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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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9 <title>D-Bus Specification</title>
10 <releaseinfo>Version 0.12</releaseinfo>
11 <date>7 November 2006</date>
14 <firstname>Havoc</firstname>
15 <surname>Pennington</surname>
17 <orgname>Red Hat, Inc.</orgname>
19 <email>hp@pobox.com</email>
24 <firstname>Anders</firstname>
25 <surname>Carlsson</surname>
27 <orgname>CodeFactory AB</orgname>
29 <email>andersca@codefactory.se</email>
34 <firstname>Alexander</firstname>
35 <surname>Larsson</surname>
37 <orgname>Red Hat, Inc.</orgname>
39 <email>alexl@redhat.com</email>
46 <sect1 id="introduction">
47 <title>Introduction</title>
49 D-Bus is a system for low-latency, low-overhead, easy to use
50 interprocess communication (IPC). In more detail:
54 D-Bus is <emphasis>low-latency</emphasis> because it is designed
55 to avoid round trips and allow asynchronous operation, much like
61 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
62 binary protocol, and does not have to convert to and from a text
63 format such as XML. Because D-Bus is intended for potentially
64 high-resolution same-machine IPC, not primarily for Internet IPC,
65 this is an interesting optimization.
70 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
71 of <firstterm>messages</firstterm> rather than byte streams, and
72 automatically handles a lot of the hard IPC issues. Also, the D-Bus
73 library is designed to be wrapped in a way that lets developers use
74 their framework's existing object/type system, rather than learning
75 a new one specifically for IPC.
82 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
83 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
84 a system for one application to talk to a single other
85 application. However, the primary intended application of the protocol is the
86 D-Bus <firstterm>message bus</firstterm>, specified in <xref
87 linkend="message-bus"/>. The message bus is a special application that
88 accepts connections from multiple other applications, and forwards
93 Uses of D-Bus include notification of system changes (notification of when
94 a camera is plugged in to a computer, or a new version of some software
95 has been installed), or desktop interoperability, for example a file
96 monitoring service or a configuration service.
100 D-Bus is designed for two specific use cases:
104 A "system bus" for notifications from the system to user sessions,
105 and to allow the system to request input from user sessions.
110 A "session bus" used to implement desktop environments such as
115 D-Bus is not intended to be a generic IPC system for any possible
116 application, and intentionally omits many features found in other
117 IPC systems for this reason.
121 At the same time, the bus daemons offer a number of features not found in
122 other IPC systems, such as single-owner "bus names" (similar to X
123 selections), on-demand startup of services, and security policies.
124 In many ways, these features are the primary motivation for developing
125 D-Bus; other systems would have sufficed if IPC were the only goal.
129 D-Bus may turn out to be useful in unanticipated applications, but future
130 versions of this spec and the reference implementation probably will not
131 incorporate features that interfere with the core use cases.
135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
137 document are to be interpreted as described in RFC 2119. However, the
138 document could use a serious audit to be sure it makes sense to do
139 so. Also, they are not capitalized.
142 <sect2 id="stability">
143 <title>Protocol and Specification Stability</title>
145 The D-Bus protocol is frozen (only compatible extensions are allowed) as
146 of November 8, 2006. However, this specification could still use a fair
147 bit of work to make interoperable reimplementation possible without
148 reference to the D-Bus reference implementation. Thus, this
149 specification is not marked 1.0. To mark it 1.0, we'd like to see
150 someone invest significant effort in clarifying the specification
151 language, and growing the specification to cover more aspects of the
152 reference implementation's behavior.
155 Until this work is complete, any attempt to reimplement D-Bus will
156 probably require looking at the reference implementation and/or asking
157 questions on the D-Bus mailing list about intended behavior.
158 Questions on the list are very welcome.
161 Nonetheless, this document should be a useful starting point and is
162 to our knowledge accurate, though incomplete.
168 <sect1 id="message-protocol">
169 <title>Message Protocol</title>
172 A <firstterm>message</firstterm> consists of a
173 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
174 think of a message as a package, the header is the address, and the body
175 contains the package contents. The message delivery system uses the header
176 information to figure out where to send the message and how to interpret
177 it; the recipient interprets the body of the message.
181 The body of the message is made up of zero or more
182 <firstterm>arguments</firstterm>, which are typed values, such as an
183 integer or a byte array.
187 Both header and body use the same type system and format for
188 serializing data. Each type of value has a wire format.
189 Converting a value from some other representation into the wire
190 format is called <firstterm>marshaling</firstterm> and converting
191 it back from the wire format is <firstterm>unmarshaling</firstterm>.
194 <sect2 id="message-protocol-signatures">
195 <title>Type Signatures</title>
198 The D-Bus protocol does not include type tags in the marshaled data; a
199 block of marshaled values must have a known <firstterm>type
200 signature</firstterm>. The type signature is made up of <firstterm>type
201 codes</firstterm>. A type code is an ASCII character representing the
202 type of a value. Because ASCII characters are used, the type signature
203 will always form a valid ASCII string. A simple string compare
204 determines whether two type signatures are equivalent.
208 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
209 the ASCII character 'i'. So the signature for a block of values
210 containing a single <literal>INT32</literal> would be:
214 A block of values containing two <literal>INT32</literal> would have this signature:
221 All <firstterm>basic</firstterm> types work like
222 <literal>INT32</literal> in this example. To marshal and unmarshal
223 basic types, you simply read one value from the data
224 block corresponding to each type code in the signature.
225 In addition to basic types, there are four <firstterm>container</firstterm>
226 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
227 and <literal>DICT_ENTRY</literal>.
231 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
232 code does not appear in signatures. Instead, ASCII characters
233 '(' and ')' are used to mark the beginning and end of the struct.
234 So for example, a struct containing two integers would have this
239 Structs can be nested, so for example a struct containing
240 an integer and another struct:
244 The value block storing that struct would contain three integers; the
245 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
250 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
251 but is useful in code that implements the protocol. This type code
252 is specified to allow such code to interoperate in non-protocol contexts.
256 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
257 followed by a <firstterm>single complete type</firstterm>. The single
258 complete type following the array is the type of each array element. So
259 the simple example is:
263 which is an array of 32-bit integers. But an array can be of any type,
264 such as this array-of-struct-with-two-int32-fields:
268 Or this array of array of integer:
275 The phrase <firstterm>single complete type</firstterm> deserves some
276 definition. A single complete type is a basic type code, a variant type code,
277 an array with its element type, or a struct with its fields.
278 So the following signatures are not single complete types:
288 And the following signatures contain multiple complete types:
298 Note however that a single complete type may <emphasis>contain</emphasis>
299 multiple other single complete types.
303 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
304 type <literal>VARIANT</literal> will have the signature of a single complete type as part
305 of the <emphasis>value</emphasis>. This signature will be followed by a
306 marshaled value of that type.
310 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
311 than parentheses it uses curly braces, and it has more restrictions.
312 The restrictions are: it occurs only as an array element type; it has
313 exactly two single complete types inside the curly braces; the first
314 single complete type (the "key") must be a basic type rather than a
315 container type. Implementations must not accept dict entries outside of
316 arrays, must not accept dict entries with zero, one, or more than two
317 fields, and must not accept dict entries with non-basic-typed keys. A
318 dict entry is always a key-value pair.
322 The first field in the <literal>DICT_ENTRY</literal> is always the key.
323 A message is considered corrupt if the same key occurs twice in the same
324 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
325 implementations are not required to reject dicts with duplicate keys.
329 In most languages, an array of dict entry would be represented as a
330 map, hash table, or dict object.
334 The following table summarizes the D-Bus types.
339 <entry>Conventional Name</entry>
341 <entry>Description</entry>
346 <entry><literal>INVALID</literal></entry>
347 <entry>0 (ASCII NUL)</entry>
348 <entry>Not a valid type code, used to terminate signatures</entry>
350 <entry><literal>BYTE</literal></entry>
351 <entry>121 (ASCII 'y')</entry>
352 <entry>8-bit unsigned integer</entry>
354 <entry><literal>BOOLEAN</literal></entry>
355 <entry>98 (ASCII 'b')</entry>
356 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
358 <entry><literal>INT16</literal></entry>
359 <entry>110 (ASCII 'n')</entry>
360 <entry>16-bit signed integer</entry>
362 <entry><literal>UINT16</literal></entry>
363 <entry>113 (ASCII 'q')</entry>
364 <entry>16-bit unsigned integer</entry>
366 <entry><literal>INT32</literal></entry>
367 <entry>105 (ASCII 'i')</entry>
368 <entry>32-bit signed integer</entry>
370 <entry><literal>UINT32</literal></entry>
371 <entry>117 (ASCII 'u')</entry>
372 <entry>32-bit unsigned integer</entry>
374 <entry><literal>INT64</literal></entry>
375 <entry>120 (ASCII 'x')</entry>
376 <entry>64-bit signed integer</entry>
378 <entry><literal>UINT64</literal></entry>
379 <entry>116 (ASCII 't')</entry>
380 <entry>64-bit unsigned integer</entry>
382 <entry><literal>DOUBLE</literal></entry>
383 <entry>100 (ASCII 'd')</entry>
384 <entry>IEEE 754 double</entry>
386 <entry><literal>STRING</literal></entry>
387 <entry>115 (ASCII 's')</entry>
388 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
390 <entry><literal>OBJECT_PATH</literal></entry>
391 <entry>111 (ASCII 'o')</entry>
392 <entry>Name of an object instance</entry>
394 <entry><literal>SIGNATURE</literal></entry>
395 <entry>103 (ASCII 'g')</entry>
396 <entry>A type signature</entry>
398 <entry><literal>ARRAY</literal></entry>
399 <entry>97 (ASCII 'a')</entry>
402 <entry><literal>STRUCT</literal></entry>
403 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
404 <entry>Struct</entry>
406 <entry><literal>VARIANT</literal></entry>
407 <entry>118 (ASCII 'v') </entry>
408 <entry>Variant type (the type of the value is part of the value itself)</entry>
410 <entry><literal>DICT_ENTRY</literal></entry>
411 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
412 <entry>Entry in a dict or map (array of key-value pairs)</entry>
421 <sect2 id="message-protocol-marshaling">
422 <title>Marshaling (Wire Format)</title>
425 Given a type signature, a block of bytes can be converted into typed
426 values. This section describes the format of the block of bytes. Byte
427 order and alignment issues are handled uniformly for all D-Bus types.
431 A block of bytes has an associated byte order. The byte order
432 has to be discovered in some way; for D-Bus messages, the
433 byte order is part of the message header as described in
434 <xref linkend="message-protocol-messages"/>. For now, assume
435 that the byte order is known to be either little endian or big
440 Each value in a block of bytes is aligned "naturally," for example
441 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
442 8-byte boundary. To properly align a value, <firstterm>alignment
443 padding</firstterm> may be necessary. The alignment padding must always
444 be the minimum required padding to properly align the following value;
445 and it must always be made up of nul bytes. The alignment padding must
446 not be left uninitialized (it can't contain garbage), and more padding
447 than required must not be used.
