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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 1.
764 <entry>1st <literal>UINT32</literal></entry>
765 <entry>Length in bytes of the message body, starting
766 from the end of the header. The header ends after
767 its alignment padding to an 8-boundary.
771 <entry>2nd <literal>UINT32</literal></entry>
772 <entry>The serial of this message, used as a cookie
773 by the sender to identify the reply corresponding
778 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
779 <entry>An array of zero or more <firstterm>header
780 fields</firstterm> where the byte is the field code, and the
781 variant is the field value. The message type determines
782 which fields are required.
790 <firstterm>Message types</firstterm> that can appear in the second byte
796 <entry>Conventional name</entry>
797 <entry>Decimal value</entry>
798 <entry>Description</entry>
803 <entry><literal>INVALID</literal></entry>
805 <entry>This is an invalid type.</entry>
808 <entry><literal>METHOD_CALL</literal></entry>
810 <entry>Method call.</entry>
813 <entry><literal>METHOD_RETURN</literal></entry>
815 <entry>Method reply with returned data.</entry>
818 <entry><literal>ERROR</literal></entry>
820 <entry>Error reply. If the first argument exists and is a
821 string, it is an error message.</entry>
824 <entry><literal>SIGNAL</literal></entry>
826 <entry>Signal emission.</entry>
833 Flags that can appear in the third byte of the header:
838 <entry>Conventional name</entry>
839 <entry>Hex value</entry>
840 <entry>Description</entry>
845 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
847 <entry>This message does not expect method return replies or
848 error replies; the reply can be omitted as an
849 optimization. However, it is compliant with this specification
850 to return the reply despite this flag and the only harm
851 from doing so is extra network traffic.
855 <entry><literal>NO_AUTO_START</literal></entry>
857 <entry>The bus must not launch an owner
858 for the destination name in response to this message.
866 <sect3 id="message-protocol-header-fields">
867 <title>Header Fields</title>
870 The array at the end of the header contains <firstterm>header
871 fields</firstterm>, where each field is a 1-byte field code followed
872 by a field value. A header must contain the required header fields for
873 its message type, and zero or more of any optional header
874 fields. Future versions of this protocol specification may add new
875 fields. Implementations must ignore fields they do not
876 understand. Implementations must not invent their own header fields;
877 only changes to this specification may introduce new header fields.
881 Again, if an implementation sees a header field code that it does not
882 expect, it must ignore that field, as it will be part of a new
883 (but compatible) version of this specification. This also applies
884 to known header fields appearing in unexpected messages, for
885 example: if a signal has a reply serial it must be ignored
886 even though it has no meaning as of this version of the spec.
890 However, implementations must not send or accept known header fields
891 with the wrong type stored in the field value. So for example a
892 message with an <literal>INTERFACE</literal> field of type
893 <literal>UINT32</literal> would be considered corrupt.
897 Here are the currently-defined header fields:
902 <entry>Conventional Name</entry>
903 <entry>Decimal Code</entry>
905 <entry>Required In</entry>
906 <entry>Description</entry>
911 <entry><literal>INVALID</literal></entry>
914 <entry>not allowed</entry>
915 <entry>Not a valid field name (error if it appears in a message)</entry>
918 <entry><literal>PATH</literal></entry>
920 <entry><literal>OBJECT_PATH</literal></entry>
921 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
922 <entry>The object to send a call to,
923 or the object a signal is emitted from.
925 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
926 implementations should not send messages with this path,
927 and the reference implementation of the bus daemon will
928 disconnect any application that attempts to do so.
932 <entry><literal>INTERFACE</literal></entry>
934 <entry><literal>STRING</literal></entry>
935 <entry><literal>SIGNAL</literal></entry>
937 The interface to invoke a method call on, or
938 that a signal is emitted from. Optional for
939 method calls, required for signals.
940 The special interface
941 <literal>org.freedesktop.DBus.Local</literal> is reserved;
942 implementations should not send messages with this
943 interface, and the reference implementation of the bus
944 daemon will disconnect any application that attempts to
949 <entry><literal>MEMBER</literal></entry>
951 <entry><literal>STRING</literal></entry>
952 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
953 <entry>The member, either the method name or signal name.</entry>
956 <entry><literal>ERROR_NAME</literal></entry>
958 <entry><literal>STRING</literal></entry>
959 <entry><literal>ERROR</literal></entry>
960 <entry>The name of the error that occurred, for errors</entry>
963 <entry><literal>REPLY_SERIAL</literal></entry>
965 <entry><literal>UINT32</literal></entry>
966 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
967 <entry>The serial number of the message this message is a reply
968 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
971 <entry><literal>DESTINATION</literal></entry>
973 <entry><literal>STRING</literal></entry>
974 <entry>optional</entry>
975 <entry>The name of the connection this message is intended for.
976 Only used in combination with the message bus, see
977 <xref linkend="message-bus"/>.</entry>
980 <entry><literal>SENDER</literal></entry>
982 <entry><literal>STRING</literal></entry>
983 <entry>optional</entry>
984 <entry>Unique name of the sending connection.
985 The message bus fills in this field so it is reliable; the field is
986 only meaningful in combination with the message bus.</entry>
989 <entry><literal>SIGNATURE</literal></entry>
991 <entry><literal>SIGNATURE</literal></entry>
992 <entry>optional</entry>
993 <entry>The signature of the message body.
994 If omitted, it is assumed to be the
995 empty signature "" (i.e. the body must be 0-length).</entry>
1004 <sect2 id="message-protocol-names">
1005 <title>Valid Names</title>
1007 The various names in D-Bus messages have some restrictions.
1010 There is a <firstterm>maximum name length</firstterm>
1011 of 255 which applies to bus names, interfaces, and members.
1013 <sect3 id="message-protocol-names-interface">
1014 <title>Interface names</title>
1016 Interfaces have names with type <literal>STRING</literal>, meaning that
1017 they must be valid UTF-8. However, there are also some
1018 additional restrictions that apply to interface names
1021 <listitem><para>Interface names are composed of 1 or more elements separated by
1022 a period ('.') character. All elements must contain at least
1026 <listitem><para>Each element must only contain the ASCII characters
1027 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1031 <listitem><para>Interface names must contain at least one '.' (period)
1032 character (and thus at least two elements).
1035 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1036 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1040 <sect3 id="message-protocol-names-bus">
1041 <title>Bus names</title>
1043 Connections have one or more bus names associated with them.
1044 A connection has exactly one bus name that is a unique connection
1045 name. The unique connection name remains with the connection for
1046 its entire lifetime.
1047 A bus name is of type <literal>STRING</literal>,
1048 meaning that it must be valid UTF-8. However, there are also
1049 some additional restrictions that apply to bus names
1052 <listitem><para>Bus names that start with a colon (':')
1053 character are unique connection names.
1056 <listitem><para>Bus names are composed of 1 or more elements separated by
1057 a period ('.') character. All elements must contain at least
1061 <listitem><para>Each element must only contain the ASCII characters
1062 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1063 connection name may begin with a digit, elements in
1064 other bus names must not begin with a digit.
1068 <listitem><para>Bus names must contain at least one '.' (period)
1069 character (and thus at least two elements).
1072 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1073 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1077 Note that the hyphen ('-') character is allowed in bus names but
1078 not in interface names.
1081 <sect3 id="message-protocol-names-member">
1082 <title>Member names</title>
1084 Member (i.e. method or signal) names:
1086 <listitem><para>Must only contain the ASCII characters
1087 "[A-Z][a-z][0-9]_" and may not begin with a
1088 digit.</para></listitem>
1089 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1090 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1091 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1095 <sect3 id="message-protocol-names-error">
1096 <title>Error names</title>
1098 Error names have the same restrictions as interface names.
1103 <sect2 id="message-protocol-types">
1104 <title>Message Types</title>
1106 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1107 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1108 This section describes these conventions.
1110 <sect3 id="message-protocol-types-method">
1111 <title>Method Calls</title>
1113 Some messages invoke an operation on a remote object. These are
1114 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1115 messages map naturally to methods on objects in a typical program.
1118 A method call message is required to have a <literal>MEMBER</literal> header field
1119 indicating the name of the method. Optionally, the message has an
1120 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1121 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1122 a method with the same name, it is undefined which of the two methods
1123 will be invoked. Implementations may also choose to return an error in
1124 this ambiguous case. However, if a method name is unique
1125 implementations must not require an interface field.
1128 Method call messages also include a <literal>PATH</literal> field
1129 indicating the object to invoke the method on. If the call is passing
1130 through a message bus, the message will also have a
1131 <literal>DESTINATION</literal> field giving the name of the connection
1132 to receive the message.
