry>
2831 <entry>(string)</entry>
2832 <entry>dns name or ip address</entry>
2836 <entry>(number)</entry>
2837 <entry>The tcp port the server will open. A zero value let the server
2838 choose a free port provided from the underlaying operating system.
2839 libdbus is able to retrieve the real used port from the server.
2843 <entry>family</entry>
2844 <entry>(string)</entry>
2845 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2852 <sect2 id="transports-nonce-tcp-sockets">
2853 <title>Nonce-secured TCP Sockets</title>
2855 The nonce-tcp transport provides a secured TCP transport, using a
2856 simple authentication mechanism to ensure that only clients with read
2857 access to a certain location in the filesystem can connect to the server.
2858 The server writes a secret, the nonce, to a file and an incoming client
2859 connection is only accepted if the client sends the nonce right after
2860 the connect. The nonce mechanism requires no setup and is orthogonal to
2861 the higher-level authentication mechanisms described in the
2862 Authentication section.
2866 On start, the server generates a random 16 byte nonce and writes it
2867 to a file in the user's temporary directory. The nonce file location
2868 is published as part of the server's D-Bus address using the
2869 "noncefile" key-value pair.
2871 After an accept, the server reads 16 bytes from the socket. If the
2872 read bytes do not match the nonce stored in the nonce file, the
2873 server MUST immediately drop the connection.
2874 If the nonce match the received byte sequence, the client is accepted
2875 and the transport behaves like an unsecured tcp transport.
2878 After a successful connect to the server socket, the client MUST read
2879 the nonce from the file published by the server via the noncefile=
2880 key-value pair and send it over the socket. After that, the
2881 transport behaves like an unsecured tcp transport.
2883 <sect3 id="transports-nonce-tcp-sockets-addresses">
2884 <title>Server Address Format</title>
2886 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2887 and support the following key/value pairs:
2894 <entry>Values</entry>
2895 <entry>Description</entry>
2901 <entry>(string)</entry>
2902 <entry>dns name or ip address</entry>
2906 <entry>(number)</entry>
2907 <entry>The tcp port the server will open. A zero value let the server
2908 choose a free port provided from the underlaying operating system.
2909 libdbus is able to retrieve the real used port from the server.
2913 <entry>family</entry>
2914 <entry>(string)</entry>
2915 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2918 <entry>noncefile</entry>
2919 <entry>(path)</entry>
2920 <entry>file location containing the secret</entry>
2928 <sect1 id="meta-transports">
2929 <title>Meta Transports</title>
2931 Meta transports are a kind of transport with special enhancements or
2932 behavior. Currently available meta transports include: autolaunch
2935 <sect2 id="meta-transports-autolaunch">
2936 <title>Autolaunch</title>
2937 <para>The autolaunch transport provides a way for dbus clients to autodetect
2938 a running dbus session bus and to autolaunch a session bus if not present.
2940 <sect3 id="meta-transports-autolaunch-addresses">
2941 <title>Server Address Format</title>
2943 Autolaunch addresses uses the "autolaunch:" prefix and support the
2944 following key/value pairs:
2951 <entry>Values</entry>
2952 <entry>Description</entry>
2957 <entry>scope</entry>
2958 <entry>(string)</entry>
2959 <entry>scope of autolaunch (Windows only)
2963 "*install-path" - limit session bus to dbus installation path.
2964 The dbus installation path is determined from the location of
2965 the shared dbus library. If the library is located in a 'bin'
2966 subdirectory the installation root is the directory above,
2967 otherwise the directory where the library lives is taken as
2970 <install-root>/bin/[lib]dbus-1.dll
2971 <install-root>/[lib]dbus-1.dll
2977 "*user" - limit session bus to the recent user.
2982 other values - specify dedicated session bus like "release",
2994 <sect3 id="meta-transports-autolaunch-windows-implementation">
2995 <title>Windows implementation</title>
2997 On start, the server opens a platform specific transport, creates a mutex
2998 and a shared memory section containing the related session bus address.
2999 This mutex will be inspected by the dbus client library to detect a
3000 running dbus session bus. The access to the mutex and the shared memory
3001 section are protected by global locks.
3004 In the recent implementation the autolaunch transport uses a tcp transport
3005 on localhost with a port choosen from the operating system. This detail may
3006 change in the future.
3009 Disclaimer: The recent implementation is in an early state and may not
3010 work in all cirumstances and/or may have security issues. Because of this
3011 the implementation is not documentated yet.
3018 <title>UUIDs</title>
3020 A working D-Bus implementation uses universally-unique IDs in two places.
3021 First, each server address has a UUID identifying the address,
3022 as described in <xref linkend="addresses"/>. Second, each operating
3023 system kernel instance running a D-Bus client or server has a UUID
3024 identifying that kernel, retrieved by invoking the method
3025 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3026 linkend="standard-interfaces-peer"/>).
3029 The term "UUID" in this document is intended literally, i.e. an
3030 identifier that is universally unique. It is not intended to refer to
3031 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3034 The UUID must contain 128 bits of data and be hex-encoded. The
3035 hex-encoded string may not contain hyphens or other non-hex-digit
3036 characters, and it must be exactly 32 characters long. To generate a
3037 UUID, the current reference implementation concatenates 96 bits of random
3038 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3042 It would also be acceptable and probably better to simply generate 128
3043 bits of random data, as long as the random number generator is of high
3044 quality. The timestamp could conceivably help if the random bits are not
3045 very random. With a quality random number generator, collisions are
3046 extremely unlikely even with only 96 bits, so it's somewhat academic.
3049 Implementations should, however, stick to random data for the first 96 bits
3054 <sect1 id="standard-interfaces">
3055 <title>Standard Interfaces</title>
3057 See <xref linkend="message-protocol-types-notation"/> for details on
3058 the notation used in this section. There are some standard interfaces
3059 that may be useful across various D-Bus applications.
3061 <sect2 id="standard-interfaces-peer">
3062 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3064 The <literal>org.freedesktop.DBus.Peer</literal> interface
3067 org.freedesktop.DBus.Peer.Ping ()
3068 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3072 On receipt of the <literal>METHOD_CALL</literal> message
3073 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3074 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3075 usual. It does not matter which object path a ping is sent to. The
3076 reference implementation handles this method automatically.
3079 On receipt of the <literal>METHOD_CALL</literal> message
3080 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3081 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3082 UUID representing the identity of the machine the process is running on.
3083 This UUID must be the same for all processes on a single system at least
3084 until that system next reboots. It should be the same across reboots
3085 if possible, but this is not always possible to implement and is not
3087 It does not matter which object path a GetMachineId is sent to. The
3088 reference implementation handles this method automatically.
3091 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3092 a virtual machine running on a hypervisor, rather than a physical machine.
3093 Basically if two processes see the same UUID, they should also see the same
3094 shared memory, UNIX domain sockets, process IDs, and other features that require
3095 a running OS kernel in common between the processes.
3098 The UUID is often used where other programs might use a hostname. Hostnames
3099 can change without rebooting, however, or just be "localhost" - so the UUID
3103 <xref linkend="uuids"/> explains the format of the UUID.