451 Given all this, the types are marshaled on the wire as follows:
456 <entry>Conventional Name</entry>
457 <entry>Encoding</entry>
458 <entry>Alignment</entry>
463 <entry><literal>INVALID</literal></entry>
464 <entry>Not applicable; cannot be marshaled.</entry>
467 <entry><literal>BYTE</literal></entry>
468 <entry>A single 8-bit byte.</entry>
471 <entry><literal>BOOLEAN</literal></entry>
472 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
475 <entry><literal>INT16</literal></entry>
476 <entry>16-bit signed integer in the message's byte order.</entry>
479 <entry><literal>UINT16</literal></entry>
480 <entry>16-bit unsigned integer in the message's byte order.</entry>
483 <entry><literal>INT32</literal></entry>
484 <entry>32-bit signed integer in the message's byte order.</entry>
487 <entry><literal>UINT32</literal></entry>
488 <entry>32-bit unsigned integer in the message's byte order.</entry>
491 <entry><literal>INT64</literal></entry>
492 <entry>64-bit signed integer in the message's byte order.</entry>
495 <entry><literal>UINT64</literal></entry>
496 <entry>64-bit unsigned integer in the message's byte order.</entry>
499 <entry><literal>DOUBLE</literal></entry>
500 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
503 <entry><literal>STRING</literal></entry>
504 <entry>A <literal>UINT32</literal> indicating the string's
505 length in bytes excluding its terminating nul, followed by
506 non-nul string data of the given length, followed by a terminating nul
513 <entry><literal>OBJECT_PATH</literal></entry>
514 <entry>Exactly the same as <literal>STRING</literal> except the
515 content must be a valid object path (see below).
521 <entry><literal>SIGNATURE</literal></entry>
522 <entry>The same as <literal>STRING</literal> except the length is a single
523 byte (thus signatures have a maximum length of 255)
524 and the content must be a valid signature (see below).
530 <entry><literal>ARRAY</literal></entry>
532 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
533 alignment padding to the alignment boundary of the array element type,
534 followed by each array element. The array length is from the
535 end of the alignment padding to the end of the last element,
536 i.e. it does not include the padding after the length,
537 or any padding after the last element.
538 Arrays have a maximum length defined to be 2 to the 26th power or
539 67108864. Implementations must not send or accept arrays exceeding this
546 <entry><literal>STRUCT</literal></entry>
548 A struct must start on an 8-byte boundary regardless of the
549 type of the struct fields. The struct value consists of each
550 field marshaled in sequence starting from that 8-byte
557 <entry><literal>VARIANT</literal></entry>
559 A variant type has a marshaled <literal>SIGNATURE</literal>
560 followed by a marshaled value with the type
561 given in the signature.
562 Unlike a message signature, the variant signature
563 can contain only a single complete type.
564 So "i" is OK, "ii" is not.
567 1 (alignment of the signature)
570 <entry><literal>DICT_ENTRY</literal></entry>
583 <sect3 id="message-protocol-marshaling-object-path">
584 <title>Valid Object Paths</title>
587 An object path is a name used to refer to an object instance.
588 Conceptually, each participant in a D-Bus message exchange may have
589 any number of object instances (think of C++ or Java objects) and each
590 such instance will have a path. Like a filesystem, the object
591 instances in an application form a hierarchical tree.
595 The following rules define a valid object path. Implementations must
596 not send or accept messages with invalid object paths.
600 The path may be of any length.
605 The path must begin with an ASCII '/' (integer 47) character,
606 and must consist of elements separated by slash characters.
611 Each element must only contain the ASCII characters
617 No element may be the empty string.
622 Multiple '/' characters cannot occur in sequence.
627 A trailing '/' character is not allowed unless the
628 path is the root path (a single '/' character).
637 <sect3 id="message-protocol-marshaling-signature">
638 <title>Valid Signatures</title>
640 An implementation must not send or accept invalid signatures.
641 Valid signatures will conform to the following rules:
645 The signature ends with a nul byte.
650 The signature is a list of single complete types.
651 Arrays must have element types, and structs must
652 have both open and close parentheses.
657 Only type codes and open and close parentheses are
658 allowed in the signature. The <literal>STRUCT</literal> type code
659 is not allowed in signatures, because parentheses
665 The maximum depth of container type nesting is 32 array type
666 codes and 32 open parentheses. This implies that the maximum
667 total depth of recursion is 64, for an "array of array of array
668 of ... struct of struct of struct of ..." where there are 32
674 The maximum length of a signature is 255.
679 Signatures must be nul-terminated.
688 <sect2 id="message-protocol-messages">
689 <title>Message Format</title>
692 A message consists of a header and a body. The header is a block of
693 values with a fixed signature and meaning. The body is a separate block
694 of values, with a signature specified in the header.
698 The length of the header must be a multiple of 8, allowing the body to
699 begin on an 8-byte boundary when storing the entire message in a single
700 buffer. If the header does not naturally end on an 8-byte boundary
701 up to 7 bytes of nul-initialized alignment padding must be added.
705 The message body need not end on an 8-byte boundary.
709 The maximum length of a message, including header, header alignment padding,
710 and body is 2 to the 27th power or 134217728. Implementations must not
711 send or accept messages exceeding this size.
715 The signature of the header is:
719 Written out more readably, this is:
721 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
726 These values have the following meanings:
732 <entry>Description</entry>
737 <entry>1st <literal>BYTE</literal></entry>
738 <entry>Endianness flag; ASCII 'l' for little-endian
739 or ASCII 'B' for big-endian. Both header and body are
740 in this endianness.</entry>
743 <entry>2nd <literal>BYTE</literal></entry>
744 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
745 Currently-defined types are described below.
749 <entry>3rd <literal>BYTE</literal></entry>
750 <entry>Bitwise OR of flags. Unknown flags
751 must be ignored. Currently-defined flags are described below.
755 <entry>4th <literal>BYTE</literal></entry>
756 <entry>Major protocol version of the sending application. If
757 the major protocol version of the receiving application does not
758 match, the applications will not be able to communicate and the
759 D-Bus connection must be disconnected. The major protocol
760 version for this version of the specification is 0.
761 FIXME this field is stupid and pointless to put in
766 <entry>1st <literal>UINT32</literal></entry>
767 <entry>Length in bytes of the message body, starting
768 from the end of the header. The header ends after
769 its alignment padding to an 8-boundary.
773 <entry>2nd <literal>UINT32</literal></entry>
774 <entry>The serial of this message, used as a cookie
775 by the sender to identify the reply corresponding
780 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
781 <entry>An array of zero or more <firstterm>header
782 fields</firstterm> where the byte is the field code, and the
783 variant is the field value. The message type determines
784 which fields are required.
792 <firstterm>Message types</firstterm> that can appear in the second byte
798 <entry>Conventional name</entry>
799 <entry>Decimal value</entry>
800 <entry>Description</entry>
805 <entry><literal>INVALID</literal></entry>
807 <entry>This is an invalid type.</entry>
810 <entry><literal>METHOD_CALL</literal></entry>
812 <entry>Method call.</entry>
815 <entry><literal>METHOD_RETURN</literal></entry>
817 <entry>Method reply with returned data.</entry>
820 <entry><literal>ERROR</literal></entry>
822 <entry>Error reply. If the first argument exists and is a
823 string, it is an error message.</entry>
826 <entry><literal>SIGNAL</literal></entry>
828 <entry>Signal emission.</entry>
835 Flags that can appear in the third byte of the header:
840 <entry>Conventional name</entry>
841 <entry>Hex value</entry>
842 <entry>Description</entry>
847 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
849 <entry>This message does not expect method return replies or
850 error replies; the reply can be omitted as an
851 optimization. However, it is compliant with this specification
852 to return the reply despite this flag and the only harm
853 from doing so is extra network traffic.
857 <entry><literal>NO_AUTO_START</literal></entry>
859 <entry>The bus must not launch an owner
860 for the destination name in response to this message.
868 <sect3 id="message-protocol-header-fields">
869 <title>Header Fields</title>
872 The array at the end of the header contains <firstterm>header
873 fields</firstterm>, where each field is a 1-byte field code followed
874 by a field value. A header must contain the required header fields for
875 its message type, and zero or more of any optional header
876 fields. Future versions of this protocol specification may add new
877 fields. Implementations must ignore fields they do not
878 understand. Implementations must not invent their own header fields;
879 only changes to this specification may introduce new header fields.
883 Again, if an implementation sees a header field code that it does not
884 expect, it must ignore that field, as it will be part of a new
885 (but compatible) version of this specification. This also applies
886 to known header fields appearing in unexpected messages, for
887 example: if a signal has a reply serial it must be ignored
888 even though it has no meaning as of this version of the spec.
892 However, implementations must not send or accept known header fields
893 with the wrong type stored in the field value. So for example a
894 message with an <literal>INTERFACE</literal> field of type
895 <literal>UINT32</literal> would be considered corrupt.
899 Here are the currently-defined header fields:
904 <entry>Conventional Name</entry>
905 <entry>Decimal Code</entry>
907 <entry>Required In</entry>
908 <entry>Description</entry>
913 <entry><literal>INVALID</literal></entry>
916 <entry>not allowed</entry>
917 <entry>Not a valid field name (error if it appears in a message)</entry>
920 <entry><literal>PATH</literal></entry>
922 <entry><literal>OBJECT_PATH</literal></entry>
923 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
924 <entry>The object to send a call to,
925 or the object a signal is emitted from.
927 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
928 implementations should not send messages with this path,
929 and the reference implementation of the bus daemon will
930 disconnect any application that attempts to do so.
934 <entry><literal>INTERFACE</literal></entry>
936 <entry><literal>STRING</literal></entry>
937 <entry><literal>SIGNAL</literal></entry>
939 The interface to invoke a method call on, or
940 that a signal is emitted from. Optional for
941 method calls, required for signals.
942 The special interface
943 <literal>org.freedesktop.DBus.Local</literal> is reserved;
944 implementations should not send messages with this
945 interface, and the reference implementation of the bus
946 daemon will disconnect any application that attempts to
951 <entry><literal>MEMBER</literal></entry>
953 <entry><literal>STRING</literal></entry>
954 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
955 <entry>The member, either the method name or signal name.</entry>
958 <entry><literal>ERROR_NAME</literal></entry>
960 <entry><literal>STRING</literal></entry>
961 <entry><literal>ERROR</literal></entry>
962 <entry>The name of the error that occurred, for errors</entry>
965 <entry><literal>REPLY_SERIAL</literal></entry>
967 <entry><literal>UINT32</literal></entry>
968 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
969 <entry>The serial number of the message this message is a reply
970 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
973 <entry><literal>DESTINATION</literal></entry>
975 <entry><literal>STRING</literal></entry>
976 <entry>optional</entry>
977 <entry>The name of the connection this message is intended for.