1135 When an application handles a method call message, it is required to
1136 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1137 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1138 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1141 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1142 are the return value(s) or "out parameters" of the method call.
1143 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1144 and the call fails; no return value will be provided. It makes
1145 no sense to send multiple replies to the same method call.
1148 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1149 reply is required, so the caller will know the method
1150 was successfully processed.
1153 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1157 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1158 then as an optimization the application receiving the method
1159 call may choose to omit the reply message (regardless of
1160 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1161 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1162 flag and reply anyway.
1165 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1166 destination name does not exist then a program to own the destination
1167 name will be started before the message is delivered. The message
1168 will be held until the new program is successfully started or has
1169 failed to start; in case of failure, an error will be returned. This
1170 flag is only relevant in the context of a message bus, it is ignored
1171 during one-to-one communication with no intermediate bus.
1173 <sect4 id="message-protocol-types-method-apis">
1174 <title>Mapping method calls to native APIs</title>
1176 APIs for D-Bus may map method calls to a method call in a specific
1177 programming language, such as C++, or may map a method call written
1178 in an IDL to a D-Bus message.
1181 In APIs of this nature, arguments to a method are often termed "in"
1182 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1183 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1184 "inout" arguments, which are both sent and received, i.e. the caller
1185 passes in a value which is modified. Mapped to D-Bus, an "inout"
1186 argument is equivalent to an "in" argument, followed by an "out"
1187 argument. You can't pass things "by reference" over the wire, so
1188 "inout" is purely an illusion of the in-process API.
1191 Given a method with zero or one return values, followed by zero or more
1192 arguments, where each argument may be "in", "out", or "inout", the
1193 caller constructs a message by appending each "in" or "inout" argument,
1194 in order. "out" arguments are not represented in the caller's message.
1197 The recipient constructs a reply by appending first the return value
1198 if any, then each "out" or "inout" argument, in order.
1199 "in" arguments are not represented in the reply message.
1202 Error replies are normally mapped to exceptions in languages that have
1206 In converting from native APIs to D-Bus, it is perhaps nice to
1207 map D-Bus naming conventions ("FooBar") to native conventions
1208 such as "fooBar" or "foo_bar" automatically. This is OK
1209 as long as you can say that the native API is one that
1210 was specifically written for D-Bus. It makes the most sense
1211 when writing object implementations that will be exported
1212 over the bus. Object proxies used to invoke remote D-Bus
1213 objects probably need the ability to call any D-Bus method,
1214 and thus a magic name mapping like this could be a problem.
1217 This specification doesn't require anything of native API bindings;
1218 the preceding is only a suggested convention for consistency
1224 <sect3 id="message-protocol-types-signal">
1225 <title>Signal Emission</title>
1227 Unlike method calls, signal emissions have no replies.
1228 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1229 It must have three header fields: <literal>PATH</literal> giving the object
1230 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1231 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1232 for signals, though it is optional for method calls.
1236 <sect3 id="message-protocol-types-errors">
1237 <title>Errors</title>
1239 Messages of type <literal>ERROR</literal> are most commonly replies
1240 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1241 to any kind of message. The message bus for example
1242 will return an <literal>ERROR</literal> in reply to a signal emission if
1243 the bus does not have enough memory to send the signal.
1246 An <literal>ERROR</literal> may have any arguments, but if the first
1247 argument is a <literal>STRING</literal>, it must be an error message.
1248 The error message may be logged or shown to the user
1253 <sect3 id="message-protocol-types-notation">
1254 <title>Notation in this document</title>
1256 This document uses a simple pseudo-IDL to describe particular method
1257 calls and signals. Here is an example of a method call:
1259 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1260 out UINT32 resultcode)
1262 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1263 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1264 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1265 characters so it's known that the last part of the name in
1266 the "IDL" is the member name.
1269 In C++ that might end up looking like this:
1271 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1272 unsigned int flags);
1274 or equally valid, the return value could be done as an argument:
1276 void org::freedesktop::DBus::StartServiceByName (const char *name,
1278 unsigned int *resultcode);
1280 It's really up to the API designer how they want to make
1281 this look. You could design an API where the namespace wasn't used
1282 in C++, using STL or Qt, using varargs, or whatever you wanted.
1285 Signals are written as follows:
1287 org.freedesktop.DBus.NameLost (STRING name)
1289 Signals don't specify "in" vs. "out" because only
1290 a single direction is possible.
1293 It isn't especially encouraged to use this lame pseudo-IDL in actual
1294 API implementations; you might use the native notation for the
1295 language you're using, or you might use COM or CORBA IDL, for example.
1300 <sect2 id="message-protocol-handling-invalid">
1301 <title>Invalid Protocol and Spec Extensions</title>
1304 For security reasons, the D-Bus protocol should be strictly parsed and
1305 validated, with the exception of defined extension points. Any invalid
1306 protocol or spec violations should result in immediately dropping the
1307 connection without notice to the other end. Exceptions should be
1308 carefully considered, e.g. an exception may be warranted for a
1309 well-understood idiosyncrasy of a widely-deployed implementation. In
1310 cases where the other end of a connection is 100% trusted and known to
1311 be friendly, skipping validation for performance reasons could also make
1312 sense in certain cases.
1316 Generally speaking violations of the "must" requirements in this spec
1317 should be considered possible attempts to exploit security, and violations
1318 of the "should" suggestions should be considered legitimate (though perhaps
1319 they should generate an error in some cases).
1323 The following extension points are built in to D-Bus on purpose and must
1324 not be treated as invalid protocol. The extension points are intended
1325 for use by future versions of this spec, they are not intended for third
1326 parties. At the moment, the only way a third party could extend D-Bus
1327 without breaking interoperability would be to introduce a way to negotiate new
1328 feature support as part of the auth protocol, using EXTENSION_-prefixed
1329 commands. There is not yet a standard way to negotiate features.
1333 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1334 commands result in an ERROR rather than a disconnect. This enables
1335 future extensions to the protocol. Commands starting with EXTENSION_ are
1336 reserved for third parties.
1341 The authentication protocol supports pluggable auth mechanisms.
1346 The address format (see <xref linkend="addresses"/>) supports new
1352 Messages with an unknown type (something other than
1353 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1354 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1355 Unknown-type messages must still be well-formed in the same way
1356 as the known messages, however. They still have the normal
1362 Header fields with an unknown or unexpected field code must be ignored,
1363 though again they must still be well-formed.
1368 New standard interfaces (with new methods and signals) can of course be added.
1378 <sect1 id="auth-protocol">
1379 <title>Authentication Protocol</title>
1381 Before the flow of messages begins, two applications must
1382 authenticate. A simple plain-text protocol is used for
1383 authentication; this protocol is a SASL profile, and maps fairly
1384 directly from the SASL specification. The message encoding is
1385 NOT used here, only plain text messages.
1388 In examples, "C:" and "S:" indicate lines sent by the client and
1389 server respectively.
1391 <sect2 id="auth-protocol-overview">
1392 <title>Protocol Overview</title>
1394 The protocol is a line-based protocol, where each line ends with
1395 \r\n. Each line begins with an all-caps ASCII command name containing
1396 only the character range [A-Z_], a space, then any arguments for the
1397 command, then the \r\n ending the line. The protocol is
1398 case-sensitive. All bytes must be in the ASCII character set.
1400 Commands from the client to the server are as follows:
1403 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1404 <listitem><para>CANCEL</para></listitem>
1405 <listitem><para>BEGIN</para></listitem>
1406 <listitem><para>DATA <data in hex encoding></para></listitem>
1407 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1410 From server to client are as follows:
1413 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1414 <listitem><para>OK <GUID in hex></para></listitem>
1415 <listitem><para>DATA <data in hex encoding></para></listitem>
1416 <listitem><para>ERROR</para></listitem>
1420 Unofficial extensions to the command set must begin with the letters
1421 "EXTENSION_", to avoid conflicts with future official commands.
1422 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1425 <sect2 id="auth-nul-byte">
1426 <title>Special credentials-passing nul byte</title>
1428 Immediately after connecting to the server, the client must send a
1429 single nul byte. This byte may be accompanied by credentials
1430 information on some operating systems that use sendmsg() with
1431 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1432 sockets. However, the nul byte must be sent even on other kinds of
1433 socket, and even on operating systems that do not require a byte to be
1434 sent in order to transmit credentials. The text protocol described in
1435 this document begins after the single nul byte. If the first byte
1436 received from the client is not a nul byte, the server may disconnect
1440 A nul byte in any context other than the initial byte is an error;
1441 the protocol is ASCII-only.
1444 The credentials sent along with the nul byte may be used with the
1445 SASL mechanism EXTERNAL.
1448 <sect2 id="auth-command-auth">
1449 <title>AUTH command</title>
1451 If an AUTH command has no arguments, it is a request to list
1452 available mechanisms. The server must respond with a REJECTED
1453 command listing the mechanisms it understands, or with an error.
