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>.
296 The D-Bus protocol does not include type tags in the marshaled data; a
297 block of marshaled values must have a known <firstterm>type
298 signature</firstterm>. The type signature is made up of <firstterm>type
299 codes</firstterm>. A type code is an ASCII character representing the
300 type of a value. Because ASCII characters are used, the type signature
301 will always form a valid ASCII string. A simple string compare
302 determines whether two type signatures are equivalent.
305 <sect2 id="basic-types">
306 <title>Basic types</title>
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.
329 <sect2 id="container-types">
330 <title>Container types</title>
333 In addition to basic types, there are four <firstterm>container</firstterm>
334 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
335 and <literal>DICT_ENTRY</literal>.
339 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
340 code does not appear in signatures. Instead, ASCII characters
341 '(' and ')' are used to mark the beginning and end of the struct.
342 So for example, a struct containing two integers would have this
347 Structs can be nested, so for example a struct containing
348 an integer and another struct:
352 The value block storing that struct would contain three integers; the
353 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
358 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
359 but is useful in code that implements the protocol. This type code
360 is specified to allow such code to interoperate in non-protocol contexts.
364 Empty structures are not allowed; there must be at least one
365 type code between the parentheses.
369 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
370 followed by a <firstterm>single complete type</firstterm>. The single
371 complete type following the array is the type of each array element. So
372 the simple example is:
376 which is an array of 32-bit integers. But an array can be of any type,
377 such as this array-of-struct-with-two-int32-fields:
381 Or this array of array of integer:
388 The phrase <firstterm>single complete type</firstterm> deserves some
389 definition. A single complete type is a basic type code, a variant type code,
390 an array with its element type, or a struct with its fields.
391 So the following signatures are not single complete types:
401 And the following signatures contain multiple complete types:
411 Note however that a single complete type may <emphasis>contain</emphasis>
412 multiple other single complete types.
416 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
417 type <literal>VARIANT</literal> will have the signature of a single complete type as part
418 of the <emphasis>value</emphasis>. This signature will be followed by a
419 marshaled value of that type.
423 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
424 than parentheses it uses curly braces, and it has more restrictions.
425 The restrictions are: it occurs only as an array element type; it has
426 exactly two single complete types inside the curly braces; the first
427 single complete type (the "key") must be a basic type rather than a
428 container type. Implementations must not accept dict entries outside of
429 arrays, must not accept dict entries with zero, one, or more than two
430 fields, and must not accept dict entries with non-basic-typed keys. A
431 dict entry is always a key-value pair.
435 The first field in the <literal>DICT_ENTRY</literal> is always the key.
436 A message is considered corrupt if the same key occurs twice in the same
437 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
438 implementations are not required to reject dicts with duplicate keys.
442 In most languages, an array of dict entry would be represented as a
443 map, hash table, or dict object.
448 <title>Summary of types</title>
451 The following table summarizes the D-Bus types.
456 <entry>Conventional Name</entry>
458 <entry>Description</entry>
463 <entry><literal>INVALID</literal></entry>
464 <entry>0 (ASCII NUL)</entry>
465 <entry>Not a valid type code, used to terminate signatures</entry>
467 <entry><literal>BYTE</literal></entry>
468 <entry>121 (ASCII 'y')</entry>
469 <entry>8-bit unsigned integer</entry>
471 <entry><literal>BOOLEAN</literal></entry>
472 <entry>98 (ASCII 'b')</entry>
473 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
475 <entry><literal>INT16</literal></entry>
476 <entry>110 (ASCII 'n')</entry>
477 <entry>16-bit signed integer</entry>
479 <entry><literal>UINT16</literal></entry>
480 <entry>113 (ASCII 'q')</entry>
481 <entry>16-bit unsigned integer</entry>
483 <entry><literal>INT32</literal></entry>
484 <entry>105 (ASCII 'i')</entry>
485 <entry>32-bit signed integer</entry>
487 <entry><literal>UINT32</literal></entry>
488 <entry>117 (ASCII 'u')</entry>
489 <entry>32-bit unsigned integer</entry>
491 <entry><literal>INT64</literal></entry>
492 <entry>120 (ASCII 'x')</entry>
493 <entry>64-bit signed integer</entry>
495 <entry><literal>UINT64</literal></entry>
496 <entry>116 (ASCII 't')</entry>
497 <entry>64-bit unsigned integer</entry>
499 <entry><literal>DOUBLE</literal></entry>
500 <entry>100 (ASCII 'd')</entry>
501 <entry>IEEE 754 double</entry>
503 <entry><literal>STRING</literal></entry>
504 <entry>115 (ASCII 's')</entry>
505 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
507 <entry><literal>OBJECT_PATH</literal></entry>
508 <entry>111 (ASCII 'o')</entry>
509 <entry>Name of an object instance</entry>
511 <entry><literal>SIGNATURE</literal></entry>
512 <entry>103 (ASCII 'g')</entry>
513 <entry>A type signature</entry>
515 <entry><literal>ARRAY</literal></entry>
516 <entry>97 (ASCII 'a')</entry>
519 <entry><literal>STRUCT</literal></entry>
520 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
521 <entry>Struct; type code 114 'r' is reserved for use in
522 bindings and implementations to represent the general
523 concept of a struct, and must not appear in signatures
524 used on D-Bus.</entry>
526 <entry><literal>VARIANT</literal></entry>
527 <entry>118 (ASCII 'v') </entry>
528 <entry>Variant type (the type of the value is part of the value itself)</entry>
530 <entry><literal>DICT_ENTRY</literal></entry>
531 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
532 <entry>Entry in a dict or map (array of key-value pairs).
533 Type code 101 'e' is reserved for use in bindings and
534 implementations to represent the general concept of a
535 dict or dict-entry, and must not appear in signatures
536 used on D-Bus.</entry>
538 <entry><literal>UNIX_FD</literal></entry>
539 <entry>104 (ASCII 'h')</entry>
540 <entry>Unix file descriptor</entry>
543 <entry>(reserved)</entry>
544 <entry>109 (ASCII 'm')</entry>
545 <entry>Reserved for <ulink
546 url="https://bugs.freedesktop.org/show_bug.cgi?id=27857">a
547 'maybe' type compatible with the one in GVariant</ulink>,
548 and must not appear in signatures used on D-Bus until
549 specified here</entry>
552 <entry>(reserved)</entry>
553 <entry>42 (ASCII '*')</entry>
554 <entry>Reserved for use in bindings/implementations to
555 represent any <firstterm>single complete type</firstterm>,
556 and must not appear in signatures used on D-Bus.</entry>
559 <entry>(reserved)</entry>
560 <entry>63 (ASCII '?')</entry>
561 <entry>Reserved for use in bindings/implementations to
562 represent any <firstterm>basic type</firstterm>, and must
563 not appear in signatures used on D-Bus.</entry>
566 <entry>(reserved)</entry>
567 <entry>64 (ASCII '@'), 38 (ASCII '&'),
568 94 (ASCII '^')</entry>
569 <entry>Reserved for internal use by bindings/implementations,
570 and must not appear in signatures used on D-Bus.
571 GVariant uses these type-codes to encode calling
581 <sect2 id="message-protocol-marshaling">
582 <title>Marshaling (Wire Format)</title>
585 Given a type signature, a block of bytes can be converted into typed
586 values. This section describes the format of the block of bytes. Byte
587 order and alignment issues are handled uniformly for all D-Bus types.
591 A block of bytes has an associated byte order. The byte order
592 has to be discovered in some way; for D-Bus messages, the
593 byte order is part of the message header as described in
594 <xref linkend="message-protocol-messages"/>. For now, assume
595 that the byte order is known to be either little endian or big
600 Each value in a block of bytes is aligned "naturally," for example
601 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
602 8-byte boundary. To properly align a value, <firstterm>alignment
603 padding</firstterm> may be necessary. The alignment padding must always
604 be the minimum required padding to properly align the following value;
605 and it must always be made up of nul bytes. The alignment padding must
606 not be left uninitialized (it can't contain garbage), and more padding
607 than required must not be used.
611 Given all this, the types are marshaled on the wire as follows:
616 <entry>Conventional Name</entry>
617 <entry>Encoding</entry>
618 <entry>Alignment</entry>
623 <entry><literal>INVALID</literal></entry>
624 <entry>Not applicable; cannot be marshaled.</entry>
627 <entry><literal>BYTE</literal></entry>
628 <entry>A single 8-bit byte.</entry>
631 <entry><literal>BOOLEAN</literal></entry>
632 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
635 <entry><literal>INT16</literal></entry>
636 <entry>16-bit signed integer in the message's byte order.</entry>
639 <entry><literal>UINT16</literal></entry>
640 <entry>16-bit unsigned integer in the message's byte order.</entry>
643 <entry><literal>INT32</literal></entry>
644 <entry>32-bit signed integer in the message's byte order.</entry>
647 <entry><literal>UINT32</literal></entry>
648 <entry>32-bit unsigned integer in the message's byte order.</entry>
651 <entry><literal>INT64</literal></entry>
652 <entry>64-bit signed integer in the message's byte order.</entry>
655 <entry><literal>UINT64</literal></entry>
656 <entry>64-bit unsigned integer in the message's byte order.</entry>
659 <entry><literal>DOUBLE</literal></entry>
660 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
663 <entry><literal>STRING</literal></entry>
664 <entry>A <literal>UINT32</literal> indicating the string's
665 length in bytes excluding its terminating nul, followed by
666 non-nul string data of the given length, followed by a terminating nul
673 <entry><literal>OBJECT_PATH</literal></entry>
674 <entry>Exactly the same as <literal>STRING</literal> except the
675 content must be a valid object path (see below).
681 <entry><literal>SIGNATURE</literal></entry>
682 <entry>The same as <literal>STRING</literal> except the length is a single
683 byte (thus signatures have a maximum length of 255)
684 and the content must be a valid signature (see below).
690 <entry><literal>ARRAY</literal></entry>
692 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
693 alignment padding to the alignment boundary of the array element type,
694 followed by each array element. The array length is from the
695 end of the alignment padding to the end of the last element,
696 i.e. it does not include the padding after the length,
697 or any padding after the last element.
698 Arrays have a maximum length defined to be 2 to the 26th power or
699 67108864. Implementations must not send or accept arrays exceeding this
706 <entry><literal>STRUCT</literal></entry>
708 A struct must start on an 8-byte boundary regardless of the
709 type of the struct fields. The struct value consists of each
710 field marshaled in sequence starting from that 8-byte
717 <entry><literal>VARIANT</literal></entry>
719 A variant type has a marshaled
720 <literal>SIGNATURE</literal> followed by a marshaled
721 value with the type given in the signature. Unlike
722 a message signature, the variant signature can
723 contain only a single complete type. So "i", "ai"
724 or "(ii)" is OK, but "ii" is not. Use of variants may not
725 cause a total message depth to be larger than 64, including
726 other container types such as structures.
729 1 (alignment of the signature)
732 <entry><literal>DICT_ENTRY</literal></entry>
740 <entry><literal>UNIX_FD</literal></entry>
741 <entry>32-bit unsigned integer in the message's byte
742 order. The actual file descriptors need to be
743 transferred out-of-band via some platform specific
744 mechanism. On the wire, values of this type store the index to the
745 file descriptor in the array of file descriptors that
746 accompany the message.</entry>
754 <sect3 id="message-protocol-marshaling-object-path">
755 <title>Valid Object Paths</title>
758 An object path is a name used to refer to an object instance.
759 Conceptually, each participant in a D-Bus message exchange may have
760 any number of object instances (think of C++ or Java objects) and each
761 such instance will have a path. Like a filesystem, the object
762 instances in an application form a hierarchical tree.
766 The following rules define a valid object path. Implementations must
767 not send or accept messages with invalid object paths.
771 The path may be of any length.
776 The path must begin with an ASCII '/' (integer 47) character,
777 and must consist of elements separated by slash characters.
782 Each element must only contain the ASCII characters
788 No element may be the empty string.
793 Multiple '/' characters cannot occur in sequence.
798 A trailing '/' character is not allowed unless the
799 path is the root path (a single '/' character).
806 Object paths are often namespaced by starting with a reversed
807 domain name and containing an interface version number, in the
809 <link linkend="message-protocol-names-interface">interface
811 <link linkend="message-protocol-names-bus">well-known
813 This makes it possible to implement more than one service, or
814 more than one version of a service, in the same process,
815 even if the services share a connection but cannot otherwise
816 co-operate (for instance, if they are implemented by different
821 For instance, if the owner of <literal>example.com</literal> is
822 developing a D-Bus API for a music player, they might use the
823 hierarchy of object paths that start with
824 <literal>/com/example/MusicPlayer1</literal> for its objects.
828 <sect3 id="message-protocol-marshaling-signature">
829 <title>Valid Signatures</title>
831 An implementation must not send or accept invalid signatures.
832 Valid signatures will conform to the following rules:
836 The signature ends with a nul byte.
841 The signature is a list of single complete types.
842 Arrays must have element types, and structs must
843 have both open and close parentheses.
848 Only type codes and open and close parentheses are
849 allowed in the signature. The <literal>STRUCT</literal> type code
850 is not allowed in signatures, because parentheses
856 The maximum depth of container type nesting is 32 array type
857 codes and 32 open parentheses. This implies that the maximum
858 total depth of recursion is 64, for an "array of array of array
859 of ... struct of struct of struct of ..." where there are 32
865 The maximum length of a signature is 255.
870 Signatures must be nul-terminated.
881 <sect1 id="message-protocol">
882 <title>Message Protocol</title>
885 A <firstterm>message</firstterm> consists of a
886 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
887 think of a message as a package, the header is the address, and the body
888 contains the package contents. The message delivery system uses the header
889 information to figure out where to send the message and how to interpret
890 it; the recipient interprets the body of the message.
894 The body of the message is made up of zero or more
895 <firstterm>arguments</firstterm>, which are typed values, such as an
896 integer or a byte array.
900 Both header and body use the D-Bus <link linkend="type-system">type
901 system</link> and format for serializing data.
904 <sect2 id="message-protocol-messages">
905 <title>Message Format</title>
908 A message consists of a header and a body. The header is a block of
909 values with a fixed signature and meaning. The body is a separate block
910 of values, with a signature specified in the header.
914 The length of the header must be a multiple of 8, allowing the body to
915 begin on an 8-byte boundary when storing the entire message in a single
916 buffer. If the header does not naturally end on an 8-byte boundary
917 up to 7 bytes of nul-initialized alignment padding must be added.
921 The message body need not end on an 8-byte boundary.
925 The maximum length of a message, including header, header alignment padding,
926 and body is 2 to the 27th power or 134217728. Implementations must not
927 send or accept messages exceeding this size.
931 The signature of the header is:
935 Written out more readably, this is:
937 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
942 These values have the following meanings:
948 <entry>Description</entry>
953 <entry>1st <literal>BYTE</literal></entry>
954 <entry>Endianness flag; ASCII 'l' for little-endian
955 or ASCII 'B' for big-endian. Both header and body are
956 in this endianness.</entry>
959 <entry>2nd <literal>BYTE</literal></entry>
960 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
961 Currently-defined types are described below.
965 <entry>3rd <literal>BYTE</literal></entry>
966 <entry>Bitwise OR of flags. Unknown flags
967 must be ignored. Currently-defined flags are described below.