3107 <sect2 id="standard-interfaces-introspectable">
3108 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3110 This interface has one method:
3112 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3116 Objects instances may implement
3117 <literal>Introspect</literal> which returns an XML description of
3118 the object, including its interfaces (with signals and methods), objects
3119 below it in the object path tree, and its properties.
3122 <xref linkend="introspection-format"/> describes the format of this XML string.
3125 <sect2 id="standard-interfaces-properties">
3126 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3128 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3129 or <firstterm>attributes</firstterm>. These can be exposed via the
3130 <literal>org.freedesktop.DBus.Properties</literal> interface.
3134 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3135 in STRING property_name,
3137 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3138 in STRING property_name,
3140 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3141 out DICT<STRING,VARIANT> props);
3145 It is conventional to give D-Bus properties names consisting of
3146 capitalized words without punctuation ("CamelCase"), like
3147 <link linkend="message-protocol-names-member">member names</link>.
3148 For instance, the GObject property
3149 <literal>connection-status</literal> or the Qt property
3150 <literal>connectionStatus</literal> could be represented on D-Bus
3151 as <literal>ConnectionStatus</literal>.
3154 Strictly speaking, D-Bus property names are not required to follow
3155 the same naming restrictions as member names, but D-Bus property
3156 names that would not be valid member names (in particular,
3157 GObject-style dash-separated property names) can cause interoperability
3158 problems and should be avoided.
3161 The available properties and whether they are writable can be determined
3162 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3163 see <xref linkend="standard-interfaces-introspectable"/>.
3166 An empty string may be provided for the interface name; in this case,
3167 if there are multiple properties on an object with the same name,
3168 the results are undefined (picking one by according to an arbitrary
3169 deterministic rule, or returning an error, are the reasonable
3173 If one or more properties change on an object, the
3174 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3175 signal may be emitted (this signal was added in 0.14):
3179 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3180 DICT<STRING,VARIANT> changed_properties,
3181 ARRAY<STRING> invalidated_properties);
3185 where <literal>changed_properties</literal> is a dictionary
3186 containing the changed properties with the new values and
3187 <literal>invalidated_properties</literal> is an array of
3188 properties that changed but the value is not conveyed.
3191 Whether the <literal>PropertiesChanged</literal> signal is
3192 supported can be determined by calling
3193 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3194 that the signal may be supported for an object but it may
3195 differ how whether and how it is used on a per-property basis
3196 (for e.g. performance or security reasons). Each property (or
3197 the parent interface) must be annotated with the
3198 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3199 annotation to convey this (usually the default value
3200 <literal>true</literal> is sufficient meaning that the
3201 annotation does not need to be used). See <xref
3202 linkend="introspection-format"/> for details on this
3207 <sect2 id="standard-interfaces-objectmanager">
3208 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3210 An API can optionally make use of this interface for one or
3211 more sub-trees of objects. The root of each sub-tree implements
3212 this interface so other applications can get all objects,
3213 interfaces and properties in a single method call. It is
3214 appropriate to use this interface if users of the tree of
3215 objects are expected to be interested in all interfaces of all
3216 objects in the tree; a more granular API should be used if
3217 users of the objects are expected to be interested in a small
3218 subset of the objects, a small subset of their interfaces, or
3222 The method that applications can use to get all objects and
3223 properties is <literal>GetManagedObjects</literal>:
3227 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3231 The return value of this method is a dict whose keys are
3232 object paths. All returned object paths are children of the
3233 object path implementing this interface, i.e. their object
3234 paths start with the ObjectManager's object path plus '/'.
3237 Each value is a dict whose keys are interfaces names. Each
3238 value in this inner dict is the same dict that would be
3239 returned by the <link
3240 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3241 method for that combination of object path and interface. If
3242 an interface has no properties, the empty dict is returned.
3245 Changes are emitted using the following two signals:
3249 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3250 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3251 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3252 ARRAY<STRING> interfaces);
3256 The <literal>InterfacesAdded</literal> signal is emitted when
3257 either a new object is added or when an existing object gains
3258 one or more interfaces. The
3259 <literal>InterfacesRemoved</literal> signal is emitted
3260 whenever an object is removed or it loses one or more
3261 interfaces. The second parameter of the
3262 <literal>InterfacesAdded</literal> signal contains a dict with
3263 the interfaces and properties (if any) that have been added to
3264 the given object path. Similarly, the second parameter of the
3265 <literal>InterfacesRemoved</literal> signal contains an array
3266 of the interfaces that were removed. Note that changes on
3267 properties on existing interfaces are not reported using this
3268 interface - an application should also monitor the existing <link
3269 linkend="standard-interfaces-properties">PropertiesChanged</link>
3270 signal on each object.
3273 Applications SHOULD NOT export objects that are children of an
3274 object (directly or otherwise) implementing this interface but
3275 which are not returned in the reply from the
3276 <literal>GetManagedObjects()</literal> method of this
3277 interface on the given object.
3280 The intent of the <literal>ObjectManager</literal> interface
3281 is to make it easy to write a robust client
3282 implementation. The trivial client implementation only needs
3283 to make two method calls:
3287 org.freedesktop.DBus.AddMatch (bus_proxy,
3288 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3289 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3293 on the message bus and the remote application's
3294 <literal>ObjectManager</literal>, respectively. Whenever a new
3295 remote object is created (or an existing object gains a new
3296 interface), the <literal>InterfacesAdded</literal> signal is
3297 emitted, and since this signal contains all properties for the
3298 interfaces, no calls to the
3299 <literal>org.freedesktop.Properties</literal> interface on the
3300 remote object are needed. Additionally, since the initial
3301 <literal>AddMatch()</literal> rule already includes signal
3302 messages from the newly created child object, no new
3303 <literal>AddMatch()</literal> call is needed.
3308 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3309 interface was added in version 0.17 of the D-Bus
3316 <sect1 id="introspection-format">
3317 <title>Introspection Data Format</title>
3319 As described in <xref linkend="standard-interfaces-introspectable"/>,
3320 objects may be introspected at runtime, returning an XML string
3321 that describes the object. The same XML format may be used in
3322 other contexts as well, for example as an "IDL" for generating
3323 static language bindings.
3326 Here is an example of introspection data:
3328 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3329 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3330 <node name="/org/freedesktop/sample_object">
3331 <interface name="org.freedesktop.SampleInterface">
3332 <method name="Frobate">
3333 <arg name="foo" type="i" direction="in"/>
3334 <arg name="bar" type="s" direction="out"/>
3335 <arg name="baz" type="a{us}" direction="out"/>
3336 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3338 <method name="Bazify">
3339 <arg name="bar" type="(iiu)" direction="in"/>
3340 <arg name="bar" type="v" direction="out"/>
3342 <method name="Mogrify">
3343 <arg name="bar" type="(iiav)" direction="in"/>
3345 <signal name="Changed">
3346 <arg name="new_value" type="b"/>
3348 <property name="Bar" type="y" access="readwrite"/>
3350 <node name="child_of_sample_object"/>
3351 <node name="another_child_of_sample_object"/>
3356 A more formal DTD and spec needs writing, but here are some quick notes.
3360 Only the root <node> element can omit the node name, as it's
3361 known to be the object that was introspected. If the root
3362 <node> does have a name attribute, it must be an absolute
3363 object path. If child <node> have object paths, they must be
3369 If a child <node> has any sub-elements, then they
3370 must represent a complete introspection of the child.
