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, an executed subprocess 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>
2927 <sect2 id="transports-exec">
2928 <title>Executed Subprocesses on Unix</title>
2930 This transport forks off a process and connects its standard
2931 input and standard output with an anonymous Unix domain
2932 socket. This socket is then used for communication by the
2933 transport. This transport may be used to use out-of-process
2934 forwarder programs as basis for the D-Bus protocol.
2937 The forked process will inherit the standard error output and
2938 process group from the parent process.
2941 Executed subprocesses are not available on Windows.
2943 <sect3 id="transports-exec-addresses">
2944 <title>Server Address Format</title>
2946 Executed subprocess addresses are identified by the "unixexec:" prefix
2947 and support the following key/value pairs:
2954 <entry>Values</entry>
2955 <entry>Description</entry>
2961 <entry>(path)</entry>
2962 <entry>Path of the binary to execute, either an absolute
2963 path or a binary name that is searched for in the default
2964 search path of the OS. This corresponds to the first
2965 argument of execlp(). This key is mandatory.</entry>
2968 <entry>argv0</entry>
2969 <entry>(string)</entry>
2970 <entry>The program name to use when executing the
2971 binary. If omitted the same value as specified for path=
2972 will be used. This corresponds to the second argument of
2976 <entry>argv1, argv2, ...</entry>
2977 <entry>(string)</entry>
2978 <entry>Arguments to pass to the binary. This corresponds
2979 to the third and later arguments of execlp(). If a
2980 specific argvX is not specified no further argvY for Y > X
2981 are taken into account.</entry>
2989 <sect1 id="meta-transports">
2990 <title>Meta Transports</title>
2992 Meta transports are a kind of transport with special enhancements or
2993 behavior. Currently available meta transports include: autolaunch
2996 <sect2 id="meta-transports-autolaunch">
2997 <title>Autolaunch</title>
2998 <para>The autolaunch transport provides a way for dbus clients to autodetect
2999 a running dbus session bus and to autolaunch a session bus if not present.
3001 <sect3 id="meta-transports-autolaunch-addresses">
3002 <title>Server Address Format</title>
3004 Autolaunch addresses uses the "autolaunch:" prefix and support the
3005 following key/value pairs:
3012 <entry>Values</entry>
3013 <entry>Description</entry>
3018 <entry>scope</entry>
3019 <entry>(string)</entry>
3020 <entry>scope of autolaunch (Windows only)
3024 "*install-path" - limit session bus to dbus installation path.
3025 The dbus installation path is determined from the location of
3026 the shared dbus library. If the library is located in a 'bin'
3027 subdirectory the installation root is the directory above,
3028 otherwise the directory where the library lives is taken as
3031 <install-root>/bin/[lib]dbus-1.dll
3032 <install-root>/[lib]dbus-1.dll
3038 "*user" - limit session bus to the recent user.
3043 other values - specify dedicated session bus like "release",
3055 <sect3 id="meta-transports-autolaunch-windows-implementation">
3056 <title>Windows implementation</title>
3058 On start, the server opens a platform specific transport, creates a mutex
3059 and a shared memory section containing the related session bus address.
3060 This mutex will be inspected by the dbus client library to detect a
3061 running dbus session bus. The access to the mutex and the shared memory
3062 section are protected by global locks.
3065 In the recent implementation the autolaunch transport uses a tcp transport
3066 on localhost with a port choosen from the operating system. This detail may
3067 change in the future.
3070 Disclaimer: The recent implementation is in an early state and may not
3071 work in all cirumstances and/or may have security issues. Because of this
3072 the implementation is not documentated yet.
3079 <title>UUIDs</title>
3081 A working D-Bus implementation uses universally-unique IDs in two places.
3082 First, each server address has a UUID identifying the address,
3083 as described in <xref linkend="addresses"/>. Second, each operating
3084 system kernel instance running a D-Bus client or server has a UUID
3085 identifying that kernel, retrieved by invoking the method
3086 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3087 linkend="standard-interfaces-peer"/>).
3090 The term "UUID" in this document is intended literally, i.e. an
3091 identifier that is universally unique. It is not intended to refer to
3092 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3095 The UUID must contain 128 bits of data and be hex-encoded. The
3096 hex-encoded string may not contain hyphens or other non-hex-digit
3097 characters, and it must be exactly 32 characters long. To generate a
3098 UUID, the current reference implementation concatenates 96 bits of random
3099 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3103 It would also be acceptable and probably better to simply generate 128
3104 bits of random data, as long as the random number generator is of high
3105 quality. The timestamp could conceivably help if the random bits are not
3106 very random. With a quality random number generator, collisions are
3107 extremely unlikely even with only 96 bits, so it's somewhat academic.
3110 Implementations should, however, stick to random data for the first 96 bits
3115 <sect1 id="standard-interfaces">
3116 <title>Standard Interfaces</title>
3118 See <xref linkend="message-protocol-types-notation"/> for details on
3119 the notation used in this section. There are some standard interfaces
3120 that may be useful across various D-Bus applications.
3122 <sect2 id="standard-interfaces-peer">
3123 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3125 The <literal>org.freedesktop.DBus.Peer</literal> interface
3128 org.freedesktop.DBus.Peer.Ping ()
3129 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3133 On receipt of the <literal>METHOD_CALL</literal> message
3134 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3135 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3136 usual. It does not matter which object path a ping is sent to. The
3137 reference implementation handles this method automatically.
3140 On receipt of the <literal>METHOD_CALL</literal> message
3141 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3142 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3143 UUID representing the identity of the machine the process is running on.
3144 This UUID must be the same for all processes on a single system at least
3145 until that system next reboots. It should be the same across reboots
3146 if possible, but this is not always possible to implement and is not
3148 It does not matter which object path a GetMachineId is sent to. The
3149 reference implementation handles this method automatically.
3152 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3153 a virtual machine running on a hypervisor, rather than a physical machine.
3154 Basically if two processes see the same UUID, they should also see the same
3155 shared memory, UNIX domain sockets, process IDs, and other features that require
3156 a running OS kernel in common between the processes.
3159 The UUID is often used where other programs might use a hostname. Hostnames
3160 can change without rebooting, however, or just be "localhost" - so the UUID
3164 <xref linkend="uuids"/> explains the format of the UUID.
3168 <sect2 id="standard-interfaces-introspectable">
3169 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3171 This interface has one method:
3173 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3177 Objects instances may implement
3178 <literal>Introspect</literal> which returns an XML description of
3179 the object, including its interfaces (with signals and methods), objects
3180 below it in the object path tree, and its properties.
3183 <xref linkend="introspection-format"/> describes the format of this XML string.
3186 <sect2 id="standard-interfaces-properties">
3187 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3189 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3190 or <firstterm>attributes</firstterm>. These can be exposed via the
3191 <literal>org.freedesktop.DBus.Properties</literal> interface.
