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2 <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
9 <title>D-Bus Specification</title>
10 <releaseinfo>Version 0.12</releaseinfo>
11 <date>7 November 2006</date>
14 <firstname>Havoc</firstname>
15 <surname>Pennington</surname>
17 <orgname>Red Hat, Inc.</orgname>
19 <email>hp@pobox.com</email>
24 <firstname>Anders</firstname>
25 <surname>Carlsson</surname>
27 <orgname>CodeFactory AB</orgname>
29 <email>andersca@codefactory.se</email>
34 <firstname>Alexander</firstname>
35 <surname>Larsson</surname>
37 <orgname>Red Hat, Inc.</orgname>
39 <email>alexl@redhat.com</email>
46 <sect1 id="introduction">
47 <title>Introduction</title>
49 D-Bus is a system for low-latency, low-overhead, easy to use
50 interprocess communication (IPC). In more detail:
54 D-Bus is <emphasis>low-latency</emphasis> because it is designed
55 to avoid round trips and allow asynchronous operation, much like
61 D-Bus is <emphasis>low-overhead</emphasis> because it uses a
62 binary protocol, and does not have to convert to and from a text
63 format such as XML. Because D-Bus is intended for potentially
64 high-resolution same-machine IPC, not primarily for Internet IPC,
65 this is an interesting optimization.
70 D-Bus is <emphasis>easy to use</emphasis> because it works in terms
71 of <firstterm>messages</firstterm> rather than byte streams, and
72 automatically handles a lot of the hard IPC issues. Also, the D-Bus
73 library is designed to be wrapped in a way that lets developers use
74 their framework's existing object/type system, rather than learning
75 a new one specifically for IPC.
82 The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
83 protocol, specified in <xref linkend="message-protocol"/>. That is, it is
84 a system for one application to talk to a single other
85 application. However, the primary intended application of the protocol is the
86 D-Bus <firstterm>message bus</firstterm>, specified in <xref
87 linkend="message-bus"/>. The message bus is a special application that
88 accepts connections from multiple other applications, and forwards
93 Uses of D-Bus include notification of system changes (notification of when
94 a camera is plugged in to a computer, or a new version of some software
95 has been installed), or desktop interoperability, for example a file
96 monitoring service or a configuration service.
100 D-Bus is designed for two specific use cases:
104 A "system bus" for notifications from the system to user sessions,
105 and to allow the system to request input from user sessions.
110 A "session bus" used to implement desktop environments such as
115 D-Bus is not intended to be a generic IPC system for any possible
116 application, and intentionally omits many features found in other
117 IPC systems for this reason.
121 At the same time, the bus daemons offer a number of features not found in
122 other IPC systems, such as single-owner "bus names" (similar to X
123 selections), on-demand startup of services, and security policies.
124 In many ways, these features are the primary motivation for developing
125 D-Bus; other systems would have sufficed if IPC were the only goal.
129 D-Bus may turn out to be useful in unanticipated applications, but future
130 versions of this spec and the reference implementation probably will not
131 incorporate features that interfere with the core use cases.
135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
137 document are to be interpreted as described in RFC 2119. However, the
138 document could use a serious audit to be sure it makes sense to do
139 so. Also, they are not capitalized.
142 <sect2 id="stability">
143 <title>Protocol and Specification Stability</title>
145 The D-Bus protocol is frozen (only compatible extensions are allowed) as
146 of November 8, 2006. However, this specification could still use a fair
147 bit of work to make interoperable reimplementation possible without
148 reference to the D-Bus reference implementation. Thus, this
149 specification is not marked 1.0. To mark it 1.0, we'd like to see
150 someone invest significant effort in clarifying the specification
151 language, and growing the specification to cover more aspects of the
152 reference implementation's behavior.
155 Until this work is complete, any attempt to reimplement D-Bus will
156 probably require looking at the reference implementation and/or asking
157 questions on the D-Bus mailing list about intended behavior.
158 Questions on the list are very welcome.
161 Nonetheless, this document should be a useful starting point and is
162 to our knowledge accurate, though incomplete.
168 <sect1 id="message-protocol">
169 <title>Message Protocol</title>
172 A <firstterm>message</firstterm> consists of a
173 <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
174 think of a message as a package, the header is the address, and the body
175 contains the package contents. The message delivery system uses the header
176 information to figure out where to send the message and how to interpret
177 it; the recipient interprets the body of the message.
181 The body of the message is made up of zero or more
182 <firstterm>arguments</firstterm>, which are typed values, such as an
183 integer or a byte array.
187 Both header and body use the same type system and format for
188 serializing data. Each type of value has a wire format.
189 Converting a value from some other representation into the wire
190 format is called <firstterm>marshaling</firstterm> and converting
191 it back from the wire format is <firstterm>unmarshaling</firstterm>.
194 <sect2 id="message-protocol-signatures">
195 <title>Type Signatures</title>
198 The D-Bus protocol does not include type tags in the marshaled data; a
199 block of marshaled values must have a known <firstterm>type
200 signature</firstterm>. The type signature is made up of <firstterm>type
201 codes</firstterm>. A type code is an ASCII character representing the
202 type of a value. Because ASCII characters are used, the type signature
203 will always form a valid ASCII string. A simple string compare
204 determines whether two type signatures are equivalent.
208 As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
209 the ASCII character 'i'. So the signature for a block of values
210 containing a single <literal>INT32</literal> would be:
214 A block of values containing two <literal>INT32</literal> would have this signature:
221 All <firstterm>basic</firstterm> types work like
222 <literal>INT32</literal> in this example. To marshal and unmarshal
223 basic types, you simply read one value from the data
224 block corresponding to each type code in the signature.
225 In addition to basic types, there are four <firstterm>container</firstterm>
226 types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
227 and <literal>DICT_ENTRY</literal>.
231 <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
232 code does not appear in signatures. Instead, ASCII characters
233 '(' and ')' are used to mark the beginning and end of the struct.
234 So for example, a struct containing two integers would have this
239 Structs can be nested, so for example a struct containing
240 an integer and another struct:
244 The value block storing that struct would contain three integers; the
245 type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
250 The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
251 but is useful in code that implements the protocol. This type code
252 is specified to allow such code to interoperate in non-protocol contexts.
256 Empty structures are not allowed; there must be at least one
257 type code between the parentheses.
261 <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
262 followed by a <firstterm>single complete type</firstterm>. The single
263 complete type following the array is the type of each array element. So
264 the simple example is:
268 which is an array of 32-bit integers. But an array can be of any type,
269 such as this array-of-struct-with-two-int32-fields:
273 Or this array of array of integer:
280 The phrase <firstterm>single complete type</firstterm> deserves some
281 definition. A single complete type is a basic type code, a variant type code,
282 an array with its element type, or a struct with its fields.
283 So the following signatures are not single complete types:
293 And the following signatures contain multiple complete types:
303 Note however that a single complete type may <emphasis>contain</emphasis>
304 multiple other single complete types.
308 <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
309 type <literal>VARIANT</literal> will have the signature of a single complete type as part
310 of the <emphasis>value</emphasis>. This signature will be followed by a
311 marshaled value of that type.
315 A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
316 than parentheses it uses curly braces, and it has more restrictions.
317 The restrictions are: it occurs only as an array element type; it has
318 exactly two single complete types inside the curly braces; the first
319 single complete type (the "key") must be a basic type rather than a
320 container type. Implementations must not accept dict entries outside of
321 arrays, must not accept dict entries with zero, one, or more than two
322 fields, and must not accept dict entries with non-basic-typed keys. A
323 dict entry is always a key-value pair.
327 The first field in the <literal>DICT_ENTRY</literal> is always the key.
328 A message is considered corrupt if the same key occurs twice in the same
329 array of <literal>DICT_ENTRY</literal>. However, for performance reasons
330 implementations are not required to reject dicts with duplicate keys.
334 In most languages, an array of dict entry would be represented as a
335 map, hash table, or dict object.
339 The following table summarizes the D-Bus types.
344 <entry>Conventional Name</entry>
346 <entry>Description</entry>
351 <entry><literal>INVALID</literal></entry>
352 <entry>0 (ASCII NUL)</entry>
353 <entry>Not a valid type code, used to terminate signatures</entry>
355 <entry><literal>BYTE</literal></entry>
356 <entry>121 (ASCII 'y')</entry>
357 <entry>8-bit unsigned integer</entry>
359 <entry><literal>BOOLEAN</literal></entry>
360 <entry>98 (ASCII 'b')</entry>
361 <entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
363 <entry><literal>INT16</literal></entry>
364 <entry>110 (ASCII 'n')</entry>
365 <entry>16-bit signed integer</entry>
367 <entry><literal>UINT16</literal></entry>
368 <entry>113 (ASCII 'q')</entry>
369 <entry>16-bit unsigned integer</entry>
371 <entry><literal>INT32</literal></entry>
372 <entry>105 (ASCII 'i')</entry>
373 <entry>32-bit signed integer</entry>
375 <entry><literal>UINT32</literal></entry>
376 <entry>117 (ASCII 'u')</entry>
377 <entry>32-bit unsigned integer</entry>
379 <entry><literal>INT64</literal></entry>
380 <entry>120 (ASCII 'x')</entry>
381 <entry>64-bit signed integer</entry>
383 <entry><literal>UINT64</literal></entry>
384 <entry>116 (ASCII 't')</entry>
385 <entry>64-bit unsigned integer</entry>
387 <entry><literal>DOUBLE</literal></entry>
388 <entry>100 (ASCII 'd')</entry>
389 <entry>IEEE 754 double</entry>
391 <entry><literal>STRING</literal></entry>
392 <entry>115 (ASCII 's')</entry>
393 <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
395 <entry><literal>OBJECT_PATH</literal></entry>
396 <entry>111 (ASCII 'o')</entry>
397 <entry>Name of an object instance</entry>
399 <entry><literal>SIGNATURE</literal></entry>
400 <entry>103 (ASCII 'g')</entry>
401 <entry>A type signature</entry>
403 <entry><literal>ARRAY</literal></entry>
404 <entry>97 (ASCII 'a')</entry>
407 <entry><literal>STRUCT</literal></entry>
408 <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
409 <entry>Struct</entry>
411 <entry><literal>VARIANT</literal></entry>
412 <entry>118 (ASCII 'v') </entry>
413 <entry>Variant type (the type of the value is part of the value itself)</entry>
415 <entry><literal>DICT_ENTRY</literal></entry>
416 <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
417 <entry>Entry in a dict or map (array of key-value pairs)</entry>
419 <entry><literal>UNIX_FD</literal></entry>
420 <entry>104 (ASCII 'h')</entry>
421 <entry>Unix file descriptor</entry>
430 <sect2 id="message-protocol-marshaling">
431 <title>Marshaling (Wire Format)</title>
434 Given a type signature, a block of bytes can be converted into typed
435 values. This section describes the format of the block of bytes. Byte
436 order and alignment issues are handled uniformly for all D-Bus types.
440 A block of bytes has an associated byte order. The byte order
441 has to be discovered in some way; for D-Bus messages, the
442 byte order is part of the message header as described in
443 <xref linkend="message-protocol-messages"/>. For now, assume
444 that the byte order is known to be either little endian or big
449 Each value in a block of bytes is aligned "naturally," for example
450 4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
451 8-byte boundary. To properly align a value, <firstterm>alignment
452 padding</firstterm> may be necessary. The alignment padding must always
453 be the minimum required padding to properly align the following value;
454 and it must always be made up of nul bytes. The alignment padding must
455 not be left uninitialized (it can't contain garbage), and more padding
456 than required must not be used.