3371 If a child <node> is empty, then it may or may
3372 not have sub-elements; the child must be introspected
3373 in order to find out. The intent is that if an object
3374 knows that its children are "fast" to introspect
3375 it can go ahead and return their information, but
3376 otherwise it can omit it.
3381 The direction element on <arg> may be omitted,
3382 in which case it defaults to "in" for method calls
3383 and "out" for signals. Signals only allow "out"
3384 so while direction may be specified, it's pointless.
3389 The possible directions are "in" and "out",
3390 unlike CORBA there is no "inout"
3395 The possible property access flags are
3396 "readwrite", "read", and "write"
3401 Multiple interfaces can of course be listed for
3407 The "name" attribute on arguments is optional.
3413 Method, interface, property, and signal elements may have
3414 "annotations", which are generic key/value pairs of metadata.
3415 They are similar conceptually to Java's annotations and C# attributes.
3416 Well-known annotations:
3423 <entry>Values (separated by ,)</entry>
3424 <entry>Description</entry>
3429 <entry>org.freedesktop.DBus.Deprecated</entry>
3430 <entry>true,false</entry>
3431 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3434 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3435 <entry>(string)</entry>
3436 <entry>The C symbol; may be used for methods and interfaces</entry>
3439 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3440 <entry>true,false</entry>
3441 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3444 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3445 <entry>true,invalidates,false</entry>
3448 If set to <literal>false</literal>, the
3449 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3451 linkend="standard-interfaces-properties"/> is not
3452 guaranteed to be emitted if the property changes.
3455 If set to <literal>invalidates</literal> the signal
3456 is emitted but the value is not included in the
3460 If set to <literal>true</literal> the signal is
3461 emitted with the value included.
3464 The value for the annotation defaults to
3465 <literal>true</literal> if the enclosing interface
3466 element does not specify the annotation. Otherwise it
3467 defaults to the value specified in the enclosing
3476 <sect1 id="message-bus">
3477 <title>Message Bus Specification</title>
3478 <sect2 id="message-bus-overview">
3479 <title>Message Bus Overview</title>
3481 The message bus accepts connections from one or more applications.
3482 Once connected, applications can exchange messages with other
3483 applications that are also connected to the bus.
3486 In order to route messages among connections, the message bus keeps a
3487 mapping from names to connections. Each connection has one
3488 unique-for-the-lifetime-of-the-bus name automatically assigned.
3489 Applications may request additional names for a connection. Additional
3490 names are usually "well-known names" such as
3491 "org.freedesktop.TextEditor". When a name is bound to a connection,
3492 that connection is said to <firstterm>own</firstterm> the name.
3495 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3496 This name routes messages to the bus, allowing applications to make
3497 administrative requests. For example, applications can ask the bus
3498 to assign a name to a connection.
3501 Each name may have <firstterm>queued owners</firstterm>. When an
3502 application requests a name for a connection and the name is already in
3503 use, the bus will optionally add the connection to a queue waiting for
3504 the name. If the current owner of the name disconnects or releases
3505 the name, the next connection in the queue will become the new owner.
3509 This feature causes the right thing to happen if you start two text
3510 editors for example; the first one may request "org.freedesktop.TextEditor",
3511 and the second will be queued as a possible owner of that name. When
3512 the first exits, the second will take over.
3516 Applications may send <firstterm>unicast messages</firstterm> to
3517 a specific recipient or to the message bus itself, or
3518 <firstterm>broadcast messages</firstterm> to all interested recipients.
3519 See <xref linkend="message-bus-routing"/> for details.
3523 <sect2 id="message-bus-names">
3524 <title>Message Bus Names</title>
3526 Each connection has at least one name, assigned at connection time and
3527 returned in response to the
3528 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3529 automatically-assigned name is called the connection's <firstterm>unique
3530 name</firstterm>. Unique names are never reused for two different
3531 connections to the same bus.
3534 Ownership of a unique name is a prerequisite for interaction with
3535 the message bus. It logically follows that the unique name is always
3536 the first name that an application comes to own, and the last
3537 one that it loses ownership of.
3540 Unique connection names must begin with the character ':' (ASCII colon
3541 character); bus names that are not unique names must not begin
3542 with this character. (The bus must reject any attempt by an application
3543 to manually request a name beginning with ':'.) This restriction
3544 categorically prevents "spoofing"; messages sent to a unique name
3545 will always go to the expected connection.
3548 When a connection is closed, all the names that it owns are deleted (or
3549 transferred to the next connection in the queue if any).
3552 A connection can request additional names to be associated with it using
3553 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3554 linkend="message-protocol-names-bus"/> describes the format of a valid
3555 name. These names can be released again using the
3556 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3559 <sect3 id="bus-messages-request-name">
3560 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3564 UINT32 RequestName (in STRING name, in UINT32 flags)
3571 <entry>Argument</entry>
3573 <entry>Description</entry>
3579 <entry>STRING</entry>
3580 <entry>Name to request</entry>
3584 <entry>UINT32</entry>
3585 <entry>Flags</entry>
3595 <entry>Argument</entry>
3597 <entry>Description</entry>
3603 <entry>UINT32</entry>
3604 <entry>Return value</entry>
3611 This method call should be sent to
3612 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3613 assign the given name to the method caller. Each name maintains a
3614 queue of possible owners, where the head of the queue is the primary
3615 or current owner of the name. Each potential owner in the queue
3616 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3617 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3618 call. When RequestName is invoked the following occurs:
3622 If the method caller is currently the primary owner of the name,
3623 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3624 values are updated with the values from the new RequestName call,
3625 and nothing further happens.
3631 If the current primary owner (head of the queue) has
3632 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3633 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3634 the caller of RequestName replaces the current primary owner at
3635 the head of the queue and the current primary owner moves to the
3636 second position in the queue. If the caller of RequestName was
3637 in the queue previously its flags are updated with the values from
3638 the new RequestName in addition to moving it to the head of the queue.
3644 If replacement is not possible, and the method caller is
3645 currently in the queue but not the primary owner, its flags are
3646 updated with the values from the new RequestName call.
3652 If replacement is not possible, and the method caller is
3653 currently not in the queue, the method caller is appended to the
3660 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3661 set and is not the primary owner, it is removed from the
3662 queue. This can apply to the previous primary owner (if it
3663 was replaced) or the method caller (if it updated the
3664 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3665 queue, or if it was just added to the queue with that flag set).