3195 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3196 in STRING property_name,
3198 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3199 in STRING property_name,
3201 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3202 out DICT<STRING,VARIANT> props);
3206 It is conventional to give D-Bus properties names consisting of
3207 capitalized words without punctuation ("CamelCase"), like
3208 <link linkend="message-protocol-names-member">member names</link>.
3209 For instance, the GObject property
3210 <literal>connection-status</literal> or the Qt property
3211 <literal>connectionStatus</literal> could be represented on D-Bus
3212 as <literal>ConnectionStatus</literal>.
3215 Strictly speaking, D-Bus property names are not required to follow
3216 the same naming restrictions as member names, but D-Bus property
3217 names that would not be valid member names (in particular,
3218 GObject-style dash-separated property names) can cause interoperability
3219 problems and should be avoided.
3222 The available properties and whether they are writable can be determined
3223 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3224 see <xref linkend="standard-interfaces-introspectable"/>.
3227 An empty string may be provided for the interface name; in this case,
3228 if there are multiple properties on an object with the same name,
3229 the results are undefined (picking one by according to an arbitrary
3230 deterministic rule, or returning an error, are the reasonable
3234 If one or more properties change on an object, the
3235 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3236 signal may be emitted (this signal was added in 0.14):
3240 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3241 DICT<STRING,VARIANT> changed_properties,
3242 ARRAY<STRING> invalidated_properties);
3246 where <literal>changed_properties</literal> is a dictionary
3247 containing the changed properties with the new values and
3248 <literal>invalidated_properties</literal> is an array of
3249 properties that changed but the value is not conveyed.
3252 Whether the <literal>PropertiesChanged</literal> signal is
3253 supported can be determined by calling
3254 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3255 that the signal may be supported for an object but it may
3256 differ how whether and how it is used on a per-property basis
3257 (for e.g. performance or security reasons). Each property (or
3258 the parent interface) must be annotated with the
3259 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3260 annotation to convey this (usually the default value
3261 <literal>true</literal> is sufficient meaning that the
3262 annotation does not need to be used). See <xref
3263 linkend="introspection-format"/> for details on this
3268 <sect2 id="standard-interfaces-objectmanager">
3269 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3271 An API can optionally make use of this interface for one or
3272 more sub-trees of objects. The root of each sub-tree implements
3273 this interface so other applications can get all objects,
3274 interfaces and properties in a single method call. It is
3275 appropriate to use this interface if users of the tree of
3276 objects are expected to be interested in all interfaces of all
3277 objects in the tree; a more granular API should be used if
3278 users of the objects are expected to be interested in a small
3279 subset of the objects, a small subset of their interfaces, or
3283 The method that applications can use to get all objects and
3284 properties is <literal>GetManagedObjects</literal>:
3288 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3292 The return value of this method is a dict whose keys are
3293 object paths. All returned object paths are children of the
3294 object path implementing this interface, i.e. their object
3295 paths start with the ObjectManager's object path plus '/'.
3298 Each value is a dict whose keys are interfaces names. Each
3299 value in this inner dict is the same dict that would be
3300 returned by the <link
3301 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3302 method for that combination of object path and interface. If
3303 an interface has no properties, the empty dict is returned.
3306 Changes are emitted using the following two signals:
3310 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3311 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3312 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3313 ARRAY<STRING> interfaces);
3317 The <literal>InterfacesAdded</literal> signal is emitted when
3318 either a new object is added or when an existing object gains
3319 one or more interfaces. The
3320 <literal>InterfacesRemoved</literal> signal is emitted
3321 whenever an object is removed or it loses one or more
3322 interfaces. The second parameter of the
3323 <literal>InterfacesAdded</literal> signal contains a dict with
3324 the interfaces and properties (if any) that have been added to
3325 the given object path. Similarly, the second parameter of the
3326 <literal>InterfacesRemoved</literal> signal contains an array
3327 of the interfaces that were removed. Note that changes on
3328 properties on existing interfaces are not reported using this
3329 interface - an application should also monitor the existing <link
3330 linkend="standard-interfaces-properties">PropertiesChanged</link>
3331 signal on each object.
3334 Applications SHOULD NOT export objects that are children of an
3335 object (directly or otherwise) implementing this interface but
3336 which are not returned in the reply from the
3337 <literal>GetManagedObjects()</literal> method of this
3338 interface on the given object.
3341 The intent of the <literal>ObjectManager</literal> interface
3342 is to make it easy to write a robust client
3343 implementation. The trivial client implementation only needs
3344 to make two method calls:
3348 org.freedesktop.DBus.AddMatch (bus_proxy,
3349 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3350 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3354 on the message bus and the remote application's
3355 <literal>ObjectManager</literal>, respectively. Whenever a new
3356 remote object is created (or an existing object gains a new
3357 interface), the <literal>InterfacesAdded</literal> signal is
3358 emitted, and since this signal contains all properties for the
3359 interfaces, no calls to the
3360 <literal>org.freedesktop.Properties</literal> interface on the
3361 remote object are needed. Additionally, since the initial
3362 <literal>AddMatch()</literal> rule already includes signal
3363 messages from the newly created child object, no new
3364 <literal>AddMatch()</literal> call is needed.
3369 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3370 interface was added in version 0.17 of the D-Bus
3377 <sect1 id="introspection-format">
3378 <title>Introspection Data Format</title>
3380 As described in <xref linkend="standard-interfaces-introspectable"/>,
3381 objects may be introspected at runtime, returning an XML string
3382 that describes the object. The same XML format may be used in
3383 other contexts as well, for example as an "IDL" for generating
3384 static language bindings.
3387 Here is an example of introspection data:
3389 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3390 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3391 <node name="/org/freedesktop/sample_object">
3392 <interface name="org.freedesktop.SampleInterface">
3393 <method name="Frobate">
3394 <arg name="foo" type="i" direction="in"/>
3395 <arg name="bar" type="s" direction="out"/>
3396 <arg name="baz" type="a{us}" direction="out"/>
3397 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3399 <method name="Bazify">
3400 <arg name="bar" type="(iiu)" direction="in"/>
3401 <arg name="bar" type="v" direction="out"/>
3403 <method name="Mogrify">
3404 <arg name="bar" type="(iiav)" direction="in"/>
3406 <signal name="Changed">
3407 <arg name="new_value" type="b"/>
3409 <property name="Bar" type="y" access="readwrite"/>
3411 <node name="child_of_sample_object"/>
3412 <node name="another_child_of_sample_object"/>
3417 A more formal DTD and spec needs writing, but here are some quick notes.
3421 Only the root <node> element can omit the node name, as it's
3422 known to be the object that was introspected. If the root
3423 <node> does have a name attribute, it must be an absolute
3424 object path. If child <node> have object paths, they must be
3430 If a child <node> has any sub-elements, then they
3431 must represent a complete introspection of the child.
3432 If a child <node> is empty, then it may or may
3433 not have sub-elements; the child must be introspected
3434 in order to find out. The intent is that if an object
3435 knows that its children are "fast" to introspect
3436 it can go ahead and return their information, but
3437 otherwise it can omit it.