460 Given all this, the types are marshaled on the wire as follows:
465 <entry>Conventional Name</entry>
466 <entry>Encoding</entry>
467 <entry>Alignment</entry>
472 <entry><literal>INVALID</literal></entry>
473 <entry>Not applicable; cannot be marshaled.</entry>
476 <entry><literal>BYTE</literal></entry>
477 <entry>A single 8-bit byte.</entry>
480 <entry><literal>BOOLEAN</literal></entry>
481 <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
484 <entry><literal>INT16</literal></entry>
485 <entry>16-bit signed integer in the message's byte order.</entry>
488 <entry><literal>UINT16</literal></entry>
489 <entry>16-bit unsigned integer in the message's byte order.</entry>
492 <entry><literal>INT32</literal></entry>
493 <entry>32-bit signed integer in the message's byte order.</entry>
496 <entry><literal>UINT32</literal></entry>
497 <entry>32-bit unsigned integer in the message's byte order.</entry>
500 <entry><literal>INT64</literal></entry>
501 <entry>64-bit signed integer in the message's byte order.</entry>
504 <entry><literal>UINT64</literal></entry>
505 <entry>64-bit unsigned integer in the message's byte order.</entry>
508 <entry><literal>DOUBLE</literal></entry>
509 <entry>64-bit IEEE 754 double in the message's byte order.</entry>
512 <entry><literal>STRING</literal></entry>
513 <entry>A <literal>UINT32</literal> indicating the string's
514 length in bytes excluding its terminating nul, followed by
515 non-nul string data of the given length, followed by a terminating nul
522 <entry><literal>OBJECT_PATH</literal></entry>
523 <entry>Exactly the same as <literal>STRING</literal> except the
524 content must be a valid object path (see below).
530 <entry><literal>SIGNATURE</literal></entry>
531 <entry>The same as <literal>STRING</literal> except the length is a single
532 byte (thus signatures have a maximum length of 255)
533 and the content must be a valid signature (see below).
539 <entry><literal>ARRAY</literal></entry>
541 A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
542 alignment padding to the alignment boundary of the array element type,
543 followed by each array element. The array length is from the
544 end of the alignment padding to the end of the last element,
545 i.e. it does not include the padding after the length,
546 or any padding after the last element.
547 Arrays have a maximum length defined to be 2 to the 26th power or
548 67108864. Implementations must not send or accept arrays exceeding this
555 <entry><literal>STRUCT</literal></entry>
557 A struct must start on an 8-byte boundary regardless of the
558 type of the struct fields. The struct value consists of each
559 field marshaled in sequence starting from that 8-byte
566 <entry><literal>VARIANT</literal></entry>
568 A variant type has a marshaled <literal>SIGNATURE</literal>
569 followed by a marshaled value with the type
570 given in the signature.
571 Unlike a message signature, the variant signature
572 can contain only a single complete type.
573 So "i", "ai" or "(ii)" is OK, but "ii" is not.
576 1 (alignment of the signature)
579 <entry><literal>DICT_ENTRY</literal></entry>
587 <entry><literal>UNIX_FD</literal></entry>
588 <entry>32-bit unsigned integer in the message's byte
589 order. The actual file descriptors need to be
590 transferred out-of-band via some platform specific
591 mechanism. On the wire, values of this type store the index to the
592 file descriptor in the array of file descriptors that
593 accompany the message.</entry>
601 <sect3 id="message-protocol-marshaling-object-path">
602 <title>Valid Object Paths</title>
605 An object path is a name used to refer to an object instance.
606 Conceptually, each participant in a D-Bus message exchange may have
607 any number of object instances (think of C++ or Java objects) and each
608 such instance will have a path. Like a filesystem, the object
609 instances in an application form a hierarchical tree.
613 The following rules define a valid object path. Implementations must
614 not send or accept messages with invalid object paths.
618 The path may be of any length.
623 The path must begin with an ASCII '/' (integer 47) character,
624 and must consist of elements separated by slash characters.
629 Each element must only contain the ASCII characters
635 No element may be the empty string.
640 Multiple '/' characters cannot occur in sequence.
645 A trailing '/' character is not allowed unless the
646 path is the root path (a single '/' character).
655 <sect3 id="message-protocol-marshaling-signature">
656 <title>Valid Signatures</title>
658 An implementation must not send or accept invalid signatures.
659 Valid signatures will conform to the following rules:
663 The signature ends with a nul byte.
668 The signature is a list of single complete types.
669 Arrays must have element types, and structs must
670 have both open and close parentheses.
675 Only type codes and open and close parentheses are
676 allowed in the signature. The <literal>STRUCT</literal> type code
677 is not allowed in signatures, because parentheses
683 The maximum depth of container type nesting is 32 array type
684 codes and 32 open parentheses. This implies that the maximum
685 total depth of recursion is 64, for an "array of array of array
686 of ... struct of struct of struct of ..." where there are 32
692 The maximum length of a signature is 255.
697 Signatures must be nul-terminated.
706 <sect2 id="message-protocol-messages">
707 <title>Message Format</title>
710 A message consists of a header and a body. The header is a block of
711 values with a fixed signature and meaning. The body is a separate block
712 of values, with a signature specified in the header.
716 The length of the header must be a multiple of 8, allowing the body to
717 begin on an 8-byte boundary when storing the entire message in a single
718 buffer. If the header does not naturally end on an 8-byte boundary
719 up to 7 bytes of nul-initialized alignment padding must be added.
723 The message body need not end on an 8-byte boundary.
727 The maximum length of a message, including header, header alignment padding,
728 and body is 2 to the 27th power or 134217728. Implementations must not
729 send or accept messages exceeding this size.
733 The signature of the header is:
737 Written out more readably, this is:
739 BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
744 These values have the following meanings:
750 <entry>Description</entry>
755 <entry>1st <literal>BYTE</literal></entry>
756 <entry>Endianness flag; ASCII 'l' for little-endian
757 or ASCII 'B' for big-endian. Both header and body are
758 in this endianness.</entry>
761 <entry>2nd <literal>BYTE</literal></entry>
762 <entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
763 Currently-defined types are described below.
767 <entry>3rd <literal>BYTE</literal></entry>
768 <entry>Bitwise OR of flags. Unknown flags
769 must be ignored. Currently-defined flags are described below.
773 <entry>4th <literal>BYTE</literal></entry>
774 <entry>Major protocol version of the sending application. If
775 the major protocol version of the receiving application does not
776 match, the applications will not be able to communicate and the
777 D-Bus connection must be disconnected. The major protocol
778 version for this version of the specification is 1.
782 <entry>1st <literal>UINT32</literal></entry>
783 <entry>Length in bytes of the message body, starting
784 from the end of the header. The header ends after
785 its alignment padding to an 8-boundary.
789 <entry>2nd <literal>UINT32</literal></entry>
790 <entry>The serial of this message, used as a cookie
791 by the sender to identify the reply corresponding
796 <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
797 <entry>An array of zero or more <firstterm>header
798 fields</firstterm> where the byte is the field code, and the
799 variant is the field value. The message type determines
800 which fields are required.
808 <firstterm>Message types</firstterm> that can appear in the second byte
814 <entry>Conventional name</entry>
815 <entry>Decimal value</entry>
816 <entry>Description</entry>
821 <entry><literal>INVALID</literal></entry>
823 <entry>This is an invalid type.</entry>
826 <entry><literal>METHOD_CALL</literal></entry>
828 <entry>Method call.</entry>
831 <entry><literal>METHOD_RETURN</literal></entry>
833 <entry>Method reply with returned data.</entry>
836 <entry><literal>ERROR</literal></entry>
838 <entry>Error reply. If the first argument exists and is a
839 string, it is an error message.</entry>
842 <entry><literal>SIGNAL</literal></entry>
844 <entry>Signal emission.</entry>
851 Flags that can appear in the third byte of the header:
856 <entry>Conventional name</entry>
857 <entry>Hex value</entry>
858 <entry>Description</entry>
863 <entry><literal>NO_REPLY_EXPECTED</literal></entry>
865 <entry>This message does not expect method return replies or
866 error replies; the reply can be omitted as an
867 optimization. However, it is compliant with this specification
868 to return the reply despite this flag and the only harm
869 from doing so is extra network traffic.
873 <entry><literal>NO_AUTO_START</literal></entry>
875 <entry>The bus must not launch an owner
876 for the destination name in response to this message.
884 <sect3 id="message-protocol-header-fields">
885 <title>Header Fields</title>
888 The array at the end of the header contains <firstterm>header
889 fields</firstterm>, where each field is a 1-byte field code followed
890 by a field value. A header must contain the required header fields for
891 its message type, and zero or more of any optional header
892 fields. Future versions of this protocol specification may add new
893 fields. Implementations must ignore fields they do not
894 understand. Implementations must not invent their own header fields;
895 only changes to this specification may introduce new header fields.
899 Again, if an implementation sees a header field code that it does not
900 expect, it must ignore that field, as it will be part of a new
901 (but compatible) version of this specification. This also applies
902 to known header fields appearing in unexpected messages, for
903 example: if a signal has a reply serial it must be ignored
904 even though it has no meaning as of this version of the spec.
908 However, implementations must not send or accept known header fields
909 with the wrong type stored in the field value. So for example a
910 message with an <literal>INTERFACE</literal> field of type
911 <literal>UINT32</literal> would be considered corrupt.
915 Here are the currently-defined header fields:
920 <entry>Conventional Name</entry>
921 <entry>Decimal Code</entry>
923 <entry>Required In</entry>
924 <entry>Description</entry>
929 <entry><literal>INVALID</literal></entry>
932 <entry>not allowed</entry>
933 <entry>Not a valid field name (error if it appears in a message)</entry>
936 <entry><literal>PATH</literal></entry>
938 <entry><literal>OBJECT_PATH</literal></entry>
939 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
940 <entry>The object to send a call to,
941 or the object a signal is emitted from.
943 <literal>/org/freedesktop/DBus/Local</literal> is reserved;
944 implementations should not send messages with this path,
945 and the reference implementation of the bus daemon will
946 disconnect any application that attempts to do so.
950 <entry><literal>INTERFACE</literal></entry>
952 <entry><literal>STRING</literal></entry>
953 <entry><literal>SIGNAL</literal></entry>
955 The interface to invoke a method call on, or
956 that a signal is emitted from. Optional for
957 method calls, required for signals.
958 The special interface
959 <literal>org.freedesktop.DBus.Local</literal> is reserved;
960 implementations should not send messages with this
961 interface, and the reference implementation of the bus
962 daemon will disconnect any application that attempts to
967 <entry><literal>MEMBER</literal></entry>
969 <entry><literal>STRING</literal></entry>
970 <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
971 <entry>The member, either the method name or signal name.</entry>
974 <entry><literal>ERROR_NAME</literal></entry>
976 <entry><literal>STRING</literal></entry>
977 <entry><literal>ERROR</literal></entry>
978 <entry>The name of the error that occurred, for errors</entry>
981 <entry><literal>REPLY_SERIAL</literal></entry>
983 <entry><literal>UINT32</literal></entry>
984 <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
985 <entry>The serial number of the message this message is a reply
986 to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
989 <entry><literal>DESTINATION</literal></entry>
991 <entry><literal>STRING</literal></entry>
992 <entry>optional</entry>
993 <entry>The name of the connection this message is intended for.
994 Only used in combination with the message bus, see
995 <xref linkend="message-bus"/>.</entry>
998 <entry><literal>SENDER</literal></entry>
1000 <entry><literal>STRING</literal></entry>
1001 <entry>optional</entry>
1002 <entry>Unique name of the sending connection.
1003 The message bus fills in this field so it is reliable; the field is
1004 only meaningful in combination with the message bus.</entry>
1007 <entry><literal>SIGNATURE</literal></entry>
1009 <entry><literal>SIGNATURE</literal></entry>
1010 <entry>optional</entry>
1011 <entry>The signature of the message body.