978 Only used in combination with the message bus, see
979 <xref linkend="message-bus"/>.</entry>
982 <entry><literal>SENDER</literal></entry>
984 <entry><literal>STRING</literal></entry>
985 <entry>optional</entry>
986 <entry>Unique name of the sending connection.
987 The message bus fills in this field so it is reliable; the field is
988 only meaningful in combination with the message bus.</entry>
991 <entry><literal>SIGNATURE</literal></entry>
993 <entry><literal>SIGNATURE</literal></entry>
994 <entry>optional</entry>
995 <entry>The signature of the message body.
996 If omitted, it is assumed to be the
997 empty signature "" (i.e. the body must be 0-length).</entry>
1006 <sect2 id="message-protocol-names">
1007 <title>Valid Names</title>
1009 The various names in D-Bus messages have some restrictions.
1012 There is a <firstterm>maximum name length</firstterm>
1013 of 255 which applies to bus names, interfaces, and members.
1015 <sect3 id="message-protocol-names-interface">
1016 <title>Interface names</title>
1018 Interfaces have names with type <literal>STRING</literal>, meaning that
1019 they must be valid UTF-8. However, there are also some
1020 additional restrictions that apply to interface names
1023 <listitem><para>Interface names are composed of 1 or more elements separated by
1024 a period ('.') character. All elements must contain at least
1028 <listitem><para>Each element must only contain the ASCII characters
1029 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1033 <listitem><para>Interface names must contain at least one '.' (period)
1034 character (and thus at least two elements).
1037 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1038 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1042 <sect3 id="message-protocol-names-bus">
1043 <title>Bus names</title>
1045 Connections have one or more bus names associated with them.
1046 A connection has exactly one bus name that is a unique connection
1047 name. The unique connection name remains with the connection for
1048 its entire lifetime.
1049 A bus name is of type <literal>STRING</literal>,
1050 meaning that it must be valid UTF-8. However, there are also
1051 some additional restrictions that apply to bus names
1054 <listitem><para>Bus names that start with a colon (':')
1055 character are unique connection names.
1058 <listitem><para>Bus names are composed of 1 or more elements separated by
1059 a period ('.') character. All elements must contain at least
1063 <listitem><para>Each element must only contain the ASCII characters
1064 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1065 connection name may begin with a digit, elements in
1066 other bus names must not begin with a digit.
1070 <listitem><para>Bus names must contain at least one '.' (period)
1071 character (and thus at least two elements).
1074 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1075 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1079 Note that the hyphen ('-') character is allowed in bus names but
1080 not in interface names.
1083 <sect3 id="message-protocol-names-member">
1084 <title>Member names</title>
1086 Member (i.e. method or signal) names:
1088 <listitem><para>Must only contain the ASCII characters
1089 "[A-Z][a-z][0-9]_" and may not begin with a
1090 digit.</para></listitem>
1091 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1092 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1093 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1097 <sect3 id="message-protocol-names-error">
1098 <title>Error names</title>
1100 Error names have the same restrictions as interface names.
1105 <sect2 id="message-protocol-types">
1106 <title>Message Types</title>
1108 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1109 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1110 This section describes these conventions.
1112 <sect3 id="message-protocol-types-method">
1113 <title>Method Calls</title>
1115 Some messages invoke an operation on a remote object. These are
1116 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1117 messages map naturally to methods on objects in a typical program.
1120 A method call message is required to have a <literal>MEMBER</literal> header field
1121 indicating the name of the method. Optionally, the message has an
1122 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1123 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1124 a method with the same name, it is undefined which of the two methods
1125 will be invoked. Implementations may also choose to return an error in
1126 this ambiguous case. However, if a method name is unique
1127 implementations must not require an interface field.
1130 Method call messages also include a <literal>PATH</literal> field
1131 indicating the object to invoke the method on. If the call is passing
1132 through a message bus, the message will also have a
1133 <literal>DESTINATION</literal> field giving the name of the connection
1134 to receive the message.
1137 When an application handles a method call message, it is required to
1138 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1139 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1140 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1143 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1144 are the return value(s) or "out parameters" of the method call.
1145 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1146 and the call fails; no return value will be provided. It makes
1147 no sense to send multiple replies to the same method call.
1150 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1151 reply is required, so the caller will know the method
1152 was successfully processed.
1155 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1159 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1160 then as an optimization the application receiving the method
1161 call may choose to omit the reply message (regardless of
1162 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1163 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1164 flag and reply anyway.
1167 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1168 destination name does not exist then a program to own the destination
1169 name will be started before the message is delivered. The message
1170 will be held until the new program is successfully started or has
1171 failed to start; in case of failure, an error will be returned. This
1172 flag is only relevant in the context of a message bus, it is ignored
1173 during one-to-one communication with no intermediate bus.
1175 <sect4 id="message-protocol-types-method-apis">
1176 <title>Mapping method calls to native APIs</title>
1178 APIs for D-Bus may map method calls to a method call in a specific
1179 programming language, such as C++, or may map a method call written
1180 in an IDL to a D-Bus message.
1183 In APIs of this nature, arguments to a method are often termed "in"
1184 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1185 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1186 "inout" arguments, which are both sent and received, i.e. the caller
1187 passes in a value which is modified. Mapped to D-Bus, an "inout"
1188 argument is equivalent to an "in" argument, followed by an "out"
1189 argument. You can't pass things "by reference" over the wire, so
1190 "inout" is purely an illusion of the in-process API.
1193 Given a method with zero or one return values, followed by zero or more
1194 arguments, where each argument may be "in", "out", or "inout", the
1195 caller constructs a message by appending each "in" or "inout" argument,
1196 in order. "out" arguments are not represented in the caller's message.
1199 The recipient constructs a reply by appending first the return value
1200 if any, then each "out" or "inout" argument, in order.
1201 "in" arguments are not represented in the reply message.
1204 Error replies are normally mapped to exceptions in languages that have
1208 In converting from native APIs to D-Bus, it is perhaps nice to
1209 map D-Bus naming conventions ("FooBar") to native conventions
1210 such as "fooBar" or "foo_bar" automatically. This is OK
1211 as long as you can say that the native API is one that
1212 was specifically written for D-Bus. It makes the most sense
1213 when writing object implementations that will be exported
1214 over the bus. Object proxies used to invoke remote D-Bus
1215 objects probably need the ability to call any D-Bus method,
1216 and thus a magic name mapping like this could be a problem.
1219 This specification doesn't require anything of native API bindings;
1220 the preceding is only a suggested convention for consistency
1226 <sect3 id="message-protocol-types-signal">
1227 <title>Signal Emission</title>
1229 Unlike method calls, signal emissions have no replies.
1230 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1231 It must have three header fields: <literal>PATH</literal> giving the object
1232 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1233 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1234 for signals, though it is optional for method calls.
1238 <sect3 id="message-protocol-types-errors">
1239 <title>Errors</title>
1241 Messages of type <literal>ERROR</literal> are most commonly replies
1242 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1243 to any kind of message. The message bus for example
1244 will return an <literal>ERROR</literal> in reply to a signal emission if
1245 the bus does not have enough memory to send the signal.
1248 An <literal>ERROR</literal> may have any arguments, but if the first
1249 argument is a <literal>STRING</literal>, it must be an error message.
1250 The error message may be logged or shown to the user
1255 <sect3 id="message-protocol-types-notation">
1256 <title>Notation in this document</title>
1258 This document uses a simple pseudo-IDL to describe particular method
1259 calls and signals. Here is an example of a method call:
1261 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1262 out UINT32 resultcode)
1264 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1265 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1266 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1267 characters so it's known that the last part of the name in
1268 the "IDL" is the member name.
1271 In C++ that might end up looking like this:
1273 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1274 unsigned int flags);
1276 or equally valid, the return value could be done as an argument:
1278 void org::freedesktop::DBus::StartServiceByName (const char *name,
1280 unsigned int *resultcode);
1282 It's really up to the API designer how they want to make
1283 this look. You could design an API where the namespace wasn't used
1284 in C++, using STL or Qt, using varargs, or whatever you wanted.
1287 Signals are written as follows:
1289 org.freedesktop.DBus.NameLost (STRING name)
1291 Signals don't specify "in" vs. "out" because only
1292 a single direction is possible.
1295 It isn't especially encouraged to use this lame pseudo-IDL in actual
1296 API implementations; you might use the native notation for the
1297 language you're using, or you might use COM or CORBA IDL, for example.
1302 <sect2 id="message-protocol-handling-invalid">
1303 <title>Invalid Protocol and Spec Extensions</title>
1306 For security reasons, the D-Bus protocol should be strictly parsed and
1307 validated, with the exception of defined extension points. Any invalid
1308 protocol or spec violations should result in immediately dropping the
1309 connection without notice to the other end. Exceptions should be
1310 carefully considered, e.g. an exception may be warranted for a
1311 well-understood idiosyncrasy of a widely-deployed implementation. In
1312 cases where the other end of a connection is 100% trusted and known to
1313 be friendly, skipping validation for performance reasons could also make
1314 sense in certain cases.
1318 Generally speaking violations of the "must" requirements in this spec
1319 should be considered possible attempts to exploit security, and violations
1320 of the "should" suggestions should be considered legitimate (though perhaps
1321 they should generate an error in some cases).
1325 The following extension points are built in to D-Bus on purpose and must
1326 not be treated as invalid protocol. The extension points are intended
1327 for use by future versions of this spec, they are not intended for third
1328 parties. At the moment, the only way a third party could extend D-Bus
1329 without breaking interoperability would be to introduce a way to negotiate new
1330 feature support as part of the auth protocol, using EXTENSION_-prefixed
1331 commands. There is not yet a standard way to negotiate features.
1335 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1336 commands result in an ERROR rather than a disconnect. This enables
1337 future extensions to the protocol. Commands starting with EXTENSION_ are
1338 reserved for third parties.
1343 The authentication protocol supports pluggable auth mechanisms.
1348 The address format (see <xref linkend="addresses"/>) supports new
1354 Messages with an unknown type (something other than
1355 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1356 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1357 Unknown-type messages must still be well-formed in the same way
1358 as the known messages, however. They still have the normal
1364 Header fields with an unknown or unexpected field code must be ignored,
1365 though again they must still be well-formed.
1370 New standard interfaces (with new methods and signals) can of course be added.
1380 <sect1 id="auth-protocol">
1381 <title>Authentication Protocol</title>
1383 Before the flow of messages begins, two applications must
1384 authenticate. A simple plain-text protocol is used for
1385 authentication; this protocol is a SASL profile, and maps fairly
1386 directly from the SASL specification. The message encoding is
1387 NOT used here, only plain text messages.
1390 In examples, "C:" and "S:" indicate lines sent by the client and
1391 server respectively.