3442 The direction element on <arg> may be omitted,
3443 in which case it defaults to "in" for method calls
3444 and "out" for signals. Signals only allow "out"
3445 so while direction may be specified, it's pointless.
3450 The possible directions are "in" and "out",
3451 unlike CORBA there is no "inout"
3456 The possible property access flags are
3457 "readwrite", "read", and "write"
3462 Multiple interfaces can of course be listed for
3468 The "name" attribute on arguments is optional.
3474 Method, interface, property, and signal elements may have
3475 "annotations", which are generic key/value pairs of metadata.
3476 They are similar conceptually to Java's annotations and C# attributes.
3477 Well-known annotations:
3484 <entry>Values (separated by ,)</entry>
3485 <entry>Description</entry>
3490 <entry>org.freedesktop.DBus.Deprecated</entry>
3491 <entry>true,false</entry>
3492 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3495 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3496 <entry>(string)</entry>
3497 <entry>The C symbol; may be used for methods and interfaces</entry>
3500 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3501 <entry>true,false</entry>
3502 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3505 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3506 <entry>true,invalidates,false</entry>
3509 If set to <literal>false</literal>, the
3510 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3512 linkend="standard-interfaces-properties"/> is not
3513 guaranteed to be emitted if the property changes.
3516 If set to <literal>invalidates</literal> the signal
3517 is emitted but the value is not included in the
3521 If set to <literal>true</literal> the signal is
3522 emitted with the value included.
3525 The value for the annotation defaults to
3526 <literal>true</literal> if the enclosing interface
3527 element does not specify the annotation. Otherwise it
3528 defaults to the value specified in the enclosing
3537 <sect1 id="message-bus">
3538 <title>Message Bus Specification</title>
3539 <sect2 id="message-bus-overview">
3540 <title>Message Bus Overview</title>
3542 The message bus accepts connections from one or more applications.
3543 Once connected, applications can exchange messages with other
3544 applications that are also connected to the bus.
3547 In order to route messages among connections, the message bus keeps a
3548 mapping from names to connections. Each connection has one
3549 unique-for-the-lifetime-of-the-bus name automatically assigned.
3550 Applications may request additional names for a connection. Additional
3551 names are usually "well-known names" such as
3552 "org.freedesktop.TextEditor". When a name is bound to a connection,
3553 that connection is said to <firstterm>own</firstterm> the name.
3556 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3557 This name routes messages to the bus, allowing applications to make
3558 administrative requests. For example, applications can ask the bus
3559 to assign a name to a connection.
3562 Each name may have <firstterm>queued owners</firstterm>. When an
3563 application requests a name for a connection and the name is already in
3564 use, the bus will optionally add the connection to a queue waiting for
3565 the name. If the current owner of the name disconnects or releases
3566 the name, the next connection in the queue will become the new owner.
3570 This feature causes the right thing to happen if you start two text
3571 editors for example; the first one may request "org.freedesktop.TextEditor",
3572 and the second will be queued as a possible owner of that name. When
3573 the first exits, the second will take over.
3577 Applications may send <firstterm>unicast messages</firstterm> to
3578 a specific recipient or to the message bus itself, or
3579 <firstterm>broadcast messages</firstterm> to all interested recipients.
3580 See <xref linkend="message-bus-routing"/> for details.
3584 <sect2 id="message-bus-names">
3585 <title>Message Bus Names</title>
3587 Each connection has at least one name, assigned at connection time and
3588 returned in response to the
3589 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3590 automatically-assigned name is called the connection's <firstterm>unique
3591 name</firstterm>. Unique names are never reused for two different
3592 connections to the same bus.
3595 Ownership of a unique name is a prerequisite for interaction with
3596 the message bus. It logically follows that the unique name is always
3597 the first name that an application comes to own, and the last
3598 one that it loses ownership of.
3601 Unique connection names must begin with the character ':' (ASCII colon
3602 character); bus names that are not unique names must not begin
3603 with this character. (The bus must reject any attempt by an application
3604 to manually request a name beginning with ':'.) This restriction
3605 categorically prevents "spoofing"; messages sent to a unique name
3606 will always go to the expected connection.
3609 When a connection is closed, all the names that it owns are deleted (or
3610 transferred to the next connection in the queue if any).
3613 A connection can request additional names to be associated with it using
3614 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3615 linkend="message-protocol-names-bus"/> describes the format of a valid
3616 name. These names can be released again using the
3617 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3620 <sect3 id="bus-messages-request-name">
3621 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3625 UINT32 RequestName (in STRING name, in UINT32 flags)
3632 <entry>Argument</entry>
3634 <entry>Description</entry>
3640 <entry>STRING</entry>
3641 <entry>Name to request</entry>
3645 <entry>UINT32</entry>
3646 <entry>Flags</entry>
3656 <entry>Argument</entry>
3658 <entry>Description</entry>
3664 <entry>UINT32</entry>
3665 <entry>Return value</entry>
3672 This method call should be sent to
3673 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3674 assign the given name to the method caller. Each name maintains a
3675 queue of possible owners, where the head of the queue is the primary
3676 or current owner of the name. Each potential owner in the queue
3677 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3678 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3679 call. When RequestName is invoked the following occurs:
3683 If the method caller is currently the primary owner of the name,
3684 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3685 values are updated with the values from the new RequestName call,
3686 and nothing further happens.
3692 If the current primary owner (head of the queue) has
3693 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3694 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3695 the caller of RequestName replaces the current primary owner at
3696 the head of the queue and the current primary owner moves to the
3697 second position in the queue. If the caller of RequestName was
3698 in the queue previously its flags are updated with the values from
3699 the new RequestName in addition to moving it to the head of the queue.
3705 If replacement is not possible, and the method caller is
3706 currently in the queue but not the primary owner, its flags are
3707 updated with the values from the new RequestName call.
3713 If replacement is not possible, and the method caller is
3714 currently not in the queue, the method caller is appended to the
3721 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3722 set and is not the primary owner, it is removed from the
3723 queue. This can apply to the previous primary owner (if it
3724 was replaced) or the method caller (if it updated the
3725 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3726 queue, or if it was just added to the queue with that flag set).
3732 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3733 queue," even if another application already in the queue had specified
3734 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3735 that does not allow replacement goes away, and the next primary owner
3736 does allow replacement. In this case, queued items that specified
3737 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3738 automatically replace the new primary owner. In other words,
3739 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3740 time RequestName is called. This is deliberate to avoid an infinite loop
3741 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3742 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3745 The flags argument contains any of the following values logically ORed
3752 <entry>Conventional Name</entry>
3753 <entry>Value</entry>
3754 <entry>Description</entry>
3759 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3763 If an application A specifies this flag and succeeds in
3764 becoming the owner of the name, and another application B
3765 later calls RequestName with the
3766 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3767 will lose ownership and receive a
3768 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3769 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3770 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3771 is not specified by application B, then application B will not replace
3772 application A as the owner.