3671 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3672 queue," even if another application already in the queue had specified
3673 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3674 that does not allow replacement goes away, and the next primary owner
3675 does allow replacement. In this case, queued items that specified
3676 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3677 automatically replace the new primary owner. In other words,
3678 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3679 time RequestName is called. This is deliberate to avoid an infinite loop
3680 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3681 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3684 The flags argument contains any of the following values logically ORed
3691 <entry>Conventional Name</entry>
3692 <entry>Value</entry>
3693 <entry>Description</entry>
3698 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3702 If an application A specifies this flag and succeeds in
3703 becoming the owner of the name, and another application B
3704 later calls RequestName with the
3705 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3706 will lose ownership and receive a
3707 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3708 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3709 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3710 is not specified by application B, then application B will not replace
3711 application A as the owner.
3716 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3720 Try to replace the current owner if there is one. If this
3721 flag is not set the application will only become the owner of
3722 the name if there is no current owner. If this flag is set,
3723 the application will replace the current owner if
3724 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3729 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3733 Without this flag, if an application requests a name that is
3734 already owned, the application will be placed in a queue to
3735 own the name when the current owner gives it up. If this
3736 flag is given, the application will not be placed in the
3737 queue, the request for the name will simply fail. This flag
3738 also affects behavior when an application is replaced as
3739 name owner; by default the application moves back into the
3740 waiting queue, unless this flag was provided when the application
3741 became the name owner.
3749 The return code can be one of the following values:
3755 <entry>Conventional Name</entry>
3756 <entry>Value</entry>
3757 <entry>Description</entry>
3762 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3763 <entry>1</entry> <entry>The caller is now the primary owner of
3764 the name, replacing any previous owner. Either the name had no
3765 owner before, or the caller specified
3766 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3767 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3770 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3773 <entry>The name already had an owner,
3774 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3775 the current owner did not specify
3776 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3777 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3781 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3782 <entry>The name already has an owner,
3783 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3784 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3785 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3786 specified by the requesting application.</entry>
3789 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3791 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3799 <sect3 id="bus-messages-release-name">
3800 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3804 UINT32 ReleaseName (in STRING name)
3811 <entry>Argument</entry>
3813 <entry>Description</entry>
3819 <entry>STRING</entry>
3820 <entry>Name to release</entry>
3830 <entry>Argument</entry>
3832 <entry>Description</entry>
3838 <entry>UINT32</entry>
3839 <entry>Return value</entry>
3846 This method call should be sent to
3847 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3848 release the method caller's claim to the given name. If the caller is
3849 the primary owner, a new primary owner will be selected from the
3850 queue if any other owners are waiting. If the caller is waiting in
3851 the queue for the name, the caller will removed from the queue and
3852 will not be made an owner of the name if it later becomes available.
3853 If there are no other owners in the queue for the name, it will be
3854 removed from the bus entirely.
3856 The return code can be one of the following values:
3862 <entry>Conventional Name</entry>
3863 <entry>Value</entry>
3864 <entry>Description</entry>
3869 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3870 <entry>1</entry> <entry>The caller has released his claim on
3871 the given name. Either the caller was the primary owner of
3872 the name, and the name is now unused or taken by somebody
3873 waiting in the queue for the name, or the caller was waiting
3874 in the queue for the name and has now been removed from the
3878 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3880 <entry>The given name does not exist on this bus.</entry>
3883 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3885 <entry>The caller was not the primary owner of this name,
3886 and was also not waiting in the queue to own this name.</entry>
3894 <sect3 id="bus-messages-list-queued-owners">
3895 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3899 ARRAY of STRING ListQueuedOwners (in STRING name)
3906 <entry>Argument</entry>
3908 <entry>Description</entry>
3914 <entry>STRING</entry>
3915 <entry>The well-known bus name to query, such as
3916 <literal>com.example.cappuccino</literal></entry>
3926 <entry>Argument</entry>
3928 <entry>Description</entry>
3934 <entry>ARRAY of STRING</entry>
3935 <entry>The unique bus names of connections currently queued
3936 for the name</entry>
3943 This method call should be sent to
3944 <literal>org.freedesktop.DBus</literal> and lists the connections
3945 currently queued for a bus name (see
3946 <xref linkend="term-queued-owner"/>).
3951 <sect2 id="message-bus-routing">
3952 <title>Message Bus Message Routing</title>
3955 Messages may have a <literal>DESTINATION</literal> field (see <xref
3956 linkend="message-protocol-header-fields"/>), resulting in a
3957 <firstterm>unicast message</firstterm>. If the
3958 <literal>DESTINATION</literal> field is present, it specifies a message
3959 recipient by name. Method calls and replies normally specify this field.
3960 The message bus must send messages (of any type) with the
3961 <literal>DESTINATION</literal> field set to the specified recipient,
3962 regardless of whether the recipient has set up a match rule matching
3967 When the message bus receives a signal, if the
3968 <literal>DESTINATION</literal> field is absent, it is considered to
3969 be a <firstterm>broadcast signal</firstterm>, and is sent to all
3970 applications with <firstterm>message matching rules</firstterm> that
3971 match the message. Most signal messages are broadcasts.
3975 Unicast signal messages (those with a <literal>DESTINATION</literal>
3976 field) are not commonly used, but they are treated like any unicast
3977 message: they are delivered to the specified receipient,
3978 regardless of its match rules. One use for unicast signals is to
3979 avoid a race condition in which a signal is emitted before the intended
3980 recipient can call <xref linkend="bus-messages-add-match"/> to
3981 receive that signal: if the signal is sent directly to that recipient
3982 using a unicast message, it does not need to add a match rule at all,
3983 and there is no race condition. Another use for unicast signals,
3984 on message buses whose security policy prevents eavesdropping, is to
3985 send sensitive information which should only be visible to one
3990 When the message bus receives a method call, if the
3991 <literal>DESTINATION</literal> field is absent, the call is taken to be
3992 a standard one-to-one message and interpreted by the message bus
3993 itself. For example, sending an
3994 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3995 <literal>DESTINATION</literal> will cause the message bus itself to
3996 reply to the ping immediately; the message bus will not make this
3997 message visible to other applications.
4001 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4002 the ping message were sent with a <literal>DESTINATION</literal> name of
4003 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4004 forwarded, and the Yoyodyne Corporation screensaver application would be
4005 expected to reply to the ping.
4009 Message bus implementations may impose a security policy which
4010 prevents certain messages from being sent or received.
4011 When a message cannot be sent or received due to a security
4012 policy, the message bus should send an error reply, unless the
4013 original message had the <literal>NO_REPLY</literal> flag.
4016 <sect3 id="message-bus-routing-eavesdropping">
4017 <title>Eavesdropping</title>
4019 Receiving a unicast message whose <literal>DESTINATION</literal>
4020 indicates a different recipient is called
4021 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4022 a security boundary (like the standard system bus), the security
4023 policy should usually prevent eavesdropping, since unicast messages
4024 are normally kept private and may contain security-sensitive
4029 Eavesdropping is mainly useful for debugging tools, such as
4030 the <literal>dbus-monitor</literal> tool in the reference
4031 implementation of D-Bus. Tools which eavesdrop on the message bus
4032 should be careful to avoid sending a reply or error in response to
4033 messages intended for a different client.
