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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
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.</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 string data of the given length, followed by a terminating nul
513 <entry><literal>OBJECT_PATH</literal></entry>
514 <entry>Exactly the same as <literal>STRING</literal> except the
515 content must be a valid object path (see below).
521 <entry><literal>SIGNATURE</literal></entry>
522 <entry>The same as <literal>STRING</literal> except the length is a single
523 byte (thus signatures have a maximum length of 255)
524 and the content must be a valid signature (see below).
530 <entry><literal>ARRAY</literal></entry>
532 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
533 alignment padding to the alignment boundary of the array element type,
534 followed by each array element. The array length is from the
535 end of the alignment padding to the end of the last element,
536 i.e. it does not include the padding after the length,
537 or any padding after the last element.
538 Arrays have a maximum length defined to be 2 to the 26th power or
539 67108864. Implementations must not send or accept arrays exceeding this
546 <entry><literal>STRUCT</literal></entry>
548 A struct must start on an 8-byte boundary regardless of the
549 type of the struct fields. The struct value consists of each
550 field marshaled in sequence starting from that 8-byte
557 <entry><literal>VARIANT</literal></entry>
559 A variant type has a marshaled <literal>SIGNATURE</literal>
560 followed by a marshaled value with the type
561 given in the signature.
562 Unlike a message signature, the variant signature
563 can contain only a single complete type.
564 So "i" is OK, "ii" is not.
567 1 (alignment of the signature)
570 <entry><literal>DICT_ENTRY</literal></entry>
583 <sect3 id="message-protocol-marshaling-object-path">
584 <title>Valid Object Paths</title>
587 An object path is a name used to refer to an object instance.
588 Conceptually, each participant in a D-Bus message exchange may have
589 any number of object instances (think of C++ or Java objects) and each
590 such instance will have a path. Like a filesystem, the object
591 instances in an application form a hierarchical tree.
595 The following rules define a valid object path. Implementations must
596 not send or accept messages with invalid object paths.
600 The path may be of any length.
605 The path must begin with an ASCII '/' (integer 47) character,
606 and must consist of elements separated by slash characters.
611 Each element must only contain the ASCII characters
617 No element may be the empty string.
622 Multiple '/' characters cannot occur in sequence.
627 A trailing '/' character is not allowed unless the
628 path is the root path (a single '/' character).
637 <sect3 id="message-protocol-marshaling-signature">
638 <title>Valid Signatures</title>
640 An implementation must not send or accept invalid signatures.
641 Valid signatures will conform to the following rules:
645 The signature ends with a nul byte.
650 The signature is a list of single complete types.
651 Arrays must have element types, and structs must
652 have both open and close parentheses.
657 Only type codes and open and close parentheses are
658 allowed in the signature. The <literal>STRUCT</literal> type code
659 is not allowed in signatures, because parentheses
665 The maximum depth of container type nesting is 32 array type
666 codes and 32 open parentheses. This implies that the maximum
667 total depth of recursion is 64, for an "array of array of array
668 of ... struct of struct of struct of ..." where there are 32
674 The maximum length of a signature is 255.
679 Signatures must be nul-terminated.
688 <sect2 id="message-protocol-messages">
689 <title>Message Format</title>
692 A message consists of a header and a body. The header is a block of
693 values with a fixed signature and meaning. The body is a separate block
694 of values, with a signature specified in the header.
698 The length of the header must be a multiple of 8, allowing the body to
699 begin on an 8-byte boundary when storing the entire message in a single
700 buffer. If the header does not naturally end on an 8-byte boundary
701 up to 7 bytes of nul-initialized alignment padding must be added.
705 The message body need not end on an 8-byte boundary.
709 The maximum length of a message, including header, header alignment padding,
710 and body is 2 to the 27th power or 134217728. Implementations must not
711 send or accept messages exceeding this size.
715 The signature of the header is:
719 Written out more readably, this is:
721 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
726 These values have the following meanings:
732 <entry>Description</entry>
737 <entry>1st <literal>BYTE</literal></entry>
738 <entry>Endianness flag; ASCII 'l' for little-endian
739 or ASCII 'B' for big-endian. Both header and body are
740 in this endianness.</entry>
743 <entry>2nd <literal>BYTE</literal></entry>
744 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
745 Currently-defined types are described below.
749 <entry>3rd <literal>BYTE</literal></entry>
750 <entry>Bitwise OR of flags. Unknown flags
751 must be ignored. Currently-defined flags are described below.
755 <entry>4th <literal>BYTE</literal></entry>
756 <entry>Major protocol version of the sending application. If
757 the major protocol version of the receiving application does not
758 match, the applications will not be able to communicate and the
759 D-Bus connection must be disconnected. The major protocol
760 version for this version of the specification is 0.
761 FIXME this field is stupid and pointless to put in
766 <entry>1st <literal>UINT32</literal></entry>
767 <entry>Length in bytes of the message body, starting
768 from the end of the header. The header ends after
769 its alignment padding to an 8-boundary.
773 <entry>2nd <literal>UINT32</literal></entry>
774 <entry>The serial of this message, used as a cookie
775 by the sender to identify the reply corresponding
780 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
781 <entry>An array of zero or more <firstterm>header
782 fields</firstterm> where the byte is the field code, and the
783 variant is the field value. The message type determines
784 which fields are required.
792 <firstterm>Message types</firstterm> that can appear in the second byte
798 <entry>Conventional name</entry>
799 <entry>Decimal value</entry>
800 <entry>Description</entry>
805 <entry><literal>INVALID</literal></entry>
807 <entry>This is an invalid type.</entry>
810 <entry><literal>METHOD_CALL</literal></entry>
812 <entry>Method call.</entry>
815 <entry><literal>METHOD_RETURN</literal></entry>
817 <entry>Method reply with returned data.</entry>
820 <entry><literal>ERROR</literal></entry>
822 <entry>Error reply. If the first argument exists and is a
823 string, it is an error message.</entry>
826 <entry><literal>SIGNAL</literal></entry>
828 <entry>Signal emission.</entry>
835 Flags that can appear in the third byte of the header:
840 <entry>Conventional name</entry>
841 <entry>Hex value</entry>
842 <entry>Description</entry>
847 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
849 <entry>This message does not expect method return replies or
850 error replies; the reply can be omitted as an
851 optimization. However, it is compliant with this specification
852 to return the reply despite this flag and the only harm
853 from doing so is extra network traffic.
857 <entry><literal>NO_AUTO_START</literal></entry>
859 <entry>The bus must not launch an owner
860 for the destination name in response to this message.
868 <sect3 id="message-protocol-header-fields">
869 <title>Header Fields</title>
872 The array at the end of the header contains <firstterm>header
873 fields</firstterm>, where each field is a 1-byte field code followed
874 by a field value. A header must contain the required header fields for
875 its message type, and zero or more of any optional header
876 fields. Future versions of this protocol specification may add new
877 fields. Implementations must ignore fields they do not
878 understand. Implementations must not invent their own header fields;
879 only changes to this specification may introduce new header fields.
883 Again, if an implementation sees a header field code that it does not
884 expect, it must ignore that field, as it will be part of a new
885 (but compatible) version of this specification. This also applies
886 to known header fields appearing in unexpected messages, for
887 example: if a signal has a reply serial it must be ignored
888 even though it has no meaning as of this version of the spec.
892 However, implementations must not send or accept known header fields
893 with the wrong type stored in the field value. So for example a
894 message with an <literal>INTERFACE</literal> field of type
895 <literal>UINT32</literal> would be considered corrupt.
899 Here are the currently-defined header fields:
904 <entry>Conventional Name</entry>
905 <entry>Decimal Code</entry>
907 <entry>Required In</entry>
908 <entry>Description</entry>
913 <entry><literal>INVALID</literal></entry>
916 <entry>not allowed</entry>
917 <entry>Not a valid field name (error if it appears in a message)</entry>
920 <entry><literal>PATH</literal></entry>
922 <entry><literal>OBJECT_PATH</literal></entry>
923 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
924 <entry>The object to send a call to,
925 or the object a signal is emitted from.
929 <entry><literal>INTERFACE</literal></entry>
931 <entry><literal>STRING</literal></entry>
932 <entry><literal>SIGNAL</literal></entry>
934 The interface to invoke a method call on, or
935 that a signal is emitted from. Optional for
936 method calls, required for signals.
940 <entry><literal>MEMBER</literal></entry>
942 <entry><literal>STRING</literal></entry>
943 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
944 <entry>The member, either the method name or signal name.</entry>
947 <entry><literal>ERROR_NAME</literal></entry>
949 <entry><literal>STRING</literal></entry>
950 <entry><literal>ERROR</literal></entry>
951 <entry>The name of the error that occurred, for errors</entry>
954 <entry><literal>REPLY_SERIAL</literal></entry>
956 <entry><literal>UINT32</literal></entry>
957 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
958 <entry>The serial number of the message this message is a reply
959 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
962 <entry><literal>DESTINATION</literal></entry>
964 <entry><literal>STRING</literal></entry>
965 <entry>optional</entry>
966 <entry>The name of the connection this message is intended for.