3777 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3781 Try to replace the current owner if there is one. If this
3782 flag is not set the application will only become the owner of
3783 the name if there is no current owner. If this flag is set,
3784 the application will replace the current owner if
3785 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3790 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3794 Without this flag, if an application requests a name that is
3795 already owned, the application will be placed in a queue to
3796 own the name when the current owner gives it up. If this
3797 flag is given, the application will not be placed in the
3798 queue, the request for the name will simply fail. This flag
3799 also affects behavior when an application is replaced as
3800 name owner; by default the application moves back into the
3801 waiting queue, unless this flag was provided when the application
3802 became the name owner.
3810 The return code can be one of the following values:
3816 <entry>Conventional Name</entry>
3817 <entry>Value</entry>
3818 <entry>Description</entry>
3823 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3824 <entry>1</entry> <entry>The caller is now the primary owner of
3825 the name, replacing any previous owner. Either the name had no
3826 owner before, or the caller specified
3827 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3828 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3831 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3834 <entry>The name already had an owner,
3835 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3836 the current owner did not specify
3837 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3838 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3842 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3843 <entry>The name already has an owner,
3844 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3845 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3846 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3847 specified by the requesting application.</entry>
3850 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3852 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3860 <sect3 id="bus-messages-release-name">
3861 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3865 UINT32 ReleaseName (in STRING name)
3872 <entry>Argument</entry>
3874 <entry>Description</entry>
3880 <entry>STRING</entry>
3881 <entry>Name to release</entry>
3891 <entry>Argument</entry>
3893 <entry>Description</entry>
3899 <entry>UINT32</entry>
3900 <entry>Return value</entry>
3907 This method call should be sent to
3908 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3909 release the method caller's claim to the given name. If the caller is
3910 the primary owner, a new primary owner will be selected from the
3911 queue if any other owners are waiting. If the caller is waiting in
3912 the queue for the name, the caller will removed from the queue and
3913 will not be made an owner of the name if it later becomes available.
3914 If there are no other owners in the queue for the name, it will be
3915 removed from the bus entirely.
3917 The return code can be one of the following values:
3923 <entry>Conventional Name</entry>
3924 <entry>Value</entry>
3925 <entry>Description</entry>
3930 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3931 <entry>1</entry> <entry>The caller has released his claim on
3932 the given name. Either the caller was the primary owner of
3933 the name, and the name is now unused or taken by somebody
3934 waiting in the queue for the name, or the caller was waiting
3935 in the queue for the name and has now been removed from the
3939 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3941 <entry>The given name does not exist on this bus.</entry>
3944 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3946 <entry>The caller was not the primary owner of this name,
3947 and was also not waiting in the queue to own this name.</entry>
3955 <sect3 id="bus-messages-list-queued-owners">
3956 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3960 ARRAY of STRING ListQueuedOwners (in STRING name)
3967 <entry>Argument</entry>
3969 <entry>Description</entry>
3975 <entry>STRING</entry>
3976 <entry>The well-known bus name to query, such as
3977 <literal>com.example.cappuccino</literal></entry>
3987 <entry>Argument</entry>
3989 <entry>Description</entry>
3995 <entry>ARRAY of STRING</entry>
3996 <entry>The unique bus names of connections currently queued
3997 for the name</entry>
4004 This method call should be sent to
4005 <literal>org.freedesktop.DBus</literal> and lists the connections
4006 currently queued for a bus name (see
4007 <xref linkend="term-queued-owner"/>).
4012 <sect2 id="message-bus-routing">
4013 <title>Message Bus Message Routing</title>
4016 Messages may have a <literal>DESTINATION</literal> field (see <xref
4017 linkend="message-protocol-header-fields"/>), resulting in a
4018 <firstterm>unicast message</firstterm>. If the
4019 <literal>DESTINATION</literal> field is present, it specifies a message
4020 recipient by name. Method calls and replies normally specify this field.
4021 The message bus must send messages (of any type) with the
4022 <literal>DESTINATION</literal> field set to the specified recipient,
4023 regardless of whether the recipient has set up a match rule matching
4028 When the message bus receives a signal, if the
4029 <literal>DESTINATION</literal> field is absent, it is considered to
4030 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4031 applications with <firstterm>message matching rules</firstterm> that
4032 match the message. Most signal messages are broadcasts.
4036 Unicast signal messages (those with a <literal>DESTINATION</literal>
4037 field) are not commonly used, but they are treated like any unicast
4038 message: they are delivered to the specified receipient,
4039 regardless of its match rules. One use for unicast signals is to
4040 avoid a race condition in which a signal is emitted before the intended
4041 recipient can call <xref linkend="bus-messages-add-match"/> to
4042 receive that signal: if the signal is sent directly to that recipient
4043 using a unicast message, it does not need to add a match rule at all,
4044 and there is no race condition. Another use for unicast signals,
4045 on message buses whose security policy prevents eavesdropping, is to
4046 send sensitive information which should only be visible to one
4051 When the message bus receives a method call, if the
4052 <literal>DESTINATION</literal> field is absent, the call is taken to be
4053 a standard one-to-one message and interpreted by the message bus
4054 itself. For example, sending an
4055 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4056 <literal>DESTINATION</literal> will cause the message bus itself to
4057 reply to the ping immediately; the message bus will not make this
4058 message visible to other applications.
4062 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4063 the ping message were sent with a <literal>DESTINATION</literal> name of
4064 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4065 forwarded, and the Yoyodyne Corporation screensaver application would be
4066 expected to reply to the ping.
4070 Message bus implementations may impose a security policy which
4071 prevents certain messages from being sent or received.
4072 When a message cannot be sent or received due to a security
4073 policy, the message bus should send an error reply, unless the
4074 original message had the <literal>NO_REPLY</literal> flag.
4077 <sect3 id="message-bus-routing-eavesdropping">
4078 <title>Eavesdropping</title>
4080 Receiving a unicast message whose <literal>DESTINATION</literal>
4081 indicates a different recipient is called
4082 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4083 a security boundary (like the standard system bus), the security
4084 policy should usually prevent eavesdropping, since unicast messages
4085 are normally kept private and may contain security-sensitive
4090 Eavesdropping is mainly useful for debugging tools, such as
4091 the <literal>dbus-monitor</literal> tool in the reference
4092 implementation of D-Bus. Tools which eavesdrop on the message bus
4093 should be careful to avoid sending a reply or error in response to
4094 messages intended for a different client.
4098 Clients may attempt to eavesdrop by adding match rules
4099 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4100 the <literal>eavesdrop='true'</literal> match. If the message bus'
4101 security policy does not allow eavesdropping, the match rule can
4102 still be added, but will not have any practical effect. For
4103 compatibility with older message bus implementations, if adding such
4104 a match rule results in an error reply, the client may fall back to
4105 adding the same rule with the <literal>eavesdrop</literal> match
4110 <sect3 id="message-bus-routing-match-rules">
4111 <title>Match Rules</title>
4113 An important part of the message bus routing protocol is match
4114 rules. Match rules describe the messages that should be sent to a
4115 client, based on the contents of the message. Broadcast signals
4116 are only sent to clients which have a suitable match rule: this
4117 avoids waking up client processes to deal with signals that are
4118 not relevant to that client.
