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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"
8 <title>D-Bus Specification</title>
9 <releaseinfo>Version 0.15</releaseinfo>
10 <date>3 November 2010</date>
13 <firstname>Havoc</firstname>
14 <surname>Pennington</surname>
16 <orgname>Red Hat, Inc.</orgname>
18 <email>hp@pobox.com</email>
23 <firstname>Anders</firstname>
24 <surname>Carlsson</surname>
26 <orgname>CodeFactory AB</orgname>
28 <email>andersca@codefactory.se</email>
33 <firstname>Alexander</firstname>
34 <surname>Larsson</surname>
36 <orgname>Red Hat, Inc.</orgname>
38 <email>alexl@redhat.com</email>
45 <revnumber>current</revnumber>
46 <date><ulink url='http://cgit.freedesktop.org/dbus/dbus/log/doc/dbus-specification.xml'>commit log</ulink></date>
47 <authorinitials></authorinitials>
48 <revremark></revremark>
51 <revnumber>0.15</revnumber>
52 <date>3 November 2010</date>
53 <authorinitials></authorinitials>
54 <revremark></revremark>
57 <revnumber>0.14</revnumber>
58 <date>12 May 2010</date>
59 <authorinitials></authorinitials>
60 <revremark></revremark>
63 <revnumber>0.13</revnumber>
64 <date>23 Dezember 2009</date>
65 <authorinitials></authorinitials>
66 <revremark></revremark>
69 <revnumber>0.12</revnumber>
70 <date>7 November, 2006</date>
71 <authorinitials></authorinitials>
72 <revremark></revremark>
75 <revnumber>0.11</revnumber>
76 <date>6 February 2005</date>
77 <authorinitials></authorinitials>
78 <revremark></revremark>
81 <revnumber>0.10</revnumber>
82 <date>28 January 2005</date>
83 <authorinitials></authorinitials>
84 <revremark></revremark>
87 <revnumber>0.9</revnumber>
88 <date>7 Januar 2005</date>
89 <authorinitials></authorinitials>
90 <revremark></revremark>
93 <revnumber>0.8</revnumber>
94 <date>06 September 2003</date>
95 <authorinitials></authorinitials>
96 <revremark>First released document.</revremark>
101 <sect1 id="introduction">
102 <title>Introduction</title>
104 D-Bus is a system for low-latency, low-overhead, easy to use
105 interprocess communication (IPC). In more detail:
109 D-Bus is <emphasis>low-latency</emphasis> because it is designed
110 to avoid round trips and allow asynchronous operation, much like
116 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
117 binary protocol, and does not have to convert to and from a text
118 format such as XML. Because D-Bus is intended for potentially
119 high-resolution same-machine IPC, not primarily for Internet IPC,
120 this is an interesting optimization.
125 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
126 of <firstterm>messages</firstterm> rather than byte streams, and
127 automatically handles a lot of the hard IPC issues. Also, the D-Bus
128 library is designed to be wrapped in a way that lets developers use
129 their framework's existing object/type system, rather than learning
130 a new one specifically for IPC.
137 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
138 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
139 a system for one application to talk to a single other
140 application. However, the primary intended application of the protocol is the
141 D-Bus <firstterm>message bus</firstterm>, specified in <xref
142 linkend="message-bus"/>. The message bus is a special application that
143 accepts connections from multiple other applications, and forwards
148 Uses of D-Bus include notification of system changes (notification of when
149 a camera is plugged in to a computer, or a new version of some software
150 has been installed), or desktop interoperability, for example a file
151 monitoring service or a configuration service.
155 D-Bus is designed for two specific use cases:
159 A "system bus" for notifications from the system to user sessions,
160 and to allow the system to request input from user sessions.
165 A "session bus" used to implement desktop environments such as
170 D-Bus is not intended to be a generic IPC system for any possible
171 application, and intentionally omits many features found in other
172 IPC systems for this reason.
176 At the same time, the bus daemons offer a number of features not found in
177 other IPC systems, such as single-owner "bus names" (similar to X
178 selections), on-demand startup of services, and security policies.
179 In many ways, these features are the primary motivation for developing
180 D-Bus; other systems would have sufficed if IPC were the only goal.
184 D-Bus may turn out to be useful in unanticipated applications, but future
185 versions of this spec and the reference implementation probably will not
186 incorporate features that interfere with the core use cases.
190 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
191 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
192 document are to be interpreted as described in RFC 2119. However, the
193 document could use a serious audit to be sure it makes sense to do
194 so. Also, they are not capitalized.
197 <sect2 id="stability">
198 <title>Protocol and Specification Stability</title>
200 The D-Bus protocol is frozen (only compatible extensions are allowed) as
201 of November 8, 2006. However, this specification could still use a fair
202 bit of work to make interoperable reimplementation possible without
203 reference to the D-Bus reference implementation. Thus, this
204 specification is not marked 1.0. To mark it 1.0, we'd like to see
205 someone invest significant effort in clarifying the specification
206 language, and growing the specification to cover more aspects of the
207 reference implementation's behavior.
210 Until this work is complete, any attempt to reimplement D-Bus will
211 probably require looking at the reference implementation and/or asking
212 questions on the D-Bus mailing list about intended behavior.
213 Questions on the list are very welcome.
216 Nonetheless, this document should be a useful starting point and is
217 to our knowledge accurate, though incomplete.
223 <sect1 id="message-protocol">
224 <title>Message Protocol</title>
227 A <firstterm>message</firstterm> consists of a
228 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
229 think of a message as a package, the header is the address, and the body
230 contains the package contents. The message delivery system uses the header
231 information to figure out where to send the message and how to interpret
232 it; the recipient interprets the body of the message.
236 The body of the message is made up of zero or more
237 <firstterm>arguments</firstterm>, which are typed values, such as an
238 integer or a byte array.
242 Both header and body use the same type system and format for
243 serializing data. Each type of value has a wire format.
244 Converting a value from some other representation into the wire
245 format is called <firstterm>marshaling</firstterm> and converting
246 it back from the wire format is <firstterm>unmarshaling</firstterm>.
249 <sect2 id="message-protocol-signatures">
250 <title>Type Signatures</title>
253 The D-Bus protocol does not include type tags in the marshaled data; a
254 block of marshaled values must have a known <firstterm>type
255 signature</firstterm>. The type signature is made up of <firstterm>type
256 codes</firstterm>. A type code is an ASCII character representing the
257 type of a value. Because ASCII characters are used, the type signature
258 will always form a valid ASCII string. A simple string compare
259 determines whether two type signatures are equivalent.
263 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
264 the ASCII character 'i'. So the signature for a block of values
265 containing a single <literal>INT32</literal> would be:
269 A block of values containing two <literal>INT32</literal> would have this signature:
276 All <firstterm>basic</firstterm> types work like
277 <literal>INT32</literal> in this example. To marshal and unmarshal
278 basic types, you simply read one value from the data
279 block corresponding to each type code in the signature.
280 In addition to basic types, there are four <firstterm>container</firstterm>
281 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
282 and <literal>DICT_ENTRY</literal>.
286 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
287 code does not appear in signatures. Instead, ASCII characters
288 '(' and ')' are used to mark the beginning and end of the struct.
289 So for example, a struct containing two integers would have this
294 Structs can be nested, so for example a struct containing
295 an integer and another struct:
299 The value block storing that struct would contain three integers; the
300 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
305 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
306 but is useful in code that implements the protocol. This type code
307 is specified to allow such code to interoperate in non-protocol contexts.
311 Empty structures are not allowed; there must be at least one
312 type code between the parentheses.
316 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
317 followed by a <firstterm>single complete type</firstterm>. The single
318 complete type following the array is the type of each array element. So
319 the simple example is:
323 which is an array of 32-bit integers. But an array can be of any type,
324 such as this array-of-struct-with-two-int32-fields:
328 Or this array of array of integer:
335 The phrase <firstterm>single complete type</firstterm> deserves some
336 definition. A single complete type is a basic type code, a variant type code,
337 an array with its element type, or a struct with its fields.
338 So the following signatures are not single complete types:
348 And the following signatures contain multiple complete types:
358 Note however that a single complete type may <emphasis>contain</emphasis>
359 multiple other single complete types.
363 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
364 type <literal>VARIANT</literal> will have the signature of a single complete type as part
365 of the <emphasis>value</emphasis>. This signature will be followed by a
366 marshaled value of that type.
370 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
371 than parentheses it uses curly braces, and it has more restrictions.
372 The restrictions are: it occurs only as an array element type; it has
373 exactly two single complete types inside the curly braces; the first
374 single complete type (the "key") must be a basic type rather than a
375 container type. Implementations must not accept dict entries outside of
376 arrays, must not accept dict entries with zero, one, or more than two
377 fields, and must not accept dict entries with non-basic-typed keys. A
378 dict entry is always a key-value pair.
382 The first field in the <literal>DICT_ENTRY</literal> is always the key.
383 A message is considered corrupt if the same key occurs twice in the same
384 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
385 implementations are not required to reject dicts with duplicate keys.
389 In most languages, an array of dict entry would be represented as a
390 map, hash table, or dict object.
394 The following table summarizes the D-Bus types.
399 <entry>Conventional Name</entry>
401 <entry>Description</entry>
406 <entry><literal>INVALID</literal></entry>
407 <entry>0 (ASCII NUL)</entry>
408 <entry>Not a valid type code, used to terminate signatures</entry>
410 <entry><literal>BYTE</literal></entry>
411 <entry>121 (ASCII 'y')</entry>
412 <entry>8-bit unsigned integer</entry>
414 <entry><literal>BOOLEAN</literal></entry>
415 <entry>98 (ASCII 'b')</entry>
416 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
418 <entry><literal>INT16</literal></entry>
419 <entry>110 (ASCII 'n')</entry>
420 <entry>16-bit signed integer</entry>
422 <entry><literal>UINT16</literal></entry>
423 <entry>113 (ASCII 'q')</entry>
424 <entry>16-bit unsigned integer</entry>
426 <entry><literal>INT32</literal></entry>
427 <entry>105 (ASCII 'i')</entry>
428 <entry>32-bit signed integer</entry>
430 <entry><literal>UINT32</literal></entry>
431 <entry>117 (ASCII 'u')</entry>
432 <entry>32-bit unsigned integer</entry>
434 <entry><literal>INT64</literal></entry>
435 <entry>120 (ASCII 'x')</entry>
436 <entry>64-bit signed integer</entry>
438 <entry><literal>UINT64</literal></entry>
439 <entry>116 (ASCII 't')</entry>
440 <entry>64-bit unsigned integer</entry>
442 <entry><literal>DOUBLE</literal></entry>
443 <entry>100 (ASCII 'd')</entry>
444 <entry>IEEE 754 double</entry>
446 <entry><literal>STRING</literal></entry>
447 <entry>115 (ASCII 's')</entry>
448 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
450 <entry><literal>OBJECT_PATH</literal></entry>
451 <entry>111 (ASCII 'o')</entry>
452 <entry>Name of an object instance</entry>
454 <entry><literal>SIGNATURE</literal></entry>
455 <entry>103 (ASCII 'g')</entry>
456 <entry>A type signature</entry>
458 <entry><literal>ARRAY</literal></entry>
459 <entry>97 (ASCII 'a')</entry>
462 <entry><literal>STRUCT</literal></entry>
463 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
464 <entry>Struct</entry>
466 <entry><literal>VARIANT</literal></entry>
467 <entry>118 (ASCII 'v') </entry>
468 <entry>Variant type (the type of the value is part of the value itself)</entry>
470 <entry><literal>DICT_ENTRY</literal></entry>
471 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
472 <entry>Entry in a dict or map (array of key-value pairs)</entry>
474 <entry><literal>UNIX_FD</literal></entry>
475 <entry>104 (ASCII 'h')</entry>
476 <entry>Unix file descriptor</entry>
485 <sect2 id="message-protocol-marshaling">
486 <title>Marshaling (Wire Format)</title>
489 Given a type signature, a block of bytes can be converted into typed
490 values. This section describes the format of the block of bytes. Byte
491 order and alignment issues are handled uniformly for all D-Bus types.
495 A block of bytes has an associated byte order. The byte order
496 has to be discovered in some way; for D-Bus messages, the
497 byte order is part of the message header as described in
498 <xref linkend="message-protocol-messages"/>. For now, assume
499 that the byte order is known to be either little endian or big
504 Each value in a block of bytes is aligned "naturally," for example
505 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
506 8-byte boundary. To properly align a value, <firstterm>alignment
507 padding</firstterm> may be necessary. The alignment padding must always
508 be the minimum required padding to properly align the following value;
509 and it must always be made up of nul bytes. The alignment padding must
510 not be left uninitialized (it can't contain garbage), and more padding
511 than required must not be used.