1456 If an AUTH command specifies a mechanism, and the server supports
1457 said mechanism, the server should begin exchanging SASL
1458 challenge-response data with the client using DATA commands.
1461 If the server does not support the mechanism given in the AUTH
1462 command, it must send either a REJECTED command listing the mechanisms
1463 it does support, or an error.
1466 If the [initial-response] argument is provided, it is intended for use
1467 with mechanisms that have no initial challenge (or an empty initial
1468 challenge), as if it were the argument to an initial DATA command. If
1469 the selected mechanism has an initial challenge and [initial-response]
1470 was provided, the server should reject authentication by sending
1474 If authentication succeeds after exchanging DATA commands,
1475 an OK command must be sent to the client.
1478 The first octet received by the client after the \r\n of the OK
1479 command must be the first octet of the authenticated/encrypted
1480 stream of D-Bus messages.
1483 The first octet received by the server after the \r\n of the BEGIN
1484 command from the client must be the first octet of the
1485 authenticated/encrypted stream of D-Bus messages.
1488 <sect2 id="auth-command-cancel">
1489 <title>CANCEL Command</title>
1491 At any time up to sending the BEGIN command, the client may send a
1492 CANCEL command. On receiving the CANCEL command, the server must
1493 send a REJECTED command and abort the current authentication
1497 <sect2 id="auth-command-data">
1498 <title>DATA Command</title>
1500 The DATA command may come from either client or server, and simply
1501 contains a hex-encoded block of data to be interpreted
1502 according to the SASL mechanism in use.
1505 Some SASL mechanisms support sending an "empty string";
1506 FIXME we need some way to do this.
1509 <sect2 id="auth-command-begin">
1510 <title>BEGIN Command</title>
1512 The BEGIN command acknowledges that the client has received an
1513 OK command from the server, and that the stream of messages
1517 The first octet received by the server after the \r\n of the BEGIN
1518 command from the client must be the first octet of the
1519 authenticated/encrypted stream of D-Bus messages.
1522 <sect2 id="auth-command-rejected">
1523 <title>REJECTED Command</title>
1525 The REJECTED command indicates that the current authentication
1526 exchange has failed, and further exchange of DATA is inappropriate.
1527 The client would normally try another mechanism, or try providing
1528 different responses to challenges.
1530 Optionally, the REJECTED command has a space-separated list of
1531 available auth mechanisms as arguments. If a server ever provides
1532 a list of supported mechanisms, it must provide the same list
1533 each time it sends a REJECTED message. Clients are free to
1534 ignore all lists received after the first.
1537 <sect2 id="auth-command-ok">
1538 <title>OK Command</title>
1540 The OK command indicates that the client has been authenticated,
1541 and that further communication will be a stream of D-Bus messages
1542 (optionally encrypted, as negotiated) rather than this protocol.
1545 The first octet received by the client after the \r\n of the OK
1546 command must be the first octet of the authenticated/encrypted
1547 stream of D-Bus messages.
1550 The client must respond to the OK command by sending a BEGIN
1551 command, followed by its stream of messages, or by disconnecting.
1552 The server must not accept additional commands using this protocol
1553 after the OK command has been sent.
1556 The OK command has one argument, which is the GUID of the server.
1557 See <xref linkend="addresses"/> for more on server GUIDs.
1560 <sect2 id="auth-command-error">
1561 <title>ERROR Command</title>
1563 The ERROR command indicates that either server or client did not
1564 know a command, does not accept the given command in the current
1565 context, or did not understand the arguments to the command. This
1566 allows the protocol to be extended; a client or server can send a
1567 command present or permitted only in new protocol versions, and if
1568 an ERROR is received instead of an appropriate response, fall back
1569 to using some other technique.
1572 If an ERROR is sent, the server or client that sent the
1573 error must continue as if the command causing the ERROR had never been
1574 received. However, the the server or client receiving the error
1575 should try something other than whatever caused the error;
1576 if only canceling/rejecting the authentication.
1579 If the D-Bus protocol changes incompatibly at some future time,
1580 applications implementing the new protocol would probably be able to
1581 check for support of the new protocol by sending a new command and
1582 receiving an ERROR from applications that don't understand it. Thus the
1583 ERROR feature of the auth protocol is an escape hatch that lets us
1584 negotiate extensions or changes to the D-Bus protocol in the future.
1587 <sect2 id="auth-examples">
1588 <title>Authentication examples</title>
1592 <title>Example of successful magic cookie authentication</title>
1594 (MAGIC_COOKIE is a made up mechanism)
1596 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1602 <title>Example of finding out mechanisms then picking one</title>
1605 S: REJECTED KERBEROS_V4 SKEY
1606 C: AUTH SKEY 7ab83f32ee
1607 S: DATA 8799cabb2ea93e
1608 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1614 <title>Example of client sends unknown command then falls back to regular auth</title>
1618 C: AUTH MAGIC_COOKIE 3736343435313230333039
1624 <title>Example of server doesn't support initial auth mechanism</title>
1626 C: AUTH MAGIC_COOKIE 3736343435313230333039
1627 S: REJECTED KERBEROS_V4 SKEY
1628 C: AUTH SKEY 7ab83f32ee
1629 S: DATA 8799cabb2ea93e
1630 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1636 <title>Example of wrong password or the like followed by successful retry</title>
1638 C: AUTH MAGIC_COOKIE 3736343435313230333039
1639 S: REJECTED KERBEROS_V4 SKEY
1640 C: AUTH SKEY 7ab83f32ee
1641 S: DATA 8799cabb2ea93e
1642 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1644 C: AUTH SKEY 7ab83f32ee
1645 S: DATA 8799cabb2ea93e
1646 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1652 <title>Example of skey cancelled and restarted</title>
1654 C: AUTH MAGIC_COOKIE 3736343435313230333039
1655 S: REJECTED KERBEROS_V4 SKEY
1656 C: AUTH SKEY 7ab83f32ee
1657 S: DATA 8799cabb2ea93e
1660 C: AUTH SKEY 7ab83f32ee
1661 S: DATA 8799cabb2ea93e
1662 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1669 <sect2 id="auth-states">
1670 <title>Authentication state diagrams</title>
1673 This section documents the auth protocol in terms of
1674 a state machine for the client and the server. This is
1675 probably the most robust way to implement the protocol.
1678 <sect3 id="auth-states-client">
1679 <title>Client states</title>
1682 To more precisely describe the interaction between the
1683 protocol state machine and the authentication mechanisms the
1684 following notation is used: MECH(CHALL) means that the
1685 server challenge CHALL was fed to the mechanism MECH, which
1691 CONTINUE(RESP) means continue the auth conversation
1692 and send RESP as the response to the server;
1698 OK(RESP) means that after sending RESP to the server
1699 the client side of the auth conversation is finished
1700 and the server should return "OK";
1706 ERROR means that CHALL was invalid and could not be
1712 Both RESP and CHALL may be empty.
1716 The Client starts by getting an initial response from the
1717 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1718 the mechanism did not provide an initial response. If the
1719 mechanism returns CONTINUE, the client starts in state
1720 <emphasis>WaitingForData</emphasis>, if the mechanism
1721 returns OK the client starts in state
1722 <emphasis>WaitingForOK</emphasis>.
1726 The client should keep track of available mechanisms and
1727 which it mechanisms it has already attempted. This list is
1728 used to decide which AUTH command to send. When the list is
1729 exhausted, the client should give up and close the
1734 <title><emphasis>WaitingForData</emphasis></title>
1742 MECH(CHALL) returns CONTINUE(RESP) → send
1744 <emphasis>WaitingForData</emphasis>
1748 MECH(CHALL) returns OK(RESP) → send DATA
1749 RESP, goto <emphasis>WaitingForOK</emphasis>
1753 MECH(CHALL) returns ERROR → send ERROR
1754 [msg], goto <emphasis>WaitingForData</emphasis>
1762 Receive REJECTED [mechs] →
1763 send AUTH [next mech], goto
1764 WaitingForData or <emphasis>WaitingForOK</emphasis>
1769 Receive ERROR → send
1771 <emphasis>WaitingForReject</emphasis>
1776 Receive OK → send
1777 BEGIN, terminate auth
1778 conversation, authenticated
1783 Receive anything else → send
1785 <emphasis>WaitingForData</emphasis>
1793 <title><emphasis>WaitingForOK</emphasis></title>
1798 Receive OK → send BEGIN, terminate auth
1799 conversation, <emphasis>authenticated</emphasis>
1804 Receive REJECT [mechs] → send AUTH [next mech],
1805 goto <emphasis>WaitingForData</emphasis> or
1806 <emphasis>WaitingForOK</emphasis>
1812 Receive DATA → send CANCEL, goto
1813 <emphasis>WaitingForReject</emphasis>
1819 Receive ERROR → send CANCEL, goto
1820 <emphasis>WaitingForReject</emphasis>
1826 Receive anything else → send ERROR, goto
1827 <emphasis>WaitingForOK</emphasis>
1835 <title><emphasis>WaitingForReject</emphasis></title>
1840 Receive REJECT [mechs] → send AUTH [next mech],
1841 goto <emphasis>WaitingForData</emphasis> or
1842 <emphasis>WaitingForOK</emphasis>
1848 Receive anything else → terminate auth
1849 conversation, disconnect
1858 <sect3 id="auth-states-server">
1859 <title>Server states</title>
1862 For the server MECH(RESP) means that the client response
1863 RESP was fed to the the mechanism MECH, which returns one of
1868 CONTINUE(CHALL) means continue the auth conversation and
1869 send CHALL as the challenge to the client;
1875 OK means that the client has been successfully
1882 REJECT means that the client failed to authenticate or
1883 there was an error in RESP.