4121 Messages that list a client as their <literal>DESTINATION</literal>
4122 do not need to match the client's match rules, and are sent to that
4123 client regardless. As a result, match rules are mainly used to
4124 receive a subset of broadcast signals.
4127 Match rules can also be used for eavesdropping
4128 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4129 if the security policy of the message bus allows it.
4132 Match rules are added using the AddMatch bus method
4133 (see <xref linkend="bus-messages-add-match"/>). Rules are
4134 specified as a string of comma separated key/value pairs.
4135 Excluding a key from the rule indicates a wildcard match.
4136 For instance excluding the the member from a match rule but
4137 adding a sender would let all messages from that sender through.
4138 An example of a complete rule would be
4139 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4142 The following table describes the keys that can be used to create
4144 The following table summarizes the D-Bus types.
4150 <entry>Possible Values</entry>
4151 <entry>Description</entry>
4156 <entry><literal>type</literal></entry>
4157 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4158 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4161 <entry><literal>sender</literal></entry>
4162 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4163 and <xref linkend="term-unique-name"/> respectively)
4165 <entry>Match messages sent by a particular sender. An example of a sender match
4166 is sender='org.freedesktop.Hal'</entry>
4169 <entry><literal>interface</literal></entry>
4170 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4171 <entry>Match messages sent over or to a particular interface. An example of an
4172 interface match is interface='org.freedesktop.Hal.Manager'.
4173 If a message omits the interface header, it must not match any rule
4174 that specifies this key.</entry>
4177 <entry><literal>member</literal></entry>
4178 <entry>Any valid method or signal name</entry>
4179 <entry>Matches messages which have the give method or signal name. An example of
4180 a member match is member='NameOwnerChanged'</entry>
4183 <entry><literal>path</literal></entry>
4184 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4185 <entry>Matches messages which are sent from or to the given object. An example of a
4186 path match is path='/org/freedesktop/Hal/Manager'</entry>
4189 <entry><literal>path_namespace</literal></entry>
4190 <entry>An object path</entry>
4193 Matches messages which are sent from or to an
4194 object for which the object path is either the
4195 given value, or that value followed by one or
4196 more path components.
4201 <literal>path_namespace='/com/example/foo'</literal>
4202 would match signals sent by
4203 <literal>/com/example/foo</literal>
4205 <literal>/com/example/foo/bar</literal>,
4207 <literal>/com/example/foobar</literal>.
4211 Using both <literal>path</literal> and
4212 <literal>path_namespace</literal> in the same match
4213 rule is not allowed.
4218 This match key was added in version 0.16 of the
4219 D-Bus specification and implemented by the bus
4220 daemon in dbus 1.5.0 and later.
4226 <entry><literal>destination</literal></entry>
4227 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4228 <entry>Matches messages which are being sent to the given unique name. An
4229 example of a destination match is destination=':1.0'</entry>
4232 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4233 <entry>Any string</entry>
4234 <entry>Arg matches are special and are used for further restricting the
4235 match based on the arguments in the body of a message. Only arguments of type
4236 STRING can be matched in this way. An example of an argument match
4237 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4241 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4242 <entry>Any string</entry>
4244 <para>Argument path matches provide a specialised form of wildcard matching for
4245 path-like namespaces. They can match arguments whose type is either STRING or
4246 OBJECT_PATH. As with normal argument matches,
4247 if the argument is exactly equal to the string given in the match
4248 rule then the rule is satisfied. Additionally, there is also a
4249 match when either the string given in the match rule or the
4250 appropriate message argument ends with '/' and is a prefix of the
4251 other. An example argument path match is arg0path='/aa/bb/'. This
4252 would match messages with first arguments of '/', '/aa/',
4253 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4254 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4256 <para>This is intended for monitoring “directories” in file system-like
4257 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4258 system. An application interested in all nodes in a particular hierarchy would
4259 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4260 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4261 represent a modification to the “bar” property, or a signal with zeroth
4262 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4263 many properties within that directory, and the interested application would be
4264 notified in both cases.</para>
4267 This match key was added in version 0.12 of the
4268 D-Bus specification, implemented for STRING
4269 arguments by the bus daemon in dbus 1.2.0 and later,
4270 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4277 <entry><literal>arg0namespace</literal></entry>
4278 <entry>Like a bus name, except that the string is not
4279 required to contain a '.' (period)</entry>
4281 <para>Match messages whose first argument is of type STRING, and is a bus name
4282 or interface name within the specified namespace. This is primarily intended
4283 for watching name owner changes for a group of related bus names, rather than
4284 for a single name or all name changes.</para>
4286 <para>Because every valid interface name is also a valid
4287 bus name, this can also be used for messages whose
4288 first argument is an interface name.</para>
4290 <para>For example, the match rule
4291 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4292 matches name owner changes for bus names such as
4293 <literal>com.example.backend.foo</literal>,
4294 <literal>com.example.backend.foo.bar</literal>, and
4295 <literal>com.example.backend</literal> itself.</para>
4297 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4300 This match key was added in version 0.16 of the
4301 D-Bus specification and implemented by the bus
4302 daemon in dbus 1.5.0 and later.
4308 <entry><literal>eavesdrop</literal></entry>
4309 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4310 <entry>Since D-Bus 1.5.6, match rules do not
4311 match messages which have a <literal>DESTINATION</literal>
4312 field unless the match rule specifically
4314 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4315 by specifying <literal>eavesdrop='true'</literal>
4316 in the match rule. <literal>eavesdrop='false'</literal>
4317 restores the default behaviour. Messages are
4318 delivered to their <literal>DESTINATION</literal>
4319 regardless of match rules, so this match does not
4320 affect normal delivery of unicast messages.
4321 If the message bus has a security policy which forbids
4322 eavesdropping, this match may still be used without error,
4323 but will not have any practical effect.
4324 In older versions of D-Bus, this match was not allowed
4325 in match rules, and all match rules behaved as if
4326 <literal>eavesdrop='true'</literal> had been used.
4335 <sect2 id="message-bus-starting-services">
4336 <title>Message Bus Starting Services</title>
4338 The message bus can start applications on behalf of other applications.
4339 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4340 An application that can be started in this way is called a
4341 <firstterm>service</firstterm>.
4344 With D-Bus, starting a service is normally done by name. That is,
4345 applications ask the message bus to start some program that will own a
4346 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4347 This implies a contract documented along with the name
4348 <literal>org.freedesktop.TextEditor</literal> for which objects
4349 the owner of that name will provide, and what interfaces those
4353 To find an executable corresponding to a particular name, the bus daemon
4354 looks for <firstterm>service description files</firstterm>. Service
4355 description files define a mapping from names to executables. Different
4356 kinds of message bus will look for these files in different places, see
4357 <xref linkend="message-bus-types"/>.