515 Given all this, the types are marshaled on the wire as follows:
520 <entry>Conventional Name</entry>
521 <entry>Encoding</entry>
522 <entry>Alignment</entry>
527 <entry><literal>INVALID</literal></entry>
528 <entry>Not applicable; cannot be marshaled.</entry>
531 <entry><literal>BYTE</literal></entry>
532 <entry>A single 8-bit byte.</entry>
535 <entry><literal>BOOLEAN</literal></entry>
536 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
539 <entry><literal>INT16</literal></entry>
540 <entry>16-bit signed integer in the message's byte order.</entry>
543 <entry><literal>UINT16</literal></entry>
544 <entry>16-bit unsigned integer in the message's byte order.</entry>
547 <entry><literal>INT32</literal></entry>
548 <entry>32-bit signed integer in the message's byte order.</entry>
551 <entry><literal>UINT32</literal></entry>
552 <entry>32-bit unsigned integer in the message's byte order.</entry>
555 <entry><literal>INT64</literal></entry>
556 <entry>64-bit signed integer in the message's byte order.</entry>
559 <entry><literal>UINT64</literal></entry>
560 <entry>64-bit unsigned integer in the message's byte order.</entry>
563 <entry><literal>DOUBLE</literal></entry>
564 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
567 <entry><literal>STRING</literal></entry>
568 <entry>A <literal>UINT32</literal> indicating the string's
569 length in bytes excluding its terminating nul, followed by
570 non-nul string data of the given length, followed by a terminating nul
577 <entry><literal>OBJECT_PATH</literal></entry>
578 <entry>Exactly the same as <literal>STRING</literal> except the
579 content must be a valid object path (see below).
585 <entry><literal>SIGNATURE</literal></entry>
586 <entry>The same as <literal>STRING</literal> except the length is a single
587 byte (thus signatures have a maximum length of 255)
588 and the content must be a valid signature (see below).
594 <entry><literal>ARRAY</literal></entry>
596 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
597 alignment padding to the alignment boundary of the array element type,
598 followed by each array element. The array length is from the
599 end of the alignment padding to the end of the last element,
600 i.e. it does not include the padding after the length,
601 or any padding after the last element.
602 Arrays have a maximum length defined to be 2 to the 26th power or
603 67108864. Implementations must not send or accept arrays exceeding this
610 <entry><literal>STRUCT</literal></entry>
612 A struct must start on an 8-byte boundary regardless of the
613 type of the struct fields. The struct value consists of each
614 field marshaled in sequence starting from that 8-byte
621 <entry><literal>VARIANT</literal></entry>
623 A variant type has a marshaled <literal>SIGNATURE</literal>
624 followed by a marshaled value with the type
625 given in the signature.
626 Unlike a message signature, the variant signature
627 can contain only a single complete type.
628 So "i", "ai" or "(ii)" is OK, but "ii" is not.
631 1 (alignment of the signature)
634 <entry><literal>DICT_ENTRY</literal></entry>
642 <entry><literal>UNIX_FD</literal></entry>
643 <entry>32-bit unsigned integer in the message's byte
644 order. The actual file descriptors need to be
645 transferred out-of-band via some platform specific
646 mechanism. On the wire, values of this type store the index to the
647 file descriptor in the array of file descriptors that
648 accompany the message.</entry>
656 <sect3 id="message-protocol-marshaling-object-path">
657 <title>Valid Object Paths</title>
660 An object path is a name used to refer to an object instance.
661 Conceptually, each participant in a D-Bus message exchange may have
662 any number of object instances (think of C++ or Java objects) and each
663 such instance will have a path. Like a filesystem, the object
664 instances in an application form a hierarchical tree.
668 The following rules define a valid object path. Implementations must
669 not send or accept messages with invalid object paths.
673 The path may be of any length.
678 The path must begin with an ASCII '/' (integer 47) character,
679 and must consist of elements separated by slash characters.
684 Each element must only contain the ASCII characters
690 No element may be the empty string.
695 Multiple '/' characters cannot occur in sequence.
700 A trailing '/' character is not allowed unless the
701 path is the root path (a single '/' character).
710 <sect3 id="message-protocol-marshaling-signature">
711 <title>Valid Signatures</title>
713 An implementation must not send or accept invalid signatures.
714 Valid signatures will conform to the following rules:
718 The signature ends with a nul byte.
723 The signature is a list of single complete types.
724 Arrays must have element types, and structs must
725 have both open and close parentheses.
730 Only type codes and open and close parentheses are
731 allowed in the signature. The <literal>STRUCT</literal> type code
732 is not allowed in signatures, because parentheses
738 The maximum depth of container type nesting is 32 array type
739 codes and 32 open parentheses. This implies that the maximum
740 total depth of recursion is 64, for an "array of array of array
741 of ... struct of struct of struct of ..." where there are 32
747 The maximum length of a signature is 255.
752 Signatures must be nul-terminated.
761 <sect2 id="message-protocol-messages">
762 <title>Message Format</title>
765 A message consists of a header and a body. The header is a block of
766 values with a fixed signature and meaning. The body is a separate block
767 of values, with a signature specified in the header.
771 The length of the header must be a multiple of 8, allowing the body to
772 begin on an 8-byte boundary when storing the entire message in a single
773 buffer. If the header does not naturally end on an 8-byte boundary
774 up to 7 bytes of nul-initialized alignment padding must be added.
778 The message body need not end on an 8-byte boundary.
782 The maximum length of a message, including header, header alignment padding,
783 and body is 2 to the 27th power or 134217728. Implementations must not
784 send or accept messages exceeding this size.
788 The signature of the header is:
792 Written out more readably, this is:
794 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
799 These values have the following meanings:
805 <entry>Description</entry>
810 <entry>1st <literal>BYTE</literal></entry>
811 <entry>Endianness flag; ASCII 'l' for little-endian
812 or ASCII 'B' for big-endian. Both header and body are
813 in this endianness.</entry>
816 <entry>2nd <literal>BYTE</literal></entry>
817 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
818 Currently-defined types are described below.
822 <entry>3rd <literal>BYTE</literal></entry>
823 <entry>Bitwise OR of flags. Unknown flags
824 must be ignored. Currently-defined flags are described below.
828 <entry>4th <literal>BYTE</literal></entry>
829 <entry>Major protocol version of the sending application. If
830 the major protocol version of the receiving application does not
831 match, the applications will not be able to communicate and the
832 D-Bus connection must be disconnected. The major protocol
833 version for this version of the specification is 1.
837 <entry>1st <literal>UINT32</literal></entry>
838 <entry>Length in bytes of the message body, starting
839 from the end of the header. The header ends after
840 its alignment padding to an 8-boundary.
844 <entry>2nd <literal>UINT32</literal></entry>
845 <entry>The serial of this message, used as a cookie
846 by the sender to identify the reply corresponding
847 to this request. This must not be zero.
851 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
852 <entry>An array of zero or more <firstterm>header
853 fields</firstterm> where the byte is the field code, and the
854 variant is the field value. The message type determines
855 which fields are required.
863 <firstterm>Message types</firstterm> that can appear in the second byte
869 <entry>Conventional name</entry>
870 <entry>Decimal value</entry>
871 <entry>Description</entry>
876 <entry><literal>INVALID</literal></entry>
878 <entry>This is an invalid type.</entry>
881 <entry><literal>METHOD_CALL</literal></entry>
883 <entry>Method call.</entry>
886 <entry><literal>METHOD_RETURN</literal></entry>
888 <entry>Method reply with returned data.</entry>
891 <entry><literal>ERROR</literal></entry>
893 <entry>Error reply. If the first argument exists and is a
894 string, it is an error message.</entry>
897 <entry><literal>SIGNAL</literal></entry>
899 <entry>Signal emission.</entry>
906 Flags that can appear in the third byte of the header:
911 <entry>Conventional name</entry>
912 <entry>Hex value</entry>
913 <entry>Description</entry>
918 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
920 <entry>This message does not expect method return replies or
921 error replies; the reply can be omitted as an
922 optimization. However, it is compliant with this specification
923 to return the reply despite this flag and the only harm
924 from doing so is extra network traffic.
928 <entry><literal>NO_AUTO_START</literal></entry>
930 <entry>The bus must not launch an owner
931 for the destination name in response to this message.
939 <sect3 id="message-protocol-header-fields">
940 <title>Header Fields</title>
943 The array at the end of the header contains <firstterm>header
944 fields</firstterm>, where each field is a 1-byte field code followed
945 by a field value. A header must contain the required header fields for
946 its message type, and zero or more of any optional header
947 fields. Future versions of this protocol specification may add new
948 fields. Implementations must ignore fields they do not
949 understand. Implementations must not invent their own header fields;
950 only changes to this specification may introduce new header fields.
954 Again, if an implementation sees a header field code that it does not
955 expect, it must ignore that field, as it will be part of a new
956 (but compatible) version of this specification. This also applies
957 to known header fields appearing in unexpected messages, for
958 example: if a signal has a reply serial it must be ignored
959 even though it has no meaning as of this version of the spec.
963 However, implementations must not send or accept known header fields
964 with the wrong type stored in the field value. So for example a
965 message with an <literal>INTERFACE</literal> field of type
966 <literal>UINT32</literal> would be considered corrupt.
970 Here are the currently-defined header fields:
975 <entry>Conventional Name</entry>
976 <entry>Decimal Code</entry>
978 <entry>Required In</entry>
979 <entry>Description</entry>
984 <entry><literal>INVALID</literal></entry>
987 <entry>not allowed</entry>
988 <entry>Not a valid field name (error if it appears in a message)</entry>
991 <entry><literal>PATH</literal></entry>
993 <entry><literal>OBJECT_PATH</literal></entry>
994 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
995 <entry>The object to send a call to,
996 or the object a signal is emitted from.
998 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
999 implementations should not send messages with this path,
1000 and the reference implementation of the bus daemon will
1001 disconnect any application that attempts to do so.
1005 <entry><literal>INTERFACE</literal></entry>
1007 <entry><literal>STRING</literal></entry>
1008 <entry><literal>SIGNAL</literal></entry>
1010 The interface to invoke a method call on, or
1011 that a signal is emitted from. Optional for
1012 method calls, required for signals.
1013 The special interface
1014 <literal>org.freedesktop.DBus.Local</literal> is reserved;
1015 implementations should not send messages with this
1016 interface, and the reference implementation of the bus
1017 daemon will disconnect any application that attempts to
1022 <entry><literal>MEMBER</literal></entry>
1024 <entry><literal>STRING</literal></entry>
1025 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
1026 <entry>The member, either the method name or signal name.</entry>
1029 <entry><literal>ERROR_NAME</literal></entry>
1031 <entry><literal>STRING</literal></entry>
1032 <entry><literal>ERROR</literal></entry>
1033 <entry>The name of the error that occurred, for errors</entry>
1036 <entry><literal>REPLY_SERIAL</literal></entry>
1038 <entry><literal>UINT32</literal></entry>
1039 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
1040 <entry>The serial number of the message this message is a reply
1041 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
1044 <entry><literal>DESTINATION</literal></entry>
1046 <entry><literal>STRING</literal></entry>
1047 <entry>optional</entry>
1048 <entry>The name of the connection this message is intended for.
1049 Only used in combination with the message bus, see
1050 <xref linkend="message-bus"/>.</entry>
1053 <entry><literal>SENDER</literal></entry>
1055 <entry><literal>STRING</literal></entry>
1056 <entry>optional</entry>
1057 <entry>Unique name of the sending connection.
1058 The message bus fills in this field so it is reliable; the field is
1059 only meaningful in combination with the message bus.</entry>
1062 <entry><literal>SIGNATURE</literal></entry>
1064 <entry><literal>SIGNATURE</literal></entry>
1065 <entry>optional</entry>
1066 <entry>The signature of the message body.
1067 If omitted, it is assumed to be the
1068 empty signature "" (i.e. the body must be 0-length).</entry>
1071 <entry><literal>UNIX_FDS</literal></entry>
1073 <entry><literal>UINT32</literal></entry>
1074 <entry>optional</entry>
1075 <entry>The number of Unix file descriptors that
1076 accompany the message. If omitted, it is assumed
1077 that no Unix file descriptors accompany the
1078 message. The actual file descriptors need to be
1079 transferred via platform specific mechanism
1080 out-of-band. They must be sent at the same time as
1081 part of the message itself. They may not be sent
1082 before the first byte of the message itself is
1083 transferred or after the last byte of the message
1093 <sect2 id="message-protocol-names">
1094 <title>Valid Names</title>
1096 The various names in D-Bus messages have some restrictions.
1099 There is a <firstterm>maximum name length</firstterm>
1100 of 255 which applies to bus names, interfaces, and members.
1102 <sect3 id="message-protocol-names-interface">
1103 <title>Interface names</title>
1105 Interfaces have names with type <literal>STRING</literal>, meaning that
1106 they must be valid UTF-8. However, there are also some
1107 additional restrictions that apply to interface names
1110 <listitem><para>Interface names are composed of 1 or more elements separated by
1111 a period ('.') character. All elements must contain at least
1115 <listitem><para>Each element must only contain the ASCII characters
1116 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1120 <listitem><para>Interface names must contain at least one '.' (period)
1121 character (and thus at least two elements).
1124 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1125 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1129 <sect3 id="message-protocol-names-bus">
1130 <title>Bus names</title>
1132 Connections have one or more bus names associated with them.
1133 A connection has exactly one bus name that is a unique connection
1134 name. The unique connection name remains with the connection for
1135 its entire lifetime.
1136 A bus name is of type <literal>STRING</literal>,
1137 meaning that it must be valid UTF-8. However, there are also
1138 some additional restrictions that apply to bus names
1141 <listitem><para>Bus names that start with a colon (':')
1142 character are unique connection names.