4037 Clients may attempt to eavesdrop by adding match rules
4038 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4039 the <literal>eavesdrop='true'</literal> match. If the message bus'
4040 security policy does not allow eavesdropping, the match rule can
4041 still be added, but will not have any practical effect. For
4042 compatibility with older message bus implementations, if adding such
4043 a match rule results in an error reply, the client may fall back to
4044 adding the same rule with the <literal>eavesdrop</literal> match
4049 <sect3 id="message-bus-routing-match-rules">
4050 <title>Match Rules</title>
4052 An important part of the message bus routing protocol is match
4053 rules. Match rules describe the messages that should be sent to a
4054 client, based on the contents of the message. Broadcast signals
4055 are only sent to clients which have a suitable match rule: this
4056 avoids waking up client processes to deal with signals that are
4057 not relevant to that client.
4060 Messages that list a client as their <literal>DESTINATION</literal>
4061 do not need to match the client's match rules, and are sent to that
4062 client regardless. As a result, match rules are mainly used to
4063 receive a subset of broadcast signals.
4066 Match rules can also be used for eavesdropping
4067 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4068 if the security policy of the message bus allows it.
4071 Match rules are added using the AddMatch bus method
4072 (see <xref linkend="bus-messages-add-match"/>). Rules are
4073 specified as a string of comma separated key/value pairs.
4074 Excluding a key from the rule indicates a wildcard match.
4075 For instance excluding the the member from a match rule but
4076 adding a sender would let all messages from that sender through.
4077 An example of a complete rule would be
4078 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4081 The following table describes the keys that can be used to create
4083 The following table summarizes the D-Bus types.
4089 <entry>Possible Values</entry>
4090 <entry>Description</entry>
4095 <entry><literal>type</literal></entry>
4096 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4097 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4100 <entry><literal>sender</literal></entry>
4101 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4102 and <xref linkend="term-unique-name"/> respectively)
4104 <entry>Match messages sent by a particular sender. An example of a sender match
4105 is sender='org.freedesktop.Hal'</entry>
4108 <entry><literal>interface</literal></entry>
4109 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4110 <entry>Match messages sent over or to a particular interface. An example of an
4111 interface match is interface='org.freedesktop.Hal.Manager'.
4112 If a message omits the interface header, it must not match any rule
4113 that specifies this key.</entry>
4116 <entry><literal>member</literal></entry>
4117 <entry>Any valid method or signal name</entry>
4118 <entry>Matches messages which have the give method or signal name. An example of
4119 a member match is member='NameOwnerChanged'</entry>
4122 <entry><literal>path</literal></entry>
4123 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4124 <entry>Matches messages which are sent from or to the given object. An example of a
4125 path match is path='/org/freedesktop/Hal/Manager'</entry>
4128 <entry><literal>path_namespace</literal></entry>
4129 <entry>An object path</entry>
4132 Matches messages which are sent from or to an
4133 object for which the object path is either the
4134 given value, or that value followed by one or
4135 more path components.
4140 <literal>path_namespace='/com/example/foo'</literal>
4141 would match signals sent by
4142 <literal>/com/example/foo</literal>
4144 <literal>/com/example/foo/bar</literal>,
4146 <literal>/com/example/foobar</literal>.
4150 Using both <literal>path</literal> and
4151 <literal>path_namespace</literal> in the same match
4152 rule is not allowed.
4157 This match key was added in version 0.16 of the
4158 D-Bus specification and implemented by the bus
4159 daemon in dbus 1.5.0 and later.
4165 <entry><literal>destination</literal></entry>
4166 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4167 <entry>Matches messages which are being sent to the given unique name. An
4168 example of a destination match is destination=':1.0'</entry>
4171 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4172 <entry>Any string</entry>
4173 <entry>Arg matches are special and are used for further restricting the
4174 match based on the arguments in the body of a message. Only arguments of type
4175 STRING can be matched in this way. An example of an argument match
4176 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4180 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4181 <entry>Any string</entry>
4183 <para>Argument path matches provide a specialised form of wildcard matching for
4184 path-like namespaces. They can match arguments whose type is either STRING or
4185 OBJECT_PATH. As with normal argument matches,
4186 if the argument is exactly equal to the string given in the match
4187 rule then the rule is satisfied. Additionally, there is also a
4188 match when either the string given in the match rule or the
4189 appropriate message argument ends with '/' and is a prefix of the
4190 other. An example argument path match is arg0path='/aa/bb/'. This
4191 would match messages with first arguments of '/', '/aa/',
4192 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4193 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4195 <para>This is intended for monitoring “directories” in file system-like
4196 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4197 system. An application interested in all nodes in a particular hierarchy would
4198 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4199 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4200 represent a modification to the “bar” property, or a signal with zeroth
4201 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4202 many properties within that directory, and the interested application would be
4203 notified in both cases.</para>
4206 This match key was added in version 0.12 of the
4207 D-Bus specification, implemented for STRING
4208 arguments by the bus daemon in dbus 1.2.0 and later,
4209 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4216 <entry><literal>arg0namespace</literal></entry>
4217 <entry>Like a bus name, except that the string is not
4218 required to contain a '.' (period)</entry>
4220 <para>Match messages whose first argument is of type STRING, and is a bus name
4221 or interface name within the specified namespace. This is primarily intended
4222 for watching name owner changes for a group of related bus names, rather than
4223 for a single name or all name changes.</para>
4225 <para>Because every valid interface name is also a valid
4226 bus name, this can also be used for messages whose
4227 first argument is an interface name.</para>
4229 <para>For example, the match rule
4230 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4231 matches name owner changes for bus names such as
4232 <literal>com.example.backend.foo</literal>,
4233 <literal>com.example.backend.foo.bar</literal>, and
4234 <literal>com.example.backend</literal> itself.</para>
4236 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4239 This match key was added in version 0.16 of the
4240 D-Bus specification and implemented by the bus
4241 daemon in dbus 1.5.0 and later.
4247 <entry><literal>eavesdrop</literal></entry>
4248 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4249 <entry>Since D-Bus 1.5.6, match rules do not
4250 match messages which have a <literal>DESTINATION</literal>
4251 field unless the match rule specifically
4253 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4254 by specifying <literal>eavesdrop='true'</literal>
4255 in the match rule. <literal>eavesdrop='false'</literal>
4256 restores the default behaviour. Messages are
4257 delivered to their <literal>DESTINATION</literal>
4258 regardless of match rules, so this match does not
4259 affect normal delivery of unicast messages.
4260 If the message bus has a security policy which forbids
4261 eavesdropping, this match may still be used without error,
4262 but will not have any practical effect.
4263 In older versions of D-Bus, this match was not allowed
4264 in match rules, and all match rules behaved as if
4265 <literal>eavesdrop='true'</literal> had been used.
4274 <sect2 id="message-bus-starting-services">
4275 <title>Message Bus Starting Services</title>
4277 The message bus can start applications on behalf of other applications.
4278 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4279 An application that can be started in this way is called a
4280 <firstterm>service</firstterm>.