971 <entry>4th <literal>BYTE</literal></entry>
972 <entry>Major protocol version of the sending application. If
973 the major protocol version of the receiving application does not
974 match, the applications will not be able to communicate and the
975 D-Bus connection must be disconnected. The major protocol
976 version for this version of the specification is 1.
980 <entry>1st <literal>UINT32</literal></entry>
981 <entry>Length in bytes of the message body, starting
982 from the end of the header. The header ends after
983 its alignment padding to an 8-boundary.
987 <entry>2nd <literal>UINT32</literal></entry>
988 <entry>The serial of this message, used as a cookie
989 by the sender to identify the reply corresponding
990 to this request. This must not be zero.
994 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
995 <entry>An array of zero or more <firstterm>header
996 fields</firstterm> where the byte is the field code, and the
997 variant is the field value. The message type determines
998 which fields are required.
1006 <firstterm>Message types</firstterm> that can appear in the second byte
1012 <entry>Conventional name</entry>
1013 <entry>Decimal value</entry>
1014 <entry>Description</entry>
1019 <entry><literal>INVALID</literal></entry>
1021 <entry>This is an invalid type.</entry>
1024 <entry><literal>METHOD_CALL</literal></entry>
1026 <entry>Method call.</entry>
1029 <entry><literal>METHOD_RETURN</literal></entry>
1031 <entry>Method reply with returned data.</entry>
1034 <entry><literal>ERROR</literal></entry>
1036 <entry>Error reply. If the first argument exists and is a
1037 string, it is an error message.</entry>
1040 <entry><literal>SIGNAL</literal></entry>
1042 <entry>Signal emission.</entry>
1049 Flags that can appear in the third byte of the header:
1054 <entry>Conventional name</entry>
1055 <entry>Hex value</entry>
1056 <entry>Description</entry>
1061 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
1063 <entry>This message does not expect method return replies or
1064 error replies; the reply can be omitted as an
1065 optimization. However, it is compliant with this specification
1066 to return the reply despite this flag and the only harm
1067 from doing so is extra network traffic.
1071 <entry><literal>NO_AUTO_START</literal></entry>
1073 <entry>The bus must not launch an owner
1074 for the destination name in response to this message.
1082 <sect3 id="message-protocol-header-fields">
1083 <title>Header Fields</title>
1086 The array at the end of the header contains <firstterm>header
1087 fields</firstterm>, where each field is a 1-byte field code followed
1088 by a field value. A header must contain the required header fields for
1089 its message type, and zero or more of any optional header
1090 fields. Future versions of this protocol specification may add new
1091 fields. Implementations must ignore fields they do not
1092 understand. Implementations must not invent their own header fields;
1093 only changes to this specification may introduce new header fields.
1097 Again, if an implementation sees a header field code that it does not
1098 expect, it must ignore that field, as it will be part of a new
1099 (but compatible) version of this specification. This also applies
1100 to known header fields appearing in unexpected messages, for
1101 example: if a signal has a reply serial it must be ignored
1102 even though it has no meaning as of this version of the spec.
1106 However, implementations must not send or accept known header fields
1107 with the wrong type stored in the field value. So for example a
1108 message with an <literal>INTERFACE</literal> field of type
1109 <literal>UINT32</literal> would be considered corrupt.
1113 Here are the currently-defined header fields:
1118 <entry>Conventional Name</entry>
1119 <entry>Decimal Code</entry>
1121 <entry>Required In</entry>
1122 <entry>Description</entry>
1127 <entry><literal>INVALID</literal></entry>
1130 <entry>not allowed</entry>
1131 <entry>Not a valid field name (error if it appears in a message)</entry>
1134 <entry><literal>PATH</literal></entry>
1136 <entry><literal>OBJECT_PATH</literal></entry>
1137 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1138 <entry>The object to send a call to,
1139 or the object a signal is emitted from.
1141 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
1142 implementations should not send messages with this path,
1143 and the reference implementation of the bus daemon will
1144 disconnect any application that attempts to do so.
1148 <entry><literal>INTERFACE</literal></entry>
1150 <entry><literal>STRING</literal></entry>
1151 <entry><literal>SIGNAL</literal></entry>
1153 The interface to invoke a method call on, or
1154 that a signal is emitted from. Optional for
1155 method calls, required for signals.
1156 The special interface
1157 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1158 implementations should not send messages with this
1159 interface, and the reference implementation of the bus
1160 daemon will disconnect any application that attempts to
1165 <entry><literal>MEMBER</literal></entry>
1167 <entry><literal>STRING</literal></entry>
1168 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1169 <entry>The member, either the method name or signal name.</entry>
1172 <entry><literal>ERROR_NAME</literal></entry>
1174 <entry><literal>STRING</literal></entry>
1175 <entry><literal>ERROR</literal></entry>
1176 <entry>The name of the error that occurred, for errors</entry>
1179 <entry><literal>REPLY_SERIAL</literal></entry>
1181 <entry><literal>UINT32</literal></entry>
1182 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1183 <entry>The serial number of the message this message is a reply
1184 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1187 <entry><literal>DESTINATION</literal></entry>
1189 <entry><literal>STRING</literal></entry>
1190 <entry>optional</entry>
1191 <entry>The name of the connection this message is intended for.
1192 Only used in combination with the message bus, see
1193 <xref linkend="message-bus"/>.</entry>
1196 <entry><literal>SENDER</literal></entry>
1198 <entry><literal>STRING</literal></entry>
1199 <entry>optional</entry>
1200 <entry>Unique name of the sending connection.
1201 The message bus fills in this field so it is reliable; the field is
1202 only meaningful in combination with the message bus.</entry>
1205 <entry><literal>SIGNATURE</literal></entry>
1207 <entry><literal>SIGNATURE</literal></entry>
1208 <entry>optional</entry>
1209 <entry>The signature of the message body.
1210 If omitted, it is assumed to be the
1211 empty signature "" (i.e. the body must be 0-length).</entry>
1214 <entry><literal>UNIX_FDS</literal></entry>
1216 <entry><literal>UINT32</literal></entry>
1217 <entry>optional</entry>
1218 <entry>The number of Unix file descriptors that
1219 accompany the message. If omitted, it is assumed
1220 that no Unix file descriptors accompany the
1221 message. The actual file descriptors need to be
1222 transferred via platform specific mechanism
1223 out-of-band. They must be sent at the same time as
1224 part of the message itself. They may not be sent
1225 before the first byte of the message itself is
1226 transferred or after the last byte of the message
1236 <sect2 id="message-protocol-names">
1237 <title>Valid Names</title>
1239 The various names in D-Bus messages have some restrictions.
1242 There is a <firstterm>maximum name length</firstterm>
1243 of 255 which applies to bus names, interfaces, and members.
1245 <sect3 id="message-protocol-names-interface">
1246 <title>Interface names</title>
1248 Interfaces have names with type <literal>STRING</literal>, meaning that
1249 they must be valid UTF-8. However, there are also some
1250 additional restrictions that apply to interface names
1253 <listitem><para>Interface names are composed of 1 or more elements separated by
1254 a period ('.') character. All elements must contain at least
1258 <listitem><para>Each element must only contain the ASCII characters
1259 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1263 <listitem><para>Interface names must contain at least one '.' (period)
1264 character (and thus at least two elements).
1267 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1268 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1273 Interface names should start with the reversed DNS domain name of
1274 the author of the interface (in lower-case), like interface names
1275 in Java. It is conventional for the rest of the interface name
1276 to consist of words run together, with initial capital letters
1277 on all words ("CamelCase"). Several levels of hierarchy can be used.
1278 It is also a good idea to include the major version of the interface
1279 in the name, and increment it if incompatible changes are made;
1280 this way, a single object can implement several versions of an
1281 interface in parallel, if necessary.
1285 For instance, if the owner of <literal>example.com</literal> is
1286 developing a D-Bus API for a music player, they might define
1287 interfaces called <literal>com.example.MusicPlayer1</literal>,
1288 <literal>com.example.MusicPlayer1.Track</literal> and
1289 <literal>com.example.MusicPlayer1.Seekable</literal>.
1293 D-Bus does not distinguish between the concepts that would be
1294 called classes and interfaces in Java: either can be identified on
1295 D-Bus by an interface name.
1298 <sect3 id="message-protocol-names-bus">
1299 <title>Bus names</title>
1301 Connections have one or more bus names associated with them.
1302 A connection has exactly one bus name that is a <firstterm>unique
1303 connection name</firstterm>. The unique connection name remains
1304 with the connection for its entire lifetime.
1305 A bus name is of type <literal>STRING</literal>,
1306 meaning that it must be valid UTF-8. However, there are also
1307 some additional restrictions that apply to bus names
1310 <listitem><para>Bus names that start with a colon (':')
1311 character are unique connection names. Other bus names
1312 are called <firstterm>well-known bus names</firstterm>.
1315 <listitem><para>Bus names are composed of 1 or more elements separated by
1316 a period ('.') character. All elements must contain at least
1320 <listitem><para>Each element must only contain the ASCII characters
1321 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1322 connection name may begin with a digit, elements in
1323 other bus names must not begin with a digit.
1327 <listitem><para>Bus names must contain at least one '.' (period)
1328 character (and thus at least two elements).
1331 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1332 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1336 Note that the hyphen ('-') character is allowed in bus names but
1337 not in interface names.
1341 Like <link linkend="message-protocol-names-interface">interface
1342 names</link>, well-known bus names should start with the
1343 reversed DNS domain name of the author of the interface (in
1344 lower-case), and it is conventional for the rest of the well-known
1345 bus name to consist of words run together, with initial
1346 capital letters. As with interface names, including a version
1347 number in well-known bus names is a good idea; it's possible to
1348 have the well-known bus name for more than one version
1349 simultaneously if backwards compatibility is required.
1353 If a well-known bus name implies the presence of a "main" interface,
1354 that "main" interface is often given the same name as
1355 the well-known bus name, and situated at the corresponding object
1356 path. For instance, if the owner of <literal>example.com</literal>
1357 is developing a D-Bus API for a music player, they might define
1358 that any application that takes the well-known name
1359 <literal>com.example.MusicPlayer1</literal> should have an object
1360 at the object path <literal>/com/example/MusicPlayer1</literal>
1361 which implements the interface
1362 <literal>com.example.MusicPlayer1</literal>.
1365 <sect3 id="message-protocol-names-member">
1366 <title>Member names</title>
1368 Member (i.e. method or signal) names:
1370 <listitem><para>Must only contain the ASCII characters
1371 "[A-Z][a-z][0-9]_" and may not begin with a
1372 digit.</para></listitem>
1373 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1374 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1375 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1380 It is conventional for member names on D-Bus to consist of
1381 capitalized words with no punctuation ("camel-case").
1382 Method names should usually be verbs, such as
1383 <literal>GetItems</literal>, and signal names should usually be
1384 a description of an event, such as <literal>ItemsChanged</literal>.
1387 <sect3 id="message-protocol-names-error">
1388 <title>Error names</title>
1390 Error names have the same restrictions as interface names.
1394 Error names have the same naming conventions as interface
1395 names, and often contain <literal>.Error.</literal>; for instance,
1396 the owner of <literal>example.com</literal> might define the
1397 errors <literal>com.example.MusicPlayer.Error.FileNotFound</literal>
1398 and <literal>com.example.MusicPlayer.Error.OutOfMemory</literal>.
1399 The errors defined by D-Bus itself, such as
1400 <literal>org.freedesktop.DBus.Error.Failed</literal>, follow a
1406 <sect2 id="message-protocol-types">
1407 <title>Message Types</title>
1409 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1410 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1411 This section describes these conventions.
1413 <sect3 id="message-protocol-types-method">
1414 <title>Method Calls</title>
1416 Some messages invoke an operation on a remote object. These are
1417 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1418 messages map naturally to methods on objects in a typical program.
1421 A method call message is required to have a <literal>MEMBER</literal> header field
1422 indicating the name of the method. Optionally, the message has an
1423 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1424 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1425 a method with the same name, it is undefined which of the two methods
1426 will be invoked. Implementations may also choose to return an error in
1427 this ambiguous case. However, if a method name is unique
1428 implementations must not require an interface field.
1431 Method call messages also include a <literal>PATH</literal> field
1432 indicating the object to invoke the method on. If the call is passing
1433 through a message bus, the message will also have a
1434 <literal>DESTINATION</literal> field giving the name of the connection
1435 to receive the message.
1438 When an application handles a method call message, it is required to
1439 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1440 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1441 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1444 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1445 are the return value(s) or "out parameters" of the method call.
1446 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1447 and the call fails; no return value will be provided. It makes
1448 no sense to send multiple replies to the same method call.
1451 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1452 reply is required, so the caller will know the method
1453 was successfully processed.
1456 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1460 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1461 then as an optimization the application receiving the method
1462 call may choose to omit the reply message (regardless of
1463 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1464 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1465 flag and reply anyway.
1468 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1469 destination name does not exist then a program to own the destination
1470 name will be started before the message is delivered. The message
1471 will be held until the new program is successfully started or has
1472 failed to start; in case of failure, an error will be returned. This
1473 flag is only relevant in the context of a message bus, it is ignored
1474 during one-to-one communication with no intermediate bus.
1476 <sect4 id="message-protocol-types-method-apis">
1477 <title>Mapping method calls to native APIs</title>
1479 APIs for D-Bus may map method calls to a method call in a specific
1480 programming language, such as C++, or may map a method call written
1481 in an IDL to a D-Bus message.
1484 In APIs of this nature, arguments to a method are often termed "in"
1485 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1486 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1487 "inout" arguments, which are both sent and received, i.e. the caller
1488 passes in a value which is modified. Mapped to D-Bus, an "inout"
1489 argument is equivalent to an "in" argument, followed by an "out"
1490 argument. You can't pass things "by reference" over the wire, so
1491 "inout" is purely an illusion of the in-process API.
1494 Given a method with zero or one return values, followed by zero or more
1495 arguments, where each argument may be "in", "out", or "inout", the
1496 caller constructs a message by appending each "in" or "inout" argument,
1497 in order. "out" arguments are not represented in the caller's message.
1500 The recipient constructs a reply by appending first the return value
1501 if any, then each "out" or "inout" argument, in order.
1502 "in" arguments are not represented in the reply message.
1505 Error replies are normally mapped to exceptions in languages that have
1509 In converting from native APIs to D-Bus, it is perhaps nice to
1510 map D-Bus naming conventions ("FooBar") to native conventions
1511 such as "fooBar" or "foo_bar" automatically. This is OK
1512 as long as you can say that the native API is one that
1513 was specifically written for D-Bus. It makes the most sense
1514 when writing object implementations that will be exported
1515 over the bus. Object proxies used to invoke remote D-Bus
1516 objects probably need the ability to call any D-Bus method,
1517 and thus a magic name mapping like this could be a problem.