1012 If omitted, it is assumed to be the
1013 empty signature "" (i.e. the body must be 0-length).</entry>
1016 <entry><literal>UNIX_FDS</literal></entry>
1018 <entry><literal>UINT32</literal></entry>
1019 <entry>optional</entry>
1020 <entry>The number of Unix file descriptors that
1021 accompany the message. If omitted, it is assumed
1022 that no Unix file descriptors accompany the
1023 message. The actual file descriptors need to be
1024 transferred via platform specific mechanism
1025 out-of-band. They must be sent at the same time as
1026 part of the message itself. They may not be sent
1027 before the first byte of the message itself is
1028 transferred or after the last byte of the message
1038 <sect2 id="message-protocol-names">
1039 <title>Valid Names</title>
1041 The various names in D-Bus messages have some restrictions.
1044 There is a <firstterm>maximum name length</firstterm>
1045 of 255 which applies to bus names, interfaces, and members.
1047 <sect3 id="message-protocol-names-interface">
1048 <title>Interface names</title>
1050 Interfaces have names with type <literal>STRING</literal>, meaning that
1051 they must be valid UTF-8. However, there are also some
1052 additional restrictions that apply to interface names
1055 <listitem><para>Interface names are composed of 1 or more elements separated by
1056 a period ('.') character. All elements must contain at least
1060 <listitem><para>Each element must only contain the ASCII characters
1061 "[A-Z][a-z][0-9]_" and must not begin with a digit.
1065 <listitem><para>Interface names must contain at least one '.' (period)
1066 character (and thus at least two elements).
1069 <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
1070 <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
1074 <sect3 id="message-protocol-names-bus">
1075 <title>Bus names</title>
1077 Connections have one or more bus names associated with them.
1078 A connection has exactly one bus name that is a unique connection
1079 name. The unique connection name remains with the connection for
1080 its entire lifetime.
1081 A bus name is of type <literal>STRING</literal>,
1082 meaning that it must be valid UTF-8. However, there are also
1083 some additional restrictions that apply to bus names
1086 <listitem><para>Bus names that start with a colon (':')
1087 character are unique connection names.
1090 <listitem><para>Bus names are composed of 1 or more elements separated by
1091 a period ('.') character. All elements must contain at least
1095 <listitem><para>Each element must only contain the ASCII characters
1096 "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
1097 connection name may begin with a digit, elements in
1098 other bus names must not begin with a digit.
1102 <listitem><para>Bus names must contain at least one '.' (period)
1103 character (and thus at least two elements).
1106 <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
1107 <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
1111 Note that the hyphen ('-') character is allowed in bus names but
1112 not in interface names.
1115 <sect3 id="message-protocol-names-member">
1116 <title>Member names</title>
1118 Member (i.e. method or signal) names:
1120 <listitem><para>Must only contain the ASCII characters
1121 "[A-Z][a-z][0-9]_" and may not begin with a
1122 digit.</para></listitem>
1123 <listitem><para>Must not contain the '.' (period) character.</para></listitem>
1124 <listitem><para>Must not exceed the maximum name length.</para></listitem>
1125 <listitem><para>Must be at least 1 byte in length.</para></listitem>
1129 <sect3 id="message-protocol-names-error">
1130 <title>Error names</title>
1132 Error names have the same restrictions as interface names.
1137 <sect2 id="message-protocol-types">
1138 <title>Message Types</title>
1140 Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
1141 <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
1142 This section describes these conventions.
1144 <sect3 id="message-protocol-types-method">
1145 <title>Method Calls</title>
1147 Some messages invoke an operation on a remote object. These are
1148 called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
1149 messages map naturally to methods on objects in a typical program.
1152 A method call message is required to have a <literal>MEMBER</literal> header field
1153 indicating the name of the method. Optionally, the message has an
1154 <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
1155 absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
1156 a method with the same name, it is undefined which of the two methods
1157 will be invoked. Implementations may also choose to return an error in
1158 this ambiguous case. However, if a method name is unique
1159 implementations must not require an interface field.
1162 Method call messages also include a <literal>PATH</literal> field
1163 indicating the object to invoke the method on. If the call is passing
1164 through a message bus, the message will also have a
1165 <literal>DESTINATION</literal> field giving the name of the connection
1166 to receive the message.
1169 When an application handles a method call message, it is required to
1170 return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
1171 indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
1172 reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
1175 If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
1176 are the return value(s) or "out parameters" of the method call.
1177 If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
1178 and the call fails; no return value will be provided. It makes
1179 no sense to send multiple replies to the same method call.
1182 Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
1183 reply is required, so the caller will know the method
1184 was successfully processed.
1187 The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
1191 If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
1192 then as an optimization the application receiving the method
1193 call may choose to omit the reply message (regardless of
1194 whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
1195 However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
1196 flag and reply anyway.
1199 Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
1200 destination name does not exist then a program to own the destination
1201 name will be started before the message is delivered. The message
1202 will be held until the new program is successfully started or has
1203 failed to start; in case of failure, an error will be returned. This
1204 flag is only relevant in the context of a message bus, it is ignored
1205 during one-to-one communication with no intermediate bus.
1207 <sect4 id="message-protocol-types-method-apis">
1208 <title>Mapping method calls to native APIs</title>
1210 APIs for D-Bus may map method calls to a method call in a specific
1211 programming language, such as C++, or may map a method call written
1212 in an IDL to a D-Bus message.
1215 In APIs of this nature, arguments to a method are often termed "in"
1216 (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
1217 returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
1218 "inout" arguments, which are both sent and received, i.e. the caller
1219 passes in a value which is modified. Mapped to D-Bus, an "inout"
1220 argument is equivalent to an "in" argument, followed by an "out"
1221 argument. You can't pass things "by reference" over the wire, so
1222 "inout" is purely an illusion of the in-process API.
1225 Given a method with zero or one return values, followed by zero or more
1226 arguments, where each argument may be "in", "out", or "inout", the
1227 caller constructs a message by appending each "in" or "inout" argument,
1228 in order. "out" arguments are not represented in the caller's message.
1231 The recipient constructs a reply by appending first the return value
1232 if any, then each "out" or "inout" argument, in order.
1233 "in" arguments are not represented in the reply message.
1236 Error replies are normally mapped to exceptions in languages that have
1240 In converting from native APIs to D-Bus, it is perhaps nice to
1241 map D-Bus naming conventions ("FooBar") to native conventions
1242 such as "fooBar" or "foo_bar" automatically. This is OK
1243 as long as you can say that the native API is one that
1244 was specifically written for D-Bus. It makes the most sense
1245 when writing object implementations that will be exported
1246 over the bus. Object proxies used to invoke remote D-Bus
1247 objects probably need the ability to call any D-Bus method,
1248 and thus a magic name mapping like this could be a problem.
1251 This specification doesn't require anything of native API bindings;
1252 the preceding is only a suggested convention for consistency
1258 <sect3 id="message-protocol-types-signal">
1259 <title>Signal Emission</title>
1261 Unlike method calls, signal emissions have no replies.
1262 A signal emission is simply a single message of type <literal>SIGNAL</literal>.
1263 It must have three header fields: <literal>PATH</literal> giving the object
1264 the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
1265 the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
1266 for signals, though it is optional for method calls.
1270 <sect3 id="message-protocol-types-errors">
1271 <title>Errors</title>
1273 Messages of type <literal>ERROR</literal> are most commonly replies
1274 to a <literal>METHOD_CALL</literal>, but may be returned in reply
1275 to any kind of message. The message bus for example
1276 will return an <literal>ERROR</literal> in reply to a signal emission if
1277 the bus does not have enough memory to send the signal.
1280 An <literal>ERROR</literal> may have any arguments, but if the first
1281 argument is a <literal>STRING</literal>, it must be an error message.
1282 The error message may be logged or shown to the user
1287 <sect3 id="message-protocol-types-notation">
1288 <title>Notation in this document</title>
1290 This document uses a simple pseudo-IDL to describe particular method
1291 calls and signals. Here is an example of a method call:
1293 org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
1294 out UINT32 resultcode)
1296 This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
1297 <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
1298 is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
1299 characters so it's known that the last part of the name in
1300 the "IDL" is the member name.
1303 In C++ that might end up looking like this:
1305 unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
1306 unsigned int flags);
1308 or equally valid, the return value could be done as an argument:
1310 void org::freedesktop::DBus::StartServiceByName (const char *name,
1312 unsigned int *resultcode);
1314 It's really up to the API designer how they want to make
1315 this look. You could design an API where the namespace wasn't used
1316 in C++, using STL or Qt, using varargs, or whatever you wanted.
1319 Signals are written as follows:
1321 org.freedesktop.DBus.NameLost (STRING name)
1323 Signals don't specify "in" vs. "out" because only
1324 a single direction is possible.
1327 It isn't especially encouraged to use this lame pseudo-IDL in actual
1328 API implementations; you might use the native notation for the
1329 language you're using, or you might use COM or CORBA IDL, for example.
1334 <sect2 id="message-protocol-handling-invalid">
1335 <title>Invalid Protocol and Spec Extensions</title>
1338 For security reasons, the D-Bus protocol should be strictly parsed and
1339 validated, with the exception of defined extension points. Any invalid
1340 protocol or spec violations should result in immediately dropping the
1341 connection without notice to the other end. Exceptions should be
1342 carefully considered, e.g. an exception may be warranted for a
1343 well-understood idiosyncrasy of a widely-deployed implementation. In
1344 cases where the other end of a connection is 100% trusted and known to
1345 be friendly, skipping validation for performance reasons could also make
1346 sense in certain cases.
1350 Generally speaking violations of the "must" requirements in this spec
1351 should be considered possible attempts to exploit security, and violations
1352 of the "should" suggestions should be considered legitimate (though perhaps
1353 they should generate an error in some cases).
1357 The following extension points are built in to D-Bus on purpose and must
1358 not be treated as invalid protocol. The extension points are intended
1359 for use by future versions of this spec, they are not intended for third
1360 parties. At the moment, the only way a third party could extend D-Bus
1361 without breaking interoperability would be to introduce a way to negotiate new
1362 feature support as part of the auth protocol, using EXTENSION_-prefixed
1363 commands. There is not yet a standard way to negotiate features.
1367 In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
1368 commands result in an ERROR rather than a disconnect. This enables
1369 future extensions to the protocol. Commands starting with EXTENSION_ are
1370 reserved for third parties.
1375 The authentication protocol supports pluggable auth mechanisms.
1380 The address format (see <xref linkend="addresses"/>) supports new
1386 Messages with an unknown type (something other than
1387 <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
1388 <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
1389 Unknown-type messages must still be well-formed in the same way
1390 as the known messages, however. They still have the normal
1396 Header fields with an unknown or unexpected field code must be ignored,
1397 though again they must still be well-formed.
1402 New standard interfaces (with new methods and signals) can of course be added.
1412 <sect1 id="auth-protocol">
1413 <title>Authentication Protocol</title>
1415 Before the flow of messages begins, two applications must
1416 authenticate. A simple plain-text protocol is used for
1417 authentication; this protocol is a SASL profile, and maps fairly
1418 directly from the SASL specification. The message encoding is
1419 NOT used here, only plain text messages.
1422 In examples, "C:" and "S:" indicate lines sent by the client and
1423 server respectively.
1425 <sect2 id="auth-protocol-overview">
1426 <title>Protocol Overview</title>
1428 The protocol is a line-based protocol, where each line ends with
1429 \r\n. Each line begins with an all-caps ASCII command name containing
1430 only the character range [A-Z_], a space, then any arguments for the
1431 command, then the \r\n ending the line. The protocol is
1432 case-sensitive. All bytes must be in the ASCII character set.
1434 Commands from the client to the server are as follows:
1437 <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
1438 <listitem><para>CANCEL</para></listitem>
1439 <listitem><para>BEGIN</para></listitem>
1440 <listitem><para>DATA <data in hex encoding></para></listitem>
1441 <listitem><para>ERROR [human-readable error explanation]</para></listitem>
1442 <listitem><para>NEGOTIATE_UNIX_FD</para></listitem>
1445 From server to client are as follows:
1448 <listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
1449 <listitem><para>OK <GUID in hex></para></listitem>
1450 <listitem><para>DATA <data in hex encoding></para></listitem>
1451 <listitem><para>ERROR</para></listitem>
1452 <listitem><para>AGREE_UNIX_FD</para></listitem>
1456 Unofficial extensions to the command set must begin with the letters
1457 "EXTENSION_", to avoid conflicts with future official commands.