4283 With D-Bus, starting a service is normally done by name. That is,
4284 applications ask the message bus to start some program that will own a
4285 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4286 This implies a contract documented along with the name
4287 <literal>org.freedesktop.TextEditor</literal> for which objects
4288 the owner of that name will provide, and what interfaces those
4292 To find an executable corresponding to a particular name, the bus daemon
4293 looks for <firstterm>service description files</firstterm>. Service
4294 description files define a mapping from names to executables. Different
4295 kinds of message bus will look for these files in different places, see
4296 <xref linkend="message-bus-types"/>.
4299 Service description files have the ".service" file
4300 extension. The message bus will only load service description files
4301 ending with .service; all other files will be ignored. The file format
4302 is similar to that of <ulink
4303 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4304 entries</ulink>. All service description files must be in UTF-8
4305 encoding. To ensure that there will be no name collisions, service files
4306 must be namespaced using the same mechanism as messages and service
4311 [FIXME the file format should be much better specified than "similar to
4312 .desktop entries" esp. since desktop entries are already
4313 badly-specified. ;-)]
4314 These sections from the specification apply to service files as well:
4317 <listitem><para>General syntax</para></listitem>
4318 <listitem><para>Comment format</para></listitem>
4322 <title>Example service description file</title>
4324 # Sample service description file
4326 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4327 Exec=/usr/libexec/gconfd-2
4332 When an application asks to start a service by name, the bus daemon tries to
4333 find a service that will own that name. It then tries to spawn the
4334 executable associated with it. If this fails, it will report an
4335 error. [FIXME what happens if two .service files offer the same service;
4336 what kind of error is reported, should we have a way for the client to
4340 The executable launched will have the environment variable
4341 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4342 message bus so it can connect and request the appropriate names.
4345 The executable being launched may want to know whether the message bus
4346 starting it is one of the well-known message buses (see <xref
4347 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4348 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4349 of the well-known buses. The currently-defined values for this variable
4350 are <literal>system</literal> for the systemwide message bus,
4351 and <literal>session</literal> for the per-login-session message
4352 bus. The new executable must still connect to the address given
4353 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4354 resulting connection is to the well-known bus.
4357 [FIXME there should be a timeout somewhere, either specified
4358 in the .service file, by the client, or just a global value
4359 and if the client being activated fails to connect within that
4360 timeout, an error should be sent back.]
4363 <sect3 id="message-bus-starting-services-scope">
4364 <title>Message Bus Service Scope</title>
4366 The "scope" of a service is its "per-", such as per-session,
4367 per-machine, per-home-directory, or per-display. The reference
4368 implementation doesn't yet support starting services in a different
4369 scope from the message bus itself. So e.g. if you start a service
4370 on the session bus its scope is per-session.
4373 We could add an optional scope to a bus name. For example, for
4374 per-(display,session pair), we could have a unique ID for each display
4375 generated automatically at login and set on screen 0 by executing a
4376 special "set display ID" binary. The ID would be stored in a
4377 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4378 random bytes. This ID would then be used to scope names.
4379 Starting/locating a service could be done by ID-name pair rather than
4383 Contrast this with a per-display scope. To achieve that, we would
4384 want a single bus spanning all sessions using a given display.
4385 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4386 property on screen 0 of the display, pointing to this bus.
4391 <sect2 id="message-bus-types">
4392 <title>Well-known Message Bus Instances</title>
4394 Two standard message bus instances are defined here, along with how
4395 to locate them and where their service files live.
4397 <sect3 id="message-bus-types-login">
4398 <title>Login session message bus</title>
4400 Each time a user logs in, a <firstterm>login session message
4401 bus</firstterm> may be started. All applications in the user's login
4402 session may interact with one another using this message bus.
4405 The address of the login session message bus is given
4406 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4407 variable. If that variable is not set, applications may
4408 also try to read the address from the X Window System root
4409 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4410 The root window property must have type <literal>STRING</literal>.
4411 The environment variable should have precedence over the
4412 root window property.
4414 <para>The address of the login session message bus is given in the
4415 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4416 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4417 "autolaunch:", the system should use platform-specific methods of
4418 locating a running D-Bus session server, or starting one if a running
4419 instance cannot be found. Note that this mechanism is not recommended
4420 for attempting to determine if a daemon is running. It is inherently
4421 racy to attempt to make this determination, since the bus daemon may
4422 be started just before or just after the determination is made.
4423 Therefore, it is recommended that applications do not try to make this
4424 determination for their functionality purposes, and instead they
4425 should attempt to start the server.</para>
4427 <sect4 id="message-bus-types-login-x-windows">
4428 <title>X Windowing System</title>
4430 For the X Windowing System, the application must locate the
4431 window owner of the selection represented by the atom formed by
4435 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4439 <para>the current user's username</para>
4443 <para>the literal character '_' (underscore)</para>
4447 <para>the machine's ID</para>
4453 The following properties are defined for the window that owns
4455 <informaltable frame="all">
4464 <para>meaning</para>
4470 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4474 <para>the actual address of the server socket</para>
4480 <para>_DBUS_SESSION_BUS_PID</para>
4484 <para>the PID of the server process</para>
4493 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4494 present in this window.
4498 If the X selection cannot be located or if reading the
4499 properties from the window fails, the implementation MUST conclude
4500 that there is no D-Bus server running and proceed to start a new
4501 server. (See below on concurrency issues)
4505 Failure to connect to the D-Bus server address thus obtained
4506 MUST be treated as a fatal connection error and should be reported
4511 As an alternative, an implementation MAY find the information
4512 in the following file located in the current user's home directory,
4513 in subdirectory .dbus/session-bus/:
4516 <para>the machine's ID</para>
4520 <para>the literal character '-' (dash)</para>
4524 <para>the X display without the screen number, with the
4525 following prefixes removed, if present: ":", "localhost:"
4526 ."localhost.localdomain:". That is, a display of
4527 "localhost:10.0" produces just the number "10"</para>
4533 The contents of this file NAME=value assignment pairs and
4534 lines starting with # are comments (no comments are allowed
4535 otherwise). The following variable names are defined:
4542 <para>Variable</para>
4546 <para>meaning</para>
4552 <para>DBUS_SESSION_BUS_ADDRESS</para>
4556 <para>the actual address of the server socket</para>
4562 <para>DBUS_SESSION_BUS_PID</para>
4566 <para>the PID of the server process</para>
4572 <para>DBUS_SESSION_BUS_WINDOWID</para>
4576 <para>the window ID</para>
4585 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4590 Failure to open this file MUST be interpreted as absence of a
4591 running server. Therefore, the implementation MUST proceed to
4592 attempting to launch a new bus server if the file cannot be
4597 However, success in opening this file MUST NOT lead to the
4598 conclusion that the server is running. Thus, a failure to connect to
4599 the bus address obtained by the alternative method MUST NOT be
4600 considered a fatal error. If the connection cannot be established,
4601 the implementation MUST proceed to check the X selection settings or
4602 to start the server on its own.