1393 <sect2 id="auth-protocol-overview">
1394 <title>Protocol Overview</title>
1396 The protocol is a line-based protocol, where each line ends with
1397 \r\n. Each line begins with an all-caps ASCII command name containing
1398 only the character range [A-Z_], a space, then any arguments for the
1399 command, then the \r\n ending the line. The protocol is
1400 case-sensitive. All bytes must be in the ASCII character set.
1402 Commands from the client to the server are as follows:
1405 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1406 <listitem><para>CANCEL</para></listitem>
1407 <listitem><para>BEGIN</para></listitem>
1408 <listitem><para>DATA <data in hex encoding></para></listitem>
1409 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1412 From server to client are as follows:
1415 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1416 <listitem><para>OK <GUID in hex></para></listitem>
1417 <listitem><para>DATA <data in hex encoding></para></listitem>
1418 <listitem><para>ERROR</para></listitem>
1422 Unofficial extensions to the command set must begin with the letters
1423 "EXTENSION_", to avoid conflicts with future official commands.
1424 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1427 <sect2 id="auth-nul-byte">
1428 <title>Special credentials-passing nul byte</title>
1430 Immediately after connecting to the server, the client must send a
1431 single nul byte. This byte may be accompanied by credentials
1432 information on some operating systems that use sendmsg() with
1433 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1434 sockets. However, the nul byte must be sent even on other kinds of
1435 socket, and even on operating systems that do not require a byte to be
1436 sent in order to transmit credentials. The text protocol described in
1437 this document begins after the single nul byte. If the first byte
1438 received from the client is not a nul byte, the server may disconnect
1442 A nul byte in any context other than the initial byte is an error;
1443 the protocol is ASCII-only.
1446 The credentials sent along with the nul byte may be used with the
1447 SASL mechanism EXTERNAL.
1450 <sect2 id="auth-command-auth">
1451 <title>AUTH command</title>
1453 If an AUTH command has no arguments, it is a request to list
1454 available mechanisms. The server must respond with a REJECTED
1455 command listing the mechanisms it understands, or with an error.
1458 If an AUTH command specifies a mechanism, and the server supports
1459 said mechanism, the server should begin exchanging SASL
1460 challenge-response data with the client using DATA commands.
1463 If the server does not support the mechanism given in the AUTH
1464 command, it must send either a REJECTED command listing the mechanisms
1465 it does support, or an error.
1468 If the [initial-response] argument is provided, it is intended for use
1469 with mechanisms that have no initial challenge (or an empty initial
1470 challenge), as if it were the argument to an initial DATA command. If
1471 the selected mechanism has an initial challenge and [initial-response]
1472 was provided, the server should reject authentication by sending
1476 If authentication succeeds after exchanging DATA commands,
1477 an OK command must be sent to the client.
1480 The first octet received by the client after the \r\n of the OK
1481 command must be the first octet of the authenticated/encrypted
1482 stream of D-Bus messages.
1485 The first octet received by the server after the \r\n of the BEGIN
1486 command from the client must be the first octet of the
1487 authenticated/encrypted stream of D-Bus messages.
1490 <sect2 id="auth-command-cancel">
1491 <title>CANCEL Command</title>
1493 At any time up to sending the BEGIN command, the client may send a
1494 CANCEL command. On receiving the CANCEL command, the server must
1495 send a REJECTED command and abort the current authentication
1499 <sect2 id="auth-command-data">
1500 <title>DATA Command</title>
1502 The DATA command may come from either client or server, and simply
1503 contains a hex-encoded block of data to be interpreted
1504 according to the SASL mechanism in use.
1507 Some SASL mechanisms support sending an "empty string";
1508 FIXME we need some way to do this.
1511 <sect2 id="auth-command-begin">
1512 <title>BEGIN Command</title>
1514 The BEGIN command acknowledges that the client has received an
1515 OK command from the server, and that the stream of messages
1519 The first octet received by the server after the \r\n of the BEGIN
1520 command from the client must be the first octet of the
1521 authenticated/encrypted stream of D-Bus messages.
1524 <sect2 id="auth-command-rejected">
1525 <title>REJECTED Command</title>
1527 The REJECTED command indicates that the current authentication
1528 exchange has failed, and further exchange of DATA is inappropriate.
1529 The client would normally try another mechanism, or try providing
1530 different responses to challenges.
1532 Optionally, the REJECTED command has a space-separated list of
1533 available auth mechanisms as arguments. If a server ever provides
1534 a list of supported mechanisms, it must provide the same list
1535 each time it sends a REJECTED message. Clients are free to
1536 ignore all lists received after the first.
1539 <sect2 id="auth-command-ok">
1540 <title>OK Command</title>
1542 The OK command indicates that the client has been authenticated,
1543 and that further communication will be a stream of D-Bus messages
1544 (optionally encrypted, as negotiated) rather than this protocol.
1547 The first octet received by the client after the \r\n of the OK
1548 command must be the first octet of the authenticated/encrypted
1549 stream of D-Bus messages.
1552 The client must respond to the OK command by sending a BEGIN
1553 command, followed by its stream of messages, or by disconnecting.
1554 The server must not accept additional commands using this protocol
1555 after the OK command has been sent.
1558 The OK command has one argument, which is the GUID of the server.
1559 See <xref linkend="addresses"/> for more on server GUIDs.
1562 <sect2 id="auth-command-error">
1563 <title>ERROR Command</title>
1565 The ERROR command indicates that either server or client did not
1566 know a command, does not accept the given command in the current
1567 context, or did not understand the arguments to the command. This
1568 allows the protocol to be extended; a client or server can send a
1569 command present or permitted only in new protocol versions, and if
1570 an ERROR is received instead of an appropriate response, fall back
1571 to using some other technique.
1574 If an ERROR is sent, the server or client that sent the
1575 error must continue as if the command causing the ERROR had never been
1576 received. However, the the server or client receiving the error
1577 should try something other than whatever caused the error;
1578 if only canceling/rejecting the authentication.
1581 If the D-Bus protocol changes incompatibly at some future time,
1582 applications implementing the new protocol would probably be able to
1583 check for support of the new protocol by sending a new command and
1584 receiving an ERROR from applications that don't understand it. Thus the
1585 ERROR feature of the auth protocol is an escape hatch that lets us
1586 negotiate extensions or changes to the D-Bus protocol in the future.
1589 <sect2 id="auth-examples">
1590 <title>Authentication examples</title>
1594 <title>Example of successful magic cookie authentication</title>
1596 (MAGIC_COOKIE is a made up mechanism)
1598 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1604 <title>Example of finding out mechanisms then picking one</title>
1607 S: REJECTED KERBEROS_V4 SKEY
1608 C: AUTH SKEY 7ab83f32ee
1609 S: DATA 8799cabb2ea93e
1610 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1616 <title>Example of client sends unknown command then falls back to regular auth</title>
1620 C: AUTH MAGIC_COOKIE 3736343435313230333039
1626 <title>Example of server doesn't support initial auth mechanism</title>
1628 C: AUTH MAGIC_COOKIE 3736343435313230333039
1629 S: REJECTED KERBEROS_V4 SKEY
1630 C: AUTH SKEY 7ab83f32ee
1631 S: DATA 8799cabb2ea93e
1632 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1638 <title>Example of wrong password or the like followed by successful retry</title>
1640 C: AUTH MAGIC_COOKIE 3736343435313230333039
1641 S: REJECTED KERBEROS_V4 SKEY
1642 C: AUTH SKEY 7ab83f32ee
1643 S: DATA 8799cabb2ea93e
1644 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1646 C: AUTH SKEY 7ab83f32ee
1647 S: DATA 8799cabb2ea93e
1648 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1654 <title>Example of skey cancelled and restarted</title>
1656 C: AUTH MAGIC_COOKIE 3736343435313230333039
1657 S: REJECTED KERBEROS_V4 SKEY
1658 C: AUTH SKEY 7ab83f32ee
1659 S: DATA 8799cabb2ea93e
1662 C: AUTH SKEY 7ab83f32ee
1663 S: DATA 8799cabb2ea93e
1664 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1671 <sect2 id="auth-states">
1672 <title>Authentication state diagrams</title>
1675 This section documents the auth protocol in terms of
1676 a state machine for the client and the server. This is
1677 probably the most robust way to implement the protocol.
1680 <sect3 id="auth-states-client">
1681 <title>Client states</title>
1684 To more precisely describe the interaction between the
1685 protocol state machine and the authentication mechanisms the
1686 following notation is used: MECH(CHALL) means that the
1687 server challenge CHALL was fed to the mechanism MECH, which
1693 CONTINUE(RESP) means continue the auth conversation
1694 and send RESP as the response to the server;
1700 OK(RESP) means that after sending RESP to the server
1701 the client side of the auth conversation is finished
1702 and the server should return "OK";
1708 ERROR means that CHALL was invalid and could not be
1714 Both RESP and CHALL may be empty.
1718 The Client starts by getting an initial response from the
1719 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1720 the mechanism did not provide an initial response. If the
1721 mechanism returns CONTINUE, the client starts in state
1722 <emphasis>WaitingForData</emphasis>, if the mechanism
1723 returns OK the client starts in state
1724 <emphasis>WaitingForOK</emphasis>.
1728 The client should keep track of available mechanisms and
1729 which it mechanisms it has already attempted. This list is
1730 used to decide which AUTH command to send. When the list is
1731 exhausted, the client should give up and close the
1736 <title><emphasis>WaitingForData</emphasis></title>
1744 MECH(CHALL) returns CONTINUE(RESP) → send
1746 <emphasis>WaitingForData</emphasis>
1750 MECH(CHALL) returns OK(RESP) → send DATA
1751 RESP, goto <emphasis>WaitingForOK</emphasis>
1755 MECH(CHALL) returns ERROR → send ERROR
1756 [msg], goto <emphasis>WaitingForData</emphasis>
1764 Receive REJECTED [mechs] →
1765 send AUTH [next mech], goto
1766 WaitingForData or <emphasis>WaitingForOK</emphasis>
1771 Receive ERROR → send
1773 <emphasis>WaitingForReject</emphasis>
1778 Receive OK → send
1779 BEGIN, terminate auth
1780 conversation, authenticated
1785 Receive anything else → send
1787 <emphasis>WaitingForData</emphasis>
1795 <title><emphasis>WaitingForOK</emphasis></title>
1800 Receive OK → send BEGIN, terminate auth
1801 conversation, <emphasis>authenticated</emphasis>
1806 Receive REJECT [mechs] → send AUTH [next mech],
1807 goto <emphasis>WaitingForData</emphasis> or
1808 <emphasis>WaitingForOK</emphasis>
1814 Receive DATA → send CANCEL, goto
1815 <emphasis>WaitingForReject</emphasis>
1821 Receive ERROR → send CANCEL, goto
1822 <emphasis>WaitingForReject</emphasis>
1828 Receive anything else → send ERROR, goto
1829 <emphasis>WaitingForOK</emphasis>
1837 <title><emphasis>WaitingForReject</emphasis></title>
1842 Receive REJECT [mechs] → send AUTH [next mech],
1843 goto <emphasis>WaitingForData</emphasis> or
1844 <emphasis>WaitingForOK</emphasis>
1850 Receive anything else → terminate auth
1851 conversation, disconnect
1860 <sect3 id="auth-states-server">
1861 <title>Server states</title>
1864 For the server MECH(RESP) means that the client response
1865 RESP was fed to the the mechanism MECH, which returns one of
1870 CONTINUE(CHALL) means continue the auth conversation and
1871 send CHALL as the challenge to the client;
1877 OK means that the client has been successfully
1884 REJECT means that the client failed to authenticate or
1885 there was an error in RESP.