1458 For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
1461 <sect2 id="auth-nul-byte">
1462 <title>Special credentials-passing nul byte</title>
1464 Immediately after connecting to the server, the client must send a
1465 single nul byte. This byte may be accompanied by credentials
1466 information on some operating systems that use sendmsg() with
1467 SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
1468 sockets. However, the nul byte must be sent even on other kinds of
1469 socket, and even on operating systems that do not require a byte to be
1470 sent in order to transmit credentials. The text protocol described in
1471 this document begins after the single nul byte. If the first byte
1472 received from the client is not a nul byte, the server may disconnect
1476 A nul byte in any context other than the initial byte is an error;
1477 the protocol is ASCII-only.
1480 The credentials sent along with the nul byte may be used with the
1481 SASL mechanism EXTERNAL.
1484 <sect2 id="auth-command-auth">
1485 <title>AUTH command</title>
1487 If an AUTH command has no arguments, it is a request to list
1488 available mechanisms. The server must respond with a REJECTED
1489 command listing the mechanisms it understands, or with an error.
1492 If an AUTH command specifies a mechanism, and the server supports
1493 said mechanism, the server should begin exchanging SASL
1494 challenge-response data with the client using DATA commands.
1497 If the server does not support the mechanism given in the AUTH
1498 command, it must send either a REJECTED command listing the mechanisms
1499 it does support, or an error.
1502 If the [initial-response] argument is provided, it is intended for use
1503 with mechanisms that have no initial challenge (or an empty initial
1504 challenge), as if it were the argument to an initial DATA command. If
1505 the selected mechanism has an initial challenge and [initial-response]
1506 was provided, the server should reject authentication by sending
1510 If authentication succeeds after exchanging DATA commands,
1511 an OK command must be sent to the client.
1514 The first octet received by the server after the \r\n of the BEGIN
1515 command from the client must be the first octet of the
1516 authenticated/encrypted stream of D-Bus messages.
1519 If BEGIN is received by the server, the first octet received
1520 by the client after the \r\n of the OK command must be the
1521 first octet of the authenticated/encrypted stream of D-Bus
1525 <sect2 id="auth-command-cancel">
1526 <title>CANCEL Command</title>
1528 At any time up to sending the BEGIN command, the client may send a
1529 CANCEL command. On receiving the CANCEL command, the server must
1530 send a REJECTED command and abort the current authentication
1534 <sect2 id="auth-command-data">
1535 <title>DATA Command</title>
1537 The DATA command may come from either client or server, and simply
1538 contains a hex-encoded block of data to be interpreted
1539 according to the SASL mechanism in use.
1542 Some SASL mechanisms support sending an "empty string";
1543 FIXME we need some way to do this.
1546 <sect2 id="auth-command-begin">
1547 <title>BEGIN Command</title>
1549 The BEGIN command acknowledges that the client has received an
1550 OK command from the server, and that the stream of messages
1554 The first octet received by the server after the \r\n of the BEGIN
1555 command from the client must be the first octet of the
1556 authenticated/encrypted stream of D-Bus messages.
1559 <sect2 id="auth-command-rejected">
1560 <title>REJECTED Command</title>
1562 The REJECTED command indicates that the current authentication
1563 exchange has failed, and further exchange of DATA is inappropriate.
1564 The client would normally try another mechanism, or try providing
1565 different responses to challenges.
1567 Optionally, the REJECTED command has a space-separated list of
1568 available auth mechanisms as arguments. If a server ever provides
1569 a list of supported mechanisms, it must provide the same list
1570 each time it sends a REJECTED message. Clients are free to
1571 ignore all lists received after the first.
1574 <sect2 id="auth-command-ok">
1575 <title>OK Command</title>
1577 The OK command indicates that the client has been
1578 authenticated. The client may now proceed with negotiating
1579 Unix file descriptor passing. To do that it shall send
1580 NEGOTIATE_UNIX_FD to the server.
1583 Otherwise, the client must respond to the OK command by
1584 sending a BEGIN command, followed by its stream of messages,
1585 or by disconnecting. The server must not accept additional
1586 commands using this protocol after the BEGIN command has been
1587 received. Further communication will be a stream of D-Bus
1588 messages (optionally encrypted, as negotiated) rather than
1592 If a client sends BEGIN the first octet received by the client
1593 after the \r\n of the OK command must be the first octet of
1594 the authenticated/encrypted stream of D-Bus messages.
1597 The OK command has one argument, which is the GUID of the server.
1598 See <xref linkend="addresses"/> for more on server GUIDs.
1601 <sect2 id="auth-command-error">
1602 <title>ERROR Command</title>
1604 The ERROR command indicates that either server or client did not
1605 know a command, does not accept the given command in the current
1606 context, or did not understand the arguments to the command. This
1607 allows the protocol to be extended; a client or server can send a
1608 command present or permitted only in new protocol versions, and if
1609 an ERROR is received instead of an appropriate response, fall back
1610 to using some other technique.
1613 If an ERROR is sent, the server or client that sent the
1614 error must continue as if the command causing the ERROR had never been
1615 received. However, the the server or client receiving the error
1616 should try something other than whatever caused the error;
1617 if only canceling/rejecting the authentication.
1620 If the D-Bus protocol changes incompatibly at some future time,
1621 applications implementing the new protocol would probably be able to
1622 check for support of the new protocol by sending a new command and
1623 receiving an ERROR from applications that don't understand it. Thus the
1624 ERROR feature of the auth protocol is an escape hatch that lets us
1625 negotiate extensions or changes to the D-Bus protocol in the future.
1628 <sect2 id="auth-command-negotiate-unix-fd">
1629 <title>NEGOTIATE_UNIX_FD Command</title>
1631 The NEGOTIATE_UNIX_FD command indicates that the client
1632 supports Unix file descriptor passing. This command may only
1633 be sent after the connection is authenticated, i.e. after OK
1634 was received by the client. This command may only be sent on
1635 transports that support Unix file descriptor passing.
1638 On receiving NEGOTIATE_UNIX_FD the server must respond with
1639 either AGREE_UNIX_FD or ERROR. It shall respond the former if
1640 the transport chosen supports Unix file descriptor passing and
1641 the server supports this feature. It shall respond the latter
1642 if the transport does not support Unix file descriptor
1643 passing, the server does not support this feature, or the
1644 server decides not to enable file descriptor passing due to
1645 security or other reasons.
1648 <sect2 id="auth-command-agree-unix-fd">
1649 <title>AGREE_UNIX_FD Command</title>
1651 The AGREE_UNIX_FD command indicates that the server supports
1652 Unix file descriptor passing. This command may only be sent
1653 after the connection is authenticated, and the client sent
1654 NEGOTIATE_UNIX_FD to enable Unix file descriptor passing. This
1655 command may only be sent on transports that support Unix file
1659 On receiving AGREE_UNIX_FD the client must respond with BEGIN,
1660 followed by its stream of messages, or by disconnecting. The
1661 server must not accept additional commands using this protocol
1662 after the BEGIN command has been received. Further
1663 communication will be a stream of D-Bus messages (optionally
1664 encrypted, as negotiated) rather than this protocol.
1667 <sect2 id="auth-command-future">
1668 <title>Future Extensions</title>
1670 Future extensions to the authentication and negotiation
1671 protocol are possible. For that new commands may be
1672 introduced. If a client or server receives an unknown command
1673 it shall respond with ERROR and not consider this fatal. New
1674 commands may be introduced both before, and after
1675 authentication, i.e. both before and after the OK command.
1678 <sect2 id="auth-examples">
1679 <title>Authentication examples</title>
1683 <title>Example of successful magic cookie authentication</title>
1685 (MAGIC_COOKIE is a made up mechanism)
1687 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1693 <title>Example of finding out mechanisms then picking one</title>
1696 S: REJECTED KERBEROS_V4 SKEY
1697 C: AUTH SKEY 7ab83f32ee
1698 S: DATA 8799cabb2ea93e
1699 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1705 <title>Example of client sends unknown command then falls back to regular auth</title>
1709 C: AUTH MAGIC_COOKIE 3736343435313230333039
1715 <title>Example of server doesn't support initial auth mechanism</title>
1717 C: AUTH MAGIC_COOKIE 3736343435313230333039
1718 S: REJECTED KERBEROS_V4 SKEY
1719 C: AUTH SKEY 7ab83f32ee
1720 S: DATA 8799cabb2ea93e
1721 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1727 <title>Example of wrong password or the like followed by successful retry</title>
1729 C: AUTH MAGIC_COOKIE 3736343435313230333039
1730 S: REJECTED KERBEROS_V4 SKEY
1731 C: AUTH SKEY 7ab83f32ee
1732 S: DATA 8799cabb2ea93e
1733 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1735 C: AUTH SKEY 7ab83f32ee
1736 S: DATA 8799cabb2ea93e
1737 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1743 <title>Example of skey cancelled and restarted</title>
1745 C: AUTH MAGIC_COOKIE 3736343435313230333039
1746 S: REJECTED KERBEROS_V4 SKEY
1747 C: AUTH SKEY 7ab83f32ee
1748 S: DATA 8799cabb2ea93e
1751 C: AUTH SKEY 7ab83f32ee
1752 S: DATA 8799cabb2ea93e
1753 C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
1759 <title>Example of successful magic cookie authentication with successful negotiation of Unix FD passing</title>
1761 (MAGIC_COOKIE is a made up mechanism)
1763 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1765 C: NEGOTIATE_UNIX_FD
1771 <title>Example of successful magic cookie authentication with unsuccessful negotiation of Unix FD passing</title>
1773 (MAGIC_COOKIE is a made up mechanism)
1775 C: AUTH MAGIC_COOKIE 3138363935333137393635383634
1777 C: NEGOTIATE_UNIX_FD
1784 <sect2 id="auth-states">
1785 <title>Authentication state diagrams</title>
1788 This section documents the auth protocol in terms of
1789 a state machine for the client and the server. This is
1790 probably the most robust way to implement the protocol.
1793 <sect3 id="auth-states-client">
1794 <title>Client states</title>
1797 To more precisely describe the interaction between the
1798 protocol state machine and the authentication mechanisms the
1799 following notation is used: MECH(CHALL) means that the
1800 server challenge CHALL was fed to the mechanism MECH, which
1806 CONTINUE(RESP) means continue the auth conversation
1807 and send RESP as the response to the server;
1813 OK(RESP) means that after sending RESP to the server
1814 the client side of the auth conversation is finished
1815 and the server should return "OK";
1821 ERROR means that CHALL was invalid and could not be
1827 Both RESP and CHALL may be empty.
1831 The Client starts by getting an initial response from the
1832 default mechanism and sends AUTH MECH RESP, or AUTH MECH if
1833 the mechanism did not provide an initial response. If the
1834 mechanism returns CONTINUE, the client starts in state
1835 <emphasis>WaitingForData</emphasis>, if the mechanism
1836 returns OK the client starts in state
1837 <emphasis>WaitingForOK</emphasis>.