1520 This specification doesn't require anything of native API bindings;
1521 the preceding is only a suggested convention for consistency
1527 <sect3 id="message-protocol-types-signal">
1528 <title>Signal Emission</title>
1530 Unlike method calls, signal emissions have no replies.
1531 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1532 It must have three header fields: <literal>PATH</literal> giving the object
1533 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1534 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1535 for signals, though it is optional for method calls.
1539 <sect3 id="message-protocol-types-errors">
1540 <title>Errors</title>
1542 Messages of type <literal>ERROR</literal> are most commonly replies
1543 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1544 to any kind of message. The message bus for example
1545 will return an <literal>ERROR</literal> in reply to a signal emission if
1546 the bus does not have enough memory to send the signal.
1549 An <literal>ERROR</literal> may have any arguments, but if the first
1550 argument is a <literal>STRING</literal>, it must be an error message.
1551 The error message may be logged or shown to the user
1556 <sect3 id="message-protocol-types-notation">
1557 <title>Notation in this document</title>
1559 This document uses a simple pseudo-IDL to describe particular method
1560 calls and signals. Here is an example of a method call:
1562 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1563 out UINT32 resultcode)
1565 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1566 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1567 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1568 characters so it's known that the last part of the name in
1569 the "IDL" is the member name.
1572 In C++ that might end up looking like this:
1574 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1575 unsigned int flags);
1577 or equally valid, the return value could be done as an argument:
1579 void org::freedesktop::DBus::StartServiceByName (const char *name,
1581 unsigned int *resultcode);
1583 It's really up to the API designer how they want to make
1584 this look. You could design an API where the namespace wasn't used
1585 in C++, using STL or Qt, using varargs, or whatever you wanted.
1588 Signals are written as follows:
1590 org.freedesktop.DBus.NameLost (STRING name)
1592 Signals don't specify "in" vs. "out" because only
1593 a single direction is possible.
1596 It isn't especially encouraged to use this lame pseudo-IDL in actual
1597 API implementations; you might use the native notation for the
1598 language you're using, or you might use COM or CORBA IDL, for example.
1603 <sect2 id="message-protocol-handling-invalid">
1604 <title>Invalid Protocol and Spec Extensions</title>
1607 For security reasons, the D-Bus protocol should be strictly parsed and
1608 validated, with the exception of defined extension points. Any invalid
1609 protocol or spec violations should result in immediately dropping the
1610 connection without notice to the other end. Exceptions should be
1611 carefully considered, e.g. an exception may be warranted for a
1612 well-understood idiosyncrasy of a widely-deployed implementation. In
1613 cases where the other end of a connection is 100% trusted and known to
1614 be friendly, skipping validation for performance reasons could also make
1615 sense in certain cases.
1619 Generally speaking violations of the "must" requirements in this spec
1620 should be considered possible attempts to exploit security, and violations
1621 of the "should" suggestions should be considered legitimate (though perhaps
1622 they should generate an error in some cases).
1626 The following extension points are built in to D-Bus on purpose and must
1627 not be treated as invalid protocol. The extension points are intended
1628 for use by future versions of this spec, they are not intended for third
1629 parties. At the moment, the only way a third party could extend D-Bus
1630 without breaking interoperability would be to introduce a way to negotiate new
1631 feature support as part of the auth protocol, using EXTENSION_-prefixed
1632 commands. There is not yet a standard way to negotiate features.
1636 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1637 commands result in an ERROR rather than a disconnect. This enables
1638 future extensions to the protocol. Commands starting with EXTENSION_ are
1639 reserved for third parties.
1644 The authentication protocol supports pluggable auth mechanisms.
1649 The address format (see <xref linkend="addresses"/>) supports new
1655 Messages with an unknown type (something other than
1656 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1657 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1658 Unknown-type messages must still be well-formed in the same way
1659 as the known messages, however. They still have the normal
1665 Header fields with an unknown or unexpected field code must be ignored,
1666 though again they must still be well-formed.
1671 New standard interfaces (with new methods and signals) can of course be added.
1681 <sect1 id="auth-protocol">
1682 <title>Authentication Protocol</title>
1684 Before the flow of messages begins, two applications must
1685 authenticate. A simple plain-text protocol is used for
1686 authentication; this protocol is a SASL profile, and maps fairly
1687 directly from the SASL specification. The message encoding is
1688 NOT used here, only plain text messages.
1691 In examples, "C:" and "S:" indicate lines sent by the client and
1692 server respectively.
1694 <sect2 id="auth-protocol-overview">
1695 <title>Protocol Overview</title>
1697 The protocol is a line-based protocol, where each line ends with
1698 \r\n. Each line begins with an all-caps ASCII command name containing
1699 only the character range [A-Z_], a space, then any arguments for the
1700 command, then the \r\n ending the line. The protocol is
1701 case-sensitive. All bytes must be in the ASCII character set.
1703 Commands from the client to the server are as follows:
1706 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1707 <listitem><para>CANCEL</para></listitem>
1708 <listitem><para>BEGIN</para></listitem>
1709 <listitem><para>DATA <data in hex encoding></para></listitem>
1710 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1711 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1714 From server to client are as follows:
1717 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1718 <listitem><para>OK <GUID in hex></para></listitem>
1719 <listitem><para>DATA <data in hex encoding></para></listitem>
1720 <listitem><para>ERROR</para></listitem>
1721 <listitem><para>AGREE_UNIX_FD</para></listitem>
1725 Unofficial extensions to the command set must begin with the letters
1726 "EXTENSION_", to avoid conflicts with future official commands.
1727 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1730 <sect2 id="auth-nul-byte">
1731 <title>Special credentials-passing nul byte</title>
1733 Immediately after connecting to the server, the client must send a
1734 single nul byte. This byte may be accompanied by credentials
1735 information on some operating systems that use sendmsg() with
1736 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1737 sockets. However, the nul byte must be sent even on other kinds of
1738 socket, and even on operating systems that do not require a byte to be
1739 sent in order to transmit credentials. The text protocol described in
1740 this document begins after the single nul byte. If the first byte
1741 received from the client is not a nul byte, the server may disconnect
1745 A nul byte in any context other than the initial byte is an error;
1746 the protocol is ASCII-only.
1749 The credentials sent along with the nul byte may be used with the
1750 SASL mechanism EXTERNAL.
1753 <sect2 id="auth-command-auth">
1754 <title>AUTH command</title>
1756 If an AUTH command has no arguments, it is a request to list
1757 available mechanisms. The server must respond with a REJECTED
1758 command listing the mechanisms it understands, or with an error.
1761 If an AUTH command specifies a mechanism, and the server supports
1762 said mechanism, the server should begin exchanging SASL
1763 challenge-response data with the client using DATA commands.
1766 If the server does not support the mechanism given in the AUTH
1767 command, it must send either a REJECTED command listing the mechanisms
1768 it does support, or an error.
1771 If the [initial-response] argument is provided, it is intended for use
1772 with mechanisms that have no initial challenge (or an empty initial
1773 challenge), as if it were the argument to an initial DATA command. If
1774 the selected mechanism has an initial challenge and [initial-response]
1775 was provided, the server should reject authentication by sending
1779 If authentication succeeds after exchanging DATA commands,
1780 an OK command must be sent to the client.
1783 The first octet received by the server after the \r\n of the BEGIN
1784 command from the client must be the first octet of the
1785 authenticated/encrypted stream of D-Bus messages.
1788 If BEGIN is received by the server, the first octet received
1789 by the client after the \r\n of the OK command must be the
1790 first octet of the authenticated/encrypted stream of D-Bus
1794 <sect2 id="auth-command-cancel">
1795 <title>CANCEL Command</title>
1797 At any time up to sending the BEGIN command, the client may send a
1798 CANCEL command. On receiving the CANCEL command, the server must
1799 send a REJECTED command and abort the current authentication
1803 <sect2 id="auth-command-data">
1804 <title>DATA Command</title>
1806 The DATA command may come from either client or server, and simply
1807 contains a hex-encoded block of data to be interpreted
1808 according to the SASL mechanism in use.
1811 Some SASL mechanisms support sending an "empty string";
1812 FIXME we need some way to do this.
1815 <sect2 id="auth-command-begin">
1816 <title>BEGIN Command</title>
1818 The BEGIN command acknowledges that the client has received an
1819 OK command from the server, and that the stream of messages
1823 The first octet received by the server after the \r\n of the BEGIN
1824 command from the client must be the first octet of the
1825 authenticated/encrypted stream of D-Bus messages.
1828 <sect2 id="auth-command-rejected">
1829 <title>REJECTED Command</title>
1831 The REJECTED command indicates that the current authentication
1832 exchange has failed, and further exchange of DATA is inappropriate.
1833 The client would normally try another mechanism, or try providing
1834 different responses to challenges.
1836 Optionally, the REJECTED command has a space-separated list of
1837 available auth mechanisms as arguments. If a server ever provides
1838 a list of supported mechanisms, it must provide the same list
1839 each time it sends a REJECTED message. Clients are free to
1840 ignore all lists received after the first.
1843 <sect2 id="auth-command-ok">
1844 <title>OK Command</title>
1846 The OK command indicates that the client has been
1847 authenticated. The client may now proceed with negotiating
1848 Unix file descriptor passing. To do that it shall send
1849 NEGOTIATE_UNIX_FD to the server.
1852 Otherwise, the client must respond to the OK command by
1853 sending a BEGIN command, followed by its stream of messages,
1854 or by disconnecting. The server must not accept additional
1855 commands using this protocol after the BEGIN command has been
1856 received. Further communication will be a stream of D-Bus
1857 messages (optionally encrypted, as negotiated) rather than
1861 If a client sends BEGIN the first octet received by the client
1862 after the \r\n of the OK command must be the first octet of
1863 the authenticated/encrypted stream of D-Bus messages.
1866 The OK command has one argument, which is the GUID of the server.
1867 See <xref linkend="addresses"/> for more on server GUIDs.
1870 <sect2 id="auth-command-error">
1871 <title>ERROR Command</title>
1873 The ERROR command indicates that either server or client did not
1874 know a command, does not accept the given command in the current
1875 context, or did not understand the arguments to the command. This
1876 allows the protocol to be extended; a client or server can send a
1877 command present or permitted only in new protocol versions, and if
1878 an ERROR is received instead of an appropriate response, fall back
1879 to using some other technique.
1882 If an ERROR is sent, the server or client that sent the
1883 error must continue as if the command causing the ERROR had never been
1884 received. However, the the server or client receiving the error
1885 should try something other than whatever caused the error;
1886 if only canceling/rejecting the authentication.
1889 If the D-Bus protocol changes incompatibly at some future time,
1890 applications implementing the new protocol would probably be able to
1891 check for support of the new protocol by sending a new command and
1892 receiving an ERROR from applications that don't understand it. Thus the
1893 ERROR feature of the auth protocol is an escape hatch that lets us
1894 negotiate extensions or changes to the D-Bus protocol in the future.
1897 <sect2 id="auth-command-negotiate-unix-fd">
1898 <title>NEGOTIATE_UNIX_FD Command</title>
1900 The NEGOTIATE_UNIX_FD command indicates that the client
1901 supports Unix file descriptor passing. This command may only
1902 be sent after the connection is authenticated, i.e. after OK
1903 was received by the client. This command may only be sent on
1904 transports that support Unix file descriptor passing.
1907 On receiving NEGOTIATE_UNIX_FD the server must respond with
1908 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1909 the transport chosen supports Unix file descriptor passing and
1910 the server supports this feature. It shall respond the latter
1911 if the transport does not support Unix file descriptor
1912 passing, the server does not support this feature, or the
1913 server decides not to enable file descriptor passing due to
1914 security or other reasons.
1917 <sect2 id="auth-command-agree-unix-fd">
1918 <title>AGREE_UNIX_FD Command</title>
1920 The AGREE_UNIX_FD command indicates that the server supports
1921 Unix file descriptor passing. This command may only be sent
1922 after the connection is authenticated, and the client sent
1923 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1924 command may only be sent on transports that support Unix file
1928 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1929 followed by its stream of messages, or by disconnecting. The
1930 server must not accept additional commands using this protocol
1931 after the BEGIN command has been received. Further
1932 communication will be a stream of D-Bus messages (optionally
1933 encrypted, as negotiated) rather than this protocol.
1936 <sect2 id="auth-command-future">
1937 <title>Future Extensions</title>
1939 Future extensions to the authentication and negotiation
1940 protocol are possible. For that new commands may be
1941 introduced. If a client or server receives an unknown command
1942 it shall respond with ERROR and not consider this fatal. New
1943 commands may be introduced both before, and after
1944 authentication, i.e. both before and after the OK command.
1947 <sect2 id="auth-examples">
1948 <title>Authentication examples</title>
1952 <title>Example of successful magic cookie authentication</title>
1954 (MAGIC_COOKIE is a made up mechanism)
1956 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1962 <title>Example of finding out mechanisms then picking one</title>
1965 S: REJECTED KERBEROS_V4 SKEY
1966 C: AUTH SKEY 7ab83f32ee
1967 S: DATA 8799cabb2ea93e
1968 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1974 <title>Example of client sends unknown command then falls back to regular auth</title>
1978 C: AUTH MAGIC_COOKIE 3736343435313230333039
1984 <title>Example of server doesn't support initial auth mechanism</title>
1986 C: AUTH MAGIC_COOKIE 3736343435313230333039
1987 S: REJECTED KERBEROS_V4 SKEY
1988 C: AUTH SKEY 7ab83f32ee
1989 S: DATA 8799cabb2ea93e
1990 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1996 <title>Example of wrong password or the like followed by successful retry</title>
1998 C: AUTH MAGIC_COOKIE 3736343435313230333039
1999 S: REJECTED KERBEROS_V4 SKEY
2000 C: AUTH SKEY 7ab83f32ee
2001 S: DATA 8799cabb2ea93e
2002 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2004 C: AUTH SKEY 7ab83f32ee
2005 S: DATA 8799cabb2ea93e
2006 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2012 <title>Example of skey cancelled and restarted</title>
2014 C: AUTH MAGIC_COOKIE 3736343435313230333039
2015 S: REJECTED KERBEROS_V4 SKEY
2016 C: AUTH SKEY 7ab83f32ee
2017 S: DATA 8799cabb2ea93e
2020 C: AUTH SKEY 7ab83f32ee
2021 S: DATA 8799cabb2ea93e
2022 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
2028 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
2030 (MAGIC_COOKIE is a made up mechanism)
2032 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2034 C: NEGOTIATE_UNIX_FD
2040 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
2042 (MAGIC_COOKIE is a made up mechanism)
2044 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
2046 C: NEGOTIATE_UNIX_FD
2053 <sect2 id="auth-states">
2054 <title>Authentication state diagrams</title>
2057 This section documents the auth protocol in terms of
2058 a state machine for the client and the server. This is
2059 probably the most robust way to implement the protocol.