1888 The server starts out in state
1889 <emphasis>WaitingForAuth</emphasis>. If the client is
1890 rejected too many times the server must disconnect the
1895 <title><emphasis>WaitingForAuth</emphasis></title>
1901 Receive AUTH → send REJECTED [mechs], goto
1902 <emphasis>WaitingForAuth</emphasis>
1908 Receive AUTH MECH RESP
1912 MECH not valid mechanism → send REJECTED
1914 <emphasis>WaitingForAuth</emphasis>
1918 MECH(RESP) returns CONTINUE(CHALL) → send
1920 <emphasis>WaitingForData</emphasis>
1924 MECH(RESP) returns OK → send OK, goto
1925 <emphasis>WaitingForBegin</emphasis>
1929 MECH(RESP) returns REJECT → send REJECTED
1931 <emphasis>WaitingForAuth</emphasis>
1939 Receive BEGIN → terminate
1940 auth conversation, disconnect
1946 Receive ERROR → send REJECTED [mechs], goto
1947 <emphasis>WaitingForAuth</emphasis>
1953 Receive anything else → send
1955 <emphasis>WaitingForAuth</emphasis>
1964 <title><emphasis>WaitingForData</emphasis></title>
1972 MECH(RESP) returns CONTINUE(CHALL) → send
1974 <emphasis>WaitingForData</emphasis>
1978 MECH(RESP) returns OK → send OK, goto
1979 <emphasis>WaitingForBegin</emphasis>
1983 MECH(RESP) returns REJECT → send REJECTED
1985 <emphasis>WaitingForAuth</emphasis>
1993 Receive BEGIN → terminate auth conversation,
2000 Receive CANCEL → send REJECTED [mechs], goto
2001 <emphasis>WaitingForAuth</emphasis>
2007 Receive ERROR → send REJECTED [mechs], goto
2008 <emphasis>WaitingForAuth</emphasis>
2014 Receive anything else → send ERROR, goto
2015 <emphasis>WaitingForData</emphasis>
2023 <title><emphasis>WaitingForBegin</emphasis></title>
2028 Receive BEGIN → terminate auth conversation,
2029 client authenticated
2035 Receive CANCEL → send REJECTED [mechs], goto
2036 <emphasis>WaitingForAuth</emphasis>
2042 Receive ERROR → send REJECTED [mechs], goto
2043 <emphasis>WaitingForAuth</emphasis>
2049 Receive anything else → send ERROR, goto
2050 <emphasis>WaitingForBegin</emphasis>
2060 <sect2 id="auth-mechanisms">
2061 <title>Authentication mechanisms</title>
2063 This section describes some new authentication mechanisms.
2064 D-Bus also allows any standard SASL mechanism of course.
2066 <sect3 id="auth-mechanisms-sha">
2067 <title>DBUS_COOKIE_SHA1</title>
2069 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2070 has the ability to read a private file owned by the user being
2071 authenticated. If the client can prove that it has access to a secret
2072 cookie stored in this file, then the client is authenticated.
2073 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2077 Authentication proceeds as follows:
2081 The client sends the username it would like to authenticate
2087 The server sends the name of its "cookie context" (see below); a
2088 space character; the integer ID of the secret cookie the client
2089 must demonstrate knowledge of; a space character; then a
2090 hex-encoded randomly-generated challenge string.
2095 The client locates the cookie, and generates its own hex-encoded
2096 randomly-generated challenge string. The client then
2097 concatenates the server's hex-encoded challenge, a ":"
2098 character, its own hex-encoded challenge, another ":" character,
2099 and the hex-encoded cookie. It computes the SHA-1 hash of this
2100 composite string. It sends back to the server the client's
2101 hex-encoded challenge string, a space character, and the SHA-1
2107 The server generates the same concatenated string used by the
2108 client and computes its SHA-1 hash. It compares the hash with
2109 the hash received from the client; if the two hashes match, the
2110 client is authenticated.
2116 Each server has a "cookie context," which is a name that identifies a
2117 set of cookies that apply to that server. A sample context might be
2118 "org_freedesktop_session_bus". Context names must be valid ASCII,
2119 nonzero length, and may not contain the characters slash ("/"),
2120 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2121 tab ("\t"), or period ("."). There is a default context,
2122 "org_freedesktop_general" that's used by servers that do not specify
2126 Cookies are stored in a user's home directory, in the directory
2127 <filename>~/.dbus-keyrings/</filename>. This directory must
2128 not be readable or writable by other users. If it is,
2129 clients and servers must ignore it. The directory
2130 contains cookie files named after the cookie context.
2133 A cookie file contains one cookie per line. Each line
2134 has three space-separated fields:
2138 The cookie ID number, which must be a non-negative integer and
2139 may not be used twice in the same file.
2144 The cookie's creation time, in UNIX seconds-since-the-epoch
2150 The cookie itself, a hex-encoded random block of bytes. The cookie
2151 may be of any length, though obviously security increases
2152 as the length increases.
2158 Only server processes modify the cookie file.
2159 They must do so with this procedure:
2163 Create a lockfile name by appending ".lock" to the name of the
2164 cookie file. The server should attempt to create this file
2165 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2166 fails, the lock fails. Servers should retry for a reasonable
2167 period of time, then they may choose to delete an existing lock
2168 to keep users from having to manually delete a stale
2169 lock. <footnote><para>Lockfiles are used instead of real file
2170 locking <literal>fcntl()</literal> because real locking
2171 implementations are still flaky on network
2172 filesystems.</para></footnote>
2177 Once the lockfile has been created, the server loads the cookie
2178 file. It should then delete any cookies that are old (the
2179 timeout can be fairly short), or more than a reasonable
2180 time in the future (so that cookies never accidentally
2181 become permanent, if the clock was set far into the future
2182 at some point). If no recent keys remain, the
2183 server may generate a new key.
2188 The pruned and possibly added-to cookie file
2189 must be resaved atomically (using a temporary
2190 file which is rename()'d).
2195 The lock must be dropped by deleting the lockfile.
2201 Clients need not lock the file in order to load it,
2202 because servers are required to save the file atomically.
2207 <sect1 id="addresses">
2208 <title>Server Addresses</title>
2210 Server addresses consist of a transport name followed by a colon, and
2211 then an optional, comma-separated list of keys and values in the form key=value.
2212 Each value is escaped.
2216 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2217 Which is the address to a unix socket with the path /tmp/dbus-test.
2220 Value escaping is similar to URI escaping but simpler.
2224 The set of optionally-escaped bytes is:
2225 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2226 <emphasis>byte</emphasis> (note, not character) which is not in the
2227 set of optionally-escaped bytes must be replaced with an ASCII
2228 percent (<literal>%</literal>) and the value of the byte in hex.
2229 The hex value must always be two digits, even if the first digit is
2230 zero. The optionally-escaped bytes may be escaped if desired.
2235 To unescape, append each byte in the value; if a byte is an ASCII
2236 percent (<literal>%</literal>) character then append the following
2237 hex value instead. It is an error if a <literal>%</literal> byte
2238 does not have two hex digits following. It is an error if a
2239 non-optionally-escaped byte is seen unescaped.
2243 The set of optionally-escaped bytes is intended to preserve address
2244 readability and convenience.
2248 A server may specify a key-value pair with the key <literal>guid</literal>
2249 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2250 describes the format of the <literal>guid</literal> field. If present,
2251 this UUID may be used to distinguish one server address from another. A
2252 server should use a different UUID for each address it listens on. For
2253 example, if a message bus daemon offers both UNIX domain socket and TCP
2254 connections, but treats clients the same regardless of how they connect,
2255 those two connections are equivalent post-connection but should have
2256 distinct UUIDs to distinguish the kinds of connection.