1841 The client should keep track of available mechanisms and
1842 which it mechanisms it has already attempted. This list is
1843 used to decide which AUTH command to send. When the list is
1844 exhausted, the client should give up and close the
1849 <title><emphasis>WaitingForData</emphasis></title>
1857 MECH(CHALL) returns CONTINUE(RESP) → send
1859 <emphasis>WaitingForData</emphasis>
1863 MECH(CHALL) returns OK(RESP) → send DATA
1864 RESP, goto <emphasis>WaitingForOK</emphasis>
1868 MECH(CHALL) returns ERROR → send ERROR
1869 [msg], goto <emphasis>WaitingForData</emphasis>
1877 Receive REJECTED [mechs] →
1878 send AUTH [next mech], goto
1879 WaitingForData or <emphasis>WaitingForOK</emphasis>
1884 Receive ERROR → send
1886 <emphasis>WaitingForReject</emphasis>
1891 Receive OK → send
1892 BEGIN, terminate auth
1893 conversation, authenticated
1898 Receive anything else → send
1900 <emphasis>WaitingForData</emphasis>
1908 <title><emphasis>WaitingForOK</emphasis></title>
1913 Receive OK → send BEGIN, terminate auth
1914 conversation, <emphasis>authenticated</emphasis>
1919 Receive REJECT [mechs] → send AUTH [next mech],
1920 goto <emphasis>WaitingForData</emphasis> or
1921 <emphasis>WaitingForOK</emphasis>
1927 Receive DATA → send CANCEL, goto
1928 <emphasis>WaitingForReject</emphasis>
1934 Receive ERROR → send CANCEL, goto
1935 <emphasis>WaitingForReject</emphasis>
1941 Receive anything else → send ERROR, goto
1942 <emphasis>WaitingForOK</emphasis>
1950 <title><emphasis>WaitingForReject</emphasis></title>
1955 Receive REJECT [mechs] → send AUTH [next mech],
1956 goto <emphasis>WaitingForData</emphasis> or
1957 <emphasis>WaitingForOK</emphasis>
1963 Receive anything else → terminate auth
1964 conversation, disconnect
1973 <sect3 id="auth-states-server">
1974 <title>Server states</title>
1977 For the server MECH(RESP) means that the client response
1978 RESP was fed to the the mechanism MECH, which returns one of
1983 CONTINUE(CHALL) means continue the auth conversation and
1984 send CHALL as the challenge to the client;
1990 OK means that the client has been successfully
1997 REJECT means that the client failed to authenticate or
1998 there was an error in RESP.
2003 The server starts out in state
2004 <emphasis>WaitingForAuth</emphasis>. If the client is
2005 rejected too many times the server must disconnect the
2010 <title><emphasis>WaitingForAuth</emphasis></title>
2016 Receive AUTH → send REJECTED [mechs], goto
2017 <emphasis>WaitingForAuth</emphasis>
2023 Receive AUTH MECH RESP
2027 MECH not valid mechanism → send REJECTED
2029 <emphasis>WaitingForAuth</emphasis>
2033 MECH(RESP) returns CONTINUE(CHALL) → send
2035 <emphasis>WaitingForData</emphasis>
2039 MECH(RESP) returns OK → send OK, goto
2040 <emphasis>WaitingForBegin</emphasis>
2044 MECH(RESP) returns REJECT → send REJECTED
2046 <emphasis>WaitingForAuth</emphasis>
2054 Receive BEGIN → terminate
2055 auth conversation, disconnect
2061 Receive ERROR → send REJECTED [mechs], goto
2062 <emphasis>WaitingForAuth</emphasis>
2068 Receive anything else → send
2070 <emphasis>WaitingForAuth</emphasis>
2079 <title><emphasis>WaitingForData</emphasis></title>
2087 MECH(RESP) returns CONTINUE(CHALL) → send
2089 <emphasis>WaitingForData</emphasis>
2093 MECH(RESP) returns OK → send OK, goto
2094 <emphasis>WaitingForBegin</emphasis>
2098 MECH(RESP) returns REJECT → send REJECTED
2100 <emphasis>WaitingForAuth</emphasis>
2108 Receive BEGIN → terminate auth conversation,
2115 Receive CANCEL → send REJECTED [mechs], goto
2116 <emphasis>WaitingForAuth</emphasis>
2122 Receive ERROR → send REJECTED [mechs], goto
2123 <emphasis>WaitingForAuth</emphasis>
2129 Receive anything else → send ERROR, goto
2130 <emphasis>WaitingForData</emphasis>
2138 <title><emphasis>WaitingForBegin</emphasis></title>
2143 Receive BEGIN → terminate auth conversation,
2144 client authenticated
2150 Receive CANCEL → send REJECTED [mechs], goto
2151 <emphasis>WaitingForAuth</emphasis>
2157 Receive ERROR → send REJECTED [mechs], goto
2158 <emphasis>WaitingForAuth</emphasis>
2164 Receive anything else → send ERROR, goto
2165 <emphasis>WaitingForBegin</emphasis>
2175 <sect2 id="auth-mechanisms">
2176 <title>Authentication mechanisms</title>
2178 This section describes some new authentication mechanisms.
2179 D-Bus also allows any standard SASL mechanism of course.
2181 <sect3 id="auth-mechanisms-sha">
2182 <title>DBUS_COOKIE_SHA1</title>
2184 The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
2185 has the ability to read a private file owned by the user being
2186 authenticated. If the client can prove that it has access to a secret
2187 cookie stored in this file, then the client is authenticated.
2188 Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
2192 Authentication proceeds as follows:
2196 The client sends the username it would like to authenticate
2202 The server sends the name of its "cookie context" (see below); a
2203 space character; the integer ID of the secret cookie the client
2204 must demonstrate knowledge of; a space character; then a
2205 randomly-generated challenge string, all of this hex-encoded into
2211 The client locates the cookie and generates its own
2212 randomly-generated challenge string. The client then concatenates
2213 the server's decoded challenge, a ":" character, its own challenge,
2214 another ":" character, and the cookie. It computes the SHA-1 hash
2215 of this composite string as a hex digest. It concatenates the
2216 client's challenge string, a space character, and the SHA-1 hex
2217 digest, hex-encodes the result and sends it back to the server.
2222 The server generates the same concatenated string used by the
2223 client and computes its SHA-1 hash. It compares the hash with
2224 the hash received from the client; if the two hashes match, the
2225 client is authenticated.
2231 Each server has a "cookie context," which is a name that identifies a
2232 set of cookies that apply to that server. A sample context might be
2233 "org_freedesktop_session_bus". Context names must be valid ASCII,
2234 nonzero length, and may not contain the characters slash ("/"),
2235 backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
2236 tab ("\t"), or period ("."). There is a default context,
2237 "org_freedesktop_general" that's used by servers that do not specify
2241 Cookies are stored in a user's home directory, in the directory
2242 <filename>~/.dbus-keyrings/</filename>. This directory must
2243 not be readable or writable by other users. If it is,
2244 clients and servers must ignore it. The directory
2245 contains cookie files named after the cookie context.
2248 A cookie file contains one cookie per line. Each line
2249 has three space-separated fields:
2253 The cookie ID number, which must be a non-negative integer and
2254 may not be used twice in the same file.
2259 The cookie's creation time, in UNIX seconds-since-the-epoch
2265 The cookie itself, a hex-encoded random block of bytes. The cookie
2266 may be of any length, though obviously security increases
2267 as the length increases.
2273 Only server processes modify the cookie file.
2274 They must do so with this procedure:
2278 Create a lockfile name by appending ".lock" to the name of the
2279 cookie file. The server should attempt to create this file
2280 using <literal>O_CREAT | O_EXCL</literal>. If file creation
2281 fails, the lock fails. Servers should retry for a reasonable
2282 period of time, then they may choose to delete an existing lock
2283 to keep users from having to manually delete a stale
2284 lock. <footnote><para>Lockfiles are used instead of real file
2285 locking <literal>fcntl()</literal> because real locking
2286 implementations are still flaky on network
2287 filesystems.</para></footnote>
2292 Once the lockfile has been created, the server loads the cookie
2293 file. It should then delete any cookies that are old (the
2294 timeout can be fairly short), or more than a reasonable
2295 time in the future (so that cookies never accidentally
2296 become permanent, if the clock was set far into the future
2297 at some point). If no recent keys remain, the
2298 server may generate a new key.
2303 The pruned and possibly added-to cookie file
2304 must be resaved atomically (using a temporary
2305 file which is rename()'d).
2310 The lock must be dropped by deleting the lockfile.
2316 Clients need not lock the file in order to load it,
2317 because servers are required to save the file atomically.
2322 <sect1 id="addresses">
2323 <title>Server Addresses</title>
2325 Server addresses consist of a transport name followed by a colon, and
2326 then an optional, comma-separated list of keys and values in the form key=value.
2327 Each value is escaped.
2331 <programlisting>unix:path=/tmp/dbus-test</programlisting>
2332 Which is the address to a unix socket with the path /tmp/dbus-test.
2335 Value escaping is similar to URI escaping but simpler.
2339 The set of optionally-escaped bytes is:
2340 <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
2341 <emphasis>byte</emphasis> (note, not character) which is not in the
2342 set of optionally-escaped bytes must be replaced with an ASCII
2343 percent (<literal>%</literal>) and the value of the byte in hex.
2344 The hex value must always be two digits, even if the first digit is
2345 zero. The optionally-escaped bytes may be escaped if desired.
2350 To unescape, append each byte in the value; if a byte is an ASCII
2351 percent (<literal>%</literal>) character then append the following
2352 hex value instead. It is an error if a <literal>%</literal> byte
2353 does not have two hex digits following. It is an error if a
2354 non-optionally-escaped byte is seen unescaped.
2358 The set of optionally-escaped bytes is intended to preserve address
2359 readability and convenience.
2363 A server may specify a key-value pair with the key <literal>guid</literal>
2364 and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
2365 describes the format of the <literal>guid</literal> field. If present,
2366 this UUID may be used to distinguish one server address from another. A
2367 server should use a different UUID for each address it listens on. For
2368 example, if a message bus daemon offers both UNIX domain socket and TCP
2369 connections, but treats clients the same regardless of how they connect,
2370 those two connections are equivalent post-connection but should have
2371 distinct UUIDs to distinguish the kinds of connection.
2375 The intent of the address UUID feature is to allow a client to avoid
2376 opening multiple identical connections to the same server, by allowing the
2377 client to check whether an address corresponds to an already-existing
2378 connection. Comparing two addresses is insufficient, because addresses
2379 can be recycled by distinct servers, and equivalent addresses may look
2380 different if simply compared as strings (for example, the host in a TCP
2381 address can be given as an IP address or as a hostname).
2385 Note that the address key is <literal>guid</literal> even though the
2386 rest of the API and documentation says "UUID," for historical reasons.
2390 [FIXME clarify if attempting to connect to each is a requirement
2391 or just a suggestion]
2392 When connecting to a server, multiple server addresses can be
2393 separated by a semi-colon. The library will then try to connect
2394 to the first address and if that fails, it'll try to connect to
2395 the next one specified, and so forth. For example
2396 <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
2401 <sect1 id="transports">
2402 <title>Transports</title>
2404 [FIXME we need to specify in detail each transport and its possible arguments]
2406 Current transports include: unix domain sockets (including
2407 abstract namespace on linux), TCP/IP, and a debug/testing transport using
2408 in-process pipes. Future possible transports include one that
2409 tunnels over X11 protocol.
2412 <sect2 id="transports-unix-domain-sockets">
2413 <title>Unix Domain Sockets</title>
2415 Unix domain sockets can be either paths in the file system or on Linux
2416 kernels, they can be abstract which are similar to paths but
2417 do not show up in the file system.
2421 When a socket is opened by the D-Bus library it truncates the path
2422 name right before the first trailing Nul byte. This is true for both
2423 normal paths and abstract paths. Note that this is a departure from
2424 previous versions of D-Bus that would create sockets with a fixed
2425 length path name. Names which were shorter than the fixed length
2426 would be padded by Nul bytes.
2431 <sect1 id="naming-conventions">
2432 <title>Naming Conventions</title>
2435 D-Bus namespaces are all lowercase and correspond to reversed domain
2436 names, as with Java. e.g. "org.freedesktop"
2439 Interface, signal, method, and property names are "WindowsStyleCaps", note
2440 that the first letter is capitalized, unlike Java.
2443 Object paths are normally all lowercase with underscores used rather than
2449 <title>UUIDs</title>
2451 A working D-Bus implementation uses universally-unique IDs in two places.
2452 First, each server address has a UUID identifying the address,
2453 as described in <xref linkend="addresses"/>. Second, each operating
2454 system kernel instance running a D-Bus client or server has a UUID
2455 identifying that kernel, retrieved by invoking the method
2456 org.freedesktop.DBus.Peer.GetMachineId() (see <xref
2457 linkend="standard-interfaces-peer"/>).