2062 <sect3 id="auth-states-client">
2063 <title>Client states</title>
2066 To more precisely describe the interaction between the
2067 protocol state machine and the authentication mechanisms the
2068 following notation is used: MECH(CHALL) means that the
2069 server challenge CHALL was fed to the mechanism MECH, which
2075 CONTINUE(RESP) means continue the auth conversation
2076 and send RESP as the response to the server;
2082 OK(RESP) means that after sending RESP to the server
2083 the client side of the auth conversation is finished
2084 and the server should return "OK";
2090 ERROR means that CHALL was invalid and could not be
2096 Both RESP and CHALL may be empty.
2100 The Client starts by getting an initial response from the
2101 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
2102 the mechanism did not provide an initial response. If the
2103 mechanism returns CONTINUE, the client starts in state
2104 <emphasis>WaitingForData</emphasis>, if the mechanism
2105 returns OK the client starts in state
2106 <emphasis>WaitingForOK</emphasis>.
2110 The client should keep track of available mechanisms and
2111 which it mechanisms it has already attempted. This list is
2112 used to decide which AUTH command to send. When the list is
2113 exhausted, the client should give up and close the
2118 <title><emphasis>WaitingForData</emphasis></title>
2126 MECH(CHALL) returns CONTINUE(RESP) → send
2128 <emphasis>WaitingForData</emphasis>
2132 MECH(CHALL) returns OK(RESP) → send DATA
2133 RESP, goto <emphasis>WaitingForOK</emphasis>
2137 MECH(CHALL) returns ERROR → send ERROR
2138 [msg], goto <emphasis>WaitingForData</emphasis>
2146 Receive REJECTED [mechs] →
2147 send AUTH [next mech], goto
2148 WaitingForData or <emphasis>WaitingForOK</emphasis>
2153 Receive ERROR → send
2155 <emphasis>WaitingForReject</emphasis>
2160 Receive OK → send
2161 BEGIN, terminate auth
2162 conversation, authenticated
2167 Receive anything else → send
2169 <emphasis>WaitingForData</emphasis>
2177 <title><emphasis>WaitingForOK</emphasis></title>
2182 Receive OK → send BEGIN, terminate auth
2183 conversation, <emphasis>authenticated</emphasis>
2188 Receive REJECT [mechs] → send AUTH [next mech],
2189 goto <emphasis>WaitingForData</emphasis> or
2190 <emphasis>WaitingForOK</emphasis>
2196 Receive DATA → send CANCEL, goto
2197 <emphasis>WaitingForReject</emphasis>
2203 Receive ERROR → send CANCEL, goto
2204 <emphasis>WaitingForReject</emphasis>
2210 Receive anything else → send ERROR, goto
2211 <emphasis>WaitingForOK</emphasis>
2219 <title><emphasis>WaitingForReject</emphasis></title>
2224 Receive REJECT [mechs] → send AUTH [next mech],
2225 goto <emphasis>WaitingForData</emphasis> or
2226 <emphasis>WaitingForOK</emphasis>
2232 Receive anything else → terminate auth
2233 conversation, disconnect
2242 <sect3 id="auth-states-server">
2243 <title>Server states</title>
2246 For the server MECH(RESP) means that the client response
2247 RESP was fed to the the mechanism MECH, which returns one of
2252 CONTINUE(CHALL) means continue the auth conversation and
2253 send CHALL as the challenge to the client;
2259 OK means that the client has been successfully
2266 REJECT means that the client failed to authenticate or
2267 there was an error in RESP.
2272 The server starts out in state
2273 <emphasis>WaitingForAuth</emphasis>. If the client is
2274 rejected too many times the server must disconnect the
2279 <title><emphasis>WaitingForAuth</emphasis></title>
2285 Receive AUTH → send REJECTED [mechs], goto
2286 <emphasis>WaitingForAuth</emphasis>
2292 Receive AUTH MECH RESP
2296 MECH not valid mechanism → send REJECTED
2298 <emphasis>WaitingForAuth</emphasis>
2302 MECH(RESP) returns CONTINUE(CHALL) → send
2304 <emphasis>WaitingForData</emphasis>
2308 MECH(RESP) returns OK → send OK, goto
2309 <emphasis>WaitingForBegin</emphasis>
2313 MECH(RESP) returns REJECT → send REJECTED
2315 <emphasis>WaitingForAuth</emphasis>
2323 Receive BEGIN → terminate
2324 auth conversation, disconnect
2330 Receive ERROR → send REJECTED [mechs], goto
2331 <emphasis>WaitingForAuth</emphasis>
2337 Receive anything else → send
2339 <emphasis>WaitingForAuth</emphasis>
2348 <title><emphasis>WaitingForData</emphasis></title>
2356 MECH(RESP) returns CONTINUE(CHALL) → send
2358 <emphasis>WaitingForData</emphasis>
2362 MECH(RESP) returns OK → send OK, goto
2363 <emphasis>WaitingForBegin</emphasis>
2367 MECH(RESP) returns REJECT → send REJECTED
2369 <emphasis>WaitingForAuth</emphasis>
2377 Receive BEGIN → terminate auth conversation,
2384 Receive CANCEL → send REJECTED [mechs], goto
2385 <emphasis>WaitingForAuth</emphasis>
2391 Receive ERROR → send REJECTED [mechs], goto
2392 <emphasis>WaitingForAuth</emphasis>
2398 Receive anything else → send ERROR, goto
2399 <emphasis>WaitingForData</emphasis>
2407 <title><emphasis>WaitingForBegin</emphasis></title>
2412 Receive BEGIN → terminate auth conversation,
2413 client authenticated
2419 Receive CANCEL → send REJECTED [mechs], goto
2420 <emphasis>WaitingForAuth</emphasis>
2426 Receive ERROR → send REJECTED [mechs], goto
2427 <emphasis>WaitingForAuth</emphasis>
2433 Receive anything else → send ERROR, goto
2434 <emphasis>WaitingForBegin</emphasis>
2444 <sect2 id="auth-mechanisms">
2445 <title>Authentication mechanisms</title>
2447 This section describes some new authentication mechanisms.
2448 D-Bus also allows any standard SASL mechanism of course.
2450 <sect3 id="auth-mechanisms-sha">
2451 <title>DBUS_COOKIE_SHA1</title>
2453 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2454 has the ability to read a private file owned by the user being
2455 authenticated. If the client can prove that it has access to a secret
2456 cookie stored in this file, then the client is authenticated.
2457 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2461 Throughout this description, "hex encoding" must output the digits
2462 from a to f in lower-case; the digits A to F must not be used
2463 in the DBUS_COOKIE_SHA1 mechanism.
2466 Authentication proceeds as follows:
2470 The client sends the username it would like to authenticate
2476 The server sends the name of its "cookie context" (see below); a
2477 space character; the integer ID of the secret cookie the client
2478 must demonstrate knowledge of; a space character; then a
2479 randomly-generated challenge string, all of this hex-encoded into
2485 The client locates the cookie and generates its own
2486 randomly-generated challenge string. The client then concatenates
2487 the server's decoded challenge, a ":" character, its own challenge,
2488 another ":" character, and the cookie. It computes the SHA-1 hash
2489 of this composite string as a hex digest. It concatenates the
2490 client's challenge string, a space character, and the SHA-1 hex
2491 digest, hex-encodes the result and sends it back to the server.
2496 The server generates the same concatenated string used by the
2497 client and computes its SHA-1 hash. It compares the hash with
2498 the hash received from the client; if the two hashes match, the
2499 client is authenticated.
2505 Each server has a "cookie context," which is a name that identifies a
2506 set of cookies that apply to that server. A sample context might be
2507 "org_freedesktop_session_bus". Context names must be valid ASCII,
2508 nonzero length, and may not contain the characters slash ("/"),
2509 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2510 tab ("\t"), or period ("."). There is a default context,
2511 "org_freedesktop_general" that's used by servers that do not specify
2515 Cookies are stored in a user's home directory, in the directory
2516 <filename>~/.dbus-keyrings/</filename>. This directory must
2517 not be readable or writable by other users. If it is,
2518 clients and servers must ignore it. The directory
2519 contains cookie files named after the cookie context.
2522 A cookie file contains one cookie per line. Each line
2523 has three space-separated fields:
2527 The cookie ID number, which must be a non-negative integer and
2528 may not be used twice in the same file.
2533 The cookie's creation time, in UNIX seconds-since-the-epoch
2539 The cookie itself, a hex-encoded random block of bytes. The cookie
2540 may be of any length, though obviously security increases
2541 as the length increases.
2547 Only server processes modify the cookie file.
2548 They must do so with this procedure:
2552 Create a lockfile name by appending ".lock" to the name of the
2553 cookie file. The server should attempt to create this file
2554 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2555 fails, the lock fails. Servers should retry for a reasonable
2556 period of time, then they may choose to delete an existing lock
2557 to keep users from having to manually delete a stale
2558 lock. <footnote><para>Lockfiles are used instead of real file
2559 locking <literal>fcntl()</literal> because real locking
2560 implementations are still flaky on network
2561 filesystems.</para></footnote>
2566 Once the lockfile has been created, the server loads the cookie
2567 file. It should then delete any cookies that are old (the
2568 timeout can be fairly short), or more than a reasonable
2569 time in the future (so that cookies never accidentally
2570 become permanent, if the clock was set far into the future
2571 at some point). If no recent keys remain, the
2572 server may generate a new key.
2577 The pruned and possibly added-to cookie file
2578 must be resaved atomically (using a temporary
2579 file which is rename()'d).
2584 The lock must be dropped by deleting the lockfile.
2590 Clients need not lock the file in order to load it,
2591 because servers are required to save the file atomically.
2596 <sect1 id="addresses">
2597 <title>Server Addresses</title>
2599 Server addresses consist of a transport name followed by a colon, and
2600 then an optional, comma-separated list of keys and values in the form key=value.
2601 Each value is escaped.
2605 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2606 Which is the address to a unix socket with the path /tmp/dbus-test.
2609 Value escaping is similar to URI escaping but simpler.
2613 The set of optionally-escaped bytes is:
2614 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2615 <emphasis>byte</emphasis> (note, not character) which is not in the
2616 set of optionally-escaped bytes must be replaced with an ASCII
2617 percent (<literal>%</literal>) and the value of the byte in hex.
2618 The hex value must always be two digits, even if the first digit is
2619 zero. The optionally-escaped bytes may be escaped if desired.
2624 To unescape, append each byte in the value; if a byte is an ASCII
2625 percent (<literal>%</literal>) character then append the following
2626 hex value instead. It is an error if a <literal>%</literal> byte
2627 does not have two hex digits following. It is an error if a
2628 non-optionally-escaped byte is seen unescaped.
2632 The set of optionally-escaped bytes is intended to preserve address
2633 readability and convenience.
2637 A server may specify a key-value pair with the key <literal>guid</literal>
2638 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2639 describes the format of the <literal>guid</literal> field. If present,
2640 this UUID may be used to distinguish one server address from another. A
2641 server should use a different UUID for each address it listens on. For
2642 example, if a message bus daemon offers both UNIX domain socket and TCP
2643 connections, but treats clients the same regardless of how they connect,
2644 those two connections are equivalent post-connection but should have
2645 distinct UUIDs to distinguish the kinds of connection.
2649 The intent of the address UUID feature is to allow a client to avoid
2650 opening multiple identical connections to the same server, by allowing the
2651 client to check whether an address corresponds to an already-existing
2652 connection. Comparing two addresses is insufficient, because addresses
2653 can be recycled by distinct servers, and equivalent addresses may look
2654 different if simply compared as strings (for example, the host in a TCP
2655 address can be given as an IP address or as a hostname).
2659 Note that the address key is <literal>guid</literal> even though the
2660 rest of the API and documentation says "UUID," for historical reasons.
2664 [FIXME clarify if attempting to connect to each is a requirement
2665 or just a suggestion]
2666 When connecting to a server, multiple server addresses can be
2667 separated by a semi-colon. The library will then try to connect
2668 to the first address and if that fails, it'll try to connect to
2669 the next one specified, and so forth. For example
2670 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2675 <sect1 id="transports">
2676 <title>Transports</title>
2678 [FIXME we need to specify in detail each transport and its possible arguments]
2680 Current transports include: unix domain sockets (including
2681 abstract namespace on linux), launchd, systemd, TCP/IP, an executed subprocess and a debug/testing transport
2682 using in-process pipes. Future possible transports include one that
2683 tunnels over X11 protocol.
2686 <sect2 id="transports-unix-domain-sockets">
2687 <title>Unix Domain Sockets</title>
2689 Unix domain sockets can be either paths in the file system or on Linux
2690 kernels, they can be abstract which are similar to paths but
2691 do not show up in the file system.
2695 When a socket is opened by the D-Bus library it truncates the path
2696 name right before the first trailing Nul byte. This is true for both
2697 normal paths and abstract paths. Note that this is a departure from
2698 previous versions of D-Bus that would create sockets with a fixed
2699 length path name. Names which were shorter than the fixed length
2700 would be padded by Nul bytes.
2703 Unix domain sockets are not available on Windows.
2705 <sect3 id="transports-unix-domain-sockets-addresses">
2706 <title>Server Address Format</title>
2708 Unix domain socket addresses are identified by the "unix:" prefix
2709 and support the following key/value pairs:
2716 <entry>Values</entry>
2717 <entry>Description</entry>
2723 <entry>(path)</entry>
2724 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2727 <entry>tmpdir</entry>
2728 <entry>(path)</entry>
2729 <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>
2732 <entry>abstract</entry>
2733 <entry>(string)</entry>
2734 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2741 <sect2 id="transports-launchd">
2742 <title>launchd</title>
2744 launchd is an open-source server management system that replaces init, inetd
2745 and cron on Apple Mac OS X versions 10.4 and above. It provides a common session
2746 bus address for each user and deprecates the X11-enabled D-Bus launcher on OSX.
2750 launchd allocates a socket and provides it with the unix path through the
2751 DBUS_LAUNCHD_SESSION_BUS_SOCKET variable in launchd's environment. Every process
2752 spawned by launchd (or dbus-daemon, if it was started by launchd) can access
2753 it through its environment.
2754 Other processes can query for the launchd socket by executing:
2755 $ launchctl getenv DBUS_LAUNCHD_SESSION_BUS_SOCKET
2756 This is normally done by the D-Bus client library so doesn't have to be done
2760 launchd is not available on Microsoft Windows.
2762 <sect3 id="transports-launchd-addresses">
2763 <title>Server Address Format</title>
2765 launchd addresses are identified by the "launchd:" prefix
2766 and support the following key/value pairs:
2773 <entry>Values</entry>
2774 <entry>Description</entry>
2780 <entry>(environment variable)</entry>
2781 <entry>path of the unix domain socket for the launchd created dbus-daemon.</entry>
2788 <sect2 id="transports-systemd">
2789 <title>systemd</title>
2791 systemd is an open-source server management system that
2792 replaces init and inetd on newer Linux systems. It supports
2793 socket activation. The D-Bus systemd transport is used to acquire
2794 socket activation file descriptors from systemd and use them
2795 as D-Bus transport when the current process is spawned by
2796 socket activation from it.
2799 The systemd transport accepts only one or more Unix domain or
2800 TCP streams sockets passed in via socket activation.
2803 The systemd transport is not available on non-Linux operating systems.
2806 The systemd transport defines no parameter keys.
2809 <sect2 id="transports-tcp-sockets">
2810 <title>TCP Sockets</title>
2812 The tcp transport provides TCP/IP based connections between clients
2813 located on the same or different hosts.