967 Only used in combination with the message bus, see
968 <xref linkend="message-bus"/>.</entry>
971 <entry><literal>SENDER</literal></entry>
973 <entry><literal>STRING</literal></entry>
974 <entry>optional</entry>
975 <entry>Unique name of the sending connection.
976 The message bus fills in this field so it is reliable; the field is
977 only meaningful in combination with the message bus.</entry>
980 <entry><literal>SIGNATURE</literal></entry>
982 <entry><literal>SIGNATURE</literal></entry>
983 <entry>optional</entry>
984 <entry>The signature of the message body.
985 If omitted, it is assumed to be the
986 empty signature "" (i.e. the body must be 0-length).</entry>
995 <sect2 id="message-protocol-names">
996 <title>Valid Names</title>
998 The various names in D-Bus messages have some restrictions.
1001 There is a <firstterm>maximum name length</firstterm>
1002 of 255 which applies to bus names, interfaces, and members.
1004 <sect3 id="message-protocol-names-interface">
1005 <title>Interface names</title>
1007 Interfaces have names with type <literal>STRING</literal>, meaning that
1008 they must be valid UTF-8. However, there are also some
1009 additional restrictions that apply to interface names
1012 <listitem><para>Interface names are composed of 1 or more elements separated by
1013 a period ('.') character. All elements must contain at least
1017 <listitem><para>Each element must only contain the ASCII characters
1018 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1022 <listitem><para>Interface names must contain at least one '.' (period)
1023 character (and thus at least two elements).
1026 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1027 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1031 <sect3 id="message-protocol-names-bus">
1032 <title>Bus names</title>
1034 Connections have one or more bus names associated with them.
1035 A connection has exactly one bus name that is a unique connection
1036 name. The unique connection name remains with the connection for
1037 its entire lifetime.
1038 A bus name is of type <literal>STRING</literal>,
1039 meaning that it must be valid UTF-8. However, there are also
1040 some additional restrictions that apply to bus names
1043 <listitem><para>Bus names that start with a colon (':')
1044 character are unique connection names.
1047 <listitem><para>Bus names are composed of 1 or more elements separated by
1048 a period ('.') character. All elements must contain at least
1052 <listitem><para>Each element must only contain the ASCII characters
1053 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1054 connection name may begin with a digit, elements in
1055 other bus names must not begin with a digit.
1059 <listitem><para>Bus names must contain at least one '.' (period)
1060 character (and thus at least two elements).
1063 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1064 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1068 Note that the hyphen ('-') character is allowed in bus names but
1069 not in interface names.
1072 <sect3 id="message-protocol-names-member">
1073 <title>Member names</title>
1075 Member (i.e. method or signal) names:
1077 <listitem><para>Must only contain the ASCII characters
1078 "[A-Z][a-z][0-9]_" and may not begin with a
1079 digit.</para></listitem>
1080 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1081 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1082 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1086 <sect3 id="message-protocol-names-error">
1087 <title>Error names</title>
1089 Error names have the same restrictions as interface names.
1094 <sect2 id="message-protocol-types">
1095 <title>Message Types</title>
1097 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1098 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1099 This section describes these conventions.
1101 <sect3 id="message-protocol-types-method">
1102 <title>Method Calls</title>
1104 Some messages invoke an operation on a remote object. These are
1105 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1106 messages map naturally to methods on objects in a typical program.
1109 A method call message is required to have a <literal>MEMBER</literal> header field
1110 indicating the name of the method. Optionally, the message has an
1111 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1112 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1113 a method with the same name, it is undefined which of the two methods
1114 will be invoked. Implementations may also choose to return an error in
1115 this ambiguous case. However, if a method name is unique
1116 implementations must not require an interface field.
1119 Method call messages also include a <literal>PATH</literal> field
1120 indicating the object to invoke the method on. If the call is passing
1121 through a message bus, the message will also have a
1122 <literal>DESTINATION</literal> field giving the name of the connection
1123 to receive the message.
1126 When an application handles a method call message, it is required to
1127 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1128 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1129 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1132 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1133 are the return value(s) or "out parameters" of the method call.
1134 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1135 and the call fails; no return value will be provided. It makes
1136 no sense to send multiple replies to the same method call.
1139 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1140 reply is required, so the caller will know the method
1141 was successfully processed.
1144 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1148 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1149 then as an optimization the application receiving the method
1150 call may choose to omit the reply message (regardless of
1151 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1152 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1153 flag and reply anyway.
1156 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1157 destination name does not exist then a program to own the destination
1158 name will be started before the message is delivered. The message
1159 will be held until the new program is successfully started or has
1160 failed to start; in case of failure, an error will be returned. This
1161 flag is only relevant in the context of a message bus, it is ignored
1162 during one-to-one communication with no intermediate bus.
1164 <sect4 id="message-protocol-types-method-apis">
1165 <title>Mapping method calls to native APIs</title>
1167 APIs for D-Bus may map method calls to a method call in a specific
1168 programming language, such as C++, or may map a method call written
1169 in an IDL to a D-Bus message.
1172 In APIs of this nature, arguments to a method are often termed "in"
1173 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1174 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1175 "inout" arguments, which are both sent and received, i.e. the caller
1176 passes in a value which is modified. Mapped to D-Bus, an "inout"
1177 argument is equivalent to an "in" argument, followed by an "out"
1178 argument. You can't pass things "by reference" over the wire, so
1179 "inout" is purely an illusion of the in-process API.
1182 Given a method with zero or one return values, followed by zero or more
1183 arguments, where each argument may be "in", "out", or "inout", the
1184 caller constructs a message by appending each "in" or "inout" argument,
1185 in order. "out" arguments are not represented in the caller's message.
1188 The recipient constructs a reply by appending first the return value
1189 if any, then each "out" or "inout" argument, in order.
1190 "in" arguments are not represented in the reply message.
1193 Error replies are normally mapped to exceptions in languages that have
1197 In converting from native APIs to D-Bus, it is perhaps nice to
1198 map D-Bus naming conventions ("FooBar") to native conventions
1199 such as "fooBar" or "foo_bar" automatically. This is OK
1200 as long as you can say that the native API is one that
1201 was specifically written for D-Bus. It makes the most sense
1202 when writing object implementations that will be exported
1203 over the bus. Object proxies used to invoke remote D-Bus
1204 objects probably need the ability to call any D-Bus method,
1205 and thus a magic name mapping like this could be a problem.
1208 This specification doesn't require anything of native API bindings;
1209 the preceding is only a suggested convention for consistency
1215 <sect3 id="message-protocol-types-signal">
1216 <title>Signal Emission</title>
1218 Unlike method calls, signal emissions have no replies.
1219 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1220 It must have three header fields: <literal>PATH</literal> giving the object
1221 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1222 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1223 for signals, though it is optional for method calls.
1227 <sect3 id="message-protocol-types-errors">
1228 <title>Errors</title>
1230 Messages of type <literal>ERROR</literal> are most commonly replies
1231 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1232 to any kind of message. The message bus for example
1233 will return an <literal>ERROR</literal> in reply to a signal emission if
1234 the bus does not have enough memory to send the signal.
1237 An <literal>ERROR</literal> may have any arguments, but if the first
1238 argument is a <literal>STRING</literal>, it must be an error message.
1239 The error message may be logged or shown to the user
1244 <sect3 id="message-protocol-types-notation">
1245 <title>Notation in this document</title>
1247 This document uses a simple pseudo-IDL to describe particular method
1248 calls and signals. Here is an example of a method call:
1250 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1251 out UINT32 resultcode)
1253 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1254 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1255 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1256 characters so it's known that the last part of the name in
1257 the "IDL" is the member name.
1260 In C++ that might end up looking like this:
1262 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1263 unsigned int flags);
1265 or equally valid, the return value could be done as an argument:
1267 void org::freedesktop::DBus::StartServiceByName (const char *name,
1269 unsigned int *resultcode);
1271 It's really up to the API designer how they want to make
1272 this look. You could design an API where the namespace wasn't used
1273 in C++, using STL or Qt, using varargs, or whatever you wanted.
1276 Signals are written as follows:
1278 org.freedesktop.DBus.NameLost (STRING name)
1280 Signals don't specify "in" vs. "out" because only
1281 a single direction is possible.
1284 It isn't especially encouraged to use this lame pseudo-IDL in actual
1285 API implementations; you might use the native notation for the
1286 language you're using, or you might use COM or CORBA IDL, for example.
1291 <sect2 id="message-protocol-handling-invalid">
1292 <title>Invalid Protocol and Spec Extensions</title>
1295 For security reasons, the D-Bus protocol should be strictly parsed and
1296 validated, with the exception of defined extension points. Any invalid
1297 protocol or spec violations should result in immediately dropping the
1298 connection without notice to the other end. Exceptions should be
1299 carefully considered, e.g. an exception may be warranted for a
1300 well-understood idiosyncrasy of a widely-deployed implementation. In
1301 cases where the other end of a connection is 100% trusted and known to
1302 be friendly, skipping validation for performance reasons could also make
1303 sense in certain cases.