2260 The intent of the address UUID feature is to allow a client to avoid
2261 opening multiple identical connections to the same server, by allowing the
2262 client to check whether an address corresponds to an already-existing
2263 connection. Comparing two addresses is insufficient, because addresses
2264 can be recycled by distinct servers, and equivalent addresses may look
2265 different if simply compared as strings (for example, the host in a TCP
2266 address can be given as an IP address or as a hostname).
2270 Note that the address key is <literal>guid</literal> even though the
2271 rest of the API and documentation says "UUID," for historical reasons.
2275 [FIXME clarify if attempting to connect to each is a requirement
2276 or just a suggestion]
2277 When connecting to a server, multiple server addresses can be
2278 separated by a semi-colon. The library will then try to connect
2279 to the first address and if that fails, it'll try to connect to
2280 the next one specified, and so forth. For example
2281 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2286 <sect1 id="transports">
2287 <title>Transports</title>
2289 [FIXME we need to specify in detail each transport and its possible arguments]
2291 Current transports include: unix domain sockets (including
2292 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2293 in-process pipes. Future possible transports include one that
2294 tunnels over X11 protocol.
2297 <sect2 id="transports-unix-domain-sockets">
2298 <title>Unix Domain Sockets</title>
2300 Unix domain sockets can be either paths in the file system or on Linux
2301 kernels, they can be abstract which are similar to paths but
2302 do not show up in the file system.
2306 When a socket is opened by the D-Bus library it truncates the path
2307 name right before the first trailing Nul byte. This is true for both
2308 normal paths and abstract paths. Note that this is a departure from
2309 previous versions of D-Bus that would create sockets with a fixed
2310 length path name. Names which were shorter than the fixed length
2311 would be padded by Nul bytes.
2316 <sect1 id="naming-conventions">
2317 <title>Naming Conventions</title>
2320 D-Bus namespaces are all lowercase and correspond to reversed domain
2321 names, as with Java. e.g. "org.freedesktop"
2324 Interface, signal, method, and property names are "WindowsStyleCaps", note
2325 that the first letter is capitalized, unlike Java.
2328 Object paths are normally all lowercase with underscores used rather than
2334 <title>UUIDs</title>
2336 A working D-Bus implementation uses universally-unique IDs in two places.
2337 First, each server address has a UUID identifying the address,
2338 as described in <xref linkend="addresses"/>. Second, each operating
2339 system kernel instance running a D-Bus client or server has a UUID
2340 identifying that kernel, retrieved by invoking the method
2341 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2342 linkend="standard-interfaces-peer"/>).
2345 The term "UUID" in this document is intended literally, i.e. an
2346 identifier that is universally unique. It is not intended to refer to
2347 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2350 The UUID must contain 128 bits of data and be hex-encoded. The
2351 hex-encoded string may not contain hyphens or other non-hex-digit
2352 characters, and it must be exactly 32 characters long. To generate a
2353 UUID, the current reference implementation concatenates 96 bits of random
2354 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2358 It would also be acceptable and probably better to simply generate 128
2359 bits of random data, as long as the random number generator is of high
2360 quality. The timestamp could conceivably help if the random bits are not
2361 very random. With a quality random number generator, collisions are
2362 extremely unlikely even with only 96 bits, so it's somewhat academic.
2365 Implementations should, however, stick to random data for the first 96 bits
2370 <sect1 id="standard-interfaces">
2371 <title>Standard Interfaces</title>
2373 See <xref linkend="message-protocol-types-notation"/> for details on
2374 the notation used in this section. There are some standard interfaces
2375 that may be useful across various D-Bus applications.
2377 <sect2 id="standard-interfaces-peer">
2378 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2380 The <literal>org.freedesktop.DBus.Peer</literal> interface
2383 org.freedesktop.DBus.Peer.Ping ()
2384 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2388 On receipt of the <literal>METHOD_CALL</literal> message
2389 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2390 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2391 usual. It does not matter which object path a ping is sent to. The
2392 reference implementation handles this method automatically.
2395 On receipt of the <literal>METHOD_CALL</literal> message
2396 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2397 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2398 UUID representing the identity of the machine the process is running on.
2399 This UUID must be the same for all processes on a single system at least
2400 until that system next reboots. It should be the same across reboots
2401 if possible, but this is not always possible to implement and is not
2403 It does not matter which object path a GetMachineId is sent to. The
2404 reference implementation handles this method automatically.
2407 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2408 a virtual machine running on a hypervisor, rather than a physical machine.
2409 Basically if two processes see the same UUID, they should also see the same
2410 shared memory, UNIX domain sockets, process IDs, and other features that require
2411 a running OS kernel in common between the processes.
2414 The UUID is often used where other programs might use a hostname. Hostnames
2415 can change without rebooting, however, or just be "localhost" - so the UUID
2419 <xref linkend="uuids"/> explains the format of the UUID.
2423 <sect2 id="standard-interfaces-introspectable">
2424 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2426 This interface has one method:
2428 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2432 Objects instances may implement
2433 <literal>Introspect</literal> which returns an XML description of
2434 the object, including its interfaces (with signals and methods), objects
2435 below it in the object path tree, and its properties.
2438 <xref linkend="introspection-format"/> describes the format of this XML string.
2441 <sect2 id="standard-interfaces-properties">
2442 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2444 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2445 or <firstterm>attributes</firstterm>. These can be exposed via the
2446 <literal>org.freedesktop.DBus.Properties</literal> interface.
2450 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2451 in STRING property_name,
2453 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2454 in STRING property_name,
2456 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
2457 out DICT<STRING,VARIANT> props);
2461 The available properties and whether they are writable can be determined
2462 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2463 see <xref linkend="standard-interfaces-introspectable"/>.
2466 An empty string may be provided for the interface name; in this case,
2467 if there are multiple properties on an object with the same name,
2468 the results are undefined (picking one by according to an arbitrary
2469 deterministic rule, or returning an error, are the reasonable
2475 <sect1 id="introspection-format">
2476 <title>Introspection Data Format</title>
2478 As described in <xref linkend="standard-interfaces-introspectable"/>,
2479 objects may be introspected at runtime, returning an XML string
2480 that describes the object. The same XML format may be used in
2481 other contexts as well, for example as an "IDL" for generating
2482 static language bindings.
2485 Here is an example of introspection data:
2487 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2488 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2489 <node name="/org/freedesktop/sample_object">
2490 <interface name="org.freedesktop.SampleInterface">
2491 <method name="Frobate">
2492 <arg name="foo" type="i" direction="in"/>
2493 <arg name="bar" type="s" direction="out"/>
2494 <arg name="baz" type="a{us}" direction="out"/>
2495 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2497 <method name="Bazify">
2498 <arg name="bar" type="(iiu)" direction="in"/>
2499 <arg name="bar" type="v" direction="out"/>
2501 <method name="Mogrify">
2502 <arg name="bar" type="(iiav)" direction="in"/>
2504 <signal name="Changed">
2505 <arg name="new_value" type="b"/>
2507 <property name="Bar" type="y" access="readwrite"/>
2509 <node name="child_of_sample_object"/>
2510 <node name="another_child_of_sample_object"/>
2515 A more formal DTD and spec needs writing, but here are some quick notes.
2519 Only the root <node> element can omit the node name, as it's
2520 known to be the object that was introspected. If the root
2521 <node> does have a name attribute, it must be an absolute
2522 object path. If child <node> have object paths, they must be
2528 If a child <node> has any sub-elements, then they
2529 must represent a complete introspection of the child.
2530 If a child <node> is empty, then it may or may
2531 not have sub-elements; the child must be introspected
2532 in order to find out. The intent is that if an object
2533 knows that its children are "fast" to introspect
2534 it can go ahead and return their information, but
2535 otherwise it can omit it.
2540 The direction element on <arg> may be omitted,
2541 in which case it defaults to "in" for method calls
2542 and "out" for signals. Signals only allow "out"
2543 so while direction may be specified, it's pointless.
2548 The possible directions are "in" and "out",
2549 unlike CORBA there is no "inout"
2554 The possible property access flags are
2555 "readwrite", "read", and "write"
2560 Multiple interfaces can of course be listed for
2566 The "name" attribute on arguments is optional.
2572 Method, interface, property, and signal elements may have
2573 "annotations", which are generic key/value pairs of metadata.