2460 The term "UUID" in this document is intended literally, i.e. an
2461 identifier that is universally unique. It is not intended to refer to
2462 RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
2465 The UUID must contain 128 bits of data and be hex-encoded. The
2466 hex-encoded string may not contain hyphens or other non-hex-digit
2467 characters, and it must be exactly 32 characters long. To generate a
2468 UUID, the current reference implementation concatenates 96 bits of random
2469 data followed by the 32-bit time in seconds since the UNIX epoch (in big
2473 It would also be acceptable and probably better to simply generate 128
2474 bits of random data, as long as the random number generator is of high
2475 quality. The timestamp could conceivably help if the random bits are not
2476 very random. With a quality random number generator, collisions are
2477 extremely unlikely even with only 96 bits, so it's somewhat academic.
2480 Implementations should, however, stick to random data for the first 96 bits
2485 <sect1 id="standard-interfaces">
2486 <title>Standard Interfaces</title>
2488 See <xref linkend="message-protocol-types-notation"/> for details on
2489 the notation used in this section. There are some standard interfaces
2490 that may be useful across various D-Bus applications.
2492 <sect2 id="standard-interfaces-peer">
2493 <title><literal>org.freedesktop.DBus.Peer</literal></title>
2495 The <literal>org.freedesktop.DBus.Peer</literal> interface
2498 org.freedesktop.DBus.Peer.Ping ()
2499 org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
2503 On receipt of the <literal>METHOD_CALL</literal> message
2504 <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
2505 nothing other than reply with a <literal>METHOD_RETURN</literal> as
2506 usual. It does not matter which object path a ping is sent to. The
2507 reference implementation handles this method automatically.
2510 On receipt of the <literal>METHOD_CALL</literal> message
2511 <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
2512 reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
2513 UUID representing the identity of the machine the process is running on.
2514 This UUID must be the same for all processes on a single system at least
2515 until that system next reboots. It should be the same across reboots
2516 if possible, but this is not always possible to implement and is not
2518 It does not matter which object path a GetMachineId is sent to. The
2519 reference implementation handles this method automatically.
2522 The UUID is intended to be per-instance-of-the-operating-system, so may represent
2523 a virtual machine running on a hypervisor, rather than a physical machine.
2524 Basically if two processes see the same UUID, they should also see the same
2525 shared memory, UNIX domain sockets, process IDs, and other features that require
2526 a running OS kernel in common between the processes.
2529 The UUID is often used where other programs might use a hostname. Hostnames
2530 can change without rebooting, however, or just be "localhost" - so the UUID
2534 <xref linkend="uuids"/> explains the format of the UUID.
2538 <sect2 id="standard-interfaces-introspectable">
2539 <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
2541 This interface has one method:
2543 org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
2547 Objects instances may implement
2548 <literal>Introspect</literal> which returns an XML description of
2549 the object, including its interfaces (with signals and methods), objects
2550 below it in the object path tree, and its properties.
2553 <xref linkend="introspection-format"/> describes the format of this XML string.
2556 <sect2 id="standard-interfaces-properties">
2557 <title><literal>org.freedesktop.DBus.Properties</literal></title>
2559 Many native APIs will have a concept of object <firstterm>properties</firstterm>
2560 or <firstterm>attributes</firstterm>. These can be exposed via the
2561 <literal>org.freedesktop.DBus.Properties</literal> interface.
2565 org.freedesktop.DBus.Properties.Get (in STRING interface_name,
2566 in STRING property_name,
2568 org.freedesktop.DBus.Properties.Set (in STRING interface_name,
2569 in STRING property_name,
2571 org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
2572 out DICT<STRING,VARIANT> props);
2576 The available properties and whether they are writable can be determined
2577 by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
2578 see <xref linkend="standard-interfaces-introspectable"/>.
2581 An empty string may be provided for the interface name; in this case,
2582 if there are multiple properties on an object with the same name,
2583 the results are undefined (picking one by according to an arbitrary
2584 deterministic rule, or returning an error, are the reasonable
2590 <sect1 id="introspection-format">
2591 <title>Introspection Data Format</title>
2593 As described in <xref linkend="standard-interfaces-introspectable"/>,
2594 objects may be introspected at runtime, returning an XML string
2595 that describes the object. The same XML format may be used in
2596 other contexts as well, for example as an "IDL" for generating
2597 static language bindings.
2600 Here is an example of introspection data:
2602 <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
2603 "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
2604 <node name="/org/freedesktop/sample_object">
2605 <interface name="org.freedesktop.SampleInterface">
2606 <method name="Frobate">
2607 <arg name="foo" type="i" direction="in"/>
2608 <arg name="bar" type="s" direction="out"/>
2609 <arg name="baz" type="a{us}" direction="out"/>
2610 <annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
2612 <method name="Bazify">
2613 <arg name="bar" type="(iiu)" direction="in"/>
2614 <arg name="bar" type="v" direction="out"/>
2616 <method name="Mogrify">
2617 <arg name="bar" type="(iiav)" direction="in"/>
2619 <signal name="Changed">
2620 <arg name="new_value" type="b"/>
2622 <property name="Bar" type="y" access="readwrite"/>
2624 <node name="child_of_sample_object"/>
2625 <node name="another_child_of_sample_object"/>
2630 A more formal DTD and spec needs writing, but here are some quick notes.
2634 Only the root <node> element can omit the node name, as it's
2635 known to be the object that was introspected. If the root
2636 <node> does have a name attribute, it must be an absolute
2637 object path. If child <node> have object paths, they must be
2643 If a child <node> has any sub-elements, then they
2644 must represent a complete introspection of the child.
2645 If a child <node> is empty, then it may or may
2646 not have sub-elements; the child must be introspected
2647 in order to find out. The intent is that if an object
2648 knows that its children are "fast" to introspect
2649 it can go ahead and return their information, but
2650 otherwise it can omit it.
2655 The direction element on <arg> may be omitted,
2656 in which case it defaults to "in" for method calls
2657 and "out" for signals. Signals only allow "out"
2658 so while direction may be specified, it's pointless.
2663 The possible directions are "in" and "out",
2664 unlike CORBA there is no "inout"
2669 The possible property access flags are
2670 "readwrite", "read", and "write"
2675 Multiple interfaces can of course be listed for
2681 The "name" attribute on arguments is optional.
2687 Method, interface, property, and signal elements may have
2688 "annotations", which are generic key/value pairs of metadata.
2689 They are similar conceptually to Java's annotations and C# attributes.
2690 Well-known annotations:
2697 <entry>Values (separated by ,)</entry>
2698 <entry>Description</entry>
2703 <entry>org.freedesktop.DBus.Deprecated</entry>
2704 <entry>true,false</entry>
2705 <entry>Whether or not the entity is deprecated; defaults to false</entry>
2708 <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
2709 <entry>(string)</entry>
2710 <entry>The C symbol; may be used for methods and interfaces</entry>
2713 <entry>org.freedesktop.DBus.Method.NoReply</entry>
2714 <entry>true,false</entry>
2715 <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
2721 <sect1 id="message-bus">
2722 <title>Message Bus Specification</title>
2723 <sect2 id="message-bus-overview">
2724 <title>Message Bus Overview</title>
2726 The message bus accepts connections from one or more applications.
2727 Once connected, applications can exchange messages with other
2728 applications that are also connected to the bus.
2731 In order to route messages among connections, the message bus keeps a
2732 mapping from names to connections. Each connection has one
2733 unique-for-the-lifetime-of-the-bus name automatically assigned.
2734 Applications may request additional names for a connection. Additional
2735 names are usually "well-known names" such as
2736 "org.freedesktop.TextEditor". When a name is bound to a connection,
2737 that connection is said to <firstterm>own</firstterm> the name.
2740 The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
2741 This name routes messages to the bus, allowing applications to make
2742 administrative requests. For example, applications can ask the bus
2743 to assign a name to a connection.
2746 Each name may have <firstterm>queued owners</firstterm>. When an
2747 application requests a name for a connection and the name is already in
2748 use, the bus will optionally add the connection to a queue waiting for
2749 the name. If the current owner of the name disconnects or releases
2750 the name, the next connection in the queue will become the new owner.
2754 This feature causes the right thing to happen if you start two text
2755 editors for example; the first one may request "org.freedesktop.TextEditor",
2756 and the second will be queued as a possible owner of that name. When
2757 the first exits, the second will take over.
2761 Messages may have a <literal>DESTINATION</literal> field (see <xref
2762 linkend="message-protocol-header-fields"/>). If the
2763 <literal>DESTINATION</literal> field is present, it specifies a message
2764 recipient by name. Method calls and replies normally specify this field.
2768 Signals normally do not specify a destination; they are sent to all
2769 applications with <firstterm>message matching rules</firstterm> that
2774 When the message bus receives a method call, if the
2775 <literal>DESTINATION</literal> field is absent, the call is taken to be
2776 a standard one-to-one message and interpreted by the message bus
2777 itself. For example, sending an
2778 <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
2779 <literal>DESTINATION</literal> will cause the message bus itself to
2780 reply to the ping immediately; the message bus will not make this
2781 message visible to other applications.
2785 Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
2786 the ping message were sent with a <literal>DESTINATION</literal> name of
2787 <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
2788 forwarded, and the Yoyodyne Corporation screensaver application would be
2789 expected to reply to the ping.
2793 <sect2 id="message-bus-names">
2794 <title>Message Bus Names</title>
2796 Each connection has at least one name, assigned at connection time and
2797 returned in response to the
2798 <literal>org.freedesktop.DBus.Hello</literal> method call. This
2799 automatically-assigned name is called the connection's <firstterm>unique
2800 name</firstterm>. Unique names are never reused for two different
2801 connections to the same bus.
2804 Ownership of a unique name is a prerequisite for interaction with
2805 the message bus. It logically follows that the unique name is always
2806 the first name that an application comes to own, and the last
2807 one that it loses ownership of.
2810 Unique connection names must begin with the character ':' (ASCII colon
2811 character); bus names that are not unique names must not begin
2812 with this character. (The bus must reject any attempt by an application
2813 to manually request a name beginning with ':'.) This restriction
2814 categorically prevents "spoofing"; messages sent to a unique name
2815 will always go to the expected connection.
2818 When a connection is closed, all the names that it owns are deleted (or
2819 transferred to the next connection in the queue if any).
2822 A connection can request additional names to be associated with it using
2823 the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
2824 linkend="message-protocol-names-bus"/> describes the format of a valid
2825 name. These names can be released again using the
2826 <literal>org.freedesktop.DBus.ReleaseName</literal> message.
2829 <sect3 id="bus-messages-request-name">
2830 <title><literal>org.freedesktop.DBus.RequestName</literal></title>
2834 UINT32 RequestName (in STRING name, in UINT32 flags)
2841 <entry>Argument</entry>
2843 <entry>Description</entry>
2849 <entry>STRING</entry>
2850 <entry>Name to request</entry>
2854 <entry>UINT32</entry>
2855 <entry>Flags</entry>
2865 <entry>Argument</entry>
2867 <entry>Description</entry>
2873 <entry>UINT32</entry>
2874 <entry>Return value</entry>
2881 This method call should be sent to
2882 <literal>org.freedesktop.DBus</literal> and asks the message bus to
2883 assign the given name to the method caller. Each name maintains a
2884 queue of possible owners, where the head of the queue is the primary
2885 or current owner of the name. Each potential owner in the queue
2886 maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
2887 DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
2888 call. When RequestName is invoked the following occurs:
2892 If the method caller is currently the primary owner of the name,
2893 the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
2894 values are updated with the values from the new RequestName call,
2895 and nothing further happens.