1307 Generally speaking violations of the "must" requirements in this spec
1308 should be considered possible attempts to exploit security, and violations
1309 of the "should" suggestions should be considered legitimate (though perhaps
1310 they should generate an error in some cases).
1314 The following extension points are built in to D-Bus on purpose and must
1315 not be treated as invalid protocol. The extension points are intended
1316 for use by future versions of this spec, they are not intended for third
1317 parties. At the moment, the only way a third party could extend D-Bus
1318 without breaking interoperability would be to introduce a way to negotiate new
1319 feature support as part of the auth protocol, using EXTENSION_-prefixed
1320 commands. There is not yet a standard way to negotiate features.
1324 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1325 commands result in an ERROR rather than a disconnect. This enables
1326 future extensions to the protocol. Commands starting with EXTENSION_ are
1327 reserved for third parties.
1332 The authentication protocol supports pluggable auth mechanisms.
1337 The address format (see <xref linkend="addresses"/>) supports new
1343 Messages with an unknown type (something other than
1344 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1345 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1346 Unknown-type messages must still be well-formed in the same way
1347 as the known messages, however. They still have the normal
1353 Header fields with an unknown or unexpected field code must be ignored,
1354 though again they must still be well-formed.
1359 New standard interfaces (with new methods and signals) can of course be added.
1369 <sect1 id="auth-protocol">
1370 <title>Authentication Protocol</title>
1372 Before the flow of messages begins, two applications must
1373 authenticate. A simple plain-text protocol is used for
1374 authentication; this protocol is a SASL profile, and maps fairly
1375 directly from the SASL specification. The message encoding is
1376 NOT used here, only plain text messages.
1379 In examples, "C:" and "S:" indicate lines sent by the client and
1380 server respectively.
1382 <sect2 id="auth-protocol-overview">
1383 <title>Protocol Overview</title>
1385 The protocol is a line-based protocol, where each line ends with
1386 \r\n. Each line begins with an all-caps ASCII command name containing
1387 only the character range [A-Z_], a space, then any arguments for the
1388 command, then the \r\n ending the line. The protocol is
1389 case-sensitive. All bytes must be in the ASCII character set.
1391 Commands from the client to the server are as follows:
1394 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1395 <listitem><para>CANCEL</para></listitem>
1396 <listitem><para>BEGIN</para></listitem>
1397 <listitem><para>DATA <data in hex encoding></para></listitem>
1398 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1401 From server to client are as follows:
1404 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1405 <listitem><para>OK <GUID in hex></para></listitem>
1406 <listitem><para>DATA <data in hex encoding></para></listitem>
1407 <listitem><para>ERROR</para></listitem>
1411 Unofficial extensions to the command set must begin with the letters
1412 "EXTENSION_", to avoid conflicts with future official commands.
1413 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1416 <sect2 id="auth-nul-byte">
1417 <title>Special credentials-passing nul byte</title>
1419 Immediately after connecting to the server, the client must send a
1420 single nul byte. This byte may be accompanied by credentials
1421 information on some operating systems that use sendmsg() with
1422 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1423 sockets. However, the nul byte must be sent even on other kinds of
1424 socket, and even on operating systems that do not require a byte to be
1425 sent in order to transmit credentials. The text protocol described in
1426 this document begins after the single nul byte. If the first byte
1427 received from the client is not a nul byte, the server may disconnect
1431 A nul byte in any context other than the initial byte is an error;
1432 the protocol is ASCII-only.
1435 The credentials sent along with the nul byte may be used with the
1436 SASL mechanism EXTERNAL.
1439 <sect2 id="auth-command-auth">
1440 <title>AUTH command</title>
1442 If an AUTH command has no arguments, it is a request to list
1443 available mechanisms. The server must respond with a REJECTED
1444 command listing the mechanisms it understands, or with an error.
1447 If an AUTH command specifies a mechanism, and the server supports
1448 said mechanism, the server should begin exchanging SASL
1449 challenge-response data with the client using DATA commands.
1452 If the server does not support the mechanism given in the AUTH
1453 command, it must send either a REJECTED command listing the mechanisms
1454 it does support, or an error.
1457 If the [initial-response] argument is provided, it is intended for use
1458 with mechanisms that have no initial challenge (or an empty initial
1459 challenge), as if it were the argument to an initial DATA command. If
1460 the selected mechanism has an initial challenge and [initial-response]
1461 was provided, the server should reject authentication by sending
1465 If authentication succeeds after exchanging DATA commands,
1466 an OK command must be sent to the client.
1469 The first octet received by the client after the \r\n of the OK
1470 command must be the first octet of the authenticated/encrypted
1471 stream of D-Bus messages.
1474 The first octet received by the server after the \r\n of the BEGIN
1475 command from the client must be the first octet of the
1476 authenticated/encrypted stream of D-Bus messages.
1479 <sect2 id="auth-command-cancel">
1480 <title>CANCEL Command</title>
1482 At any time up to sending the BEGIN command, the client may send a
1483 CANCEL command. On receiving the CANCEL command, the server must
1484 send a REJECTED command and abort the current authentication
1488 <sect2 id="auth-command-data">
1489 <title>DATA Command</title>
1491 The DATA command may come from either client or server, and simply
1492 contains a hex-encoded block of data to be interpreted
1493 according to the SASL mechanism in use.
1496 Some SASL mechanisms support sending an "empty string";
1497 FIXME we need some way to do this.
1500 <sect2 id="auth-command-begin">
1501 <title>BEGIN Command</title>
1503 The BEGIN command acknowledges that the client has received an
1504 OK command from the server, and that the stream of messages
1508 The first octet received by the server after the \r\n of the BEGIN
1509 command from the client must be the first octet of the
1510 authenticated/encrypted stream of D-Bus messages.
1513 <sect2 id="auth-command-rejected">
1514 <title>REJECTED Command</title>
1516 The REJECTED command indicates that the current authentication
1517 exchange has failed, and further exchange of DATA is inappropriate.
1518 The client would normally try another mechanism, or try providing
1519 different responses to challenges.
1521 Optionally, the REJECTED command has a space-separated list of
1522 available auth mechanisms as arguments. If a server ever provides
1523 a list of supported mechanisms, it must provide the same list
1524 each time it sends a REJECTED message. Clients are free to
1525 ignore all lists received after the first.
1528 <sect2 id="auth-command-ok">
1529 <title>OK Command</title>
1531 The OK command indicates that the client has been authenticated,
1532 and that further communication will be a stream of D-Bus messages
1533 (optionally encrypted, as negotiated) rather than this protocol.
1536 The first octet received by the client after the \r\n of the OK
1537 command must be the first octet of the authenticated/encrypted
1538 stream of D-Bus messages.
1541 The client must respond to the OK command by sending a BEGIN
1542 command, followed by its stream of messages, or by disconnecting.
1543 The server must not accept additional commands using this protocol
1544 after the OK command has been sent.
1547 The OK command has one argument, which is the GUID of the server.
1548 See <xref linkend="addresses"/> for more on server GUIDs.
1551 <sect2 id="auth-command-error">
1552 <title>ERROR Command</title>
1554 The ERROR command indicates that either server or client did not
1555 know a command, does not accept the given command in the current
1556 context, or did not understand the arguments to the command. This
1557 allows the protocol to be extended; a client or server can send a
1558 command present or permitted only in new protocol versions, and if
1559 an ERROR is received instead of an appropriate response, fall back
1560 to using some other technique.
1563 If an ERROR is sent, the server or client that sent the
1564 error must continue as if the command causing the ERROR had never been
1565 received. However, the the server or client receiving the error
1566 should try something other than whatever caused the error;
1567 if only canceling/rejecting the authentication.
1570 If the D-Bus protocol changes incompatibly at some future time,
1571 applications implementing the new protocol would probably be able to
1572 check for support of the new protocol by sending a new command and
1573 receiving an ERROR from applications that don't understand it. Thus the
1574 ERROR feature of the auth protocol is an escape hatch that lets us
1575 negotiate extensions or changes to the D-Bus protocol in the future.
1578 <sect2 id="auth-examples">
1579 <title>Authentication examples</title>
1583 <title>Example of successful magic cookie authentication</title>
1585 (MAGIC_COOKIE is a made up mechanism)
1587 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1593 <title>Example of finding out mechanisms then picking one</title>
1596 S: REJECTED KERBEROS_V4 SKEY
1597 C: AUTH SKEY 7ab83f32ee
1598 S: DATA 8799cabb2ea93e
1599 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1605 <title>Example of client sends unknown command then falls back to regular auth</title>
1609 C: AUTH MAGIC_COOKIE 3736343435313230333039
1615 <title>Example of server doesn't support initial auth mechanism</title>
1617 C: AUTH MAGIC_COOKIE 3736343435313230333039
1618 S: REJECTED KERBEROS_V4 SKEY
1619 C: AUTH SKEY 7ab83f32ee
1620 S: DATA 8799cabb2ea93e
1621 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1627 <title>Example of wrong password or the like followed by successful retry</title>
1629 C: AUTH MAGIC_COOKIE 3736343435313230333039
1630 S: REJECTED KERBEROS_V4 SKEY
1631 C: AUTH SKEY 7ab83f32ee
1632 S: DATA 8799cabb2ea93e
1633 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1635 C: AUTH SKEY 7ab83f32ee
1636 S: DATA 8799cabb2ea93e
1637 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1643 <title>Example of skey cancelled and restarted</title>
1645 C: AUTH MAGIC_COOKIE 3736343435313230333039
1646 S: REJECTED KERBEROS_V4 SKEY
1647 C: AUTH SKEY 7ab83f32ee
1648 S: DATA 8799cabb2ea93e
1651 C: AUTH SKEY 7ab83f32ee
1652 S: DATA 8799cabb2ea93e
1653 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1660 <sect2 id="auth-states">
1661 <title>Authentication state diagrams</title>
1664 This section documents the auth protocol in terms of
1665 a state machine for the client and the server. This is
1666 probably the most robust way to implement the protocol.