2574 They are similar conceptually to Java's annotations and C# attributes.
2575 Well-known annotations:
2582 <entry>Values (separated by ,)</entry>
2583 <entry>Description</entry>
2588 <entry>org.freedesktop.DBus.Deprecated</entry>
2589 <entry>true,false</entry>
2590 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2593 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2594 <entry>(string)</entry>
2595 <entry>The C symbol; may be used for methods and interfaces</entry>
2598 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2599 <entry>true,false</entry>
2600 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2606 <sect1 id="message-bus">
2607 <title>Message Bus Specification</title>
2608 <sect2 id="message-bus-overview">
2609 <title>Message Bus Overview</title>
2611 The message bus accepts connections from one or more applications.
2612 Once connected, applications can exchange messages with other
2613 applications that are also connected to the bus.
2616 In order to route messages among connections, the message bus keeps a
2617 mapping from names to connections. Each connection has one
2618 unique-for-the-lifetime-of-the-bus name automatically assigned.
2619 Applications may request additional names for a connection. Additional
2620 names are usually "well-known names" such as
2621 "org.freedesktop.TextEditor". When a name is bound to a connection,
2622 that connection is said to <firstterm>own</firstterm> the name.
2625 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
2626 This name routes messages to the bus, allowing applications to make
2627 administrative requests. For example, applications can ask the bus
2628 to assign a name to a connection.
2631 Each name may have <firstterm>queued owners</firstterm>. When an
2632 application requests a name for a connection and the name is already in
2633 use, the bus will optionally add the connection to a queue waiting for
2634 the name. If the current owner of the name disconnects or releases
2635 the name, the next connection in the queue will become the new owner.
2639 This feature causes the right thing to happen if you start two text
2640 editors for example; the first one may request "org.freedesktop.TextEditor",
2641 and the second will be queued as a possible owner of that name. When
2642 the first exits, the second will take over.
2646 Messages may have a <literal>DESTINATION</literal> field (see <xref
2647 linkend="message-protocol-header-fields"/>). If the
2648 <literal>DESTINATION</literal> field is present, it specifies a message
2649 recipient by name. Method calls and replies normally specify this field.
2653 Signals normally do not specify a destination; they are sent to all
2654 applications with <firstterm>message matching rules</firstterm> that
2659 When the message bus receives a method call, if the
2660 <literal>DESTINATION</literal> field is absent, the call is taken to be
2661 a standard one-to-one message and interpreted by the message bus
2662 itself. For example, sending an
2663 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
2664 <literal>DESTINATION</literal> will cause the message bus itself to
2665 reply to the ping immediately; the message bus will not make this
2666 message visible to other applications.
2670 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
2671 the ping message were sent with a <literal>DESTINATION</literal> name of
2672 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
2673 forwarded, and the Yoyodyne Corporation screensaver application would be
2674 expected to reply to the ping.
2678 <sect2 id="message-bus-names">
2679 <title>Message Bus Names</title>
2681 Each connection has at least one name, assigned at connection time and
2682 returned in response to the
2683 <literal>org.freedesktop.DBus.Hello</literal> method call. This
2684 automatically-assigned name is called the connection's <firstterm>unique
2685 name</firstterm>. Unique names are never reused for two different
2686 connections to the same bus.
2689 Ownership of a unique name is a prerequisite for interaction with
2690 the message bus. It logically follows that the unique name is always
2691 the first name that an application comes to own, and the last
2692 one that it loses ownership of.
2695 Unique connection names must begin with the character ':' (ASCII colon
2696 character); bus names that are not unique names must not begin
2697 with this character. (The bus must reject any attempt by an application
2698 to manually request a name beginning with ':'.) This restriction
2699 categorically prevents "spoofing"; messages sent to a unique name
2700 will always go to the expected connection.
2703 When a connection is closed, all the names that it owns are deleted (or
2704 transferred to the next connection in the queue if any).
2707 A connection can request additional names to be associated with it using
2708 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
2709 linkend="message-protocol-names-bus"/> describes the format of a valid
2710 name. These names can be released again using the
2711 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
2714 <sect3 id="bus-messages-request-name">
2715 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
2719 UINT32 RequestName (in STRING name, in UINT32 flags)
2726 <entry>Argument</entry>
2728 <entry>Description</entry>
2734 <entry>STRING</entry>
2735 <entry>Name to request</entry>
2739 <entry>UINT32</entry>
2740 <entry>Flags</entry>
2750 <entry>Argument</entry>
2752 <entry>Description</entry>
2758 <entry>UINT32</entry>
2759 <entry>Return value</entry>
2766 This method call should be sent to
2767 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2768 assign the given name to the method caller. Each name maintains a
2769 queue of possible owners, where the head of the queue is the primary
2770 or current owner of the name. Each potential owner in the queue
2771 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
2772 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
2773 call. When RequestName is invoked the following occurs:
2777 If the method caller is currently the primary owner of the name,
2778 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
2779 values are updated with the values from the new RequestName call,
2780 and nothing further happens.
2786 If the current primary owner (head of the queue) has
2787 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
2788 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
2789 the caller of RequestName replaces the current primary owner at
2790 the head of the queue and the current primary owner moves to the
2791 second position in the queue. If the caller of RequestName was
2792 in the queue previously its flags are updated with the values from
2793 the new RequestName in addition to moving it to the head of the queue.
2799 If replacement is not possible, and the method caller is
2800 currently in the queue but not the primary owner, its flags are
2801 updated with the values from the new RequestName call.
2807 If replacement is not possible, and the method caller is
2808 currently not in the queue, the method caller is appended to the
2815 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
2816 set and is not the primary owner, it is removed from the
2817 queue. This can apply to the previous primary owner (if it
2818 was replaced) or the method caller (if it updated the
2819 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
2820 queue, or if it was just added to the queue with that flag set).
2826 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
2827 queue," even if another application already in the queue had specified
2828 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
2829 that does not allow replacement goes away, and the next primary owner
2830 does allow replacement. In this case, queued items that specified
2831 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
2832 automatically replace the new primary owner. In other words,
2833 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
2834 time RequestName is called. This is deliberate to avoid an infinite loop
2835 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2836 and DBUS_NAME_FLAG_REPLACE_EXISTING.
2839 The flags argument contains any of the following values logically ORed
2846 <entry>Conventional Name</entry>
2847 <entry>Value</entry>
2848 <entry>Description</entry>
2853 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
2857 If an application A specifies this flag and succeeds in
2858 becoming the owner of the name, and another application B
2859 later calls RequestName with the
2860 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
2861 will lose ownership and receive a
2862 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
2863 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2864 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
2865 is not specified by application B, then application B will not replace
2866 application A as the owner.
2871 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
2875 Try to replace the current owner if there is one. If this
2876 flag is not set the application will only become the owner of
2877 the name if there is no current owner. If this flag is set,
2878 the application will replace the current owner if
2879 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
2884 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
2888 Without this flag, if an application requests a name that is
2889 already owned, the application will be placed in a queue to
2890 own the name when the current owner gives it up. If this
2891 flag is given, the application will not be placed in the
2892 queue, the request for the name will simply fail. This flag
2893 also affects behavior when an application is replaced as
2894 name owner; by default the application moves back into the
2895 waiting queue, unless this flag was provided when the application
2896 became the name owner.
2904 The return code can be one of the following values:
2910 <entry>Conventional Name</entry>
2911 <entry>Value</entry>
2912 <entry>Description</entry>
2917 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
2918 <entry>1</entry> <entry>The caller is now the primary owner of
2919 the name, replacing any previous owner. Either the name had no
2920 owner before, or the caller specified
2921 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
2922 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
2925 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
2928 <entry>The name already had an owner,
2929 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
2930 the current owner did not specify
2931 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
2932 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
2936 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
2937 <entry>The name already has an owner,
2938 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
2939 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
2940 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
2941 specified by the requesting application.</entry>
2944 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
2946 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
2954 <sect3 id="bus-messages-release-name">
2955 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
2959 UINT32 ReleaseName (in STRING name)
2966 <entry>Argument</entry>
2968 <entry>Description</entry>
2974 <entry>STRING</entry>
2975 <entry>Name to release</entry>
2985 <entry>Argument</entry>
2987 <entry>Description</entry>
2993 <entry>UINT32</entry>
2994 <entry>Return value</entry>
3001 This method call should be sent to
3002 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3003 release the method caller's claim to the given name. If the caller is
3004 the primary owner, a new primary owner will be selected from the
3005 queue if any other owners are waiting. If the caller is waiting in
3006 the queue for the name, the caller will removed from the queue and
3007 will not be made an owner of the name if it later becomes available.
3008 If there are no other owners in the queue for the name, it will be
3009 removed from the bus entirely.