1890 The server starts out in state
1891 <emphasis>WaitingForAuth</emphasis>. If the client is
1892 rejected too many times the server must disconnect the
1897 <title><emphasis>WaitingForAuth</emphasis></title>
1903 Receive AUTH → send REJECTED [mechs], goto
1904 <emphasis>WaitingForAuth</emphasis>
1910 Receive AUTH MECH RESP
1914 MECH not valid mechanism → send REJECTED
1916 <emphasis>WaitingForAuth</emphasis>
1920 MECH(RESP) returns CONTINUE(CHALL) → send
1922 <emphasis>WaitingForData</emphasis>
1926 MECH(RESP) returns OK → send OK, goto
1927 <emphasis>WaitingForBegin</emphasis>
1931 MECH(RESP) returns REJECT → send REJECTED
1933 <emphasis>WaitingForAuth</emphasis>
1941 Receive BEGIN → terminate
1942 auth conversation, disconnect
1948 Receive ERROR → send REJECTED [mechs], goto
1949 <emphasis>WaitingForAuth</emphasis>
1955 Receive anything else → send
1957 <emphasis>WaitingForAuth</emphasis>
1966 <title><emphasis>WaitingForData</emphasis></title>
1974 MECH(RESP) returns CONTINUE(CHALL) → send
1976 <emphasis>WaitingForData</emphasis>
1980 MECH(RESP) returns OK → send OK, goto
1981 <emphasis>WaitingForBegin</emphasis>
1985 MECH(RESP) returns REJECT → send REJECTED
1987 <emphasis>WaitingForAuth</emphasis>
1995 Receive BEGIN → terminate auth conversation,
2002 Receive CANCEL → send REJECTED [mechs], goto
2003 <emphasis>WaitingForAuth</emphasis>
2009 Receive ERROR → send REJECTED [mechs], goto
2010 <emphasis>WaitingForAuth</emphasis>
2016 Receive anything else → send ERROR, goto
2017 <emphasis>WaitingForData</emphasis>
2025 <title><emphasis>WaitingForBegin</emphasis></title>
2030 Receive BEGIN → terminate auth conversation,
2031 client authenticated
2037 Receive CANCEL → send REJECTED [mechs], goto
2038 <emphasis>WaitingForAuth</emphasis>
2044 Receive ERROR → send REJECTED [mechs], goto
2045 <emphasis>WaitingForAuth</emphasis>
2051 Receive anything else → send ERROR, goto
2052 <emphasis>WaitingForBegin</emphasis>
2062 <sect2 id="auth-mechanisms">
2063 <title>Authentication mechanisms</title>
2065 This section describes some new authentication mechanisms.
2066 D-Bus also allows any standard SASL mechanism of course.
2068 <sect3 id="auth-mechanisms-sha">
2069 <title>DBUS_COOKIE_SHA1</title>
2071 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2072 has the ability to read a private file owned by the user being
2073 authenticated. If the client can prove that it has access to a secret
2074 cookie stored in this file, then the client is authenticated.
2075 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2079 Authentication proceeds as follows:
2083 The client sends the username it would like to authenticate
2089 The server sends the name of its "cookie context" (see below); a
2090 space character; the integer ID of the secret cookie the client
2091 must demonstrate knowledge of; a space character; then a
2092 hex-encoded randomly-generated challenge string.
2097 The client locates the cookie, and generates its own hex-encoded
2098 randomly-generated challenge string. The client then
2099 concatenates the server's hex-encoded challenge, a ":"
2100 character, its own hex-encoded challenge, another ":" character,
2101 and the hex-encoded cookie. It computes the SHA-1 hash of this
2102 composite string. It sends back to the server the client's
2103 hex-encoded challenge string, a space character, and the SHA-1
2109 The server generates the same concatenated string used by the
2110 client and computes its SHA-1 hash. It compares the hash with
2111 the hash received from the client; if the two hashes match, the
2112 client is authenticated.
2118 Each server has a "cookie context," which is a name that identifies a
2119 set of cookies that apply to that server. A sample context might be
2120 "org_freedesktop_session_bus". Context names must be valid ASCII,
2121 nonzero length, and may not contain the characters slash ("/"),
2122 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2123 tab ("\t"), or period ("."). There is a default context,
2124 "org_freedesktop_general" that's used by servers that do not specify
2128 Cookies are stored in a user's home directory, in the directory
2129 <filename>~/.dbus-keyrings/</filename>. This directory must
2130 not be readable or writable by other users. If it is,
2131 clients and servers must ignore it. The directory
2132 contains cookie files named after the cookie context.
2135 A cookie file contains one cookie per line. Each line
2136 has three space-separated fields:
2140 The cookie ID number, which must be a non-negative integer and
2141 may not be used twice in the same file.
2146 The cookie's creation time, in UNIX seconds-since-the-epoch
2152 The cookie itself, a hex-encoded random block of bytes. The cookie
2153 may be of any length, though obviously security increases
2154 as the length increases.
2160 Only server processes modify the cookie file.
2161 They must do so with this procedure:
2165 Create a lockfile name by appending ".lock" to the name of the
2166 cookie file. The server should attempt to create this file
2167 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2168 fails, the lock fails. Servers should retry for a reasonable
2169 period of time, then they may choose to delete an existing lock
2170 to keep users from having to manually delete a stale
2171 lock. <footnote><para>Lockfiles are used instead of real file
2172 locking <literal>fcntl()</literal> because real locking
2173 implementations are still flaky on network
2174 filesystems.</para></footnote>
2179 Once the lockfile has been created, the server loads the cookie
2180 file. It should then delete any cookies that are old (the
2181 timeout can be fairly short), or more than a reasonable
2182 time in the future (so that cookies never accidentally
2183 become permanent, if the clock was set far into the future
2184 at some point). If no recent keys remain, the
2185 server may generate a new key.
2190 The pruned and possibly added-to cookie file
2191 must be resaved atomically (using a temporary
2192 file which is rename()'d).
2197 The lock must be dropped by deleting the lockfile.
2203 Clients need not lock the file in order to load it,
2204 because servers are required to save the file atomically.
2209 <sect1 id="addresses">
2210 <title>Server Addresses</title>
2212 Server addresses consist of a transport name followed by a colon, and
2213 then an optional, comma-separated list of keys and values in the form key=value.
2214 Each value is escaped.
2218 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2219 Which is the address to a unix socket with the path /tmp/dbus-test.
2222 Value escaping is similar to URI escaping but simpler.
2226 The set of optionally-escaped bytes is:
2227 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2228 <emphasis>byte</emphasis> (note, not character) which is not in the
2229 set of optionally-escaped bytes must be replaced with an ASCII
2230 percent (<literal>%</literal>) and the value of the byte in hex.
2231 The hex value must always be two digits, even if the first digit is
2232 zero. The optionally-escaped bytes may be escaped if desired.
2237 To unescape, append each byte in the value; if a byte is an ASCII
2238 percent (<literal>%</literal>) character then append the following
2239 hex value instead. It is an error if a <literal>%</literal> byte
2240 does not have two hex digits following. It is an error if a
2241 non-optionally-escaped byte is seen unescaped.
2245 The set of optionally-escaped bytes is intended to preserve address
2246 readability and convenience.
2250 A server may specify a key-value pair with the key <literal>guid</literal>
2251 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2252 describes the format of the <literal>guid</literal> field. If present,
2253 this UUID may be used to distinguish one server address from another. A
2254 server should use a different UUID for each address it listens on. For
2255 example, if a message bus daemon offers both UNIX domain socket and TCP
2256 connections, but treats clients the same regardless of how they connect,
2257 those two connections are equivalent post-connection but should have
2258 distinct UUIDs to distinguish the kinds of connection.
2262 The intent of the address UUID feature is to allow a client to avoid
2263 opening multiple identical connections to the same server, by allowing the
2264 client to check whether an address corresponds to an already-existing
2265 connection. Comparing two addresses is insufficient, because addresses
2266 can be recycled by distinct servers, and equivalent addresses may look
2267 different if simply compared as strings (for example, the host in a TCP
2268 address can be given as an IP address or as a hostname).
2272 Note that the address key is <literal>guid</literal> even though the
2273 rest of the API and documentation says "UUID," for historical reasons.
2277 [FIXME clarify if attempting to connect to each is a requirement
2278 or just a suggestion]
2279 When connecting to a server, multiple server addresses can be
2280 separated by a semi-colon. The library will then try to connect
2281 to the first address and if that fails, it'll try to connect to
2282 the next one specified, and so forth. For example
2283 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2288 <sect1 id="transports">
2289 <title>Transports</title>
2291 [FIXME we need to specify in detail each transport and its possible arguments]
2293 Current transports include: unix domain sockets (including
2294 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2295 in-process pipes. Future possible transports include one that
2296 tunnels over X11 protocol.
2299 <sect2 id="transports-unix-domain-sockets">
2300 <title>Unix Domain Sockets</title>
2302 Unix domain sockets can be either paths in the file system or on Linux
2303 kernels, they can be abstract which are similar to paths but
2304 do not show up in the file system.
2308 When a socket is opened by the D-Bus library it truncates the path
2309 name right before the first trailing Nul byte. This is true for both
2310 normal paths and abstract paths. Note that this is a departure from
2311 previous versions of D-Bus that would create sockets with a fixed
2312 length path name. Names which were shorter than the fixed length
2313 would be padded by Nul bytes.
2318 <sect1 id="naming-conventions">
2319 <title>Naming Conventions</title>
2322 D-Bus namespaces are all lowercase and correspond to reversed domain
2323 names, as with Java. e.g. "org.freedesktop"
2326 Interface, signal, method, and property names are "WindowsStyleCaps", note
2327 that the first letter is capitalized, unlike Java.