1669 <sect3 id="auth-states-client">
1670 <title>Client states</title>
1673 To more precisely describe the interaction between the
1674 protocol state machine and the authentication mechanisms the
1675 following notation is used: MECH(CHALL) means that the
1676 server challenge CHALL was fed to the mechanism MECH, which
1682 CONTINUE(RESP) means continue the auth conversation
1683 and send RESP as the response to the server;
1689 OK(RESP) means that after sending RESP to the server
1690 the client side of the auth conversation is finished
1691 and the server should return "OK";
1697 ERROR means that CHALL was invalid and could not be
1703 Both RESP and CHALL may be empty.
1707 The Client starts by getting an initial response from the
1708 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1709 the mechanism did not provide an initial response. If the
1710 mechanism returns CONTINUE, the client starts in state
1711 <emphasis>WaitingForData</emphasis>, if the mechanism
1712 returns OK the client starts in state
1713 <emphasis>WaitingForOK</emphasis>.
1717 The client should keep track of available mechanisms and
1718 which it mechanisms it has already attempted. This list is
1719 used to decide which AUTH command to send. When the list is
1720 exhausted, the client should give up and close the
1725 <title><emphasis>WaitingForData</emphasis></title>
1733 MECH(CHALL) returns CONTINUE(RESP) → send
1735 <emphasis>WaitingForData</emphasis>
1739 MECH(CHALL) returns OK(RESP) → send DATA
1740 RESP, goto <emphasis>WaitingForOK</emphasis>
1744 MECH(CHALL) returns ERROR → send ERROR
1745 [msg], goto <emphasis>WaitingForData</emphasis>
1753 Receive REJECTED [mechs] →
1754 send AUTH [next mech], goto
1755 WaitingForData or <emphasis>WaitingForOK</emphasis>
1760 Receive ERROR → send
1762 <emphasis>WaitingForReject</emphasis>
1767 Receive OK → send
1768 BEGIN, terminate auth
1769 conversation, authenticated
1774 Receive anything else → send
1776 <emphasis>WaitingForData</emphasis>
1784 <title><emphasis>WaitingForOK</emphasis></title>
1789 Receive OK → send BEGIN, terminate auth
1790 conversation, <emphasis>authenticated</emphasis>
1795 Receive REJECT [mechs] → send AUTH [next mech],
1796 goto <emphasis>WaitingForData</emphasis> or
1797 <emphasis>WaitingForOK</emphasis>
1803 Receive DATA → send CANCEL, goto
1804 <emphasis>WaitingForReject</emphasis>
1810 Receive ERROR → send CANCEL, goto
1811 <emphasis>WaitingForReject</emphasis>
1817 Receive anything else → send ERROR, goto
1818 <emphasis>WaitingForOK</emphasis>
1826 <title><emphasis>WaitingForReject</emphasis></title>
1831 Receive REJECT [mechs] → send AUTH [next mech],
1832 goto <emphasis>WaitingForData</emphasis> or
1833 <emphasis>WaitingForOK</emphasis>
1839 Receive anything else → terminate auth
1840 conversation, disconnect
1849 <sect3 id="auth-states-server">
1850 <title>Server states</title>
1853 For the server MECH(RESP) means that the client response
1854 RESP was fed to the the mechanism MECH, which returns one of
1859 CONTINUE(CHALL) means continue the auth conversation and
1860 send CHALL as the challenge to the client;
1866 OK means that the client has been successfully
1873 REJECT means that the client failed to authenticate or
1874 there was an error in RESP.
1879 The server starts out in state
1880 <emphasis>WaitingForAuth</emphasis>. If the client is
1881 rejected too many times the server must disconnect the
1886 <title><emphasis>WaitingForAuth</emphasis></title>
1892 Receive AUTH → send REJECTED [mechs], goto
1893 <emphasis>WaitingForAuth</emphasis>
1899 Receive AUTH MECH RESP
1903 MECH not valid mechanism → send REJECTED
1905 <emphasis>WaitingForAuth</emphasis>
1909 MECH(RESP) returns CONTINUE(CHALL) → send
1911 <emphasis>WaitingForData</emphasis>
1915 MECH(RESP) returns OK → send OK, goto
1916 <emphasis>WaitingForBegin</emphasis>
1920 MECH(RESP) returns REJECT → send REJECTED
1922 <emphasis>WaitingForAuth</emphasis>
1930 Receive BEGIN → terminate
1931 auth conversation, disconnect
1937 Receive ERROR → send REJECTED [mechs], goto
1938 <emphasis>WaitingForAuth</emphasis>
1944 Receive anything else → send
1946 <emphasis>WaitingForAuth</emphasis>
1955 <title><emphasis>WaitingForData</emphasis></title>
1963 MECH(RESP) returns CONTINUE(CHALL) → send
1965 <emphasis>WaitingForData</emphasis>
1969 MECH(RESP) returns OK → send OK, goto
1970 <emphasis>WaitingForBegin</emphasis>
1974 MECH(RESP) returns REJECT → send REJECTED
1976 <emphasis>WaitingForAuth</emphasis>
1984 Receive BEGIN → terminate auth conversation,
1991 Receive CANCEL → send REJECTED [mechs], goto
1992 <emphasis>WaitingForAuth</emphasis>
1998 Receive ERROR → send REJECTED [mechs], goto
1999 <emphasis>WaitingForAuth</emphasis>
2005 Receive anything else → send ERROR, goto
2006 <emphasis>WaitingForData</emphasis>
2014 <title><emphasis>WaitingForBegin</emphasis></title>
2019 Receive BEGIN → terminate auth conversation,
2020 client authenticated
2026 Receive CANCEL → send REJECTED [mechs], goto
2027 <emphasis>WaitingForAuth</emphasis>
2033 Receive ERROR → send REJECTED [mechs], goto
2034 <emphasis>WaitingForAuth</emphasis>
2040 Receive anything else → send ERROR, goto
2041 <emphasis>WaitingForBegin</emphasis>
2051 <sect2 id="auth-mechanisms">
2052 <title>Authentication mechanisms</title>
2054 This section describes some new authentication mechanisms.
2055 D-Bus also allows any standard SASL mechanism of course.
2057 <sect3 id="auth-mechanisms-sha">
2058 <title>DBUS_COOKIE_SHA1</title>
2060 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2061 has the ability to read a private file owned by the user being
2062 authenticated. If the client can prove that it has access to a secret
2063 cookie stored in this file, then the client is authenticated.
2064 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2068 Authentication proceeds as follows:
2072 The client sends the username it would like to authenticate
2078 The server sends the name of its "cookie context" (see below); a
2079 space character; the integer ID of the secret cookie the client
2080 must demonstrate knowledge of; a space character; then a
2081 hex-encoded randomly-generated challenge string.
2086 The client locates the cookie, and generates its own hex-encoded
2087 randomly-generated challenge string. The client then
2088 concatenates the server's hex-encoded challenge, a ":"
2089 character, its own hex-encoded challenge, another ":" character,
2090 and the hex-encoded cookie. It computes the SHA-1 hash of this
2091 composite string. It sends back to the server the client's
2092 hex-encoded challenge string, a space character, and the SHA-1
2098 The server generates the same concatenated string used by the
2099 client and computes its SHA-1 hash. It compares the hash with
2100 the hash received from the client; if the two hashes match, the
2101 client is authenticated.
2107 Each server has a "cookie context," which is a name that identifies a
2108 set of cookies that apply to that server. A sample context might be
2109 "org_freedesktop_session_bus". Context names must be valid ASCII,
2110 nonzero length, and may not contain the characters slash ("/"),
2111 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2112 tab ("\t"), or period ("."). There is a default context,
2113 "org_freedesktop_general" that's used by servers that do not specify
2117 Cookies are stored in a user's home directory, in the directory
2118 <filename>~/.dbus-keyrings/</filename>. This directory must
2119 not be readable or writable by other users. If it is,
2120 clients and servers must ignore it. The directory
2121 contains cookie files named after the cookie context.
2124 A cookie file contains one cookie per line. Each line
2125 has three space-separated fields:
2129 The cookie ID number, which must be a non-negative integer and
2130 may not be used twice in the same file.
2135 The cookie's creation time, in UNIX seconds-since-the-epoch
2141 The cookie itself, a hex-encoded random block of bytes. The cookie
2142 may be of any length, though obviously security increases
2143 as the length increases.