4360 Service description files have the ".service" file
4361 extension. The message bus will only load service description files
4362 ending with .service; all other files will be ignored. The file format
4363 is similar to that of <ulink
4364 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4365 entries</ulink>. All service description files must be in UTF-8
4366 encoding. To ensure that there will be no name collisions, service files
4367 must be namespaced using the same mechanism as messages and service
4372 [FIXME the file format should be much better specified than "similar to
4373 .desktop entries" esp. since desktop entries are already
4374 badly-specified. ;-)]
4375 These sections from the specification apply to service files as well:
4378 <listitem><para>General syntax</para></listitem>
4379 <listitem><para>Comment format</para></listitem>
4383 <title>Example service description file</title>
4385 # Sample service description file
4387 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4388 Exec=/usr/libexec/gconfd-2
4393 When an application asks to start a service by name, the bus daemon tries to
4394 find a service that will own that name. It then tries to spawn the
4395 executable associated with it. If this fails, it will report an
4396 error. [FIXME what happens if two .service files offer the same service;
4397 what kind of error is reported, should we have a way for the client to
4401 The executable launched will have the environment variable
4402 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4403 message bus so it can connect and request the appropriate names.
4406 The executable being launched may want to know whether the message bus
4407 starting it is one of the well-known message buses (see <xref
4408 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4409 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4410 of the well-known buses. The currently-defined values for this variable
4411 are <literal>system</literal> for the systemwide message bus,
4412 and <literal>session</literal> for the per-login-session message
4413 bus. The new executable must still connect to the address given
4414 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4415 resulting connection is to the well-known bus.
4418 [FIXME there should be a timeout somewhere, either specified
4419 in the .service file, by the client, or just a global value
4420 and if the client being activated fails to connect within that
4421 timeout, an error should be sent back.]
4424 <sect3 id="message-bus-starting-services-scope">
4425 <title>Message Bus Service Scope</title>
4427 The "scope" of a service is its "per-", such as per-session,
4428 per-machine, per-home-directory, or per-display. The reference
4429 implementation doesn't yet support starting services in a different
4430 scope from the message bus itself. So e.g. if you start a service
4431 on the session bus its scope is per-session.
4434 We could add an optional scope to a bus name. For example, for
4435 per-(display,session pair), we could have a unique ID for each display
4436 generated automatically at login and set on screen 0 by executing a
4437 special "set display ID" binary. The ID would be stored in a
4438 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4439 random bytes. This ID would then be used to scope names.
4440 Starting/locating a service could be done by ID-name pair rather than
4444 Contrast this with a per-display scope. To achieve that, we would
4445 want a single bus spanning all sessions using a given display.
4446 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4447 property on screen 0 of the display, pointing to this bus.
4452 <sect2 id="message-bus-types">
4453 <title>Well-known Message Bus Instances</title>
4455 Two standard message bus instances are defined here, along with how
4456 to locate them and where their service files live.
4458 <sect3 id="message-bus-types-login">
4459 <title>Login session message bus</title>
4461 Each time a user logs in, a <firstterm>login session message
4462 bus</firstterm> may be started. All applications in the user's login
4463 session may interact with one another using this message bus.
4466 The address of the login session message bus is given
4467 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4468 variable. If that variable is not set, applications may
4469 also try to read the address from the X Window System root
4470 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4471 The root window property must have type <literal>STRING</literal>.
4472 The environment variable should have precedence over the
4473 root window property.
4475 <para>The address of the login session message bus is given in the
4476 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4477 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4478 "autolaunch:", the system should use platform-specific methods of
4479 locating a running D-Bus session server, or starting one if a running
4480 instance cannot be found. Note that this mechanism is not recommended
4481 for attempting to determine if a daemon is running. It is inherently
4482 racy to attempt to make this determination, since the bus daemon may
4483 be started just before or just after the determination is made.
4484 Therefore, it is recommended that applications do not try to make this
4485 determination for their functionality purposes, and instead they
4486 should attempt to start the server.</para>
4488 <sect4 id="message-bus-types-login-x-windows">
4489 <title>X Windowing System</title>
4491 For the X Windowing System, the application must locate the
4492 window owner of the selection represented by the atom formed by
4496 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4500 <para>the current user's username</para>
4504 <para>the literal character '_' (underscore)</para>
4508 <para>the machine's ID</para>
4514 The following properties are defined for the window that owns
4516 <informaltable frame="all">
4525 <para>meaning</para>
4531 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4535 <para>the actual address of the server socket</para>
4541 <para>_DBUS_SESSION_BUS_PID</para>
4545 <para>the PID of the server process</para>
4554 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4555 present in this window.
4559 If the X selection cannot be located or if reading the
4560 properties from the window fails, the implementation MUST conclude
4561 that there is no D-Bus server running and proceed to start a new
4562 server. (See below on concurrency issues)
4566 Failure to connect to the D-Bus server address thus obtained
4567 MUST be treated as a fatal connection error and should be reported
4572 As an alternative, an implementation MAY find the information
4573 in the following file located in the current user's home directory,
4574 in subdirectory .dbus/session-bus/:
4577 <para>the machine's ID</para>
4581 <para>the literal character '-' (dash)</para>
4585 <para>the X display without the screen number, with the
4586 following prefixes removed, if present: ":", "localhost:"
4587 ."localhost.localdomain:". That is, a display of
4588 "localhost:10.0" produces just the number "10"</para>
4594 The contents of this file NAME=value assignment pairs and
4595 lines starting with # are comments (no comments are allowed
4596 otherwise). The following variable names are defined:
4603 <para>Variable</para>
4607 <para>meaning</para>
4613 <para>DBUS_SESSION_BUS_ADDRESS</para>
4617 <para>the actual address of the server socket</para>
4623 <para>DBUS_SESSION_BUS_PID</para>
4627 <para>the PID of the server process</para>
4633 <para>DBUS_SESSION_BUS_WINDOWID</para>
4637 <para>the window ID</para>
4646 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4651 Failure to open this file MUST be interpreted as absence of a
4652 running server. Therefore, the implementation MUST proceed to
4653 attempting to launch a new bus server if the file cannot be
4658 However, success in opening this file MUST NOT lead to the
4659 conclusion that the server is running. Thus, a failure to connect to
4660 the bus address obtained by the alternative method MUST NOT be
4661 considered a fatal error. If the connection cannot be established,
4662 the implementation MUST proceed to check the X selection settings or
4663 to start the server on its own.