4606 If the implementation concludes that the D-Bus server is not
4607 running it MUST attempt to start a new server and it MUST also
4608 ensure that the daemon started as an effect of the "autolaunch"
4609 mechanism provides the lookup mechanisms described above, so
4610 subsequent calls can locate the newly started server. The
4611 implementation MUST also ensure that if two or more concurrent
4612 initiations happen, only one server remains running and all other
4613 initiations are able to obtain the address of this server and
4614 connect to it. In other words, the implementation MUST ensure that
4615 the X selection is not present when it attempts to set it, without
4616 allowing another process to set the selection between the
4617 verification and the setting (e.g., by using XGrabServer /
4624 On Unix systems, the session bus should search for .service files
4625 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4627 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4628 Implementations may also search additional locations, which
4629 should be searched with lower priority than anything in
4630 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4631 for example, the reference implementation also
4632 looks in <literal>${datadir}/dbus-1/services</literal> as
4633 set at compile time.
4636 As described in the XDG Base Directory Specification, software
4637 packages should install their session .service files to their
4638 configured <literal>${datadir}/dbus-1/services</literal>,
4639 where <literal>${datadir}</literal> is as defined by the GNU
4640 coding standards. System administrators or users can arrange
4641 for these service files to be read by setting XDG_DATA_DIRS or by
4642 symlinking them into the default locations.
4646 <sect3 id="message-bus-types-system">
4647 <title>System message bus</title>
4649 A computer may have a <firstterm>system message bus</firstterm>,
4650 accessible to all applications on the system. This message bus may be
4651 used to broadcast system events, such as adding new hardware devices,
4652 changes in the printer queue, and so forth.
4655 The address of the system message bus is given
4656 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4657 variable. If that variable is not set, applications should try
4658 to connect to the well-known address
4659 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4662 The D-Bus reference implementation actually honors the
4663 <literal>$(localstatedir)</literal> configure option
4664 for this address, on both client and server side.
4669 On Unix systems, the system bus should default to searching
4670 for .service files in
4671 <literal>/usr/local/share/dbus-1/system-services</literal>,
4672 <literal>/usr/share/dbus-1/system-services</literal> and
4673 <literal>/lib/dbus-1/system-services</literal>, with that order
4674 of precedence. It may also search other implementation-specific
4675 locations, but should not vary these locations based on environment
4679 The system bus is security-sensitive and is typically executed
4680 by an init system with a clean environment. Its launch helper
4681 process is particularly security-sensitive, and specifically
4682 clears its own environment.
4687 Software packages should install their system .service
4688 files to their configured
4689 <literal>${datadir}/dbus-1/system-services</literal>,
4690 where <literal>${datadir}</literal> is as defined by the GNU
4691 coding standards. System administrators can arrange
4692 for these service files to be read by editing the system bus'
4693 configuration file or by symlinking them into the default
4699 <sect2 id="message-bus-messages">
4700 <title>Message Bus Messages</title>
4702 The special message bus name <literal>org.freedesktop.DBus</literal>
4703 responds to a number of additional messages.
4706 <sect3 id="bus-messages-hello">
4707 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4718 <entry>Argument</entry>
4720 <entry>Description</entry>
4726 <entry>STRING</entry>
4727 <entry>Unique name assigned to the connection</entry>
4734 Before an application is able to send messages to other applications
4735 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4736 to the message bus to obtain a unique name. If an application without
4737 a unique name tries to send a message to another application, or a
4738 message to the message bus itself that isn't the
4739 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4740 disconnected from the bus.
4743 There is no corresponding "disconnect" request; if a client wishes to
4744 disconnect from the bus, it simply closes the socket (or other
4745 communication channel).
4748 <sect3 id="bus-messages-list-names">
4749 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4753 ARRAY of STRING ListNames ()
4760 <entry>Argument</entry>
4762 <entry>Description</entry>
4768 <entry>ARRAY of STRING</entry>
4769 <entry>Array of strings where each string is a bus name</entry>
4776 Returns a list of all currently-owned names on the bus.
4779 <sect3 id="bus-messages-list-activatable-names">
4780 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4784 ARRAY of STRING ListActivatableNames ()
4791 <entry>Argument</entry>
4793 <entry>Description</entry>
4799 <entry>ARRAY of STRING</entry>
4800 <entry>Array of strings where each string is a bus name</entry>
4807 Returns a list of all names that can be activated on the bus.
4810 <sect3 id="bus-messages-name-exists">
4811 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4815 BOOLEAN NameHasOwner (in STRING name)
4822 <entry>Argument</entry>
4824 <entry>Description</entry>
4830 <entry>STRING</entry>
4831 <entry>Name to check</entry>
4841 <entry>Argument</entry>
4843 <entry>Description</entry>
4849 <entry>BOOLEAN</entry>
4850 <entry>Return value, true if the name exists</entry>
4857 Checks if the specified name exists (currently has an owner).
4861 <sect3 id="bus-messages-name-owner-changed">
4862 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4866 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4873 <entry>Argument</entry>
4875 <entry>Description</entry>
4881 <entry>STRING</entry>
4882 <entry>Name with a new owner</entry>
4886 <entry>STRING</entry>
4887 <entry>Old owner or empty string if none</entry>
4891 <entry>STRING</entry>
4892 <entry>New owner or empty string if none</entry>
4899 This signal indicates that the owner of a name has changed.
4900 It's also the signal to use to detect the appearance of
4901 new names on the bus.
4904 <sect3 id="bus-messages-name-lost">
4905 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4909 NameLost (STRING name)
4916 <entry>Argument</entry>
4918 <entry>Description</entry>
4924 <entry>STRING</entry>
4925 <entry>Name which was lost</entry>
4932 This signal is sent to a specific application when it loses
4933 ownership of a name.
4937 <sect3 id="bus-messages-name-acquired">
4938 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4942 NameAcquired (STRING name)
4949 <entry>Argument</entry>
4951 <entry>Description</entry>
4957 <entry>STRING</entry>
4958 <entry>Name which was acquired</entry>
4965 This signal is sent to a specific application when it gains
4966 ownership of a name.
4970 <sect3 id="bus-messages-start-service-by-name">
4971 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4975 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4982 <entry>Argument</entry>
4984 <entry>Description</entry>
4990 <entry>STRING</entry>
4991 <entry>Name of the service to start</entry>
4995 <entry>UINT32</entry>
4996 <entry>Flags (currently not used)</entry>
5006 <entry>Argument</entry>
5008 <entry>Description</entry>
5014 <entry>UINT32</entry>
5015 <entry>Return value</entry>
5020 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5024 The return value can be one of the following values:
5029 <entry>Identifier</entry>
5030 <entry>Value</entry>
5031 <entry>Description</entry>
5036 <entry>DBUS_START_REPLY_SUCCESS</entry>
5038 <entry>The service was successfully started.</entry>
5041 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5043 <entry>A connection already owns the given name.</entry>
5052 <sect3 id="bus-messages-update-activation-environment">
5053 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5057 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5064 <entry>Argument</entry>
5066 <entry>Description</entry>
5072 <entry>ARRAY of DICT<STRING,STRING></entry>
5073 <entry>Environment to add or update</entry>
5078 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5081 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5084 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.