2149 Only server processes modify the cookie file.
2150 They must do so with this procedure:
2154 Create a lockfile name by appending ".lock" to the name of the
2155 cookie file. The server should attempt to create this file
2156 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2157 fails, the lock fails. Servers should retry for a reasonable
2158 period of time, then they may choose to delete an existing lock
2159 to keep users from having to manually delete a stale
2160 lock. <footnote><para>Lockfiles are used instead of real file
2161 locking <literal>fcntl()</literal> because real locking
2162 implementations are still flaky on network
2163 filesystems.</para></footnote>
2168 Once the lockfile has been created, the server loads the cookie
2169 file. It should then delete any cookies that are old (the
2170 timeout can be fairly short), or more than a reasonable
2171 time in the future (so that cookies never accidentally
2172 become permanent, if the clock was set far into the future
2173 at some point). If no recent keys remain, the
2174 server may generate a new key.
2179 The pruned and possibly added-to cookie file
2180 must be resaved atomically (using a temporary
2181 file which is rename()'d).
2186 The lock must be dropped by deleting the lockfile.
2192 Clients need not lock the file in order to load it,
2193 because servers are required to save the file atomically.
2198 <sect1 id="addresses">
2199 <title>Server Addresses</title>
2201 Server addresses consist of a transport name followed by a colon, and
2202 then an optional, comma-separated list of keys and values in the form key=value.
2203 Each value is escaped.
2207 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2208 Which is the address to a unix socket with the path /tmp/dbus-test.
2211 Value escaping is similar to URI escaping but simpler.
2215 The set of optionally-escaped bytes is:
2216 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2217 <emphasis>byte</emphasis> (note, not character) which is not in the
2218 set of optionally-escaped bytes must be replaced with an ASCII
2219 percent (<literal>%</literal>) and the value of the byte in hex.
2220 The hex value must always be two digits, even if the first digit is
2221 zero. The optionally-escaped bytes may be escaped if desired.
2226 To unescape, append each byte in the value; if a byte is an ASCII
2227 percent (<literal>%</literal>) character then append the following
2228 hex value instead. It is an error if a <literal>%</literal> byte
2229 does not have two hex digits following. It is an error if a
2230 non-optionally-escaped byte is seen unescaped.
2234 The set of optionally-escaped bytes is intended to preserve address
2235 readability and convenience.
2239 A server may specify a key-value pair with the key <literal>guid</literal>
2240 and the value a hex-encoded 16-byte sequence. This globally unique ID must
2241 be created by filling the first 4 bytes with a 32-bit UNIX time since the
2242 epoch, and the remaining 12 bytes with random bytes. If present, the GUID
2243 may be used to distinguish one server from another. A server should use a
2244 different GUID for each address it listens on. For example, if a message
2245 bus daemon offers both UNIX domain socket and TCP connections, but treats
2246 clients the same regardless of how they connect, those two connections are
2247 equivalent post-connection but should have distinct GUIDs to distinguish
2248 the kinds of connection.
2252 The intent of the GUID feature is to allow a client to avoid opening
2253 multiple identical connections to the same server, by allowing the client
2254 to check whether an address corresponds to an already-existing connection.
2255 Comparing two addresses is insufficient, because addresses can be recycled
2256 by distinct servers.
2260 [FIXME clarify if attempting to connect to each is a requirement
2261 or just a suggestion]
2262 When connecting to a server, multiple server addresses can be
2263 separated by a semi-colon. The library will then try to connect
2264 to the first address and if that fails, it'll try to connect to
2265 the next one specified, and so forth. For example
2266 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2271 <sect1 id="transports">
2272 <title>Transports</title>
2274 [FIXME we need to specify in detail each transport and its possible arguments]
2276 Current transports include: unix domain sockets (including
2277 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2278 in-process pipes. Future possible transports include one that
2279 tunnels over X11 protocol.
2282 <sect2 id="transports-unix-domain-sockets">
2283 <title>Unix Domain Sockets</title>
2285 Unix domain sockets can be either paths in the file system or on Linux
2286 kernels, they can be abstract which are similar to paths but
2287 do not show up in the file system.
2291 When a socket is opened by the D-Bus library it truncates the path
2292 name right before the first trailing Nul byte. This is true for both
2293 normal paths and abstract paths. Note that this is a departure from
2294 previous versions of D-Bus that would create sockets with a fixed
2295 length path name. Names which were shorter than the fixed length
2296 would be padded by Nul bytes.
2301 <sect1 id="naming-conventions">
2302 <title>Naming Conventions</title>
2305 D-Bus namespaces are all lowercase and correspond to reversed domain
2306 names, as with Java. e.g. "org.freedesktop"
2309 Interface, signal, method, and property names are "WindowsStyleCaps", note
2310 that the first letter is capitalized, unlike Java.
2313 Object paths are normally all lowercase with underscores used rather than
2318 <sect1 id="standard-interfaces">
2319 <title>Standard Interfaces</title>
2321 See <xref linkend="message-protocol-types-notation"/> for details on
2322 the notation used in this section. There are some standard interfaces
2323 that may be useful across various D-Bus applications.
2325 <sect2 id="standard-interfaces-peer">
2326 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2328 The <literal>org.freedesktop.DBus.Peer</literal> interface
2331 org.freedesktop.DBus.Peer.Ping ()
2332 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2336 On receipt of the <literal>METHOD_CALL</literal> message
2337 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2338 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2339 usual. It does not matter which object path a ping is sent to. The
2340 reference implementation handles this method automatically.
2343 On receipt of the <literal>METHOD_CALL</literal> message
2344 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2345 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2346 UUID representing the identity of the machine the process is running on.
2347 This UUID must be the same for all processes on a single system at least
2348 until that system next reboots. It should be the same across reboots
2349 if possible, but this is not always possible to implement and is not
2351 It does not matter which object path a GetMachineId is sent to. The
2352 reference implementation handles this method automatically.
2355 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2356 a virtual machine running on a hypervisor, rather than a physical machine.
2357 Basically if two processes see the same UUID, they should also see the same
2358 shared memory, UNIX domain sockets, process IDs, and other features that require
2359 a running OS kernel in common between the processes.
2362 The UUID is often used where other programs might use a hostname. Hostnames
2363 can change without rebooting, however, or just be "localhost" - so the UUID
2367 The UUID must contain 128 bits of data and be hex-encoded (meaning, the hex
2368 string contains 32 ASCII characters). The hex-encoded string may not contain
2369 hyphens or other non-hex-digit characters, and it must be exactly 32 characters long.
2370 To generate a UUID, the recommended algorithm is to put the current time in seconds
2371 since the UNIX epoch in the last 32 bits of the UUID, and to put randomly-generated bits
2372 in the first 96 bits of the UUID.
2376 <sect2 id="standard-interfaces-introspectable">
2377 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2379 This interface has one method:
2381 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2385 Objects instances may implement
2386 <literal>Introspect</literal> which returns an XML description of
2387 the object, including its interfaces (with signals and methods), objects
2388 below it in the object path tree, and its properties.
2391 <xref linkend="introspection-format"/> describes the format of this XML string.
2394 <sect2 id="standard-interfaces-properties">
2395 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2397 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2398 or <firstterm>attributes</firstterm>. These can be exposed via the
2399 <literal>org.freedesktop.DBus.Properties</literal> interface.
2403 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2404 in STRING property_name,
2406 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2407 in STRING property_name,
2412 The available properties and whether they are writable can be determined
2413 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2414 see <xref linkend="standard-interfaces-introspectable"/>.
2417 An empty string may be provided for the interface name; in this case,
2418 if there are multiple properties on an object with the same name,
2419 the results are undefined (picking one by according to an arbitrary
2420 deterministic rule, or returning an error, are the reasonable
2426 <sect1 id="introspection-format">
2427 <title>Introspection Data Format</title>
2429 As described in <xref linkend="standard-interfaces-introspectable"/>,
2430 objects may be introspected at runtime, returning an XML string
2431 that describes the object. The same XML format may be used in
2432 other contexts as well, for example as an "IDL" for generating
2433 static language bindings.
2436 Here is an example of introspection data:
2438 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2439 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2440 <node name="/org/freedesktop/sample_object">
2441 <interface name="org.freedesktop.SampleInterface">
2442 <method name="Frobate">
2443 <arg name="foo" type="i" direction="in"/>
2444 <arg name="bar" type="s" direction="out"/>
2445 <arg name="baz" type="a{us}" direction="out"/>
2446 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2448 <method name="Bazify">
2449 <arg name="bar" type="(iiu)" direction="in"/>
2450 <arg name="bar" type="v" direction="out"/>
2452 <method name="Mogrify">
2453 <arg name="bar" type="(iiav)" direction="in"/>
2455 <signal name="Changed">
2456 <arg name="new_value" type="b"/>
2458 <property name="Bar" type="y" access="readwrite"/>
2460 <node name="child_of_sample_object"/>
2461 <node name="another_child_of_sample_object"/>
2466 A more formal DTD and spec needs writing, but here are some quick notes.
2470 Only the root <node> element can omit the node name, as it's
2471 known to be the object that was introspected. If the root
2472 <node> does have a name attribute, it must be an absolute
2473 object path. If child <node> have object paths, they must be
2479 If a child <node> has any sub-elements, then they
2480 must represent a complete introspection of the child.