1145 <listitem><para>Bus names are composed of 1 or more elements separated by
1146 a period ('.') character. All elements must contain at least
1150 <listitem><para>Each element must only contain the ASCII characters
1151 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1152 connection name may begin with a digit, elements in
1153 other bus names must not begin with a digit.
1157 <listitem><para>Bus names must contain at least one '.' (period)
1158 character (and thus at least two elements).
1161 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1162 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1166 Note that the hyphen ('-') character is allowed in bus names but
1167 not in interface names.
1170 <sect3 id="message-protocol-names-member">
1171 <title>Member names</title>
1173 Member (i.e. method or signal) names:
1175 <listitem><para>Must only contain the ASCII characters
1176 "[A-Z][a-z][0-9]_" and may not begin with a
1177 digit.</para></listitem>
1178 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1179 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1180 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1184 <sect3 id="message-protocol-names-error">
1185 <title>Error names</title>
1187 Error names have the same restrictions as interface names.
1192 <sect2 id="message-protocol-types">
1193 <title>Message Types</title>
1195 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1196 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1197 This section describes these conventions.
1199 <sect3 id="message-protocol-types-method">
1200 <title>Method Calls</title>
1202 Some messages invoke an operation on a remote object. These are
1203 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1204 messages map naturally to methods on objects in a typical program.
1207 A method call message is required to have a <literal>MEMBER</literal> header field
1208 indicating the name of the method. Optionally, the message has an
1209 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1210 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1211 a method with the same name, it is undefined which of the two methods
1212 will be invoked. Implementations may also choose to return an error in
1213 this ambiguous case. However, if a method name is unique
1214 implementations must not require an interface field.
1217 Method call messages also include a <literal>PATH</literal> field
1218 indicating the object to invoke the method on. If the call is passing
1219 through a message bus, the message will also have a
1220 <literal>DESTINATION</literal> field giving the name of the connection
1221 to receive the message.
1224 When an application handles a method call message, it is required to
1225 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1226 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1227 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1230 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1231 are the return value(s) or "out parameters" of the method call.
1232 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1233 and the call fails; no return value will be provided. It makes
1234 no sense to send multiple replies to the same method call.
1237 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1238 reply is required, so the caller will know the method
1239 was successfully processed.
1242 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1246 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1247 then as an optimization the application receiving the method
1248 call may choose to omit the reply message (regardless of
1249 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1250 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1251 flag and reply anyway.
1254 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1255 destination name does not exist then a program to own the destination
1256 name will be started before the message is delivered. The message
1257 will be held until the new program is successfully started or has
1258 failed to start; in case of failure, an error will be returned. This
1259 flag is only relevant in the context of a message bus, it is ignored
1260 during one-to-one communication with no intermediate bus.
1262 <sect4 id="message-protocol-types-method-apis">
1263 <title>Mapping method calls to native APIs</title>
1265 APIs for D-Bus may map method calls to a method call in a specific
1266 programming language, such as C++, or may map a method call written
1267 in an IDL to a D-Bus message.
1270 In APIs of this nature, arguments to a method are often termed "in"
1271 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1272 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1273 "inout" arguments, which are both sent and received, i.e. the caller
1274 passes in a value which is modified. Mapped to D-Bus, an "inout"
1275 argument is equivalent to an "in" argument, followed by an "out"
1276 argument. You can't pass things "by reference" over the wire, so
1277 "inout" is purely an illusion of the in-process API.
1280 Given a method with zero or one return values, followed by zero or more
1281 arguments, where each argument may be "in", "out", or "inout", the
1282 caller constructs a message by appending each "in" or "inout" argument,
1283 in order. "out" arguments are not represented in the caller's message.
1286 The recipient constructs a reply by appending first the return value
1287 if any, then each "out" or "inout" argument, in order.
1288 "in" arguments are not represented in the reply message.
1291 Error replies are normally mapped to exceptions in languages that have
1295 In converting from native APIs to D-Bus, it is perhaps nice to
1296 map D-Bus naming conventions ("FooBar") to native conventions
1297 such as "fooBar" or "foo_bar" automatically. This is OK
1298 as long as you can say that the native API is one that
1299 was specifically written for D-Bus. It makes the most sense
1300 when writing object implementations that will be exported
1301 over the bus. Object proxies used to invoke remote D-Bus
1302 objects probably need the ability to call any D-Bus method,
1303 and thus a magic name mapping like this could be a problem.
1306 This specification doesn't require anything of native API bindings;
1307 the preceding is only a suggested convention for consistency
1313 <sect3 id="message-protocol-types-signal">
1314 <title>Signal Emission</title>
1316 Unlike method calls, signal emissions have no replies.
1317 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1318 It must have three header fields: <literal>PATH</literal> giving the object
1319 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1320 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1321 for signals, though it is optional for method calls.
1325 <sect3 id="message-protocol-types-errors">
1326 <title>Errors</title>
1328 Messages of type <literal>ERROR</literal> are most commonly replies
1329 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1330 to any kind of message. The message bus for example
1331 will return an <literal>ERROR</literal> in reply to a signal emission if
1332 the bus does not have enough memory to send the signal.
1335 An <literal>ERROR</literal> may have any arguments, but if the first
1336 argument is a <literal>STRING</literal>, it must be an error message.
1337 The error message may be logged or shown to the user
1342 <sect3 id="message-protocol-types-notation">
1343 <title>Notation in this document</title>
1345 This document uses a simple pseudo-IDL to describe particular method
1346 calls and signals. Here is an example of a method call:
1348 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1349 out UINT32 resultcode)
1351 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1352 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1353 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1354 characters so it's known that the last part of the name in
1355 the "IDL" is the member name.
1358 In C++ that might end up looking like this:
1360 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1361 unsigned int flags);
1363 or equally valid, the return value could be done as an argument:
1365 void org::freedesktop::DBus::StartServiceByName (const char *name,
1367 unsigned int *resultcode);
1369 It's really up to the API designer how they want to make
1370 this look. You could design an API where the namespace wasn't used
1371 in C++, using STL or Qt, using varargs, or whatever you wanted.
1374 Signals are written as follows:
1376 org.freedesktop.DBus.NameLost (STRING name)
1378 Signals don't specify "in" vs. "out" because only
1379 a single direction is possible.
1382 It isn't especially encouraged to use this lame pseudo-IDL in actual
1383 API implementations; you might use the native notation for the
1384 language you're using, or you might use COM or CORBA IDL, for example.
1389 <sect2 id="message-protocol-handling-invalid">
1390 <title>Invalid Protocol and Spec Extensions</title>
1393 For security reasons, the D-Bus protocol should be strictly parsed and
1394 validated, with the exception of defined extension points. Any invalid
1395 protocol or spec violations should result in immediately dropping the
1396 connection without notice to the other end. Exceptions should be
1397 carefully considered, e.g. an exception may be warranted for a
1398 well-understood idiosyncrasy of a widely-deployed implementation. In
1399 cases where the other end of a connection is 100% trusted and known to
1400 be friendly, skipping validation for performance reasons could also make
1401 sense in certain cases.
1405 Generally speaking violations of the "must" requirements in this spec
1406 should be considered possible attempts to exploit security, and violations
1407 of the "should" suggestions should be considered legitimate (though perhaps
1408 they should generate an error in some cases).
1412 The following extension points are built in to D-Bus on purpose and must
1413 not be treated as invalid protocol. The extension points are intended
1414 for use by future versions of this spec, they are not intended for third
1415 parties. At the moment, the only way a third party could extend D-Bus
1416 without breaking interoperability would be to introduce a way to negotiate new
1417 feature support as part of the auth protocol, using EXTENSION_-prefixed
1418 commands. There is not yet a standard way to negotiate features.
1422 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1423 commands result in an ERROR rather than a disconnect. This enables
1424 future extensions to the protocol. Commands starting with EXTENSION_ are
1425 reserved for third parties.
1430 The authentication protocol supports pluggable auth mechanisms.
1435 The address format (see <xref linkend="addresses"/>) supports new
1441 Messages with an unknown type (something other than
1442 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1443 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1444 Unknown-type messages must still be well-formed in the same way
1445 as the known messages, however. They still have the normal
1451 Header fields with an unknown or unexpected field code must be ignored,
1452 though again they must still be well-formed.
1457 New standard interfaces (with new methods and signals) can of course be added.
1467 <sect1 id="auth-protocol">
1468 <title>Authentication Protocol</title>
1470 Before the flow of messages begins, two applications must
1471 authenticate. A simple plain-text protocol is used for
1472 authentication; this protocol is a SASL profile, and maps fairly
1473 directly from the SASL specification. The message encoding is
1474 NOT used here, only plain text messages.
1477 In examples, "C:" and "S:" indicate lines sent by the client and
1478 server respectively.
1480 <sect2 id="auth-protocol-overview">
1481 <title>Protocol Overview</title>
1483 The protocol is a line-based protocol, where each line ends with
1484 \r\n. Each line begins with an all-caps ASCII command name containing
1485 only the character range [A-Z_], a space, then any arguments for the
1486 command, then the \r\n ending the line. The protocol is
1487 case-sensitive. All bytes must be in the ASCII character set.
1489 Commands from the client to the server are as follows:
1492 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1493 <listitem><para>CANCEL</para></listitem>
1494 <listitem><para>BEGIN</para></listitem>
1495 <listitem><para>DATA <data in hex encoding></para></listitem>
1496 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1497 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1500 From server to client are as follows:
1503 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1504 <listitem><para>OK <GUID in hex></para></listitem>
1505 <listitem><para>DATA <data in hex encoding></para></listitem>
1506 <listitem><para>ERROR</para></listitem>
1507 <listitem><para>AGREE_UNIX_FD</para></listitem>
1511 Unofficial extensions to the command set must begin with the letters
1512 "EXTENSION_", to avoid conflicts with future official commands.
1513 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1516 <sect2 id="auth-nul-byte">
1517 <title>Special credentials-passing nul byte</title>
1519 Immediately after connecting to the server, the client must send a
1520 single nul byte. This byte may be accompanied by credentials
1521 information on some operating systems that use sendmsg() with
1522 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1523 sockets. However, the nul byte must be sent even on other kinds of
1524 socket, and even on operating systems that do not require a byte to be
1525 sent in order to transmit credentials. The text protocol described in
1526 this document begins after the single nul byte. If the first byte
1527 received from the client is not a nul byte, the server may disconnect
1531 A nul byte in any context other than the initial byte is an error;
1532 the protocol is ASCII-only.
1535 The credentials sent along with the nul byte may be used with the
1536 SASL mechanism EXTERNAL.
1539 <sect2 id="auth-command-auth">
1540 <title>AUTH command</title>
1542 If an AUTH command has no arguments, it is a request to list
1543 available mechanisms. The server must respond with a REJECTED
1544 command listing the mechanisms it understands, or with an error.
1547 If an AUTH command specifies a mechanism, and the server supports
1548 said mechanism, the server should begin exchanging SASL
1549 challenge-response data with the client using DATA commands.
1552 If the server does not support the mechanism given in the AUTH
1553 command, it must send either a REJECTED command listing the mechanisms
1554 it does support, or an error.
1557 If the [initial-response] argument is provided, it is intended for use
1558 with mechanisms that have no initial challenge (or an empty initial
1559 challenge), as if it were the argument to an initial DATA command. If
1560 the selected mechanism has an initial challenge and [initial-response]
1561 was provided, the server should reject authentication by sending
1565 If authentication succeeds after exchanging DATA commands,
1566 an OK command must be sent to the client.
1569 The first octet received by the server after the \r\n of the BEGIN
1570 command from the client must be the first octet of the
1571 authenticated/encrypted stream of D-Bus messages.
1574 If BEGIN is received by the server, the first octet received
1575 by the client after the \r\n of the OK command must be the
1576 first octet of the authenticated/encrypted stream of D-Bus
1580 <sect2 id="auth-command-cancel">
1581 <title>CANCEL Command</title>
1583 At any time up to sending the BEGIN command, the client may send a
1584 CANCEL command. On receiving the CANCEL command, the server must
1585 send a REJECTED command and abort the current authentication
1589 <sect2 id="auth-command-data">
1590 <title>DATA Command</title>
1592 The DATA command may come from either client or server, and simply
1593 contains a hex-encoded block of data to be interpreted
1594 according to the SASL mechanism in use.
1597 Some SASL mechanisms support sending an "empty string";
1598 FIXME we need some way to do this.
1601 <sect2 id="auth-command-begin">
1602 <title>BEGIN Command</title>
1604 The BEGIN command acknowledges that the client has received an
1605 OK command from the server, and that the stream of messages
1609 The first octet received by the server after the \r\n of the BEGIN
1610 command from the client must be the first octet of the
1611 authenticated/encrypted stream of D-Bus messages.
1614 <sect2 id="auth-command-rejected">
1615 <title>REJECTED Command</title>
1617 The REJECTED command indicates that the current authentication
1618 exchange has failed, and further exchange of DATA is inappropriate.
1619 The client would normally try another mechanism, or try providing
1620 different responses to challenges.