2330 Object paths are normally all lowercase with underscores used rather than
2336 <title>UUIDs</title>
2338 A working D-Bus implementation uses universally-unique IDs in two places.
2339 First, each server address has a UUID identifying the address,
2340 as described in <xref linkend="addresses"/>. Second, each operating
2341 system kernel instance running a D-Bus client or server has a UUID
2342 identifying that kernel, retrieved by invoking the method
2343 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2344 linkend="standard-interfaces-peer"/>).
2347 The term "UUID" in this document is intended literally, i.e. an
2348 identifier that is universally unique. It is not intended to refer to
2349 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2352 The UUID must contain 128 bits of data and be hex-encoded. The
2353 hex-encoded string may not contain hyphens or other non-hex-digit
2354 characters, and it must be exactly 32 characters long. To generate a
2355 UUID, the current reference implementation concatenates 96 bits of random
2356 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2360 It would also be acceptable and probably better to simply generate 128
2361 bits of random data, as long as the random number generator is of high
2362 quality. The timestamp could conceivably help if the random bits are not
2363 very random. With a quality random number generator, collisions are
2364 extremely unlikely even with only 96 bits, so it's somewhat academic.
2367 Implementations should, however, stick to random data for the first 96 bits
2372 <sect1 id="standard-interfaces">
2373 <title>Standard Interfaces</title>
2375 See <xref linkend="message-protocol-types-notation"/> for details on
2376 the notation used in this section. There are some standard interfaces
2377 that may be useful across various D-Bus applications.
2379 <sect2 id="standard-interfaces-peer">
2380 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2382 The <literal>org.freedesktop.DBus.Peer</literal> interface
2385 org.freedesktop.DBus.Peer.Ping ()
2386 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2390 On receipt of the <literal>METHOD_CALL</literal> message
2391 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2392 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2393 usual. It does not matter which object path a ping is sent to. The
2394 reference implementation handles this method automatically.
2397 On receipt of the <literal>METHOD_CALL</literal> message
2398 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2399 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2400 UUID representing the identity of the machine the process is running on.
2401 This UUID must be the same for all processes on a single system at least
2402 until that system next reboots. It should be the same across reboots
2403 if possible, but this is not always possible to implement and is not
2405 It does not matter which object path a GetMachineId is sent to. The
2406 reference implementation handles this method automatically.
2409 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2410 a virtual machine running on a hypervisor, rather than a physical machine.
2411 Basically if two processes see the same UUID, they should also see the same
2412 shared memory, UNIX domain sockets, process IDs, and other features that require
2413 a running OS kernel in common between the processes.
2416 The UUID is often used where other programs might use a hostname. Hostnames
2417 can change without rebooting, however, or just be "localhost" - so the UUID
2421 <xref linkend="uuids"/> explains the format of the UUID.
2425 <sect2 id="standard-interfaces-introspectable">
2426 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2428 This interface has one method:
2430 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2434 Objects instances may implement
2435 <literal>Introspect</literal> which returns an XML description of
2436 the object, including its interfaces (with signals and methods), objects
2437 below it in the object path tree, and its properties.
2440 <xref linkend="introspection-format"/> describes the format of this XML string.
2443 <sect2 id="standard-interfaces-properties">
2444 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2446 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2447 or <firstterm>attributes</firstterm>. These can be exposed via the
2448 <literal>org.freedesktop.DBus.Properties</literal> interface.
2452 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2453 in STRING property_name,
2455 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2456 in STRING property_name,
2458 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
2459 out DICT<STRING,VARIANT> props);
2463 The available properties and whether they are writable can be determined
2464 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2465 see <xref linkend="standard-interfaces-introspectable"/>.
2468 An empty string may be provided for the interface name; in this case,
2469 if there are multiple properties on an object with the same name,
2470 the results are undefined (picking one by according to an arbitrary
2471 deterministic rule, or returning an error, are the reasonable
2477 <sect1 id="introspection-format">
2478 <title>Introspection Data Format</title>
2480 As described in <xref linkend="standard-interfaces-introspectable"/>,
2481 objects may be introspected at runtime, returning an XML string
2482 that describes the object. The same XML format may be used in
2483 other contexts as well, for example as an "IDL" for generating
2484 static language bindings.
2487 Here is an example of introspection data:
2489 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2490 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2491 <node name="/org/freedesktop/sample_object">
2492 <interface name="org.freedesktop.SampleInterface">
2493 <method name="Frobate">
2494 <arg name="foo" type="i" direction="in"/>
2495 <arg name="bar" type="s" direction="out"/>
2496 <arg name="baz" type="a{us}" direction="out"/>
2497 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2499 <method name="Bazify">
2500 <arg name="bar" type="(iiu)" direction="in"/>
2501 <arg name="bar" type="v" direction="out"/>
2503 <method name="Mogrify">
2504 <arg name="bar" type="(iiav)" direction="in"/>
2506 <signal name="Changed">
2507 <arg name="new_value" type="b"/>
2509 <property name="Bar" type="y" access="readwrite"/>
2511 <node name="child_of_sample_object"/>
2512 <node name="another_child_of_sample_object"/>
2517 A more formal DTD and spec needs writing, but here are some quick notes.
2521 Only the root <node> element can omit the node name, as it's
2522 known to be the object that was introspected. If the root
2523 <node> does have a name attribute, it must be an absolute
2524 object path. If child <node> have object paths, they must be
2530 If a child <node> has any sub-elements, then they
2531 must represent a complete introspection of the child.
2532 If a child <node> is empty, then it may or may
2533 not have sub-elements; the child must be introspected
2534 in order to find out. The intent is that if an object
2535 knows that its children are "fast" to introspect
2536 it can go ahead and return their information, but
2537 otherwise it can omit it.
2542 The direction element on <arg> may be omitted,
2543 in which case it defaults to "in" for method calls
2544 and "out" for signals. Signals only allow "out"
2545 so while direction may be specified, it's pointless.
2550 The possible directions are "in" and "out",
2551 unlike CORBA there is no "inout"
2556 The possible property access flags are
2557 "readwrite", "read", and "write"
2562 Multiple interfaces can of course be listed for
2568 The "name" attribute on arguments is optional.
2574 Method, interface, property, and signal elements may have
2575 "annotations", which are generic key/value pairs of metadata.
2576 They are similar conceptually to Java's annotations and C# attributes.
2577 Well-known annotations:
2584 <entry>Values (separated by ,)</entry>
2585 <entry>Description</entry>
2590 <entry>org.freedesktop.DBus.Deprecated</entry>
2591 <entry>true,false</entry>
2592 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2595 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2596 <entry>(string)</entry>
2597 <entry>The C symbol; may be used for methods and interfaces</entry>
2600 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2601 <entry>true,false</entry>
2602 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2608 <sect1 id="message-bus">
2609 <title>Message Bus Specification</title>
2610 <sect2 id="message-bus-overview">
2611 <title>Message Bus Overview</title>
2613 The message bus accepts connections from one or more applications.
2614 Once connected, applications can exchange messages with other
2615 applications that are also connected to the bus.
2618 In order to route messages among connections, the message bus keeps a
2619 mapping from names to connections. Each connection has one
2620 unique-for-the-lifetime-of-the-bus name automatically assigned.
2621 Applications may request additional names for a connection. Additional
2622 names are usually "well-known names" such as
2623 "org.freedesktop.TextEditor". When a name is bound to a connection,
2624 that connection is said to <firstterm>own</firstterm> the name.
2627 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
2628 This name routes messages to the bus, allowing applications to make
2629 administrative requests. For example, applications can ask the bus
2630 to assign a name to a connection.
2633 Each name may have <firstterm>queued owners</firstterm>. When an
2634 application requests a name for a connection and the name is already in
2635 use, the bus will optionally add the connection to a queue waiting for
2636 the name. If the current owner of the name disconnects or releases
2637 the name, the next connection in the queue will become the new owner.
2641 This feature causes the right thing to happen if you start two text
2642 editors for example; the first one may request "org.freedesktop.TextEditor",
2643 and the second will be queued as a possible owner of that name. When
2644 the first exits, the second will take over.
2648 Messages may have a <literal>DESTINATION</literal> field (see <xref
2649 linkend="message-protocol-header-fields"/>). If the
2650 <literal>DESTINATION</literal> field is present, it specifies a message
2651 recipient by name. Method calls and replies normally specify this field.
2655 Signals normally do not specify a destination; they are sent to all
2656 applications with <firstterm>message matching rules</firstterm> that
2661 When the message bus receives a method call, if the
2662 <literal>DESTINATION</literal> field is absent, the call is taken to be
2663 a standard one-to-one message and interpreted by the message bus
2664 itself. For example, sending an
2665 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
2666 <literal>DESTINATION</literal> will cause the message bus itself to
2667 reply to the ping immediately; the message bus will not make this
2668 message visible to other applications.
2672 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
2673 the ping message were sent with a <literal>DESTINATION</literal> name of
2674 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
2675 forwarded, and the Yoyodyne Corporation screensaver application would be
2676 expected to reply to the ping.
2680 <sect2 id="message-bus-names">
2681 <title>Message Bus Names</title>
2683 Each connection has at least one name, assigned at connection time and
2684 returned in response to the
2685 <literal>org.freedesktop.DBus.Hello</literal> method call. This
2686 automatically-assigned name is called the connection's <firstterm>unique
2687 name</firstterm>. Unique names are never reused for two different
2688 connections to the same bus.
2691 Ownership of a unique name is a prerequisite for interaction with
2692 the message bus. It logically follows that the unique name is always
2693 the first name that an application comes to own, and the last
2694 one that it loses ownership of.
2697 Unique connection names must begin with the character ':' (ASCII colon
2698 character); bus names that are not unique names must not begin
2699 with this character. (The bus must reject any attempt by an application
2700 to manually request a name beginning with ':'.) This restriction
2701 categorically prevents "spoofing"; messages sent to a unique name
2702 will always go to the expected connection.
2705 When a connection is closed, all the names that it owns are deleted (or
2706 transferred to the next connection in the queue if any).
2709 A connection can request additional names to be associated with it using
2710 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
2711 linkend="message-protocol-names-bus"/> describes the format of a valid
2712 name. These names can be released again using the
2713 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
2716 <sect3 id="bus-messages-request-name">
2717 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
2721 UINT32 RequestName (in STRING name, in UINT32 flags)
2728 <entry>Argument</entry>
2730 <entry>Description</entry>
2736 <entry>STRING</entry>
2737 <entry>Name to request</entry>
2741 <entry>UINT32</entry>
2742 <entry>Flags</entry>
2752 <entry>Argument</entry>
2754 <entry>Description</entry>
2760 <entry>UINT32</entry>
2761 <entry>Return value</entry>
2768 This method call should be sent to
2769 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2770 assign the given name to the method caller. Each name maintains a
2771 queue of possible owners, where the head of the queue is the primary
2772 or current owner of the name. Each potential owner in the queue
2773 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
2774 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
2775 call. When RequestName is invoked the following occurs:
2779 If the method caller is currently the primary owner of the name,
2780 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
2781 values are updated with the values from the new RequestName call,
2782 and nothing further happens.