2481 If a child <node> is empty, then it may or may
2482 not have sub-elements; the child must be introspected
2483 in order to find out. The intent is that if an object
2484 knows that its children are "fast" to introspect
2485 it can go ahead and return their information, but
2486 otherwise it can omit it.
2491 The direction element on <arg> may be omitted,
2492 in which case it defaults to "in" for method calls
2493 and "out" for signals. Signals only allow "out"
2494 so while direction may be specified, it's pointless.
2499 The possible directions are "in" and "out",
2500 unlike CORBA there is no "inout"
2505 The possible property access flags are
2506 "readwrite", "read", and "write"
2511 Multiple interfaces can of course be listed for
2517 The "name" attribute on arguments is optional.
2523 Method, interface, property, and signal elements may have
2524 "annotations", which are generic key/value pairs of metadata.
2525 They are similar conceptually to Java's annotations and C# attributes.
2526 Well-known annotations:
2533 <entry>Values (separated by ,)</entry>
2534 <entry>Description</entry>
2539 <entry>org.freedesktop.DBus.Deprecated</entry>
2540 <entry>true,false</entry>
2541 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2544 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2545 <entry>(string)</entry>
2546 <entry>The C symbol; may be used for methods and interfaces</entry>
2549 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2550 <entry>true,false</entry>
2551 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2557 <sect1 id="message-bus">
2558 <title>Message Bus Specification</title>
2559 <sect2 id="message-bus-overview">
2560 <title>Message Bus Overview</title>
2562 The message bus accepts connections from one or more applications.
2563 Once connected, applications can exchange messages with other
2564 applications that are also connected to the bus.
2567 In order to route messages among connections, the message bus keeps a
2568 mapping from names to connections. Each connection has one
2569 unique-for-the-lifetime-of-the-bus name automatically assigned.
2570 Applications may request additional names for a connection. Additional
2571 names are usually "well-known names" such as
2572 "org.freedesktop.TextEditor". When a name is bound to a connection,
2573 that connection is said to <firstterm>own</firstterm> the name.
2576 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
2577 This name routes messages to the bus, allowing applications to make
2578 administrative requests. For example, applications can ask the bus
2579 to assign a name to a connection.
2582 Each name may have <firstterm>queued owners</firstterm>. When an
2583 application requests a name for a connection and the name is already in
2584 use, the bus will optionally add the connection to a queue waiting for
2585 the name. If the current owner of the name disconnects or releases
2586 the name, the next connection in the queue will become the new owner.
2590 This feature causes the right thing to happen if you start two text
2591 editors for example; the first one may request "org.freedesktop.TextEditor",
2592 and the second will be queued as a possible owner of that name. When
2593 the first exits, the second will take over.
2597 Messages may have a <literal>DESTINATION</literal> field (see <xref
2598 linkend="message-protocol-header-fields"/>). If the
2599 <literal>DESTINATION</literal> field is present, it specifies a message
2600 recipient by name. Method calls and replies normally specify this field.
2604 Signals normally do not specify a destination; they are sent to all
2605 applications with <firstterm>message matching rules</firstterm> that
2610 When the message bus receives a method call, if the
2611 <literal>DESTINATION</literal> field is absent, the call is taken to be
2612 a standard one-to-one message and interpreted by the message bus
2613 itself. For example, sending an
2614 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
2615 <literal>DESTINATION</literal> will cause the message bus itself to
2616 reply to the ping immediately; the message bus will not make this
2617 message visible to other applications.
2621 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
2622 the ping message were sent with a <literal>DESTINATION</literal> name of
2623 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
2624 forwarded, and the Yoyodyne Corporation screensaver application would be
2625 expected to reply to the ping.
2629 <sect2 id="message-bus-names">
2630 <title>Message Bus Names</title>
2632 Each connection has at least one name, assigned at connection time and
2633 returned in response to the
2634 <literal>org.freedesktop.DBus.Hello</literal> method call. This
2635 automatically-assigned name is called the connection's <firstterm>unique
2636 name</firstterm>. Unique names are never reused for two different
2637 connections to the same bus.
2640 Ownership of a unique name is a prerequisite for interaction with
2641 the message bus. It logically follows that the unique name is always
2642 the first name that an application comes to own, and the last
2643 one that it loses ownership of.
2646 Unique connection names must begin with the character ':' (ASCII colon
2647 character); bus names that are not unique names must not begin
2648 with this character. (The bus must reject any attempt by an application
2649 to manually request a name beginning with ':'.) This restriction
2650 categorically prevents "spoofing"; messages sent to a unique name
2651 will always go to the expected connection.
2654 When a connection is closed, all the names that it owns are deleted (or
2655 transferred to the next connection in the queue if any).
2658 A connection can request additional names to be associated with it using
2659 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
2660 linkend="message-protocol-names-bus"/> describes the format of a valid
2661 name. These names can be released again using the
2662 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
2665 <sect3 id="bus-messages-request-name">
2666 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
2670 UINT32 RequestName (in STRING name, in UINT32 flags)
2677 <entry>Argument</entry>
2679 <entry>Description</entry>
2685 <entry>STRING</entry>
2686 <entry>Name to request</entry>
2690 <entry>UINT32</entry>
2691 <entry>Flags</entry>
2701 <entry>Argument</entry>
2703 <entry>Description</entry>
2709 <entry>UINT32</entry>
2710 <entry>Return value</entry>
2717 This method call should be sent to
2718 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2719 assign the given name to the method caller. Each name maintains a
2720 queue of possible owners, where the head of the queue is the primary
2721 or current owner of the name. Each potential owner in the queue
2722 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
2723 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
2724 call. When RequestName is invoked the following occurs:
2728 If the method caller is currently the primary owner of the name,
2729 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
2730 values are updated with the values from the new RequestName call,
2731 and nothing further happens.
2737 If the current primary owner (head of the queue) has
2738 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
2739 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
2740 the caller of RequestName replaces the current primary owner at
2741 the head of the queue and the current primary owner moves to the
2742 second position in the queue. If the caller of RequestName was
2743 in the queue previously its flags are updated with the values from
2744 the new RequestName in addition to moving it to the head of the queue.
2750 If replacement is not possible, and the method caller is
2751 currently in the queue but not the primary owner, its flags are
2752 updated with the values from the new RequestName call.
2758 If replacement is not possible, and the method caller is
2759 currently not in the queue, the method caller is appended to the
2766 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
2767 set and is not the primary owner, it is removed from the
2768 queue. This can apply to the previous primary owner (if it
2769 was replaced) or the method caller (if it updated the
2770 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
2771 queue, or if it was just added to the queue with that flag set).
2777 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
2778 queue," even if another application already in the queue had specified
2779 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
2780 that does not allow replacement goes away, and the next primary owner
2781 does allow replacement. In this case, queued items that specified
2782 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
2783 automatically replace the new primary owner. In other words,
2784 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
2785 time RequestName is called. This is deliberate to avoid an infinite loop
2786 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2787 and DBUS_NAME_FLAG_REPLACE_EXISTING.
2790 The flags argument contains any of the following values logically ORed
2797 <entry>Conventional Name</entry>
2798 <entry>Value</entry>
2799 <entry>Description</entry>
2804 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
2808 If an application A specifies this flag and succeeds in
2809 becoming the owner of the name, and another application B
2810 later calls RequestName with the
2811 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
2812 will lose ownership and receive a
2813 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
2814 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2815 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
2816 is not specified by application B, then application B will not replace
2817 application A as the owner.
2822 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
2826 Try to replace the current owner if there is one. If this
2827 flag is not set the application will only become the owner of
2828 the name if there is no current owner. If this flag is set,
2829 the application will replace the current owner if
2830 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
2835 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
2839 Without this flag, if an application requests a name that is
2840 already owned, the application will be placed in a queue to
2841 own the name when the current owner gives it up. If this
2842 flag is given, the application will not be placed in the
2843 queue, the request for the name will simply fail. This flag
2844 also affects behavior when an application is replaced as
2845 name owner; by default the application moves back into the
2846 waiting queue, unless this flag was provided when the application
2847 became the name owner.
2855 The return code can be one of the following values:
2861 <entry>Conventional Name</entry>
2862 <entry>Value</entry>
2863 <entry>Description</entry>
2868 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
2869 <entry>1</entry> <entry>The caller is now the primary owner of
2870 the name, replacing any previous owner. Either the name had no
2871 owner before, or the caller specified
2872 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
2873 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
2876 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
2879 <entry>The name already had an owner,
2880 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
2881 the current owner did not specify
2882 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
2883 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
2887 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
2888 <entry>The name already has an owner,
2889 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
2890 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
2891 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
2892 specified by the requesting application.</entry>
2895 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
2897 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
2905 <sect3 id="bus-messages-release-name">
2906 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
2910 UINT32 ReleaseName (in STRING name)
2917 <entry>Argument</entry>
2919 <entry>Description</entry>
2925 <entry>STRING</entry>
2926 <entry>Name to release</entry>
2936 <entry>Argument</entry>
2938 <entry>Description</entry>
2944 <entry>UINT32</entry>
2945 <entry>Return value</entry>
2952 This method call should be sent to
2953 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2954 release the method caller's claim to the given name. If the caller is
2955 the primary owner, a new primary owner will be selected from the
2956 queue if any other owners are waiting. If the caller is waiting in
2957 the queue for the name, the caller will removed from the queue and
2958 will not be made an owner of the name if it later becomes available.