1622 Optionally, the REJECTED command has a space-separated list of
1623 available auth mechanisms as arguments. If a server ever provides
1624 a list of supported mechanisms, it must provide the same list
1625 each time it sends a REJECTED message. Clients are free to
1626 ignore all lists received after the first.
1629 <sect2 id="auth-command-ok">
1630 <title>OK Command</title>
1632 The OK command indicates that the client has been
1633 authenticated. The client may now proceed with negotiating
1634 Unix file descriptor passing. To do that it shall send
1635 NEGOTIATE_UNIX_FD to the server.
1638 Otherwise, the client must respond to the OK command by
1639 sending a BEGIN command, followed by its stream of messages,
1640 or by disconnecting. The server must not accept additional
1641 commands using this protocol after the BEGIN command has been
1642 received. Further communication will be a stream of D-Bus
1643 messages (optionally encrypted, as negotiated) rather than
1647 If a client sends BEGIN the first octet received by the client
1648 after the \r\n of the OK command must be the first octet of
1649 the authenticated/encrypted stream of D-Bus messages.
1652 The OK command has one argument, which is the GUID of the server.
1653 See <xref linkend="addresses"/> for more on server GUIDs.
1656 <sect2 id="auth-command-error">
1657 <title>ERROR Command</title>
1659 The ERROR command indicates that either server or client did not
1660 know a command, does not accept the given command in the current
1661 context, or did not understand the arguments to the command. This
1662 allows the protocol to be extended; a client or server can send a
1663 command present or permitted only in new protocol versions, and if
1664 an ERROR is received instead of an appropriate response, fall back
1665 to using some other technique.
1668 If an ERROR is sent, the server or client that sent the
1669 error must continue as if the command causing the ERROR had never been
1670 received. However, the the server or client receiving the error
1671 should try something other than whatever caused the error;
1672 if only canceling/rejecting the authentication.
1675 If the D-Bus protocol changes incompatibly at some future time,
1676 applications implementing the new protocol would probably be able to
1677 check for support of the new protocol by sending a new command and
1678 receiving an ERROR from applications that don't understand it. Thus the
1679 ERROR feature of the auth protocol is an escape hatch that lets us
1680 negotiate extensions or changes to the D-Bus protocol in the future.
1683 <sect2 id="auth-command-negotiate-unix-fd">
1684 <title>NEGOTIATE_UNIX_FD Command</title>
1686 The NEGOTIATE_UNIX_FD command indicates that the client
1687 supports Unix file descriptor passing. This command may only
1688 be sent after the connection is authenticated, i.e. after OK
1689 was received by the client. This command may only be sent on
1690 transports that support Unix file descriptor passing.
1693 On receiving NEGOTIATE_UNIX_FD the server must respond with
1694 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1695 the transport chosen supports Unix file descriptor passing and
1696 the server supports this feature. It shall respond the latter
1697 if the transport does not support Unix file descriptor
1698 passing, the server does not support this feature, or the
1699 server decides not to enable file descriptor passing due to
1700 security or other reasons.
1703 <sect2 id="auth-command-agree-unix-fd">
1704 <title>AGREE_UNIX_FD Command</title>
1706 The AGREE_UNIX_FD command indicates that the server supports
1707 Unix file descriptor passing. This command may only be sent
1708 after the connection is authenticated, and the client sent
1709 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1710 command may only be sent on transports that support Unix file
1714 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1715 followed by its stream of messages, or by disconnecting. The
1716 server must not accept additional commands using this protocol
1717 after the BEGIN command has been received. Further
1718 communication will be a stream of D-Bus messages (optionally
1719 encrypted, as negotiated) rather than this protocol.
1722 <sect2 id="auth-command-future">
1723 <title>Future Extensions</title>
1725 Future extensions to the authentication and negotiation
1726 protocol are possible. For that new commands may be
1727 introduced. If a client or server receives an unknown command
1728 it shall respond with ERROR and not consider this fatal. New
1729 commands may be introduced both before, and after
1730 authentication, i.e. both before and after the OK command.
1733 <sect2 id="auth-examples">
1734 <title>Authentication examples</title>
1738 <title>Example of successful magic cookie authentication</title>
1740 (MAGIC_COOKIE is a made up mechanism)
1742 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1748 <title>Example of finding out mechanisms then picking one</title>
1751 S: REJECTED KERBEROS_V4 SKEY
1752 C: AUTH SKEY 7ab83f32ee
1753 S: DATA 8799cabb2ea93e
1754 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1760 <title>Example of client sends unknown command then falls back to regular auth</title>
1764 C: AUTH MAGIC_COOKIE 3736343435313230333039
1770 <title>Example of server doesn't support initial auth mechanism</title>
1772 C: AUTH MAGIC_COOKIE 3736343435313230333039
1773 S: REJECTED KERBEROS_V4 SKEY
1774 C: AUTH SKEY 7ab83f32ee
1775 S: DATA 8799cabb2ea93e
1776 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1782 <title>Example of wrong password or the like followed by successful retry</title>
1784 C: AUTH MAGIC_COOKIE 3736343435313230333039
1785 S: REJECTED KERBEROS_V4 SKEY
1786 C: AUTH SKEY 7ab83f32ee
1787 S: DATA 8799cabb2ea93e
1788 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1790 C: AUTH SKEY 7ab83f32ee
1791 S: DATA 8799cabb2ea93e
1792 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1798 <title>Example of skey cancelled and restarted</title>
1800 C: AUTH MAGIC_COOKIE 3736343435313230333039
1801 S: REJECTED KERBEROS_V4 SKEY
1802 C: AUTH SKEY 7ab83f32ee
1803 S: DATA 8799cabb2ea93e
1806 C: AUTH SKEY 7ab83f32ee
1807 S: DATA 8799cabb2ea93e
1808 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1814 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1816 (MAGIC_COOKIE is a made up mechanism)
1818 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1820 C: NEGOTIATE_UNIX_FD
1826 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1828 (MAGIC_COOKIE is a made up mechanism)
1830 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1832 C: NEGOTIATE_UNIX_FD
1839 <sect2 id="auth-states">
1840 <title>Authentication state diagrams</title>
1843 This section documents the auth protocol in terms of
1844 a state machine for the client and the server. This is
1845 probably the most robust way to implement the protocol.
1848 <sect3 id="auth-states-client">
1849 <title>Client states</title>
1852 To more precisely describe the interaction between the
1853 protocol state machine and the authentication mechanisms the
1854 following notation is used: MECH(CHALL) means that the
1855 server challenge CHALL was fed to the mechanism MECH, which
1861 CONTINUE(RESP) means continue the auth conversation
1862 and send RESP as the response to the server;
1868 OK(RESP) means that after sending RESP to the server
1869 the client side of the auth conversation is finished
1870 and the server should return "OK";
1876 ERROR means that CHALL was invalid and could not be
1882 Both RESP and CHALL may be empty.
1886 The Client starts by getting an initial response from the
1887 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1888 the mechanism did not provide an initial response. If the
1889 mechanism returns CONTINUE, the client starts in state
1890 <emphasis>WaitingForData</emphasis>, if the mechanism
1891 returns OK the client starts in state
1892 <emphasis>WaitingForOK</emphasis>.
1896 The client should keep track of available mechanisms and
1897 which it mechanisms it has already attempted. This list is
1898 used to decide which AUTH command to send. When the list is
1899 exhausted, the client should give up and close the
1904 <title><emphasis>WaitingForData</emphasis></title>
1912 MECH(CHALL) returns CONTINUE(RESP) → send
1914 <emphasis>WaitingForData</emphasis>
1918 MECH(CHALL) returns OK(RESP) → send DATA
1919 RESP, goto <emphasis>WaitingForOK</emphasis>
1923 MECH(CHALL) returns ERROR → send ERROR
1924 [msg], goto <emphasis>WaitingForData</emphasis>
1932 Receive REJECTED [mechs] →
1933 send AUTH [next mech], goto
1934 WaitingForData or <emphasis>WaitingForOK</emphasis>
1939 Receive ERROR → send
1941 <emphasis>WaitingForReject</emphasis>
1946 Receive OK → send
1947 BEGIN, terminate auth
1948 conversation, authenticated
1953 Receive anything else → send
1955 <emphasis>WaitingForData</emphasis>
1963 <title><emphasis>WaitingForOK</emphasis></title>
1968 Receive OK → send BEGIN, terminate auth
1969 conversation, <emphasis>authenticated</emphasis>
1974 Receive REJECT [mechs] → send AUTH [next mech],
1975 goto <emphasis>WaitingForData</emphasis> or
1976 <emphasis>WaitingForOK</emphasis>
1982 Receive DATA → send CANCEL, goto
1983 <emphasis>WaitingForReject</emphasis>
1989 Receive ERROR → send CANCEL, goto
1990 <emphasis>WaitingForReject</emphasis>
1996 Receive anything else → send ERROR, goto
1997 <emphasis>WaitingForOK</emphasis>
2005 <title><emphasis>WaitingForReject</emphasis></title>
2010 Receive REJECT [mechs] → send AUTH [next mech],
2011 goto <emphasis>WaitingForData</emphasis> or
2012 <emphasis>WaitingForOK</emphasis>
2018 Receive anything else → terminate auth
2019 conversation, disconnect
2028 <sect3 id="auth-states-server">
2029 <title>Server states</title>
2032 For the server MECH(RESP) means that the client response
2033 RESP was fed to the the mechanism MECH, which returns one of
2038 CONTINUE(CHALL) means continue the auth conversation and
2039 send CHALL as the challenge to the client;
2045 OK means that the client has been successfully
2052 REJECT means that the client failed to authenticate or
2053 there was an error in RESP.
2058 The server starts out in state
2059 <emphasis>WaitingForAuth</emphasis>. If the client is
2060 rejected too many times the server must disconnect the
2065 <title><emphasis>WaitingForAuth</emphasis></title>
2071 Receive AUTH → send REJECTED [mechs], goto
2072 <emphasis>WaitingForAuth</emphasis>
2078 Receive AUTH MECH RESP
2082 MECH not valid mechanism → send REJECTED
2084 <emphasis>WaitingForAuth</emphasis>
2088 MECH(RESP) returns CONTINUE(CHALL) → send
2090 <emphasis>WaitingForData</emphasis>
2094 MECH(RESP) returns OK → send OK, goto
2095 <emphasis>WaitingForBegin</emphasis>
2099 MECH(RESP) returns REJECT → send REJECTED
2101 <emphasis>WaitingForAuth</emphasis>
2109 Receive BEGIN → terminate
2110 auth conversation, disconnect
2116 Receive ERROR → send REJECTED [mechs], goto
2117 <emphasis>WaitingForAuth</emphasis>
2123 Receive anything else → send
2125 <emphasis>WaitingForAuth</emphasis>
2134 <title><emphasis>WaitingForData</emphasis></title>
2142 MECH(RESP) returns CONTINUE(CHALL) → send
2144 <emphasis>WaitingForData</emphasis>
2148 MECH(RESP) returns OK → send OK, goto
2149 <emphasis>WaitingForBegin</emphasis>
2153 MECH(RESP) returns REJECT → send REJECTED
2155 <emphasis>WaitingForAuth</emphasis>
2163 Receive BEGIN → terminate auth conversation,
2170 Receive CANCEL → send REJECTED [mechs], goto
2171 <emphasis>WaitingForAuth</emphasis>
2177 Receive ERROR → send REJECTED [mechs], goto
2178 <emphasis>WaitingForAuth</emphasis>
2184 Receive anything else → send ERROR, goto
2185 <emphasis>WaitingForData</emphasis>
2193 <title><emphasis>WaitingForBegin</emphasis></title>
2198 Receive BEGIN → terminate auth conversation,
2199 client authenticated
2205 Receive CANCEL → send REJECTED [mechs], goto
2206 <emphasis>WaitingForAuth</emphasis>
2212 Receive ERROR → send REJECTED [mechs], goto
2213 <emphasis>WaitingForAuth</emphasis>
2219 Receive anything else → send ERROR, goto
2220 <emphasis>WaitingForBegin</emphasis>
2230 <sect2 id="auth-mechanisms">
2231 <title>Authentication mechanisms</title>
2233 This section describes some new authentication mechanisms.
2234 D-Bus also allows any standard SASL mechanism of course.
2236 <sect3 id="auth-mechanisms-sha">
2237 <title>DBUS_COOKIE_SHA1</title>
2239 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2240 has the ability to read a private file owned by the user being
2241 authenticated. If the client can prove that it has access to a secret
2242 cookie stored in this file, then the client is authenticated.
2243 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2247 Throughout this description, "hex encoding" must output the digits
2248 from a to f in lower-case; the digits A to F must not be used
2249 in the DBUS_COOKIE_SHA1 mechanism.
2252 Authentication proceeds as follows:
2256 The client sends the username it would like to authenticate
2262 The server sends the name of its "cookie context" (see below); a
2263 space character; the integer ID of the secret cookie the client
2264 must demonstrate knowledge of; a space character; then a
2265 randomly-generated challenge string, all of this hex-encoded into
2271 The client locates the cookie and generates its own
2272 randomly-generated challenge string. The client then concatenates
2273 the server's decoded challenge, a ":" character, its own challenge,
2274 another ":" character, and the cookie. It computes the SHA-1 hash
2275 of this composite string as a hex digest. It concatenates the
2276 client's challenge string, a space character, and the SHA-1 hex
2277 digest, hex-encodes the result and sends it back to the server.