2788 If the current primary owner (head of the queue) has
2789 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
2790 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
2791 the caller of RequestName replaces the current primary owner at
2792 the head of the queue and the current primary owner moves to the
2793 second position in the queue. If the caller of RequestName was
2794 in the queue previously its flags are updated with the values from
2795 the new RequestName in addition to moving it to the head of the queue.
2801 If replacement is not possible, and the method caller is
2802 currently in the queue but not the primary owner, its flags are
2803 updated with the values from the new RequestName call.
2809 If replacement is not possible, and the method caller is
2810 currently not in the queue, the method caller is appended to the
2817 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
2818 set and is not the primary owner, it is removed from the
2819 queue. This can apply to the previous primary owner (if it
2820 was replaced) or the method caller (if it updated the
2821 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
2822 queue, or if it was just added to the queue with that flag set).
2828 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
2829 queue," even if another application already in the queue had specified
2830 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
2831 that does not allow replacement goes away, and the next primary owner
2832 does allow replacement. In this case, queued items that specified
2833 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
2834 automatically replace the new primary owner. In other words,
2835 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
2836 time RequestName is called. This is deliberate to avoid an infinite loop
2837 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2838 and DBUS_NAME_FLAG_REPLACE_EXISTING.
2841 The flags argument contains any of the following values logically ORed
2848 <entry>Conventional Name</entry>
2849 <entry>Value</entry>
2850 <entry>Description</entry>
2855 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
2859 If an application A specifies this flag and succeeds in
2860 becoming the owner of the name, and another application B
2861 later calls RequestName with the
2862 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
2863 will lose ownership and receive a
2864 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
2865 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2866 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
2867 is not specified by application B, then application B will not replace
2868 application A as the owner.
2873 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
2877 Try to replace the current owner if there is one. If this
2878 flag is not set the application will only become the owner of
2879 the name if there is no current owner. If this flag is set,
2880 the application will replace the current owner if
2881 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
2886 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
2890 Without this flag, if an application requests a name that is
2891 already owned, the application will be placed in a queue to
2892 own the name when the current owner gives it up. If this
2893 flag is given, the application will not be placed in the
2894 queue, the request for the name will simply fail. This flag
2895 also affects behavior when an application is replaced as
2896 name owner; by default the application moves back into the
2897 waiting queue, unless this flag was provided when the application
2898 became the name owner.
2906 The return code can be one of the following values:
2912 <entry>Conventional Name</entry>
2913 <entry>Value</entry>
2914 <entry>Description</entry>
2919 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
2920 <entry>1</entry> <entry>The caller is now the primary owner of
2921 the name, replacing any previous owner. Either the name had no
2922 owner before, or the caller specified
2923 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
2924 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
2927 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
2930 <entry>The name already had an owner,
2931 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
2932 the current owner did not specify
2933 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
2934 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
2938 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
2939 <entry>The name already has an owner,
2940 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
2941 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
2942 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
2943 specified by the requesting application.</entry>
2946 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
2948 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
2956 <sect3 id="bus-messages-release-name">
2957 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
2961 UINT32 ReleaseName (in STRING name)
2968 <entry>Argument</entry>
2970 <entry>Description</entry>
2976 <entry>STRING</entry>
2977 <entry>Name to release</entry>
2987 <entry>Argument</entry>
2989 <entry>Description</entry>
2995 <entry>UINT32</entry>
2996 <entry>Return value</entry>
3003 This method call should be sent to
3004 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3005 release the method caller's claim to the given name. If the caller is
3006 the primary owner, a new primary owner will be selected from the
3007 queue if any other owners are waiting. If the caller is waiting in
3008 the queue for the name, the caller will removed from the queue and
3009 will not be made an owner of the name if it later becomes available.
3010 If there are no other owners in the queue for the name, it will be
3011 removed from the bus entirely.
3013 The return code can be one of the following values:
3019 <entry>Conventional Name</entry>
3020 <entry>Value</entry>
3021 <entry>Description</entry>
3026 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3027 <entry>1</entry> <entry>The caller has released his claim on
3028 the given name. Either the caller was the primary owner of
3029 the name, and the name is now unused or taken by somebody
3030 waiting in the queue for the name, or the caller was waiting
3031 in the queue for the name and has now been removed from the
3035 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3037 <entry>The given name does not exist on this bus.</entry>
3040 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3042 <entry>The caller was not the primary owner of this name,
3043 and was also not waiting in the queue to own this name.</entry>
3052 <sect2 id="message-bus-routing">
3053 <title>Message Bus Message Routing</title>
3057 <sect3 id="message-bus-routing-match-rules">
3058 <title>Match Rules</title>
3060 An important part of the message bus routing protocol is match
3061 rules. Match rules describe what messages can be sent to a client
3062 based on the contents of the message. When a message is routed
3063 through the bus it is compared to clients' match rules. If any
3064 of the rules match, the message is dispatched to the client.
3065 If none of the rules match the message never leaves the bus. This
3066 is an effective way to control traffic over the bus and to make sure
3067 only relevant message need to be processed by the client.
3070 Match rules are added using the AddMatch bus method
3071 (see xref linkend="bus-messages-add-match"/>). Rules are
3072 specified as a string of comma separated key/value pairs.
3073 Excluding a key from the rule indicates a wildcard match.
3074 For instance excluding the the member from a match rule but
3075 adding a sender would let all messages from that sender through.
3076 An example of a complete rule would be
3077 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3080 The following table describes the keys that can be used to create
3082 The following table summarizes the D-Bus types.
3088 <entry>Possible Values</entry>
3089 <entry>Description</entry>
3094 <entry><literal>type</literal></entry>
3095 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3096 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3099 <entry><literal>sender</literal></entry>
3100 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3101 and <xref linkend="term-unique-name"/> respectively)
3103 <entry>Match messages sent by a particular sender. An example of a sender match
3104 is sender='org.freedesktop.Hal'</entry>
3107 <entry><literal>interface</literal></entry>
3108 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3109 <entry>Match messages sent over or to a particular interface. An example of an
3110 interface match is interface='org.freedesktop.Hal.Manager'.
3111 If a message omits the interface header, it must not match any rule
3112 that specifies this key.</entry>
3115 <entry><literal>member</literal></entry>
3116 <entry>Any valid method or signal name</entry>
3117 <entry>Matches messages which have the give method or signal name. An example of
3118 a member match is member='NameOwnerChanged'</entry>
3121 <entry><literal>path</literal></entry>
3122 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3123 <entry>Matches messages which are sent from or to the given object. An example of a
3124 path match is path='/org/freedesktop/Hal/Manager'</entry>
3127 <entry><literal>destination</literal></entry>
3128 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3129 <entry>Matches messages which are being sent to the given unique name. An
3130 example of a destination match is destination=':1.0'</entry>
3133 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3134 <entry>Any string</entry>
3135 <entry>Arg matches are special and are used for further restricting the
3136 match based on the arguments in the body of a message. As of this time
3137 only string arguments can be matched. An example of an argument match
3138 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3147 <sect2 id="message-bus-starting-services">
3148 <title>Message Bus Starting Services</title>
3150 The message bus can start applications on behalf of other applications.
3151 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3152 An application that can be started in this way is called a
3153 <firstterm>service</firstterm>.
3156 With D-Bus, starting a service is normally done by name. That is,
3157 applications ask the message bus to start some program that will own a
3158 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3159 This implies a contract documented along with the name
3160 <literal>org.freedesktop.TextEditor</literal> for which objects
3161 the owner of that name will provide, and what interfaces those
3165 To find an executable corresponding to a particular name, the bus daemon
3166 looks for <firstterm>service description files</firstterm>. Service
3167 description files define a mapping from names to executables. Different
3168 kinds of message bus will look for these files in different places, see
3169 <xref linkend="message-bus-types"/>.
3172 [FIXME the file format should be much better specified than "similar to
3173 .desktop entries" esp. since desktop entries are already
3174 badly-specified. ;-)] Service description files have the ".service" file
3175 extension. The message bus will only load service description files
3176 ending with .service; all other files will be ignored. The file format
3177 is similar to that of <ulink
3178 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
3179 entries</ulink>. All service description files must be in UTF-8
3180 encoding. To ensure that there will be no name collisions, service files
3181 must be namespaced using the same mechanism as messages and service
3185 <title>Example service description file</title>
3187 # Sample service description file
3189 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3190 Exec=/usr/libexec/gconfd-2
3195 When an application asks to start a service by name, the bus daemon tries to
3196 find a service that will own that name. It then tries to spawn the
3197 executable associated with it. If this fails, it will report an
3198 error. [FIXME what happens if two .service files offer the same service;
3199 what kind of error is reported, should we have a way for the client to
3203 The executable launched will have the environment variable
3204 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3205 message bus so it can connect and request the appropriate names.
3208 The executable being launched may want to know whether the message bus
3209 starting it is one of the well-known message buses (see <xref
3210 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3211 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3212 of the well-known buses. The currently-defined values for this variable
3213 are <literal>system</literal> for the systemwide message bus,
3214 and <literal>session</literal> for the per-login-session message
3215 bus. The new executable must still connect to the address given
3216 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3217 resulting connection is to the well-known bus.
3220 [FIXME there should be a timeout somewhere, either specified
3221 in the .service file, by the client, or just a global value
3222 and if the client being activated fails to connect within that
3223 timeout, an error should be sent back.]
3226 <sect3 id="message-bus-starting-services-scope">
3227 <title>Message Bus Service Scope</title>
3229 The "scope" of a service is its "per-", such as per-session,
3230 per-machine, per-home-directory, or per-display. The reference
3231 implementation doesn't yet support starting services in a different
3232 scope from the message bus itself. So e.g. if you start a service
3233 on the session bus its scope is per-session.
3236 We could add an optional scope to a bus name. For example, for
3237 per-(display,session pair), we could have a unique ID for each display
3238 generated automatically at login and set on screen 0 by executing a
3239 special "set display ID" binary. The ID would be stored in a
3240 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3241 random bytes. This ID would then be used to scope names.
3242 Starting/locating a service could be done by ID-name pair rather than
3246 Contrast this with a per-display scope. To achieve that, we would
3247 want a single bus spanning all sessions using a given display.
3248 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3249 property on screen 0 of the display, pointing to this bus.
3254 <sect2 id="message-bus-types">
3255 <title>Well-known Message Bus Instances</title>
3257 Two standard message bus instances are defined here, along with how
3258 to locate them and where their service files live.