5089 <sect3 id="bus-messages-get-name-owner">
5090 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5094 STRING GetNameOwner (in STRING name)
5101 <entry>Argument</entry>
5103 <entry>Description</entry>
5109 <entry>STRING</entry>
5110 <entry>Name to get the owner of</entry>
5120 <entry>Argument</entry>
5122 <entry>Description</entry>
5128 <entry>STRING</entry>
5129 <entry>Return value, a unique connection name</entry>
5134 Returns the unique connection name of the primary owner of the name
5135 given. If the requested name doesn't have an owner, returns a
5136 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5140 <sect3 id="bus-messages-get-connection-unix-user">
5141 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5145 UINT32 GetConnectionUnixUser (in STRING bus_name)
5152 <entry>Argument</entry>
5154 <entry>Description</entry>
5160 <entry>STRING</entry>
5161 <entry>Unique or well-known bus name of the connection to
5162 query, such as <literal>:12.34</literal> or
5163 <literal>com.example.tea</literal></entry>
5173 <entry>Argument</entry>
5175 <entry>Description</entry>
5181 <entry>UINT32</entry>
5182 <entry>Unix user ID</entry>
5187 Returns the Unix user ID of the process connected to the server. If
5188 unable to determine it (for instance, because the process is not on the
5189 same machine as the bus daemon), an error is returned.
5193 <sect3 id="bus-messages-get-connection-unix-process-id">
5194 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5198 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5205 <entry>Argument</entry>
5207 <entry>Description</entry>
5213 <entry>STRING</entry>
5214 <entry>Unique or well-known bus name of the connection to
5215 query, such as <literal>:12.34</literal> or
5216 <literal>com.example.tea</literal></entry>
5226 <entry>Argument</entry>
5228 <entry>Description</entry>
5234 <entry>UINT32</entry>
5235 <entry>Unix process id</entry>
5240 Returns the Unix process ID of the process connected to the server. If
5241 unable to determine it (for instance, because the process is not on the
5242 same machine as the bus daemon), an error is returned.
5246 <sect3 id="bus-messages-add-match">
5247 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5251 AddMatch (in STRING rule)
5258 <entry>Argument</entry>
5260 <entry>Description</entry>
5266 <entry>STRING</entry>
5267 <entry>Match rule to add to the connection</entry>
5272 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5273 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5277 <sect3 id="bus-messages-remove-match">
5278 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5282 RemoveMatch (in STRING rule)
5289 <entry>Argument</entry>
5291 <entry>Description</entry>
5297 <entry>STRING</entry>
5298 <entry>Match rule to remove from the connection</entry>
5303 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5304 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5309 <sect3 id="bus-messages-get-id">
5310 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5314 GetId (out STRING id)
5321 <entry>Argument</entry>
5323 <entry>Description</entry>
5329 <entry>STRING</entry>
5330 <entry>Unique ID identifying the bus daemon</entry>
5335 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5336 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5337 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5338 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5339 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5340 by org.freedesktop.DBus.Peer.GetMachineId().
5341 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5349 <appendix id="implementation-notes">
5350 <title>Implementation notes</title>
5351 <sect1 id="implementation-notes-subsection">
5359 <glossary><title>Glossary</title>
5361 This glossary defines some of the terms used in this specification.
5364 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5367 The message bus maintains an association between names and
5368 connections. (Normally, there's one connection per application.) A
5369 bus name is simply an identifier used to locate connections. For
5370 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5371 name might be used to send a message to a screensaver from Yoyodyne
5372 Corporation. An application is said to <firstterm>own</firstterm> a
5373 name if the message bus has associated the application's connection
5374 with the name. Names may also have <firstterm>queued
5375 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5376 The bus assigns a unique name to each connection,
5377 see <xref linkend="term-unique-name"/>. Other names
5378 can be thought of as "well-known names" and are
5379 used to find applications that offer specific functionality.
5383 See <xref linkend="message-protocol-names-bus"/> for details of
5384 the syntax and naming conventions for bus names.
5389 <glossentry id="term-message"><glossterm>Message</glossterm>
5392 A message is the atomic unit of communication via the D-Bus
5393 protocol. It consists of a <firstterm>header</firstterm> and a
5394 <firstterm>body</firstterm>; the body is made up of
5395 <firstterm>arguments</firstterm>.
5400 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5403 The message bus is a special application that forwards
5404 or routes messages between a group of applications
5405 connected to the message bus. It also manages
5406 <firstterm>names</firstterm> used for routing
5412 <glossentry id="term-name"><glossterm>Name</glossterm>
5415 See <xref linkend="term-bus-name"/>. "Name" may
5416 also be used to refer to some of the other names
5417 in D-Bus, such as interface names.
5422 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5425 Used to prevent collisions when defining new interfaces, bus names
5426 etc. The convention used is the same one Java uses for defining
5427 classes: a reversed domain name.
5428 See <xref linkend="message-protocol-names-bus"/>,
5429 <xref linkend="message-protocol-names-interface"/>,
5430 <xref linkend="message-protocol-names-error"/>,
5431 <xref linkend="message-protocol-marshaling-object-path"/>.
5436 <glossentry id="term-object"><glossterm>Object</glossterm>
5439 Each application contains <firstterm>objects</firstterm>, which have
5440 <firstterm>interfaces</firstterm> and
5441 <firstterm>methods</firstterm>. Objects are referred to by a name,
5442 called a <firstterm>path</firstterm>.
5447 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5450 An application talking directly to another application, without going
5451 through a message bus. One-to-one connections may be "peer to peer" or
5452 "client to server." The D-Bus protocol has no concept of client
5453 vs. server after a connection has authenticated; the flow of messages
5454 is symmetrical (full duplex).
5459 <glossentry id="term-path"><glossterm>Path</glossterm>
5462 Object references (object names) in D-Bus are organized into a
5463 filesystem-style hierarchy, so each object is named by a path. As in
5464 LDAP, there's no difference between "files" and "directories"; a path
5465 can refer to an object, while still having child objects below it.
5470 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5473 Each bus name has a primary owner; messages sent to the name go to the
5474 primary owner. However, certain names also maintain a queue of
5475 secondary owners "waiting in the wings." If the primary owner releases
5476 the name, then the first secondary owner in the queue automatically
5477 becomes the new owner of the name.
5482 <glossentry id="term-service"><glossterm>Service</glossterm>
5485 A service is an executable that can be launched by the bus daemon.
5486 Services normally guarantee some particular features, for example they
5487 may guarantee that they will request a specific name such as
5488 "org.freedesktop.Screensaver", have a singleton object
5489 "/org/freedesktop/Application", and that object will implement the
5490 interface "org.freedesktop.ScreensaverControl".
5495 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5498 ".service files" tell the bus about service applications that can be
5499 launched (see <xref linkend="term-service"/>). Most importantly they
5500 provide a mapping from bus names to services that will request those
5501 names when they start up.
5506 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5509 The special name automatically assigned to each connection by the
5510 message bus. This name will never change owner, and will be unique
5511 (never reused during the lifetime of the message bus).
5512 It will begin with a ':' character.