2816 Using tcp transport without any additional secure authentification mechanismus
2817 over a network is unsecure.
2820 Windows notes: Because of the tcp stack on Windows does not provide sending
2821 credentials over a tcp connection, the EXTERNAL authentification
2822 mechanismus does not work.
2824 <sect3 id="transports-tcp-sockets-addresses">
2825 <title>Server Address Format</title>
2827 TCP/IP socket addresses are identified by the "tcp:" prefix
2828 and support the following key/value pairs:
2835 <entry>Values</entry>
2836 <entry>Description</entry>
2842 <entry>(string)</entry>
2843 <entry>dns name or ip address</entry>
2847 <entry>(number)</entry>
2848 <entry>The tcp port the server will open. A zero value let the server
2849 choose a free port provided from the underlaying operating system.
2850 libdbus is able to retrieve the real used port from the server.
2854 <entry>family</entry>
2855 <entry>(string)</entry>
2856 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2863 <sect2 id="transports-nonce-tcp-sockets">
2864 <title>Nonce-secured TCP Sockets</title>
2866 The nonce-tcp transport provides a secured TCP transport, using a
2867 simple authentication mechanism to ensure that only clients with read
2868 access to a certain location in the filesystem can connect to the server.
2869 The server writes a secret, the nonce, to a file and an incoming client
2870 connection is only accepted if the client sends the nonce right after
2871 the connect. The nonce mechanism requires no setup and is orthogonal to
2872 the higher-level authentication mechanisms described in the
2873 Authentication section.
2877 On start, the server generates a random 16 byte nonce and writes it
2878 to a file in the user's temporary directory. The nonce file location
2879 is published as part of the server's D-Bus address using the
2880 "noncefile" key-value pair.
2882 After an accept, the server reads 16 bytes from the socket. If the
2883 read bytes do not match the nonce stored in the nonce file, the
2884 server MUST immediately drop the connection.
2885 If the nonce match the received byte sequence, the client is accepted
2886 and the transport behaves like an unsecured tcp transport.
2889 After a successful connect to the server socket, the client MUST read
2890 the nonce from the file published by the server via the noncefile=
2891 key-value pair and send it over the socket. After that, the
2892 transport behaves like an unsecured tcp transport.
2894 <sect3 id="transports-nonce-tcp-sockets-addresses">
2895 <title>Server Address Format</title>
2897 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2898 and support the following key/value pairs:
2905 <entry>Values</entry>
2906 <entry>Description</entry>
2912 <entry>(string)</entry>
2913 <entry>dns name or ip address</entry>
2917 <entry>(number)</entry>
2918 <entry>The tcp port the server will open. A zero value let the server
2919 choose a free port provided from the underlaying operating system.
2920 libdbus is able to retrieve the real used port from the server.
2924 <entry>family</entry>
2925 <entry>(string)</entry>
2926 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2929 <entry>noncefile</entry>
2930 <entry>(path)</entry>
2931 <entry>file location containing the secret</entry>
2938 <sect2 id="transports-exec">
2939 <title>Executed Subprocesses on Unix</title>
2941 This transport forks off a process and connects its standard
2942 input and standard output with an anonymous Unix domain
2943 socket. This socket is then used for communication by the
2944 transport. This transport may be used to use out-of-process
2945 forwarder programs as basis for the D-Bus protocol.
2948 The forked process will inherit the standard error output and
2949 process group from the parent process.
2952 Executed subprocesses are not available on Windows.
2954 <sect3 id="transports-exec-addresses">
2955 <title>Server Address Format</title>
2957 Executed subprocess addresses are identified by the "unixexec:" prefix
2958 and support the following key/value pairs:
2965 <entry>Values</entry>
2966 <entry>Description</entry>
2972 <entry>(path)</entry>
2973 <entry>Path of the binary to execute, either an absolute
2974 path or a binary name that is searched for in the default
2975 search path of the OS. This corresponds to the first
2976 argument of execlp(). This key is mandatory.</entry>
2979 <entry>argv0</entry>
2980 <entry>(string)</entry>
2981 <entry>The program name to use when executing the
2982 binary. If omitted the same value as specified for path=
2983 will be used. This corresponds to the second argument of
2987 <entry>argv1, argv2, ...</entry>
2988 <entry>(string)</entry>
2989 <entry>Arguments to pass to the binary. This corresponds
2990 to the third and later arguments of execlp(). If a
2991 specific argvX is not specified no further argvY for Y > X
2992 are taken into account.</entry>
3000 <sect1 id="meta-transports">
3001 <title>Meta Transports</title>
3003 Meta transports are a kind of transport with special enhancements or
3004 behavior. Currently available meta transports include: autolaunch
3007 <sect2 id="meta-transports-autolaunch">
3008 <title>Autolaunch</title>
3009 <para>The autolaunch transport provides a way for dbus clients to autodetect
3010 a running dbus session bus and to autolaunch a session bus if not present.
3012 <sect3 id="meta-transports-autolaunch-addresses">
3013 <title>Server Address Format</title>
3015 Autolaunch addresses uses the "autolaunch:" prefix and support the
3016 following key/value pairs:
3023 <entry>Values</entry>
3024 <entry>Description</entry>
3029 <entry>scope</entry>
3030 <entry>(string)</entry>
3031 <entry>scope of autolaunch (Windows only)
3035 "*install-path" - limit session bus to dbus installation path.
3036 The dbus installation path is determined from the location of
3037 the shared dbus library. If the library is located in a 'bin'
3038 subdirectory the installation root is the directory above,
3039 otherwise the directory where the library lives is taken as
3042 <install-root>/bin/[lib]dbus-1.dll
3043 <install-root>/[lib]dbus-1.dll
3049 "*user" - limit session bus to the recent user.
3054 other values - specify dedicated session bus like "release",
3066 <sect3 id="meta-transports-autolaunch-windows-implementation">
3067 <title>Windows implementation</title>
3069 On start, the server opens a platform specific transport, creates a mutex
3070 and a shared memory section containing the related session bus address.
3071 This mutex will be inspected by the dbus client library to detect a
3072 running dbus session bus. The access to the mutex and the shared memory
3073 section are protected by global locks.
3076 In the recent implementation the autolaunch transport uses a tcp transport
3077 on localhost with a port choosen from the operating system. This detail may
3078 change in the future.
3081 Disclaimer: The recent implementation is in an early state and may not
3082 work in all cirumstances and/or may have security issues. Because of this
3083 the implementation is not documentated yet.
3090 <title>UUIDs</title>
3092 A working D-Bus implementation uses universally-unique IDs in two places.
3093 First, each server address has a UUID identifying the address,
3094 as described in <xref linkend="addresses"/>. Second, each operating
3095 system kernel instance running a D-Bus client or server has a UUID
3096 identifying that kernel, retrieved by invoking the method
3097 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
3098 linkend="standard-interfaces-peer"/>).
3101 The term "UUID" in this document is intended literally, i.e. an
3102 identifier that is universally unique. It is not intended to refer to
3103 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
3106 The UUID must contain 128 bits of data and be hex-encoded. The
3107 hex-encoded string may not contain hyphens or other non-hex-digit
3108 characters, and it must be exactly 32 characters long. To generate a
3109 UUID, the current reference implementation concatenates 96 bits of random
3110 data followed by the 32-bit time in seconds since the UNIX epoch (in big
3114 It would also be acceptable and probably better to simply generate 128
3115 bits of random data, as long as the random number generator is of high
3116 quality. The timestamp could conceivably help if the random bits are not
3117 very random. With a quality random number generator, collisions are
3118 extremely unlikely even with only 96 bits, so it's somewhat academic.
3121 Implementations should, however, stick to random data for the first 96 bits
3126 <sect1 id="standard-interfaces">
3127 <title>Standard Interfaces</title>
3129 See <xref linkend="message-protocol-types-notation"/> for details on
3130 the notation used in this section. There are some standard interfaces
3131 that may be useful across various D-Bus applications.
3133 <sect2 id="standard-interfaces-peer">
3134 <title><literal>org.freedesktop.DBus.Peer</literal></title>
3136 The <literal>org.freedesktop.DBus.Peer</literal> interface
3139 org.freedesktop.DBus.Peer.Ping ()
3140 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
3144 On receipt of the <literal>METHOD_CALL</literal> message
3145 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
3146 nothing other than reply with a <literal>METHOD_RETURN</literal> as
3147 usual. It does not matter which object path a ping is sent to. The
3148 reference implementation handles this method automatically.
3151 On receipt of the <literal>METHOD_CALL</literal> message
3152 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
3153 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
3154 UUID representing the identity of the machine the process is running on.
3155 This UUID must be the same for all processes on a single system at least
3156 until that system next reboots. It should be the same across reboots
3157 if possible, but this is not always possible to implement and is not
3159 It does not matter which object path a GetMachineId is sent to. The
3160 reference implementation handles this method automatically.
3163 The UUID is intended to be per-instance-of-the-operating-system, so may represent
3164 a virtual machine running on a hypervisor, rather than a physical machine.
3165 Basically if two processes see the same UUID, they should also see the same
3166 shared memory, UNIX domain sockets, process IDs, and other features that require
3167 a running OS kernel in common between the processes.
3170 The UUID is often used where other programs might use a hostname. Hostnames
3171 can change without rebooting, however, or just be "localhost" - so the UUID
3175 <xref linkend="uuids"/> explains the format of the UUID.
3179 <sect2 id="standard-interfaces-introspectable">
3180 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
3182 This interface has one method:
3184 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
3188 Objects instances may implement
3189 <literal>Introspect</literal> which returns an XML description of
3190 the object, including its interfaces (with signals and methods), objects
3191 below it in the object path tree, and its properties.
3194 <xref linkend="introspection-format"/> describes the format of this XML string.
3197 <sect2 id="standard-interfaces-properties">
3198 <title><literal>org.freedesktop.DBus.Properties</literal></title>
3200 Many native APIs will have a concept of object <firstterm>properties</firstterm>
3201 or <firstterm>attributes</firstterm>. These can be exposed via the
3202 <literal>org.freedesktop.DBus.Properties</literal> interface.
3206 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
3207 in STRING property_name,
3209 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
3210 in STRING property_name,
3212 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
3213 out DICT<STRING,VARIANT> props);
3217 It is conventional to give D-Bus properties names consisting of
3218 capitalized words without punctuation ("CamelCase"), like
3219 <link linkend="message-protocol-names-member">member names</link>.
3220 For instance, the GObject property
3221 <literal>connection-status</literal> or the Qt property
3222 <literal>connectionStatus</literal> could be represented on D-Bus
3223 as <literal>ConnectionStatus</literal>.
3226 Strictly speaking, D-Bus property names are not required to follow
3227 the same naming restrictions as member names, but D-Bus property
3228 names that would not be valid member names (in particular,
3229 GObject-style dash-separated property names) can cause interoperability
3230 problems and should be avoided.
3233 The available properties and whether they are writable can be determined
3234 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
3235 see <xref linkend="standard-interfaces-introspectable"/>.
3238 An empty string may be provided for the interface name; in this case,
3239 if there are multiple properties on an object with the same name,
3240 the results are undefined (picking one by according to an arbitrary
3241 deterministic rule, or returning an error, are the reasonable
3245 If one or more properties change on an object, the
3246 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3247 signal may be emitted (this signal was added in 0.14):
3251 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
3252 DICT<STRING,VARIANT> changed_properties,
3253 ARRAY<STRING> invalidated_properties);
3257 where <literal>changed_properties</literal> is a dictionary
3258 containing the changed properties with the new values and
3259 <literal>invalidated_properties</literal> is an array of
3260 properties that changed but the value is not conveyed.
3263 Whether the <literal>PropertiesChanged</literal> signal is
3264 supported can be determined by calling
3265 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
3266 that the signal may be supported for an object but it may
3267 differ how whether and how it is used on a per-property basis
3268 (for e.g. performance or security reasons). Each property (or
3269 the parent interface) must be annotated with the
3270 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
3271 annotation to convey this (usually the default value
3272 <literal>true</literal> is sufficient meaning that the
3273 annotation does not need to be used). See <xref
3274 linkend="introspection-format"/> for details on this
3279 <sect2 id="standard-interfaces-objectmanager">
3280 <title><literal>org.freedesktop.DBus.ObjectManager</literal></title>
3282 An API can optionally make use of this interface for one or
3283 more sub-trees of objects. The root of each sub-tree implements
3284 this interface so other applications can get all objects,
3285 interfaces and properties in a single method call. It is
3286 appropriate to use this interface if users of the tree of
3287 objects are expected to be interested in all interfaces of all
3288 objects in the tree; a more granular API should be used if
3289 users of the objects are expected to be interested in a small
3290 subset of the objects, a small subset of their interfaces, or
3294 The method that applications can use to get all objects and
3295 properties is <literal>GetManagedObjects</literal>:
3299 org.freedesktop.DBus.ObjectManager.GetManagedObjects (out DICT<OBJPATH,DICT<STRING,DICT<STRING,VARIANT>>> objpath_interfaces_and_properties);
3303 The return value of this method is a dict whose keys are
3304 object paths. All returned object paths are children of the
3305 object path implementing this interface, i.e. their object
3306 paths start with the ObjectManager's object path plus '/'.
3309 Each value is a dict whose keys are interfaces names. Each
3310 value in this inner dict is the same dict that would be
3311 returned by the <link
3312 linkend="standard-interfaces-properties">org.freedesktop.DBus.Properties.GetAll()</link>
3313 method for that combination of object path and interface. If
3314 an interface has no properties, the empty dict is returned.
3317 Changes are emitted using the following two signals:
3321 org.freedesktop.DBus.ObjectManager.InterfacesAdded (OBJPATH object_path,
3322 DICT<STRING,DICT<STRING,VARIANT>> interfaces_and_properties);
3323 org.freedesktop.DBus.ObjectManager.InterfacesRemoved (OBJPATH object_path,
3324 ARRAY<STRING> interfaces);
3328 The <literal>InterfacesAdded</literal> signal is emitted when
3329 either a new object is added or when an existing object gains
3330 one or more interfaces. The
3331 <literal>InterfacesRemoved</literal> signal is emitted
3332 whenever an object is removed or it loses one or more
3333 interfaces. The second parameter of the
3334 <literal>InterfacesAdded</literal> signal contains a dict with
3335 the interfaces and properties (if any) that have been added to
3336 the given object path. Similarly, the second parameter of the
3337 <literal>InterfacesRemoved</literal> signal contains an array
3338 of the interfaces that were removed. Note that changes on
3339 properties on existing interfaces are not reported using this
3340 interface - an application should also monitor the existing <link
3341 linkend="standard-interfaces-properties">PropertiesChanged</link>
3342 signal on each object.
3345 Applications SHOULD NOT export objects that are children of an
3346 object (directly or otherwise) implementing this interface but
3347 which are not returned in the reply from the
3348 <literal>GetManagedObjects()</literal> method of this
3349 interface on the given object.