3260 <sect3 id="message-bus-types-login">
3261 <title>Login session message bus</title>
3263 Each time a user logs in, a <firstterm>login session message
3264 bus</firstterm> may be started. All applications in the user's login
3265 session may interact with one another using this message bus.
3268 The address of the login session message bus is given
3269 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3270 variable. If that variable is not set, applications may
3271 also try to read the address from the X Window System root
3272 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3273 The root window property must have type <literal>STRING</literal>.
3274 The environment variable should have precedence over the
3275 root window property.
3278 [FIXME specify location of .service files, probably using
3279 DESKTOP_DIRS etc. from basedir specification, though login session
3280 bus is not really desktop-specific]
3283 <sect3 id="message-bus-types-system">
3284 <title>System message bus</title>
3286 A computer may have a <firstterm>system message bus</firstterm>,
3287 accessible to all applications on the system. This message bus may be
3288 used to broadcast system events, such as adding new hardware devices,
3289 changes in the printer queue, and so forth.
3292 The address of the system message bus is given
3293 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3294 variable. If that variable is not set, applications should try
3295 to connect to the well-known address
3296 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3299 The D-Bus reference implementation actually honors the
3300 <literal>$(localstatedir)</literal> configure option
3301 for this address, on both client and server side.
3306 [FIXME specify location of system bus .service files]
3311 <sect2 id="message-bus-messages">
3312 <title>Message Bus Messages</title>
3314 The special message bus name <literal>org.freedesktop.DBus</literal>
3315 responds to a number of additional messages.
3318 <sect3 id="bus-messages-hello">
3319 <title><literal>org.freedesktop.DBus.Hello</literal></title>
3330 <entry>Argument</entry>
3332 <entry>Description</entry>
3338 <entry>STRING</entry>
3339 <entry>Unique name assigned to the connection</entry>
3346 Before an application is able to send messages to other applications
3347 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
3348 to the message bus to obtain a unique name. If an application without
3349 a unique name tries to send a message to another application, or a
3350 message to the message bus itself that isn't the
3351 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
3352 disconnected from the bus.
3355 There is no corresponding "disconnect" request; if a client wishes to
3356 disconnect from the bus, it simply closes the socket (or other
3357 communication channel).
3360 <sect3 id="bus-messages-list-names">
3361 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
3365 ARRAY of STRING ListNames ()
3372 <entry>Argument</entry>
3374 <entry>Description</entry>
3380 <entry>ARRAY of STRING</entry>
3381 <entry>Array of strings where each string is a bus name</entry>
3388 Returns a list of all currently-owned names on the bus.
3391 <sect3 id="bus-messages-list-activatable-names">
3392 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
3396 ARRAY of STRING ListActivatableNames ()
3403 <entry>Argument</entry>
3405 <entry>Description</entry>
3411 <entry>ARRAY of STRING</entry>
3412 <entry>Array of strings where each string is a bus name</entry>
3419 Returns a list of all names that can be activated on the bus.
3422 <sect3 id="bus-messages-name-exists">
3423 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
3427 BOOLEAN NameHasOwner (in STRING name)
3434 <entry>Argument</entry>
3436 <entry>Description</entry>
3442 <entry>STRING</entry>
3443 <entry>Name to check</entry>
3453 <entry>Argument</entry>
3455 <entry>Description</entry>
3461 <entry>BOOLEAN</entry>
3462 <entry>Return value, true if the name exists</entry>
3469 Checks if the specified name exists (currently has an owner).
3473 <sect3 id="bus-messages-name-owner-changed">
3474 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
3478 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
3485 <entry>Argument</entry>
3487 <entry>Description</entry>
3493 <entry>STRING</entry>
3494 <entry>Name with a new owner</entry>
3498 <entry>STRING</entry>
3499 <entry>Old owner or empty string if none</entry>
3503 <entry>STRING</entry>
3504 <entry>New owner or empty string if none</entry>
3511 This signal indicates that the owner of a name has changed.
3512 It's also the signal to use to detect the appearance of
3513 new names on the bus.
3516 <sect3 id="bus-messages-name-lost">
3517 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
3521 NameLost (STRING name)
3528 <entry>Argument</entry>
3530 <entry>Description</entry>
3536 <entry>STRING</entry>
3537 <entry>Name which was lost</entry>
3544 This signal is sent to a specific application when it loses
3545 ownership of a name.
3549 <sect3 id="bus-messages-name-acquired">
3550 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
3554 NameAcquired (STRING name)
3561 <entry>Argument</entry>
3563 <entry>Description</entry>
3569 <entry>STRING</entry>
3570 <entry>Name which was acquired</entry>
3577 This signal is sent to a specific application when it gains
3578 ownership of a name.
3582 <sect3 id="bus-messages-start-service-by-name">
3583 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
3587 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
3594 <entry>Argument</entry>
3596 <entry>Description</entry>
3602 <entry>STRING</entry>
3603 <entry>Name of the service to start</entry>
3607 <entry>UINT32</entry>
3608 <entry>Flags (currently not used)</entry>
3618 <entry>Argument</entry>
3620 <entry>Description</entry>
3626 <entry>UINT32</entry>
3627 <entry>Return value</entry>
3632 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
3636 The return value can be one of the following values:
3641 <entry>Identifier</entry>
3642 <entry>Value</entry>
3643 <entry>Description</entry>
3648 <entry>DBUS_START_REPLY_SUCCESS</entry>
3650 <entry>The service was successfully started.</entry>
3653 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
3655 <entry>A connection already owns the given name.</entry>
3664 <sect3 id="bus-messages-get-name-owner">
3665 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
3669 STRING GetNameOwner (in STRING name)
3676 <entry>Argument</entry>
3678 <entry>Description</entry>
3684 <entry>STRING</entry>
3685 <entry>Name to get the owner of</entry>
3695 <entry>Argument</entry>
3697 <entry>Description</entry>
3703 <entry>STRING</entry>
3704 <entry>Return value, a unique connection name</entry>
3709 Returns the unique connection name of the primary owner of the name
3710 given. If the requested name doesn't have an owner, returns a
3711 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
3715 <sect3 id="bus-messages-get-connection-unix-user">
3716 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
3720 UINT32 GetConnectionUnixUser (in STRING connection_name)
3727 <entry>Argument</entry>
3729 <entry>Description</entry>
3735 <entry>STRING</entry>
3736 <entry>Name of the connection to query</entry>
3746 <entry>Argument</entry>
3748 <entry>Description</entry>
3754 <entry>UINT32</entry>
3755 <entry>unix user id</entry>
3760 Returns the unix uid of the process connected to the server. If unable to
3761 determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
3766 <sect3 id="bus-messages-add-match">
3767 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
3771 AddMatch (in STRING rule)
3778 <entry>Argument</entry>
3780 <entry>Description</entry>
3786 <entry>STRING</entry>
3787 <entry>Match rule to add to the connection</entry>
3792 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
3793 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
3797 <sect3 id="bus-messages-remove-match">
3798 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
3802 RemoveMatch (in STRING rule)
3809 <entry>Argument</entry>
3811 <entry>Description</entry>
3817 <entry>STRING</entry>
3818 <entry>Match rule to remove from the connection</entry>
3823 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
3824 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
3833 <appendix id="implementation-notes">
3834 <title>Implementation notes</title>
3835 <sect1 id="implementation-notes-subsection">
3843 <glossary><title>Glossary</title>
3845 This glossary defines some of the terms used in this specification.
3848 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
3851 The message bus maintains an association between names and
3852 connections. (Normally, there's one connection per application.) A
3853 bus name is simply an identifier used to locate connections. For
3854 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
3855 name might be used to send a message to a screensaver from Yoyodyne
3856 Corporation. An application is said to <firstterm>own</firstterm> a
3857 name if the message bus has associated the application's connection
3858 with the name. Names may also have <firstterm>queued
3859 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
3860 The bus assigns a unique name to each connection,
3861 see <xref linkend="term-unique-name"/>. Other names
3862 can be thought of as "well-known names" and are
3863 used to find applications that offer specific functionality.
3868 <glossentry id="term-message"><glossterm>Message</glossterm>
3871 A message is the atomic unit of communication via the D-Bus
3872 protocol. It consists of a <firstterm>header</firstterm> and a
3873 <firstterm>body</firstterm>; the body is made up of
3874 <firstterm>arguments</firstterm>.
3879 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
3882 The message bus is a special application that forwards
3883 or routes messages between a group of applications
3884 connected to the message bus. It also manages
3885 <firstterm>names</firstterm> used for routing
3891 <glossentry id="term-name"><glossterm>Name</glossterm>
3894 See <xref linkend="term-bus-name"/>. "Name" may
3895 also be used to refer to some of the other names
3896 in D-Bus, such as interface names.
3901 <glossentry id="namespace"><glossterm>Namespace</glossterm>
3904 Used to prevent collisions when defining new interfaces or bus
3905 names. The convention used is the same one Java uses for defining
3906 classes: a reversed domain name.
3911 <glossentry id="term-object"><glossterm>Object</glossterm>
3914 Each application contains <firstterm>objects</firstterm>, which have
3915 <firstterm>interfaces</firstterm> and
3916 <firstterm>methods</firstterm>. Objects are referred to by a name,
3917 called a <firstterm>path</firstterm>.
3922 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
3925 An application talking directly to another application, without going
3926 through a message bus. One-to-one connections may be "peer to peer" or
3927 "client to server." The D-Bus protocol has no concept of client
3928 vs. server after a connection has authenticated; the flow of messages
3929 is symmetrical (full duplex).
3934 <glossentry id="term-path"><glossterm>Path</glossterm>
3937 Object references (object names) in D-Bus are organized into a
3938 filesystem-style hierarchy, so each object is named by a path. As in
3939 LDAP, there's no difference between "files" and "directories"; a path
3940 can refer to an object, while still having child objects below it.
3945 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
3948 Each bus name has a primary owner; messages sent to the name go to the
3949 primary owner. However, certain names also maintain a queue of
3950 secondary owners "waiting in the wings." If the primary owner releases
3951 the name, then the first secondary owner in the queue automatically
3952 becomes the new owner of the name.
3957 <glossentry id="term-service"><glossterm>Service</glossterm>
3960 A service is an executable that can be launched by the bus daemon.
3961 Services normally guarantee some particular features, for example they
3962 may guarantee that they will request a specific name such as
3963 "org.freedesktop.Screensaver", have a singleton object
3964 "/org/freedesktop/Application", and that object will implement the
3965 interface "org.freedesktop.ScreensaverControl".
3970 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
3973 ".service files" tell the bus about service applications that can be
3974 launched (see <xref linkend="term-service"/>). Most importantly they
3975 provide a mapping from bus names to services that will request those
3976 names when they start up.
3981 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
3984 The special name automatically assigned to each connection by the
3985 message bus. This name will never change owner, and will be unique
3986 (never reused during the lifetime of the message bus).
3987 It will begin with a ':' character.
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