4667 If the implementation concludes that the D-Bus server is not
4668 running it MUST attempt to start a new server and it MUST also
4669 ensure that the daemon started as an effect of the "autolaunch"
4670 mechanism provides the lookup mechanisms described above, so
4671 subsequent calls can locate the newly started server. The
4672 implementation MUST also ensure that if two or more concurrent
4673 initiations happen, only one server remains running and all other
4674 initiations are able to obtain the address of this server and
4675 connect to it. In other words, the implementation MUST ensure that
4676 the X selection is not present when it attempts to set it, without
4677 allowing another process to set the selection between the
4678 verification and the setting (e.g., by using XGrabServer /
4685 On Unix systems, the session bus should search for .service files
4686 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4688 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4689 Implementations may also search additional locations, which
4690 should be searched with lower priority than anything in
4691 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4692 for example, the reference implementation also
4693 looks in <literal>${datadir}/dbus-1/services</literal> as
4694 set at compile time.
4697 As described in the XDG Base Directory Specification, software
4698 packages should install their session .service files to their
4699 configured <literal>${datadir}/dbus-1/services</literal>,
4700 where <literal>${datadir}</literal> is as defined by the GNU
4701 coding standards. System administrators or users can arrange
4702 for these service files to be read by setting XDG_DATA_DIRS or by
4703 symlinking them into the default locations.
4707 <sect3 id="message-bus-types-system">
4708 <title>System message bus</title>
4710 A computer may have a <firstterm>system message bus</firstterm>,
4711 accessible to all applications on the system. This message bus may be
4712 used to broadcast system events, such as adding new hardware devices,
4713 changes in the printer queue, and so forth.
4716 The address of the system message bus is given
4717 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4718 variable. If that variable is not set, applications should try
4719 to connect to the well-known address
4720 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4723 The D-Bus reference implementation actually honors the
4724 <literal>$(localstatedir)</literal> configure option
4725 for this address, on both client and server side.
4730 On Unix systems, the system bus should default to searching
4731 for .service files in
4732 <literal>/usr/local/share/dbus-1/system-services</literal>,
4733 <literal>/usr/share/dbus-1/system-services</literal> and
4734 <literal>/lib/dbus-1/system-services</literal>, with that order
4735 of precedence. It may also search other implementation-specific
4736 locations, but should not vary these locations based on environment
4740 The system bus is security-sensitive and is typically executed
4741 by an init system with a clean environment. Its launch helper
4742 process is particularly security-sensitive, and specifically
4743 clears its own environment.
4748 Software packages should install their system .service
4749 files to their configured
4750 <literal>${datadir}/dbus-1/system-services</literal>,
4751 where <literal>${datadir}</literal> is as defined by the GNU
4752 coding standards. System administrators can arrange
4753 for these service files to be read by editing the system bus'
4754 configuration file or by symlinking them into the default
4760 <sect2 id="message-bus-messages">
4761 <title>Message Bus Messages</title>
4763 The special message bus name <literal>org.freedesktop.DBus</literal>
4764 responds to a number of additional messages.
4767 <sect3 id="bus-messages-hello">
4768 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4779 <entry>Argument</entry>
4781 <entry>Description</entry>
4787 <entry>STRING</entry>
4788 <entry>Unique name assigned to the connection</entry>
4795 Before an application is able to send messages to other applications
4796 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4797 to the message bus to obtain a unique name. If an application without
4798 a unique name tries to send a message to another application, or a
4799 message to the message bus itself that isn't the
4800 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4801 disconnected from the bus.
4804 There is no corresponding "disconnect" request; if a client wishes to
4805 disconnect from the bus, it simply closes the socket (or other
4806 communication channel).
4809 <sect3 id="bus-messages-list-names">
4810 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4814 ARRAY of STRING ListNames ()
4821 <entry>Argument</entry>
4823 <entry>Description</entry>
4829 <entry>ARRAY of STRING</entry>
4830 <entry>Array of strings where each string is a bus name</entry>
4837 Returns a list of all currently-owned names on the bus.
4840 <sect3 id="bus-messages-list-activatable-names">
4841 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4845 ARRAY of STRING ListActivatableNames ()
4852 <entry>Argument</entry>
4854 <entry>Description</entry>
4860 <entry>ARRAY of STRING</entry>
4861 <entry>Array of strings where each string is a bus name</entry>
4868 Returns a list of all names that can be activated on the bus.
4871 <sect3 id="bus-messages-name-exists">
4872 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4876 BOOLEAN NameHasOwner (in STRING name)
4883 <entry>Argument</entry>
4885 <entry>Description</entry>
4891 <entry>STRING</entry>
4892 <entry>Name to check</entry>
4902 <entry>Argument</entry>
4904 <entry>Description</entry>
4910 <entry>BOOLEAN</entry>
4911 <entry>Return value, true if the name exists</entry>
4918 Checks if the specified name exists (currently has an owner).
4922 <sect3 id="bus-messages-name-owner-changed">
4923 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4927 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4934 <entry>Argument</entry>
4936 <entry>Description</entry>
4942 <entry>STRING</entry>
4943 <entry>Name with a new owner</entry>
4947 <entry>STRING</entry>
4948 <entry>Old owner or empty string if none</entry>
4952 <entry>STRING</entry>
4953 <entry>New owner or empty string if none</entry>
4960 This signal indicates that the owner of a name has changed.
4961 It's also the signal to use to detect the appearance of
4962 new names on the bus.
4965 <sect3 id="bus-messages-name-lost">
4966 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4970 NameLost (STRING name)
4977 <entry>Argument</entry>
4979 <entry>Description</entry>
4985 <entry>STRING</entry>
4986 <entry>Name which was lost</entry>
4993 This signal is sent to a specific application when it loses
4994 ownership of a name.
4998 <sect3 id="bus-messages-name-acquired">
4999 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5003 NameAcquired (STRING name)
5010 <entry>Argument</entry>
5012 <entry>Description</entry>
5018 <entry>STRING</entry>
5019 <entry>Name which was acquired</entry>
5026 This signal is sent to a specific application when it gains
5027 ownership of a name.
5031 <sect3 id="bus-messages-start-service-by-name">
5032 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5036 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5043 <entry>Argument</entry>
5045 <entry>Description</entry>
5051 <entry>STRING</entry>
5052 <entry>Name of the service to start</entry>
5056 <entry>UINT32</entry>
5057 <entry>Flags (currently not used)</entry>
5067 <entry>Argument</entry>
5069 <entry>Description</entry>
5075 <entry>UINT32</entry>
5076 <entry>Return value</entry>
5081 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5085 The return value can be one of the following values:
5090 <entry>Identifier</entry>
5091 <entry>Value</entry>
5092 <entry>Description</entry>
5097 <entry>DBUS_START_REPLY_SUCCESS</entry>
5099 <entry>The service was successfully started.</entry>
5102 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5104 <entry>A connection already owns the given name.</entry>
5113 <sect3 id="bus-messages-update-activation-environment">
5114 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5118 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5125 <entry>Argument</entry>
5127 <entry>Description</entry>
5133 <entry>ARRAY of DICT<STRING,STRING></entry>
5134 <entry>Environment to add or update</entry>
5139 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5142 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5145 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.