2282 The server generates the same concatenated string used by the
2283 client and computes its SHA-1 hash. It compares the hash with
2284 the hash received from the client; if the two hashes match, the
2285 client is authenticated.
2291 Each server has a "cookie context," which is a name that identifies a
2292 set of cookies that apply to that server. A sample context might be
2293 "org_freedesktop_session_bus". Context names must be valid ASCII,
2294 nonzero length, and may not contain the characters slash ("/"),
2295 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2296 tab ("\t"), or period ("."). There is a default context,
2297 "org_freedesktop_general" that's used by servers that do not specify
2301 Cookies are stored in a user's home directory, in the directory
2302 <filename>~/.dbus-keyrings/</filename>. This directory must
2303 not be readable or writable by other users. If it is,
2304 clients and servers must ignore it. The directory
2305 contains cookie files named after the cookie context.
2308 A cookie file contains one cookie per line. Each line
2309 has three space-separated fields:
2313 The cookie ID number, which must be a non-negative integer and
2314 may not be used twice in the same file.
2319 The cookie's creation time, in UNIX seconds-since-the-epoch
2325 The cookie itself, a hex-encoded random block of bytes. The cookie
2326 may be of any length, though obviously security increases
2327 as the length increases.
2333 Only server processes modify the cookie file.
2334 They must do so with this procedure:
2338 Create a lockfile name by appending ".lock" to the name of the
2339 cookie file. The server should attempt to create this file
2340 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2341 fails, the lock fails. Servers should retry for a reasonable
2342 period of time, then they may choose to delete an existing lock
2343 to keep users from having to manually delete a stale
2344 lock. <footnote><para>Lockfiles are used instead of real file
2345 locking <literal>fcntl()</literal> because real locking
2346 implementations are still flaky on network
2347 filesystems.</para></footnote>
2352 Once the lockfile has been created, the server loads the cookie
2353 file. It should then delete any cookies that are old (the
2354 timeout can be fairly short), or more than a reasonable
2355 time in the future (so that cookies never accidentally
2356 become permanent, if the clock was set far into the future
2357 at some point). If no recent keys remain, the
2358 server may generate a new key.
2363 The pruned and possibly added-to cookie file
2364 must be resaved atomically (using a temporary
2365 file which is rename()'d).
2370 The lock must be dropped by deleting the lockfile.
2376 Clients need not lock the file in order to load it,
2377 because servers are required to save the file atomically.
2382 <sect1 id="addresses">
2383 <title>Server Addresses</title>
2385 Server addresses consist of a transport name followed by a colon, and
2386 then an optional, comma-separated list of keys and values in the form key=value.
2387 Each value is escaped.
2391 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2392 Which is the address to a unix socket with the path /tmp/dbus-test.
2395 Value escaping is similar to URI escaping but simpler.
2399 The set of optionally-escaped bytes is:
2400 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2401 <emphasis>byte</emphasis> (note, not character) which is not in the
2402 set of optionally-escaped bytes must be replaced with an ASCII
2403 percent (<literal>%</literal>) and the value of the byte in hex.
2404 The hex value must always be two digits, even if the first digit is
2405 zero. The optionally-escaped bytes may be escaped if desired.
2410 To unescape, append each byte in the value; if a byte is an ASCII
2411 percent (<literal>%</literal>) character then append the following
2412 hex value instead. It is an error if a <literal>%</literal> byte
2413 does not have two hex digits following. It is an error if a
2414 non-optionally-escaped byte is seen unescaped.
2418 The set of optionally-escaped bytes is intended to preserve address
2419 readability and convenience.
2423 A server may specify a key-value pair with the key <literal>guid</literal>
2424 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2425 describes the format of the <literal>guid</literal> field. If present,
2426 this UUID may be used to distinguish one server address from another. A
2427 server should use a different UUID for each address it listens on. For
2428 example, if a message bus daemon offers both UNIX domain socket and TCP
2429 connections, but treats clients the same regardless of how they connect,
2430 those two connections are equivalent post-connection but should have
2431 distinct UUIDs to distinguish the kinds of connection.
2435 The intent of the address UUID feature is to allow a client to avoid
2436 opening multiple identical connections to the same server, by allowing the
2437 client to check whether an address corresponds to an already-existing
2438 connection. Comparing two addresses is insufficient, because addresses
2439 can be recycled by distinct servers, and equivalent addresses may look
2440 different if simply compared as strings (for example, the host in a TCP
2441 address can be given as an IP address or as a hostname).
2445 Note that the address key is <literal>guid</literal> even though the
2446 rest of the API and documentation says "UUID," for historical reasons.
2450 [FIXME clarify if attempting to connect to each is a requirement
2451 or just a suggestion]
2452 When connecting to a server, multiple server addresses can be
2453 separated by a semi-colon. The library will then try to connect
2454 to the first address and if that fails, it'll try to connect to
2455 the next one specified, and so forth. For example
2456 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2461 <sect1 id="transports">
2462 <title>Transports</title>
2464 [FIXME we need to specify in detail each transport and its possible arguments]
2466 Current transports include: unix domain sockets (including
2467 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2468 in-process pipes. Future possible transports include one that
2469 tunnels over X11 protocol.
2472 <sect2 id="transports-unix-domain-sockets">
2473 <title>Unix Domain Sockets</title>
2475 Unix domain sockets can be either paths in the file system or on Linux
2476 kernels, they can be abstract which are similar to paths but
2477 do not show up in the file system.
2481 When a socket is opened by the D-Bus library it truncates the path
2482 name right before the first trailing Nul byte. This is true for both
2483 normal paths and abstract paths. Note that this is a departure from
2484 previous versions of D-Bus that would create sockets with a fixed
2485 length path name. Names which were shorter than the fixed length
2486 would be padded by Nul bytes.
2489 Unix domain sockets are not available on windows.
2491 <sect3 id="transports-unix-domain-sockets-addresses">
2492 <title>Server Address Format</title>
2494 Unix domain socket addresses are identified by the "unix:" prefix
2495 and support the following key/value pairs:
2502 <entry>Values</entry>
2503 <entry>Description</entry>
2509 <entry>(path)</entry>
2510 <entry>path of the unix domain socket. If set, the "tmpdir" and "abstract" key must not be set.</entry>
2513 <entry>tmpdir</entry>
2514 <entry>(path)</entry>
2515 <entry>temporary directory in which a socket file with a random file name starting with 'dbus-' will be created by the server. This key can only be used in server addresses, not in client addresses. If set, the "path" and "abstract" key must not be set.</entry>
2518 <entry>abstract</entry>
2519 <entry>(string)</entry>
2520 <entry>unique string (path) in the abstract namespace. If set, the "path" or "tempdir" key must not be set.</entry>
2527 <sect2 id="transports-tcp-sockets">
2528 <title>TCP Sockets</title>
2530 The tcp transport provides TCP/IP based connections between clients
2531 located on the same or different hosts.
2534 Using tcp transport without any additional secure authentification mechanismus
2535 over a network is unsecure.
2538 Windows notes: Because of the tcp stack on windows does not provide sending
2539 credentials over a tcp connection, the EXTERNAL authentification
2540 mechanismus does not work.
2542 <sect3 id="transports-tcp-sockets-addresses">
2543 <title>Server Address Format</title>
2545 TCP/IP socket addresses are identified by the "tcp:" prefix
2546 and support the following key/value pairs:
2553 <entry>Values</entry>
2554 <entry>Description</entry>
2560 <entry>(string)</entry>
2561 <entry>dns name or ip address</entry>
2565 <entry>(number)</entry>
2566 <entry>The tcp port the server will open. A zero value let the server
2567 choose a free port provided from the underlaying operating system.
2568 libdbus is able to retrieve the real used port from the server.
2572 <entry>family</entry>
2573 <entry>(string)</entry>
2574 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2581 <sect2 id="transports-nonce-tcp-sockets">
2582 <title>Nonce-secured TCP Sockets</title>
2584 The nonce-tcp transport provides a secured TCP transport, using a
2585 simple authentication mechanism to ensure that only clients with read
2586 access to a certain location in the filesystem can connect to the server.
2587 The server writes a secret, the nonce, to a file and an incoming client
2588 connection is only accepted if the client sends the nonce right after
2589 the connect. The nonce mechanism requires no setup and is orthogonal to
2590 the higher-level authentication mechanisms described in the
2591 Authentication section.
2595 On start, the server generates a random 16 byte nonce and writes it
2596 to a file in the user's temporary directory. The nonce file location
2597 is published as part of the server's D-Bus address using the
2598 "noncefile" key-value pair.
2600 After an accept, the server reads 16 bytes from the socket. If the
2601 read bytes do not match the nonce stored in the nonce file, the
2602 server MUST immediately drop the connection.
2603 If the nonce match the received byte sequence, the client is accepted
2604 and the transport behaves like an unsecured tcp transport.
2607 After a successful connect to the server socket, the client MUST read
2608 the nonce from the file published by the server via the noncefile=
2609 key-value pair and send it over the socket. After that, the
2610 transport behaves like an unsecured tcp transport.
2612 <sect3 id="transports-nonce-tcp-sockets-addresses">
2613 <title>Server Address Format</title>
2615 Nonce TCP/IP socket addresses uses the "nonce-tcp:" prefix
2616 and support the following key/value pairs:
2623 <entry>Values</entry>
2624 <entry>Description</entry>
2630 <entry>(string)</entry>
2631 <entry>dns name or ip address</entry>
2635 <entry>(number)</entry>
2636 <entry>The tcp port the server will open. A zero value let the server
2637 choose a free port provided from the underlaying operating system.
2638 libdbus is able to retrieve the real used port from the server.
2642 <entry>family</entry>
2643 <entry>(string)</entry>
2644 <entry>If set, provide the type of socket family either "ipv4" or "ipv6". If unset, the family is unspecified.</entry>
2647 <entry>noncefile</entry>
2648 <entry>(path)</entry>
2649 <entry>file location containing the secret</entry>
2659 <sect1 id="naming-conventions">
2660 <title>Naming Conventions</title>
2663 D-Bus namespaces are all lowercase and correspond to reversed domain
2664 names, as with Java. e.g. "org.freedesktop"
2667 Interface, signal, method, and property names are "WindowsStyleCaps", note
2668 that the first letter is capitalized, unlike Java.
2671 Object paths are normally all lowercase with underscores used rather than
2677 <title>UUIDs</title>
2679 A working D-Bus implementation uses universally-unique IDs in two places.
2680 First, each server address has a UUID identifying the address,
2681 as described in <xref linkend="addresses"/>. Second, each operating
2682 system kernel instance running a D-Bus client or server has a UUID
2683 identifying that kernel, retrieved by invoking the method
2684 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2685 linkend="standard-interfaces-peer"/>).
2688 The term "UUID" in this document is intended literally, i.e. an
2689 identifier that is universally unique. It is not intended to refer to
2690 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2693 The UUID must contain 128 bits of data and be hex-encoded. The
2694 hex-encoded string may not contain hyphens or other non-hex-digit
2695 characters, and it must be exactly 32 characters long. To generate a
2696 UUID, the current reference implementation concatenates 96 bits of random
2697 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2701 It would also be acceptable and probably better to simply generate 128
2702 bits of random data, as long as the random number generator is of high
2703 quality. The timestamp could conceivably help if the random bits are not
2704 very random. With a quality random number generator, collisions are
2705 extremely unlikely even with only 96 bits, so it's somewhat academic.
2708 Implementations should, however, stick to random data for the first 96 bits
2713 <sect1 id="standard-interfaces">
2714 <title>Standard Interfaces</title>
2716 See <xref linkend="message-protocol-types-notation"/> for details on
2717 the notation used in this section. There are some standard interfaces
2718 that may be useful across various D-Bus applications.
2720 <sect2 id="standard-interfaces-peer">
2721 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2723 The <literal>org.freedesktop.DBus.Peer</literal> interface
2726 org.freedesktop.DBus.Peer.Ping ()
2727 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2731 On receipt of the <literal>METHOD_CALL</literal> message
2732 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2733 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2734 usual. It does not matter which object path a ping is sent to. The
2735 reference implementation handles this method automatically.
2738 On receipt of the <literal>METHOD_CALL</literal> message
2739 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2740 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2741 UUID representing the identity of the machine the process is running on.
2742 This UUID must be the same for all processes on a single system at least
2743 until that system next reboots. It should be the same across reboots
2744 if possible, but this is not always possible to implement and is not
2746 It does not matter which object path a GetMachineId is sent to. The
2747 reference implementation handles this method automatically.
2750 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2751 a virtual machine running on a hypervisor, rather than a physical machine.
2752 Basically if two processes see the same UUID, they should also see the same
2753 shared memory, UNIX domain sockets, process IDs, and other features that require
2754 a running OS kernel in common between the processes.
2757 The UUID is often used where other programs might use a hostname. Hostnames
2758 can change without rebooting, however, or just be "localhost" - so the UUID
2762 <xref linkend="uuids"/> explains the format of the UUID.