2959 If there are no other owners in the queue for the name, it will be
2960 removed from the bus entirely.
2962 The return code can be one of the following values:
2968 <entry>Conventional Name</entry>
2969 <entry>Value</entry>
2970 <entry>Description</entry>
2975 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
2976 <entry>1</entry> <entry>The caller has released his claim on
2977 the given name. Either the caller was the primary owner of
2978 the name, and the name is now unused or taken by somebody
2979 waiting in the queue for the name, or the caller was waiting
2980 in the queue for the name and has now been removed from the
2984 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
2986 <entry>The given name does not exist on this bus.</entry>
2989 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
2991 <entry>The caller was not the primary owner of this name,
2992 and was also not waiting in the queue to own this name.</entry>
3001 <sect2 id="message-bus-routing">
3002 <title>Message Bus Message Routing</title>
3006 <sect3 id="message-bus-routing-match-rules">
3007 <title>Match Rules</title>
3009 An important part of the message bus routing protocol is match
3010 rules. Match rules describe what messages can be sent to a client
3011 based on the contents of the message. When a message is routed
3012 through the bus it is compared to clients' match rules. If any
3013 of the rules match, the message is dispatched to the client.
3014 If none of the rules match the message never leaves the bus. This
3015 is an effective way to control traffic over the bus and to make sure
3016 only relevant message need to be processed by the client.
3019 Match rules are added using the AddMatch bus method
3020 (see xref linkend="bus-messages-add-match"/>). Rules are
3021 specified as a string of comma separated key/value pairs.
3022 Excluding a key from the rule indicates a wildcard match.
3023 For instance excluding the the member from a match rule but
3024 adding a sender would let all messages from that sender through.
3025 An example of a complete rule would be
3026 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3029 The following table describes the keys that can be used to create
3031 The following table summarizes the D-Bus types.
3037 <entry>Possible Values</entry>
3038 <entry>Description</entry>
3043 <entry><literal>type</literal></entry>
3044 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3045 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3048 <entry><literal>sender</literal></entry>
3049 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3050 and <xref linkend="term-unique-name"/> respectively)
3052 <entry>Match messages sent by a particular sender. An example of a sender match
3053 is sender='org.freedesktop.Hal'</entry>
3056 <entry><literal>interface</literal></entry>
3057 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3058 <entry>Match messages sent over or to a particular interface. An example of an
3059 interface match is interface='org.freedesktop.Hal.Manager'.
3060 If a message omits the interface header, it must not match any rule
3061 that specifies this key.</entry>
3064 <entry><literal>member</literal></entry>
3065 <entry>Any valid method or signal name</entry>
3066 <entry>Matches messages which have the give method or signal name. An example of
3067 a member match is member='NameOwnerChanged'</entry>
3070 <entry><literal>path</literal></entry>
3071 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3072 <entry>Matches messages which are sent from or to the given object. An example of a
3073 path match is path='/org/freedesktop/Hal/Manager'</entry>
3076 <entry><literal>destination</literal></entry>
3077 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3078 <entry>Matches messages which are being sent to the given unique name. An
3079 example of a destination match is destination=':1.0'</entry>
3082 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3083 <entry>Any string</entry>
3084 <entry>Arg matches are special and are used for further restricting the
3085 match based on the arguments in the body of a message. As of this time
3086 only string arguments can be matched. An example of an argument match
3087 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3096 <sect2 id="message-bus-starting-services">
3097 <title>Message Bus Starting Services</title>
3099 The message bus can start applications on behalf of other applications.
3100 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3101 An application that can be started in this way is called a
3102 <firstterm>service</firstterm>.
3105 With D-Bus, starting a service is normally done by name. That is,
3106 applications ask the message bus to start some program that will own a
3107 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3108 This implies a contract documented along with the name
3109 <literal>org.freedesktop.TextEditor</literal> for which objects
3110 the owner of that name will provide, and what interfaces those
3114 To find an executable corresponding to a particular name, the bus daemon
3115 looks for <firstterm>service description files</firstterm>. Service
3116 description files define a mapping from names to executables. Different
3117 kinds of message bus will look for these files in different places, see
3118 <xref linkend="message-bus-types"/>.
3121 [FIXME the file format should be much better specified than "similar to
3122 .desktop entries" esp. since desktop entries are already
3123 badly-specified. ;-)] Service description files have the ".service" file
3124 extension. The message bus will only load service description files
3125 ending with .service; all other files will be ignored. The file format
3126 is similar to that of <ulink
3127 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
3128 entries</ulink>. All service description files must be in UTF-8
3129 encoding. To ensure that there will be no name collisions, service files
3130 must be namespaced using the same mechanism as messages and service
3134 <title>Example service description file</title>
3136 # Sample service description file
3138 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3139 Exec=/usr/libexec/gconfd-2
3144 When an application asks to start a service by name, the bus daemon tries to
3145 find a service that will own that name. It then tries to spawn the
3146 executable associated with it. If this fails, it will report an
3147 error. [FIXME what happens if two .service files offer the same service;
3148 what kind of error is reported, should we have a way for the client to
3152 The executable launched will have the environment variable
3153 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3154 message bus so it can connect and request the appropriate names.
3157 The executable being launched may want to know whether the message bus
3158 starting it is one of the well-known message buses (see <xref
3159 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3160 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3161 of the well-known buses. The currently-defined values for this variable
3162 are <literal>system</literal> for the systemwide message bus,
3163 and <literal>session</literal> for the per-login-session message
3164 bus. The new executable must still connect to the address given
3165 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3166 resulting connection is to the well-known bus.
3169 [FIXME there should be a timeout somewhere, either specified
3170 in the .service file, by the client, or just a global value
3171 and if the client being activated fails to connect within that
3172 timeout, an error should be sent back.]
3175 <sect3 id="message-bus-starting-services-scope">
3176 <title>Message Bus Service Scope</title>
3178 The "scope" of a service is its "per-", such as per-session,
3179 per-machine, per-home-directory, or per-display. The reference
3180 implementation doesn't yet support starting services in a different
3181 scope from the message bus itself. So e.g. if you start a service
3182 on the session bus its scope is per-session.
3185 We could add an optional scope to a bus name. For example, for
3186 per-(display,session pair), we could have a unique ID for each display
3187 generated automatically at login and set on screen 0 by executing a
3188 special "set display ID" binary. The ID would be stored in a
3189 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3190 random bytes. This ID would then be used to scope names.
3191 Starting/locating a service could be done by ID-name pair rather than
3195 Contrast this with a per-display scope. To achieve that, we would
3196 want a single bus spanning all sessions using a given display.
3197 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3198 property on screen 0 of the display, pointing to this bus.
3203 <sect2 id="message-bus-types">
3204 <title>Well-known Message Bus Instances</title>
3206 Two standard message bus instances are defined here, along with how
3207 to locate them and where their service files live.
3209 <sect3 id="message-bus-types-login">
3210 <title>Login session message bus</title>
3212 Each time a user logs in, a <firstterm>login session message
3213 bus</firstterm> may be started. All applications in the user's login
3214 session may interact with one another using this message bus.
3217 The address of the login session message bus is given
3218 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3219 variable. If that variable is not set, applications may
3220 also try to read the address from the X Window System root
3221 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3222 The root window property must have type <literal>STRING</literal>.
3223 The environment variable should have precedence over the
3224 root window property.
3227 [FIXME specify location of .service files, probably using
3228 DESKTOP_DIRS etc. from basedir specification, though login session
3229 bus is not really desktop-specific]
3232 <sect3 id="message-bus-types-system">
3233 <title>System message bus</title>
3235 A computer may have a <firstterm>system message bus</firstterm>,
3236 accessible to all applications on the system. This message bus may be
3237 used to broadcast system events, such as adding new hardware devices,
3238 changes in the printer queue, and so forth.
3241 The address of the system message bus is given
3242 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3243 variable. If that variable is not set, applications should try
3244 to connect to the well-known address
3245 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3248 The D-Bus reference implementation actually honors the
3249 <literal>$(localstatedir)</literal> configure option
3250 for this address, on both client and server side.
3255 [FIXME specify location of system bus .service files]
3260 <sect2 id="message-bus-messages">
3261 <title>Message Bus Messages</title>
3263 The special message bus name <literal>org.freedesktop.DBus</literal>
3264 responds to a number of additional messages.
3267 <sect3 id="bus-messages-hello">
3268 <title><literal>org.freedesktop.DBus.Hello</literal></title>
3279 <entry>Argument</entry>
3281 <entry>Description</entry>
3287 <entry>STRING</entry>
3288 <entry>Unique name assigned to the connection</entry>
3295 Before an application is able to send messages to other applications
3296 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
3297 to the message bus to obtain a unique name. If an application without
3298 a unique name tries to send a message to another application, or a
3299 message to the message bus itself that isn't the
3300 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
3301 disconnected from the bus.