5150 <sect3 id="bus-messages-get-name-owner">
5151 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5155 STRING GetNameOwner (in STRING name)
5162 <entry>Argument</entry>
5164 <entry>Description</entry>
5170 <entry>STRING</entry>
5171 <entry>Name to get the owner of</entry>
5181 <entry>Argument</entry>
5183 <entry>Description</entry>
5189 <entry>STRING</entry>
5190 <entry>Return value, a unique connection name</entry>
5195 Returns the unique connection name of the primary owner of the name
5196 given. If the requested name doesn't have an owner, returns a
5197 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5201 <sect3 id="bus-messages-get-connection-unix-user">
5202 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5206 UINT32 GetConnectionUnixUser (in STRING bus_name)
5213 <entry>Argument</entry>
5215 <entry>Description</entry>
5221 <entry>STRING</entry>
5222 <entry>Unique or well-known bus name of the connection to
5223 query, such as <literal>:12.34</literal> or
5224 <literal>com.example.tea</literal></entry>
5234 <entry>Argument</entry>
5236 <entry>Description</entry>
5242 <entry>UINT32</entry>
5243 <entry>Unix user ID</entry>
5248 Returns the Unix user ID of the process connected to the server. If
5249 unable to determine it (for instance, because the process is not on the
5250 same machine as the bus daemon), an error is returned.
5254 <sect3 id="bus-messages-get-connection-unix-process-id">
5255 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5259 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5266 <entry>Argument</entry>
5268 <entry>Description</entry>
5274 <entry>STRING</entry>
5275 <entry>Unique or well-known bus name of the connection to
5276 query, such as <literal>:12.34</literal> or
5277 <literal>com.example.tea</literal></entry>
5287 <entry>Argument</entry>
5289 <entry>Description</entry>
5295 <entry>UINT32</entry>
5296 <entry>Unix process id</entry>
5301 Returns the Unix process ID of the process connected to the server. If
5302 unable to determine it (for instance, because the process is not on the
5303 same machine as the bus daemon), an error is returned.
5307 <sect3 id="bus-messages-add-match">
5308 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5312 AddMatch (in STRING rule)
5319 <entry>Argument</entry>
5321 <entry>Description</entry>
5327 <entry>STRING</entry>
5328 <entry>Match rule to add to the connection</entry>
5333 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5334 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5338 <sect3 id="bus-messages-remove-match">
5339 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5343 RemoveMatch (in STRING rule)
5350 <entry>Argument</entry>
5352 <entry>Description</entry>
5358 <entry>STRING</entry>
5359 <entry>Match rule to remove from the connection</entry>
5364 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5365 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5370 <sect3 id="bus-messages-get-id">
5371 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5375 GetId (out STRING id)
5382 <entry>Argument</entry>
5384 <entry>Description</entry>
5390 <entry>STRING</entry>
5391 <entry>Unique ID identifying the bus daemon</entry>
5396 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5397 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5398 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5399 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5400 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5401 by org.freedesktop.DBus.Peer.GetMachineId().
5402 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5410 <appendix id="implementation-notes">
5411 <title>Implementation notes</title>
5412 <sect1 id="implementation-notes-subsection">
5420 <glossary><title>Glossary</title>
5422 This glossary defines some of the terms used in this specification.
5425 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5428 The message bus maintains an association between names and
5429 connections. (Normally, there's one connection per application.) A
5430 bus name is simply an identifier used to locate connections. For
5431 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5432 name might be used to send a message to a screensaver from Yoyodyne
5433 Corporation. An application is said to <firstterm>own</firstterm> a
5434 name if the message bus has associated the application's connection
5435 with the name. Names may also have <firstterm>queued
5436 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5437 The bus assigns a unique name to each connection,
5438 see <xref linkend="term-unique-name"/>. Other names
5439 can be thought of as "well-known names" and are
5440 used to find applications that offer specific functionality.
5444 See <xref linkend="message-protocol-names-bus"/> for details of
5445 the syntax and naming conventions for bus names.
5450 <glossentry id="term-message"><glossterm>Message</glossterm>
5453 A message is the atomic unit of communication via the D-Bus
5454 protocol. It consists of a <firstterm>header</firstterm> and a
5455 <firstterm>body</firstterm>; the body is made up of
5456 <firstterm>arguments</firstterm>.
5461 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5464 The message bus is a special application that forwards
5465 or routes messages between a group of applications
5466 connected to the message bus. It also manages
5467 <firstterm>names</firstterm> used for routing
5473 <glossentry id="term-name"><glossterm>Name</glossterm>
5476 See <xref linkend="term-bus-name"/>. "Name" may
5477 also be used to refer to some of the other names
5478 in D-Bus, such as interface names.
5483 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5486 Used to prevent collisions when defining new interfaces, bus names
5487 etc. The convention used is the same one Java uses for defining
5488 classes: a reversed domain name.
5489 See <xref linkend="message-protocol-names-bus"/>,
5490 <xref linkend="message-protocol-names-interface"/>,
5491 <xref linkend="message-protocol-names-error"/>,
5492 <xref linkend="message-protocol-marshaling-object-path"/>.
5497 <glossentry id="term-object"><glossterm>Object</glossterm>
5500 Each application contains <firstterm>objects</firstterm>, which have
5501 <firstterm>interfaces</firstterm> and
5502 <firstterm>methods</firstterm>. Objects are referred to by a name,
5503 called a <firstterm>path</firstterm>.
5508 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5511 An application talking directly to another application, without going
5512 through a message bus. One-to-one connections may be "peer to peer" or
5513 "client to server." The D-Bus protocol has no concept of client
5514 vs. server after a connection has authenticated; the flow of messages
5515 is symmetrical (full duplex).
5520 <glossentry id="term-path"><glossterm>Path</glossterm>
5523 Object references (object names) in D-Bus are organized into a
5524 filesystem-style hierarchy, so each object is named by a path. As in
5525 LDAP, there's no difference between "files" and "directories"; a path
5526 can refer to an object, while still having child objects below it.
5531 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5534 Each bus name has a primary owner; messages sent to the name go to the
5535 primary owner. However, certain names also maintain a queue of
5536 secondary owners "waiting in the wings." If the primary owner releases
5537 the name, then the first secondary owner in the queue automatically
5538 becomes the new owner of the name.
5543 <glossentry id="term-service"><glossterm>Service</glossterm>
5546 A service is an executable that can be launched by the bus daemon.
5547 Services normally guarantee some particular features, for example they
5548 may guarantee that they will request a specific name such as
5549 "org.freedesktop.Screensaver", have a singleton object
5550 "/org/freedesktop/Application", and that object will implement the
5551 interface "org.freedesktop.ScreensaverControl".
5556 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5559 ".service files" tell the bus about service applications that can be
5560 launched (see <xref linkend="term-service"/>). Most importantly they
5561 provide a mapping from bus names to services that will request those
5562 names when they start up.
5567 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5570 The special name automatically assigned to each connection by the
5571 message bus. This name will never change owner, and will be unique
5572 (never reused during the lifetime of the message bus).
5573 It will begin with a ':' character.