2766 <sect2 id="standard-interfaces-introspectable">
2767 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2769 This interface has one method:
2771 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2775 Objects instances may implement
2776 <literal>Introspect</literal> which returns an XML description of
2777 the object, including its interfaces (with signals and methods), objects
2778 below it in the object path tree, and its properties.
2781 <xref linkend="introspection-format"/> describes the format of this XML string.
2784 <sect2 id="standard-interfaces-properties">
2785 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2787 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2788 or <firstterm>attributes</firstterm>. These can be exposed via the
2789 <literal>org.freedesktop.DBus.Properties</literal> interface.
2793 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2794 in STRING property_name,
2796 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2797 in STRING property_name,
2799 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
2800 out DICT<STRING,VARIANT> props);
2804 The available properties and whether they are writable can be determined
2805 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2806 see <xref linkend="standard-interfaces-introspectable"/>.
2809 An empty string may be provided for the interface name; in this case,
2810 if there are multiple properties on an object with the same name,
2811 the results are undefined (picking one by according to an arbitrary
2812 deterministic rule, or returning an error, are the reasonable
2816 If one or more properties change on an object, the
2817 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
2818 signal may be emitted (this signal was added in 0.14):
2822 org.freedesktop.DBus.Properties.PropertiesChanged (STRING interface_name,
2823 DICT<STRING,VARIANT> changed_properties,
2824 ARRAY<STRING> invalidated_properties);
2828 where <literal>changed_properties</literal> is a dictionary
2829 containing the changed properties with the new values and
2830 <literal>invalidated_properties</literal> is an array of
2831 properties that changed but the value is not conveyed.
2834 Whether the <literal>PropertiesChanged</literal> signal is
2835 supported can be determined by calling
2836 <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>. Note
2837 that the signal may be supported for an object but it may
2838 differ how whether and how it is used on a per-property basis
2839 (for e.g. performance or security reasons). Each property (or
2840 the parent interface) must be annotated with the
2841 <literal>org.freedesktop.DBus.Property.EmitsChangedSignal</literal>
2842 annotation to convey this (usually the default value
2843 <literal>true</literal> is sufficient meaning that the
2844 annotation does not need to be used). See <xref
2845 linkend="introspection-format"/> for details on this
2851 <sect1 id="introspection-format">
2852 <title>Introspection Data Format</title>
2854 As described in <xref linkend="standard-interfaces-introspectable"/>,
2855 objects may be introspected at runtime, returning an XML string
2856 that describes the object. The same XML format may be used in
2857 other contexts as well, for example as an "IDL" for generating
2858 static language bindings.
2861 Here is an example of introspection data:
2863 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2864 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2865 <node name="/org/freedesktop/sample_object">
2866 <interface name="org.freedesktop.SampleInterface">
2867 <method name="Frobate">
2868 <arg name="foo" type="i" direction="in"/>
2869 <arg name="bar" type="s" direction="out"/>
2870 <arg name="baz" type="a{us}" direction="out"/>
2871 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2873 <method name="Bazify">
2874 <arg name="bar" type="(iiu)" direction="in"/>
2875 <arg name="bar" type="v" direction="out"/>
2877 <method name="Mogrify">
2878 <arg name="bar" type="(iiav)" direction="in"/>
2880 <signal name="Changed">
2881 <arg name="new_value" type="b"/>
2883 <property name="Bar" type="y" access="readwrite"/>
2885 <node name="child_of_sample_object"/>
2886 <node name="another_child_of_sample_object"/>
2891 A more formal DTD and spec needs writing, but here are some quick notes.
2895 Only the root <node> element can omit the node name, as it's
2896 known to be the object that was introspected. If the root
2897 <node> does have a name attribute, it must be an absolute
2898 object path. If child <node> have object paths, they must be
2904 If a child <node> has any sub-elements, then they
2905 must represent a complete introspection of the child.
2906 If a child <node> is empty, then it may or may
2907 not have sub-elements; the child must be introspected
2908 in order to find out. The intent is that if an object
2909 knows that its children are "fast" to introspect
2910 it can go ahead and return their information, but
2911 otherwise it can omit it.
2916 The direction element on <arg> may be omitted,
2917 in which case it defaults to "in" for method calls
2918 and "out" for signals. Signals only allow "out"
2919 so while direction may be specified, it's pointless.
2924 The possible directions are "in" and "out",
2925 unlike CORBA there is no "inout"
2930 The possible property access flags are
2931 "readwrite", "read", and "write"
2936 Multiple interfaces can of course be listed for
2942 The "name" attribute on arguments is optional.
2948 Method, interface, property, and signal elements may have
2949 "annotations", which are generic key/value pairs of metadata.
2950 They are similar conceptually to Java's annotations and C# attributes.
2951 Well-known annotations:
2958 <entry>Values (separated by ,)</entry>
2959 <entry>Description</entry>
2964 <entry>org.freedesktop.DBus.Deprecated</entry>
2965 <entry>true,false</entry>
2966 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2969 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2970 <entry>(string)</entry>
2971 <entry>The C symbol; may be used for methods and interfaces</entry>
2974 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2975 <entry>true,false</entry>
2976 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2979 <entry>org.freedesktop.DBus.Property.EmitsChangedSignal</entry>
2980 <entry>true,invalidates,false</entry>
2983 If set to <literal>false</literal>, the
2984 <literal>org.freedesktop.DBus.Properties.PropertiesChanged</literal>
2986 linkend="standard-interfaces-properties"/> is not
2987 guaranteed to be emitted if the property changes.
2990 If set to <literal>invalidates</literal> the signal
2991 is emitted but the value is not included in the
2995 If set to <literal>true</literal> the signal is
2996 emitted with the value included.
2999 The value for the annotation defaults to
3000 <literal>true</literal> if the enclosing interface
3001 element does not specify the annotation. Otherwise it
3002 defaults to the value specified in the enclosing
3011 <sect1 id="message-bus">
3012 <title>Message Bus Specification</title>
3013 <sect2 id="message-bus-overview">
3014 <title>Message Bus Overview</title>
3016 The message bus accepts connections from one or more applications.
3017 Once connected, applications can exchange messages with other
3018 applications that are also connected to the bus.
3021 In order to route messages among connections, the message bus keeps a
3022 mapping from names to connections. Each connection has one
3023 unique-for-the-lifetime-of-the-bus name automatically assigned.
3024 Applications may request additional names for a connection. Additional
3025 names are usually "well-known names" such as
3026 "org.freedesktop.TextEditor". When a name is bound to a connection,
3027 that connection is said to <firstterm>own</firstterm> the name.
3030 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
3031 This name routes messages to the bus, allowing applications to make
3032 administrative requests. For example, applications can ask the bus
3033 to assign a name to a connection.
3036 Each name may have <firstterm>queued owners</firstterm>. When an
3037 application requests a name for a connection and the name is already in
3038 use, the bus will optionally add the connection to a queue waiting for
3039 the name. If the current owner of the name disconnects or releases
3040 the name, the next connection in the queue will become the new owner.
3044 This feature causes the right thing to happen if you start two text
3045 editors for example; the first one may request "org.freedesktop.TextEditor",
3046 and the second will be queued as a possible owner of that name. When
3047 the first exits, the second will take over.
3051 Messages may have a <literal>DESTINATION</literal> field (see <xref
3052 linkend="message-protocol-header-fields"/>). If the
3053 <literal>DESTINATION</literal> field is present, it specifies a message
3054 recipient by name. Method calls and replies normally specify this field.
3055 The message bus must send messages (of any type) with the
3056 <literal>DESTINATION</literal> field set to the specified recipient,
3057 regardless of whether the recipient has set up a match rule matching
3062 Signals normally do not specify a destination; they are sent to all
3063 applications with <firstterm>message matching rules</firstterm> that
3068 When the message bus receives a method call, if the
3069 <literal>DESTINATION</literal> field is absent, the call is taken to be
3070 a standard one-to-one message and interpreted by the message bus
3071 itself. For example, sending an
3072 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
3073 <literal>DESTINATION</literal> will cause the message bus itself to
3074 reply to the ping immediately; the message bus will not make this
3075 message visible to other applications.
3079 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
3080 the ping message were sent with a <literal>DESTINATION</literal> name of
3081 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
3082 forwarded, and the Yoyodyne Corporation screensaver application would be
3083 expected to reply to the ping.
3087 <sect2 id="message-bus-names">
3088 <title>Message Bus Names</title>
3090 Each connection has at least one name, assigned at connection time and
3091 returned in response to the
3092 <literal>org.freedesktop.DBus.Hello</literal> method call. This
3093 automatically-assigned name is called the connection's <firstterm>unique
3094 name</firstterm>. Unique names are never reused for two different
3095 connections to the same bus.
3098 Ownership of a unique name is a prerequisite for interaction with
3099 the message bus. It logically follows that the unique name is always
3100 the first name that an application comes to own, and the last
3101 one that it loses ownership of.
3104 Unique connection names must begin with the character ':' (ASCII colon
3105 character); bus names that are not unique names must not begin
3106 with this character. (The bus must reject any attempt by an application
3107 to manually request a name beginning with ':'.) This restriction
3108 categorically prevents "spoofing"; messages sent to a unique name
3109 will always go to the expected connection.
3112 When a connection is closed, all the names that it owns are deleted (or
3113 transferred to the next connection in the queue if any).
3116 A connection can request additional names to be associated with it using
3117 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
3118 linkend="message-protocol-names-bus"/> describes the format of a valid
3119 name. These names can be released again using the
3120 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
3123 <sect3 id="bus-messages-request-name">
3124 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
3128 UINT32 RequestName (in STRING name, in UINT32 flags)
3135 <entry>Argument</entry>
3137 <entry>Description</entry>
3143 <entry>STRING</entry>
3144 <entry>Name to request</entry>
3148 <entry>UINT32</entry>
3149 <entry>Flags</entry>
3159 <entry>Argument</entry>
3161 <entry>Description</entry>
3167 <entry>UINT32</entry>
3168 <entry>Return value</entry>
3175 This method call should be sent to
3176 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3177 assign the given name to the method caller. Each name maintains a
3178 queue of possible owners, where the head of the queue is the primary
3179 or current owner of the name. Each potential owner in the queue
3180 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
3181 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
3182 call. When RequestName is invoked the following occurs:
3186 If the method caller is currently the primary owner of the name,
3187 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
3188 values are updated with the values from the new RequestName call,
3189 and nothing further happens.
3195 If the current primary owner (head of the queue) has
3196 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
3197 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
3198 the caller of RequestName replaces the current primary owner at
3199 the head of the queue and the current primary owner moves to the
3200 second position in the queue. If the caller of RequestName was
3201 in the queue previously its flags are updated with the values from
3202 the new RequestName in addition to moving it to the head of the queue.
3208 If replacement is not possible, and the method caller is
3209 currently in the queue but not the primary owner, its flags are
3210 updated with the values from the new RequestName call.
3216 If replacement is not possible, and the method caller is
3217 currently not in the queue, the method caller is appended to the
3224 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
3225 set and is not the primary owner, it is removed from the
3226 queue. This can apply to the previous primary owner (if it
3227 was replaced) or the method caller (if it updated the
3228 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
3229 queue, or if it was just added to the queue with that flag set).
3235 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
3236 queue," even if another application already in the queue had specified
3237 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
3238 that does not allow replacement goes away, and the next primary owner
3239 does allow replacement. In this case, queued items that specified
3240 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
3241 automatically replace the new primary owner. In other words,
3242 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
3243 time RequestName is called. This is deliberate to avoid an infinite loop
3244 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3245 and DBUS_NAME_FLAG_REPLACE_EXISTING.
3248 The flags argument contains any of the following values logically ORed
3255 <entry>Conventional Name</entry>
3256 <entry>Value</entry>
3257 <entry>Description</entry>
3262 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
3266 If an application A specifies this flag and succeeds in
3267 becoming the owner of the name, and another application B
3268 later calls RequestName with the
3269 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
3270 will lose ownership and receive a
3271 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
3272 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
3273 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
3274 is not specified by application B, then application B will not replace
3275 application A as the owner.
3280 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
3284 Try to replace the current owner if there is one. If this
3285 flag is not set the application will only become the owner of
3286 the name if there is no current owner. If this flag is set,
3287 the application will replace the current owner if
3288 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
3293 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3297 Without this flag, if an application requests a name that is
3298 already owned, the application will be placed in a queue to
3299 own the name when the current owner gives it up. If this
3300 flag is given, the application will not be placed in the
3301 queue, the request for the name will simply fail. This flag
3302 also affects behavior when an application is replaced as
3303 name owner; by default the application moves back into the
3304 waiting queue, unless this flag was provided when the application
3305 became the name owner.