2901 If the current primary owner (head of the queue) has
2902 DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
2903 invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
2904 the caller of RequestName replaces the current primary owner at
2905 the head of the queue and the current primary owner moves to the
2906 second position in the queue. If the caller of RequestName was
2907 in the queue previously its flags are updated with the values from
2908 the new RequestName in addition to moving it to the head of the queue.
2914 If replacement is not possible, and the method caller is
2915 currently in the queue but not the primary owner, its flags are
2916 updated with the values from the new RequestName call.
2922 If replacement is not possible, and the method caller is
2923 currently not in the queue, the method caller is appended to the
2930 If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
2931 set and is not the primary owner, it is removed from the
2932 queue. This can apply to the previous primary owner (if it
2933 was replaced) or the method caller (if it updated the
2934 DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
2935 queue, or if it was just added to the queue with that flag set).
2941 Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
2942 queue," even if another application already in the queue had specified
2943 DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
2944 that does not allow replacement goes away, and the next primary owner
2945 does allow replacement. In this case, queued items that specified
2946 DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
2947 automatically replace the new primary owner. In other words,
2948 DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
2949 time RequestName is called. This is deliberate to avoid an infinite loop
2950 anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2951 and DBUS_NAME_FLAG_REPLACE_EXISTING.
2954 The flags argument contains any of the following values logically ORed
2961 <entry>Conventional Name</entry>
2962 <entry>Value</entry>
2963 <entry>Description</entry>
2968 <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
2972 If an application A specifies this flag and succeeds in
2973 becoming the owner of the name, and another application B
2974 later calls RequestName with the
2975 DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
2976 will lose ownership and receive a
2977 <literal>org.freedesktop.DBus.NameLost</literal> signal, and
2978 application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
2979 is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
2980 is not specified by application B, then application B will not replace
2981 application A as the owner.
2986 <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
2990 Try to replace the current owner if there is one. If this
2991 flag is not set the application will only become the owner of
2992 the name if there is no current owner. If this flag is set,
2993 the application will replace the current owner if
2994 the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
2999 <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
3003 Without this flag, if an application requests a name that is
3004 already owned, the application will be placed in a queue to
3005 own the name when the current owner gives it up. If this
3006 flag is given, the application will not be placed in the
3007 queue, the request for the name will simply fail. This flag
3008 also affects behavior when an application is replaced as
3009 name owner; by default the application moves back into the
3010 waiting queue, unless this flag was provided when the application
3011 became the name owner.
3019 The return code can be one of the following values:
3025 <entry>Conventional Name</entry>
3026 <entry>Value</entry>
3027 <entry>Description</entry>
3032 <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
3033 <entry>1</entry> <entry>The caller is now the primary owner of
3034 the name, replacing any previous owner. Either the name had no
3035 owner before, or the caller specified
3036 DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
3037 DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
3040 <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
3043 <entry>The name already had an owner,
3044 DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
3045 the current owner did not specify
3046 DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
3047 application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
3051 <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
3052 <entry>The name already has an owner,
3053 DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
3054 DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
3055 current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
3056 specified by the requesting application.</entry>
3059 <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
3061 <entry>The application trying to request ownership of a name is already the owner of it.</entry>
3069 <sect3 id="bus-messages-release-name">
3070 <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
3074 UINT32 ReleaseName (in STRING name)
3081 <entry>Argument</entry>
3083 <entry>Description</entry>
3089 <entry>STRING</entry>
3090 <entry>Name to release</entry>
3100 <entry>Argument</entry>
3102 <entry>Description</entry>
3108 <entry>UINT32</entry>
3109 <entry>Return value</entry>
3116 This method call should be sent to
3117 <literal>org.freedesktop.DBus</literal> and asks the message bus to
3118 release the method caller's claim to the given name. If the caller is
3119 the primary owner, a new primary owner will be selected from the
3120 queue if any other owners are waiting. If the caller is waiting in
3121 the queue for the name, the caller will removed from the queue and
3122 will not be made an owner of the name if it later becomes available.
3123 If there are no other owners in the queue for the name, it will be
3124 removed from the bus entirely.
3126 The return code can be one of the following values:
3132 <entry>Conventional Name</entry>
3133 <entry>Value</entry>
3134 <entry>Description</entry>
3139 <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
3140 <entry>1</entry> <entry>The caller has released his claim on
3141 the given name. Either the caller was the primary owner of
3142 the name, and the name is now unused or taken by somebody
3143 waiting in the queue for the name, or the caller was waiting
3144 in the queue for the name and has now been removed from the
3148 <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
3150 <entry>The given name does not exist on this bus.</entry>
3153 <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
3155 <entry>The caller was not the primary owner of this name,
3156 and was also not waiting in the queue to own this name.</entry>
3165 <sect2 id="message-bus-routing">
3166 <title>Message Bus Message Routing</title>
3170 <sect3 id="message-bus-routing-match-rules">
3171 <title>Match Rules</title>
3173 An important part of the message bus routing protocol is match
3174 rules. Match rules describe what messages can be sent to a client
3175 based on the contents of the message. When a message is routed
3176 through the bus it is compared to clients' match rules. If any
3177 of the rules match, the message is dispatched to the client.
3178 If none of the rules match the message never leaves the bus. This
3179 is an effective way to control traffic over the bus and to make sure
3180 only relevant message need to be processed by the client.
3183 Match rules are added using the AddMatch bus method
3184 (see xref linkend="bus-messages-add-match"/>). Rules are
3185 specified as a string of comma separated key/value pairs.
3186 Excluding a key from the rule indicates a wildcard match.
3187 For instance excluding the the member from a match rule but
3188 adding a sender would let all messages from that sender through.
3189 An example of a complete rule would be
3190 "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
3193 The following table describes the keys that can be used to create
3195 The following table summarizes the D-Bus types.
3201 <entry>Possible Values</entry>
3202 <entry>Description</entry>
3207 <entry><literal>type</literal></entry>
3208 <entry>'signal', 'method_call', 'method_return', 'error'</entry>
3209 <entry>Match on the message type. An example of a type match is type='signal'</entry>
3212 <entry><literal>sender</literal></entry>
3213 <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
3214 and <xref linkend="term-unique-name"/> respectively)
3216 <entry>Match messages sent by a particular sender. An example of a sender match
3217 is sender='org.freedesktop.Hal'</entry>
3220 <entry><literal>interface</literal></entry>
3221 <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
3222 <entry>Match messages sent over or to a particular interface. An example of an
3223 interface match is interface='org.freedesktop.Hal.Manager'.
3224 If a message omits the interface header, it must not match any rule
3225 that specifies this key.</entry>
3228 <entry><literal>member</literal></entry>
3229 <entry>Any valid method or signal name</entry>
3230 <entry>Matches messages which have the give method or signal name. An example of
3231 a member match is member='NameOwnerChanged'</entry>
3234 <entry><literal>path</literal></entry>
3235 <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
3236 <entry>Matches messages which are sent from or to the given object. An example of a
3237 path match is path='/org/freedesktop/Hal/Manager'</entry>
3240 <entry><literal>destination</literal></entry>
3241 <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
3242 <entry>Matches messages which are being sent to the given unique name. An
3243 example of a destination match is destination=':1.0'</entry>
3246 <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
3247 <entry>Any string</entry>
3248 <entry>Arg matches are special and are used for further restricting the
3249 match based on the arguments in the body of a message. As of this time
3250 only string arguments can be matched. An example of an argument match
3251 would be arg3='Foo'. Only argument indexes from 0 to 63 should be
3255 <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
3256 <entry>Any string</entry>
3257 <entry>Argument path matches provide a specialised form of wildcard
3258 matching for path-like namespaces. As with normal argument matches,
3259 if the argument is exactly equal to the string given in the match
3260 rule then the rule is satisfied. Additionally, there is also a
3261 match when either the string given in the match rule or the
3262 appropriate message argument ends with '/' and is a prefix of the
3263 other. An example argument path match is arg0path='/aa/bb/'. This
3264 would match messages with first arguments of '/', '/aa/',
3265 '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
3266 messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</entry>
3274 <sect2 id="message-bus-starting-services">
3275 <title>Message Bus Starting Services</title>
3277 The message bus can start applications on behalf of other applications.
3278 In CORBA terms, this would be called <firstterm>activation</firstterm>.
3279 An application that can be started in this way is called a
3280 <firstterm>service</firstterm>.
3283 With D-Bus, starting a service is normally done by name. That is,
3284 applications ask the message bus to start some program that will own a
3285 well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
3286 This implies a contract documented along with the name
3287 <literal>org.freedesktop.TextEditor</literal> for which objects
3288 the owner of that name will provide, and what interfaces those
3292 To find an executable corresponding to a particular name, the bus daemon
3293 looks for <firstterm>service description files</firstterm>. Service
3294 description files define a mapping from names to executables. Different
3295 kinds of message bus will look for these files in different places, see
3296 <xref linkend="message-bus-types"/>.
3299 [FIXME the file format should be much better specified than "similar to
3300 .desktop entries" esp. since desktop entries are already
3301 badly-specified. ;-)] Service description files have the ".service" file
3302 extension. The message bus will only load service description files
3303 ending with .service; all other files will be ignored. The file format
3304 is similar to that of <ulink
3305 url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
3306 entries</ulink>. All service description files must be in UTF-8
3307 encoding. To ensure that there will be no name collisions, service files
3308 must be namespaced using the same mechanism as messages and service
3312 <title>Example service description file</title>
3314 # Sample service description file
3316 Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
3317 Exec=/usr/libexec/gconfd-2
3322 When an application asks to start a service by name, the bus daemon tries to
3323 find a service that will own that name. It then tries to spawn the
3324 executable associated with it. If this fails, it will report an
3325 error. [FIXME what happens if two .service files offer the same service;
3326 what kind of error is reported, should we have a way for the client to
3330 The executable launched will have the environment variable
3331 <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
3332 message bus so it can connect and request the appropriate names.
3335 The executable being launched may want to know whether the message bus
3336 starting it is one of the well-known message buses (see <xref
3337 linkend="message-bus-types"/>). To facilitate this, the bus must also set
3338 the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
3339 of the well-known buses. The currently-defined values for this variable
3340 are <literal>system</literal> for the systemwide message bus,
3341 and <literal>session</literal> for the per-login-session message
3342 bus. The new executable must still connect to the address given
3343 in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
3344 resulting connection is to the well-known bus.
3347 [FIXME there should be a timeout somewhere, either specified
3348 in the .service file, by the client, or just a global value
3349 and if the client being activated fails to connect within that
3350 timeout, an error should be sent back.]
3353 <sect3 id="message-bus-starting-services-scope">
3354 <title>Message Bus Service Scope</title>
3356 The "scope" of a service is its "per-", such as per-session,
3357 per-machine, per-home-directory, or per-display. The reference
3358 implementation doesn't yet support starting services in a different
3359 scope from the message bus itself. So e.g. if you start a service
3360 on the session bus its scope is per-session.
3363 We could add an optional scope to a bus name. For example, for
3364 per-(display,session pair), we could have a unique ID for each display
3365 generated automatically at login and set on screen 0 by executing a
3366 special "set display ID" binary. The ID would be stored in a
3367 <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
3368 random bytes. This ID would then be used to scope names.
3369 Starting/locating a service could be done by ID-name pair rather than
3373 Contrast this with a per-display scope. To achieve that, we would
3374 want a single bus spanning all sessions using a given display.
3375 So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
3376 property on screen 0 of the display, pointing to this bus.
3381 <sect2 id="message-bus-types">
3382 <title>Well-known Message Bus Instances</title>
3384 Two standard message bus instances are defined here, along with how
3385 to locate them and where their service files live.
3387 <sect3 id="message-bus-types-login">
3388 <title>Login session message bus</title>
3390 Each time a user logs in, a <firstterm>login session message
3391 bus</firstterm> may be started. All applications in the user's login
3392 session may interact with one another using this message bus.