3352 The intent of the <literal>ObjectManager</literal> interface
3353 is to make it easy to write a robust client
3354 implementation. The trivial client implementation only needs
3355 to make two method calls:
3359 org.freedesktop.DBus.AddMatch (bus_proxy,
3360 "type='signal',name='org.example.App',path_namespace='/org/example/App'");
3361 objects = org.freedesktop.DBus.ObjectManager.GetManagedObjects (app_proxy);
3365 on the message bus and the remote application's
3366 <literal>ObjectManager</literal>, respectively. Whenever a new
3367 remote object is created (or an existing object gains a new
3368 interface), the <literal>InterfacesAdded</literal> signal is
3369 emitted, and since this signal contains all properties for the
3370 interfaces, no calls to the
3371 <literal>org.freedesktop.Properties</literal> interface on the
3372 remote object are needed. Additionally, since the initial
3373 <literal>AddMatch()</literal> rule already includes signal
3374 messages from the newly created child object, no new
3375 <literal>AddMatch()</literal> call is needed.
3380 The <literal>org.freedesktop.DBus.ObjectManager</literal>
3381 interface was added in version 0.17 of the D-Bus
3388 <sect1 id="introspection-format">
3389 <title>Introspection Data Format</title>
3391 As described in <xref linkend="standard-interfaces-introspectable"/>,
3392 objects may be introspected at runtime, returning an XML string
3393 that describes the object. The same XML format may be used in
3394 other contexts as well, for example as an "IDL" for generating
3395 static language bindings.
3398 Here is an example of introspection data:
3400 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
3401 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
3402 <node name="/org/freedesktop/sample_object">
3403 <interface name="org.freedesktop.SampleInterface">
3404 <method name="Frobate">
3405 <arg name="foo" type="i" direction="in"/>
3406 <arg name="bar" type="s" direction="out"/>
3407 <arg name="baz" type="a{us}" direction="out"/>
3408 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
3410 <method name="Bazify">
3411 <arg name="bar" type="(iiu)" direction="in"/>
3412 <arg name="bar" type="v" direction="out"/>
3414 <method name="Mogrify">
3415 <arg name="bar" type="(iiav)" direction="in"/>
3417 <signal name="Changed">
3418 <arg name="new_value" type="b"/>
3420 <property name="Bar" type="y" access="readwrite"/>
3422 <node name="child_of_sample_object"/>
3423 <node name="another_child_of_sample_object"/>
3428 A more formal DTD and spec needs writing, but here are some quick notes.
3432 Only the root <node> element can omit the node name, as it's
3433 known to be the object that was introspected. If the root
3434 <node> does have a name attribute, it must be an absolute
3435 object path. If child <node> have object paths, they must be
3441 If a child <node> has any sub-elements, then they
3442 must represent a complete introspection of the child.
3443 If a child <node> is empty, then it may or may
3444 not have sub-elements; the child must be introspected
3445 in order to find out. The intent is that if an object
3446 knows that its children are "fast" to introspect
3447 it can go ahead and return their information, but
3448 otherwise it can omit it.
3453 The direction element on <arg> may be omitted,
3454 in which case it defaults to "in" for method calls
3455 and "out" for signals. Signals only allow "out"
3456 so while direction may be specified, it's pointless.
3461 The possible directions are "in" and "out",
3462 unlike CORBA there is no "inout"
3467 The possible property access flags are
3468 "readwrite", "read", and "write"
3473 Multiple interfaces can of course be listed for
3479 The "name" attribute on arguments is optional.
3485 Method, interface, property, and signal elements may have
3486 "annotations", which are generic key/value pairs of metadata.
3487 They are similar conceptually to Java's annotations and C# attributes.
3488 Well-known annotations:
3495 <entry>Values (separated by ,)</entry>
3496 <entry>Description</entry>
3501 <entry>org.freedesktop.DBus.Deprecated</entry>
3502 <entry>true,false</entry>
3503 <entry>Whether or not the entity is deprecated; defaults to false</entry>
3506 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
3507 <entry>(string)</entry>
3508 <entry>The C symbol; may be used for methods and interfaces</entry>
3511 <entry>org.freedesktop.DBus.Method.NoReply</entry>
3512 <entry>true,false</entry>
3513 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
3516 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
3517 <entry>true,invalidates,false</entry>
3520 If set to <literal>false</literal>, the
3521 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
3523 linkend="standard-interfaces-properties"/> is not
3524 guaranteed to be emitted if the property changes.
3527 If set to <literal>invalidates</literal> the signal
3528 is emitted but the value is not included in the
3532 If set to <literal>true</literal> the signal is
3533 emitted with the value included.
3536 The value for the annotation defaults to
3537 <literal>true</literal> if the enclosing interface
3538 element does not specify the annotation. Otherwise it
3539 defaults to the value specified in the enclosing
3548 <sect1 id="message-bus">
3549 <title>Message Bus Specification</title>
3550 <sect2 id="message-bus-overview">
3551 <title>Message Bus Overview</title>
3553 The message bus accepts connections from one or more applications.
3554 Once connected, applications can exchange messages with other
3555 applications that are also connected to the bus.
3558 In order to route messages among connections, the message bus keeps a
3559 mapping from names to connections. Each connection has one
3560 unique-for-the-lifetime-of-the-bus name automatically assigned.
3561 Applications may request additional names for a connection. Additional
3562 names are usually "well-known names" such as
3563 "org.freedesktop.TextEditor". When a name is bound to a connection,
3564 that connection is said to <firstterm>own</firstterm> the name.
3567 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3568 This name routes messages to the bus, allowing applications to make
3569 administrative requests. For example, applications can ask the bus
3570 to assign a name to a connection.
3573 Each name may have <firstterm>queued owners</firstterm>. When an
3574 application requests a name for a connection and the name is already in
3575 use, the bus will optionally add the connection to a queue waiting for
3576 the name. If the current owner of the name disconnects or releases
3577 the name, the next connection in the queue will become the new owner.
3581 This feature causes the right thing to happen if you start two text
3582 editors for example; the first one may request "org.freedesktop.TextEditor",
3583 and the second will be queued as a possible owner of that name. When
3584 the first exits, the second will take over.
3588 Applications may send <firstterm>unicast messages</firstterm> to
3589 a specific recipient or to the message bus itself, or
3590 <firstterm>broadcast messages</firstterm> to all interested recipients.
3591 See <xref linkend="message-bus-routing"/> for details.
3595 <sect2 id="message-bus-names">
3596 <title>Message Bus Names</title>
3598 Each connection has at least one name, assigned at connection time and
3599 returned in response to the
3600 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3601 automatically-assigned name is called the connection's <firstterm>unique
3602 name</firstterm>. Unique names are never reused for two different
3603 connections to the same bus.
3606 Ownership of a unique name is a prerequisite for interaction with
3607 the message bus. It logically follows that the unique name is always
3608 the first name that an application comes to own, and the last
3609 one that it loses ownership of.
3612 Unique connection names must begin with the character ':' (ASCII colon
3613 character); bus names that are not unique names must not begin
3614 with this character. (The bus must reject any attempt by an application
3615 to manually request a name beginning with ':'.) This restriction
3616 categorically prevents "spoofing"; messages sent to a unique name
3617 will always go to the expected connection.
3620 When a connection is closed, all the names that it owns are deleted (or
3621 transferred to the next connection in the queue if any).
3624 A connection can request additional names to be associated with it using
3625 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3626 linkend="message-protocol-names-bus"/> describes the format of a valid
3627 name. These names can be released again using the
3628 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3631 <sect3 id="bus-messages-request-name">
3632 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3636 UINT32 RequestName (in STRING name, in UINT32 flags)
3643 <entry>Argument</entry>
3645 <entry>Description</entry>
3651 <entry>STRING</entry>
3652 <entry>Name to request</entry>
3656 <entry>UINT32</entry>
3657 <entry>Flags</entry>
3667 <entry>Argument</entry>
3669 <entry>Description</entry>
3675 <entry>UINT32</entry>
3676 <entry>Return value</entry>
3683 This method call should be sent to
3684 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3685 assign the given name to the method caller. Each name maintains a
3686 queue of possible owners, where the head of the queue is the primary
3687 or current owner of the name. Each potential owner in the queue
3688 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3689 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3690 call. When RequestName is invoked the following occurs:
3694 If the method caller is currently the primary owner of the name,
3695 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3696 values are updated with the values from the new RequestName call,
3697 and nothing further happens.
3703 If the current primary owner (head of the queue) has
3704 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3705 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3706 the caller of RequestName replaces the current primary owner at
3707 the head of the queue and the current primary owner moves to the
3708 second position in the queue. If the caller of RequestName was
3709 in the queue previously its flags are updated with the values from
3710 the new RequestName in addition to moving it to the head of the queue.
3716 If replacement is not possible, and the method caller is
3717 currently in the queue but not the primary owner, its flags are
3718 updated with the values from the new RequestName call.
3724 If replacement is not possible, and the method caller is
3725 currently not in the queue, the method caller is appended to the
3732 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3733 set and is not the primary owner, it is removed from the
3734 queue. This can apply to the previous primary owner (if it
3735 was replaced) or the method caller (if it updated the
3736 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3737 queue, or if it was just added to the queue with that flag set).
3743 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3744 queue," even if another application already in the queue had specified
3745 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3746 that does not allow replacement goes away, and the next primary owner
3747 does allow replacement. In this case, queued items that specified
3748 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3749 automatically replace the new primary owner. In other words,
3750 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3751 time RequestName is called. This is deliberate to avoid an infinite loop
3752 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3753 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3756 The flags argument contains any of the following values logically ORed
3763 <entry>Conventional Name</entry>
3764 <entry>Value</entry>
3765 <entry>Description</entry>
3770 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3774 If an application A specifies this flag and succeeds in
3775 becoming the owner of the name, and another application B
3776 later calls RequestName with the
3777 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3778 will lose ownership and receive a
3779 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3780 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3781 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3782 is not specified by application B, then application B will not replace
3783 application A as the owner.
3788 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3792 Try to replace the current owner if there is one. If this
3793 flag is not set the application will only become the owner of
3794 the name if there is no current owner. If this flag is set,
3795 the application will replace the current owner if
3796 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3801 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3805 Without this flag, if an application requests a name that is
3806 already owned, the application will be placed in a queue to
3807 own the name when the current owner gives it up. If this
3808 flag is given, the application will not be placed in the
3809 queue, the request for the name will simply fail. This flag
3810 also affects behavior when an application is replaced as
3811 name owner; by default the application moves back into the
3812 waiting queue, unless this flag was provided when the application
3813 became the name owner.
3821 The return code can be one of the following values:
3827 <entry>Conventional Name</entry>
3828 <entry>Value</entry>
3829 <entry>Description</entry>
3834 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3835 <entry>1</entry> <entry>The caller is now the primary owner of
3836 the name, replacing any previous owner. Either the name had no
3837 owner before, or the caller specified
3838 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3839 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3842 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3845 <entry>The name already had an owner,
3846 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3847 the current owner did not specify
3848 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3849 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3853 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3854 <entry>The name already has an owner,
3855 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3856 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3857 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3858 specified by the requesting application.</entry>
3861 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3863 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3871 <sect3 id="bus-messages-release-name">
3872 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3876 UINT32 ReleaseName (in STRING name)
3883 <entry>Argument</entry>
3885 <entry>Description</entry>
3891 <entry>STRING</entry>
3892 <entry>Name to release</entry>
3902 <entry>Argument</entry>
3904 <entry>Description</entry>
3910 <entry>UINT32</entry>
3911 <entry>Return value</entry>
3918 This method call should be sent to
3919 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3920 release the method caller's claim to the given name. If the caller is
3921 the primary owner, a new primary owner will be selected from the
3922 queue if any other owners are waiting. If the caller is waiting in
3923 the queue for the name, the caller will removed from the queue and
3924 will not be made an owner of the name if it later becomes available.
3925 If there are no other owners in the queue for the name, it will be
3926 removed from the bus entirely.
3928 The return code can be one of the following values:
3934 <entry>Conventional Name</entry>
3935 <entry>Value</entry>
3936 <entry>Description</entry>
3941 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3942 <entry>1</entry> <entry>The caller has released his claim on
3943 the given name. Either the caller was the primary owner of
3944 the name, and the name is now unused or taken by somebody
3945 waiting in the queue for the name, or the caller was waiting
3946 in the queue for the name and has now been removed from the
3950 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3952 <entry>The given name does not exist on this bus.</entry>
3955 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3957 <entry>The caller was not the primary owner of this name,
3958 and was also not waiting in the queue to own this name.</entry>
3966 <sect3 id="bus-messages-list-queued-owners">
3967 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3971 ARRAY of STRING ListQueuedOwners (in STRING name)
3978 <entry>Argument</entry>
3980 <entry>Description</entry>
3986 <entry>STRING</entry>
3987 <entry>The well-known bus name to query, such as
3988 <literal>com.example.cappuccino</literal></entry>
3998 <entry>Argument</entry>
4000 <entry>Description</entry>
4006 <entry>ARRAY of STRING</entry>
4007 <entry>The unique bus names of connections currently queued
4008 for the name</entry>
4015 This method call should be sent to
4016 <literal>org.freedesktop.DBus</literal> and lists the connections
4017 currently queued for a bus name (see
4018 <xref linkend="term-queued-owner"/>).
4023 <sect2 id="message-bus-routing">
4024 <title>Message Bus Message Routing</title>
4027 Messages may have a <literal>DESTINATION</literal> field (see <xref
4028 linkend="message-protocol-header-fields"/>), resulting in a
4029 <firstterm>unicast message</firstterm>. If the
4030 <literal>DESTINATION</literal> field is present, it specifies a message
4031 recipient by name. Method calls and replies normally specify this field.
4032 The message bus must send messages (of any type) with the
4033 <literal>DESTINATION</literal> field set to the specified recipient,
4034 regardless of whether the recipient has set up a match rule matching
4039 When the message bus receives a signal, if the
4040 <literal>DESTINATION</literal> field is absent, it is considered to
4041 be a <firstterm>broadcast signal</firstterm>, and is sent to all
4042 applications with <firstterm>message matching rules</firstterm> that
4043 match the message. Most signal messages are broadcasts.
4047 Unicast signal messages (those with a <literal>DESTINATION</literal>
4048 field) are not commonly used, but they are treated like any unicast
4049 message: they are delivered to the specified receipient,
4050 regardless of its match rules. One use for unicast signals is to
4051 avoid a race condition in which a signal is emitted before the intended
4052 recipient can call <xref linkend="bus-messages-add-match"/> to
4053 receive that signal: if the signal is sent directly to that recipient
4054 using a unicast message, it does not need to add a match rule at all,
4055 and there is no race condition. Another use for unicast signals,
4056 on message buses whose security policy prevents eavesdropping, is to
4057 send sensitive information which should only be visible to one
4062 When the message bus receives a method call, if the
4063 <literal>DESTINATION</literal> field is absent, the call is taken to be
4064 a standard one-to-one message and interpreted by the message bus
4065 itself. For example, sending an
4066 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
4067 <literal>DESTINATION</literal> will cause the message bus itself to
4068 reply to the ping immediately; the message bus will not make this
4069 message visible to other applications.
4073 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
4074 the ping message were sent with a <literal>DESTINATION</literal> name of
4075 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
4076 forwarded, and the Yoyodyne Corporation screensaver application would be
4077 expected to reply to the ping.