3304 There is no corresponding "disconnect" request; if a client wishes to
3305 disconnect from the bus, it simply closes the socket (or other
3306 communication channel).
3309 <sect3 id="bus-messages-list-names">
3310 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
3314 ARRAY of STRING ListNames ()
3321 <entry>Argument</entry>
3323 <entry>Description</entry>
3329 <entry>ARRAY of STRING</entry>
3330 <entry>Array of strings where each string is a bus name</entry>
3337 Returns a list of all currently-owned names on the bus.
3340 <sect3 id="bus-messages-list-activatable-names">
3341 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
3345 ARRAY of STRING ListActivatableNames ()
3352 <entry>Argument</entry>
3354 <entry>Description</entry>
3360 <entry>ARRAY of STRING</entry>
3361 <entry>Array of strings where each string is a bus name</entry>
3368 Returns a list of all names that can be activated on the bus.
3371 <sect3 id="bus-messages-name-exists">
3372 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
3376 BOOLEAN NameHasOwner (in STRING name)
3383 <entry>Argument</entry>
3385 <entry>Description</entry>
3391 <entry>STRING</entry>
3392 <entry>Name to check</entry>
3402 <entry>Argument</entry>
3404 <entry>Description</entry>
3410 <entry>BOOLEAN</entry>
3411 <entry>Return value, true if the name exists</entry>
3418 Checks if the specified name exists (currently has an owner).
3422 <sect3 id="bus-messages-name-owner-changed">
3423 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
3427 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
3434 <entry>Argument</entry>
3436 <entry>Description</entry>
3442 <entry>STRING</entry>
3443 <entry>Name with a new owner</entry>
3447 <entry>STRING</entry>
3448 <entry>Old owner or empty string if none</entry>
3452 <entry>STRING</entry>
3453 <entry>New owner or empty string if none</entry>
3460 This signal indicates that the owner of a name has changed.
3461 It's also the signal to use to detect the appearance of
3462 new names on the bus.
3465 <sect3 id="bus-messages-name-lost">
3466 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
3470 NameLost (STRING name)
3477 <entry>Argument</entry>
3479 <entry>Description</entry>
3485 <entry>STRING</entry>
3486 <entry>Name which was lost</entry>
3493 This signal is sent to a specific application when it loses
3494 ownership of a name.
3498 <sect3 id="bus-messages-name-acquired">
3499 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
3503 NameAcquired (STRING name)
3510 <entry>Argument</entry>
3512 <entry>Description</entry>
3518 <entry>STRING</entry>
3519 <entry>Name which was acquired</entry>
3526 This signal is sent to a specific application when it gains
3527 ownership of a name.
3531 <sect3 id="bus-messages-start-service-by-name">
3532 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
3536 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
3543 <entry>Argument</entry>
3545 <entry>Description</entry>
3551 <entry>STRING</entry>
3552 <entry>Name of the service to start</entry>
3556 <entry>UINT32</entry>
3557 <entry>Flags (currently not used)</entry>
3567 <entry>Argument</entry>
3569 <entry>Description</entry>
3575 <entry>UINT32</entry>
3576 <entry>Return value</entry>
3581 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
3585 The return value can be one of the following values:
3590 <entry>Identifier</entry>
3591 <entry>Value</entry>
3592 <entry>Description</entry>
3597 <entry>DBUS_START_REPLY_SUCCESS</entry>
3599 <entry>The service was successfully started.</entry>
3602 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
3604 <entry>A connection already owns the given name.</entry>
3613 <sect3 id="bus-messages-get-name-owner">
3614 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
3618 STRING GetNameOwner (in STRING name)
3625 <entry>Argument</entry>
3627 <entry>Description</entry>
3633 <entry>STRING</entry>
3634 <entry>Name to get the owner of</entry>
3644 <entry>Argument</entry>
3646 <entry>Description</entry>
3652 <entry>STRING</entry>
3653 <entry>Return value, a unique connection name</entry>
3658 Returns the unique connection name of the primary owner of the name
3659 given. If the requested name doesn't have an owner, returns a
3660 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
3664 <sect3 id="bus-messages-get-connection-unix-user">
3665 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
3669 UINT32 GetConnectionUnixUser (in STRING connection_name)
3676 <entry>Argument</entry>
3678 <entry>Description</entry>
3684 <entry>STRING</entry>
3685 <entry>Name of the connection to query</entry>
3695 <entry>Argument</entry>
3697 <entry>Description</entry>
3703 <entry>UINT32</entry>
3704 <entry>unix user id</entry>
3709 Returns the unix uid of the process connected to the server. If unable to
3710 determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
3715 <sect3 id="bus-messages-add-match">
3716 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
3720 AddMatch (in STRING rule)
3727 <entry>Argument</entry>
3729 <entry>Description</entry>
3735 <entry>STRING</entry>
3736 <entry>Match rule to add to the connection</entry>
3741 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
3742 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
3746 <sect3 id="bus-messages-remove-match">
3747 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
3751 RemoveMatch (in STRING rule)
3758 <entry>Argument</entry>
3760 <entry>Description</entry>
3766 <entry>STRING</entry>
3767 <entry>Match rule to remove from the connection</entry>
3772 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
3773 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
3782 <appendix id="implementation-notes">
3783 <title>Implementation notes</title>
3784 <sect1 id="implementation-notes-subsection">
3792 <glossary><title>Glossary</title>
3794 This glossary defines some of the terms used in this specification.
3797 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
3800 The message bus maintains an association between names and
3801 connections. (Normally, there's one connection per application.) A
3802 bus name is simply an identifier used to locate connections. For
3803 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
3804 name might be used to send a message to a screensaver from Yoyodyne
3805 Corporation. An application is said to <firstterm>own</firstterm> a
3806 name if the message bus has associated the application's connection
3807 with the name. Names may also have <firstterm>queued
3808 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
3809 The bus assigns a unique name to each connection,
3810 see <xref linkend="term-unique-name"/>. Other names
3811 can be thought of as "well-known names" and are
3812 used to find applications that offer specific functionality.
3817 <glossentry id="term-message"><glossterm>Message</glossterm>
3820 A message is the atomic unit of communication via the D-Bus
3821 protocol. It consists of a <firstterm>header</firstterm> and a
3822 <firstterm>body</firstterm>; the body is made up of
3823 <firstterm>arguments</firstterm>.
3828 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
3831 The message bus is a special application that forwards
3832 or routes messages between a group of applications
3833 connected to the message bus. It also manages
3834 <firstterm>names</firstterm> used for routing
3840 <glossentry id="term-name"><glossterm>Name</glossterm>
3843 See <xref linkend="term-bus-name"/>. "Name" may
3844 also be used to refer to some of the other names
3845 in D-Bus, such as interface names.
3850 <glossentry id="namespace"><glossterm>Namespace</glossterm>
3853 Used to prevent collisions when defining new interfaces or bus
3854 names. The convention used is the same one Java uses for defining
3855 classes: a reversed domain name.
3860 <glossentry id="term-object"><glossterm>Object</glossterm>
3863 Each application contains <firstterm>objects</firstterm>, which have
3864 <firstterm>interfaces</firstterm> and
3865 <firstterm>methods</firstterm>. Objects are referred to by a name,
3866 called a <firstterm>path</firstterm>.
3871 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
3874 An application talking directly to another application, without going
3875 through a message bus. One-to-one connections may be "peer to peer" or
3876 "client to server." The D-Bus protocol has no concept of client
3877 vs. server after a connection has authenticated; the flow of messages
3878 is symmetrical (full duplex).
3883 <glossentry id="term-path"><glossterm>Path</glossterm>
3886 Object references (object names) in D-Bus are organized into a
3887 filesystem-style hierarchy, so each object is named by a path. As in
3888 LDAP, there's no difference between "files" and "directories"; a path
3889 can refer to an object, while still having child objects below it.
3894 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
3897 Each bus name has a primary owner; messages sent to the name go to the
3898 primary owner. However, certain names also maintain a queue of
3899 secondary owners "waiting in the wings." If the primary owner releases
3900 the name, then the first secondary owner in the queue automatically
3901 becomes the new owner of the name.
3906 <glossentry id="term-service"><glossterm>Service</glossterm>
3909 A service is an executable that can be launched by the bus daemon.
3910 Services normally guarantee some particular features, for example they
3911 may guarantee that they will request a specific name such as
3912 "org.freedesktop.Screensaver", have a singleton object
3913 "/org/freedesktop/Application", and that object will implement the
3914 interface "org.freedesktop.ScreensaverControl".
3919 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
3922 ".service files" tell the bus about service applications that can be
3923 launched (see <xref linkend="term-service"/>). Most importantly they
3924 provide a mapping from bus names to services that will request those
3925 names when they start up.
3930 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
3933 The special name automatically assigned to each connection by the
3934 message bus. This name will never change owner, and will be unique
3935 (never reused during the lifetime of the message bus).
3936 It will begin with a ':' character.