3395 The address of the login session message bus is given
3396 in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
3397 variable. If that variable is not set, applications may
3398 also try to read the address from the X Window System root
3399 window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
3400 The root window property must have type <literal>STRING</literal>.
3401 The environment variable should have precedence over the
3402 root window property.
3405 [FIXME specify location of .service files, probably using
3406 DESKTOP_DIRS etc. from basedir specification, though login session
3407 bus is not really desktop-specific]
3410 <sect3 id="message-bus-types-system">
3411 <title>System message bus</title>
3413 A computer may have a <firstterm>system message bus</firstterm>,
3414 accessible to all applications on the system. This message bus may be
3415 used to broadcast system events, such as adding new hardware devices,
3416 changes in the printer queue, and so forth.
3419 The address of the system message bus is given
3420 in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
3421 variable. If that variable is not set, applications should try
3422 to connect to the well-known address
3423 <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
3426 The D-Bus reference implementation actually honors the
3427 <literal>$(localstatedir)</literal> configure option
3428 for this address, on both client and server side.
3433 [FIXME specify location of system bus .service files]
3438 <sect2 id="message-bus-messages">
3439 <title>Message Bus Messages</title>
3441 The special message bus name <literal>org.freedesktop.DBus</literal>
3442 responds to a number of additional messages.
3445 <sect3 id="bus-messages-hello">
3446 <title><literal>org.freedesktop.DBus.Hello</literal></title>
3457 <entry>Argument</entry>
3459 <entry>Description</entry>
3465 <entry>STRING</entry>
3466 <entry>Unique name assigned to the connection</entry>
3473 Before an application is able to send messages to other applications
3474 it must send the <literal>org.freedesktop.DBus.Hello</literal> message
3475 to the message bus to obtain a unique name. If an application without
3476 a unique name tries to send a message to another application, or a
3477 message to the message bus itself that isn't the
3478 <literal>org.freedesktop.DBus.Hello</literal> message, it will be
3479 disconnected from the bus.
3482 There is no corresponding "disconnect" request; if a client wishes to
3483 disconnect from the bus, it simply closes the socket (or other
3484 communication channel).
3487 <sect3 id="bus-messages-list-names">
3488 <title><literal>org.freedesktop.DBus.ListNames</literal></title>
3492 ARRAY of STRING ListNames ()
3499 <entry>Argument</entry>
3501 <entry>Description</entry>
3507 <entry>ARRAY of STRING</entry>
3508 <entry>Array of strings where each string is a bus name</entry>
3515 Returns a list of all currently-owned names on the bus.
3518 <sect3 id="bus-messages-list-activatable-names">
3519 <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
3523 ARRAY of STRING ListActivatableNames ()
3530 <entry>Argument</entry>
3532 <entry>Description</entry>
3538 <entry>ARRAY of STRING</entry>
3539 <entry>Array of strings where each string is a bus name</entry>
3546 Returns a list of all names that can be activated on the bus.
3549 <sect3 id="bus-messages-name-exists">
3550 <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
3554 BOOLEAN NameHasOwner (in STRING name)
3561 <entry>Argument</entry>
3563 <entry>Description</entry>
3569 <entry>STRING</entry>
3570 <entry>Name to check</entry>
3580 <entry>Argument</entry>
3582 <entry>Description</entry>
3588 <entry>BOOLEAN</entry>
3589 <entry>Return value, true if the name exists</entry>
3596 Checks if the specified name exists (currently has an owner).
3600 <sect3 id="bus-messages-name-owner-changed">
3601 <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
3605 NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
3612 <entry>Argument</entry>
3614 <entry>Description</entry>
3620 <entry>STRING</entry>
3621 <entry>Name with a new owner</entry>
3625 <entry>STRING</entry>
3626 <entry>Old owner or empty string if none</entry>
3630 <entry>STRING</entry>
3631 <entry>New owner or empty string if none</entry>
3638 This signal indicates that the owner of a name has changed.
3639 It's also the signal to use to detect the appearance of
3640 new names on the bus.
3643 <sect3 id="bus-messages-name-lost">
3644 <title><literal>org.freedesktop.DBus.NameLost</literal></title>
3648 NameLost (STRING name)
3655 <entry>Argument</entry>
3657 <entry>Description</entry>
3663 <entry>STRING</entry>
3664 <entry>Name which was lost</entry>
3671 This signal is sent to a specific application when it loses
3672 ownership of a name.
3676 <sect3 id="bus-messages-name-acquired">
3677 <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
3681 NameAcquired (STRING name)
3688 <entry>Argument</entry>
3690 <entry>Description</entry>
3696 <entry>STRING</entry>
3697 <entry>Name which was acquired</entry>
3704 This signal is sent to a specific application when it gains
3705 ownership of a name.
3709 <sect3 id="bus-messages-start-service-by-name">
3710 <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
3714 UINT32 StartServiceByName (in STRING name, in UINT32 flags)
3721 <entry>Argument</entry>
3723 <entry>Description</entry>
3729 <entry>STRING</entry>
3730 <entry>Name of the service to start</entry>
3734 <entry>UINT32</entry>
3735 <entry>Flags (currently not used)</entry>
3745 <entry>Argument</entry>
3747 <entry>Description</entry>
3753 <entry>UINT32</entry>
3754 <entry>Return value</entry>
3759 Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
3763 The return value can be one of the following values:
3768 <entry>Identifier</entry>
3769 <entry>Value</entry>
3770 <entry>Description</entry>
3775 <entry>DBUS_START_REPLY_SUCCESS</entry>
3777 <entry>The service was successfully started.</entry>
3780 <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
3782 <entry>A connection already owns the given name.</entry>
3791 <sect3 id="bus-messages-update-activation-environment">
3792 <title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
3796 UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
3803 <entry>Argument</entry>
3805 <entry>Description</entry>
3811 <entry>ARRAY of DICT<STRING,STRING></entry>
3812 <entry>Environment to add or update</entry>
3817 Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
3820 Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
3825 <sect3 id="bus-messages-get-name-owner">
3826 <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
3830 STRING GetNameOwner (in STRING name)
3837 <entry>Argument</entry>
3839 <entry>Description</entry>
3845 <entry>STRING</entry>
3846 <entry>Name to get the owner of</entry>
3856 <entry>Argument</entry>
3858 <entry>Description</entry>
3864 <entry>STRING</entry>
3865 <entry>Return value, a unique connection name</entry>
3870 Returns the unique connection name of the primary owner of the name
3871 given. If the requested name doesn't have an owner, returns a
3872 <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
3876 <sect3 id="bus-messages-get-connection-unix-user">
3877 <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
3881 UINT32 GetConnectionUnixUser (in STRING connection_name)
3888 <entry>Argument</entry>
3890 <entry>Description</entry>
3896 <entry>STRING</entry>
3897 <entry>Name of the connection to query</entry>
3907 <entry>Argument</entry>
3909 <entry>Description</entry>
3915 <entry>UINT32</entry>
3916 <entry>unix user id</entry>
3921 Returns the unix uid of the process connected to the server. If unable to
3922 determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
3927 <sect3 id="bus-messages-add-match">
3928 <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
3932 AddMatch (in STRING rule)
3939 <entry>Argument</entry>
3941 <entry>Description</entry>
3947 <entry>STRING</entry>
3948 <entry>Match rule to add to the connection</entry>
3953 Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
3954 If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
3958 <sect3 id="bus-messages-remove-match">
3959 <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
3963 RemoveMatch (in STRING rule)
3970 <entry>Argument</entry>
3972 <entry>Description</entry>
3978 <entry>STRING</entry>
3979 <entry>Match rule to remove from the connection</entry>
3984 Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
3985 If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
3990 <sect3 id="bus-messages-get-id">
3991 <title><literal>org.freedesktop.DBus.GetId</literal></title>
3995 GetId (out STRING id)
4002 <entry>Argument</entry>
4004 <entry>Description</entry>
4010 <entry>STRING</entry>
4011 <entry>Unique ID identifying the bus daemon</entry>
4016 Gets the unique ID of the bus. The unique ID here is shared among all addresses the
4017 bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
4018 <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
4019 ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
4020 There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
4021 by org.freedesktop.DBus.Peer.GetMachineId().
4022 For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
4030 <appendix id="implementation-notes">
4031 <title>Implementation notes</title>
4032 <sect1 id="implementation-notes-subsection">
4040 <glossary><title>Glossary</title>
4042 This glossary defines some of the terms used in this specification.
4045 <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
4048 The message bus maintains an association between names and
4049 connections. (Normally, there's one connection per application.) A
4050 bus name is simply an identifier used to locate connections. For
4051 example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
4052 name might be used to send a message to a screensaver from Yoyodyne
4053 Corporation. An application is said to <firstterm>own</firstterm> a
4054 name if the message bus has associated the application's connection
4055 with the name. Names may also have <firstterm>queued
4056 owners</firstterm> (see <xref linkend="term-queued-owner"/>).
4057 The bus assigns a unique name to each connection,
4058 see <xref linkend="term-unique-name"/>. Other names
4059 can be thought of as "well-known names" and are
4060 used to find applications that offer specific functionality.
4065 <glossentry id="term-message"><glossterm>Message</glossterm>
4068 A message is the atomic unit of communication via the D-Bus
4069 protocol. It consists of a <firstterm>header</firstterm> and a
4070 <firstterm>body</firstterm>; the body is made up of
4071 <firstterm>arguments</firstterm>.
4076 <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
4079 The message bus is a special application that forwards
4080 or routes messages between a group of applications
4081 connected to the message bus. It also manages
4082 <firstterm>names</firstterm> used for routing
4088 <glossentry id="term-name"><glossterm>Name</glossterm>
4091 See <xref linkend="term-bus-name"/>. "Name" may
4092 also be used to refer to some of the other names
4093 in D-Bus, such as interface names.
4098 <glossentry id="namespace"><glossterm>Namespace</glossterm>
4101 Used to prevent collisions when defining new interfaces or bus
4102 names. The convention used is the same one Java uses for defining
4103 classes: a reversed domain name.
4108 <glossentry id="term-object"><glossterm>Object</glossterm>
4111 Each application contains <firstterm>objects</firstterm>, which have
4112 <firstterm>interfaces</firstterm> and
4113 <firstterm>methods</firstterm>. Objects are referred to by a name,
4114 called a <firstterm>path</firstterm>.
4119 <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
4122 An application talking directly to another application, without going
4123 through a message bus. One-to-one connections may be "peer to peer" or
4124 "client to server." The D-Bus protocol has no concept of client
4125 vs. server after a connection has authenticated; the flow of messages
4126 is symmetrical (full duplex).
4131 <glossentry id="term-path"><glossterm>Path</glossterm>
4134 Object references (object names) in D-Bus are organized into a
4135 filesystem-style hierarchy, so each object is named by a path. As in
4136 LDAP, there's no difference between "files" and "directories"; a path
4137 can refer to an object, while still having child objects below it.
4142 <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
4145 Each bus name has a primary owner; messages sent to the name go to the
4146 primary owner. However, certain names also maintain a queue of
4147 secondary owners "waiting in the wings." If the primary owner releases
4148 the name, then the first secondary owner in the queue automatically
4149 becomes the new owner of the name.
4154 <glossentry id="term-service"><glossterm>Service</glossterm>
4157 A service is an executable that can be launched by the bus daemon.
4158 Services normally guarantee some particular features, for example they
4159 may guarantee that they will request a specific name such as
4160 "org.freedesktop.Screensaver", have a singleton object
4161 "/org/freedesktop/Application", and that object will implement the
4162 interface "org.freedesktop.ScreensaverControl".
4167 <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
4170 ".service files" tell the bus about service applications that can be
4171 launched (see <xref linkend="term-service"/>). Most importantly they
4172 provide a mapping from bus names to services that will request those
4173 names when they start up.
4178 <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
4181 The special name automatically assigned to each connection by the
4182 message bus. This name will never change owner, and will be unique
4183 (never reused during the lifetime of the message bus).
4184 It will begin with a ':' character.
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