4081 Message bus implementations may impose a security policy which
4082 prevents certain messages from being sent or received.
4083 When a message cannot be sent or received due to a security
4084 policy, the message bus should send an error reply, unless the
4085 original message had the <literal>NO_REPLY</literal> flag.
4088 <sect3 id="message-bus-routing-eavesdropping">
4089 <title>Eavesdropping</title>
4091 Receiving a unicast message whose <literal>DESTINATION</literal>
4092 indicates a different recipient is called
4093 <firstterm>eavesdropping</firstterm>. On a message bus which acts as
4094 a security boundary (like the standard system bus), the security
4095 policy should usually prevent eavesdropping, since unicast messages
4096 are normally kept private and may contain security-sensitive
4101 Eavesdropping is mainly useful for debugging tools, such as
4102 the <literal>dbus-monitor</literal> tool in the reference
4103 implementation of D-Bus. Tools which eavesdrop on the message bus
4104 should be careful to avoid sending a reply or error in response to
4105 messages intended for a different client.
4109 Clients may attempt to eavesdrop by adding match rules
4110 (see <xref linkend="message-bus-routing-match-rules"/>) containing
4111 the <literal>eavesdrop='true'</literal> match. If the message bus'
4112 security policy does not allow eavesdropping, the match rule can
4113 still be added, but will not have any practical effect. For
4114 compatibility with older message bus implementations, if adding such
4115 a match rule results in an error reply, the client may fall back to
4116 adding the same rule with the <literal>eavesdrop</literal> match
4121 <sect3 id="message-bus-routing-match-rules">
4122 <title>Match Rules</title>
4124 An important part of the message bus routing protocol is match
4125 rules. Match rules describe the messages that should be sent to a
4126 client, based on the contents of the message. Broadcast signals
4127 are only sent to clients which have a suitable match rule: this
4128 avoids waking up client processes to deal with signals that are
4129 not relevant to that client.
4132 Messages that list a client as their <literal>DESTINATION</literal>
4133 do not need to match the client's match rules, and are sent to that
4134 client regardless. As a result, match rules are mainly used to
4135 receive a subset of broadcast signals.
4138 Match rules can also be used for eavesdropping
4139 (see <xref linkend="message-bus-routing-eavesdropping"/>),
4140 if the security policy of the message bus allows it.
4143 Match rules are added using the AddMatch bus method
4144 (see <xref linkend="bus-messages-add-match"/>). Rules are
4145 specified as a string of comma separated key/value pairs.
4146 Excluding a key from the rule indicates a wildcard match.
4147 For instance excluding the the member from a match rule but
4148 adding a sender would let all messages from that sender through.
4149 An example of a complete rule would be
4150 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
4153 The following table describes the keys that can be used to create
4155 The following table summarizes the D-Bus types.
4161 <entry>Possible Values</entry>
4162 <entry>Description</entry>
4167 <entry><literal>type</literal></entry>
4168 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
4169 <entry>Match on the message type. An example of a type match is type='signal'</entry>
4172 <entry><literal>sender</literal></entry>
4173 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
4174 and <xref linkend="term-unique-name"/> respectively)
4176 <entry>Match messages sent by a particular sender. An example of a sender match
4177 is sender='org.freedesktop.Hal'</entry>
4180 <entry><literal>interface</literal></entry>
4181 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
4182 <entry>Match messages sent over or to a particular interface. An example of an
4183 interface match is interface='org.freedesktop.Hal.Manager'.
4184 If a message omits the interface header, it must not match any rule
4185 that specifies this key.</entry>
4188 <entry><literal>member</literal></entry>
4189 <entry>Any valid method or signal name</entry>
4190 <entry>Matches messages which have the give method or signal name. An example of
4191 a member match is member='NameOwnerChanged'</entry>
4194 <entry><literal>path</literal></entry>
4195 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
4196 <entry>Matches messages which are sent from or to the given object. An example of a
4197 path match is path='/org/freedesktop/Hal/Manager'</entry>
4200 <entry><literal>path_namespace</literal></entry>
4201 <entry>An object path</entry>
4204 Matches messages which are sent from or to an
4205 object for which the object path is either the
4206 given value, or that value followed by one or
4207 more path components.
4212 <literal>path_namespace='/com/example/foo'</literal>
4213 would match signals sent by
4214 <literal>/com/example/foo</literal>
4216 <literal>/com/example/foo/bar</literal>,
4218 <literal>/com/example/foobar</literal>.
4222 Using both <literal>path</literal> and
4223 <literal>path_namespace</literal> in the same match
4224 rule is not allowed.
4229 This match key was added in version 0.16 of the
4230 D-Bus specification and implemented by the bus
4231 daemon in dbus 1.5.0 and later.
4237 <entry><literal>destination</literal></entry>
4238 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
4239 <entry>Matches messages which are being sent to the given unique name. An
4240 example of a destination match is destination=':1.0'</entry>
4243 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
4244 <entry>Any string</entry>
4245 <entry>Arg matches are special and are used for further restricting the
4246 match based on the arguments in the body of a message. Only arguments of type
4247 STRING can be matched in this way. An example of an argument match
4248 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
4252 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
4253 <entry>Any string</entry>
4255 <para>Argument path matches provide a specialised form of wildcard matching for
4256 path-like namespaces. They can match arguments whose type is either STRING or
4257 OBJECT_PATH. As with normal argument matches,
4258 if the argument is exactly equal to the string given in the match
4259 rule then the rule is satisfied. Additionally, there is also a
4260 match when either the string given in the match rule or the
4261 appropriate message argument ends with '/' and is a prefix of the
4262 other. An example argument path match is arg0path='/aa/bb/'. This
4263 would match messages with first arguments of '/', '/aa/',
4264 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
4265 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
4267 <para>This is intended for monitoring “directories” in file system-like
4268 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
4269 system. An application interested in all nodes in a particular hierarchy would
4270 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
4271 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
4272 represent a modification to the “bar” property, or a signal with zeroth
4273 argument <literal>"/ca/example/"</literal> to represent atomic modification of
4274 many properties within that directory, and the interested application would be
4275 notified in both cases.</para>
4278 This match key was added in version 0.12 of the
4279 D-Bus specification, implemented for STRING
4280 arguments by the bus daemon in dbus 1.2.0 and later,
4281 and implemented for OBJECT_PATH arguments in dbus 1.5.0
4288 <entry><literal>arg0namespace</literal></entry>
4289 <entry>Like a bus name, except that the string is not
4290 required to contain a '.' (period)</entry>
4292 <para>Match messages whose first argument is of type STRING, and is a bus name
4293 or interface name within the specified namespace. This is primarily intended
4294 for watching name owner changes for a group of related bus names, rather than
4295 for a single name or all name changes.</para>
4297 <para>Because every valid interface name is also a valid
4298 bus name, this can also be used for messages whose
4299 first argument is an interface name.</para>
4301 <para>For example, the match rule
4302 <literal>member='NameOwnerChanged',arg0namespace='com.example.backend'</literal>
4303 matches name owner changes for bus names such as
4304 <literal>com.example.backend.foo</literal>,
4305 <literal>com.example.backend.foo.bar</literal>, and
4306 <literal>com.example.backend</literal> itself.</para>
4308 <para>See also <xref linkend='bus-messages-name-owner-changed'/>.</para>
4311 This match key was added in version 0.16 of the
4312 D-Bus specification and implemented by the bus
4313 daemon in dbus 1.5.0 and later.
4319 <entry><literal>eavesdrop</literal></entry>
4320 <entry><literal>'true'</literal>, <literal>'false'</literal></entry>
4321 <entry>Since D-Bus 1.5.6, match rules do not
4322 match messages which have a <literal>DESTINATION</literal>
4323 field unless the match rule specifically
4325 (see <xref linkend="message-bus-routing-eavesdropping"/>)
4326 by specifying <literal>eavesdrop='true'</literal>
4327 in the match rule. <literal>eavesdrop='false'</literal>
4328 restores the default behaviour. Messages are
4329 delivered to their <literal>DESTINATION</literal>
4330 regardless of match rules, so this match does not
4331 affect normal delivery of unicast messages.
4332 If the message bus has a security policy which forbids
4333 eavesdropping, this match may still be used without error,
4334 but will not have any practical effect.
4335 In older versions of D-Bus, this match was not allowed
4336 in match rules, and all match rules behaved as if
4337 <literal>eavesdrop='true'</literal> had been used.
4346 <sect2 id="message-bus-starting-services">
4347 <title>Message Bus Starting Services</title>
4349 The message bus can start applications on behalf of other applications.
4350 In CORBA terms, this would be called <firstterm>activation</firstterm>.
4351 An application that can be started in this way is called a
4352 <firstterm>service</firstterm>.
4355 With D-Bus, starting a service is normally done by name. That is,
4356 applications ask the message bus to start some program that will own a
4357 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
4358 This implies a contract documented along with the name
4359 <literal>org.freedesktop.TextEditor</literal> for which objects
4360 the owner of that name will provide, and what interfaces those
4364 To find an executable corresponding to a particular name, the bus daemon
4365 looks for <firstterm>service description files</firstterm>. Service
4366 description files define a mapping from names to executables. Different
4367 kinds of message bus will look for these files in different places, see
4368 <xref linkend="message-bus-types"/>.
4371 Service description files have the ".service" file
4372 extension. The message bus will only load service description files
4373 ending with .service; all other files will be ignored. The file format
4374 is similar to that of <ulink
4375 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
4376 entries</ulink>. All service description files must be in UTF-8
4377 encoding. To ensure that there will be no name collisions, service files
4378 must be namespaced using the same mechanism as messages and service
4383 [FIXME the file format should be much better specified than "similar to
4384 .desktop entries" esp. since desktop entries are already
4385 badly-specified. ;-)]
4386 These sections from the specification apply to service files as well:
4389 <listitem><para>General syntax</para></listitem>
4390 <listitem><para>Comment format</para></listitem>
4394 <title>Example service description file</title>
4396 # Sample service description file
4398 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
4399 Exec=/usr/libexec/gconfd-2
4404 When an application asks to start a service by name, the bus daemon tries to
4405 find a service that will own that name. It then tries to spawn the
4406 executable associated with it. If this fails, it will report an
4407 error. [FIXME what happens if two .service files offer the same service;
4408 what kind of error is reported, should we have a way for the client to
4412 The executable launched will have the environment variable
4413 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
4414 message bus so it can connect and request the appropriate names.
4417 The executable being launched may want to know whether the message bus
4418 starting it is one of the well-known message buses (see <xref
4419 linkend="message-bus-types"/>). To facilitate this, the bus must also set
4420 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
4421 of the well-known buses. The currently-defined values for this variable
4422 are <literal>system</literal> for the systemwide message bus,
4423 and <literal>session</literal> for the per-login-session message
4424 bus. The new executable must still connect to the address given
4425 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
4426 resulting connection is to the well-known bus.
4429 [FIXME there should be a timeout somewhere, either specified
4430 in the .service file, by the client, or just a global value
4431 and if the client being activated fails to connect within that
4432 timeout, an error should be sent back.]
4435 <sect3 id="message-bus-starting-services-scope">
4436 <title>Message Bus Service Scope</title>
4438 The "scope" of a service is its "per-", such as per-session,
4439 per-machine, per-home-directory, or per-display. The reference
4440 implementation doesn't yet support starting services in a different
4441 scope from the message bus itself. So e.g. if you start a service
4442 on the session bus its scope is per-session.
4445 We could add an optional scope to a bus name. For example, for
4446 per-(display,session pair), we could have a unique ID for each display
4447 generated automatically at login and set on screen 0 by executing a
4448 special "set display ID" binary. The ID would be stored in a
4449 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
4450 random bytes. This ID would then be used to scope names.
4451 Starting/locating a service could be done by ID-name pair rather than
4455 Contrast this with a per-display scope. To achieve that, we would
4456 want a single bus spanning all sessions using a given display.
4457 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
4458 property on screen 0 of the display, pointing to this bus.
4463 <sect2 id="message-bus-types">
4464 <title>Well-known Message Bus Instances</title>
4466 Two standard message bus instances are defined here, along with how
4467 to locate them and where their service files live.
4469 <sect3 id="message-bus-types-login">
4470 <title>Login session message bus</title>
4472 Each time a user logs in, a <firstterm>login session message
4473 bus</firstterm> may be started. All applications in the user's login
4474 session may interact with one another using this message bus.
4477 The address of the login session message bus is given
4478 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
4479 variable. If that variable is not set, applications may
4480 also try to read the address from the X Window System root
4481 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
4482 The root window property must have type <literal>STRING</literal>.
4483 The environment variable should have precedence over the
4484 root window property.
4486 <para>The address of the login session message bus is given in the
4487 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
4488 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
4489 "autolaunch:", the system should use platform-specific methods of
4490 locating a running D-Bus session server, or starting one if a running
4491 instance cannot be found. Note that this mechanism is not recommended
4492 for attempting to determine if a daemon is running. It is inherently
4493 racy to attempt to make this determination, since the bus daemon may
4494 be started just before or just after the determination is made.
4495 Therefore, it is recommended that applications do not try to make this
4496 determination for their functionality purposes, and instead they
4497 should attempt to start the server.</para>
4499 <sect4 id="message-bus-types-login-x-windows">
4500 <title>X Windowing System</title>
4502 For the X Windowing System, the application must locate the
4503 window owner of the selection represented by the atom formed by
4507 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
4511 <para>the current user's username</para>
4515 <para>the literal character '_' (underscore)</para>
4519 <para>the machine's ID</para>
4525 The following properties are defined for the window that owns
4527 <informaltable frame="all">
4536 <para>meaning</para>
4542 <para>_DBUS_SESSION_BUS_ADDRESS</para>
4546 <para>the actual address of the server socket</para>
4552 <para>_DBUS_SESSION_BUS_PID</para>
4556 <para>the PID of the server process</para>
4565 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
4566 present in this window.
4570 If the X selection cannot be located or if reading the
4571 properties from the window fails, the implementation MUST conclude
4572 that there is no D-Bus server running and proceed to start a new
4573 server. (See below on concurrency issues)
4577 Failure to connect to the D-Bus server address thus obtained
4578 MUST be treated as a fatal connection error and should be reported
4583 As an alternative, an implementation MAY find the information
4584 in the following file located in the current user's home directory,
4585 in subdirectory .dbus/session-bus/:
4588 <para>the machine's ID</para>
4592 <para>the literal character '-' (dash)</para>
4596 <para>the X display without the screen number, with the
4597 following prefixes removed, if present: ":", "localhost:"
4598 ."localhost.localdomain:". That is, a display of
4599 "localhost:10.0" produces just the number "10"</para>
4605 The contents of this file NAME=value assignment pairs and
4606 lines starting with # are comments (no comments are allowed
4607 otherwise). The following variable names are defined:
4614 <para>Variable</para>
4618 <para>meaning</para>
4624 <para>DBUS_SESSION_BUS_ADDRESS</para>
4628 <para>the actual address of the server socket</para>
4634 <para>DBUS_SESSION_BUS_PID</para>
4638 <para>the PID of the server process</para>
4644 <para>DBUS_SESSION_BUS_WINDOWID</para>
4648 <para>the window ID</para>
4657 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
4662 Failure to open this file MUST be interpreted as absence of a
4663 running server. Therefore, the implementation MUST proceed to
4664 attempting to launch a new bus server if the file cannot be
4669 However, success in opening this file MUST NOT lead to the
4670 conclusion that the server is running. Thus, a failure to connect to
4671 the bus address obtained by the alternative method MUST NOT be
4672 considered a fatal error. If the connection cannot be established,
4673 the implementation MUST proceed to check the X selection settings or
4674 to start the server on its own.