3313 The return code can be one of the following values:
3319 <entry>Conventional Name</entry>
3320 <entry>Value</entry>
3321 <entry>Description</entry>
3326 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3327 <entry>1</entry> <entry>The caller is now the primary owner of
3328 the name, replacing any previous owner. Either the name had no
3329 owner before, or the caller specified
3330 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3331 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3334 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3337 <entry>The name already had an owner,
3338 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3339 the current owner did not specify
3340 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3341 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3345 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3346 <entry>The name already has an owner,
3347 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3348 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3349 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3350 specified by the requesting application.</entry>
3353 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3355 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3363 <sect3 id="bus-messages-release-name">
3364 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3368 UINT32 ReleaseName (in STRING name)
3375 <entry>Argument</entry>
3377 <entry>Description</entry>
3383 <entry>STRING</entry>
3384 <entry>Name to release</entry>
3394 <entry>Argument</entry>
3396 <entry>Description</entry>
3402 <entry>UINT32</entry>
3403 <entry>Return value</entry>
3410 This method call should be sent to
3411 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3412 release the method caller's claim to the given name. If the caller is
3413 the primary owner, a new primary owner will be selected from the
3414 queue if any other owners are waiting. If the caller is waiting in
3415 the queue for the name, the caller will removed from the queue and
3416 will not be made an owner of the name if it later becomes available.
3417 If there are no other owners in the queue for the name, it will be
3418 removed from the bus entirely.
3420 The return code can be one of the following values:
3426 <entry>Conventional Name</entry>
3427 <entry>Value</entry>
3428 <entry>Description</entry>
3433 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3434 <entry>1</entry> <entry>The caller has released his claim on
3435 the given name. Either the caller was the primary owner of
3436 the name, and the name is now unused or taken by somebody
3437 waiting in the queue for the name, or the caller was waiting
3438 in the queue for the name and has now been removed from the
3442 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3444 <entry>The given name does not exist on this bus.</entry>
3447 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3449 <entry>The caller was not the primary owner of this name,
3450 and was also not waiting in the queue to own this name.</entry>
3458 <sect3 id="bus-messages-list-queued-owners">
3459 <title><literal>org.freedesktop.DBus.ListQueuedOwners</literal></title>
3463 ARRAY of STRING ListQueuedOwners (in STRING name)
3470 <entry>Argument</entry>
3472 <entry>Description</entry>
3478 <entry>STRING</entry>
3479 <entry>The well-known bus name to query, such as
3480 <literal>com.example.cappuccino</literal></entry>
3490 <entry>Argument</entry>
3492 <entry>Description</entry>
3498 <entry>ARRAY of STRING</entry>
3499 <entry>The unique bus names of connections currently queued
3500 for the name</entry>
3507 This method call should be sent to
3508 <literal>org.freedesktop.DBus</literal> and lists the connections
3509 currently queued for a bus name (see
3510 <xref linkend="term-queued-owner"/>).
3515 <sect2 id="message-bus-routing">
3516 <title>Message Bus Message Routing</title>
3520 <sect3 id="message-bus-routing-match-rules">
3521 <title>Match Rules</title>
3523 An important part of the message bus routing protocol is match
3524 rules. Match rules describe what messages can be sent to a client
3525 based on the contents of the message. When a message is routed
3526 through the bus it is compared to clients' match rules. If any
3527 of the rules match, the message is dispatched to the client.
3528 If none of the rules match the message never leaves the bus. This
3529 is an effective way to control traffic over the bus and to make sure
3530 only relevant message need to be processed by the client.
3533 Match rules are added using the AddMatch bus method
3534 (see <xref linkend="bus-messages-add-match"/>). Rules are
3535 specified as a string of comma separated key/value pairs.
3536 Excluding a key from the rule indicates a wildcard match.
3537 For instance excluding the the member from a match rule but
3538 adding a sender would let all messages from that sender through.
3539 An example of a complete rule would be
3540 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3543 The following table describes the keys that can be used to create
3545 The following table summarizes the D-Bus types.
3551 <entry>Possible Values</entry>
3552 <entry>Description</entry>
3557 <entry><literal>type</literal></entry>
3558 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3559 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3562 <entry><literal>sender</literal></entry>
3563 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3564 and <xref linkend="term-unique-name"/> respectively)
3566 <entry>Match messages sent by a particular sender. An example of a sender match
3567 is sender='org.freedesktop.Hal'</entry>
3570 <entry><literal>interface</literal></entry>
3571 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3572 <entry>Match messages sent over or to a particular interface. An example of an
3573 interface match is interface='org.freedesktop.Hal.Manager'.
3574 If a message omits the interface header, it must not match any rule
3575 that specifies this key.</entry>
3578 <entry><literal>member</literal></entry>
3579 <entry>Any valid method or signal name</entry>
3580 <entry>Matches messages which have the give method or signal name. An example of
3581 a member match is member='NameOwnerChanged'</entry>
3584 <entry><literal>path</literal></entry>
3585 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3586 <entry>Matches messages which are sent from or to the given object. An example of a
3587 path match is path='/org/freedesktop/Hal/Manager'</entry>
3590 <entry><literal>destination</literal></entry>
3591 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3592 <entry>Matches messages which are being sent to the given unique name. An
3593 example of a destination match is destination=':1.0'</entry>
3596 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3597 <entry>Any string</entry>
3598 <entry>Arg matches are special and are used for further restricting the
3599 match based on the arguments in the body of a message. As of this time
3600 only string arguments can be matched. An example of an argument match
3601 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3605 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
3606 <entry>Any string</entry>
3608 <para>Argument path matches provide a specialised form of wildcard
3609 matching for path-like namespaces. As with normal argument matches,
3610 if the argument is exactly equal to the string given in the match
3611 rule then the rule is satisfied. Additionally, there is also a
3612 match when either the string given in the match rule or the
3613 appropriate message argument ends with '/' and is a prefix of the
3614 other. An example argument path match is arg0path='/aa/bb/'. This
3615 would match messages with first arguments of '/', '/aa/',
3616 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
3617 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</para>
3619 <para>This is intended for monitoring “directories” in file system-like
3620 hierarchies, as used in the <citetitle>dconf</citetitle> configuration
3621 system. An application interested in all nodes in a particular hierarchy would
3622 monitor <literal>arg0path='/ca/example/foo/'</literal>. Then the service could
3623 emit a signal with zeroth argument <literal>"/ca/example/foo/bar"</literal> to
3624 represent a modification to the “bar” property, or a signal with zeroth
3625 argument <literal>"/ca/example/"</literal> to represent atomic modification of
3626 many properties within that directory, and the interested application would be
3627 notified in both cases.</para>
3636 <sect2 id="message-bus-starting-services">
3637 <title>Message Bus Starting Services</title>
3639 The message bus can start applications on behalf of other applications.
3640 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3641 An application that can be started in this way is called a
3642 <firstterm>service</firstterm>.
3645 With D-Bus, starting a service is normally done by name. That is,
3646 applications ask the message bus to start some program that will own a
3647 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3648 This implies a contract documented along with the name
3649 <literal>org.freedesktop.TextEditor</literal> for which objects
3650 the owner of that name will provide, and what interfaces those
3654 To find an executable corresponding to a particular name, the bus daemon
3655 looks for <firstterm>service description files</firstterm>. Service
3656 description files define a mapping from names to executables. Different
3657 kinds of message bus will look for these files in different places, see
3658 <xref linkend="message-bus-types"/>.
3661 [FIXME the file format should be much better specified than "similar to
3662 .desktop entries" esp. since desktop entries are already
3663 badly-specified. ;-)] Service description files have the ".service" file
3664 extension. The message bus will only load service description files
3665 ending with .service; all other files will be ignored. The file format
3666 is similar to that of <ulink
3667 url="http://standards.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html">desktop
3668 entries</ulink>. All service description files must be in UTF-8
3669 encoding. To ensure that there will be no name collisions, service files
3670 must be namespaced using the same mechanism as messages and service
3674 <title>Example service description file</title>
3676 # Sample service description file
3678 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3679 Exec=/usr/libexec/gconfd-2
3684 When an application asks to start a service by name, the bus daemon tries to
3685 find a service that will own that name. It then tries to spawn the
3686 executable associated with it. If this fails, it will report an
3687 error. [FIXME what happens if two .service files offer the same service;
3688 what kind of error is reported, should we have a way for the client to
3692 The executable launched will have the environment variable
3693 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3694 message bus so it can connect and request the appropriate names.
3697 The executable being launched may want to know whether the message bus
3698 starting it is one of the well-known message buses (see <xref
3699 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3700 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3701 of the well-known buses. The currently-defined values for this variable
3702 are <literal>system</literal> for the systemwide message bus,
3703 and <literal>session</literal> for the per-login-session message
3704 bus. The new executable must still connect to the address given
3705 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3706 resulting connection is to the well-known bus.
3709 [FIXME there should be a timeout somewhere, either specified
3710 in the .service file, by the client, or just a global value
3711 and if the client being activated fails to connect within that
3712 timeout, an error should be sent back.]
3715 <sect3 id="message-bus-starting-services-scope">
3716 <title>Message Bus Service Scope</title>
3718 The "scope" of a service is its "per-", such as per-session,
3719 per-machine, per-home-directory, or per-display. The reference
3720 implementation doesn't yet support starting services in a different
3721 scope from the message bus itself. So e.g. if you start a service
3722 on the session bus its scope is per-session.
3725 We could add an optional scope to a bus name. For example, for
3726 per-(display,session pair), we could have a unique ID for each display
3727 generated automatically at login and set on screen 0 by executing a
3728 special "set display ID" binary. The ID would be stored in a
3729 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3730 random bytes. This ID would then be used to scope names.
3731 Starting/locating a service could be done by ID-name pair rather than
3735 Contrast this with a per-display scope. To achieve that, we would
3736 want a single bus spanning all sessions using a given display.
3737 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3738 property on screen 0 of the display, pointing to this bus.
3743 <sect2 id="message-bus-types">
3744 <title>Well-known Message Bus Instances</title>
3746 Two standard message bus instances are defined here, along with how
3747 to locate them and where their service files live.
3749 <sect3 id="message-bus-types-login">
3750 <title>Login session message bus</title>
3752 Each time a user logs in, a <firstterm>login session message
3753 bus</firstterm> may be started. All applications in the user's login
3754 session may interact with one another using this message bus.
3757 The address of the login session message bus is given
3758 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3759 variable. If that variable is not set, applications may
3760 also try to read the address from the X Window System root
3761 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3762 The root window property must have type <literal>STRING</literal>.
3763 The environment variable should have precedence over the
3764 root window property.
3766 <para>The address of the login session message bus is given in the
3767 <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment variable. If
3768 DBUS_SESSION_BUS_ADDRESS is not set, or if it's set to the string
3769 "autolaunch:", the system should use platform-specific methods of
3770 locating a running D-Bus session server, or starting one if a running
3771 instance cannot be found. Note that this mechanism is not recommended
3772 for attempting to determine if a daemon is running. It is inherently
3773 racy to attempt to make this determination, since the bus daemon may
3774 be started just before or just after the determination is made.
3775 Therefore, it is recommended that applications do not try to make this
3776 determination for their functionality purposes, and instead they
3777 should attempt to start the server.</para>
3779 <sect4 id="message-bus-types-login-x-windows">
3780 <title>X Windowing System</title>
3782 For the X Windowing System, the application must locate the
3783 window owner of the selection represented by the atom formed by
3787 <para>the literal string "_DBUS_SESSION_BUS_SELECTION_"</para>
3791 <para>the current user's username</para>
3795 <para>the literal character '_' (underscore)</para>
3799 <para>the machine's ID</para>
3805 The following properties are defined for the window that owns
3807 <informaltable frame="all">
3816 <para>meaning</para>
3822 <para>_DBUS_SESSION_BUS_ADDRESS</para>
3826 <para>the actual address of the server socket</para>
3832 <para>_DBUS_SESSION_BUS_PID</para>
3836 <para>the PID of the server process</para>
3845 At least the _DBUS_SESSION_BUS_ADDRESS property MUST be
3846 present in this window.
3850 If the X selection cannot be located or if reading the
3851 properties from the window fails, the implementation MUST conclude
3852 that there is no D-Bus server running and proceed to start a new
3853 server. (See below on concurrency issues)
3857 Failure to connect to the D-Bus server address thus obtained
3858 MUST be treated as a fatal connection error and should be reported
3863 As an alternative, an implementation MAY find the information
3864 in the following file located in the current user's home directory,
3865 in subdirectory .dbus/session-bus/:
3868 <para>the machine's ID</para>
3872 <para>the literal character '-' (dash)</para>
3876 <para>the X display without the screen number, with the
3877 following prefixes removed, if present: ":", "localhost:"
3878 ."localhost.localdomain:". That is, a display of
3879 "localhost:10.0" produces just the number "10"</para>
3885 The contents of this file NAME=value assignment pairs and
3886 lines starting with # are comments (no comments are allowed
3887 otherwise). The following variable names are defined:
3894 <para>Variable</para>
3898 <para>meaning</para>
3904 <para>DBUS_SESSION_BUS_ADDRESS</para>
3908 <para>the actual address of the server socket</para>
3914 <para>DBUS_SESSION_BUS_PID</para>
3918 <para>the PID of the server process</para>
3924 <para>DBUS_SESSION_BUS_WINDOWID</para>
3928 <para>the window ID</para>
3937 At least the DBUS_SESSION_BUS_ADDRESS variable MUST be present
3942 Failure to open this file MUST be interpreted as absence of a
3943 running server. Therefore, the implementation MUST proceed to
3944 attempting to launch a new bus server if the file cannot be
3949 However, success in opening this file MUST NOT lead to the
3950 conclusion that the server is running. Thus, a failure to connect to
3951 the bus address obtained by the alternative method MUST NOT be
3952 considered a fatal error. If the connection cannot be established,
3953 the implementation MUST proceed to check the X selection settings or
3954 to start the server on its own.