3011 The return code can be one of the following values:
3017 <entry>Conventional Name</entry>
3018 <entry>Value</entry>
3019 <entry>Description</entry>
3024 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3025 <entry>1</entry> <entry>The caller has released his claim on
3026 the given name. Either the caller was the primary owner of
3027 the name, and the name is now unused or taken by somebody
3028 waiting in the queue for the name, or the caller was waiting
3029 in the queue for the name and has now been removed from the
3033 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3035 <entry>The given name does not exist on this bus.</entry>
3038 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3040 <entry>The caller was not the primary owner of this name,
3041 and was also not waiting in the queue to own this name.</entry>
3050 <sect2 id="message-bus-routing">
3051 <title>Message Bus Message Routing</title>
3055 <sect3 id="message-bus-routing-match-rules">
3056 <title>Match Rules</title>
3058 An important part of the message bus routing protocol is match
3059 rules. Match rules describe what messages can be sent to a client
3060 based on the contents of the message. When a message is routed
3061 through the bus it is compared to clients' match rules. If any
3062 of the rules match, the message is dispatched to the client.
3063 If none of the rules match the message never leaves the bus. This
3064 is an effective way to control traffic over the bus and to make sure
3065 only relevant message need to be processed by the client.
3068 Match rules are added using the AddMatch bus method
3069 (see xref linkend="bus-messages-add-match"/>). Rules are
3070 specified as a string of comma separated key/value pairs.
3071 Excluding a key from the rule indicates a wildcard match.
3072 For instance excluding the the member from a match rule but
3073 adding a sender would let all messages from that sender through.
3074 An example of a complete rule would be
3075 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3078 The following table describes the keys that can be used to create
3080 The following table summarizes the D-Bus types.
3086 <entry>Possible Values</entry>
3087 <entry>Description</entry>
3092 <entry><literal>type</literal></entry>
3093 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3094 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3097 <entry><literal>sender</literal></entry>
3098 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3099 and <xref linkend="term-unique-name"/> respectively)
3101 <entry>Match messages sent by a particular sender. An example of a sender match
3102 is sender='org.freedesktop.Hal'</entry>
3105 <entry><literal>interface</literal></entry>
3106 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3107 <entry>Match messages sent over or to a particular interface. An example of an
3108 interface match is interface='org.freedesktop.Hal.Manager'.
3109 If a message omits the interface header, it must not match any rule
3110 that specifies this key.</entry>
3113 <entry><literal>member</literal></entry>
3114 <entry>Any valid method or signal name</entry>
3115 <entry>Matches messages which have the give method or signal name. An example of
3116 a member match is member='NameOwnerChanged'</entry>
3119 <entry><literal>path</literal></entry>
3120 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3121 <entry>Matches messages which are sent from or to the given object. An example of a
3122 path match is path='/org/freedesktop/Hal/Manager'</entry>
3125 <entry><literal>destination</literal></entry>
3126 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3127 <entry>Matches messages which are being sent to the given unique name. An
3128 example of a destination match is destination=':1.0'</entry>
3131 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3132 <entry>Any string</entry>
3133 <entry>Arg matches are special and are used for further restricting the
3134 match based on the arguments in the body of a message. As of this time
3135 only string arguments can be matched. An example of an argument match
3136 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3145 <sect2 id="message-bus-starting-services">
3146 <title>Message Bus Starting Services</title>
3148 The message bus can start applications on behalf of other applications.
3149 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3150 An application that can be started in this way is called a
3151 <firstterm>service</firstterm>.
3154 With D-Bus, starting a service is normally done by name. That is,
3155 applications ask the message bus to start some program that will own a
3156 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3157 This implies a contract documented along with the name
3158 <literal>org.freedesktop.TextEditor</literal> for which objects
3159 the owner of that name will provide, and what interfaces those
3163 To find an executable corresponding to a particular name, the bus daemon
3164 looks for <firstterm>service description files</firstterm>. Service
3165 description files define a mapping from names to executables. Different
3166 kinds of message bus will look for these files in different places, see
3167 <xref linkend="message-bus-types"/>.
3170 [FIXME the file format should be much better specified than "similar to
3171 .desktop entries" esp. since desktop entries are already
3172 badly-specified. ;-)] Service description files have the ".service" file
3173 extension. The message bus will only load service description files
3174 ending with .service; all other files will be ignored. The file format
3175 is similar to that of <ulink
3176 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
3177 entries</ulink>. All service description files must be in UTF-8
3178 encoding. To ensure that there will be no name collisions, service files
3179 must be namespaced using the same mechanism as messages and service
3183 <title>Example service description file</title>
3185 # Sample service description file
3187 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3188 Exec=/usr/libexec/gconfd-2
3193 When an application asks to start a service by name, the bus daemon tries to
3194 find a service that will own that name. It then tries to spawn the
3195 executable associated with it. If this fails, it will report an
3196 error. [FIXME what happens if two .service files offer the same service;
3197 what kind of error is reported, should we have a way for the client to
3201 The executable launched will have the environment variable
3202 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3203 message bus so it can connect and request the appropriate names.
3206 The executable being launched may want to know whether the message bus
3207 starting it is one of the well-known message buses (see <xref
3208 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3209 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3210 of the well-known buses. The currently-defined values for this variable
3211 are <literal>system</literal> for the systemwide message bus,
3212 and <literal>session</literal> for the per-login-session message
3213 bus. The new executable must still connect to the address given
3214 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3215 resulting connection is to the well-known bus.
3218 [FIXME there should be a timeout somewhere, either specified
3219 in the .service file, by the client, or just a global value
3220 and if the client being activated fails to connect within that
3221 timeout, an error should be sent back.]
3224 <sect3 id="message-bus-starting-services-scope">
3225 <title>Message Bus Service Scope</title>
3227 The "scope" of a service is its "per-", such as per-session,
3228 per-machine, per-home-directory, or per-display. The reference
3229 implementation doesn't yet support starting services in a different
3230 scope from the message bus itself. So e.g. if you start a service
3231 on the session bus its scope is per-session.
3234 We could add an optional scope to a bus name. For example, for
3235 per-(display,session pair), we could have a unique ID for each display
3236 generated automatically at login and set on screen 0 by executing a
3237 special "set display ID" binary. The ID would be stored in a
3238 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3239 random bytes. This ID would then be used to scope names.
3240 Starting/locating a service could be done by ID-name pair rather than
3244 Contrast this with a per-display scope. To achieve that, we would
3245 want a single bus spanning all sessions using a given display.
3246 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3247 property on screen 0 of the display, pointing to this bus.
3252 <sect2 id="message-bus-types">
3253 <title>Well-known Message Bus Instances</title>
3255 Two standard message bus instances are defined here, along with how
3256 to locate them and where their service files live.
3258 <sect3 id="message-bus-types-login">
3259 <title>Login session message bus</title>
3261 Each time a user logs in, a <firstterm>login session message
3262 bus</firstterm> may be started. All applications in the user's login
3263 session may interact with one another using this message bus.
3266 The address of the login session message bus is given
3267 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3268 variable. If that variable is not set, applications may
3269 also try to read the address from the X Window System root
3270 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3271 The root window property must have type <literal>STRING</literal>.
3272 The environment variable should have precedence over the
3273 root window property.
3276 [FIXME specify location of .service files, probably using
3277 DESKTOP_DIRS etc. from basedir specification, though login session
3278 bus is not really desktop-specific]
3281 <sect3 id="message-bus-types-system">
3282 <title>System message bus</title>
3284 A computer may have a <firstterm>system message bus</firstterm>,
3285 accessible to all applications on the system. This message bus may be
3286 used to broadcast system events, such as adding new hardware devices,
3287 changes in the printer queue, and so forth.
3290 The address of the system message bus is given
3291 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3292 variable. If that variable is not set, applications should try
3293 to connect to the well-known address
3294 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3297 The D-Bus reference implementation actually honors the
3298 <literal>$(localstatedir)</literal> configure option
3299 for this address, on both client and server side.
3304 [FIXME specify location of system bus .service files]
3309 <sect2 id="message-bus-messages">
3310 <title>Message Bus Messages</title>
3312 The special message bus name <literal>org.freedesktop.DBus</literal>
3313 responds to a number of additional messages.
3316 <sect3 id="bus-messages-hello">
3317 <title><literal>org.freedesktop.DBus.Hello</literal></title>
3328 <entry>Argument</entry>
3330 <entry>Description</entry>
3336 <entry>STRING</entry>
3337 <entry>Unique name assigned to the connection</entry>
3344 Before an application is able to send messages to other applications
3345 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
3346 to the message bus to obtain a unique name. If an application without
3347 a unique name tries to send a message to another application, or a
3348 message to the message bus itself that isn't the
3349 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
3350 disconnected from the bus.
3353 There is no corresponding "disconnect" request; if a client wishes to
3354 disconnect from the bus, it simply closes the socket (or other
3355 communication channel).