4678 If the implementation concludes that the D-Bus server is not
4679 running it MUST attempt to start a new server and it MUST also
4680 ensure that the daemon started as an effect of the "autolaunch"
4681 mechanism provides the lookup mechanisms described above, so
4682 subsequent calls can locate the newly started server. The
4683 implementation MUST also ensure that if two or more concurrent
4684 initiations happen, only one server remains running and all other
4685 initiations are able to obtain the address of this server and
4686 connect to it. In other words, the implementation MUST ensure that
4687 the X selection is not present when it attempts to set it, without
4688 allowing another process to set the selection between the
4689 verification and the setting (e.g., by using XGrabServer /
4696 On Unix systems, the session bus should search for .service files
4697 in <literal>$XDG_DATA_DIRS/dbus-1/services</literal> as defined
4699 <ulink url="http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html">XDG Base Directory Specification</ulink>.
4700 Implementations may also search additional locations, which
4701 should be searched with lower priority than anything in
4702 XDG_DATA_HOME, XDG_DATA_DIRS or their respective defaults;
4703 for example, the reference implementation also
4704 looks in <literal>${datadir}/dbus-1/services</literal> as
4705 set at compile time.
4708 As described in the XDG Base Directory Specification, software
4709 packages should install their session .service files to their
4710 configured <literal>${datadir}/dbus-1/services</literal>,
4711 where <literal>${datadir}</literal> is as defined by the GNU
4712 coding standards. System administrators or users can arrange
4713 for these service files to be read by setting XDG_DATA_DIRS or by
4714 symlinking them into the default locations.
4718 <sect3 id="message-bus-types-system">
4719 <title>System message bus</title>
4721 A computer may have a <firstterm>system message bus</firstterm>,
4722 accessible to all applications on the system. This message bus may be
4723 used to broadcast system events, such as adding new hardware devices,
4724 changes in the printer queue, and so forth.
4727 The address of the system message bus is given
4728 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
4729 variable. If that variable is not set, applications should try
4730 to connect to the well-known address
4731 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
4734 The D-Bus reference implementation actually honors the
4735 <literal>$(localstatedir)</literal> configure option
4736 for this address, on both client and server side.
4741 On Unix systems, the system bus should default to searching
4742 for .service files in
4743 <literal>/usr/local/share/dbus-1/system-services</literal>,
4744 <literal>/usr/share/dbus-1/system-services</literal> and
4745 <literal>/lib/dbus-1/system-services</literal>, with that order
4746 of precedence. It may also search other implementation-specific
4747 locations, but should not vary these locations based on environment
4751 The system bus is security-sensitive and is typically executed
4752 by an init system with a clean environment. Its launch helper
4753 process is particularly security-sensitive, and specifically
4754 clears its own environment.
4759 Software packages should install their system .service
4760 files to their configured
4761 <literal>${datadir}/dbus-1/system-services</literal>,
4762 where <literal>${datadir}</literal> is as defined by the GNU
4763 coding standards. System administrators can arrange
4764 for these service files to be read by editing the system bus'
4765 configuration file or by symlinking them into the default
4771 <sect2 id="message-bus-messages">
4772 <title>Message Bus Messages</title>
4774 The special message bus name <literal>org.freedesktop.DBus</literal>
4775 responds to a number of additional messages.
4778 <sect3 id="bus-messages-hello">
4779 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4790 <entry>Argument</entry>
4792 <entry>Description</entry>
4798 <entry>STRING</entry>
4799 <entry>Unique name assigned to the connection</entry>
4806 Before an application is able to send messages to other applications
4807 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4808 to the message bus to obtain a unique name. If an application without
4809 a unique name tries to send a message to another application, or a
4810 message to the message bus itself that isn't the
4811 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4812 disconnected from the bus.
4815 There is no corresponding "disconnect" request; if a client wishes to
4816 disconnect from the bus, it simply closes the socket (or other
4817 communication channel).
4820 <sect3 id="bus-messages-list-names">
4821 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4825 ARRAY of STRING ListNames ()
4832 <entry>Argument</entry>
4834 <entry>Description</entry>
4840 <entry>ARRAY of STRING</entry>
4841 <entry>Array of strings where each string is a bus name</entry>
4848 Returns a list of all currently-owned names on the bus.
4851 <sect3 id="bus-messages-list-activatable-names">
4852 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4856 ARRAY of STRING ListActivatableNames ()
4863 <entry>Argument</entry>
4865 <entry>Description</entry>
4871 <entry>ARRAY of STRING</entry>
4872 <entry>Array of strings where each string is a bus name</entry>
4879 Returns a list of all names that can be activated on the bus.
4882 <sect3 id="bus-messages-name-exists">
4883 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4887 BOOLEAN NameHasOwner (in STRING name)
4894 <entry>Argument</entry>
4896 <entry>Description</entry>
4902 <entry>STRING</entry>
4903 <entry>Name to check</entry>
4913 <entry>Argument</entry>
4915 <entry>Description</entry>
4921 <entry>BOOLEAN</entry>
4922 <entry>Return value, true if the name exists</entry>
4929 Checks if the specified name exists (currently has an owner).
4933 <sect3 id="bus-messages-name-owner-changed">
4934 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4938 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4945 <entry>Argument</entry>
4947 <entry>Description</entry>
4953 <entry>STRING</entry>
4954 <entry>Name with a new owner</entry>
4958 <entry>STRING</entry>
4959 <entry>Old owner or empty string if none</entry>
4963 <entry>STRING</entry>
4964 <entry>New owner or empty string if none</entry>
4971 This signal indicates that the owner of a name has changed.
4972 It's also the signal to use to detect the appearance of
4973 new names on the bus.
4976 <sect3 id="bus-messages-name-lost">
4977 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4981 NameLost (STRING name)
4988 <entry>Argument</entry>
4990 <entry>Description</entry>
4996 <entry>STRING</entry>
4997 <entry>Name which was lost</entry>
5004 This signal is sent to a specific application when it loses
5005 ownership of a name.
5009 <sect3 id="bus-messages-name-acquired">
5010 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
5014 NameAcquired (STRING name)
5021 <entry>Argument</entry>
5023 <entry>Description</entry>
5029 <entry>STRING</entry>
5030 <entry>Name which was acquired</entry>
5037 This signal is sent to a specific application when it gains
5038 ownership of a name.
5042 <sect3 id="bus-messages-start-service-by-name">
5043 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
5047 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
5054 <entry>Argument</entry>
5056 <entry>Description</entry>
5062 <entry>STRING</entry>
5063 <entry>Name of the service to start</entry>
5067 <entry>UINT32</entry>
5068 <entry>Flags (currently not used)</entry>
5078 <entry>Argument</entry>
5080 <entry>Description</entry>
5086 <entry>UINT32</entry>
5087 <entry>Return value</entry>
5092 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
5096 The return value can be one of the following values:
5101 <entry>Identifier</entry>
5102 <entry>Value</entry>
5103 <entry>Description</entry>
5108 <entry>DBUS_START_REPLY_SUCCESS</entry>
5110 <entry>The service was successfully started.</entry>
5113 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
5115 <entry>A connection already owns the given name.</entry>
5124 <sect3 id="bus-messages-update-activation-environment">
5125 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
5129 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
5136 <entry>Argument</entry>
5138 <entry>Description</entry>
5144 <entry>ARRAY of DICT<STRING,STRING></entry>
5145 <entry>Environment to add or update</entry>
5150 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
5153 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
5156 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.
5161 <sect3 id="bus-messages-get-name-owner">
5162 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
5166 STRING GetNameOwner (in STRING name)
5173 <entry>Argument</entry>
5175 <entry>Description</entry>
5181 <entry>STRING</entry>
5182 <entry>Name to get the owner of</entry>
5192 <entry>Argument</entry>
5194 <entry>Description</entry>
5200 <entry>STRING</entry>
5201 <entry>Return value, a unique connection name</entry>
5206 Returns the unique connection name of the primary owner of the name
5207 given. If the requested name doesn't have an owner, returns a
5208 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
5212 <sect3 id="bus-messages-get-connection-unix-user">
5213 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
5217 UINT32 GetConnectionUnixUser (in STRING bus_name)
5224 <entry>Argument</entry>
5226 <entry>Description</entry>
5232 <entry>STRING</entry>
5233 <entry>Unique or well-known bus name of the connection to
5234 query, such as <literal>:12.34</literal> or
5235 <literal>com.example.tea</literal></entry>
5245 <entry>Argument</entry>
5247 <entry>Description</entry>
5253 <entry>UINT32</entry>
5254 <entry>Unix user ID</entry>
5259 Returns the Unix user ID of the process connected to the server. If
5260 unable to determine it (for instance, because the process is not on the
5261 same machine as the bus daemon), an error is returned.
5265 <sect3 id="bus-messages-get-connection-unix-process-id">
5266 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
5270 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
5277 <entry>Argument</entry>
5279 <entry>Description</entry>
5285 <entry>STRING</entry>
5286 <entry>Unique or well-known bus name of the connection to
5287 query, such as <literal>:12.34</literal> or
5288 <literal>com.example.tea</literal></entry>
5298 <entry>Argument</entry>
5300 <entry>Description</entry>
5306 <entry>UINT32</entry>
5307 <entry>Unix process id</entry>
5312 Returns the Unix process ID of the process connected to the server. If
5313 unable to determine it (for instance, because the process is not on the
5314 same machine as the bus daemon), an error is returned.
5318 <sect3 id="bus-messages-add-match">
5319 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
5323 AddMatch (in STRING rule)
5330 <entry>Argument</entry>
5332 <entry>Description</entry>
5338 <entry>STRING</entry>
5339 <entry>Match rule to add to the connection</entry>
5344 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
5345 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
5349 <sect3 id="bus-messages-remove-match">
5350 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
5354 RemoveMatch (in STRING rule)
5361 <entry>Argument</entry>
5363 <entry>Description</entry>
5369 <entry>STRING</entry>
5370 <entry>Match rule to remove from the connection</entry>
5375 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
5376 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
5381 <sect3 id="bus-messages-get-id">
5382 <title><literal>org.freedesktop.DBus.GetId</literal></title>
5386 GetId (out STRING id)
5393 <entry>Argument</entry>
5395 <entry>Description</entry>
5401 <entry>STRING</entry>
5402 <entry>Unique ID identifying the bus daemon</entry>
5407 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
5408 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
5409 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
5410 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
5411 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
5412 by org.freedesktop.DBus.Peer.GetMachineId().
5413 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
5421 <appendix id="implementation-notes">
5422 <title>Implementation notes</title>
5423 <sect1 id="implementation-notes-subsection">
5431 <glossary><title>Glossary</title>
5433 This glossary defines some of the terms used in this specification.
5436 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
5439 The message bus maintains an association between names and
5440 connections. (Normally, there's one connection per application.) A
5441 bus name is simply an identifier used to locate connections. For
5442 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
5443 name might be used to send a message to a screensaver from Yoyodyne
5444 Corporation. An application is said to <firstterm>own</firstterm> a
5445 name if the message bus has associated the application's connection
5446 with the name. Names may also have <firstterm>queued
5447 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
5448 The bus assigns a unique name to each connection,
5449 see <xref linkend="term-unique-name"/>. Other names
5450 can be thought of as "well-known names" and are
5451 used to find applications that offer specific functionality.
5455 See <xref linkend="message-protocol-names-bus"/> for details of
5456 the syntax and naming conventions for bus names.
5461 <glossentry id="term-message"><glossterm>Message</glossterm>
5464 A message is the atomic unit of communication via the D-Bus
5465 protocol. It consists of a <firstterm>header</firstterm> and a
5466 <firstterm>body</firstterm>; the body is made up of
5467 <firstterm>arguments</firstterm>.
5472 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
5475 The message bus is a special application that forwards
5476 or routes messages between a group of applications
5477 connected to the message bus. It also manages
5478 <firstterm>names</firstterm> used for routing
5484 <glossentry id="term-name"><glossterm>Name</glossterm>
5487 See <xref linkend="term-bus-name"/>. "Name" may
5488 also be used to refer to some of the other names
5489 in D-Bus, such as interface names.
5494 <glossentry id="namespace"><glossterm>Namespace</glossterm>
5497 Used to prevent collisions when defining new interfaces, bus names
5498 etc. The convention used is the same one Java uses for defining
5499 classes: a reversed domain name.
5500 See <xref linkend="message-protocol-names-bus"/>,
5501 <xref linkend="message-protocol-names-interface"/>,
5502 <xref linkend="message-protocol-names-error"/>,
5503 <xref linkend="message-protocol-marshaling-object-path"/>.
5508 <glossentry id="term-object"><glossterm>Object</glossterm>
5511 Each application contains <firstterm>objects</firstterm>, which have
5512 <firstterm>interfaces</firstterm> and
5513 <firstterm>methods</firstterm>. Objects are referred to by a name,
5514 called a <firstterm>path</firstterm>.
5519 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
5522 An application talking directly to another application, without going
5523 through a message bus. One-to-one connections may be "peer to peer" or
5524 "client to server." The D-Bus protocol has no concept of client
5525 vs. server after a connection has authenticated; the flow of messages
5526 is symmetrical (full duplex).
5531 <glossentry id="term-path"><glossterm>Path</glossterm>
5534 Object references (object names) in D-Bus are organized into a
5535 filesystem-style hierarchy, so each object is named by a path. As in
5536 LDAP, there's no difference between "files" and "directories"; a path
5537 can refer to an object, while still having child objects below it.
5542 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
5545 Each bus name has a primary owner; messages sent to the name go to the
5546 primary owner. However, certain names also maintain a queue of
5547 secondary owners "waiting in the wings." If the primary owner releases
5548 the name, then the first secondary owner in the queue automatically
5549 becomes the new owner of the name.
5554 <glossentry id="term-service"><glossterm>Service</glossterm>
5557 A service is an executable that can be launched by the bus daemon.
5558 Services normally guarantee some particular features, for example they
5559 may guarantee that they will request a specific name such as
5560 "org.freedesktop.Screensaver", have a singleton object
5561 "/org/freedesktop/Application", and that object will implement the
5562 interface "org.freedesktop.ScreensaverControl".
5567 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
5570 ".service files" tell the bus about service applications that can be
5571 launched (see <xref linkend="term-service"/>). Most importantly they
5572 provide a mapping from bus names to services that will request those
5573 names when they start up.
5578 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
5581 The special name automatically assigned to each connection by the
5582 message bus. This name will never change owner, and will be unique
5583 (never reused during the lifetime of the message bus).
5584 It will begin with a ':' character.