3958 If the implementation concludes that the D-Bus server is not
3959 running it MUST attempt to start a new server and it MUST also
3960 ensure that the daemon started as an effect of the "autolaunch"
3961 mechanism provides the lookup mechanisms described above, so
3962 subsequent calls can locate the newly started server. The
3963 implementation MUST also ensure that if two or more concurrent
3964 initiations happen, only one server remains running and all other
3965 initiations are able to obtain the address of this server and
3966 connect to it. In other words, the implementation MUST ensure that
3967 the X selection is not present when it attempts to set it, without
3968 allowing another process to set the selection between the
3969 verification and the setting (e.g., by using XGrabServer /
3976 [FIXME specify location of .service files, probably using
3977 DESKTOP_DIRS etc. from basedir specification, though login session
3978 bus is not really desktop-specific]
3982 <sect3 id="message-bus-types-system">
3983 <title>System message bus</title>
3985 A computer may have a <firstterm>system message bus</firstterm>,
3986 accessible to all applications on the system. This message bus may be
3987 used to broadcast system events, such as adding new hardware devices,
3988 changes in the printer queue, and so forth.
3991 The address of the system message bus is given
3992 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3993 variable. If that variable is not set, applications should try
3994 to connect to the well-known address
3995 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3998 The D-Bus reference implementation actually honors the
3999 <literal>$(localstatedir)</literal> configure option
4000 for this address, on both client and server side.
4005 [FIXME specify location of system bus .service files]
4010 <sect2 id="message-bus-messages">
4011 <title>Message Bus Messages</title>
4013 The special message bus name <literal>org.freedesktop.DBus</literal>
4014 responds to a number of additional messages.
4017 <sect3 id="bus-messages-hello">
4018 <title><literal>org.freedesktop.DBus.Hello</literal></title>
4029 <entry>Argument</entry>
4031 <entry>Description</entry>
4037 <entry>STRING</entry>
4038 <entry>Unique name assigned to the connection</entry>
4045 Before an application is able to send messages to other applications
4046 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
4047 to the message bus to obtain a unique name. If an application without
4048 a unique name tries to send a message to another application, or a
4049 message to the message bus itself that isn't the
4050 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
4051 disconnected from the bus.
4054 There is no corresponding "disconnect" request; if a client wishes to
4055 disconnect from the bus, it simply closes the socket (or other
4056 communication channel).
4059 <sect3 id="bus-messages-list-names">
4060 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
4064 ARRAY of STRING ListNames ()
4071 <entry>Argument</entry>
4073 <entry>Description</entry>
4079 <entry>ARRAY of STRING</entry>
4080 <entry>Array of strings where each string is a bus name</entry>
4087 Returns a list of all currently-owned names on the bus.
4090 <sect3 id="bus-messages-list-activatable-names">
4091 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
4095 ARRAY of STRING ListActivatableNames ()
4102 <entry>Argument</entry>
4104 <entry>Description</entry>
4110 <entry>ARRAY of STRING</entry>
4111 <entry>Array of strings where each string is a bus name</entry>
4118 Returns a list of all names that can be activated on the bus.
4121 <sect3 id="bus-messages-name-exists">
4122 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
4126 BOOLEAN NameHasOwner (in STRING name)
4133 <entry>Argument</entry>
4135 <entry>Description</entry>
4141 <entry>STRING</entry>
4142 <entry>Name to check</entry>
4152 <entry>Argument</entry>
4154 <entry>Description</entry>
4160 <entry>BOOLEAN</entry>
4161 <entry>Return value, true if the name exists</entry>
4168 Checks if the specified name exists (currently has an owner).
4172 <sect3 id="bus-messages-name-owner-changed">
4173 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
4177 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
4184 <entry>Argument</entry>
4186 <entry>Description</entry>
4192 <entry>STRING</entry>
4193 <entry>Name with a new owner</entry>
4197 <entry>STRING</entry>
4198 <entry>Old owner or empty string if none</entry>
4202 <entry>STRING</entry>
4203 <entry>New owner or empty string if none</entry>
4210 This signal indicates that the owner of a name has changed.
4211 It's also the signal to use to detect the appearance of
4212 new names on the bus.
4215 <sect3 id="bus-messages-name-lost">
4216 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
4220 NameLost (STRING name)
4227 <entry>Argument</entry>
4229 <entry>Description</entry>
4235 <entry>STRING</entry>
4236 <entry>Name which was lost</entry>
4243 This signal is sent to a specific application when it loses
4244 ownership of a name.
4248 <sect3 id="bus-messages-name-acquired">
4249 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
4253 NameAcquired (STRING name)
4260 <entry>Argument</entry>
4262 <entry>Description</entry>
4268 <entry>STRING</entry>
4269 <entry>Name which was acquired</entry>
4276 This signal is sent to a specific application when it gains
4277 ownership of a name.
4281 <sect3 id="bus-messages-start-service-by-name">
4282 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
4286 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
4293 <entry>Argument</entry>
4295 <entry>Description</entry>
4301 <entry>STRING</entry>
4302 <entry>Name of the service to start</entry>
4306 <entry>UINT32</entry>
4307 <entry>Flags (currently not used)</entry>
4317 <entry>Argument</entry>
4319 <entry>Description</entry>
4325 <entry>UINT32</entry>
4326 <entry>Return value</entry>
4331 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
4335 The return value can be one of the following values:
4340 <entry>Identifier</entry>
4341 <entry>Value</entry>
4342 <entry>Description</entry>
4347 <entry>DBUS_START_REPLY_SUCCESS</entry>
4349 <entry>The service was successfully started.</entry>
4352 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
4354 <entry>A connection already owns the given name.</entry>
4363 <sect3 id="bus-messages-update-activation-environment">
4364 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
4368 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
4375 <entry>Argument</entry>
4377 <entry>Description</entry>
4383 <entry>ARRAY of DICT<STRING,STRING></entry>
4384 <entry>Environment to add or update</entry>
4389 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
4392 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
4395 Note, both the environment variable names and values must be valid UTF-8. There's no way to update the activation environment with data that is invalid UTF-8.
4400 <sect3 id="bus-messages-get-name-owner">
4401 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
4405 STRING GetNameOwner (in STRING name)
4412 <entry>Argument</entry>
4414 <entry>Description</entry>
4420 <entry>STRING</entry>
4421 <entry>Name to get the owner of</entry>
4431 <entry>Argument</entry>
4433 <entry>Description</entry>
4439 <entry>STRING</entry>
4440 <entry>Return value, a unique connection name</entry>
4445 Returns the unique connection name of the primary owner of the name
4446 given. If the requested name doesn't have an owner, returns a
4447 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
4451 <sect3 id="bus-messages-get-connection-unix-user">
4452 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
4456 UINT32 GetConnectionUnixUser (in STRING bus_name)
4463 <entry>Argument</entry>
4465 <entry>Description</entry>
4471 <entry>STRING</entry>
4472 <entry>Unique or well-known bus name of the connection to
4473 query, such as <literal>:12.34</literal> or
4474 <literal>com.example.tea</literal></entry>
4484 <entry>Argument</entry>
4486 <entry>Description</entry>
4492 <entry>UINT32</entry>
4493 <entry>Unix user ID</entry>
4498 Returns the Unix user ID of the process connected to the server. If
4499 unable to determine it (for instance, because the process is not on the
4500 same machine as the bus daemon), an error is returned.
4504 <sect3 id="bus-messages-get-connection-unix-process-id">
4505 <title><literal>org.freedesktop.DBus.GetConnectionUnixProcessID</literal></title>
4509 UINT32 GetConnectionUnixProcessID (in STRING bus_name)
4516 <entry>Argument</entry>
4518 <entry>Description</entry>
4524 <entry>STRING</entry>
4525 <entry>Unique or well-known bus name of the connection to
4526 query, such as <literal>:12.34</literal> or
4527 <literal>com.example.tea</literal></entry>
4537 <entry>Argument</entry>
4539 <entry>Description</entry>
4545 <entry>UINT32</entry>
4546 <entry>Unix process id</entry>
4551 Returns the Unix process ID of the process connected to the server. If
4552 unable to determine it (for instance, because the process is not on the
4553 same machine as the bus daemon), an error is returned.
4557 <sect3 id="bus-messages-add-match">
4558 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
4562 AddMatch (in STRING rule)
4569 <entry>Argument</entry>
4571 <entry>Description</entry>
4577 <entry>STRING</entry>
4578 <entry>Match rule to add to the connection</entry>
4583 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
4584 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
4588 <sect3 id="bus-messages-remove-match">
4589 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
4593 RemoveMatch (in STRING rule)
4600 <entry>Argument</entry>
4602 <entry>Description</entry>
4608 <entry>STRING</entry>
4609 <entry>Match rule to remove from the connection</entry>
4614 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
4615 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
4620 <sect3 id="bus-messages-get-id">
4621 <title><literal>org.freedesktop.DBus.GetId</literal></title>
4625 GetId (out STRING id)
4632 <entry>Argument</entry>
4634 <entry>Description</entry>
4640 <entry>STRING</entry>
4641 <entry>Unique ID identifying the bus daemon</entry>
4646 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
4647 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
4648 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
4649 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
4650 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
4651 by org.freedesktop.DBus.Peer.GetMachineId().
4652 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
4660 <appendix id="implementation-notes">
4661 <title>Implementation notes</title>
4662 <sect1 id="implementation-notes-subsection">
4670 <glossary><title>Glossary</title>
4672 This glossary defines some of the terms used in this specification.
4675 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
4678 The message bus maintains an association between names and
4679 connections. (Normally, there's one connection per application.) A
4680 bus name is simply an identifier used to locate connections. For
4681 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
4682 name might be used to send a message to a screensaver from Yoyodyne
4683 Corporation. An application is said to <firstterm>own</firstterm> a
4684 name if the message bus has associated the application's connection
4685 with the name. Names may also have <firstterm>queued
4686 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
4687 The bus assigns a unique name to each connection,
4688 see <xref linkend="term-unique-name"/>. Other names
4689 can be thought of as "well-known names" and are
4690 used to find applications that offer specific functionality.
4695 <glossentry id="term-message"><glossterm>Message</glossterm>
4698 A message is the atomic unit of communication via the D-Bus
4699 protocol. It consists of a <firstterm>header</firstterm> and a
4700 <firstterm>body</firstterm>; the body is made up of
4701 <firstterm>arguments</firstterm>.
4706 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
4709 The message bus is a special application that forwards
4710 or routes messages between a group of applications
4711 connected to the message bus. It also manages
4712 <firstterm>names</firstterm> used for routing
4718 <glossentry id="term-name"><glossterm>Name</glossterm>
4721 See <xref linkend="term-bus-name"/>. "Name" may
4722 also be used to refer to some of the other names
4723 in D-Bus, such as interface names.
4728 <glossentry id="namespace"><glossterm>Namespace</glossterm>
4731 Used to prevent collisions when defining new interfaces or bus
4732 names. The convention used is the same one Java uses for defining
4733 classes: a reversed domain name.
4738 <glossentry id="term-object"><glossterm>Object</glossterm>
4741 Each application contains <firstterm>objects</firstterm>, which have
4742 <firstterm>interfaces</firstterm> and
4743 <firstterm>methods</firstterm>. Objects are referred to by a name,
4744 called a <firstterm>path</firstterm>.
4749 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
4752 An application talking directly to another application, without going
4753 through a message bus. One-to-one connections may be "peer to peer" or
4754 "client to server." The D-Bus protocol has no concept of client
4755 vs. server after a connection has authenticated; the flow of messages
4756 is symmetrical (full duplex).
4761 <glossentry id="term-path"><glossterm>Path</glossterm>
4764 Object references (object names) in D-Bus are organized into a
4765 filesystem-style hierarchy, so each object is named by a path. As in
4766 LDAP, there's no difference between "files" and "directories"; a path
4767 can refer to an object, while still having child objects below it.
4772 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
4775 Each bus name has a primary owner; messages sent to the name go to the
4776 primary owner. However, certain names also maintain a queue of
4777 secondary owners "waiting in the wings." If the primary owner releases
4778 the name, then the first secondary owner in the queue automatically
4779 becomes the new owner of the name.
4784 <glossentry id="term-service"><glossterm>Service</glossterm>
4787 A service is an executable that can be launched by the bus daemon.
4788 Services normally guarantee some particular features, for example they
4789 may guarantee that they will request a specific name such as
4790 "org.freedesktop.Screensaver", have a singleton object
4791 "/org/freedesktop/Application", and that object will implement the
4792 interface "org.freedesktop.ScreensaverControl".
4797 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
4800 ".service files" tell the bus about service applications that can be
4801 launched (see <xref linkend="term-service"/>). Most importantly they
4802 provide a mapping from bus names to services that will request those
4803 names when they start up.
4808 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
4811 The special name automatically assigned to each connection by the
4812 message bus. This name will never change owner, and will be unique
4813 (never reused during the lifetime of the message bus).
4814 It will begin with a ':' character.