3358 <sect3 id="bus-messages-list-names">
3359 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
3363 ARRAY of STRING ListNames ()
3370 <entry>Argument</entry>
3372 <entry>Description</entry>
3378 <entry>ARRAY of STRING</entry>
3379 <entry>Array of strings where each string is a bus name</entry>
3386 Returns a list of all currently-owned names on the bus.
3389 <sect3 id="bus-messages-list-activatable-names">
3390 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
3394 ARRAY of STRING ListActivatableNames ()
3401 <entry>Argument</entry>
3403 <entry>Description</entry>
3409 <entry>ARRAY of STRING</entry>
3410 <entry>Array of strings where each string is a bus name</entry>
3417 Returns a list of all names that can be activated on the bus.
3420 <sect3 id="bus-messages-name-exists">
3421 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
3425 BOOLEAN NameHasOwner (in STRING name)
3432 <entry>Argument</entry>
3434 <entry>Description</entry>
3440 <entry>STRING</entry>
3441 <entry>Name to check</entry>
3451 <entry>Argument</entry>
3453 <entry>Description</entry>
3459 <entry>BOOLEAN</entry>
3460 <entry>Return value, true if the name exists</entry>
3467 Checks if the specified name exists (currently has an owner).
3471 <sect3 id="bus-messages-name-owner-changed">
3472 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
3476 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
3483 <entry>Argument</entry>
3485 <entry>Description</entry>
3491 <entry>STRING</entry>
3492 <entry>Name with a new owner</entry>
3496 <entry>STRING</entry>
3497 <entry>Old owner or empty string if none</entry>
3501 <entry>STRING</entry>
3502 <entry>New owner or empty string if none</entry>
3509 This signal indicates that the owner of a name has changed.
3510 It's also the signal to use to detect the appearance of
3511 new names on the bus.
3514 <sect3 id="bus-messages-name-lost">
3515 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
3519 NameLost (STRING name)
3526 <entry>Argument</entry>
3528 <entry>Description</entry>
3534 <entry>STRING</entry>
3535 <entry>Name which was lost</entry>
3542 This signal is sent to a specific application when it loses
3543 ownership of a name.
3547 <sect3 id="bus-messages-name-acquired">
3548 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
3552 NameAcquired (STRING name)
3559 <entry>Argument</entry>
3561 <entry>Description</entry>
3567 <entry>STRING</entry>
3568 <entry>Name which was acquired</entry>
3575 This signal is sent to a specific application when it gains
3576 ownership of a name.
3580 <sect3 id="bus-messages-start-service-by-name">
3581 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
3585 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
3592 <entry>Argument</entry>
3594 <entry>Description</entry>
3600 <entry>STRING</entry>
3601 <entry>Name of the service to start</entry>
3605 <entry>UINT32</entry>
3606 <entry>Flags (currently not used)</entry>
3616 <entry>Argument</entry>
3618 <entry>Description</entry>
3624 <entry>UINT32</entry>
3625 <entry>Return value</entry>
3630 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
3634 The return value can be one of the following values:
3639 <entry>Identifier</entry>
3640 <entry>Value</entry>
3641 <entry>Description</entry>
3646 <entry>DBUS_START_REPLY_SUCCESS</entry>
3648 <entry>The service was successfully started.</entry>
3651 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
3653 <entry>A connection already owns the given name.</entry>
3662 <sect3 id="bus-messages-get-name-owner">
3663 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
3667 STRING GetNameOwner (in STRING name)
3674 <entry>Argument</entry>
3676 <entry>Description</entry>
3682 <entry>STRING</entry>
3683 <entry>Name to get the owner of</entry>
3693 <entry>Argument</entry>
3695 <entry>Description</entry>
3701 <entry>STRING</entry>
3702 <entry>Return value, a unique connection name</entry>
3707 Returns the unique connection name of the primary owner of the name
3708 given. If the requested name doesn't have an owner, returns a
3709 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
3713 <sect3 id="bus-messages-get-connection-unix-user">
3714 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
3718 UINT32 GetConnectionUnixUser (in STRING connection_name)
3725 <entry>Argument</entry>
3727 <entry>Description</entry>
3733 <entry>STRING</entry>
3734 <entry>Name of the connection to query</entry>
3744 <entry>Argument</entry>
3746 <entry>Description</entry>
3752 <entry>UINT32</entry>
3753 <entry>unix user id</entry>
3758 Returns the unix uid of the process connected to the server. If unable to
3759 determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
3764 <sect3 id="bus-messages-add-match">
3765 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
3769 AddMatch (in STRING rule)
3776 <entry>Argument</entry>
3778 <entry>Description</entry>
3784 <entry>STRING</entry>
3785 <entry>Match rule to add to the connection</entry>
3790 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
3791 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
3795 <sect3 id="bus-messages-remove-match">
3796 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
3800 RemoveMatch (in STRING rule)
3807 <entry>Argument</entry>
3809 <entry>Description</entry>
3815 <entry>STRING</entry>
3816 <entry>Match rule to remove from the connection</entry>
3821 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
3822 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
3831 <appendix id="implementation-notes">
3832 <title>Implementation notes</title>
3833 <sect1 id="implementation-notes-subsection">
3841 <glossary><title>Glossary</title>
3843 This glossary defines some of the terms used in this specification.
3846 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
3849 The message bus maintains an association between names and
3850 connections. (Normally, there's one connection per application.) A
3851 bus name is simply an identifier used to locate connections. For
3852 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
3853 name might be used to send a message to a screensaver from Yoyodyne
3854 Corporation. An application is said to <firstterm>own</firstterm> a
3855 name if the message bus has associated the application's connection
3856 with the name. Names may also have <firstterm>queued
3857 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
3858 The bus assigns a unique name to each connection,
3859 see <xref linkend="term-unique-name"/>. Other names
3860 can be thought of as "well-known names" and are
3861 used to find applications that offer specific functionality.
3866 <glossentry id="term-message"><glossterm>Message</glossterm>
3869 A message is the atomic unit of communication via the D-Bus
3870 protocol. It consists of a <firstterm>header</firstterm> and a
3871 <firstterm>body</firstterm>; the body is made up of
3872 <firstterm>arguments</firstterm>.
3877 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
3880 The message bus is a special application that forwards
3881 or routes messages between a group of applications
3882 connected to the message bus. It also manages
3883 <firstterm>names</firstterm> used for routing
3889 <glossentry id="term-name"><glossterm>Name</glossterm>
3892 See <xref linkend="term-bus-name"/>. "Name" may
3893 also be used to refer to some of the other names
3894 in D-Bus, such as interface names.
3899 <glossentry id="namespace"><glossterm>Namespace</glossterm>
3902 Used to prevent collisions when defining new interfaces or bus
3903 names. The convention used is the same one Java uses for defining
3904 classes: a reversed domain name.
3909 <glossentry id="term-object"><glossterm>Object</glossterm>
3912 Each application contains <firstterm>objects</firstterm>, which have
3913 <firstterm>interfaces</firstterm> and
3914 <firstterm>methods</firstterm>. Objects are referred to by a name,
3915 called a <firstterm>path</firstterm>.
3920 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
3923 An application talking directly to another application, without going
3924 through a message bus. One-to-one connections may be "peer to peer" or
3925 "client to server." The D-Bus protocol has no concept of client
3926 vs. server after a connection has authenticated; the flow of messages
3927 is symmetrical (full duplex).
3932 <glossentry id="term-path"><glossterm>Path</glossterm>
3935 Object references (object names) in D-Bus are organized into a
3936 filesystem-style hierarchy, so each object is named by a path. As in
3937 LDAP, there's no difference between "files" and "directories"; a path
3938 can refer to an object, while still having child objects below it.
3943 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
3946 Each bus name has a primary owner; messages sent to the name go to the
3947 primary owner. However, certain names also maintain a queue of
3948 secondary owners "waiting in the wings." If the primary owner releases
3949 the name, then the first secondary owner in the queue automatically
3950 becomes the new owner of the name.
3955 <glossentry id="term-service"><glossterm>Service</glossterm>
3958 A service is an executable that can be launched by the bus daemon.
3959 Services normally guarantee some particular features, for example they
3960 may guarantee that they will request a specific name such as
3961 "org.freedesktop.Screensaver", have a singleton object
3962 "/org/freedesktop/Application", and that object will implement the
3963 interface "org.freedesktop.ScreensaverControl".
3968 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
3971 ".service files" tell the bus about service applications that can be
3972 launched (see <xref linkend="term-service"/>). Most importantly they
3973 provide a mapping from bus names to services that will request those
3974 names when they start up.
3979 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
3982 The special name automatically assigned to each connection by the
3983 message bus. This name will never change owner, and will be unique
3984 (never reused during the lifetime of the message bus).
3985 It will begin with a ':' character.
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